[Pw_forum] about energy gap

[Stefano de Gironcoli]

1) LDA strongly underestimate band gaps
how large are your gaps for the parents materials compared to literature
values (Exp and Theory)

2) N doping is known to produce large reduction (giant negative bowing)
in GaAs.
A few % of N actually reduces the gap.

3) disorder tends to close the gaps

4) are your valence-band top and conduction -band bottom states
localized on different atoms species.
is your cell a kind of nano-heterojunction ? in that case you might have
some band-lineup effects that
could reduce the gap

Dig better in the literature and in your calculations.
Compare with known results in similar systems.

best regards,
Stefano de Gironcoli


sun jason wrote:

> Dear all,
>
> I'm a freshman in computational materials, I'm puzzled by a simple
> question about energy gap, when I calculated the C2-BN system which
> was construtured by mixing cubic BN and diamond.
> Well, the computed LDA band gap of c-BN and diamond is around 4.5 and
> 4.1 eV, respectively. Therefore, by mixing two compounds with such a
> wide band gap (roughly of the order of 4
> eV) one might expect to get a BC2N compound with an Eg between 4.5 and
> 4.1eV. Hence, why is our calculated Eg (about 1.0 to 2.0 eV) is much
> lower than expected for a C2-BN system?
> the unit cell I calculated is crystallined in diamond-like structure
> and the atoms are sp3 hybridization.
>
> Thank you very much!
>
>
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