Surface Morphologies and Optical Properties of Homoepitaxial ZnO Grown by Close-Spaced Chemical Vapor Transport
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1012-Y03-25
Surface Morphologies and Optical Properties of Homoepitaxial ZnO Grown by Close-Spaced Chemical Vapor Transport Koji Abe, Tetsuya Tokuda, Yuta Banno, and Osamu Eryu Department of Electrical and Electronic Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya, 466-8555, Japan
ABSTRACT Chemical vapor transport (CVT) is studied for homoepitaxial growth on O-polar ZnO substrates. Characterization of the surface morphology, crystal quality and luminescent properties of the epilayers is presented. To increase growth rate at high temperatures, we keep a substrate close to ZnO source powder. Surface smoothness and crystal quality of epilayers are remarkably improved by increasing a substrate temperature. Smooth surfaces are observed on the epilayer grown at substrate temperatures above 920∞C. INTRODUCTION Zinc oxide (ZnO) is expected as a material for transparent electrodes and light-emitting devices, owing to its attractive properties such as wide bandgap energy of 3.37 eV and large exciton binding energy of 60 meV. However, fabrication of ZnO-based devices is still limited because the formation of reproducible p-type ZnO layer is difficult, despite much progress in epitaxial growth techniques [1-3]. Crystal defects and unintentional impurities enhance difficulties in p-type doping. Epitaxial growth of ZnO on various substrates has been studied extensively. Homoepitaxy is expected to realize high-quality ZnO films because ZnO films grown on lattice-matched ScAlMgO4 have shown high crystallinity [1]. Many techniques such as molecular beam epitaxy, pulsed laser deposition, and chemical vapor transport (CVT) have been used to perform growth of ZnO. CVT has advantages of being able to achieve high-quality ZnO homoepitaxial films with high growth rate since this growth technique has been employed for growth of ZnO bulk single crystals [4,5]. In this study, homoepitaxial growth of ZnO has been carried out with close-spaced CVT (CS-CVT). We have investigated the surface morphologies, optical and electrical properties of ZnO films grown on O-polar ZnO substrates. EXPERIMENTAL DETAILS Substrates used in this study were commercial O-polar ZnO (000-1) grown by a hydrothermal technique. ZnO epilayers were deposited on the ZnO substrates at various substrate temperatures by CS-CVT. Schematic diagram of the CS-CVT system used in this study is illustrated in figure 1. The substrate was mounted on the lid of the graphite crucible filled with source ZnO powder (99.999%). The diameter and the height of the crucible were 20 mm and
Pyrometer
ZnO substrate
Graphite crucible
Ar CO2
ZnO powder Inner ring
Work coil Pump Figure 1. Schematic diagram of the CS-CVT system used in this study. 15 mm, respectively. The source and substrate temperature were monitored by an optical pyrometer. The inner ring was used to keep a constant substrate temperature. The growth time was 1 h. During the growth, the distance between the substrate and the source powder increased from 2 mm to 5 mm with decreasing source powder. In a co
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