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

[stefano de gironcoli]

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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