INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS J. Phys. D: Appl. Phys. 40 (2007) R75–R92 doi:10.1088/0022-3727/40/4/R01 TOPICAL REVIEW Elastically strain-sharing nanomembranes: flexible and transferable strained silicon and silicon–germanium alloys Shelley A Scott and Max G Lagally University of Wisconsin, Madison, WI 53706, USA Received 20 September 2006, in final form 19 December 2006 Published 2 February 2007 Online at stacks.iop.org/JPhysD/40/R75 Abstract The emerging field of strained-Si based nanomembranes is reviewed, including fabrication techniques, strain-induced band structure engineering, electronic applications and three-dimensional membrane architectures. Elastic strain sharing between thin heteroepitaxial Si and SiGe films, enabled by techniques that allow release of these films from a handling substrate, creates a new material: freestanding, single-crystal, strained nanomembranes. These flexible nanomembranes are virtually dislocation-free and have many potential new applications. Strain engineering also provides opportunities for massively parallel self-assembly of a wide variety of three-dimensional nanostructures. (Some figures in this article are in colour only in the electronic version) 1. Introduction The remarkable enhancement in silicon-based device performance over recent decades has resulted primarily from scaling of device dimensions. Because of the impending difficulties in further miniaturization of features [1], many complementary or alternative approaches are under consideration. A class of such approaches uses strain as a controllable parameter that can enhance performance. Consequently the materials and processing science of strained Si and SiGe has been the focus of accelerating interest in recent years. Strain engineering of Si offers the potential to increase carrier mobility, and hence device performance [2], in appli- cations spanning areas as diverse as nanoelectromechanical systems (NEMS), nanophotonics and high-speed electronics. Strain significantly modifies the band structure of Si [3]. In the technologically important case of Si(001), tensile strain splits the 6-fold degenerate conduction band minimum into a higher-energy 4-fold degenerate valley and a lower-energy 2-fold degenerate valley. The valence band is altered by lifting the degeneracy of the heavy and light-hole bands [4]. These two effects suppress intervalley/band scattering and reduce the effective transport mass, resulting in significant carrier mobil- ity enhancement [5, 6]. Improvements of 125% in electron mobility [7] and 120% in hole mobility [8] have been demon- strated in strained-Si metal-oxide-semiconductor field effect transistors (MOSFETs). In addition to sub-band splitting, strain also changes the Si band gap [3]. In multilayer het- erostructures strain engineering can be used to tune band offsets, and hence carrier confinement, adding an additional parameter for manipulation in device design [9]. Fabricating tensilely strained Si (or compressively strained SiGe) is conceptually not difficult; however, doing so without introducing undesirable dislocation defects or complex fabrication techniques is challenging. Dislocations behave as scattering sites and diminish carrier mobility, thus counteracting the potential enhancements offered by strain engineering. The most widely used technique to minimize dislocation generation in strained-Si films involves epitaxial growth of Si on a strain-relaxed SiGe virtual substrate [10]. A lattice mismatch of 4.2% between Si and Ge allows 0022-3727/07/040075+18$30.00 © 2007 IOP Publishing Ltd Printed in the UK R75