Investigation of van der Waals crystals of GaSe and GaSe and GaS$_x$Se$_{1-x}$ by photoreflectance method

Photoreflectance spectra of layered undoped GaSe and GaS$_x$Se$_{1-x}$ 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 GaS$_x$Se$_{1-x}$ 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 GaS$_x$Se$_{1-x}$ 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 GaS$_x$Se$_{1-x}$ 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.