Epitaxial GaN Layer Growth Using Nitrogen Enriched TiN Buffer Layers
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0916-DD04-06
Epitaxial GaN Layer Growth Using Nitrogen Enriched TiN Buffer Layers Kazuhiro Ito1, Yu Uchida1, Sang-jin Lee1, Susumu Tsukimoto1, Yuhei Ikemoto2, Koji Hirata2, Naoki Shibata2, and Masanori Murakami1 1 Department of Materials Science and Engineering, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto, 606-8501, Japan 2 Optoelectronics Division, Toyoda Gosei Co., Ltd., Inazawa, Aichi, 490-1312, Japan
ABSTRACT About 20 years ago, the discovery of an AlN buffer layer lead to the breakthrough in epitaxial growth of GaN layers with mirror-like surface, using a metal organic chemical vapor deposition (MOCVD) technique on sapphire substrates. Since then, extensive efforts have been continued to develop a conductive buffer layer/substrate for MOCVD-grown GaN layers to improve light emission of GaN light-emitting diodes. In the present study, we produced MOCVD-grown, continuous, flat epitaxial GaN layers on nitrogen enriched TiN buffer layers with the upper limit of the nitrogen content of TiN deposited at room temperature (RT) on sapphire substrates. It was concluded that the nitrogen enrichment would reduce significantly the TiN/GaN interfacial energy. The RT deposition of the TiN buffer layers suppresses their grain growth during the nitrogen enrichment and the grain size refining must increase nucleation site of GaN. In addition, threading dislocation density in the GaN layers grown on TiN was much lower than that in the GaN layers grown on AlN.
INTRODUCTION GaN-based semiconductors with wide-band gaps have been used as the key elements for blue light-emitting diodes (LEDs) and laser diodes. Since bulk GaN was not commercially available, non-conductive sapphires were extensively used as substrates for GaN layers grown by a metal organic chemical vapor deposition (MOCVD) technique. A lattice mismatch of about 15% between GaN and sapphire means a buffer layer is needed for lateral growth of GaN on the sapphire substrate [1]. Until now, the best buffer layer discovered was an AlN layer deposited at relatively low temperatures, and the use of this layer made it possible for growth of GaN with mirror surface [2]. Although a lattice mismatch of about 2.5% exists between GaN and AlN, the AlN buffer layers are believed to enhance GaN nucleation and growth due to a relatively low AlN/GaN interface energy [3]. However, conductive substrates are desirable to increase the light emitting efficiency of the diodes [4]. Thus, GaN growth on the conductive substrates such as Si, GaAs, and SiC has been studied extensively, and conductive SiC was used as substrate in commercial GaN LEDs. However, the cost of the SiC wafers is much higher than that of sapphire wafers, an undesirable situation from a viewpoint of mass production. As the first step to achieve our goal to develop conductive substrates with relatively low cost, we ventured development of conductive buffer layers for epitaxial GaN layer growth.
We recently reported that epitaxial, continuous, flat GaN layers were successfully grown on a metallic TiN buffer layer
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