Growth and characterization of semi-polar (11-22) GaN with in-situ SiN x interlayers
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Growth and characterization of semi-polar (11-22) GaN with in-situ SiNx interlayers Jonathan Hollander, Clifford McAleese, Menno Kappers, and Colin Humphreys Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, United Kingdom
ABSTRACT Semi-polar (112¯ 2) GaN films were grown on (1¯ 100) m-plane sapphire substrates. Growth demonstrated surface striations aligned perpendicular to the in-plane GaN m-axis. SiNx interlayers were incorporated into the as-grown films with the purpose of decreasing the density of defects in the material. Inclusion of interlayers increased the characteristic length of surface striations and feature size. X-ray rocking curves widths are shown to be correlated to specific threading dislocation geometry. Skew-symmetric omega scan peak broadening suggests a decrease in the proportion of screw-type dislocations to edge-type dislocations with increasing number of interlayers. INTRODUCTION The majority of commercial optoelectronic GaN devices are grown with respect to the cplane, which has a [0001] surface normal. While these devices are the easiest to produce with repeatable results, they are influenced by spontaneous and piezoelectric polarization effects at interfaces within the active layers, decreasing device efficiency and luminescent output due to the quantum-confined Stark effect (QCSE). Films orientated with the growth surface tilted away from the basal plane have decreased polar characteristic. It has been shown that the radiative recombination lifetimes are longer in quantum wells on polar, c-plane GaN than non-polar GaN films [1]. These results signify that the effects of charge segregation are circumvented in such structures. Such studies have prompt further investigation into the manner of GaN growth on a variety of available sapphire substrates with different orientations to produce films of semi-polar characteristic. Conceivably, such orientations should also mitigate the QCSE [2]. Semi-polar (112¯ 2) GaN has been grown on m-plane (1¯ 100) sapphire by way of hydride vapor-phase epitaxy (HVPE) [3] and by metal-organic vapor-phase epitaxy (MOVPE) [4]. The epitaxial relationship between the film and the substrate is [1¯ 100] GaN || [112¯ 0] sapphire in the plane of the wafer, as indicated by the schematic representation in Figure 1. The GaN [112¯ 0] a-axis is tilted 32 ˚ away from the growth direction toward the sapphire [0001] c-axis. As-grown films have a high (~1010 cm-2) density of threading dislocations. A technique which has been shown to lower the threading dislocation density of c-plane GaN films involves a two-step process: initial growth under conditions resulting in roughening of the surface, and then coalescing with lateral growth. The first step will promote the formation of faceted islands, while the second step takes advantage of lateral growth from inclined side facets. Threading dislocations, which propagate along the growth direction in c-plane GaN, bend over during lateral growth of island side facets and annih
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