Stability of the Surface Electron Accumulation Layers on the Nonpolar (101̅0) and (112̅0) Faces of ZnO

Abstract

The stability of the ubiquitous hydroxyl termination and downward band bending on the m-plane () and a-plane () faces of ZnO single crystals was investigated using synchrotron and real-time x-ray photoelectron spectroscopy. On these non-polar surfaces, a strong correlation was found between the surface band bending and surface OH coverage, both of which could be modified via heat treatment in ultra high vacuum (UHV). On the m-plane () face, a threshold temperature of ~400 o C was observed, after which there was a sudden increase in OH desorption and upwards movement of the near-surface bands, leading to a metallic-to-semiconductor transition in the electronic nature of the surface, and a change from surface electron accumulation to depletion. This loss of surface metallicity is associated with the disruption of a stable monolayer of chemisorbed hydroxyl groups that form a closed hydrogen-bonded network, across the rows of Zn-O dimers, on the m-plane () face. The downward band bending and surface electron accumulation layers on both the m-plane () and a-plane () faces could be modified and eventually removed by simple UHV heat treatment, with important implications for the processing and electrical performance of ZnO nanostructures and catalytic ZnO nanopowders, which usually contain a high proportion of these non-polar surfaces.

Local aspects of hydrogen-induced metallization of theZnO(101¯0)surface

“…Previous ZnO(1010) studies have focused on surfaces with H adsorbed either exclusively on surface O sites, 17 or have used comparably high H dosages that lead to a saturation of all energetically possible surface sites, 17,18,28 These studies do therefore not investigate the competition of H adsorption at the two different sites, especially in the regime of low coverages. Although the different effects of O-H vs. Zn-H bond formation have been recognized previously, a detailed microscopic view of how single H atoms interact with the pristine or slightly Hcovered ZnO(1010) surface is not established.…”

Band alignment at Ag/ZnO(0001) interfaces: A combined soft and hard x-ray photoemission study

“…The increase in carrier concentration in the accumulation layer was correlated (i) to a variation of the band bending that induces quasi-bidimensional confined trapped states 29,[33][34][35][36]42 and (ii) to a decrease in work function. These adsorbate-dependent 43 evolutions of the band levels were characterized by optical measurements 5 , macroscopic 6,44 or local Kelvin probes 45 , ultraviolet 43,46 , laboratory X-ray 40,41,[47][48][49] or synchrotron [34][35][36]41,42 photoemission. Atomic H adsorption systematically diminishes the work function, bends bands downward to create an accumulation layer 6,[33][34][35][36]42 by creating H-related levels that donate electrons to the conduction band.…”

“…The origin of this accumulation/depletion zone has been intensively revisited in the past years through photoemission spectroscopy [34][35][36][40][41][42]48,49 .…”

“…The upward band bending of ZnO-Zn was more resilient to annealing. However, these studies 41,48,49 were performed on just cleaned as-loaded surfaces that kept inherent polishing damages and showed poor Low-Energy Electron Diffraction (LEED) patterns. Generally speaking, the origin of the observed changes in work function, band bending and conductivity remains unclear although there are compelling evidence of a correlation with surface hydroxylation; surface states 53,54 , donor characters of surface hydroxyl groups [34][35][36][40][41][42]45,48,49 , hydrogen diffusion in the subsurface, subsurface defects diffusion 6,43 have all been invoked.…”

Controlling the Electrical Transport Properties of Nanocontacts to Nanowires

“…The plots (b) and (c) show the spatial distribution of the conduction band minimum energy (relative to the Fermi level) when there is an acceptor charge density of 1x10 12 cm -2 and donor charge of density 1x10 13 cm -2 , respectively.Although no surface charge was required to provide agreement between the simulated and experimental results here, the electrostatic condition of the semiconductor surface can affect the transport in nanostructures. This is highly debated for ZnO with many reports showing polar and non-polar ZnO facets in accumulation, while studies of large-area nanowire arrays show a generalized depletion [42][43][44]. The effect on the I-V characteristics of accumulation or depletion of the side and top surfaces of the nanowires is explored through simulations.…”