Crystallinity Improvement of ZnO Thin Film on Different Buffer Layers Grown by MBE

Shao-Ying Ting,Hsiang-Chen Wang,Che-Hao Liao,C C Yang,Ya-Ping Hsieh,Po-Ju Chen,Wen-Ming Chang

doi:10.1155/2012/929278

Shao-Ying Ting, Hsiang-Chen Wang + Show 5 more

Open Access

https://doi.org/10.1155/2012/929278

Copy DOI

Abstract

The material and optical properties of ZnO thin film samples grown on different buffer layers on sapphire substrates through a two-step temperature variation growth by molecular beam epitaxy were investigated. The thin buffer layer between the ZnO layer and the sapphire substrate decreased the lattice mismatch to achieve higher quality ZnO thin film growth. A GaN buffer layer slightly increased the quality of the ZnO thin film, but the threading dislocations still stretched along thec-axis of the GaN layer. The use of MgO as the buffer layer decreased the surface roughness of the ZnO thin film by 58.8% due to the suppression of surface cracks through strain transfer of the sample. From deep level emission and rocking curve measurements it was found that the threading dislocations play a more important role than oxygen vacancies for high-quality ZnO thin film growth.

Highlights

ZnO has received considerable attention due to its excellent emission properties in the UV region
A GaN buffer layer slightly increased the quality of the ZnO thin film, but the threading dislocations still stretched along the c-axis of the GaN layer
These results indicate the higher quality of ZnO growth on an MgO buffer layer

Summary

Introduction

ZnO has received considerable attention due to its excellent emission properties in the UV region. When ZnO films are grown on c-Al2O3 substrates without a buffer layer, the ZnO films generally show poor crystalline quality with a high dislocation density because of the 18% lattice mismatch between ZnO and c-Al2O3. The improvement in growth quality by using a buffer layer to decrease the lattice mismatch between ZnO and c-Al2O3 is a well-known method. These thin buffer layers for growing ZnO are GaN, MgO, ZnS, or SiC [13,14,15,16]. The growth of ZnO thin films on c-plane sapphire substrates by a two-step temperature variation was demonstrated using different MBE-deposited buffer layers. The ZnO thin film with a 7 nm thick MgO buffer layer, 18 nm thick low-temperature-grown ZnO layer, and 100 nm thick high-temperature-grown ZnO layer showed the best crystallization properties in this study

Growth Conditions and Structures of the Growth Samples

XRD and XRCs Measurements

Deep Level and Edge Band Emission

SEM and AFM Images

Conclusion