What is the mechanism of Burstein - Moss blue shift in heavily doped p-type INDIRECT BAND semiconductor?
Initially in p - type semiconductor, the Fermi level is closer to valence band.
Due to heavy doping, there will be closely packed localised band system within the indirect bandgap.
Where does this localised states exist in the bandgap and how does it contribute to the Burstein - Moss blue shift in absorption spectroscopy?
Dear M N H Liton
The effect of apparent increase in the energy gap due to degeneracy is explained by dr.Kadri where the transfer of electrons from the valence band to the conduction band occurs from the top of the valence band to the Fermi level which lies above the conduction band in the heavily doped semiconductors.
There is also at the same time the rigid bandgap narrowing because of the increases electrostatic attraction between holes in the valence band and electrons in the conduction band as their concentration increases.
It is found that for silicon that narrowing overweigh the enlarging of the energy gap so that one has a net narrowing of the energy gap.
For more information please follow the link:Article A New and Simple Model for Plasma and Doping induced Band Ga...
Best wishes
The Moss-Burstein shift is an apparent increase of the energy gap (blue shift) of a heavily doped semiconductor (either n-type or p-type) as a result of the shift of the Fermi energy towards higher energies deep in allowed bands (either conduction band for n-type doped SC or valence band for p-type doped SC). The result is :
Apparent Eg = Nominal Eg + Moss burstein shift (delta Eg).
This effect appears as a result of heavy doping no matter the SC is with direct or indirect energy bandgap.
The (delta Eg) Moss-Burstein shift is due to the fact that the carriers must have an energy higher than the nominal Eg in order to be excited in allowed bands (BC for n-type and BV for p-type). Because they have to be excited not to the bottom of the allowed band but to EF which lies much higher (several kT) than this bottom of allowed bands ! This happens no matter the energy bandgap is direct as in GaAs or indirect as in Si.
The heavy doping induces band tails in the density of states (DOS) of allowed bands in your case, the valence band.
These DOS tail states lie in the forbidden bandgap states but very close to the bottom of allowed bands.
Band states in these DOS tails are localized because of disorder and correlations between doping impurities.
Due to impurity (donor/acceptor) doping, when does we say, it is bandgap narrowing or Burestine-Moss effect ? is there any limitation?
Dear M N H Liton
The effect of apparent increase in the energy gap due to degeneracy is explained by dr.Kadri where the transfer of electrons from the valence band to the conduction band occurs from the top of the valence band to the Fermi level which lies above the conduction band in the heavily doped semiconductors.
There is also at the same time the rigid bandgap narrowing because of the increases electrostatic attraction between holes in the valence band and electrons in the conduction band as their concentration increases.
It is found that for silicon that narrowing overweigh the enlarging of the energy gap so that one has a net narrowing of the energy gap.
For more information please follow the link:Article A New and Simple Model for Plasma and Doping induced Band Ga...
Best wishes
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