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We investigated by the thermally stimulated currents (TSC) high purity high resistivity GaAs films, grown by vapor phase epitaxy (VPE). To reveal the single levels the thermal emptying of the defects by fractional heating was used. We also investigated the thermally stimulated depolarization currents (TSD) in samples, which were excited and polarized by light in the presence of an electric field. The following prevailing levels below the conductivity band were found: 0.21-0.23, 0.31-0.33, 0.35-0.38, 0.45-0.49 and 0.54 eV. We demonstrate that the TSC and TSD current spectra depend sensitively on the excitation conditions. So light excitation with quantum energy higher than 0.83 eV reveals the level with an activation energy of 0.21-0.23 eV, which dominates over the temperature range from about 95 to 200 K. Most probably these defects are located within the VPE layer, and have relatively small effective electron capture and generation coefficients. Moreover, at low temperatures their effective ionization energy is supposed to increase because of the existence of potential barriers. By contrast, if the quantum energy is 0.5-0.83 eV the influence of this level diminishes, and the contribution of donor levels at about 0.13 and 0.17 becomes visible. This is evidence of carrier redistribution between different defects. Furthermore, peculiarities of the TSC were observed which could not be explained by a homogeneous semiconductor model. The existence of different polarization sources in different temperature ranges is demonstrated by TSD. In particular, the inhomogeneous sample polarization causes the scattering of the activation energy values. Our results prove the influence of the potential relief of the band gap, appearing due to microinhomogeneities of the samples. Furthermore, excitation by 0.5-0.83 eV light could lead to the formation of different (p-type) conductivity channels around dislocations through the metastable transformation of the EL2 level.
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