p型及n型半導體之反向自旋霍爾效應

Abstract

矽和砷化鎵是很重要的自旋半導體材料因為它們具有較大的自旋生命週期(spin lifetime)或反向自旋霍爾角(inverse spin Hall angle)，同時他們也是現今電子元件中不可或缺的材料之二。然而關於這些半導體材料的自旋傳輸特性及傳輸機制尚未被完整研究及討論。從過去研究結果發現，各種半導體材料的反向自旋霍爾角符號及大小差異是由於n型和p型半導體中的摻雜濃度及摻雜原子不同所導致。為了理解半導體材料中自旋傳輸現象以及摻雜效應對自旋軌道交互作用的影響，我們利用鐵磁共振式自旋幫浦的技術來對矽和砷化鎵材料中的摻雜效應進行有系統的研究，並且精準量測到幾個關鍵的自旋傳輸參數，例如:自旋生命週期、自旋擴散長度(spin diffusion length)及反向自旋霍爾角。 藉由改變矽半導體中的載子種類(n型和p型)及摻雜濃度(1×1013 to 1×1019 cm-3)觀察到自旋擴散長度和反向自旋霍爾角之間的關聯性，也證明了可以藉由改變摻雜濃度及摻雜原子來有效調控自旋-電荷間的轉換效率。同時計算出矽的自旋擴散長度和自旋霍爾角乘積為17.8 nm，該值略大於白金的實驗結果(12.8 nm)。此結果顯示矽半導體的大範圍可調控性將成為具有發展潛力的自旋電子應用材料。此外，反向自旋霍爾角的大小與摻雜原子序(Z)相關但其符號與載子種類無關。從砷化鎵的研究中觀察到反向自旋霍爾角的數值與Z^2成正比。該結果與Landau-Lifshitz Z^2 scaling 的理論模型相符。該模型考慮了各種元素外層電子對自旋偶合效應的影響而非只討論特定元素如過渡金屬。藉由該研究結果可以深入了解到半導體材料中摻雜效應對自旋-電荷間轉換效率的影響。由於自旋電子流的節能特性使自旋電子元件成為下世代的發展主軸，因此這研究成果將有助於半導體自旋電子元件的開發與設計，如新型自旋場效電晶體(spin-based CMOS)。

Silicon (Si) and gallium arsenide (GaAs) are two important semiconducting materials for the applications of spintronics due to either long spin lifetime (for Si) or large inverse spin Hall angle (for GaAs) compared to those of other semiconductors (SC). Meanwhile, they are already the essential parts of modern electronic devices. However, for practical applications, the complete knowledge about the spintronic properties and the related mechanism in SC is still not established yet. In particular, the reported results on the magnitude and the sign of θ_ISHE in the various SCs are rather diversed due to the fact that the doping concentration and dopants atom in n-type and p-type sample are always different. In order to understand the behavior of spintronic transport and the dopant effect on spin-orbit interaction in SCs, we adopted the technique of ferromagnetic resonance driven spin pumping (FMR-SP) to accurately determine several critical parameters, including the τ_s, the spin diffusion length (λ_s) and the θ_ISHE in the Si and GaAs single crystal. By changing the type (n- and p-type) of carrier and the doping concentration (1×1013 to 1×1019 cm-3) for Si, a correlation between λ_s and θ_ISHE is found, demonstrating an effective route to tune the efficiency of spin-charge conversion by changing the doping concentration and dopant atoms. A constant value of λ_s θ_ISHE (17.2 nm) for Si is found to be larger than Py (12.8 nm), indicating that Si is as effective as Pt in terms of converting the spin current to charge current for the application of spintronic devices. Furthermore, the magnitude of θ_ISHE is sensitive to the atomic number of dopant (Z), but the sign of θ_ISHE is independent on the carrier type. The relationship between θ_ISHE of GaAs and Z follows the Landau-Lifshitz Z^2 scaling, in agreement with the model considering only the outmost electron for non-specific atoms. The overall results of this study provide an in-depth understanding for the influence of dopant effect on the spin-charge conversion rate of SC, which benefits the future applications of energy-saving spintronic devices such as new type of field emission spin-based MOSFET.