Optimizing and Understanding Semiconductor Gas Sensors

An interesting class of gas sensors is based on the resistivity of metal oxide semiconductor thin films. Gases adsorb onto the surface of these metal oxide semiconductor thin films and alter the film resistivity. The surface adsorption perturbs the underlying charge carriers in the film or the electron conduction between individual crystalline grains in the film. However, the exact echanism of the effect of surface adsorption is not clearly known. Optimization of these metal oxide semiconductor gas sensors must await additional understanding.

Our gas sensor work has focused on ZnO films. We have grown ZnO films using ALD techniques and simultaneously measured their resistivity [1]. Sensitivity is extremely high for ultrathin ZnO films. The ZnO film resistivity also varies dramatically with adsorbed surface species. ZnO films terminated with Zn-CH3* species have a much higher sensitivity that ZnO films terminated with Zn-OH* species. Resistivity results for ZnO films during ZnO ALD with diethyl zinc (DEZ) and H2O are shown in the accompanying figure. Future research will explore the effect gas adsorption and attempt to understand and optimize the sensor sensitivity.

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