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

The 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 were studied. Sensors made of randomly oriented polycrystalline films (deposited at 725 C on alumina) showed a sensitivity of about 90% for 50 ppm of H2 at sensor operating temperatures above 240 C with a good response (about 30 s) and retracing times (180 s). Sensors made of a axis oriented films (deposited on LaAlO3(100) at 525 C) showed an exceptionally high sensitivity of 30 to 40% even for 1 ppm of H2 at 310 C with a shorter response time of about 15 s. However, the retrace time was very long (about 20 min). Sensors made of predominantly (101) oriented films (grown at 525 C on sapphire (1102)) showed an exceptional sensitivity of 90%. Thin films with exceptionally high sensitivity showed large changes in electrical conductivity and activation energy as a function of oxygen partial pressure. AFM showed that the films are highly granular with an average size of about 150-200 nm which is ten times larger than the critical size of 8 nm. 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. 28 refs.