Raman-active phonon line profiles in semiconducting nanowires

Results of Raman scattering investigations of optical phonons confined in the cross section of small diameter Si nanowires are discussed. Using low excitation intensity at 514.53 nm to study Si as a prototypical material, we first demonstrate that the outcome of the phonon confinement is an asymmetric broadening and downshifting of the LO-TO phonon line observed at 520 cm^-1 in the bulk. The effect is important in Si wires with diameter d < 10 nm. We find good agreement between our data and an early theory due to Richter et al., provided we introduce an additional factor that sets the phonon confinement length. Furthermore, we examine the effects on the 520 cm^-1 lineshape from increasing the laser power density in the tight microRaman focal spot size (∼1 μm). We find that a second asymmetric line broadening mechanism is also present that can be identified with an inhomogeneous temperature distribution set up in the nanowires. This distribution is driven by the Gaussian spatial intensity of the laser beam in the focal plane of the microRaman instrument. Using the thermal properties of the phonons in Si, we can explain semi-quantitatively the complex changes in the lineshape that we observe in both small and large diameter Si nanowires, i.e., with or without phonon confinement as an active consideration.