2. Articole
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Item STRUCTURAL AND OPTICAL PROPERTIES OF ZNO:GA THIN FILMS DEPOSITED ON ITO/GLASS SUBSTRATES FOR OPTOELECTRONIC APPLICATIONS(Academia de Ştiinţe a Moldovei, 2021) Rusnac, Dumitru; Lungu, Ion; Colibaba, Gleb; Potlog, TamaraDoped (with GaCl3), undoped ZnO and ITO/ZnO:Ga nanostructured thin films are synthesized using the spray pyrolysis method. The doped ZnO thin films are synthesized at the atomic ratio of Ga/Zn added in the starting solution fixed at 1, 2, 3, and 5. Gallium-doped ZnO films synthesized on glass/ITO substrates are annealed at 4500C in different environments: vacuum, oxygen, and hydrogen. X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM), and current–voltage (I–V) measurements are applied to characterize the structural properties, composition, surface morphology, and electrical properties of ZnO:Ga nanostructured thin films. X-ray diffraction analysis shows that ZnO:Ga films deposited on glass substrates have a dense and homogeneous surface with a hexagonal structure. The ZnO:Ga films deposited on glass/ITO substrates are composed of two phases, namely, hexagonal ZnO and cubic ITO. The I–V characteristics show the presence of good ohmic contacts between Al and In metals and ZnO:Ga thin films regardless of the nature of the substrate and the annealing atmosphere.Item ZnO NANOMETRIC LAYERS USED IN PHOTOVOLTAIC CELLS(Springer Nature, 2020) Botnariuc, Vasile; Gorceac, Leonid; Coval, Andrei; Cinic, Boris; Gaugas, Petru; Chetrus, Petru; Lungu, Ion; Raevschi, SimionThe ZnO thin layers were grown on glass, InP and pInP-nCdS substrates from zinc acetate dissolved in water-acetic acid-methanol solution having a molarity of 0.2 M by using the spray method in the argon flow in the temperature range of (250–450) °C. The dependence of optical properties of ZnO layers on growth temperature have been investigated. The optical transmittance has values of 80–85% in the wavelength range of (200–1000) nm. The using of ZnO of the thickness of (60–80) nm as antireflective layers in nCdS-pInP structures allowed to increase the photovoltaic cell efficiency by 3%. The photosensitivity of the fabricated nZnO-pInP structures covers the wavelength region from 450 nm up to 1100 nm and allows the more efficient utilization of the incident light.Item LUMINESCENT PROPERTIES ON ZnO:Cr NANOCRYSTALS AND THIN LAYERS(Springer Nature, 2020) Goglidze, Tatiana; Dementiev, Igor; Goncearenco, Evghenii; Iurieva, Tatiana; Nedeoglo, Dumitru; Nedeoglo, NataliaBoth undoped and chromium doped zinc oxide nanocrystal powders are obtained by chemical deposition and hydrothermal methods. ZnO and ZnO:Cr thin layers on the surface of ZnSe and ZnSe:Cr samples, respectively, are obtained by isovalent substitution of selenium by oxygen in the process of thermal treatment of the samples in air. Photoluminescence spectra of the ZnO and ZnO:Cr nanopowders and thin layers obtained by various techniques are investigated at room temperature. Cr doped ZnO powders obtained by chemical deposition and hydrothermal methods improves the powder quality, as evidenced exciton emission is more intensive. New emission bands are found in visible (615 nm, 625 nm) and infrared (925 nm, ~2000 nm) spectral ranges for ZnO:Cr nanopowders and thin films. The contribution of Cr impurity to the formation of emission bands is discussed.Item SINTERING HIGHLY CONDUCTIVE ZnO:HCl CERAMICS BY MEANS OF CHEMICAL VAPOR TRANSPORT REACTIONS(Elsevier, 2019) Colibaba, GlebThe features of sintering ZnO ceramics by means of chemical vapor transport (CVT) in sealed quartz chambers were investigated. CO, C, H2, HCl, HCl + H2, HCl + C, HCl + Zn and HCl + H2+ C mixtures were used as transport agents (TAs) at the 925–1070 °C temperature range. The exact thermodynamic analysis of compound CVT systems, carried out for wide temperature and loaded TA pressure ranges, was applied to establish the relation between medium composition and the characteristics of sintered materials. The advantages of some compound TAs based on HCl were shown. ZnO:HCl ceramics with a diameter of 25 mm (99 ± 1% of the initial diameter of the sintering powder), a density of 5.1 ± 0.3 g/cm3, a hardness of 2.0 ± 0.2 GPa, a resistivity of 2.4⋅10-2 Ω cm, and a controllable stoichiometric deviation, were obtained. The investigated materials have no contamination from metallic Zn, ZnCl2 or solid C. The doping efficiency of ZnO ceramics by oxides of various metals, by means of low temperature CVT reactions with ZnCl2 vapors as a TA, was calculated for oxides of all non-radioactive metals of the periodic table.Item RECENT ADVANCES IN TERAHERTZ TECHNOLOGY FOR SECURITY AND BIOMEDICAL APPLICATIONS(Universitatea de Stat din Tiraspol, 2021) Sirkeli, VadimTerahertz waves have low photon energies (~ 4.1 meV for 1 THz), which is about 1 million times weaker than the energy of X-ray photons. They do not cause any harmful ionization in biological tissues. The terahertz radiation is strongly attenuated by water and is very sensitive to water content. This paper provides current status and recent advances in terahertz technology for security and medical applications. In particular, we report on our designs of THz quantum cascade lasers to identify cancerous tissues and other medical issues.Item EFFECT OF p-NiO INTERLAYER ON INTERNAL QUANTUM EFFICIENCY OF p-GaN/n-ZnO LIGHT-EMITTING DEVICES(American Scientific Publishers, 2015) Sirkeli, Vadim; Yilmazoglu, Oktay; Küppers, Franko; Hartnagel, HansWe report on numerical investigations of p-GaN/n-ZnO light-emitting devices with p-NiO interlayer, and on LED design optimization which includes bandgap engineering, thickness and doping of constituent layers. The current–voltage dependences of investigated LEDs show a threshold voltage of 3.1 V and 5.4 V for the LED devices without and with presence of p-NiO interlayer, respectively. It is found that p-NiO layer act as electron blocking layer, that lead to the enhance of charge carriers confinement in active region, and to the increasing of internal quantum efficiency (IQE) of LED device up to 0.5%, that in four times higher in compare with that for original p-GaN/n-ZnO LED device.Item EFFECT OF p-NiO AND n-ZnSe INTERLAYERS ON THE EFFICIENCY OF p-GaN/n-ZnO LIGHT-EMITTING DIODE STRUCTURES(IOP Publishing Ltd, 2015) Sirkeli, Vadim; Yilmazoglu, Oktay; Küppers, Franko; Hartnagel, HansWe report on a numerical study of the characteristics of p-GaN/n-ZnO light-emitting diodes (LEDs) with p-NiO and n-ZnSe interlayers, and on LED design optimization which includes bandgap engineering, thickness and doping of constituent layers. The current-voltage dependences of investigated LEDs show a threshold voltage of 3.1 V, 5.4 V and 5.6 V for LED devices without and with the presence of p-NiO and n-ZnSe interlayers, respectively. It is found that p-NiO, n-ZnSe and n-ZnO interlayers act as an electron blocking layer, active media layer, and electron transport layer, respectively. It is established that the insertion of both p-NiO and n-ZnSe interlayers leads to the enhancement of charge carrier-confinement in the active region and to the significant increase of internal quantum efficiency (IQE) of the LED device up to 82%, which is comparable with IQE values in order to obtain better AlGaN- and InGaN-based LEDs. It is found that the efficiency of LED devices at 100 A cm−2 is equal to 0.024, 0.09 and 16.4% of external quantum efficiency (EQE), 1.3 × 10−4, 1.6 × 10−4, and 6.4 lm W−1 of PE, and 1.3 × 10−4, 2.9 × 10−4, and 12 cd A−1 of CE for p-GaN/n-ZnO, p-GaN/p-NiO/n-ZnO, and p-GaN/p-NiO/n-ZnSe/n-ZnO LED devices, respectively.Item ЛЮМИНЕСЦЕНТНЫЕ ХАРАКТЕРИСТИКИ ТОНКИХ ПЛЁНОК ОКСИДА ЦИНКА, ПОЛУЧЕННЫХ МЕТОДОМ ЭЛЕКТРОФОРЕЗА(CEP USM, 2012) Гоглидзе, Т.И.; Дементьев, И.В.; Задорожный, А.П.; Коваль, А.В.; Петренко, П.А.; Гашин, П.А.Показана возможность осаждения тонких однородных слоёв оксида цинка с низкой проводимостью электрофоретическим методом. Представлены результаты измерений фотолюминесценции тонких слоёв ZnO при температурах 300 К и 77 К.Item ZnO DOPING EFFICIENCY BY MULTIVALENT METALS IN COMPLEX CVT REACTIONS(Elsevier, 2019) Colibaba, GlebThe present investigation is a theoretical study of doping efficiency of ZnO crystals grown by using compound chemical vapor transport (CVT) agents based on HCl and doped by oxides of various metals in the growth process. The thermodynamic analysis for compositions of MexOy−ZnO–ZnCl2–CO CVT systems in the closed growth chambers was carried out for oxides (MexOy) of all non-radioactive metals of the periodic table, taking into account various types of chloride species. The influence of temperature (1000–1500 K), ZnCl2 pressure (10−2−10 atm) and Zn pressure (10−5−3 atm) on the total pressure and mass transport rate of doping species (MeCln) were investigated. The possibility of increase in the doping efficiency of ZnO by MexOy + ZnCl2 + Zn CVT reactions is predicted for some multivalent metals. Some calculation results are confirmed experimentally.Item HALIDE-CARBON VAPOR TRANSPORT OF ZnO AND ITS APPLICATION PERSPECTIVES FOR DOPING WITH MULTIVALENT METALS(Elsevier, 2018) Colibaba, GlebThe growth of ZnO single crystals in closed ampoules using HCl+C mixture as a chemical vapor transport agent (TA) was studied. The influence of growth temperature, TA density, C/HCl ratio, and undercooling on the ZnO mass transport rate was investigated theoretically and experimentally. The influence of growth medium composition on the features of crystal growth, such as the minimization of growth nucleus density, the increase in the lateral growth rate of up to 1 mm per day, the suppression of the attachment effect, and stable growth of non-polar, semi-polar and polar planes of a hexagonal structure was analyzed. The structural, photoluminescent, optical and electrical properties were investigated. The doping efficiency of ZnO by oxides of metals was analyzed for various TAs and Zn pressure in the growth medium. The possibility of increase in the doping efficiency by several orders of magnitude for multivalent metals was predicted for HCl+C TA.