Theoretical Study On ZnO Based Semiconductor Heterostructure Materials And Devices

ZnO is one of the most promising semiconductor oxides.It is a typical direct wide band gap semiconductor material with a large band gap of 3.37 eV and an exciton binding energy of up to 60 meV.The most stable three-dimensional ZnO structure at room temperature is hexagonal wurtzite structure,which has excellent piezoelectric properties along the c-axis direction.These characteristics make ZnO show wide and superior application prospects in the fields of ultraviolet light emitting devices,high temperature microelectronic devices,optoelectronic devices,solar cells,piezoelectric devices and so on.In addition,the successful fabrication of two-dimensional ZnO（g-ZnO）in 2013 has promoted the research of ZnO into the low-dimensional fields,providing a basis for miniaturization,integration and multi-function in the application of ZnO-based electronic devices.In order to obtain more novel and excellent properties,constructing heterostructure is one of the most effective research methods at present.Generally speaking,heterostructure usually show more superb properties and application values than its individuals.At present,researches on ZnO-based heterostructures include three-dimensional and two-dimensional heterostructures.Besides,more and more new physical properties in ZnO and ZnO-based heterostructures have been discovered.From the first-principles based on the density functional theory,this thesis theoretically studies the local polarization,energy band structure and optical properties of three-dimensional ZnO-based heterostructures and g-ZnO-based heterostructures.The effects of strain and external electric field on these physical properties are also considered.The calculated results might give some theoretical guidances for the design of ZnO（g-ZnO）-based semiconductor optoelectronic devices.The main research contents and results are as follows:Firstly,the lattice constants and the fraction of Fock exchange-AEXX of three-dimensional wurtzite ZnO and cubic phase PbTiO₃ are calculated.The lattice constants of ZnO and PbTiO₃ are calculated as a=3.278?,c=5.330?,and a=3.973?,respectively.It is found that the band gaps of ZnO and PbTiO₃ are 3.40 eV and 3.38 eV respectively（with the parameter AEXX=0.39）,which is in good agreement with their experimental values.Therefore,AEXX=0.39 is suitable for performing accurate electronic structure based on ZnO and PbTiO₃.Then,the atomic structure,local polarization,and band offsets of heterostructures composed of ZnO（11^-2 0）and PbTiO₃（001）are studied by us.The effects of different numbers of ZnO（11^-2 0）layers and strains on the local polarization in PbTiO₃（001）substrate and the band offsets of ZnO（11^-2 0）/PbTiO₃（001）heterostructure are investigated.The results show that as the number of ZnO（11^-2 0）layers increases,the local polarization in PbTiO₃（001）substrate and the band offsets of the ZnO（11^-2 0）/PbTiO₃（001）heterostructure tend to be stable.The polarization intensity in the PbO layer generally larger than that in the TiO₂ layer.Strain has an obvious regulation effect on the local polarization and band offsets.With the application of strain,the intensity of local polarization can be significantly improved,whereas the band offset increases after an initial decrease with the increase of strain.This work provides a theoretical basis for new polarization materials.Then,the lattice constants,energy band structure and optical properties of two-dimensional g-ZnO and 2H-TiS₂ are calculated.After constructing the g-ZnO/2H-TiS₂ van der Waals heterostructures,the structural properties,local polarization,optical properties,and band structure are calculated and analyzed.The effects of strain and external electric field are considered too.A promising out of plane polarization spontaneously induced by 2H-TiS₂ is found for the g-ZnO monolayer,which can be significantly tuned in a wide range of 9.929μC/cm to33.271μC/cm by the in-plane strain.A type-II band alignment and an indirect band gap of 0.05 eV are presented in stable g-ZnO/2H-TiS₂ heterostructure.Although the type of band alignment keeps steady,the band gap value can be tuned by strain,which results in the transformation of the g-ZnO/2H-TiS₂ heterostructure from semiconductor to conductor.Furthermore,the g-ZnO/2H-TiS₂ heterostructure exhibits extraordinary absorption properties in the visible and ultraviolet regions.The absorption peaks in visible region present red shift under the compressive strain and blue shift under the tension.In addition,the out-of-plane polarization modulation is observed in g-ZnO/2H-TiS₂ heterostructures under the external electric field.For the positive electric field,the out-of-plane polarization for g-ZnO and band gap for g-ZnO/2H-TiS₂ heterostructure increase significantly.While for the negative electric field,both out-of-plane polarization for g-ZnO and band gap for g-ZnO/2H-TiS₂heterostructure are obviously reduced.Our investigation might give some theoretical guidances for the development and application of the g-ZnO/2H-TiS₂ heterostructure in sensors,ferroelectric,and optoelectronic devices.