(Invited) Strain and Phonon-Carrier Interactions in Ge-Si0.5Ge0.5 Core-Shell Nanowires Probed Using Tip-Enhanced Raman Spectroscopy

Group IV nanowires and nanowire heterostructures are of current interest for a broad range of applications including field-effect transistors with enhanced carrier mobility, and thermoelectric devices with engineered thermal and carrier transport properties.  The energy band alignments among Si, Ge, and Si_xGe_1-x alloys enable quantum confinement of either holes or electrons, while compositional variations and structural morphology at the nanoscale can lead to new phonon scattering and confinement effects.  Because of the lattice mismatch between Si and Ge, however, the dimensions and alloy compositions in nanowire heterostructures must be carefully controlled and characterized to maintain coherent strain.  Here we describe the use of tip-enhanced Raman spectroscopy to characterize nanoscale correlations between strain and nanowire dimensions in Ge-Si_xGe_1-x core-shell nanowires grown by ultrahigh-vacuum chemical vapor deposition, and to elucidate phonon-carrier interactions that are manifested via shifts in Raman peaks originating from Ge-Ge vibrational modes within the quantum-confined hole gas formed at the Ge-Si_xGe_1-x interface.  Typical nanowire structures consisted of a Ge core 35-55nm in diameter surrounded by a 5nm thick Si_0.5Ge_0.5 shell, leading to compressive strain in the Ge core and formation of a quantum-confined hole gas at the Ge-Si_0.5Ge_0.5 interface.  Tip-enhanced Raman spectroscopy (TERS) was performed under ambient atmospheric conditions on nanowires dispersed on an Au surface using an Au-coated atomic force microscope probe tip and 633nm laser excitation in a side illumination geometry.  Tip-enhanced Raman spectra exhibit two distinct peaks associated with Ge-Ge vibrational modes – one arising from Ge-Ge vibrations in the nanowire Ge core region, and the other from Ge-Ge vibrations occurring within the quantum-confined hole gas at the Ge-Si_0.5Ge_0.5 interface. Both peaks exhibit a dependence of Raman shift on local nanowire diameter consistent with coherent strain in the nanowire, demonstrating that the TERS measurements provide the ability to probe strain at the nanoscale.  The difference in Raman shift associated with these peaks is shown to be a consequence of coupling between intervalence band transitions and the Ge-Ge phonon mode that leads to Fano resonance behavior and a shift in the phonon self-energy.  In addition, positioning of the Au-coated probe tip in closer proximity to the nanowire leads directly to an increase in the separation between these peaks and in the relative amplitude of the peak from the Ge-Si_0.5Ge_0.5 interface region, confirming that the TERS measurement is sensitive to phenomena within an extremely localized region in the vicinity of the probe tip apex.  Computational electromagnetic simulations confirm that proximity to the probe tip leads to a large increase in electromagnetic field amplitude in the nanowire within ~10-20nm of the probe tip surface, and provide insight into the dependence of coupling to intervalence-band carrier transitions on proximity to the probe tip during the TERS measurement.