The effect of Ge implantation dose on the optical properties of Ge nanocrystals in SiO₂

Ge nanocrystallites (Ge-nc) embedded in a SiO₂ matrix are investigated using Raman spectroscopy, photoluminescence and Fourier transform infrared spectroscopy. The samples were prepared by ion implantation with different implantation doses (0.5, 0.8, 1, 2, 3 and 4) × 10 ¹⁶cm^-2 using 250keV energy. After implantation, the samples were annealed at 1000 °C in a forming gas atmosphere for 1h. All samples show a broad Raman spectrum centred at w≈304cm^-1 with a slight shift depending on the implantation doses. The Raman intensity also depends on the Ge⁷⁴⁺ dose. A maximum photoluminescence intensity is observed for the sample implanted at room temperature with a dose of 2 × 10¹⁶cm^-2 at 3.2eV. Infrared spectroscopy shows that the SiO₂ films moved off stoichiometry due to Ge⁷⁴⁺ ion implantation, and Ge oxides are formed in it. This result is shown as a reduction of GeO_x at exactly the dose corresponding to the maximum blue-violet PL emission and the largest Raman emission at 304cm^-1. Finally, the Raman spectra were fitted with a theoretical expression to evaluate the average size, full-width at half-maximum (FWHM) and dispersion of Ge-nc size.