Effect of oxygen in photoluminescence from chromium-doped semi-insulating GaAs

Photoluminescence measurements have been performed on chromiumdoped semi-insulating GaAs as a function of temperature. The effect of oxygen is explained in terms of the relative strength of the oxygen-emission at ～ 0.62 eV and the chromium-emission at ～ 0.81 eV. An explanation of chromium emission at ～ 0.81 and 0.54 eV in terms of a static Jahn-Teller distortion in the ground state of Cr²⁺ is proposed.     

Annealing behavior of the 0.8 eV luminescence in undoped semiinsulating GaAs

The annealing behavior of the 0.8 eV luminescence band in undoped semiinsulating GaAs has been investigated. It is found to be fully analogous to that of the As_Ga antisite electron-paramagnetic-resonance signal. The radiative recombination of electrons with the doubly ionized As_Ga double donor is discussed as the origin for the 0.8 eV band.     

Optical properties and origin of infrared light scattering centers in undoped semi-insulating GaAs crystals

The wavelength dependence and polarization characteristics of the infrared light scattered from an undoped GaAs crystal were investigated in the 90° angle infrared light scattering configuration. The scattering is Rayleigh scattering from scatterers which are always associated with the dislocations, and they are classified into three types,S, L _A , andL _G scatterers, according to their polarization characteristics. TheS, L _A , andL _G -scatterers are thought to be small As clusters, large As precipitates and large Ga precipitates, respectively.     

Surface space-charge layer analysis in semi-insulating GaAs containing deep levels in bulk

A computer simulation study of space-charge layers at the surface of semiinsulating GaAs containing deep EL2 and Cr centers in bulk is presented. Substantial influence of the deep bulk levels on the main characteristics of the surface space charge layers, is demonstrated. The special features of these characteristics and the conditions of their arising are discussed.     

Thermal quenching of the 1–1.35 eV extrinsic photoconductivity in semi-insulating GaAs (Cr,O)

We report in this work, photoconductivity measurements between 77 and 300 K, in semi-insulating GaAs samples doped with chromium and oxygen. These samples exhibit a striking photoconductivity behaviour, which is characterized by a strong increase of the photoconductivity after long time excitation with photons in the 1–1.35 eV spectral range at 77 K. This increase is thermally quenched at 135 K. We explain this anomalous behaviour by means of a generation of traps by light within this spectral range; these photogenerated traps being annealed at 135 K. To our knowledge this is a phenomenon which has not yet been observed in GaAs.     

Electromagnetically induced transparency due to intervalence band coherence in a GaAs quantum well

We demonstrate electromagnetically induced transparency in the transient optical response in a GaAs quantum well by using the nonradiative coherence between the heavy-hole and the light-hole valence bands.     

Temperature dependence of 1.98 eV photoluminescence band in Cd1-xMnxTe semiconductor alloys

The 1.98 eV photoluminescence (PL) band in Cd_1-xMn_xTe has been studied over the composition range 0.4 ≤ x ≤ 0.7. This emission is shown to be insensitive to magnetic phase transitions. Above ～ 70 K, the emission intensity displays two temperature activated regions which appear to be intrinsic to the manganese system. The band maximum undergoes a red shift on cooling down to 60 K, followed by a nearly equal blue shift down to 1.8 K: models explaining these behaviors are discussed.     

Space-charge layers in semi-insulating GaAs: Photoexcited two-dimensional electron systems

Resonant Raman spectra of photoexcited semi-insulating GaAs and Fe/GaAs show features characteristic of two-dimensional electron plasmas. The results are ascribed to the presence of a space-charge layer at the surface, originating in a slight mismatch of Fermi-level positions at the vacuum (or metal) interface and in the bulk. Calculations using values of intersubband transition energies from the data give an estimated shift of ～0.04 eV for the Fermi-level position at T = 85K.     

Low frequency oscillations in semi-insulating GaAs: a nonlinear analysis

We have observed low frequency current oscillations in a semi-insulating GaAs sample grown by low temperature molecular beam epitaxy. For this, an experimental setup proper to measure high impedance samples with small external noise was developed. Spontaneous oscillations in the current were observed for some bias conditions. Although measurements were carried out from room temperature down to liquid helium, the dynamical analysis was carried out around 200 K where the signal to noise ratio was fairly favorable. To increase the data quality we have also used a noise reduction algorithm suitably developed for nonlinear systems. We observed attractors having low embedding dimension, limit cycle bifurcations, and chaotic behavior characteristic of nonlinear dynamical processes in route to chaos. Attractor reconstruction, Poincare sections, Lyapunov exponents, and correlation dimension were also analyzed.     

Upconversion of photoluminescence due to subband resonances in a GaAs/AlAs multiple quantum well structure

We have investigated the upconversion of photoluminescence (PL) due to subband resonances in a simple GaAs(15.3 nm)/AlAs(4.5 nm) multiple quantum well embedded in a p–i–n diode structure. The systematic measurements of the PL spectra and the calculated results of the interband transition energies as a function of electric field strength reveal that the PL bands from the electron subbands with n=3 (E3) and n=4 (E4) sharply appear under the first-nearest-neighbor resonance conditions between the E1 and E3 subbands and the E1 and E4 subbands, respectively, owing to the carrier injection to the E3 and E4 subbands from the E1 subband. This result indicates that the resonant tunneling due to the subband resonance is a dominant mechanism for the carrier population in the higher lying subbands. Utilizing these subband resonances, we have demonstrated the upconversion of PL from the E3 and E4 subbands under the excitation condition of the fundamental interband transition between the E1 and the n=1 heavy-hole subbands.     

On the d.c. conductivity in semi-insulating GaAs

We investigate the temperature dependence of electrical conductivity of SI-GaAs. We have shown that the surface conductivity obeys the Berthelot type of dependence, the bulk component, on the other hand, fulfills the Arrhenius dependence.     

Bulk and near-surface annealing behavior of the 0.8 eV luminescence in semi-insulating gallium arsenide

An annealing study of the 0.8 eV photoluminescence band in LEC GaAs has been performed, using a combination of 10 min annealing steps over a temperature range of 800–1100° C and one hour furnace annealing steps at 500–700° C. Results show that the defect responsible for the luminescence is stable in bulk material under all annealing conditions that have been investigated. In combination with earlier results, this demonstrates a stability of the defect to temperatures in excess of 900° C and indicates a parallel with the annealing behavior of the 0.67 eV band in semi-insulating GaAs. The observed annealing behavior is very different in the near-surface region, indicating an important role of As out-diffusion.     

On the interpretation of photoconductivity and photo-hall spectra in semi-insulating GaAs:Cr

A mixed-conductivity analysis is used to separate the contributions of n, p. and μ_n to photoconductivity and photo-Hall spectra in semi-insulating GaAs:Cr. It is shown that the spectral dependence of μ_n is often important in the interpretation of these data. A room-temperature energy diagram is presented, and includes chromium-related centers at 0.73 and 0.52 eV from the conduction band.     

Zeeman studies of the 0.839eV emission in SI GaAs:Cr

The multi-structured zero phonon transitions of the 0.839eV emission observed in SI GaAs:Cr have been investigated by Zeeman measurements. Magnetic splittings for the directions [111], [110] and [100] as well as angular dependence studies in the (110) plane show that the centre has a symmetry axis along [111] with a small orthorhombic distortion. The emissions cannot be due to the isolated [100] Jahn-Teller distorted Cr²⁺ ions observed in magnetic resonance but to [111] centres such as the (Cr²⁺-donor) pairs as suggested by White.     

EL2 related levels in semi-insulating GaAs: Recapture and thermal regeneration

Recently Levinson proposed a dipolar model for the EL2 center in GaAs; this defect could appear in two different configurations (O) and (O¹) following the electrical history of the center. We identified three TSC low temperature peaks as characteristic of the (O) configuration. The conversion and regeneration of the (O) configuration are observed through these (TSC) peak strengths. It is proposed that the “thermal” regeneration process could be triggered by thermal emission of holes from acceptor levels; we also confirm that EL2 centers are not unique but more likely belong to a family of similar defects.     

Photo-Hall study of the optically enhanced photocurrent in semi-insulating LEC GaAs

The photocurrent of semi-insulating GaAs is enhanced by long time excitation with photons of energy higher than 1 eV. Photo-Hall experiments on the enhanced photocurrent reveal that this is mainly due to holes, and that the mobility is increased in conditions of strong photocurrent enhancement. This is discussed on the basis of metastability of midgap levels in GaAs. Thermal annealing experiments showed that likely EL6 is involved in this phenomenon.