Origin of p -type conduction in single-crystal CuAlO2

We report measurements of the structural, optical, transport, and magnetic properties of single crystals of the anisotropic p -type transparent semiconductor CuAlO2. 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 cm2 V-1 s-1, and we infer a c -axis mobility of 0.12 cm2 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 3.4× 1020 cm-3. These results suggest that the conduction mechanism for p -type carriers in CuAlO2 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.