光学共振纳米结构GaAs基自旋极化电子源研究

Gallium arsenide (GaAs) semiconductor strained thin film photocathodes can realize the high spin polarized (>90%) electron source, which have been widely used in high energy physics, accelerator, advanced light source, etc. However, the limit on quantum efficiency (QE) is still an open question for the GaAs-based spin polarized photocathodes. It is well known that the QE of the GaAs-based strained thin film structured photocathodes was mainly restrained by the low absorptance of the incident light. The optically Mie Resonant (MR) effect of the semiconductor nanostructure has been confirmed in most of the optoelectronic devices. By tailoring the MR properties, controlled by the physical dimensions of the nanostructures, one can greatly enhance light absorption and localize the optical and electric field intensity, and also excite the multi-photon effects. In this project, the MR effect was used to develop the new generation of the nanostructured GaAs-based photocathode electron source with the high QE, high spin polarization and long lifetime. In this project, we will establish an electron emission theory model based on the MR effect for this spin polarized electron source, and then use the model to analyze the effects of space distributions of light field and charge, multi-photon effect, quantum size effect, and strain on the generation, transport, and emission of spin polarized electrons in different nanostructure electron sources. The optimum GaAs-based nanostructured materials will be designed and then be fabricated by epitaxy, and nanoimprinting or nanosphere etching processes. The electron source based on nanostructured GaAs/GaAsP strain-compensated superlattice with high QE, high polarization, and long lifetime will be prepared in an ultra-high vacuum activation chamber. Through this work, the advanced spin polarized electron source and the related technical reserves will be developed, and lay the foundation for the application of polarized electron source in the accelerator in china.

GaAs应变薄膜光阴极能产生自旋极化度高于90%的电子束，广泛应用于加速器等领域，然而应变薄膜有源区极低的光吸收率导致的低量子效率成为限制应用的主要因素。本项目提出用GaAs基纳米结构作为光电发射有源区，通过局域米氏光学共振增强有源区光吸收，利用其对光场与电荷的空间限制能力提高发射电子的量子效率和极化度，并利用纳米结构延长电子源工作寿命，利用其非线性光学效应降低光电发射阈值。项目拟通过建立自旋极化电子源发射物理模型，获得不同纳米结构光学共振增强效应下光场与电荷的空间分布、多光子效应、量子尺寸效应和应变等对自旋极化电子的产生、输运及发射过程的影响规律，优化设计GaAs基纳米结构材料。外延生长GaAs基应变超晶格薄膜，并采用纳米压印/纳米球刻蚀等工艺制备应变GaAs/GaAsP纳米结构。激活实现高量子效率、高极化度、长寿命的纳米结构GaAs基电子源，为我国极化电子源在加速器中的应用奠定基础。

项目基于时域有限差分法及半导体载流子输运理论，建立了纳米结构GaAs基自旋极化电子源的光电发射物理模型。利用该模型仿真发现，GaAs纳米柱可激发很强的Mie散射共振效应，能显著提升光子吸收和电子输运效率，从而提高电子源量子效率和自旋极化率。通过纳米压印与自组装纳米球刻蚀工艺制备了不同尺寸的GaAs纳米柱阵列，并实现了在700-800nm波段激发偶极子共振，在500-800nm波段激发四极子共振。GaAs纳米柱阵列电子源在共振波长处光子吸收率可达95%以上，偶极子和四极子共振分别将量子效率提升至平面型电子源的1.8倍和3.5倍，寿命提升至11.3倍。分析了纳米结构尺寸对电子源自旋极化率的影响，发现纳米结构由于尺寸小，发射电子自旋极化率远高于平面结构电子源。利用XPS原位测试了电子源在不同阶段表面的元素含量变化，并分析了其原因。项目研究验证了纳米结构是提升GaAs电子源量子效率、工作寿命和自旋极化率的有效手段，该类电子源对于丰富电子源类型和电子发射理论具有积极意义，在电子加速器等领域具有重要的应用前景。