Soumendra N. Basu

PHASE SEPARATION AND ORDERING IN INGAN ALLOYS GROWN BY MOLECULAR BEAM EPITAXY

In this study, we investigated phase separation and long-range atomic ordering phenomena in InGaN alloys produced by molecular beam epitaxy. Films grown at substrate temperatures of 700–750 °C with indium concentration higher than 35% showed phase separation, in good agreement with thermodynamic predictions for spinodal decomposition. Films grown at lower substrate temperatures (650–675 °C) revealed compositional inhomogeneity when the indium content was larger than 25%. These films, upon annealing to 725 °C, underwent phase separation, similar to those grown at the same temperature. The InGaN films also exhibited long-range atomic ordering. The ordering parameter was found to increase with the growth rate of the films, consistent with the notion that ordering is induced at the growth surface. The ordered phase was found to be stable up to annealing temperatures of 725 °C. A competition between ordering and phase separation has been observed, suggesting that the driving force for both phenomena is lattice...

Oxidation behavior of WC-Co

Abstract The high temperature oxidation behavior of WC-Co samples has been investigated. The oxidation rate of WC-Co was found to increase rapidly with temperature above 600 °C and with the oxygen content of the atmosphere. Increasing the Co content led to an improvement in the oxidation behavior. In all cases, only WO3 and CoWO4 phases were present in the oxide. Although the morphology of the two oxide phases depended on the oxidation conditions, their volume fractions were determined solely by the Co content of the sample. The WO3 formed during the initial stages of oxidation, changed from a strong {{001}} texture at 600 °C to a weak {{200}} texture at 800 °C. As the flow rate of the oxidizing gas was increased, the oxidation rate of WC-Co increased at lower flow rates due to an increase in the oxygen supply to the oxide/sample interface. However, at higher flow rates, the oxidation rates decreased with increasing flow rate.

Electroluminescence from silicon-rich nitride/silicon superlattice structures

Luminescent silicon-rich nitride/silicon superlattice structures (SRN/Si-SLs) with different silicon concentrations were fabricated by direct magnetron cosputtering deposition. Rapid thermal annealing at 700 °C resulted in the nucleation of small amorphous Si clusters that emit at 800 nm under both optical and electrical excitations. The electrical transport mechanism and the electroluminescence (EL) of SRN/Si-SLs have been investigated. Devices with low turn-on voltage (6 V) have been demonstrated and the EL mechanism has been attributed to bipolar recombination of electron-hole pairs at Si nanoclusters. Our results demonstrate that amorphous Si clusters in SRN/Si-SLs provide a promising route for the fabrication of Si-compatible optical devices.

MICROSTRUCTURES OF GAN FILMS DEPOSITED ON (001) AND (111) SI SUBSTRATES USING ELECTRON CYCLOTRON RESONANCE ASSISTED-MOLECULAR BEAM EPITAXY

The microstructures of GaN films, grown on (001) and (111) Si substrates by a two-step method using Electron Cyclotron Resonance assisted-Molecular Beam Epitaxy (ECR-MBE), were studied by electron microscopy techniques. Films grown on (001) Si had a predominantly zinc-blende structure. The GaN buffer layer, grown in the first deposition step, accommodated the 17% lattice mismatch between the film and substrate by a combination of misoriented domains and misfit dislocations. Beyond the buffer layer, the film consisted of highly oriented domains separated by inversion domain boundaries, with a substantial decrease in the defect density away from the interface. The majority of defects in the film were stacking faults, microtwins, and localized regions having the wurtzite structure. The structure of the GaN films grown on (111) Si was found to be primarily wurtzite, with a substantial fraction of twinned zinc-blende phase. Occasional wurtzite grains, misoriented by a 30°twist along the [0001] axis, were also observed. A substantial diffusion of Si was seen in films grown on both substrates.

Sensitized erbium emission from silicon-rich nitride/silicon superlattice structures

Erbium-doped silicon-rich nitride/silicon superlattice structures were fabricated by direct magnetron cosputtering deposition on Si substrates. Rapid thermal annealing resulted in the nucleation of small amorphous Si clusters, which efficiently sensitize 1.54μm emission via a nanosecond-fast nonresonant energy transfer process, providing an alternative route toward the fabrication of Si-compatible devices based on Er sensitization.

Epitaxial growth of gallium nitride thin films on A-Plane sapphire by molecular beam epitaxy

In this article, we propose a crystallographic model to describe epitaxy of GaN on (1120) sapphire (A plane). The (1102) cleavage plane in sapphire is shown to extend to the GaN lattice as the (1120) plane, facilitating the formation of cleaved facets. It is shown that, although the lattice mismatch is much smaller than in the case of epitaxy on (0001), the difference in the planar symmetry in this case results in high-strained bonds near the interface. The use of nitridation and a low temperature buffer is therefore necessary. A systematic study of GaN growth on the A-plane sapphire by plasma-assisted molecular beam epitaxy was carried out to study the effects of plasma nitridation of the substrate and the growth of a low temperature GaN buffer on the structure and optoelectronic properties of the films. Transmission electron microscopy (TEM) studies indicate that films grown on substrates which were not nitridated prior to growth have a significant fraction of zinc-blende domains and poor orientation...

Energy transfer and 1.54 μm emission in amorphous silicon nitride films

Er-doped amorphous silicon nitride films with various Si concentrations (Er:SiNx) were fabricated by reactive magnetron cosputtering followed by thermal annealing. The effects of Si concentrations and annealing temperatures were investigated in relation to Er emission and excitation processes. Efficient excitation of Er ions was demonstrated within a broad energy spectrum and attributed to disorder-induced localized transitions in amorphous Er:SiNx. A systematic optimization of the 1.54 μm emission was performed and a fundamental trade-off was discovered between Er excitation and emission efficiency due to excess Si incorporation. These results provide an alternative approach for the engineering of sensitized Si-based light sources and lasers.

Domain structure in chemically ordered InxGa1−xN alloys grown by molecular beam epitaxy

Observation of long range atomic ordering in InGaN films grown by molecular beam epitaxy on A-plane sapphire is reported, based on x-ray diffraction and transmission electron microscopy studies. The InGaN films have a domain structure, with alternating domains of ordered and disordered phases, close to the film/substrate interface. Closer to the film surface, disordered domains are no longer observed. The degree of ordering was found to increase with growth rate (at the low growth rates used in these materials), which is consistent with ordering being a surface phenomenon.

A TEM study of microstructures of YBa2Cu3O7−x thin films deposited on LaAlO3 by laser ablation

The microstructures of YBa{sub 2}Cu{sub 3}O{sub 7{minus}}{sub {ital x}} thin films deposited by laser ablation on single crystal (001) LaAlO{sub 3} substrates have been investigated. The orientation of the YBa{sub 2}Cu{sub 3}O{sub 7{minus}}{sub {ital x}} layer next to the interface is found to be completely {ital c}-perpendicular, with a high degree of epitaxy between the film and the substrate. Misfit dislocations, with a periodic spacing of around 13 nm, are present at the interface. Two distinct interfacial structures are seen in these films. At a film thickness of around 400 nm, nucleation of {ital c}-parallel grains occurs, leading to a switchover from a {ital c}-perpendicular to a {ital c}-parallel microstructure. Amorphous particulates, ejected from the target during processing, lead to the formation of misoriented grains, giving rise to high-angle grain boundaries in the film.

Structure and high-temperature stability of compositionally graded CVD mullite coatings

Abstract Dense, uniform and crack-free mullite (3Al 2 O 3 ·2SiO 2 ) coatings were deposited on SiC by chemical vapor deposition. The coatings were compositionally graded, with the Al/Si ratio increasing towards the outer surface of the coatings for improved corrosion resistance. The coatings were found to start out as a nanocrystalline layer, which is an intimate mixture of γ-Al 2 O 3 nanocrystallites imbedded in a vitreous silica-rich matrix at the substrate/coating interface. Mullite grains nucleated when the surface composition of the growing coating was in a narrow range close to that of stoichiometric mullite. The phase transformations occurring in these coatings during high-temperature anneals in the range 1100–1400 °C were studied. These phase transformations, which include a tetragonal-to-orthorhombic transformation, mullitization and devitrification of silica in the nanocrystalline layer, and α-alumina precipitation and twinning of the alumina-rich mullite, are discussed in light of the adhesion and corrosion resistance of the coatings.