Nea Gan Photocathode Quantum Efficiency Research

In UV spectroscopy and low-light-level UV-imaging applications, there are strong demands for improved detectors which have higher quantum efficiency, low dark current, sharper wavelength cut-off response, and stable and robust characteristics. GaN is one of the promising candidate materials to meet these demands. In this thesis, the researches were made on such aspects as depuration method, activation technique, test of spectral response and stability performance for NEA GaN photocathode.1. 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 methods were given in detail. After the effective chemical cleaning and the heating of 710℃about 20 minutes in the ultrahigh vacuum system, the oxides and carbon contaminations on GaN (0001) surface can be effectively removed. And the ideal atom clean surface can be obtained. The many activation experiments 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.2. The photocurrent curves during either Cs or Cs/O activation process for 3 reflection-mode GaN photocathode samples of different doping concentrations were tested by using dedicated experimental system for activating and evaluating of NEA photocathode. The photocurrent of 3 samples during the Cs,O activation shows that grad doping sample can produce much higher value than uniform doping samples. The QE curves indicate that the grad doping structure can improve the photoemission of GaN cathode dramatically. The reason for this is that the diffuse length of grad doping sample is far longer that of uniform doping samples.3. 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 for reflection-mode NEA GaN photocathode was studied. 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 the 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 the different wave bands.4. The spectral response of fully activated 2 NEA GaN photocathode samples (one is opaque grad-doping, the other is transparent uniform doping) working under transmission mode were measured by using dedicated ultraviolet spectral response measurement instrument. The quantum efficiency of tranparent NEA GaN photocathode in transmission mode reaches up to 6.5% at 280nm, a "door" shape curve was observed from 240nm to 380nm has been observed. Based on the former research results, the factors influencing quantum efficiency were also comprehensively analyzed.