First principles calculation of electron-phonon and alloy scattering in strained SiGe

Abstract

First-principles electronic structure methods are used to predict the mobility of n-type carrier scattering in strained SiGe. We consider the effects of strain on the electron-phonon deformation potentials and the alloy scattering parameters. We calculate the electron-phonon matrix elements and fit them up to second order in strain. We find, as expected, that the main effect of strain on mobility comes from the breaking of the degeneracy of the six Δ and L valleys, and the choice of transport direction. The non-linear effects on the electron-phonon coupling of the Δ valley due to shear strain are found to reduce the mobility of Si-like SiGe by 50% per % strain. We find increases in mobility between 2 and 11 times that of unstrained SiGe for certain fixed Ge compositions, which should enhance the thermoelectric figure of merit in the same order, and could be important for piezoresistive applications

Giant piezoresistance in silicon-germanium alloys

“…The acoustic electron-phonon scattering is calculated using density functional theory (DFT) and the frozen phonon method 15 accounting for the effects of strain on the matrix elements. 16 The intervalley and optical electron-phonon scattering is calculated with density functional perturbation theory, considering the effects of alloy disorder within the random mass approximation. 15,16 Alloy disorder scattering is calculated using the supercell approach and DFT as reported in Ref.…”

“…16 The intervalley and optical electron-phonon scattering is calculated with density functional perturbation theory, considering the effects of alloy disorder within the random mass approximation. 15,16 Alloy disorder scattering is calculated using the supercell approach and DFT as reported in Ref. 13.…”

“…These calculations yielded previously unknown scattering and transport parameters that were used to predict the mobility in strained Ge 19 and strained SiGe alloys. 16 Similar ab initio methods have also been used to calculate the electron-phonon scattering parameters in Ge, 20 GaAs, and GaP, 21 with excellent agreement with experiments.…”

“…In this work we use ab initio theoretical methods [13][14][15][16] to explore the parameter space in alloy composition, strain configuration, and current direction to determine the best possible strain gauge factors for single crystalline n-type bulk silicon-germanium alloys. We find ambient temperature gauge factors as high as G = 500, which are a factor of three larger than the maximum possible for single-crystalline silicon, and over five times that of germanium.…”

Phonon-assisted optical absorption in germanium

“…As we will see later, the value of D ΓL is critical for the assessment of any theory of indirect eV/cm [28]. Using instead a frozen-phonon approach, Murphy-Armando and Fahy [29] found D ΓL = 4.1×10 8 eV/cm. In our fits of experimental data to be discussed below, D ΓL will be taken as an adjustable parameter, and the agreement with these well-established values will be an important criterion to decide on the absorption model validity.…”