Invited Paper EM-ThM1
Bulk and Surface Impurities and Point Defects in ZnO

Thursday, October 18, 2007, 8:00 am, Room 612

ZnO is a popular material at present because of potential photonic and electronic applications, such as UV LEDs and transparent transistors. As with most semiconductors, the bulk optical and electrical properties are controlled by impurities and defects acting as donors, acceptors, traps, and recombination centers. However, ZnO is somewhat unusual in that the surface also has donors and acceptors that produce electrical activity and can affect Ohmic and Schottky contact formation, among other things. Bulk donors and acceptors can be quantified by temperature-dependent Hall-effect (T-Hall) measurements, and their identities can often be determined by correlation with low-temperature photoluminescence (PL) and SIMS measurements in conjunction with doping and electron-irradiation experiments. Surface donors and acceptors, however, are much more difficult to quantify, although in this work we show that fairly reliable concentrations of surface donors (but not acceptors) can be determined by means of a low-temperature Hall-effect measurement and a room-temperature electrochemical C-V (ECV) measurement. Information on the possible identities of these surface donors can be obtained with XPS, SIMS, and other surface-sensitive techniques. We present T-Hall, PL, and ECV data on bulk, commercially available, ZnO samples grown by the vapor-phase, melt, and hydrothermal methods. Accurate concentrations of bulk donors and acceptors are obtained in all cases; the donors are tentatively assigned to various combinations of H, Al, Ga, and a Zn-interstitial complex, and the acceptors, to Li and the Zn vacancy. Fairly accurate densities of surface donors are also obtained in each case, and it is speculated that they are due to H complexes since their concentrations can be significantly increased by forming-gas anneals. To our knowledge, this is one of the first quantifications of surface donors in any semiconductor material, and the methodology should have general applicability.