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Journal Articles Scientific Reports Year : 2018

New strategies for producing defect free SiGe strained nanolayers

Abstract

Strain engineering is seen as a cost-effective way to improve the properties of electronic devices. However, this technique is limited by the development of the Asarro Tiller Grinfeld growth instability and nucleation of dislocations. Two strain engineering processes have been developed, fabrication of stretchable nanomembranes by deposition of SiGe on a sacrificial compliant substrate and use of lateral stressors to strain SiGe on Silicon On Insulator. Here, we investigate the influence of substrate softness and pre-strain on growth instability and nucleation of dislocations. We show that while a soft pseudo-substrate could significantly enhance the growth rate of the instability in specific conditions, no effet is seen for SiGe heteroepitaxy, because of the normalized thickness of the layers. Such results were obtained for substrates up to 10 times softer than bulk silicon. The theoretical predictions are supported by experimental results obtained first on moderately soft Silicon On Insulator and second on highly soft porous silicon. On the contrary, the use of a tensily pre-strained substrate is far more efficient to inhibit both the development of the instability and the nucleation of misfit dislocations. Such inhibitions are nicely observed during the heteroepitaxy of SiGe on pre-strained porous silicon. During the past decades the field of IV-IV SiGe/Si heterostructures has witnessed dramatic advances thanks to extensive research along new pathways to enhance their electronic properties, in particular using strain engineering nanostructures to largely confine the carriers 1-6. The increasing role of group IV nanostructrues in quantum electronics and photonic devices is due to their excellent compatibility with CMOS technology, ease of integration and perfect strain control 7-10. The epitaxial strain induced by the mismatch between SiGe and Si is still considered as the most efficient way to produce strain engineering that is a promising path for drive current enhancement by improving the electron and hole mobility in Si-based devices 11,12. Furthermore, most of the physical and structural properties of SiGe can be continuously tuned with composition from 0 to 100% Ge concentration. This is particularly true for the lattice constant and epitaxial strain which varies linearly with Ge concentration offering a cost-efficient way to produce controlled strain-engineering systems in order to controllably enhance electronic transport properties like charge carrier mobility, carrier confinement effects etc 13,14. Various systems including superlattices, core-shell nanowires, quantum dots, nanocrystals that have been suggested as high-performance materials have failed to meet that challenges mainly because of the strain-related behaviours such as elastic relaxation by Asarro Tiller Grinfled (ATG) growth instability or Ge hut islands nucle-ation, plastic relaxation by nucleation of dislocations and chemical relaxation by strain-driven interdiffusion 15. The use of a compliant substrate for the epitaxy of SiGe layers could be an elegant and low cost solution to control the strain-related behaviour 16-18. In this context, the role of a template layer on the strain distribution and as a consequence on the ATG development and the nucleation of dislocations should be fully understood both theoretically and experimentally. The template layer with specific elastic properties could be either soft/rigid and/ or pre-strained, acting as a compliant substrate. Alternative approaches have been explored to enhance devices performances. In particular, nMOS and pMOS devices are now built on Silicon On Insulator (SOI) substrates. Their improved performances result mainly from the reduction of parasitic device capacitance, and leakage current. In addition, the introduction of strained silicon , in particular with the epitaxial growth of Si on Silicon Germanium (SiGe) fully strained layers, could significantly improve the mobility of both n and p channel devices. Moreover, fabrication of ultra-thin Germanium On Insulator (GOI) substrates plays also an important role in <10 nm technology node and beyond. In such approaches, it is mandatory to determine the level of biaxial strain in Si and SiGe layers and to predict the strain relaxation (Ge interdiffusion, dislocation nucleation etc.) induced by subsequent processing steps. In parallel, it
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hal-01794555 , version 1 (25-10-2019)

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Thomas David, Jean-Noël Aqua, Kailang Liu, Luc Favre, Antoine Ronda, et al.. New strategies for producing defect free SiGe strained nanolayers. Scientific Reports, 2018, 8 (1), pp.476 - 483. ⟨10.1038/s41598-018-21299-9⟩. ⟨hal-01794555⟩
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