Paper SS-ThM3
Evolution and Growth of Polar ZnO Nanostructures and their Correlation with Native Point Defects

Thursday, October 21, 2010, 8:40 am, Room Picuris

We have used a complement of depth-resolved cathodoluminescence spectroscopy (DRCLS), Kelvin probe force microscopy (KPFM), atomic force microscopy (AFM), and surface photovoltage spectroscopy (SPS) to measure how polarity, morphology, and nanoscale growth on ZnO surfaces correlate with electrically-active native point defects. Previous DRCLS showed that ZnO nanostructures can grow spontaneously on bare, air-exposed ZnO surfaces and produce electronically-active defects. SPS measures the filling and emptying of these states and thereby their energy level position in the band gap. CLS associates these optical transitions with V_Zn and V_O-related (V­_O-R) defects, respectively. Positron annihilation spectroscopy shows that 2.1eV DRCLS emission correlates with V_Zn vs. depth, anticorrelating with 2.5 eV V_O-R emission. SPS correlates these emissions with optical unfilling and filling transitions, respectively, within the same near-surface region on a nanometer scale. The SPS V_Zn trap state features vary laterally, increasing in nanostructured regions common to ZnO surfaces versus atomically-flat regions. Near both hexagonal pits on (0001) and individual ZnO nano-mounds on both polar faces, 2.1 eV trap densities increase with radial proximity. KPFM maps before and after 2.25 eV illumination show increased potential that reveals large concentrations of V_Zn distributed non-uniformly around and extending away from the hexagonal surface pits. These observations all suggest that ZnO nanostructures grow by oxidation of Zn at the free ZnO surface due to mobile Zn atoms extracted from the underlying lattice.

We measured the morphology, potential, and defect distributions of these structures vs. annealing temperature in flowing oxygen over a 400ºC range. AFM/KPFM maps of room temperature and annealed (0001) surfaces reveal deep (~150 nm) hexagonal pits with spoke-like trenches extending from both the corners and faces of the hexagonal formation and >50 meV positive potentials that increase with temperature. Conversely, (000-1) surfaces initially show few pits with random geometry and raised spoke-like ridges extending from pits. After 1 hr at 200 ºC, pitting increases with strong (~150 mV) negative potentials and newly formed spoke-like trenches. At 300ºC, nanoscale mounds grow 5 – 50 nm high and potentials vary further. From AFM increases in nanorod mass vs. temperature, Arrhenius plots yield 130+10 meV (0001) and 150+10 meV(000-1) activation energies, consistent with the low (0.57 eV) activation energy for Zn interstitial diffusion. Overall, this complement of techniques reveals the interplay between ZnO surface nanostructure, polarity, and electronic defects.