2. Articole
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Item OPTICAL AND PHOTOELECTRICAL PROPERTIES OF GaS AND CdTe THIN FILMS, COMPONENTS OF GaS/CdTe HETEROJUNCTIONS(2006) Cuculescu, Elmira; Evtodiev, Igor; Caraman, Mihail; Rusu, MihaelaIn 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.Item ABSORPTION SPECTRA AND EXTRINSIC PHOTOCONDUCTIVITY OF Cu AND Cd DOPED GaSe SINGLE – CRYSTAL FILMS(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.Item STUDY OF GENERATION-RECOMBINATION PROCESSES OF NON- EQUILIBRIUM CHARGE CARRIERS IN SINGLE CRYSTALLINE THIN GaSe(Cu) FILMS(2006) Cuculescu, Elmira; Evtodiev, Igor; Caraman, Mihail; Leontie, LiviuThe 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.%.Item OPTICAL PROPERTIES OF BISMUTH OXIDE THIN FILMS PREPARED BY REACTIVE D.C. MAGNETRON SPUTTERING ONTO p-GaSe (Cu)(2008) Leontie, Liviu; Caraman, Mihail; Evtodiev, Igor; Cuculescu, Elmira; Mija, AnaBismuth 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.Item UNELE PROPRIETĂŢI OPTICE ALE STRATURILOR SUBŢIRI DE GaSe ŞI GaS DOPATE CU Cu(CEP USM, 2007) Gaşin, Petru; Cuculescu, Elmira; Evtodiev, Igor; Anghel, Sergiu; Caraman, MihailGaS and GaSe doped with Cu thin films have been deposited by "flash" evaporation. The thickness of the layers varied from 138 to 1450 nm. The absorption spectra in the fundamental band edge region have been studied. The optical band gap has been determined as follows: 2.42 eV for GaS (Cu) and 1.92 eV for GaSe (Cu). The fabrication temperature for obtaining amorphous and nanocrystalline GaSe has been determined as 390 K and 623 K respectively.Item ELLIPSOMETRIC STUDIES OF NANOMETRIC CdS AND CdTe FILMS(2005) Caraman, Mihail; Evtodiev, Igor; Cuculescu, Elmira; Rusu, Marin; Salaoru, IurieThe thickness of the layers CdS and CdTe grown on the Si, Ge, GaAs was determined from the analysis of the polarization ellipse of the reflected light from the surfaces of structures thin layers substrate. The thermal treatment of the samples CdS-CdTe in the presence of CdCl2 leads to the increasing of the crystalline phase from layers.