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First-principles electronic structure methods are used to predict the rate of n-type carrier scattering due to phonons in highly-strained Ge. We show that strains achievable in nanoscale structures, where Ge becomes a direct bandgap semiconductor, cause the phonon-limited mobility to be enhanced by hundreds of times that of unstrained Ge, and over a thousand times that of Si. This makes highly tensile strained Ge a most promising material for the construction of channels in CMOS devices, as well as for Si-based photonic applications. Biaxial (001) strain achieves mobility enhancements of 100 to 1000 with strains over 2%. Low temperature mobility can be increased by even larger factors. Second order terms in the deformation potential of the Γ valley are found to be important in this mobility enhancement. Although they are modified by shifts in the conduction band valleys, which are caused by carrier quantum confinement, these mobility enhancements persist in strained nanostructures down to sizes of 20 nm.

The role of reduced dimensionality and of the surface on electron-phonon (e-ph) coupling in silicon nanowires is determined from first principles. Surface termination and chemistry is found to have a relatively small influence, whereas reduced dimensionality fundamentally alters the behavior of deformation potentials. As a consequence, electron coupling to 'breathing modes' emerges that can not be described by conventional treatments of e-ph coupling. The consequences for physical properties such as scattering lengths and mobilities is significant: the mobilities for [110] grown wires is 6 times larger than for [100] wires, an effect that can not be predicted without the form we find for Si nanowire deformation potentials.Silicon nanowires, beyond having been successfully demonstrated as conventional semiconductor devices, 1-4 are beginning to be seen as the important blocks for novel energy harvesting applications, such as solar cells 5,6 and efficient thermoelectric devices. 7,8 All these applications rely on a high electronic conductivity of electrons, while in the latter a low phonon conductivity is also essential. 9,10 At the heart of understanding the interrelation between these properties is the interaction of phonons and electrons: scattering of electrons with phonons reduces the conductivity of these devices, increasing heating and reducing their efficiency, while the scattering

We calculate the uniaxial and dilatation acoustic deformation potentials, Ξ L u and Ξ L d , of the conduction band L valleys of PbTe from first principles, using the local density approximation (LDA) and hybrid functional (HSE03) exchange-correlation functionals. We find that the choice of a functional does not substantially affect the effective band masses and deformation potentials as long as a physically correct representation of the conduction band states near the band gap has been obtained. Fitting of the electron-phonon matrix elements obtained in density functional perturbation theory (DFPT) with the LDA excluding spin orbit interaction (SOI) gives Ξ L u = 7.0 eV and Ξ L d = 0.4 eV. Computing the relative shifts of the L valleys induced by strain with the HSE03 functional including SOI gives Ξ L u = 5.5 eV and Ξ L d = 0.8 eV, in good agreement with the DFPT values. Our calculated values of Ξ L u agree fairly well with experiment (∼ 3 − 4.5 eV). The computed values of Ξ L d are substantially smaller than those obtained by fitting electronic transport measurements (∼ 17 − 22 eV), indicating that intravalley acoustic phonon scattering in PbTe is much weaker than previously thought.

The growth, structural and optical properties, and energy band alignments of tensile-strained germanium (ε-Ge) epilayers heterogeneously integrated on silicon (Si) were demonstrated for the first time. The tunable ε-Ge thin films were achieved using a composite linearly graded InxGa1-xAs/GaAs buffer architecture grown via solid source molecular beam epitaxy. High-resolution X-ray diffraction and micro-Raman spectroscopic analysis confirmed a pseudomorphic ε-Ge epitaxy whereby the degree of strain varied as a function of the In(x)Ga(1-x)As buffer indium alloy composition. Sharp heterointerfaces between each ε-Ge epilayer and the respective In(x)Ga(1-x)As strain template were confirmed by detailed strain analysis using cross-sectional transmission electron microscopy. Low-temperature microphotoluminescence measurements confirmed both direct and indirect bandgap radiative recombination between the Γ and L valleys of Ge to the light-hole valence band, with L-lh bandgaps of 0.68 and 0.65 eV demonstrated for the 0.82 ± 0.06% and 1.11 ± 0.03% strained Ge on Si, respectively. Type-I band alignments and valence band offsets of 0.27 and 0.29 eV for the ε-Ge/In(0.11)Ga(0.89)As (0.82%) and ε-Ge/In(0.17)Ga(0.83)As (1.11%) heterointerfaces, respectively, show promise for ε-Ge carrier confinement in future nanoscale optoelectronic devices. Therefore, the successful heterogeneous integration of tunable tensile-strained Ge on Si paves the way for the design and implementation of novel Ge-based photonic devices on the Si technology platform.

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