Paper EM1-ThA8
Creating a Responsive SiGe Substrate to Form 2D Array of Ge Quantum Dots Using Stress-induced Near-surface Compositional Redistribution

Thursday, November 13, 2014, 4:40 pm, Room 311

A well-defined array of Ge quantum structures possesses unique electronic properties for a variety of applications, including quantum-computers and infrared photodetectors. Herein, we use simulation to predict and experiment to demonstrate the compositional redistribution of Si and Ge in the near-surface region of Si_0.8Ge_0.2 substrates by applying a spatially structured compressive stress to the substrate and thermally annealing the substrate under stress. The primary advantage of the proposed approach is that a single, reusable template is used to induce the compositional variation for multiple substrates. The compositional redistribution of Ge is predicted under purely elastic deformation, using a lattice kinetic Monte-Carlo simulation that accounts for the influence of composition, temperature, and stress on the diffusion kinetics of Ge in SiGe alloy. Atomistic stress field in a SiGe slab is computed using the Tersoff empirical potential and static relaxation. This compositional variation in turn can be used to selectively grow a 2D array of Ge quantum dots upon Ge exposure. To complement the computational prediction, the compressive stress is applied by pressing a 2D array of Si pillars against the Si_0.8Ge_0.2 substrate. Hertz contact model is used to calculate the compressive stress applied to the Si_0.8Ge_0.2 substrate under the Si nanopillars. We observe that the magnitude of compressive stress and annealing temperature determine the nature of deformation (elastic or plastic) in the Si_0.8Ge_0.2 substrate. Corresponding energy dispersive x-ray spectroscopy (EDS) shows that the compositional redistribution of Si and Ge in the near-surface region of Si_0.8Ge_0.2 substrates results from elastic deformation within a thermal annealing temperature range of 950 to 1000 °C and an applied stress range of 15 to 18 GPa. Based on nano-probe EDS, the elastically deformed compressed region shows near-complete Ge depletion and Si enrichment in atomic concentration. However, the temperature and stress exceeding the aforementioned ranges result in plastic deformation with no compositional variation. The plastic deformation depth is ~30 nm according to scanning transmission electron microscope images. We attribute the plastic deformation to (1) the localized pressure applied to the substrate under the contact area, (2) the near-surface substrate stiffness at substrate temperature, and (3) the tensile biaxial stress under the compressed region due to different thermal expansion rates of Si vs. Si_0.8Ge_0.2.