The effect of electron-phonon and electron-impurity scattering on the electronic transport properties of silicon/germanium superlattices

Semiconductor superlattices have been extensively investigated for thermoelectric applications, to explore the effects of compositions, interface structures, and lattice strain environments on the reduction of thermal conductivity, and improvement of efficiency. Most studies assumed that the electronic properties of superlattices remain unaffected compared to those of their bulk counterparts. However, recent studies demonstrated that the electronic properties of silicon (Si)/germanium (Ge) superlattices show significant variations depending on compositions and growth substrates. These studies used a constant relaxation time approximation, and ignored the effects of electron scattering processes. Here, we consider electron scattering with phonons and ionized impurities, and report first-principles calculations of the electronic transport properties including the scattering rates. We investigate two classes of Si/Ge short-period superlattices: superlattices with varied compositions grown on identical substrates and with identical compositions but grown on different substrates. We illustrate the relationship between the energy bands of the superlattices and the electron-phonon relaxation times. We model the electron-ionized impurity interaction potentials by explicitly accounting for the in-plane and the cross-plane structural anisotropy of the configurations. Our analysis reveals that the inclusion of electron-phonon and electron-impurity scattering processes can lead to an similar to 1.56-fold improved peak power-factor of superlattices, compared to that of bulk Si. We observe that superlattices can also display dramatically reduced power-factors for certain lattice strain environments. Such reduction could cancel out potential thermoelectric efficiency improvements due to reduced thermal conductivities. Our study provides insight to predict the variation of electronic properties due to changes in lattice strain environments, essential for designing superlattices with optimized electronic properties.