Study of the Epitaxial Lateral Overgrowth (ELO) Process for GaN on Sapphire Using Scanning Electron Microscopy and Monoc
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Internet Journal Nitride Semiconductor Research
Study of the Epitaxial Lateral Overgrowth (ELO) Process for GaN on Sapphire Using Scanning Electron Microscopy and Monochromatic Cathodoluminescence Zhonghai Yu1, M.A.L. Johnson1, T. Mcnulty1, J.D. Brown1, J.W. Cook,Jr1 and J.F. Schetzina1 1Department
of Physics, North Carolina State University,
(Received Monday, February 23, 1998; accepted Tuesday, March 24, 1998)
Growth of GaN by MOVPE on mismatched substrates such as sapphire or SiC produces a columnar material consisting of many hexagonal grains ~1 µm across. In contrast, the epitaxial-lateralovergrowth (ELO) process creates a new material — single-crystal GaN. We have studied the ELO process using GaN/sapphire layers patterned with SiO2 stripes. SEM images show that the (0001) GaN surface remains very flat as the ELO progresses. Cathodoluminescence images at 590 nm reveal spotty yellow-green emission from the columnar GaN as it emerges from the window areas. Very bright 590 nm emission occurs as the ELO process begins. We associate this deep-level cathodoluminescence with the strain field that accompanies the conversion of columnar GaN into single-crystal GaN via the ELO process. As the ELO process continues across the SiO2 stripes, the 590 nm emission disappears and is replaced with pure band edge cathodoluminescence at 365 nm which is maintained until coalescence of adjacent ELO layers occurs near the centers of the SiO2 stripes.
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Introduction
The development of III-V nitride materials and devices has suffered from the lack of availability of low-dislocation-density, lattice-matched native nitride substrates for device synthesis by MOVPE or MBE. As a consequence, 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, as is illustrated in Figure 1, which gives rise to very large dislocation densities. In spite of this, there have been demonstrations of very bright LEDs and laser diodes at Nichia Chemical [2] [3] [4] [5] and elsewhere [6] [7] [8] [9]. These light-emitting devices, all prepared by MOVPE, show dislocation densities of 109-1010 per cm2 but function as 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] [11] [12] [13] and SiC [14] [15] [16] using an epitaxial lateral overgrowth (ELO) technique. The ELO technique is illustrated schematically in Figure 2. The ELO process produces stripes of GaN (5–10 µm wide) with a remarkable reduction in dislocation density to about 104 per cm2 or less. It is on these low-dislocation-density single-crystal GaN stripes that Nichia Chemical has produced laser diodes having very long CW lifetimes (10,000 hrs) [17]. In this pape
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