Study on photoemission mechanism for negative electron affinity GaN vacuum electron source

Abstract

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. NEA GaN photocathode can perfectly meet the requirement for vacuum electron source. But many of problems about NEA GaN photocathode are still unsolved at present. The photoemission mechanism for NEA GaN vacuum electron source is not completely clear yet.According to W.E. Spicer "three-step model", the first step is the absorption of incidence light.The incidence photon energy is absorbed by the electrons in the valence band, and this establishes the foundation that the electrons in the valence band are stimulated to the conduction band. The stimulation course relate to the band structure of photocathode material and the absorption coefficient of the material.The second step of Spicer "three-step model" is the transportation of photoelectron, the excitated electrons in the conduction band will move from bulk to surface by diffusing or drifting.This transporting course of the carriers can be divided into two steps: bulk transportation and surface band bend region transportation. Firstly, it is the course of transporting in cathode bulk. During the forming course of NEA surface state, valence band electrons reach to conductor band by excitation of photons, and become the over hot electrons at first, the over hot electrons interact with the crystal lattice by releasing phonons, losing energy 30-60meV each time, the stimulated photoelectrons fall the range several kT above conductor band bottom quickly. Secondly, it is the course of transporting in surface band bend region. The structure of surface band bend region is more complicated than the bulk. There are surface potential reflection, phonon scattering and strong electric field in this area, thus the transportation in surface band bend region is more complex. According to semiconductor theory, the energy distribution of electron thermalized in r valley or M-L valley is considered to satisfy Boltzmann distribution [1,2]. (1) Where k is Boltzmann constant, T is the absolute temperature. Suppose the energy of thermal electron is Et, for r valley, M= E t -Ec' where Ec is the energy level of r valley conduction band minimum, and for M -L valley, M= E t -E M -L' where EM-L is the energy level of M-L valley, the values of Et, Ec and EM-L are all relative to valence band maximum Ev, namely assume 978-1-4244-6644-3/10/$26.00 ©2010 IEEE 309

Quantum Yield of Reflection Mode Varied Doping GaN Photocathode

“…The main source of NEA photocathode photoemission is the escape of thermalization electron [10] . From equation (4),the factors affecting the quantum yield for varied doping NEA GaN photocathode includes the electron diffuse length L D , the escape probability of electron P, the photocathode absorption coefficient to the incident light D , the reflectance of cathode materials for incident light R etc..…”

Study on photoemission surface of varied doping GaN photocathode

“…It has many virtues, such as high quantum efficiency, low dark current, concentrated electron energy distribution and angle distribution, adjustive threshold and so on [1][2][3][4] . How to improve the quantum efficiency is always the important research content for GaN photocathode.…”