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Dr. Claudia MIHAI

Scientific Researcher

Laboratory of Optical Processes in Nanostructured Materials

claudia[DOT]mihai[AT]infim[DOT]ro

Education

Swiss Federal Institute of Technology (ETH), Zurich, Switzerland - Postdoctoral Fellowship - “Complex systems physics”, 2013 – 2014

University of Bucharest, Faculty of Physics, Romania - Ph.D. Fellowship (POSDRU) - Biophysics, Thesis: “Modeling structures of biological interest generated by hydrophobic interaction”, 2009 – 2012

University of Bucharest, Faculty of Physics, Romania - M.S. - Theoretical Physics, 2007 – 2009

University of Bucharest, Faculty of Physics, Romania - B.A. - Physics, 2003 – 2007

Positions

Scientific Researcher: National Institute of Materials Physics, Romania, 2020 – present

Search for memristor materials using machine learning models;

Synthesis and characterization of 2D monochalcogenides;

 

Assistant Researcher: National Institute of Materials Physics, Romania, 2016 – 2020

Phase change modeling in stacked chalcogenide layers of GeTe, GaSb and SnSe using cellular automata;

Prediction of new threshold switching ovonic materials (OTS) using machine learning algorithms;

 

Postdoctoral research: Swiss Federal Institute of Technology (ETH), Zurich, Switzerland, 2013 – 2014

Study the dynamics of autocatalytic systems with diffusive reactants in the context of biological populations;

Investigation of scaling relationships in the growth dynamics of startup companies;

 

Doctoral Research: Faculty of Physics, University of Bucharest, Romania, 2008-2011

Cellular Automata modeling of  structures generated by hydrophobic interaction; 

 

Assistant Researcher: Horia Hulubei National Institute for R&D in Physics and Nuclear Engineering (IFIN-HH), Romania, 2007 – 2009

Measurements of the proton structure in electron-proton collisions at the H1 detector at HERA accelerator in DESY - Hamburg;

Search for excited quarks at HERA in the q* q channel.

Publications

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.
9. 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.
10. 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.
11. 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.
12. 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.
13. 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.
14. 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.
15. 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/).
16. 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.
17. 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.
18. 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.
19. 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.
20. 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.
21. 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.
22. 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.
23. 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.
24. 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.
25. 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.
26. 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

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.
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