Methods of Analyzing Deep Levels in GaN

Abstract

Analytical methods for investigating deep levels in GaN are reviewed. Quantitative accuracy of deep-level transient spectroscopy (DLTS) technology is discussed. The isothermal method allows a quick measurement and is useful to characterize trap states for the case that a temperature scan causes reaction of defects or the occupancy condition for each filling pulse needs to be kept constant. The DLTS measurement for hole traps in p-type GaN needs special considerations due to the relatively large ionization energy of magnesium acceptors, where the low-frequency DLTS system is utilized. For investigation of minority carrier traps, DLTS using injection bias pulses for p-n junctions is the most reliable method for the quantitative evaluation. DLTS methods using light filling pulses such as minority carrier transient spectroscopy and optical DLTS are applied for samples without p-n junctions, which needs considerations of light absorption, the minority carrier diffusion length, and the optical capture cross sections of deep levels. Investigation of deep levels far from both band edges requires the approaches without the use of thermal emission process. For this aim, this chapter overviews deep-level optical spectroscopy, photoluminescence involving a time-resolved method, positron annihilation spectroscopy, and an analysis of recombination current in a p-n junction.