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

[Stefano de Gironcoli]

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)
>>> University of California, San Diego
>>> 
>>> 
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>>> Pw_forum mailing list
>>> Pw_forum at pwscf.org
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>> 
>> 
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> 
> 
> -- 
> Mohammad Abu Raihan Miah
> PhD Student
> Department of Electrical and Computer Engineering (ECE)
> University of California, San Diego
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