Evolution Process of Wurtzite ZnO Films on Cubic MgO (001) Substrates: a Structural, Optical and Electronic Investigation of the Misfit Structures

TL;DR

This study investigates the structural, optical, and electronic evolution of Wurtzite ZnO films on MgO substrates, revealing growth behaviors, domain structures, and electronic state changes relevant for optoelectronic applications.

Contribution

It provides detailed insights into the growth mechanisms, domain formations, and electronic structure evolution of ZnO films on MgO substrates, supported by experimental and theoretical analysis.

Findings

Growth mode depends on substrate temperature.

Double-domain features with 30-degree rotation are observed.

Optical and electronic properties are influenced by growth directions.

Abstract

The interface between hexagonal ZnO films and cubic MgO (001) substrates, fabricated through molecular beam epitaxy, are thoroughly investigated. X-ray diffraction and (scanning) transmission electron microscopy reveal that, at the substrate temperature above 200 degree C, the growth follows the single [0001] direction; while at the substrate below 150 degree C, the growth is initially along [0001] and then mainly changes to [0-332] variants beyond the thickness of about 10 nm. Interestingly, a double-domain feature with a rotational angle of 30 degree appears for the growth along [0001] regardless of the growth temperature, experimentally demonstrated the theoretical predictions for occurrence of double rotational domains in such a heteroepitaxy [Grundmann et al, Phys. Rev. Lett. 105, 146102 (2010)]. It is also found that, the optical transmissivity of the ZnO film is greatly influenced by the mutation of growth directions, stimulated by the bond-length modulations, as further determined by X-ray absorption Spectra (XAS) at Zn K edge. The XAS results also show the evolution of 4pxy and 4pz states in the conduction band as the growth temperature increases. The results obtained from this work can hopefully promote the applications of ZnO in advanced optoelectronics for which its integration with other materials of different phases is desirable.