Unexpected influence of substrate temperature on the properties of P-doped ZnO

The electrical conductivity of phosphorus-doped ZnO (P:ZnO) was found to change with sputter substrate temperature. Unexpectedly, n-type conductivity was obtained for samples deposited at substrate temperatures of 200, 250 and 300 °C, but p-type conductivity was achieved for samples deposited at room temperature (RT). The type of a ZnO sample was determined and cross-checked by using three different techniques: the Hall effect, the Seebeck effect and magnetic field-dependent Hall voltage measurements. The n-type samples had conductivities of 11–45 S cm−1 and the p-type sample possessed a conductivity of 0.74 S cm−1. Furthermore, significantly different activation energies were found for the n-type (10 meV) and p-type (134 meV) P:ZnO samples, which were in good agreement with the positions of the donor and acceptor levels in the band structure reported in the literature. SIMS profiling measurement revealed that the oxygen concentration in the P:ZnO film deposited at RT was clearly higher than those in the films deposited at higher substrate temperatures. The results and relevant mechanisms were discussed. It appears that the n-type or p-type conductivity of a P:ZnO sample was determined mainly by the dominance of the n-type defect donors or the P induced acceptors.