鉍硒拓樸元件中間相之研究

Abstract

摘要 拓樸絕緣體(topological insulator， TI)為一新奇的材料其內部為絕緣體，但其外部表面態(surface state, SS)卻為導體，這代表電子只能在拓樸的表面態中流動，且表面態具有自旋動量鎖相(spin momentum locking, SML)特性能限制電子的流向和自旋方向成一垂直關係，有效的產生自旋電流(spin current)，達到能利用電流控制自旋方向的可能性，這是以往的半導體元件所無法做到的。而拓樸與鐵磁性層的結合使拓樸的潛力得以發揮，能將拓樸產生的自旋電流透過自旋軌道矩 (spin/orbital torque, SOT)高效率的翻轉鐵磁層磁矩，不但省下能量的消耗且不會產生過多的熱，非常適合來製作積體電路(integrated circuits,IC)，因此成為自旋電子元件的明日之星。 目前的研究方向以拓樸與磁性層異質元件的自旋傳輸效率與原理為主流，較少文獻對拓樸和磁性層異質結構之介面中間相(intermediate phase)進行探討，吾人認為介面的中間相研究是必須的，因為中間相的出現可能會影響到拓樸本身的良好特性如自旋動量鎖相和自旋軌道矩，而大大地降低元件的自旋傳輸效能，這些效應都是拓樸元件的核心， 吾人以上述原因為動機選擇以拓樸和鐵磁層之介面中間相的結構為研究方向，分析不同拓樸厚度(5 nm、20 nm與40 nm)的鉍硒(Bi2Se3)拓樸絕緣體與鎳鐵合金Ni80Fe20 (permalloy , Py )異質結構。結論指出，介面對硒化鐵中間相的出現相當敏感且對磁性層造成難軸(hard axis)與易軸(eas -y axis)的轉換和電性上的變化，而硒化鐵中間相的穩定度和其對鎳鐵層造的改變階皆與拓樸厚度相關。經此研究後，吾人發現異質介面的中間相對拓樸的介面性質扮演重要的腳色，因此在製作拓樸與磁性層異質元件時，需要選擇適當的磁性材料作為磁性層以防止中間相的產生而影響了元件的運作機制和效應。

Abstract A topological insulator (TI) is a material that behaves as an insulator in its interior but whose surface contains conducting states. This means that electrons can only move along the surface of the material. The momentum and spin of electrons in the topological insulators are constrained to be perpendicular due to strong spin-orbit coupling. This unique coupling between current and spin-polarization may enable a host of devices impossible with other semiconductor materials. The combination of TI and ferromagnet (FM) can enable ultrahigh efficiency of the TI in converting electrical charge current into spin accumulation by transfer/orbital torque. This opens many opportunities for spintronic applications. With the TI/FM’s electrical charge/spin accumulation effect, many ground-breaking studies have demonstrated possible device operation with reduced power consumption, and the compatibility of electrically controlled devices with semiconductor integrated circuits. Despite extensive studies about this great excitement, the existence, as well as the effects, of intermediate phase in the vicinity of the TI-FM epitaxial junction is never explored. This issue is essential because the intermediate phase, if exists, would tremendously affect the TI’s electronic behavior such as the fascinating spin transfer/orbital torque and spin-momentum locking effects. These physical characteristics are at the heart for a high charge/spin current converting efficiency of the TI/FM spintronic devices. Thus a close study of interfacial configurations of TI/FM heterostructure is essential for how to improve and tailor them for charge/spin current converting. With this motivation we thoroughly investigated the intermediate phase of a Ni80Fe20/Bi2Se3 heterostructure. The results point to strong sensitivity of the intermediate phase (Fe-Se compound) to the interfacial interactions and the attendant phenomenon, such as switching of magnetic easy-axis and transport properties. The stability of the Fe-Si intermediate phase, together with those physical characteristics, was found to be variable with TI thickness. Our work suggests that the emergence of the intermediate phase plays a critical role in determining the surface state of the TI itself, as well as the functionality of the TI/FM device. Therefore, the fabrication of TI/FM devices needs to be carefully handled in order to prevent the formation of the intermediate phase. This also means that the selection of the FM is another important factor responsible for above facts.