GaN光电阴极负电子亲和势形成机理研究

Vacuum ultraviolet detectors based on GaN photocathodes are very promising in space exploration, missile approach warning and corona detection. Research on the negative electron affinity formation mechanism of GaN photocathode to further improve its performance becomes one of the hotspots in the field. Although several surface models are already proposed about this problem, they show obvious deficiencies for lack of experimental research foundations. Based on the research of surface cleaning and activation, this project proposes that it is highly possible that gallium oxide plays a positive role in the NEA formation of GaN photocathode, and a further analysis about this problem will be done with research on the atomic components and proportions of the surface barriers. The main contents of this project are as followings:(1) the role of gallium oxide during the activation of GaN photocathode,(2) the variations of surface components, downward band bending and decrease of the surface vacuum level at each period of the GaN photocathode preparation (3) proposal of surface dipole model based on the above experimental results. This project will demonstrate important direction of the exploration of photoemssion nature, further improvement of characteristics of GaN photocathode.

以GaN光电阴极为主要探测单元的真空紫外探测器在空间探测、导弹逼近告警、电晕检测等领域具有广泛的应用前景，探究GaN光电阴极表面负电子亲和势(NEA)形成机理以进一步提高其性能成为该领域的研究热点之一。目前针对该问题尽管提出了几个表面模型，但因缺乏实验研究基础而存在明显的不足。本项目根据GaN材料的表面净化和激活研究结果，提出镓的氧化物有可能在GaN光电阴极NEA形成过程中起到积极作用这一假设，拟通过研究表面势垒原子组成、比例等，对GaN光电阴极NEA形成机理进行深入剖析。项目的主要研究内容包括：镓的氧化物在GaN光电阴极激活过程中的地位和作用研究；GaN光电阴极在不同制备阶段表面成分、表面能带弯曲量和表面真空能级下降量研究；基于上述实验结果的NEA GaN光电阴极表面偶极层模型研究等。该课题的研究对探索GaN光电阴极光电发射本质、提高其性能有重要的理论指导和现实意义。

为探究GaN和AlGaN光电阴极光电发射机理，本项目主要研究了材料表面是否存在固有的Ga2O3以及Ga2O3是否在光电阴极负电子亲和势形成中起到积极作用这一课题。通过对材料表面处理过程中的XPS测试，验证了表面Ga2O3的存在，而且采用Ar+离子溅射的方式比常规化学清洗和热清洗更能有效去除表面污染物。对于变Al组分AlGaN为缓冲层的GaN光电阴极出现的特殊光电流变化过程，采用提出的Ga-O-Cs模型给与了解释。第一性原理计算结果显示，Ga-O-Cs的吸附结构可以比Ga-Cs结构获得更低的表面功函数，显示出表面氧化物对NEA形成的积极意义。同时计算表明，随着Cs、O覆盖度的增加，吸附原子在表面的吸附位置会发生改变，而最终NEA的形成是表面多种原子与Cs、O原子共同作用的结果。该研究结果对促进超宽禁带半导体Ga2O3为基础的光电阴极研究奠定了基础。