Interplay of native point defects with ZnO Schottky barriers and doping

Abstract

A combination of depth-resolved electronic and structural techniques reveals that native point defects can play a major role in ZnO Schottky barrier formation and charged carrier doping. Previous work ignored these lattice defects at metal–ZnO interfaces due to relatively low point defect densities in the bulk. At higher densities, however, they may account for the wide range of Schottky barrier results in the literature. Similarly, efforts to control doping type and density usually treat native defects as passive, compensating donors or acceptors. Recent advances provide a deeper understanding of the interplay between native point defects and electronic properties at ZnO surfaces, interfaces, and epitaxial films. Key to ZnO Schottky barrier formation is a massive redistribution of native point defects near its surfaces and interfaces. It is now possible to measure the energies, densities, and in many cases the type of point defects below the semiconductor-free surface and its metal interface with nanoscale precision. Depth-resolved cathodoluminescence spectroscopy of deep level emissions calibrated with electrical techniques show that native point defects can (1) increase by orders of magnitude in densities within tens of nanometers of the semiconductor surface, (2) alter free carrier concentrations and band profiles within the surface space charge region, (3) dominate Schottky barrier formation for metal contacts to ZnO, and (4) play an active role in semiconductor doping. The authors address these issues by clearly identifying transition energies of leading native point defects and defect complexes in ZnO and the effects of different annealing methods on their spatial distributions on a nanoscale. These results reveal the interplay between ZnO electronic defects, dopants, polarity, and surface nanostructure, highlighting new ways to control ZnO Schottky barriers and doping.