[QE-users] Silicon gap vs. temperature

[David Fernandes Machado]

Hello,
I performed an MD calculation at different temperatures using a supercell with 512 silicon atoms. After stabilizing the temperature, I extracted 160 structures for each temperature and conducted SCF calculations only at the gamma point for each extracted structure, which worked. I observed that the band gap decreases as the temperature increases. I am still waiting for all the SCF calculations to be completed, but currently, the average band gap is 0.54624 eV at 250K and 0.65271 eV at 4K.
Thank you for your help, Hasan.

Le Mardi, Août 13, 2024 15:26 CEST, "Md. Jahid Hasan Sagor" <md.sagor at maine.edu> a écrit:

 
Have you considered the lattice vibration (phonon) contribution? You need different atomic positions at different temperatures to include the phononic effect. BestM J Hasan
Mechanical Engineering
University of Maine

 On Tue, Aug 13, 2024 at 8:17 AM David Fernandes Machado <dfernandes at laas.fr> wrote:
 
Hello all,
 
I am trying to determine the dependence of the energy gap of silicon as a function of temperature. In the literature, it is stated that the decrease in the energy gap of silicon with increasing temperature can be explained by thermal expansion and electron-phonon interaction.
 
First, I used the thermo_pw library (which uses the QHA approximation) to determine the lattice parameter of silicon as a function of temperature. Then, I ran the following calculations: SCF, NSCF, DOS, band, and finally plotband. I performed these calculations using the lattice parameters of Si corresponding to temperatures in a range from 4K to 800K. For this simulation, I am using PBE pseudopotentials, an ecutwfc of 25 Ry, and a unit cell with 2 atoms.
 
The problem is that the gap increases with temperature instead of decreasing. I obtained a gap of 0.6187 eV at 4K and 0.6315 eV at 800K.
 
I also tried calculating the band structure considering electron-phonon coupling using the EPW library, but the gap still increases with temperature.
 
Has anyone already tried to calculate the silicon gap as a function of temperature? What am I doing wrong?
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