Investigating Carrier Transport through Extended Defects in (Bi_1-xSb_x)₂Te₃ Thin Films

(Bi_1-xSb_x)₂Te₃ alloys are topological insulators whose Dirac point can be tuned within the bulk bandgap by varying the composition, effectively reducing bulk conduction while allowing surface carrier conduction. We performed temperature-dependent magnetotransport measurements on a series of (Bi_1-xSb_x)₂Te₃ thin films to characterize carrier transport in these materials. The magnetoconductivity exhibits weak anti-localization behavior below 10 K, which originates from destructive interference of time-reversed scattering paths of carriers with spin-locked momentum. Single-channel analysis of Hall measurements reveals carrier concentrations on the order of 10¹³ cm^-2, and fits to the Hikami-Larkin-Nagaoka formula yield prefactors, α, ranging from 0.8 to 1.5 and phase coherence lengths, l_φ, on the order of hundreds of nanometers. Transmission electron microscopy images of the thin films indicate the presence of twin boundaries with a spacing of about 100 nm. Surprisingly, images also show flat antimonene monolayers with a lattice spacing of about 4.7 Å above the sapphire substrate. Kim et al. have reported that 60° twin boundaries in Bi₂Te₃ can act as free electron sources due to the addition of defect levels caused by atomic bond distortions at the boundary surface. Our density functional theory calculations also show an increase of carrier density of states with the presence of 60° twin boundaries, suggesting a similar process. We will discuss the effects the twin boundaries and antimonene monolayers may have on carrier transport.