Igor Aksenov

GaN heteroepitaxial growth on silicon nitride buffer layers formed on Si (111) surfaces by plasma-assisted molecular beam epitaxy

Wurtzite GaN films were grown on silicon nitride buffer layers formed on Si (111) substrates by radio frequency plasma-assisted molecular beam epitaxy. Reflection high energy electron diffraction, Auger electron spectroscopy, transmission electron microscopy, and photoluminescence results indicate that the single crystalline wurtzite GaN was grown on the buffer layers of amorphouslike silicon nitride formed on Si (111) substrates by taking the following relationship with the substrate: GaN [0001]//Si [111] and GaN (1120)//Si (110). Both faces of the silicon nitride buffer layer were found to be flat and sharp, the thickness of the buffer layer (1–1.5 nm) being constant across the interface. Efficient bound exciton emission was observed at 3.46 eV. The growth technique described was found to be simple but very powerful for growing high quality GaN films on Si substrates.

Scanning tunneling microscopy studies of formation of 8×5 reconstructed structure of Ga on the Si(001) surface

The behavior of Ga on the Si(001) 2×1 surface has been studied for Ga coverage of 0.8 monolayers and annealing temperatures in the range 623–863 K by means of scanning tunneling microscopy. Annealing at the lowest annealing temperature Ta leads to the formation of the Ga 2×2 phase, which is a result of self-arrangement of Ga dimers, as well as irregular Ga clusters. An increase in Ta to 673 K results in the formation of a partially ordered metastable Ga 5×2 phase, whereas further increase in Ta leads to the transformation of the 5×2 structure into 8×n (n=4,5) structure, the degree of order of which gradually increases with an increase in the annealing temperature. At Ta=833 K the surface is uniformly covered by the periodically aligned arrays of 8×5 units, this 8×5 structure is believed to consist of the ordered double-layered Ga clusters having 24–26 Ga atoms in the first layer and four Ga atoms in the second layer. The 8×5 units are out of registry with the underlying Si surface in one direction, this u...

Effect of Fermi Level Motion on ESR and Optical Properties of CuAlS2

Optical absorption and electron spin resonance (ESR) spectra have been studied in as-grown, Cr-doped, and annealed in different atmospheres CuAlS2 crystals. It has been found that the major influence on the optical and ESR properties of CuAlS2 crystals, obtained by chemical vapour transport, arises from the transition ions Fe3+ and Cr2+, which are present in the crystal lattice as residual impurities. Thermal treated crystals exhibit drastic changes in their absorption and ESR spectra depending on the annealing atmosphere, which is explained by the process of the Fermi level motion in the vicinity of the Fe2+/Fe3+ and Cr+/Cr2+ demarcation levels. The results obtained show that the valence states of Fe and Cr ions and their relative amounts in each valence state are controlled by the position of the Fermi level in the band gap of the host crystal, and are stoichiometry on dependent.

Visible photoluminescence of Zn-doped CuAlS2

The observation of a very intense blue and green photoluminescence from low‐resistivity CuAlS2 crystals, grown by chemical vapor transport and subsequently doped with Zn, is reported. The emission is strong even at room temperature, the color of emission being dependent on Zn‐doping conditions. Results obtained suggest that the CuAlS2 compound can be considered as a perspective material for blue and green light‐emitting device realization.

Auger electron spectroscopy studies of nitridation of the GaAs(001) surface

Auger electron spectroscopy has been used to investigate the processes taking place during the initial stages of nitridation of the As-stabilized GaAs(001)-2×4 surface by active nitrogen species generated by a radio-frequency plasma source. The results of analysis of the spectral shape of core-level Auger electron signals from Ga, As, and N, as well as dependencies of the intensities of those signals on the duration of nitridation combined with reflection high-energy electron diffraction results show that nitridation occurs in two distinct steps: the first step (with duration of only a few minutes) being the formation of 1 ML of nitrogen (partially mixed with arsenic) on the surface, and the second stage being the formation of the disordered GaAsN phase, which may be the GaAsxN1−x surface phase. The subsequent thermal annealing for several minutes at 600 °C leads to the desorption of arsenic and the resulting crystallization of the GaAsN phase into a cubic GaN layer of about 20 A thickness.

Nitridation of GaAs(001) surface: Auger electron spectroscopy and reflection high-energy electron diffraction

Auger electron spectroscopy (AES) and reflection high-energy electron diffraction (RHEED) have been used to investigate the processes taking place during the initial stages of nitridation of GaAs(001)–2×4 surface by active nitrogen species. The results of analysis of the spectral shapes and the spectral positions of the Auger electron signals from Ga, As, and N, as well as their dependencies on the nitrogen exposure combined with RHEED results show that the processes taking place during nitridation greatly differ depending on the nitridation temperature. On the one hand, at low temperatures (Ts⩽200 °C) nitridation is hindered by kinetic restrictions on the atomic migration, leading to island growth with formation of the disordered GaAsN phase in the subsurface region, which cannot be completely re-crystallized even after prolonged annealing. On the other hand, at high temperatures (Ts⩾500 °C) the process of nitridation takes place simultaneously with etching of the surface due to decomposition of the subs...

Photoluminescence Studies in CuAlS2 Crystals

Photoluminescence (PL) measurements have been carried out at low temperature (77 and 10 K) on CuAlS2 crystals grown by the chemical vapor transport method. Seven sharp PL lines have been observed near the band edge. Based on the photoreflectance measurements, the PL line at 3.550 eV has been assigned to a free exciton emission. The lines at 3.540, 3.532, 3.500 and 3.475 eV are tentatively assigned to the bound excitons, and they are discussed in terms of the crystal composition and the annealing conditions. This study also refers to the PL lines and peaks at about 2.9 eV.

Optical Absorption Spectra in CuAlS2 Doped with Vanadium

Optical absorption spectra were measured at room temperature in undoped and V-doped single crystals of CuAlS2 grown by the chemical vapour transport technique. Intense absorption bands characteristic of vanadium have been observed. These bands were attributed to the d-d type transitions between 3d-originated orbitals of V3+ ion as well as to the charge-transfer type transitions from the valence band of the host crystal to the 3d-shell orbitals of V3+ ion.

Pressure Induced Phase Transitions in Spinel and Wurtzite Phases of ZnAl2S4 Compound

ZnAl2S4 single crystals with spinel (α-phase) and wurtzite (w-phase) structures have been studied by Raman spectroscopy under hydrostatic pressures of up to 300 kbar. Significant changes in the phonon spectrum of the α-phase have been observed at the critical pressure of 230 kbar, which are attributed to a reversible phase transition to a denser high-pressure phase, having a similar structure to that of calcium ferrite. In the pressure interval of 180 to 230 kbar, the two phases coexist. The irreversible disappearance of the Raman signal of w-ZnAl2S4 doped by Cd at pressures above 90 kbar was attributed to a phase transition to a rocksalt-type structure. This critical pressure is 40 kbar lower than that in undoped w-ZnAl2S4 and is explained on the basis of crystal structure quality. Different structures were realized upon removing the pressure, depending on the highest pressure previously reached, such as a mixture of wurtzite and spinel phases, a spinel quasi-crystalline structure, or a pressure-induced amorphous phase. The behavior of the quasi-crystalline spinel structure upon repeating the pressure cycle was found to be different from that of the α-phase single crystal.

Optical Absorption of Co2+ in CuAlS2

The optical absorption spectra of CuAlS2:Co were studied at RT and 70 K in near IR and visible regions. The spectra exhibit two fine structured bands around 1450 nm and 750 nm, as well as a large red shift of the absorption edge of CuAlS2, which has been assumed to occur due to Co3+→Co2+ charge transfer type transitions. The fine structures are attributed to the transitions in the 3d-shell originated orbitals of Co2+ ion, and are analyzed in the framework of the crystal field theory in the first approximation.