Origin of $p$-type conduction in single-crystal ${\text{CuAlO}}_2$

We report measurements of the structural, optical, transport, and magnetic properties of single crystals of the anisotropic $p$-type transparent semiconductor ${\text{CuAlO}}_2$. 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 $3{\text{cm}}^2{\text{V}}^{-1}{\text{s}}^{-1}$, and we infer a $c$-axis mobility of $0.12{\text{cm}}^2{\text{V}}^{-1}{\text{s}}^{-1}$. 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 $3.4\times {10}^{20}{\text{cm}}^{-3}$. These results suggest that the conduction mechanism for $p$-type carriers in ${\text{CuAlO}}_2$ 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.

Figure 1

(Color online) The crystal structure of the 3R polytype of ${\text{CuAlO}}_2$. Alternating layers of Cu atoms and ${\text{AlO}}_6$ octahedra stacked along the $c$ axis give the structure its anisotropic character.

Figure 2

(Color online) Powder x-ray diffraction pattern of ${\text{CuAlO}}_2$ (upper trace, logarithmic scale). The powder was made from grinding single crystals similar to those depicted in the inset. The lower trace (linear scale) is the reference pattern for the 3R polytype.

Figure 3

(Color online) Temperature-dependent transport properties of single-crystal ${\text{CuAlO}}_2$ measured in the $ab$ plane. (a) The hole carrier density $p$ determined from in-plane Hall coefficient measurements. (b) The in-plane resistivity ${\rho }_{ab}$. (c) The mobility $\mu$ derived from ${\rho }_{ab}$ and $p$. Insets show the linearity of $\text{log}p$ and $\text{log}{\rho }_{ab}$ vs $1/T$, and of $\text{log}\mu$ vs $\text{log}T$, and the van der Pauw contact configuration.

Figure 4

(Color online) (a) Resistivity of a single crystal of ${\text{CuAlO}}_2$ from 180 to 290 K measured in the $ab$ plane and in the $c$-axis direction. The insets show the contact configuration. (b) The temperature-dependent resistivity anisotropy.

Figure 5

Seebeck coefficient of a ${\text{CuAlO}}_2$ single crystal. The sharp increase at low temperatures is indicative of semiconducting behavior. Inset: power factor $S^2\sigma$. The data for $T<400\text{K}$ are from this work, while the higher temperature data are from Ref. 7.

Figure 6

Optical absorption at room temperature of a $20\mu \text{m}$-thick ${\text{CuAlO}}_2$ single crystal with light incident along the $c$ axis. Two defect absorptions at 0.85 and 1.75 eV are evident. At higher energies band-to-band absorption occurs, with an indirect gap at 2.97 eV and a direct gap at 3.47 eV. The absorption values are systematically high because reflection from the faces of the crystal is not accounted for.

Figure 7

(Color online) Magnetic susceptibility of 186 mg of ${\text{CuAlO}}_2$ single crystals from 5 to 300 K. Inset: magnetic susceptibility of ${\text{CuAlO}}_2$ powder.

Figure 8

Energy-level diagram showing acceptor levels about 700 meV above the valence-band maximum. The donor density is much lower than the acceptor density, and the Fermi level is approximately half way between ${\epsilon }_A$ and the valence-band maximum. Direct and indirect band gaps are 3.47 and 2.97 eV, respectively.