Liberation of Ion Implanted Ge Nanocrystals from a Silicon Dioxide Matrix via Hydrofluoric Acid Vapor Etching

Abstract

A method to liberate germanium (Ge) nanocrystals from silicon dioxide (SiO2) thin films by hydrofluoric acid (HF) vapor etching is presented. Multi-energy implantation of mass separated Ge ions into 500-nm-thick wet oxide layers on silicon (Si) substrates followed by thermal annealing produces nanocrystals that are 2 to 8 nm in diameter. Raman spectra exhibit the expected asymmetric line shapes due to the phonon confinement effect, but with a higher peak frequency than predicted. To free the nanocrystals, samples are etched in HF vapor to selectively remove the SiO2 matrix and expose the nanocrystal surfaces. Raman spectra of etched samples display peak frequencies consistent with relief of compressive stress. The liberated nanocrystals show long-term stability under ambient atmospheric conditions. Ge nanocrystals can be removed from etched surfaces using an ultrasonic methanol cleaning procedure. The nanocrystal-containing solution is applied to a TEM grid and the solvent is evaporated. Subsequently obtained electron diffraction patterns confirm that the nanocrystals survive this transfer step. Thus, liberated Ge nanocrystals are expected to be accessible for a wide range of manipulation processes and direct characterization techniques.