A Tem Study of the Microstructural Evolution of MBE-Grown GaN
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RESULTS Nucleation and growth of GaN on (001) GaAs RHEED patterns taken from the surface of nitrided (001) GaAs (specimens MG154 and MG155) show rather diffuse spots which indicate the presence of only one phase, the spacings of which index as cubic GaN (fig. 1). The pattern does not correspond to GaAs, since the lattice parameters of GaN and Such RHEED GaAs differ by some 20%. patterns therefore demonstrate that a thin layer of cubic GaN has formed on the substrate surface during nitridation (ie before 'growth' of GaN), presumably with N displacing As from the substrate. For a smooth surface, the RHEED penetration depth is only a few monolayers, Fig. 1: RHEED pattern from MG155, which indexes despite the 100kV electron beam energy, and no as GaN. contribution from the GaAs substrate could be detected here. Diffraction from a plan view TEM specimen of a nominally 36A thick GaN layer grown on (001) GaAs (fig. 2) confirms that the growing zinc-blende GaN adopts a cube-cube relationship with the substrate. Measurement of this pattern indicates that at this stage, the GaN is already fully relaxed. Significant streaking of the GaN spots can be seen, which in this projection appears to run parallel to the two orthogonal and directions (significantly, the diffraction has clear four-fold not two-fold symmetry). Tilting of the specimen away from the [001] orientation increases the contrast from these streaks, indicating that they are in fact inclined. The streaking is thus attributable to the presence of inclined {11 1} and {I 1 ) faults. Such faulting is clearly seen both in fringe contrast in plan view images (fig. 3) and when the specimen is viewed in cross-section (figs 4 and 5). Figure 3 is a dark field image of the same TEM foil as in figure 2, taken using a GaN spot, and it reveals the presence of individual islands of GaN of dimension approximately 10nm. The variation in contrast from island to island shows that they vary significantly in their orientation. The GaN coverage at this stage is almost completely continuous, as witnessed by the continuity of moir6 fringes, and confirmed by cross-sectional observations (figs 4 and 5).
Fig. 2: Diffraction pattern from a plan view specimen Fig. 3: Dark field image of a plan view specimen of of NA239, nominally 36A of GaN on (001) GaAs. NA239. In addition to very fine moir6 fringes, whose spacing indicates complete GaN relaxation, broader GaN H GaAs substrate. fringes are seen which are due to inclined faults. 89
Fig. 4: Dark field image from a cross-sectional Fig. 6: Cross-section of NA243 (nominally 250oA GaN specimen of NA239 (nominally 36A of CaN on (001) on (001) GaMs), dark field image, showing inclined GaAs) showing a near continu
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