(Color online) Atomic force microscope images of van Hoof epitaxial structures grown (a) on GaN/sapphire and (b) on high-pressure bulk GaN substrates. On the GaN/sapphire substrate the dislocation density is found to be 4 × 108 cm−2. The small pits where the dislocations exit on the surface are marked with arrows. No pits are visible on the structure grown on bulk HP GaN.
(Color online) Room temperature contactless electroreflectance spectra for as-grown (dashed) and etched (red, solid line) c-plane GaN van Hoof structures of various thicknesses of undoped GaN layers for (a) the first series grown on GaN/sapphire and (b) the second series grown on high-pressure bulk GaN.
Analysis of the built-in electric field in the undoped GaN layer for van Hoof structures for (a) the first series and (b) the second series. Open and solid symbols represent results for the as-grown surface and surfaces etched in Piranha solution, respectively.
Built-in field determined from FKO analysis for both series of van Hoof samples. The experimental data for as-grown (open squares) and etched (solid squares) samples with the best fits from the function F = Φ/d (dashed and solid lines). The insets show the schematic of the electric field distribution in the GaN cap layer of the van Hoof structure. The results are those obtained for (a) the first series and (b) the second series.
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