Paper EM-ThM12
Preparation of p-type ZnO by Codoping Method using Gallium and Arsenic

Thursday, November 12, 2009, 11:40 am, Room B1

In this research, we successfully prepared p-type ZnO films by rapid thermal annealing (RTA) of Ga and As codoped ZnO films deposited by magnetron DC sputtering using target made of ZnO mixed with Ga₂O₃ and Zn₃As₂ powders. These codoped p-type ZnO films were deposited on undoped silicon substrates at 300^oC and have hole concentrations of 10¹⁸~ 10¹⁹ cm^-3 and resistivities of 10^-1~10^-2 ohm-cm. We changed the oxygen flow rate from 0 to 50 sccm and the RTA time for 30 seconds, 2 minutes, and 5 minutes. From the Hall measurement, the electrical conduction type of codoped ZnO film converted from n-type to p-type after RTA of the films at 1000^oC for 30 seconds. But it converted to n-type conduction again for longer annealing time of 2 minutes and 5 minutes. The X-ray diffraction (XRD) data indicates the crystallinity of the codoped films is improved by increased RTA time. From the field emission scanning electronic microscopy (FESEM) observation, we found the grain size of ZnO increases with annealing time, but there is less significant change of grain size after 30 seconds of RTA. The result indicates the grain boundary may not affect the electrical conduction type. According to the room temperature photoluminescence (PL) spectra, we found that the sample prepared with oxygen flow rate of 30 sccm followed by RTA at 1000^oC for 30 seconds in O₂ ambient had the highest near band edge (NBE) emission intensity. It indicates this RTA condition is the optimum RTA condition among other conditions. We found that the p-type ZnO films can be obtained with the same RTA parameter even though the ZnO films are prepared with different oxygen flow rate during deposition, i.e., the p-type conduction is independent of the oxygen flow rate in the deposition process and the ZnO films need an RTA process to activate the p-type conduction. From the X-ray photoelectron spectroscopy (XPS), we can not find the Ga-As bonding either in Ga 3d peak or As 3d peak. It indicates the Ga and As do not bond together in the ZnO film. This fact agrees with the codoping theory proposed by Yamamoto and Katayama-Yoshida. According to others researches and calculations, doped p-type ZnO films are produced in the oxygen-rich growth condition. We think the codoping method, which can lower the Madelung energy and activate the deep acceptor state may enhance p-type formation. Thus even in the zinc-rich condition, we can still obtain p-type ZnO thin films at appropriate RTA condition by codoping method.