The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

/pdf/a-comprehensive-review-of-zno-materials-and-devices-21a3zjadem.pdf

A comprehensive review of zno materials and devices

We report a size dependence of Young's modulus in [0001] oriented ZnO nanowires (NWs) with diameters ranging from 17 to 550 nm for the first time. The measured modulus for NWs with diameters smaller than about 120 nm is increasing dramatically with the decreasing diameters, and is significantly higher than that of the larger ones whose modulus tends to that of bulk ZnO. A core-shell composite NW model in terms of the surface stiffening effect correlated with significant bond length contractions occurred near the {1010} free surfaces (which extend several layers deep into the bulk and fade off slowly) is proposed to explore the origin of the size dependence, and present experimental result is well explained. Furthermore, it is possible to estimate the size-related elastic properties of GaN nanotubes and relative nanostructures by using this model.

Size dependence of Young's modulus in ZnO nanowires.

For n- and p-doped III-V compounds, Fermi-level pinning and accompanying phenomena of the (110) cleavage surface have been studied carefully using photoemission at hv≲ 300 eV (so that core as well as valence band levels could be studied). Both the clean surfaces and the changes produced, as metals or oxygen are added to those surfaces in submonolayer quantities, have been examined. It is found that, in general, the Fermi level stabilizes after a small fraction of a monolayer of either metal or oxygen atoms have been placed on the surface. Most strikingly, Fermi-level pinning produced on a given semiconductor by metals and oxygen are similar. However, there is a strong difference in these pinning positions depending on the semiconductor: The pinning position is near (1) the conduction band maximum (CBM) for InP, (2) midgap for GaAs, and (3) the valence band maximum (VBM) for GaSb. The similarity in the pinning position on a given semiconductor produced by both metals and oxygen suggests that the states responsible for the pinning resulted from interaction between the adatoms and the semiconductor. Support for formation of defect levels in the semiconductor at or near the surface is found in the appearance of semiconductor atoms in the metal and in disorder in the valence band with a few percent of oxygen. Based on the available information on Fermi energy pinning, a model is developed for each semiconductor with two different electronic levels which are produced by removal of anions or cations from their normal positions in the surface region of the semiconductors. The pinning levels have the following locations, with respect to the VBM: GaAs, 0.75 and 0.5 eV; InP, 0.9 and 1.2 eV (all levels + 0.1 eV).

New and unified model for Schottky barrier and III–V insulator interface states formation

http://www.ruhr-uni-bochum.de/pc1/sfb/publikationen/Abstract_97.pdf

The chemistry and physics of zinc oxide surfaces

The structure and properties of metal-semiconductor interfaces

Dynamical calculations of the intensities of normally-incident low-energy electrons diffracted from $\mathrm{ZnO}(10\overline{1}0)$, performed using an "exact" matrix-inversion method, are compared both with earlier calculations based on the renormalized-forward-scattering (RFS) method and with measured intensities. The sensitivity of the calculated intensities to the choice of model potential and the magnitude of thermal atomic vibrations is displayed within the context of examining the implications of uncertainties in nonstructural model parameters on surface-structure determinations via elastic low-energy-electron diffraction (ELEED) intensity analyses. We extend our earlier analysis of the surface structure of $\mathrm{ZnO}(10\overline{1}0)$ by utilizing the matrix inversion rather than RFS method, a recently revised bulk geometry for ZnO, an improved model potential, and a consideration of second-as well as top-layer structural distortions. The combination of these four improvements lead to the selection of the most probable surface structure for $\mathrm{ZnO}(10\overline{1}0)$ as one in which the top-layer oxygen is displaced vertically downward by $\ensuremath{\Delta}{d}_{\ensuremath{\perp}}(\mathrm{O})=\ensuremath{-}0.05\ifmmode\pm\else\textpm\fi{}0.1$ \AA{} and the top-layer zinc likewise by $\ensuremath{\Delta}{d}_{\ensuremath{\perp}}(\mathrm{Zn})=\ensuremath{-}0.45\ifmmode\pm\else\textpm\fi{}0.1$ \AA{}. No compelling evidence either for lateral distortions within the top layer or for second-layer distortions is obtained, although small improvements in the agreement between the calculated and observed intensities can be achieved by considering them. Our major conclusion is that given the limitations in the available ELEED intensity data and the uncertainties in the model potential and surface atomic vibrations, the vertical distortion cited above constitute the maximum structural information that can be extracted unambiguously via ELEED intensity analysis at the present time.

Calculation of low-energy-electron-diffraction intensities from ZnO (101̄0). II. Influence of calculational procedure, model potential, and second-layer structural distortions

Dynamical calculations of the intensities of normally incident low-energy electrons diffracted from GaAs(110), performed using a matrix-inversion method, are compared both with earlier kinematical calculations and with measured intensities. The insensitivity of the calculated intensities to the choice of exchange potential and vacuum-solid boundary conditions is displayed. Surface lattice vibrations are found to be adequately described by the bulk Debye temperature. We consider second- and third-layer structural distortions as well as top-layer reconstructions. This analysis leads to the selection of the most probable surface structure for GaAs(110) as one in which the top layer undergoes both a rigid rotation of 27.4\ifmmode^\circ\else\textdegree\fi{} and a 0.05-\AA{} contraction with the As atoms moving outward and the Ga atoms inward, giving a relative vertical shear of 0.65 \AA{}. In the second layer the Ga moves outward 0.06 \AA{} and the second-layer As moves inward 0.06 \AA{}. The dynamical analysis reported herein shows no evidence for third-layer distortions.

Dynamical calculation of low-energy electron diffraction intensities from GaAs(110): Influence of boundary conditions, exchange potential, lattice vibrations, and multilayer reconstructions

A brief synopsis is presented of low‐energy electron diffraction structure analyses reported for the cleavage faces of zincblende and wurtzite compound semiconductor surfaces, specifically GaAs(110), InSb(110), ZnTe(110), InP(110), and ZnO(1010). In all of these cases the surface atomic geometry is distorted from that of a truncated bulk solid, although the space group symmetries at the surface are indistinguishable from those of the corresponding bulk materials. The surface reconstructions involve large (Δd∠0.5 A) displacements of the species in the uppermost atomic layer and often smaller displacements (Δd∠0.1 A) in the layer beneath. The character of these reconstructions depends, however, on the nature of the chemical bonding in the bulk materials. For the most covalent materials, i.e., GaAs and InSb, the anion rotates outward and the cation inward in a fashion such that bond lengths remain unaltered to within 1% except for a 5% contraction of the bond between the uppermost anion and the cation in th...

Trends in surface atomic geometries of compound semiconductors

Dynamical analysis of low-energy-electron diffraction intensities from InP (110)

Low‐energy electron diffraction (LEED) intensity–voltage data for a complete set of first‐order diffraction beams of the GaAs (110) surface is averaged at constant momentum transfer and compared with kinematical and dynamical model calculations for several surface atomic geometries. The atomic displacements in the surface and first two subsurface layers are obtained via this analysis. The influence of multiple scattering is examined by computing the single and multiple scattering contributions to the intensity–voltage profiles associated with atomic distortions within the first three lattice layers. This procedure allows the identification of the major features of the structure in the LEED–CMTA (constant momentum transfer average) profiles either with multiple scattering effects or with single scattering resonances derived from the lattice geometry.

R. J. Meyer

Papers

Calculation of low-energy-electron-diffraction intensities from ZnO (101̄0). II. Influence of calculational procedure, model potential, and second-layer structural distortions

Dynamical calculation of low-energy electron diffraction intensities from GaAs(110): Influence of boundary conditions, exchange potential, lattice vibrations, and multilayer reconstructions

Trends in surface atomic geometries of compound semiconductors

Dynamical analysis of low-energy-electron diffraction intensities from InP (110)

SURFACE AND NEAR-SURFACE ATOMIC STRUCTURE OF GaAs (110).