Facultatea de Fizică şi Inginerie / Faculty of Physics and Engineering

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Author: search.filters.author.Evtodiev, IgorEnd date: 2011

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    Metode experimentale de studiere a stărilor de suprafaţă în compuşii stratificaţi de tip AIII BVI [Articol]
    (Tipografia Universității de Stat „Alecu Russo” din Bălți, 2005-10-05) Blaj, Octavian; Scurtu, Roman; Evtodiev, Igor
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    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, Valentin
    In 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.
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    Optical and photoelectrical properties of GaS and CdTe thin FILMS, components of GaS/CdTe heterojunctions [Articol]
    (2006) Cuculescu, Elmira; Evtodiev, Igor; Caraman, Mihail; Rusu, Mihaela
    In the paper, experimental results concerning the absorption and photosensibility spectral dependence of GaS and CdTe thin film components of GaS/CdTe heterojunctions are presented. GaS films (d = 0.06 μm – 2.8 μm) were deposited onto ITO substrate by laser light pulses. CdTe films (d = 3.6 μm) were deposited onto GaS films by close-spaced sublimation technique. At the fundamental absorption edge, the absorption coefficient of CdTe layers increases of five orders of magnitude in a narrow 40 – 100 meV energy range, up to 4 104cm-1. At room temperature, the energy band of GaS/CdTe heterojunctions photosensitivity is 1.45 eV-2.75 eV, the photocurrent having a constant value up to 2.0 eV photon energy.
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    Absorption spectra and extrinsic photoconductivity of Cu and Cd doped GaSe single - Crystal films [Articol]
    (2006) Cuculescu, Elmira; Evtodiev, Igor; Caraman, Mihail; Rusu, George G.
    GaSe single crystal films were doped during growth process with Cu or Cd atoms with concentration ranged between 0.05 and 0.50 at. %. Single crystal films (with thickness about 0.5 μm) were obtained by mechanical splitting of bulk single crystals. Impurity concentration was determined using atomic emission spectroscopy. Spectral dependences of absorption coefficient and photoconductivity were studied in the range 1.50 eV – 3.70 eV. It was experimentally established that the absorption spectra have an additional absorption band and its corresponding energy depends on the nature (Cu or Cd) and concentration of the doping atoms. Also, independently on the presence of the dopant, other two absorption bands in the IR region are present.
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    Study of generation-recombination processes of non-equilibrium charge carriers in single crystalline thin GaSe(Cu) films [Articol]
    (2006) Cuculescu, Elmira; Evtodiev, Igor; Caraman, Mihail; Leontie, Liviu
    The generation-recombination processes of non-equilibrium charge carriers in undoped and Cu-doped (in the range 0.1-0.5 at.%.) single crystalline GaSe films with thickness d in the range 1.5-225 μm are investigated. Cu doping of GaSe crystals up to 0.5 at.% leads to an increase of electrical conductivity by over 4 orders of magnitude, as well to enhancement of impurity luminescence band (PL) and extension of photoconductivity spectral range. By studying PL and photonductivity spectra, for different excitation (photon) energies in temperature range (78-420) K, energies of localized states due to both Cu and accidental impurities are determined. By analysing temperature dependence of electrical conductivity and photoconductivity for undoped and Cu-doped films, the activation energy of acceptor levels in doped films was determined as 0.058 and 0.025 eV. Increasing Cu doping from 0.1 to 0.5 at.%. results in decreasing energy of acceptor levels up to ∼0.02 eV. By analysing the impurity absorption and photoconduction at 78 K the energy of acceptor levels was determined as 12-15 meV greater than previously evaluated, depending on Cu concentration. For films with d<5 μm, the surface states concentration increased for Cu doping over 0.3 at.%.
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    Optical properties of p -GaSe single crystals doped with Te [Articol]
    (2009) Evtodiev, Igor; Leontie, Liviu; Caraman, Mihail; Stamate, Marius D.; Aramă, Efim
    Optical absorption in the region of fundamental absorption edge and photoluminescence at 78 and 293 K of p-GaSe crystals doped with Te 0.05, 0.10, and 0.05 at. % have been studied. At low concentrations 0.05 at. %, Te atoms liquidate structural defects in GaSe single crystals and modify their excitonic absorption and photoluminescence. For increased concentrations of 0.10 and 0.50 at. %, Te creates impurity states responsible for the red shift of the fundamental absorption edge of GaSe and appearance of new photoluminescence bands, B at 2.000 eV and C at 1.700 eV. The activation energies of the thermal quenching of Te-related PL bands B and C was found to be 28 and 32 meV for T 150 K and 84 and 62 meV for T 150 K, respectively.
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    Optical properties of bismuth oxide thin films prepared by reactive d.c. magnetron sputtering onto p-GaSe (Cu) [Articol]
    (2008) Leontie, Liviu; Caraman, Mihail; Evtodiev, Igor; Cuculescu, Elmira; Mija, Ana
    Bismuth oxide (Bi2O3) thin films with thickness in the range 20 – 160 nm have been deposited by d.c. reactive magnetron sputtering of Bi in an atmosphere Ar : O2 (1 : 1), onto single crystalline p-GaSe (Cu) substrates. The optical constants, n and k, of oxide films have been determined from the analysis of the polarization ellipse of the reflected radiation from outer surface of Bi2O3/p-GaSe structures. In the wavelength range 400 – 800 nm the refractive index of nanometric Bi2O3 films onto GaSe(Cu) decreases from 2.10 to 1.78 and it seen to in- crease at decreasing sample thickness. In order to determine the interaction mechanism between semiconducting oxide film and GaSe surface, the spectral characteristics of photocurrent through Bi2O3/p-GaSe junc- tion and optical absorption in the range 400 – 800 nm have been examined. As resulted from respective analyses, Bi2O3 film generates new valence bonds, which contribute to the in- crease in the density of localized states at Bi2O3/p-GaSe (Cu) junction interface.
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    Probleme propuse la Olimpiada Republicană de Fizică – 2003 [Articol]
    (2003) Evtodiev, Igor; Cliucanov, Alexandr; Cojuhari, Dumitru; Andronic, Iuliana
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    Proprietăţile electrice şi fotoelectrice ale monoseleniurii de galiu dopat cu Cd [Articol]
    (CEP USM, 2011) Dmitroglo, Liliana; Vatavu, Elmira; Evtodiev, Igor; Caraman, Mihail
    The paper is focused on analysis of structural transformations and investigations on electrical and photoelectrical properties of GaSe and GaSe: Cd semiconductors. The XRD analysis demonstrated that Cd atoms are homogeneously distributed in GaSe. It has been also shown that Cd atoms form localized levels which increase the absorption for hν
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    Anisotropy of the Exciton Processes in GaSe Crystals with Low S and Te Concentrations [Articol]
    (American Scientific Publishers, 2009) Evtodiev, Igor
    The anisotropy of the excitonical processes in the GaSe crystals and GaSe with small quantities of GaS(GaSe0.99S0.01) crystals has been studied through the optical specters (SO) and through the photoluminescence (PL) from the perpendicular surface on the symmetry axis C6 (E⊥C polarization) and from the flat surface parallel with the C6axis (E∥C and E⊥C polarization). The edge of the fundamental band of the GaSe crystals as well as of the GaSe0.99S0.01 and GaSe0.99Te0.01 crystals is formed at T = 78 K of the direct excitons' band. The width of the free excitons' band is determined by the processes of interaction between the excitons and optical and acoustic phonons. Phonons with energy of 17 meV and 27 meV participate to the formation of the edge towards small energies of the excitonic band in the GaSe crystals. The average energy of the phonons that participate to the formation of the excitonic absorbtion band in the GaSe crystals with small concentrations of S and Te equals 17 meV. Due to the mechanism of interaction of the excitons and phonons the integral absorption coefficient for the studied crystals (polarized E⊥C) is in small increase once with the temperature whilst the integral absorption coefficient in the maximum of the direct excitons' band. The n = 1 state is in diminution. For example, for the GaSe0.99Te0.01 crystals, α increases from 2700 at T = 78 K to 2025 cm−1 at 220 K. The edge towards small energies of the free excitons' band in the GaSe crystals and GaSe crystals with small quantities of S and Te is in a great concordance with Toyozowa's theory. The constant of interaction between the free excitons with phonons with an average energy of 135 cm−1 equals 0.9. Using the spectral characteristic of the reflection coefficient from the surface parallel to the C6 axis, there has been determined the refraction index placed in the center of the excitons n = 1 which equals 2.62 for GaSe and 2.58 and 2.55 respectively for the GaSe0.99S0.01 and GaSe0.99Te0.01 crystals. The shifting of the reflection specters towards big energies like ∼10 meV in a E⊥C polarization comparing to E∥C is determined by the difference of the oscillators' strength in these polarizations. The PL at T = 78 K specters from the surface parallel with the C6 axis (polarized E∥C) confirm the difference between the forces of the excitons' oscillators in the E∥C and E⊥C polarization. The intensity of PL bands, at the (001) surface as well as at the (100) surface depends on the excitation intensity by a function of a I = Ln force towards the emission bands of the direct and indirect free excitons the force factor is overlinear, and for the impurity nature bands it represents ∼0.5. The parameters that determine the width of the bands of excitonic PL is determined, considering the strong concentration of the structural faults at the (100) surface of the GaSe and GaSe0.99Te0.01 and GaSe0.99S0.01 GaSe crystals. Out of the spectral analysis I(L) the nature of the impurity bands has been determined, and from the PL specter structure there has been determined the energy of the accepting level which equals 93 meV from the maximum of the valence band of the GaSe crystals. Out of the analysis of the PL specter in a E∥C and E⊥C polarization (the (100) surface) it was stated that the process of emissional annihilation of the indirect excitons in the E∥C polarization takes place once with the emission of the phonons of a 38 meV energy whilst at the E⊥C polarization there are emitted phonons with an energy of 17 meV. GaSe with small concentrations of GaS and GaTe leads to the forming of a considerable concentration of localizing centers of the direct excitons and at the same time to the shifting towards small and big energies of the excitonic emission band (state n = 1) comparing to the GaSe crystals with a stoikiometric composition.