Quantum efficiency for reflection-mode varied doping negative-electron-affinity GaN photocathode

Abstract

As a new kind of ultraviolet photocathode material, the negative-electron-affinity (NEA) GaN photocathode needs to further improve its photoemission performance and the stable performance in practical applications. Under the limit of GaN photocathode material growth level, how to further improve the quantum efficiency of cathode is an important problem. The varied doping technology can help to solve the problem under such circumstances. According to the photoemission mechanism of varying doping NEA GaN photocathode material, the built-in electric field formulas and the quantum efficiency formulas for reflection-mode varied doping NEA GaN photocathode are given. The preliminary structure of varied doping NEA GaN photocathode is designed. The varied doping material sample is divided into four layers according to the doping concentration. Using the self-developed experimental equipment, the varied doping GaN photocathode sample is activated with Cs/O. The activation process and the change characteristics of photocurrent for varied doping NEA GaN photocathode are discussed. At the beginning, the photocurrent is increased steady with the introduction of Cs, then the Cs kill phenomenon appears in the presence of excessive Cs. After the introduction of O, the photocurrent value starts to rise again. The spectral response of varied doping GaN photocathode is tested in situ after activation, and the quantum efficiency values ranging from 240 nm to 354 nm are obtained. On the basis of the obtained experimental results of quantum efficiency, combining to the typical quantum efficiency curve from University of California, the characteristics of quantum efficiency curves are analyzed. The results show that the quantum efficiency value for reflection-mode varied doping NEA GaN photocathode can reach 56% at 240 nm because of the built-in electric field, yet the quantum efficiency maximum value for uniform doping GaN photocathode is only 37% at 230 nm. The tested quantum efficiency maximum value of varied doping NEA GaN photocathode is improved much more than that of the uniform doping GaN photocathode. In a wider range of the incident light wavelength, the quantum efficiency of varied doping NEA GaN photocathode is relatively stable, and the excellent properties of varied doping GaN photocathode are confirmed. The reason why the value of quantum efficiency decreases with the increase of incident light wavelength is given. First, the photon energy decreases with the increase of incident light wavelength. Second, the incident light is absorbed from the front surface of cathode for reflection mode. In addition, the quantum efficiency curves of varied doping GaN photocathode show obvious sharp cut-off characteristics near the threshold, and the sharp cut-off characteristic is necessary for high detection sensitivity. The property of negative electron affinity for varied doping GaN cathode material after successful activation is also proved by the sharp cut-off feature.