Rashba半導體碘碲化鉍及溴碲化鉍之表面電子結構研究

Abstract

自旋軌道交互作用是藉著電子自旋以及外加電場產生出一個基本的耦合效應。具有強自旋分裂現象的材料，像是藉由材料的Rashba效應，使我們可以利用電場控制自旋電流，來調節許多自旋電子學的應用。鉍碲和鹵素的化合物具有強自旋軌道分裂，提供了一個廣大的平台供我們作關於強自旋軌道分裂之載子的研究。另外，鉍碲和鹵素的化合物具有非中央對稱的極化結構，破壞了空間的反轉對稱性，在導電帶和價電帶產生出巨大的Rashba自旋分裂；因著此結構在其兩種不同的表面帶有兩種相反的電荷，使帶有相反電荷的兩種表面產生強的極化效應和強的能帶彎曲，分別產生二維的累積層與空乏層。在本篇論文中，我們結合了實驗上藉由掃描式穿隧電子顯微鏡測得的表面地形圖、掃描式穿隧電子能譜和模擬計算的結果，以及碘碲化鉍和溴碲化鉍的化學性質及電性結構。在碘碲化鉍表面，我們在碲和碘的極化表面發現類似局域化的區域，也藉由X射線光電子能譜、穿隧能譜和ab initio的計算結果明白了在兩種表面產生的兩種相反的能帶彎曲效應。我們也發現因著表面能帶彎曲使電荷在不同表面間的側向邊界遷移，產生出從類似p型的電子結構反轉至n型的電子結構。這種接面可藉由空間解析的穿隧能譜視覺化的呈現，這樣的邊界也顯示了第一個發現的二維pn接面。在溴碲化鉍表面，藉由表面的地形圖、穿隧電子能譜和X射線光電子能譜發現尺度超過1公釐的廣大極化表面。其中，在碲表面產生的累積層發現布洛赫波函數的準粒子干涉表明了因著表面缺陷產生的電子駐波。藉由能量相關的探索，確認了類似電子的高線性相關之色散關係。在碲的導電帶，6p軌域的布洛赫電子散射對於因著內層的鉍缺失造成的缺陷顯示為最為強烈，使我們得知原子等級缺線的化學性質以及彈道傳輸之特性之間的交互作用。這些探索提供人們一個平台去明瞭以及如何去應用一個真實材料的二維、相對論性自旋軌道分裂載子的表面系統。

The spin orbit interaction provides the most fundamental coupling of the electron spin to applied electric fields. Materials with strong spin-orbit splitting phenomena such as the Rashba effect offer a path towards the all-electric control of spin currents and a myriad of other spintronic functions. Semiconductor materials with giant spin orbit splitting, such as the family of compounds BiTeX, provide a versatile platform for the investigation of strongly spin-orbit split carriers. The non-centrosymmetric polar structure of BiTeX strongly breaks spatial inversion symmetry, driving a giant Rashba spin splitting in both the conduction and valence bands. This structure also features strong polarization induced band bending, with opposite signs at its two different polar terminations, creating a two-dimensional accumulation or depletion layer depending on the termination. In this work, combined microscopic, spectroscopic and computational investigations of the surface morphology, chemistry and electronic structure are presented for the compounds BiTeI and BiTeBr. For BiTeI, domain-like regions of polar Te- and I-terminated surfaces are observed using scanning tunneling microscopy. Opposite polarization induced band bending effects, understood with the aid of ab initio calculations, are observed in scanning tunneling spectroscopy measurements and in core level x-ray photoemission spectra. At the lateral boundary between regions of different termination, a reversal of the surface band bending leading to a transition at the boundary from p-like to n-like electronic structure is revealed. This type of junction, previously speculated upon, and dubbed the `Rashba p-n junction' due to the strong Rashba splitting of electrons and holes at either side, is directly visualized using spatially resolved tunneling spectroscopy. This boundary represents the first two-dimensional p-n junction discovered. For BiTeBr, scanning tunneling microscopy images, tunneling spectra and also x-ray photoemission spectra reveal uniform polar surfaces on scales exceeding 1 mm. In the accumulation layer confined to the Te terminated surface of BiTeBr, quasi-particle interference of Bloch wavefunctions is observed to manifest as standing electron waves around surface defects. Energy dependent observations are used to confirm the expected highly linear electron-like dispersion. Scattering of Bloch electrons in the Bi 6p-derived conduction band is shown to be strongest for defects in the Bi sublattice of the crystal, revealing the interplay between atomic scale defect chemistry and ballistic transport properties. These investigations provide a platform for the understanding and harnessing of two dimensional, relativistic and strongly spin-orbit split carriers at the surfaces of real materials systems.