Patterning of sapphire substrates via a solid state conversion process
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Nanopatterned sapphire substrates offer the potential for improved performance of devices based on III-V nitrides, e.g., light-emitting diodes and laser diodes. Due to the chemical stability and hardness of sapphire, however, surface patterning is a time-consuming and expensive process. Therefore, a novel method was utilized, whereby a surface coating of Al was deposited on a sapphire substrate and patterned into an array of square mesas using e-beam lithography. The lateral dimensions of each mesa were approximately 400 × 400 nm, and the average height was approximately 100 nm. The metallic film was subsequently subjected to an oxidation treatment at 450 °C for 24 h (a heat treatment which had previously been shown to minimize hillock formation). For the second heat treatment, which is necessary to induce migration of the sapphire interface and hence achieve solid state conversion, a range of temperatures (800–1350 °C) was explored. Results showed that for a heat-treatment time of 1 h, pattern retention was achieved for annealing temperatures less than or equal to 1250 °C. Successful epitaxial conversion of the patterned mesas to sapphire was confirmed using electron backscatter diffraction.
I. INTRODUCTION
Sapphire is widely used as a substrate material for light-emitting diode (LED) and laser diode (LD) applications using III-V nitrides because of its physical robustness and high-temperature stability.1,2 Following from the pioneering work of Akasaki and his group,3,4 today nitride-based LED and LD devices have extensive applications in full-color electroluminescent displays, and high-efficiency light sources for high-density information storage on magnetic and optical media.5,6 Commercially, these devices are typically fabricated by metalorganic chemical vapor deposition (MOCVD) growth of a series of nitride based layers on a sapphire substrate. When GaN is epitaxially grown on sapphire, studies have shown that the resulting optical device performance is highly dependent on the quality of the substrate surface finish.7–10 Furthermore, line defects that limit performance are generated due to the large differences in the thermal expansion coefficient and lattice constant between the nitride layer and the sapphire substrate. The threading dislocation density of conventional GaN-based LEDs and LDs on a polished sapphire substrate ranges from 108–1012 cm−2 in the GaN epitaxial layer.11 For optimal device performance of nitride-based LEDs and DOI: 10.1557/JMR.2005.0050 J. Mater. Res., Vol. 20, No. 2, Feb 2005
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LDs it is necessary to further reduce the threading dislocation density. Selective lateral growth methods such as LEO (lateral epitaxial overgrowth)9,12–15 and PE (pendeo-epitaxy)16,17 have been demonstrated to achieve lower dislocation densities, and improved operating characteristics and lifetimes in laser devices.9,14 These techniques have several variations, but the underlying principle is that growth of an initial GaN layer is interrupted, and the l
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