Epitaxial Lateral Overgrowth of GaN on SiC and Sapphire Substrates
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Department of Physics, North Carolina State University, Raleigh NC 27695, [email protected] Department of Materials Science and Engineering, North Carolina State University, Raleigh NC 27695 *** Cree Research Inc, 4600 Silicon Drive, Durham NC 27703 Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, G4.3 (1999)
ABSTRACT The epitaxial lateral overgrowth (ELO) process for GaN has been studied using SiC and sapphire substrates. Both MBE and MOVPE growth processes were employed in the study. The use of SiO2 versus SiNx insulator stripes was investigated using window/stripe widths ranging from 2 µm/4 µm to 3 µm/15 µm. GaN film depositions were completed at temperatures ranging from 800 oC to 1120 oC. Characterization experiments included RHEED, TEM, SEM and cathodolumenescence studies. The MBE growth experiments produced polycrystalline GaN over the insulator stripes even at deposition temperatures as high as 990 oC. In contrast, MOVPE growth produced single-crystal GaN stripes with no observable threading dislocations. INTRODUCTION Growth of GaN and other III-V nitrides on mismatched substrates such as sapphire or SiC produces a columnar material consisting of many small hexagonal grains [1]. The individual grains have a distribution of tilt and rotation within the GaN film that gives rise to very large dislocation densities and broad x-ray rocking curves (300-400 arc sec). In spite of this high degree of disorder, there have been demonstrations of very bright blue and green LEDs and blue/violet laser diodes at Nichia Chemical [2-5] and elsewhere [6-9]. These light-emitting devices, all prepared by MOVPE, exhibit dislocation densities of 109-1010 per cm2 but function as very bright light emitters as though these internal disruptions to periodicity are virtually absent – perhaps due to some unknown passivation process associated with the MOVPE growth process itself. Recently, however, there have been demonstrations of defect reduction in GaN layers grown on sapphire [10-16] and SiC [17-19] using an epitaxial lateral overgrowth (ELO) technique. The ELO technique is illustrated schematically in Fig. 2. First, a high quality 1-2 µm thick layer of GaN is grown by MOVPE. Next, standard deposition and photolithographic techniques are employed to prepare a set of parallel SiO2 stripes oriented along a [1 1 00] GaN crystal direction separated by window areas which expose the underlying GaN. A second deposition of GaN onto this patterned substrate is then initiated. During the initial GaN growth, the SiO2 stripes function as non-wettable surfaces and no GaN deposition occurs on them. However, once the GaN film growth from the window stripes reaches the tops of the SiO2 stripes, epitaxial lateral overgrowth of GaN commences. The ELO process produces stripes of GaN (5–10 µm wide) over the original SiO2 stripes which display a remarkable reduction in dislocation density to ≤106 per cm2. It is on these low-dislocation-density single-crystal GaN stripes that Nichia Chemical has
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