Structural Properties of Laterally Overgrown GaN
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ABSTRACT Structural properties of epitaxially laterally overgrown (ELO) GaN on patterned GaN 'substrates' by hydride vapor phase epitaxy (HVPE) have been investigated. The epitaxially lateral overgrowth of GaN on SiO 2 areas is realized and a planar ELO GaN film is obtained. Scanning electron microscope, transmission electron microscope (TEM) and atomic force microscope (AFM) are used to study the structure and surface morphology of the ELO GaN materials. AFM images indicate that no observable step termination is detected over a 4 µm2 area in the ELO region. TEM observations indicate that the dislocation density is very low in the ELO region. No void at the coalescence interface is observed. Lattice bending as high as 3.3o is observed and attributed to pileup of threading dislocations coming from the underlying GaN “seeding layer” and tilting horizontally and quenching at the coalescence interface. I.
INTRODUCTION
GaN and related compound materials have been intensively studied due to their important practical applications and promising for potential applications [1-3]. The existence of high density threading dislocations in GaN films is one of major factors to impede further development of GaN optoelectronic and electronic devices [4]. So far the most successful and practical technique for reducing the threading dislocation (TD) density of GaN epilayers is the epitaxial lateral overgrowth (ELO). The TD density of the GaN ELO region could be 4 or more orders lower than the conventionally grown GaN films [5]. Both metalorganic vapor phase epitaxy (MOVPE)[6-9] and hydride vapor phase epitaxy (HVPE)[10-11] have been used to produce GaN ELO materials with high crystal quality. HVPE is a very useful technique to grow GaN films via both large-area epitaxy and ELO. In HVPE ELO GaN materials, no observable void is found in the coalescence interface and a variety of growth front geometries are observed [10]. Compared to a non-halide process, a halide process principally offers an advantage of a high lateral-to-vertical growth rate ratio [12], which is a key factor influencing the growth front and related dislocation motion in the GaN epilayer grown by ELO. HVPE offers a high growth rate and high material quality for GaN growth [13,14]. The typical growth rate can get 100-200 µm/hr. GaN produced by the HVPE technique does result in a greatly reduced intensity of the defect-based luminescence referred to as the yellow band (YL) when compared to the trimethyl gallium (TMG)-based MOVPE materials. The ability to grow thick GaN film with high quality and obtaining low TD density in ELO GaN films make it possible to get high quality self-standing GaN wafer in a single HVPE
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growth chamber. In this paper the structural properties of HVPE ELO GaN films, especially coalesced flat-top ELO GaN film
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