Facultatea de Fizică şi Inginerie / Faculty of Physics and Engineering
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Item Magnetotransport properties of ultrathin LaMnO3 layers [Articol](Technical University of Moldova, 2011-07-07) Șapoval, Oleg; Belenciuc, Alexandr; Zasavițchi, Efim; Canțer, Valeriu; Moșneaga, VasiliiWe report the transport and magnetic properties of La-deficient ultrathin films of La1-MnO3 (LMO) grown on SrTiO3 (STO) and engineered by using different interfacial layers. LMO layer and adjusting interface oxide (LaO-STO and SMO) layers were grown by a metalorganic aerosol deposition technique with monolayer accuracy. The role of LaO-TiO2 interface in the formation of ferromagnetic metallic state in very thin LMO films was demonstrated. Ferromagnetic metallic ground state in LMO films with the thickness down to 6 monolayers is stabilized by a combination of a La-deficiency and the interface-induced doping.Item Ultrathin manganite layers [Articol](2011) Șapoval, OlegEpitaxial La-deficient ultrathin films of La1-MnO3 (LMO) were grown by a metalorganic aerosol deposition technique with monolayer accuracy on SrTiO3 (STO) substrates and engineered by using different interfacial layers. The role of LaO-TiO2 interface in the formation of the ferromagnetic metallic state in very thin LMO films was demonstrated. The ferromagnetic metallic ground state in LMO films with the thickness down to 6 monolayers is stabilized by a combination of a La-deficiency and the interface-induced doping.Item Fotoluminescenţa straturilor nanolamelare de GaSe obţinute prin intercalarea cu Cd [Articol](2012) Dmitroglo, Liliana; Evtodiev, Igor; Caraman, Iuliana; Nedeff, Valentin; Dafinei, AdrianS-au analizat spectrele de emisie fotoluminescentă a lamelor monocristaline p-GaSe cu concentraţia golurilor3•1014 cm-3 şi a lamelor intercalate cu Cd în fază de vapori la temperatura 500 °C. Durata tratamentului termic a fost de 20 şi 24 ore. Spectrul de emisie al cristalelor de GaSe conţine liniile excitonilor direcţi localizaţi cu energia de legătură ~6 meV, prima repetare fononică a acestora (ħωf =20 meV) şi banda de emisie a excitonilor indirecţi cu emisia fononilor cu energia 15 meV. Spectrul de emisie a compozitelor obţinute prin intercalarea lamelor de GaSe cu Cd se obţine în rezultatul suprapunerii benzilor de emisie a compusului CdSe şi banda impuritară a monoseleniurii de galiu. Structura spectrului FL depinde de durata tratamentului termic. La majorarea timpului de tratament se amplifică subbanda corespunzătoare compusului CdSe.Item Thin layer photovoltaic modules based on CdS-CdTe heterojunctions [Articol](2011) Fiodorov, Vladimir; Gașin, Petru; Gagara, Ludmila; Chetruș, PetruA technology of thin layer photovoltaic modules with dimensions of 65 x 80 mm2 based on CdS-CdTe heterojunctions prepared via the quasi-closed volume technique was developed. The modules were made on a single glass plate; each element was positioned along the given plate so that the resulting module contains nine solar cells with an area of 3 cm2. The design of the module made it possible to study the energy parameters of each solar cell as well as the entire module. The study of the photovoltaic module based on CdS-CdTe heterojunctions showed the following parameters: short circuit current Isc = 270 mA; open circuit voltage Ucd = 0.786 V; FF =0.46, power discharged in the load Pmax ≈ 100 mW, and efficiency η ≈ 8.2% at the radiation power of 53 mW/cm2.Item Nanolamellar structures of Oxide-AIIIBVI: Cd semiconductors type for use as detectors of radiation in the UV spectral region [Articol](Technical University of Moldova, 2011-07-07) Dmitroglo, Liliana; Untila, Dumitru; Chetruș, Petru; Evtodiev, Igor; Caraman, Iuliana; Lazăr, Gabriel; Nedeff, ValentinIn the paper, optical and photoelectrical properties of GaSe and InSe single crystal films of 10-5÷10-7 m submicron thickness and of semiconductor-native oxide structures obtained by annealing at (450÷700)°C in a normal atmosphere, are studied. The absorption spectrum of InSe lamella as well as of GaSe lamella in the energetic range from the red threshold up to 4,5 eV contains three bands with a rapid increase of the absorption coefficient which varies in the limits of (100÷106) cm-1 At the energies higher than 1,25 eV and 2,01 eV for InSe and GaSe respectively the light absorption are determined by the direct optical transitions in the centre of the Brillouin zone and at the energies higher than. At the absorption coefficients of (100÷102)cm-1 the indirect optic transitions are present. 3,0 eV also by the direct optical transitions in the points of the bands high symmetry. The resistive photosensitivity bands cover the spectral range Eg≤ hν ≤ 4,5 eV for lamellar photoresistors in which electric field EC6. The resistive photosensitivity band width could be controlled by the lamella thickness for d ≥1μm. The open circuit voltage spectral distribution is analysed from which results that at the oxidation temperature of 700°C in GaSe layer at the heterojunction interface the defects are formed on which the charge carriers, collected in the junction, are dissipated. The noneequilibrium charge carrier free path is of 0,8 μm.Item Electrical properties of thermally annealed CdS thin films obtained by chemical bath deposition [Articol](Institute of Electrical and Electronics Engineers Inc., 2011-10-17) Scorțescu, Dumitru; Maticiuc, Natalia; Nicorici, Valentina; Spalatu, Nicolae; Potlog, Tamara; Hiie, Jaan; Valdna, VelloElectrical conductivity and the Hall-effect are investigated in the temperature interval (80-400) K on thermally annealed in H 2 CdS thin films obtained by chemical bath deposition. Different characters of the temperature dependence of conductibility are observed in the CdS films annealed at different temperatures. The Hall measurements allow calculating the values of the NA , N D , n ex and ED . According to Hall measurements the CdS films show several donor levels at different energetic depths in dependence of the annealing temperature. The sample annealed at high temperatures than 350oC proves to be compensated with a sharply decreasing electrical conductivity with the temperature decrease.Item Morphology and luminescence properties of ZnO layers produced by magnetron spattering [Articol](Technical University of Moldova, 2011-07-07) Rusu, Emil; Ghițu, Irina; Prilepov, Vladimir; Zamalai, Victor; Ursachi, VeaceslavWe show that the morphology and the luminescence properties of ZnO layers produced by magnetron sputtering can be controlled by technological parameters of sputtering, particularly by the ratio of argon to oxygen gases in the gas flow during the growth process. Smooth and flat layers were produced with a high Ar/O ratio, while porous layers with various morphologies were obtained with a low Ar/O ratio. The layers produced with O/Ar ration equal to 10 exhibit extremely high near-bandgap luminescence intensity even higher in comparison with bulk ZnO single crystals. The free carrier density estimated from the analysis of photoluminescence spectra is also very high in these samples suggesting that these technological conditions promote both optical and electrical activation of the doping Al impurity. The samples grown with high Ar/O ratios exhibit strong visible emission which is controlled by the technological conditions.Item High temperature ultra-fast gallium arsenide rectifier devices [Articol](Institute of Electrical and Electronics Engineers Inc., 2012-10-20) Baranov, Simion; Gorceac, Leonid; Cheibaș, VictorThe report is related to the high temperature electronics field. The industrial developed country practice demonstrates that up to 70% of the fabricated electrical energy passes through semiconductor converters. Only in the field of electrical drive, which uses about 50% from the produced electrical energy, passing to the frequency converter of the asynchronous drives speed control, brings an energy economy up to 25%. In the electrical transport with the energy braking recovering the economy is more than 30%. The present elaboration is foreseeing the improvement of the technological production route of gallium arsenide (GaAs) power semiconductor devices (PSD), by introducing some innovative technological processes as: (1) GaAs epitaxial technology in the Ga-AsCl 3-H2 system; (2) epitaxial structures divided into crystals by chemical method; (3) GaAs p-n junction passivation by own oxide (Ga2 O3) precipitate on the divided surface of the crystal. The goal of the project is increasing manufacturing efficiency (ME) at the GaAs power devices with more than 600 V of voltage fabrication. The proposed improvements are increasing ME with 40% and are expanding the blocking voltage interval up to 1000 V. As a result a new product was elaborated and proposed to the manufacturing– the ultra-fast current converter with 80 ns of recovery time, stable at high temperature (200 oC) and 4.5 kV of blocking voltage, that outrun the characteristics of commercial product ESJC30-05(0.3 s, 110 oC).Item Automation control system of the alloying process of gallium arsenide layers growth by epitaxial technology [Articol](Institute of Electrical and Electronics Engineers Inc., 2012-10-20) Baranov, Simion; Cojuhari, Irina; Izvoreanu, Bartolomeu; Gorceac, LeonidThis paper proposes to control the thermal alloying process of GaAs epilayers growth by transport reaction in Ga- AsCl 3-H2 system. The thermal alloying process is controlled by using the universal two-channel programmable PID-controller TRM151, which permits automatic control of complicated objects with high precision. The thermal process was identified using the parametric model ARX (Auto-Regressive eXogenous) from System Identification Toolbox from MATLAB. The experimental technologist’s program contains steps of p +- po –n + photovoltaic structures achievement with damper layer on p-GaAs substrate. There is a possibility to obtain multiple periodical epitaxial layers structures with different dimensions and electro-physical properties, including nano-dimension structures, changing different steps of production. The tuning controller was performed using the maximal stability degree method. This control system minimizes the new technologies elaboration terms, accelerates the implementation of the elaborated technology in industry by reducing production costs, improving the product quality and its market competitiveness.Item The caracterization of the CdS-based solar cell heterojunctions [Articol](Institute of Electrical and Electronics Engineers Inc., 2010-10-11) Potlog, Tamara; Botnariuc, Vasile; Gorceac, Leonid; Spalatu, Nicolae; Maticiuc, Natalia; Raevschi, SimionThe CdS-based solar cell heterojunctions (HJ) have been obtained by growth of CdS at relatively lower temperature using the close space sublimation method(CSS). Investigation of the photovoltaic characteristics shows an efficiency of about 12 % for InP/CdS and 9.6 % for CdS/CdTe solar cell HJ. The analysis of the forward dark current-voltage and the capacitance-voltage characteristics indicate a tunnelling recombination current which flows through states near or at the interfaces of a thermal energy of about 0.62 eV for CdS/CdTe and 0.42 eV for InP/CdS solar cell heterojunctions. The solar energy conversion efficiency is influenced by the interface states through the open circuit voltage and the fill factor.