Two-dimensional growth of ZnO epitaxial films on c-Al2O3(0001) substrates with optimized growth temperature and low-temperature buffer layer by plasma-assisted molecular beam epitaxy

High-quality ZnO thin films were deposited on c-plane sapphire substrates with the low-temperature (LT) ZnO homo-buffer layer by plasma-assisted molecular beam epitaxy. LT ZnO buffer layer with the thickness of 15nm was grown at 500degreesC. After high-temperature annealing at 800degreesC for 30min, the growth of ZnO with about 800nm thickness was restarted at different temperatures from 680 to 800degreesC. Although the surface of the LT-buffer layer was three-dimensional, appropriate subsequent growth temperature facilitated two-dimensional growth. The smallest full-width at half-maximum (FWHM) of X-ray omega-rocking for ZnO(0 0 0 2) diffraction was 85arcsec and then slightly increased with the increament of the deposition temperature. The RHEED pattern over the surface of ZnO film grown at 720degreesC showed very streaky lines, while streaky lines superimposed with spotty patterns were obtained at other temperatures. From the Hall measurement. the mobility values for the ZnO films deposited at 720 and 760degreesC were 103 and 105 cm(2)/V s. and the carrier concentration was 2.45 x 10(17) and 2.21 x 10(17)/cm(3), respectively. In low-temperature photoluminescence measurement at 10K, most of the ZnO thin films showed neutral donor-bound exciton, I-4(D-0, X) at 3.362eV and acceptor-bound exciton, I-10(A(0), X) at 3.3497eV were clearly observed with the phonon replica at 3.308eV, and the lowest FWFM of I-10 peak was found to be 8.4meV for the ZnO grown at 720degreesC. (C) 2004 Elsevier B.V. All rights reserved.