A new method of reflection anisotropy spectroscopy (RAS) with increased mid-IR efficiency owing to the use of a Fourier-transform infrared (FT-IR) spectrometer has been developed. An optical setup was implemented using a photoelastic modulator (PEM) to modulate the direction of linear polarization of the probe beam originating from the Michelson interferometer. An original measurement algorithm was proposed to eliminate the influence of spectral inhomogeneity of the PEM efficiency on the obtained spectra via appropriate calibration. It was shown that to preserve the sign of the RAS signal, it is necessary to use a specialized procedure for phase correction of the interferogram registered by the FT-IR spectrometer. In the visible range, good agreement was confirmed between the obtained reflection anisotropy (RA) spectra of a semiconductor crystal and the results of independent measurements using a conventional diffraction grating spectrometer-based setup. The RA spectrum of a III-V semiconductor heterostructure in the mid-infrared range (λ up to 8 µm) is demonstrated. Application of the developed FT-IR RAS method to layered black phosphorus has enabled characterization of anisotropic interband transitions in a graphene-like semiconductor crystal.
Photoreflectance spectra of layered undoped GaSe and GaSxSe1-x crystals present Franz--Keldysh oscillations indicating the near-surface built-in electric field, that can participate in the separation of photoinduced charge carriers in ultrahigh-sensitive photodetectors based on these materials. The measured value of the field strength in GaSxSe1-x turned out to be almost 1.5 times less than in GaSe, that may indicate a smaller number of free charge carriers in the solid solution. The broadening parameter of GaSxSe1-x spectral lines is also significantly lower than in the case of GaSe. This is due to the fact that isovalent atoms, being added into the GaSe, fill Ga vacancies, reducing the number of defects and the concentration of intrinsic charge carriers. The high-field modulation mode observed in the photoreflectance spectrum of GaSxSe1-x doped with an Al donor impurity indicates a relatively small thickness of the depletion region due to the presence of a large number of free electrons. Keywords: photoreflectance, Franz--Keldysh oscillations, GaSe, gallium monoselenide, van der Waals crystals, layered crystals.
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