Photoexcited Free Carrier Dynamics in Bi₂Se₃, (Bi_0.75In_0.25)₂Se₃, and (Bi_0.50In_0.50)₂Se₃: From Topological to Band Insulator

Replacing some Bi with In in Bi2Se3 transforms it from a topological insulator to a band insulator. Here, we have used time-resolved terahertz spectroscopy to investigate photoexcited carrier dynamics in (Bi1-xInx)2Se3 films with indium concentration x = 0%, 25%, and 50%. In Bi2Se3, optically excited carriers scatter from the bulk conduction band states into high mobility topological surface states within picoseconds after excitation. We demonstrate that a second set of Dirac surface states, located ∼1.5-1.8 eV above the conduction band minimum and accessible to carriers excited by 3.1 eV pulses, is characterized by a higher mobility than the surface states within the band gap that dominate equilibrium conductivity. In (Bi0.75In0.25)2Se3 and (Bi0.50In0.50)2Se3, which are insulating without photoexcitation, the dynamics of photoexcited free carriers are affected by the twin domain boundaries and are sensitive to the disorder introduced by indium substitution. Transient conductivity rise time, as well as the mobility and lifetime of the photoexcited carriers in (Bi1-xInx)2Se3 films, can be tuned by the indium content, enabling tailoring of band insulators that have the desired optoelectronic properties and are fully structurally compatible with the topological insulator Bi2Se3 for applications in high-speed photonic devices based on topological insulator/band insulator heterostructures.