Investigations of the Surface Electronic Structure of the Rashba Semiconductors BiTeI and BiTeBr
Date Issued
2015
Date
2015
Author(s)
Butler, Christopher John
Abstract
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.
Subjects
Rashba effect
polar terminations
BiTeI
BiTeBr
scanning tunneling microscopy
scanning tunneling spectroscopy
photo-emission spectroscopy
quasi-particle interference
Type
thesis