Plasma Immersion Ion Implantation Preparation Of P-type Zno Thin Film Materials

A new hot spot in the field of optoelectronics, zinc oxide (ZnO), has numerous application prospects and enormous potential market, to which world-wide science and technology groups attach great importance. However, the main challenge during the research of ZnO is to fabricate repeated, steady and device-level p-type ZnO of low resistivity. In this paper, a new technique of p-type ZnO film preparation is introduced, which is plasma immersion ion implantation (PⅢ) assisted with co-doping method. First, AI doped ZnO (AZO) films are grown by radio-frequency magnetron sputtering and later laid into the vacuum PⅢchamber. The input gas (N source) is discharged by a 600W radio-frequency power supply and uniform inductively coupled bulk plasma is generated. The parameters of the generated plasma are diagnosed by Langmuir dual probe and multi-channel optical emission spectroscopy (OES). The substrate holder connects to a negative pulsed high voltage bias with a rise time less than 10-6s. The pulse duration and frequency are adjustable and the maximum amplitude of the bias is 65kV. The high voltage bias can effectively break former bonding configurations during the implantation and enable N element incorporate into AZO films, which forms N-Al co-doped ZnO. Affected by radiation diffusion caused by ion bombardment, the ion penetration depth can reach several hundred nanometers. The density distribution of implanted ions can be modified by ion bombardment energy (also the negative bias). After the implantation, implanted N can be activated by controlling the annealing process, which improves the crystallinity and makes p-type conversion of ZnO achievable. Using this method, we have managed to fabricate p-type ZnO.Characterization of prepared ZnO films is carried out, including electrical, optical and morphological properties. The mechanism of p-type conversion of ZnO films and the influence of experimental parameters are thoroughly discussed, including implantation gas, implantation dose, annealing temperature and Al content in AZO films.(i) The key of p-type conversion of ZnO films is the control of defects in ZnO films. When acceptors are dominant, ZnO shows p-type property. On the contrary, if donors exceed acceptors in the films, ZnO therefore becomes n-type. By changing the implantation gas, we can control the forms of N defects in ZnO films. The plasma generated need to provide abundant N+ rather than N2+ to form much more donors in the films because No are donor-like defects but (N2)o are acceptor-like defects. Such plasma with abundant N+ can be achieved when the implantation gas is composed of 25％NO and 75% O2. By the post-implant annealing, implanted N can occupy O vacancies and provide holes, contributing to p-type conduction. In other cases, such as pure N2 or pure NO, the number of N2+ in the plasma largely exceeds that of N+, therefore inhibiting the p-type conversion of ZnO films.(ⅱ) The influence of implantation dose to p-type conversion of ZnO is studied when the implantation gas is 25% NO+75% O2. Implanted N tend to occupy O vacancies. When most of the O vacancies are filled, N is likely to substitute O to bond with Zn and Al. Too much or less implantation dose cannot achieve p-type ZnO because too less dose cannot fully compensate the intrinsic defects in the AZO films while excessive dose introduces new defects.(ⅲ) Annealing process is a vital ring in the whole chain of p-ZnO film fabrication. Only when the annealing temperature exceeds a threshold (850℃in our work), p-type ZnO films can be achievable. That is because the implanted N prefer to act as interstitial defects before annealing and implanted N will occupy O vacancies and provide holes only certain temperature of N activation is reached.(ⅳ) Al content in the AZO films can also affect the defect composition and further influence the electrical property. When Al content is low, there is not enough Al to bond with implanted N, so the solubility of N in ZnO films is low and it is impossible to achieve p-type conversion. But excessive N also prohibit p-type conversion because of the large amount of acceptors Al introduces. Only proper Al content can enhance the effect of donor-acceptor co-doping, promoting more N incorporating into ZnO films.