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Dr. Catalin PALADE

Scientific Researcher

Laboratory of Atomic Structures and Defects in Advanced Materials (LASDAM)

catalin[DOT]palade[AT]infim[DOT]ro

Projects

1. Memory capacitors based on ZrO2 with Ge nanocrystals or traps as charge storage nodes

Project Type: PD, Start Date: 2020-09-01 End Date: 2022-08-31

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Publications

1. Annealing Effects on the Charging-Discharging Mechanism in Trilayer Al2O3/Ge/Al2O3 Memory Structures
Authors: Stavarache, I; Palade, C; Maraloiu, VA; Teodorescu, VS; Stoica, T; Ciurea, ML

Published: 2024 JAN 17 2024, ACS APPLIED ELECTRONIC MATERIALS, DOI: 10.1021/acsaelm.3c01454

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.
2. Enhancing SiGeSn nanocrystals SWIR photosensing by high passivation in nanocrystalline HfO2 matrix
Authors: Dascalescu, I; Palade, C; Slav, A; Stavarache, I; Cojocaru, O; Teodorescu, VS; Maraloiu, VA; Lepadatu, AM; Ciurea, ML; Stoica, T

Published: FEB 12 2024, SCIENTIFIC REPORTS, 14, 3532, DOI: 10.1038/s41598-024-53845-z

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.
3. Atomically Thin MoS2 Layers Selectively Grown on Mo Patterned Substrates for Field-Effect-Controlled Photosensors
Authors: Stavarache, I; Palade, C; Slav, A; Dascalescu, I; Lepadatu, AM; Trupina, L; Matei, E; Ciurea, ML; Stoica, T

Published: FEB 28 2024, ACS APPLIED NANO MATERIALS, 7, DOI: 10.1021/acsanm.3c05809

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.
4. Annealing Effects on the Charging-Discharging Mechanism in Trilayer Al2O3/Ge/Al2O3 Memory Structures
Authors: Stavarache, I; Palade, C; Maraloiu, VA; Teodorescu, VS; Stoica, T; Ciurea, ML

Published: JAN 17 2024, ACS APPLIED ELECTRONIC MATERIALS, 6, DOI: 10.1021/acsaelm.3c01454

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.
5. Enhancing Short-Wave Infrared Photosensitivity of SiGe Nanocrystals-Based Films through Embedding Matrix-Induced Passivation, Stress, and Nanocrystallization
Authors: Lepadatu, AM; Stavarache, I; Palade, C; Slav, A; Dascalescu, I; Cojocaru, O; Maraloiu, VA; Teodorescu, VS; Stoica, T; Ciurea, ML

Published: 2024 MAR 4 2024, JOURNAL OF PHYSICAL CHEMISTRY C, DOI: 10.1021/acs.jpcc.3c06996

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.
6. Memory Properties of Zr-Doped ZrO2 MOS-like Capacitor
Authors: Palade, C; Slav, A; Stavarache, I; Maraloiu, VA; Negrila, C; Ciurea, ML

Published: SEP 2022, COATINGS, 12, 1369, DOI: 10.3390/coatings12091369

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.
7. SiGeSn Quantum Dots in HfO2 for Floating Gate Memory Capacitors
Authors: Palade, C; Slav, A; Cojocaru, O; Teodorescu, VS; Stoica, T; Ciurea, ML; Lepadatu, AM

Published: MAR 2022, COATINGS, 12, 348, DOI: 10.3390/coatings12030348

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.
8. Graphene/Ferroelectric (Ge-Doped HfO2) Adaptable Transistors Acting as Reconfigurable Logic Gates
Authors: Dragoman, M; Dinescu, A; Dragoman, D; Palade, C; Teodorescu, VS; Ciurea, ML

Published: JAN 2022, NANOMATERIALS, 12, 279, DOI: 10.3390/nano12020279

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.
9. A nanoscale continuous transition from the monoclinic to ferroelectric orthorhombic phase inside HfO2 nanocrystals stabilized by HfO2 capping and self-controlled Ge doping
Authors: Palade, C; Lepadatu, AM; Slav, A; Cojocaru, O; Iuga, A; Maraloiu, VA; Moldovan, A; Dinescu, M; Teodorescu, VS; Stoica, T; Ciurea, ML

Published: SEP 28 2021, JOURNAL OF MATERIALS CHEMISTRY C, 9, 12366, DOI: 10.1039/d1tc02921e

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.
10. Nanocrystallized Ge-Rich SiGe-HfO2 Highly Photosensitive in Short-Wave Infrared
Authors: Palade, C; Lepadatu, AM; Slav, A; Teodorescu, VS; Stoica, T; Ciurea, ML; Ursutiu, D; Samoila, C

Published: NOV 2021, MATERIALS, 14, DOI: 10.3390/ma14227040

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.
11. GeSn/SiO2 Multilayers by Magnetron Sputtering Deposition for Short-Wave Infrared Photonics
Authors: Slav, A; Dascalescu, I; Lepadatu, AM; Palade, C; Zoita, NC; Stroescu, H; Iftimie, S; Lazanu, S; Gartner, M; Buca, D; Teodorescu, VS; Ciurea, ML; Braic, M; Stoica, T

Published: DEC 16 2020, ACS APPLIED MATERIALS & INTERFACES, 12, 56171, DOI: 10.1021/acsami.0c15887

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.
12. GeSi Nanocrystals Photo-Sensors for Optical Detection of Slippery Road Conditions Combining Two Classification Algorithms
Authors: Palade, C; Stavarache, I; Stoica, T; Ciurea, ML

Published: NOV 2020, SENSORS, 20, DOI: 10.3390/s20216395

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.
13. Epitaxial GeSn Obtained by High Power Impulse Magnetron Sputtering and the Heterojunction with Embedded GeSn Nanocrystals for Shortwave Infrared Detection
Authors: Dascalescu, I; Zoita, NC; Slav, A; Matei, E; Iftimie, S; Comanescu, F; Lepadatu, AM; Palade, C; Lazanu, S; Buca, D; Teodorescu, VS; Ciurea, ML; Braic, M; Stoica, T

Published: JUL 29 2020, ACS APPLIED MATERIALS & INTERFACES, 12, 33886, DOI: 10.1021/acsami.0c06212

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.
14. Influence of SiGe Nanocrystallization on Short-Wave Infrared Sensitivity of SiGe-TiO2 Films and Multilayers
Authors: Lepadatu, AM; Palade, C; Slav, A; Cojocaru, O; Maraloiu, VA; Iftimie, S; Comanescu, F; Dinescu, A; Teodorescu, VS; Stoica, T; Ciurea, ML

Published: NOV 12 2020, JOURNAL OF PHYSICAL CHEMISTRY C, 124, 25053, DOI: 10.1021/acs.jpcc.0c06290

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.
15. Wafer-scale graphene-ferroelectric HfO2/Ge-HfO2/HfO(2)transistors acting as three-terminal memristors
Authors: Dragoman, M; Dinescu, A; Dragoman, D; Palade, C; Moldovan, A; Dinescu, M; Teodorescu, VS; Ciurea, ML

Published: DEC 4 2020, NANOTECHNOLOGY, 31, DOI: 10.1088/1361-6528/abb2bf

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.
16. SWIR photoresponse of SiGe/TiO2 multilayers with Ge-rich SiGe nanocrystals
Authors: Lepadatu, AM; Palade, C; Slav, A; Dascalescu, I; Cojocaru, O; Iftimie, S; Teodorescu, VS; Stoica, T; Ciurea, ML

Published: 2020, CAS 2020 PROCEEDINGS: 2020 INTERNATIONAL SEMICONDUCTOR CONFERENCE, 238, DOI:

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.
17. GeSn Nanocrystals in GeSnSiO2 by Magnetron Sputtering for Short-Wave Infrared Detection
Authors: Slav, A; Palade, C; Logofatu, C; Dascalescu, I; Lepadatu, AM; Stavarache, I; Comanescu, F; Iftimie, S; Antohe, S; Lazanu, S; Teodorescu, VS; Buca, D; Ciurea, ML; Braic, M; Stoica, T

Published: JUN 2019, ACS APPLIED NANO MATERIALS, 2, +, DOI: 10.1021/acsanm.9b00571

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.
18. Controlling SWIR photosensitivity limit by composition engineering: from Ge to GeSi nanocrystals embedded in TiO2
Authors: Dascalescu, I; Cojocaru, O; Lalau, I; Palade, C; Slav, A; Lepadatu, AM; Lazanu, S; Stoica, T; Ciurea, ML

Published: 2019, 2019 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS 2019), 42ND EDITION, 40, DOI:

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).
19. Orthorhombic HfO2 with embedded Ge nanoparticles in nonvolatile memories used for the detection of ionizing radiation
Authors: Palade, C; Slav, A; Lepadatu, AM; Stavarache, I; Dascalescu, I; Maraloiu, AV; Negrila, C; Logofatu, C; Stoica, T; Teodorescu, VS; Ciurea, ML; Lazanu, S

Published: NOV 1 2019, NANOTECHNOLOGY, 30, DOI: 10.1088/1361-6528/ab352b

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.
20. Enhanced photoconductivity of SiGe-trilayer stack by retrenching annealing conditions
Authors: Sultan, MT; Gudmundsson, JT; Manolescu, A; Ciurea, ML; Palade, C; Maraloiu, AV; Svavarsson, HG

Published: 2018, CAS 2018 PROCEEDINGS: 2018 INTERNATIONAL SEMICONDUCTOR CONFERENCE, 64, DOI:

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.
21. Enhanced photocurrent in GeSi NCs/TiO2 multilayers
Authors: Palade, C; Slav, A; Cojocaru, O; Teodorescu, VS; Lazanu, S; Stoica, T; Sultan, MT; Svavarsson, HG; Ciurea, ML

Published: 2018, CAS 2018 PROCEEDINGS: 2018 INTERNATIONAL SEMICONDUCTOR CONFERENCE, 76, DOI:

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).
22. Dense Ge nanocrystals embedded in TiO2 with exponentially increased photoconduction by field effect
Authors: Lepadatu, AM; Slav, A; Palade, C; Dascalescu, I; Enculescu, M; Iftimie, S; Lazanu, S; Teodorescul, VS; Ciurea, ML; Stoica, T

Published: MAR 20 2018, SCIENTIFIC REPORTS, 8, DOI: 10.1038/s41598-018-23316-3

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.
23. Enhanced photocurrent in GeSi NCs / TiO2 multilayers
Authors: Palade, C; Slav, A; Cojocaru, O; Teodorescu, VS; Lazanu, S; Stoica, T; Sultan, MT; Svavarsson, HG; Ciurea, ML

Published: 2018, , DOI:

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).
24. Material parameters from frequency dispersion simulation of floating gate memory with Ge nanocrystals in HfO2
Authors: Palade, C; Lepadatu, AM; Slav, A; Lazanu, S; Teodorescu, VS; Stoica, T; Ciurea, ML

Published: JAN 15 2018, APPLIED SURFACE SCIENCE, 428, 702, DOI: 10.1016/j.apsusc.2017.09.038

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.
25. Optoelectric charging-discharging of Ge nanocrystals in floating gate memory
Authors: Palade, C; Slav, A; Lepadatu, AM; Maraloiu, AV; Dascalescu, I; Iftimie, S; Lazanu, S; Ciurea, ML; Stoica, T

Published: NOV 19 2018, APPLIED PHYSICS LETTERS, 113, DOI: 10.1063/1.5039554

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.
26. MOS DOSIMETER BASED ON Ge NANOCRYSTALS IN HfO2
Authors: Palade, C; Slav, A; Lepadatu, AM; Stavarache, I; Dascalescu, I; Cojocaru, O; Stoica, T; Ciurea, ML; Lazanu, S

Published: 2018, CAS 2018 PROCEEDINGS: 2018 INTERNATIONAL SEMICONDUCTOR CONFERENCE, 90, DOI:

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.
27. Influence of preparation conditions on structure and photosensing properties of GeSi/TiO2 multilayers
Authors: Slav, A; Palade, C; Stavarache, I; Teodorescu, VS; Ciurea, ML; Muller, R; Dinescu, A; Sultan, MT; Manolescu, A; Gudmundsson, JT; Svavarsson, HG

Published: 2017, 2017 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), 40TH EDITION, 66, DOI:

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.
28. Single layer of Ge quantum dots in HfO2 for floating gate memory capacitors
Authors: Lepadatu, AM; Palade, C; Slav, A; Maraloiu, AV; Lazanu, S; Stoica, T; Logofatu, C; Teodorescu, VS; Ciurea, ML

Published: APR 28 2017, NANOTECHNOLOGY, 28, DOI: 10.1088/1361-6528/aa66b7

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.
29. Light illumination effects on floating gate memory with Ge nanocrystals in HfO2
Authors: Palade, C; Slav, A; Lepadatu, AM; Lazanu, S; Ciurea, ML; Stoica, T

Published: 2017, 2017 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), 40TH EDITION, 90, DOI:

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.
30. Photosensitive GeSi/TiO2 multilayers in VIS-NIR
Authors: Palade, C; Dascalescu, I; Slav, A; Lepadatu, AM; Lazanu, S; Stoica, T; Teodorescu, VS; Ciurea, ML; Comanescu, F; Muller, R; Dinescu, A; Enuica, A

Published: 2017, 2017 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), 40TH EDITION, 70, DOI:

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.
31. Correlation between strain and defects in Bi implanted Si
Authors: Palade, C; Lepadatu, AM; Slav, A; Ciurea, ML; Lazanu, S

Published: JUN 2016, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 93, 32, DOI: 10.1016/j.jpcs.2016.02.005

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.
32. Non-volatile memory devices based on Ge nanocrystals
Authors: Vasilache, D; Cismaru, A; Dragoman, M; Stavarache, I; Palade, C; Lepadatu, AM; Ciurea, ML

Published: FEB 2016, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 213, 259, DOI: 10.1002/pssa.201532376

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.
33. How morphology determines the charge storage properties of Ge nanocrystals in HfO2
Authors: Slav, A; Palade, C; Lepadatu, AM; Ciurea, ML; Teodorescu, VS; Lazanu, S; Maraloiu, AV; Logofatu, C; Braic, M; Kiss, A

Published: MAR 1 2016, SCRIPTA MATERIALIA, 113, 138, DOI: 10.1016/j.scriptamat.2015.10.028

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.
34. Non-volatile memory structures with Ge NCs-HfO2 intermediate layer
Authors: Palade, C; Slav, A; Lepadatu, AM; Maraloiu, AV; Lazanu, S; Logofatu, C; Teodorescu, VS; Ciurea, ML

Published: 2016, 2016 39TH INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), 166, DOI:

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.
35. HfO2 with embedded Ge nanocrystals with memory effects
Authors: Palade, C; Slav, A; Lepadatu, AM; Maraloiu, AV; Teodorescu, VS; Ciurea, ML

Published: 2015, 2015 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), 48, DOI:

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.
36. STRAIN DRIVEN CHANGES OF DEFECT PARAMETERS IN HEAVY ION IMPLANTED Si
Authors: Palade, C; Lepadatu, AM; Slav, A; Ciurea, ML; Lazanu, S

Published: OCT-DEC 2015, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 10, 1381, DOI:

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 <100> 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.
37. Influence of strain field on nanoscale electronic processes in silicon-based semiconductors
Authors: Lepadatu, AM; Palade, C; Slav, A; Lazanu, S

Published: 2015, 2015 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), 44, DOI:

The effects of strain field are studied in Si wafers implanted with heavy iodine and bismuth ions and in multi-quantum well structures. The experimental method of thermally stimulated currents without applied bias is used, and the trapping centres parameters are determined by modelling the discharge curves. In both cases, the strain field produces temperature-dependent parameters of trapping levels. So, due to the high strain field in the neighbourhood of implanted ions, energy levels broaden and cross sections become temperature dependent. In multilayer structures, for the trapping centres corresponding to strain-induced defects both concentrations and capture cross sections are temperature dependent.
38. Transition in conduction mechanism in GeSi nanostructures
Authors: Palade, C; Lepadatu, AM; Stavarache, I; Teodorescu, VS; Ciurea, ML

Published: 2014, 2014 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), 58, DOI:

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.
39. Charge storage properties of HfO2/Ge-HfO2/SiO2 trilayer structures
Authors: Palade, C; Slav, A; Lepadatu, AM; Teodorescu, VS; Ciurea, ML

Published: 2014, 2014 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), 62, DOI:

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.
40. Trapping centers in heavy ion irradiated silicon
Authors: Palade, C; Lazanu, S; Ciurea, ML

Published: 2014, 2014 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), 128, DOI:

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.
41. Strain-induced modification of trap parameters due to the stopped ions in Bi-irradiated Si
Authors: Ciurea, ML; Lazanu, S; Slav, A; Palade, C

Published: NOV 2014, EPL, 108, DOI: 10.1209/0295-5075/108/36004

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
42. Effect of Bismuth Irradiation on Crystalline Silicon
Authors: Palade, C; Slav, A; Ciurea, ML; Lazanu, S

Published: 2013, 2013 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), VOLS 1-2, 76, DOI:

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.
43. Conduction mechanism versus annealing in SiO2 films with Ge nanoparticles
Authors: Palade, C; Lepadatu, AM; Stavarache, I; Teodorescu, VS; Ciurea, ML

Published: 2013, 2013 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), VOLS 1-2, 34, DOI:

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.
44. IODINE IRRADIATION INDUCED DEFECTS IN CRYSTALLINE SILICON
Authors: Slav, A; Lepadatu, AM; Palade, C; Stavarache, I; Iordache, G; Ciurea, ML; Lazanu, S; Mitroi, MR

Published: 2012, 2012 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), VOLS 1 AND 2, 2, 276, DOI:

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.
45. TRANSPORT MECHANISMS IN SiO2 FILMS WITH EMBEDDED GERMANIUM NANOPARTICLES
Authors: Palade, C; Lepadatu, AM; Stavarache, I; Maraloiu, AV; Teodorescu, VS; Ciurea, ML

Published: 2012, 2012 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), VOLS 1 AND 2, 2, 94, DOI:

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.
46. Effects produced by iodine irradiation on high resistivity silicon
Authors: Lazanu, S; Slav, A; Lepadatu, AM; Stavarache, I; Palade, C; Iordache, G; Ciurea, ML

Published: DEC 10 2012, APPLIED PHYSICS LETTERS, 101, DOI: 10.1063/1.4772015

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]
47. ELECTRICAL BEHAVIOUR RELATED TO STRUCTURE OF NANOSTRUCTURED GeSi FILMS ANNEALED AT 700 degrees C
Authors: Lepadatu, AM; Stavarache, I; Maraloiu, A; Palade, C; Teodorescu, VS; Ciurea, ML

Published: 2012, 2012 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), VOLS 1 AND 2, 2, 112, DOI:

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.
48. A nanoscale continuous transition from the monoclinic to ferroelectric orthorhombic phase inside HfO2 nanocrystals stabilized by HfO2 capping and self-controlled Ge doping
Authors: Palade, C; Lepadatu, AM; Slav, A; Cojocaru, O; Iuga, A; Maraloiu, VA; Moldovan, A; Dinescu, M; Teodorescu, VS; Stoica, T; Ciurea, ML

Published: , JOURNAL OF MATERIALS CHEMISTRY C, DOI: 10.1039/d1tc02921e

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