GaN: From Selective Area to Epitaxial Lateral Overgrowth
- PDF / 2,375,856 Bytes
- 6 Pages / 417.6 x 639 pts Page_size
- 4 Downloads / 247 Views
explored the spatially resolved optical properties at each ELO stage." 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. (a)
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 NH 3 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 4.tm thick GaN epilayer or directly on a GaN buffer layer. Patterns studied include stripes (50 120 ltm) spaced by 350 l.tm, arrays of narrow stripes (2- 6 Itm wide) spaced by 2 - 10 xtm between stripes and by 350 ltm between stripe arrays. A cross structure (25 x 80 [tm) 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 NH 3 in the MOCVD chamber to -1 020'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 topography10measurements. Details of the growth condition and the CL setup have been described 12 previously. ' RESULTS AND DISCUSSION 1. Tovo2raphy Evolution 1.1 Topography Evolution with stripe width We have grown GaN wide stripes (25 - 120 ptm) spaced by 350 pm on a 3 [tm thick GaN epilayer. Figure 2 shows an AFM image of a 75 ýtm 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 [tm. The enhancement factor is in the range of 5 - 10 from the stripe center to the edge. Under the specified experimental condition, 5 [tm-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 on a low-temperature-grown buffer layer do not form wide plateaus, probably because of the smaller diffusion length due to rough surface.
-
l
,Figure
2AFM image of a75 jam GaN stipe grown on a 3 pm GaN epilayer.
Figure 3 shows a SEM image of a cross structure and an AFM image of part of the cross. As in the case of stripes, plateaus are observed at the ed
Data Loading...
