Theoretical Study On The Photoemission Mechanism Of NEA GaN Photocathode

GaN photocathode material has great application potential in the field of blue light and"solar blind"detector.The research of Ga N photocathode materials started earlier in foreign countries.The highest quantum conversion efficiency of Ga N photocathode material can reach80%and the highest level of 71.9%in China.The breakthrough of quantum conversion efficiency of GaN photocathode material can greatly improve the performance of the device.Simulation with theoretical calculation method can effectively reduce the cost of research and improve the efficiency of scientific research.Based on the first principle calculation method,the Ga N bulk structure model is established in the Material Studio software and simulated in the CASTEP module.The basic parameters of the calculation are determined.On this basis,the Cs/O activation of GaN?0001?polar surface is carried out.Based on the slab model,the values of adsorption energy of?1×1?GaN?0001?oxidation surface are-0.1919eV,-3.0683eV,-1.9574e V,-1.948eV and-4.6394eV.The theoretical model is reasonable.The charge transfer shows that Cs and O atoms are easy to combine with Ga and N atoms on the surface.During the whole activation process,the value of work function is 3.974eV,9.624eV,1.663eV,2.481eV,8.583eV and 1.927eV.The final value of work function is much lower than 3.974eV of clean surface.This value is positively related to the electron affinity.This shows that the electron affinity is also smaller,the difficulty of electron photoelectric conversion escaping to vacuum is reduced,and the negative electron affinity state is more easily obtained,and the electron photoelectric conversion efficiency will be improved.This is consistent with the trend of data obtained in the experiment.The band structure and density of state diagram show that the surface of GaN?0001?shows metallicity,and that of O atom is stronger after adsorption.A?2×2?Ga_0.9375Mg_0.0625N?0001?surface atomic model is established.Cs/O activation was carried out on the surface.The calculated adsorption energy was the lowest,the value was-1.45eV,and the adsorption was the most stable.According to the population distribution of mulliken,the dipole moment produced by charge transfer corresponds to the numerical variation of the work function.Compared with the activation process of Cs/O on the O/GaN?0001?surface,it is found that the work function of the system with p-type doping only decreases more than that of the system with oxidation surface.The final work function of activation is 0.925eV,which is in line with the phenomenon that the negative electron affinity state can be obtained only after p-type doping in the experiment.The calculated band structure and density of states show that the metallicity of p-type doped surface will be enhanced.The oxidation surface of?2×2?Ga_0.9375Mg_0.0625N?0001?is established.The model fully considered the surface oxidation and p-type doping.According to the principle of the lowest energy,the total energy of the system is the lowest when the adsorption position is above Mg atom,the value is-4.9313 eV.The next is N atom,the value is-4.7733eV.When two Cs atoms are adsorbed on the top of Mg and N atoms,the adsorption energy of N atoms becomes the most stable.The value is-5.2713eV,which is caused by the mutual repulsion between the atoms.The calculated adsorption energy is negative,and the model is reasonable.According to the statistical surface charge,the charge transfer between Ga,O and Cs atoms leads to the dipole moment which affects the efficiency of electron moving into vacuum.The variation trend of the work function values obtained in each adsorption process corresponds to the dipole moment generated by charge transfer.The density of states for each activation process can more accurately show the contribution of the outermost electrons of each atom to the total energy level.At the same time,compared with the p-doped surface,it is found that after the surface oxidation is introduced into the system,the value of the work function decreases lower after the Cs/O activation.The final value of the work function is 0.743eV,which shows that the hard to remove surface Ga₂O₃ plays a positive role in the formation of negative electron affinity.