Anomalous capacitance effects in GaN/Al$_{0.026}$Ga$_{0.974}$N structures*

The effects of interface defect states on the capacitance characteristics of an $n^{+}${\-}GaN/Al$_{0.026}$Ga$_{0.974}$N/$i${\-}GaN/$n^{+}${\-}GaN structure are reported. An anomalous high-frequency capacitance peak was observed in the capacitance-frequency (C-f) profiles. Using IR spectroscopy, the defect related absorption centers with activation energies of 125~$\pm $~1 and 139~$\pm $~2~meV were attributed to C-donor/N-vacancy and Si-donor states pinned to the $n^{+}${\-}GaN layer, respectively. Si defect states at the $i${\-}GaN/$n^{+}${\-}GaN interface were found to produce the high-frequency capacitance peak. The peak can result from resonance scattering due to the hybridization of localized Si-donor states in the band gap (with electronic levels above the conduction band minimum) and continuous conduction band states at the $i${\-}GaN/$n^{+}${\-}GaN interface.