V. E. Sedov

Polarization-resolved photoluminescence piezospectroscopy of GaAs/Al0.35Ga0.65As:Be quantum wells

The photoluminescence (PL) of GaAs/Al0.35Ga0.65As:Be quantum wells is studied at temperatures of 77 and 300 K under conditions of uniaxial compression along the [110] direction. There are two main lines in the PL spectra; at zero pressure and T = 77 K, the peaks appear at 1.517 and 1.532 eV. Comparison of the pressure dependences of the peak positions and the polarization of the PL measured experimentally with those calculated theoretically gives evidence that, at T ≥ 77 K, these bands originate from the recombination of free electrons with heavy and light holes in the GaAs valence band.

On the origin of the 1-eV band in photoluminescence from Cd1−xZnxTe

Photoluminescence (PL) at 77 K from Cd1−xZnxTe samples (x = 0, 0.005 and 0.01) annealed at 900°C and cadmium vapor pressure PCd = 3 × 104−2 × 105 Pa has been studied. It was found that the contribution of the 1-eV band to the spectrum-integrated PL from these samples is independent of PCd, in contrast to Cd0.95Zn0.05Te samples in which this contribution increases up to ∼90% as PCd grows. The band is not shifted to shorter wavelengths as x becomes larger. The conclusion that Zn vacancies are involved in the formation of Cd1−xZnxTe properties is confirmed. The 1-eV band is attributed to capture of free holes to acceptor levels related to vacancies of both cadmium and zinc. These levels are closely spaced and, therefore, are difficult to resolve.

Exact self-compensation of conduction in Cd0.95Zn0.05Te:Cl crystals in a wide range of Cd vapor pressures

The process of self-compensation in Cd0.95Zn0.05Te:Cl solid-solution crystals has been studied by annealing single crystals under a controlled Cd vapor pressure, with subsequent measurements of the Hall effect, photoluminescence, carrier lifetime and mobility, and photocurrent memory in the annealed crystals. By means of this annealing, conditions of thermal treatment that make it possible to fabricate low-conductivity samples with a low carrier density, 107–1011 cm−3, are defined. In these samples, a p → n conduction inversion is observed at a higher free-carrier density (n, p ≈ 109 cm−3) and the dependence of the electron density on the Cd vapor pressure exhibits a more gentle slope than in the case of CdTe:Cl crystals. The obtained data are discussed in terms of a self-compensation model in which intrinsic point defects act as acceptors with deep levels. This level is attributed to a Zn vacancy, which remains active at high Cd pressure.

Photoluminescence of CuGaTeAs and CuGaSnGa complexes in n-GaAs under resonance polarized excitation

Photoluminescence (PL) of n-type GaAs:Te:Cu and GaAs:Sn:Cu with an electron density of about 1018 cm−3 was studied at 77 K. A broad band with a peak at the photon energy near 1.30 eV (GaAs:Te:Cu) or 1.27 eV (GaAs:Sn:Cu) was dominant in the PL spectrum under interband excitation. This band arose from the recombination of electrons with holes trapped by CuGaTeAs or CuGaSnGa complexes. It has been found that the low-energy edge of the excitation spectrum of this PL band at photon energies below ∼1.4 eV is controlled by the optical ejection of electrons from a complex into the conduction band or to a shallow excited state. The PL polarization factors upon excitation by polarized light from this spectral range suggest that the complexes have no additional distortions caused by an interaction of a hole bound at the center in the light-emitting state with local phonons of low symmetry. This feature makes CuGaTeAs and CuGaSnGa complexes different from those with the Ga vacancy (VGa) instead of CuGa. The dissimilarity arises from the difference in the intensity of interaction of a hole localized at the orbital of an isolated deep-level acceptor in the state corresponding to its preemission state in the complex (CuGa− and VGa2−) with low-symmetry vibrations of atoms. The perturbation of the hole orbital induced by the donor in the complex practically does not affect this interaction.

Optical characteristics of 1.18-eV luminescence band complexes in n-GaAs:Sn(Si): Results of a photoluminescence study with polarized resonant excitation

Experimental values of the polarization of the low-temperature luminescence from the VGaSnGa and VGaSiGa complexes in n-GaAs under conditions of resonant excitation by polarized light propagating along the [110] or [100] crystal axis are compared with expressions obtained in the classical dipole approximation for defects with triclinic or monoclinic symmetry. It is shown that the rotator fraction in the superposition of rotator and oscillator contributions to the emission of the complexes is 17–18%. The direction of the axis of these dipoles, which matches the experimental data, is consistent with the assumption that the effect of the donor on the vacancy orbitals of a hole localized in the complex is lower than that of the Jahn-Teller effect. The resulting symmetry of the complex may be monoclinic or triclinic. In either case, deviation of the optical dipole axis of the complex from the dipole axis of an isolated VGa vacancy distorted as a result of the Jahn-Teller effect is lower for the VGaSnGa and VGaSiGa complexes than for VGaTeAs complexes. This means that the effect of the donor on the electron structure of the VGaTeAs complexes is greater than in the VGaSnGa and VGaSiGa complexes. This correlates with the difference in the donor position in these complexes.

Polarization photoluminescence study of the complex VGaTeAs in n-type GaAs in the temperature range 77–230 K

The polarization of the photoluminescence band with a maximum near 1.18 eV, which is formed as a result of the resonance excitation of VGaTeAs complexes by polarized light, is investigated for GaAs:Te with various electron densities in the temperature range 77–230 K. Based on a previously developed model of these defects, theoretical equations for the polarization of their luminescence are derived in the one-dipole approximation, taking into account the possible reorientation of Jahn-Teller distortions of the complexes. It is shown that the temperature dependence of the polarization of the investigated band is well described by these equations, and the parameters characterizing the optical dipoles of the complexes are estimated. A decrease in the degree of polarization at temperatures above ∼ 120 K is explained by the transfer of excitation to complexes with any possible orientations of the initial axis and by Jahn-Teller distortion (owing to thermal-field emission and retrapping of holes by the photoexcited complexes). The decrease in polarization can also be partially linked to the reorientation of distortions during the lifetime of the emitting state of the complex. The height of the energy barrier for such reorientation is at least ∼200 meV.

Fine Structure of Levels and Piezospectroscopy of A + Centers in GaAs/AlGaAs Quantum Wells

Experimental and theoretical piezospectroscopic investigation of A+ centers in GaAs/AlGaAs quantum wells doped with beryllium is presented. Spectra of linearly polarized photoluminescence are studied experimentally depending on applied uniaxial pressure. A model of the A+ center in the quantum well in the presence of uniaxial deformation in the plane of the quantum well has been constructed. Analytical expressions for the level energy, optical transition intensities, and polarization ratio have been obtained. In the framework of the proposed theory, the experimentally observed change in the polarization ratio depending on pressure and the shift of the line maximum towards short waves are explained.

Optical study of the 2D A+ center system

We report photoluminescence (PL) measurements carried out on the 2D A+ center system, that was prepared in GaAs/AlGaAs quantum wells by means of doping wells and barriers with beryllium. In general, four PL lines were observed in our samples depending on doping concentration, temperature and pump intensity. Two lines are related to recombination of free electrons with heavy and light holes, another two are due to recombination of A+ center holes with free and localized electrons. Measurements of polarized PL spectra in applied magnetic field are in agreement with this interpretation.

Growth and annealing of CdZnTe:Cl crystals with different content of Zn for nuclear detectors

The methods of photoluminescence, Hall effect, extrinsic photoconductivity, and transit time have been used to study the influence of the pressure of cadmium vapor (PCd) in the course of postgrowth annealing of ingots and annealing of semi-insulating crystals Cd1 − xZnxTe:Cl (x = 0.005, 0.01, 0.05, 0.1) on compensation of conductivity of the material used for fabrication of nuclear detectors. It is established that free holes are captured by acceptor levels of both cadmium and zinc vacancies. Studies of electrical properties of crystals after annealing at various pressures of cadmium vapors showed that, at low zinc content (x = 0.005 and 0.01), the determining effect on the conductivity compensation in Cd1 − xZnxTe:Tl is exhibited by the VCd−2 cadmium point defects. However, even at the zinc content x ≥ 0.05, it is necessary to take into account the effect of zinc point defects VZn−2; in order to obtain such crystals with the best transport characteristics it is necessary to control not only the pressure of cadmium vapors in the course of growth of the material but also the zinc vapor pressure. It is established that it is possible to control the main electrical characteristics of the semi-insulating material with the zinc content x ≤ 0.01 grown by horizontal planar crystallization and designed for nuclear detectors by varying the pressure of cadmium vapors in the course of the postgrowth annealing.

Comparison of the polarizations of the 1.2-eV photoluminescence band in n-GaAs:Te under uniaxial pressure and resonance polarized excitation

It is shown that the photoluminescence (PL) band at 1.2 eV in n-GaAs:Te, which is associated with emission from VGaTeAs complexes with reorienting Jahn-Teller distortions, also includes a contribution from nonreorienting defects. The optical dipole parameters are almost the same for both types of defects. Expressions relating the polarization of the PL band at 1.2 eV under uniaxial pressure and polarized resonant excitation to dipole parameters and to relative contributions to emission from reorienting and nonreorienting defects are derived. A procedure is developed for evaluating these characteristics by analyzing experimental data, and the contributions from each kind of defects to the PL band at 1.2 eV were found to be comparable, even though they vary from sample to sample. The obtained angles characterizing the position of the axes of optical dipoles associated with the defects in light-absorbing and light-emitting states indicate that, in the former state, the effects of donors and the Jahn-Teller distortion on the vacancy orbitals of the VGaTeAs complex are comparable, while in the latter, the effect of distortion is dominant.