In Situ Raman Analysis of a Bulk GaN-Based Schottky Rectifier Under Operation
We have fabricated vertical Schottky rectifiers based on a free-standing GaN substrate and have measured the temperature of the device under operation in situ using micro-Raman spectroscopy. The n-type bulk GaN wafer with 500 mu m thickness was prepared using hydride vapor-phase epitaxy. The carrier concentration of the wafer was similar to 2.4 x 10(16) cm(-3). Semitransparent Ni and multilayered Ti/Al/Pt/Au were used to make a Schottky and a full backside ohmic contact, respectively. In this investigation, Raman spectra were collected as a function of the forward power applied to the Schottky diode. A systematic shift and broadening of the Raman E(2) peak were observed as a function of increasing bias. This was caused by device heating due to the increase in current as the forward bias was increased. It was demonstrated that micro-Raman spectroscopy can serve as an excellent in situ diagnostic tool for analyzing thermal characteristics of the GaN Schottky diode. Moreover, the strain caused by the piezoelectric effect was calculated to lead to a shift of the Raman peak at the level of 0.001 cm(-1). This confirmed that the observed Raman peak shift was predominantly produced by a thermal not piezoelectric effect.