Control of the Ge nanocrystal synthesis by co-implantation of Si

The synthesis of Ge nanocrystals (Ge-nc) prepared by Ge implantation into fused silica followed by co-implantation of Si has been investigated for annealing temperatures varying between 850 and 1150 °C. By limiting the thermal diffusion of Ge, co-implanting Si reduces the Ge desorption and affects the growth of Ge-nc, through a Ge trapping mechanism involving the formation of Ge-Si chemical bonds. This is supported by Raman analysis, providing information regarding the material composition for a large variety of fabrication parameters, as well as high resolution scanning electron microscopy imaging, indicating that the average dimension of the synthesized Ge-nc decreases for increasing doses of co-implanted Si. From the spectral analysis of Raman measurements, a systematic evolution of the Ge-Ge, Ge-Si, and Si-Si bond concentrations is characterized as a function of the co-implantation fluences. Two different regimes are clearly identified for each annealing temperature. The first is associated with a linear increase of the residual Ge content with respect to the co-implanted Si, having a slope of ∼1, independent of the annealing temperature. Here, the nucleation of pure Ge-nc and Ge-nc containing Si impurities occurs at similar rates, for co-implanted Si fluences generally lower than the dose of implanted Ge. The second regime occurs for greater co-implantation fluence thresholds that depend on the annealing temperature. It is related to the saturation of the Ge trapping efficiency. In this regime, the formation of Si-Ge bonds dominates, sufficiently reducing the diffusion of Ge to prevent the formation of pure Ge-nc. In addition to limiting the unwanted and critical Ge desorption effects, Si co-implantation is a promising technique for precisely controlling the Ge-nc density, diameter, and uniformity at nanoscale dimensions, parameters which cannot be solely set from the local Ge concentration and/or the annealing parameters due to the high thermal diffusivity of Ge.