Development of Polarized Electron Source for Linear Colliders by Using Superlattice Photocathhode

我们在1991年初从ALGA-As-GaAs超晶格中获得了715极化的光发射。下面描述的研究是为了进一步推进成果，旨在将超晶格应用于未来的线性对撞机（例如JLC）。我们研究了电子在导带中扩散过程中的自旋退极化。对超晶格进行p掺杂以实现负电子亲和，这是将超晶格用于光电阴极所必需的。我们在超晶格的大部分区域应用了较低的受体浓度，而在表面附近应用了正常浓度（5 × 10^<18>/cm^3），甚至更高的浓度。我们发现这种类型的调制掺杂提高了光电阴极的极化和量子效率。我们已经研究了极化如何依赖于超晶格的总厚度。外推到零厚度，极化率为-75%。我们测试了具有各种表面条件的样品。它们都没有表现出明显的极化改善。我们已经得出结论，在地表附近没有大的去极化源。因此，在导带中射出的电子极化率为-75%。我们已经测试了从超晶格中抽取大量电荷。这个测试是作为日本-SLAC合作的一部分在SLAC进行的。结果是成功的；2.3x10^<11>电子在2.5nsec内被提取。没有观察到光电二极管材料的电荷限制。生成的电子数受到火炮空间电荷极限的限制。这一结果是非常令人鼓舞的，因为电荷极限现象被认为是光电阴极枪在未来线性对撞机中最严重的问题。

We achieved 715 polarization of photoemission from the ALGA-As-GaAs superlattice in early 1991. The study described below has been performed to proceed the fruits further aimed to apply the superlattice in future linear colliders (JLC for example).We have studied the spin depolarization during the diffusion of electrons in the conduction band. The superlattice is p-doped to achieve negative-electron-affinity, which is necessary to use the superlattice for a photocathode. We applied lower acceptor concentration in most part of a superlattice, on the other hand normal(5x10^<18>/cm^3), or even higher, concentration near the surface. We have found this type of modulation doping improves both polarization and quantum efficiency of a photocathode.We have studied that how polarization depends on the total thickness of the superlattice. The extrapolation to zero thickness gave a polarization of -75%. We tested sample which has various surface conditions. Non of them showed significant improvement of the polarization. We have concluded there was no source of large depolarization near the surface. Therefore the polarization of electrons exited in the conduction band was -75%.We have tested large charge-extraction from a superlattice. This test was performed at SLAC as a part of the Japan-SLAC collaboration. The result was successful ; 2.3x10^<11>electrons were extracted in 2.5nsec. No charge Limit of the photochatode material was observed. The resultant number of electrons was limited by Space Charge Limit of the gun. This result is very encouraging becaouse the charge Limit phenomenon is supposed to be the most serious problem of a photocathode gun in a future linear collider.