Defects in bulk ZnO studied by steady-state and time-resolved photoluminescence
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1035-L11-15
Defects in bulk ZnO studied by steady-state and time-resolved photoluminescence M. A. Reshchikov Department of Physics, Virginia Commonwealth University, Richmond, VA, 23284 ABSTRACT Unintentionally doped bulk ZnO samples were grown by hydrothermal method in Tokyo Denpa Co. Ltd. (Japan) and MTI Corporation. At low temperatures the PL spectrum contained a very broad band with the peak position (between 2.0 and 2.4 eV) depending on the excitation intensity. Evolution of the PL spectrum after a pulse excitation revealed that the broad band is composed of an orange (OL) and green (GL) luminescence bands having maxima at 1.96 and ~2.35 eV, respectively. The GL band dominated at times up to 1 ms and then disappeared. The OL band decayed as approximately t -1 over a wide time interval, and its spectrum could be recorded even 24 hours after the excitation source was switched off. The slow nonexponential decay of the OL band is attributed to transitions from shallow donors to a deep acceptor (donoracceptor pair transitions). INTRODUCTION In spite of considerable attention to defect-related broad photoluminescence (PL) bands in ZnO, the origin of the deep-level defects in this semiconductor remains uncertain, and transitions causing these bands are not well identified. One of the most studied defects in ZnO is the CuZn acceptor, which is responsible for the green band peaking at 2.45 eV with a characteristic phonon-related fine structure and zero-phonon line at 2.859 eV at low temperature [1]. The LiZn deep acceptor is responsible for the orange luminescence (OL) band peaking at 2.0 eV in ZnO:Li [2,3,4]. In fact many more broad bands can be observed in the visible part of the PL spectrum from undoped n-type ZnO [5]. 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. Time-resolved PL (TRPL) and temperature-dependent steady-state PL (SSPL) enable one to resolve PL bands related to different defects and determine types of optical transitions. We studied in detail the OL band and PL bands contributing in green part of the emission spectrum in high-quality ZnO bulk crystals grown by hydrothermal method. EXPERIMENTAL DETAILS Unintentionally doped bulk ZnO samples were grown by hydrothermal method in Tokyo Denpa Co. Ltd., Japan (TD) and MTI Corporation. Three TD and four MTI samples (from both Zn and O faces) were studied by SSPL, and selected samples were studied by TRPL. The SSPL and TRPL were excited with a cw He-Cd laser (50 mW, photon energy 3.81 eV) and a pulsed nitrogen laser (5-ns pulses with repetition frequency 6 Hz and photon energy 3.68 eV), respectively. The PL signal was dispersed by a 1200 rules/mm grating in a 0.3 m monochromator and detected by a cooled photomultiplier tube. Calibrated neutral-density filters were used to attenuate the excitation power density (Pexc) in the range from 10-7 to 100 W/cm2 in the SSPL experiments. A closed-cycle optical cryostat was used for temperatures between 1
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