To accurately analyze the deep states at the insulator/wide-bandgap semiconductor interface, this study reassessed and improved the conventional photoassisted capacitance-voltage (PACV) method. First, as previously pointed out, the illumination time under depletion should be long enough that the voltage shift caused by interface-state depopulation (in n-type semiconductors) saturates. Excessive illumination, however, causes insulator charging, thereby apparently increasing estimated values. To solve this problem, this study proposes to measure reference characteristics just after postillumination ones. Secondly, the postillumination measurements should be started without delay after turning off the light or may be carried out with the samples being illuminated. Thirdly, the depletion should be deep enough that the magnitude of band bending in the substrate at the beginning of the postillumination measurements is larger than 1 V. This guideline considerably relaxes a previous one that required a band bending of bandgap or larger. Furthermore, this study developed a method for compensating the interface-state depopulation (in n-type) during the reference measurements, in which the depopulation causes the so-called stretch-out. The results thus obtained from an Al/Al2O3/GaN capacitor agreed fairly well with those by a recently developed transient photoassisted capacitance method, supporting the validity of both methods. Being less sensitive to the gate-insulator charging, the advanced PACV method developed here has an advantage over the transient method and, therefore, will help advance the technology for fabricating high-performance, high-reliability insulator/wide-bandgap semiconductor insulators.
ASJC Scopus subject areas