Pt/NiO ring gate based Schottky diode hydrogen sensors with enhanced sensitivity and thermal stability

Abstract

This article reports on how an additional interfacial nickel oxide (NiO) influenced the hydrogen sensory performance of a Pt/AlGaN/GaN Schottky gate diode of a circular HEMT device. The oxide was formed by the oxidation of a thin Ni layer on top of the Pt/AlGaN gate interface at a high temperature. We propose that the oxide should provide a thermally stable interface, and its nanocrystalline surface morphology should be a favourable platform for the subsequent deposition of a catalytic Pt absorption layer on top. We analyzed the electrical properties of this interface and studied the sensory performance of such Schottky gate diode sensors to hydrogen versus the varied thickness of the NiO layer. It is shown that the parameters of the gate interface (barrier height, ideality factor) and the hydrogen sensory performance (sensitivity, time response) can be tuned with the thickness of the NiO interfacial layer. The diode exhibited a 60-fold increase in sensitivity for an optimized 20nm thick oxide interlayer compared with that of a conventional Pt/AlGaN/GaN gate diode sensor. This can be explained considering that hydrogen dissociated more efficiently at the nanocrystalline surface of the Pt/NiO stack. However, the transient characteristics of the sensors with NiO showed a longer response time due to a longer diffusion path for hydrogen. We show that the optimal operation temperature of the sensor shifted considerably from 250°C (reference Pt/AlGaN/GaN sensor) to as low as 50°C (sensor with the NiO layer). This temperature shift is desirable for battery-powered hydrogen sensors.