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Zinc oxide (ZnO) has emerged as a promising wide bandgap material (3.35eV at 300K) for use in next-generation nanoelectronics and photonics, with important piezoelectric, pyroelectric, sensing, and optoelectronic properties. ZnO has seen specific application in ultraviolet (UV) photodetectors, UV lasers [1], hydrogen gas sensors [2, 3], surface acoustic wave devices, piezoelectric generators [4], and transparent thin-film transistors for displays [5]. Various forms of ZnO nanostructures, such as nanobelts, nanobows, and nanowires, and have all attracted significant attention due to their ease of fabrication, remarkable relative surface area, and low-dimensional nature [6, 7]. Nanowires of ZnO in particular can exhibit pinch-off of electrical current with surface charge-sensitive depletion depths that are on the order of the wire radius [8, 9]. In bulk ZnO, defects have been shown to strongly affect the behavior of metal contacts, by modifying band bending and allowing trap-assisted tunneling transport through the metal-ZnO Schottky barrier [10]. The electronic impact of native point defects becomes critical at the nanoscale, since their physical properties can dominate charge carrier transport and especially electronic contact behavior. In order to control the distribution of defects at the metal-nanowire interface, various forms of surface modification were investigated. We report the in-situ fabrication of both Ohmic and Schottky platinum (Pt) metal contacts to single ZnO nanowires prepared by pulsed laser deposition (PLD) and carbothermal vapor phase transport, using Ga-ion surface modification and both furnace and electron beam annealing. A Ga focused ion beam (FIB) was operated at 30 keV to implant nanowire surfaces before metallization for production of Ohmic contacts, and at 5 keV to gently mill the defect-rich outer annulus, promoting formation of Schottky contacts. Electron beam induced deposition (EBID) was used to pattern Pt metal contacts to the wires. The optical properties of defects at the nanowire surface and metal-nanowire interface were probed using depth-resolved cathodoluminesence spectroscopy (DRCLS). Results demonstrated that differences in native point defect distributions under the 30nm Pt contacts and into the bulk were correlated directly with Schottky versus Ohmic behavior exhibited in measured current-voltage characteristics. Depth profiles of DRCL spectra at and under Ga-implanted Ohmic contacts revealed interfacial segregation of copper on zinc site defects (CuZn, 2.34eV), as well as zinc vacancies (VZn, 1.80eV). A depth profile of DRCL spectra at the interface of a Ga-milled area and 30nm Pt contact demonstrated that milling of the nanowire surface decreased the concentration of CuZn by an order of magnitude, promoting the formation of Schottky contacts. A Schottky contact with 2 orders of magnitude rectification was fabricated to the thin end of a 50µm long tapered NW whose diameter increased linearly end-to-end, from 400nm to 1µm. To cause pinch-off, the depletion width must be comparable to the NW diameter [10]. Investigation of defect dependence on nanowire diameter also demonstrated a 2x linear increase in CuZn from 500nm to 1µm diameter. Thus, thinner wires are inherently easier to pinch-off and have a lower native concentration of surface defects, promoting formation of Schottky contacts.The interfacial physics of contacts to nanowires is influenced by the diameter of the wire and its defect profile at the metal interface. Through the control of defects in these nanowires by Ga-ion surface modification, Ohmic and Schottky contacts can be fabricated in-situ using only a single metal. |
