Revealing Optical Transitions and Carrier Recombination Dynamics within the Bulk Band Structure of Bi2Se3

TL;DR

This study investigates the ultrafast dynamics of electrons and holes within the bulk band structure of Bi2Se3, revealing rapid thermalization and recombination processes crucial for understanding its electronic properties.

Contribution

It provides the first detailed experimental and theoretical analysis of bulk carrier dynamics in Bi2Se3, focusing on ultrafast thermalization and recombination within the extended bulk bands.

Findings

Electrons and holes are photoexcited within a dense, cold electron gas.

Carrier thermalization occurs within a couple of picoseconds.

Minority carriers recombine within 150 ps at 10 K and 50 ps at 300 K.

Abstract

Bismuth selenide (Bi2Se3) is a prototypical three-dimensional topological insulator whose Dirac surface states have been extensively studied theoretically and experimentally. Surprisingly little, however, is known about the energetics and dynamics of electrons and holes within the bulk band structure of the semiconductor. We use mid-infrared femtosecond transient reflectance measurements in a thick exfoliated nanoflake to study the ultrafast thermalization and recombination dynamics of photoexcited electrons and holes within the extended bulk band structure over a wide energy range (0.3-1.2 eV). Theoretical modeling of the reflectivity spectral lineshapes at 10 K demonstrates that the electrons and holes are photoexcited within a dense and cold electron gas with a Fermi level positioned well above the bottom of the lowest conduction band. Direct optical transitions from the first and the second spin-orbit split valence bands to the Fermi level above the lowest conduction band minimum are identified. The photoexcited carriers thermalize rapidly to the lattice temperature within a couple of picoseconds due to optical phonon emission and scattering with the cold electron gas. The minority carrier holes recombine with the dense electron gas within 150 ps at 10 K and 50 ps at 300 K. Such knowledge of interaction of electrons and holes within the bulk band structure provides a foundation for understanding of how such states interact dynamically with the topologically protected Dirac surface states.