Chemical Sensor Research
In a collaboration with groups at Alabama A&M University and the NASA Lewis Research Center, recent work in chemical sensor research has centered on Schottky-type diode sensors composed of silicon carbide. Since SiC crystals and films are not adversely affected by high temperatures up to 900 C, it should be possible to fabricate a SiC surface that can be employed for high temperature sensing applications. At present, we are looking at the deposition of catalytic metals and alloys that are stable at these elevated temperatures. A catalytic metal is necessary for the dissociative adsorption of hydrogen and hydrogen containing combustible gases. The transduction mechanisim is shown schematically in Figure 1.
Figure 1. Schematic depiction
of the mechanism for hydrogen detection using Pd-SiC Schottky
type sensors. Hydrogen adsorbs to Pd and dissociates. The
H-atoms diffuse across the surface and
through the Pd to the Pd-SiC interface affecting surface charge.
This change is detectable by current-voltage
measurements and can be correlated to hydrogen concentration.
Both diamond films and silicon carbide have been shown that they can
be employed as oxygen and hydrogen sensors that operate in a temperature
regime considerably higher than conventional sensors such as tin oxide.
This of course can be very useful in hostile environments such as in engine
exhaust or in incinerators to sample the smoke and off gas products. The
adsorbing gas changes the conductivity and this change in conductivity is
measured and can be correlated to surface concentrations and to the levels
of the sampled gas in the ambient as depicted in figure 1. Typically, Schottky
type sensors are monitored by measuring the I-V characteristics of the SiC
sensor before and after exposure to hydrogen. Variations in the behavior
of the diode upon exposure to hydrogen can be measured by monitoring the
current of the device as a function of time, at fixed forward-bias voltage,
as plotted in Figure 2.
Figure 2. Current-time plot showing the response of a Pd-SiC sensor to hydrogen exposure as a function of temperature.
In addition to looking for stable catalytic metals to increase the effective temperature range, we ar producing layered arrays of SiC and we are examining novel sensor preparations employing various state-of-the-art materials preparation techniques. These include the use of laser assisted deposition for the preparation of catalytic sites on the surfaces of SiC, and ion beam surface modification. The later will involve the use of ion implantation and etching processes. By employing implantation, it may be possible to produce stable sub-surface deposits that enhance catalytic properties while keeping Schottky properties in tact at elevated temperatures.
It is anticipated that this study will lead to the development of a wide range of elevated temperature applications including chemical and optical sensors. It may also be possible to produce a sensor that employs a completely different transducing mechanism. Implantation of an optically active layer just below the surface can result in materials that can be employed for remote sensing in hostile environments. It may also be possible to utilize the resonance properties of SiC to produce microelectromechanical (MEMs) type cantilever sensors that are sensitive to extremely low concentrations of adsorbates on their surfaces or small fluctuations in thermal conditions.