【作者】 乔建良；

【导师】 常本康；

【作者基本信息】 南京理工大学 ， 电子科学与技术， 2010， 博士

【摘要】 负电子亲和势(NEA) GaN光电阴极具有灵敏度高、暗发射小、发射电子能量分布集中等优点,是非常理想的新型紫外光电阴极。针对目前NEA GaN光电阴极的基础理论、制备方法与评估手段研究的不足,围绕反射式GaN光电阴极的光电发射机理、净化方法、激活工艺、光谱响应测试以及阴极的稳定性能等方面开展研究。根据W.E.Spicer提出的光电发射的“三步模型”,详细分析了NEA GaN光电阴极从光电子的激发、体内到表面的输运到穿越表面势垒逸出到真空的全过程,导出了光电子隧穿阴极表面势垒的透射系数。通过求解非平衡载流子的扩散方程导出了反射式NEA GaN光电阴极的量子效率公式。结合阴极的激活过程及充分激活后的NEA特性,给出了NEA GaN光电阴极铯(Cs)氧(O)激活后的表面模型[GaN(Mg):Cs]:O-Cs。利用NEA光电阴极激活系统和XPS表面分析系统研究了GaN光电阴极的净化方法,给出了具体的化学清洗和加热净化工艺。经过有效化学清洗后,超高真空中GaN样品在700℃下加热20分钟,可以有效去除阴极表面的氧化物以及C杂质,获得较为理想的原子级清洁表面。利用自行研制的光电阴极激活评估实验系统,给出了反射式GaN光电阴极Cs激活及C_S/O激活的光电流曲线。针对GaN光电阴极NEA特性的成因,结合激活过程中光电流变化规律和成功激活后阴极表面模型,研究了NEA GaN光电阴极激活机理,得到了阴极激活时光电流的变化规律和激活过程中电子亲和势的变化之间的关系。实验表明：GaN光电阴极在单独导入Cs激活时就可获得明显的NEA特性,C_S/O激活时引入O后光电流的增长幅度不大。用双偶极层模型[GaN(Mg):Cs]:O-Cs较好地解释了激活成功后GaN光电阴极NEA特性的成因。利用自行研制的紫外光谱响应测试仪器,测试了成功激活的反射式GaN光电阴极的光谱响应,给出了230nm-400nm波段内反射式NEA GaN光电阴极量子效率曲线。在230nm处得到了反射式GaN光电阴极高达37.40%的量子效率,230nm和400nm之间的锐截止比率超过2个数量级。结合国外对GaN光电阴极量子效率的研究结果,综合分析了影响量子效率的因素,得到了量子效率与入射光波长、阴极材料特性以及阴极制备水平之间的关系。以反射式NEA GaN光电阴极激活的光电流曲线和充分激活后的量子效率曲线为依据,针对阴极量子效率的衰减以及不同波段对应量子效率衰减速度的不同,论述了NEA GaN光电阴极量子效率的衰减机理。得到了反射式NEA GaN光电阴极量子效率的衰减现象与有效偶极子数量减小之间的关系以及量子效率曲线的衰减特点与表面势垒形状改变之间的关系。结合GaN光电阴极铯氧激活后的表面模型[GaN(Mg):Cs]:O-Cs,通过对量子效率衰减过程中阴极的能带与表面势垒结构变化的分析,得出结论：有效偶极子数量的减小是造成量子效率降低的根本原因,表面Ⅰ、Ⅱ势垒形状的变化造成了不同波段对应的量子效率下降速度的不同。更多还原

【Abstract】 As a new type ultraviolet (UV) photocathode, 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. Aiming at investigative scarcity of basic theory, preparation method and evaluation means for NEA GaN photocathode at present, the researches were made on such aspects as photoemission mechanism, depuration method, activation technique, test of spectral response and stability performance for reflection-mode NEA GaN photocathode.According to W.E.Spicer photoemission "3-step model" theory, the whole process including photoelectron excitation, transportation from bulk to surface and escape to the vacuum by traversing surface barrier was analysed in detail. The transmission coefficient that photoelectrons traverse cathode surface barrier was educed. The quantum yield formula of reflection-mode NEA GaN photocathode was gotten by solving the diffuse equation of non-equilibrium carriers. According to GaN photocathode NEA property during activation process and after being fully activated, the surface model [GaN(Mg):Cs]:O-Cs for NEA GaN photocathode after Cs/O activation was given.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 method were given in detail. After the effective chemical cleaning and the heating of 700℃about 20 minutes in ultrahigh vacuum system, the oxides and carbon impurities on cathode surface can be effectively removed, and the ideal atom clean surface can be obtained.The photocurrent curves of Cs and Cs/O activation for reflection-mode GaN photocathode were tested by using dedicated experimental system for activating and evaluating of NEA photocathode. Aiming at the formation cause of NEA property for GaN photocathode and according to the rule of photocurrent change during activation period and the surface model of a fully activated photocathode, the activation mechanism of NEA GaN photocathode was studied. The relation of the rule of photocurrent change during activation period and the change of electron affinity during activation period was gotten. The experiment 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. The NEA property formation reasons of GaN photocathode after being fully activated successfully can be well explained using the double dipole layer model [GaN(Mg):Cs]:O-Cs.The spectral response of fully activated reflection-mode NEA GaN photocathode was measured by using dedicated ultraviolet spectral response measurement instrument. The quantum efficiency curves of reflection-mode NEA GaN photocathode were given in the band region from 230nm to 400nm. The quantum efficiency of reflection-mode NEA GaN photocathode reaches up to 37.40% at 230nm, a sharp cutoff characteristic with over two orders of magnitude degradation from 230nm to 400nm has been observed. Based on the former research results, the factors influencing quantum efficiency were also comprehensively analyzed. The relation of quantum efficiency and incidence light wavelength, cathode material characteristic and cathode preparation technique was gotten.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 of reflection-mode NEA GaN photocathode was studied. The relation of the quantum efficiency decay of reflection-mode NEA GaN photocathode and the reduction of effective dipole quantity was gotten, and the relation of the decay characteristic of quantum efficiency curves and the change of surface barrier shape was also gotten. 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 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 different wave bands.更多还原