V. V. Ratnikov

Strains and crystal lattice defects arising in macroporous silicon under oxidation

The deformation behavior of a macroporous silicon wafer subjected to high-temperature oxidation has been studied, and the basic parameters describing the sample bending and subsequent stress relaxation when oxide is removed are determined. X-ray diffractometry and topography were used to determine the sample bending radius and lattice parameters, and to reveal the areas of dislocation generation. The strain of a silicon lattice in oxidized macroporous Si is about 10−4, and it decreases by an order of magnitude after oxide dissolution. The plastic part of the strain is accompanied by the generation of dislocations in the most strained regions of a structure, i.e., at the interfaces between the porous layer and substrate in the vertical direction and between the central porous region and the pore-free edge in the horizontal plane. The dislocation density is ∼104 cm−2.

X-ray diffraction diagnostics methods as applied to highly doped semiconductor single crystals

Si(As, P, B) and GaSb(Si) single crystals are used as examples to demonstrate the possibilities of methods of X-ray diffraction for the diagnostics (examination of a real structure) of highly doped semiconductor crystals. Prominence is given to characterizing the state of impurity: whether it is in a solid solution or at a certain stage of its decomposition. An optimum combination of X-ray diffraction methods is found to obtain the most complete information on the microsegregation and structural heterogeneity in crystals with low and high X-ray absorption. This combination is based on X-ray diffraction topography and X-ray diffractometry methods having an increased sensitivity to lattice strains.

Microstructure and strain of ZnO molecular-beam epitaxial layers on sapphire

Strains and crystalline perfection of ZnO epitaxial layers on sapphire grown by molecular-beam epitaxy were studied using high-resolution X-ray diffrac tometry. The strain state of samples was determined by curvature measurements. The structural quality of layers was analyzed by diffraction measurements in the Bragg and Laue geometry using θ and (θ-2θ) scanning modes. It was found that the grown layers (Zn/O > 1) are subject to biaxial tensile stresses, while stresses are absent for Zn/O < 1. The density and depth of different dislocations were calculated from the diffraction peak broadening.

MOSAIC STRUCTURE AND SI DOPING RELATED PECULIARITIES OF CHARGE CARRIER TRANSPORT IN III-V NITRIDES

Studies of the charge carrier transport in undoped, moderately and heavily Si doped GaN and AlxGa1—xN (x = 0.1 to 0.2) epilayers with different mosaic structure are presented. The epilayers were grown by low-pressure MOCVD on (0001) sapphire substrates. The mosaic (columnar) structure of the epilayers has been characterized by X-ray diffraction and AFM surface topography studies. Application of electrostatic force microscopy (EFM) permitted to reveal irregular potential barriers at the mosaic domain boundaries in undoped layers and their reduction in Si doped layers. For undoped GaN and AlGaN epilayers unconventional transport of electrons (dependence σ ∼ exp (—1/T)) and a low RT mobility (20 to 70 cm2 V—1 s—1) have been found. We relate the peculiarities in the electron transport in undoped samples with additional carrier scattering on the potential barriers at domain boundaries. Si doping reduces the potential barriers. Moderate Si doping causes a considerable increase of the electron RT mobility up to 600 cm2 V—1 s—1 and restores the dependence μ ∼ T3/2.

Spontaneous and stimulated emission in the mid-ultraviolet range of quantum-well heterostructures based on AlGaN compounds grown by molecular beam epitaxy on c-sapphire substrates

This paper reports on the results of investigations of the spontaneous and stimulated luminescence in AlGaN heterostructures with a single quantum well and a high Al content (up to ∼80 mol % in barrier layers), which were grown by plasma assisted molecular beam epitaxy (PAMBE) on c-sapphire substrates. It has been demonstrated that the stimulated emission occurs in the mid-ultraviolet range of the spectrum at wavelengths of 259, 270, and 289 nm with threshold excitation power densities of 1500, 900, and 700 kW/cm2, respectively. It has been shown that there exists a possibility of TE polarization (E ⊥ c) of both stimulated and spontaneous luminescence down to wavelengths of 259 nm.

Concentration and structural inhomogeneities in highly doped GaSb(Si) single crystals

Concentration and structural inhomogeneities in highly doped GaSb(Si) single crystals grown under various conditions of heat and mass transfer are studied by methods of X-ray topography, high-resolution X-ray diffractometry, and digital image processing. It is established that the inhomogeneity of crystals is determined by specific features of impurity microsegregation during growth under conditions of nonstationary convection in a melt and by peculiarities of the dislocation structure of crystals. The processes related to the initial stage of the decay of the Si supersaturated solid solution in GaSb contribute considerably to the inhomogeneity of crystals on the micro- and macrolevels.

Nanometric-scale fluctuations of intrinsic electric fields in GaN/AlGaN quantum wells with inversion domains

Strain and electric field fluctuations in regions of different polarities in GaN/AlGaN quantum well (QW) structures of dominant N-polarity with inversion domains (IDs) split the photoluminescence (PL) emission into two bands. Micro-PL and time-resolved PL studies reveal strong inhomogeneity of the array of the IDs, where essential parameters, such as strain, electric fields, and sizes are fluctuating quantities. We demonstrate also that the ID formation decreases the intrinsic electric field magnitudes.

X-ray diffraction study of changes in the CdTe monocrystal real structure induced by laser radiation

The changes in the real structure of CdTe monocrystals caused by the thermal action of a high-power laser pulse (1.6–1.97 J/cm2) were studied by high-resolution x-ray topography and diffractometry methods. It was shown that, under our experimental conditions, in a thin surface layer within the crystal region exposed to the radiation, a dislocation cell structure with an increased dislocation density and with considerable micromisorientations, in comparison with the crystal region unexposed to the radiation, was formed. The characteristics of this modified crystal region were determined, and the thickness of the layer with the changed structure was estimated.

Estimation of quality of GaAs substrates used for constructing semiconductor power devices

X-ray topography and high-resolution diffractometry methods are used for testing GaAs wafers from different manufacturers, which are used as substrates for epitaxial growth in construction of power semiconductor devices. Typical features of such wafers are a distorted surface layer, bent, and growth dislocations with two types of distribution with a density of (1–2) × 104 cm−2. The best and worse substrates are determined from the finishing of the working surface, and the optimal combination of X-ray methods for estimating the quality of finishing of the working surface of the crystals with a high level of X-ray absorption is established.

X-ray diffractometry and electron microscopy of porous silicon layers at different stages of oxidation in air

Porous silicon layers prepared by anodic etching under different conditions have been systematically studied in the course of their natural oxidation (aging) in air by many-crystal X-ray diffractometry and transmission electron microscopy. Quantitative information on the strains and crystal structure of layers has been obtained using a combination of measurements of the 004 symmetric and 224 asymmetric reflections in the Bragg geometry on double-crystal and triple-crystal X-ray diffractometers. It has been revealed that the aging of porous silicon is accompanied by an increase in macrostrains and microstrains, as well as in micro-misorientations of crystal fragments, which lead to gradual destruction of porous layers up to their complete amorphization.