| Gas sensors have been used to detect hazardous gases, combustible gases and various kind of odor for several critical applications such as industry, health and safety and homeland security. However, currently available gas sensors have poor sensitivity, selectivity and stability. Gas sensing based on inelastic electron tunneling spectroscopy (IETS) is a highly selective and sensitive technique, which detects the vibrational modes of gas molecules. Studies of IETS based sensors as gas detection technique indicated feasibility but room temperature spectra were not obtained and poor fabrication yield and unstable sensor were of major concern. The objectives of this work were to fabricate and characterize ultra smooth Hafnium and Hafnium oxide films for improved robustness and stability of IETS junctions, obtain room temperature IETS gas spectra and study the failure mechanism of Al2O3 junctions using scanning probe microscopy. Al/Al2O 3/Al and Hf/HfO2/Hf metal-insulator-metal junctions were studied. IETS gas sensing was feasible at room temperature using Al/Al 2O3/Al tunnel junction, but stability and robustness of the junction due to its non uniform oxide layer were of concern as the junction failed during prolonged IETS tests at 77K and room temperature. Hf/HfO 2/Hf tunnel junctions did not fail while Al/Al2O3/Al tunnel junctions did fail. This data indicates that HfO2 is more robust and stable compared to Al2O3 as an insulator layer; however, inability to obtain room temperature IETS spectra peaks is of concern. Change density and surface potential image of the top electrode of tunnel junction was used to study the quality of the junctions but breakdown mechanism could not be analyzed with certainty. Future work should address: measurement of IETS response of Hf/HfO2/Hf tunnel junctions at 4 K; identify actual peaks of HfO2; analyze room temperature spectra and optimize the top metal electrode design to maximize the absorption of gas molecule into the insulator layer. |
