Development of Gallium Nitride Substrates

Prominent progress has been made in nitride semiconductor since high bright blue LED has developed in 1993. It has also expanded to an industry after applied to white LED. These LEDs are produced by the epitaxial growth of nitride semiconductor layers on sapphire (a Al2O3) substrate. On the other hand, recording density in optical disks has increased from CD in 1980s to DVD in latter 1990s. Laser diode is used for reading and recording data in optical disks. For the higher recording density, the shorter wavelength of laser is required. Infra-red laser of 780 nm made of AlGaAs epitaxial layers on GaAs substrate is used for CD, whereas red laser of 650 nm made of AlGaInP epitaxial layers is used for DVD. Furthermore, for a higher recording optical disk which can memorize two-hour long Hi-Vision movie, violet laser diode with wavelength 405 nm had been required. InGaN nitride semiconductor epitaxial layers are used for violet laser diode. In the early stage of the research for violet laser diode, sapphire substrates were used as same as LEDs. However, it turned to be difficult to use sapphire substrates for the high quality violet laser diodes. One of the main reasons is the formidable difficulties in obtaining excellent cleavage surface for resonator mirror by using sapphire substrates. This is because cleavage surface of epitaxial layers and sapphire do not match. Another reason is the numerous crystal defects (line defects called dislocation) which exist in gallium nitride (GaN) epitaxial layers. These dislocations are generated in GaN crystal from the interface of GaN and sapphire because of the 16% mismatch in crystal constants between GaN and sapphire. They cause no problem for the application to LED. However, they are not acceptable for the laser diode which needs much higher current density for operation because of short lifetime due to the high density of dislocations. In order to solve these issues and to realize violet laser diode, GaN single crystal substrate with large size and high quality is proved to be indispensable. At that time, although research for GaN bulk crystal under a high temperature and a high pressure had been done by Polish group, only thin and a small sized GaN crystal was obtained. The industrialization of GaN substrates was thought to be difficult. Under these situations, Sumitomo Electric Industries, Ltd. had started research and development for GaN substrates. At last Sumitomo Electric had succeeded in the development of GaN substrates which meet all the requirements for the application of violet laser diodes. This paper reviews the results in the early stage of the R&D at Sumitomo Electric.