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

Tue Aug 2 19:13:15 CEST 2016

Dear Stefano,

Thanks for your insightful question. They should be same.

Value of the bandgap is almost same. (Same upto hundredth). But the shape
is not exactly similar. But almost same.

We chose k points according to our computation resource, so that may be a
reason. I used 5*5*5 k points (10 k points) for 54 atoms as brillouin zone
is smaller now, so less number of k point is needed.  As I use the
supercell size as three times the original unit cell, so zone folding
hopefully won't be a issue that much.

I would appreciate any correction to my understanding.

Thank you very much.

On Mon, Aug 1, 2016 at 7:46 PM, Stefano de Gironcoli <degironc at sissa.it>
wrote:

> Dear Mohammad,
>   How can be possible that the two Si calculations (54 and 2-atom cell)
> give different band structure ?
> Aren't they both silicon ? With the same cutoff and equivalent kpoints?
> Shouldn't they give the same result ?
>
> stefano
> (sent from my phone)
>
> On 02 Aug 2016, at 04:45, Mohammad Abu Raihan Miah <mraihanm at eng.ucsd.edu>
> wrote:
>
> 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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-- 
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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