Preparation Of Al-N Codoped P-type ZnO Thin Films And Investigation On Its Property

Zinc Oxide with a direct wide bandgap of 3.3 eV at room temperature has recently attracted considerable attention because of its many potential applications. Comparing with other wide bandgap semiconductor materials, ZnO has many promising advantages: ZnO has large excitonic binding energy(60meV at RT), which promises strong photoluminescence from bound excitonic emissions at room temperature; it is more resistant to radiation damage; high- quality ZnO with very low defect densities can be synthesized at relatively low temperature. For these advantages, ZnO is promised to be widely used in the photoelectricity devices such as UV light-emitting diodes (LEDs) and lasers. To realize such optoelectronic devices, it is required to control the electronic properties, such as the nature of conduction as well as its carrier density and mobility. The naturally occurring ZnO has n-type conductivity which can be enhanced by doping with group-Ill elements ( Al, Ga, etc. ). However, ZnO has suffered from one major disadvantage: the lack of good, reliable, p-type material. There are several possbile reasons for this dilemma: (1) lack of shallow-acceptor dopants; (2) low solubility of such dopants; and (3) compensation by increased levels of impurity and native defect donor species. Several groups have achieved p-type ZnO with group-I or group-V elements, but it either had low carrier concentration and high resistivity, or was unstable. Theoretical calculations of the electronic band structure predicted that nitrogen is the best candidate for producing a shallow acceptor level in ZnO. But N doping increases the Madelung energy of ZnO, which resulting in the instability of the ionic charge distributions. The codoping method using acceptors and reactive donors in the ratio of 2:1 simultaneously was proposed to increase the solubility of nitrogen in ZnO and decrease the Madelung energy, On the basis of this idea, p-type ZnO was prepared in N₂O-O₂ atmospheres by Al-N codoping method using DC reactive magnetron sputtering in this paper. The structure and properties are characterized by XRD, SEM, Transmission spectra and Hall. Effects of growth parameters, such as substrate temperatures and Al contents in sputtering targets, on the properties of as-grown co-doped ZnO were investigated. Results indicate that co-doped ZnO films with a thickness of about 270nm exhibit polycrystalline structure with the preferential orientation of (002) plane, smooth dense surface, and good grain size uniformity. Compared with N doping alone, Al-N co-doped p-type ZnO films have greatly higher carrier concentration and lower resistivity.The best result is: the highest carrier concentration of the as-grown ZnO is 1.3 × 10¹⁸ cm^-3 , with the lowest resistivity of 54 Ω cm. The low carrier mobility of the co-doped ZnO is likely due to the scattering of ionic impurity and the poor crystal quality of ZnO. If we can improve the crystal quality of ZnO, the carrier mobility of the co-doped ZnO will greatly increase, thereby p-type ZnO with high carrier concentration and low resistivity can be realized using co-doping technique.