Large On-Off Enhancement of Au Nanocatalyst Contacts to ZnO Nanowires with Bulk and Surface Oxygen Modification.

Schottky diodes have been a fundamental component of electrical circuits for many decades, and intense research continues to this day on planar materials with increasingly exotic compounds. With the birth of nanotechnology, a paradigm shift occurred with Schottky contacts proving to be essential for enabling nanodevice inventions and increasing their performance by many orders of magnitude, particularly in the fields of piezotronics and piezoelectric energy harvesting. ZnO nanomaterials have proven to be the most popular materials in those devices as they possess high piezoelectric coefficients, high surface sensitivity, and low resistivity due to the high native n-type doping and low hole concentration. ZnO nanowires grown by vapor phase techniques with the aid of a metal catalyst provide a ready-made epitaxial Schottky contact free from interfacial layers and major defects. We show here with the most comprehensive experimental investigation to-date of Au nanocontacts to ZnO nanowires that the modulation of bulk and surface oxygen can dramatically increase the rectifying quality of these contacts when applied in the metal-semiconductor-metal (M-S-M) device configuration with potential barriers approaching the performance of planar contacts on single crystal ZnO. Before modification, the Au-ZnO nanowire contacts in a rectifying-nanowire-ohmic M-S-M device configuration typically show limited current rectification and electrical transport properties dominated by surface effects and tunneling at the contact edge. Interestingly, the oxygen modulation only has a minor effect on the resistivity as the high-resolution cathodoluminescence spectroscopy shows that the dominant donors are In, Ga, and Al with no visible band emissions often associated with detrimental point defects. The spectroscopy also revealed that carbon is incorporated into the bulk that may present interesting magnetic properties for future spintronics applications. Atomic-resolution electron microscopy confirms the Zn-polar orientation of the high-quality single crystal nanowires used for the electrical measurements. X-ray photoelectron spectroscopy shows oxygen-annealed nanowires have fewer surface oxygen defects, and when that difference is coupled with a reduction in surface oxygen vacancies via oxygen plasma treatment, the current rectification can increase by several orders of magnitude with a much lower dispersion in the effective potential barrier properties when compared to those that are not annealed. This study concludes after the electrical measurements of 66 nanowire contacts/M-S-M structures with diameters as small as 25 nm using a scanning tunneling microscopy probe that effective device potential barrier heights of 0.65 eV and on-off ratios of 3 orders of magnitude can be achieved. Interestingly, this change in contact properties is transient in nature, revealing dynamic surface effects can govern the rectifying behavior and surface passivation techniques are desirable to achieve consistent performance. This work shows the overriding effects of surface defects and adsorbates on the sloping facets near the Au contact edge and the potential for this effect to be used to control the electrical transport properties and produce molecular-scale sensors to greatly enhance the performance of many piezotronic and energy harvesting devices.