Paper SS-TuP15
Experimental and Theoretical Investigation of Stress-induced Near-surface Compositional Redistribution on Si_0.8Ge_0.2 Substrates for 2D Array Growth of Ge Quantum Dots

Tuesday, October 29, 2013, 6:00 pm, Room Hall B

Long-range, ordered, three-dimensional micro-/nano-structures of Si/Ge heteroepitaxial systems possess unique electronic properties for a wide range of applications, including quantum computers, photodetectors, and solar cells. 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 stress is applied by a mechanical assembly that presses a 2D array of Si pillars (40 nm diameter and 400 nm pitch) fabricated on a Si substrate against the Si_1-xGe_x substrate. Based on energy dispersive x-ray spectroscopy, the compressed region shows Ge depletion (8%), while the surrounding areas show an increase in Ge concentration (3%). This compositional variation in turn can be used to selectively grow a 2D array of Ge quantum dots upon Ge exposure. When the annealing temperature is above 950 °C, the compressive stress under each pillar (> 1 GPa) is found to generate plastic deformation, while no plastic deformation is observed below 950 °C. The depth of the plastically deformed regions increases from 15 to 30 nm as the annealing temperature, held constant for each annealing experiment, increases from 950 to 1000 °C. We attribute the plastic deformation to (1) the localized pressure applied to the substrate under the contact area (4.3 to 15 GPa), (2) the near-surface substrate stiffness decreasing with increasing substrate temperature, and (3) the tensile biaxial stress (86 to 135 GPa) under the compressed region due to different thermal expansion rates of Si vs. Si_0.8Ge_0.2. In particular, the biaxial stress exceeds the reduced Young’s modulus (75.5 GPa) at the annealing temperature range. The compositional redistribution of Ge under purely elastic deformation conditions is also predicted, using a kinetic Monte Carlo simulation that accounts for the influence of composition, temperature, and stress on the diffusion kinetics of Ge in SiGe alloy. We will compare the prediction with experimental results in this presentation.