A. A. Gutkin

Statistical analysis of AFM topographic images of self-assembled quantum dots

To obtain statistical data on quantum-dot sizes, AFM topographic images of the substrate on which the dots under study are grown are analyzed. Due to the nonideality of the substrate containing height differences on the order of the size of nanoparticles at distances of 1–10 μm and the insufficient resolution of closely arranged dots due to the finite curvature radius of the AFM probe, automation of the statistical analysis of their large dot array requires special techniques for processing topographic images to eliminate the loss of a particle fraction during conventional processing. As such a technique, convolution of the initial matrix of the AFM image with a specially selected matrix is used. This makes it possible to determine the position of each nanoparticle and, using the initial matrix, to measure their geometrical parameters. The results of statistical analysis by this method of self-assembled InAs quantum dots formed on the surface of an AlGaAs epitaxial layer are presented. It is shown that their concentration, average size, and half-width of height distribution depend strongly on the In flow and total amount of deposited InAs which are varied within insignificant limits.

Local triboelectrification of an n-GaAs surface using the tip of an atomic-force microscope

The method of scanning Kelvin-probe microscopy is used to show that the effect of triboelectrification is observed when the tip of an atomic-force microscope interacts with the surface of n-GaAs epitaxial layers. The sign of the change in the potential indicates that the sample surface after triboelectrification becomes more negative. The observed specific features of the phenomena can be attributed to the thermally activated generation of point defects in the vicinity of the sample surface due to deformation caused by the tip.

Electrochemical capacitance-voltage profiling of the free-carrier concentration in HEMT heterostructures based on InGaAs/AlGaAs/GaAs compounds

The depth distribution of free carriers over the HEMT structures with quantum-well layers is studied by electrochemical capacitance-voltage profiling. It is shown that the actual distribution of the concentration of free carriers and their energy spectrum in the HEMT structure channel can be obtained by numerical simulation of the results of profiling based on the self-consistent solution of one-dimensional Schrödinger and Poisson equations.

Electron emission from multilayer ensembles of vertically coupled InAs quantum dots in an n-GaAs matrix

Electron emission from multilayer arrays of vertically coupled InAs quantum dots into the n-GaAs matrix in Schottky-barrier structures (electron concentration n ≈ 2 × 1016 cm−3) is studied by admittance spectroscopy. It is established that, in the temperature region below ∼70 K, electron emission in a rate range of 3 × 104–3 × 106 s−1 proceeds via thermally activated tunneling through intermediate virtual states. As the number of layers in the quantum dot array increases from three to ten, a decrease in the electron emission rate is observed.

An X-ray diffraction and electron-microscopic study of the influence of gamma radiation on multilayer AlGaAs/InGaAs/GaAs heterostructures

The influence of gamma radiation on multilayer AlGaAs/InGaAs/GaAs transistor heterostructures has been studied by means of X-ray diffraction analysis and transmission electron microscopy. It was found that irradiation with doses exceeding ∼3 × 107 rad leads to destruction of the GaAs layer on the surface of these structures. An irradiation dose of 108 rad significantly deteriorates the surface planarity, with the surface roughness reaching values of several nanometers. In addition, dislocations are formed in the cap layer of the structure. Such a behavior of the cap layer may be due to the existence of an oxide layer on its free surface and to the possible chemical reactions, induced by gamma radiation, between atoms of the cap layer and free radicals formed in the oxide and in the ambient atmosphere. No noticeable changes in the structure and composition of the thin InGaAs channel layer occur at doses lower than 108 rad.

Integrated diagnostics of heterostructures with QW layers

Potential of the integrated diagnostics of III–V complex quantum-well (QW) structures used for fabrication of power lasers is demonstrated for the example of GaInP/GaAs/AlGaAs QW structures grown by the metal-organic chemical vapor deposition method. An analysis of cathodoluminescence spectra enabled estimation of the composition of barrier layers, confirmed the existence of a QW, and detected anomalies in its emission associated with compositional changes. An examination of the structures with a transmission electron microscope determined the thickness of barrier layers and the width and composition of the QW, and discovered transition layers near interfaces. The composition of the barrier layers was precisely determined by electron probe microanalysis. The resulting values of the parameters were used in interpretation of X-ray diffraction data, which confirmed the existence of transition regions and revealed composition gradients and partial relaxation in the main part of barrier layers. The integrated study yielded consistent and substantiated data on the layer thickness and composition and data on the quality of interfaces, partial relaxation of the layers, and existence of transition layers and composition gradients.

Photoluminescence of CuGaTeAs and CuGaSnGa complexes in n-GaAs under resonance polarized excitation

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 (CuGa− and VGa2−) 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.

Change in the energy of Jahn-Teller configurations of vacancy-donor complexes induced by uniaxial strain

Estimates are obtained for the energy splitting of equivalent Jahn-Teller configurations of the light-absorbing state of 〈vacancy Ga (VGa)〉-〈tellurium (TeAs)〉 complexes in n-GaAs generated by uniaxial stress along the directions [111] and [001]. These estimates are based on measuring the stress dependence of the polarization of photoluminescence associated with these complexes at T⋍2 and 77 K. A phenomenological model of the complexes, which describes how the donor (TeAs) and the Jahn-Teller effect modify the initial t2-orbitals of the vacancy component of the complex, is discussed as the effect of uniaxial strain. This model makes it possible to relate measured values of the energy splitting of equivalent configurations relative to the splitting of the original t2-state of the vacancy arising from the presence of the donor and the Jahn-Teller effect. Comparison of calculations with experimental data shows that the contribution of the Jahn-Teller effect to the formation of the light-absorbing state of the complex VGaTeAs exceeds the contribution due to the donor effect, although the two effects are comparable.

Optical characteristics of 1.18-eV luminescence band complexes in n-GaAs:Sn(Si): Results of a photoluminescence study with polarized resonant excitation

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.

Surface electrostatic potential of inn epitaxial layers and its changes during anodic oxidization

An estimation carried out via scanning Kelvin probe microscopy (SKPM) confirms that valleys on the initial surface of n-type InN layers correspond to a decrease in electrostatic potential by at least several millivolts. At the same time, surfaces subjected to anodic oxidization (the oxide thickness is no less than 10 nm) do not support this correspondence in remarkable number of cases. This is apparently caused by fluctuations in the oxide’s charge. Strong oxidization is found to lead to a substantial increase in the energy of the conduction-band bottom on the InN surface. The average potential of an oxidized surface is demonstrated to exceed that of the initial one and be positive with respect to the SKPM probe. The measured data enable us to infer that the electron work function of InN anodic oxide is less than 5 eV.