Donors and Acceptors in Bulk ZnO Grown by the Hydrothermal, Vapor-Phase, and Melt Processes
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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. An interesting result of this study is a close correlation between the PL-derived Al (I6) donor energy of 51 meV, and the T-Hall-derived energy of 52 meV.
Look, D. C.
(2007). Donors and Acceptors in Bulk ZnO Grown by the Hydrothermal, Vapor-Phase, and Melt Processes. MRS Proceedings, 957, 127-136.