Microsecond carrier lifetime measurement using extended time-resolved THz spectroscopy

Abstract

This study presents a terahertz (THz) spectroscopy platform for the comprehensive characterization of charge carrier dynamics in photovoltaic (PV) materials. The system integrates steady-state conductivity measurements with electronically synchronized diode pump–THz probe measurements, enabling both equilibrium and non-equilibrium analyses of carrier behavior in silicon (Si) and germanium (Ge). In steady-state mode, frequency-dependent conductivity is extracted via Drude model analysis to obtain carrier scattering time and mobility. From the equilibrium carrier distribution, we determine an effective carrier lifetime that reflects the balance between the generation, diffusion, and recombination processes. The time-resolved component extends the measurements to the microsecond regime using pulsed diode laser excitation synchronized with the THz probe. This enables direct observation of carrier decay dynamics and reveals significantly longer lifetimes than those obtained from steady-state measurements, indicating fundamentally different physical processes governing each approach. Numerical simulations reconcile this discrepancy by capturing how spatial carrier distributions differ under steady-state and transient conditions. Under steady-state conditions, the effective lifetime is reduced due to constant carrier generation near the surface and continuous surface recombination. After pulsed-laser excitation, carrier diffusion toward the bulk reduces surface recombination and yields a longer transient lifetime. This shift in the spatial carrier distribution leads to differences in the observed lifetime and highlights the value of combined steady-state and time-resolved THz approaches for semiconductor characterization. The platform provides detailed insights into both microscopic transport properties and macroscopic carrier dynamics, supporting the optimized design of PV devices.