Effect Of Doping On Defects And Optoelectronic Properties Of ZnO

Zinc oxide (ZnO) is an common II-VI semiconductor, with direct gap of3.4eV and exciton binding energy of60meV. ZnO has been found to have potential application to short-wave optoelectronic devices. Large numbers of defects that existed in ZnO due to limit of doping asymmetry strongly influence the performance of ZnO-base optoelectronic devices. A combination of positron annihilation spectroscopy, X-ray diffraction, transmission electron microscopy, Raman scattering, photoluminescence and Hall effect was used to study the effect of doping on defects and optoelectronic properties of ZnO in this thesis, and the research content is followed:1?Hydrothermal grown ZnO single crystals were annealed in N2or O2between900?and1300?. Positron lifetime measurements reveal a single lifetime in all the ZnO samples before and after annealing. The positron lifetime is about181ps after annealing at900?in either N2or O2atmosphere. However, increase of the positron lifetime is observed after further annealing the sample at higher temperatures up to1300?, and it has a faster increase in O2ambient. Temperature dependence measurements show that the positron lifetime has very slight increase with temperature for the900?annealed sample, while it shows notable variation for the sample annealed at1300?. This implied that annealing1300?introduces additional vacancy in ZnO single crystal. These defects are supposed to be Zn vacancy-related defects. Cathodoluminescence (CL) measurements indicates enhancement of both UV and green emission after annealing, and the enhancement of green emission is much stronger for the samples annealed in O2ambient. The possible origin of green emission maybe related to OZn and Oi.2?Undoped ZnO single crystals were implanted with300keV Si+ions to a dose of6X1016cm-2. A combination of X-ray diffraction (XRD), positron annihilation, Raman scattering, transmission electron microscopy (TEM), and photoluminescence (PL) was used to study the microstructure evolution after implantation and subsequent annealing. A very large increase of Doppler broadening S parameters in Si+-implanted region was detected by using a slow positron beam, indicating that vacancy clusters or microvoids are induced by implantation. The S parameters increase further after annealing up to700"C, suggesting agglomeration of these vacancies or microvoids to larger size. Most of these defects are removed after annealing up to1100?. The other measurements such as XRD, Raman scattering, and PL all indicate severe damage and even disordered structure induced by Si+implantation. The damage and disordered lattice shows recovery after annealing above700?. Amorphous regions are observed by TEM measurement, which directly testify that amorphous phase is induced by Si+implantation in ZnO. Analysis of the S-W correlation and the coincidence Doppler broadening spectra gives direct evidence of SiO2precipitates in the sample annealed at700?, which strongly supports the chemical effect of Si ions on the amorphization of ZnO lattice.3?Abstract ZnO single crystals were implanted with280keV C+to a dose of6X1016cm-2. Positron annihilation measurements reveal a large number of vacancy clusters in the implanted sample. They further agglomerate into larger size or even microvoids after annealing up to700?, and are fully removed at1200"C. X-ray diffraction, photoluminescence, and Raman scattering measurements all indicate severe damage introduced by implantation, and the damaged lattice is partially recovered after annealing above500?. From room temperature photoluminescence measurements, an additional peak at around3.235eV appears in the implanted sample after annealing at1100?, which is much stronger than that of the free exciton. From the analysis of low temperature photoluminescence spectra, this peak is mostly a free electron to acceptor (e,A0) line which is probably associated with Co.4?Li-doped ZnO polycrystals with Li2CO3concentration between0and7.5at.%have been prepared using a solid state reaction method. After annealing of the mixed samples at1200"C, Li2CO3completely decomposed to Li2O and the lattice parameters a/c first increase with Li2CO3concentration up to2.5at.%, and then decrease with increasing Li2CO3concentration. This indicates that Li atoms prefer interstitial sites at low concentration and substitute Zn sites when Li concentration is beyond a certain threshold. Positron annihilation measurements were performed for undoped ZnO and Li2CO3-doped ZnO with7.5at.%samples as a function of annealing temperature. A high concentration of vacancy defects is observed in the undoped and as-prepared Li2CO3-doped samples, and they are full removed after annealing at1200?. The average positron lifetimes of all Li2CO3-doped samples after annealing at1200?are the same, suggesting that Li doping has little effect on the positron lifetime in ZnO. This might be due to the invisibility of positron to Lii or Lii-LiZn complex, which is further proved by theoretical calculation.