First-principles Study On The Electronic Structure And Optical Properties Of La-doped ZnO

ZnO,as a new type of II-VI direct band gap semiconductor material,has a band gap of3.37eV and a high exciton binding energy of 60meV at room temperature and pressure.With excellent physical properties and chemical properties,it has a wide range of applications in the fields of transparent conductive films,photocatalysts,solar cells and ultraviolet semiconductor lasers.Rare earth elements have excellent electronic,optical and catalytic properties due to their unique electronic structure and rich separation energy levels.At present,a large number of experimental and theoretical studies have been carried out on the doping of rare earth element ZnO in the field of photocatalysis.The results show that the doping of rare earth elements can generate impurity energy levels in the energy band gap.By adjusting the physical properties of ZnO,the visible light response of the band gap is enlarged,promote the luminescence characteristics of ZnO to change,so as to effectively enhance the photocatalytic performance of ZnO semiconductor.Therefore,the electronic structure and optical properties of La-doped ZnO materials are predicted theoretically through first-principles calculation data,and the feasibility of the rare-earth element La doping is confirmed therefrom.In this paper,the CASTEP module of Materials Studio 2017 is used,and the first-principles plane wave ultrasoft pseudopotential method based on density functional theory is used to calculate and study the different La doping concentrations and the composite co-doping of ZnO with La and intrinsic defects.Five supercell models?2×2×2?of pure ZnO,La:ZnO?6.25%?,2La:ZnO?12.5%?,La:ZnO+V_Znn and La:ZnO+V_O were constructed,among which five doping configurations of 2La:ZnO were constructed according to the difference of La-La spacing.After using GGA and GGA+U methods to optimize the geometric structure of all the constructed supercell models,and the most stable structure is selected through energy calculation.The stability,Mulliken bond population,band structure,density of states and optical properties of the system before and after doping were calculated and analyzed.The main conclusions are as follows:?1?Structural optimization and energy calculation results show that as the amount of La doped and the distance between La-La atoms increase,the volume of the doped system increases and lattice distortion occurs,the formation energy of the system increases and the stability decrease.?2?The calculation results of the electronic structure show that the population of La-O bonds in the doping system decreases,and La³⁺and O^2-ion hybridization is weakened,covalentity is weakened,Zn-O bond length increases,bond energy decreases,and system stability decreases.The band gap of La-doped ZnO remains a direct band gap and the band structure is more compact.The Fermi energy level moves upward into the conduction band,and the phenomenon of carrier degeneration occurs,showing stronger metallicity and exhibits n-type conductivity.The existence of an oxygen vacancy causes defect levels in the doped system near the Fermi level.?3?The calculation results of optical properties show that the incorporation of La makes the minimum optical band gap of the doping system wider,causing a blue shift in the light absorption edge.Compared with pure ZnO,the doped system has a new absorption peak in the low energy region,which originated from the in-band electronic transition in the d-d orbital of the La atom.In the vacuum ultraviolet region,due to the electronic transition in the La-5p orbital,the peak of the high energy region is red shift and the peak intensity decreases as the La doping concentration increases.The energy loss peak moves toward the low energy direction and the peak intensity is significantly weakened.The existence of vacancy defects makes the reflection peak of the doping system significantly increase in the low energy region.