Surface-charge Lithography for the Fabrication of Gallium Nitride Based Gas Sensors

he development of electronic gas sensors capable of operation in extreme environmental conditions (notably high temperature and chemically aggressive ambients) is an area of considerable interest. The outstanding stability of Gallium Nitride (GaN) together with a remarkable progress in GaN device technologies in recent years makes this material an ideal candidate for these applications. Surface-adsorption-induced conductivity change has been previously proposed to be responsible for the transducing mechanism of GaN resistive gas sensors. Therefore, to improve the sensitivity of these devices an enhanced surface to volume ratio has to be achieved compared to a bulk or thin film topology. This is possible by integration of micro- or nanostructure into a GaN sensor. Here we demonstrate the use of novel surface-charge lithography (SCL) for the controlled fabrication of nanostructures from n-type GaN, for gas sensing applications. Surface-charge lithography combines focused Ga ion-beam (FIB) treatment with photoelectrochemical (PEC) etching. This quick and maskless approach directly writes patterns of negative surface charge, which are shielded from etching when the sample is placed in KOH (0.01-0.1M) solution and exposed to UV optical excitation from a mercury lamp. Regions which have not been exposed to the ion beam are quickly etched due to the oxidising action of the photogenerated holes and electrolyte. Thanks the FIB instrument’s superior resolution it is possible to etch structures with lateral dimensions as small as ~100nm using this process, limited by the surface depletion region width in our samples. Resistive hydrogen gas sensors are fabricated by depositing a pair of Ti/Al/Ti/Au ohmic contacts about 200 µm apart onto an n-type ~4 µm thick epitaxial GaN layer (Si doped; 1.2x1018 cm-3) grown by molecular organic chemical vapour deposition on a sapphire substrate. We report on resistive gas sensing devices consisting of various types of electrically connected GaN nano-wires, fabricated in a controlled manner using the SCL method. These devices do not rely on the thermally unstable catalytic Schottky gate contacts, therefore have a potential to be operated at relatively high temperatures (up to 500-700°C).