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.

AbstractExperimental 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.

Transition-metal-doped semiconductor GaAs crystals are used as model systems for spintronic research, as well as in other applications. To explore the structure and properties of such impurities, we extended the methodology of ultrasonic investigation of the Jahn-Teller effect in dielectric impurity-centers to the study of significantly different semiconductor impurities using the GaAs:Cu crystal as an example. Phase velocity and attenuation of ultrasound in this system were measured in the temperature interval of 1.9–80 K at 52 MHz and 156 MHz. The anomaly in the velocity and a peak of attenuation found for the longitudinal and slow shear waves indicate the presence of the Jahn-Teller effect with the e-type local distortions of the CuGa4As impurity complex. The temperature dependence of the elastic modulus and relaxation time shows that below 5 K, the thermal activation mechanism of relaxation is possibly replaced by resonance type transitions. The main parameters of the Jahn-Teller effect, stabilization energy, minima positions and the barrier between them, frequency of pseudorotation of the distortions, and the tunneling splitting of the ground state energy, as well as the constant of exchange interaction between the two holes in Cu2+ centers and the concentration of the centers were estimated.

The interaction of ultrasound with CuGa4As in a GaAs:Cu crystal has been experimentally studied. The temperature dependences of the attenuation of all normal ultrasonic modes propagating in the 〈110〉 direction both in doped copper and in nominally pure gallium arsenide crystals have been measured. In the GaAs:Cu crystal, the attenuation peak has been revealed for a transverse wave polarized along the 〈110〉 axis whose elastic shifts correspond to the symmetry of the tetragonal mode of the Jahn-Teller effect. The temperature dependence of the attenuation of this wave indicates that two types of attenuation—relaxation and resonance—occur. The constructed temperature dependence of the relaxation time indicates that tunneling through the potential barrier between the minima of the adiabatic potential energy is the main relaxation mechanism at temperatures below 10 K. Tunneling splitting estimated from experimental data is in good agreement with the theoretical estimate.

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.

Conductance compensation of Cd1-xZnxTe:Cl crystals with variable content of Zn (x = 0,0002, 0,005, 0,01 and 0,05) for nuclear detectors both during postgrowth annealing of ingots and at annealing of separate samples at different vapor pressures of Cd was investigated. It is established that to obtain the Cd1-xZnxTe:Cl crystals (x≥0.05) with the best transport performances, it is necessary to control cadmium vapour pressure as well as zinc vapour pressure of during material growing. On crystals with Zn content x=0.0002; 0.005; 0.01 with annealing of conditions change it was possible to achieve high values of μeτeand μhτh, comparable with these values for CdTe:Cl. For crystals with x=0.05; 0.1 μeτeand μhτh practically did not depend on conditions of postgrowth annealing (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The methods of photoluminescence, Hall effect, extrinsic photoconductivity, and transit time have been used to study the influence of the pressure of cadmium vapor ( P Cd ) in the course of postgrowth annealing of ingots and annealing of semi-insulating crystals Cd 1 − x Zn x Te: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 Cd 1 − x Zn x Te:Tl is exhibited by the V Cd −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 V Zn −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.

We report measurements of photoluminescence (PL) spectra due to A+-centers in GaAs/AlGaAs quantum well (QW). We observe temperature and pump dependence of the PL spectra as well as polarized PL spectra taken under an applied magnetic field and uniaxial stress. It was shown that hole–hole exchange interaction brings the A+-centers into a state characterized by J=2 split by QW potential in two states: ground mJ=0 and excited mJ=±1,±2 ones. Coulomb interaction changes the density of states (DOS) in the valence band thus a Coulomb gap appears. Our Monte-Carlo calculations of DOS demonstrate good agreement with the measured temperature PL dependence.

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.

Photoluminescence (PL) at 77 K from Cd1−x Zn x Te samples (x = 0, 0.005 and 0.01) annealed at 900°C and cadmium vapor pressure P Cd = 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 P Cd, in contrast to Cd0.95Zn0.05Te samples in which this contribution increases up to ∼90% as P Cd 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−x Zn x Te 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.

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, 10E7-10E11 cm-3, are denned. In these samples, a p -- n conduction inversion is observed at a higher free-carrier density ((n, p) near 10E9 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 dis-cussed 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.

The process of self-compensation in Cd 0.95 Zn 0.05 Te: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, 10 7 –10 11 cm −3 , are defined. In these samples, a p → n conduction inversion is observed at a higher free-carrier density ( n, p ≈ 10 9 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.

Studies of n-CdZnTe crystals (photoluminescence, extrinsic photoconductivity, Hall effect, time-of-flight technique) have shown that the excess concentration of vacancies of cadmium (Vcd) is the main reason of low, as a rule, values of product of mobility to life time of holes (mhth). The reduction of the concentration of cadmium vacancies (decreasing of the intensity of near 1eV photoluminescence band and an intensity of the (0.9-1.3) eV extrinsic photoconductivity band) by annealing of the crystals at 600 C results in increasing of value of mhth. Influence of Zn on formation of the basic photoelectric properties of CdZnTe crystals has been explained by "self-control" of a concentration of cadmium vacancies Vcd due to addition of Zn results in formation of divacancies of metal, which in part dissociate and provide a crystal with necessary quantity of monovacancies for processes of complex formation. That makes process of obtaining of semi-insulating CdZnTe crystals less dependent from pressure Pcd in comparison with CdTe. However with the purpose of obtaining CdZnTe crystals with high value of mhth (i.e. with small concentration of the free vacancies of cadmium) it is necessary to control Pcd above the crystal at stages of its growth and annealing.

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 (Cu Ga − and V Ga 2− ) 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.

It is shown that the photoluminescence (PL) band at 1.2 eV in n -GaAs:Te, which is associated with emission from V Ga Te As 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 V Ga Te As complex are comparable, while in the latter, the effect of distortion is dominant.

The excitation and induced polarization spectra of the photoluminescence band with the maximum near a photon energy of 1.18 eV in Sn-or Si-doped n -GaAs with an electron density ∼ 10 18 cm −3 are measured at various temperatures. It is shown that the temperature dependence of the induced polarization of this photoluminescence due to V Ga Sn Ga or V Ga Si Ga complexes in the temperature range 77–230K is close to the corresponding dependence for V Ga Te As complexes. In addition, a slight decrease in the induced polarization as the temperature is increased, not observed for V Ga Te As complexes, is observed in the range 77–125 K for the investigated complexes. It is hypothesized that the difference is attributable to the existence of excited configurations in the absorbing and emitting states of the V Ga Sn Ga and V Ga Si Ga complexes, where the populations of these configurations in the absorbing state increase with the temperature. The difference between the total energies of the excited and ground configurations of the absorbing state is 10–20 meV for V Ga Sn Ga complexes and 15–30 meV for V Ga Si Ga complexes.

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.