Improved silicon carbide-based Schottky diodes are undergoing development for use in detecting hydrocarbon (CxHy) and nitrogen oxide (NOx) gases at high temperatures. In comparison with gas sensors of other types, Schottky-diode gas sensors exhibit relatively high sensitivity -- an advantage for monitoring engine-exhaust emissions and hazardous gases emitted in low concentrations. SiC-based Schottky diodes can function at temperatures too high for Si-based devices; beyond the obvious advantage of a wider operating-temperature range, this opens the possibility of incorporating into sensors materials that react with CxHy and NOxat high temperatures in ways that enhance sensitivity, selectivity, and/or stability.
The simplest Schottky-diode gas sensor is a metal/semiconductor (MS) device; it includes an electrode made of a catalytic metal (in this case, Pd) deposited on a semiconductor (in this case, SiC). For hydrogen and hydrocarbons, the gas dissociates on the exposed surface of the metal and diffuses to the metal/semiconductor or metal/insulator interface, forming a dipole layer that changes the electronic properties of the Schottky diode in proportion to the amount of the gas.
The SiC-based Schottky-diode gas sensors that are the subjects of the present development efforts are metal/reactive-insulator/semiconductor (MRIS) devices. In contrast to standard metal/insulator/semiconductor (MIS) structures, the insulations in a MRIS structure are chosen for their reactivity to the gas of interest. In the fabrication of an MRIS device, an electrically insulating layer of metal oxide (for example, SnO2) is incorporated between the catalytic electrode and the SiC semiconductor layer to obtain the advantageous high-temperature characteristics mentioned above. More specifically:
- Sensitivity is increased because the CxHyor NOx gas reacts with the reactive oxide insulating layer as well as with the catalytic metal;
- The sensor can be made more stable in that the oxide layer can act as a barrier between the metal and the SiC; and
- The selectivity of the sensor can be varied by varying the oxide layer.
In preparation for comparative experiments, both Pd/SiC and Pd/SnO2/SiC Schottky diodes were fabricated on the same chip. The two diodes were the same except that Pd/SnO2/SiC diodes included sputter-deposited layers of SnO2 about 50 Å thick.
The current-vs.-voltage responses of these diodes were measured during exposures to air and to a mixture of 400 parts per million (ppm) H2 in N2, at a temperature of 350 °C. The results of these measurements (see Figure 1) on the Pd/SnO2/SiC diode show parallel shunt resistance at potentials below 1.0 V; proceeding upward from 1.0 V, the data show exponential Schottky behavior at first, with series resistance increasing and beginning to dominate at the high end of the voltage range. Analysis of the exponential portion of the curve leads to the conclusion that the SnO2layer increased the Schottky barrier height. The effect of the 400 ppm H2in N2 was to increase the current at a given voltage. One of the effects of the SnO2 layer was to impart higher sensitivity to H2in the Schottky-like conduction region. The Pd/SiC diode behaved differently, exhibiting an exponential response in the low-voltage region and series-resistance effect at higher voltages. These results clearly show that the SnO2layer changed the basic electronic behavior and the response to H2.
The stability and sensitivity of the sensors were also improved using the MRIS structure. Pd/SiC and Pd/SnO2/SiC diodes were aged for several weeks at a temperature of 350 °C. In the experiments, these diodes were exposed, variously, to H2, methane, or propylene at a concentration of 400 ppm in a carrier gas mixture of N2 and air, at a temperature of 350 °C. During these exposures, diode currents were measured as a function of time at a fixed applied potential of 0.5 V. The results (see Figure 2) indicate that the Pd/SiC diode had been degraded during the aging process, in that this diode did not respond to H2 in the air/N2 carrier gas at the end of the experiment. It also did not respond to propylene or methane (not plotted in the figure). In contrast, the Pd/SnO2/ SiC diode responded to hydrogen, methane, and propylene. Thus, the SnO2 layer made it possible to detect gases that would otherwise not be detected and enabled the desired longer-term operation of the sensor: Variation of the type of reactive insulator is expected to produce different gas sensitivities.
This work was done by Gary W. Hunter and Philip G. Neudeck of Lewis Research Center, Dak Knight of Cortez III, Chung-Chium Liu and Qing-Hai Wu of Case Western Reserve University, and Liang-Yu Chen of National Research Council. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Lewis Research Center
Commercial Technology Office
Attn: Tech Brief Patent Status
Mail Stop 7-3
21000 Brookpark Road
Refer to LEW-16544