Electron-dependent thermoelectric properties in Si/Si _{1- x}Ge _x heterostructures and Si _{1- x}Ge _x alloys from first-principles

Unlike phononic thermal conductivity (which is shown in the literature to be reduced due to alloying and has a nearly constant value over a range of compositional variations), electron-dependent thermoelectric properties are shown here, from first-principles, to vary nonlinearly with composition. Of the Si/Si _1-xGe _x systems considered, the maximum thermopower observed, which is 10 higher than that of crystalline Si, is obtained for a Si _0.875Ge _0.125 alloy. Also, heterostructuring is shown to reduce thermopower, electrical conductivity, and electron thermal conductivity. Additionally, neither Lorenz number nor Seebeck coefficient shows oscillations for heterostructures, regardless of electron/hole energies, contradicting the conclusions obtained with miniband approximations.