M. A. Yagovkina

Dependence of the efficiency of III-N blue LEDs on the structural perfection of GaN epitaxial buffer layers

III-N blue LED structures with active regions based on InGaN nanoislands are studied. The structures are grown by metalorganic vapor-phase epitaxy (MOVPE) on GaN layers deposited by various methods for the initial formation of an epitaxial layer. It is shown that, due to strong carrier localization in narrow-gap InGaN nanoislands, the electroluminescence efficiency is independent of the crystal perfection of the material.

Effect of annealing on the nonequilibrium carrier lifetime in GaAs grown at low temperatures

GaAs samples grown by molecular-beam epitaxy at low (230°C) temperatures are investigated. One of the samples is subjected to aftergrowth annealing at 600°C. Using an unconventional pump-probe scheme for measuring the dynamic variation in the light refractive index, the nonequilibrium charge-carrier lifetime (275 ± 30 fs before annealing) is determined. Such a short carrier lifetime in the unannealed material is due to the high concentration of point defects, mainly AsGa antisite defects. According to X-ray diffraction and steady-state optical absorption data, the AsGa concentration in the samples is 3 × 1019 cm−3, which corresponds to an arsenic excess of 0.26 at %. Upon annealing at 600°C, the superstoichiometric As defects self-organize and form As nanoinclusions in the GaAs crystal matrix. It is shown that in this case the nonequilibrium charge-carrier lifetime increases to 452 ± 5 fs. This lifetime is apparently ensured by the capture of non-equilibrium charge carriers at metal As nanoinclusions.

Optical and structural properties of InGaN/GaN short-period superlattices for the active region of light- emitting diodes

The results of the study of structural and optical properties of short-period InGaN/GaN superlattices synthesized by MOCVD on sapphire substrates are presented. To form the superlattices, the method of periodic interruption of the growth of the InGaN layer with hydrogen supply into the reactor was used. It is shown that, with the use of the suggested method, an InGaN/GaN periodic structure with the developed interfaces and regions of joining the neighboring InGaN layers not correlated in a vertical direction is formed. The formation of such regions leads to a heavy dependence of the shape of the emission spectra of the super-lattices on the number of periods in the range of 400–470 nm.

X-ray diffraction study of short-period AlN/GaN superlattices

The structure of short-period hexagonal GaN/AlN superlattices (SLs) has been investigated by X-ray diffraction. The samples have been grown by metalorganic vapor-phase epitaxy (MOVPE) in a horizontal reactor at a temperature of 1050°C on (0001)Al2O3 substrates using GaN and AlN buffer layers. The SL period changes from 2 to 6 nm, and the thickness of the structure varies in a range from 0.3 to 1 μm. The complex of X-ray diffraction techniques includes a measurement of θ-2θ rocking curves of symmetric Bragg reflection, the construction of intensity maps for asymmetric reflections, a measurement and analysis of peak broadenings in different diffraction geometries, a precise measurement of lattice parameters, and the determination of radii of curvature. The thickness and strain of separate SL layers are determined by measuring the θ-2θ rocking curves subsequent simulation. It is shown that most SL samples are completely relaxed as a whole. At the same time, relaxation is absent between sublayers, which is why strains in the AlN and GaN sublayers (on the order of 1.2 × 10−2) have different signs. An analysis of diffraction peak half-widths allows us to determine the densities of individual sets of dislocations and observe their change from buffer layers to SLs.

Experimental study of cyclic action of plasma on tungsten

We report on experimental results on multiple action of hydrogen, deuterium, and helium plasmas produced by a plasma gun and the Globus-M tokamak on tungsten. The surface temperature in the course of irradiation is measured with a bichromatic pyrometer with a time resolution of ⩾1 μs. The morphology of the surface layer is investigated and X-ray structure analysis of tungsten exposed to multiple radiations by the plasma under various conditions is carried out. A slight decrease in the lattice parameter in the sample subjected to the maximal number of irradiation cycles is detected. It is shown that the morphology of the tungsten surface irradiated by the hydrogen plasma from the gun and by the deuterium plasma from the Globus-M tokamak changes (the structure becomes smoother). The characteristic depth of the layer in which impurities have been accumulated exceeds 0.5 μm. This depth was the largest for the sample exposed to 1000 shots from the gun and 2370 shots from the tokamak. It is shown that the helium jet from the plasma gun makes it possible to simulate the action of helium ions on the International Thermonuclear Experimental Reactor (ITER) diverter, producing a layer of submicrometer particles (bubbles).

Nanostructured Ge2Sb2Te5 chalcogenide films produced by laser electrodispersion

Amorphous nanostructured films of a complex chalcogenide (Ge2Sb2Te5) are produced by laser electrodispersion and their structural and electrical properties are studied. It is found that the characteristic size of Ge2Sb2Te5 nanoparticles in the structure of the films is 1.5–5 nm.

Semi-insulating GaN:C epilayers grown by metalorganic vapor phase epitaxy using propane as a carbon source

The influence of propane present in a reactor at various stages of GaN growth by metalorganic vapor phase epitaxy (MOVPE) on sapphire substrates on the character of epitaxial process and the properties of epilayers has been studied. Doped GaN epilayers with carbon concentration 5 × 1018 cm–3 characterized by high crystalline perfection, an atomically smooth surface, and electric breakdown voltage above 500 V at a doped layer thickness of 4 μm have been obtained.

Complex use of the diffraction techniques in depth profiling of the crystal lattice parameter and composition of InGaAs/GaAs gradient layers

A technique is proposed for testing thick (1 μm and larger) gradient layers with the composition and relaxation degree alternating over the layer depth on the basis of comparative analysis of X-ray scattered intensity maps in the reciprocal space and depth profiles of the crystal lattice parameters obtained by electron microdiffraction. The informativity of the proposed technique is demonstrated using the example of an InxGa1–xAs/GaAs layer with linear depth variation in x. Complex representation of the diffraction data in the form of the depth-profiled reciprocal space map allows taking into account the additional relaxation caused by thinning electron microscopy specimens.

Vapor phase epitaxy of aluminum nitride from trimethylaluminum and molecular nitrogen

We have studied the pyrolysis of trimethylaluminum (TMA) in a nitrogen-containing atmosphere of a vapor phase epitaxy reactor. It is established that, in the presence of gallium nitride coatings in the reactor, the main product of TMA pyrolysis in a nitrogen-hydrogen atmosphere is aluminum nitride. Using this process (without introducing ammonia), we obtained perfect epitaxial aluminum nitride layers.

Properties of InGaN/GaN heterostructures obtained using growth interruption under various conditions

Conversion of the near-surface regions of InGaN into GaN during growth interruption has been studied. It has been established that the process exhibits saturation in time, develops at a depth below 2 nm, and proceeds in a similar way in the presence and absence of hydrogen, although hydrogen supply significantly accelerates the conversion process. InGaN/GaN heterostructures obtained with interruption of growth exhibit an additional photoluminescence line with a longer wavelength as compared to that for an otherwise analogous continuous InGaN layer. It is established that growth interruption in a hydrogen-free atmosphere is more favorable for obtaining light-emitting structures for the green spectral range.