Band Meandering due to Charged Impurity Effects and Carrier Transport in Ternary Topological Insulators

Abstract

Controlling charged impurity disorder is a critical challenge for realizing the promise of topological insulator (TI) surfaces in devices. While doping is often used to tune the chemical potential, its impact on the fundamental disorder landscape remains poorly understood. Here, we investigate this effect in ternary (Bi,Sb)$_2$Te$_3$ (BST) thin films and their indium-doped (IBST) counterparts. Gate-dependent transport reveals that indium doping increases charged impurity density by an order of magnitude, which in turn reduces the characteristic size of disorder-induced charge puddles from $\sim$91 nm to $\sim$38 nm. This amplified disorder enhances Coulomb scattering and suppresses field-effect mobility, directly demonstrating how doping-induced compensation degrades surface transport. Our work establishes doping as a powerful method to probe the limits of topological protection and underscores that defect suppression, not just compensation, is essential for developing high-performance TI devices.

Figures (5)

• Figure 1: (a) illustrates a thin film of topological insulator placed on a Si/SiO2 substrate, with a positive or negative gate voltage (+/- V) applied to the Si++ wafer. The current through the topological insulator is denoted as I, and G represents the common ground. (b) Shows resistance vs temperature at different gate voltages denoted by $V_{gs}$ in volts. (c) Shows the density of states (DOS) of the valence band versus energy (E) with chemical potential $E_F$ at the band tail.
• Figure 2: (a) presents the resistance–temperature (R–T) behavior at $V_{gs} = 10$ volts, fitted using two different approaches depicted in (b) and (c). (b) applies the Mott variable range hopping model, utilizing Equation 1 for fitting. (c) employs the parallel resistor model, based on Equation 2. (d) displays activation energies at elevated temperatures for various gate voltages, where their slopes can be utilized to determine the density of states around $E_F$.
• Figure 3: (a) depicts the relationship between drain-source current and gate voltage at various temperatures. (b) presents the field-effect mobility as a function of temperature, derived using Equation 3. (c,d) shows the decreasing behaviour of mobility while going to either side of the band.
• Figure 4: (a) Shows R-T of thick-BST and thin-IBST. (b) The R-Vgs plot demonstrates a peak in resistance, corresponding to the charge neutrality point for BST. (c,d,e) depicts the relationship between drain-source current and gate voltage at various temperatures for BST. (f) depicts the electron-hole puddles.
• Figure 5: (a,b) illustrates the temperature variation of the magnitude of four-probe field-effect mobility for BST and IBST respectively.