A miniaturized Schottky diode hydrogen and hydrocarbon sensor with the structure of catalytic metal-metal oxide-silicon carbide (SiC) has been developed. The major innovation of this work is the use of the metal oxide, palladium oxide (PdOx), as a barrier layer between the catalytic metal and the SiC in the gas-sensing structure. A major advantage of adding a PdOx barrier layer between the gate metal and the SiC is to prevent and alleviate chemical reactions between the gate metal and the SiC. Without the PdOx barrier layer, the gate metal and the substrate can easily form metal silicides at high temperature, leading to diode structure disruption. The metal oxide barrier layer can be incorporated into a gas-sensing structure by standard deposition techniques in a controlled manner. This oxide naturally forms with Pd in Pd-based gas sensor systems and can disrupt the gas sensor structure when formed in situ in an uncontrolled manner. However, purposely including this oxide in the Schottky barrier structure produces a stable barrier layer that enables a stable and sensitive gas sensor structure.
A test sensor remained sensitive and stable after 778 hours of gas testing at temperatures ranging from 450 to 600 °C. The sensor has the advantages of being simple to fabricate, small in size, and fast in response. With the presence of the PdOx interlayer, the developed sensor is expected to survive for extended periods of time and much longer than a catalytic metal/SiC system without this barrier layer. Simultaneously, the PdOx layer typically maintains high sensor sensitivity. The PdOx interlayer could also be combined with a variety of metal or metal alloy gate layers to fabricate high-temperature Schottky diode sensors, capacitors, transistors, or other gas-sensitive structures.
The unique or novel feature of the innovation is the application of a barrier layer of PdOx that prevents/minimizes chemical reaction between the catalytic sensing layer (metal or metal alloy gate) and the substrate layer (SiC). This approach uses the reaction product of PdOx, whose uncontrolled formation previously from a palladium chrome barrier layer contributed to the disruption of the sensor structure. By controlling the formation of PdOx and its position in the gas sensor structure, it is possible to improve sensor stability combined with sensitivity. Hydrogen or hydrocarbons could potentially reduce PdOx to Pd, but this would likely increase sensitivity of the sensor by increasing the amount of catalytic sensing metal. Oxidation of the barrier layer, which can be problematic with some barrier layers, is not an issue with PdOx because it is already oxidized.
The sensor and associated supporting electronics (power supply and heating elements) could be integrated on a postage-stamp-sized system. Its compact size is suitable for semiconductor wafer chip mass-fabrication, and makes it highly affordable and integratable into a wide array of locations and applications.