NIMP| Alin VELEA, Author at NIMP

Education

Interuniversity Microelectronics Center (IMEC), KU Leuven, Belgium - Postdoctoral Fellowship - 2015 – 2016, Project: “Material selection and engineering for selector applications for future high density Resistive RAM Memories”

Paul Scherrer Institute (PSI), Switzerland - Sciex Postdoctoral Fellowship, Scientific Exchange Programme NMS-CH - 2013 – 2014, Project: “In situ investigations of phase change in chalcogenide thin films using synchrotron techniques”

University of Bucharest, Faculty of Physics, Romania - Ph.D. Fellowship (POSDRU) - Materials Science, 2008 – 2011, Thesis: “Phase change nanomaterials”

University of Bucharest, Faculty of Physics, Romania - M.S. - Materials Science, 2008 – 2010

University of Bucharest, Faculty of Physics, Romania - B.A. - Physics-Informatics, 2003 – 2008

Positions

Scientific Researcher I: National Institute of Materials Physics (NIMP), 2021 – present

Study of phase change in memristor devices;

Structural and optical properties of amorphous ternary chalcogenides thin films prepared by combinatorial sputtering;

Synthesis of Cu2S-ZnS-SnS2 thin films for solar cells;

Synthesis and characterization of 2D monochalcogenides;

Scientific Researcher II: National Institute of Materials Physics (NIMP), 2016 – 2021

Materials engineering for Ovonic Threshold Switching by combinatorial magnetron sputtering deposition and high throughput investigations;

Synthesis and characterization of 2D transition metal dichalcogenides;

Development of magnetron sputtering and chemical vapor transport deposition tools for obtaining 2D TMDCs and semiconductor thin films.

Postdoctoral research: IMEC, Belgium, 2015 – 2016

Preparation and electrical assessment of crossbar selector devices built by e-beam lithography;

Modeling subthreshold conduction in materials exhibiting ovonic threshold switching (OTS);

Scientific Researcher III: National Institute of Materials Physics, 2014 – 2015

X-ray diffraction and X-ray reflectometry investigations of phase change in GeTe, SnSe and GaSb mono and multilayers thin films;

Electrical resistance vs. temperature measurements and I-V characteristics of PCM.

Study of optical properties of chalcogenide phase change films by thermo-ellipsometry;

Simulation of the structure of Ge-As-Te chalcogenide materials during memory switching;

Relationship between the glass-formation domain and the memory-switching phenomena;

Postdoctoral research: Paul Scherrer Institute, Switzerland, 2013 – 2014

Study of phase change in chalcogenide thin films using synchrotron techniques (X-ray diffraction and X-ray spectroscopy);

In situ investigations of amorphous chalcogenide layers crystallization;

Scientific Researcher: National Institute of Materials Physics, Romania, 2009 – 2013

Arsenic trisulphide microlenses and photonic structures obtained through femtosecond laser irradiation of vacuum thermal deposited/pulsed laser deposited As2S3 thin films; Silver diffusion study in arsenic trisulphide matrix obtained by ultraviolet, green light and broadband light irradiation of As2S3/Ag heterostructures;

Doctoral Research: National Institute of Materials Physics, Romania, 2008-2011

Study of undoped and metal doped chalcogenide phase change materials from the Ge – Sb –Te and Sn – Se systems for applications in data storage;

Cellular Automata and Monte Carlo modeling of the switching process in chalcogenide phase change materials;

Assistant Researcher: National Institute of Materials Physics, Romania, 2007 – 2009

As2S3 spherical microlenses production and couplings of optical fibers to laser diodes through chalcogenide microlenses; Study of the modifications induced by UV radiation in spherical microlenses made of glassy arsenic sulphide.

Awards

“Radu Grigorovici” prize awarded by the Romanian Academy in 2020 for my work in chalcogenide materials.

Patents

US11386953B2 Multiple memory states device and method of making same

US10902914B2 Programmable Resistive Memory Element and a Method of Making the Same

Projects

1. Functional 2D materials and heterostructures for hybrid spintronic-memristive devices

Project Type: M-ERA.NET, Start Date: 2019-09-01 End Date: 2023-12-31

2. A map for ovonic threshold switching materials (AMOS)

Project Type: TE, Start Date: 2018-05-02 End Date: 2020-04-30

3. Investigation of phase change in stacked chalcogenide thin films for multistate memory cells

Project Type: TE, Start Date: 2015-10-01 End Date: 2017-11-30

Publications

1. Synthesis of WS₂ Ultrathin Films by Magnetron Sputtering Followed by Sulfurization in a Confined Space

Authors: Sava, F; Simandan, ID; Buruiana, AT; Bocirnea, AE; El Khouja, O; Tite, T; Zaki, MY; Mihai, C; Velea, A

Published: MAR 2024, SURFACES, 7, DOI: 10.3390/surfaces7010008

In the quest for advanced materials suitable for next-generation electronic and optoelectronic applications, tungsten disulfide (WS2) ultrathin films have emerged as promising candidates due to their unique properties. However, obtaining WS2 directly on the desired substrate, eliminating the need for transfer, which produces additional defects, poses many challenges. This paper aims to explore the synthesis of WS2 ultrathin films via physical vapor deposition (PVD) followed by sulfurization in a confined space, addressing the challenge of film formation for practical applications. Precursor layers of tungsten and WS2 were deposited by RF magnetron sputtering. Subsequent sulfurization treatments were conducted in a small, closed, graphite box to produce WS2 films. The physical and chemical properties of these precursor and sulfurized layers were thoroughly characterized using techniques such as X-ray reflectometry (XRR), X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The findings reveal notable distinctions in film thickness, structural orientation, and chemical composition, attributable to the different precursor used. Particularly, the sulfurized layers from the tungsten precursor exhibited a preferred orientation of WS2 crystallites with their (00L) planes parallel to the substrate surface, along with a deviation from parallelism in a small angular range. This study highlights the necessity of precise control over deposition and sulfurization parameters to tailor the properties of WS2 films for specific technological applications.

2. Fs Laser Patterning of Amorphous As₂S₃ Thin Films

Authors: Mihai, C; Jipa, F; Socol, G; Kiss, AE; Zamfirescu, M; Velea, A

Published: FEB 2024, MATERIALS, 17, 798, DOI: 10.3390/ma17040798

This study investigates the morphological changes induced by femtosecond (fs) laser pulses in arsenic trisulfide (As2S3) thin films and gold-arsenic trisulfide (Au\As2S3) heterostructures, grown by pulsed laser deposition (PLD). By means of a direct laser writing experimental setup, the films were systematically irradiated at various laser power and irradiation times to observe their effects on surface modifications. AFM was employed for morphological and topological characterization. Our results reveal a clear transition threshold between photoexpansion and photoevaporation phenomena under different femtosecond laser power regimes, occurring between 1 and 1.5 mW, irrespective of exposure time. Notably, the presence of a gold layer in the heterostructure minimally influenced this threshold. A maximum photoexpansion of 5.2% was obtained in As2S3 films, while the Au\As2S3 heterostructure exhibited a peak photoexpansion of 0.8%. The study also includes a comparative analysis of continuous-wave (cw) laser irradiation, confirming the efficiency of fs laser pulses in inducing photoexpansion effects.

3. First Sharp Diffraction Peak features of the intermediate phase glasses and amorphous thin films in the non-stoichiometric (GeS4)x(AsS3)1-x system

Authors: Ciobanu, M; Galca, AC; Sava, F; Zaki, MY; Velea, A; Tsiulyanu, D

Published: MAY 31 2023, THIN SOLID FILMS, 773, 139828, DOI: 10.1016/j.tsf.2023.139828

Grazing incidence X-ray scattering (GIXRS) patterns of thin solid films based on non-stoichiometric chalcogenide glasses (ChG) from the pseudo - binary system (GeS4)x(AsS3)1-x were studied with a focus on the First Sharp Diffraction Peak (FSDP), assigned to the middle range order (MRO) of the glassy material. The films were grown using explosive thermal evaporation in vacuum (10-4 Pa) of pulverized ChG, prepared from previously synthesized bulk glasses, onto mono-crystalline silicon substrates. Scanning Electron Microscopy (SEM) and EnergyDispersive X-ray (EDX) spectroscopy were used to examine the morphology and elemental composition of the films, which were found to have similar composition to the bulk glasses. However, it was revealed that the molecular structure of the grown amorphous films differs from that of the initial ChG bulk material, as indicated by changes in the composition-dependent position and width of the FSDP. Additionally, the intensities of the FSDP in the films were higher compared to those of the bulk samples, suggesting that the molecular-like structure of ChGs is more pronounced in the form of thin films grown from the vapor phase.

4. From non-stoichiometric CTSe to single phase and stoichiometric CZTSe films by annealing under Sn plus Se atmosphere

Authors: Zaki, MY; Sava, F; Simandan, ID; Buruiana, AT; Bocirnea, AE; Stavarache, I; Velea, A; Galca, AC; Pintilie, L

Published: NOV 1 2023, CERAMICS INTERNATIONAL, 49, DOI: 10.1016/j.ceramint.2023.08.056

One of the new materials for next-generation thin film solar cells is Cu2ZnSnSe4 (CZTSe). However, achieving a single-phase CZTSe compound remains a challenge. This study describes the development of Cu2ZnSnSe4 thin films through the sequential deposition of stacked films of non-stoichiometric Cu2SnSe3 (CTSe) and ZnSe by magnetron sputtering. The structural, morphological, and electrical properties as well as the surface chemistry of the films were investigated and compared depending on the growth sequence of the thin films. By using Raman spectroscopy and grazing incidence X-ray diffraction, the tetragonal CZTSe structure was confirmed. Scanning electron microscopy and energy-dispersive spectroscopy measurements of the morphological and compositional properties indicated large grains and dense surfaces with an elemental composition close to the desired stoichiometry in SLG\SnSe2\Cu\ZnSe and SLG\SnSe2\Cu2Se\ZnSe stacks. To ascertain the surface chemistry and unique characteristics of the produced films, additional X-ray photoemission spectroscopy experiments were carried out. The optimal band gap values for the absorber layers were found using conventional spectroscopy, and they ranged from 0.88 to 1.47 eV. According to the electrical measurements, all the films were p-type and have high carrier concentrations between 1016 and 1020 cm-3. Our findings demonstrate that employing a sequential deposition approach and annealing in different atmospheres can yield CZTSe absorber layers with desirable properties, overcoming the challenge of non-stoichiometric CTSe precursors.

5. Understanding the Effects of Post-Deposition Sequential Annealing on the Physical and Chemical Properties of Cu₂ZnSnSe₄ Thin Films

Authors: Catana, DS; Zaki, MY; Simandan, ID; Buruiana, AT; Sava, F; Velea, A

Published: DEC 2023, SURFACES, 6, DOI: 10.3390/surfaces6040031

Cu2ZnSnSe4 thin films have been synthesized by employing two magnetron-sputtering depositions, interlaced with two sequential post-deposition heat treatments in low vacuum, Sn+Se and Se-rich atmospheres at 550 degrees C. By employing successive structural analysis methods, namely Grazing Incidence X-Ray Diffraction (GIXRD) and Raman Spectroscopy, secondary phases such as ZnSe coexisting with the main kesterite phase have been identified. SEM peered into the surface morphology of the samples, detecting structural defects and grain profiles, while EDS experiments showed off-stoichiometric elemental composition. The optical bandgaps in our samples were calculated by a widely used extrapolation method from recorded transmission spectra, holding values from 1.42 to 2.01 eV. Understanding the processes behind the appearance of secondary phases and occurring structural defects accompanied by finding ways to mitigate their impact on the solar cells' properties is the prime goal of the research beforehand.

6. Cu2SnSe3 phase formation from different metallic and binary chalcogenides stacks using magnetron sputtering

Authors: Zaki, MY; Sava, F; Simandan, ID; Buruiana, AT; Mihai, C; Velea, A; Galca, AC

Published: JAN 2023, MATERIALS SCIENCE IN SEMICONDUCTOR PROCESSING, 153, 107195, DOI: 10.1016/j.mssp.2022.107195

Cu2SnSe3 (CTSe) is a polyvalent material that can be used as an absorber layer for thin film solar cells or as a starting layer for the synthesis of CZTSe or CZTSSe compounds. Obtaining CTSe single phase films with opti-mized properties for thin film solar cells is a difficult task. A systematic study using both metallic and binary chalcogenides precursors for the formation of the CTSe phase was not performed. The films consisting of four different stacks (Sn\Cu, SnSe2\Cu, Sn\Cu2Se, and SnSe2\Cu2Se) were prepared by magnetron sputtering on soda lime glass (SLG) and molybdenum (Mo) coated SLG substrates, followed by annealing at 550 degrees C under Sn + Se atmosphere. X-ray diffraction and Raman spectroscopy results indicated the formation of a single CTSe phase in most of the stacks deposited on both substrates. Scanning electron microscopy images showed compact surfaces with large grains in the films deposited on Mo substrate, while the films on SLG have more voids on their sur-faces. The elemental analysis measured by energy dispersive spectroscopy revealed stoichiometric films on Mo, and copper and tin rich compositions on SLG substrates. The band gap values inferred by conventional spec-troscopy are between 0.81 and 1.95 eV. It was found that the SnSe2\Cu and Sn\Cu2Se stacks are preferred for the formation of a single CTSe phase, with dense surface morphology, a stoichiometric composition, and an optimal absorber layer band gap. This study opens the way to comprehend the formation reactions during the seleni-zation of metallic and binary chalcogenides precursors towards the optimization of kesterite absorber for photovoltaic device fabrication.

7. 'Put variety in White': Multi-analytical investigation of the white pigments inlaid on Early Chalcolithic pottery from Southern Romania

Authors: Opris, V; Velea, A; Secu, M; Rostas, AM; Buruiana, AT; Simion, CA; Mirea, DA; Matei, E; Bartha, C; Dimache, M; Lazar, C

Published: APR 2022, JOURNAL OF ARCHAEOLOGICAL SCIENCE-REPORTS, 42, 103402, DOI: 10.1016/j.jasrep.2022.103402

The current study aims to present and discuss the results obtained by complementary archaeometric methods applied for the first time on white pigments inlaid on excised pottery of the Boian-Vidra tradition (Early Chalcolithic, c. 4900-4600 BCE). The samples came from three settlements located in Southern Romania (Sultana-Ghetarie, Vidra, and Vladiceasca). They were selected considering that the pottery was produced in approximately contemporary sites, located relatively close to each other in the same geographical region, namely the Romanian Plain. The experimental part included the analysis of local samples of carbonate concretions and prehistoric animal bone ash as reference materials. Archaeometric investigations consisted in applying "in-air" PIXE and EDX methods for the chemical composition, XRD and FTIR for mineralogical data, SEM for microstructure observation, and EPR for the characterisation of the paramagnetic centres. Calcite, bone ash, and silica rich sediments were identified as the primary decorating pigments. The mixtures of calcite and bone-ash observed in 13 samples were specific to the sites at Vidra and Vladiceasca. Silica-rich sediments from distant sources were the main whitening materials in two samples from Vladiceasca, while for the samples from Sultana-Ghetarie, calcite was the only whitening mineral. The results show with a high degree of confidence the use of both local (i.e., carbonate neo-formations and bone ash) and exotic (silica-rich sediments) raw materials to obtain the white pigment applied to Boian-Vidra pottery. Thus, the current data show the adaptability of the potters with respect to the surrounding resources and also provide new evidence for a vast trade network of raw materials and/or finished products in the Lower Danube area during the Early Chalcolithic. The deliberate mixing of two whitening materials from different sources could be a technological choice and may highlight complex symbolic behaviours.

8. Synthesis and characterization of some C-Ti based multilayer and composite nanostructures

Authors: Ciupina, V; Lungu, CP; Vladoiu, R; Prodan, GC; Porosnicu, C; Vasile, E; Prodan, M; Nicolescu, V; Dinca, V; Cupsa, O; Velea, A; Manu, R

Published: MAR-APR 2022, JOURNAL OF OVONIC RESEARCH, 18, DOI: 10.15251/JOR.2022.182.177

Carbon-Titanium multilayer and composite thin films were obtained by Thermionic Vacuum Arc (TVA) method. The nanostructured films consisted by a carbon base layer and seven alternatively Titanium and Carbon layers were deposed on Silicon substrate. As well, to give C-Ti multilayer films with different percentages in Ti and C of layers, a thick Carbon base layer was deposed on Si substrate, and then seven Ti-C layers. In order to achieve the successively layers with C, and Ti different percentages, were adjusted the discharge parameters of C and Ti plasma sources to obtain the desired composition of layers. By changing of substrate temperature, and on the other hand the bias potential up to ???700V, different batches of samples were obtained. Characterization of structural properties of films was achieved by Grazing Incidence X-ray Diffraction (GIXRD) and Electron Microscopy technique (TEM). The measurements show that increase of the substrate temperature reveal the changes in TixCy lattice parameters. The tribological measurements were performed using a ball-on-disk system with normal forces of 0.5, 1, 2, 3N respectively and a Bruker Hystrion TI 980 TriboIndenter. Was found that the coefficient of friction depends on the synthesis temperature, bias voltage and also by the C content, Ti content and amount of TiC nanocrystallites. To characterize the electrical conductive properties, the electrical surface resistance versus temperature have been measured, and then the electrical conductivity is calculated. Using the Wiedeman-Frantz law was obtained the thermal conductivity.

9. A Two-Step Magnetron Sputtering Approach for the Synthesis of Cu2ZnSnS4 Films from Cu2SnS3\ZnS Stacks

Authors: Zaki, MY; Sava, F; Simandan, ID; Buruiana, AT; Stavarache, I; Bocirnea, AE; Mihai, C; Velea, A; Galca, AC

Published: JUN 2022, ACS OMEGA, DOI: 10.1021/acsomega.2c02475

Cu2ZnSnS4 (CZTS) is regarded as one of the emerging materials for next-generation thin film solar cells. However, its synthesis is complex, and obtaining a single-phase CZTS thin film is difficult. This work reports the elaboration of Cu2ZnSnS4 thin films by a sequential magnetron sputtering deposition of Cu2SnS3 (CTS) and ZnS as stacked films. Initially, the CTS films were prepared on a soda lime glass substrate by annealing Cu and SnS2 stacked layers. Second, ZnS was deposited by magnetron sputtering on the CTS films. The CTS\ZnS stacks were then annealed in Sn + S or S atmospheres. The tetragonal CZTS structure was obtained and confirmed by grazing incidence X-ray diffraction and Raman spectroscopy. The morphological and compositional characteristics, measured by scanning electron microscopy and energy-dispersive spectroscopy, revealed large grains and dense surfaces with the elemental composition close to the intended stoichiometry. Additional X-ray photoemission spectroscopy measurements were performed to determine the surface chemistry and particularities of the obtained films. The optical properties, determined using conventional spectroscopy, showed optimal absorber layer band gap values ranging between 1.38 and 1.50 eV. The electrical measurements showed that all the films are p-type with high carrier concentrations in the range of 10(15) to 10(20) cm(-3). This new synthesis route for CZTS opens the way to obtain high-quality films by an industry-compatible method.

10. Effect of the stacking order, annealing temperature and atmosphere on crystal phase and optical properties of Cu2SnS3

Authors: Zaki, MY; Sava, F; Simandan, ID; Buruiana, AT; Mihai, C; Velea, A; Galca, AC

Published: MAY 13 2022, SCIENTIFIC REPORTS, 12, 7958, DOI: 10.1038/s41598-022-12045-3

Cu2SnS3 (CTS) is emerging as a promising absorber for the next generation thin film solar cells (TFSC) due to its excellent optical and electronic properties, earth-abundance and eco-friendly elemental composition. In addition, CTS can be used as precursor films for the Cu2ZnSnS4 (CZTS) synthesis. The optical properties of CTS are influenced by stoichiometry, crystalline structure, secondary phases and crystallite size. Routes for obtaining CTS films with optimized properties for TFSC are still being sought. Here, the CTS thin films synthesized by magnetron sputtering on soda lime glass (SLG) using Cu and SnS2 targets in two different stacks, were studied. The SLG\Cu\SnS2 and SLG\SnS2\Cu stacks were annealed in S and Sn + S atmospheres, at various temperatures. Both stacks show a polymorphic structure, and higher annealing temperatures favor the monoclinic CTS phase formation. Morphology is influenced by the stacking order since a SnS2 top layer generates several voids on the surface due to the evaporation of SnS, while a Cu top layer provides uniform and void-free surfaces. The films in the copper-capped stack annealed under Sn + S atmosphere have the best structural, morphological, compositional and optical properties, with tunable band gaps between 1.18 and 1.37 eV. Remarkably, secondary phases are present only in a very low percent (< 3.5%) in samples annealed at higher temperatures. This new synthesis strategy opens the way for obtaining CTS thin films for solar cell applications, that can be used also as intermediary stage for CZTS synthesis.

11. Layered SnSe nanoflakes with anharmonic phonon properties and memristive characteristics

Authors: Buruiana, AT; Bocirnea, AE; Kuncser, AC; Tite, T; Matei, E; Mihai, C; Zawadzka, N; Olkowska-Pucko, K; Kipczak, L; Babinski, A; Molas, MR; Velea, A; Galca, AC

Published: OCT 15 2022, APPLIED SURFACE SCIENCE, 599, 153983, DOI: 10.1016/j.apsusc.2022.153983

Understanding the phonon anharmonicity and temperature-dependent behavior of phonons that affect the thermal transport properties in 2D materials is crucial for developing efficient thermoelectric and memristor devices. SnSe has attracted significant interest because of its potential applications for developing such novel devices. Here, orthorhombic SnSe nanoflakes with a thickness of less than 100 nm and oriented along the [100] crystal axis were obtained using physical vapor transport at atmospheric pressure. Polarization-resolved Raman spectroscopy of SnSe nanoflakes was performed at a temperature of 5 K. Temperature-dependent frequencies and linewidths of Raman modes in tin selenide were fitted according to the anharmonic phonon coupling theory. The results indicate that both two and three order processes are responsible for the phonon decay in tin selenide. The memristive property was confirmed by electrical measurements of SnSe devices. SnSe memristors have an operating current of 10-4 A, similar to other transition-metal dichalcogenide memristors, but are more energy efficient than memristors based on defect migration, with a threshold voltage of 3 V.

12. New Chalcogenide Glass-Ceramics Based on Ge-Zn-Se for IR Applications

Authors: Velea, A; Sava, F; Badica, P; Burdusel, M; Mihai, C; Galca, AC; Matei, E; Buruiana, AT; El Khouja, O; Calvez, L

Published: JUL 2022, MATERIALS, 15, 5002, DOI: 10.3390/ma15145002

The consumer market requests infrared (IR) optical components, made of relatively abundant and environmentally friendly materials, to be integrated or attached to smartphones. For this purpose, three new chalcogenides samples, namely Ge23.3Zn30.0Se46.7 (d_GZSe-1), Ge26.7Zn20.0Se53.3 (d_GZSe-2) and Ba4.0Ge12.0Zn17.0Se59.0I8.0 (d_GZSe-3) were obtained by mechanical alloying and processed by spark plasma sintering into dense bulk disks. Obtaining a completely amorphous and homogeneous material proved to be difficult. d_GZSe-2 and d_GZSe-3 are glass-ceramics with the amount of the amorphous phase being 19.7 and 51.4 wt. %, while d_GZSe-1 is fully polycrystalline. Doping with barium and iodine preserves the amorphous phase formed by milling and lowers the sintering temperature from 350 degrees C to 200 degrees C. The main crystalline phase in all of the prepared samples is cubic ZnSe or cubic Zn0.5Ge0.25Se, while in d_GZSe-3 the amorphous phase contains GeSe4 clusters. The color of the first two sintered samples is black (the band gap values are 0.42 and 0.79 eV), while d_GZSe-3 is red (E-g is 1.37 eV) and is transparent in IR domain. These results are promising for future research in IR materials and thin films.

13. Enhanced photoelectrochemical activity of WO3-decorated native titania films by mild laser treatment

Authors: Spataru, T; Mihai, MA; Preda, L; Marcu, M; Radu, MM; Becherescu, ND; Velea, A; Zaki, MY; Udrea, R; Satulu, V; Spataru, N

Published: SEP 15 2022, APPLIED SURFACE SCIENCE, 596, 153682, DOI: 10.1016/j.apsusc.2022.153682

The effectiveness of the electrochemical WO3-modification as a method for improving the photoactivity of native air-formed TiO2 layers was assessed. The way in which a mild laser treatment influences the photo-electrochemical performances of the thus obtained WO3/TiO2 systems was also investigated. At laser-treated electrodes (L-WO3/TiO2), the melting-solidification process induced by the treatment led to a smaller size of the deposited WO3 particles and to their better dispersion on the surface. The treatment also enhanced the surface oxygen deficiency and ensured better relative absorptivity of the oxygenated species on the surface. These features, together with the intrinsic narrower bandgap of the WO3/TiO2 composites, the higher donor density and the lower flat band potential of L-WO3/TiO2 enabled faster kinetic of the oxygen photoanodic evolution. Importantly, the same process exhibited a cathodic shift of its onset potential. The laser treatment also strongly enhanced the photoelectrocatalytic performances for UV-assisted methanol anodic oxidation.

14. Synthesis and Characterization of Cu2ZnSnS4 Thin Films Obtained by Combined Magnetron Sputtering and Pulsed Laser Deposition

Authors: Zaki, MY; Sava, F; Buruiana, AT; Simandan, ID; Becherescu, N; Galca, AC; Mihai, C; Velea, A

Published: SEP 2021, NANOMATERIALS, 11, DOI: 10.3390/nano11092403

Cu2ZnSnS4 (CZTS) is a complex quaternary material, and obtaining a single-phase CZTS with no secondary phases is known to be challenging and dependent on the production technique. This work involves the synthesis and characterization of CZTS absorber layers for solar cells. Thin films were deposited on Si and glass substrates by a combined magnetron sputtering (MS) and pulsed laser deposition (PLD) hybrid system, followed by annealing without and with sulfur powder at 500 degrees C under argon (Ar) flow. Three different Cu2S, SnS2, and ZnS targets were used each time, employing a different target for PLD and the two others for MS. The effect of the different target arrangements and the role of annealing and/or sulfurization treatment were investigated. The characterization of the absorber films was performed by grazing incidence X-ray diffraction (GIXRD), X-ray reflectometry (XRR), Raman spectroscopy, scanning electron microscopy, and regular transmission spectroscopy. The film with ZnS deposited by PLD and SnS2 and Cu2S by MS was found to be the best for obtaining a single CZTS phase, with uniform surface morphology, a nearly stoichiometric composition, and an optimal band gap of 1.40 eV. These results show that a new method that combines the advantages of both MS and PLD techniques was successfully used to obtain single-phase Cu2ZnSnS4 films for solar cell applications.

15. The Effect of the Deposition Method on the Structural and Optical Properties of ZnS Thin Films

Authors: Simandan, ID; Sava, F; Buruiana, AT; Burducea, I; Becherescu, N; Mihai, C; Velea, A; Galca, AC

Published: SEP 2021, COATINGS, 11, DOI: 10.3390/coatings11091064

ZnS is a wide band gap material which was proposed as a possible candidate to replace CdS as a buffer layer in solar cells. However, the structural and optical properties are influenced by the deposition method. ZnS thin films were prepared using magnetron sputtering (MS), pulsed laser deposition (PLD), and a combined deposition technique that uses the same bulk target for sputtering and PLD at the same time, named MSPLD. The compositional, structural, and optical properties of the as-deposited and annealed films were inferred from Rutherford backscattering spectrometry, X-ray diffraction, X-ray reflectometry, Raman spectroscopy, and spectroscopic ellipsometry. PLD leads to the best stoichiometric transfer from target to substrate, MS makes fully amorphous films, whereas MSPLD facilitates obtaining the densest films. The study reveals that the band gap is only slightly influenced by the deposition method, or by annealing, which is encouraging for photovoltaic applications. However, sulphur vacancies contribute to lowering the bandgap and therefore should be controlled. Moreover, the results add valuable information towards the understanding of ZnS polymorphism. The combined MSPLD method offers several advantages such as an increased deposition rate and the possibility to tune the optical properties of the obtained thin films.

16. Micrometer Sized Hexagonal Chromium Selenide Flakes for Cryogenic Temperature Sensors

Authors: Buruiana, AT; Sava, F; Iacob, N; Matei, E; Bocirnea, AE; Onea, M; Galca, AC; Mihai, C; Velea, A; Kuncser, V

Published: DEC 2021, SENSORS, 21, DOI: 10.3390/s21238084

Nanoscale thermometers with high sensitivity are needed in domains which study quantum and classical effects at cryogenic temperatures. Here, we present a micrometer sized and nanometer thick chromium selenide cryogenic temperature sensor capable of measuring a large domain of cryogenic temperatures down to tenths of K. Hexagonal Cr-Se flakes were obtained by a simple physical vapor transport method and investigated using scanning electron microscopy, energy dispersive X-ray spectrometry and X-ray photoelectron spectroscopy measurements. The flakes were transferred onto Au contacts using a dry transfer method and resistivity measurements were performed in a temperature range from 7 K to 300 K. The collected data have been fitted by exponential functions. The excellent fit quality allowed for the further extrapolation of resistivity values down to tenths of K. It has been shown that the logarithmic sensitivity of the sensor computed over a large domain of cryogenic temperature is higher than the sensitivity of thermometers commonly used in industry and research. This study opens the way to produce Cr-Se sensors for classical and quantum cryogenic measurements.

17. Multilevel Memristive GeTe Devices

Authors: Velea, A; Dumitru, V; Sava, F; Galca, AC; Mihai, C

Published: MAR 2021, PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS, 15, DOI: 10.1002/pssr.202000475

Phase-change memories have reached an advanced degree of maturity, although, to be able to meet the increasing storage demand, multilevel capability is needed. A GeTe memristor is obtained in an amorphous state and it is subjected to a specific thermal treatment which initiates the transition toward the crystalline state. It is found that this crystalline state initialization process is highly beneficial for subsequently obtaining a large number of intermediate resistive states between the high and low resistive states. Multiple resistance levels are achieved by operating the devices in both DC sweeps and rectangular pulse modes in the low-voltage subthreshold regime. The conduction is modeled using a space charge limited conduction model, showing three distinct conduction regions in the high resistive state which merge toward a single conduction region as the low resistive state is approached. The obtained memristors can be used as multilevel nonvolatile memories or as synapses in neuromorphic computing.

18. Influence of Deposition Method on the Structural and Optical Properties of Ge2Sb2Te5

Authors: Simandan, ID; Sava, F; Buruiana, AT; Galca, AC; Becherescu, N; Burducea, I; Mihai, C; Velea, A

Published: JUL 2021, MATERIALS, 14, DOI: 10.3390/ma14133663

Ge2Sb2Te5 (GST-225) is a chalcogenide material with applications in nonvolatile memories. However, chalcogenide material properties are dependent on the deposition technique. GST-225 thin films were prepared using three deposition methods: magnetron sputtering (MS), pulsed laser deposition (PLD) and a deposition technique that combines MS and PLD, namely MSPLD. In the MSPLD technique, the same bulk target is used for sputtering but also for PLD at the same time. The structural and optical properties of the as-deposited and annealed thin films were characterized by Rutherford backscattering spectrometry, X-ray reflectometry, X-ray diffraction, Raman spectroscopy and spectroscopic ellipsometry. MS has the advantage of easily leading to fully amorphous films and to a single crystalline phase after annealing. MS also produces the highest optical contrast between the as-deposited and annealed films. PLD leads to the best stoichiometric transfer, whereas the annealed MSPLD films have the highest mass density. All the as-deposited films obtained with the three methods have a similar optical bandgap of approximately 0.7 eV, which decreases after annealing, mostly in the case of the MS sample. This study reveals that the properties of GST-225 are significantly influenced by the deposition technique, and the proper method should be selected when targeting a specific application. In particular, for electrical and optical phase change memories, MS is the best suited deposition method.

19. Thermal stability of amorphous metal chalcogenide thin films

Authors: Sava, F; Simandan, ID; Stavarache, I; Porosnicu, C; Mihai, C; Velea, A

Published: MAY 1 2021, JOURNAL OF NON-CRYSTALLINE SOLIDS, 559, DOI: 10.1016/j.jnoncrysol.2021.120663

Amorphous metal chalcogenides have good switching properties for resistive memories, but have low thermal stability. In this work, the response to rapid thermal stress, as high as 550 degrees C, of amorphous Cu-GeSe, Ag-GeSe, Cu-GeTe, Ag-GeTe thin films, is investigated. Metal-GeTe films, which are amorphous up to 280 degrees C, are the most stable. Metal-GeSe films start to crystallize at 190 degrees C and a Cu1.59Se phase, with 20.5% Cu vacancies and a structure similar to the c-Cu2-xSe superionic conductor, is formed. This might boost the performance of memory devices. Silver atoms migration is facilitated in Ag-GeSe by poor crystallization (below 5%, at all temperatures). Difussion of Ag is enhanced in Ag-GeTe, due to the crystallization of the cubic (Ag2Te)(4)-GeTe2 (Ag8GeTe6) phase, which has Ag+ vacancies. In Cu-GeTe, the formation of stoichiometric polycrystalline Cu0.67Ge0.33 Te might hinder diffusion. An unusual anisotropic behaviour (increase in thickness, simultaneously with contraction of surface) is observed at 100 degrees C in Cu-GeSe and Cu-GeTe thin films, which suggests the orientation of the amorphous clusters package along a preferential direction.

20. Structural and optical properties of amorphous Si-Ge-Te thin films prepared by combinatorial sputtering

Authors: Mihai, C; Sava, F; Simandan, ID; Galca, AC; Burducea, I; Becherescu, N; Velea, A

Published: JUN 3 2021, SCIENTIFIC REPORTS, 11, DOI: 10.1038/s41598-021-91138-x

The lack of order in amorphous chalcogenides offers them novel properties but also adds increased challenges in the discovery and design of advanced functional materials. The amorphous compositions in the Si-Ge-Te system are of interest for many applications such as optical data storage, optical sensors and Ovonic threshold switches. But an extended exploration of this system is still missing. In this study, magnetron co-sputtering is used for the combinatorial synthesis of thin film libraries, outside the glass formation domain. Compositional, structural and optical properties are investigated and discussed in the framework of topological constraint theory. The materials in the library are classified as stressed-rigid amorphous networks. The bandgap is heavily influenced by the Te content while the near-IR refractive index dependence on Ge concentration shows a minimum, which could be exploited in applications. A transition from a disordered to a more ordered amorphous network at 60 at% Te, is observed. The thermal stability study shows that the formed crystalline phases are dictated by the concentration of Ge and Te. New amorphous compositions in the Si-Ge-Te system were found and their properties explored, thus enabling an informed and rapid material selection and design for applications.

21. Simple and clean method for obtaining Sn nanoparticles for hydrophobic coatings

Authors: Buruiana, AT; Sava, F; Matei, E; Zgura, I; Burdusel, M; Mihai, C; Velea, A

Published: NOV 1 2020, MATERIALS LETTERS, 278, DOI: 10.1016/j.matlet.2020.128419

Sn nanoparticles (NPs) are usually obtained by difficult chemical routes in several steps followed by thermal treatments. Here, a simple and clean method, to obtain Sn NPs directly on the substrate, is developed based on a vapor transport technique. The method is versatile, thus can be easily adjusted to obtain Sn NPs of different size, areal density and morphology, by controlling the deposition conditions. NPs are grown on Si/SiO2 substrate and characterized. Water contact angle measurements show that Sn nanoparticles increase the surface hydrophobicity by 20%. Thus, NPs cleanly obtained from a low-cost, earth-abundant, and environmentally friendly material, can be used to modulate the wettability of surfaces. (C) 2020 Elsevier B.V. All rights reserved.

22. Characterization of C-Ti multilayer thin films obtained by TVA technology

Authors: Ciupina, V; Lungu, CP; Vladoiu, R; Prodan, GC; Porosnicu, C; Vasile, E; Prodan, M; Nicolescu, V; Dinca, V; Manu, R; Cupsa, O; Velea, A

Published: 2020, NANOENGINEERING: FABRICATION, PROPERTIES, OPTICS, THIN FILMS, AND DEVICES XVII, 11467, DOI: 10.1117/12.2567927

Carbon-Titanium multilayer thin films were obtained by Thermionic Vacuum Arc (TVA) method. The nanostructured films consisting by 100nm Carbon base layer and seven 40nm alternatively Titanium and Carbon layers were deposed on Silicon substrate. As well, to give C-Ti multilayer films with different percentages in Ti and C of layers, a 100nm thick Carbon base layer was deposed on Si substrate, and then seven Ti-C layers, each of these having thickness of 40nm. In order to achieve the successively layers with C, and Ti different percentages, were adjusted the discharge parameters of C and Ti plasma sources to obtain the desired composition of layers. Also, were obtained composite films having a variable C: Ti atomic ratio 9:1 at interface to 1:9 at the surface. By changing of substrate temperature from room temperature to 100 degrees C, 200 degrees C, 300 degrees C, 400 degrees C respectively, and on the other hand the bias potential up to -700V, different batches of samples were obtained. Characterization of structural properties of films was achieved by Electron Microscopy technique (TEM, STEM) and GIXRD techniques. The measurements show that increase of the substrate temperature reveal changes in TixCy lattice parameters. Thus, according to GIXRD analysis it was found out that the Ti:C atomic ratio changes with increase of synthesis temperature. Also, in the case of composite films an increase of amount and sizes of TiC nanocrystals with the increase of energy of Ti ions determined by increase of bias voltage was observed. The tribological measurements were performed using a ball-on-disk system with normal forces of 0.5, 1, 2, 3N respectively. Was found that the coefficient of friction depends on the synthesis temperature and on the bias voltage. It is also noted that the friction coefficient depends on the pure C content, Ti content and amount of TiC nanocrystallites. These results are due to atomic diffusion at Ti/C interfaces and also are associated with amount of TiC nanocrystallites. To characterize the electrical conductive properties, the electrical surface resistance versus temperature have been measured, and then the electrical conductivity is calculated. Using the Wiedemann-Frantz law was obtained the thermal conductivity.

23. Chalcogenide Science in Romania

Authors: Lorinczi, A; Badica, P; Botila, T; Ciurea, M; Velea, A; Popescu, A; Socol, G; Antohe, S; Nedelcu, N; Sobetkii, A

Published: NOV 2020, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 257, DOI: 10.1002/pssb.202000284

Almost six decades ago, in Romania a small group of physicists begun to study chalcogenide compositions, motivated primarily by the desire to understand the phase-change phenomenon in these materials, discovered recently, at that time, by Stanford R. Ovshinsky. It took not too long for them to realize the challenges these materials set to the research. With newcomers to the field, the research was broadened. In some cases just for basic research, to model, and to understand the chalcogenide materials, whereas in other cases, the applicative potential was revealed and used. Herein, the evolution of the field of these somewhat exotic materials is followed, listing the main contributions done in Romania, both in basic and applied research.

24. Secondary Crystalline Phases Influence on Optical Properties in Off-Stoichiometric Cu2S-ZnS-SnS2 Thin Films

Authors: Sava, F; Diagne, O; Galca, AC; Simandan, ID; Matei, E; Burdusel, M; Becherescu, N; Becherescu, V; Mihai, C; Velea, A

Published: OCT 2020, MATERIALS, 13, DOI: 10.3390/ma13204624

Cu2ZnSnS4 (CZTS) is an economically and environmentally friendly alternative to other toxic and expensive materials used for photovoltaics, however, the variation in the composition during synthesis is often followed by the occurrence of the secondary binary and ternary crystalline phases. These phases produce changes in the optical absorption edge important in cell efficiency. We explore here the secondary phases that emerge in a combinatorial Cu2S-ZnS-SnS2 thin films library. Thin films with a composition gradient were prepared by simultaneous magnetron sputtering from three binary chalcogenide targets (Cu2S, SnS2 and ZnS). Then, the samples were crystallized by sulfurization annealing at 450 degrees C under argon flow. Their composition was measured by energy dispersive X-ray spectroscopy (EDX), whereas the structural and optical properties were investigated by grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy and optical transmission measurements. As already known, we found that annealing in a sulfur environment is beneficial, increasing the crystallinity of the samples. Raman spectroscopy revealed the presence of CZTS in all the samples from the library. Secondary crystalline phases such as SnS2, ZnS and Cu-S are also formed in the samples depending on their proximity to the binary chalcogenide targets. The formation of ZnS or Cu-S strongly correlates with the Zn/Sn and Cu/Zn ratio of the total sample composition. The presence of these phases produces a variation in the bandgap between 1.41 eV and 1.68 eV. This study reveals that as we go further away from CZTS in the composition space, in the quasi-ternary Cu2S-ZnS-SnS2 diagram, secondary crystalline phases arise and increase in number, whereas the bandgap takes values outside the optimum range for photovoltaic applications.

25. Low power non-volatile memory switching in monolayer-rich 2D WS2 and MoS2 devices

Authors: Mihai, C; Sava, F; Galca, AC; Velea, A

Published: FEB 1 2020, AIP ADVANCES, 10, DOI: 10.1063/1.5140717

Memristors characterized by non-volatile memory resistance switching are promising candidates for building brain inspired computing architectures. However, existing memristive devices are still far from the energy efficiency of petaflops per joule exhibited by biological neural networks. Therefore, to achieve the goal of ultra-low power operation, it is necessary to develop new materials for the active layer in memristors. Here, we show highly energy efficient memristive devices built from liquid-exfoliated 2D WS2 and MoS2 nanosheets, enriched in monolayers using a cascade centrifugation method. Lateral devices with electrochemically inert electrodes were built using the drop casting method. The devices show non-volatile resistive switching with a remarkable low energy consumption. This work contributes to the realization of energy efficient and high performance neuromorphic computing applications. (c) 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

26. Effect of the process control agent in the ball-milled powders and SPS-consolidation temperature on the grain refinement, density and Vickers hardness of Fe14Cr ODS ferritic alloys

Authors: Mihalache, V; Mercioniu, I; Velea, A; Palade, P

Published: APR 1 2019, POWDER TECHNOLOGY, 347, 113, DOI: 10.1016/j.powtec.2019.02.006

Fe-14Cr-0.4Ti-0.25Y(2)O(3) ferritic steels were produced by varying the amount of residual process control agent, PCA (ethanol), in the ball-milled powders and changing the spark-plasma-sintering, SPS, temperature. Near the-oretical density (99.3%), high Vickers hardness (501-920 HV, measured by applying a load of 100 g for 5 s) and fine grain size (26-36 nm), very stable against heating, can be achieved on ODS ferritic steels, consolidated from powders with a low amount of PCA and processing temperature in the range of 1000 degrees C-1100 degrees C. Additional grain refinement occurs near alpha -> gamma transition which is generated by the reaction of the traces of PCA with the ferritic matrix upon heating. High local temperatures and the evolved thermally activated processes, at the contact points between particles/at the particle surfaces during SPS-consolidation, were demonstrated to be the main factors responsible for improved densities and hardness. The role of PCA in the sintering, thermal and microstructure particularities and its impact on the quality of the final steel was thoroughly analysed throughout the work. (C) 2019 Elsevier B.V. All rights reserved.

27. Structural characterisation and thermal stability of SnSe\GaSb stacked films

Authors: Sava, F; Borca, CN; Galca, AC; Socol, G; Grolimund, D; Mihai, C; Velea, A

Published: JAN 2 2019, PHILOSOPHICAL MAGAZINE, 99, 72, DOI: 10.1080/14786435.2018.1529442

We have investigated the effect of thermal annealing on the structure of single and stacked phase change memory films based on SnSe and GaSb. Samples were prepared by pulsed laser deposition and investigated by X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) methods. Electrical resistance versus temperature investigations showed crystallisation temperatures of 292 degrees C and 198 degrees C for SnSe and GaSb single films, respectively. Above the transition temperature, GaSb crystallises into a face-centered cubic structure, whereas SnSe has an orthorhombic arrangement. Annealing at three temperatures (160 degrees C, 250 degrees C and 350 degrees C) of the SnSe\GaSb stacked films promotes bond breaking, atom diffusion between the two layers and formation of new phases. At 160 degrees C, GaSb films crystallise partially and no effect is observed on the crystallinity of SnSe films. After 250 degrees C, rhombohedral SnSb emerges in addition to GaSb complete crystallisation. A major, completely new, body-centered orthorhombic unindexed quaternary Ga-Sn-Sb-Se phase formation was observed in the samples annealed at 350 degrees C. The GaSb crystallites are fully dissolved and we have observed the formation of a minor hexagonal SnSe2 phase. The analysis of EXAFS data, measured at Se and Ga K-edges, revealed changes in the local atomic environment as a function of the annealing temperature. A tetrahedral configuration is obtained for the Ga atoms in both as-deposited and annealed samples, whereas Se is mostly bivalent in the amorphous samples and has an octahedral arrangement in crystalline SnSe. Our results show that inter-layer diffusion should always be considered and evaluated when designing memory cells composed of stacked phase change chalcogenide films.

28. Nonvolatile resistance switching in monolayer transition metal dichalcogenides: an explanation

Authors: Mihai, C; Velea, A; Sava, F

Published: DEC 2019, SEMICONDUCTOR SCIENCE AND TECHNOLOGY, 34, DOI: 10.1088/1361-6641/ab4b85

Monolayers of transition metal (from the group VI B) dichalcogenides (MoS2, MoSe2, WS2 and WSe2) show nonvolatile resistance switching: a transition from a high to a low resistance state. Here we propose two explanations for this behaviour. The first one is that the transition metals swaps from a trigonal prismatic to an octahedral coordination (due to a high applied electric field and pressure) and thus the monolayer switches from a semiconducting to a metallic phase. The second one is a two-step process where the high electric field and pressure break the M-X bonds and the transition metal atoms become firstly tetrahedrally coordinated and afterwards square-planar coordinated. Thus, all transition metal and chalcogen atoms are in the same plane, and the transition metal atoms are in contact with the atoms of the top and bottom electrodes.

29. Structural, Compositional, and Mechanical Characterization of WxCryFe1-x-y Layers Relevant to Nuclear Fusion, Obtained with TVA Technology

Authors: Lungu, M; Porosnicu, I; Dinca, P; Velea, A; Baiasu, F; Butoi, B; Pompilian, OG; Staicu, C; Constantina, PA; Porosnicu, C; Lungu, C; Tiseanu, I

Published: DEC 2 2019, MATERIALS, 12, DOI: 10.3390/ma12244072

Reduced activation ferritic and martensitic steel like EUROFER (9Cr-1W) are considered as potential structural materials for the first wall of the future next-generation DEMOnstration Power Station (DEMO) fusion reactor and as a reference material for the International Thermonuclear Experimental Reactor (ITER) test blanket module. The primary motivation of this work is to study the re-deposition of the main constituent materials of EUROFER, namely tungsten (W), iron (Fe), and chromium (Cr), in a DEMO type reactor by producing and analyzing complex WxCryFe1-x-y layers. The composite layers were produced in laboratory using the thermionic vacuum arc (TVA) method, and the morphology, crystalline structure, elemental composition, and mechanical properties were studied using scanning electron microscopy (SEM), X-ray diffraction (XRD), micro-X-ray fluorescence (micro-XRF), and glow discharge optical emission spectrometry (GDOES), as well as nanoindentation and tribology measurements. The results show that the layer morphology is textured and is highly dependent on sample positioning during the deposition process. The formation of polycrystalline WxCryFe1-x-y was observed for all samples with the exception of the sample positioned closer to Fe anode during deposition. The crystalline grain size dimension varied between 10 and 20 nm. The composition and thickness of the layers were strongly influenced by the in-situ coating position, and the elemental depth profiles show a non-uniform distribution of Fe and Cr in the layers. The highest hardness was measured for the sample positioned near the Cr anode, 6.84 GPa, and the lowest was 4.84 GPa, measured for the sample positioned near the W anode. The tribology measurements showed an abrasive sliding wear behavior for most of the samples with a reduction of the friction coefficient with the increase of the normal load.

30. Energy-enhanced deposition of copper thin films by bipolar high power impulse magnetron sputtering

Authors: Velicu, IL; Ianos, GT; Porosnicu, C; Mihaila, I; Burducea, I; Velea, A; Cristea, D; Munteanu, D; Tiron, V

Published: FEB 15 2019, SURFACE & COATINGS TECHNOLOGY, 359, 107, DOI: 10.1016/j.surfcoat.2018.12.079

Bipolar Pulse High Power Impulse Magnetron Sputtering (BP-HiPIMS) was investigated and used in this work to control the ion bombardment process of growing thin films and to improve their structure and properties. Energy-resolving mass spectroscopy was used to investigate the effect of reverse target voltage on the ion energies and fluxes during BP-HiPIMS of a high-purity copper target, in argon gas. It was found that the reverse target voltage provides a wide range of ion energies and fluxes incident to the growing film, which, in turn, produce a wide variety of effects during the deposition process, improving the adhesion strength and influencing both surface and bulk properties. Fast ICCD imaging was used to investigate both HiPIMS and BP-HiPIMS plasma dynamics. The temporal and spatial distributions of plasma potential measurements were performed in order to explain the mechanisms for accelerating the ions. The topological, structural and mechanical properties of the deposited coatings were investigated using atomic force microscopy (AFM), X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), thermal desorption spectroscopy (TDS), scanning electron microscopy (SEM), nanoindentation and scratch tests. The obtained results indicate an energy-enhanced deposition process during BP-HiPIMS, the deposited films being characterized by smooth surfaces, dense microstructure, small inert gas inclusions, high elastic strain to failure, scratch resistance and good adhesion to the substrate. These improvements in the films' structure and properties may be attributed to the intense and energetic ion bombardment taking place during the deposition process. During BP-HiPIMS operation, there is no net increase in the deposition rate as compared to the monopolar regime due to the re-sputtering process.

31. Negative ion-induced deuterium retention in mixed W-Al layers co-deposited in dual-HiPIMS

Authors: Dinca, P; Tiron, V; Velicu, IL; Porosnicu, C; Butoi, B; Velea, A; Grigore, E; Costin, C; Lungu, CP

Published: APR 15 2019, SURFACE & COATINGS TECHNOLOGY, 363, 281, DOI: 10.1016/j.surfcoat.2019.02.019

Co-sputtering of tungsten-aluminum fusion relevant materials in a dual-High Power Impulse Magnetron Sputtering discharge, operated in different Ar-D-2 gas mixtures, was investigated in gas phase by means of energy-resolving mass spectrometry. Experimental results indicate that the total ion flux and its composition are strongly dependent on sputtering gas composition and the average power applied to the targets. During single HiPIMS operation with W target, the D- ions are the most abundant species. The measured D- ion flux shows an increase with the rising of D-2 content in Ar-D-2 gas mixture and a linear increase with the power applied to the W target. In contrast, during dual-HiPIMS operation, a decrease of D- ion flux was observed when the input power applied to the Al target was increased. The origin of different deuterium ion species and retention mechanisms are discussed. The surface morphology, microstructure and chemical composition of the W-Al coatings obtained in Ar-D-2, were investigated by means of, Atomic Force Microscopy, X-ray diffraction and Glow Discharge Optical Emission Spectroscopy. GDOES depth profiles show the presence of a large amount of deuterium (up to 21 at.%) in the mixed W-Al layers and indicate that the D retention in the mixed W-Al layers is mainly related to the W in-depth concentration and less dependent on the Al one. The intense and energetic bombardment of the growing film with D- ions seems to be responsible for the large amount of D retained in the W-Al layers.

32. Thermal stability of phase change GaSb\GeTe, SnSe\GeTe and GaSb\SnSe double stacked films revealed by X-ray reflectometry and X-ray diffraction

Authors: Velea, A; Sava, F; Socol, G; Vlaicu, AM; Mihai, C; Lorinczi, A; Simandan, ID

Published: JUL 15 2018, JOURNAL OF NON-CRYSTALLINE SOLIDS, 492, 17, DOI: 10.1016/j.jnoncrysol.2018.02.033

We report a study related to the influence of heat treatment (up to 300 degrees C) on the structure of GaSb \GeTe, SnSe\GeTe and GaSb\SnSe stacked phase change memory films and of their counterparts with Hf thin film barrier between the layers. Samples were prepared by pulsed laser deposition and investigated by X-ray reflectometry and X-ray diffraction in order to evaluate the inter-films diffusion and the temperature threshold where this process is initiated. The thickness and mass density variations of films after each heat treatment, as well as the efficiency of hafnium barrier film, to eliminate potential atomic diffusion issues, were investigated.

33. Structural and optical properties of optimized amorphous GeTe films for memory applications

Authors: Galca, AC; Sava, F; Simandan, ID; Bucur, C; Dumitru, V; Porosnicu, C; Mihai, C; Velea, A

Published: NOV 1 2018, JOURNAL OF NON-CRYSTALLINE SOLIDS, 499, 7, DOI: 10.1016/j.jnoncrysol.2018.07.007

Chalcogenide amorphous materials, such as GeTe, are known to exhibit deposition dependent optical and structural properties. The formation of a single and homogeneous amorphous GeTe (a-GeTe) phase is questionable since the deposited films can be mixtures of monoelemental nanoclusters. In this work, we employed two deposition techniques, pulsed laser deposition from a polycrystalline GeTe target and co-sputtering from two distinct Ge and Te targets, respectively, to obtain a-GeTe films. To improve the homogeneity of the amorphous phase obtained by magnetron sputtering, the substrate temperature was varied from room temperature up to 180 degrees C. The samples were investigated by X-ray diffraction, X-ray reflectometry, X-ray photoelectron spectroscopy and spectroscopic ellipsometry. It was found that the film mass density, optical bandgap, refractive index and absolute reflectivity become progressively larger with increasing substrate temperature, due to the minimization of voids fraction and the number of dangling bonds in the amorphous structure. Moreover, X-ray photoelectron spectroscopy results prove the formation of Ge-Te bonds and therefore of the GeTe alloy at the optimal substrate temperature of 180 degrees C. This study reveals the importance of optimizing the deposition conditions for obtaining a specific amorphous phase, which enables the atomic rearrangements responsible for fast phase-change needed in memory applications.

34. Thermal Stress Effect on the Structure and Properties of Single and Double Stacked Films of GeTe and SnSe

Authors: Sava, F; Borca, CN; Galca, AC; Socol, G; Grolimund, D; Mihai, C; Velea, A

Published: JUN 2018, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 255, DOI: 10.1002/pssb.201700552

The thermal stress effect on the structure of phase change memory materials, namely single films and double stacked films of GeTe and SnSe, is evaluated. The crystallization temperatures of GeTe and SnSe single films are 138 degrees C and 292 degrees C, respectively. The films are amorphous before annealing and crystallize in rhombohedral and orthorhombic structures afterwards. Ge is tetrahedrally bonded and Se is bivalent after deposition. Both Ge and Se have an octahedral configuration after annealing. The double stacked structure is studied in the as-deposited state and after annealing at temperatures of 100, 210, and 350 degrees C. Pulsed laser deposition produces the crystallization of both as-deposited layers when stacked, mostly of SnSe, but also some crystalline GeTe is present. GeTe fully crystallizes after annealing at 210 degrees C, in the face-centred cubic structure. Annealing at 350 degrees C leads to the evaporation of a significant quantity of Se and to the formation of a cubic Ge0.75Sn0.25Te solid solution. Ge has an octahedral coordination, while Se is tetrahedrally bonded as a result of a combination of bivalent amorphous Se and octahedral Se from crystalline SnSe. The study shows that diffusion between layers at high annealing temperatures might suppress the memory property and determines the formation of irreversible solid solutions.

35. Phase change cellular automata modeling of GeTe, GaSb and SnSe stacked chalcogenide films

Authors: Mihai, C; Velea, A

Published: JUN 2018, MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING, 26, DOI: 10.1088/1361-651X/aab62f

Data storage needs are increasing at a rapid pace across all economic sectors, so the need for new memory technologies with adequate capabilities is also high. Phase change memories (PCMs) are a leading contender in the emerging race for non-volatile memories due to their fast operation speed, high scalability, good reliability and low power consumption. However, in order to meet the present and future storage demands, PCM technologies must further increase the storage density. Here, we employ a probabilistic cellular automata approach to explore the multi-step threshold switching from the reset (off) to the set (on) state in chalcogenide stacked structures. Simulations have shown that in order to obtain multi-step switching with high contrast among different resistance states, the stacked structure needs to contain materials with a large difference among their crystallization temperatures and careful tuning of strata thicknesses. The crystallization dynamics can be controlled through the external energy pulses applied to the system, in such a way that a balance between nucleation and growth in phase change behavior can be achieved, optimized for PCMs.

36. Te-based chalcogenide materials for selector applications

Authors: Velea, A; Opsomer, K; Devulder, W; Dumortier, J; Fan, J; Detavernier, C; Jurczak, M; Govoreanu, B

Published: AUG 14 2017, SCIENTIFIC REPORTS, 7, DOI: 10.1038/s41598-017-08251-z

The implementation of dense, one-selector one-resistor (1S1R), resistive switching memory arrays, can be achieved with an appropriate selector for correct information storage and retrieval. Ovonic threshold switches (OTS) based on chalcogenide materials are a strong candidate, but their low thermal stability is one of the key factors that prevents rapid adoption by emerging resistive switching memory technologies. A previously developed map for phase change materials is expanded and improved for OTS materials. Selected materials from different areas of the map, belonging to binary Ge-Te and Si-Te systems, are explored. Several routes, including Si doping and reduction of Te amount, are used to increase the crystallization temperature. Selector devices, with areas as small as 55 x 55 nm(2), were electrically assessed. Sub-threshold conduction models, based on Poole-Frenkel conduction mechanism, are applied to fresh samples in order to extract as-processed material parameters, such as trap height and density of defects, tailoring of which could be an important element for designing a suitable OTS material. Finally, a glass transition temperature estimation model is applied to Te-based materials in order to predict materials that might have the required thermal stability. A lower average number of p-electrons is correlated with a good thermal stability.

37. Photoexpansion in amorphous As2S3: A new explanation

Authors: Lorinczi, A; Sava, F; Simandan, ID; Velea, A; Popescu, M

Published: SEP 1 2016, JOURNAL OF NON-CRYSTALLINE SOLIDS, 447, 125, DOI: 10.1016/j.jnoncrysol.2016.06.010

Thin amorphous As2S3 films show a giant photoexpansion of similar to 5% upon femtosecond laser illumination and this expansion remains after switching off the laser beam. To understand the structural modifications which occur during the photoexpansion process we assumed a molecular cluster structure of the amorphous As2S3. The focused femtosecond laser beam induces electrical charge redistribution on the sulfur atoms of each cluster, which increases the electrical repulsion between sulfur atoms, and thus induces an expansion of the clusters, by network reconfiguration, without breaking the bonds. An increase of the van der Waals distance between molecular clusters also takes place. (C) 2016 Elsevier B.V. All rights reserved.

38. Amorphous thin films in the gallium-chalcogen system

Authors: Popescu, M; Sava, F; Lorinczi, A; Velea, A; Simandan, ID; Galca, AC; Matei, E; Socol, G; Gherendi, F; Savastru, D; Miclos, S

Published: JUN 2016, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 253, 1037, DOI: 10.1002/pssb.201552551

Thin amorphous films based on gallium-chalcogen (Ga-Ch), namely Ga2S3, Ga2Se3, Ga2Te3, and GaTe have been prepared by pulsed laser deposition (PLD). The films were characterized by X-ray diffraction, extended X-ray absorption fine structure (EXAFS), energy-dispersive X-ray spectroscopy (EDX), optical transmission spectroscopy, ellipsometry, and electrical measurements. Structural measurements showed that Ga is threefold coordinated, except the Te-based alloys were, it seems, only twofold coordinated, while the chalcogen is usually twofold coordinated. In all the compositions, layered and chain-like structures are assumed. The bandgaps range between 1.09 eV for Ga2Te3 and 2.21 eV for Ga2Se3. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

39. Thin films of amorphous Ga2S3 and rare-earth sulphides

Authors: Popescu, M; Lorinczi, A; Sava, F; Velea, A; Simandan, ID; Badica, P; Burdusel, M; Galca, AC; Socol, G; Jipa, F; Zamfirescu, M

Published: MAR 1 2015, MATERIALS LETTERS, 142, 231, DOI: 10.1016/j.matlet.2014.12.028

The aim of this research is to prepare amorphous thin films of undoped gallium sulphide and doped with rare-earth sulphides, of rare-earth sulphides and to investigate their physical properties. We have prepared thin amorphous films of Ga2S3, EuS, Er2S3, Gd2S3, and Ga2S3 doped with rare-earth sulphides (Ga2S3:EuS, Ga2S3:Er2S3, Ga2S3:Gd2S3) by Pulsed laser Deposition (PLD). The corresponding targets for preparation of amorphous thin films were obtained by Spark Plasma Sintering (SPS) from commercially available powders of binary sulphides. The structural results for the undoped and doped Ga2S3 thin films indicate a packing of disordered layers similar to that of amorphous As2S3. Femtosecond laser irradiation of the Ga2S3 thin films shows a photoexpansion effect at low laser power (85-100 mW) and an ablation effect at higher laser power (above 105 mW). The threshold between low power and high power pulses is situated at higher value for Ga2S3 (100 mW) in comparison with the case of As2S3 thin films (20 mW). (C) 2014 Elsevier B.V. All rights reserved.

40. In-situ characterization of the optical and electronic properties in GeTe and GaSb thin films

Authors: Velea, A; Socol, G; Popescu, M; Galca, AC

Published: OCT 7 2015, JOURNAL OF APPLIED PHYSICS, 118, DOI: 10.1063/1.4932666

GeTe and GaSb thin films obtained by pulsed laser deposition were investigated by spectroscopic ellipsometry at controlled temperatures. The GeTe films were fully amorphous, while the GaSb films were partially crystalized in the as-deposited state. The Tauc-Lorentz model was employed to fit the experimental data. From the temperature study of the optical constants, it was observed the crystallization in the 150-160 degrees C range of GeTe amorphous films and between 230 and 240 degrees C of GaSb amorphous phase. A second transition in the resonance energy and the broadening parameter of the Lorentz oscillator was observed due to the crystallization of Sb after 250 degrees C. The temperatures of 85 degrees C and 130 degrees C are noticed as the start of the relaxation of the amorphous GeTe phase and as-deposited GaSb. The peaks of the imaginary part of the dielectric function red shifted after the phase change, while the variation with temperature of the crystalline phase follows the Varshni law. The electron-phonon coupling constants are 2.88 and 1.64 for c-GeTe and c-GaSb, respectively. An optical contrast up to 60% was obtained for GeTe films and a maximum value of 7.5% is revealed in the case GaSb, which is altered by the partial crystallinity of the as-deposited films. (C) 2015 AIP Publishing LLC.

41. Ceramics and amorphous thin films based on gallium sulphide doped by rare-earth sulphides

Authors: Popescu, M; Sava, F; Lorinczi, A; Velea, A; Simandan, ID; Badica, P; Burdusel, M; Galca, AC; Matei, E; Preda, N; Secu, M; Socol, G; Jipa, F; Zamfirescu, M; Balan, A

Published: APR 2015, SEMICONDUCTOR SCIENCE AND TECHNOLOGY, 30, DOI: 10.1088/0268-1242/30/4/044001

Bulk ceramics of Ga2S3 and rare-earth sulfides (EuS, Gd2S3, Er2S3) as well as combinations thereof have been prepared by spark plasma sintering (SPS). The disk-shaped ceramics were used as targets for pulsed laser deposition (PLD) experiments to obtain amorphous thin films. The properties of these new bulks and amorphous thin films have been investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), optical transmission spectroscopy, and atomic force microscopy (AFM). In order to test the photoexpansion effect in Gd2S3 and the possibility to create planar arrays of microlenses, the film was irradiated with femtosecond laser pulses at different powers. For low laser power pulses (up to 100mW power per pulse) a photoexpansion effect was observed, which leads to formation of hillocks with a height of 40-50 nm. EuS doped Gd2S3 thin film shows luminescence properties, which recommend them for optoelectronic applications.

42. In-situ crystallization of GeTe\GaSb phase change memory stacked films

Authors: Velea, A; Borca, CN; Socol, G; Galca, AC; Grolimund, D; Popescu, M; van Bokhoven, JA

Published: DEC 21 2014, JOURNAL OF APPLIED PHYSICS, 116, DOI: 10.1063/1.4904741

Single and double layer phase change memory structures based on GeTe and GaSb thin films were deposited by pulsed laser deposition (PLD). Their crystallization behavior was studied using in-situ synchrotron techniques. Electrical resistance vs. temperature investigations, using the four points probe method, showed transition temperatures of 138 degrees C and 198 degrees C for GeTe and GaSb single films, respectively. It was found that after GeTe crystallization in the stacked films, Ga atoms from the GaSb layer diffused in the vacancies of the GeTe crystalline structure. Therefore, the crystallization temperature of the Sb-rich GaSb layer is decreased by more than 30 degrees C. Furthermore, at 210 degrees C, the antimony excess from GaSb films crystallizes as a secondary phase. At higher annealing temperatures, the crystalline Sb phase increased on the expense of GaSb crystalline phase which was reduced. Extended X-ray absorption fine structure (EXAFS) measurements at the Ga and Ge K-edges revealed changes in their local atomic environments as a function of the annealing temperature. Simulations unveil a tetrahedral configuration in the amorphous state and octahedral configuration in the crystalline state for Ge atoms, while Ga is four-fold coordinated in both as-deposited and annealed samples. (C) 2014 AIP Publishing LLC.

43. Chalcogenide systems at the border of the glass-formation domain: A key for understanding the memory-switching phenomena

Authors: Popescu, M; Velea, A; Sava, F; Lorinczi, A

Published: JUL 2014, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 251, 1339, DOI: 10.1002/pssb.201350104

The compositions in the ternary chalcogenide systems from the demarcation region between the glass-formation domain (GFD) and the partially or fully crystalline formation domain seem to exhibit outstanding properties. We have shown in this paper that the compositions with the best memory-switching properties are situated at the border of the GFD in many systems. One can use this correlation to find new phase-change materials with better switching properties or to discover GFDs that were not observed yet. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

44. Self-organization and Size Effects in Amorphous Silicon

Authors: Popescu, M; Sava, F; Velea, A

Published: 2014, SIZE EFFECTS IN NANOSTRUCTURES: BASICS AND APPLICATIONS, 205, 45, DOI: 10.1007/978-3-662-44479-5_2

Self-organization and size effects in amorphous silicon have been investigated by modelling of the structure at nanoscale. The size effect related to the disorder in silicon is treated by the free energy balance in nanometric clusters using valence force field theory. The computed structural and energetical parameters of three continuous random network (CRN) models of amorphous silicon with 2,052, 156 and 155 atoms are compared with the experimental values. In order to show the importance of the interfaces between different a-Si clusters, two networks of 200 and 205 atoms were modelled separately and then linked using an amorphous and a crystalline interface. Also the voids in the a-Si clusters are investigated.

45. Simulation of the structure of GeAs4Te7 chalcogenide materials during memory switching

Authors: Popescu, M; Sava, F; Velea, A; Lorinczi, A; Simandan, ID

Published: JUL 2014, CANADIAN JOURNAL OF PHYSICS, 92, 680, DOI: 10.1139/cjp-2013-0561

The complex chalcogenides with excellent memory switching properties are mainly situated close to the border of glass formation domain. The simulation of the structural changes occurring during the memory switching process of a ternary chalcogenide composition has been carried out. The transition of a high resistivity GeAs4Te7 amorphous cluster with 120 atoms to a low resistivity crystalline cluster was analyzed. The coordination of atoms changes from that corresponding to 8-N coordination rule (two for tellurium, three for arsenic, and four for germanium) in the amorphous phase to six (the same for all atoms) in metastable crystalline phase. Because of spatial constraints exercised by the amorphous matrix, the amorphous cluster cannot expand. In these circumstances Te atoms seem to be over-coordinated (up to sixfold-coordinated). During the switching process, the atoms are moving on distances up to 4.0 angstrom. The average displacement is of 2.36 angstrom.

46. UV SENSING EFFECT IN LANGMUIR-BLODGETT COMPLEX FILMS CONTAINING A NOVEL SYNTHESIZED Fe(III) PORPHYRIN

Authors: Baschir, L; Fagadar-Cosma, E; Creanga, I; Palade, A; Lascu, A; Birdeanu, M; Savastru, D; Savu, V; Antohe, S; Velea, A; Fagadar-Cosma, G; Popescu, M; Simandan, ID

Published: APR-JUN 2014, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 9, 857, DOI:

A novel structure of metalloporphyrin, namely: 5,10,15,20-tetrakis(3,4-dimethoxyphenyl)-porphyrin Fe(III) chloride, was successfully synthesized. UV sensitive structure based on barium stearate functionalized with the novel synthesized porphyrin and carbon nanotubes was obtained. A five monolayers structure was deposited from solution by Langmuir - Blodgett technique onto a ceramic substrate with interdigital platinum electrodes. The complex film shows sensitivity to UV radiation due to the fact that an electron is excited from a Fe d orbital into a porphyrin antibonding pi orbital and is moved onto the ligand that is attached, facilitating the charge separation, while the presence of SWCNT activates the transport of the charge carriers.

47. Optics of microlenses created by irradiation of As2S3 amorphous chalcogenide films with femtosecond laser pulses

Authors: Popescu, M; Velea, A; Miclos, S; Savastru, D

Published: APR 1 2013, PHILOSOPHICAL MAGAZINE LETTERS, 93, 220, DOI: 10.1080/09500839.2012.760058

Arrays of microlenses have been created on the surface of thin amorphous films of As2S3 by localized photoexpansion induced by femtosecond laser pulses. The profile of a typical microlens has been mathematically described. Ray-tracing analysis has been performed to determine the focal points in the red and near-infrared spectrum of electromagnetic radiation. These lie at distances of 1.21m at =650nm and 1.33m at =1.5m above the top of the microlens. It is suggested that the microlenses could be used in two-dimensional optoelectronic circuits or in fibre optics.

48. EFFECT OF THERMAL ANNEALING ON THE STRUCTURAL AND OPTICAL PROPERTIES OF Ag/As2S3 MULTILAYERS

Authors: Sava, F; Lorinczi, A; Velea, A; Simandan, ID; Preda, N; Socol, G; Mihailescu, IN; Zamfira, CS; Cretu, NC; Popescu, M

Published: NOV 2013, CHALCOGENIDE LETTERS, 10, 471, DOI:

A complex structure of four double layers of Ag / As2S3 has been deposited by Pulsed Laser Deposition method on a glass substrate. The effects of thermal annealing on the structural and optical properties were investigated. An effect of layer mixing has been evidenced.

49. Effect of broadband light on Ag/As2S3 multilayers

Authors: Sava, F; Velea, A; Popescu, M; Lorinczi, A; Simandan, ID; Vlaicu, AM; Socol, G; Mihailescu, IN; Stefan, N

Published: OCT 1 2013, JOURNAL OF NON-CRYSTALLINE SOLIDS, 377, 161, DOI: 10.1016/j.jnoncrysol.2013.02.023

A sandwich structure of four double layers of Ag (5 nm)/As2S3 (82.7 nm) has been prepared by Pulsed Laser Deposition (PLD) method. The effect of broadband light on the multilayer structure has been investigated. The X-ray reflectivity (XRR) patterns after different irradiation times have been measured. Although the fully intermixing of Ag and As2S3 layers should be expected during irradiation with visible light, however even after 5 h of irradiation the intermixing is not completed. The characteristic features of XRR diagrams for long irradiation times have been interpreted by scattering of X-ray radiation on clusters of Ag or Ag-As2S3. (c) 2013 Elsevier B.V. All rights reserved.

50. Possible mechanism of Ag photodiffusion in a-As2S3 thin films

Authors: Sava, F; Popescu, M; Lorinczi, A; Velea, A

Published: MAY 2013, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 250, 1003, DOI: 10.1002/pssb.201248517

Monitoring the silver photodiffusion in thin amorphous As2S3 film is addressed with a new experimental setup. A possible photo-diffusion mechanism of silver into the a-As2S3 thin film under green laser diode light (=532nm) irradiation is proposed. The proposed mechanism is based on a gradual filling of the structural voids existing in the network of the thin chalcogenide layer. This mechanism is supported by XRD measurements, optical absorption, and modeling data.

51. Disorder in order: silicon versus graphene

Authors: Popescu, M; Sava, F; Lorinczi, A; Velea, A

Published: MAY 2013, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 250, 1010, DOI: 10.1002/pssb.201248516

The topological transition from order to disorder in crystalline silicon was investigated by a computer simulation procedure. The gradually introduction of topological WootenWinerWeaire defect states makes the crystal change in a more and more disordered assembly of atoms. The characterization of deformation energy around a single defect state is analyzed. The topological transition from graphene structure to an amorphous carbon layer, by introduction of a high number of StoneWales defect-type states was evidenced. The comparison of the disordered structure in tetrahedrally bonded semiconductors (silicon) and a two-dimensional network based on graphene structure was made.

52. Modeling the slaving of structural fluctuations in bio-molecules to those of nearby water

Authors: Mihai, C; Velea, A; Roman, N; Tugulea, L; Moldovan, NI

Published: JUL-SEP 2012, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 7, 915, DOI:

The functions of bio-molecules depend on their structural fluctuations, which are thought to be 'slaved' (i.e. coupled) to those of nearby bulk water. Slaving was suggested to explain the universal pattern of propagation of mechanical excitations through concentrated water-containing solutions of polymers, such as the cytoplasm. To obtain a simplified model of this phenomenon, we propose to use a cellular automaton of water-based solutions. During iterations, the model computes the compactness of both the solvent and the solute (equivalent of their density), as the average number of neighbors of each class of particles. As a surrogate of slaving, we studied the temporal co-variation of these variables, using the Pearson correlation coefficient (S-av). We found that S-av depends in a biochemically-meaningful manner on the concentration of solute, on its hydrophatic character and on molecular flexibility. The simulations also show that S-av is robust to mild hypothermia. In conclusion, our cellular automaton is capable to generate a slaving-like behavior of solutes in water, as an emergent phenomenon occurring in dissolved molecular systems.

53. IMPACT OF SOLUTE MOLECULAR PROPERTIES ON THE ORGANIZATION OF NEARBY WATER: A CELLULAR AUTOMATA MODEL

Authors: Mihai, C; Velea, A; Roman, N; Tugulea, L; Moldovan, NI

Published: APR-JUN 2012, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 7, 475, DOI:

The goal of this study was the creation of a model to understand how solute properties influence the structure of nearby water. To this end, we used a two-dimensional cellular automaton model of aqueous solutions. The probabilities of translocation of water and solute molecules to occupy nearby sites, and their momentary distributions (including that of vacancies), are considered indicative of solute molecular mechanics and hydrophatic character, and are reflected in water molecules packing, i.e. 'organization'. We found that in the presence of hydrophilic solutes the fraction of water molecules with fewer neighbors was dominant, and inverse-proportionally dependent on their relative concentration. Hydrophobic molecules induced water organization, but this effect was countered by their own flexibility. These results show the emergence of cooperative effects in the manner the molecular milieu affects local organization of water, and suggests a mechanism through which molecular mechanics and crowding add a defining contribution to the way the solute impacts on nearby water.

54. Possible mechanisms of switching in amorphous chalcogenides

Authors: Popescu, M; Velea, A

Published: OCT 2012, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 249, 1971, DOI: 10.1002/pssb.201200351

A new description of the switching phenomenon is given. The switching is regarded as due to the formation and breaking of the links between the dendrites of crystalline nuclei in bulk materials, as a consequence of the energy pumped by an electrical field. This mechanism explains the very short switching time (<20?ns), the possibility to get smart memories based on multisteps of resistivity and the high number of cycles supported by the cell (1016). A cellular automaton mechanism was created on the basis of this model for switching. Multistage memory in chalcogenide materials has also been explained by this mechanism.

55. LUMINESCENCE OF EUROPIUM IN ARSENIC SULPHIDE MATRIX

Authors: Popescu, M; Lorinczi, A; Velea, A; Simandan, ID; Sava, F; Pavelescu, G; Niciu, GH; Niciu, DO; Mihailescu, IN; Socol, G; Stefan, N

Published: NOV 2011, CHALCOGENIDE LETTERS, 8, 702, DOI:

Thin films of arsenic sulphide have been obtained by Pulsed Laser Deposition from bulk As(2)S(3). A very thin layer of Eu(2)O(3) was deposited by PLD from a different target in the next deposition process. After a heat treatment at 180 degrees C for 45 minutes in inert atmosphere a structural transformation of the amorphous As(2)S(3) to realgar (As(2)S(2)) occurred and the film develops a strong luminescence effect.

56. SENSOR OF NITROGEN DIOXIDE BASED ON SINGLE WALL CARBON NANOTUBES AND MANGANESE-PORPHYRIN

Authors: Popescu, M; Simandan, ID; Sava, F; Velea, A; Fagadar-Cosma, E

Published: JUL-SEP 2011, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 6, 1256, DOI:

New sensors based on single wall carbon nanotubes (SWCNTs) and porphyrins have been tested for sensitivity to NO(2) gas. SWCNTs embedded in barium stearate multilayers were deposited with the help of Langmuir-Blodgett technique. It was demonstrated a sensitive effect of the sensor for NO(2) gas. The coverage of the sensor with metalloporphyrin (Mn) leads to strong enhancing of the sensitive effect to NO(2), besides, the selectivity was preserved both around room temperature and at larger operating temperature (up to 200 degrees C). No sensitivity to CO and CH(4) was evidenced. The NO(2) sensitivity strongly increased at 100 degrees C but at high operation temperatures (150-200 degrees C) a different behaviour is observed, probably due to the structural change of porphyrin or porphyrin-SWCNT interaction. The reversibility of the sensor resistivity is reasonably good at the optimum operating temperature (100 degrees C).

57. Optical properties of phase change memory Ge1Sb2Te4 glasses

Authors: Iovu, MS; Colomeico, EP; Benea, VG; Popescu, M; Lorinczi, A; Velea, A

Published: NOV-DEC 2011, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 13, 1486, DOI:

Phase change memory materials are promising for the next-generation of non-volatile flash memory that will serve in new mobile computing, entertainment and other handheld electronics. Among them are chalcogenide glasses Ge-Sb-Te (GST) which can exist in two separates structural states - amorphous and cristalline. Switching of the material from one to another state can be done by heating applying an electrical pulse or by exposure to intense laser beam. We report the changes of optical parameters of amorphous Ge1Sb2Te4 films under heat treatment and light exposure.

58. Eu3+ LUMINESCENCE IN As2S3 DOT NETWORK

Authors: Georgescu, S; Popescu, M; Sava, F; Velea, A; Pavelescu, G

Published: DEC 2011, CHALCOGENIDE LETTERS, 8, 738, DOI:

Thin films of As2S3 doped with Eu3+ were prepared by vacuum evaporation. It was measured the fluorescence spectra before and after a heat treatment at 180 degrees C for 45 minutes in inert atmosphere. After the heat treatment the transitions D-5(1) -> F-7(1), D-5(0) -> F-7(1) and D-5(0) -> F-7(2) were evidenced in the luminescence spectrum.

59. Nanocarbon embedded chalcogenides. Onion-like model

Authors: Popescu, M; Sava, F; Lorinczi, A; Velea, A; Simandan, ID

Published: NOV-DEC 2011, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 13, 1453, DOI:

We modeled the equilibrium structure, at 0 K temperature, of some spherical As(2)S(3) molecules with only 12 rings of ten member (alternating As and S atoms) and 20 triangular faces: As(60)S(90) (like C(60)), As(140)S(210) (like C(140)), As(320)S(480) (like C(320)), grown on carbon fullerene and As(20)S(30) (like C(20)). We used a Monte-Carlo relaxation procedure in the frame of valence force fields theory. The onion-like configurations of the arsenic sulphide was demonstrated as possible. The deposited films of As(2)S(3) seen in the cross-section prove the presence of onion-like configurations even in pure As(2)S(3) material.

60. Silver doped As2S3 chalcogenide films: A diffusion study

Authors: Lorinczi, A; Popescu, M; Sava, F; Velea, A; Simandan, ID

Published: 2011, PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 8, NO 9, 8, 2620, DOI: 10.1002/pssc.201084096

Photo-diffusion of silver across an As2S3 thin film layer is studied, as continuing some earlier work about surface morphology modifications in double-layer structures of Ag and As2S3, and some qualitative preliminary results on lateral Ag diffusion in As2S3 thin films. An estimation of the Ag film's dissolution rate and the diffusion front's advancement rate in As,S3 is given based on crosssection measurements. In order to understand better the diffusion mechanism of Ag in As2S3, a structural model of the As2S3 matrix is used to estimate the amount of silver which might be hosted by an As2S3 layer in terms of molar ratio, and an interaction mechanism of the Ag with the chalcogenide matrix is proposed. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

61. COMPLEX PHOTOCONDUCTORS CHALCOGEN-POLYMER

Authors: Simandan, ID; Velea, A; Popescu, M

Published: OCT 2011, CHALCOGENIDE LETTERS, 8, 614, DOI:

There was discovered a material with strong photoconduction effect. Langmuir-Blodgett film with barium stearate, and selenium carbazole has been prepared and tested for the change of conductivity (decrease) under the influence of UV-radiation. Change of resistance in a time scale of 0-400 seconds was evidenced. The UV radiation determines an increase of the electrical resistence and after switching off the illumination a slow decrease of the resistance occurs.

62. NON-LINEAR BEHAVIOUR IN SILVER - ARSENIC SULFIDE SYSTEM

Authors: Popescu, M; Lorinczi, A; Gherendi, F; Nistor, M; Velea, A; Simandan, ID; Sava, F

Published: NOV-DEC 2011, JOURNAL OF OVONIC RESEARCH, 7, 133, DOI:

Non-linear phenomena have been observed in the Ag(paint)-As2S3 system. This behaviour is useful in rectifier devices and in dynamical switching devices based on chalcogenides.

63. LUMINESCENCE OF ARSENIC SULPHIDE DOTS DOPED WITH EUROPIUM, PREPARED BY THERMAL EVAPORATION AND PULSED LASER DEPOSITION METHODS

Authors: Popescu, M; Velea, A; Simandan, ID; Sava, F; Lorinczi, A; Ghervase, L; Pavelescu, G; Mihailescu, IN; Socol, G; Georgescu, S

Published: DEC 2011, CHALCOGENIDE LETTERS, 8, 724, DOI:

Networks of dots made of As2S3 chalcogenide has been obtained by Vacuum Evaporation and Pulsed Laser Deposition from bulk glassy As2S3 by the adapted method of SCEV (screen evaporation method). A very thin layer of Eu2O3 was deposited from a different target in the final deposition process. After a heat treatment at 180 degrees C for 90 minutes in nitrogen atmosphere a structural transformation of the amorphous As2S3 to realgar (AsS) occurred in the PLD deposited dots and the network of dots develops a luminescence effect. (Received November 25, 2011; accepted December 2, 2011)

64. Influence of defects on the switching speed of Ge2Sb2Te5

Authors: Velea, A; Popescu, M; Lorinczi, A; Sava, F; Simandan, ID; Mihai, C

Published: NOV-DEC 2011, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 13, 1596, DOI:

Chalcogenide phase change materials are one of the major contenders for the new non-volatile memory applications. Here is reported that the switching speed of Ge2Sb2Te5 is strongly dependent on the percent of defects in the material. Using cellular automata simulations it was shown that the size of the percolation cluster is minimum, thus the switching speed is maximum, for a percent of around 25% defects in the material. This is a property of the Ge2Sb2Te5 that can be useful for new phase change materials and devices design with better switching properties.

65. New insight into phase change memories

Authors: Velea, A

Published: JUL 1 2011, JOURNAL OF NON-CRYSTALLINE SOLIDS, 357, 2631, DOI: 10.1016/j.jnoncrysol.2011.02.014

The switching mechanism in phase change memories was described on the basis of minimum switching unit: the commuton. A commuton is a minimum cluster of atoms that supports a reversible phase change from high to low electrical conduction state and back under the influence of an external signal. The switching process in a phase change chalcogenide film was modeled using two dimensional cellular automata approach. A system of 50 x 50 cells, each cell containing a commuton, was simulated. In the particular case of Ge(2)Sb(2)Te(5) (investigated here) this system corresponds to a 30 x 30 nm area. The formation of the percolation path as a function of phase change induced in commutons explains the switching phenomenon. The influence of the percent of defects in the material on the percolation threshold has been studied. (C) 2011 Elsevier B.V. All rights reserved.

66. TWO DIMENSIONAL PHOTONIC STRUCTURES BASED ON As-S CHALCOGENIDE GLASS

Authors: Popescu, M; Velea, A; Lorinczi, A; Zamfirescu, M; Jipa, F; Miclos, S; Popescu, A; Ciobanu, M; Savastru, D

Published: OCT-DEC 2010, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 5, 1105, DOI:

Two-dimensional photonic structure has been imprinted on the surface of arsenic sulphide glass using the pulses of a femtosecond laser. Due to the interaction of the laser beam with the glass, the laser traces were obtained as hillocks of around 150-200 nanometer in height, or holes of depth around 100-300 nm, without the need for a later etching stage as in the usual practice. The calculation of the band structure shows a photonic band gap between 0.9 and 1.0 c/a. By varying the energy of the laser pulse we have the possibility to choose between the two possibilities to produce a photonic structure made-up of hillocks or a photonic structure consisting of holes. The value of the threshold pulse power which controls the resulting surface morphology and the obtained photonic structure has found to be around 15 mW. The surface topography of the structures obtained has been investigated by atomic force microscopy (AFM).

67. SYNTHESIS AND LUMINESCENCE PROPERTIES OF Nd-DOPED CHALCOGENIDE GLASS (Ge5As2S13) (NdCl3) x (x=0.3%, 0.5%)

Authors: Georgescu, S; Voiculescu, AM; Salamu, S; Niciu, H; Niciu, D; Popescu, M; Lorinczi, A; Velea, A; Simandan, ID

Published: NOV 2010, CHALCOGENIDE LETTERS, 7, 624, DOI:

The synthesis of the chalcogenide glass (Ge5As2S13) doped by Nd3+ has been carried out. The bulk glass develops a small amount of Nd2O3 and As2O3 crystallites besides some amount of (NdCl3)*6H(2)O crystalline phase initially introduced during synthesis. The optical absorption spectra demonstrate the presence of the Nd3+ both in the amorphous phase and in crystalline combinations. The formation of Nd2O3 crystallites was explained by the reaction of water with neodymium chloride. The presence of As2O3 is due to the oxidation of As from the glass during preparation.

68. STRUCTURAL DETAILS OF THE Ag-As2S3 INTERFACE OBTAINED BY VACUUM THERMAL EVAPORATION FOLLOWED BY GREEN LASER IRRADIATION

Authors: Lorinczi, A; Velea, A; Matei, E; Simandan, ID; Popescu, M

Published: NOV 2010, CHALCOGENIDE LETTERS, 7, 612, DOI:

As2S3 on Ag heterostructures deposited onto microscope glass substrates have been prepared. The quality of the hetero-structures has been investigated by careful analysis of the scanning electron microscope pictures taken on the native cross-sections, produced after the vacuum deposition sequence and various green laser irradiation times. The As2S3 layer is not uniformly grown, but reveals a columnar-like structure and multi-scale aggregation.

69. PREPARATION AND PROPERTIES OF BARIUM STEARATE MULTILAYERS WITH CARBON NANOTUBES, MANGANESE PORPHYRIN AND SILVER NITRATE

Authors: Simandan, ID; Popescu, M; Lorinczi, A; Velea, A; Fagadar-Cosma, E

Published: OCT-DEC 2010, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 5, 1033, DOI:

Barium stearate multilayer samples have been deposited by the Langmuir-Blodgett technique. Carbon nanotubes have been added in the deposition solution. The structural properties have been studied. The effect of the doped multilayers has been tested against the solution of ammonium nitrate in water. The effect of (5,10,15,20-tetraphenyl)-porphinato manganese (III) chloride, (MnTPP)Cl, on the multilayer sample has been tested. The change of resistivity of the multilayer samples modified by manganese porphyrin, as a function of the ultraviolet radiation has been discovered and investigated. The increase of the resistance during irradiation is a reversible process, although a slow one. This effect could give a basis for applications in UV sensors and switches. Finally, a photo-resistive effect (induced by UV light) has been discovered in Barium stearate layers doped by silver nitrate.

70. Langmuir and Langmuir-Blodgett films based on stearic acid, barium stearate and carbon nanotubes

Authors: Simandan, ID; Sava, F; Velea, A; Lorinczi, A; Popescu, M

Published: AUG 2010, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 4, 1181, DOI:

Langmuir films and Langmuir-Blodgett multilayers based on fatty acid (stearic acid, barium stearate) mixed with carbon nanotubes have been prepared and investigated by isothermal compression (for Langmuir films) and X-ray diffraction (for Langmuir-Blodgett multilayers). An irreversible modification has been observed in the case of Langmuir films doped by carbon nanotubes, due to reordering of the nanotubes in the film matrix.

71. Structure and Mossbauer measurements on SnSe2 bulk and thin films

Authors: Bibicu, I; Lorinczi, A; Velea, A; Sava, F; Popescu, M

Published: OCT 2010, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 4, 1571, DOI:

Thin films samples of SnSe2 deposited by PLD and PED have been prepared and studied by XRD and Mossbauer spectroscopy. The films are crystalline with a major phase of SnSe2 and minor phase of amorphous SnSe2. An oxidation process lead to the appearance of a thin layer of SnO2 on the surface of the PLD samples.

72. MOULDING PROCEDURE FOR THE PREPARATION OF INFRARED GLASSY MICROLENSES AND PRISMS BASED ON ARSENIC SULPHIDE CHALCOGENIDE GLASS

Authors: Niciu, H; Popescu, M; Velea, A; Lorinczi, A; Manea, A; Niciu, D; Lazarescu, M

Published: NOV 2010, CHALCOGENIDE LETTERS, 7, 629, DOI:

A new procedure for the production of infrared lenses and prisms based on As(2)S(3) chalcogeniude glass has been devised. Infrared components are obtained with the size in the range 100-1500 micrometers. The procedure allows for getting various curvature radii of the micrometer lenses, ranging from planno-convex to spherical ones.

73. Structure and properties of silver doped SnSe2 and Ge2Sb2Te5 thin films prepared by pulsed laser deposition

Authors: Popescu, M; Velea, A; Sava, F; Lorinczi, A; Tomescu, A; Simion, C; Matei, E; Socol, G; Mihailescu, IN; Andonie, A; Stamatin, I

Published: MAR 2010, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 207, 520, DOI: 10.1002/pssa.200982900

Amorphous thin films of SnSe2 and Ge2Sb2Te5 doped by different amount of silver (0.1, 0.2, 0.5 and 1 Ag atoms per formula unit) have been prepared by pulsed laser deposition (PLD) starting from solid polycrystalline targets. The films were investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The good gas sensing properties for CO, as well as the sensitivity for CH4 and NO of the Ag; doped SnSe2 films have been demonstrated in the composition SnSe2Ag0.2. The structural effect of silver introduced in Ge2Sb2Te5 matrix has been investigated. The freshly deposited thin films doped by various amount of Ag develop three phases: an amorphous one, and two crystalline phases consisting of a major fcc cubic phase of AgSbTe2 and a minor cubic phase of composition Ag2Te. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

74. SIMULATION OF PHASE CHANGE MATERIALS USING CELLULAR AUTOMATA

Authors: Velea, A

Published: OCT-DEC 2010, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 5, 1027, DOI:

The most promising media for rewritable applications are the phase change materials [ 1]. For the modelling of the phase transition processes in phase change materials it has been considered the switching unit called "commuton" [2]. A commuton is a minimum cluster of atoms that supports the reversible change from OFF to ON state under the influence of an external applied energy pulse. It was developed a bidimensional model based on the cellular automata approach [3] to predict switching behavior in a Ge2Sb2Te5 phase change chalcogenide film. We suppose that the formation of the percolation paths as a function of phase change induced in commutons can explain the switching phenomenon. The percolation thresholds of the switching were calculated for different sizes of the simulated system and the percolation kinetics was investigated.

75. QUANTITATIVE STRUCTURE - ACTIVITY RELATIONSHIP IN ANTIDIABETIC DRUGS BY USING TOPOLOGICAL DESCRIPTORS

Authors: Popescu, M; Velea, A; Mihai, C; Tivadar, S

Published: JUL-SEP 2010, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 5, 633, DOI:

The quantitative structure - activity relationship in antidiabetic oral drugs has been analyzed on the basis of topological indices that allow to discriminate the structure of different molecules either small or large. The overall correlation structure - activity allows to find the best antidiabetic drugs and to predict the activity of new compound proposed as oral antidiabetic. The screened procedure based on topological indices prevents the expensive and long testing of a high number of compounds.

76. BIOGENIC PRODUCTION OF NANOPARTICLES

Authors: Popescu, M; Velea, A; Lorinczi, A

Published: OCT-DEC 2010, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 5, 1040, DOI:

Nanoparticles of interest in nanotechnology (design of nanodevices) and medicine (controlled-release of drugs) can be produced both by anorganic and organic synthesis. The biogenetic production is now of high interest due to simplicity of the procedures and their versatility. Several species of bacteria and plants are able to synthesize nanoparticles or to help in the process of their production. The paper gives an overview on the biogenetic production of nanomaterials.

77. Tellurium based phase change materials

Authors: Velea, A

Published: DEC 2009, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 11, 1987, DOI:

Phase change materials are the most promising materials for innovation in the computer memory industry. Of great importance is the understanding of the processes that take place during the switching from the high resistivity state to the low resistivity state. In this paper we have tried to find out the correlations between the crystallo-chemical parameters and the switching properties of these materials. The crystallo-chemical parameters, average electronegativity, and glass forming ability (n) over bar(k) over bar/(Z) over bar have been calculated for a large number of tellurium chalcogenides. The switching parameters have been correlated with the physical properties of these materials: resistivity, activation energy, and average electronegativity. Change of the chemical bonding during switching would be responsible for the outstanding properties of the phase change materials.

78. Optical fiber coupling to a laser diode through chalcogenide microlenses

Authors: Velea, A; Rusu, M; Popescu, M

Published: OCT 2009, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 11, 1383, DOI:

Microspherical lenses (50 divided by 800 mu m in diameter) based on glassy As(2)S(3), have been produced by a special flame melting technique. The microlenses have been used for focusing the red - infrared laser light at the end of an optical fiber. Microlens arrays have been produced and applied in coupling optical fibers to a light remitting diode (LED), in order to increase the intensity. A double ball lens coupling scheme has been designed and manufactured.

79. Direct laser writing of two-dimensional photonic structures in amorphous As2S3 thin films

Authors: Popescu, A; Miclos, S; Savastru, D; Savastru, R; Ciobanu, M; Popescu, M; Lorinczi, A; Sava, F; Velea, A; Jipa, F; Zamfirescu, M

Published: NOV 2009, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 11, 1880, DOI:

A method for building photonic structures in As2S3 amorphous chalcogenide films has been developed. 2-D photonic configurations characterized by a regular assembly of rods (triangular lattice) or gratings with traces of micrometer period have been obtained. A femto-second laser has been used in photonic crystal registration. The simulation performed on these photonic configuration shows that the obtained chalcogenide structures could be efficient for operation in the infrared range around several micrometers wavelength.

80. Crystalline-amorphous and amorphous-amorphous transitions in phase-change materials

Authors: Popescu, M; Sava, F; Velea, A; Lorinczi, A

Published: OCT 1 2009, JOURNAL OF NON-CRYSTALLINE SOLIDS, 355, 1823, DOI: 10.1016/j.jnoncrysol.2009.04.053

The transition from the crystalline state to amorphous state and back has been studied in the particular case of the GeSb2Te4 phase-change material by a computer simulation procedure. Modelling at the nanoscale indicates specific structural characteristics, especially the multiplicity of the amorphous phase as opposite to the uniqueness of the crystalline phase. in the particular case of the Si(12)Ge(10)AS(30)Te(48) switching glass two types of ordering have been pointed out and characterized. (C) 2009 Elsevier B.V. All rights reserved.

81. Silver/amorphous As2S3 heterostructure

Authors: Popescu, M; Sava, F; Lorinczi, A; Velea, A; Leonovici, M; Zamfira, S

Published: NOV 2009, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 11, 1594, DOI:

The heterostructure Ag/As2S3 was deposited on glass substrate in two configurations: Ag/As2S3/glass and As2S3/Ag/glass. The effect of light emitted by a halogen lamp has been investigated by optical microscopy and X-ray diffraction. Particular morphological and structural aspects were revealed. Lateral diffusion rate of Ag has been determined.

82. Modelling of dissolution kinetics of thin amorphous chalcogenide films

Authors: Popescu, M; Sava, F; Lorinczi, A; Velea, A; Vlcek, M; Jain, H

Published: 2009, PHILOSOPHICAL MAGAZINE LETTERS, 89, 376, DOI: 10.1080/09500830902960117

A theoretical relationship for the dissolution kinetics of thin chalcogenide films is proposed. The influence of light irradiation on the film dissolution is taken into account. The theoretical curves are in good agreement with the experimental dissolution results obtained for As2S3 thin amorphous films.

83. Matrix-assisted photo-amorphization effect in As40S30Se30 films with silver

Authors: Popescu, M; Petkov, K; Sava, F; Taseeva, J; Lorinczi, A; Velea, A

Published: DEC 2009, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 11, 2003, DOI:

Matrix assisted photo-amorphization effect has been observed in thin chalcogenide films of composition As40S30Se30 deposited by thermal evaporation on silicon wafer substrate covered by silver. The initial films contain Ag2S and Ag4SeS crystallites embedded in an amorphous matrix. Illumination by a cold light halogen lamp induces the disappearance of the crystalline fraction and leads to significant changes in the diffraction pattern of the amorphous films.

84. COMMUTON - A NEW CONCEPT IN SWITCHING MATERIALS

Authors: Popescu, M; Velea, A; Lorinczi, A; Sava, F

Published: MAR-APR 2009, JOURNAL OF OVONIC RESEARCH, 5, 34, DOI:

A switching unit can be defined as the smallest unit that preserves the property of switching in memory materials. Atomic switching unit is discussed, molecular switching units are described and supramolecular units in phase change materials are evidenced and discussed. The "commuton", as basic switching unit, could give more insight into the particular properties of the switching phenomena in various solid materials.

85. PHASE CHANGE MATERIALS: CHEMICAL BONDING AND STRUCTURAL PROPERTIES

Authors: Velea, A; Popescu, M

Published: SEP-DEC 2009, PROCEEDINGS OF THE ROMANIAN ACADEMY SERIES A-MATHEMATICS PHYSICS TECHNICAL SCIENCES INFORMATION SCIENCE, 10, 236, DOI:

The phase change materials are the most important materials in the class of chalcogenides (combination of chalcogens (S, Se and Te) with metalloids or metals). The outstanding property of these materials is the switching from a high electrical resistivity state to low electrical resistivity state and back under a moderate voltage. The thin film materials are used in computer memories, CD and DVD devices with performing speed and storage capacity. We have studied several thin solid films made of Ge-Sb-Te in order to assess the switching quality of different compositions. In order to systemize the whole class of chalcogenide phase change materials we have investigated the correlation between different crystallo-chemical parameters and the ionicity of the elements. Binary and ternary compounds are distributed into several distinct groups. The most favorable phase change materials are situated in a specific range of mean ionicity and mean glass formation ability. The results give the possibility to design new compositions with better switching properties.

86. Chalcogenide photonic structures

Authors: Popescu, M; Savastru, D; Popescu, A; Miclos, S; Lorinczi, A; Sava, F; Velea, A; Baschir, L; Ciobanu, M; Matei, E; Socol, G; Mihailescu, IN; Niciu, H

Published: SEP 2009, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 3, 859, DOI:

Chalcogenide glasses can be tailored for getting special configurations with photonic crystal properties A review on the advances in chalcogenide glass photonics is given Several methods to prepare two-dimensional photonic As-S glass and three-dimensional packing have been developed, and the photonic structures have been characterized Numerical simulations have also been performed The chalcogenide photonic structures fabricated by micro-technological procedures are described and tuned to work in the far infrared region of the electromagnetic spectrum

87. Phase-change electrical memory elements and devices

Authors: Popescu, M; Lorinczi, A; Sava, F; Velea, A; Matei, E; Socol, G; Mihailescu, IN

Published: OCT 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 2621, DOI:

Electrical memory elements based on Ge-Sb-Te pure and doped by Sn-Se have been obtained by pulsed laser deposition on special substrates covered by gold as well as on common glass. A set of electrical memory elements in a 4x4 matrix structure on the glass substrate has been produced. Devices with 3 and 10 memory elements have been constructed and tested for their memory properties. The special features of the voltage-current characteristics have been revealed.

88. The modification induced by UV radiation in spherical microlenses made of glassy arsenic sulphide

Authors: Rusu, M; Velea, A; Popescu, M

Published: DEC 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 3171, DOI:

Spherical arsenic chalcogenide glass microlenses have been attached to optical fibers. The assembly was subjected to UV-radiation for different time intervals in different conditions of temperature. As a consequence, the focusing of the beam changes due to the modification of the refractive index, and of the structure, induced by photo-melting followed by crystallization. In the same time for high irradiation times the habitus of the lens changes into a prismatic one due probably to photomelting of the glass accompanied by a crystallization process.

89. Alternating barium stearate and copper stearate LB thin films

Authors: Popescu, M; Lorinczi, A; Velea, A; Sava, F

Published: DEC 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 3415, DOI:

Alternating layers based on copper and barium stearate have been prepared. The interlayer distance for both type of stearates have been determined by X-ray diffraction. It was found that the interlayer distance for an alternating assembly depends on the actual type of the stearate layer. Thus we demonstrated the possibility to control the inter-layer distance by choosing an appropriate sequency of layers. It is suggested that such layers may serve as high precision thickness standard at the angstrom level.

90. Ceramic materials Ba(1-x)SrxTiO3 for electronics - Synthesis and characterization

Authors: Berbecaru, C; Alexandru, HV; Porosnicu, C; Velea, A; Ioachim, A; Nedelcu, L; Tonsan, M

Published: SEP 30 2008, THIN SOLID FILMS, 516, 8214, DOI: 10.1016/j.tsf.2008.04.031

Ceramics (Ba1-xSrx)TiO3, (BST) with x=0.25; 0.35; 0.40; 0.50; 0.60; 0.75; 0.90 were prepared by sintering method from BaCO3, SrCO3 and TiO2 of high purity (99.98%). The solid-state reaction was employed to obtain pure and doped BST ceramic samples. In order to improve the sintering process the following additives were added: 1.0 wt.% MgO and 1 wt.% MnO2. The sintering treatment was performed in die temperature range 1200 to 1260 degrees C, for 2 h. Archimedean method was used for ceramic density measurements, which was found to depend linearly on strontium content: rho(x)=5.4(2)-0.8(8).x, (g/cm(3)). SEM, EDX and XRD methods were used for sample characterization. Morphology, pores and grain size distribution of both pure and doped ceramics were investigated by SEM analysis. A bimodal grain distribution could be observed in pure BST for x <= 0.25, whereas a uniform one was noticed for doped samples of x = 0.25. Well faceted grains of similar to 5 pm size, Could be seen for x=0.75 samples. Pores size has grown with the increase of Sr content. X-ray diffraction patterns have shown a dominant perovskite structure and some other minor phases. Dielectric permittivity and loss were investigated at 1kHz, in the temperature range - 150 to + 150 degrees C, at a pace of similar to 2 degrees C /min. Sharp transitions, with higher peaks values of permittivity were noticed with the increase of the sintering temperature. A shift Of the Curie points to lower values was noticed with the increase of Sr content, according to the equation: Tc = 127.4-331.x. Measured at room temperature, in the frequency range of similar to 1 GHz, permittivity was found to increase and losses to decrease with the sintering temperature. For x <= 0.35 permittivity of the samples, being in the ferroelectric phase could not be measured in microwave domain at room temperature. Our results point to important application of these BST compositions for microwave devices. (C) 2008 Elsevier B.V. All rights reserved.

91. DC conductivity in GeSb2Te4 and (GeSb2Te4)(90)(SnSe2)(10) phase change materials

Authors: Popescu, M; Kubliha, M; Kaluzny, J; Velea, A; Lorinczi, A

Published: DEC 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 3953, DOI:

The dc electrical conductivity of the bulk amorphous GeSb2Te4 material has been investigated. Pure and samples doped by 10 at. % SnSe2 have been measured. The conductivity in the samples has been compared with that of SnSe2 bulk sample. The activation energy of the doped sample is 0.165 eV. During heating the conductivity of doped material increases, reaches a maximum and then decreases. The comparison with the pure SnSe2 samples allows to explain this behavior by the release above 148 degrees C of a small amount of selenium not bonded in the network.

92. Crystallization of PLD deposited ITO thin films by thermal treating in various gaseous environments

Authors: Viespe, C; Grigoriu, C; Popescua, M; Sava, F; Lorinczi, A; Velea, A; Zamfira, S

Published: NOV 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 3566, DOI:

ITO films of composition (In2O3)(0.9) - (SnO2)(0.1) have been prepared by pulsed laser deposition (PLD).The films were found to be amorphous. The structure and evolution of the films during annealing in air, oxygen atmosphere and reducing atmosphere (CO2) have been investigated by X-ray diffraction. The films annealed in ambient atmosphere start to crystallize under 250 degrees C annealing temperature. The films treated in oxygen atmosphere crystallize at higher temperature (similar to 300 degrees C).

93. Multilevel Memristive GeTe Devices

Authors: Velea, A; Dumitru, V; Sava, F; Galca, AC; Mihai, C

Published: , PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS, 2000475, DOI: 10.1002/pssr.202000475

94. Chalcogenide Science in Romania

Authors: Lorinczi, A; Badica, P; Botila, T; Ciurea, M; Velea, A; Popescu, A; Socol, G; Antohe, S; Nedelcu, N; Sobetkii, A

Published: , , DOI: 10.1002/pssb.202000284

Dr. Alin VELEA

Scientific Researcher I

Education

Positions

Awards

Patents

Projects

1. Functional 2D materials and heterostructures for hybrid spintronic-memristive devices

2. A map for ovonic threshold switching materials (AMOS)

3. Investigation of phase change in stacked chalcogenide thin films for multistate memory cells

Publications

1. Synthesis of WS2 Ultrathin Films by Magnetron Sputtering Followed by Sulfurization in a Confined Space

2. Fs Laser Patterning of Amorphous As2S3 Thin Films

3. First Sharp Diffraction Peak features of the intermediate phase glasses and amorphous thin films in the non-stoichiometric (GeS4)x(AsS3)1-x system

4. From non-stoichiometric CTSe to single phase and stoichiometric CZTSe films by annealing under Sn plus Se atmosphere

5. Understanding the Effects of Post-Deposition Sequential Annealing on the Physical and Chemical Properties of Cu2ZnSnSe4 Thin Films

6. Cu2SnSe3 phase formation from different metallic and binary chalcogenides stacks using magnetron sputtering

7. 'Put variety in White': Multi-analytical investigation of the white pigments inlaid on Early Chalcolithic pottery from Southern Romania

8. Synthesis and characterization of some C-Ti based multilayer and composite nanostructures

9. A Two-Step Magnetron Sputtering Approach for the Synthesis of Cu2ZnSnS4 Films from Cu2SnS3\ZnS Stacks

10. Effect of the stacking order, annealing temperature and atmosphere on crystal phase and optical properties of Cu2SnS3

11. Layered SnSe nanoflakes with anharmonic phonon properties and memristive characteristics

12. New Chalcogenide Glass-Ceramics Based on Ge-Zn-Se for IR Applications

13. Enhanced photoelectrochemical activity of WO3-decorated native titania films by mild laser treatment

14. Synthesis and Characterization of Cu2ZnSnS4 Thin Films Obtained by Combined Magnetron Sputtering and Pulsed Laser Deposition

15. The Effect of the Deposition Method on the Structural and Optical Properties of ZnS Thin Films

16. Micrometer Sized Hexagonal Chromium Selenide Flakes for Cryogenic Temperature Sensors

17. Multilevel Memristive GeTe Devices

18. Influence of Deposition Method on the Structural and Optical Properties of Ge2Sb2Te5

19. Thermal stability of amorphous metal chalcogenide thin films

20. Structural and optical properties of amorphous Si-Ge-Te thin films prepared by combinatorial sputtering

21. Simple and clean method for obtaining Sn nanoparticles for hydrophobic coatings

22. Characterization of C-Ti multilayer thin films obtained by TVA technology

23. Chalcogenide Science in Romania

24. Secondary Crystalline Phases Influence on Optical Properties in Off-Stoichiometric Cu2S-ZnS-SnS2 Thin Films

25. Low power non-volatile memory switching in monolayer-rich 2D WS2 and MoS2 devices

26. Effect of the process control agent in the ball-milled powders and SPS-consolidation temperature on the grain refinement, density and Vickers hardness of Fe14Cr ODS ferritic alloys

27. Structural characterisation and thermal stability of SnSe\GaSb stacked films

28. Nonvolatile resistance switching in monolayer transition metal dichalcogenides: an explanation

29. Structural, Compositional, and Mechanical Characterization of WxCryFe1-x-y Layers Relevant to Nuclear Fusion, Obtained with TVA Technology

30. Energy-enhanced deposition of copper thin films by bipolar high power impulse magnetron sputtering

31. Negative ion-induced deuterium retention in mixed W-Al layers co-deposited in dual-HiPIMS

32. Thermal stability of phase change GaSb\GeTe, SnSe\GeTe and GaSb\SnSe double stacked films revealed by X-ray reflectometry and X-ray diffraction

33. Structural and optical properties of optimized amorphous GeTe films for memory applications

34. Thermal Stress Effect on the Structure and Properties of Single and Double Stacked Films of GeTe and SnSe

35. Phase change cellular automata modeling of GeTe, GaSb and SnSe stacked chalcogenide films

36. Te-based chalcogenide materials for selector applications

37. Photoexpansion in amorphous As2S3: A new explanation

38. Amorphous thin films in the gallium-chalcogen system

39. Thin films of amorphous Ga2S3 and rare-earth sulphides

40. In-situ characterization of the optical and electronic properties in GeTe and GaSb thin films

41. Ceramics and amorphous thin films based on gallium sulphide doped by rare-earth sulphides

42. In-situ crystallization of GeTe\GaSb phase change memory stacked films

43. Chalcogenide systems at the border of the glass-formation domain: A key for understanding the memory-switching phenomena

44. Self-organization and Size Effects in Amorphous Silicon

45. Simulation of the structure of GeAs4Te7 chalcogenide materials during memory switching

46. UV SENSING EFFECT IN LANGMUIR-BLODGETT COMPLEX FILMS CONTAINING A NOVEL SYNTHESIZED Fe(III) PORPHYRIN

47. Optics of microlenses created by irradiation of As2S3 amorphous chalcogenide films with femtosecond laser pulses

48. EFFECT OF THERMAL ANNEALING ON THE STRUCTURAL AND OPTICAL PROPERTIES OF Ag/As2S3 MULTILAYERS

49. Effect of broadband light on Ag/As2S3 multilayers

50. Possible mechanism of Ag photodiffusion in a-As2S3 thin films

51. Disorder in order: silicon versus graphene

52. Modeling the slaving of structural fluctuations in bio-molecules to those of nearby water

53. IMPACT OF SOLUTE MOLECULAR PROPERTIES ON THE ORGANIZATION OF NEARBY WATER: A CELLULAR AUTOMATA MODEL

54. Possible mechanisms of switching in amorphous chalcogenides

55. LUMINESCENCE OF EUROPIUM IN ARSENIC SULPHIDE MATRIX

56. SENSOR OF NITROGEN DIOXIDE BASED ON SINGLE WALL CARBON NANOTUBES AND MANGANESE-PORPHYRIN

57. Optical properties of phase change memory Ge1Sb2Te4 glasses

58. Eu3+ LUMINESCENCE IN As2S3 DOT NETWORK

59. Nanocarbon embedded chalcogenides. Onion-like model

60. Silver doped As2S3 chalcogenide films: A diffusion study

61. COMPLEX PHOTOCONDUCTORS CHALCOGEN-POLYMER

62. NON-LINEAR BEHAVIOUR IN SILVER - ARSENIC SULFIDE SYSTEM

63. LUMINESCENCE OF ARSENIC SULPHIDE DOTS DOPED WITH EUROPIUM, PREPARED BY THERMAL EVAPORATION AND PULSED LASER DEPOSITION METHODS

64. Influence of defects on the switching speed of Ge2Sb2Te5

65. New insight into phase change memories

66. TWO DIMENSIONAL PHOTONIC STRUCTURES BASED ON As-S CHALCOGENIDE GLASS

67. SYNTHESIS AND LUMINESCENCE PROPERTIES OF Nd-DOPED CHALCOGENIDE GLASS (Ge5As2S13) (NdCl3) x (x=0.3%, 0.5%)

68. STRUCTURAL DETAILS OF THE Ag-As2S3 INTERFACE OBTAINED BY VACUUM THERMAL EVAPORATION FOLLOWED BY GREEN LASER IRRADIATION

69. PREPARATION AND PROPERTIES OF BARIUM STEARATE MULTILAYERS WITH CARBON NANOTUBES, MANGANESE PORPHYRIN AND SILVER NITRATE

1. Synthesis of WS₂ Ultrathin Films by Magnetron Sputtering Followed by Sulfurization in a Confined Space

2. Fs Laser Patterning of Amorphous As₂S₃ Thin Films

5. Understanding the Effects of Post-Deposition Sequential Annealing on the Physical and Chemical Properties of Cu₂ZnSnSe₄ Thin Films