Research On Activation And Evaluation Of Reflection-mode NEA GaN Photocathode

As a new type ultraviolet (UV) photocathode, negative electron affinity (NEA) gallium nitride (GaN) photocathode has many good performance characteristics, such as high quantum efficiency, low dark current and concentrated electron energy distribution and so on. Aiming at investigative scarcity of basic theory, preparation method and evaluation means for NEA GaN photocathode at present, the researches were made on such aspects as photoemission mechanism, depuration method, activation technique, test of spectral response and stability performance for reflection-mode NEA GaN photocathode.According to W.E.Spicer photoemission "3-step model" theory, the whole process including photoelectron excitation, transportation from bulk to surface and escape to the vacuum by traversing surface barrier was analysed in detail. The transmission coefficient that photoelectrons traverse cathode surface barrier was educed. The quantum yield formula of reflection-mode NEA GaN photocathode was gotten by solving the diffuse equation of non-equilibrium carriers. According to GaN photocathode NEA property during activation process and after being fully activated, the surface model [GaN(Mg):Cs]:O-Cs for NEA GaN photocathode after Cs/O activation was given.The depuration method for GaN photocathode was studied by using NEA photocathode activation system and XPS surface analysis system. The chemical cleaning and the heating depuration method were given in detail. After the effective chemical cleaning and the heating of 700℃about 20 minutes in ultrahigh vacuum system, the oxides and carbon impurities on cathode surface can be effectively removed, and the ideal atom clean surface can be obtained.The photocurrent curves of Cs and Cs/O activation for reflection-mode GaN photocathode were tested by using dedicated experimental system for activating and evaluating of NEA photocathode. Aiming at the formation cause of NEA property for GaN photocathode and according to the rule of photocurrent change during activation period and the surface model of a fully activated photocathode, the activation mechanism of NEA GaN photocathode was studied. The relation of the rule of photocurrent change during activation period and the change of electron affinity during activation period was gotten. The experiment results show:the obvious NEA property can be achieved for GaN photocathode mainly by activating with Cs. The increase extent of photocurrent is not large after introducing O during Cs/O activation process for GaN photocathode. The NEA property formation reasons of GaN photocathode after being fully activated successfully can be well explained using the double dipole layer model [GaN(Mg):Cs]:O-Cs.The spectral response of fully activated reflection-mode NEA GaN photocathode was measured by using dedicated ultraviolet spectral response measurement instrument. The quantum efficiency curves of reflection-mode NEA GaN photocathode were given in the band region from 230nm to 400nm. The quantum efficiency of reflection-mode NEA GaN photocathode reaches up to 37.40% at 230nm, a sharp cutoff characteristic with over two orders of magnitude degradation from 230nm to 400nm has been observed. Based on the former research results, the factors influencing quantum efficiency were also comprehensively analyzed. The relation of quantum efficiency and incidence light wavelength, cathode material characteristic and cathode preparation technique was gotten.According to the photocurrent curves and the quantum efficiency curves of fully activated reflection-mode NEA GaN photocathode, aiming at the decay tendency for reflection-mode NEA GaN photocathode and the different decay speeds of quantum efficiency corresponding to the different wave bands, the quantum efficiency decay mechanism of reflection-mode NEA GaN photocathode was studied. The relation of the quantum efficiency decay of reflection-mode NEA GaN photocathode and the reduction of effective dipole quantity was gotten, and the relation of the decay characteristic of quantum efficiency curves and the change of surface barrier shape was also gotten. The surface model [GaN (Mg):Cs]:O-Cs for GaN photocathode after being activated with cesium and oxygen was used, and the change of energy band and surface barrier in the decay course of quantum efficiency was considered. The conclusions show:the reduction of effective dipole quantity is the basic reason causing quantum efficiency to reduce, and it is the change of surface I. II barrier shape that causes the difference of dropping speeds of quantum efficiency corresponding to different wave bands.