Properties of nano-scale artificial structures of magnetic semiconductors and search for new functionalities

在这个研究项目中，我们一直致力于开发在室温下表现出铁磁性的新型半导体材料，这被认为是未来利用电子自旋自由度的自旋电子学所不可或缺的。我们研究了（Zn,Cr）Te，它作为本征室温铁磁性半导体的候选材料之一而备受关注。具体来说，我们研究了p型和n型电荷杂质的共掺杂如何改变磁性，旨在阐明铁磁性的起源和器件应用。结果发现，氮作为p型掺杂剂共掺杂抑制了（Zn,Cr）Te的铁磁性，而碘作为n型掺杂剂共掺杂则显著增强了（Zn,Cr）Te的铁磁性。透射电子显微镜（TEM）和能谱分析（EDS）的结构和化学分析表明，共掺杂改变了生长膜中Cr的分布；Cr在碘掺杂薄膜中的分布极不均匀，形成了典型尺寸为30 ~ 50 nm的富Cr区，而未掺杂和氮掺杂薄膜中的Cr分布几乎均匀。这些结果表明，在碘掺杂（Zn,Cr）Te中铁磁转变温度Tc的显著升高是由高Cr含量的铁磁团簇的形成引起的。作为Cr分布均匀性不同的一个原因，我们提出了一个模型，即由于带电杂质的共掺杂导致Cr价态的变化影响了Cr离子之间的吸引相互作用，从而形成富Cr簇。

In this research project, we have been engaged in the exploitation of novel semiconducting materials exhibiting ferromagnetism at room temperature, which are considered to be indispensable for future spintronics utilizing spin degree of freedom of electrons. We have studied (Zn,Cr)Te, which has been attracting attention as o ne of the candidates of intrinsic room-temperature ferromagnetic semiconductors. Specifically, we have investigated how the magnetic properties are altered by co-doping of charge impurities of both p-type and n-type, aiming at the elucidation of the origin of ferromagnetism and the device application. As a result, we have found that the ferromagnetism of (Zn,Cr)Te is suppressed due to the co-doping of nitrogen as a p-type dopant while the ferromagnetism is significantly enhanced due to the co-doping of iodine as an n-type dopant. The structural and chemical analyses using transmission electron microscope (TEM) and energy-dispersive X-ray spectroscopy (EDS) have revealed that the Cr distribution in the grown films is modified due to the co-doping ; the Cr distribution in the iodine-doped films is strongly inhomogeneous, with the formation of Cr-rich regions of a typical size of 30〜50 nm, while the Cr distribution in the undoped and nitrogen-doped films is almost homogeneous. These results indicate that an significant increase of ferromagnetic transition temperature Tc in the iodine-doped (Zn,Cr)Te is caused by the formation of ferromagnetic clusters containing high Cr contents. As an origin of different homogeneities of the Cr distribution depending on the co-doping, we propose a model that the change in Cr valence due to the co-doping of charge impurities affects the attractive interaction between Cr ions, resulting in the formation of Cr-rich clusters.