Polarity Control of ZnO Films Grown on Ferroelectric (0001) LiNbO3 Substrates without Buffer Layers by Pulsed-Laser Deposition.

Im Taek Yoon,Juwon Lee,Woochul Yang,Ngoc Cuong Tran

doi:10.3390/nano10020380

Abstract

For this study, polarity-controlled ZnO films were grown on lithium niobate (LiNbO3) substrates without buffer layers using the pulsed-laser deposition technique. The interfacial structure between the ZnO films and the LiNbO3 was inspected using high-resolution transmission electron microscopy (HR-TEM) measurements, and X-ray diffraction (XRD) measurements were performed to support these HR-TEM results. The polarity determination of the ZnO films was investigated using piezoresponse force microscopy (PFM) and a chemical-etching analysis. It was verified from the PFM and chemical-etching analyses that the ZnO film grown on the (+z) LiNbO3 was Zn-polar ZnO, while the O-polar ZnO occurred on the (-z) LiNbO3. Further, a possible mechanism of the interfacial atomic configuration between the ZnO on the (+z) LiNbO3 and that on the (-z) LiNbO3 was suggested. It appears that the electrostatic stability at the substrate surface determines the initial nucleation of the ZnO films, leading to the different polarities in the ZnO systems.

Highlights

The controlling of the crystal polarity in wurtzite-structure semiconductors, such as ZnO and gallium nitride (GaN), has been enormously exploited owing to the great importance of the effects on the crystal-growth modes, material property, impurity incorporation, surface stability, and electric polarization [1,2,3,4,5,6]
A clear interfacial layer with a well-ordered structure was obtained for the ZnO films grown on (+z) LiNbO3 (Figure 1c), while an unclear interface with a thin transition layer that was approximately 3 nm thick was formed for the ZnO films grown on (-z) LiNbO3 substrate (Figure 1d)
These results imply that the atomic-stacking order along the growth direction of the ZnO films grown on the different-polarity LiNbO3 substrates is different leading to a different ZnO polarity

Summary

Introduction

The controlling of the crystal polarity in wurtzite-structure semiconductors, such as ZnO and GaN, has been enormously exploited owing to the great importance of the effects on the crystal-growth modes, material property, impurity incorporation, surface stability, and electric polarization [1,2,3,4,5,6]. The techniques that have been used for the polarity control in previous studies, have been very complicated due to additional processes, such as the substrate treatment, the insertion of additional layers, and the necessity regarding special equipment; these methods create crystalline defects that lead to disordered interfaces between the ZnO, buffer layer, and substrate, and they alsocause complications for the device applications [14]. The additional growth processes, defects, and dislocations can be minimized, and the polarity can be controlled directly in the ZnO films using different-polarity ferroelectric-LiNbO3 substrates. Considering the previous perspectives, ZnO films were grown on (+z) LiNbO3 and (-z) LiNbO3 for this study to develop the polarity-control technique of the wurtzite-structure semiconductors according to the facile method. To the authors’ knowledge, an exploration of the direct polarity-control growth in ZnO films for which different-polarity ferroelectric-LiNbO3 substrates are used has not been conducted by any research group

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