Modification of electronic and thermoelectric properties of InSe/GaSe superlattices by strain engineering

In recent years, superlattices and layered materials have been highlighted as potential candidates for thermoelectric applications, this thanks to their low thermal conductivity. Moreover, external applied pressure and biaxial strain can be used to enhance their properties by achieving a band engineering and electronic tuning. With this in mind, we performed an $ab$ initio based study on InSe/GaSe superlattices under biaxial strain along the layer planes. Layers of InSe and GaSe with ${D}_{3h}$ symmetry were stacked along the $c$ axis to create the superlattice. The atomic stacking along the $c$ axis is Se-Ga-Ga-Se-Se-In-In-Se, which corresponds to the space group #187. Our ab initio calculations predict the superlattice to be a semiconductor with an electronic band gap of ${E}_{g}$ = 0.54 eV. With the aim to increase thermoelectric performance, we apply positive and negative biaxial strain on the ab plane. Under compressive strain, the electronic structure evolves to an insulating behavior by increasing the band gap. When tensile strain is applied, we observe a transition towards a metallic character with a systematic reduction of the band gap. Interestingly, the semiconductor-metal transition only occurs when spin-orbit coupling (SOC) is switched off. With the inclusion of SOC, the system experiences an electronic topological transition around 3% tensile strain, with a double band gap along the $K\text{\ensuremath{-}}\mathrm{\ensuremath{\Gamma}}\text{\ensuremath{-}}M$ high-symmetry paths. We have found that for both, $n$-type and $p$-type doping, compressive strain improves the electronic figure of merit ($Z{T}_{e}$ under constant relaxation time approximation). Not only the electronic part increases thermoelectric performance, but also the lattice contribution to the thermal conductivity decreases with compressive strain.