利用半金屬鉍將自旋電流轉換成電荷電流之研究

Abstract

由於鉍(Bismuth)和以鉍元素為基礎的拓譜絕緣體可能擁有很大的自旋軌道耦合(spin-orbit coupling)效應，因此在自旋電子學(spintronics)中被廣泛地研究。然而，鉍的自旋流轉換成電子流之效率，在不同文獻中卻有高達三個數量級的差別。在本研究中，我們發現成長薄膜的方法和保護層的有無，都對鉍的品質和介面狀況有很大的影響，進而影響鉍的自旋流轉換成電子流之效率。 我們使用 X-射線繞射儀(XRD)和球面像差修正掃描穿透式電子顯微鏡(Cs-STEM)，對鉍/釔鐵石瑠石(Bi/YIG)樣品的結晶度和橫截面進行系統性的研究。其中，鉍薄膜分別由射頻(RF)和直流(DC)兩種不同的磁控濺鍍(magnetron sputtering)方式成長。我們發現不論是由射頻或是直流成長的鉍薄膜，都呈現多晶性，並分別具有不同的結晶傾向：射頻鉍膜為(012)方向，而直流鉍膜是(003)方向。除此之外，用射頻濺鍍的鉍/釔鐵石瑠石樣品，能形成高品質鉍膜，擁有平坦的表面和清晰的介面。因此，在射頻濺鍍的鉍/釔鐵石瑠石系統中，當我們用熱梯度或是微波驅動自旋流，能夠量測到穩定的反自旋霍爾效應(inverse spin Hall effect)訊號。另一方面， 使用直流濺鍍方式製備鉍/釔鐵石瑠石樣品，鉍膜會形成粗糙的表面以及無晶向的介面層，導致量測不到反自旋霍爾訊號。另外，我們發現保護層(二氧化矽)對於鉍的自旋流轉換成電子流之效率是很重要的。 沒有保護層的情況下，鉍膜會漸漸變得無晶向，且在介面形成一層鉍和釔鐵石瑠石混合的無晶向層，使得反自旋霍爾訊號衰減。 總結來說，我們提供嚴謹的證據，證明鉍存在自旋流轉換電子流之效率，此效率與鉍薄膜的品質和介面狀況有顯著關係。

Bismuth (Bi) and Bi-based topological insulators attract extensive interests in the society of spintronics due to the large spin-orbit coupling. However, the spin-to-charge conversion efficiency of Bi in different reports have spanned three order differences. In this work, we found that the deposition methods and the protecting layer could strongly influence the quality of Bi layer and the interface, resulting in the modulation of spin-tocharge conversion efficiency. By using XRD and Cs-STEM, we systematically studied the crystallinity and the cross section of Bi/YIG samples with Bi film grown by RF and DC sputtering. Both RFand DC-sputtered Bi were polycrystalline with crystal preferred orientation of (012) and (003), respectively. In the RF-Bi/YIG samples, the Bi films showed high quality with the flat surface, and the sharp interface. Therefore, the RF-Bi/YIG systems exhibited large inverse spin Hall effect (ISHE) signal driven by either temperature gradient or microwave.While in the DC-Bi/YIG samples, the Bi films formed rough surface and an amorphous nterfacial layer. As a result, there’s no detectable ISHE signal. Furthermore, we also demonstrated that the capping layer (SiO2) was robust to generate the spin-to-charge conversion in Bi. Without the capping layer, Bi film gradually became disordered and formed a mixed amorphous layer at the interface of Bi and YIG, which significantly reduced the ISHE signal. Our studies provided the rigorous evidence of spin-to-charge conversion in Bi.The spin-to-charge conversion could strongly dependent on the film quality of Bi and the interface condition.