ZnO has received renewed interest in recent years due to its exciting semiconductor properties and remarkable ability to grow nanostructures. As a wide band gap semiconductor, ZnO has many potential future applications including blue/UV light emitters, transparent conductors, biosensors, and electronic nanoscale devices. While the versatility of ZnO is exciting, many hurdles keep it from reaching full device potential. Chief among them are the role of native point defects and impurities in the fabrication of high quality contacts and high, yet controllable, n- and p-type doping. The scope of this work explores the electronic properties of ZnO surfaces and interfaces and the impact of native point defects on Schottky barrier formation and doping.
The results presented here use a complement of depth-resolved cathodoluminescence spectroscopy (DRCLS), atomic force microscopy (AFM), Kelvin probe force microscopy (KPFM), and surface photovoltage spectroscopy (SPS) to show that surface treatment and processing plays a significant role in the quality, stability, and efficiency of potential next generation devices. This is evident in our results showing that the Zn-polar surface is more stable and capable of forming higher quality Au Schottky barriers as compared to the O-polar surface. We go on to reveal a significant metal sensitivity and surface polarity dependence that correlates with defects and interface chemistry on ZnO. We’ve also shown the significant impact of surface preparation and post processing techniques on the optical efficiency and stability of ZnO surfaces. Our measurements reveal that remote oxygen plasma (ROP) processing is capable of decreasing oxygen vacancy related defects (VO-R) on the O-polar surfaces as well as creating new zinc vacancy related (VZn-R) defects on the Zn-polar surface. Furthermore, we have correlated the formation of native point defects with interface chemical reactions and surface morphology on ZnO. With this, we were able to determine the relationship between the strength of near band edge to deep level defect emissions finding a threshold dependence on surface roughness that can serve as a figure of merit for substrate polishing and etching. Further experiments reveal that ZnO nanostructures grow spontaneously on ZnO polar surfaces in air that generate strong potential variations which correlate with native defects. Using a staged annealing process we were able to determine the activation energy for ZnO nanostructured growth revealing that Zn-interstitial diffusion is the dominant mechanism feeding growth. In a separate study, we observed the strong interplay between VZn-R defects and dopants in degenerately Ga-doped ZnO films. In doing so we found that the DRCLS Fermi level thresholds provide a useful indicator of carrier density, revealing depth variations that anticorrelate with VZn-R densities.
The ability to understand and control native defects as well as surface and interface chemical reactions in ZnO could allow for efficient and stable n- and p-type doping and serve to making higher quality Schottky diodes for future device applications.