Interface Engineering in ZnO Epitaxy

Abstract

We report interface engineering in ZnO epitaxy to grow high-quality layers by plasma-assisted molecular beam epitaxy. Through interface engineering, we have succeeded in two-dimensional layer-by-layer growth of ZnO both on sapphire and GaN, and control of lattice polarity of ZnO films on a Ga-polar GaN template. MgO buffer has been used to convert the growth mode from a three-dimensional to a two-dimensional mode in ZnO epitaxy on sapphire. O-plasma pre-exposure on Ga-polar GaN templates has been employed to form a monoclinic Ga 2 O 3 interface layer at the ZnO/GaN heterointerface, while Zn pre-exposure prevents oxidation of the GaN surface resulting in a ZnO/GaN heterointerface without an interface layer. The lattice polarity of ZnO films on Ga-polar GaN templates with and without a Ga 2 O 3 interface layer with inversion symmetry have been revealed as Zn-and O-polar, respectively, by coaxial impact collision ion scattering spectroscopy. Structural properties of ZnO films grown with MgO buffer on sapphire or with Zn pre-exposure on GaN templates are better than those grown without MgO buffer or with an O-plasma preexposure, respectively. A buffer mechanism of MgO is discussed based on reflection high-energy electron diffraction and high-resolution X-ray diffraction analyses, while the mechanism for controlling the polarity in ZnO epitaxy is discussed using an interface layer with inversion symmetry. It is suggested that the present method offers a general approach to control the crystal polarity.

Enhancement of the surface and structural properties of ZnO epitaxial films grown on Al2O3 substrates utilizing annealed ZnO buffer layers

“…The improvement of the surface and the structural properties of the ZnO epilayer grown on Al 2 O 3 substrates utilizing ZnO buffer layers annealed in a vacuum might be attributed to the formation of the Zn-face ZnO buffers due to thermal treatment. The achievement Zn polarity of the surface in the ZnO buffer layer due to thermal annealing enhances the crystal quality of the ZnO films [18].…”

Properties of ZnO/ZnMgO nanostructures grown on r-plane Al2O3 substrates by molecular beam epitaxy

“…Many researchers used MgO buffer layers with success (eg. [10]). We undertook attempts to obtain planar ZnMgO layers using ZnO layer as a buffer because of a large structural misfit of MgO (cubic NaCl structure) and wurtzite ZnO.…”

Polarity identification of ZnO(0001) surface by reflection high-energy electron diffraction

“…14,15) Convergent beam electron diffraction (CBED), etching, coaxial impact collision ion scattering spectroscopy (CAICISS), and X-ray photoelectron spectroscopy (XPS) are widely used to determine the polarity of ZnO crystals. [16][17][18][19] However, Ullah et al reported that polarity inversion of ZnO films occurs during the growth of a thin nucleation layer. 20) Thus, a method for in situ monitoring during ZnO crystal growth is desired.…”