Photoluminescence and Raman spectra in Ga-doped ZnO layers on sapphire
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1035-L13-12
Photoluminescence and Raman spectra in Ga-doped ZnO layers on sapphire M. A. Reshchikov1, S. Nagata1, J. Xie2, B. Hertog2, and A. Osinsky2 1 Department of Physics, Virginia Commonwealth University, Richmond, VA, 23284 2 SVT Associates, Inc., Eden Prairie, MN, 55344 ABSTRACT Ga-doped ZnO layers were grown on sapphire substrates by molecular beam epitaxy (MBE). Low-temperature photoluminescence (PL) and room-temperature Raman spectra were investigated. Defect-related modes at 277 and 510 cm-1 appeared in the Raman spectrum for Gadoped layers. The PL spectrum is dominated by a donor-bound exciton peak at 3.356 eV. A weak yellow luminescence (YL) band peaking at 2.1-2.2 eV was studied in detail. It shifted to higher photon energies (up to 0.1 eV) with increasing excitation intensity. The YL band is attributed to transitions from shallow donors to a deep acceptor. The acceptor is thought to be a Zn vacancy-related defect because the intensity of the YL band decreased dramatically with Ga doping. INTRODUCTION In spite of impressive progress in the growth of high-quality ZnO [1], the origin and characteristics of point defects in this wide-bandgap semiconductor material are not well understood. Photoluminescence (PL) and Raman spectroscopy are powerful tools for the study of point defects in wide-bandgap semiconductors [2,3]. The analysis of defect-related PL in ZnO is complicated by the fact that broad PL bands from different defects have similar shapes and positions or essentially overlap. The PL bands in yellow and green range of the spectrum are often attributed to native defects such as VZn, VO, and Oi [4,5], but this remains highly controversial. The large number of PL bands observed in the PL spectrum of undoped ZnO indicates that native defects are not the only source of the spectral features. Other sources such as unintentionally introduced impurities and defect complexes formed by interactions between native defects and dopants should also be considered as possible candidates for the source of dominant point defects. In this work, we present an analysis of the optical properties of Ga-doped ZnO layers grown on sapphire substrate by molecular beam epitaxy (MBE). The yellow luminescence (YL) band with a maximum at 2.1-2.2 eV was studied in detail. EXPERIMENTAL DETAILS Six Ga-doped ZnO layers and a few undoped control layers, all ~1 µm-thick, were deposited on c-plane sapphire substrate at SVT Associates by RF-plasma MBE technique (Table I). The samples were annealed in air during 10 min at temperatures between 500 and 700°C. Electron concentration (n0) was determined from Hall effect measurements at room temperature. Raman spectra were recorded in backscattering configuration using the 632.8 nm line of a HeNe laser and Horiba Jobin Yvon LabRAM HR spectrograph equipped with a liquid nitrogen-cooled CCD detector. PL was excited with a cw He-Cd laser (50 mW, photon energy 3.81 eV), dispersed by a 1200 rules/mm grating in a 0.3 m monochromator and detected by a cooled photomultiplier tube.
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