AVS 58th Annual International Symposium and Exhibition
    Thin Film Division Thursday Sessions
       Session TF2-ThM

Paper TF2-ThM10
Experimental and Theoretical Investigations Using SiO2 Nanotemplates to Relieve Stress Caused by Thermal Expansion Coefficient Mismatch in Epitaxial Germanium Grown on Silicon

Thursday, November 3, 2011, 11:00 am, Room 110

Session: Modeling and Analysis of Thin Films
Presenter: Swapnadip Ghosh, University of New Mexico
Authors: S. Ghosh, University of New Mexico
D. Leonhardt, University of New Mexico
S.M. Han, University of New Mexico
Correspondent: Click to Email
High-quality Ge-on-Si (GoS) heterostructures are pursued for many applications, including near-infrared photodetectors, high-mobility devices with Si-based integrated circuits, and virtual substrates for III-V multijunction solar cells. Growing low-dislocation-density GoS and subsequently integrating III-V layers present two significant engineering challenges: lattice mismatch and thermal expansion coefficient mismatch. The materials engineering solutions to circumvent the lattice mismatch include metamorphic growth, graded buffer layers, selective epitaxial overgrowth, aspect ratio trapping (ART), and a variety of defect filtering strategies. The ART technique, in particular, utilizes high-aspect-ratio holes or trenches etched through dielectric films to trap dislocations, greatly reducing the dislocation density. However, one shortcoming of ART is that it has been demonstrated to be effective only for small holes or narrow strips with dimensions less than 1 µm. In this study, we demonstrate that a combination of ART with selective epitaxial growth can produce large areas of high-quality GoS. We focus on the use of SiO2-based templates with nanoscale windows placed on GoS to relieve the thermal stress. We observe that voids form around the top and sidewalls of SiO2 template deposited by chemical vapor deposition, further relieving the thermal stress. The same templates also filter threading dislocations propagating from the underlying Ge-Si interface. The Ge layer grown and coalesced over the template is analyzed by transmission electron microscopy and etch pit density measurements. When the template is used, the threading dislocation density near the Ge film surface is approximately <107 cm-2, while the twin defect density is approximately <5x107 cm-2. Finite element modeling based on a commercial software package COMSOL is used to calculate the thermal stress occurring in the epitaxial Ge due to differences in thermal expansion coefficients among Ge, Si, and SiO2. The simulation results, comparing Ge grown on Si with and without SiO2 templates, show that the nanoscale templates can effectively reduce the thermal stress. The resulting stress results obtained using the simulation model corroborate the experimental observations. In summary, the simulation results suggest that the SiO2 nanotemplates can reduce the stress caused by the thermal expansion coefficient mismatch, while simultaneously reducing the lattice-mismatch-induced dislocations in Ge grown on Si.