Electrical Structure And Optical Properties Of Doped Zno And In₂O₃by First-principles And Experimental Investigation

Transparent conductive oxides thin films (TCO) due to the excellent performance, havebecome the key research projects of academe and industry. Through the low resistivities andhigh optical transparency in visible region, they have been widely used for optoelectronicdevices such as solar cells, flat panel displays, or light emitting diodes. In2O3and ZnObelong to the prototypical n-type TCO. Nevertheless, problems of limited properties havebecome an important issue to expand technological applications, hence the urgent need toseek new materials. In fabrication methodology, doping has become a new trend as itselectronic and optical properties can be greatly improved. The current industry standardn-type In2O3:Sn (ITO) can support very high carrier concentration and still maintain highoptical transparency. Thus, theoretical study of In2O3and ZnO-based semiconductor oxidesis still an essential work.To sum up, this thesis has three research purposes: Firstly, how to adjust the optical andcatalytic properties by adjusting concentration of the intrinsic defects of ZnO; Secondly,doping ZnO with Cd, Mg, Ca and Y elements, we try change the electronic and opticalproperties; Finally, how to further improve the electronic and optical properties of In2O3arestudied by using the first-principles. The theoretical calculations are performed on CASTEPcode which is based on density functional theory (DFT) using ultrasoft pseudopotentialsmethod. Exchange and correlation effects are described by the generalized gradientapproximation (GGA) and the local density approximation (LDA). The on-site Coulombinteraction is used, which can improve the GGA (or LDA) to predict the electronic propertiesand band gap. Experimental works are done to verify or support the theoretical results.Examinations with x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), Ramanspectroscopy, transmission electron microscope (TEM) are used to investigate the structureproperties; Photoluminescence (PL) spectra and UV-Visible spectrophotometer are recordedto investigate the optical properties; Photocatalytic activities are estimated by thedegradation of20mg/L Methylene Blue (MB) solution.The main results obtained in the thesis are divided into four parts as following: 1. Core-shell nested structure of amorphous carbon coated on crystalline ZnO(carbonized ZnO) is synthesized by sol-gel method as precursor. Disordered ZnOnanoparticles are successfully prepared by annealing the carbonized ZnO precursor. HRTEM,Raman and XPS analyses show that the ZnO nanoparticles contain high concentration ofoxygen-deficiency defects. The ZnO nanoparticles exhibit an abnormal intense deep levelemission centered at green luminescence and spreading over a wide wave length range of420nm~660nm, and a negligible UV emission in its photo luminescence spectrum. Theyalso show an enhanced photocatalytic activity in degradation of MB solution. Experimentsand theoretical simulation based on density functional theory show that the intense deeplevel emission and improved photocatalytic ability should attributed to their abundantoxygen-deficiency defects in the ZnO nanoparticles introduced by the two-step preparationmethod.2. Zn1-xCdxO thin films are deposited on quartz substrate by pulse laser deposition. Byvarying Cd concentration, the band gap of Zn1-xCdxO films can be adjusted in a wide range.Simultaneity, the electronic structure and optical properties of Zn1-xCdxO alloys areinvestigated by the density functional theory with a combined generalized gradientapproximation plus Hubbard U approach, which precisely predicts the band-gaps of ZnO andZn1-xCdxO alloys. For example, the band gap of Zn1-xCdxO alloys can be adjusted in a widerange from3.219eV for ZnO to2.197eV for Zn0.5Cd0.5O, which produces differentemissions from ultraviolet to Kelly light in their photoluminescence spectra. Both theexperimental results and theoretical simulation reveal that with increasing Cd concentrationin Zn1-xCdxO alloys, their absorption coefficients in visible light range are evidentlyenhanced. The adjustable photoluminescence emission and enhanced visible light absorptionendow Zn1-xCdxO alloys potential applications in optoelectronic and photocatalytic fields.3. The electronic and optical properties of wurtzite ZnXO (X=Mg,Ca,Y) ternary alloysare calculated by using first-principles based on the framework of generalized gradientapproximation with introducing the on-site Coulomb interaction. The structures of thesealloys in wurtzite phase are analyzed by examining their lattice constants and internalstructural parameter u. Firstly, the electronic structure of wurtzite ZnMgO ternary alloys iscalculated. The calculated band gap of wurtzite ZnMgO ternary alloys shows a significantexpansion with increasing Mg concentration. The calculated optical absorption spectra alsoshow good agreement with the experimental spectra in their shape and shift trend withvarying Mg concentration. Secondly, the electronic structure of wurtzite ZnCaO ternary alloys is calculated. By comparing the formation energies of three different Ca doping sites,it is found that Ca at Zn site (CaZn) is the most favored doping site. The calculated band gapof wurtzite ZnCaO ternary alloys shows a significant increase with increasing Caconcentration. Finally, the electronic structure of wurtzite ZnYO ternary alloys is calculated.It has been found that replacement of Zn by Y should generate a shallow donor. The Y-dopedZnO would be a material with enhanced mobility and hence improved electrical conductivitywithout sacrificing optical transparency. Therefore, the theoretical results show that thewurtzite ZnXO (X=Mg,Ca,Y) ternary alloys are the potential candidate alloys foroptoelectronic materials.4. In2O3doped with yttrium or stannum shows improved optoelectronic efficiency. Herethe structural properties and electronic structures of Sn or Y-doped In2O3are investigated byfirst-principles calculations. Various doping sites of Sn in In2O3system have been modeled.The energy favorable site of Sn atom in In2O3is found to be on the b-site of In atom. Bycomparing the formation energies of different configuration, it is found that the doping of Snatom into interstitial sites (Sni) is experimentally possible due to the fluctuation in energy,and the intrinsic oxygen vacancies (VO) in In2O3favorite the formation of Sni. However, wefound that substitutional YInof d-site is more stable site, and could lead to the blue-shift ofoptical band gap. The Yiis treble donors with low formation energy and defect transitionenergy, it is hence likely to incorporate during growth. The complex defect configurationsare calculated. By comparing the formation energies of complex defect configurations, it isdetermined that the Yi-VOconfiguration also has high formation energy, and this suggests anunfavorable role during prepared process. Therefore, the theoretical results show that theInXO (X=Sn or Y) ternary alloys are the potential candidate alloys for optoelectronicmaterials.