Acceptors in ZnO Studied by Photoluminescence
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Acceptors in ZnO Studied by Photoluminescence Michael A. Reshchikov1, James Garbus1, Gabriel Lopez1, Monica Ruchala1, Bill Nemeth2, and Jeff Nause2 1 Physics, Virginia Commonwealth University, 1020 West Main St., Richmond, VA, 23284 2 Cermet, Inc., Atlanta, GA, 30318 ABSTRACT We studied photoluminescence from melt-grown ZnO crystals annealed in air ambient at different temperatures. Along with intense and sharp excitonic lines, we observed several broad bands presumably related to deep acceptors. Two luminescence bands peaking at 1.95 and 2.15 eV at 10 K were studied in detail at different temperatures. The 1.95 eV band has been attributed to transitions from shallow donors to yet unidentified deep acceptor. Very slow non-exponential decay of this band at low temperatures supports such an assumption. INTRODUCTION Due to unique optical properties, ZnO is considered as an alternative to GaN in optoelectronic devices. While structural quality of bulk and epitaxial ZnO is commonly very good, uncontrolled point defects can notably affect the electrical and optical properties of the material as well as degrade the performance and reliability of devices based on ZnO. Photoluminescence (PL) is a powerful tool for the study of point defects, in particular deep acceptors in n-type semiconductors [1]. Such characteristics as PL band width, peak position, presence of phonon replicas, activation energy of temperature-induced PL quenching, shifts of the bands with temperature or excitation intensity, type of PL decay, and others provide a useful guide to identifying the defects. The analysis of defect-related PL in ZnO is complicated by the fact that PL bands from different defects have similar shapes and positions or essentially overlap. For example, a notorious green luminescence (GL) band peaking at 2.45 eV has been assigned to various defects, among which attributions to Cu acceptor [2] and oxygen vacancy (VO) [3] appear to be the most reliable. The Cu- and VO-related PL bands have nearly the same widths and positions. However, the Cu-related band can be recognized at low temperature by a characteristic phonon-related fine structure and exponential decay after pulse excitation with the characteristic time of 440±10 ns [2]. There are many reports on other PL bands in undoped ZnO, including bands peaking at 1.94-1.97 eV [4,5], 2.00-2.05 eV [6-8], 2.17-2.21 eV [7,8], 2.25-2.35 eV [9,10]. All these bands are commonly attributed to native defects such as VZn and Oi [6,8,10]. However, almost no systematic studies of these PL bands can be found. In this work, we present analysis of PL spectra from four undoped ZnO layers studied at different temperatures and excitation intensities, using continuous and pulse excitation. The obtained characteristic properties of defect-related PL bands can be useful for future attributions. EXPERIMENTAL DETAILS A melt grown single ZnO crystal was prepared at Cermet, Inc. in an oxygen overpressure. The crystal was oriented along the c axis and cut into 1×1×0.055 cm3 wafers. Four as-cut wa
