Recombination at mixed-valence impurity centers in PbTe(Ga) epitaxial layers

The photoconductivity kinetics in PbTe(Ga) epitaxial films prepared by the hot-wall method is studied. The recombination of nonequilibrium photoexcited electrons at low temperatures was found to proceed in two stages, with a period of relatively fast relaxation followed by delayed photoconductivity. The temperature at which delayed photoconductivity appears increases with decreasing film thickness. The relaxation rate over the period of fast relaxation depends on film thickness and is the lowest in the thinnest layers. In semi-insulating films, photoconductivity is always positive, whereas in samples with lower electrical resistivity positive and negative photoconductivities are observed to coexist. The data obtained are discussed in terms of a model in which the impurity gallium atom can be in more than one charged state.     

Magnetic-field-induced terahertz photogeneration in PbTe(Ga)

It has been shown that a magnetic field in PbTe(Ga) single crystals induces the appearance of a positive photoresponse in the terahertz spectral range. Nonequilibrium charge carriers are generated from states located against the background of the continuum of the conduction band near the quasi-Fermi level. The density of states on the quasi-Fermi level is a critical parameter responsible for the process of generation.     

Selective photoconductivity induced in PbTe(Ga) by a local phonon mode

Selective photoconductivity at frequency ω=155 cm⁻ was discovered in PbTe(Ga) narrow-gap semiconductors at liquid-nitrogen temperatures. The corresponding energy is much lower than all characteristic energies of the electronic spectrum of the semiconductor. The effect is attributed to optical excitation of a local vibrational mode of an impurity center leading to delocalization of the electrons. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 5, 342–346 (10 March 1996)     

Effect of phonon scattering from neutral and charged impurity centers on the lattice heat conductivity of PbTe:(Tl, Na)

Experimental data on the effect of thallium and sodium impurities on the lattice heat conductivity of PbTe at room temperature are reported. Because the lattice of lead chalcogenides is strongly polarized near charged impurities, the effect of impurities on the lattice heat conductivity depends substantially on their charge state. This property of the material has been used to determine the charge state of the thallium impurity in PbTe. The results obtained argue for a model of quasi-local thallium-impurity states which assumes low electron-correlation energy at an impurity center. Fiz. Tverd. Tela (St. Petersburg) 40, 1206–1208 (July 1998)     

Far-infrared study of impurity local modes in gallium-doped PbTe

We present far-infrared reflection spectra and results of galvanomagnetic measurements of PbTe single crystals doped with gallium between 10 and 300 K. The analysis of the far-infrared reflection spectra was made by a fitting procedure based on the model of coupled oscillators. Together with the strong plasmon–phonon coupling we obtain three local modes of gallium at about 122, 166 and 192 cm⁻¹. The position of these modes depends of impurity center charge, and their intensity depends of temperature and of gallium concentration. Persistent photoconductivity effect was registered in the sample with 0.4 at.% Ga by galvanomagnetic and far-infrared measurements.     

Aggregation of impurity centers in ionic crystals

Results of modeling of the aggregation process in complex centers of ionic crystals are presented. Concentration dependences of the number of paired centers are obtained. A nature of the paired luminescence centers is discussed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 62–65, February, 2007.     

Optical spectroscopy of single impurity centers in semiconductors

Using optical spectroscopy with diffraction limited spatial resolution, the possibility of measuring the luminescence from single impurity centers in a semiconductor is demonstrated. Selectively studying individual centers that are formed by two neighboring nitrogen atoms in GaAs makes it possible to unveil their otherwise concealed polarization anisotropy, analyze their selection rules, identify their particular configuration, map their spatial distribution, and demonstrate the presence of a diversity of local environments. Circumventing the limitation imposed by ensemble averaging and the ability to discriminate the individual electronic responses from discrete emitters provides an unprecedented perspective on the nanoscience of impurities.     

IR-spectroscopy of crystals containing Jahn-Teller impurity centers

Using a novel IR-technique, the low-temperature spectra of CdTe, ZnTe and Cd_0.95Mn_0.05Te crystals, doped by Jahn-Teller Co²⁺ impurity ions, were studied in the spectral range from 1 to 400 μm. The absorption spectrum regions and the energy positions of ZPL, corresponding to the transitions ⁴A₂(⁴F)→⁴T₁(P), ⁴T₁(F), ⁴T₂(F) and ²G were detected. The parameters of the spin-orbit interaction λ, the crystal field parameter D_q, the parameters of electron-electron interaction B and C were obtained. It was found, that Jahn-Teller interaction differently affects the parameter λ for ⁴A₂(F)→⁴T₁(F) and ⁴T₂(F) transitions and, consequently, the energy positions of ZPL. The electron-vibronic spectra were compared with phonon spectra.     

Shallow impurity centers in semiconductor quantum well structures

Interest in the study of the behavior of shallow impurity centers in superlattices and quantum well structures is fairly recent. This paper reviews briefly both the theoretical and experimental work done in this field in the last few years. Several recent calculations of the energy levels of hydrogenic impurity states in quantum well structures, such as Ga_1?xAl_xAsGaAsGa_1?xAl_xAs, are reviewed. The behavior of these levels as a function of the quantum well size is discussed. Recent experimental data concerning the variations of the binding energies of shallow donors and acceptors as a function of the GaAs quantum well size are reviewed. A comparison between these experimental measurements and the results of recent calculations is presented.     

Linearly polarized terahertz radiation in uniaxially deformed Ge(Ga) upon the electric breakdown of an impurity

The polarization spectra of spontaneous terahertz radiation in uniaxially deformed germanium have been measured upon the electric breakdown of shallow acceptors. Lines with various degrees of polarization with respect to the compression axis have been observed in the radiation spectrum. These lines are associated with the optical transitions of holes between the excited and ground states of the acceptor, as well as with the transitions of holes from the valence band to the ground state of the impurity. At a pressure of about 3 ± 0.3 kbar in the [111] direction near the impurity breakdown, the linear polarization degree reaches ～80–90% in the main lines of terahertz radiation. As the electric field intensity increases, the depolarization of radiation is observed, which is caused by the heating of nonequilibrium holes by the electric field.     

Microscopic analysis of the valence band and impurity band theories of (Ga,Mn)As

We analyze microscopically the valence and impurity band models of ferromagnetic (Ga,Mn)As. We find that the tight-binding Anderson approach with conventional parametrization and the full potential local-density approximation+U calculations give a very similar band structure whose microscopic spectral character is consistent with the physical premise of the k·p kinetic-exchange model. On the other hand, the various models with a band structure comprising an impurity band detached from the valence band assume mutually incompatible microscopic spectral character. By adapting the tight-binding Anderson calculations individually to each of the impurity band pictures in the single Mn impurity limit and then by exploring the entire doping range, we find that a detached impurity band does not persist in any of these models in ferromagnetic (Ga,Mn)As.     

Ground-state structure of beryllium-related acceptor impurity centers in silicon

The infrared absorption spectra of some beryllium-related acceptor impurity centers in silicon have been measured. Their Fourier self-deconvoluted spectra have been obtained clearly demonstrating that the ground states of some impurity centers have a splitting feature while some consist of only a single component.     

Dependence of the lowest impurity state on alloy composition in GaAs-(Al,Ga)As quantum dots

The ground state donor binding energy is estimated using the simple first order perturbation method for a GaAs-Al _x Ga_1−x As spherical quantum dot. The calculated energy is computed as a function of Al-concentration. Donor binding energy is found to be quite sensitive to Al-concentration (x), specifically for smallx. Furthermore, the binding energy is found to be highest for the smallest and the center-doped dot indicating the strongest confinement in those cases.     

Certain characteristics of the energy spectra of impurity centers and charge-carrier scattering processes in high-resistivity GaAs(O)

The behavior of electron and hole mobilities in illuminated high-resistivity single crystals of oxygen-doped gallium arsenide has been investigated and compared with the behavior of these parameters in the unilluminated specimen. A complete liquidation of charged centers was observed at temperatures below 50°K which shows that these crystals do not contain shallow donors or acceptors. The electron-to-hole mobility ratio in the neutral-center scattering region has also been determined.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No.2, pp.25–34, February, 1976.In conclusion the authors express their appreciation to V. B. Dik for designing and constructing the helium cryostat and to M. S. Al' Kuveiti for performing some of the measurements of dark characteristics of some of the samples.     

Scattering of an exciton polariton by impurity centers in GaAs

Scattering of an exciton polariton by impurity centers at low temperatures has not been investigated comprehensively in spite of its significant role in processes accompanying Bose–Einstein condensation of an exciton polariton. For studying the peculiarities of the interaction of an exciton polariton with impurity centers, we have studied the integrated absorption of the ground state (n ₀ = 1) of the exciton in GaAs in thin (micrometer-thick) wafers with an appreciable optical transmission. Comparative analysis of the transmission in the vicinity of the exciton resonance performed on 15 samples of crystalline GaAs wafers with different concentrations N of impurity has revealed an unexpected regularity. The value of N increases by almost five decimal orders of magnitude, while the normalized spectrally integrated absorption of light exhibits a slight increase, following the power dependence N ^m on the concentration, where m = 1/6. It has been shown that this dependence indicates the diffusion mechanism of propagation of the exciton polaritons through the bulk of the semiconductor, which is present along with the ballistic propagation of light through the sample.