Abstract

We investigate the ultrafast carrier, coherent acoustic phonons (CAPs), and acoustic strain pulse dynamics in topological insulator BiSbTe1.5Se1.5 (BSTS) thin films of varying thickness using degenerate pump-probe reflection spectroscopy. Sapphire has been chosen as the main substrate due to its maximum acoustic reflectivity at the BSTS-sapphire interface compared to BSTS-GaAs, BSTS-Si, and BSTS-MgO interfaces. For the films with thickness more than twice the penetration depth, the transient reflectivity data predominantly exhibits travelling acoustic strain pulses (TASP) on top of single-exponential electronic decay (~2 ps). In contrast, films with thickness less than penetration depth are dominated by CAPs and a bi-exponential electronic background with decay constants τ1 ~2 ps and τ2 ~260-380 ps. The observed TASP dynamics are well-described by a theoretical acoustic strain model. Carrier density- and temperature-dependent studies on 22 nm and 192 nm BSTS films show that τ1 and τ2 depend on carrier density and temperature, with evidence for electron-phonon scattering, carrier diffusion, and defect-/phonon-assisted recombination. Temperature also softens the phonon frequency by ~14% and decreases the phonon damping parameter by ~48%, while the 192 nm film shows a ~7% reduction in sound velocity.

Abstract

We investigate the ultrafast carrier, coherent acoustic phonons (CAPs), and acoustic strain pulse dynamics in topological insulator BiSbTe1.5Se1.5 (BSTS) thin films of varying thickness using degenerate pump-probe reflection spectroscopy. Sapphire has been chosen as the main substrate due to its maximum acoustic reflectivity at the BSTS-sapphire interface compared to BSTS-GaAs, BSTS-Si, and BSTS-MgO interfaces. For the films with thickness more than twice the penetration depth, the transient reflectivity data predominantly exhibits travelling acoustic strain pulses (TASP) on top of single-exponential electronic decay (~2 ps). In contrast, films with thickness less than penetration depth are dominated by CAPs and a bi-exponential electronic background with decay constants τ1 ~2 ps and τ2 ~260-380 ps. The observed TASP dynamics are well-described by a theoretical acoustic strain model. Carrier density- and temperature-dependent studies on 22 nm and 192 nm BSTS films show that τ1 and τ2 depend on carrier density and temperature, with evidence for electron-phonon scattering, carrier diffusion, and defect-/phonon-assisted recombination. Temperature also softens the phonon frequency by ~14% and decreases the phonon damping parameter by ~48%, while the 192 nm film shows a ~7% reduction in sound velocity.

Abstract

We investigate the ultrafast carrier, coherent acoustic phonons (CAPs), and acoustic strain pulse dynamics in topological insulator BiSbTe1.5Se1.5 (BSTS) thin films of varying thickness using degenerate pump-probe reflection spectroscopy. Sapphire has been chosen as the main substrate due to its maximum acoustic reflectivity at the BSTS-sapphire interface compared to BSTS-GaAs, BSTS-Si, and BSTS-MgO interfaces. For the films with thickness more than twice the penetration depth, the transient reflectivity data predominantly exhibits travelling acoustic strain pulses (TASP) on top of single-exponential electronic decay (~2 ps). In contrast, films with thickness less than penetration depth are dominated by CAPs and a bi-exponential electronic background with decay constants τ1 ~2 ps and τ2 ~260-380 ps. The observed TASP dynamics are well-described by a theoretical acoustic strain model. Carrier density- and temperature-dependent studies on 22 nm and 192 nm BSTS films show that τ1 and τ2 depend on carrier density and temperature, with evidence for electron-phonon scattering, carrier diffusion, and defect-/phonon-assisted recombination. Temperature also softens the phonon frequency by ~14% and decreases the phonon damping parameter by ~48%, while the 192 nm film shows a ~7% reduction in sound velocity.

Abstract

We investigate the ultrafast carrier, coherent acoustic phonons (CAPs), and acoustic strain pulse dynamics in topological insulator BiSbTe1.5Se1.5 (BSTS) thin films of varying thickness using degenerate pump-probe reflection spectroscopy. Sapphire has been chosen as the main substrate due to its maximum acoustic reflectivity at the BSTS-sapphire interface compared to BSTS-GaAs, BSTS-Si, and BSTS-MgO interfaces. For the films with thickness more than twice the penetration depth, the transient reflectivity data predominantly exhibits travelling acoustic strain pulses (TASP) on top of single-exponential electronic decay (~2 ps). In contrast, films with thickness less than penetration depth are dominated by CAPs and a bi-exponential electronic background with decay constants τ1 ~2 ps and τ2 ~260-380 ps. The observed TASP dynamics are well-described by a theoretical acoustic strain model. Carrier density- and temperature-dependent studies on 22 nm and 192 nm BSTS films show that τ1 and τ2 depend on carrier density and temperature, with evidence for electron-phonon scattering, carrier diffusion, and defect-/phonon-assisted recombination. Temperature also softens the phonon frequency by ~14% and decreases the phonon damping parameter by ~48%, while the 192 nm film shows a ~7% reduction in sound velocity.