2. Articole

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    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.
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    ELECTRONIC TRANSPORT AND PHOTOCONDUCTIVITY OF POLYCRYSTALLINE CdSe THIN FILMS
    (2006) Baban, Cristian Ioan; Caraman, Mihail; Rusu, Gheorghe Ioan
    In this paper we have studied the electrical properties and photoconductivity of CdSe thin films, prepared by vacuum evaporation using the quasi-closed volume technique. It was found that the films were polycrystalline with a hexagonal (würtzite) structure and the crystallites are highly oriented with the (002) planes paralel to substrate. The film surface was investigated by means of SEM. The temperature dependence of electrical conductivity and Seebeck coefficient was studied. The films have n-type conduction. The spectral characteristics of photoconductivity were studied at 78 K and 300 K in the wavelength range 300–1100 nm. The bandgap energy (1.53–1.63 eV) calculated by using the Moss rule is smaller than those determined from absorption spectra. For the diffusion length, determined form absorption spectra and spectral dependence of photoconductivity, values between 0.22 and 0.29 μm were found.
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    ON THE PHOTOMAGNETIC EFFECT IN CdTe THIN FILMS EVAPORATED ONTO UNHEATED SUBSTRATES
    (2005) Rusu, George G.; Rusu, Mihaela; Caraman, Mihail
    Cadmium telluride (CdTe) thin films (d=500–700 nm) were deposited onto unheated glass substrates by thermal evaporation under vacuum. The dependence of the photomagnetic voltage on the magnetic induction for the as deposited and heat-treated films was investigated. The spectral dependence of the photomagnetic voltage was also investigated. The results are discussed in relation with structural characteristics of the studied films.
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    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, 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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    ON THE PHOTOCONDUCTIVITY OF Bi2 O3 IN THIN FILMS
    (2000) Leontie, Liviu; Caraman, Mihail; Rusu, Gheorghe Ioan
    The spectral characteristics of photoconductivity for Bi2 O3 thin films were investigated. The films were prepared by thermal oxidation in air of Bi evaporated films. As revealed by X-ray diffraction and polarizing microscopy studies, polycrystalline and multiphasic films were obtained. From the photoconductivity spectral curves, the bandgap energy values were determined, by using the Moss criterion. The influence of oxidation (preparation) conditions on the Eg values is discussed. The photoconductivity of Al-Bi 2 O3 -Al, Cu-Bi 2 O3 -Cu and Al- Bi 2 O3-In 2 O3 structures is also investigated.