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Two-dimensional (2D) materials exhibit unique properties due to quantum confinement and large surface-to-volume ratios. Thin bismuth (Bi) layers are promising for topological insulators, thermoelectric devices, sensors, and ultrafast optoelectronics. While bulk Bi is a semimetal, ultrathin films become semiconducting. Femtosecond excitation of Bi films can generate terahertz (THz) radiation through photocurrent and nonlinear optical effects, while THz excitation spectroscopy (TES) provides insight into carrier dynamics and band structure. In this work, Bi layers were grown by molecular beam epitaxy on (100)-oriented GaAs, a previously unexplored substrate orientation. THz emission was investigated using wavelength-tunable femtosecond excitation from an OPA seeded by a Yb:KGW laser.
Heterostructures containing 9 nm Bi layers were fabricated on semi-insulating, n-type, and p-type GaAs substrates. TES measurements were performed in reflection geometry using p-polarized femtosecond pulses incident at 45°, with constant average excitation power of 20 mW. Strong THz emission appeared only when photon energy exceeded the GaAs bandgap (1.42 eV), indicating that emission mainly originates from processes in the GaAs substrate. Below the bandgap, THz emission was observed only for Bi/n-GaAs structures.
Opposite THz pulse polarities from n- and p-type substrates were attributed to reversed internal electric fields at the Bi/GaAs interface. The Bi optical absorption coefficient was estimated as 3×10⁵–4×10⁵ cm⁻¹ in the 1.5–1.7 eV range, exceeding that of GaAs by more than an order of magnitude. Bi/SI-GaAs structures showed polarity reversal near 1.7 eV, suggesting an additional interface-related THz generation mechanism. Enhanced THz emission from Bi/p-GaAs heterostructures was linked to hole drift velocity overshoot in strong internal electric fields, demonstrating the potential of Bi/GaAs heterostructures as efficient THz emitters.
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