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1035-L03-05

Visible Luminescence Related to Defects in ZnO M. A. Reshchikov1, B. Nemeth2, J. Nause2, J. Xie3, B. Hertog3, A. Osinsky3, V. Avrutin4, N. Izyumskaya4, R. Shimada4, and H. Morkoc4 1 Department of Physics, Virginia Commonwealth University, Richmond, VA, 23284 2 Cermet, Inc., Atlanta, GA, 30318 3 SVT Associates, Inc., Eden Prairie, MN, 55344 4 Department of Electrical Engineering, Virginia Commonwealth University, Richmond, VA, 23284 ABSTRACT We studied several photoluminescence (PL) bands in undoped, Li-, Ga-, and N-doped highquality ZnO thin films grown by molecular beam epitaxy (MBE) and bulk crystals. By analyzing PL in a wide range of excitation power densities, sample temperature, and decay time following a laser pulse excitation, we delineated and analyzed more than 10 broad bands with unique luminescence properties. Among these bands, only the Cu-related green band with a characteristic fine structure and the Li-related orange band were well-studied and reliably identified in the past. INTRODUCTION First-principles calculations indicate that only the formation of zinc vacancies (VZn) is energetically favorable in considerable amount in n-type ZnO, while the formation of oxygen vacancies (VO) and other native defects are unlikely in as-grown ZnO [1]. This prediction and results of optically detected electron paramagnetic resonance (EPR) experiments [2] contrast the widespread belief that the green luminescence band in ZnO is caused by VO (Refs. [3] and [4] which have been cited some 1000 times in the last 5 years). In spite of numerous reports on defect-related PL in ZnO, only a few defects have been reliably identified by PL and EPR methods, which include shallow donors [5] and transition elements [6]. As for the broad bands commonly observed in the visible part of the PL spectrum, only a few of them are more or less reliably identified. One of the most studied defects in ZnO is the CuZn acceptor, which is in part responsible for the green band peaking at 2.45 eV with a characteristic phonon-related fine structure and zerophonon line at 2.859 eV at low temperature. This structured PL band has been assigned to the internal transition of a hole from the excited state at ~ EV + 0.4 eV to the ground state at ~ EC − (0.1 − 0.2) eV [7,8]. Two other well-studied defects are LiZn and NaZn centers which are deep acceptors responsible for the orange luminescence (OL) band peaking at 2.0 eV (615 nm) in ZnO:Li and the yellow luminescence (YL) band peaking at 2.17 eV (570 nm) in ZnO:Na, respectively [9-12]. In particular, recombination of an electron from the shallow donor with a hole trapped at the LiZn acceptor gives rise to the partially polarized OL band having very long afterglow at helium temperature, on the order of a few hours [9]. In fact many more broad bands can be observed in the visible part of the PL spectrum from undoped n-type ZnO. By analyzing the literature one notes that the term “yellow” or “green”

luminescence is applied to numerous PL bands with positions of maxima ranging from 2.0 to 2