absorption below band gap

[burkzdemir]

Dear developers,

I calculated the optical absorption of Si depending on the thickness of the material. I found that there is significant optical absorption below the optical gap of Si for 100 micrometer thick materials. However, experimentally, Si is mostly transparent (rest is reflection) below the optical gap. I think something is wrong with the modeling of imaginary dielectric function below the optical gap. As I understand it, imaginary dielectric function goes to small values below the optical gap but instead it should quickly become zero otherwise small imaginary dielectric function is causing significant absorption for thick enough materials. Below, I shared a gdrive link to my results.

https://drive.google.com/file/d/1ZrbjqI ... sp=sharing

Best regards,

[Daniele Varsano]

Dear Burak,

As I understand it, imaginary dielectric function goes to small values below the optical gap but instead it should quickly become zero otherwise small imaginary dielectric function is causing significant absorption for thick enough materials.

the imaginary part of the dielectric function is modelled adding an artificial smearing/damping so that you have Lorentzian peaks on top of the calculated excitation energies. You can reduce that smearing in input by setting the damping range:
BDmRange if you are calculating the spectrum in BSE or DmRngeXd in linear response calculations.
You should indicate two numbers (in eV) which are the initial and final damping values along your sepctrum range. Use the same value for a constant smearing for all the energy range.

Best,
Daniele

[burkzdemir]

Dear Daniele,

I did the calculation with reduced damping which helped transmittance onset to shift to higher energies. But, when I compare the dielectric function that I calculate with BSE, the first sharp increase of the dielectric function (eps2) is 0.5 eV red shifted with respect to literature (both BSE and experiment reported in the literature). So, what is it that I am doing wrong?

optics # [R OPT] Optics
bss # [R BSS] Bethe Salpeter Equation solver
bse # [R BSE] Bethe Salpeter Equation.
bsk # [R BSK] Bethe Salpeter Equation kernel
em1d # [R Xd] Dynamical Inverse Dielectric Matrix
ppa # [R Xp] Plasmon Pole Approximation
Chimod= "HARTREE" # [X] IP/Hartree/ALDA/LRC/PF/BSfxc
BSEmod= "retarded" # [BSE] resonant/retarded/coupling
BSKmod= "SEX" # [BSE] IP/Hartree/HF/ALDA/SEX
BSSmod= "h" # [BSS] (h)aydock/(d)iagonalization/(i)nversion/(t)ddft`
BSENGexx= 50541 RL # [BSK] Exchange components
BSENGBlk= 1 RL # [BSK] Screened interaction block size
#WehCpl # [BSK] eh interaction included also in coupling
% BEnRange
0.300000 | 5.00000 | eV # [BSS] Energy range
%
% BDmRange
0.0250000 | 0.0250000 | eV # [BSS] Damping range
%
BEnSteps= 100 # [BSS] Energy steps
% BLongDir
1.000000 | 0.000000 | 0.000000 | # [BSS] [cc] Electric Field
%
% BSEBands
11 | 21 | # [BSK] Bands range
%
BSHayTrs= -0.002000 # [BSS] Relative [o/o] Haydock threshold. Strict(>0)/Average(<0)
% BndsRnXp
1 | 200 | # [Xp] Polarization function bands
%
NGsBlkXp= 1 RL # [Xp] Response block size
% LongDrXp
1.000000 | 0.000000 | 0.000000 | # [Xp] [cc] Electric Field
%
PPAPntXp= 27.21138 eV # [Xp] PPA imaginary energy

[Daniele Varsano]

Dear Burak,

I do not know exactly the system you are studying, anyway in your input I can see two drawbacks:
1)You are doing BSE calculations on top of KS eigenvalues instead of quasi-particle energies and this is not consistent and gives you redshifted spectra.
Now you can include quasi-particle corrections or by evaluating them with previous GW calculations. Yambo will produce a database that should be then provided in the BSE input file via:

Code: Select all

KfnQPdb="E < ./SAVE/ndb.QP"

or you can also model the QP gap providing empirical corrections (e.g. taken from experiments) using a scissor operator:

Code: Select all

% KfnQP_E
 0.000000 | 1.000000 | 1.000000 |        # [EXTQP BSK BSS] E parameters  (c/v) eV|adim|adim
%

These two variables will appear in the input file if you add verbosity in the yambo input building command line (-V qp).

2) In the BSE kernel you are practically not considering the electron-hole interaction as you include the screened interaction potential using just 1 reciprocal space vector:

Code: Select all

BSENGBlk= 1 RL
NGsBlkXp= 1 RL 

The block size in the screening it is a parameter that has to be converged and 1RL is surely not converged.

Here you can find a tutorial on how to converge the absorption spectra in BSE.

Best,

Daniele