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

[Mohammad Abu Raihan Miah]

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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