Yu. G. Sadof’ev

High-resolution emission spectra of pulsed terahertz quantum-cascade lasers

The spectra of pulsed terahertz quantum-cascade lasers were measured with high spectral resolution. The characteristic line width at half maximum was 0.01 cm−1; it is controlled by laser temperature variations during the supply voltage pulse. It was shown that an increase in the laser temperature leads to a decrease in the emission frequency, which is caused by an increase in the effective refractive index of the active region. It was also found that a decrease in the supply voltage results in a decrease in the emission frequency, which is caused by a change in the energy of diagonal transitions between lasing levels.

Emission from rare-earth centers in (ZnTe:Yb):O/GaAs

Layers of Yb-doped ZnTe were grown by molecular-beam epitaxy, and the photoluminescence of ZnTe:Yb structures was studied. It was found that additional doping with O should be performed to activate emission from Yb ions. The necessary conditions for intense emission from Yb3+ ions were determined. Stark splitting of levels of Yb3+ ions in the crystal field was measured experimentally.

Band offsets in Zn 1−x Cd x Te/ZnTe single-quantum-well structures grown by molecular-beam epitaxy on GaAs(001) substrates

ZnTe heteroepitaxial layers and ZnTe/Zn1−xCdxTe/ZnTe strained quantum-confinement structures grown by molecular-beam epitaxy on GaAs(001) were studied by low-temperature cathodoluminescence spectroscopy and current-relaxation deep-level transient spectroscopy (DLTS). A peak related to electron emission from the ground size-quantization level in the conduction-band was observed in the DLTS spectra of quantum-confinement structures. The conduction-band offset parameter QC was determined from the DLTS and cathodoluminescence data. For Zn1−xCdxTe/ZnTe single-quantum-well structures with x=0.2–0.22, QC equals 0.82 ± 0.05. The effect of internal elastic strain on the band offsets and QC at the CdxZn1−xTe quantum well interfaces was calculated; the results of calculations agree well with experimental data.

Effect of elastic stresses on the IR spectra of lattice vibrations in epitaxial ZnSe films on (001)GaAs substrates

The lattice IR reflection spectra of epitaxial ZnSe films are studied for different thicknesses on a (001)GaAs substrate. The frequency of the TO mode is found to increase for films with thicknesses of 0.8 and 1.2 µm that exceed the critical value dcr≈0.1 µm for ZnSe/GaAs pairs. The effect is explained by the existence of regions with residual stress in the film.

Infrared lattice-reflection spectroscopy of Zn1−xCdxSe epitaxial layers grown on a GaAs substrate by molecular-beam epitaxy

Results are presented of the first measurements of infrared reflection spectra of Zn1−xCdxSe films (x=0–0.55; 1) grown on a GaAs substrate by molecular-beam epitaxy. It is shown by a mathematical analysis of the experimental spectra that the investigated Zn1−xCdxSe alloy system manifests a unimodal rearrangement of its vibrational spectrum as the composition is varied.

Variation of the emission frequency of a terahertz quantum cascade laser

The variation of the emission frequency of a quantum cascade laser of the 3.2 THz range within a single radiation pulse was studied using high-resolution Fourier spectroscopy. A significant (108 MHz) increase in frequency in the first 4 μs of a pulse was found. This increase is attributed to a change in the electron part of permittivity of the laser active region coinciding with a rise in the emission intensity. The frequency dropped by 81 MHz in the following 40 μs of a radiation pulse due to a change in the lattice part of permittivity occurring as the laser active region was heated during a radiation pulse.

Long-wavelength optical phonons in the ZnTe/Zn0.8Cd0.2Te superlattice

The lattice IR reflection spectra of a ZnTe/Zn0.8Cd0.2Te superlattice measured at temperatures of 300 and 10 K are analyzed. The ZnTe/Zn0.8Cd0.2Te superlattice is grown by molecular-beam epitaxy on a GaAs substrate with a ZnTe buffer layer. It is found that the lattice IR reflection spectra of the studied structure exhibit only one reflection band. Dispersion analysis of the experimental spectrum has revealed the presence of one lattice TO mode close in frequency to the mode of pure ZnTe. This result is explained by a shift in the frequency of the lattice modes of the ZnTe and Zn0.8Cd0.2Te layers of the superlattice toward each other. In turn, this shift is caused by internal elastic stresses in the superlattice due to a mismatch between the lattice parameters of the materials of these layers.

Investigation of free and bound excitons in strained ZnTe films grown by MBE on GaAs(100) substrates

A study is reported of the reflectance and low-temperature photoluminescence (PL) spectra of ZnTe films grown by molecular-beam epitaxy (MBE) on GaAs substrates [(100) orientation, 3° deflection toward 〈110〉]. It is shown that the strain-induced splitting of the free-exciton energy level (ΔEex) does not depend on ZnTe film thickness within the 1–5.7 µm range and is due to biaxial in-plane film tension. The stresses are primarily determined by the difference between the thermal expansion coefficients of the film and the substrate. It is also shown that the residual stresses originating from incomplete relaxation of the film lattice parameter to its equilibrium value at the growth temperature likewise provide a certain contribution. The position of the spectral line of an exciton bound to a neutral acceptor (As) is well approximated in terms of the present models, taking into account the stresses calculated using the value of ΔEex.

Zinc telluride epilayers and CdZnTe/ZnTe quantum wells grown by molecular-beam epitaxy on GaAs(100) substrates using solid-phase crystallization of an amorphous ZnTe seed layer

The deposition of an amorphous ZnTe seed layer of thickness 10 nm and its subsequent solid-phase crystallization are implemented prior to the start of molecular-beam epitaxy of ZnTe layers on GaAs(100) substrates. RHEED patterns from the growth surface of the samples during epitaxy confirm that the formation of three-dimensional nucleation centers is successfully eliminated by this technique, and two-dimensional growth in the early stage of epitaxy is achieved. Cathodoluminescence and x-ray structural analyses indicate a higher quality on the part of ZnTe layers grown with the application of an amorphous ZnTe layer. Quantum-well ZnCdTe/ZnTe structures that emit efficiently in the green region of the spectrum can be grown by optimizing the epitaxy regimes.

Long-term relaxation of the photovoltage in a heteroepitaxial structure

The distribution of the transverse photovoltage has been investigated in samples of a Ge-GaAs heterostructure with an oxide layer (∼15 Å) at the interface. A method of simultaneous excitation with modulated and unmodulated radiation is used to observe the long-term relaxation of the photovoltage in these heterostructures. The energy barriers responsible for this effect are localized at the surface of the Ge film and the GaAs substrate, adjacent to the interlying oxide. It is shown that the gradients of the dark carrier concentration in the film and in the substrate are in opposite directions. A transition layer is formed during the growth process and not as a result of the classical heterodiffusion of the components in the film and the substrate. It is noted that the photoconductivity signal observed at the modulation frequency in the field formed at the sample as a result of the unmmodulated illumination must be taken into account in the recorded voltage signal.