Silicon Carbide Interfaces for Chemical Sensing

Ruby Ghosh, Brage Golding and Roger Tobin
Pollution control and combustion efficiency for automobiles and power plants require high temperature gas sensors to detect hydrocarbons and nitrogen oxides. Devices based on the wide bandgap semiconductor, SiC, can operate up to 900° C in these chemically reactive environments.
The photograph shows an array of catalytic gate SiC devices, fabricated in the Keck Microfabrication Facility, mounted on a header prior to high temperature gas measurements. The (1cm)2 blue/green square is the SiC substrate with 200 to 1000 µm diameter Pt capacitor dots. Gold wire bonds connect the Pt capacitors to the gold pads on the underlying alumina header. Not visible in the photograph are the electrical feedthroughs for connecting the capacitors to the testing equipment.
A cross sectional view (not to scale) of our catalytic gate field-effect SiC device is shown on the right. At temperatures above 400° C the refractory metal gate (Pt) dehydrogenates long chain hydrocarbons, the resultant hydrogen is then detected as a shift in the device potential. We are using these structures to investigate the chemical and electronic properties of the metal/oxide and oxide/semiconductor interfaces in complex gas environments at elevated temperatures.
A unique measurement system allows us to perform capacitance-voltage (C-V) spectroscopy in-situ in a controlled gas environment, this is in contrast to traditional ex-situ measurements performed after time/temperature/gas cycling. Shown on the left is a high frequency C-V curve of our Pt/SiO2/SiC device in a hydrogen ambient at 530° C. Changing the ambient gas shifts the flat band voltage (sensor signal) as shown. We are using C-V spectroscopy to study how the interface states at the oxide-semiconductor interface effect sensor performance.

Reference:
"Profiling of the SiO2/SiC Interface Using X-ray Photoelectron Spectroscopy",
R. N. Ghosh, S. Ezhilvalavan, B. Golding, S. M. Mukhopadhyay, N. Mahadev, P. Joshi, M. K. Das and J. A. Cooper, Jr. Mat. Res. Soc. Symp. 640, H3.7.1 (2001).