Structure And Electrical Transport Properties Of ZnO And Ga:ZnO Single Crystals

As the third-generation semiconductor material,zinc oxide(ZnO)has always been the focus of material.The bandgap of ZnO is 3.34 e V,and the exciton binding energy is 60 me V.Compared with indium tin oxide(ITO)and SnO₂,ZnO has the advantages of abundant reserves,non-toxicity,high melting point,and good thermal stability.Therefore,it is still one of the leading research materials in the field of optoelectronic information materials.At present,the primary method to prepare ZnO single crystal materials is a hydrothermal method.However,more mineralizers will be added to the growth process,resulting in more Li⁺,Na⁺,and other impurity ions.It affects the photoelectric properties and crystal quality of ZnO single crystal and can not meet the application's essential requirements in optoelectronic information.Other methods for preparing ZnO single crystal materials include the cosolvent,solid,and solvent floating zone methods.However,they also have high impurity content and high lattice defect density.To reduce the impurity content and improve the crystal quality of ZnO single crystal materials,the chemical vapor transport(CVT)method was used to prepare ZnO single crystal.The main results are as follows:(1)Pure ZnO single crystals were prepared by the CVT method.The effects of annealing on the crystal structure and electrical transport properties were studied.The results show that the growth direction of ZnO single crystal is(002)plane,and the growth direction of ZnO single crystal is(002)layer by layer.The FWHM of?rocking curve of annealed ZnO single crystal is 39?.The results show that the quality of the ZnO single crystal is improved by annealing.XPS analysis shows that there are many oxygen vacancy(V_O)defects in the unannealed ZnO single crystal.After annealing in a high-temperature oxygen atmosphere,the V_O defect density of the ZnO single crystal decreases obviously,and the quality of the single-crystal is improved obviously.XPS and EDS spectra show that the percentage of Zn and O elements on the surface of ZnO single crystal gradually approaches the theoretical value after annealing.Raman analysis shows apparent differences in Raman vibration modes before and after annealing.The575 cm^-1 peak belongs to A₁-(1LO)mode and only exists before annealing,and the peak indicates that the mode is affected by annealing.Before and after annealing,the UV-vis spectrum analysis shows that the optical band gaps of ZnO single crystal are 3.05 e V and 3.2e V,respectively.Constant temperature Hall effect and R-T test show that high-temperature oxygen atmosphere annealing can improve the electrical properties of ZnO single crystal.(2)GZO single crystals were prepared by the CVT method.The effects of annealing temperature on the structure and electrical transport properties of single crystals were studied.The results show that the GZO single crystal is hexagonal wurtzite structure and grows along(002)direction in a laminated manner.The FWHM increases with the increase of annealing temperature.XPS analysis shows that gallium exists in the form of metallic state(Ga⁰)and changes to oxidized state(Ga^x+)after annealing.Raman analysis shows that the Raman vibrational modes of GZO single-crystal annealed at different temperatures are obviously different,and the Raman peaks of 631 cm^-1 and 633 cm^-1 for Ga-O bond are obtained.The UV-vis spectrum shows that the optical transmittance of GZO single-crystal annealed at different temperatures is from opaque to 57%,which indirectly indicates that the transmission spectrum of GZO single crystal is related to the existing form of gallium.Constant temperature Hall effect and low temperature electrical platform test show that different annealing temperatures have effects on the electrical properties of GZO single crystal at low temperature.The depth study of the work in this paper will be helpful to understand the relationship between the structure and electrical transport parameters of ZnO and GZO single crystals after annealing.These data have a high reference value for the research of ZnO-based single crystals in the field of optoelectronic devices.