Martin Eickhoff

Effect of Water Vapor and Surface Morphology on the Low Temperature Response of Metal Oxide Semiconductor Gas Sensors

In this work the low temperature response of metal oxide semiconductor gas sensors is analyzed. Important characteristics of this low-temperature response are a pronounced selectivity to acid- and base-forming gases and a large disparity of response and recovery time constants which often leads to an integrator-type of gas response. We show that this kind of sensor performance is related to the trend of semiconductor gas sensors to adsorb water vapor in multi-layer form and that this ability is sensitively influenced by the surface morphology. In particular we show that surface roughness in the nanometer range enhances desorption of water from multi-layer adsorbates, enabling them to respond more swiftly to changes in the ambient humidity. Further experiments reveal that reactive gases, such as NO2 and NH3, which are easily absorbed in the water adsorbate layers, are more easily exchanged across the liquid/air interface when the humidity in the ambient air is high.

8.1.1 InGaN/GaN nanowire based opto-chemical sensor for detecting hydrogen and hydrocarbons at low temperature

We report on InGaN/GaN nanowires (NWs) as opto-chemical transducers for the detection of H2 and hydrocarbons. The NWs exhibit a strong photoluminescence (PL) which persists up to temperatures of 150°C and above. The GaN/InGaN NWs were grown by plasma-assisted molecular beam epitaxy on low-resistivity n-type Si (111) substrates and covered with a semitransparent catalytic Pt-coating after deposition. With the thin Pt coating on top, the PL intensity is dependent on the concentration of hydrogen or hydrocarbons in the surrounding atmosphere. Contrary to commercially available hydrogen sensors e.g. Pd or Pt gated high electron mobility transistors or catalytically activated metaloxide-silicon-carbide FETs our all-optical sensor system allows detection of ppm concentrations of H2 at low operating temperatures (30°C-120°C).