Two Band Model Interpretation of the p to n Transition in Ternary Tetradymite Topological Insulators

The requirement for large bulk resistivity in topological insulators has led to the design of complex ternary and quaternary phases with balanced donor and acceptor levels. A common feature of the optimized phases is that they lie close to the p to n transition. The tetradymite Bi2Te3_xSex system exhibits minimum bulk conductance at the ordered composition Bi2Te2Se. By combining local and integral measurements of the density of states, we find that the point of minimum electrical conductivity at x=1.0 where carriers change from hole-like to electron-like is characterized by conductivity of the mixed type. Our experimental findings, which are interpreted within the framework of a two band model for the different carrier types, indicate that the mixed state originates from different type of native defects that strongly compensate at the crossover point.