First-principles Study Of ?-Ga₂O₃ For Solar-blind Ultraviolet Photocathodes

Solar blind UV detection technology has important application value in national defense and industrial fields.With the continuous improvement of requirements,the traditional UV photocathode materials can no longer meet the actual needs,therefore,this means that a new type of ultraviolet photocathode material is needed.?-Ga₂O₃ is a kind of ultra-wide band gap semiconductor material.Because of its excellent optoelectronic properties and mature growth process,it has become a research hotspot in the field of optoelectronic detection.However,the theoretical research on the application of?-Ga₂O₃ to ultraviolet photocathode is not sufficient.In this thesis,the optoelectronic properties of?-Ga₂O₃ materials are calculated by using the first-principles method based on density functional theory,and the activation process of?-Ga₂O₃ by cesium oxygen is simulated.This work lays a theoretical foundation for the development of?-Ga₂O₃ ultraviolet photocathode.In order to study the properties of?-Ga₂O₃ crystal plane and explore its potential as photoemission surface,three kinds of surface models are set up.They are respectively(100-A),(100-B)and(010).On the basis of DFT+U algorithm,the electronic structure and optical properties of the model are calculated and compared.The results show that the(100-A)surface has the minimum work function,and the value is 6.429e V.The formation can reflect that the structure of the(100-B)surface is the most stable and is not easy to interact with foreign atoms.The energy band structures and densities of states of the three models are calculated.The results show that the surfaces of(100-A)and(010)belong to indirect band gap,while(100-B)belong to direct band gap.These evidences prove that(100-A)plane has the potential of photoemission surface.In this thesis,we carried out a study on the adsorption of single cesium during the activation of?-Ga₂O₃ cesium oxygen.(100-A)as the adsorption surface,eight different adsorption sites are selected.The adsorption energy and work function of eight single cesium adsorption models are calculated respectively,the results show that the adsorption energy of O-bridge site is the lowest,thus showing the most stable adsorption performance.After adsorbing cesium atoms,the surface work function decreases by 3.5e V,which proves that the method of reducing the surface work function by adsorbing alkaline metal elements on the surface is also effective for?-Ga₂O₃.The influence of cesium with different coverage on the work function is calculated.The results show that the surface work function decreases with the increase of coverage,and the inflection point appears when the coverage reaches 0.75ML.The energy band structures of different coverage models are calculated.It is found that with the increase of coverage,the width of the band gap decreases gradually,and the surface shows metal properties.The theoretical study of Mg-N co-doped?-Ga2O3 materials was carried out to verify the theoretical feasibility of realizing the characteristics of?-Ga2O3 P-type semiconductors by doping metal-non-metallic elements.The results show that the band gap of Mg-N co-doped?-Ga2O3 crystal decreases from 4.906e V to 3.420e V,and metal-non-metallic element co-doping is beneficial to the transition probability of electrons from valence band to conduction band.There is a deep level acceptor impurity state density peak in the band gap of?-Ga2O3doped with N element,which provides more hole positions for electron transition.The doping of Mg broadens the width of the conduction band region of?-Ga2O3 and provides more acceptance sites for the excited transition of electrons.After Mg-N co-doping,?-Ga2O3crystals show more obvious p-type semiconductor properties.