GaN: From Selective Area to Epitaxial Lateral Overgrowth
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explored the spatially resolved optical properties at each ELO stage.11 In this paper, we explore the evolution of topography of GaN structures grown by SAE and subsequent ELO as a function of pattern geometry and growth parameters. We also discuss the origin of different emission bands of GaN through spatially resolved cathodoluminescence study.
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(b) (c) Figure 1 A schematic of three types of growth topography (cross section) for stripe patterns (a) SAE with vertical sidewalls. (b) ELO with vertical sidewalls (c) SAE with triangular cross section.
EXPERIMENTAL The GaN growth was carried out in a vertical configuration atmospheric pressure MOCVD reactor. TMGa and NH3 were used as Ga and N precursors, respectively. H2 was used as a carrier gas. A buffer layer grown at ~ 550°C was deposited on (0001) sapphire substrate before the growth of GaN epi-layer. Lithographically defined patterns were formed either on a 3 µm thick GaN epilayer or directly on a GaN buffer layer. Patterns studied include stripes (50 – 120 µm) spaced by 350 µm, arrays of narrow stripes (2- 6 µm wide) spaced by 2 – 10 µm between stripes and by 350 µm between stripe arrays. A cross structure (25 x 80 µm) is also studied in this report. The stripes were oriented either perpendicular or parallel to the sapphire (1120) flat. Under our typical growth condition, the lateral growth rate for these two directions differ by a factor of 1.5. The patterned samples were then heated under NH3 in the MOCVD chamber to ~1020°C for the growth of the GaN overlayer. A Zeiss SEM equipped with Oxford MonoCL setup was used for morphology and optical characterization. All cathodoluminescence (CL) spectra and images were taken at room temperature. A DI nanoscope III AFM was used for topography measurements. Details of the growth condition and the CL setup have been described previously.10,12 RESULTS AND DISCUSSION 1. Topography Evolution 1.1 Topography Evolution with stripe width We have grown GaN wide stripes (25 – 120 µm) spaced by 350 µm on a 3 µm thick GaN epilayer. Figure 2 shows an AFM image of a 75 µm wide stripe. Growth rate enhancement is a common phenomenon in selective area epitaxy, due to the lateral diffusion of source materials from the masked to the open area. The nominal growth thickness in this case is 0.2 µm. The enhancement factor is in the range of 5 – 10 from the stripe center to the edge. Under the specified experimental condition, 5 µm-wide plateaus are observed at the edges. As previously reported, the width of the terraces do not appear to change with stripe width and presumably is limited by the surface diffusion length of the Ga species.10 Note that wide stripes grown directly
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