2. Articole
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Item Structural and optical properties of ZnO:Ga thin films deposited on ito/glass substrates for optoelectronic applications [Articol](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 [Articol](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 [Articol](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 [Articol](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 [Articol](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 ZnO doping efficiency by multivalent metals in complex CVT reactions [Articol](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 [Articol](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.Item Halide-oxide carbon vapor transport of ZnO: Novel approach for unseeded growth of single crystals with controllable growth direction [Articol](Elsevier, 2018) Colibaba, GlebThe thermodynamic analysis of using HCl + CO gas mixture as a chemical vapor transport agent (TA) for ZnO single crystal growth in closed ampoules, including 11 chemical species, is carried out for wide temperature and loaded TA pressure ranges. The advantages of HCl + CO TA for faster and more stable growth are shown theoretically in comparison with HCl, HCl + H2 and CO. The influence of the growth temperature, of the TA density, of the HCl/CO ratio, and of the undercooling on the ZnO mass transport rate was investigated theoretically and experimentally. The HCl/CO ratios favorable for the growth of m planes and (0001)Zn surface were found. It was shown that HCl + CO TA provides: (i) a rather high growth rate (up to 1.5 mm per day); (ii) a decrease of wall adhesion effect and an etch pit density down to 103 cm−2; (iii) a minimization of growth nucleus quantity down to 1; (iv) stable unseeded growth of the high crystalline quality large single crystals with a controllable preferred growth direction. The characterization by the photoluminescence spectra, the transmission spectra and the electrical properties are analyzed.Item Acid-base properties of the surface of zinc oxide powders subjected to milling in the attritor [Articol](IOP Publishing Ltd, 2020) Korotcenkov, GhenadiiUsing the method of acid-base indicators adsorption, the influence of mechanical stresses and defects, arising in the milling process, on the surface properties of ZnO powders is analyzed. An explanation of the changes in acid-base properties is given.Item Straturi subţiri de oxid de zinc crescute prin metoda pulverizării în flux de argon [Articol](CEP USM, 2018) Botnariuc, Vasile; Gorceac, Leonid; Coval, Andrei; Cinic, Boris; Gaugaș, Petru; Chetruș, Petru M.; Moldovanu, Sergiu; Raevschi, SimionStraturile subţiri de ZnO au fost crescute din acetat de zinc dizolvat în apă-acid acetic-metanol cu o molaritate de 0,2 M folosind metoda pulverizării în intervalul de temperaturi (250-450)°C în flux de argon. Au fost cercetate proprietăţile optice şi electrice ale acestor straturi în dependenţă de temperatura de creștere. Transmitanţa în diapazonul de lungimi de undă (300-1000) nm are valori de 80-85%. Rezistenţa specifică a straturilor subțiri de ZnO crescute la 450°C după tratarea termică în vid şi hidrogen la 450°C timp de o oră se micșorează de la 33 •cm până la 0,028 •cm. Recombinarea radiantă în straturile de ZnO este însoțită de tranziții pe niveluri adânci și de tranziții bandă-bandă la interacțiunea purtătorilor de sarcină cu fononii de tip LO.