Direct observation of a multiple-peak structure in the Raman spectra of ⁷⁴Ge and ⁷⁰Ge nanocrystals

Raman scattering (RS) spectra were measured in samples of germanium nanocrystals (Ge-NC) prepared by ion-implantation of ⁷⁴Ge and ⁷⁰Ge isotopes into an amorphous SiO₂ layer initially grown on a Si 〈 100 〉 surface. A rich multiple-peak structure has been detected in addition to the well-known Ge-Ge peak at about 300 cm^-1 of bulk Ge. Specifically, new ⁷⁴Ge-NC-related peaks centered around 202 cm^-1, 217 cm^-1, and 317 cm^-1 were directly observed as verified by isotopic Raman shift in ⁷⁰Ge-NC samples. Lorentzian line shape fit of the RS spectra implies the existence of two additional Ge-related vibration modes around 260 cm^-1 and 360 cm ^-1. The origin of each of the Ge related peaks was determined using a theoretical calculation based on the harmonic approximation.