Room-temperature-operated fast and reversible vertical-heterostructure-diode gas sensor composed of reduced graphene oxide and AlGaN/GaN

A vertical heterostructure diode (VHD) based on a van der Waals heterojunction between reduced graphene oxide (rGO) and Al_0.3Ga_0.7N/GaN/sapphire was fabricated for use in the chemical sensing of toxic gases. Target gases interacted with the atomically thin rGO layer, which served as a contact and sensing material; this interaction induced a change in the forward bias current of the VHD through modulation of the effective Schottky barrier height (SBH). The VHD gas sensor showed fast, repeatable, reproducible, recoverable, and stable room-temperature (RT)-operable gas-sensing performance for toxic gases, including nitrogen dioxide, sulfur dioxide, and ammonia. The variations of the SBH, ideality factor and series resistance of the VHD gas sensor upon gas exposure were systematically analyzed by studying the changes in the current transport mechanism through the vertical junction due to the presence of various gases. The analysis revealed that the variation of the SBH upon gas exposure is the primary sensing mechanism of the VHD gas sensor. The VHD device has great promise as the fundamental structure of simple, low-power, low-noise, and RT-operable chemical sensors.