NIMP| Toma STOICA, Author at NIMP

Education

1982- PhD in Physics, Central Institute of Physics, Bucharest- Magurele

Positions

1972- present, positions in NIMP

1990-present, Senior Researcher Ist degree

1990&1993- Associate Professor, Bucharest University, Faculty of Physics

1992&1996- Humboldt Research Fellowship (1992): solid state of physics, KFA Julich Germany

1996-2001- Head of laboratory of "Physics of Low Dimensional Systems"

2001-2014- Senior Researcher, Forschungszentrum Julich, Peter Grunberg Institut, Nanoelectronics Institute, PGI-9, Juelich, Germany

2015-present- Senior Researcher, NIMP

Awards

member Humboldt Foundation Club
Romanian Academy Prize for Physics, "Constantin Miculescu" 1993 for studies on quantum SiGe structures

Research interests

Research contributions on topics of:

Physics and application of amorphous semiconductors: transport, optical absorption, field and surface effect, termostimulated depolarization, photoelectric and photovoltaic properties, amorphous chalcogenide EPROM memories, a-Si:H solar cells.
Theoretical investigation of the injection phenomena in high resistivity semiconductors, relaxation semiconductors; Modeling of electronic transport, electronic state distribution, optical transmission and ellipsometric investigations; Quantum effects in low dimensional structures, DBRT structures;
LPCVD selective epitaxy, growth, plastic and elastic relaxation of the strain, photoelectrical properties of SiGe epitaxial layers, SiGe and Ge islands LEDs, formation and ordering of Ge islands, ordered Ge dots in SiO2 for floating gate memories;
Self-assembling of GaN and InN nanowires grown by MBE, morphological, optical and electro-optical properties of III-nitride nanowires.
Studies of sol-gel derived oxides and bioactive films.

Expertize

Electric, photoelectric and optical investigations of semiconductor layers and devices
LPCVD Epitaxial growth of Si and SiGe structures
MBE growth studies of GaN-InN nanowires and their properties
AFM, SEM, UV-VIS-IR photoluminescence, Raman, single nanodevices measurements

Books

C. Popescu, T. Stoica, chapter: 2B, "Photoresistive sensors with amorphous thin films", pgs. 37-112, in "Thin film resistive sensors", editors P. Ciureanu and S. Middelhoek, Institute of Physics Publishing Bristol Philadelphia, 1992, ISBN: 0750301732
L. Vescan, T. Stoica, E. Sutter, chapter: "Structural and luminescence properties of order Ge islands on patterned substrates", pgs. 543-567, in "Lateral alignment of epitaxial quantum dots", Editor O. G. Schmidt, Springer Series on Nanoscience and Technology, 2007, ISBN: 978-3-540-46935-3
T. Stoica, E. Sutter, R. Calarco, chapter "GaN and InN nanowires: grown and optoelectronic properties", pgs. 73-96, in "Trends in Nanophysics Theory, Experiment and Tehnology", Editors: A. Aldea, V. Barsan, Springer 2010, ISBN: 978-3-642-12069-5
A. Haab, M. Mikulisc, T. Stoica, B. Kardynal, A. Winden, H. Hardtdegen, D. Grutzmachet, E. Sutter, chapter IV- Physics Frontiers: "An alternative path for the fabrication of self-assembled III-nitride nanowires", pgs. 340-350, in "Future Trends in Microelectronics: Frontiers and Innovations", Edited by S. Luryi, J. Xu, A. Zaslavsky, Wiley-IEEE Press., ISBN: 978-1-118-44216-6

Patents

OSIM patent application no. A/00193 (Publ. date 04-22-2021), "Film de GeSi-HfO2 nanostructurat fotosensibil in domeniul fereastra de lungimi de unda 1200...1600 nm", C. Palade, I. Stavarache, A. Slav, A.-M. Lepadatu, I. Dascalescu, T. Stoica, M.-L. Ciurea
OSIM patent application no. A/00193 (Publ. date 04-22-2021), "Film de SiGeSn nanocristalin fotosensibil in VIS-SWIR si procedeu de realizare a acestuia", M.-L. Ciurea, I. Stavarache, A.-M. Lepadatu, S. Lazanu, T. Stoica
OSIM patent application no. RO133299-A0 (Publ. date 04-30-2019), "Structura pe baza de nanocristale de GeSi in TiO2 pentru fotodetectori in VIS-NIR si procedeu de realizare a acesteia", M.-L. Ciurea, A. Slav, C. Palade, S. Lazanu, A.-M. Lepadatu, T. Stoica
OSIM patent application no. RO133300-A0 (Publ. date 04-30-2019), "Structura fotosensibila in domeniul SWIR, pe baza de nanocristale de Germaniu aliat cu Staniu si procedeu de realizare a acesteia" , T. Stoica, M. Braic, A. Slav, A. E. Kiss, C. Palade, S. Lazanu, A.-M. Lepadatu, M.-L. Ciurea
OSIM patent application no. RO133227-A0 (Publ. date 03-29-2019), T. Stoica, C. Palade, A. Slav, A.-M. Lepadatu
OSIM patent no. RO83541-A (Publ. date 05-30-1984), "Amorphous silicon solar element- consist of quasi-intrinsic semiconductor and insulated layers with transparent or metallic electrodes", T. Stoica, C. Popescu, R. C. Ionescu, A. Dragomir, T. F. Stoica, M. M. Lazarescu, G. Codreanu, N. Munteanu, A. Serpescu
OSIM patent no. RO83540-A (Publ. date 05-30-1984), "Amorphous silicon alloy has stepped conductivity deposit produced by luminescent discharge with supplementary nitrogen addn.", C. Popescu, T. Stoica, R. C. Ionescu, A. Dragomir, M. M. Lazarescu, T. F. Stoica, A. Nendiu, G. Codreanu, N. Munteanu, A. Serpescu

Projects

1. Multifunctional optoelectrical sensor based on two-dimensional MoS2 atomically thin layers grown by selective nucleation

Project Type: PED, Start Date: 2022-06-24 End Date: 2024-06-23

2. Nano-Structured GeSn Coatings for Photonics – GESNAPHOTO_M-ERA.NET project

Project Type: M-ERA.NET, Start Date: 2016-09-01 End Date: 2019-08-31

3. Photo-Electric Capacitor Memory based on Ge-Nanocrystals

Project Type: PED, Start Date: 2017-03-01 End Date: 2018-02-07

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

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. 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.

7. Bandgap atomistic calculations on hydrogen-passivated GeSi nanocrystals

Authors: Cojocaru, O; Lepadatu, AM; Nemnes, GA; Stoica, T; Ciurea, ML

Published: JUN 30 2021, SCIENTIFIC REPORTS, 11, DOI: 10.1038/s41598-021-92936-z

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.

8. 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.

9. Thermoelectric Efficiency of Epitaxial GeSn Alloys for Integrated Si-Based Applications: Assessing the Lattice Thermal Conductivity by Raman Thermometry

Authors: Spirito, D; Driesch, NV; Manganelli, CL; Zoellner, MH; Corley-Wiciak, AA; Ikonic, Z; Stoica, T; Grutzmacher, D; Buca, D; Capellini, G

Published: JUL 26 2021, ACS APPLIED ENERGY MATERIALS, 4, 7392, DOI: 10.1021/acsaem.1c01576

Energy harvesting for Internet of Things applications, comprising sensing, life sciences, wearables, and communications, requires efficient thermoelectric (TE) materials, ideally semiconductors compatible with Si technology. In this work, we investigate the potential of GeSn/Ge layers, a group IV material system, as TE material for low-grade heat conversion. We extract the lattice thermal conductivity, by developing an analytical model based on Raman thermometry and heat transport model, and use it to predict thermoelectric performances. The lattice thermal conductivity decreases from 56 W/(m.K) for Ge to 4 W/(m.K) by increasing the Sn atomic composition to 14%. The bulk cubic Ge0.86Sn0.14 alloy features a TE figure of merit of ZT similar to 0.4 at 300 K and an impressive 1.04 at 600 K. These values are extremely promising in view of the use of GeSn/Ge layers operating in the typical on-chip temperature range.

10. Effects of Ge-related storage centers formation in Al2O3 enhancing the performance of floating gate memories

Authors: Stavarache, I; Cojocaru, O; Maraloiu, VA; Teodorescu, VS; Stoica, T; Ciurea, ML

Published: MAR 15 2021, APPLIED SURFACE SCIENCE, 542, DOI: 10.1016/j.apsusc.2020.148702

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.

11. 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.

12. 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.

13. 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.

14. 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.

15. 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.

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. Enhanced photoconductivity of embedded SiGe nanoparticles by hydrogenation

Authors: Sultan, MT; Gudmundsson, JT; Manolescu, A; Stoica, T; Ciurea, ML; Svavarsson, HG

Published: JUN 15 2019, APPLIED SURFACE SCIENCE, 479, 409, DOI: 10.1016/j.apsusc.2019.02.096

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.

20. 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.

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:

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. 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.

28. Short-wave infrared LEDs from GeSn/SiGeSn multiple quantum wells

Authors: Stange, D; den Driesch, NV; Rainko, D; Roesgaard, S; Povstugar, I; Hartmann, JM; Stoica, T; Ikonic, Z; Mantl, S; Grutzmacher, D; Buca, D

Published: FEB 20 2017, OPTICA, 4, 188, DOI: 10.1364/OPTICA.4.000185

Group IV photonics is on its way to be integrated with electronic circuits, making information transfer and processing faster and more energy efficient. Light sources, a critical component of photonic integrated circuits, are still in development. Here, we compare multi-quantum-well (MQW) light-emitting diodes (LEDs) with Ge0.915Sn0.085 wells and Si0.1Ge0.8Sn0.1 barriers to a reference Ge0.915Sn0.085 homo-junction LED. Material properties as well as band structure calculations are discussed, followed by optical investigations. Electroluminescence spectra acquired at various temperatures indicate effective carrier confinement for electrons and holes in the GeSn quantum wells and confirm the excellent performance of GeSn/SiGeSn MQW light emitters. (C) 2017 Optical Society of America

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. Vapor transport growth of MoS2 nucleated on SiO2 patterns and graphene flakes

Authors: Stoica, T; Stoica, M; Duchamp, M; Tiedemann, A; Mantl, S; Grutzmacher, D; Buca, D; Kardynal, BE

Published: NOV 2016, NANO RESEARCH, 9, 3514, DOI: 10.1007/s12274-016-1227-2

Vapor transport growth of atomically thin MoS2 layers on patterned substrates is investigated, as it is a step towards the self-aligned growth and formation of heterojunctions, which could be useful in future applications. Enhanced formation of MoS2 flakes at the pattern edges is observed on both the substrates examined, namely, patterned thermal SiO2 on Si(100) and graphene flakes on SiO2. The diffusion driven growth leads to the formation of MoS2 monolayers (MLs) with sizes of tens of micrometers around the edges of SiO2 patterns. The growth mode and the optical quality of the MoS2 flakes can be controlled by varying the substrate temperature. Besides the lateral growth, 3R-type pyramids are obtained on prolonging the growth. Lateral MoS2-graphene heterostructures are obtained by using graphene flakes on SiO2 as a substrate.

32. Optically Pumped GeSn Microdisk Lasers on Si

Authors: Stange, D; Wirths, S; Geiger, R; Schulte-Braucks, C; Marzban, B; von den Driesch, N; Mussler, G; Zabel, T; Stoica, T; Hartmann, JM; Mantl, S; Ikonic, Z; Gruetzmacher, D; Sigg, H; Witzens, J; Buca, D

Published: JUL 2016, ACS PHOTONICS, 3, 1285, DOI: 10.1021/acsphotonics.6b00258

The strong correlation between advancing the performance of Si microelectronics and their demand of low power consumption requires new ways of data communication. Photonic circuits on Si are already highly developed except for an eligible on-chip laser source integrated monolithically. The recent demonstration of an optically pumped waveguide laser made from the Si-congruent GeSn alloy, monolithical laser integration has taken a big step forward on the way to an all-inclusive nanophotonic platform in CMOS. We present group IV microdisk lasers with significant improvements in lasing temperature and lasing threshold compared to the previously reported nonundercut Fabry Perot type lasers. Lasing is observed up to 130 K with optical excitation density threshold of 220 kW/cm(2) at 50 K. Additionally the influence of strain relaxation on the band structure of undercut resonators is discussed and allows the proof of laser emission for a just direct Ge0.915Sn0.085 alloy where Gamma and L valleys have the same energies. Moreover, the observed cavity modes are identified and modeled.

33. Dense, Regular GaAs Nanowire Arrays by Catalyst-Free Vapor Phase Epitaxy for Light Harvesting

Authors: Jin, JH; Stoica, T; Trellenkamp, S; Chen, Y; Anttu, N; Migunov, V; Kawabata, RMS; Buenconsejo, PJS; Lam, YM; Haas, F; Hardtdegen, H; Grutzmacher, D; Kardynal, BE

Published: AUG 31 2016, ACS APPLIED MATERIALS & INTERFACES, 8, 22492, DOI: 10.1021/acsami.6b05581

Density dependent growth and optical properties of periodic arrays of GaAs nanowires (NWs) by fast selective area growth MOVPE are investigated. As the period of the arrays is decreased from 500 nm down to 100 nm, a volume growth enhancement by a factor of up to four compared with the growth of a planar layer is observed. This increase is explained as resulting from increased collection of precursors on the side walls of the nanowires due to the gas flow redistribution in the space between the NWs. Normal spectral reflectance of the arrays is strongly reduced compared with a flat substrate surface in all fabricated arrays. Electromagnetic modeling reveals that this reduction is caused by antireflective action of the nanowire arrays and nanowire-diameter dependent light absorption. Irrespective of the periodicity and diameter, Raman scattering and grazing angle X-ray diffraction show signal from zinc blende and wurtzite phases, the latter-originating from stacking faults, as observed by high resolution transmission electron microscopy. Raman spectra contain intense surface phonons peaks, whose intensity depends strongly on the nanowire diameters as a result of potential structural changes and as well as variations of optical field distribution in the nanowires.

34. Study of GeSn based heterostructures: towards optimized group IV MQW LEDs

Authors: Stange, D; von den Driesch, N; Rainko, D; Schulte-Braucks, C; Wirths, S; Mussler, G; Tiedemann, AT; Stoica, T; Hartmann, JM; Ikonic, Z; Mantl, S; Grutzmacher, D; Buca, D

Published: JAN 25 2016, OPTICS EXPRESS, 24, 1367, DOI: 10.1364/OE.24.001358

We present results on CVD growth and electro-optical characterization of Ge0.92Sn0.08/Ge p-i-n heterostructure diodes. The suitability of Ge as barriers for direct bandgap GeSn active layers in different LED geometries, such as double heterostructures and multi quantum wells is discussed based on electroluminescence data. Theoretical calculations by effective mass and 6 band k center dot p method reveal low barrier heights for this specific structure. Best configurations offer only a maximum barrier height for electrons of about 40 meV at the Gamma point at room temperature (e.g. 300 K), evidently insufficient for proper light emitting devices. An alternative solution using SiGeSn as barrier material is introduced, which provides appropriate band alignment for both electrons and holes resulting in efficient confinement in direct bandgap GeSn wells. Finally, epitaxial growth of such a complete SiGeSn/GeSn/SiGeSn double heterostructure including doping is shown. (C)2016 Optical Society of America

35. Direct bandgap GeSn light emitting diodes for short-wave infrared applications grown on Si

Authors: von den Driesch, N; Stange, D; Wirths, S; Rainko, D; Mussler, G; Stoica, T; Ikonic, Z; Hartmann, JM; Grutzmacher, D; Mantl, S; Buca, D

Published: 2016, SILICON PHOTONICS XI, 9752, DOI: 10.1117/12.2211641

The experimental demonstration of fundamental direct bandgap, group IV GeSn alloys has constituted an important step towards realization of the last missing ingredient for electronic-photonic integrated circuits, i.e. the efficient group IV laser source. In this contribution, we present electroluminescence studies of reduced-pressure CVD grown, direct bandgap GeSn light emitting diodes (LEDs) with Sn contents up to 11 at.%. Besides homojunction GeSn LEDs, complex heterojunction structures, such as GeSn/Ge multi quantum wells (MQWs) have been studied. Structural and compositional investigations confirm high crystalline quality, abrupt interfaces and tailored strain of the grown structures. While also being suitable for light absorption applications, all devices show light emission in a narrow short-wave infrared (SWIR) range. Temperature dependent electroluminescence (EL) clearly indicates a fundamentally direct bandgap in the 11 at.% Sn sample, with room temperature emission at around 0.55 eV (2.25 mu m). We have, however, identified some limitations of the GeSn/Ge MQW approach regarding emission efficiency, which can be overcome by introducing SiGeSn ternary alloys as quantum confinement barriers.

36. Direct Bandgap Group IV Epitaxy on Si for Laser Applications

Authors: von den Driesch, N; Stange, D; Wirths, S; Mussler, G; Hollander, B; Ikonic, Z; Hartmann, JM; Stoica, T; Mantl, S; Grutzmacher, D; Buca, D

Published: JUL 14 2015, CHEMISTRY OF MATERIALS, 27, 4702, DOI: 10.1021/acs.chemmater.5b01327

The recent observation of a fundamental direct bandgap for GeSn group W alloys and the demonstration of low temperature lasing provide new perspectives on the fabrication of Si photonic circuits. This work addresses the progress in GeSn alloy epitaxy aiming at room temperature GeSn lasing. Chemical vapor deposition of direct bandgap GeSn alloys with a high Gamma- to L-valley energy separation and large thicknesses for efficient optical mode confinement is presented and discussed. Up to 1 mu m thick GeSn layers with Sn contents up to 14 at. % were grown on thick relaxed Ge buffers, using Ge2H6 and SnCl4 precursors. Strong strain relaxation (up to 81%) at 12.5 at. % Sn concentration, translating into an increased separation between Gamma- and L-valleys of about 60 meV, have been obtained without crystalline structure degradation, as revealed by Rutherford backscattering spectroscopy/ion channeling and transmission electron microscopy. Room temperature reflectance and photoluminescence measurements were performed to probe the optical properties of these alloys. The emission/absorption limit of GeSn alloys can be extended up to 3.5 mu m (0.35 eV), making those alloys ideal candidates for optoelectronics in the mid-infrared region. Theoretical net gain calculations indicate that large room temperature laser gains should be reachable even without additional doping.

37. Optical Transitions in Direct-Bandgap Ge1-xSnx Alloys

Authors: Stange, D; Wirths, S; den Driesch, NV; Mussler, G; Stoica, T; Ikonic, Z; Hartmann, JM; Mantl, S; Grutzmacher, D; Buca, D

Published: NOV 2015, ACS PHOTONICS, 2, 1545, DOI: 10.1021/acsphotonics.5b00372

A comprehensive study of optical transitions in direct-bandgap Ge0.875Sn0.125 group IV alloys via photo-luminescence measurements as a function of temperature, compressive strain and excitation power is performed. The analysis of the integrated emission intensities reveals a strain-dependent indirect-to-direct bandgap transition, in good agreement with band structure calculations based on the 8-band k.p and deformation potential methods. We have observed and quantified Gamma valley-heavy hole and Gamma valley-light hole transitions at low pumping power and low temperatures in order to verify the splitting of the valence band due to strain. We will demonstrate that the intensity evolution of these transitions supports the conclusion about the fundamental direct bandgap in compressively strained GeSn alloys. The presented investigation, thus, demonstrates that direct-bandgap group IV alloys can be directly grown on Ge-buffered Si(001) substrates despite their residual compressive strain.

38. Annealing temperature effect on structure and electrical properties of films formed of Ge nanoparticles in SiO2

Authors: Stavarache, I; Lepadatu, AM; Stoica, T; Ciurea, ML

Published: NOV 15 2013, APPLIED SURFACE SCIENCE, 285, 179, DOI: 10.1016/j.apsusc.2013.08.031

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.

39. Dense Ge nanocrystal layers embedded in oxide obtained by controlling the diffusion-crystallization process

Authors: Lepadatu, AM; Stoica, T; Stavarache, I; Teodorescu, VS; Buca, D; Ciurea, ML

Published: SEP 15 2013, JOURNAL OF NANOPARTICLE RESEARCH, 15, DOI: 10.1007/s11051-013-1981-y

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.

40. Structural investigations of Ge nanoparticles embedded in an amorphous SiO2 matrix

Authors: Stavarache, I; Lepadatu, AM; Gheorghe, NG; Costescu, RM; Stan, GE; Marcov, D; Slav, A; Iordache, G; Stoica, TF; Iancu, V; Teodorescu, VS; Teodorescu, CM; Ciurea, ML

Published: JAN 2011, JOURNAL OF NANOPARTICLE RESEARCH, 13, 232, DOI: 10.1007/s11051-010-0021-4

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.

41. STUDY OF Ge NANOPARTICLES EMBEDDED IN AN AMORPHOUS SiO2 MATRIX WITH PHOTOCONDUCTIVE PROPERTIES

Authors: Lepadatu, AM; Stavarache, I; Stoica, TF; Ciurea, ML

Published: JAN-MAR 2011, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 6, 73, DOI:

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.

42. COLUMN STUDY OF CADMIUM ADSORPTION ONTO POLYACRYLONITRILE/HYDROXYAPATITE COMPOSITE BEAD

Authors: Wang, X; Min, BG; Lyoo, WS; Chihaia, V; Gartner, M; Stoica, TF; Bae, JY; Suh, SH

Published: AUG 2010, REVUE ROUMAINE DE CHIMIE, 55, 447, DOI:

Macroporous polyacrylonitrile/hydroxyapatite (PAN/HAp) composite bead was studied for cadmium removal in a fixed-bed column. The morphology of PAN/HAp composite bead was measured. The adsorption performance of PAN/HAp composite bead in the column was examined by varying bed height and HAp amount in PAN bead. The maximum adsorption capacity and the exhaustion time were determined from the breakthrough curves. Experimental data was described using the Adams-Bohart model. Bed height did not exert large influence on the maximum adsorption capacity and the exhaustion time. The kinetic rate constant and the adsorption capacity of the bed were found to be affected by the total HAp amount.

43. INFLUENCE OF PREPARATION METHOD ON STRUCTURAL PROPERTIES OF GeSiO NANOSYSTEMS

Authors: Stavarache, I; Lepadatu, AM; Teodorescu, V; Stoica, T; Pasuk, I; Stan, G; Iancu, V; Ciurea, ML

Published: 2010, 2010 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), VOLS 1 AND 2, 80, DOI: 10.1109/SMICND.2010.5650255

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.

44. THE SIMULATION IN THE REAL CONDITIONS OF ANTIBACTERIAL ACTIVITY OF TiO2 (Fe) FILMS WITH OPTIMIZED MORPHOLOGY

Authors: Gartner, M; Anastasescu, C; Zaharescu, M; Enache, M; Dumitru, L; Stoica, T; Stoica, TF; Trapalis, C

Published: 2009, NANOSTRUCTURED MATERIALS AND NANOTECHNOLOGY II, 29, +, DOI:

The aim of this study was to provide and characterize nanostructured TiO2 films for application in the depollution of the contaminated water against Escherichia coli (E. coli). The nanometric size of particles and the porosity of the films have been correlated with the technological parameters of the deposition process. For this aim three-layer TiO2 films were deposited by sol-gel and dip-coating method on SiO2/glass substrate. Iron (1 and 7%) and polyethylene glycol (PEG(600)) (0.014-0.110 M) were added to the coating solutions to study their effect on the porosity of the samples and respectively on the amorphous to anatase phase transition, as well as on the antibacterial activity. The samples were characterized using different complementary methods such as X-ray Diffraction (XRD), Spectroscopic Ellipsometry (SE), Atomic Force Microscopy (AFM), dynamics (growth and adherence) of E. coli development versus nanofilm composition (where the amount of bacterial inoculums used was closer to those Found in wasted waters). The obtained films have shown low toughness (values bellow 0.74 nm) and the particle size in nanometric range. The direct optical band gap values increases with the Fe content tip to 2.64 eV (470 nm), leading to an active photocatalyst tinder visible light (in our case under natural illumination condition). The inhibitory effect towards the E. Coli is Correlated with PEG and Fe concentration and was found to he the strongest for the sample containing 0.069 M PEG.

45. STRUCTURAL AND OPTICAL PROPERTIES OF SOL-GEL DERIVED HYDROXYAPATITE FILMS IN DIFFERENT STAGES OF CRYSTALLIZATION AND DENSIFICATION PROCESSES

Authors: Tionica, S; Gartner, M; Ianculescu, A; Anastasescu, M; Slav, A; Pasuk, I; Stoica, T; Zaharescu, M

Published: 2009, NANOSTRUCTURED MATERIALS AND NANOTECHNOLOGY II, 29, +, DOI:

Hydroxyapatite (HAp) films, widely used as a biocompatible material for hard tissues repairing. are studied in this paper for a better control and understanding of the crystallization and densification of sol-gel deposited layers. Calcium nitrate and triethyl phosphite diluted in alcohols were used as calcium and phosphorus precursors. HAp coatings were obtained by spinning method, followed by drying and annealing in the range 130-750 degrees C. Specific temperatures of the chemical reactions of HAp formation have been revealed by thermogravimetric analysis (TG-DTA). Structural. chemical and optical characterizations of the films were performed by Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Analysis (EDX) facilities, X-ray diffraction (XRD). Fourier Transform Infrared Spectroscopy (FTIR) and Spectral Ellipsometry (SE). The film crystallinity increases with file annealing temperature above 550 degrees C. XRD lines of HAp crystallographic planes become narrow at 750 degrees. and correspond to a single crystallographic phase. The Film morphology depends not only on annealing history but also on the synthesis parameters. Thus. layers with large grains of well sinterized HAp nano-crystals can be obtained. Using EDX measurements, the Ca/P ratio was found close to the stoichiometric value in the well sinterized HAp grain regions and slightly increased 1.82 for boundary regions between grains. The density of pores usually observed in sol-gel derived films was estimated for different samples using modeling of SE data.

46. Atomic force microscopy study of TiO2 sol-gel films thermally treated under NH3 atmosphere

Authors: Trapalis, C; Todorova, N; Anastasescu, M; Anastasescu, C; Stoica, M; Gartner, M; Zaharescu, M; Stoica, T

Published: OCT 1 2009, THIN SOLID FILMS, 517, 6247, DOI: 10.1016/j.tsf.2009.02.070

Multilayered TiO2 films were obtained by sol-gel and dipping deposition on quartz substrate followed by thermal treatment under NH3 atmosphere. In an attempt to understand the close relationship between microstructural characteristics and the synthesis parameters, a systematic research of the structure and the morphology of NH3 modified TiO2 sol-gel films by XRD and Atomic Force Microscopy is reported. The surface morphology has been evaluated in terms of grains size, fractal dimension and surface roughness. For each surface, it was found a self-similar behavior (with mean fractal dimension in the range of 2.67-3.00) related to an optimum morphology favorable to maintain a nano-size distribution of the grains. The root mean square (RMS) roughness of the samples was found to be in the range of 0.72-6.02 nm. (C) 2009 Elsevier B.V. All rights reserved.

47. Investigation on the nitrogen doping of multilayered, porous TiO2 thin films

Authors: Gartner, M; Osiceanu, P; Anastasescu, M; Stoica, T; Stoica, TF; Trapalis, C; Glannakopoulou, T; Todorova, N; Lagoyannis, A

Published: SEP 30 2008, THIN SOLID FILMS, 516, 8189, DOI: 10.1016/j.tsf.2008.04.027

N-doping is often used to improve the photocatalytic properties of TiO2, films in order to achieve visible light response. In this work, we study the effect of annealing treatment (temperature and atmosphere) on the structural and optical properties of undoped and N-doped TiO2, films by means of X-ray diffraction (XRD), Atomic Force Microscopy (AFM), Rutherford backscattering (RBS), X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE) and UV-VIS optical transmission spectroscopy. Porous five-layer TiO2, films were deposited by sol-gel method on quartz substrate and thermally treated in oxygen or NH3 flow at 500 and 600 degrees C. Significant doping effect was achieved after annealing at 600 degrees C in NH3 and a shift in optical bandgap value down to visible range (2.69 eV) was observed. (C) 2008 Elsevier B.V. All rights reserved.

48. Hydroxyapatite films obtained by sol-gel and sputtering

Authors: Stoica, TF; Morosanu, C; Slav, A; Stoica, T; Osiceanu, P; Anastasescu, C; Gartner, M; Zaharescu, A

Published: SEP 30 2008, THIN SOLID FILMS, 516, 8116, DOI: 10.1016/j.tsf.2008.04.071

Hydroxyapatite (HA) biocoatings were obtained using two different deposition methods: sol-gel and sputtering. A direct comparison of films obtained using different techniques is necessary because of possible formation of multi-phases and components in HA films. HA films were investigated using spectroscopic ellipsometry (SE) as a powerful method to evaluate composition and structural properties of the films. For both types of films, sputtering and sol-gel, a weak dispersion of the refractive index within the wavelength range 0.4-0.7 pm was found. This corresponds to a high energy bandgap and a high optical transparency in the UV-VIS range. The internal porosity varying in a wide range in the sol-gel films was evaluated using a modeling of SE data. Additionally to SE Studies, structural, chemical and optical characterizations of the Films were performed using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Two absorption bands observed in the wavelength ranges 550-600 cm(-1) and 1000-1200 cm(-1) can be attributed to the major absorption modes associated with HA in agreement with published data. Prom XPS analysis, the atom ratio Ca/P was found for sol-gel films close to the stoichiometric value 1.67. For sputtering films, a higher Ca/P ratio of about 2.1 was estimated. However, similar FTIR spectra have been recorded for both sputtering and sol-gel films. XRD Studies show also similar crystallization conditions by annealing at temperatures higher than 500 degrees C. (C) 2008 Elsevier B.V. All rights reserved.

49. Doped sol-gel TiO2 films for biological applications

Authors: Gartner, M; Trapalis, C; Todorova, N; Giannakopoulou, T; Dobrescu, G; Anastasescu, M; Osiceanu, P; Ghita, A; Enache, M; Dumitru, L; Stoica, T; Zaharescu, M; Bae, JY; Suh, SH

Published: MAY 20 2008, BULLETIN OF THE KOREAN CHEMICAL SOCIETY, 29, 1042, DOI:

Mono and multilayer TiO2(Fe, PEG(600)) films were deposited by the dip-coating on SiO2/glass substrate using sol-gel method. In an attempt to improve the antibacterial properties of doped TiO2 films, the influence of the iron oxides and polyethilenglycol (PEG(600)) on the morphological, optical, surface chemical composition and biological properties of nanostructured layers was studied. Complementary measurements were performed including Spectroscopic Ellipsometry (SE), Scanning Electron Microscopy (SEM) coupled with the fractal analysis, X-Ray Photoelectron Spectroscopy (XPS) and antibacterial tests. It was found that different concentrations of Fe and PEG(600) added to coating solution strongly influence the porosity and morphology at nanometric scale related to fractal behaviour and the elemental and chemical states of the surfaces as well. The thermal treatment under oxidative atmosphere leads to films densification and oxides phase stabilization. The antibacterial activity of coatings against Escherichia Coli bacteria was examined by specific antibacterial tests.

50. Ge dots embedded in silicon dioxide using sol-gel deposition

Authors: Stoica, TF; Gartner, M; Teodorescu, VS; Stoica, T

Published: OCT 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 3274, DOI:

A versatile method, that of sol-gel deposition was used to produce Si1-xGexO2 films on Si substrates. Samples after annealing at different temperatures are investigated by different techniques including high-resolution transmission electron microscopy and spectral ellipsometry. It was revealed that, by annealing at high temperatures, Ge accumulates in small clusters (5-10nm) inside of the oxide layer. By ellipsometric investigations, valuable data about the temperature evolution of the composition and optical constants were obtained.

51. Growth and characterization of high density stoichiometric SiO2 dot arrays on Si through an anodic porous alumina template

Authors: Kokonou, M; Nassiopoulou, AG; Giannakopoulos, KP; Travlos, A; Stoica, T; Kennou, S

Published: MAY 14 2006, NANOTECHNOLOGY, 17, 2151, DOI: 10.1088/0957-4484/17/9/011

In this work, we present the fabrication and full characterization of stoichiometric SiO2 nanoisland arrays ( dots) on silicon, grown through an anodic porous alumina template. Atomic force and transmission electron microscopy (AFM, TEM) were used to characterize the morphology, size, size distribution and density of the dots as a function of the anodization time used. It was found that dot density is lower for very short anodization times, and it stabilizes after a certain time. The dot height increases rapidly after nucleation, reaching values of 8 - 10 nm. With prolonged oxidation the dots continue to nucleate to fill the available area on the silicon surface underneath the porous alumina, while the well developed dots grow in height and width, reaching saturation values at 14 and 55 nm respectively. X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy ( EELS) were used to investigate the stoichiometry and surface coverage of the dots.

52. Properties of high-porosity sol-gel derived indium-tin oxide films

Authors: Stoica, TF; Gartner, M; Stoica, T; Losurdo, M; Teodorescu, VS; Blanchin, MG; Zaharescu, M

Published: OCT 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 2358, DOI:

Indium-tin oxide (ITO) sol-gel films have been obtained with the void concentration up to 50%. The films are nanostructured with nanocrystals of In2O3:Sn and nanovoids. The information obtained from derivative thermo-gravimetry was used to design the annealing program for ITO film formation with a high void concentration. Multilayer films were obtained by successive deposition. The thickness of one layer was about 9 nm. By successive depositions, the void density of the film is reduced. Quantitative analysis of the void density has been performed by spectroscopic ellipsometry. The conductivity of the films can be varied in a large range by annealing in vacuum or in air, at temperature higher than 200 degrees C.

53. Piezoresponse force microscopy for imaging of GaN surfaces

Authors: Calarco, R; Meijers, R; Stoica, T; Luth, H

Published: APR 2005, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 202, 789, DOI: 10.1002/pssa.200461298

Surfaces of GaN films have been investigated by Atomic Force Microscopy (AFM) with implemented Piezoelectric Force Microscopy (PFM) technique. The GaN layers were grown by molecular beam epitaxy (MBE) on Si(111) substrates with an AlN buffer layer. Simultaneous imaging of surface morphology in the AIM and PFM response was performed. The PFM imaging is sensitively dependent on measurement conditions. By increasing the ac voltage the contrast of the PFM images is increased. Using the Voltage-Modulated Force Microscopy (VMFM) regime, with the frequency close to the first resonant frequency of the piezoelectric signal, PFM images with high contrast and high resolution on the nanometer scale were obtained. Regions of opposite piezoresponse were observed and explained by the presence of inversion domains. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

54. Spectroellipsometric study of the sol-gel nanocrystalline ITO multilayer films

Authors: Stoica, TF; Gartner, M; Losurdo, M; Teodorescu, V; Blanchin, M; Stoica, T; Zaharescu, M

Published: MAY 1 2004, THIN SOLID FILMS, 455, 512, DOI: 10.1016/j.tsf.2003.11.251

Tin-doped indium oxide (ITO) thin films have been deposited by sol-gel process using 'sols' of indium and tin isopropoxides. The thickness of one deposited ITO layer is approximately 50 nm. The desired thickness was obtained by 1-5 successive depositions. The XTEM cross-sectional view of an ITO sample with five depositions showed a clear delimitation of the layers with an alternating structure dense/porous ITO layers. The void fraction in porous regions varies between 20 and 25%. Cubic bixbyite In2O3 nanocrystals with size of 10-20 nm and no phases separation of tin oxide were observed. The optical properties of the films have been investigated by optical transmission and spectroscopic ellipsometry. Reliable optical constants and porosity are obtained only with the model of internal structure based on XTEM results. (C) 2003 Elsevier B.V. All rights reserved.

55. Absence of island-island interaction during formation of isolated Ge islands in small windows

Authors: Stoica, T; Vescan, L; Sutter, E

Published: JUN 15 2004, JOURNAL OF APPLIED PHYSICS, 95, 7711, DOI: 10.1063/1.1736312

Isolated Ge islands, i.e., islands not connected by a wetting layer can be obtained by selective epitaxial growth in voids of ultrathin oxides of thickness 1-2 nm. Voids of 30-600 nm size were created before epitaxy during a high temperature anneal of the ultrathin oxide. The formation of one island per window was investigated at 700 degreesC as a function of Ge thickness and void size. Islands nucleate mainly at the edge of the void and for this reason they have an anisotropic shape. In voids smaller than 300 nm only one island is nucleated. Islands form only in voids greater than a critical size (30-80 nm) which depends on the total amount of Ge deposited. We observe that height, width, and aspect ratio of isolated islands increase with void size for a given Ge thickness. A metastable state of Ge in small windows was observed. Moreover, the Si interdiffusion is strongly reduced with decreasing island size (i.e., with void size) reaching only similar to10% in comparison with similar to50% in islands on large areas. (C) 2004 American Institute of Physics.

56. Two-dimensional arrays of nanometre scale holes and nano-V-grooves in oxidized Si wafers for the selective growth of Ge dots or Ge/Si hetero-nanocrystals

Authors: Olzierski, A; Nassiopoulou, AG; Raptis, I; Stoica, T

Published: NOV 2004, NANOTECHNOLOGY, 15, 1700, DOI: 10.1088/0957-4484/15/11/056

Two-dimensional (2D) arrays of nanometre scale holes were opened in thin SiO2 layers on silicon by electron beam lithography and chemical etching. Oxidized silicon wafers with a 5 nm thick SiO2 layer on top were used in this respect. Pattern transfer involved either only removal of SiO2 or a two-step process of oxide removal and anisotropic silicon chemical etching to form nanometre scale silicon V-grooves. The size of the holes in the photoresist layer varied in the range 40-80 nm, depending on the exposure dose used. The smallest holes in the oxide were about 50 nm in diameter, while in V-grooves the smallest width was approximate to70 nm. 2D arrays of Ge dots or Ge/Si hetero-nanocrystals were selectively grown on these patterned silicon wafers. In small windows only one Ge island per hole was nucleated.

57. Morphology, structure and optical properties of sol-gel ITO thin films

Authors: Stoica, TF; Teodorescu, VS; Blanchin, MG; Stoica, TA; Gartner, M; Losurdo, M; Zaharescu, M

Published: AUG 15 2003, MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY, 101, 226, DOI: 10.1016/S0921-5107(02)00667-0

The alkoxidic route and the spinning deposition were used to prepare monolayer sol-gel indium tin oxide (ITO) films. The morphology and crystalline structure were investigated by cross-section transmission electron microscopy (XTEM) and atomic force microscopy (AFM). The ITO sol-gel mono-layer contains three regions of different porosities. The basic crystalline structure is that of the In2O3 lattice. The optical properties have been studied by optical transmission and spectroscopic ellipsometry. (C) 2003 Elsevier Science B.V. All rights reserved.

58. Internal structure of the nanosized sol-gel ITO thin films

Authors: Stoica, TF; Teodorescu, VS; Blanchin, MG; Stoica, TA; Gartner, M; Zaharescu, M

Published: 2001, 2001 INTERNATIONAL SEMICONDUCTOR CONFERENCE, VOL 1 & 2, PROCEEDINGS, 66, DOI:

The alkoxide route and the spinning deposition were used to prepare sol gel ITO films. The morphology and crystalline structure were investigated by Cross-section Transmission Electron Microscopy and Atomic Force Microscopy. The inside of the ITO layer has three regions of different porosity and the basic crystalline structure is that of the In2O3 lattice. Three types of surface morphological structures were found from AFM images.

59. PbSe1-xTex thick thermoelectric films obtained by electrochemical deposition from aqueous solutions

Authors: Nedelcu, M; Sima, M; Stoica, T; Manea, AS; Lazarescu, MF; Visan, T

Published: DEC 2001, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 3, 914, DOI:

It is presented a study of PbSe1-xTex thick films electrodeposition on glassy carbon and ITO electrodes in acid solutions (0.1M HNO3), using cyclic voltammetry and rotating-disk techniques. Electrochemical impedance spectroscopy study of PbSe1-xTex film in 0.1M HNO3 is reported.

60. Optical characterization of dielectric borophosphosilicate glass

Authors: Gartner, M; Modreanu, M; Bosch, S; Stoica, T

Published: APR-MAY 2000, MICROELECTRONICS RELIABILITY, 40, 620, DOI: 10.1016/S0026-2714(99)00291-7

Atmospheric pressure chemical vapour deposition films of borophosphosilicate glass with different contents of phosphorus and boron can be successfully used as masks for etching of silicon in KOH solution. The optical properties of these films were pointed out by spectroellipsometric measurements, using different theoretical models (such as Cauchy and Wemple-DiDomenico models). (C) 2000 Elsevier Science Ltd. All rights reserved.

61. Size distribution and optical properties of self-assembled Ge on Si

Authors: Vescan, L; Goryll, M; Stoica, T; Gartner, P; Grimm, K; Chretien, O; Mateeva, E; Dieker, C; Hollander, B

Published: OCT 2000, APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING, 71, 432, DOI: 10.1007/s003390000555

The distribution of Ge islands is analysed in order to understand their optical behaviour. The Ce islands described in this paper were deposited by low-pressure chemical vapour deposition at relatively high temperature (700 degrees C), therefore the diffusion length of adatoms is high (similar to 100 mu m) and thus, not the limiting factor for nucleation. By changing the deposition time and the coverage, square-based pyramids, domes and relaxed domes are nucleated. Mainly domes emit light, the emission being in the wavelength range 1.38-1.55 mu m. When pyramids or relaxed domes are present, the photoluminescence broadens and decreases in intensity. The electroluminescence of vertically correlated islands increases with the number of layers, i.e. with the number of islands. The nucleation of islands on patterned (001) Si is changed when the deposition is performed on Si mesas with high index facets. The size distribution becomes narrower when the mesa size is decreased. An intermixing of up Bo 40% Si in the 2D layer was determined from photoluminescence data. PIN diodes fabricated on patterned wafers show an area-dependent electroluminecence related to a different microstructure of islands on large and small mesas. Finally, the lateral ordering on {hkl} facets is discussed.

62. Characterization of ITO thin films prepared by spinning deposition starting from a sol-gel process

Authors: Stoica, TF; Stoica, TA; Zaharescu, M; Popescu, M; Sava, F; Popescu-Pogrion, N; Frunza, L

Published: 2000, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 2, 688, DOI:

ITO thin films were deposited by spin coating method starting from indium and tin propoxide in the frame of a sol-gel process. Films with the thickness corresponding to 68 nm for one layer deposition were produced and characterized by X-ray diffraction, transmission electron microscopy and infrared reflection spectroscopy. Low conductivity polycrystalline films exhibiting cubical structure (bixbyite-type) and good adherence were obtained.

63. Electrical and photovoltaic properties of photosensitised ITO/a-Si : H p-i-n/TPyP/Au cells

Authors: Antohe, S; Ion, L; Tomozeiu, N; Stoica, T; Barna, E

Published: MAY 15 2000, SOLAR ENERGY MATERIALS AND SOLAR CELLS, 62, 216, DOI: 10.1016/S0927-0248(99)00127-0

The a-Si:H p-i-n photovoltaic cells photosensitized with an organic layer of 5,10,15,20-tetra (4-pyrydil) 21H,23H porphine (TPyP) have been prepared. The action spectrum of these cells was extended by approximately 30 nm to longer wavelengths, with respect to non-sensitized cells. The presence of the organic layer gives some modifications in the electrical and photovoltaic behavior of the cells, that is why an analysis of dark current-voltage characteristics of the cells at room temperature, is also presented. (C) 2000 Elsevier Science B.V. All rights reserved.

64. Room-temperature light-emitting diodes with Ge islands

Authors: Vescan, L; Chretien, O; Stoica, T; Mateeva, E; Muck, A

Published: OCT-DEC 2000, MATERIALS SCIENCE IN SEMICONDUCTOR PROCESSING, 3, 387, DOI: 10.1016/S1369-8001(00)00059-7

There is an increasing interest in Si-based optoelectronics using Si1-xGex nanostructures due to the possibility of their integration with the Si technology. To overcome the problem of the indirect character of SiGe one is looking for possibilities to increase the contribution of the radiative recombination to the emission. One possible approach involves self-organised growth of lattice-mismatched layers. In the present paper, p-i-n structures, using one layer with Ge islands and which emit in the near infrared up to room temperature were fabricated. The self-organised growth of Ge was performed at 700 degreesC with a small coverage (9 ML) so as to avoid plastic relaxation of the islands, but with a high growth rate (0.3 ML/s) which leads to the formation of a broad bimodal island distribution (small- and medium-sized islands). The diode structure including the Ge islands was deposited in the form of mesas using selective epitaxial growth by low-pressure chemical vapour deposition. The mesa areas were Varied with the aim of demonstrating the influence of size distribution of the islands on the light emission. At low current density the emission is dominated by islands with smaller band gap (larger valence band offset) while at higher currents emission from islands with larger band gap takes place. From the comparison of single diodes with arrays of small-area diodes with the same total area it is found that the arrays emit three times more light due to the lower total number of deep traps in each diode. (C) 2001 Published by Elsevier Science Ltd.

65. Size distribution and electroluminescence of self-assembled Ge dots

Authors: Vescan, L; Stoica, T; Chretien, O; Goryll, M; Mateeva, E; Muck, A

Published: MAY 15 2000, JOURNAL OF APPLIED PHYSICS, 87, 7282, DOI: 10.1063/1.372980

In this article we study the electroluminescence of p-i-n diode structures with Ge dots consisting of coherent three-dimensional small (pyramids) and larger (dome) islands. The Ge dots are formed through strain-induced islanding. The diode structures, including one layer with Ge dots, were deposited on Si mesas with variable areas in order to study the influence of limited area deposition on self-assembling. It was observed that the reduction of deposited area improves island uniformity. The combined analysis of island distribution and electroluminescence spectra has lead to the conclusion that domes in small diodes have a smaller Si content or are less relaxed than domes in larger diodes. The diodes are found to emit up to room temperature near the optical communication wavelength of 1.3 microns. (C) 2000 American Institute of Physics. [S0021-8979(00)04610-7].

66. Influence of the mesa size on Ge island electroluminescence properties

Authors: Chretien, O; Stoica, T; Dentel, D; Mateeva, E; Vescan, L

Published: SEP 2000, SEMICONDUCTOR SCIENCE AND TECHNOLOGY, 15, 925, DOI: 10.1088/0268-1242/15/9/308

The influence of the mesa size on the structural and luminescence properties of Ge island layers deposited by low-pressure chemical vapour deposition was studied. Diode structures containing one Ge island layer were grown selectively on patterned wafers with variable area. The analysis shows that the electroluminescence spectra could be separated into three contributions lying at low-, medium and high-energy positions, respectively. The absolute peak intensity of each signal depends on the mesa size and on the injected current density. The signal at the low-energy position was assigned to an effect due to the facets of the diodes; the two other signals originate from islands on the (001) plane having different Si contents or strain states on the (001) plane.

67. Colloidal sol-gel ITO films on tube grown silicon

Authors: Stoica, TF; Stoica, TA; Vanca, V; Lakatos, E; Zaharescu, M

Published: JUL 6 1999, THIN SOLID FILMS, 348, 278, DOI: 10.1016/S0040-6090(99)00136-4

Electrical and optical properties of colloidal PTO sol-gel films on glass substrates have been investigated. The optical gap of 3.14 eV and room temperature conductivity of 4 Ohm(-1) cm(-1) were obtained on ITO layers of about 500 Angstrom thickness. Heterostructures of ITO/n-Si have been obtained by colloidal sol-gel deposition of ITO. In this deposition method, the oxidation of silicon during the heterostructure formation can be avoided. The silicon substrates were cut out of silicon tubes. Electrical and photoelectric properties of these structures have been studied in comparison with those of p-n tube-Si junctions obtained by thermal boron diffusion. On ITO/n-Si heterojunctions, the spectral quantum efficiency has a maximum value of about two times higher and a blue shift of the maximum position from 0.75 to 0.6 mu m, than those for p-n junctions. A theoretical investigation of the I-V curves for ITO/n-Si heterojunctions under various illumination levels has been performed and contribution of the tunneling and series resistance phenomena were evaluated. (C) 1999 Elsevier Science S.A. All rights reserved.

68. Weak localization effects in ZnO surface wells

Authors: Goldenblum, A; Bogatu, V; Stoica, T; Goldstein, Y; Many, A

Published: AUG 15 1999, PHYSICAL REVIEW B, 60, 5838, DOI: 10.1103/PhysRevB.60.5832

Hall effect, magnetoresistance, and electrical conductivity measurements, carried out on ZnO surface wells created by a large variety of methods, are analyzed in the frame of the weak-localization theory. The ZnO surface wells have some unique features that allow the investigation of the weak-localization effects: ZnO has a single valley conduction band; the Thouless length is much larger than the elastic mean-free path even at room temperature; the well accumulates the largest surface electron concentration obtained up to now in a surface quantum well; there are a large variety of preparation methods, some of them making it possible to modify independently both the width and the depth of the surface wells. These features allowed us to investigate: the presence of the weak-localization effect in the largest range of temperatures(1.6-300 K) reported up to now for a quantum well; the influence on the transport properties of the increase in the number of subbands in the well; the effect of the presence of more inelastic scattering mechanisms and their weights in the entire scattering process; and the passage from a quasi-two-dimensional system to a three-dimensional one. [S0163-1829(99)01932-3].

69. Selective epitaxial growth of strained SiGe/Si for optoelectronic devices

Authors: Vescan, L; Stoica, T; Goryll, M; Grimm, K

Published: FEB 27 1998, MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY, 51, 169, DOI: 10.1016/S0921-5107(97)00253-5

Two aspects of the selective epitaxial growth of Si and Si(1-x)Ge(x) will be discussed. First the facet formation as dependent on the oxide wall orientation and the lateral size of oxide openings. Besides the often cited {111}, {113} and {110} planes additional planes were observed, the {119} and the {0 1 12} planes. Second, the reduction of misfit dislocation density by reducing the area of the pads allows strained Si(1-x)Ge(x) layers to grow much thicker than the critical thickness. As an application for the latter the electroluminescence of forward biased PIN diodes with strained Si(0.80)Ge(0.20)/Si(001) will be discussed as being dependent on the thickness of the SiGe layer. It was found that in thicker strained samples the band edge electroluminescence persists up to room temperature. Quantitative modelling of the electroluminescence could explain the temperature and SiGe thickness dependence of the electroluminescence. (C) 1998 Elsevier Science S.A. All rights reserved.

70. Room-temperature SiGe light-emitting diodes

Authors: Vescan, L; Stoica, T

Published: DEC 1998, JOURNAL OF LUMINESCENCE, 80, 489, DOI: 10.1016/S0022-2313(98)00160-4

In the present paper the electroluminescence of PIN diodes with either strained SiGe/Si or Ge islands in the i-region has been investigated experimentally and by quantitative modelling. The modelling helped to improve the diode structure. Consequently, diodes with strained Si(0.80)Ge(0.20) could be improved so as to reveal emission up to room temperature, if the thickness was high enough. To overcome the thickness limitation due to plastic relaxation, we used selective epitaxy on small areas. We also present results for diodes with Ge islands in the active region. The internal quantum efficiency of light emitting diodes with strained SiGe was at room temperature similar to 10(-4), while diodes with islands emitted ten times less light. (C) 1999 Elsevier Science B.V. AII rights reserved.

71. Electroluminescence of strained SiGe/Si selectively grown above the critical thickness for plastic relaxation

Authors: Stoica, T; Vescan, L; Goryll, M

Published: MAR 15 1998, JOURNAL OF APPLIED PHYSICS, 83, 3373, DOI: 10.1063/1.367104

Electroluminescence of strained Si0.80Ge0.20Si(001) pin diodes has been investigated experimentally and by quantitative modeling. The key aspect of this investigation was that by selective epitaxial growth the experimental critical thickness for plastic relaxation (80 nm at T-epi=700 degrees C and large areas) could be increased in finite pads. SiGe layers with thickness of 60, 72 or 370 nm have been grown within the intrinsic i region of pin structures. Samples free of misfit dislocations revealed electroluminescence with the SiGe no-phonon peak and its transversal optical-phonon replica corresponding to interband transitions. It was found that by increasing the thickness of the SiGe layer the drop in the electroluminescence with increasing temperature could be shifted to higher temperature, so that for the 370 nm thick SiGe sample the emission was observed to persist still at 300 K. Modeling based on drift-diffusion and carrier recombination equations was used to simulate the current-voltage characteristics of the pin diodes and their band gap electroluminescence. It was found that the modeling results can account for the temperature and thickness dependence of the electroluminescence. Hole and electron Shockley-Read-Hall recombination times could be evaluated. (C) 1998 American Institute of Physics.

72. Physics of optimal resonant tunneling

Authors: Racec, PN; Stoica, T; Popescu, C; Lepsa, M; vandeRoer, TG

Published: AUG 15 1997, PHYSICAL REVIEW B, 56, 3597, DOI: 10.1103/PhysRevB.56.3595

The optimal resonant tunneling, or the complete tunneling transparence of a biased double-barrier resonant-tunneling (DBRT) structure, is discussed. It is shown that its physics does not rest on the departure from the constant potential within the barriers and well, due to the applied electric field, but on the effective symmetry of the rectangular-barrier profile, which approximates the real potential profile for the corresponding applied bias.

73. Amorphous rf-sputtered Si100-xNix thin films with O<=x<=15 at.%: Structural, optical and electrical properties

Authors: BeluMarian, A; Serbanescu, MD; Manaila, R; Stoica, T; Dragomir, A; Zavaliche, F; Tanase, M; Devenyi, A

Published: JUN 1 1997, THIN SOLID FILMS, 301, 202, DOI: 10.1016/S0040-6090(97)00007-2

The temperature dependence of the conductivity (between 15 and 300 K) and optical transmission spectra (between 0.8-3.5 mu m) have been measured on r.f.-sputtered Si100-xNix with 0 less than or equal to x less than or equal to 15. The films were characterized by Rutherford backscattering spectroscopy and X-ray diffraction. Changes in the optical sub-bandgap structure, with corresponding changes in the conduction mechanism take place by varying the Ni content. The films are amorphous, showing a main broad diffraction peak. Its position deviates, for this concentrationrange, from 2(kF) (k(F) = Fermi radius), the alloy losing the Hume-Rothery character present for 50 less than or equal to x less than or equal to 60.

74. EFFECTS OF ANNEALING ON THE CONDUCTIVITY OF C-60 THIN-FILMS

Authors: BELUMARIAN, A; MANAILA, R; STOICA, T; DRAGOMIR, A; MANCIU, M; DEVENYI, A; BRAUN, T

Published: 1995, FULLERENE SCIENCE AND TECHNOLOGY, 3, 509, DOI: 10.1080/153638X9508543803

Films of C-60, at different stages of annealing of T-c = 200 degrees and 300 degrees C have been electrical characterized over the temperature domain from -130 degrees C to T-c. X-ray diffraction revealed a random polycrystalline fee structure with stacking defects of an intrinsic nature, due to deposition conditions. The value of room-temperature conductivity was found to be in the range (6.3-1.0)*10(-10) (Omega cm)(-1). In the stable annealed state the conductivity showed an activated temperature dependence above 423 K and a non-activated dependence below 330-280 K. The activation energies E(2) 0.8 eV (film thickness 0.70 mu m) and E(2) = 1.0 eV (film thickness 2.40 mu m) were in good agreement with the energy gap values (1.63 eV and 2.08 eV) which were deduced from the al;sorption spectral dependence. Annealing decreased the non-activated contribution to conduction, extending the intrinsic conduction temperature range.

75. HIGH GAP SPUTTERED DLC LAYERS

Authors: MOROSANU, CO; STOICA, T; DEMARTINO, C; DEMICHELIS, F; TAGLIAFERRO, A

Published: APR 1994, DIAMOND AND RELATED MATERIALS, 3, 816, DOI: 10.1016/0925-9635(94)90275-5

Adherent and transparent unhydrogenated DLC layers were deposited by means of r.f. and d.c. magnetron sputtering at low deposition temperatures. The optical gap value increases from 0.7 to 2.2 eV when deposition pressure is increased from 10(-2) to 10(-1) Torr. In high gap materials, large amounts of sp3 hybridized carbon atoms are present. When Al contacts are evaporated in coplanar configuration on these high gap materials, rectifying and photoelectrical effects are observed.

76. MAGNETOTRANSPORT AND PHOTOLUMINESCENCE OF 2-DIMENSIONAL HOLE GASES IN SI/SI1-XGEX/SI HETEROSTRUCTURES

Authors: LOO, R; VESCAN, L; HARTMANN, A; APETZ, R; ZASTROW, U; SCHAPERS, T; LEUTHER, A; DIEKER, C; LUTH, H; GARTNER, P; STOICA, T

Published: DEC 15 1994, PHYSICAL REVIEW B, 50, 18123, DOI: 10.1103/PhysRevB.50.18113

77. LINE-SHAPE MODEL FOR THE BROAD PHOTOLUMINESCENCE BAND FROM SI1-XGEX/SI HETEROSTRUCTURES

Authors: HARTMANN, A; VESCAN, L; DIEKER, C; STOICA, T; LUTH, H

Published: DEC 15 1993, PHYSICAL REVIEW B, 48, 18279, DOI: 10.1103/PhysRevB.48.18276

Photoluminescence studies on Si1-xGex layers exceeding the critical thickness show not only dislocation-related peaks D1,...,D4, but also in addition a broad band (T band), centered at similar to 110 meV below the strained alloy band gap. Depth profiling studies show that the T band is due to centers in the Si1-xGex. The excitation power dependence and high quantum efficiency of the T band can be explained by assuming that it is due to an isoelectronically trapped exciton. T-band line shapes were fitted by a model based on statistical fluctuations of the Ge content and their effect on the local gap energy.

78. MEYER-NELDEL CORRELATION IN SEMICONDUCTORS AND MOTT MINIMUM METALLIC CONDUCTIVITY

Authors: POPESCU, C; STOICA, T

Published: DEC 15 1992, PHYSICAL REVIEW B, 46, 15071, DOI: 10.1103/PhysRevB.46.15063

The paper outlines conditions under which the statistical shift of the Fermi level within the density-of-states (DOS) distribution may result in the Meyer-Neldel correlation. Sufficient conditions have been found: (i) The DOS should have, around the Fermi level, two competing exponential slopes; (ii) The temperature range for the statistical shift should correspond to kT values higher than at least one of these two characteristic energies associated with the slopes. Under such conditions, the Fermi level is no longer the dominant energy for carrier-concentration changes. Relating the Mayer-Neldel correlation to DOS parameters is an essential step in a correct deduction of Mott's minimum-metallic-conductivity value from experiment.

79. GD AMORPHOUS-SILICON OBTAINED FROM TRICHLORSILANE

Authors: POPESCU, C; STOICA, T; DRAGOMIR, A; NENCIU, A; NICOLAE, M; IVAN, P; IONESCU, S

Published: DEC 1987, JOURNAL OF NON-CRYSTALLINE SOLIDS, 97-8, 1434, DOI: 10.1016/0022-3093(87)90343-7

80. AN EQUATION FOR THE CURRENT VOLTAGE CURVE OF THE A-SI-H SOLAR-CELLS

Authors: POPESCU, C; STOICA, T; SAVANE, AG

Published: JUN 16 1987, PHYSICA STATUS SOLIDI A-APPLIED RESEARCH, 101, 477, DOI: 10.1002/pssa.2211010219

81. EVALUATION OF THE OPTICAL-CONSTANTS AND THICKNESSES OF WEAKLY ABSORBING NONUNIFORM THIN-FILMS

Authors: GRIGOROVICI, R; STOICA, T; VANCU, A

Published: 1982, THIN SOLID FILMS, 97, 185, DOI: 10.1016/0040-6090(82)90226-7

82. LIGHT-INDUCED-CHANGES IN A-SI-H ANALYZED BY FIELD-EFFECT MEASUREMENTS

Authors: STOICA, T

Published: 1981, JOURNAL DE PHYSIQUE, 42, 410, DOI: 10.1051/jphyscol:1981488

83. BULK BOUNDARY-CONDITIONS FOR INJECTION AND EXTRACTION IN TRAP-FREE LIFETIME AND RELAXATION SEMICONDUCTORS

Authors: STOICA, T; POPESCU, C

Published: 1978, PHYSICAL REVIEW B, 17, 3983, DOI: 10.1103/PhysRevB.17.3972

84. PHOTOCONDUCTIVITY IN A-GESE

Authors: VESCAN, L; STOTZEL, H; STOICA, T; KOTTWITZ, A

Published: 1978, PHYSICA STATUS SOLIDI B-BASIC RESEARCH, 87, 246, DOI: 10.1002/pssb.2220870128

85. A nanoscale continuous transition from the monoclinic to ferroelectric orthorhombic phase inside HfO2 nanocrystals stabilized by HfO2 capping and self-controlled Ge doping