摘要
Depth-resolved electronic and structural techniques reveal that native point defects play a major role in ZnO Schottky barrier formation and free carrier doping. Previous work ignored these lattice defects at metal-ZnO interfaces due to relatively low point defect densities in the bulk. 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. Using depth-resolved cathodoluminescence spectroscopy (DRCLS) of deep level emissions calibrated with electrical techniques, we find that native point defects can (i) increase by orders-of-magnitude in densities within tens of nanometers of the semiconductor surface, (ii) alter free carrier concentrations and band profiles within the surface space charge region, (iii) dominate the Schottky barrier formation for metal contacts to ZnO, and (iv) play an active role in semiconductor doping. Among major roadblocks to ZnO optoelectronics have been the difficulty of both n- and p-type doping. Oxygen vacancies (VO), VO complexes, Zn interstitial-related complexes, and residual impurities such as H and Al are all believed to be shallow donors in ZnO, while Zn vacancies (VZn) and their complexes are acceptors. While their impact on free carrier compensation and recombination is recognized, the physical nature of the donors and acceptors dominating carrier densities in ZnO and their effect of carrier injection at contacts is unresolved. Furthermore, how these defects impact ZnO optoelectronics at the nanoscale is only now being explored. We used a combination of depth-resolved and scanned probe techniques to clearly identify the optical transitions and energies of VZn and VZn clusters, Li on Zn sites, Ga on Zn site donors, the effects of different annealing methods on their spatial distributions in ion-implanted as well as Ga grown-in ZnO, and how VZn, VZn clusters, and VO complexes contribute to near- and sub-surface carrier density. Defects also couple to nanostructures, which form spontaneously on ZnO polar surfaces and create sub-surface VZn locally with Zn diffusion that feeds the growth. These results reveal the interplay between ZnO electronic defects, dopants, polarity, and surface nanostructure, and they highlight new ways to control ZnO Schottky barriers and doping.
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