ZnO has a strong potential for various short-wavelength optoelectronic devices such as light emitting diodes (LED) and laser dioeds (LD), because of its wide band gap energy of 3.37 eV and large exciton binding energy of 60 meV at room temperature, which is one of the advantages over GaN. In order to exploit the potential offered by ZnO, both high-quality n- and p-type ZnO are necessary. ZnO crystallographically has a wurtzite structure and is composed of tetrahedrally-coordinated Zn2+ and O2？ ions, where the large difference in ionic sizes of Zn (0.83 Å) and O (1.40 Å) somewhat tends to break the tetrahedral lattice symmetry. This leads to the formation of non-stoichiometric defects and results in an ionic charge imbalance that makes ZnO naturally conductive even without intentional doping. N-type is easy to realize because ZnO occurs naturally as n-type conduction due to the presence of intrinsic donor defects, such as oxygen vacancy (VO) and zinc interstitial (Zni). There have been many reports for growth of high quality n-type ZnO using group-Ⅲ elements such as B, Al, Ga and In as dopants.
However, the realization of p-type ZnO has been proven difficult and is thought to the bottleneck due to high self-compensation, low solubility of the dopant and deep acceptor level. P-type dopants in ZnO may be possible by substituting either group-Ⅰ elements (Li, Na, K) for Zn sites or group-Ⅴ elements (N, P, As) for O sites.
In this study, Li-doped ZnO thin films were prepared by RF magnetron sputtering system using a 1 at.% Li doped ZnO target. The structural and electrical properties of the ZnO thin films were characterized by using scanning electron microscopy (SEM), x-ray diffraction (XRD), and Hall effect measurement. Also the vibrational modes of the samples were examined by using Raman spectroscopy, and the LiZn related optical emission properties were characterized by means of photoluminescence (PL) measurements. The variation of p-type conductivity in Li-doped ZnO thin films are analyzed in detail, and the mechanisms for the p-type behaviors are discussed.