| AVS 54th International Symposium | |
| Thin Film | Thursday Sessions |
| Session TF2-ThA |
| Session: | Computational Aspects of Thin Films |
| Presenter: | S.G. Turner, Imperial College London, UK |
| Authors: | S.G. Turner, Imperial College London, UK L. Yu, Chinese Academy of Sciences, China K. Chua, Imperial College London, UK M.R. Levy, Imperial College London, UK X. Li, University of Oxford, UK J. Zhang, Imperial College London, UK |
| Correspondent: | Click to Email |
The Si/SiGe material system on a Si platform gives opportunities to develop Si-based optoelectronic devices, allowing monolithic integration.1 The 4% mismatch requires strain to be properly managed in structures that exceed the critical thickness for relaxation. Growing a stress-balanced structure on a virtual substrate (VS) allows such control. This work characterises the composition, tilt and strain state of the VS, and that of thin layers. Growth was carried out under ULPCVD & GSMBE modes in a modified VG Semicon system. We grew a fully-relaxed SiGe VS on Si(001), compositionally graded from pure Si. We show that a composition overshoot in the graded layer is needed to achieve the required in-plane lattice parameter, and determine its value. We examined x-ray reciprocal space maps (RSM) about the 004 and 224 reflections, obtaining average tilt of the VS with respect to the substrate and tilt distribution. Results from misoriented substrates show that the VS tilts to reduce the difference between low-index planes and the physical surface by >5%, explained by the preferential introduction of misfits with certain Burgers vectors. A fully-relaxed VS provides a good template for stress-balance between thin metastable layers by considering average force in the structure.2 Predictions from linear elasticity (LE) theory (assuming linear interpolation of alloy elastic constants) were tested by depositing a DBR on a fully-relaxed VS.3 The DBR consisted of 25 repeats of a Si/SiGe (32%) bilayer of ~190nm thickness, grown at 590°C and 520°C respectively. RSMs (giving lattice parameters) revealed the bilayer was coherent to the VS, supporting the validity of the linear extrapolation. This was compared to density functional theory calculations of the elastic constants. Reflectivity was 75% at 1.4 microns, and x-ray analysis showed fringes typical of well-ordered interfaces, suggesting a good stress-balance was realised. The composition overshoot part of the VS remains under compressive strain and coherent to the subsequent grown layers. We propose to compensate this using tensile Si, obtaining a true stress-free structure. We demonstrated this for varying VS compositions. We thus show that the precise strain state of thin films can be accurately manipulated.
1Kuo et al., Nature Vol. 437 p1334
2Ekins-Daukes et al., Crystal Growth & Design Vol. 2 p287.
3Kawaguchi et al., APL Vol. 79 p476.