Research On Engineering Of Defects In ZnO And Its Spectrum Characteristics

ZnO is a promising wide-band-gap multifunctional semiconductor with a directwide band gap(3.37 e V) and high exciton binding energy(60me V) at roomtemperature. It has a wide range of potential applications, including thermal controlcoatings, solar cells, UV laser diodes, photodetectors, piezo-electric transducers,varistors, gas-sensors, flat panel displays and hydrogen storage. Optoelectronicproperties strongly depend on a firm understanding of the physics of defects andimpurities in ZnO. However, in spite of several decades of effort, the relationshipbetween the defect structures, processing, and properties is not well understood.Some of the basic properties of ZnO remain unclear, which hold back thedevelopment of ZnO based devices.Therefore, it is useful to gain a detailed understanding of the nature of theintrinsic defects as well as their evolution on the optical properties of ZnO. As iswell known, doping, ion implantation and ambient annealing are three effectivetools to control the optical properties of semiconductor materials. Thus, in this work,the influences of point defect evolution on the optical properties of ZnO aresystematic studied by varying the chemical equilibrium, combined withcomprehensive and sensitive defects characterization. The main contents of thisdissertation are listed as follows:The discrepancy between the sub-band gap absorption in ZnO induced bythermal treatment and H+ implantation are studied herein for the first time. It isfound that thermal treatment induced blue absorption band is centered at 395 nm,while the absorption band caused by H+and electron implantation is peaked at420 nm and 430 nm, respectively. Results indicate that the thermal annealing inducedoptical absorption is independent of the annealing ambient, and can be assigned toVO, while the source of visible absorption in H+-implanted and H2 ambient annealedZnO is primarily associated with VO and ionized Zni. Interestingly, it is also foundthat Al doping can apparently eliminate optical degradation induced by thermalannealing, however, can not alleviate optical absorption caused by H+implantation,suggesting Al doping can introduce extra oxygen atoms and then compensate VOdefect.The position and intensity of the green luminescence in ZnO is an overalleffect, depending on the relative concentration of these defect centers. Byemploying a combination of typical treatments and sensitive defect characterization,we discriminate between the roles of different kinds of intrinsic defects in ZnO.Thereby we offer convincing experimental evidence that the green luminescencecan originate from VO, VZn and Oi-related defects, corresponding to the 2.51 e V,2.35 e V and 2.26 e V emissions, respectively. The 2.26 e V peak is likely caused by anAl Zn-Oi complex. As for the 503 nm green emission in pristine ZnO, it can beassigned to a VO-Cu donor-acceptor pair recombination.A series of IR peaks in the range of 3400~3700cm-1 can be always observed inpolycrystalline ZnO, which was supposed to be a signal of hydrogenic zinc vacancypreviously. We found these peaks were independent of particle size and impurities.By annealing in D2 ambient, an apparent isotope shift of H was detected, thusimplying these IR peaks are caused by absorbed OH or H2 O groups on the surfaceof ZnO rather than defects.Controllable introduction of acceptor defects into ZnO under O-richenvironment is potentially an initial step toward achieving p-type conductivity. Theeffects of oxygen annealing, H2O2 treatment as well as Li doping on the defectevolution of both ZnO nanocrystals and bulk crystal are studied. We illustrated thatthe luminescence peak around 414nm(~3.0e V), which has been observed before inLi- and Na-doped ZnO, is originated from VZn defect rather than groupⅠAsubstitutes. Results also indicated that the 2.35 e V luminescence peak, obtained byhigh temperature O2 annealing as well as H2O2 etching, is caused by a VZn defectwith a different charge state which differs from that of the 414 nm emission. Inaddition, the configuration of Li atoms in ZnO lattice was also studied by varyingthermal treatment temperature. Meaningfully, indirect evidences indicated thatp-type ZnO was achieved by annealing Li-doped ZnO at 700℃.