[Pw_forum] Problem regarding transition from indirect to direct bandgap for P doped Si

[Mohammad Abu Raihan Miah]

Hi Stefano,

When I simulated for 54 atom Silicon and also for B doped Silicon(53+1),
indirect bandgap was found. Though in the result, the conduction band
minima is not in the correct position (i.e. not in 0.85 X, approximately in
0.65X).

For the simulation of 2 atoms, indirect bandgap is also found. The result
is correct (i.e. CBM is at 0.85X)

As underestimation of the bandgap value is well-known problem, it is also
found in both cases. But we are more interested in direct bandgap issue now.

Any suggestion regarding this will be a great help for me.

Thank you very much.

On Mon, Aug 1, 2016 at 11:40 AM, stefano de gironcoli <degironc at sissa.it>
wrote:

> Dear Mohammad,
>
>  I suggest you do the following exercise:
>  Compute the band structure of 54 atom of Si with a similar setting to the
> one of your present calculation. Does it have direct gap ? compute the DOS.
>  Compute the band structure of Silicon in the 2-atom unit cell
>  with celldm(1) = 10.2623467,  k_points (automatic) set to  15 15 15 0 0 0
>  Does it have a direct gap ? compute the DOS
>  Compare total energy, band structure and DOS in the two cases.
>  Which calculation is correct ?
>
> stefano
>
>
>
> On 01/08/2016 20:19, Mohammad Abu Raihan Miah wrote:
>
> Hi all,
>
> I am currently simulating band diagram for Phosphorus doped Silicon. I am
> using 53 Si atoms and 1 P atom for computation. When I computed the band
> diagram, it resulted in direct band gap, which is not true case for
> Silicon. I have not relaxed the system.
>
> My input file is as follows,
>
> &control
>  calculation = 'scf',
>  restart_mode='from_scratch',
>  verbosity = 'high',
>  prefix = 'Silicon_P_54_relax',
>  tstress = .true.,
>  tprnfor = .true.,
>  pseudo_dir = '.',
>  outdir = 'out_P_rel',
>  wf_collect=.true.,
>  max_seconds=10800,
> /
>
> &system
>  ibrav=2,
>  celldm(1) = 30.787040,
>  nat=54,
>  ntyp=2,
>  ecutwfc=50,
>  ecutrho=400.0d0,
>  input_dft='PBE',
>  nbnd=120,
>  occupations='smearing',
>  degauss=0.05,
> /
>
> &electrons
>  diagonalization='david',
>  mixing_mode = 'plain',
>  mixing_beta = 0.7,
>  conv_thr =  1.0d-6,
> /
>
> ATOMIC_SPECIES
>  Si 28.085500 Si.pbe-n-rrkjus_psl.0.1.UPF
>  P 30.973800 P.pbe-n-rrkjus_psl.0.1.UPF
>
> ATOMIC_POSITIONS (alat)
>  Si 0.000000 0.000000 0.000000
> ..................................
>  P 0.083333 0.083333 0.083333
> .............................
> Si 0.750000 0.750000 0.750000
>
> K_POINTS (automatic)
>  5 5 5 0 0 0
>
> Could anyone please explain the reason behind this change from indirect to
> direct bandgap? Or any suggestion to solve this problem?
>
> Thank you very much.
>
> Best,
> --
> Mohammad Abu Raihan Miah
> PhD Student
> Department of Electrical and Computer Engineering (ECE)
> <http://www.ece.ucsd.edu/>
> University of California, San Diego <http://www.ucsd.edu/>
>
>
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-- 
Mohammad Abu Raihan Miah
PhD Student
Department of Electrical and Computer Engineering (ECE)
<http://www.ece.ucsd.edu/>
University of California, San Diego <http://www.ucsd.edu/>
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