Though both are opposite to each other, but are equally probable.Depending upon the nature of the band gap, temperature and the mole % of the dopant, we can make a choice between the absorption and the emission methods as follows:
1. In direct band gaps, the emission peaks are usually narrow but are broadened at high temperatures while the absorption specta are invariably broad.So the emission spectra should give more accurate values of lambda (max) in direct band materials.
2. The absorption spectrum of an indirect band gap material usually depends more on temperature than that of a direct material, because at low temperatures there are fewer phonons, and, therefore, it is less likely that a photon and phonon be simultaneously absorbed to create an indirect transition.As more photons are awailable at high temperatures, the absorption coefficient (alpha) increases with temperature. So if the choice is possible, one should opt for absorption spectrum for indirect band materials at high temperatures.
3. The shift of the absorption to higher energies due to doping induced band is called Burstein-Moss shift.In such cases, we should prefer absorption techniques.
Bandgap energy is an energy range in a solid where no electron states can exist. This is equivalent to the energy required to free an outer shell electron from its orbit about the nucleus to become a mobile charge carrier, able to move freely within the solid material. So the band gap is a major factor determining the electrical conductivity of a solid. Both the methods are useful to calculate the bandgap energy of semiconductor. For details pls refer thesis mentioned below: http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/20605/ThesisFormated.pdf?sequence=1
Though both are opposite to each other, but are equally probable.Depending upon the nature of the band gap, temperature and the mole % of the dopant, we can make a choice between the absorption and the emission methods as follows:
1. In direct band gaps, the emission peaks are usually narrow but are broadened at high temperatures while the absorption specta are invariably broad.So the emission spectra should give more accurate values of lambda (max) in direct band materials.
2. The absorption spectrum of an indirect band gap material usually depends more on temperature than that of a direct material, because at low temperatures there are fewer phonons, and, therefore, it is less likely that a photon and phonon be simultaneously absorbed to create an indirect transition.As more photons are awailable at high temperatures, the absorption coefficient (alpha) increases with temperature. So if the choice is possible, one should opt for absorption spectrum for indirect band materials at high temperatures.
3. The shift of the absorption to higher energies due to doping induced band is called Burstein-Moss shift.In such cases, we should prefer absorption techniques.
Here you can find good explanation and band gap calculator,
http://www.instanano.com/characterization/theoretical/uv-vis-spectroscopy-band-gap-calculation/
hi every body . in usual energy gap is calculated by Quantum Mechanics , but i like to know what are the other methods for calculating energy gap in a structure with a large number of atoms(macromoleculs)?
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