We report measurements of the structural, optical, transport, and magnetic properties of single crystals of the anisotropic p-type transparent semiconductor CuAlO 2. The indirect and direct band gaps are 2.97 and 3.47 eV, respectively. Temperature-dependent Hall measurements yield a positive Hall coefficient in the measured range and an activated carrier temperature dependence. The resistivity is anisotropic, with the ab-plane resistivity about 25 times smaller than the c-axis resistivity at room temperature. Both are activated with similar activation energies. The room-temperature ab-plane mobility is relatively large at 3 cm 2 V −1 s −1 , and we infer a c-axis mobility of 0.12 cm 2 V −1 s −1. The Seebeck coefficient is positive at all measured temperatures, and has a T −1 dependence over most of the measured range. The low-temperature paramagnetic moment is consistent with a spin-1/2 defect with a density of 3.4ϫ 10 20 cm −3. These results suggest that the conduction mechanism for p-type carriers in CuAlO 2 is charge transport in the valence band and that the holes are thermally activated from copper-vacancy acceptor states located about 700 meV above the valence-band maximum.

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X-ray photoelectron spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) measurements of the optimized magnetic tunnel junctions (MTJs) with AlO and AlN barriers have been performed to study the chemical structures of the barrier and the underlying layer. These MTJs with AlO and AlN barriers exhibited increased tunneling magnetoresistance (TMR) after annealing at 200°C from 27% to 45% and from 25% to 33%, respectively. Surprisingly, the XPS and XAS measurements confirmed that both the as-grown and the annealed MTJs had metallic Co and Fe at the interface between the barrier and the underlying CoFe layer. After annealing, under-stoichiometric AlOx and AlNx phases in MTJs with AlO and AlN barriers partially transformed into stoichiometric Al2O3 and AlN phases, respectively. Thus the increase in TMR after annealing for MTJs with clean interface between the barrier and the underlying layer is believed due to the anion redistribution inside the barrier layer, not from back diffusion from pinned magnetic layer to barrier layer.

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