NEA GaN和GaAs光电阴极的比较

Comparison of NEA GaN and GaAs Photocathodes

针对GaN基光电阴极激活过程中Cs-O交替存在的光电流的增幅问题,本文主要比较了GaN和GaAs材料性质、表面结构以及激活过程中光电阴极的光电流。发现GaN的熔点高于GaAs,在制备GaN基光电阴极时则需要更高的热清洗温度;如果用双偶极子模型描述GaN(1000)和GaAs(100)表面的光电发射机理,GaN(1000)表面Cs原子与O原子形成第二偶极矩O-Cs,几乎"平躺"在表面,对光电发射贡献不大;GaAs(100)表面Cs原子与O原子形成第二偶极矩O-Cs几乎"垂直"于表面,降低了表面功函数,对光电发射贡献很大;Cs-O激活过程中,对于GaAs光电阴极,Cs、O交替过程形成的光电流与单纯Cs激活时的光电流相比,有几倍甚至上百倍的增长;GaN只提高了20%左右。通过第一性原理计算,与现在的GaN基(1000)面相比,GaN基的(11 2 0)和(10 1 0)面是极具潜力的光电发射面;预计闪锌矿GaN基(100)面会取得更好的结果。

In view of the increase in photocurrent in the alternation of Cs-O in the activation process of GaN-based photocathode, in this paper, the properties of GaN and GaAs materials, the surface structure, and the photocurrent of the photocathode during the activation process are compared. It is found that the melting point of GaN is higher than that of GaAs, and a higher thermal cleaning temperature is necessary in the preparation of GaN-based photocathode. Using the double dipole model to describe the photoelectric emission mechanism of the GaN（1000） and GaAs（100） surface, the GaN（1000） surface Cs atoms and 0 atoms form the second dipole moment of O-Cs that is almost flat on the surface and contribute little to the photoelectric emission; the GaAs（100） surface Cs atoms and O atoms form the second dipole moment of O-Cs that is almost vertical to the surface, reducing the surface work function of the photoelectric emission contribution. In the Cs-O activation, the photocurrent formed by the alternating process of Cs and 0 for the GaAs photocathode could be several times even more than 100 times higher than that of pure Cs activation, whereas the GaN photocathode only increases by ~20%. By first principle calculations, compared to the GaN（1000） surface ratio, GaN base（11 20） and （10 1 0） are the photoelectric emission potential surface; and the sphalerite GaN base（100） surface is expected to achieve better results.