Semiconductor Device-Based Sensors for Gas, Chemical, and Biomedical Applications

ZnO Thin-Film and Nanowire-Based Sensor Applications 169

© 2011 by Taylor & Francis Group, LLC

impurities present in the as-grown ZnO epitaxial layers.

1

An alternative explanation is that the

hydrogen induces a donor state and thereby increases the free electron concentration.

2

Subsequent

annealing reduces the carrier density to slightly below the initial value in the as-received ZnO,

which may indicate that it contained hydrogen as a result of the growth process. We emphasize that

the n-type conductivity probably arises from multiple impurity sources,

131,132

and we cannot unam-

biguously assign all of the changes to hydrogen.