Effect of Thermal Annealing on the Characteristics of Phosphorus-Implanted ZnO Crystals

Abstract

A P-doped ZnO surface layer on undoped ZnO wafers was prepared by phosphorus (P) ion implantation. Hall effect measurement revealed p-type conduction in such layers annealed at 800°C. This indicates that acceptor levels are present in P-doped ZnO, even though the ZnO is still n-type. Micro-Raman scattering in −z(xy)z geometry was conducted on P-implanted ZnO. The E 2 high mode shift observed toward the high-energy region was related to compressive stress as a result of P-ion implantation. This compressive stress led to the appearance of an A 1(LO) peak, which is an inactive mode. This A 1(LO) peak relaxed during thermal annealing in ambient oxygen at temperatures higher than 700°C. The P2p3/2 peak observed at 135.6 eV by x-ray photoelectron spectroscopy is associated with chemical bond formation leading to 2(P2O5) molecules. This indicates that implanted P ions substituted Zn sites in the ZnO layer. In photoluminescence spectroscopy, the P-related peaks observed at energies ranging between 3.1 and 3.5 eV originated from (A0, X) emission, because of PZn-2VZn complexes acting as shallow acceptors. The acceptor level was observed to be 126.9 meV above the valence band edge. Observation of this P-related emission indicates that ion implantation results in acceptor levels in the P-doped ZnO layer. This suggests that the P2O5 bonds are responsible for the p-type activity of P-implanted ZnO.