The Effect of Magnesium Oxide Buffer on the Epitaxial Growth of Zinc Oxide on Sapphire
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Institute for Materials Research (IMR), Tohoku University, Sendai 980-8577, Japan
** Photodynamics
Research Center, The Institute for Physical and Chemical Research (RIKEN), Sendai 980-0868, Japan ABSTRACT ZnO single crystal thin film is grown on A120 3(0001) substrate by plasma-assisted molecular beam epitaxy employing a thin MgO buffer layer. Using reflection high-energy electron diffraction (RHEED), we investigated the surface morphology evolution during the buffer and ZnO growth. We found that a few nanometers thick MgO layer deposited on A120 3(0001) substrate strongly influences the subsequent growth of ZnO by promoting lateral epitaxial growth, which eventually leads to an atomically flat surface. As a result, (3x3) surface reconstruction of ZnO is observed and RHEED intensity oscillations are recorded. Structural investigations indicate that MgO with rock-salt crystallographic structure forms on the A120 3(0001) surface as a template between the substrate and ZnO epilayer. Above that, the ZnO epilayer grows with little strain. The mosaicity in the ZnO film is suppressed by more than two orders as indicated by both symmetric and asymmetric X-ray rocking curves. The twin defect with a 30' in-plane crystal orientation difference is completely eliminated. Free exciton emissions at 3.3774 eV (XA) and 3.383 eV (XB) are observed in photoluminescence at 4.2 K further indicating the high quality of the resulting ZnO epilayers. INTRODUCTION Recently, ZnO based II-VI semiconductor oxides emerge to be an attractive candidate for applications to excitonic optoelectronic devices because of the extremely large exciton binding energy of ZnO and the advantage of bandgap engineering by alloying with MgO and CdO [1]. Similar as its III-V neighbor GaN, due to the absence of suitable substrate materials, main efforts on ZnO epitaxial growth has been concentrated on A120 3 (0001) substrate. However, due to the large lattice mismatch, ZnO films grown on A12 0 3 (0001) showed rough surface morphology and poor crystal quality [2-4]. In the present paper, we demonstrate how to improve of surface morphology and crystal quality by using a thin MgO buffer in plasma-assisted molecular beam epitaxy (MBE) of ZnO on A12 0 3. Since MgO itself is an II-VI oxide compound, the growth is carried out in the same chamber of ZnO growth without introducing inter-contamination problem. Also there is no additional problem in device design caused by an MgO layer, which is completely transparent at the wavelength where ZnO based devices work.
123 Mat. Res. Soc. Symp. Proc. Vol. 619 ©2000 Materials Research Society
EXPERIMENT The growth was carried out on a custom designed MBE system equipped with a rf-plasma source for producing reactive oxygen radicals and effusion cells for evaporating Zn and Mg metal sources. The details of the growth system has been described elsewhere [5]. Commercial A12 0 3 (0001) substrates were used for the growth after conventional degreasing and chemical etching. Before the growth, the substrates were annealed at 800 °C
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