Bulk-Induced Correlated Mobility-Number Density Fluctuations in Molecular Beam Epitaxy Grown Topological Insulators

We present a detailed study of 1/f noise in large-area molecular beam epitaxy grown thin (~10 nm) films of the topological insulator (Bi,Sb)₂Te₃ on strontium titanate (STO) substrate as a function of temperature (T), gate voltage and magnetic field [1]. The temperature dependence of 1/f noise displays a sharp cusp at T = 50 K which can be attributed to generation-recombination processes in the impurity band in the bulk band gap. The gate voltage dependence of noise reveals that both bulk and surface states are influenced by correlated mobility-number density fluctuations caused by defects in the bulk with a density D_it = 3.2×10¹⁷ cm^-2eV^-1. In the presence of magnetic field, the 1/f noise follows a parabolic dependence which is qualitatively similar to mobility and charge density fluctuation noise in nondegenerate semiconductors. Our studies reveal that even in thin films of (Bi,Sb)₂Te₃ with thickness as low as 10 nm, the bulk defects are the dominant source of noise. (1) S. Islam et al. APL 111, 062107 (2017)