Stable p-type conductivity in Bi-doped ZnO (Vol. 42, No. 6)

Zinc Oxide (ZnO) has potential applications in varistors, light emitting diodes and photo detectors. The primary obstacle against its use in optoelectronic devices is the lack of stable p-type material. ZnO is naturally an n-type material, its majority carriers being electrons and it must be doped to become p-type. Most p-type dopants introduce either deep or shallow acceptors and the resulting material is p-type with either very low carrier concentration or unstable conductivity. Many previous efforts to synthesize p-type ZnO used N, As and P as dopants. In the present study bismuth has been chosen.

Thin films of Bi-doped ZnO were grown with a pulsed laser deposition system using a homogeneous target. Sample thickness was about 150 nm. XRD results showed the wurtzite structure of the films and XPS confirmed the presence of Bi. No evidence of secondary phases was found. From Hall measurements made over repeated cycles, in-situ annealed 3 and 5% Bi-doped samples are p-type. However as-grown 3% doped films showed unstable p-type conductivity, which suggests that some form of activation of Bi in ZnO occurs during the post-growth annealing leading to the p-type conduction. In the as grown 5% doped samples, the Bi concentration seems high enough to impose p-type conduction. Bi substitution in ZnO lattice is known to produce acceptors; this is the case here as shown by photoluminescence experiments. Carrier concentrations were 5.4 x 1018 and 4.8 x 1019 cm-3 in annealed 3% and 5% Bi-doped samples, respectively. Temperature-dependent photoluminescence leads to an acceptor energy level at about 0.13 eV above the valence band. The p-type conductivity of these Bi-doped ZnO thin films is stable under oxygen-rich ambient or upon annealing. Thus, this study suggests a possible pathway for developing ZnO based optoelectronic devices.