Donors and Acceptors in Bulk ZnO Grown by the Hydrothermal, Vapor-phase, and Melt Processes
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Donors and Acceptors in Bulk ZnO Grown by the Hydrothermal, Vapor-Phase, and Melt Processes David Look Semiconductor Research Center, Wright State University, 3640 Colonel Glenn Highway, Dayton, OH, 45435 ABSTRACT The optical and electrical properties of commercially available bulk ZnO samples grown by the hydrothermal, vapor-phase, and melt methods are compared. Low-temperature photoluminescence (PL) is used to identify the presence of common impurities, such as H, Al, Ga, and In, and temperature-dependent Hall-effect (T-Hall) measurements are used to obtain donor energies and to quantify donor and acceptor concentrations. All three types of material produce sharp donor-bound-exciton (D0X) PL lines, I4, I6, I8, and I9, generally associated with H, Al, Ga, and In, respectively. However, the I4 and I9 lines are weak in hydrothermal ZnO, and the I4 line is also weak in melt material. Another D0X line I5, possibly associated with donors near the surface, appears in some hydrothermal samples. Electrically, hydrothermal ZnO has a much lower bulk conductivity than vapor-phase or melt ZnO, partly because the donor concentration is lower, but also because the acceptor concentration is higher. A consequence of the low bulk conductance is that surface conductance becomes much more important and must be included in the Hall-effect modeling. INTRODUCTION Most of the bulk ZnO samples available commercially are grown by vapor-phase (VP), melt (MLT), or hydrothermal (HYD) processes. In the last few years, VP and MLT samples have been characterized most thoroughly, partly because of their ready availability from ZN Technology, Inc. [1], and Cermet, Inc. [2], respectively. However, recently, high-quality HYD material has been marketed by Tokyo Denpa, Ltd. [3], and other companies, so that a comparison of the optical and electrical properties of all three types of materials would now seem to be in order. We have chosen one VP sample from ZN Technology, one MLT sample from Cermet, and two HYD samples from Tokyo Denpa, one obtained about 1 year ago (HYD1), and the other, about two years ago (HYD-2). The two HYD samples differ from each other in their low-temperature (but not room-temperature) electrical characteristics, and in their nearband-edge plotoluminescence properties at 4.2 K. Each of the four samples of this study is of high quality, but not necessarily the highest quality available from that particular supplier. For example, we have studied VP and MLT samples with higher peak mobilities that those presented here. More importantly, the present samples display some of the unique features peculiar to each growth method. The main characterization techniques employed in this investigation are temperature-dependent Hall-effect (T-Hall) and low-temperature photoluminescence (PL)
measurements, because these methods provide detailed, quantitative information on donors and acceptors [4,5].
HALL-EFFECT MEASUREMENTS Temperature-dependent Hall-effect measurements were performed on an automated LakeShore 7507 system, using
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