Table of contents

Recent results on recombination photoluminescence of hot electrons and electron-hole pairs in quantum wells of the GaAs/AlGaAs type are discussed. It is shown that linear polarization of hot photoluminescence is due to the optical alignment of two-dimensional (2D) electron momenta by linearly polarized light. The times of intra- and intersubband scattering of 2D electrons were determined from hot luminescence depolarization in a magnetic field. The intrasubband scattering times are found to be close to 150 fs for the well widths from 50 to 100 AA. This agrees with a calculation based on the assumption that 2D electrons are scattered by bulk phonons, which is consistent with a prediction from a dielectric continuum model. Experimental dispersion dependences in the valence subbands of quantum wells were first obtained from the hot photoluminescence spectra. In a perpendicular magnetic field, which restricts hot carrier motion in the quantum-well planes, radiation flares up due to recombination of hot electrons and holes created by the same light quantum (geminate recombination). Its spectral and polarization properties depend noticeably on the magnetic field and initial energy of hot carriers.

An analysis is made of the slow and fast components found in the ²⁰³Hg and ¹⁰⁹Cd self-diffusion profiles in Hg_0.8Cd_0.2Te. It is shown that arguments attributing both components to volume diffusion processes are incorrect and disagree with experimental evidence. Satisfactory fits to experimental profiles are obtained using the dislocation and grain boundary analyses of LeClaire and Rabinovitch (1981, 1982) and LeClaire in which the slow component represents volume self-diffusion and the fast component represents dislocation or grain boundary diffusion. Discussion of the evidence leads to the conclusions that (sub) grain boundaries rather than dislocations are probably responsible for the fast diffusion tails.

Presents the results of the DLTS study of irradiation-induced defects in n-GaAs irradiated by 6.7 MeV protons at T=300 K. It has been found that the irradiation produces the EL2 defect showing the persistent photoquenching (PPQ) effect. With increasing irradiation dose Phi _p>or=1*10¹¹ cm^-2 the concentration of irradiation-induced defects grows and their interaction becomes stronger. This changes the electrical properties of the EL2 defect: the thermal activation energy and the charge carrier cross section change. The experimental data permit the EL2 to be identified as the isolated antisite defect As_Ga.

Unintentionally doped epitaxial GaAs grown by low-pressure metalorganic vapour phase epitaxy has been characterized using both conventional electrical deep-level transient spectroscopy (DLTS) and optical DLTS (ODLTS). From this study it is shown that the choice of substrate material is important when characterizing material to be used for process-induced defect characterization. Material grown on silicon-doped n⁺ (10¹⁸ cm^-3) GaAs substrate material has an electron defect, a copper defect and two hole traps that have not yet been reported. The three hole traps, presumably introduced by the n⁺ substrate, compensate the material, causing a reduction in the carrier concentration from 2*10¹⁴ cm^-3 when the epitaxial layer is grown on semi-insulating (SI) substrate material to 4*10¹³ cm^-3 when grown on n⁺ (10¹⁸ cm^-3) substrate material.

The ionization energy of DX centres in Ga(As,P) is shown to depend upon the chemical nature of the substitutional n-type impurity. Tellurium in Ga(As,P) introduces a DX centre lying at about 100 meV higher than the DX centres associated with other n-type impurities (Si or Sn). A comparative study between Ga(As,P) and (Al,Ga)As alloys confirms that the ionization energy of the tellurium-related DX centre is very sensitive to the local environment of the impurity.

Warm-electron transport is studied for a two-dimensional electron gas coupled to LO phonons. The authors found that the mobility can be expanded as mu (F)/ mu ₀=(1+ gamma mod F mod + beta F²) with F the electric field. The warm-electron transport coefficients, gamma and beta , are calculated within: (1) the diffusion-to-streaming transition model, (2) the Monte Carlo simulation method and (3) the momentum-balance equation. For a non-degenerate electron gas they found gamma >0 and beta <0.

Fast photoconductivity relaxation and photoconductivity against light excitation intensity in single-crystal 3C-SiC films grown on n-Si substrates have been investigated. The measurements of as-grown and polished samples in a wide range of excitation levels showed the dominant influence of surface recombination and its concurrence with carrier diffusion toward the surface on the relaxation of non-equilibrium carrier concentration. The evaluated non-equilibrium carrier mobility at concentrations of 10¹⁷-10²⁰ cm^-3 changes in a range from 630 cm² V^-1 s^-1 to 6 cm² V^-1 s^-1.

Investigations of the electron transport in Si(100) high-mobility inversion layers were carried out up to extreme carrier concentrations N_s approximately=1.5*10¹⁷ m^-2. The authors are able to verify the existence of the population of a second subband in the Shubnikov-de Haas oscillation spectra of the longitudinal resistance R_xx directly for the first time. The mobilities in the subbands are obtained by combination of conductivity and Hall effect measurements using the concentrations determined in the Shubnikov-de Haas effect. According to the results the second occupied subband is identified as the lowest of the fourfold degenerate ladder.

Luminescence from a highly photo-excited semi-insulating GaAs is investigated from 2 to 4.2 K. The spontaneous emission from an electron-hole plasma is clearly observed in the bulk sample. Electron-hole drops below 4.2 K under high excitation up to 10 MW cm^-2 were not observed. The electron-hole plasma expands with the diffusion constant which depends on excitation intensity and temperature. The dependence supports phonon wind driven expansion of the electron-hole plasma.

No-phonon luminescence from excitons bound to shallow donors and acceptors in crystalline germanium is found to move to higher energy with increasing mass number A of the germanium at the rate dE/dA=0.35+or-0.02 meV. The authors show that it is possible to predict this shift, to a good approximation, from the temperature dependence of the energy gap and the effect on the lattice parameter of the different isotopes.

The authors have studied the cyclotron resonance (CR) of high-mobility 2D electron systems in the extreme quantum limit. Beyond the fractional quantum Hall regime, below a critical filling factor (v_c approximately=1/9), an additional CR line with a mass completely independent of temperature is observed on the high-energy side of the conventional CR. A total condensation into the novel state occurs at temperatures below 1 K. The determined energy gap ( approximately=4 K) separating this lowered ground state from the free carrier state breaks down above v_c. This new phase is discussed in terms of a magnetically induced Wigner crystal.

Shubnikov-de Haas and Van der Pauw Hall effect measurements on strained InAs_0.6P_0.4/InP one-side-modulation-doped quantum wells grown by metalorganic chemical vapour deposition have been performed in order to investigate the properties of an electron gas in an InAs_0.6P_0.4 single quantum well. Hall effect measurements showed that the mobility and the carrier concentration were 40800 cm² V^-1 s^-1 and 1.43*10¹² cm^-2 at 1.5 K, respectively. The Shubnikov-de Haas measurements and fast-Fourier analyses clearly showed an oscillation frequency, which changes with the angle between the magnetic field and the surface normal, indicative of the occupation of the InAs_0.6P_0.4 potential well by two-dimensional electrons. The subband energies and energy wavefunctions were determined using the experimental results and a self-consistent method taking into account the exchange-correlation effects.