Gas Sensitive Porous Silver-Rutile High-Temperature Schottky Diode on Thermally Oxidized Titanium

The fabrication of porous silver-${rm TiO}_{2}$ gas sensitive Schottky diodes by utilizing thermally grown oxide layer on titanium substrates is described. The junction was formed by the partial sintering of silver particles on the oxidized metal substrate. The connection between titanium metal and its native oxide is ohmic and the substrate performs as a stable back-contact to the oxide semiconductor. For the characterization of the device, electrical contacts were made by connecting silver wires to the titanium substrate and the silver aggregate. Operating at elevated temperatures, the device behaved as a Schottky diode of high junction energy barrier in clean air, while in highly reducing atmospheres the barrier height dropped to zero and the junction was characterized as an ohmic contact. This reversible transition afforded a broad dynamic range and high sensitivity for chemical detection: Operating at 300 $~^{circ}$C, the reverse current of the diode increased by six orders of magnitude in response to the presence of 1 w-% 1-butanol vapor in the surrounding air, and a single diode was able to detect hydrogen in the 50 ppm to 8% concentration range. The observed electronic features of the device were described through a model constructed based on the work function of silver varying from 4.3 to $sim$6.5 eV with oxygen adsorption.