W. Swider

Breakdown of crystallinity in low‐temperature‐grown GaAs layers

A systematic study of the change in structural quality of as‐grown GaAs layers deposited at temperatures between 180 and 210 °C by molecular beam epitaxy was performed using transmission electron microscopy, double‐crystal x‐ray rocking curves, and particle‐induced x‐ray emission. We found that the crystal quality was correlated strongly with growth temperature near 200 °C. The lattice parameter and the amount of As incorporated in the layer were observed to increase at lower growth temperatures. After exceeding a certain growth‐temperature‐dependent layer thickness, large densities of pyramidal‐type defects are formed, which at lowest growth temperature result in the breakdown of crystallinity and in columnar polycrystalline growth. The lattice expansion is ascribed to the excess As in the layers. The mechanisms of breakdown of crystallinity are discussed.

Mg-doped GaN: Similar defects in bulk crystals and layers grown on Al2O3 by metal–organic chemical-vapor deposition

Defects were observed in GaN:Mg grown on sapphire substrates using metal–organic chemical-vapor deposition (MOCVD) with Mg-delta doping similar to those previously observed in bulk GaN:Mg grown from Ga solution under high hydrostatic pressure of nitrogen. Pyramidal defects (pinholes) with (1100) hexagonal facets on the (0001) base plane and six {1122} side facets, and defects with a rectangular shape also delineated by planar facets on the basal (0001) planes, were observed for growth with Ga polarity for both of these very different growth methods. The Mg dopant is apparently responsible for their formation since the oxygen concentration in the MOCVD-grown samples was orders of magnitude lower than in the bulk samples. Mg segregation on these planes apparently does not allow uniform continuous growth on these planes leading to these hollow defects. Some defects in the heterolayers also develop into longer nanotubes elongated along the c axis. Change of polarity from Ga to N followed by a change back to...

Crystal structure of κ-In2Se3

Structural properties of single-phase films of {kappa}-In{sub 2}Se{sub 3} and {gamma}-In{sub 2}Se{sub 3} were investigated. Both films were polycrystalline but their microstructure differed considerably. The a-lattice parameter of {kappa}-In{sub 2}Se{sub 3} has been measured. Comparison between these two materials indicates that {kappa}-In{sub 2}Se{sub 3} has a significantly larger unit cell ({Delta}c = 2.5 {+-} 0.2 % and {Delta}a = 13.5 {+-} 0.5%) and a structure more similar to the {alpha}-phase of In{sub 2}Se{sub 3}.

Structural defects in heteroepitaxial and homoepitaxial GaN

The microstructure and characteristic defects of heteroepitaxial GaN films grown on sapphire using molecular beam epitaxy (MBE) and metal-organic-chemical-vapor-deposition (MOCVD) methods and of homoepitaxial GaN grown on bulk substrates are described based on transmission electron microscopy (TEM), x-ray diffraction, and cathodoluminescence (CL) studies. The difference in arrangement of dislocations along grain boundaries and die influence of buffer layers on the quality of epitaxial films is described. The structural quality of GaN epilayers is compared to diat of bulk GaN crystals grown from dilute solution of atomic nitrogen in liquid gallium. The full width at half maximum (FWHM) of the x-ray rocking curves for these crystals was in the range of 20–30 arc sec, whereas for the heteroepitaxially grown GaN the FWHM was in the range of 5–20 arc min. Homoepitaxial MBE grown films had FWHMs of about 40 arc sec. The best film quality was obtained for homoepitaxial films grown using MOCVD; these samples were almost free from extended defects. For the bulk GaN crystals a substantial difference in crystal perfection was observed for the opposite sides of the plates shaped normal to the c direction. On one side the surface was almost atomically flat, and the underlying material was free of any extended structural defects, while the other side was rough, with a high density of planar defects. This difference was related to the polarity of the crystal. A large difference in crystal stoichiometry was also observed within different sublayers of the crystals. Based on convergent beam electron diffraction and cathodoluminescence, it is proposed that Ga N antisite defects are related to the yellow luminescence observed in these crystals.

Structural characterization of low‐temperature molecular beam epitaxial In0.52Al0.48As/InP heterolayers

A systematic study of the structural quality and arsenic content of as‐grown In0.52Al0.48As/InP layers deposited on InP by molecular beam epitaxy at temperatures between 150 and 450 °C was performed using transmission electron microscopy and particle‐induced x‐ray emission. We found that the amount of As incorporated in the layers generally increases with decreasing growth temperature, with the crystalline quality of the layers being good at growth temperatures higher than 200 °C. At 150 °C, a large density of pyramidal defects is formed, the defects are related to the very large amount of excess As incorporated into the layer. The mechanisms leading to the formation of these defects are discussed. At 200 °C, however, the amount of excess As is lower than expected, and wavy streaks of diffuse scattering are seen in electron diffraction. It is shown that small ordered domains of the CuPt type on the group III atoms are responsible for these features.

Nano-tubes in GaN

Gallium nitride is one of the promising semiconductors for laser diodes for the blue and UV wavelength regions. Even though recent progress has greatly improved the crystal quality of GaN films grown on various substrates, its practical application is still hampered by the high defect density. Formation of vertical hollow nano-tubes in GaN grown on different substrates using different growth methods is described. These defects are shown to be of several different types, some related to threading dislocations, but others originating at tubular inversion domains, or crystal inhomogeneities. Kinetic mechanism based on slow growth rate on polar (0{bar 1}11) surfaces is proposed to explain the origin of these defects.

Study of high-quality GaN grown by OMVPE using an intermediate layer

Abstract The crystalline quality of heteroepitaxial GaN can be significantly improved if the threading dislocations originating from the interface are not allowed to reach the layer surface where they can propagate to the active device areas and act as harmful defects. Incorporation of an intermediate layer grown at low temperature has been shown to limit this defect propagation. This method has been proved to be effective but several interlayers have been required in order to reach dislocation density lower than 109/cm2. In this priority communication, we report on high-quality GaN layers grown with the use of only one intermediate layer. The defect analysis shows that the density of dislocation is only 8×107/cm2, compared to over 1010/cm2 for layers grown without the intermediate layer. Electron microscopy on cross-section samples shows that deposition under certain low-temperature conditions directly benefits the quality of the subsequently deposited GaN layer. The growth of the GaN top layer appears to be similar to growth observed for lateral epitaxial overgrowth layers. This observation opens the possibility for using standard GaN growth methods to achieve a dislocation density comparable to that achieved with lateral overgrowth epitaxy.

Effect of Si Doping on The Structure of Gan

The influence of Si doping on the structure of GaN grown by metal-organic chemicalvapor deposition (MOCVD) has been studied using transmission electron microscopy (TEM), x-ray diffraction and Raman spectroscopy. Undoped and low Si doped samples were compared with samples of increased dopant concentration. In addition, defect reduction due to different buffer layers (AIN and GaN) is discussed. Silicon doping improves surface morphology and influences threading dislocation arrangement. High doping leads to a more random distribution of dislocations. Based on this study it appears (for the same dopant concentration) that an AIN buffer layer can significantly reduce the number of threading dislocations, leaving the samples more strained. However, no significant reduction of threading dislocations could be observed in the samples with GaN buffer layer. These samples are the least strained.

Structural analysis of interfacial layers in Ti/Ta/Al ohmic contacts to n-AlGaN

Detailed structure of the interfacial layers of Ti/Ta/Al ohmic contacts to n-type AlGaN/GaN/sapphire are investigated by means of transmission electron microscopy. High-resolution electron microscopy (HREM), optical diffractograms, and computer simulations confirmed that TiN (∼10.0 nm) and Ti3AlN (∼1.4 nm) interfacial layers form at the interface between the Ti layer and the Al0.35Ga0.65N substrate by a solid state reaction during annealing for 3 min in N2 at 950 °C. The orientation relationship between Ti3AlN and Al0.35Ga0.65N was found to be: [011]Ti3AlN∥[2110]Al0.35Ga0.65N and (111)Ti3AlN∥(0001)Al0.35Ga0.65N. The cubic Ti3AlN interfacial layer has a lattice parameter of 0.411±0.003 nm with the space group Pm3m matching that of Al0.35Ga0.65N. A model of the atomic configurations of the Ti3AlN/Al0.35Ga0.65N interface is proposed. This model is supported by a good match between the simulated and the experimental HREM image of the Ti3AlN/Al0.35Ga0.65N interface. The formation of TiN and Ti3AlN interfaci...

Interfacial defects and morphology of InGaAs epitaxial layers grown on tilted GaAs substrates

Systematic transmission electron microscopy studies of In0.2Ga0.8As layers grown by molecular‐beam epitaxy on [001] GaAs substrates and on substrates tilted up to 10° toward [110], [120], [100], [110], and [120] are described. Three different layer thicknesses were investigated: 6, 20, and 40 nm. In the 40 nm layers misfit dislocations were formed which partially relaxed the elastic strain. Three‐dimensional growth with an indication of microscale In segregation (dendritic growth) occurred for substrates tilted toward [110] and [120] directions. Smooth growth surfaces were observed when the substrates were tilted toward [110] and [120] showing that the chemistry of the predominant surface growth steps can play an important role in the growth mode and quality of the epitaxial layer. Indium segregation to misfit dislocation cores, the presence of In‐rich platelets near the interface, and diffuseness of the interface are considered as evidence for short range migration of In during and after growth.