ADVANCED SOLID STATE SENSORS FOR VISION 21 SYSTEMS

Silicon carbide (SiC) is a high temperature semiconductor with the potential to meet the gas and temperature sensor needs in both present and future power generation systems. These devices have been and are currently being investigated for a variety of high temperature sensing applications. These include leak detection, fire detection, environmental control, and emissions monitoring. Electronically these sensors can be very simple Schottky diode structures that rely on gas-induced changes in electrical characteristics at the metal-semiconductor interface. In these devices, thermal stability of the interfaces has been shown to be an essential requirement for improving and maintaining sensor sensitivity and lifetime. In this report, we describe device fabrication and characterization studies relevant to the development of SiC based gas and temperature sensors. Specifically, we have investigated the use of periodically stepped surfaces to improve the thermal stability of the metal semiconductor interface for simple Pd-SiC Schottky diodes. These periodically stepped surfaces have atomically flat terraces on the order of 200 nm wide separated by steps of 1.5 nm height. It should be noted that 1.5 nm is the unit cell height for the 6H-SiC (0001) substrates used in these studies. These surfaces contrast markedly with the ''standard'' SiC surfaces normally used in device fabrication. Obvious scratches and pots as well as subsurface defects characterize these standard surfaces. This research involved ultrahigh vacuum deposition and characterization studies to investigate the thermal stability of Pd-SiC Schottky diodes on both the stepped and standard surfaces, high temperature electrical characterization of these device structures, and high temperature electrical characterization of diodes under wet and dry oxidizing conditions. To our knowledge, these studies have yielded the first electrical characterization of actual sensor device structures fabricated under ultrahigh vacuum conditions. The results demonstrate that the Pd-SiC interfaces formed on the stepped surface are remarkably stable at temperatures up to 670 C and that there is a definite improvement in the electrical characteristics. This temperature, though lower than DOE target temperatures is still 100 C higher than that used in reported field studies. The Pd films studied here ranged in thickness from the monolayer level ({approx}0.4 nm) to actual device dimensions ({approx}46.5 nm) and are deposited under ultrahigh vacuum conditions at {approx}50 C. The films were characterized in-situ using Auger electron spectroscopy both before and after annealing at 670 C. The Auger lineshapes were used to provide quantitative information on the chemistry of the reaction products. Ex-situ atomic force microscopy was used to characterize changes in surface morphology. Current-voltage (I-V) measurements were made as a function of temperature to further characterize the devices. Additional studies were performed to gain an understanding of the effects of wet and dry oxidizing environments on device performance. These measurements were performed for temperatures up to 325 C, the highest temperature attainable with our current apparatus. Our results clearly show a significant benefit in thermal stability and electrical characteristics associated with the stepped surface. These results are quite promising but much development and testing work remains to be done