Influence of current conduction paths and native defects on gas sensing properties of polar and non-polar GaN

Abstract

The CO gas sensing characteristics of polar GaN (P–GaN) and non-polar GaN (NP–GaN) thin-films grown by RF–plasma assisted molecular beam epitaxy on c–Al2O3 and r–Al2O3 substrate is analyzed. The temperature-dependent (27–300 °C) current-voltage (I–V) measurements were performed for both (P–GaN & NP–GaN) Schottky barrier diodes having identical device dimensions with Au as the metal contact. The Schottky barrier height increases with the increment in temperature, while vice versa was perceived for the ideality factor and series resistance. The I–V characteristics dictated that the terminal current for P–GaN is 25 times that of NP-GaN, further corroborated by simulation results. The I-V curve fitting suggests the initial emission of charge carrier from a trapped state to a continuum of electronic state, following the Frenkel-Poole emission model for P-GaN. The NP–GaN demonstrated a higher surface-to-volume ratio and native (shallow and deep level) defects corresponding to VGa and ON as compared to the P–GaN. The sensing response obtained for the NP–GaN for 100 ppm CO gas at 300 °C is ~ 33%, which is about seven times the response in the case of P–GaN. The role of native defects and the potentiality of the fabricated NP–GaN over P–GaN films in providing a better sensing characteristic by reducing the current conduction paths are elaborated.