Miniaturized hydrogen- and hydrocarbon- gas sensors, heretofore often consisting of Schottky diodes based on catalytic metal in contact with SiC, can be improved by incorporating palladium oxide (PdOx, where 0≤x≤1) between the catalytic metal and the SiC.

In prior such sensors in which the catalytic metal was the alloy PdCr, diffusion and the consequent formation of oxides and silicides of Pd and Cr during operation at high temperature were observed to cause loss of sensitivity. However, it was also observed that any PdOx layers that formed and remained at PdCr/SiC interfaces acted as barriers to diffusion, preventing further deterioration by preventing the subsequent formation of metal silicides.

In the present improvement, the lesson learned from these observations is applied by placing PdOx at the catalyticmetal/ SiC interfaces in a controlled and uniform manner to form stable diffusion barriers that prevent formation of metal silicides. A major advantage of PdOx over other candidate diffusionbarrier materials is that PdOx is a highly stable oxide that can be incorporated into gas-sensor structures by use of deposition techniques that are standard in the semiconductor industry.

The PdOx layer can be used in a gas sensor structure for improved sensor stability, while maintaining sensitivity. For example, in proof-of-concept experiments, Pt/PdOx/SiC Schottky-diode gas sensors were fabricated and tested. The fabrication process included controlled sputter deposition of PdOx to a thickness of ≈50 Å on a 400- µm-thick SiC substrate, followed by deposition of Pt to a thickness of ≈450 Å on the PdOx. The SiC substrate (400 microns in thickness) was patterned with photoresist and a Schottky-diode photomask. A lift-off process completed the definition of the Schottky-diode pattern.

The sensors were tested by measuring changes in forward currents at a bias potential of 1 V during exposure to H2 in N2 at temperatures ranging from 450 to 600 °C for more than 750 hours. The sensors were found to be stable after a break-in time of nearly 200 hours. The sensors exhibited high sensitivity: sensor currents changed by factors ranging from 300 to 800 when the gas was changed from pure N2 to 0.5 percent H2 in N2. The high sensitivity and stability of these Pt/PdOx/SiC sensors were found to represent a marked improvement over comparable Pt/SiC sensors. Moreover, surface analysis showed that there was no significant formation of silicides in the Pt/PdOx/SiC sensors.

This work was done by Gary W. Hunter and Jennifer C. Xu of Glenn Research Center and Dorothy Lukco of QSS Group, Inc. For further information, access the Technical Support Package (TSP) free online at under the Physical Sciences category.

Inquiries concerning rights for the commercial use of this invention should be addressed to:

NASA Glenn Research Center
Innovative Partnerships Office
Attn: Steve Fedor
Mail Stop 4–8
21000 Brookpark Road
Cleveland, Ohio 44135.

Refer to LEW-17859-1.

NASA Tech Briefs Magazine

This article first appeared in the April, 2006 issue of NASA Tech Briefs Magazine.

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