Epitaxy of highly optical efficient GaN on O and Zn face ZnO
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Epitaxy of highly optical efficient GaN on O and Zn face ZnO Xing Gu,1 Michael A. Reshchikov,1 Lei He,1 Ali Teke, 1,2 Feng Yun,1 Daniel K. Johnstone,1 Bill Nemeth,3 Jeff Nause,3 and Hadis Morkoç1 1 Department of Electrical Engineering, Virginia Commonwealth University, Richmond, Virginia 23220 2
Also with Balikesir University, Faculty of Art & Science, Department of Physics, 10100 Balikesir, Turkey
3
Cermet, Inc., Atlanta, GA 30318
ABSTRACT ZnO is a highly efficient photon emitter, has optical and piezoelectric properties that are attractive for a variety of applications. Due to its stacking order and close lattice to GaN, it is also considered as a substrate material for GaN epitaxy. In the past the poor preparation of ZnO surface has been a major handicap to GaN epitaxy. However, proper treatment we developed recently can make both faces of ZnO smooth with atomic level terraces. Epitaxy of GaN on O-face and Zn-face ZnO by reactive molecular beam epitaxy was performed. We used low-temperature RF growth of GaN buffer layer on ZnO surface to protect it from both ammonia and Ga. No Ga2ZnO4, an oxide with the spinel structure formed due to reaction of ZnO with Ga, was found, in contrast to earlier reports. The low-temperature photoluminescence (PL) indicates that both faces of ZnO can provide a high quality GaN with high radiative efficiency. In previous research it has been reported that O-face ZnO is slightly better for GaN epitaxy. Our new finding demonstrates that high-quality GaN epilayers can be grown on Zn face of ZnO.
INTRODUCTION Wide band-gap semiconductors such as those based on GaN and SiC have come to the forefront in the past decade because of an increasing need for short wavelength photonic devices and high-power, high-frequency electronic devices [1]. Another wide band-gap semiconductor, ZnO, has also received extensive attention recently as a candidate material for opto-electronic devices such as ultraviolet (UV) laser diodes and UV light emitting diodes, mainly due to its relatively large exciton binding energy (60 meV) compared with GaN (25 meV) and ZnSe (19 meV), which makes ZnO a highly efficient photon emitter. ZnO is also being considered as a promising substrate for GaN epitaxy because of its stacking order match and close lattice match (1.9% lattice mismatch). Moreover, ZnO can be wet chemically processed and removed easily. Furthermore, due to good conductivity of ZnO, contacts can be formed on both faces of the grown structure to reduce current crowding, which exacerbates the efficiency of high power light-emitting diodes and lasers [2,3]. Planar defects such as stacking mismatch boundaries (SMB), and inversion domain boundaries (IBD) are inevitable for GaN grown on non-wurtzite substrates such as sapphire and SiC [4].
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Due to the lack of large area and affordable GaN bulk substrates, alternative approaches such as ZnO, which is the only isomorphic substrate for GaN epitaxy, are being explored. Mixed success for GaN epitaxy on ZnO effort has already been noted in the pas
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