NIMP| Ana Maria Lepadatu, Author at NIMP

Education

2012	PhD in Physics
	Faculty of Physics / University of Bucharest / Romania
	PhD Superviser Prof. Magdalena Lidia Ciurea (https://www.brainmap.ro/lidia-magdalena-ciurea)
	Thesis “Study of the electrical properties of some nanostructures based on group IV elements”
2010	Master - Physics of Polymers and Technology of Materials Faculty of Physics / University of Bucharest / Romania

Positions

2020–	Senior Researcher rank 2 and
2016–	Group leader -https://infim.ro/lab30/2020/12/09/group-of-nanomaterials-and-nanostructures-based-on-sigesn/
2016–	Group of “Nanomaterials and Nanostructures based on SiGeSn” (shortly called SiGeSn Group)/ Laboratory 30 “Nanoscale Condensed Matter” /National Institute of Materials Physics (NIMP) / Romania
2016–2020	Senior Researcher rank 3
2016–2020	Group of “Nanomaterials and Nanostructures based on SiGeSn” / Laboratory 30 “Nanoscale Condensed Matter” / NIMP
2014–2016	Senior Researcher rank 3
2014–2016	“Si- and Ge-based Nanomaterials and Nanostructures” Team Laboratory 30 “Nanoscale Condensed Matter” / NIMP
2011–2014	Researcher
2011–2014	“Si- and Ge-based Nanomaterials and Nanostructures” Team Laboratory 30 “Nanoscale Condensed Matter” / NIMP
2008–2011	Assistant Researcher in NIMP, Group of “Silicon-Based Micro- and Nanosystems”
2006–2008	Technician (being student) in NIMP, Group of “Silicon-Based Micro- and Nanosystems”

Awards

2013

“Radu Grigorovici” Award of the Romanian Academy

2019

Diploma of Excellence & ProInvent Medal - ProInvent (International Exhibition of Research, Innovations and Inventions), XVII^th edition, Cluj-Napoca 2019

2013

Best Poster presented during E-MRS 2013 Spring Meeting (Strasbourg, France), Symposium J Prize for early career researchers presenting their work at conf.

2018, 2017

2016, 2013

2011, 2009

Best Paper Awards at IEEE International Semiconductor Conference) CAS 2018, 2017, 2016, 2013, 2011, 2009 editions (from which 4 awards for papers presented by Dr. Lepadatu)

2019

Diploma of Excellence - EuroInvent (European Exhibition of Creativity and Innovation), 11^th edition, Iasi 2019

Research interests

Field of nanostructured materials based on Si-Ge-Sn, i.e. alloy SiGeSn, SiGe, GeSn, Ge, Si nanocrystals/ quantum dots/ nanoparticles (NCs/QDs/NPs) embedded in oxides (HfO₂, TiO₂, SiO₂): #preparation; #characterization correlated with structure and morphology: charge storage, electrical transport, phototransport; #modeling; #quantum confinement (QC) effects and contribution of localized states (LSs);#applications - nonvolatile memories and optical sensors with Ge, SiGe NCs in oxides;

Structures based on ferroelectric HfO₂: #preparation; #characterization (C – V, P – V, PFM) correlated with structure and morphology (GI-XRD, Raman, TEM)

Trapping processes in monocrystals and multilayered nanostructures based on Si; #Nanoscale strain and changes induced in electrical parameters

Expertize

Magnetron sputtering deposition, nanostructuring by thermal annealing (thin films, multilayered structures)

Characterization: ●electrical properties (I–V, I–T), ●memory properties (C–V loops, C–t retention), ●phototransport (I_ph–λ, I_ph–V), ●ferroelectric properties (P – V hysteresis loops), ●Raman spectroscopy, ●photoluminescence

Modeling: ●electrical transport mechanisms in nanostructures with Ge NCs/NPs in oxides: tunneling, hopping; ●charge storage mechanisms; ●electrical behavior of films of Si QDs in SiO₂: percolation processes, electrical transitions between QC energy levels; ●energy structure (QC energy levels/quasibands) of QDs with different shapes, embedded in amorphous matrix; ●spectral quantum efficiency of light absorption on QC levels and absorption threshold in QDs; ●Raman (phonon QC in NCs, composition and strain effects) and photoluminescence spectra (recombination on QC levels in NCs)

Books

1. Chapter “Electrical Characterization Techniques for Porous Silicon” (pp. 655-672, vol. 1), M.L. Ciurea, A.M. Lepadatu in Handbook of Porous Silicon, 2nd edition, edited by L. Canham, Springer International Publishing AG, Cham (2018)

2. Chapter 3 “GeSiO Based Nanostructures: Electrical Behaviour Related to Morphology and Preparation Method” (pp. 47-73), M.L. Ciurea, V.S. Teodorescu, I. Stavarache, A.M. Lepadatu in Size Effects in Nanostructures, Basics and Applications, Springer Ser. Materials, Vol. 205, Eds. V. Kuncser and L. Miu, Springer-Verlag Berlin Heidelberg (2014)

3. Chapter 3 “Quantum Well Solar Cells- Physics, Materials and Technology” (pp. 33-47), M.L. Ciurea, A.M. Lepadatu, I. Stavarache in Advanced Solar Cell Materials, Technology, Modeling and Simulation, Eds. L. Fara and M. Yamaguchi, IGI Global, Hershey PA (2013)

4. “Ge nanodots embedded in a silica matrix” (pp. 193-212), I. Stavarache, A.M. Lepadatu, M.L. Ciurea in Nanomaterials and Nanostructures for Various Applications, Series in Micro and Nanoengineering, Vol. 19, Eds. G. Brezeanu, H. Iovu, C. Cobianu and D. Dascalu, The Publishing House of the Romanian Academy, Bucharest (2012)

5. “Modelling internal quantum efficiency for quantum dots in photovoltaic cells” (pp. 11-22), V. Iancu, M.R. Mitroi, A.M. Lepadatu, I. Stavarache, M.L. Ciurea in Nanostructuring and Nanocharacterization, Series in Micro and Nanoengineering, Vol. 16, Eds. M. Zaharescu, M. Ciurea, I. Kleps and D. Dascalu, The Publishing House of the Romanian Academy, Bucharest (2010)

Patents

1. OSIM patent no. RO131968-B1 (Publ. date May 30, 2018), “Matrice capacitiva pentru memorie nevolatila si procedeu de realizare a acesteia”, A. Slav, C. Palade, A.-M. Lepadatu, S. Lazanu, L.M. Ciurea, D. Vasilache, M. Dragoman

2. OSIM patent application filled in 2021 (A/00193 Apr 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

3. OSIM patent application no. RO134049-A0 (Publ. date Apr 30, 2020), “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

4. OSIM patent application no. RO133300-A0 (Publ. date Apr 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

5. OSIM patent application no. RO133299-A0 (Publ. date Apr 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

6. OSIM patent application no. RO133227-A0 (Publ. date Mar 29, 2019), “Structura de memorie optoelectrica cu poarta flotanta, din nanocristale de germaniu, si procedeu de realizare a acesteia”, T. Stoica, C. Palade, A. Slav, A. Lepadatu

7. OSIM patent application no. RO132946-A0 (Publ. date Nov 29, 2018), “Structura de dozimetru pe baza de capacitor MOS cu poarta flotanta din nanocristale de germaniu, si procedeu de realizare a acesteia”, S. Lazanu, C. Palade, A.M. Lepadatu, I. Stavarache, M.L. Ciurea

Projects

1. SiGeSn nanocrystals with charge storage properties at nanoscale – SIGESNANOMEM

Project Type: TE, Start Date: 2019-05-01 End Date: 2021-04-30

Publications

1. 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.

2. 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.

3. 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: MAR 4 2024, JOURNAL OF PHYSICAL CHEMISTRY C, 128, 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.

4. 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

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. 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).

15. 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.

16. 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.

17. 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.

18. 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.

19. 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.

20. 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.

21. Isotactic polypropylene-vapor grown carbon nanofibers composites: Electrical properties

Authors: Aldica, GV; Ciurea, ML; Chipara, DM; Lepadatu, AM; Lozano, K; Stavarache, I; Popa, S; Chipara, M

Published: OCT 10 2017, JOURNAL OF APPLIED POLYMER SCIENCE, 134, DOI: 10.1002/APP.45297

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

22. 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.

23. 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.

24. Correlation between strain and defects in Bi implanted Si

Authors: Palade, C; Lepadatu, AM; Slav, A; Ciurea, ML; Lazanu, S

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

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

25. Non-volatile memory devices based on Ge nanocrystals

Authors: Vasilache, D; Cismaru, A; Dragoman, M; Stavarache, I; Palade, C; Lepadatu, AM; Ciurea, ML

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

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

26. MEMORY PROPERTIES OF GE NANOCRYSTALS-BASED CAPACITORS WITH DIFFERENT COMPOSITION OF INTERMEDIATE LAYER

Authors: Lepadatu, AM

Published: OCT-DEC 2016, PROCEEDINGS OF THE ROMANIAN ACADEMY SERIES A-MATHEMATICS PHYSICS TECHNICAL SCIENCES INFORMATION SCIENCE, 17, 327, DOI:

In this work, the charge storage properties of trilayers with Ge nanocrystals (NCs) embedded in HfO2 matrix prepared by using two approaches for obtaining the intermediate layer with Ge NCs as nodes are investigated. Trilayer structures with tunnel and control oxides of HfO2, but with different intermediate layers of Ge and Ge-HfO2, respectively, were deposited by magnetron sputtering. Ge nanostructuring was achieved by subsequent rapid thermal annealing. Charge storage behaviour was studied by measurements of capacitance-voltage characteristics on MOS-like capacitors based on the annealed trilayers. The MOS samples with intermediate layer obtained by depositing a continuous Ge layer show hysteresis with memory windows up to similar to 1.3 V, while the ones with co-deposited Ge and HfO2 as intermediate layer present higher memory windows, similar to 1.8 V. In both cases, the memory windows are independent on the frequency demonstrating that the memory effect is due to storing the charge only inside Ge NCs. These show that the approach based on co-deposition of a Ge-HfO2 intermediate layer is more effective for charge storage with direct application in memory devices.

27. How morphology determines the charge storage properties of Ge nanocrystals in HfO2

Authors: Slav, A; Palade, C; Lepadatu, AM; Ciurea, ML; Teodorescu, VS; Lazanu, S; Maraloiu, AV; Logofatu, C; Braic, M; Kiss, A

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

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

28. Non-volatile memory structures with Ge NCs-HfO2 intermediate layer

Authors: Palade, C; Slav, A; Lepadatu, AM; Maraloiu, AV; Lazanu, S; Logofatu, C; Teodorescu, VS; Ciurea, ML

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

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

29. Fast atomic diffusion in amorphous films induced by laser pulse annealing

Authors: Teodorescu, VS; Ghica, C; Maraloiu, AV; Kuncser, A; Lepadatu, AM; Stavarache, I; Ciurea, ML; Scarisoreanu, ND; Andrei, A; Dinescu, M

Published: 2016, 2016 39TH INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), 158, DOI:

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

30. HfO2 with embedded Ge nanocrystals with memory effects

Authors: Palade, C; Slav, A; Lepadatu, AM; Maraloiu, AV; Teodorescu, VS; Ciurea, ML

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

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

31. Nanostructuring of GeTiO amorphous films by pulsed laser irradiation

Authors: Teodorescu, VS; Ghica, C; Maraloiu, AV; Vlaicu, M; Kuncser, A; Ciurea, ML; Stavarache, I; Lepadatu, AM; Scarisoreanu, ND; Andrei, A; Ion, V; Dinescu, M

Published: APR 7 2015, BEILSTEIN JOURNAL OF NANOTECHNOLOGY, 6, 900, DOI: 10.3762/bjnano.6.92

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

32. Tuning the properties of Ge and Si nanocrystals based structures by tailoring the preparation conditions Review

Authors: Ciurea, ML; Lepadatu, AM

Published: JAN-MAR 2015, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 10, 87, DOI:

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

33. STRAIN DRIVEN CHANGES OF DEFECT PARAMETERS IN HEAVY ION IMPLANTED Si

Authors: Palade, C; Lepadatu, AM; Slav, A; Ciurea, ML; Lazanu, S

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

We analyse the influence of the strain field on the parameters of trapping centres. The system under study is high resistivity Si implanted with Bi6+ and I6+ ions respectively, of 28 MeV kinetic energy, 3(O) off <100> axis orientation and 5x10(11) ions/cm(2) fluence. The strain field is the consequence of size and mass difference of the irradiation ions in respect to the atoms of the lattice, and the defects are produced during the slowing-down of ions, as a result of the energy transfer from the ion to Si atoms. These results are of interest for the design and manufacturing of microelectronic devices incorporating strain, particularly for quantum computers with qubits based on the interaction of electronic and nuclear spins of group-V donors in Si.

34. Influence of strain field on nanoscale electronic processes in silicon-based semiconductors

Authors: Lepadatu, AM; Palade, C; Slav, A; Lazanu, S

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

The effects of strain field are studied in Si wafers implanted with heavy iodine and bismuth ions and in multi-quantum well structures. The experimental method of thermally stimulated currents without applied bias is used, and the trapping centres parameters are determined by modelling the discharge curves. In both cases, the strain field produces temperature-dependent parameters of trapping levels. So, due to the high strain field in the neighbourhood of implanted ions, energy levels broaden and cross sections become temperature dependent. In multilayer structures, for the trapping centres corresponding to strain-induced defects both concentrations and capture cross sections are temperature dependent.

35. Transition in conduction mechanism in GeSi nanostructures

Authors: Palade, C; Lepadatu, AM; Stavarache, I; Teodorescu, VS; Ciurea, ML

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

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

36. Annealing induced changes in the structure, optical and electrical properties of GeTiO2 nanostructured films

Authors: Stavarache, I; Lepadatu, AM; Teodorescu, VS; Galca, AC; Ciurea, ML

Published: AUG 1 2014, APPLIED SURFACE SCIENCE, 309, 174, DOI: 10.1016/j.apsusc.2014.04.212

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

37. GeSiO Based Nanostructures: Electrical Behaviour Related to Morphology and Preparation Method

Authors: Ciurea, ML; Teodorescu, VS; Stavarache, I; Lepadatu, AM

Published: 2014, SIZE EFFECTS IN NANOSTRUCTURES: BASICS AND APPLICATIONS, 205, 73, DOI: 10.1007/978-3-662-44479-5_3

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

38. Electrical properties related to the structure of GeSi nanostructured films

Authors: Ciurea, ML; Stavarache, I; Lepadatu, AM; Pasuk, I; Teodorescu, VS

Published: JUL 2014, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 251, 1346, DOI: 10.1002/pssb.201350112

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

39. Charge storage properties of HfO2/Ge-HfO2/SiO2 trilayer structures

Authors: Palade, C; Slav, A; Lepadatu, AM; Teodorescu, VS; Ciurea, ML

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

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

40. TRANSMISSION ELECTRON MICROSCOPY STUDY OF Ge NANOPARTICLES FORMED IN GeSiO FILMS BY ANNEALING IN HYDROGEN

Authors: Teodorescu, VS; Maraloiu, AV; Stavarache, I; Lepadatu, AM; Ciurea, ML

Published: OCT-DEC 2013, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 8, 1780, DOI:

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

41. Quantum Well Solar Cells: Physics, Materials and Technology

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

Published: 2013, ADVANCED SOLAR CELL MATERIALS, TECHNOLOGY, MODELING, AND SIMULATION, 47, DOI: 10.4018/978-1-4666-1927-2.ch003

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

42. Conduction mechanism versus annealing in SiO2 films with Ge nanoparticles

Authors: Palade, C; Lepadatu, AM; Stavarache, I; Teodorescu, VS; Ciurea, ML

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

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

43. 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.

44. 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.

45. Structure and electrical transport in films of Ge nanoparticles embedded in SiO2 matrix

Authors: Stavarache, I; Lepadatu, AM; Maraloiu, AV; Teodorescu, VS; Ciurea, ML

Published: JUL 2012, JOURNAL OF NANOPARTICLE RESEARCH, 14, DOI: 10.1007/s11051-012-0930-5

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

46. IODINE IRRADIATION INDUCED DEFECTS IN CRYSTALLINE SILICON

Authors: Slav, A; Lepadatu, AM; Palade, C; Stavarache, I; Iordache, G; Ciurea, ML; Lazanu, S; Mitroi, MR

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

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

47. TRANSPORT MECHANISMS IN SiO2 FILMS WITH EMBEDDED GERMANIUM NANOPARTICLES

Authors: Palade, C; Lepadatu, AM; Stavarache, I; Maraloiu, AV; Teodorescu, VS; Ciurea, ML

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

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

48. Effects produced by iodine irradiation on high resistivity silicon

Authors: Lazanu, S; Slav, A; Lepadatu, AM; Stavarache, I; Palade, C; Iordache, G; Ciurea, ML

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

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

49. ELECTRICAL BEHAVIOUR RELATED TO STRUCTURE OF NANOSTRUCTURED GeSi FILMS ANNEALED AT 700 degrees C

Authors: Lepadatu, AM; Stavarache, I; Maraloiu, A; Palade, C; Teodorescu, VS; Ciurea, ML

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

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

50. 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.

51. PREPARATION AND ELECTRICAL CHARACTERIZATION OF SiGe NANOSTRUCTURES

Authors: Stavarache, I; Lepadatu, AM; Pasuk, I; Teodorescu, VS; Ciurea, ML

Published: 2011, 2011 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS 2011), 34TH EDITION, VOLS 1 AND 2, 52, DOI:

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

52. Stress-induced traps in multilayered structures

Authors: Ciurea, ML; Lazanu, S; Stavarache, I; Lepadatu, AM; Iancu, V; Mitroi, MR; Nigmatullin, RR; Baleanu, CM

Published: JAN 1 2011, JOURNAL OF APPLIED PHYSICS, 109, DOI: 10.1063/1.3525582

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

53. Calculation of the quantum efficiency for the absorption on confinement levels in quantum dots

Authors: Iancu, V; Mitroi, MR; Lepadatu, AM; Stavarache, I; Ciurea, ML

Published: APR 2011, JOURNAL OF NANOPARTICLE RESEARCH, 13, 1612, DOI: 10.1007/s11051-010-9913-6

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

54. 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.

55. PREPARATION INDUCED ELECTRICAL BEHAVIOUR OF GeSiO NANOSTRUCTURES

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

Published: 2011, 2011 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS 2011), 34TH EDITION, VOLS 1 AND 2, 34, DOI:

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

56. Electrical behavior of multi-walled carbon nanotube network embedded in amorphous silicon nitride

Authors: Stavarache, I; Lepadatu, AM; Teodorescu, VS; Ciurea, ML; Iancu, V; Dragoman, M; Konstantinidis, G; Buiculescu, R

Published: 2011, NANOSCALE RESEARCH LETTERS, 6, DOI: 10.1186/1556-276X-6-88

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

57. TEMPERATURE DEPENDENCE OF CAPTURE COEFFICIENTS IN TRAPPING PHENOMENA

Authors: Lepadatu, AM; Stavarache, I; Lazanu, S; Iancu, V; Mitroi, MR; Nigmatulin, RR; Ciurea, ML

Published: 2010, 2010 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), VOLS 1 AND 2, 374, DOI:

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

58. The influence of shape and potential barrier on confinement energy levels in quantum dots

Authors: Lepadatu, AM; Stavarache, I; Ciurea, ML; Iancu, V

Published: FEB 2010, JOURNAL OF APPLIED PHYSICS, 107, DOI: 10.1063/1.3284083

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

59. 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.

60. INVESTIGATION OF ELECTRICAL PROPERTIES OF CARBON NANOTUBES

Authors: Ciurea, ML; Stavarache, I; Lepadatu, AM; Iancu, V; Dragoman, M; Konstantinidis, G; Buiculescu, R

Published: 2009, CAS: 2009 INTERNATIONAL SEMICONDUCTOR CONFERENCE, VOLS 1 AND 2, PROCEEDINGS, +, DOI: 10.1109/SMICND.2009.5336593

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

61. MODELING OF TRAP DISCHARGING PROCESSES IN MULTIPLE QUANTUM WELL STRUCTURES

Authors: Ciurea, ML; Iancu, V; Stavarache, I; Lepadatu, AM; Rusnac, E

Published: 2008, CAS: 2008 INTERNATIONAL SEMICONDUCTOR CONFERENCE, PROCEEDINGS, +, DOI: 10.1109/SMICND.2008.4703332

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

62. Defects in silicon: From bulk crystals to nanostructures

Authors: Ciurea, ML; Iancu, V; Lazanu, S; Lepadatu, AM; Rusnac, E; Stavarache, I

Published: 2008, ROMANIAN REPORTS IN PHYSICS, 60, 748, DOI:

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

63. Percolation phenomena in silicon - Based nanocrystalline systems

Authors: Lepadatu, AM; Rusnac, E; Stavarache, I

Published: 2007, CAS 2007 INTERNATIONAL SEMICONDUCTOR CONFERENCE, VOLS 1 AND 2, PROCEEDINGS, 578, DOI: 10.1109/SMICND.2007.4519789

The present paper analyzes the appearance of voltage percolation thresholds in the current - voltage characteristics measured on silicon-based nanocrystalline systems that present random space distribution of the nanocrystallites. This percolation phenomenon is explained on the basis of the probability of tunneling under applied bias and is related to the samples microstructure.

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