TransOpt. A code to solve electrical transport properties of semiconductors in constant electron–phonon coupling approximation

Abstract

A Fortran code, TransOpt (formerly named Transoptic) is presented. This code calculates electrical transport coefficients of semiconductor materials based on Boltzmann transport theory in the relaxation time approach with the recently developed constant electron–phonon coupling approximation. The code interfaces with the Vienna ab initio Simulation Package (VASP). The band structure related scattering phase space is calculated and used in determining the effective carrier relaxation time. The electronic structure part of the relaxation time is treated explicitly with using the detailed first-principles, while the electron–phonon coupling matrix (EPCM) part is treated as a constant. This constant EPCM can be parameterized using the deformation potential method in semiconductors, and the absolute value of electrical conductivity can thus be obtained. The code can also use full electron–phonon data from Quantum Espresso. Transport properties, including the electrical conductivity, the Seebeck coefficient, electronic thermal conductivity, Lorenz number, power factor, and electronic fitness function, can be calculated by TransOpt. The electron group velocities vnk as a function of the position in the Brillouin zone, k, can be determined in two different ways: 1) The momentum matrix method, which naturally avoids the “band crossing” problem, and yields better convergence with the number of first-principles k points and 2) the gradient method, where vnk is solved by the gradient in reciprocal space of the dispersion relation, which avoids the need to compute momentum matrix. Several examples are presented to highlight the major features of TransOpt.