RF-MBE Growth of InN Dots on N-polar GaN Grown on Vicinal c-plane Sapphire
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RF-MBE Growth of InN Dots on N-polar GaN Grown on Vicinal c-plane Sapphire Naoki Hashimoto1, Naohiro Kikukawa1, Song-Bek Che1,2,3, Yoshihiro Ishitani1,2,3, and Akihiko Yoshikawa1,2,3 1 Department of Electronics and Mechanical Engineering, 2Center for Frontier Electronics and Photonics, and 3InN-project as a CREST program of JST, Chiba University 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
ABSTRACT We have grown InN quantum dots (QDs) on nitrogen-polarity (N-polarity) GaN under-layer by the radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE), and systematically investigated growth mechanism of the InN dots. The InN QDs with the N-polarity could be grown at about 500oC, which was much higher than that of previous reports on InN dots grown by MBE. When the nominal coverage of InN became more than 1 mono-layer (ML), lattice relaxation of InN occurred and high density InN dots were uniformly formed. These results indicated that InN dots were formed by Stranski-Krastanov (S-K) growth mode. For the InN deposition above about 8ML, InN dots tended to coalesce and resulted in remarakable decrease of the dots density.
INTRODUCTION Among the group III-nitrides, indium nitride (InN) is recently getting much attention due to its interesting properties: for instance, the small bandgap energy of InN, 0.6~0.8eV, which corresponds to the optical devices operating in near infrared region [1-5]. This indicate that InN-based III-nitrides are applicable to attractive photonic devices such as ultra high-speed optical modulator operating in the optical communication wavelength. In the development of the InN-based nano-structure devices, lattice mismatch with GaN or AlN would be a serious problem. The large lattice mismatch, over 10%, may result in the formation of misfit dislocations which significantly affect their epitaxy process and device performance. However, in the viewpoint of the quantum-dots (QDs) growth, it can be regarded as an advantage. The large lattice mismatch often induce three-dimentional growth of InN on GaN or AlN, which is a preferable growth condition for InN QDs by self-assembled growth mode. Up to the present, only a few papers have been reported on the crystal growth of InN dots by molecular beam epitaxy (MBE) and metal organic vapor phase epitaxy (MOVPE) [6-8]. However, growth temperatures of the InN dots by were below 400oC in the MBE. This is because these InN dots were grown on Ga-polarity GaN, i.e. the InN dots had In-polarity, resulting in low temperature growth. Generally, at low growth temperatures, it is difficult to obtain a high quality crystal and to grow high quality GaN or AlN cap layers at the same temperature. In this study, we grew InN QDs by paying strong attention on the growth with N-polarity. We have already reported the effects of the film polarity of InN on its epitaxy and found that the growth temperature of N-polarity InN (600oC) could be higher about 100oC than that of In-polarity, which resulted in remarkable improvement of InN crystalline quality [9,10].
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