梯度掺杂与均匀掺杂GaN光电阴极的对比研究

为了提高负电子亲和势(NEA)GaN光电阴极的量子效率,利用金属有机化合物化学气相淀积(MOCVD)外延生长了梯度掺杂反射式GaN光电阴极,其掺杂浓度由体内到表面依次为1×10¹⁸ cm^-3,4×10¹⁷ cm^-3,2×10¹⁷ cm^-3和6×10¹⁶ cm^-3,每个掺杂浓度区域的厚度约为45 nm,总的厚度为180 nm.在超高真 关键词： NEA GaN光电阴极 梯度掺杂 量子效率 能带结构

Comparison between gradient-doping and uniform-doping GaN photocathodes

In order to enhance the quantum efficiencies of negative electron affinity (NEA) GaN photocathodes, gradient-doping reflection-mode GaN photocathodes are grown by metal organic chemical vapor deposition (MOCVD)at doping concentrations of 1×10¹⁸cm^-3, 4×10¹⁷cm^-3, 2×10¹⁷cm^-3 and 6×10¹⁶cm^-3 from the body to the surface, with the thickness of each doping region being about 45nm and the total thickness of GaN 180 nm. The gradient-doping GaN photocathodes are activated in an ultra-high vacuum system and are compared with two kinds of uniform-doping GaN photocathodes whose thicknesses are both 150 nm and doping concentrations are 1.6×10¹⁷cm^-3 and 3×10¹⁸cm^-3 separately. The results show that both the photocurrent growth rate and the maximum photocurrent of the gradient-doping GaN photocathodes are greater than those of the uniform-doping GaN in the Cs/O activation process, and the multi-test system measured maximum quantum efficiency of the gradient-doping NEA GaN photocathode is about 56% which is as high as the double of the uniform-doping. Calculations show that the energy band bendings of the gradient-doping GaN photocathodes are 0.024eV, 0.018eV and 0.031eV from the body to the surface, a larger electron drift and diffusion length are gained due to the built-in electric field formed by the energy band bending, because of the 0.073eV total energy band bending the photoelectrons reaching the surface have higher energies and pass through the surface barrier more easily. Therefore the gradient-doping NEA GaN photocathodes have greater quantum efficiencies.