Ion-implantation- and thermal-anneal-induced intermixing in thin Si/Ge superlattices

Superlattices (SL’s) composed of thin Si and Ge layers (${\mathrm{Si}}_{12}$${\mathrm{Ge}}_{12}$, ${\mathrm{Si}}_{19}$${\mathrm{Ge}}_9$) have been implanted with As, Ge, and Ga ions with doses ranging from 1×${10}^{13}$ to 1×${10}^{14}$ ions ${\mathrm{cm}}^{\mathrm{-}2}$, and thermally annealed at 600 °C for 30 min. The disordering and the intermixing of these SL’s have been studied by the Raman-scattering technique and model calculations. The damage created by ion implantation has been estimated using trim simulations and a model. We found that when a thin symmetric ${\mathrm{Si}}_{12}$${\mathrm{Ge}}_{12}$ SL was rendered amorphous by ion implantation at high doses ≥5×${10}^{13}$ ions ${\mathrm{cm}}^{\mathrm{-}2}$, a mixed ${\mathrm{Si}}_{0.5}$${\mathrm{Ge}}_{0.5}$ material was produced by thermal annealing, but the crystalline structure of the asymmetric ${\mathrm{Si}}_{19}$${\mathrm{Ge}}_9$ SL equally disordered and annealed returns to a different SL structure with very little intermixing between the layers. Using a kinetic model, we calculated the interdiffusion coefficients and it was found that the recrystallization of the Ge layer is a fast process but that of the Si one is slow with respect to the time needed for intermixing. As a result, Ge diffuses mainly in disordered Si layers and Si in ordered Ge layers. In order to explain our experimental results, we equate the diffusion of Si into crystalline Ge to that of Ge into amorphous Si to minimize the effect of interlayer stress. Model calculations explain the difference in behavior between the two types of SL’s, and are in good agreement with the Raman data.