Abstract
We report on the polarity control of GaN regrown on pulsed-laser-deposition-grown N-polar AlN on a metalorganic-vapor-phase-epitaxy-grown Ga-polar GaN template. The polarity of the regrown GaN, which was confirmed using aqueous KOH solutions, can be inverted from that of AlN by inserting a low-temperature GaN (LT-GaN) buffer layer. We hypothetically ascribe the Ga-polarity selection of GaN on the LT-GaN buffer layer to the mixed polarity of LT-GaN grains and higher growth rate of the Ga-polar grain, which covers up the N-polar grain during the initial stage of the high-temperature growth. The X-ray rocking curve analysis revealed that the edge-dislocation density in the N-polar regrown GaN is 5 to 8 times smaller than that in the Ga-polar regrown GaN. N-polar GaN grows directly on N-polar AlN at higher temperatures. Therefore, nucleus islands grow larger than those of LT-GaN and the area fraction of coalescence boundaries between islands, where edge dislocations emerge, becomes smaller.
Introduction
ZnO has been proposed as a novel substrate for group-III nitride semiconductors because of a small lattice mismatch and the same crystal structure.1–3) Up to now, bulk crystals of ZnO have been successfully grown by the same hydrothermal method as used for the growth of quartz, and its diameter has reached 3 inches.4) There exist only a few candidates for substrates with close lattice-matching to group-III nitride semiconductors. Because of these situations, the growth of group-III nitride semiconductors of high quality on a ZnO substrate and device applications have been strongly desired to open the window to next-generation devices. Most of the devices based on group-III nitride semiconductors, including commercially available light-emitting diodes, and laser diodes have been grown by metalorganic vapor-phase epitaxy (MOVPE) because of its several advantages such as easy control of composition, high uniformity, and large throughput. However, the MOVPE growth of group-III nitride semiconductors on a ZnO substrate suffered from the peeling-off of grown films and the severe diffusion of Zn and O into group-III nitride semiconductors layers owing to the chemical and thermal instabilities of ZnO.5–7) To realize reproducible growth, an adhesive layer between ZnO and nitride semiconductors, and a stopper layer that suppresses the diffusion of Zn and O into the epitaxial growth layer must be developed. We have already reported GaN growth on a ZnO layer without the diffusion of Zn and O through a ZnAl2O4 spinel interlayer.8) This ZnAl2O4layer was formed by a simple evaporation of Al and a solid-phase reaction between the ZnO substrate and the evaporated Al layer. It was difficult to control the crystallographic orientation of GaN on the ZnAl2O4 layer because this layer was polycrystalline. In this study, we propose to use an AlN layer epitaxially grown at low temperatures (LT-AlN) as a protection layer that is resistant to the harsh MOVPE environment. For the AlN growth, pulsed laser deposition (PLD) method is chosen since the epitaxial growth of AlN/ZnO of high quality has already been demonstrated.9) The growth conditions for GaN on an AlN layer are optimized, and the polarity and dislocation density of GaN, which are important issues in the account of the epitaxial growth of GaN, are discussed.10)
Source:iopscience
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