GaN for short-wavelength light emitting devices: growth kinetics and techniques

Abstract

In the recent year, great steps have been achieved in the development of blue-green semiconductor diode lasers and LEDs, however material issues in the wide-bandgap semiconductors still limit practical device applications. The II-VI compounds ZnSe and ZnS and the III-V semiconductors GaN and AIN are the two material system being investigated for green to ultra- violet light emitting devices, with most research to date concentrating on the II-VI materials. Problems with electromigration, heating, and subsequent processing in early ZnSe/ZnS laser diodes has led many researchers to investigate the more robust material GaN for commercially viable short wavelength LEDs and laser diodes. GaN blue LEDs at room temperature have been demonstrated by Akasaki et al. [1], but p-type doping levels are not yet sufficiently high for laser diodes. The main obstacle in fabricating high quality GaN for device applications has been controlling background impurity and dopant concentrations during crystal growth and finding suitable substrates with closely matching lattice and thermal expansion coefficients. The dilemma in obtaining p-type conduction in GaN is hypothesized to be due to a large concentration of nitrogen vacancies[2] acting as shallow donors making compensation difficult. Aside from Akasaki's and Nakamura's work in Japan [1,13], all GaN material by any growth techniques has been n-type. Advances in the last few years, have produced undoped material with a reduced concentration using Metal Organic Chemical Vapor Deposition (MOCVD) (4x1016 cm-3) [3] and plasma/ion-assisted Molecular Beam Epitaxy (MBE) (8 x 1013 cm-3)[4].