Dr. Magdalena Lidia CIUREA

The gas sensitivity of field-effect structures with 2D-MoS2 channels selectively grown between Mo electrodes using the Mo-CVD method was investigated by measuring the effect of molecular adsorption from air on the device source-drain current (Isd). The channels were composed of interconnected atomically thin MoS2 grains, with their density and average thickness varied by choosing two different distances (15 and 20 mu m) between the Mo contacts. A high response to the tested stimuli, including molecule adsorption, illumination and gate voltage changes, was observed. A significant, persistent photoconduction was induced by positive charge accumulation on traps, most likely at grain boundaries and associated defects. Isd increased under high vacuum, both in the dark and under illumination. The relative dark current response to the transition from air to high vacuum reached up to 1000% at the turn-on voltage. When monitored during the gradual change in air pressure, Isd exhibited a non-monotonic function, sharply peaking at about 10-2 mbar, suggesting molecular adsorption on different defect sites and orientations of adsorbed H2O molecules, which were capable of inducing electron accumulation or depletion. Despite the screening of disorder by extra electrons, the #20 mu m sample remained more sensitive to air molecules on its surface. The high vacuum state was also investigated by annealing devices at temperatures up to 340 K in high vacuum, followed by measurements down to 100 K. This revealed thermally stimulated currents and activation energies of trapping electronic states assigned to sulfur vacancies (230 meV) and other shallow levels (85-120 meV), possibly due to natural impurities, grain boundaries or disorder defects. The results demonstrate the high sensitivity of these devices to molecular adsorption, making the technology promising for the easy fabrication of chemical sensors.

This study investigates the fabrication of short-wavelength infrared (SWIR) photosensitive amorphous and nanocrystalline Ge1-xSnx:H thin films by magnetron sputtering from separate Ge and Sn targets using different Ar: H mixing ratios as working gas. Amorphous Ge1-xSnx:H films have been obtained on both c-Si and fused quartz substrates at ambient temperature, while dynamic nanocrystallization occurs in-situ when the substrate temperature during deposition is raised to 200 degrees C. Fourier-transform infrared spectroscopy has shown the hydrogen incorporation by detecting an absorption line at 1873 cm(-1), close to the value corresponding to Ge-H bonding, only in the room temperature amorphous films. Based on that, we infer that the hydrogen concentration is very low in the films deposited at high temperature. The higher concentration of hydrogen in the amorphous samples is associated with an increase of the absorption gap to 0.5 eV compared to 0.3 eV in the 200 degrees C samples. In-situ (during deposition) and ex-situ (by subsequent rapid thermal annealing) nanocrystallization have been analyzed by high-resolution transmission electron microscopy, X-ray diffraction and micro-Raman spectroscopy. SWIR spectral photosensitivity up to 2.4 mu m was found to be more than two orders of magnitude improved in hydrogenated amorphous films with high hydrogen content, compared to the nanocrystalline ones that are weakly hydrogenated. These findings demonstrate the potential of hydrogenation to enhance the photoelectric properties of GeSn sputtering films for optoelectronic SWIR infrared applications.

The development of new materials for short-wavelength infrared (SWIR) optical sensors is of high importance for the fast development of different applications, as, for example, Internet of Things, road safety, and pollution monitoring. Group IV SiGe provides more sustainable As-, Cd-, and Pb-free nanomaterials that are cheaper and ecologic and offer easy integration with CMOS technology. This Review is on Ge and SiGe quantum dots/nanocrystals (QDs/NCs) embedded in dielectrics for VIS-SWIR photodetection, in which we highlight and discuss photocurrent mechanisms, correlation of photodetection parameters and characteristics with crystalline structure, morphology and energy bandgap, and applications as photodetectors, optical sensors, phototransistors, and solar cells. The embedding matrix induces NC surface passivation, stress field, and nanocrystallization effects and brings specific advantages depending on the matrix material. SiGe NCs in oxides for VIS-SWIR sensing represents a niche domain, showing high photosensitivity (photocurrent) in SWIR up to 1.8 mu m at room temperature and 2 mu m at 100 K, deeper in SWIR than Ge. By alloying Ge with a small content of Si, NC thermal stability is much improved as the detrimental Ge fast diffusion in oxides is hindered and SWIR photosensing is enhanced due to light absorption in Ge-rich SiGe NCs.

Selective growth of 2D MoS2 layers on patterned substrates is highly desired for easy fabrication of devices. Selectively grown 2D MoS2 on Mo patterned substrates for the formation of intimate metallic contact was obtained by a Mo-CVD method in which MoO2 from an oxidized Mo pattern and S powder are the growth precursors. Mo films were deposited by magnetron sputtering on SiO2(300 nm)/c-Si substrates and patterned by photolithography techniques for obtaining Mo strips and finger contact structures, with the gap between the strips and finger varied from 5 to 20 mu m. The filling of the gap by selectively grown atomically thin MoS2 plates of 1-2 monolayers (MLs) was demonstrated by scanning electron microscopy and atomic force microscopy imaging. Field effect devices for the characterization of the photosensitivity of selectively grown MoS2 have been fabricated from finger contact structures. The dark current is drastically reduced from 10(-9) to 10(-13)-10(-14) A by varying the gate voltage from +7 to -7 V, showing the n-type semiconductor behavior of the selectively grown 2D MoS2. High photosensitivity of 10(5) (%) was obtained for 4.5 x 10(-4) mW/cm(2) at 650 nm wavelength illumination. The spectral responsivity reaches values of 15-25 A/W at 600 nm wavelength and shows an energy onset of 1.72-1.77 eV corresponding to about 2 ML MoS2. The carrier-trapping effect responsible for the slow part of the device response can be caused by structural defects and also by adsorbed molecules like in gas sensors.

We study the dependence of photo-spectral intensity on tri- and multilayers of SiO2/[SiGe [ dSiGe]/SiO2]N with repetitions N = 1 to 10 and thicknesses dSiGe=5-100 nm. Photocurrent analysis reveals a bimodal spectral feature. A comparison of the photocurrent analysis between tri- and multilayers shows that in the multilayer structures, the photo-spectral intensity increases with increasing repetition N. The change in intensity could then be further tuned by changing the thickness of the SiGe layers dSiGe. We attribute the change in intensity to an increase in tensile strain, along with increased Ge atomic concentration and reduced SiGe-nano cluster size.

SiGeSn nanocrystals (NCs) in oxides are of considerable interest for photo-effect applications due to the fine-tuning of the optical bandgap by quantum confinement in NCs. We present a detailed study regarding the silicon germanium tin (SiGeSn) NCs embedded in a nanocrystalline hafnium oxide (HfO2) matrix fabricated by using magnetron co-sputtering deposition at room temperature and rapid thermal annealing (RTA). The NCs were formed at temperatures in the range of 500-800 degrees C. RTA was performed to obtain SiGeSn NCs with surfaces passivated by the embedding HfO2 matrix. The formation of NCs and beta-Sn segregation were discussed in relation to the deposition and processing conditions by employing HRTEM, XRD and Raman spectroscopy studies. The spectral photosensitivity exhibited up to 2000 nm in short-wavelength infrared (SWIR) depending on the Sn composition was obtained. Comparing to similar results on GeSn NCs in SiO2 matrix, the addition of Si offers a better thermal stability of SiGeSn NCs, while the use of HfO2 matrix results in better passivation of NCs increasing the SWIR photosensitivity at room temperature. These results suggest that SiGeSn NCs embedded in an HfO2 matrix are a promising material for SWIR optoelectronic devices.

For the development of memory devices for the continuous advancement of IT engineering, a good understanding of the charging-discharging mechanisms in nanocrystalline floating gate memories is crucial to overcoming the current limitations. The charging-discharging mechanism in Al2O3/Ge/Al2O3 trilayer memory structures obtained by magnetron sputtering deposition is investigated as a function of the postdeposition annealing temperature, up to 900 degrees C. The change by annealing of C-V hysteresis curves from a clockwise type at low temperatures to counterclockwise one in a sample annealed within the intermediary temperature range of 550 to 650 degrees C, and then, a return to a clockwise type for annealing within the higher temperature range of 800-900 degrees C was observed. Up to 700 degrees C, memory performances are constantly improved reaching for 600 degrees C annealed samples, a memory window of 5.6 V for voltage sweep in the range -1 to +15 V, and good retention characteristics for 650 degrees C annealed structures, in which the charge loss is only similar to 2% after 10(8) s. When the annealing temperature was increased above 700 degrees C, a rapid decrease in the memory performance takes place. The annealing-induced changes are explained based on the Ge fast diffusion and nanocrystallization process, in correlation with morphological and structural high-resolution transmission electron microscopy results.

In this paper, short-wave infrared (SWIR) photosensing of GeSn-HfO2 films with small Si amount is studied in correlation with structure and composition of films. SiGeSnHfO2 films are deposited by magnetron sputtering and nanostructured by subsequent rapid thermal annealing. XRD and Raman spectroscopy investigations are carried out revealing the SiGeSn nanocrystallization in annealed films. Spectral responsivity shows enhanced sensitivity up to 2 mu m due to SiGeSn nanocrystals (NCs) and clusters with contribution from disorder.

SiGe-SiO2-based structures present high interest for their high photosensitivity from visible to short-wavelength infrared. Herein, two postdeposition annealing procedures, that is, rapid thermal annealing (RTA) and rapid-like furnace annealing (FA), are compared. Both RTA and FA are performed at 600 degrees C for 1 min for SiGe nanocrystals (NCs) formation in SiO2 matrix in Si/SiO2/SiGe/SiO2 structures deposited by magnetron sputtering. The FA imitates RTA resulting in enhanced spectral response. X-ray diffraction, transmission electron microscopy, and Raman spectroscopy are carried out showing Ge-rich SiGe NCs with 11.3 +/- 1.2 nm size for RTA and 9.4 +/- 0.8 nm for FA. Photocurrent spectra for both structures show several peaks that are annealing dependent. The photocurrent intensity for FA samples is approximate to 7 times higher than RTA samples while cutoff wavelengths are slightly different, that is, 1365 nm for FA and 1375 nm for RTA. The FA structures show (at -1.5 V) over 4 A W-1 responsivity at 730 nm, 6.4 x 10(7) Jones detectivity at 735 nm, and 2.2 x 10(7) Jones at about 1210 nm. FA structures contain small SiGe NCs with incorporated residual strain, while RTA ones are formed of columnar SiGe NCs separated by SiGeOx amorphous regions and show increased tensile strain in the SiGe.

This study examines the opto-electric characteristics of Ge:SiO2 composite films produced via magnetron sputtering at substrate temperatures of 300 degrees C, 400 degrees C, and 500 degrees C, with varying Ge concentrations. We employed x-ray diffraction, current-voltage measurements, and spectral photocurrent analysis to investigate the films structural, optical, and opto-electrical properties. Illumination of the samples resulted in a marked increase in current compared to dark conditions. Spectral photocurrent measurements revealed cutoff wavelengths of 1300 nm for films with 25:75 vol% Ge:SiO2 ratio, extending to 1320 nm for compositions with higher Ge content (60:40 vol%). These findings align with observations from I-V curve analyses. Our research highlights the potential of Ge:SiO2 composites for enhancing optoelectronic device performance. The results underscore the importance of continued investigation and innovative applications in this field to drive technological advancements.

Group IV quantum dots (QDs) in HfO2 are attractive for non-volatile memories (NVMs) due to complementary metal-oxide semiconductor (CMOS) compatibility. Besides the role of charge storage centers, SiGeSn QDs have the advantage of a low thermal budget for formation, because Sn presence decreases crystallization temperature, while Si ensures higher thermal stability. In this paper, we prepare MOS capacitors based on 3-layer stacks of gate HfO2/floating gate of SiGeSn QDs in HfO2/tunnel HfO2/p-Si obtained by magnetron sputtering deposition followed by rapid thermal annealing (RTA) for nanocrystallization. Crystalline structure, morphology, and composition studies by cross-section transmission electron microscopy and X-ray diffraction correlated with Raman spectroscopy and C-V measurements are carried out for understanding RTA temperature effects on charge storage behavior. 3-layer morphology and Sn content trends with RTA temperature are explained by the strongly temperature-dependent Sn segregation and diffusion processes. We show that the memory properties measured on Al/3-layer stack/p-Si/Al capacitors are controlled by SiGeSn-related trapping states (deep electronic levels) and low-ordering clusters for RTA at 325-450 degrees C, and by crystalline SiGeSn QDs for 520 and 530 degrees C RTA. Specific to the structures annealed at 520 and 530 degrees C is the formation of two kinds of crystalline SiGeSn QDs, i.e., QDs with low Sn content (2 at.%) that are positioned inside the floating gate, and QDs with high Sn content (up to 12.5 at.%) located at the interface of floating gate with adjacent HfO2 layers. The presence of Sn in the SiGe intermediate layer decreases the SiGe crystallization temperature and induces the easier crystallization of the diamond structure in comparison with 3-layer stacks with Ge-HfO2 intermediate layer. High frequency-independent memory windows of 3-4 V and stored electron densities of 1-2 x 10(13) electrons/cm(2) are achieved.

The high-k-based MOS-like capacitors are a promising approach for the domain of non-volatile memory devices, which currently is limited by SiO2 technology and cannot face the rapid downsizing of the electronic device trend. In this paper, we prepare MOS-like trilayer memory structures based on high-k ZrO2 by magnetron sputtering, with a 5% and a 10% concentrations of Zr in the Zr-ZrO2 floating gate layer. For crystallization of the memory structure, rapid thermal annealing at different temperatures between 500 degrees C and 700 degrees C was performed. Additionally, Al electrodes were deposited in a top-down configuration. High-resolution transmission electron microscopy reveals that ZrO2 has a polycrystalline-columnar crystallization and a tetragonal crystalline structure, which was confirmed by X-ray diffraction measurements. It is shown that the tetragonal phase is stabilized during the crystallization by the fast diffusion of oxygen atoms. The capacitance-voltage characteristics show that the widest memory window (Delta V = 2.23 V) was obtained for samples with 10% Zr annealed at 700 degrees C for 4 min. The charge retention characteristics show a capacitance decrease of 36% after 10 years.

We present an array of 225 field-effect transistors (FETs), where each of them has a graphene monolayer channel grown on a 3-layer deposited stack of 22 nm control HfO2/5 nm Ge-HfO2 intermediate layer/8 nm tunnel HfO2/p-Si substrate. The intermediate layer is ferroelectric and acts as a floating gate. All transistors have two top gates, while the p-Si substrate is acting as a back gate. We show that these FETs are acting memtransistors, working as two-input reconfigurable logic gates with memory, the type of the logic gate depending only on the values of the applied gate voltages and the choice of a threshold current.

Orthorhombic HfO2 exhibits nanoscale ferroelectricity that opens the perspective of ultra-scalable CMOS integration of ferroelectric memories. However, many aspects of the metastable orthorhombic crystallization mechanisms still need to be elucidated and new fabrication methods are of high interest. In this paper, the atomically resolved crystal structure of HfO2 is a 3-layer structure with a Ge-rich HfO2 intermediate layer capped by a top (cap) HfO2 layer and cladded by a bottom HfO2 layer. There is a continuity of crystal growth from the top and bottom HfO2 layers into the intermediate layer. A spatial transition from a monoclinic phase to an orthorhombic phase was revealed within a region of a few atomic layers at the interface between capped and intermediate HfO2 layers. This result suggests the mechanism of orthorhombic and monoclinic phase formation by a martensitic-like transformation of the initially grown tetragonal phase. The sample fabrication method we used involved magnetron sputtering deposition of the 3-layer structures, i.e. a stack of top HfO2/Ge-rich HfO2 intermediate/bottom HfO2 layers, followed by rapid thermal annealing. It results in self-optimized orthorhombic crystallization of HfO2 by Ge nanoparticle segregation in the intermediate layer. The ferroelectric effects are revealed by polarization-voltage hysteresis loops and piezoresponse force microscopy measurements. The atomistic computations performed by using the density functional theory support the experimental results by showing that the Ge doping of HfO2 leads to orthorhombic phase stabilization and increased Berry phase polarization.

Group IV nanocrystals (NCs), in particular from the Si-Ge system, are of high interest for Si photonics applications. Ge-rich SiGe NCs embedded in nanocrystallized HfO2 were obtained by magnetron sputtering deposition followed by rapid thermal annealing at 600 & DEG;C for nanostructuring. The complex characterization of morphology and crystalline structure by X-ray diffraction, mu-Raman spectroscopy, and cross-section transmission electron microscopy evidenced the formation of Ge-rich SiGe NCs (3-7 nm diameter) in a matrix of nanocrystallized HfO2. For avoiding the fast diffusion of Ge, the layer containing SiGe NCs was cladded by very thin top and bottom pure HfO2 layers. Nanocrystallized HfO2 with tetragonal/orthorhombic structure was revealed beside the monoclinic phase in both buffer HfO2 and SiGe NCs-HfO2 layers. In the top part, the film is mainly crystallized in the monoclinic phase. High efficiency of the photocurrent was obtained in a broad spectral range of curves of 600-2000 nm at low temperatures. The high-quality SiGe NC/HfO2 matrix interface together with the strain induced in SiGe NCs by nanocrystallization of both HfO2 matrix and SiGe nanoparticles explain the unexpectedly extended photoelectric sensitivity in short-wave infrared up to about 2000 nm that is more than the sensitivity limit for Ge, in spite of the increase of bandgap by well-known quantum confinement effect in SiGe NCs.

In this paper, we report studies on Al2O3/Ge/Al2O3 trilayer memory structures deposited by magnetron sputtering at room temperature on p-Si substrates coated with 3 nm SiO2. The changes of the structure, morphology and memory properties induced by rapid thermal annealing (RTA) in a broad temperature range 550-900 degrees C have been carefully investigated. High resolution transmission electron microscopy (HRTEM) revealed the existence of distinct RTA effects for different temperature ranges, in correlation with memory properties measured on Al/Al2O3/Ge/Al2O3/SiO2/p-Si/Al devices. Thus, at temperatures smaller than 650 degrees C, Ge diffuses into adjacent Al2O3, the layers remaining amorphous. The memory window increases from as-deposited samples to those annealed at 600 degrees C reaching the maximum of 5.4 V. After RTA at 700 degrees C, Ge nanocrystals (NCs) in intermediate Ge layer and Ge-rich amorphous nanoparticles in Al2O3 tunnel oxide are formed. Increasing RTA temperature to 800 and 900 degrees C, Ge NCs are no longer formed due to Ge strong diffusion. Instead, Ge-rich mixed GeAl oxide NCs of unknown crystalline structure are evidenced by HRTEM. The memory window continuously decreases with annealing temperature in the range 650-900 degrees C. The ON (OFF) charge loss of only 11% (9.8%) was found by extrapolation to 10 years.

We present a detailed study regarding the bandgap dependence on diameter and composition of spherical Ge-rich GexSi1-x nanocrystals (NCs). For this, we conducted a series of atomistic density functional theory (DFT) calculations on H-passivated NCs of Ge-rich GeSi random alloys, with Ge atomic concentration varied from 50 to 100% and diameters ranging from 1 to 4 nm. As a result of the dominant confinement effect in the DFT computations, a composition invariance of the line shape of the bandgap diameter dependence was found for the entire computation range, the curves being shifted for different Ge concentrations by Delta E(eV)=0.651(1-x). The shape of the dependence of NCs bandgap on the diameter is well described by a power function 4.58/d(1.25) for 2-4 nm diameter range, while for smaller diameters, there is a tendency to limit the bandgap to a finite value. By H-passivation of the NC surface, the effect of surface states near the band edges is excluded aiming to accurately determine the NC bandgap. The number of H atoms necessary to fully passivate the spherical GexSi1-x NC surface reaches the total number atoms of the Ge+Si core for smallest NCs and still remains about 25% from total number of atoms for bigger NC diameters of 4 nm. The findings are in line with existing theoretical and experimental published data on pure Ge NCs and allow the evaluation of the GeSi NCs behavior required by desired optical sensor applications for which there is a lack of DFT simulation data in literature.

We investigate the photoluminescence properties of structures comprising of Si1-xGex nanoparticles (NPs) within SiO2, GeO2, TiO2 and Ta2O5 oxide matrices. Of the investigated structures, it was observed that the structures with GeO2 and TiO2 matrices provide increased spectral response (at similar to 907 and 844 nm respectively) and increased PL intensity. The improved PL characteristic have been attributed to increased diffusion barrier against oxygen which otherwise would result in formation of unwanted oxide at the film-oxide interface, thereby deteriorating the optical properties.

Deposition of Ge nanoparticles in Si3N4 films by heating Si and quartz substrates at 500 degrees C were obtained using co-sputtering Ge, and Si3N4. Their structure and photo-electrical behaviour were investigated by transmission electron microscopy, current - voltage and spectral photo-current investigations, respectively. The spectral photoresponse were correlated with microscopy results. Depending on the measuring temperature, the current under illumination increases with about five orders of magnitude compared with the dark one. The photo-current spectra measured in photovoltaic regime and at -1 V show a single cut-off wavelength in near infrared domain at about 1362 nm.

The memory properties of a trilayer structure of ZrO2/Ge-ZrO2/ZrO2/Si-p were investigated. The trilayer was prepared by magnetron sputtering deposition followed by a rapid thermal annealing process for obtaining Ge nanocrystals embedded in ZrO2 matrix. The X-ray diffraction patterns obtained on annealed structures show that both trilayer structures are cristallized, with narrow diffraction peaks. Ge as a separated phase is not evidenced in the diffractograms, instead is possible that Ge atoms enters in ZrO2 lattice structure and form a compound of ZrO3GeO8 crystallized in the tetragonal phase. In the case of ZrO2 control structure, the diffractograms show that the ZrO2 layer is crystallized in tetragonal phase. The memory properties are evidenced by C-V characteristics with counterclockwise hysteresis loop and a memory window of Delta V=1.1V for the structure annealed at 570 degrees C and Delta V=0.8 V for the structure annealed at 650 degrees C. The influence of the interface SiO2 layer on the frequency dependence of capacitance was evidenced by C-f characteristics.

The development of short-wave infrared (SWIR) photonics based on GeSn alloys is of high technological interest for many application fields, such as the Internet of things or pollution monitoring. The manufacture of crystalline GeSn is a major challenge, mainly because of the low miscibility of Ge and Sn. The use of embedded GeSn nanocrystals (NCs) by magnetron sputtering is a cost-effective and efficient method to relax the growth conditions. We report on the use of GeSn/SiO2 multilayer deposition as a way to control the NC size and their insulation. The in situ prenucleation of NCs during deposition was followed by ex situ rapid thermal annealing. The nanocrystallization of 20X(11nm_Ge0.865S0.135/1.5nm_SiO2) multilayers leads to formation of GeSn NCs with similar to 16% Sn concentration and similar to 9 nm size. Formation of GeSn domes that are vertically correlated contributes to the nanocrystallization process. The absorption limit of similar to 0.4 eV in SWIR found by ellipsometry is in agreement with the spectral photosensitivity. The ITO/20x(GeSn NC/SiO2)/p-Si/Al diodes show a maximum value of the SWIR photosensitivity at a reverse voltage of 0.5 V, with extended sensitivity to wavelengths longer than 2200 nm. The multilayer diodes have higher photocurrent efficiency compared to diodes based on a thick monolayer of GeSn NCs.

The 1600 nm-extended SWIR photoresponse of SiGe/TiO2 multilayers with Ge-rich SiGe nanocrystals (NCs) is demonstrated. The SiGe NCs based multilayers are obtained by magnetron sputtering deposition of TiO2/ 6x(Ge/SiGe/Ge/TiO2) layers on heated p-Si substrate followed by rapid thermal annealing (RTA). Grazing incidence X-ray diffraction and Raman spectroscopy evidence the formation of cubic Ge-rich SiGe NCs and anatase TiO2. ITO/Ge-rich SiGe NCs based multilayer /p-Si heterostructure diodes, fabricated by depositing top ITO and bottom Al contacts, show n-p behavior. Photocurrent-voltage characteristics measured at 100 K under integral light illumination of reverse biased diode present a photocurrent higher with up to 2 orders of magnitude than the dark current. Spectral photocurrent increases with bias voltage increase and presents a bandgap-related cutoff wavelength of similar to 1600 nm due to the high Ge content of SiGe NCs.

In this paper we report a set of experiments at the wafer level regarding field-effect transistors with a graphene monolayer channel transferred on the ferroelectric HfO2/Ge-HfO2/HfO(2)three-layer structure. This kind of transistor has a switching ratio of 10(3)between on and off states due to the bandgap in graphene induced by the ferroelectric structure. Both top and back gates effectively control the carriers' charge flow in graphene. The transistor acts as a three-terminal memristor, termed a memtransistor, with applications in neuromorphic computation.

Formation of SiGe nanocrystals in an oxide matrix via deposition and subsequent annealing is a widely applied approach as it gives good control over optical properties by varying the Ge atomic fraction, the size, shape and crystallinity of the nanocrystals. A common drawback of annealing is a strain relaxation in the structure creating dislocations, point defects, dangling bonds, Ge clustering and altered interface morphology. All these phenomena are well-known to degrade the optoelectronic and electrical properties of the structure. As a proof of concept, in this study we have utilized a modern technique of high impulse power magnetron sputtering (HiPIMS) to obtain a crystalline TiO2/SiGe/TiO2 structure without any pre-/post-annealing. It is furthermore demonstrated how a control of the nano-crystallite size is obtained by altering the HiPIMS discharge power alone. Grazing incidence X-ray diffraction analysis was carried out for the structural characterization, while photocurrent measurements were utilized to access the role of TiO2 structural morphology over interface integrity in determining spectral feature and sensitivity. An increase of 1 - 2 orders magnitude in spectral intensity was achieved for as-grown structures fabricated via HiPIMS in comparison to annealed structure, sputtered with conventional direct current magnetron sputtering.

Films of SiGe nanocrystals (NCs) in oxide have the advantage of tuning the energy band gap by adjusting SiGe NC s composition and size. In this study, SiGe-SiO2 amorphous films were deposited by magnetron sputtering on Si substrate followed by rapid thermal annealing at 700, 800 and 1000 degrees C. We investigated films with Si:Ge:SiO2 compositions of 25:25:50 vol.% and 5:45:50 vol.%. TEM investigations reveal the major changes in films morphology (SiGe NCs with different sizes and densities) produced by Si:Ge ratio and annealing temperature. XPS also show that the film depth profile of SiGe content is dependent on the annealing temperature. These changes strongly influence electrical and photoconduction properties. Depending on annealing temperature and Si:Ge ratio, photocurrents can be 10(3) times higher than dark currents. The photocurrent cutoff wavelength obtained on samples with 25:25 vol% SiGe ratio decreases with annealing temperature increase from 1260 nm in SWIR for 700 degrees C annealed films to 1210 nm for those at 1000 degrees C. By increasing Ge content in SiGe (5:45 vol%) the cutoff wavelength significantly shifts to 1345 nm (800 degrees C annealing). By performing measurements at 100 K, the cutoff wavelength extends in SWIR to 1630 nm having high photoresponsivity of 9.35 AW(-1).

Continuous development of Si photonics requires ecological and cost-effective materials. In this work, SiGe nanocrystals (NCs) embedded in TiO2 are investigated as a photosensitive material for visible (VIS) to short-wave infrared (SWIR) broad-range detection. The TiO2 matrix has the advantage of a lower band gap than SiO2, facilitating transport of photogenerated carriers in NCs. The advantage of SiGe NCs over Ge NCs is emphasized by elucidating the mechanisms involved in rapid thermal annealing (RTA)-induced nanocrystallization. An efficiently increased NC stabilization is achieved by avoiding the detrimental fast Ge diffusion. For this, the structure, morphology, and composition were carefully characterized by high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and Raman spectroscopy. Two types of structures were investigated, a film of SiGe-TiO2 alloy and a multilayer of a stack of six SiGe/TiO2 pairs. The layers have been deposited on Si wafers using magnetron sputtering of Si, Ge, and TiO2 followed by RTA in an inert atmosphere. The stabilization of SiGe NCs is achieved by the formation during RTA of protective SiO2 thin layers through Si oxidation at the SiGe NC surface, acting as a barrier for Ge diffusion. Thus, embedded Ge-rich SiGe NCs are obtained, resulting in the SWIR extension of the spectral photocurrent up to 1700 nm for films and 1600 nm for multilayers. This study has shown that in multilayers, the local anisotropy of crystallization is compensated by the stress field developed in the SiGe lattice, highly visible in the bottom part. Also, SiGe crystallizes faster than TiO2 in the rutile phase, and therefore, TiO2 remains mainly amorphous.

GeSn alloys have the potential of extending the Si photonics functionality in shortwave infrared (SWIR) light emission and detection. Epitaxial GeSn layers were deposited on a relaxed Ge buffer on Si(100) wafer by using high power impulse magnetron sputtering (HiPI-MS). Detailed X-ray reciprocal space mapping and HRTEM investigations indicate higher crystalline quality of GeSn epitaxial layers deposited by Ge HiPI-MS compared to commonly used radio frequency magnetron sputtering (RF-MS). To obtain a rectifying heterostructure for SWIR light detection, a layer of GeSn nanocrystals (NCs) embedded in oxide was deposited on the epitaxial GeSn one. Embedded GeSn NCs are obtained by cosputtering deposition of (Ge1-xSnx)(1-y)(SiO2)(y) layers and subsequent rapid thermal annealing at a low temperature of 400 degrees C. Intrinsic GeSn structural defects give p-type behavior, while the presence of oxygen leads to the n-character of the embedded GeSn NCs. Such an embedded NCs/epitaxial GeSn p-n heterostructure shows superior photoelectrical response up to 3 orders of magnitude increase in the 1.2-2.5 mu m range, as compared to performances of diode based only on embedded NCs.

One of the key elements in assessing traffic safety on the roads is the detection of asphalt conditions. In this paper, we propose an optical sensor based on GeSi nanocrystals embedded in SiO2 matrix that discriminates between different slippery road conditions (wet and icy asphalt and asphalt covered with dirty ice) in respect to dry asphalt. The sensor is fabricated by magnetron sputtering deposition followed by rapid thermal annealing. The photodetector has spectral sensitivity in the 360-1350 nm range and the signal-noise ratio is 10(2)-10(3). The working principle of sensor setup for detection of road conditions is based on the photoresponse (photocurrent) of the sensor under illumination with the light reflected from the asphalt having different reflection coefficients for dry, wet, icy and dirty ice coatings. For this, the asphalt is illuminated sequentially with 980 and 1064 nm laser diodes. A database of these photocurrents is obtained for the different road conditions. We show that the use of both k-nearest neighbor and artificial neural networks classification algorithms enables a more accurate recognition of the class corresponding to a specific road state than in the case of using only one algorithm. This is achieved by comparing the new output sensor data with previously classified data for each algorithm and then by performing an intersection of the algorithms' results.

SiGe nanoparticles dispersed in a dielectric matrix exhibit properties different from those of bulk and have shown great potential in devices for application in advanced optoelectronics. Annealing is a common fabrication step used to increase crystallinity and to form nanoparticles in such a system. A frequent downside of such annealing treatment is the formation of insulating SiO2 layer at the matrix/SiGe interface, degrading the optical properties of the structure. An annealing process that could bypass this downside would therefore be of great interest. In this work, a short-time furnace annealing of a SiGe/TiO2 system is applied to obtain SiGe nanoparticles without formation of the undesired SiO2 layer between the dielectric matrix (TiO2) and SiGe. The structures were prepared by depositing alternate layers of TiO2 and SiGe films, using direct-current magnetron sputtering technique. A wide range spectral response with a response-threshold up to similar to 1300 nm was obtained, accompanied with an increase in photo-response of more than two-orders of magnitude. Scanning electron microscopy, transmission electron microscopy, energy-dispersive x-ray spectroscopy and grazing incidence x-ray diffraction were used to analyze the morphological changes in respective structures. Photoconductive properties were studied by measuring photocurrent spectra using applied dc-voltages at various temperatures.

Detection in short-wave infrared (SWIR) has become a very stringent technology requirement for developing fields like hyperspectral imaging or climate changes. In a market dominated by III-V materials, GeSn, a Si compatible semiconductor, has the advantage of cost efficiency and inerrability by using the mature Si technology. Despite the recent progress in material growth, the easy fabrication of crystalline GeSn still remains a major challenge, and different methods are under investigation. We present the formation of GeSn nanocrystals (NCs) embedded in oxide matrix and their SWIR characterization. The simple and cost-effective fabrication method is based on thermal treatment of amorphous (Ge1-xSnx)(y)(SiO2)(1-y) layers deposited by magnetron sputtering. The nanocrystallization for Ge1-xSnx with 9-22 at. % Sn composition in SiO2 matrix with 9% to 15% mole percent was studied under low thermal budget annealing in the 350-450 degrees C temperature range. While the Sn at.% content is the main parameter influencing the band-structure of the NCs, the SWIR sensitivity can be optimized by SiO2 content and H-2 gas component in the deposition atmosphere. Their role is not only changing the crystallization parameters but also to reduce the carrier recombination by passivation of NCs defects. The experiments indicate a limited composition dependent temperature range for GeSn NCs formation before beta-Sn phase segregation occurs. NCs with an average size of 6 nm are uniformly distributed in the film, except the surface region where larger GeSn NCs are formed. Spectral photovoltaic current measured on SiO2 embedded GeSn NCs deposited on p-Si substrate shows extended SWIR sensitivity up to 2.4 mu m for 15 at. % Sn in GeSn NCs. The large extension of the SWIR detection is a result of many factors related to the growth parameters and also to the in situ or ex situ annealing procedures that influence the uniformity and size distribution of NCs.

Films of amorphous Ge nanoparticles in Si3N4 on heated Si and quartz substrates at 300 degrees C were obtained by co-sputtering Ge, and Si3N4. The films structure and photo-electrical behaviour were studied through transmission electron microscopy and, current voltage and spectral photo-current investigations, respectively. The spectral photo-current were correlated with results obtained from transmission electron microscopy. Under illumination the current present a high increase with about one order of magnitude compared with the dark one. The photo-current spectra show a widening in near infrared to 0.97eV. Internal quantum efficiency values for -1V and 0V were determined.

The VIS-SWIR photosensing properties of Ge and GeSi NCs embedded in TiO2 films are investigated. For this, we deposit GeTiO2 and GeSiTiO2 films, respectively by magnetron sputtering and then we perform rapid thermal annealing (RTA) for Ge NCs and GeSi NCs formation, respectively. Raman studies and spectral photocurrent measurements were carried out. Ge NCs formation is evidenced in the Raman spectrum of GeTiO2 film annealed at 550 degrees C. The photocurrent spectra measured on the Ge NCs-TiO2 film present four peaks separated by deconvolution. The broad peaks at similar to 700, 890, 1010 nm are due to photo-effects in the Ge NCs-TiO2 film. More than that, the photocurrent increases exponentially with the increase of bias voltage. The cut-off wavelength is similar to 1240 nm. We achieve the extension of the photosensitivity limit to similar to 1310 nm in GeSi NCs-TiO2 films (800 degrees C RTA).

Multilayer structures comprising of SiO2/SiGe/SiO2 and containing SiGe nanoparticles were obtained by depositing SiO2 layers using reactive direct current magnetron sputtering (dcMS), whereas, Si and Ge were co-sputtered using dcMS and high-power impulse magnetron sputtering (HiPIMS). The as-grown structures subsequently underwent rapid thermal annealing (550-900 degrees C for 1 min) in N-2 ambient atmosphere. The structures were investigated using X-ray diffraction, high-resolution transmission electron microscopy together with spectral photocurrent measurements, to explore structural changes and corresponding properties. It is observed that the employment of HiPIMS facilitates the formation of SiGe nanoparticles (2.1 +/- 0.8 nm) in the as-grown structure, and that presence of such nanoparticles acts as a seed for heterogeneous nucleation, which upon annealing results in the periodically arranged columnar self-assembly of SiGe core-shell nanocrystals. An increase in photocurrent intensity by more than an order of magnitude was achieved by annealing. Furthermore, a detailed discussion is provided on strain development within the structures, the consequential interface characteristics and its effect on the photocurrent spectra.

In this work we prepared films of amorphous germanium nanoparticles embedded in SiO2 deposited by magnetron sputtering on Si and quartz heated substrates at 300, 400 and 500 degrees C. Structure, morphology, optical, electrical and photoconduction properties of all films were investigated. The Ge concentration in the depth of the films is strongly dependent on the deposition temperature. In the films deposited at 300 degrees C, the Ge content is constant in the depth, while films deposited at 500 degrees C show a significant decrease of Ge content from interface of the film with substrate towards the film free surface. From the absorption curves we obtained the Ge band gap of 1.39 eV for 300 degrees C deposited films and 1.44 eV for the films deposited at 500 degrees C. The photocurrents are higher with more than one order of magnitude than the dark ones. The photocurrent spectra present different cutoff wavelengths depending on the deposition temperature, i.e. 1325 nm for 300 degrees C and 1267 nm for 500 degrees C. These films present good responsivities of 2.42 AW(-1) (52 mu W incident power) at 300 degrees C and 0.69 AW(-1) (57 mW) at 500 degrees C and high internal quantum efficiency of similar to 445% for 300 degrees C and similar to 118% for 500 degrees C.

We investigate the effect of room-temperature hydrogen-plasma treatment on the photoconductivity of SiGe nanoparticles sandwiched within SiO2 layers. An increase in photocurrent intensity of more than an order magnitude is observed after the hydrogen plasma treatment. The enhancement is attributed to neutralization of dangling bonds at the nanoparticles and to passivation of nonradiative defects in the oxide matrix and at SiGe/matrix interfaces. We find that increasing the partial pressure of hydrogen to pressures where H-3(+) and H-2(+) were the dominant ions results in increased photocurrent.

Trilayer memory capacitors of control HfO2/floating gate of Ge nanoparticles in HfO2/tunnel HfO2/Si substrate deposited by magnetron sputtering and subsequently annealed are investigated for the first time for applications in radiation dosimetry. In the floating gate (FG), amorphous Ge nanoparticles (NPs) are arranged in two rows inside the HfO2 matrix. The HfO2 matrix is formed of orthorhombic/tetragonal nanocrystals (NCs). The adjacent thin films to the FG are also formed of orthorhombic/tetragonal HfO2 NCs. This phase is formed during annealing, in samples with thick control HfO2, in the presence of Ge, being induced by the stress. In the rest of the control oxide, HfO2 NCs are monoclinic. Orthorhombic HfO2 has ferroelectric properties and therefore enhances the memory window produced by charge storage in Ge NPs to above 6 V. The high sensitivity of 0.8 mV Gy(-1) to a particle irradiation from a Am-241 source was measured by monitoring the flatband potential during radiation exposure after electrical writing of the memory.

Photosensitive films based on finely dispersed semiconductor nanocrystals (NCs) in dielectric films have great potential for sensor applications. Here we report on preparation and characterization of photosensitive Si1-xGex NCs sandwiched between SiO2 matrix. A radio-frequency magnetron sputtering was applied to obtain a multilayer-structures (MLs) by depositing SiO2/SiGe/SiO2 films on Si (0 0 1) substrate. The Si1-xGex NCs were formed by a post-deposition annealing at 100-700 degrees C for 1-5 min. The effect of annealing temperature and time on MLs morphology and NCs size and density was studied using grazing incidence X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy and measurements of spectral distribution of photocurrent. It is demonstrated how the photoconductive properties of the MLs can be enhanced and tailored by controlling the NCs formation conditions and the presence of stress field in MLs and defects acting as traps and recombination centers. All these features can be adjusted/controlled by altering the annealing conditions (temperature and time). The MLs photosensitivity was increased of more than an order of magnitude by the annealing process. A mechanism, where a competition between crystallization process (NCs formation and evolution i.e. size and shapes) and stress field appearance determines the peak position in the photocurrent spectra, was identified.

We studied the effect of short term furnace annealing over the photoconductive properties of tristacked layer i.e. TiO2/(SiGe/TiO2)(3). The structure was prepared by depositing alternate layers of TiO2 and SiGe films, using direct-current magnetron sputtering technique. A transmission electron microscopy and grazing incidence spectroscopy was used to analyze the morphology of the structure. Photoconductive properties were studied by measuring photocurrent spectra at different applied voltages and temperatures. Tristack layers were obtained with 5-10 nm SiGe nanocrystals (NCs) by annealing at 600 degrees C for 5 min. No sign of SiO2 formation was found inside stacked layers. A maximum in the photocurrent spectra was observed at 994 nm at 300 K but it red-shifted gradually to 1045 nm with decrease in temperature to 100 K. This transition in peak maxima is attributed to SiGe NCs, due to lattice vibration and to contribution of non-radiative recombination at low temperatures.

GeSi NCs / TiO2 multilayers with enhanced photocurrent properties were prepared and studied. Multilayers of TiO2 /(GeSi/TiO2)x2 /Si-p were deposited by magnetron sputtering and annealed by RTA at 700 degrees C for GeSi NCs formation. A post-annealing hydrogenation in plasma was performed on multilayers for healing of defects acting as traps and/or recombination centers and consequently producing the photocurrent enhancement. We studied the electrical and photoconductive properties of multilayers annealed by RTA and post-annealing hydrogenated. The current - temperature dependence reveals the conduction mechanisms in GeSi NCs / TiO2 multilayers RTA annealed, i.e. thermal activation of carriers to extended states (0.31 eV activation energy), the electron tunneling mechanism to nearest neighbors (T-1/2 behavior) and Mott variable range hopping (T-1/4 dependence). The photocurrent spectra made on multilayers structures hydrogenated for 10, 20 and 30 min evidence the photocurrent increasing up to 50%, showing that the hydrogenation is a suitable treatment for enhancing photocurrent. All photocurrent spectra present a dominant maximum (920 nm) and two shoulders (similar to 770 and similar to 1060 nm).

Si and Ge nanocrystals in oxides are of a large interest for photo-effect applications due to the fine-tuning of the optical bandgap by quantum confinement in nanocrystals. In this work, dense Ge nanocrystals suitable for enhanced photoconduction were fabricated from 60% Ge in TiO2 amorphous layers by low temperature rapid thermal annealing at 550 degrees C. An exponential increase of the photocurrent with the applied voltage was observed in coplanar structure of Ge nanocrystals composite films deposited on oxidized Si wafers. The behaviour was explained by field effect control of the Fermi level at the Ge nanocrystals-TiO2 layer/substrate interfaces. The blue-shift of the absorption gap from bulk Ge value to 1.14 eV was evidenced in both photocurrent spectra and optical reflection-transmission experiments, in good agreement with quantum confinement induced bandgap broadening in Ge nanocrystal with sizes of about 5 nm as found from HRTEM and XRD investigations. A nonmonotonic spectral dependence of the refractive index is associated to the Ge nanocrystals formation. The nanocrystal morphology is also in good agreement with the Coulomb gap hopping mechanism of T-1/2 -type explaining the temperature dependence of the dark conduction.

GeSi NCs / TiO2 multilayers with enhanced photocurrent properties were prepared and studied. Multilayers of TiO2 /(GeSi/TiO2)x2 /Si-p were deposited by magnetron sputtering and annealed by RTA at 700 degrees C for GeSi NCs formation. A post-annealing hydrogenation in plasma was performed on multilayers for healing of defects acting as traps and/or recombination centers and consequently producing the photocurrent enhancement. We studied the electrical and photoconductive properties of multilayers annealed by RTA and post-annealing hydrogenated. The current - temperature dependence reveals the conduction mechanisms in GeSi NCs / TiO2 multilayers RTA annealed, i.e. thermal activation of carriers to extended states (0.31 eV activation energy), the electron tunneling mechanism to nearest neighbors (T-1/2 behavior) and Mott variable range hopping (T-1/4 dependence). The photocurrent spectra made on multilayers structures hydrogenated for 10, 20 and 30 min evidence the photocurrent increasing up to 50%, showing that the hydrogenation is a suitable treatment for enhancing photocurrent. All photocurrent spectra present a dominant maximum (920 nm) and two shoulders (similar to 770 and similar to 1060 nm).

Trilayer memory capacitors with Ge nanocrystals (NCs) floating gate in HfO2 were obtained by magnetron sputtering deposition on p-type Si substrate followed by rapid thermal annealing at relatively low temperature of 600 degrees C. The frequency dispersion of capacitance and resistance was measured in accumulation regime of Al/HfO2 gate oxide/Ge NCs in HfO2 floating gate/HfO2 tunnel oxide/SiOx/p-Si/Al memory capacitors. For simulation of the frequency dispersion a complex circuit model was used considering an equivalent parallel RC circuit for each layer of the trilayer structure. A series resistance due to metallic contacts and Si substrate was necessary to be included in the model. A very good fit to the experimental data was obtained and the parameters of each layer in the memory capacitor, i.e. capacitances and resistances were determined and in turn the intrinsic material parameters, i.e. dielectric constants and resistivities of layers were evaluated. The results are very important for the study and optimization of the hysteresis behaviour of floating gate memories based on NCs embedded in oxide. (C) 2017 Published by Elsevier B.V.

The films of SiGe nanocrystals in SiO2 on Si substrate were obtained by co-sputtering Si, Ge, and SiO2 followed by rapid thermal annealing. The films structure and morphology together with electrical and photoelectrical properties were studied by x-ray diffraction, transmission electron microscopy, current - voltage and spectral photocurrent measurements. The photocurrent spectra at 300, 200 and 100 K were correlated with results obtained from X-ray diffractograms and transmission electron microscopy. The photocurrent spectra show an extension in near infrared due to the enriching SiGe nanocrystals in Ge.

Photo-induced effects on charging and discharging of nanocrystals (NCs) in capacitor memories with Ge NCs in an HfO2 matrix as a floating gate layer are studied. The sequence of layers HfO2/Ge-HfO2/ HfO2 was deposited on a p-Si substrate using magnetron sputtering. Well separated Ge NCs are obtained after rapid thermal annealing at 600 degrees C. The optoelectric capacitor memories were fabricated with a semi-transparent electrode on top of the structure and an Al electrode on the back side of the Si substrate. Light illumination effects on hysteresis curves were investigated using different operation modes. The hysteresis window increases by increasing the light exposure time. The spectral dependence of the hysteresis window shows the maximum contribution of the light in the wavelength range of 950-1000 nm, corresponding to both contributions from the Si substrate and from Ge NCs. The stored information about the electrical and optical pulses is also investigated in the regime of the flat band potential measurements (retention measurements). It is shown that in our memory structure, the photo-effect on the memory retention corresponds to a tunnelling transfer of negative charges from the Si substrate to Ge NCs, up to a mean value of 1.6 electrons per NC. Published by AIP Publishing.

The effect of room temperature hydrogen plasma treatment on the photoconductive properties of the SiO2 matrix containing SiGe nanoparticles is investigated. A considerable increase in photocurrent intensity is observed after plasma treatment. The increase is partly attributed to neutralization of dangling bonds around the nanoparticles and partly to passivation of non-radiative centers and defects in the matrix and at the nanoparticles-matrix interfaces.

Trilayer MOS capacitors gate HfO2 / floating gate of Ge nanocrystals in HfO2 / tunnel HfO2 / Si substrate were prepared in the aim to be used for the detection of ionizing radiation. Magnetron sputtering and rapid thermal annealing were used for their fabrication. Capacitance-voltage measurements showed that Ge nanocrystals are the most important charge storage centres in our structure. The possibility to use these trilayer MOS capacitors as dosimeters was investigated, and the sensitivity to alpha particle irradiation was extracted.

The photosensing properties related to the structure of GeSi/TiO2 multilayers prepared under different conditions are studied. TiO2 cap/(GeSi/TiO2)(2) multilayers (ML) were deposited by magnetron sputtering (MS) and annealed by rapid thermal annealing. Trilayers of TiO2 cap/GeSi/TiO2 (TL) were also deposited using reactive high power impulse MS (HiPIMS) for TiO2 layers and dc MS for the GeSi layer. For TL samples a two-step annealing was employed, one before and the second after depositing TiO2 cap. Structure and morphology characterization (X-ray diffraction, scanning and transmission electron microscopy) was carried out and photocurrent measurements (voltage dependences, spectral curves) were performed. The annealed ML samples are formed of GeSi NCs with 5 - 10 nm sizes, while in the annealed TL samples, the GeSi NCs are larger (20 - 30 nm). These morphologies determine the multilayers photosensing properties in VIS-NIR of ML structures and in UV in TL ones, respectively.

High performance trilayer memory capacitors with a floating gate of a single layer of Ge quantum dots (QDs) in HfO2 were fabricated using magnetron sputtering followed by rapid thermal annealing (RTA). The layer sequence of the capacitors is gate HfO2/floating gate of single layer of Ge QDs in HfO2/tunnel HfO2/p-Si wafers. Both Ge and HfO2 are nanostructured by RTA at moderate temperatures of 600-700 degrees C. By nanostructuring at 600 degrees C, the formation of a single layer of well separated Ge QDs with diameters of 2-3 nm at a density of 4-5 x 1015 m(-2) is achieved in the floating gate (intermediate layer). The Ge QDs inside the intermediate layer are arranged in a single layer and are separated from each other by HfO2 nanocrystals (NCs) about 8 nm in diameter with a tetragonal/orthorhombic structure. The Ge QDs in the single layer are located at the crossing of the HfO2 NCs boundaries. In the intermediate layer, besides Ge QDs, a part of the Ge atoms is segregated by RTA at the HfO2 NCs boundaries, while another part of the Ge atoms is present inside the HfO2 lattice stabilizing the tetragonal/orthorhombic structure. The fabricated capacitors show a memory window of 3.8. +/-. 0.5 V and a capacitance-time characteristic with 14% capacitance decay in the first 3000-4000 s followed by a very slow capacitance decrease extrapolated to 50% after 10 years. This high performance is mainly due to the floating gate of a single layer of well separated Ge QDs in HfO2, distanced from the Si substrate by the tunnel oxide layer with a precise thickness.

Nanocomposites have been obtained by dispersing various amounts of vapor grown carbon nanofibers within isotactic polypropylene. Thermal investigations done by differential scanning calorimetry and dynamic mechanical analysis revealed the effect of the vapor grown carbon nanofibers on the melting, crystallization, alpha, and beta relaxations. Direct current electrical features of these nanocomposites have been investigated and related to the thermal features of these nanocomposites. The effect of the loading with carbon nanofibers on the electrical properties of these nanocomposites is discussed within the percolation theory. The percolation threshold was estimated at about 5.5% wt carbon nanofibers. The temperature dependence of the direct current conductivity is analyzed in detail and it is concluded that the electronic hopping is the dominant transport mechanism. A transition from one-dimensional hopping towards a three-dimensional hopping was noticed as the concentration of carbon nanofibers was increased from 10% wt to 20% wt carbon nano-fiber. The possibility of a differential negative resistivity is suggested. (C) 2017 Wiley Periodicals, Inc.

The influence of light illumination on the programming of a capacitor floating gate memory based on Ge nanocrystals in HfO2 was studied. The capacitor was fabricated on a c-Si substrate by magnetron sputtering deposition of a layer sequence of HfO2/Ge-HfO2/HfO2 and post-growth rapid thermal annealing for nanocrystals formation at 600 degrees C. The illumination of the structure was performed through a semi-transparent Au contact (20% transparency). A maximum value of the light- induced change of 90% in C-V curve was obtained for 5 mW/cm(2) illumination during +5 V writing programming. The effect of the light exposure on the relative change of the C-V curve can be increased by reducing the writing time at 1 min.

The electrical and photosensing properties correlated with structure and morphology of TiO2/(GeSi/TiO2)(2) multilayers are investigated. The multilayers are prepared by magnetron sputtering followed by rapid thermal annealing. Studies of Raman spectroscopy, transmission electron microscopy and X-ray diffraction are carried out. Measurements of dark current versus voltage and temperature are done. The photosensing properties are studied by measuring photocurrent spectra at different temperatures. We obtain multilayers with 10 - 15 nm Ge0.6Si0.4 nanocrystals (NCs) by annealing at 800 degrees C. We evidence the tunneling mechanism between neighbor NCs (T-1/2 law) in the dark current-temperature dependence. The photocurrent spectrum has a maximum with position shifting from 940 to 980 nm when the measurement temperature increases from 150 to 300 K, being due to the GeSi NCs.

The strain in Si containing group-V impurities is a topical subject of study due to its potential applications in quantum computing. In this paper we study Bi-209 implanted Si concerning the correlation between the strain produced by stopped Bi ions and trapping characteristics of the defects resulted from implantation. The depths distributions of stopped ions and primary defects are simulated and the distributions of permanent defects are modelled for Si implanted with low fluence Bi-209 ions of 28 MeV kinetic energy. For comparison, these depths distributions were similarly calculated for I-127 ions with the same fluence and energy, implanted in Si. The results are compared with each other and correlated with the characteristics of traps in these systems, previously obtained. We demonstrate that the intensity of the strain field is the most important factor in changing of trap parameters, while the superposition between the region with strain and the region where defects are located is a second order effect. (C) 2016 Elsevier Ltd. All rights reserved.

The article presents the fabrication and characterization of NV (non-volatile) memory devices based on SiO2/Ge/SiO2 trilayer structures on Si wafers. The trilayer structures were obtained by using the magnetron sputtering method for the deposition of gate SiO2 and intermediate Ge layers and the rapid thermal oxidation for the growth of tunnel SiO2 layer. Rapid thermal annealing was performed for obtaining Ge nanocrystals embedded in the SiO2 gate oxide, as charge-storage elements. Two NV cross bar memory structures based on two cell sizes of 300x300 and 100x100m(2) were manufactured. Capacity-voltage curves were measured on the memory devices, at different frequencies in the 1kHz-10MHz range at room temperature (RT) for evidencing the hysteresis loops and for showing that the devices keep memory in time at these frequencies. We have obtained capacity-voltage hysteresis curves with large memory window up to 2V. We demonstrate that the trilayer structure SiO2/Ge/SiO2/on Si with Ge NCs embedded in the SiO2 gate oxide is suitable for NV memory applications having a large number of cells.

The strong correlation between morphology and charge storage properties of HfO2/Ge/HfO2/Si trilayer structures was evidenced. The morphology of structures deposited by magnetron sputtering and electron beam evaporation was tailored by rapid thermal annealing and investigated by transmission electron microscopy, Raman and X-ray photoelectron spectroscopies. The best hysteresis loops (capacitance-voltage characteristics) were obtained for trilayers with high density Ge nanocrystals located in the position of as-deposited Ge layer. The decrease of Ge nanocrystals density narrows the memory window, by spreading Ge atoms into HfO2 matrix (sputtered trilayers), or by Ge atoms expulsion toward HfO2 nanocrystals surface (evaporated trilayers). (C) 2015 Elsevier Ltd. All rights reserved.

The structure and charge storage properties of a trilayer structure with Ge nanocrystals embedded in HfO2 oxide were studied. The trilayer structure HfO2/Ge-HfO2/HfO2/p-Si was prepared by magnetron sputtering and subsequent rapid thermal annealing at 600 degrees C. The TEM investigations reveal the formation of Ge NCs embedded in crystalline HfO2 at the position of the Ge-HfO2 layer. The capacitors were made by Al evaporation on both front and backside of the trilayer structure. The C-V characteristics show a counterclockwise hysteresis with large memory window of 0.85 V which is given only by the contribution of the Ge NCs embedded in HfO2. The I-V characteristics show an asymmetric behavior, the currents are three times higher for the negative voltage than the positive one.

Fast atomic diffusion was evidenced in the surface layer of amorphous thin films of oxides and semiconductors irradiated with low fluence UV pulse laser. This process takes place in a surface layer with a thickness related to the laser radiation absorption depth in the target material and was revealed by the cross section transmission electron microscopy studies. These high values of diffusivity can be explained by supposing the glass transition transformation in the amorphous structure, triggered by the action of the laser pulse field. This effect can have application for controlling structural modifications at nanoscale of the thin films surface and also for inducing structural modification of interfaces between the film and substrate.

The charge storage properties of Ge nanocrystals-based MOS-like capacitors with tunnel and gate HfO2 are studied. HfO2/Ge/HfO2/Si trilayer structures were prepared by magnetron sputtering (in Ar) and subsequent rapid thermal annealing (650 degrees C). HfO2/Si structures were also prepared, some under similar conditions, while others were deposited in Ar:O-2. TEM investigations and C-V measurements were performed. TEM on annealed trilayers evidences the formation of ordered and precisely positioned array of Ge nanocrystals embedded in crystalline HfO2. The annealed Al/HfO2/Ge/HfO2/p-Si/Al capacitors present counterclockwise C-V hysteresis (0.8 V memory window) mainly given by Ge nanocrystals, with negligible contribution from crystallized-HfO2 traps.

Laser pulse processing of surfaces and thin films is a useful tool for amorphous thin films crystallization, surface nanostructuring, phase transformation and modification of physical properties of thin films. Here we show the effects of nanostructuring produced at the surface and under the surface of amorphous GeTiO films through laser pulses using fluences of 10-30 mJ/cm(2). The GeTiO films were obtained by RF magnetron sputtering with 50:50 initial atomic ratio of Ge:TiO2. Laser irradiation was performed by using the fourth harmonic (266 nm) of a Nd:YAG laser. The laser-induced nanostructuring results in two effects, the first one is the appearance of a wave-like topography at the film surface, with a periodicity of 200 nm and the second one is the structure modification of a layer under the film surface, at a depth that is related to the absorption length of the laser radiation. The periodicity of the wave-like relief is smaller than the laser wavelength. In the modified layer, the Ge atoms are segregated in spherical amorphous nanoparticles as a result of the fast diffusion of Ge atoms in the amorphous GeTiO matrix. The temperature estimation of the film surface during the laser pulses shows a maximum of about 500 degrees C, which is much lower than the melting temperature of the GeTiO matrix. GeO gas is formed at laser fluences higher than 20 mJ/cm(2) and produces nanovoids in the laser-modified layer at the film surface. A glass transition at low temperatures could happen in the amorphous GeTiO film, which explains the formation of the wave-like topography. The very high Ge diffusivity during the laser pulse action, which is characteristic for liquids, cannot be reached in a viscous matrix. Our experiments show that the diffusivity of atomic and molecular species such as Ge and GeO is very much enhanced in the presence of the laser pulse field. Consequently, the fast diffusion drives the formation of amorphous Ge nanoparticles through the segregation of Ge atoms in the GeTiO matrix. The nanostructuring effects induced by the laser irradiation can be used in functionalizing the surface of the films.

Ge and Si nanocrystals (NCs), quantum dots (QDs), and amorphous nanoparticles (NPs) have a significant role in the developement of micro- and nanoelectronic devices due to capability to tune their electrical and photoconductive properties by tailoring the morphology and structure parameters. Ge and Si NCs/QDs/NPs are zero-dimensional (0D) systems and the SiO2 films containing them are percolative systems. In this review, the role of deposition and annealing conditions in the morphology and structure of Ge and Si NCs/QDs/NPs embedded in amorphous SiO2 matrix is discussed for a wide variety of films and multilayered structures. The electrical and photoconductive properties of nanostructures deposited by different techniques as magnetron sputtering, ion implantation, chemical vapour deposition, sol-gel and molecular beam epitaxy, and subsequently annealed in conventional furnace or by rapid thermal annealing under different conditions are analised. We demonstrate how the electrical and photoconductive properties of nanostructures can be tuned by varying the deposition and annealing parameters. The role of Si-rich oxide and defects in the formation of Ge and Si NCs is shown and the role of defects in improving electrical properties and enhancing the photoconductivity of films and multilayered structures is highlighted. We evidence the contribution of quantum confinement effect and show that the most probable transport mechanisms in these percolative systems are tunnelling and hopping.

We analyse the influence of the strain field on the parameters of trapping centres. The system under study is high resistivity Si implanted with Bi6+ and I6+ ions respectively, of 28 MeV kinetic energy, 3(O) off axis orientation and 5x10(11) ions/cm(2) fluence. The strain field is the consequence of size and mass difference of the irradiation ions in respect to the atoms of the lattice, and the defects are produced during the slowing-down of ions, as a result of the energy transfer from the ion to Si atoms. These results are of interest for the design and manufacturing of microelectronic devices incorporating strain, particularly for quantum computers with qubits based on the interaction of electronic and nuclear spins of group-V donors in Si.

GeSi-based nanostructures show unique properties which make them suitable for integrated circuit technology. The strong interest is to enhance their electronic properties in order to improve the device performance. In order to obtain fundamental knowledge on the electrical transport taking place in GeSi nanostructures we have investigated the effects of different microstructures on the electrical behavior of GeSi nanostructured films, by modifying the annealing conditions. We manufactured GeSi nanostructured films with equiatomic composition and different structures by co-sputtering followed by adequate annealing under different temperatures. For determining the electrical behavior we performed and modeled current-temperature I - T characteristics taking into account the films structures. We found that the electrical behavior changes with the film structure by evidencing a transition in conduction mechanism. In films that are almost crystallized, being formed of small GeSi nanocrystals separated by thin amorphous regions, the I - T dependence at low temperature is due to thermally activated tunneling of carriers between neighboring nanocrystals. In contrast, in the completely crystallized films with big GeSi nanocrystals and crystallized borders between them, the electrical behavior is a typical polycrystalline one. Our findings help to clarify the conduction mechanisms taking place in GeSi nanostructures and to provide a route to electronic devices with high performance based on these materials.

We propose a numerical procedure consisting of a simplified physical model and a numerical method with the aim of optimizing the performance parameters of dye-sensitized solar cells (DSSCs). We calculate the real rate of absorbed photons (in the dye spectral range) G(real)(x) by introducing a factor beta < 1 in order to simplify the light absorption and reflection on TCO electrode. We consider the electrical transport to be purely diffusive and the recombination process only to occur between electrons from the TiO2 conduction band and anions from the electrolyte. The used numerical method permits solving the system of differential equations resulting from the physical model. We apply the proposed numerical procedure on a classical DSSC based on Ruthenium dye in order to validate it. For this, we simulate the J-V characteristics and calculate the main parameters: short-circuit current density J(sc), open circuit voltage V-oc, fill factor FF, and power conversion efficiency eta. We analyze the influence of the nature of semiconductor (TiO2) and dye and also the influence of different technological parameters on the performance parameters of DSSCs. The obtained results show that the proposed numerical procedure is suitable for developing a numerical simulation platform for improving the DSSCs performance by choosing the optimal parameters.

The GeTiO2 amorphous films were deposited by magnetron sputtering and subsequently annealed at 400, 550, 600 and 700 degrees C for nanostructuring. The structure of annealed films was investigated by X-ray diffraction and transmission electron microscopy. The transmittance spectra of all annealed GeTiO2 films were measured and simulated by using Bruggeman effective medium approximation considering components of TiO2 anatase, crystalline Ge, GeO2 and voids determined from the structure investigations. The electrical behavior of 400, 600 and 700 degrees C annealed films was studied by measuring current-voltage characteristics. We found that by increasing the annealing temperature the films thickness decreases from 330 nm (as-deposited films) to 290 nm (700 degrees C annealed films). The 400 degrees C annealed films are amorphous, while all the others annealed at higher temperatures are crystallized (X-ray diffraction and transmission electron microscopy). In the 550 and 600 degrees C annealed films we found the (TiGe)O-2 rutile structure which is formed by starting from the GeO2 tetragonal structure with high Ti content. Additionally, these films contain TiO2 anatase structure and cubic Ge nanocrystals. At 700 degrees C annealing temperature, a surface layer of GeO2 tetragonal nanocrystals is formed by Ge diffusion and a part of Ge is lost. The experimental transmittance spectra indicate a broadening of the transparency range by increasing the annealing temperature, and the simulated ones also indicate this behavior with the decrease of Ge content, the experimental and simulated spectra being in good agreement. Also, the increase of annealing temperature produces an increase of electrical conductivity. (C) 2014 Elsevier B.V. All rights reserved.

The structure of GeSiO films resulted from deposition and annealing conditions draws their electrical behaviour. GeSiO films prepared either by magnetron sputtering or sol-gel method and subsequently annealed are formed of Ge nanocrystals and/or amorphous Ge nanoparticles embedded in amorphous SiO2 matrix. Firstly, the size effect which is the main effect in these systems produces specific quantum confinement energy levels in the enlarged forbidden energy band gap in nanocrystals. Secondly, these films are percolative systems, so that the main conduction mechanisms which govern the electrical behaviour are the tunnelling and hopping between neighbouring Ge nanocrystals or amorphous nanoparticles. Accordingly, the charge transport is strongly determined by the structure and morphology of films.

GeSi nanostructured films were obtained by cosputtering from two Ge and Si targets and subsequent annealing in furnace in N-2 for 5 h at 700, 800 and 900 degrees C, with the aim to show the correlation between electrical properties and crystalline structure of the films. The as-deposited films are amorphous, have a Ge: Si composition of 55: 45 and 185 nm thickness. The film structure was investigated by XRD and TEM, and the electrical behaviour was studied by measuring current-voltage (I-V) and current-temperature (I-T) characteristics and by discussing them in correlation with the structure. Different electrical behaviours of GeSi films corresponding to different structures have been evidenced. The 700 degrees C annealed GeSi films are formed of nanocrystals (7-15 nm) separated by amorphous regions (1-2 nm). These films present a superlinear I-V characteristic typical of high field-assisted tunnelling through potential barriers (amorphous regions) between nanocrystals. The I-T characteristic at low temperature follows a T-1/2 law showing a thermally activated tunnelling of carriers between neighboring GeSi nanocrystals. The films annealed at 800 and 900 degrees C have a like behavior, they are completely crystallized and present linear I-V and Arrhenius I-T dependences reflecting the polycrystalline behaviour. [GRAPHICS] . (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We report on the charge storage properties of trilayer structures consisting in sputtered gate HfO2/co-sputtered Ge-HfO2 layer/rapid thermal tunneling SiO2 oxide. Investigations of transmission electron microscopy and X-ray diffraction evidence the formation of HfO2 with mixed structure of monoclinic and tetragonal in the annealed structures. Capacitance-voltage (C-V) characteristics were measured on Al/HfO2/Ge-HfO2/SiO2/Si/Al metal-oxide-semiconductor capacitors based on as-deposited and annealed structures. Large C-V hysteresis is observed for the as-deposited structures and is controlled by traps present in oxide and interface. The annealing yields a C-V hysteresis with smaller memory window being due to injected charges in Ge nanocrystals.

Low fluence heavy ions incident on high resistivity Si produce lattice defects which act as trapping centers, and produce also an important local field of strain. The strain field intensity increases with the increase of the difference in atomic size and mass between the ion and the Si atom host. We investigate the correlation between the change of trapping parameters and the strain field. The strain field produced by Bi ions in Si is two times more intense than in Si irradiated with I ions, and effects the Gaussian broadening of trapping levels and the temperature dependence of cross sections.

Multi-quantum well structures and Si wafers implanted with heavy iodine and bismuth ions are studied in order to evaluate the influence of stress on the parameters of trapping centers. The experimental method of thermostimullatedcurrents without applied bias is used, and the trapping centers are filled by illumination. By modeling the discharge curves, we found in multilayered structures the parameters of both 'normal' traps and 'stress-induced' ones, the last having a Gaussian-shaped temperature dependence of the cross section. The stress field due to the presence of stopped heavy ions implanted into Si was modeled by a permanent electric field. The increase of the strain from the neighborhood of I ions to the neighborhood of Bi ions produces the broadening of some energy levels and also a temperature dependence of the cross sections for all levels.

By implanting Bi in Si, a strong strain field due to the bigger atomic mass and size of stopped Bi ions than Si lattice ones is produced, together with irradiation defects. The controlled doping of Si with Bi leads to modern applications such as quantum information processing. Here we show that the parameters of trapping centres are modified under the strain field. In the literature there are no reports on this subject. We irradiated Si wafers with Bi6+ ions of 28MeV kinetic energy, 3 degrees off [100] axis orientation. The depth distributions of stopped ions and of primary defects were simulated. The traps produced by irradiation were investigated using the thermally stimulated currents method without bias. We recorded and modelled the discharge current curves. The strain field was modelled as a permanent and temperature-independent electric field. The traps of V-2, VO/CiCs, and CiOi and other two not assigned were evidenced. We have found that all trapping levels are broadened, and all capture cross-sections are temperature dependent which we attribute to the strong strain field produced by Bi. These results are important and must be taken into account in designing and manufacturing microelectronic devices incorporating strain, including the topical spin qubit ones. Copyright (C) EPLA, 2014

N-type silicon single crystals with resistivity higher than 8000 Omega cm were irradiated with ions of Bi6+, of 28 MeV kinetic energy. At this energy, the ions are stopped into the wafer. Being much heavier and bigger than the host atoms, they produce major disturbances into the lattice. On the other hand, the produced collision cascade is the source of lattice defects which act as traps. We investigated them using the method of thermally stimulated currents without applied bias. The results are compared with those obtained from the analysis of silicon irradiated with I6+ ions.

This paper presents a detailed transmission electron microscopy (TEM) study of GeSiO films with Ge nanoparticles. The films with 2.5 mu m thickness were deposited by magnetron sputtering and subsequently annealed in H-2 at 2 atm and 500 degrees C for 2 h for nanostructuring. After H-2 annealing, the majority of the resulted Ge nanoparticles are amorphous, less than 5 nm in size, forming a uniform network in the film volume. Big Ge nanoparticles with sizes between 20 and 50 nm are also formed. Some of them are identified to be crystallized in the (Ge-III/ST12) tetragonal phase. The high resolution TEM observation induces the amorphysation of the Ge tetragonal phase, followed by the crystallization of the amorphous Ge phase in the cubic diamond structure (Ge-I), as an effect of electron irradiation. A secondary annealing performed in N-2 at 800 degrees C and 1 atm for 2 h induces formation of faceted cubic Ge nanoparticles distributed in the SiO2 matrix.

Quantum well solar cells with p-i-n structure are presented. The physical processes in multiple quantum well solar cells, the materials commonly used for photovoltaic applications, and technological aspects are analyzed. The quantum confinement effect produces resonant energy levels located in the valence and conduction bands of well layers. In addition, it produces energy quantum confinement levels located in the energy band gap of both well and barrier layers. The absorption on both resonant and quantum confinement levels leads to an extension of the internal quantum efficiency in near infrared domain. Several structures with different absorbers from 3-5 and 4 groups are described and discussed. Various technological and design solutions, such as multiple quantum well solar cells with graded band gap, with tandem configurations, with strain-balanced structure, and strain-balanced structure improved with nanoparticles deposited atop are analyzed. The cell parameters are discussed and related to the materials and technology.

In this article, we present and discuss how the stress influences the trapping phenomena in both the bulk Si and nanostructures based on Si. For this, we use single crystal Si irradiated with 28 MeV iodine ions of fluence (5 +/- 0.5) x 10(11) ions/cm(2) and 2D Si based nanostructures of (nc-Si/CaF2)(50) multilayers. The method of thermally stimulated discharge currents without bias was employed to investigate the trapping phenomena and the modeling of the discharge current curves was carried out. In the case of irradiated Si, we have modeled the discharge currents considering six trapping levels and a supplementary stress electric field independent of temperature superposed on the frozen-in electric field produced by the charged traps at low temperature. The stress electric field is due to the stopped iodine ions which are heavier than those of Si host. Six traps are evidenced and their parameters are calculated. In the (nc-Si/CaF2)(50) multilayer structures, we experimentally evidenced traps which produce in the discharge current maxima with two shapes. Three maxima have normal shape and the others five maxima are narrow and sharp looking like spikes. We modeled the discharge current curves taking for the concentration a power temperature dependence law, and for the cross section a Gaussian temperature dependence.

This paper analyses and discusses the effect of Ge/Si atomic ratio and annealing temperature on the conduction mechanisms governing the electrical behavior of Ge-SiO2 films containing Ge nanoparticles embedded in amorphous SiO2 matrix. For this, the experimental conductance-temperature curves are modeled in correlation with the microstructure findings for two types of films. One type of films has a lower Ge/Si ratio of 0.73 and was obtained by magnetron sputtering followed by annealing in H-2, at 500 degrees C, while the second one has a higher ratio of 1.86 and was obtained also by sputtering, but was annealed in N-2, at 800 degrees C. Both types of films show an electrical behavior with a T-1/4 conductance dependence on temperature, typical for hopping mechanism.

Ge-SiO2 films with high Ge/Si atomic ratio of about 1.86 were obtained by co-sputtering of Ge and SiO2 targets and subsequently annealed at different temperatures between 600 and 1000 C in a conventional furnace in order to show how the annealing process influences the film morphology concerning the Ge nanocrystal and/or amorphous nanoparticle formation and to study their electrical behaviour. Atomic force microscopy (AFM) imaging, Raman spectroscopy and electrical conductance measurements were performed in order to find out the annealing effect on the film surface morphology, as well as the Ge nanoparticle formation in correlation with the hopping conductivity of the films. AFM images show that the films annealed at 600 and 700 C present a granular surface with particle height of about 15 nm, while those annealed at higher temperatures have smoother surface. The Raman investigations evidence Ge nanocrystals (including small ones) coexisting with amorphous Ge in the films annealed at 600 C and show that almost all Ge is crystallized in the films annealed at 700 C. The annealing at 800 C disadvantages the Ge nanocrystal formation due to the strong Ge diffusion. This transition in Ge nanocrystals formation process by annealing temperature increase from 700 to 800 C revealed by AFM and Raman spectroscopy measurements corresponds to a change in the electrical transport mechanism. Thus, in the 700 C annealed films, the current depends on temperature according to a T-1/2 law which is typical for a tunnelling mechanism between neighbour Ge nanocrystals. In the 800C annealed films, the current-temperature characteristic has a T-114 dependence showing a hopping mechanism within an electronic band of localized states related to diffused Ge in SiO2. (C) 2013 Elsevier B.V. All rights reserved.

Amorphous Ge/SiO2 multilayer structures deposited by magnetron sputtering have been annealed at different temperatures between 650 and 800 degrees C for obtaining Ge nanocrystals in oxide matrix. The properties of the annealed structures were investigated by transmission electron microscopy, Raman spectroscopy, and low temperature photoluminescence. The Ge crystallization is partially achieved at 650 degrees C and increases with annealing temperature. Insight of the Ge nanocrystal formation was acquired by comparing two annealing procedures, i.e., in a conventional tube furnace and by a rapid thermal annealing. By rapid thermal annealing in comparison to conventional furnace one, the Ge crystallization process is faster than Ge diffusion, resulting in the formation of more compact layers of Ge nanocrystals with 8-9.5-nm size as Raman spectroscopy reveals. These findings are important to improve the annealing efficiency in the nanocrystals formation for a precise control of their sizes and location in oxide matrix and for the possibility to create systems with interacting nanoparticles for charge or excitonic transfer. The infrared photoluminescence of Ge nanocrystals at low temperatures shows strong emission with two sharp peaks at about 1,000 meV.

The cryogenic detectors in the form of bolometers are presently used for different applications, in particular for very rare or hypothetical events associated with new forms of matter, specifically related to searches for dark matter. In the detection of particles with a semiconductor as target and detector, usually two signals are measured: ionization and heat. The amplification of the thermal signal is obtained with the prescriptions from the Luke-Neganov effect. The energy deposited in the semiconductor lattice as stable defects in the form of Frenkel pairs at cryogenic temperatures, following the interaction of a dark matter particle, is evaluated and consequences for measured quantities are discussed. This contribution is included in the energy balance of the Luke effect. Applying the present model to germanium and silicon, we found that for the same incident weakly interacting massive particle the energy deposited in defects in germanium is about twice the value for silicon. (C) 2013 Elsevier B.V. All rights reserved.

The films containing Ge nanoparticles embedded in SiO2 matrix were prepared by RF magnetron sputtering and subsequently by thermal annealing. Their structure was investigated by conventional transmission electron microscopy and high resolution transmission electron microscopy together with energy-dispersive X-ray spectroscopy. The electrical behavior of films was studied by measuring current-temperature and current-voltage characteristics. The structure investigation reveals two kinds of features: a low density of big Ge nanoparticles with sizes from 20 to 50 nm and a network of small amorphous Ge nanoregions/nanoparticles (5 nm size or less) with high density, both being embedded in amorphous SiO2 matrix. The electrical transport was shown to take place through the network of amorphous Ge nanoregions. At low temperature, the T-1/4 dependence of the current was evidenced, while at high temperature, the T-1 Arrhenius dependence was found. At both low and high temperatures, the conductivity is nearly constant. The behavior at low temperature was explained by the hopping mechanism on localized states located in a band near the Fermi energy, while at high temperature by the charge excitation to the extended states.

N-type P-doped silicon single crystals with resistivity higher than 8000 Omega cm were irradiated with I-127(6+) ions of 28 MeV kinetic energy. The penetration of the ions through the target and the processes of energy loss were simulated using the CTRIM Monte Carlo code, and point defect production was calculated in the frame of our diffusion-reaction model. Trapping phenomena were investigated using the method of thermally stimulated currents without applied bias. The modeling of the current-temperature curves takes into consideration both point defects and stress-type trapping centers, produced by the ions stopped into the crystal.

This paper reports on the conduction mechanisms in amorphous SiO2 films with embedded Ge nanoparticles. For this, measurements of current-temperature and current-voltage were employed and correlated with the microstructure results obtained from transmission electron microscopy (TEM). TEM images reveal that our films contain big Ge nanoparticles with low density and small Ge nanoparticles with high density, the last ones being the only responsible for the electrical transport. Two transport mechanisms were found at low and high temperature respectively, namely hopping on localized states in a band near Fermi level and charge excitation to the extended states at mobility edge.

The effects of 5 x 10(11) cm(-2 6+)I(127) ions of 28 MeV kinetic energy on high resistivity (100) Si were studied. The profile of primary defects was simulated. The defects produced by irradiation which act as traps were investigated. Thermally stimulated current measurements without externally applied bias were used, and for this the traps were charged by illuminating samples with 1000, 800, and 400 nm wavelengths. The discharge currents were recorded and modeled, and therefore the parameters of the traps were determined. The presence of I ions, heavier than Si, stopped into the target was modeled as a temperature independent electric field. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4772015]

In this paper we continue the previous investigations on nanostructured GexSi1-x films. The films were deposited by magnetron sputtering and annealed in N-2 atmosphere at 700 degrees C. Their structure was investigated and correlated with the electrical behavior. For this, conventional and high-resolution transmission electron microscopy together with selected area electron diffraction was used. Electrical measurements of current-voltage and current-temperature curves were made. The majority of crystallites that forms the films have the composition Ge50Si50 and 15 - 30 nm size. The I - T characteristics have Arrhenius dependence, with two activation energies interpreted as transitions between quantum confinement levels.

Transmission electron microscopy and X-ray photoelectron spectroscopy analyses are performed to investigate Ge nanoparticles embedded in an amorphous SiO2 matrix. GeSiO thin films are prepared by two methods, sol-gel and radio frequency magnetron sputtering. After the deposition, the sol-gel films are annealed in either N-2 (at 1 atm and 800 A degrees C) or H-2 (at 2 atm and 500 A degrees C), and the sputtered films in H-2 (at 2 atm and 500 A degrees C), to allow Ge segregation. Amorphous Ge-rich nanoparticles (3-7 nm size) are observed in sol-gel films. Crystalline Ge nanoparticles in the high pressure tetragonal phase (10-50 nm size) are identified in the sputtered films. The size of the nanoparticles increases with Ge concentration in the volume of the film. At the film surface, the Ge concentration is much larger that in the volume for both sol-gel and sputtered films. At the same time, at the film surface, only oxidized Ge is observed.

This paper presents the preparation and investigation of structure and electrical properties of nanostructures consisting of Si1-xGex nanocrystals. Nanostructures were prepared by RF magnetron sputtering, followed by thermal annealing. X-ray diffraction, TEM, high resolution TEM and SAED measurements were performed. Current-voltage and current-temperature characteristics were taken. Nanostructures have different electrical behavior. Current-voltage curves are linear, while current-temperature curves are dependent on the annealing temperature. In films annealed at 650 degrees C, electrical transport is controlled by quantum confinement effect and localized states (current-temperature characteristics), while in films annealed at 900 degrees C it is controlled by tunneling of thermally activated carriers between neighboring nanocrystals.

Trapping aspects in two typical silicon-based nanostructures are investigated. Both 1D nanocrystalline porous silicon structures and 2D (nc-Si/CaF2)50 multilayered structures are considered. The curves of relaxation current versus temperature were obtained using the thermally stimulated currents method without external bias. Two types of maxima (rather broad maxima and spikes) were experimentally evidenced. The spikes are due to the stress-induced traps and their parameters are temperature dependent. To describe the trapping-detrapping-retrapping processes, a general model was proposed. The temperature dependence of stress-induced trap concentrations was described by a power law and the corresponding temperature dependence of cross-sections by a Gaussian law. The model was successfully applied on available experimental data.

A numerical method to determine the cell parameters from the analysis of the J-V characteristics of a polymer solar cell is proposed. This method uses the equations given by the diode model, experimental data from the literature, and an adequate fitting procedure with seven fit parameters. Different aspects of the obtained results are discussed. Information concerning cell design optimization is also obtained. The method is of general application in the field of polymer solar cells, as well as to any kind of diode-like cell. Copyright (C) 2010 John Wiley & Sons, Ltd.

The trap parameters of defects in Si/CaF2 multilayered structures were determined from the analysis of optical charging spectroscopy measurements. Two kinds of maxima were observed. Some of them were rather broad, corresponding to "normal" traps, while the others, very sharp, were attributed to stress-induced traps. A procedure of optimal linear smoothing the noisy experimental data has been developed and applied. This procedure is based on finding the minimal value of the relative error with respect to the value of the smoothing window. In order to obtain a better accuracy for the description of the trapping-detrapping process, a Gaussian temperature dependence of the capture cross-sections characterizing the stress-induced traps was introduced. Both the normal and the stress-induced traps have been characterized, including some previously considered as only noise features. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3525582]

The quantum efficiency of the absorption on quantum confinement levels is investigated. This is achieved by modeling the electron confinement in a spherical quantum dot (QD). The confinement levels are calculated using both infinite and finite rectangular quantum wells. The spectral internal quantum efficiency is evaluated within both the models, by computing Einstein's coefficients for the transitions between confinement levels. The size of QDs (1-3 nm radius) leads to negligible many body effects. The nature of the QD material and of the matrix embedding is taken into account in the finite rectangular quantum well approximation and introduces only a small correction. The temperature dependence of the efficiency is also taken into account. A numerical application is performed for a silicon QD of 2.5 nm radius, embedded in amorphous silica. It is proved that the absorption threshold shifts toward the far infrared limit and that the spectral internal quantum efficiency reaches 4-5% at the threshold.

Electrical and photoconductive properties of films consisting of amorphous Ge nanoparticles uniformly distributed in amorphous SiO2 were studied. These films were prepared by sol-gel method and treated by rapid thermal annealing technique. Measurements of current-voltage and conductance-temperature characteristics, spectral and bias dependences of the photocurrent on samples with coplanar geometry of electrodes, were performed. The current-voltage characteristics have a weak rectifying behaviour. The variable range hopping transport mechanism, described by the Mott law, in amorphous materials, in the absence of dominant Coulomb interactions, was evidenced in the temperature dependence of the dark current. The samples exhibit very good photoconductive properties, explained by taking into account the Ge clusters and defects, produced by the rapid thermal annealing.

GeSiO nanostructures obtained by using two different preparation methods, sol-gel and magnetron-sputtering were studied. They are formed from Ge nanoparticles dispersed in amorphous matrix, with different morphology depending on the preparation method. Electrical investigations (current-voltage and current-temperature measurements) were performed and experimental results were modelled. It was shown that preparation conditions induce the electrical behaviour of GeSiO films, so that the electrical transport in sputtered films is governed by a hopping mechanism, while in sol-gel ones it is dominated by the junction formed at the interface with Si substrate. Also, the temperature dependence of current shows different hopping mechanisms.

The electrical behavior of multi-walled carbon nanotube network embedded in amorphous silicon nitride is studied by measuring the voltage and temperature dependences of the current. The microstructure of the network is investigated by cross-sectional transmission electron microscopy. The multi-walled carbon nanotube network has an uniform spatial extension in the silicon nitride matrix. The current-voltage and resistance-temperature characteristics are both linear, proving the metallic behavior of the network. The I-V curves present oscillations that are further analyzed by computing the conductance-voltage characteristics. The conductance presents minima and maxima that appear at the same voltage for both bias polarities, at both 20 and 298 K, and that are not periodic. These oscillations are interpreted as due to percolation processes. The voltage percolation thresholds are identified with the conductance minima.

The temperature dependence of the capture coefficients in trapping phenomena is investigated. It is proved that, besides the dependence induced by the thermal velocity of the carriers, the stress-induced traps at the interfaces of the multi-layered structures present a supplementary temperature dependence. This dependence is found to be of Gaussian type and is in a good agreement with the experimental results.

This paper discusses the quantum confinement effects in nanometric structures that form low dimensional systems. In such systems, each surface/interface acts like a potential barrier, i.e. the wall of a quantum well, generating new energy levels. These levels are computed in a model that uses the approximation of the infinite rectangular quantum wells. The model is adapted for 2D, 1D and 0D systems, respectively. Different applications are discussed. The differences between the model results and the experimental data are proved to be of the same order of magnitude as the differences between the levels computed within the frame of infinite and finite quantum well approximations.

The influence of the shape of silicon quantum dots embedded in an amorphous silica matrix on the quantum confinement energy levels, as well as that of the Si/SiO2 potential barrier, are studied. The energy levels are computed using both the infinite and finite rectangular quantum well models for spherical quantum dots and the infinite rectangular quantum well for prolate spheroidal quantum dots. The results are compared with each other and also with the experimental activation energies obtained from the temperature dependence of the dark current. These activation energies are identified with the differences between the quantum confinement energies, subject to the selection rules. The finite rectangular quantum well model takes into account the experimental value of the finite potential barrier and the matrix-to-dot electron mass ratio. The energy levels are smaller than those for the infinite rectangular quantum well case; they decrease when the potential barrier decreases and the mass ratio increases. Different aspects of the models are discussed. All the errors are less than about 4%. The spheroidal shape lifts the degeneracy on the magnetic quantum number. The energy levels can decrease or increase with eccentricity as a consequence of the different quantum confinement effects along the major and minor axes. The supplementary information on the magnetic quantum number is beneficial for optical applications.

GeSiO nanosystems were obtained using two different preparation methods, sol-gel and magnetron-sputtering. Transmission electron microscopy measurements were performed to investigate the films structure. Amorphous and crystalline Ge dots embedded in amorphous silicon dioxide were observed. The Ge concentration in the GeSiO films was by Energy-dispersive X-ray spectroscopy.

This paper presents the investigation of electrical properties of carbon nanotubes (CNT). A sandwich configuration, quartz substrate/Cr/Al/CNT (partially immersed in SiN)/Cr/Al was investigated. The CNT are mainly oriented parallel with the electrodes. Current - voltage characteristics were taken at 20 K and room temperature and a current - temperature characteristic was taken at constant voltage (20 mV). The I - V characteristics are almost linear, while the G - V characteristics present some peaks and dips, interpreted as voltage percolation thresholds. The I - T and R - T characteristics are also linear (except at low temperatures). The investigated structures have a high electrical polarizability.

The interactions by which ions lose their energy in silicon are investigated at the microscopic level. this theoretical study put in evidence that the ionization could also represent a source of structural defects, and, contrary to the processes initiated by elementary particles, could generate extended primary defects, as, e.g. four-fold coordinated defects, due to the simultaneous breaking of bond for more neighbouring atoms in the lattice in a single interaction, in the ionisation core. The average energy transferred per atom is calculated, and also the energy spent by ionisation. The time dependence of the temperature in the ionisation core is derived in some simplifying assumptions. These contributions not considered yet in the studies of degradation of silicon detectors in radiation fields and could account for the discrepancies observed between models and measurements at microscopic versus device level for hadrons and ions. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The knowledge of the effects of radiation in semiconductor devices, in particular in detectors, represents an important and active field of research. The influence of isovalent impurities, carbon and germanium, on the radiation damage of silicon for detectors is investigated in the frame of a quantitative phenomenological model for defect kinetics, developed previously by the authors. The concentrations of defects induced by irradiation in materials with different doping levels are calculated, as well as the leakage current and effective carrier concentrations in p-n junction detectors made from these materials. The beneficial effect of Ge on the radiation damage of silicon is deduced.

The nanostructure of Si-x(SiO2)(1-x) films deposited on quartz substrate, where x varies from 0 to 1, was determined by high-resolution transmission electron microscopy in the sample regions with x approximate to 0.1, 0.2. 0.5. and 0.75. In the Si-0.5(SiO2)(0.5) region, the formation of a Si nanocrystallite network was established. At high concentrations of Si nanocrystallites, nanotwins and stacking faults occurred in the crystallites. Large Si crystallites appeared at x >= 0.5 in the quartz Substrate under the interface, while the film presented nanopores over the interface. The mechanisms for the formation of the nanocrystallites were discussed and correlated with the film properties.

In this work we report the synthesis and characterization of the polypyrrole - porous silicon nanocomposites. Our main goal is to control the growth of conducting polypyrrole into the alveolar pores of porous silicon, in order to get a functionalized nanocomposite with the required properties for application as electrode in microfluidic devices. Porous silicon was electrochemically etched from (100) p-type silicon wafers. Different silicon resistivities (from 1.8 to 11 Omega cm) and preparation conditions (different electrolyte concentrations and current densities) were used. The electrodeposition of polypyrrole into porous silicon was carried out galvanostatically, at a constant current density 2.5 mA/cm(2) from a solution containing acetonitrile, the monomer, pyrrole and p-toluensulfonic acid as electrolyte. The polymerization time was varied in order to investigate the growing process of polypyrrole into the pores. The morphology and of the as-prepared nanocomposites was investigated by SEM. The existence of PPY into porous silicon is evidenced by the EDX spectra of the nanocomposite. The electrical characterization evidences the relative contributions of the polypyrrole and the substrate resistivity, respectively.

The paper presents the modeling of trap discharging processes in Multiple Quantum Well nanostructures. The coupling between trapping and detrapping phenomena, due to the CaF2 buffer layers is discussed. The relative role of tunneling and displacement currents is also analyzed. The model allows the determination of trap parameters that are not directly measurable. The results are in good agreement with the experimental data.

Defects in silicon are studied as function of the dimensionality of the investigated structures. Defects produced by strong irradiation in bulk crystals, like vacancies or interstitial defects, induce other defects, so that the irradiated devices are irreversibly damaged. Defects in nanostructures are shown to be produced mainly by surface/interface states and strains, and are therefore specific to the investigated structure. The modeling of the experimental methods allows the determination of defect parameters that are not directly measurable. The carriers capture on quantum confinement levels in OD systems has a similar behavior with the trapping phenomena.

In the present paper, we intend to bring some clarifications related to the role played by primary defects in long term degradation, to the dependence of degradation on the orientation of the wafer, as well as on the spatial distribution of the degradation produced by low energy particle irradiation.

The paper presents an analysis of the electrical transport, phototransport and photoluminescence measurements performed on silicon-based nanocrystalline systems. The experimental results are discussed within the frame of a quantum confinement model. It is proved that the differences between the energy levels identified in the measurements are related to the quantum selection rules specific to each process. It is also proved that, at nanometric scale, the nature of the atoms composing the nanocrystal represents a first order correction, while the size represents a zero order factor.

The paper presents a quantum confinement model which describes the phototransport processes in nanocrystalline porous silicon. The model supposes that the nanocrystallites surface/interface acts as the walls of an infinite rectangular quantum well, introducing discrete confinement energy levels. This model is proved able to also describe the photoluminescence and the temperature dependence of the dark current in the same samples. Its results are in agreement with the Microstructure investigations.

In this paper, the trapping phenomena in silicon-based nanocrystalline semiconductors are studied. We propose a general and complete model for optical charging spectroscopy measurements, which takes into account the trapping-detrapping-retrapping processes and all the types of discharge currents that can occur in a nanocrystalline system. The model was applied to the measurements performed on multi-quantum well structures, (nc-Si/CaF2)(50), as well as on nanocrystalline porous silicon. This way, the trap parameters that are not directly measurable were determined. (C) 2007 Elsevier Ltd. All rights reserved.

The paper presents the phototransport and photoluminescence investigations on nanocrystalline porous silicon. Spectral dependence curves of the phototransport and photoluminescence were taken at room temperature. A typical phototransport curve presents several maxima and shoulders, while the photoluminescence curves present only one maximum. The temperature dependence of the phototransport was measured in the 85 - 250 K range for different wavelengths. All these curves present only one activation energy on the whole temperature interval. The experimental results are interpreted taking into account a quantum confinement model. Previous microstructure investigations proved that the studied nanocrystalline porous silicon is formed by a nanowire network. Then, the electron Hamiltonian can be split into the sum of a 1D longitudinal Bloch part and a 2D transversal part, the nanowire wall acting like a potential well. Thus, a number of discrete energy levels will appear into the band gap. Both phototransport and photoluminescence maxima are due to the transitions between these levels, corresponding to a mean nanowire diameter of 3.2 nm, in good agreement with the previous microstructure and electrical transport investigations. On the contrary, the temperature dependence of the phototransport is determined by the surface states located on the internal surface and/or the interface between nanocrystallites.

Current-voltage characteristics of samples containing silicon nanodots embedded in an amorphous silicon dioxide matrix were investigated. A percolation mechanism was evidenced by the dependence on the concentration of the initial differential conductance and by the appearance of several thresholds in the I-V characteristics.

We report experimental investigations and modeling of the electronic transport in Si-SiO2 nanocomposite films. The current-voltage characteristics measured at room temperature are interpreted as due to high field-assisted tunneling. The activation energies from the current-temperature curves,are given by the energy separations between quantum confinement electronic states, determined from a quantum well model. Consequently, the calculated mean diameter of a nanodot (5.2 nm) is in good agreement with the microstructure data (5 nm). Also, the potential barrier between nanocrystalline Si and amorphous SiO2, previously obtained for nanocrystalline oxidized porous Si (2.2 eV), is confirmed. (c) 2006 Elsevier B.V. All rights reserved.

The transport properties of nanocrystalline silicon films are presented. The studies were performed on fresh and stabilized nanocrystalline porous silicon (nc-PS) and on Si/SiO2 nanocomposite films. Current-voltage characteristics and the temperature dependence of the dark current were measured on both systems. From the current-voltage characteristics of the oxidized nc-PS, the Si-SiO2 potential barrier was determined (2.2 eV). The Si/SiO2 measurements proved that the main conduction mechanism for the nanodot systems is the field-assisted tunneling under Coulomb blockade. The quantum confinement was evidenced by the activation energies observed in the current-temperature characteristics and it was modeled by means of an infinite quantum well. The trapping-detrapping processes were studied by using the Optical Charging Spectroscopy (OCS) method: the traps are filled by illumination with a suitable wavelength at low temperature and the detrapping is produced by heating at a constant rate. The experimental investigations evidenced six trapping levels in nc-PS, four located at the surface/interface and two deep ones located in the bulk. The deepest level was observed only in oxidized samples. The modeling of the OCS trapping-detrapping processes allows for the determination of the characteristic parameters that cannot be directly obtained from the measurements.

The paper presents a quantum confinement model for the electrical transport, the phototransport and the photoluminescence phenomena in nanocrystalline silicon. The infinite rectangular quantum well was proved to be the best choice for the investigated systems - nanocrystalline porous silicon and silicon nanodots embedded in an amorphous silicon dioxide matrix. Previous microstructure investigations have shown that the nanocrystalline porous silicon is formed by a nanowire network, so that the electron Hamiltonian is the sum of a one-dimensional Bloch-like Hamiltonian and a two-dimensional infinite rectangular quantum well. In the case of the silicon nanodots, the quantum well is three-dimensional. In both cases, the quantum well introduces quantum confinement levels in the band gap, the investigated phenomena being related with transitions between these levels.

Quantum confinement effects in different kinds of nanocrystalline silicon systems are experimentally and theoretically investigated. Porous silicon structured as a nanowire network and silicon nanodots embedded in amorphous silicon dioxide are studied. The main quantum confinement effect in both cases is represented by the appearance of new energy levels in the silicon band gap. The corresponding energies can be experimentally determined from the current - temperature characteristics, which show an Arrhenius-like behavior. The curves present several activation energies between liquid nitrogen temperature and room temperature. The energy levels can be evaluated from a quantum well model. The fundamental level is located at the top of the valence band. The change of the activation energy is then related with the filling of the levels. The ratios of the consecutive activation energies in the current - temperature characteristics prove that the excitation undergoes the angular momentum conservation law imposed by the applied electric field. The estimation of the mean size of the nanocrystals from the values of the activation energies is in good agreement with the microstructure investigations performed on the samples. The confinement levels are also in good agreement with the photoluminescence measurements..

The paper presents the photoluminescence of nanocrystalline porous silicon. Two maxima were observed for rather fresh samples, one located at the infrared edge of the visible range and the other one in red. After ageing, the first maximum vanishes, suggesting its relation with the surface states, while the red one undergoes a significant blue shift. A simple quantum confinement model allows to correlate the photon energy of the red maximum with a transition between two confinement levels and to interpret the blue shift in terms of size reduction by oxidation. These results are in good agreement with previous microstructure measurements.

Electrical conduction mechanisms in Si-SiO2 nanocomposite films were experimentally investigated and theoretically modelled. The films with silicon volume concentration varying from 0% to 100% were deposited on rectangular quartz substrates. They were prepared by co-sputtering CVD from two targets, one of silicon and one of silicon dioxide. Their middle part is formed by Si nanodots embedded in SiO2. 50 parallel aluminium electrodes were deposited, forming 49 coplanar samples, centred around x = 45% Si concentration. The microstructure investigations proved that the silicon nanodots have diameters between 4 nm (x = 23%) and 36 nm (x = 81%). The current-voltage characteristics were measured at room temperature, in the -25 divided by +25 V interval. The obtained curves are symmetric and superlinear. As the current - voltage characteristic is determined by the maximum resistance met by the carriers in their path, the experimental curve is modelled by the field-assisted tunnelling under Coulomb blockade. The used parameters are correlated with previous results obtained on nanocrystalline porous silicon. The correlation coefficient between the theoretical formula and the experimental data is greater than 99.95%.

The paper presents the inicrostructure of the Si/SiO2 nanocomposite films prepared by co-sputtering from Si and SiO2 targets. High resolution transmission electron microscopy and electron diffraction were used to reveal and compare the structures with different silicon/oxygen (SiOx, 0 < x < 2) coinposition. Si nanocrystallites have been evidenced clearly in the SiO1.6 areas. Percolation limits between the Si crystallites was observed in the x=1 areas. The morphology of the Si nanocrystallites and the amorphous SiO2 zones were compared in regions with different composition.

Silicon detectors represent an important option for the Large Hadron Collider and for its upgrades in luminosity and energy, especially for the tracking system. The main limitation in their utilisation comes from the degradation in the hostile radiation environment where they, will work for long time, without the possibility to be changed. The main goal of this paper is to study, the role of growth technology on stable defect concentrations produced by long-time continuous irradiation in the radiation field estimated for LHC and SLHC.

A model for trapping phenomena in nanocrystalline silicon investigated by the optical charging spectroscopy method is proposed. The model takes into account all the possible contributions to the discharge current. The results previously obtained on fresh and passivated porous silicon samples are interpreted within the framework of the model, which provides a good fit to the experimental data. The role played by the different kinds of trapping centers (donors and acceptors, surface and bulk) and the different trap parameters is analyzed. (C) 2003 American Institute of Physics.

In this paper we report correlations between the structure, the photoluminescence and the transport properties of luminescent porous silicon. These correlations combined with the observed temperature dependence of tunneling characteristics yield quite a wholesome (pea-pod-like) model for this system. (C) 2002 Elsevier Science B.V. All rights reserved.

Nanocrystalline silicon is studied with a view to obtaining a new photonic material. Non-equilibrium electronic processes in such materials play a significant role. We have studied trapping phenomena in nanocrystalline porous silicon and nanocrystalline silicon-based Multi-Quantum Wells structures by means of Optical Charging Spectroscopy method, which is a very good and sensitive method. We have also analyzed the modeling of the processes that occur during our measurements. This modeling allows us to separate the relative contribution of the different types of discharge currents that can appear: ohmic conduction currents of either equilibrium or non-equilibrium carriers, displacement currents, diffusion currents and tunneling currents through insulating layers (in Multi-Quantum Wells structures). It also allows us to increase the accuracy of the determination of the experimental trap parameters and to determine parameters that are not directly measurable. The model can be applied to other nanocrystalline semiconductors and can be easily generalized for other high resistivity materials.

Trapping levels which govern the vertical transport in (nc-Si/CaF2)(n) multi-quantum wells were investigated using two different techniques: (a) the temperature dependence of the dark current and (b) the thermally stimulated depolarization current technique (TSDC). Measurements by the first technique showed that the conduction mechanism was thermally activated above 200 K with activation energies of 0.35-0.7 eV, These activation energies were found to increase with increasing electric field. TSDC measurements showed also the existence of at least one broad peak above 200 K with estimated activation energies in the range of 0.4-0.45 eV. Analysis of the peak by the fractional heating method showed a continuous distribution of defect states from 0.3 to 0.83 eV. (C) 2001 Elsevier Science B.V. All rights reserved.

Traps in Multi-Quantum Wells (nc-Si/CaF2)(50) nanostructures using Optical Charging Spectroscopy method were investigated. Three maxima and a final abrupt increase were found in the discharge current versus temperature characteristics. The relative contributions of the tunneling and displacement currents through the insulating calcium fluoride were analyzed.

Conduction properties as well as trapping phenomena in nanocrystalline porous silicon films are investigated in relation to their microstructure. Theoretical models to describe the phenomena are proposed. The main results presented in this paper are: the two-scale porosity of our porous silicon films, the decisive role of the quantum confinement to the conduction phenomena and the prominent part played by the oxidation on the surface effects.

Trapping levels in fresh (one month) and naturally aged (one year) nanocrystalline porous silicon have been investigated using the optical charging spectroscopy method. Four significant maxima and/or shoulders were observed for fresh samples and five for aged ones. They have been attributed to five and six trapping levels, respectively. The trapping centers corresponding to the most shallow four levels are situated at or nearby the internal surface of the porous silicon films. (C) 2000 American Institute of Physics. [S0003-6951(00)01621-1].

The changes produced by anodic and natural oxidation upon the trap parameters in nanocrystalline porous silicon were compared. To put them in evidence, we used optical charging spectroscopy The same trapping It levels (with the same activation energies) were observed after both oxidation processes. In comparison with flesh samples, a new trapping level (the deepest one) appears and the concentrations of the surface traps strongly diminish.

A microstructural investigation of porous silicon films, related with a study of carriers transport. is presented. Scanning electron microscopy and conventional transmission electron microscopy with selected area electron diffraction were performed. The films show a double scale porosity: the macroporosity forms an alveolar structure at a micrometric scale and the nanoporosity forms a silicon nanowires network in the alveolar walls and at their surface. Current-voltage characteristics in the -5 to +5 V range and the temperature dependence of dark current in the 150-330 K interval at low voltage were measured. The transport properties are explained by means of a simplified quantum confinement model. Two symmetries of the nanowires, cylindrical and square, are investigated and the differences between them are shown to be insignificant. (C) 1999 The Electrochemical Society. S0013-4651(98)12-109-2. All rights reserved.

We have measured I-V and C-V characteristics, the temperature dependence of dark currents, and thermally stimulated depolarisation currents on fresh and stored samples of photoluminescent porous silicon, By storage in ambient, the low rectifying I-V curves become strong rectifying, and C-V curves become MIS-like. I-T characteristics for fresh samples have only one activation energy, in the 0.49-0.55 eV range. After storage, a slightly modified value, of about 0.50-0.60 eV is observed at low temperatures only. At about 280 K, the activation energy suddenly changes to 1.20-1.80 eV. Also, both the number and the positions of maxima in thermally stimulated depolarisation currents change by storage. The annealing at about 50 degrees C induces small reversible changes in I-T characteristics and strong irreversible ones in thermally stimulated depolarisation currents, both for fresh and stored samples. A simplified quantum confinement model is proposed to explain the main aspects of the electrical behaviour of porous silicon films. The surface and/or interface contributions are observed especially in thermally stimulated depolarisation currents. The changes induced by storage are attributed to the oxidation process of the internal surface of porous silicon films. (C) 1998 Elsevier Science S.A. All rights reserved.

A quantum confinement model is proposed to explain the electrical transport properties in fresh and stored porous silicon (PS) films. In the present paper, the model is verified for the temperature dependence of the dark current. The studied samples are Formed mainly by a network of nanowires, suggesting the separation of the electron Hamiltonian into a longitudinal and a transversal part. The last one can be well described by a two-dimensional (2D) cylindrical infinite quantum well? whose levels determine the dark current activation energies. The storage oxidation induces modifications both in the number and the values of the activation energies, which are in excellent agreement with our model. (C) 1998 Elsevier Science Ltd. All rights reserved.

A quantum confinement model is proposed to explain the carriers transport in a nanowire network of porous silicon. The electron Hamiltonian is written as the sum of the longitudinal and the transversal contributions. The last one corresponds to a two-dimensional infinite cylindrical quantum well, whose energy levels determine the activation energies observed in the temperature dependence of the dark current. The model is in excellent agreement with the experimental data.

Optical properties of as prepared porous silicon (PS) were compared with those of thermally treated and chemically cleaned samples. The change of the photoluminescence (PL) spectra correlated with ellipsometric. reflectance and Raman measurements leads us to conclude that the adsorbed species in the PS nanocrystalline structure play an important role in the origin of the PI emission, at least for as - anodized samples.

The photoluminescence (PL) decay measurements were performed on porous silicon films. It was observed that the two components of FL, one of them fast (ns) and the other slow (mu s or ms sometimes) have different contributions to PL signal, depending on the wavelength of the excitation light. The slow component of PL was in details investigated. Time decay cures for different excitation (337.1 nm, 470 nm and 550nm) and emission (550, 650, 700, 800 and 860 nm) wavelengths and also for different excitation intensities were taken. All decay curves were fitted with a stretched exponential. The slow component of PL was proposed to be attributed to the radiative recombination on surfaces.

The electrical transport of stabilised porous silicon layers was investigated. The samples were stored in ambient for 1,5-2 years, A MIS-like C-V characteristic, a strong rectifying I-V curve and I-T dependence with two activation energies were obtained, Thermally stimulated depolarization currents have an activation energy of 0.81-0.87 eV. The electrical properties are discussed in the frame of a quantum confinement model, keeping into account the surface component.

Measurements of the visible photoluminescence and the decay of the photoluminescence, performed at different temperatures on porous silicon films, prepared by the anodization process under different conditions on (100) p-type silicon wafers are reported. A separation of the broad visible band of the photoluminescence in two subbands with maxima situated at 1.54 and 1.72 eV at room temperature related to the preparation conditions is obtained. This separation is still present at low temperatures but is very weak. The PL decay process is well described by a bimolecular recombination. The decay time alpha(-1) is in the order of microseconds at room temperature and nanoseconds at liquid nitrogen temperature.

Optical charging spectroscopy and thermally stimulated depolarization measurements were performed on metal-insulator-semiconductor-insulator-metal structures with a ZnS:Mn active layer. Two groups of trapping centres situated at every insulator-semiconductor interface were in evidence. Their depths were found to be centred at 0.55 eV and 0.875 eV respectively in the forbidden gap of a ZnS:Mn film. A partial thermal healing of these interface states was observed.

Localized levels play a major role in the electro-optic properties of Bi12SiO2O (BSO) crystals. The activation energy of trapping levels was studied by different laboratories using various methods (e.g., thermally stimulated currents and photoinduced current transient spectroscopy). A more sensitive investigation of traps in undoped BSO single crystals has been performed by optical charging spectroscopy. The presence of traps in the energy range 0.2-1.1 eV was found, and the results are in good agreement with previous studies. On the other hand, this method led us to suggest that the trapping levels observed can be both electron traps and hole traps. For deep trapping levels at higher temperatures, a strong temperature dependence of the cross section was observed.