P-type Nitrogen Doped ZnO Films Grown By Thermal Evaporation
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P-type Nitrogen Doped ZnO Films Grown By Thermal Evaporation Wei Mu, Lei Guo and Lei L. Kerr1 Dept. of Paper and Chemical Engineering, Miami University, Oxford, OH, 45056 David C. Look Semiconductor Research Center, Wright State University, Dayton, OH, 45435 We report the formation of p-type nitrogen doped ZnO (ZnO:N) grown by thermal-evaporation deposition. The effects of nitrogen precursors on the electrical and optical properties of ZnO:N were investigated. This study shows that growth process plays a critical role in the electrical properties of ZnO:N. The chemical reaction mechanism was analyzed. Introduction For many applications, ZnO has certain advantages over GaN, such as a high exciton binding energy (60 meV), low cost, and the availability of large-area substrates [1-3]. However, the development of ZnO for optoelectronic devices is hindered due to the difficulty of synthesizing p-type ZnO to make a p-n homojunction. At this time, in spite of great efforts, there is no recipe for making good p-type ZnO in a reproducible manner [4-10]. Group-V elements are predicted to be the most promising dopant candidates, especially those involving nitrogen [11-18]. Many techniques have been explored to deposit ZnO:N including MOCVD [19-23], sputtering [24-27], pulsed laser deposition [28], molecular beam epitaxy [29], etc. In this work, a low-cost thermal evaporation technique is developed to grow ZnO:N. The nitrogen doping chemistry in ZnO:N is unknown. The origin of the typically weak p-type electrical properties is far from being fully understood, partly because the results obtained from different groups are rather inconsistent [30-31]. It has been suggested that the NO or NO2 molecule is a more efficient N source than the N2 or N2O molecule. This is because the diatom precursors contains pairs of N and can easily incorporate (N-N)O into the film [30], possibly contributing to the n-type behavior of ZnO:N. However, Xu et. al. observed that native donor-type defects (e.g. Zni or VO) can be reduced by using an appropriate amount of N2O and thus lead to an improvement of the p-type conductivity [31]. Therefore, in this study, two different N doping precursors (pure NO and 5 vol% NO and N2 mixture) were used to investigate the effects of single-atom and diatom N on film conductivity. Experiments ZnO is a naturally n-type material, mainly due to donor-type impurities such as Al, but also partially due to native defects, in particular, the zinc interstitial [32-33]. 1
Corresponding author:
Dr. Lei L. Kerr, Dept. of Paper and Chemical Engineering, Miami
University, Oxford, OH 45056. (Tel) 513-529-0768. (Fax) 513-529-0761. 1
[email protected].
Thus, theoretically, if such impurities and defects could be eliminated, and if acceptors such as N could be incorporated, then a high hole concentration would be possible. In this study, two separate processes of thermal evaporation deposition were designed to determine the effects of oxygen nonstoichiometry. All the samples are grown on glass substrates. Process I
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