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Milanova M.,Central Laboratory of Applied Physics | Koleva G.,Central Laboratory of Applied Physics | Kakanakov R.,Central Laboratory of Applied Physics | Vitanov P.,Bulgarian Academy of Science | And 5 more authors.
Energy Procedia | Year: 2010

The present work demonstrates the possibility to use liquid phase epitaxy to incorporate nitrogen in epitaxial GaAsN/GaAs and GaInAsN/GaAs heterostructures, including nanoscaled ones. The structures are grown from Ga - and GaIn - melts containing polycrystalline GaN as a nitrogen source. The red shift of the absorption spectra corresponds to nitrogen content in the epitaxial layers near or less than 0.2 at %. Photoluminescence spectra of dilute nitride GaAsN and GaInAsN show emission from localized nitrogen states - N-nanoclusters of more than two N atoms. These studies show that the melt grown dilute GaAsN and GaInAsN nanostructures can be used for solar cells with extended long wavelength edge. © 2009 Published by Elsevier Ltd.


Milanova M.,Central Laboratory of Applied Physics | Vitanov P.,Bulgarian Academy of Science | Sendova-Vassileva M.,Bulgarian Academy of Science | Koleva G.,Central Laboratory of Applied Physics | Popov G.,Central Laboratory of Applied Physics
Energy Procedia | Year: 2011

This work demonstrates the possibility for low-temperature Liquid-Phase Epitaxy (LPE) growth of lattice matched to GaAs substrate dilute nitride InGaAsN layers with good crystalline quality and high Hall electron mobility. X-ray microanalysis and X-ray diffraction methods have been used to determine the composition and crystalline quality of the grown InGaAsN layers. Surface roughness is examined by atomic force microscopy. N-related local vibration modes are observed by Raman scattering. The Hall electron mobility and free carrier concentration have been measured in the temperature range 80-300K by conventional Van der Pauw method. © 2011 Published by Elsevier Ltd.


Milanova M.,Central Laboratory of Applied Physics | Vitanov P.,Bulgarian Academy of Science | Terziyska P.,Lakehead University | Koleva G.,Central Laboratory of Applied Physics | Popov G.,Central Laboratory of Applied Physics
Physica Status Solidi (C) Current Topics in Solid State Physics | Year: 2013

This paper presents the comparison of nitrogen incorporation in GaAsN and InGaAsN layers grown on GaAs substrate from Ga- and In-rich solution, respectively, by liquid-phase epitaxy. Polycrystalline GaN has been used as a source of nitrogen in two cases. The initial epitaxy temperature has been varied in the temperature range 600-550 °C. Nitrogen content in Ga1-xAsNx grown layers has been determined to be in the range 0.1-0.5%. Higher nitrogen incorporation efficiency has been found for quaternary InGaAsN layers grown under carefully chosen lattice matched conditions. The incorporation of nitrogen into GaAsN and InGaAsN layers has been study by vibrational mode absorption spectroscopy. Nitrogen-induced vibration mode near 472 cm-1 has been registered in GaAsN samples. Preferential In-N bonds and the formation of N-centred In3Ga1 clusters have been identified for lattice matched to GaAs epitaxial InGaAsN layers. Electrical properties of the samples have been characterized by temperature-dependent Hall effect measurements. Nominally undoped GaAsN and InGaAsN grown layers are n-type with Hall concentration about one order of magnitude higher in comparison to layers not containing nitrogen. Thermally activated increase in the free carrier concentration at temperatures higher than 150 K is observed which indicates the presence of N-related deep donor levels below dilute nitride conduction band edge. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Milanova M.,Central Laboratory of Applied Physics | Kakanakov R.,Central Laboratory of Applied Physics | Koleva G.,Central Laboratory of Applied Physics | Vitanov P.,Bulgarian Academy of Science | And 3 more authors.
Solid State Phenomena | Year: 2010

GaSb based III-V heterostuctures are attractive for optoelectronic devices such as midinfrared lasers, detectors, and thermophotovoltaics (TPVs). In this paper the growth and characterization of GaInAsSb and GaAlAsSb quaternary layers, lattice-matched to GaSb substrate, are reported, with a particular focus on these alloys for TPV devices. High-quality with a mirror-like surface morphology epilayers Ga1-x InxAsy Sb 1-y with In content x in the range 0.1-0.22 and Ga 1-xAlxAsySb1-y layers with Al content up to 0.3 in the solid are grown by Liquid-Phase Epitaxy (LPE) from In- and Ga-rich melt, respectively. The compositions of the quaternary compounds are determined by X-ray microanalysis. The crystalline quality of GaInAsSb/GaSb and GaAlAsSb/GaSb heterostuctures is studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. © (2010) Trans Tech Publications.


Vitanov P.,Bulgarian Academy of Science | Milanova M.,Central Laboratory of Applied Physics | Arnaudov B.,Sofia University | Evtimova S.,Sofia University | And 4 more authors.
Journal of Physics: Conference Series | Year: 2010

This paper presents investigation of electronic transport properties of GaAsN and InGaAsN epitaxial layers with low nitrogen content, the so called dilute nitrides, grown by liquid-phase epitaxy (LPE). The layers up to 2 microns thick have been grown from Ga- and In-rich melt at different initial epitaxy temperatures in the range 660-620 °C. Polycrystalline GaN has been used as a source for nitrogen. As grown, unintentionally doped GaAsN and InGaNAs are n-type with free carrier concentrations one order of magnitude higher than those for the reference nitrogen free undoped GaAs and InGaAs layers. Lattice matched to GaAs substrate InGaAsN layers exhibit Hall mobility values higher than 2000 V/cm2.s © 2010 IOP Publishing Ltd.


Vitanov P.,Bulgarian Academy of Science | Milanova M.,Central Laboratory of Applied Physics | Goranova E.,Bulgarian Academy of Science | Dikov C.,Bulgarian Academy of Science | And 2 more authors.
Journal of Physics: Conference Series | Year: 2010

Low-cost single-junction GaAs solar cells on the base of p-AlGaAs/p-GaAs/ n-GaAs heterostructures grown by low-temperature liquid-phase epitaxy have been fabricated. For analyzing the factors contributing to solar cell efficiency the program products PC1D have been used. The optimal doping concentrations and thicknesses of the layers which correspond to the maximum values of the solar cell output parameters have been determined. The multilayer solar cell structure has been optimized on the base of this simulation. Low cost technological processes and masks, necessary for fabrication of the cells on the base of the grown heterostructures, have been developed, too. The front and back metallization of the cell is Ni/Al system deposited by magnetron sputtering. A double layer Al2O3 /ZrO2 antireflection coating is deposited on the front side of a solar cell structure by the spin coating technique. The conversion efficiencies of 19 % at one sun AM1.5 conditions have been measured. © 2010 IOP Publishing Ltd.


Milanova M.,Central Laboratory of Applied Physics | Vitanov P.,Bulgarian Academy of Science | Terziyska P.,Lakehead University | Popov G.,Central Laboratory of Applied Physics | Koleva G.,Central Laboratory of Applied Physics
Journal of Crystal Growth | Year: 2012

Crystallographic and transport properties of nominally undoped and Sn-doped InGaAsN layers grown by low-temperature LPE have been studied and related to the growth conditions. In the case of lattice matching, flat and uniform mirror-like layers of 8-10 μm in thickness are obtained. The compositions of the layers under study have been determined by combination of X-ray microanalysis and X-ray diffraction methods to be In 0.035Ga 0.065As 0.086N 0.014. The lattice mismatch between layer and substrate Δa l/a s calculated from X-ray diffraction curves is less than-7×10 -4 for all samples. The layers grown at lower epitaxy temperatures exhibit the highest crystalline quality, better lattice match and better homogeneity. This is in good agreement with the results of morphological study by atomic force microscopy which show root mean-square surface roughness of 0.18 nm for the best layers. CV and Hall measurements reveal that intentionally undoped InGaAsN layers are n-type with free carrier concentration about one order of magnitude higher in comparison to layers not containing nitrogen and high electron mobility values over 2000 cm 2/Vs. A dramatic reduction in the free carrier concentration and slightly increase in mobility are observed for Sn-doped InGaAsN layers. © 2012 Elsevier B.V. All rights reserved.


Arnaudov B.,Sofia University | Domanevskii D.S.,Belarusian National Technical University | Evtimova S.,Sofia University | Ivanov C.,Sofia University | Kakanakov R.,Central Laboratory of Applied Physics
Solid State Phenomena | Year: 2010

We investigate light emission spectra at different excitation levels of nanoscale thin InGaN film participating in an InGaN/GaN quantum well (QW) with heavily doped barriers for green and blue light emitting diodes (LEDs). We model the spectral shape and energy position in frames of the free electron recombination model created first for highly doped 3D direct gap III-V semiconductor films and applied for QWs at low excitation. The model accounts for the influence on the potential width of the QW of the random impurity potential of the doped barriers which penetrates into the QW. The blue shift at high excitation is supposed to be due to the filling of the conduction band with degenerate 2D nonequilibrium electrons. A structure in the emission bands is observed and it is assumed to be a result from step-like 2D density-of-states (DOS) in the QW. A good agreement is obtained between the calculated and experimental spectra assuming that the barriers are graded. © (2010) Trans Tech Publications.


Chitanov V.,Central Laboratory of Applied Physics | Cholakova T.,Central Laboratory of Applied Physics | Kolaklieva L.,Central Laboratory of Applied Physics | Kakanakov R.,Central Laboratory of Applied Physics | And 2 more authors.
Journal of Chemical Technology and Metallurgy | Year: 2013

This work presents a study on the influence of the deposition and annealing temperatures on the mechanical properties of quaternary TiAlSiN-based thin films. The coatings are deposited by cathodic arc evaporation onto substrates of cemented carbide at deposition temperatures of 470° and 500°. A part of them is post-deposition annealed at 550° and 525°. The investigation of the mechanical propertied showed strong dependence on the thermal treatment. Highest nanohardness of 45 GPa was obtained at deposition temperature of 500°C and post-annealing at 525°C. The results from the scratch test revealed very good adhesion to the substrates as no delamination was observed, and a relatively low friction coefficient of 0.05-0.07 was obtained.


Chitanov V.,Central Laboratory of Applied Physics | Kolaklieva L.,Central Laboratory of Applied Physics | Kakanakov R.,Central Laboratory of Applied Physics
Solid State Phenomena | Year: 2010

The real electro-discharge polishing (EDP) system has been presented by an equivalent electrical scheme and described by a corresponded equation system. The Runge-Kutta-Merson method with automatically changed step is used for the numerical solution the equation system. The current through the resistor equivalent to the steam gas wrapper is defined with an I-V characteristic obtained by the method of multi-interval quadratic interpolation-approximation. A mathematical model of the power supply-load system has been realized in Basic and Matlab® languages. On the base of the developed modelling conditions limiting the current and voltage overload in the EDP system have been determined depending on the maximum polished area and the electrolyte temperature. © (2010) Trans Tech Publications.

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