Optical vibrational modes of Ge nanowires: A computational approach

Although Ge nanowires (GeNWs) have been extensively studied in the last decade the information about their vibrational modes is still scarce, their correct comprehension could hasten the development of new microelectronic technologies, therefore, in this work we aimed to study the vibrational properties, Raman and IR and spectrum of GeNWs using the first principles density functional perturbation theory. The nanowires are modelled in the [001] direction and all dangling bonds are passivated with H and Cl atoms. Results show that the vibrational modes can be classified in three frequency intervals, a low frequency one (between 0 and 300 cm^{- 1}) of mainly Ge-Ge vibrations, and two of Ge-H bending and stretching vibrations (400-500 cm^{- 1} and 2000 cm^{- 1}, respectively). There is a shift of the highest optical modes of Ge-Ge vibrations compared to their bulk counterparts due to phonon confinement effects, however it is masked by some Ge-H bond bending modes as demonstrated by the IR and Raman responses. The Cl passivated case shows a larger number of modes at lower frequencies due to the higher mass of Cl compared to H, which in turn reduces the red shift of the highest optical modes frequencies. These results could be important for the characterization of GeNWs with different surface passivations.