Stress effects on electrostatic potential barriers in piezoelectric semiconductors.

摘要

Electrically active interfaces are integral to and often dictate solid state device properties. These interfaces are engineered with ever greater complexity and precision to yield improved electronics and novel classes of devices. In the case of polycrystalline semiconductors, the materials can be fabricated into bulk devices through ceramic processing routes, and the electrical behavior may be strongly dependent on electrostatic potential barriers at the grain boundaries. Zinc oxide varistors represent a commercially successful example of these electroceramics. ZnO, as with all tetrahedrally coordinated compound semiconductors, is piezoelectric owing to its lack of a center of symmetry. The present work aims to establish the effects of stress on grain boundary potentials as coupled through the piezoelectric effect.; First, a stochastic model is presented which addresses piezoelectric modifications of barrier heights in polycrystalline ZnO. The change in macroscopic transport behavior is correlated to the influence of an internal stress distribution on the barrier height distribution via these modifications. It was found that a broadening of the stress distribution causes a degradation in varistor switching behavior.; This work motivated a study of epitaxial ZnO film growth for constructing bicrystalline varistors. Though it was not possible to control the morphology to an extent amenable for grain boundary investigations, a model was developed to explain the evolution of morphologies observed. In a kinetically limited growth regime, c axis oriented ZnO exhibits a “cratered” surface topography due to geometrical growth and lateral impingement of hexagonal pyramids. As elucidated by the model, the resultant surface morphology suggests a probabilistic distribution of nuclei in both time and space, with facetting of the crater dictated by the in-plane orientation distribution of the crystallites.; Lastly, in an attempt to investigate the interaction of piezoelectric charges and the potential barrier at a well defined, flat boundary, the effects of stress on GaN Schottky diodes were explored. The diode behavior was seen to weakly depend on stress, suggesting that piezoelectric charges are neutralized by the metal rather than by charge redistribution in the semiconductor.