Role of grain boundaries in exceptionally H2 sensitive highly oriented laser ablated thin films of SnO2

A comparative study on growth, electrical conductivity, and sensor characteristics of highly oriented (transparent) and randomly oriented thin films of SnO2 grown by pulsed laser (KrF; lambda = 248 nm) ablation technique have been carried out. Sensors made of randomly oriented polycrystalline films (deposited at 725 degreesC on alumina) exhibited a sensitivity of about 90% for 50 ppm of H-2 at sensor operating temperatures above 240 degreesC with a good response (similar to 30 s) and retracing times (180 s). Sensors made of a axis orientated films [deposited on LaAlO3(100) at 525 degreesC] exhibited an exceptionally high sensitivity of 30 to 40% even for 1 ppm of H-2 at 310 degreesC with a shorter response time of about 15 s. However, the retrace time was very long (about 20 min). Sensors made of predominantly (101) orientated films [grown at 525 degreesC on sapphire (1102)] exhibited an exceptional sensitivity of 90% even for 5 ppm H-2 at 300 degreesC also had remarkably short response times of a axis oriented films as well as the quick retracing times of polycrystalline films. Thin films, which exhibited exceptionally high sensitivity showed large changes in electrical conductivity and activation energy as function of oxygen partial pressure. Atomic force microscopy investigation reveals that the films are highly granular with average size of about 150-200 nm which is ten times larger than the critical size of 8 nm (2 x L-D, as L-D being 8 nm for SnO2). Analysis of results based on the model for carrier transport across the grain boundaries in polycrystalline semiconductors reveals that the surface barrier height of the grain boundaries is responsible for the large variation in activation energy and sensitivity. (C) 2001 The Electrochemical Society.