Evolution of defect states in C2H2-doped GaN:C studied by optical deep-level transient spectroscopy

Carbon-doped GaN (GaN:C) is widely used as a semi-insulating buffer layer in GaN-based power devices. Although the electrical properties of GaN:C grown by carbon autodoping have been extensively studied, the influence of the carbon incorporation method remains unclear. In this study, we investigate whether the conventional assumption of discrete, quasi-isolated carbon-related deep levels remains sufficient to describe defect states when carbon is introduced via an external hydrocarbon source. GaN:C layers grown using C2H2 gas were characterized by current–voltage (I–V), capacitance–voltage (C–V), and optical deep-level transient spectroscopy (ODLTS). The samples exhibit a resistive and effectively fully depleted state at high carbon concentrations. ODLTS measurements reveal an evolution of trap responses from quasi-isolated states at ∼1017 cm−3 to broad and distributed responses at ∼1019 cm−3. These results indicate that the carbon incorporation pathway influences the nature of electrically active defect states. By comparing our results with previous reports on autodoped GaN:C, this study clarifies the distinct evolution of defect states induced by C2H2 doping. These findings provide insight into the realization of stable semi-insulating GaN buffer layers for power device applications.