A comprehensive study of the p-type conductivity formation in radio frequency magnetron sputtering grown arsenic-doped ZnO film

As-doped ZnO films were grown by the radio frequency magnetron sputtering method at different substrate temperature TS. For each of the TS’s, the average carrier concentration and mobility were obtained from samples grown by eight independent runs of growth through the Hall measurement. ZnO films grown on SiO2, Si and glass exhibited similar conductivity dependence on TS. As a typical example shown in the figure illustrating the carrier concentration of ZnO:As/SiO2, n-type high resistive films were yielded at low TS (i.e. 200°C). Those grown at TS=350°C and 400°C were p-type, though had large deviations from individual runs of growth. Reproducible p-type films having h~6x10¹⁷cm-3 and µ~6 cm2V-1s-1 were fabricated at high enough TS of 450°C. Conversion of the n-type ZnO:As film grown at TS=200°C to p-type film (p~10¹⁷cm-3) was observed upon the 450°C post-growth annealing in Ar.
The ZnO:As films grown at different TS’s were studied by X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), positron annihilation spectroscopy (PAS) and nuclear reaction analysis (NRA). The XPS and the 10K PL results showed that majority of the As-atoms occupied the Zn-site of the lattice and the As-related acceptor had an activation energy Ea=155 meV. The correlated increase of S-parameter (obtained from the PAS study) showed that the p-type conductivity was associated with the Vzn-related defect and/or its increased open volume. These results were in consistent with Limijumnong et al’s AsZn-2VZn shallow acceptor model [Phys. Rev. Lett. 92, 155504 (2004)], which proposed an Ea=0.15 eV and a VZn volume increase due to atomic relaxation. An anti-correlation between the H-density and the p-type conductivity was also observed in the NRA study. Our results thus suggested that the p-type conductivity was associated with the thermally induced AsZn-2VZn shallow acceptor formation and the H-donor reduction.

This work was supported by the GRF (No.7031/08P) of RGC HKSAR, G_HK026/07 of RGC HKSAR and DAAD Germany, and the UDF of HKU HKSAR.