Converting an indirect gap material to direct one

  1. For which reason some of materials have direct gap and the other have indirect gap?
    In other words, what we have to do to convert a direct gap material to indirect one?
     
  2. jcsd
  3. DrDu

    DrDu 4,461
    Science Advisor

    This is a question of the electronic structure. In direct band gap materials, the valence and the conduction band have a maximum and minimum at k=0, respectively. In the semiconductors Si or Ge, both the valence and the conduction bands are made up of p orbitals (this is exact at k=0, at other values of k, other orbitals like s, will contribute, too), which leads to a direct band gap. In other materials, the valence band may be s but the conduction band p type, which leads to a different picture. Also polar bonding as in III-V semiconductors may lead to a shift of band maxima and minima.
     
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  4. They can change indirect band gap silicon (single crystal and polycrystalline) into direct bandgap by making it amorphous
     
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