Zeinab Shiolashvili

The purpose of this work was to study the ability of an NH3 + HCl gaseous mixture to produce copper-based nanomaterials. A gas mixture containing both reactants can be easily produced by heating the solid powder of NH4Cl at temperatures exceeding 340 °C. The vertical quartz reactor with CuO source on its bottom was first evacuated and then heated up to 750 °C. After reacting with gas mixture the volatile species were formed and condensed on the Si substrate placed in the cold zone above the CuO source. The obtained nanomaterials were analyzed by the transmission and scanning electron microscopy, X-ray diffraction and energy dispersive spectroscopy. Nanomaterials with different shapes and compositions were synthesized depending on the Si substrate temperature, which was changed in the range of 250 – 500 °C. These materials include CuO, CuCl, and CuCl2 nanocrystals and dendrite-like structures. The most interesting synthesized nanostructures were the elongated surface nanobubbles with the average wall thickness of 80 nm

and heights up to tens of micrometers. They were formed of CuCl, which is strongly hygroscopic and degrades in the atmosphere. The surface bubbles may be blown out due to the decomposition of two chemicals that are formed in the reactor, namely, Cu2OCl2 and CuCl2. The detailed thermochemical analyses of reactions led us to the conclusion that the thermal decomposition of CuCl2, accompanied by the release of Cl2, is the most probable reason for the formation of surface bubbles. It was shown that the chemical activity of the NH3 + HCl gaseous mixture can be farther increased by introducing hydrazine (N2H4) vapor into the reactor, which easily decomposes to nitriding (NH,NH2) and reducing (H2, atomic H) species

The vapor growth of ZnO microcrystals was studied using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The microcrystals were grown by annealing ZnO, CuO and ammonium chloride powders. As a result, the ZnCl and condensed on Si substrate, which was gradually heated from room to preset temperature selected in the range of 220 – 350 °C. The process lasted for 2 h. The growth proceeded in two stages. In the first stage, at ∼ 240 °C, the CuCl droplet with a low melting point was formed. With time, the droplet was oversaturated with ZnO and the solid nuclei of ZnO were precipitating from it. They served as seeds for the formation of hexagonal ZnO microrods, which were growing along the c-axis ([0001] direction) by the slow, thermodynamically driven Vapor–Solid mechanism. As a result, the rod-like ZnO microstructures were produced on Si substrate. The second stage of growth started when the substrate 300°C. At this temperature the secondary nucleation took place on the prism surfaces of ZnO microrods, causing the formation of brunched structure. The ZnO brunches were growing again along the c-axes, forming elongated 1D type microcrystals. In o the slow growth of primer ZnO microrods, at elevated temperatures the ZnO brunches were growing significantly faster. This kinetically driven process caused the vanishing of the fast growing (0001) plains, resulting in the tapering of ZnO microcrystal tips. ZnO has a high melting point and its vapor-phase synthesis normally needs a high temperature The results of this work demonstrated that ZnO micro together with brunched ZnO structures, can be grown from the vapor phase at a low temperature, close to 300 °C. 

Gas sensors employing nanowires attracted great interest during the last two decades due to their extremely high sensitivity, which for some gases reach even tens of ppb levels. The purpose of this work was to develop new technology for the growth of In2Ge2O7 nanowires. The second goal was to fabricate the nanowire networkbased gas sensors on these nanowires and examine their characteristics. The nanowires were synthesized using the vapour-solid growth mechanism realized in the gaseous ambient formed after pyrolytic decomposition of the moistened hydrazine (N2H4), containing water (3-10 mol.%). After pyrolysis, the ambient was simultaneously consisting of such oxidizing, reducing and nitriding species like O2, H2O, NH and NH2 radicals, atomic hydrogen, ammonia and hydrogen molecules. Annealing of In+Ge solid sources in such ambient caused the formation of volatile suboxide precursors (In2O and GeO) that were accomplishing the mass transfer to the substrate, located in the cold zone, and subsequent growth of nanowires. It was shown that the process temperature has a great influence on the composition and morphology of nanowires. At C, only indium oxide nanowires were formed that were growing from the In self°temperatures below 400 catalyst. The mixture of pure Ge and In2O3 nanowires were obtained when the substrate temperature was in C the In2Ge2O7 nanowires were synthesized, which were decorated with InN°C. At 450°the range of 400-420 nanocrystals. Scanning and transmission electron microscopy, together with X-ray diffraction and energy dispersive X-ray spectroscopy were used to study the composition, structure and morphology of synthesized nanowires. The gas sensor was fabricated by forming the nanowire networks on areas between interdigitated gold electrodes, deposited onto the Pyrex glass substrate. The initial experiments revealed that for such sensor C, while the response and recovery times were 15 and 9°the detection limit for ammonia was 0.8 ppm at 300 seconds respectively

A vast amount of nanoparticles has been developed and proposed for the local hyperthermia of cancer during the last decades, but only a few of them correspond to the mandatory requirements of having therapeutic range Curie temperature (T_C= 41-45⁰C), high-rate crystallinity and “strong” magnetic properties, strictly controlled homogeneity and dispersion of the nanoparticles, good biocompatibility and harmless decomposition products. Among them are the nickel-copper (Ni-Cu) and silver doped lanthanum manganite (AgxLa1-xMnO3) nanoparticles. The developed research showed that the materials obtained at lower than usual temperatures using microwave enhanced synthesizes and annealing can be successfully used for local hyperthermia revealing high magnetic properties. Mixtures of Nanoparticles with liquid crystals can play an important role as effective and non-toxic material for modalities of the Curie point controlled strongly localized magnetic hyperthermia with a precisely tunned prolonged release of nanoparticles into tissue cells.Therefore, the preparation of mixtures of liquid crystals with a transition temperature from the nematic to the dispersed phase in the range of 41-45⁰C, and their comprehensive study is an important practical task. This problem is complicated by the fact that the transition temperature to the dispersed phase depends essentially on the concentration of nanoparticles in the mixture. The reported research deals with preparation of mixtures of Ni-Cu and AgxLa1-xMnO3 nanoparticles and compositions of 12 commercially available liquid crystals.

The vapor-phase synthesis of Cu-based nanomaterials using inorganic volatile Cu precursors is a key for controlling the composition, morphology and structure of copper containing nanomaterials. In this paper, we have shown that annealing of a solid Cu or CuO sources in the ambient of ammonium chloride and hydrazine decomposition products leads to the formation of volatile CuCl species. The mass transfer from source to the substrate, which was located in the “cold” zone of the reactor, was accomplished by these CuCl species. After condensation on Si substrate heated up to 400°C, they were interacting with hydrazine and ammonium chloride decomposition products forming, the agglomerated Cu microcrystals in case of Cu source. Different nanomaterials were synthesized when CuO was used as a source. These nanomaterials included Cu-based nanocrystals, nanowires and elongated microbubbles. Further investigations are planned to determine the

composition and structure of these nanomaterials.

The research is dedicated to microwave and conventional methods of solution combustion synthesis of the relatively new nanomaterial proposed for magnetic hyperthermia of cancer cells and preliminary assessment of the toxicity of developed materials based on the behavioral methods and techniques at the levels far below of commonly registered by means of usualy and widely applied assays for humans.Farther research is needed to optimize the methods of synthesis of silver doped lanthanum manganites with required characteristics.

The formation of crystalline nitride materials is a complicated task, which usually needs temperature processes and the application of active nitriding precursors. Previously,we have developed the hydrazine-based technology for producing such nitride nanomaterials as germanium and indium nitrides. The purpose of this work was to further improve this technology by adding the ammonium chloride (NH4Cl) to hydrazine (N2H4), and to investigate the formation of Ge and boron nitrides using this technology. The germanium nitride was chosen as a model material, because its formation in a poor hydrazine was studied in details, allowing for the comparative study of differences between the hydrazine-assisted and hydrazine+ammonium chloride-based processes. The interest for the growth of BN nanomaterials was caused by its unique physical properties and the ability of hBN to form the layered 2D nanostructures. The grown nanomaterials were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS). The analysis of thermochemical reactions that involved the precursor existing in the reactor revealed that in the case of Ge and B sources the reactions that lead to the formation of Ge3N4 and BN are thermodynamically most favorable. The nanostructures containing BN layers were synthesized on Si substrate during 20 hours at 700°C. They were also subjected to Rapid Thermal Annealing. The formation of h-BN was confirmed by XRD and EDS analysis. The thickness of BN layers was well below 100 nm. As for germanium nitride, the α-Ge3N4 nanowires were synthesized on Ge substrate at 440 °C. This synthetic temperature is by 60 and 410 °C lower than the growth temperature of the same

material using hydrazine or ammonia. The obtained results clearly demonstrated that in the vapor of a mixture of NH4Cl+ N2H4 the active nitriding precursors are formed, which enable the low temperature growth of nitride

nanomaterials.

Gas sensors based on oxide-semiconductor nanowires have been a subject of extensive research because of their potential application in detecting several inflammable, toxic and odorless gases. Among them, In2O3 has been found to have a pronounced sensitivity to such gases as NO2, NH3, O3, Cl2, CO, H2, C2H5OH and other species. Sensing NO2 in the atmosphere has assumed great importance because of the serious problem of atmospheric air pollution caused by car exhaust and other sources. Recent developments showed that In2O3 nanowires doped with different atoms exhibit superior selectivity to NO2, H2S and some other gases with short response and recovery times [1 – 4]. In this work we describe the low-temperature synthesis of In2O3 nanowires, fabrication of a simple nanowire-network based gas sensor and its application for sensing NO2

The germanium nitride and InP nanowires were grown using the pyrolytic decomposition products of hydrazine (N2H4), which was containing 3 mol.% H2O. In separate set of experiments the quartz microbalance was used to study the interaction of water containing hydrazine with Ge sample in the temperature range of 450-650°C. It was established that up to 500°C only water molecules interact with Ge, forming volatile suboxide GeO. At higher temperatures GeO molecules and nitrogen precursors, produced after decomposition of hydrazine, form crystalline Ge3N4 nanowires on the Ge surface. Analysis of thermo-chemical reactions reveal that in the presence of water molecules and nitrogen precursors the formation of nitride is thermodynamically favourable than the synthesis of germanium dioxide. When InP was annealed in hydrazine at 440°C the water molecules were producing volatile In2O. After reaching the Si substrate these molecules were interacting with phosphorus vapor producing InP nanowires

The InP based nanowires were produced by direct annealing of crystalline InP sources in hydrazine (N_2H_4) vapor and subsequent condensation of volatile spices onto the substrates. The morphology and sizes of nanowires showed strong dependence on the growth temperature. In the temperature range of 440 – 540 °C the morphology of InP nanostructures were changed from true nanowires with minimum diameters of ca. 25 nm formed at 440 °C, to faceted, several micrometer size large crystalline blocks of InP growing at 540 °C simultaneously with the rhombus decorated zigzag shaped InP nanowires with extended surfaces. The nanowires growth mechanism also varied with the temperature. In the range of 440 – 500 °C they were growing through the Vapor–Solid mechanism. At 540 °C the Vapor–Solid and Vapor–Liquid–Solid mechanisms coexisted forming large elongated blocks of indium phosphide together with zigzag shaped InP nanowires.

The tapered single-crystalline α-Ge_3N_4 nanowires were grown simultaneously on the surfaces of crystalline Ge source and Si substrate located at 3 mm above it. The growth was performed at 500 – 560 °C in the presence of hydrazine (N_2H_4) vapor containing 3 mol. % water. The nanowires were grown through the vapor–liquid–solid mechanism using Ge catalyst. Produced nanowires were tapered. However, the direction of taper was different for nanowires grown on Ge and Si. The difference in tapering was explained by differences in the fluxes of volatile GeO molecules at the beginning of growth process and at the stage of temperature stabilization. It was found that at the surface of Si substrate a part of GeO molecules was reduced to pure Ge due to the presence of hydrogen in the pyrolytic decomposition products of hydrazine. As a result the chain-like Ge nanostructures were formed together with Ge_3N_4 nanowires.

The composite nanowires comprising crystalline semiconductor cores surrounded with amorphous shells are considered as promising materials for the fabrication of nanowire based transistors, photovoltaics, gas sensors, catalysts for direct water splitting by sunlight etc. Recently we have developed the new pyrolytic technology and produced some core–shell 1D nanomaterials using the sublimation of products, formed after thermal annealing of InP + Zn source in the N2H4 + 3 mol. % H2O vapor [1 – 3]. The nanowires were synthesized from the gaseous phase on the Si substrate that was located in the “cold zone” just above the source.. The purpose of this work was to study the mechanism of the formation of shells in these composite nanowires. 

The development of new gas sensors and sensor materials is an important issue for different fields of human activities, including medicine, science, technology, environment protection etc. Nanowires are considered as one of the most suitable materials for the fabrication of modern gas sensors because of their unique physical and chemical properties, together with clearly manifested quantum features. One of the technical solutions for the fabrication of gas sensors is the formation of “nanowire network-based” sensor. In this work, we present the data on the growth of In₂O₃ nanowires using the pyrolytic technology, which was performed in the presence of hydrazine vapor diluted with 5 mol.% water. The nanowire network was deposited on the interdigitated gold electrodes with 5 mkm gapping that were deposited onto the glass substrate. It was found, that the fabricated gas sensor can detect ammonia at the level of hundred ppb-s.

The single crystal In₂O₃ and Ge₃N₄ nanowires were synthesized using the indium or germanium sources in new ambient comprising hydrazine decomposition products diluted with 3 mol.% water. This ambient was simultaneously containing oxidizing, nitriding and reducing active precursors. In spite of this only In₂O₃ nanowires were produced in case of In source, and only α-Ge₃N₄ nanowires were formed when Ge source was used. These active precursors provided formation of volatile suboxides of source materials, their flow to the Si substrate while a part of them was reduced to In or Ge catalyst droplets, and another part fed catalyst to grow the nanowiress trough the Vapor-Liquid-Solid mechanism. The growth temperature for In₂O₃ nanowires was lowered down to 420°C, while the Ge₃N₄nanowires were grown at 480°C which is by 370°C lower than the temperature indicated in the literature. The nanowires were characterized by a high crystallinity and the minimum thickness of 7 nm.

The hidrazine (N₂H₄) vapor was used to produce 1D nanomaterials. Hydrazine easily decomposes at the surfaces of semiconductors producing chemically active NH₂ species, because these surfaces serve as catalysts for the decomposition. This process is quite complicated and byproducts include H₂, NH₃, NH and other active species. The water content in hydrazine determines the composition of a final product. In₂O₃nanowires were used for the fabrication of nanowire network based gas sensors. It was able to detect the ammonia trases at the level of hundreds of ppb.The main result of this work: the developed hydrazine based pyrolytic technology is a viable method for producing oxide, nitride and phosphide 1D nanomaterials (nanowires, nanobelts, core-shell structures and nanotubes);

Indium phosphide nanowires (InP NWs) are accessible at 440 °C from a novel vapor phase deposition approach from crystalline InP sources in hydrazine atmospheres containing 3 mol % H2O. Uniform zinc blende (ZB) InP NWs with diameters around 20 nm and lengths up to several tens of micrometers are preferably deposited on Si substrates. InP particle sizes further increase with the deposition temperature. The straightforward protocol was extended on the one-step formation of new core-shell InP–Ga NWs from mixed InP/Ga source materials. Composite nanocables with diameters below 20 nm and shells of amorphous gallium oxide are obtained at low deposition temperatures around 350 °C. Furthermore, InP/Zn sources afford InP NWs with amorphous Zn/P/O-coatings at slightly higher temperatures (400 °C) from analogous setups. At 450 °C, the smooth outer layer of InP-Zn NWs is transformed into bead-shaped coatings. The novel combinations of the key semiconductor InP with isotropic insulator shell materials open up nteresting application perspectives in nanoelectronics.

The purpose of this work was to show the ability of hydrazine vapor to produce nitride, oxynitride and oxide 1D Nanomaterials. Hidrazine (N2H4) is a chemically very active reagent, which finds different applications. We used hydrazine for producing nitride nanomaterials by the pyrolysis at 500°C, in combination with Ge, InP and In source materials. It was found that the water content in hydrazine determines the composition of a final product. When the H2O content does not exceed 3 mol.%, the pure nitride nanowires of Ge3N4 and InN were produced. At elevated water concentrations the oxide and oxinitride nanomaterials were fabricated. The main result of this work: the developed hydrazine based pyrolytic technology provides a simple way to fabricate different ID nanostructures (nanowires, nanobelts, nanocables, nanotubes etc.). The synthesized materials include Ge, Ge3N4, InN, In2Ge2O7, In2O3, Ga2O3, InP, Zn3(PO4)2.

The morphology of NWs strongly depends on the growth methods, process parameters and substrates. The purpose of this work was to study the shape and tapering of nitride NWs grown by vapor–liquid–solid (VLS) mechanism on Ge, Si and glass substrates, located in different zones of the pyrolytic reactor tube. The single-crystalline germanium nitride (α-Ge3N4) nanowires were chosen as model material. They were grown directly on the surface of a crystalline Ge source, together with glass and Si substrates located at 200 – 500 μm above it. During the first 15 min the temperature of Ge source was raised up to 500 – 585 °C and then stabilized, while the temperature of substrates above was kept by 100 – 150 °C lower. The nitration was carried out in the hydrazine (N2H4) vapor containing 3 mol. % of water. The mass transfer was performed by volatile GeO molecules formed by the interaction of Ge source and water. The nitration of GeO ensured the growth of Ge3N4 nanowires.

The Ge nanocrystals with tetragonal structure and maximum diameters of 12 nm were synthesized on the tips of Ge3N4 nanowires. These nanowires were grown by the Vapor-Liquid-Solid technology using the Ge self-catalyst. After the process was over, the cooling of a molten Ge droplet caused the formation of a solid natural oxide shell, which was covering the liquid Ge droplet core. The further solidification of a Ge core proceeded in a fixed volume. As is known, the Ge expands during solidification. Accordingly, the internal stress of 1.5 GPa appeared in the Ge core during its solidification. This resulted in the formation of tetragonal Ge particles with 12 atoms in the unit cell, instead of the diamond type Ge nanocrystals with 8 atoms per unitl cell. It was found that if the size of the droplet exceeded 12 nm, then it produced the amorphous Ge nanoparticle.

Gas sensors based on oxide-semiconductor nanowires have been a subject of extensive research because of their potential application in detecting several inflammable, toxic and odorless gases. Among them, In2O3 has been found to have a pronounced sensitivity to such gases as NO2, NH3, O3, Cl2, CO, H2, C2H5OH and other species. Sensing NO2 in the atmosphere has assumed great importance because of the serious problem of atmospheric air pollution caused by car exhaust and other sources. Recent developments showed that In2O3 nanowires doped with different atoms exhibit superior selectivity to NO2, H2S and some other gases with short response and recovery times. In this work we describe the low-temperature synthesis of In2O3 nanowires, fabrication of a simple nanowire-network based gas sensor and its application for sensing NO2. The simple gas sensor was fabricated on Si + SiO2 substrate by depositing Ti / Au interdigitated electrodes and connecting the electrodes by bridging nanowire networks, formed after drying the droplet of acetone with dissolved In2O3 nanowires. The fabricated gas sensor was able to detect the concentration of NO2 molecules down to 5 ppm level at 200°C, with quite short and stable response and recovery time

One dimensional nanomaterials (nanotubes, nanowires, nanofibers, nanobelts etc.) are considered as the main building blocks of modern nanodevices which can serve as transient materials for the future quantum dot based advanced nanocircuits and devices. Except laser ablation, the synthetic methods for producing one dimensional (1D) oxides, nitrides, phosphides usually rely on the use of corresponding vapor flows and in most cases need relatively high temperatures exceeding ~600°C. The purpose of this work was to develop a hydrazine-based simple and efficient new technology for fabricating new 1D nanomaterials (nitrides, oxynitrides, oxides, phosphides) of Ge, In, Ga and Ge-In, Ge-Zn, In-Ga, In-Zn, In-Ga-Zn systems. Another purpose was to investigate the properties of the emerging novel nanomaterials in order to evaluate their application potential in different nanodevices. The composition and structure of 1D nanomaterials were analyzed by X-ray diffraction, Energy Dispersive Spectroscopy, Transmission and Raster Electron Microscopy.

The hydrazine (N2H4) vapor was chosen as a chemically active ambient due to its low pyrolysis temperature and ability to form active radicals and reducing agents like hydrogen. For producing oxide and oxinitride nanowires (NWs) the hydrazine was diluted with water (up to 3 mol.%). 1D nanomaterials were grown in the vertical quartz reactor. The solid source materials were placed on its heated bottom. After evaporation of sources and chemical reactions the NWs were growing on the substrates located at some distance above the source. In contrast to traditional growth methods with dynamic gas flows, the static pressure of hydrazine (~10 Torr) was preserved during the whole nanowire growth time (0.5 – 5.0 hours).

Using this technology following 1D nanomaterials were synthesized: Ge3N4, In2Ge2O7, InN, InP, InP:Zn, InxGa1-xP. The initial experiments clearly show that during the NW growth processes the hydrazine vapor can serve a variety of purposes.

The nanowire-based gas sensors are considered as the most advanced technical solutions for increasing the sensitivity far below ppm levels. There is a great need for such sensors in the industry, medicine, environmental protection, etc. The purpose of this work was to develop new technology for the fabrication of indium oxide and indium nitride one-dimensional nanomaterials and to test the gas sensor properties fabricated on the bases of these nanomaterials.  In₂O₃, InN, InN and In₂Ge₂O₇ nanowires were synthesized using the developed hydrazine-based technology. After fabricating and testing of gas sensors, it was established that the gas sensor on In₂O₃ nanowires had the best characteristics because it allowed the reliable detection of CH₄ molecules at the level of 10 ppm

One of the major challenges of modern ecology is the precise control of environmental pollution. The solution to this problem is associated with the fabrication of sensors for various gases that can detect harmful impurities at the ppb level. Recently, it was demonstrated that nanowire-based gas sensors can readily perform this heavy task. The purpose of this work was to synthesize the indium oxide (In₂O₃) and germanium nitride (Ge₃N₄) nanowires and to fabricate the nanowire area-based gas sensors on these materials. The nanowires were grown pyrolytically in the presence of hydrazine vapor, which was diluted with 3 mol.% water molecules. The nanowires were grown through the Vapor-Liquid-Solid technology using Ge and In self catalysts. SEM, TEM, and X-ray diffraction were used for the characterization of grown material. The gas sensors were fabricated by scrapping off the nanowires from Si substrate, dissolving them in alcohol, and depositing nanowires on the interdigitated gold electrode with 10 μm gapping. The best results were obtained when using In₂O₃ nanowire sensor, which was able to detect ammonia (NH₃) at a level of several hundreds of ppb.

Previously it was established that the Vapor-Liquid-Solid growth of Ge3N^T4 nanowire in the hydrazine vapor proceeds with the assistance of Ge boll-tip catalyst. In the present work we found that for the Ge catalyst sizes not exceeding 12 nm the post-growth cooling causes its crystallization in the high-pressure tetragonal form. At larger sizes of catalyst droplet the solidification proceeds with the formation of an amorphous Ge nanocrystal. We suppose, that during the cooling of a tip the amorphous Ge02 sheath around the Ge core is first solidified encapsulating liquid Ge droplet in a fixed volume and restricting its volume expansion during solidification (the volume of molten Ge increases by 6% when it crystallizes in the diamond structure 1). Upon further cooling the crystallizing Ge core embedded in GeCL sheath influences the high compressive pressure which is sufficient to form a tetragonal structure in droplets with sizes up to 12 nm.

The two types of single-crystalline alpha-Ge3N4 nanowires (NWs) were synthesized at 550degC by annealing the crystalline Ge sample in the hydrazine vapor containing 3 mol.% of water. The mass transfer was accomplished by volatile GeO molecules. The tapered NWs were grown by the vapor-liquid-solid method with ~8 nm Ge catalyst droplet surrounded by ~5 nm thick GeOx shell. NWs with uniform diameters were formed on the same sample by the oxide-assisted growth method. In the photoluminescence spectra of NWs the five peaks were observed in the blue-green region with energies close to the photoluminescence peaks of germanium suboxides.

Nanowires are considered as one of the most suitable materials for the fabrication of modern nanodevices because of their unique physical and chemical properties, together with clearly manifested quantum features. The Vapor-Liquid-Solid (VLS) mechanism of their growth is a well-established and popular method of their synthesis. Under some growth conditions, the tapering of VLS-grown nanowires is observed, which a technological drawback is. In this work, we have studied the reasons for the tapering of Ge3N4 nanowires grown by the self catalyzed VLS method. The technology was based on the application of active species, formed after pyrolytic decomposition of N2H4 3 mol.% water. These spaces were able to form volatile GeO molecules together with reducing NH radicals and hydrogen. It was found, that in Ge3N4 nanowires, grown on Ge source, the diameters were gradually decreased with growth time. The inverse tapering was observed in nanowires grown by the same VLS method on the Si substrate. It was established that during the growth process, which started at 350°C, the temperature was permanently raising with time, finally reaching the value of 500°C. This caused the time-dependent redaction of catalyst diameters for nanowires grown on Si substrate and their tapering. In contrast to this, for nanowires growing on Si substrate, the catalyst diameter was increased with time, due to more intense evaporation of GeO species and their reduction on the catalyst surface. As a result, the catalyst of nanowires that were growing on Si substrate was gradually increasing, thus increasing the diameter of Ge3N4 nanowires.

Germanium Nitride Nanowires (Ge₃N₄) were synthesized by the developed hydrazine (N₂H₄)-based technology. Annealing of germanium or Ge source in the vapor of N₂H₄+3 mol.% H₂O caused the formation of volatile GeO molecules in the hot zone. These molecules were transferred to the Si substrate, which was placed in the cold zone of a reactor. After interacting with hydrazine decomposition products (NH₃, NH₂, NH, H₂, H) and water, Ge₃N₄ nanowires and nanobelts were produced on the Ge source in the temperature range of 500–520 ºC. The growth temperature of Ge₃N₄ nanowires in hydrazine vapor was by 350 ºC lower than the temperature reported in the literature. The possible chemical reactions for the synthesis of these nanostructures were evaluated. The results of this work clearly demonstrate that the hydrazine vapor has a high chemical activity and can be successfully used for the low temperature production of nitride nanomaterials.

Nanoporous materials attract great interest due to their increased surface area and hence, their increased chemical activity. Our strategy for the formation of nanoporous films was based on the deposition of two-phased Ge/GeO₂ film and its subsequent etching. The films were deposited by magnetron sputtering of Ge target in the plasma of a mixture of O₂ and Ar gases at a total pressure of 2´10^-3 Torr. The partial pressure of oxygen was kept sufficiently low (8´10^-4 Torr) to ensure the formation of a phase-separated film comprising Ge nanocrystals, embedded in the amorphous GeO₂ film. The structure, morphology, and phase content of films were analyzed by SEM, TEM, and X-ray diffraction methods. It was shown that the nonhomogeneous internal strains are built-up in the film, which causes the formation of the nanoporous film after its etching in water

The amorphous Al2O3 films with embedded Ge or Si nanocrystals were produced by the magnetron sputtering of Al+Si(Ge) targets in a mixture of Ar+02. The value of the oxygen partial pressure was selected in such a way, that the Al target was sputtered in the "oxide" mode, while the neighboring Si (Ge) target was operating in the "metallic" mode. Annealing of these films using photon pulses with 6 kW power, cause formation of Ge or Si nanocrystals in the amorphous Ab03 matrix. The presence of Al2O3 and elemental Si(Ge) phases was proved by Auger spectroscopy and Transmission Electron Microscopy.

The films with regular and randomly distributed pores are challenging materials for membranes, photonic crystals, thermal insulation, surface protection, light coatings, and several other applications. The purpose of this work was to develop the technology for the deposition of phase-separated Ge:GeO₂ film using the magnetron sputtering of Ge target in the Oxygen deficient atmosphere. IR, SEM and X-ray diffraction confirmed the simultaneous presence of Ge and GeO₂ phases. As is well known, GeO₂ easily dissolves in water. After drying the water-treated phase-separated Ge:GeO­₂ films, the pores appeared with sizes in the range of 80-450 nm. The sizeable difference in pore sizes was attributed to the large deviations in Ge islands that were distributed in the amorphous GeO₂ matrices. It was concluded that the pore diameters decreased when the film deposition rate was increased due to the formation of smaller, densely distributed Ge dots. The increase in temperature caused the coalescence of neighboring Ge clusters, forming large Ge spots and larger pores. It was shown that the pore sizes and their distribution can be controlled by properly adjusting the process parameters of film deposition.  

      The nanocrystals embedded in insulating matrices are considered as prospective materials for the fabrication of different nanodevices including sensitive gas-sensors, light emitting diods, hard coatings etc.  The results of this work clearly show that germanium quantum dots embedded in GeO₂ can be fabricated by DC magnetron sputtering of Ge target in the mixture of Ar and oxygen. The IR bands at 880 and 680 cm ^{- 1} confirm the presence of GeO₂ phase and also show the existence of Ge-O-Ge bonds at the boundaries between Ge and GeO₂ phases. The optical absorption and Auger electron spectroscopy data prove that elemental Ge presents in the magnetron sputtered CeO_x film. TEM experiments show that crystallization takes place after Rapid Thermal Annealing (RTA) of films. Ge quantum dots with average size of 12 nm and different shapes are formed in GeO₂ matrix after 11 annealing cycles

The sublimation of GeO molecules formed by the solid phase reaction between a-Ge and a-GeO2 films (Ge+GeO2=2Ge) was studied using vacuum microbalance. The c-Si/Ge/GeO2, cSi/GeO2/Ge, c-Ge/GeO2 and c-Si/GeO/GeO2 structures, fabricated by the ion-plasma deposition were annealed in the temperature range of 650-725°C. The diffusion of GeO molecules through the amorphous GeO2 and Ge films was investigated. The calculated activation energy indicated that the sublimated molecules were (GeO)2 dimmers. Dimeric (GeO)2 molecules were formed directly in the reaction zone at the Ge/GeO2 interface and after diffusion through the overlayer they were sublimated without decomposition. The sublimation rate of (GeO)2 was influenced by the degree of crystallinity of the Ge film. The Arrhenius expressions were obtained for calculation of diffusivities and permeability. Examination of the diffusion and permeation data suggests, that the process of diffusion through the GeO2 or Ge films limits the evaporation rate of(GeO)2. © 2002 IEEE.

Auger electron spectroscopy (AES) was used to study the solid phase surface reactions and Ge diffusion process during the thermal treatment (500-750°C) of GeO₂/(111)GaAs structures. Different results were obtained for the A and B surfaces of (111)GaAs. The diffusivity data for A and B surfaces could be described by the following Arrhenius expressions: A-D(cm² s^-1)=1.16×10² exp(-3.54/kT); B-D(cm² s^-1)=2.16×10^-5 exp(-2.12/kT)

 According to modern understanding, high-quality surface passivation of GaAs can be achieved only after the removal of its thin native oxide layer, which covers the surface and forms the interface with a high interface defect density (>1012 cm2). This work was aimed at the development of such a technique, which can create an intermediate layer between GaAs and the outer insulating film. This layer, called the “interface control layer” (ICL), must form high-quality interfaces with both neighboring materials. A thin, ~20 nm thick, Ge layer was chosen to accomplish this task.

 The results reported in this paper clearly show that pyrolytic GeOxNy film can be used for high-quality surface passivation of GaAs and fabrication of an effective MIS transistor on this semiconductor.

One of the main directions of modern microelectronics concentrates on the development of high­ performance radiation-hardened electronic parts to be used in harsh environments. The ionizing radiation primarily affects the gate-insulator of MIS (Metal-Insulator-Semiconductor) devices. Our new approach is based on the assumption that the stability of MIS structures will increase if an insulating film will be substituted by an amorphous 0 and N doped Germanium film (GeOxNy) film) with a wide energy gap and a high resistivity. In such material a certain amount of free carriers exists and they have an anomalous low mobility. The movement of free carriers, whose relaxation times are significantly shorter, compensates the long relaxation time instabilities caused by irradiation. The developed new type film forms a high quality interface with Si (Nss=2´10 cm-2 eV-1), which is close to, the best Si-Si02 interface.

The results of our preliminary experiments prove that our approach has a sound physical basis. It can be used to develop an essentially new type of effective, simple and inexpensive technology for the high-quality surface passivation of Si, GaAs and other semiconductors. It can be also used for the fabrication of microwave MIS Integrated Circuits on GaAs with all inherent advantages of this type of devices.

 Zn and Ge diffusion in (n)GaAs from the zinc-silicate glass (ZSG) and Ge02 solid dopant sources was investigated. After pulse photon annealing of the ZSG/n-GaAs(lnP)/Ge02 system the p-n-n+ structures were formed during a single photon flash. Due to the amphoteric nature of Ge atoms their diffusion in the polar          (111) GaAs surfaces results in formation of a p-type layer in case of B-surface and n-layer - when A-surface is used.

Doctoral Thesis Referee

Master Theses Supervisor

Doctoral Thesis Supervisor/Co-supervisor

Scientific editor of monographs in foreign languages

Scientific editor of a monograph in Georgian

Editor-in-Chief of a peer-reviewed or professional journal / proceedings

Review of a scientific professional journal / proceedings

Member of the editorial board of a peer-reviewed scientific or professional journal / proceedings

Participation in a project / grant funded by an international organization

Participation in a project / grant funded from the state budget

Patent authorship

Membership of the Georgian National Academy of Science or Georgian Academy of Agricultural Sciences

Membership of an international professional organization

Membership of the Conference Organizing / Program Committee

National Award / Sectoral Award, Order, Medal, etc.

Honorary title

Monograph

Handbook

Research articles in high impact factor and local Scientific Journals

Publication in Scientific Conference Proceedings Indexed in Web of Science and Scopus