Studies of Metal/Gallium Nitride Gas Sensors: Sensing Response, Morphology and Sensing Applications

Reliable gas sensors with excellent sensitivity and robustness are important for the development of advanced technological applications while ensuring a safe environment in both industrial and household security. The chemically and mechanically robust gallium nitride (GaN) is a promising semiconductor for these important applications, especially for use at high temperatures and in extreme environments.

When a metal is in contact with a semiconductor surface, a space charge region and Schottky barrier are formed on the semiconductor side. In this thesis, the sensing response of Pt and GaN to gaseous H2 and CO and the dependence of the response on Pt and GaN surface morphologies are explored. The sensing opportunities are expanded when GaN is decorated with Ag and the structure is used for small molecule analysis using surface enhanced Raman scattering (SERS).

Combining the high surface area of nanoporous GaN with Pt nanoparticles deposited by electroless chemical deposition, the sensing performance of the well-known H-mediated Schottky barrier based on the Pt/GaN sensor is studied. The H2 sensing performance of, as defined by the limit of detection (LOD), Pt-decorated porous GaN measured by AC four-point probe resistance measurements is more than an order of magnitude better than planar GaN sensors based on the same Pt/GaN Schottky barrier height concept. The potential utility of high surface area porous GaN was realized by decorating the confined nanopores with metal (Pt), thus increasing the surface area available for sensing and lowering the LOD.

Pt/GaN structures can also be used to detect CO at high temperature. The CO sensing response is also dependent on the Pt morphology. For continuous films, CO signal increases as the thickness of the metal film decreases. In discontinuous Pt films, increasing Pt surface area also increases the CO signal when the Pt/GaN interfacial area remains constant. A model is proposed, in which the influence of the adsorbed CO on Pt surface is inversely proportional to the distance of the surface dipoles to the polarization sites at the Pt/GaN interface. The model provides good qualitative match to the experimental data.

The high surface area porous GaN can also be decorated with Ag through electroless deposition to form a three-dimensional Ag-coated porous GaN substrate for SERS experiments. Due to the optical transparency of GaN in the visible spectrum and the high surface area available for the Ag nanoparticles, the intensities of the Raman signal from the Ag-coated porous GaN are two orders of magnitude higher than Ag-coated planar GaN substrate.

In summary, the work conveyed here demonstrates that the low-cost and ability to modulate the deposited metal morphology and coverage in confined nanopores offered by electroless chemical deposition, significant advantages to large surface area nanoporous GaN for hydrogen sensing, CO detection, and SERS studies. The CO response on the Pt/GaN is consistent with the sensing model developed for Pt/GaN structures, which reveals that CO sensing is strongly dependent on the surface area and thickness of the metal.