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 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 3.4 x 10(20) 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.