Terahertz spectroscopy of individual donors in silicon by low-temperature s-SNOM

Isolated atoms or ions, typically confined in traps, are ideal systems for studying fascinating coherent quantum effects such as photon echoes. Likewise, isolated donor impurity atoms in semiconductors like silicon show a hydrogen-like spectrum, shifted to the far infrared due to the small effective mass and high dielectric constant [1]. Excited Rydberg states are of particular interest for quantum information, because they allow one to prepare long-living microscopic polarization states.

In contrast to previous far-field spectroscopic studies which probed ensembles of many impurities, we aim here at studying individual impurity centers. To this end, low-temperature scattering-type scanning near-field optical microscopy (s-SNOM) is employed and a free-electron laser is used as a precisely tunable terahertz source [2]. Our silicon samples contain different donors (P, Bi) with different defect densities, respectively, and are pre-characterized by conventional Fourier transform infrared spectroscopy.

[1] Greenland et al., Nature 465, 1057 (2010).
[2] Döring et al., Appl. Phys. Lett. 105, 053109 (2014).