Facultatea de Fizică şi Inginerie / Faculty of Physics and Engineering
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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 Nuclear quadrupole resonance of mixed-valence charge-ordered dimers [Articol](1999) Clochișner, Sofia; Gorceac, LeonidThe influence of an external magnetic field on the conditions of charge ordering of mixed-valence dimer clusters Ni 2q–Niq is considered. The manifestations of charge ordering in the spectra of nuclear quadrupole resonance are elucidated. These spectra are shown to give information on the key parameters of charge-ordered crystals.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.Item About the edge luminescence of cadmium sulphide thin layers grown on molybdenum [Articol](Institute of Electrical and Electronics Engineers, 1995-11-11) Raevschi, Simion; Gorceac, Leonid; Coval, Andrei; Chetruș, PetruCdS layers on to molybdenum were synthesized from separate elements in an open flowing hydrogen set. The surface micromorphology and some electrical and photoluminescent properties were investigated. The nature of layers edge photoluminescence peaks are discussed in the frames of the excitated exciton model.Item CdS nanometric layers grown on SnO2 coated glass substrates for photovoltaic devices [Articol](2012) Botnariuc, Vasile; Gorceac, Leonid; Coval, Andrei; Chetruș, Petru; Cinic, Boris; Raevschii, Simion; Micli, ValdecCdS layer thickness decreases at high substrate temperatures due to the fact that a part of CdCl2/(NH2)2 CS aqueous solution evaporates without reaching the substrate surface. CdS layers deposited at the substrate temperature from 250oC to 450oC are growing with a deficit of sulfur. CdS layers with better stoichiometry were grown in the conditions when in the solutions, used for CdS layers growing, there is an excess of thiourea (CdCl2/ TU =1:2). The high charge carrier concentration of 1020cm-3 in CdS layers grown on glass substrates coated with SnO2 layer is related to Sn doping of the layers from SnO2 layer.Item Thin AIN films growth on Si (III) by hydride vapor phase epitaxy [Articol](2008) Raevschi, Simion; Davydov, Valerii; Zhilaev, Yurii; Gorceac, Leonid; Botnariuc, VasileThin AlN layers have been grown on Si (111) by hydride vapor phase epitaxy (HVPE) method in a horizontal quartz reactor. The surface of layers has been studied by scanning electron microscopy and by the Raman spectroscopy method and found to have the structured morphology. It has been determined that the layers have high specific electrical resistance and are strained in the plane of the substrate.Item Proprietăţi electrofizice ale straturilor de CdS obţinute prin metoda pulverizării cu tratare termică în hidrogen [Articol](CEP USM, 2012) Botnariuc, Vasile; Gorceac, Leonid; Coval, Andrei; Cinic, Boris; Chetruș, Petru; Raevschi, Semion; Banu, SimionPrin metoda pulverizării chimice au fost crescute straturi de CdS din soluţii apoase de CdCl2/(NH2)2 CS cu molaritatea de 0,1 M în intervalul de temperaturi (250...450)°C. Straturile au fost tratate termic în flux de hidrogen timp de 20 min la temperaturile de 350 şi 450°C. Au fost cercetate proprietăţile electrofizice şi fotoluminescenţa acestor straturi. În straturile depuse la 450°C cu mărirea temperaturii de tratare până la 450°C se observă o uşoară micşorare a concentraţiei purtătorilor de sarcină. Spectrul fotoluminescenţei prezintă o fâşie largă în intervalul de energii 1,6...2,6 eV. Se observă un vârf al fotoluminescenţei cu energia de 1,95 eV, care se deplasează cu mărirea temperaturii de creştere a straturilor de CdS şi atinge valoarea de 2,5 eV pentru straturile crescute la temperatura de 450°C.Item Straturi CdS crescute pe suporturi de sticlă prin metoda pulverizării [Articol](CEP USM, 2012) Botnariuc, Vasile; Gorceac, Leonid; Coval, Andrei; Chetruș, Petru; Cinic, Boris; Raevschi, Simion; Micli, ValdecCdS layers were grown from aqueous solutions of cadmium chlorine (CdCl2) and thyourine (NH2)2 CS with the molarity of 0,1 M by pulverization method in the temperature range of (250...450)°C. CdS layers were grown on glass substrates covered with a previously deposited SnO2 layer. The deposited CdS layers morphology, atomic weight and composition were studied biasing a sunning electron microscope (SEM). The morphology, atomic weight and composition of the deposited CdS layers considerably changes with the increase of the deposition temperature. The charge carriers’ concentration and their mobility in CdS layers deposited at different temperatures were measured and estimated.Item Straturi subţiri de CdS depuse din soluţii lichide (baie chimică) [Articol](CEP USM, 2011) Botnariuc, Vasilii; Gorceac, Leonid; Coval, Andrei; Raevschi, Simion; Micli, Valdec; Cinic, BorisThin layers of CdS were deposited on InP (100) substrates with a (3...5) arc degrees misorientation relative to (110) using water solutions of CdSO4, (NH4)2SO4, NH4OH, NH4Cl and CS(NH2)2 for synthesis. The morphology, atomic composition, photoluminescence and electrical properties of the deposited layers were investigated. The morphology of the CdS layers is characterized by a granular structure that is not changing under thermic treatment. A band in the energy interval (1,55 – 3,1) eV at 77 K with the maximum at 2,282 eV is observed in the photoluminescence spectra of the thin layers. Under thermic treatment from 200°C to 500°C in hydrogen the concentration of charge carriers is increasing from 2⋅1017 cm-3 to 2⋅1018 cm-3.