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

Abstract

Bismuth selenide (BiSe) is a prototypical 3D 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 on a single 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 to 1.2 eV). Theoretical modeling of the reflectivity spectral line shapes 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 how such states interact dynamically with the topologically protected Dirac surface states.

Clarifying Ultrafast Carrier Dynamics in Ultrathin Films of the Topological Insulator Bi2Se3 Using Transient Absorption Spectroscopy

“…25 On the contrary, FCA in the mid-IR/THz range is due to more common Drude-type absorption, nevertheless, also providing a positive contribution to the transient reflectivity and transient conductivity traces. 20,22 In high-quality molecular beam epitaxy-grown films of Bi 2 Se 3 , bandgap renormalization is expected to be mainly governed by interaction with LO-phonons (polaronic bandgap renormalization), 25,33 whereas FCA should occur when the local density of carriers becomes high enough owing to their accumulation in some states. Using the pumping power dependences of the pump−probe trace peak intensity, one can set the bandgap renormalization and FCA processes apart, since they are differently dependent on the photoexcited carrier density (n 1/3 versus n, respectively).…”

“…The ultrafast carrier dynamics in the TI Bi2Se3 have been studied using a variety of techniques such as time-resolved and angle-resolved photoemission spectroscopy (TrARPES) 6,[9][10][11][12] , pump-probe reflectivity [13][14][15][16][17][18] , pump-probe second harmonic generation (SHG) 8,14 , transient reflectivity (TR) spectroscopy [19][20][21] , and transient conductivity (TC) spectroscopy 22,23 . In TrARPES experiments, the pumping beam of a commercial ultrafast Ti:Sapphire laser (~800 nm -1.55 eV photon energy) is usually applied, whereas the UV range probing beam allows photoemitting electrons in either the one-photon or two-photon regime depending on the probing beam power 6,[9][10][11][12] .…”

“…Similarly, the identical wavelength pumping and probing beams of Ti:Sapphire laser have been used in the most common pump-probe reflection and SHG configurations 8,[13][14][15][16][17][18] . On the contrary, in TR spectroscopy, the visible range pumping and either the visible or mid-IR/THz range probing were applied [19][20][21] . Finally, the visible/mid-IR/THz range pumping and the THz range probing have been used in TC spectroscopy 22,23 .…”

“…The pump-probe SHG technique is also surface sensitive, probing the charge-separation dynamics within a few atomic layers through the electric-field-induced SHG response 8,14,24 . In contrast, the visible and mid-IR/THz range TR and TC techniques monitor the transiently excited carrier population on the much longer length scale of a few tens of nanometers through the Pauli blocking mechanism (photoinduced bleaching) and through either the bandgap renormalization (BGR) or free carrier absorption (FCA) mechanism (photoinduced absorption) [13][14][15][16][17][18][19][20][21][22][23]25 . The FCA mechanism, however, is known to appear differently in the mid-IR range (Drude absorption) and in the visible range (inverse bremsstrahlung absorption) 25 .…”

Fabrication and Characterization of a Self-Powered n-Bi2Se3/p-Si Nanowire Bulk Heterojunction Broadband Photodetector

“…In order to facilitate the collection of a photo-generated carrier and reduce the recombination, in the present study, we plan to deposit a thick Bi 2 Se 3 film of 100 nm onto Si NWs to form bulk heterojunction to shorten the diffuse distance of minority carriers to the dissociating interface. In the bulk heterostructure, two materials form interpenetrating networks minimize the distance that excitons must travel before dissociating at a heterointerface [ 26 , 27 ]. As a result, the photo-generated carriers are efficiently collected before they are recombined.…”