Strain-free epitaxy of germanium film on mica

  • Germanium, an elemental semiconductor, was the material of choice in the early history of electronic devices, before it was largely replaced by silicon.
  • But due to its high charge carrier mobility, higher than silicon by threefold, the semiconductor is making a comeback.
  • Germanium (Ge) is generally grown on expensive single-crystal substrates, adding another challenge to making it sustainably viable for most applications.
  • This incorporates van der Waals’ forces to grow Ge on mica.  This is the first time strain free van der Waals epitaxy of an elemental semiconductor has been produced.
  • Growing crystalline film layers on crystalline substrates (called epitaxy) is ubiquitous in semiconductor fabrication.
  • If the film and substrate materials are the same, then the perfectly matched layers form strong chemical bonds for optimal charge carrier mobility.
  • Layering different materials effectively, however, is a challenge because the crystal lattices typically don’t align. To get around this, researchers employed vdW forces, phenomena that are based on the probabilistic nature of electrons, which are not in a fixed position around a nucleus.
  • Rather, the electrons can be anywhere, and the probability that they will be unevenly distributed exists almost all the time. When this happens, there is an induced dipole: a slight positive charge on one side and a slight negative charge opposite.
  • This produces weakly attractive interactions between neutral atoms.
  • The mica is chosen as the substrate on which to grow the Ge film because of its atomically smooth surface, which is free of dangling bonds (unpaired valence electrons). This ensured that no chemical bonding would take place during the vdW epitaxy process.
  • The vdW epitaxy allows the Ge film to be mechanically exfoliated from the mica surface and stand alone as a substrateless film.
SUBSTRATE TEMPERATURE-300-500 degree Celsius
  • Optoelectronics
  • Solar cells
  • Transistors

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