High-resolution, background-free spectroscopy of shallow-impurity transitions in semiconductors with a terahertz photomixer source

We report on a method which enables background-free spectroscopy of the narrow absorption lines of shallow impurity transitions in low-doped and high-purity semiconductors, providing a much better spectral resolution than standard Fourier-transform spectrometers. To achieve this, we combined high-resolution continuous-wave terahertz spectroscopy based on a photomixer system with a bandgap excitation scheme for varying the concentration of neutral impurities. For the minority doping species, the lifetime of the excited states can be directly inferred from such a measurement by analyzing the line shape with respect to the width of the lifetime-related Lorentzian contribution. For a germanium sample with residual impurity concentrations of the order 10¹² cm⁻³, we obtained lifetime values of 110 and 130 ps for the $2Γ8−$ excited state of the acceptors B and Al, respectively. The method complements time-domain pump-probe measurements for moderately and highly doped samples.