L. P. Avakyants

Electrical and optical properties of near-surface AlGaAs/InGaAs/AlGaAs quantum wells with different quantum-well depths

A series of AlGaAs/InGaAs/AlGaAs quantum-well heterostructures with different quantum-well depths and approximately the same concentrations of two-dimensional electrons is grown by molecular-beam epitaxy. The built-in electric field in the grown samples is determined from the photoreflectance data and, on this basis, the energy-band structure in the quantum-well region is calculated. It is found that the highest mobility μe of two-dimensional electrons is attained in the sample with a barrier-layer thickness of Lb = 11 nm. Measurements of the photoluminescence spectra and the band-structure calculations demonstrate that, as the quantum well becomes closer to the surface, the doping profile broadens due to diffusion and segregation processes. The nonmonotonic dependence of μe on the distance between the surface and the quantum well is explained.

Computerized Setup for Double-Monochromator Photoreflectance Spectroscopy

An experimental setup for studying semiconductor structures by photoreflectance spectroscopy is designed. The double-monochromator-based optical scheme of the setup makes it possible to depress uncontrolled heating of the sample and diminishes a bending of the energy bands due to charge carrier photogeneration. Accordingly, the photoreflectance spectra are detected with a minimal influence of the modulating and probe radiations on the sample. With this setup, the room-temperature photoreflectance spectra from GaAs/GaAsP superlattices are taken and the interband transition energies, as well as a potential step in the conduction band of these superlattices, are measured.

Electroreflectance spectra of InGaN/AlGaN/GaN quantum-well heterostructures

Abstractp−n InGaN/AlGaN/GaN heterostructures with InGaN/AlGaN multiple quantum wells are studied by electroreflectance spectroscopy. The structures are grown by metal—organic epitaxy and arranged with the p region in contact with the heat sink. Light is incident on and reflected from the structures through the sapphire substrate. To modulate the reflectivity, rectangular voltage pulses and a dc reverse bias are applied to the p−n junction. A line corresponding to interband transitions in the region of InGaN/AlGaN multiple quantum wells is observed in the electroreflectance spectra. The peak of this line is shifted to shorter wavelengths from the peak of injection luminescence of the light-emitting diode structures. The low-field model developed by Aspnes is used to describe the electroreflectance spectra. By choosing the parameters of the model to fit the experimental data, the effective band gap of the active region of the structure, Eg*, is determined at 2.76–2.78 eV. The experimental dependence of Eg* on the applied voltage is attributed to the effect of piezoelectric fields in the InGaN quantum wells. In the electroreflectance spectra, an interference pattern is observed in the wide spectral range from 1.4 to 3.2 eV. The interference is due to the dependence of the effective refractive index on the electric field.

Interband optical transitions in GaAs modulation-doped quantum wells: photoreflectance experiment and self-consistent calculations

Photoreflectance spectra of AlGaAs/GaAs/AlGaAs wide quantum wells doped by Si up to Nd = 2 x 1018cm-3 were investigated at room temperature. Three kinds of spectral peculiarities were observed in photoreflectance spectra: a spectral line due to the GaAs band gap (1.42 eV), short-period Frantz?Keldysh oscillations originating from the barrier band gap (1.71 eV) and lines of subbands in the quantum well. The energies of optical transitions are determined by means of the least-squares approximation of experimental data by the sum of Aspnes relations. The experimental results are in good agreement with the self-consistent subband structure calculation. It is shown that with an increase of the doping level the energies of the interband transitions in the quantum well are changed.

Photoreflection studies of band offsets at the heterojunction in strained short-period GaAs/GaAsP superlattices

Energies of band-to-band transitions with the involvement of the quantum-confinement subbands are determined from the photoreflection spectra of the strained short-period GaAs/GaAs0.6P0.4 superlattice. Strains caused by the mismatch of the crystal lattice in the GaAs and GaAs0.6P0.4 layers are calculated on the basis of the observed shift of the fundamental-transition energy in GaAs0.6P0.4. Positions of minibands in the superlattice are simulated in relation to the potential jump at the heteroboundary; the Kronig-Penney model is used in the calculations. Comparison of the results of simulation with experimental data shows that the studied superlattice is of type I with weakly localized electrons and light holes. The potential jump at the heteroboundary in the conduction band amounts to ΔEc/ΔEg=0.15.

Photoreflectance spectroscopy of delta-doped GaAs layers

Delta-doped GaAs layers have been studied by photoreflectance spectroscopy. The built-in electric fields and interband transition energies in the semiconductor structure have been evaluated from analysis of Franz-Keldysh oscillations. The delta-doped region is found to have an increased interband transition energy, which is interpreted in terms of the Burstein-Moss effect and carrier photogeneration.

Study of the effects of size quantization in coupled AlxGa1−x As/GaAs/Alx Ga1−x as quantum wells by means of photoreflectance spectroscopy

Photoreflectance spectra are measured in heterostructures with coupled quantum wells at room temperature. The energies of the optical transitions are determined, and their variation with the well width and barrier thickness is studied. The experimental results are compared with the theoretically calculated electron-hole transition energies. Good agreement is obtained for narrow wells.

Photoreflectance spectroscopy of electron-hole states in a graded-width GaAs/InGaAs/GaAs quantum well

The spectrum of electron-hole states in a GaAs/In0.5Ga0.5As quantum well with a width graded in the range from 1.1 to 3.6 nm is studied by photoreflectance spectroscopy. The energies of the size-quantization levels of electrons and holes are calculated taking into account the strain-induced changes in the band structures of the quantum well. It is shown that the best fit of the experimental data to the results of calculations is attained if the ratio between the offset of the conduction band and that of the valence band at the heterojunction is Q = ΔEc/ΔEv = 0.62/0.38. A photoreflectance signal is detected in the region of the shadow of modulating radiation beam at a spacing between the spots produced by probing and modulating radiation shorter than 6 mm.

Determination of the carrier concentration in doped n-GaAs layers by Raman and light reflection spectroscopies

The carrier concentrations in Si-doped n-GaAs films have been determined by Raman and light reflection spectroscopies. The data obtained are in good agreement with the results of Hall measurements. It is shown that the light reflection and Raman spectroscopies supplement each other in determination of carrier concentrations in the range 1017−1019 cm−3.

Photoreflection studies of the dopant activation in InP implanted with Be+ ions

Photoreflection spectroscopy is used to study the activation of impurity in InP crystals implanted with 100-keV Be+ ions at a dose of 1013 cm−2 and then subjected to thermal annealing for 10 s. After annealing at temperatures no higher than 400°C, lines characteristic of crystalline InP are not observed in the photoreflection spectrum, which indicates that the crystal lattice has become disordered as a result of the ion implantation. If the annealing temperature is in the range from 400 to 700°C, the lines related to the fundamental transition in InP (1.34 eV) and the transition between the conduction band and the subband, which has split off from the valence band owing to a spin-orbit interaction (1.44 eV), are observed in the spectrum, which indicates that the InP crystal structure is restored. The dopant is activated in samples annealed at 800°C, as indicated by the Franz-Keldysh oscillations observed in the corresponding photoreflection spectra. Free-carrier concentration is determined from the oscillation period and is found to be equal to 2.2 × 1016 cm−3.