First-principles Study Of The Zno Electronic Structure

Zinc oxide (ZnO) is a novel II - IV compound semiconductor which is widely used in UV light-emitters, varistors, surface acoustic wave devices (SAW), piezoelectric transducers, transparent electrodes and so on. It has a large room temperature band gap of 3.37eV and a high binding energy of 60meV. As such, ZnO is a potential candidate for applications in optoelectronic device and receiving more and more important attention.The mechanism of native point defects in ZnO is complex. At present, there are lots of divarications about accurate levels of native point defects and the main mechanism of native point defects in ZnO. ZnO:Al thin film has been studied, although there is a lack of theory support about the mechanism of doped Al and proportion of doped Al. It is hard to fabricate p-type ZnO. The mechanism of doped acceptor needs more theory support, especially the mechanism of Li-N doping. Electronic structures were caculated by using a first-principles ultrasoft pseudo-potential of the plane based upon the Density Functional Theory (DFT) to investigate native point defects and doping. The main contents are as the following:1. We caculated electronic structures of perfect ZnO and ZnO with native point defects such as Zn_i,O_i,V_Zn and V_O. The results indicated that ZnO was a direct wide band gap semiconductor material. The bottommost conduction band and the top of valence band were located in the G point with direct gap 1.2eV in the Brillouin zone. Vacant Zn (Vzn) produced a acceptor level above the top of the valence band (VB) interstitial O(O_i) produced a acceptor level and a donor level, while interstitial Zn (Zn_i) produced a shallow donor level bellow the bottom of the conduction band(CB), and vacant O (Vo) produced a deep donor level below the bottom of CB. On the basis of these results, we confirm that Zn_i is the main factor to induce the native n-type conductivity in ZnO.2. Different mechanisms and proportions of Al doped ZnO were caculated. The calculated conclusions revealed that interstitial Al caused the formation of deep donor level, while substitute Al producted shallow donor level and broaden optical absorption edg, and optical absorption edg was found to be wider with increasing dopant concentration. High dopant concentration caused formation of deep donor level.3. Electronic structures of respectively doping Li N and Li-N co-doped in ZnO were caculated. In single doping models, substitute Li producted a weak shallow acceptor level, which was easily to be compensated by a shallow donor level producted by interstitial Li. Doping N producted a narrow deep donor level, which was hardly localized near the top of the valence band. This fact caused that it was hard to dope N in ZnO. Therefor, respectively doping Li and N maked against to fabricate high quality p-type ZnO film. In Li-N co-doped models, Doping Li_Zn combined VB and acceptor level by making VB shift upon and broadening the acceptor level producted by doping N, which actived N actom and increased the concentration of N. Doping donor as Lij could decrease Mudelumn energy in the system, and active N. Doping Lij made the the acceptor level producted by doping N shift toward lower-energy region and shallow acceptor level, which was showed deloealized characters. On the basis of these results, we confirm that high quality p-type ZnO thin film can be fabricated by co-doping Li-N.