Abstract
A terahertz scattering-type scanning near-field optical microscope is used for nano-scale non-invasive conductivity measurements on bulk silicon samples. We first investigate the case where the density of charge carriers is determined by optical interband excitation. We show that the amplitude and phase of the near-field signal are reproduced by simulations based on an established near-field interaction model, which takes the Drude conductivity, ambipolar carrier diffusion, and known recombination properties of photo-excited carrier pairs in Si into account. This study is then extended to impurity-doped Si. We demonstrate that the phase of the near-field signal, which can easily be measured in absolute terms, allows us to quantitatively determine the conductivity of the specimens, from which the carrier density is derived based on the known carrier momentum relaxation time. A measurement at a single properly chosen terahertz frequency is sufficient. The technique proposed here holds promise for the spatially resolved quantitative characterization of micro- and nanoelectronic materials and devices.
Highlights
Scattering-type Scanning Near-Field Optical Microscopy (s-SNOM) is a near-field microscopy technique able to overcome the classical diffraction limit of the spatial resolution of light propagating in free space
The THz range is very well suited to investigate phenomena related to mobile charge carriers in√semiconductors, as the plasma frequency fp = ωp/(2π)—with ωp = ne2/(meff εLε0), εL being the static background dielectric constant, n being the charge carrier density, and meff being the effective mass of a charge carrier—lies in this frequency regime and strongly influences the dielectric response
We have shown the capability of single-frequency homodyne s-SNOM measurements to derive the conductivity and, from it, the density of mobile charge carriers, in the volume close to the surface of bulk silicon, both optically doped and homogeneously impurity-doped
Summary
Scattering-type Scanning Near-Field Optical Microscopy (s-SNOM) is a near-field microscopy technique able to overcome the classical diffraction limit of the spatial resolution of light propagating in free space. It is based on tapping-mode Atomic Force Microscopy (AFM), where the oscillating tip (frequency Ω) is illuminated with electromagnetic radiation, which induces a near-field interaction between the tip apex and the sample. the spatial resolution does not scale with the size of the illuminated spot anymore but rather depends on the tip apex geometry. We use photo-excitation of a Si sample to continuously tune the charge carrier density by adjusting the incident power (for related near-field investigations of optically excited GaAs, in this case, with time resolution, see Ref. 26). This part of the study gives us confidence about the validity of the near-field interaction model with regard to the extraction of dielectric information from the single-frequency measurements over the carrier density range specified above. Depletion or accumulation of carriers in any surface space–charge field does not seem to play a major role in CW excitation
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