Silicon Carbide-Based Hydrogen and Hydrocarbon Gas Detection

Hydrogen and hydrocarbon detection in aerosafe use, storage and handling. Detection of low nautical applications is important for reasons of concentrations of these gases is a vital issue in consafety and emissions control The use of silicon trolling and monitoring emissions from, for example, carbide as a semiconductor in a metal-semlconducan aircraft engine. NASA Lewis Research Center tor or metal-insulator-semiconductor structure (LeRC), in conjunction with Case Western Reserve opens opportunities to measure hydrogen and University (CWRU), has been developing hydrogen hydrocarbons in high temperature environments and hydrocarbon sensor technologies intended to beyond the capabilities of silicon-based devices. The meet a wide variety of needs. One component of this purpose of this paper is to explore the response and program is the development of gas sensor technolstability of Pd-SiC Schottky diodes as gas sensors in ogy using silicon carbide (SIC) as a semiconductor in the temperature range from 100 °C to 400 ° C. The Metal-Insulator-Semiconductor (MIS) or Metal effect of heat treating on the diode properties as Semiconductor (MS) sensors. This work adapts measured at 100 °C is explored. Subsequent operapreviously developed silicon-based hydrogen sensing tion at 400 °C demonstrates the diode's sensitivity technology to SiC. to hydrogen and hydrocarbons. It Is concluded that the Pd-SiC Schottky diode has potential as a hydroThere are significant advantages in a number gen and hydrocarbon sensor over a wide range of of gas sensing applications to the use of SiC as the temperatures but further studies are necessary to semiconductor rather than silicon (Si). These addetermine the diode's long term stability, vantages are due to the improved material properties of SiC over Si such as high temperature semiconductor operation, superior mechanical INTRODUCTION toughness, and increased thermal conductivity. For example, the recovery back to its baseline after exDetection of hydrogen and hydrocarbons over posure to hydrogen of a sensor operated at near a wide range of concentrations h important for a room temperature can be slow in an inert environnumber of aeronautical and commercial ment. 1 This recovery time could be greatly accelerapplications. This is due to their use both as a fuel ated by heating the sensor to high temperatures and their presence as a by-product of the use of without damage to the sensor. Further, detection of fuels. Monitoring of flammable or explosive gases such as hydrocarbons, which decompose at concentrations of these gases is necessary In their high temperatures, could be facilitated by heating the sensor to the gas decomposition temperature. Recently, the capabilities of gas sensors using SiC have been explored. Schottky diodes composed ' of palladium (Pd) and palladium-silver (PdAg) deposited on SiC have been shown to be sensitive to 5000 ppm hydrogen in helium or nitrogen at near room temperature. 1"=SiC-based capacitors using platinum (Pt) as the gas sensitive metal have decomplete sensor package. The development of a tected hydrogen concentrations as low as 2.5 ppm complete sensor package for high temperature operand have operated at temperatures as high as 800 ation involves different processing than a room tem°C? Further, sensitivity to hydrocarbons such as perature sensor package. In conclusion, this techmethane, ethane, and propane at 457 °C has been nology shows promise for aeronautical applications demonstrated? MIS SIC-based structures using although further development is necessary. several different types of contacts have, with varying sensitivities, measured hydrogen at temperatures from near room temperature to 627 °C. s FabricaDEVICE FABRICATION AND TESTING tion of a complete SiC-based sensor with temperature detector and heater has been attempted with Palladium MS Schottky diodes with SiC as the mixed results. I semiconductor were prepared in the following manner. An 4-5/_m thick 4H-SiC epilayer was grown by A possible mechanism for hydrogen and hychemical vapor deposition on a commercially availdrocarbon detection in SiC-based devices using Pd able 4H silicon-face SiC substrate." Approximately is the same as that proposed for Pd/Si-based devices: 400 angstroms of palladium metal were sputter the dissociation of hydrogen or hydrocarbons on the deposited onto the as-grown 4H-SiC epilayer surface metal surface leads to the formation of a dipole layer and patterned by a lift-off technique into circular Pd composed of hydrogen at the metal-semiconductor Schottky patterns of diameter 200/_m. Device or metal-insulator interface. This dipole layer at'fabrication was completed by sputtering Pd onto the fects the electronic properties of the device in probottom of the wafer to form a backside contact. portion to the amount of hydrogen and other gas species (especially oxygen) present in the surroundThe facility used for sensor testing at NASA ing ambient atmosphere. _'v LeRC is shown schematically in Figure 1. The facility can supply a continuous flow of gaseous hydroNASA LeRC and CWRU are presently develgen (H2), helium (He), nitrogen (N2) , or air, either oping a complete hydrogen and hydrocarbon gas individually or as a mixture, to a chamber containsensor to be operational to temperatures of at least ing the sensor under test. The temperature (T) and 400 °C. This effort involves two major components, pressure (P) ofthe gas are measured as the gas One component is the fabrication of a stable gas enters the test chamber. The composition of the gas sensing element which can withstand high temperais monitored by a mass spectrometer. The gas ture operation and be sensitive to hydrogen bearing leaves the test chamber and is sent through a flame gases in a variety of ambients. The sensing element before being vented. will use Pd or a Pd-alioy as the hydrogen or hydrocarbon sensitive metal The first structure being examined is a Pd-SIC Schottky diode. The second (_ To Vent major component in the SiC sensor development program is the fabrication of a complete sensor package which can monitor and control the temperFlame ature of the hydrogen or hydrocarbon sensing element. This paper discusses the progress of SiC-based __[_]-_ _)_) _ t_/ hydrogen and hydrocarbon sensor development at NASA LeRC and CWRU. We will first discuss the fabrication and testing ofthe Pd-SIC Schottky diode _ sensing elements. We will then focus on two elements of the characterization ofthe diodes: 1) El'ee ay Valve fects of heat treatments on diode operation at 100 Solenoid IMass Spectrometer[ °C. 2) Sensor operation at 400 °C. It is shown that :-_M assF ow Con_o ers Pd-SiC Schottky diodes can detect hydrogen at 100 1 11 °C even after a range of heat treatments, and at 400 °C. Hydrocarbon detection, specifically proFigure 1. Schematic diagram of the NASA LeRC H_ pylene, is also demonstrated at 400 °C. We will then sensor testing facility. T and P represent temperadiscuss the present development at CWRU of a ture and pressure measurements.