Research On Optoelectronic Properties And Cesium/Oxygen Activation Mechanism Of Ga_1-xAl_xN Photocathodes

The negative electron affinity?NEA?Ga_1-xAl_xN photocathode is an important part of an ultraviolet image intensifier which is widely used in astronomical observation,aerospace applications,missile early warning systems and so on.At present,theoretical research on the optoelectronic properties of Ga_1-xAl_xN photocathodes is incomplete.Using first-principles with a plane wave ultrasoft pseudopotential method based on the density functional theory,the electronic structure and optical properties of Ga_1-xAl_xN were studied,and the mechanism of Cs,O activation was examined at the electronic and atomic level,which helped to provide theoretical guidance for the design and preparation of NEA Ga_1-xAl_xN photocathodes.Optical properties of Ga_1-xAl_xN with varying Al contents were studied based on first-principles calculations.The results showed that the edge of the absorption coefficient shifts to higher energy levels with the increase of Al content.The optical model of a transmission-mode Ga_1-xAl_xN photocathodes was constructed,and the reflectivity,transmittance,absorptivity,and quantum efficiency of this transmission-mode Ga_1-xAl_xN photocathodes were simulated.The results indicated that the AlN buffer layer mainly influenced the absorptivity and quantum efficiency at wavelengths of 200?240nm and the thickness of the buffer layer should not be too thick.When the Al component was 0.375,the cut-off wavelength of the photocathode was found to be 294.lnm,which satisfied the requirements of "solar blind" detectors.The optimum thickness of the Ga_1-xAl_xN active layer,which allowed for full absorption of the photons and caused the photoelectrons to escape successfully,was found to be 0.05?0.1?m.Comparative research on the atomic geometry and electronic structures of Mg and Be doped Ga_1-xAl_xN was also conducted.The results showed that the ionized energy of Be atom is less than that of Mg atom.Meanwhile it is easier for Be to act as a n-type donor,so Mg atom is more appropriate to be a p-type dopant.Hydrogen atoms,which were introduced in the materials growth process,passivated the Mg atoms by providing electrons.The formation energy of the Mg-H complex is smaller than that of Mg atoms.Consequently,Mg-H codoping was accomplished initially and the doped Ga_1-xAl_xN photocathode was then annealed at high temperature to force the release of the H atoms,was a desirable method for growing p-type Ga_0.75Al_0.25N with a high Mg dopant concentration.Based on first-principles calculations,we calculated the atomic and electronic structures of Ga_0.75Al_0.25N with various point defects,including vacancy defects,interstitial defects,and anti-site defects.The results showed that the N vacancy defect?VN?has the smallest formation energy in the p-type condition and it always acts as a donor.There are two charged states for VN:3+ and,and the transition level from VN3+ to VN+ was found to be 0.70eV.The band gaps of Ga_0.75Al_0.25N?0001?,?10???0?,and?11 2 0?surfaces were found to be 0.086eV,1.357eV,and 1.630eV,respectively,which were smaller than 2.261eV of the bulk Ga_0.75Al_0.25N.So the corresponding cut-off wavelength near the surface is longer than that in bulk.This provided a theoretical explanation for the longer wavelength shift of the quantum efficiency.The adsorption energies of GaO,AlO,Ga₂O₃ and Al2O3 on Ga?Mg?0.75Al0 25N?0001?surface were calculated and they were found to be-1.754eV,-2.383eV,-1.926eV,and-2.922eV,respectively.This showed that it is more difficult to remove AI2O3 than Ga₂O₃ because of the smaller adsorption energy.This also suggest that the heating cleaning temperature of Ga1-xAlXN photocathodes should be higher than that of the GaN photocathodes.The work functions of Ga?Mg?_0.75Al_0.25N?0001?surfaces with various coverages of Cs atoms were calculated.Initially,the work function decreased with the increase of Cs coverage.Meanwhile it rebounded when the coverage reached 0.75ML,which was in good agreement with the "Cs-kill" phenomenon demonstrated in activation experiments.As the distance between Mg and Cs atoms increased,the work functions of surfaces increased,showing that the surface dipole moments were weaker.So the photoemission was not uniform and there were many photoemission peaks,which corresponded to the presence of the Mg atoms near the surface,which is referred to as the "scale-like" effect.By comparing Cs,O coadsorption on the Ga?Mg?_0.75Al_0.25N?0001?surfaces with and without vacancy defects,the smallest work functions of the two surfaces were obtained.It was 1.431eV for the surface with Ga vacancy defect while it was 1.822eV for the pure surface.Research on Cs,O coadsorption on the Ga?Mg?_0.75Al_0.25N?10 1 0?and?11 2 0?nonpolar surfaces were also carried out.Results showed that the smallest work functions of the two surfaces were 1.332eV and 1.515eV,respectively,which demonstrated that the two surfaces have potential to act as photoemission surfaces.