Converting an indirect gap material to direct one

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SUMMARY

This discussion focuses on the conversion of indirect band gap materials to direct band gap materials, emphasizing the electronic structure's role. In direct band gap materials, such as silicon (Si) and germanium (Ge), the valence and conduction bands reach their maximum and minimum at k=0 due to the presence of p orbitals. The discussion highlights that polar bonding in III-V semiconductors can influence band structure, allowing for the transformation of indirect band gap silicon, both single crystal and polycrystalline, into a direct band gap by creating an amorphous structure.

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hokhani
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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?
 
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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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They can change indirect band gap silicon (single crystal and polycrystalline) into direct bandgap by making it amorphous
 

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