摘要
GaN常温下禁带宽度为3.4eV,光谱响应的闽值波长为365 nm,不吸收可见光,制成的紫外探测器可以做到可见光盲,不需要滤光系统。这样可以大大提高量子效率,满足紫外探测的需求。由GaN材料构成的负电子亲和势(NEA)光电阴极可以达到很高的量子效率,同时耐高温、耐腐蚀、抗辐射性能也更好,因而是研制真空紫外探测器的最理想材料之一。本文围绕GaN光电阴极的制备工艺、梯度掺杂阴极样品的量子效率、阴极光电发射量子效率的衰减与恢复及阴极在透射模式下的发射量子效率等方面开展研究。
利用紫外光电阴极在线激活与测试系统和X射线光电子光谱仪(XPS)研究了GaN光电阴极的制备工艺,给出了具体的化学清洗工艺和加热净化过程。利用XPS分别分析了化学清洗和加热处理后的GaN(0001)表面的化学成分,并研究了GaN阴极Cs,O激活步骤及方法。
为了获得更好的量子效率,设计了一种梯度掺杂结构的GaN光电阴极样品,利用紫外光谱响应测试仪器对三种样品(其他两种均匀掺杂样品作为参照)一起进行了净化及激活,测得三者的量子效率,实验结构证明变掺杂结构可以有效的提高GaN光电阴极的量子效率,在230nm处取得的峰值可达60%。
测试了均匀掺杂及梯度掺杂两种样品在真空系统中300nm处量子效率的衰减,并从双偶极层模型角度分析了量子效率衰减的原因,运用补Cs激活,可以使GaN光电阴极的量子效率得到恢复甚至达到比第一次激活更高的值,说明真空系统中GaN光电阴极量子效率的衰减是由于阴极表面Cs的吸附量减少造成。
测试了激活后的透射式均匀掺杂光电阴极和反射式梯度掺杂阴极在透射模式下的光电发射量子效率。结果表明,发射层的厚度及样品的导电性将影响GaN透射式工作模式下的量子效率。通过推导透射式光电阴极的量子效率公式,讨论了材料的特性对GaN光电阴极量子效率的影响。
In UV spectroscopy and low-light-level UV-imaging applications, there are strong demands for improved detectors which have higher quantum efficiency, low dark current, sharper wavelength cut-off response, and stable and robust characteristics. GaN is one of the promising candidate materials to meet these demands. In this thesis, the researches were made on such aspects as depuration method, activation technique, test of spectral response and stability performance for NEA GaN photocathode.
1. 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 methods were given in detail. After the effective chemical cleaning and the heating of 710℃about 20 minutes in the ultrahigh vacuum system, the oxides and carbon contaminations on GaN (0001) surface can be effectively removed. And the ideal atom clean surface can be obtained. The many activation experiments 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.
2. The photocurrent curves during either Cs or Cs/O activation process for 3 reflection-mode GaN photocathode samples of different doping concentrations were tested by using dedicated experimental system for activating and evaluating of NEA photocathode. The photocurrent of 3 samples during the Cs,O activation shows that grad doping sample can produce much higher value than uniform doping samples. The QE curves indicate that the grad doping structure can improve the photoemission of GaN cathode dramatically. The reason for this is that the diffuse length of grad doping sample is far longer that of uniform doping samples.
3. 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 for reflection-mode NEA GaN photocathode was studied. 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 the 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 the different wave bands.
4. The spectral response of fully activated 2 NEA GaN photocathode samples (one is opaque grad-doping, the other is transparent uniform doping) working under transmission mode were measured by using dedicated ultraviolet spectral response measurement instrument. The quantum efficiency of tranparent NEA GaN photocathode in transmission mode reaches up to 6.5% at 280nm, a "door" shape curve was observed from 240nm to 380nm has been observed. Based on the former research results, the factors influencing quantum efficiency were also comprehensively analyzed.
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