M. Benamara

Evolution of deep centers in GaN grown by hydride vapor phase epitaxy

Evolution of deep centers in GaN grown by hydride vapor phase epitaxy Z-Q. Fang and D.C. Look Semiconductor Research Center, Wright State University, Dayton, Ohio 45435 J. Jasinski, M. Benamara, and Z Liliental-Weber Lawrence Berkeley National Laboratory, Berkeley, California 94720 R.J. Molnar Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Massachusetts Abstract Deep centers and dislocation densities in undoped n GaN, grown by hydride vapor phase epitaxy (HVPE), were characterized as a function of the layer thickness by deep level transient spectroscopy and transmission electron microscopy, respectively. As the layer thickness decreases, the variety and concentration of deep centers increase, in conjunction with the increase of dislocation density. Based on comparison with electron- irradiation induced centers, some dominant centers in HVPE GaN are identified as possible point defects.

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...

Electron Beam and Optical Depth Profiling of Quasibulk GaN

Electron beam and optical depth profiling of thick (5.5–64 μm) quasibulk n-type GaN samples, grown by hydride vapor-phase epitaxy, were carried out using electron beam induced current (EBIC), microphotoluminescence (PL), and transmission electron microscopy (TEM). The minority carrier diffusion length, L, was found to increase linearly from 0.25 μm, at a distance of about 5 μm from the GaN/sapphire interface, to 0.63 μm at the GaN surface, for a 36-μm-thick sample. The increase in L was accompanied by a corresponding increase in PL band-to-band radiative transition intensity as a function of distance from the GaN/sapphire interface. We attribute the latter changes in PL intensity and minority carrier diffusion length to a reduced carrier mobility and lifetime at the interface, due to scattering at threading dislocations. The results of EBIC and PL measurements are in good agreement with the values for dislocation density obtained using TEM.

Correlations between spatially resolved Raman shifts and dislocation density in GaN films

Spatially resolved Raman spectra were measured on thick GaN samples with known dislocation density grown by hydride vapor phase epitaxy. The frequencies of the E2 (high) and E1 (transverse optical) phonons shift to lower wave number over a distance of 30 μm from the sapphire substrate/GaN interface. The shifts are linearly correlated with the dislocation density suggesting that the strain due to the lattice mismatch at the interface determines both quantities.

Reduction of threading dislocation density in GaN using an intermediate temperature interlayer

GaN thin films with a reduced threading dislocation density have been produced by organometallic vapor phase epitaxy using an intermediate temperature interlayer. A description of the growth process is presented with characterization results. Reduction of the dislocation density was obtained by insertion of a single thin interlayer grown at an intermediate temperature after the initial growth at high temperature. A large percentage of the threading dislocations present in the first GaN epilayer are found to bend in the interlayer and do not propagate in the top layer grown at higher temperature in a lateral growth mode.

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.

Influence of dopants on defect formation in GaN

Influence of p-dopants (Mg and Be) on the structure of GaN has been studied using Transmission Electron Microscopy (TEM). Bulk GaN:Mg and GaN:Be crystals grown by a high pressure and high temperature process and GaN:Mg grown by metal-organic chemical-vapor deposition (MOCVD) have been studied. Structural dependence on growth polarity was observed in the bulk crystals. Spontaneous ordering in bulk GaN:Mg on c-plane (formation of Mg-rich planar defects with characteristics of inversion domains) was observed for growth in the N to Ga polar direction (N polarity). On the opposite site of the crystal (growth in the Ga to N polar direction) Mg-rich pyramidal defects empty inside (pinholes) were observed. Both these defects were also observed in MOCVD grown crystals. Pyramidal defects were also observed in the bulk GaN:Be crystals.

Atomic Scale Analysis of InGaN Multi-Quantum Wells

InGaN multiquantum wells grown by MOCVD on GaN have been investigated by transmission electron microscopy techniques and numerical analysis of high resolution (HREM) images. One objective of this research was to correlate the atomic structure and emission mechanisms of InGAN quantum well. The studied layers contained 13% or 20% In. It was shown that GaN/InGaN interfaces are rather rough and exhibit an oscillating contrast. Structural defects were found on these interfaces. The relative c-lattice parameter variation in the well was determined using numerical processing of HREM images. The lattice spacings appear to be larger than that expected from Vegards law suggesting the presence of a biaxial strain. Further observations also revealed a redistribution of In within the well. Instead of a continuous In-rich layer, quantum dots were often observed along the well with a regular spacing. The formation of these In-rich dots was not intended and their presence suggests either a periodic modulation of strain along the well or In-rich cluster formation.

Tem Study of Defects in Laterally Overgrown GaN Layers

Transmission electron microscopy was applied to study defects in laterally overgrown GaN layers, with initial growth on A1 2 O 3 substrates followed by further growth over SiO 2 masks. Dislocations found in the overgrown areas show changes in line direction. Most dislocations propagate along c-planes. In the overgrown material planar defects (faulted loops) are present on c-planes and their presence is most probably related to segregation of excess point defects and impurities present in this material. They appear to be initiated by the fast lateral growth. Some dislocations with screw orientation become helical resulting from climb motion. Formation of voids and also a high dislocation density was observed at the boundaries where two overgrowing fronts meet. Tilt and twist components were observed for these boundaries that were different for different overgrown strips grown in the same crystallographic direction suggesting that the GaN subgrain orientations on the two sides of a SiO 2 mask are responsible for the final tilt and twist value.