Michelle M. Roberts

Elastically relaxed free-standing strained-silicon nanomembranes

Strain plays a critical role in the properties of materials. In silicon and silicon–germanium, strain provides a mechanism for control of both carrier mobility and band offsets. In materials integration, strain is typically tuned through the use of dislocations and elemental composition. We demonstrate a versatile method to control strain by fabricating membranes in which the final strain state is controlled by elastic strain sharing, that is, without the formation of defects. We grow Si/SiGe layers on a substrate from which they can be released, forming nanomembranes. X-ray-diffraction measurements confirm a final strain predicted by elasticity theory. The effectiveness of elastic strain to alter electronic properties is demonstrated by low-temperature longitudinal Hall-effect measurements on a strained-silicon quantum well before and after release. Elastic strain sharing and film transfer offer an intriguing path towards complex, multiple-layer structures in which each layer’s properties are controlled elastically, without the introduction of undesirable defects.

High-speed strained-single-crystal-silicon thin-film transistors on flexible polymers

We fabricate thin-film transistors (TFTs) on both strained and unstrained single-crystal Si membranes transferred to flexible-polymer substrates. The active layer is transferred from the starting silicon on insulator (SOI) using a simple, fast, and reliable dry-printing method. When a multilayer Si∕SiGe∕Si structure is pseudomorphically grown on SOI and the buried oxide is selectively removed, strained Si with a negligible density of dislocations is achieved via elastic strain sharing between the SiGe alloy layer and the Si layers. Both the drain current and the transconductance of TFTs fabricated on this strained Si∕SiGe∕Si membrane after its transfer to the flexible polymer are much higher than of TFTs fabricated on the unstrained-Si counterpart.

Single-crystal silicon/silicon dioxide multilayer heterostructures based on nanomembrane transfer

A method to fabricate single-crystal Si∕SiO2 multilayer heterostructures is presented. Heterostructures are fabricated by repeated transfer of single crystal silicon nanomembranes alternating with deposition of spin-on-glass. Nanomembrane transfer produces multilayers with low surface roughness and smooth interfaces. To demonstrate interface quality, the specular reflectivities of one-, two-, and three-membrane heterostructures are measured. Comparison of the measured reflectivity with theoretical calculations shows good agreement. Nanomembrane stacking allows for the preprocessing of individual membranes with a high thermal budget before the low thermal budget assembly of the stack, suggesting a new avenue for the three dimensional integration of integrated circuits.

Formation of microtubes from strained SiGe/Si heterostructures

We report the formation of micrometre-sized SiGe/Si tubes by releasing strained SiGe/Si bilayers from substrates in a wet chemical-etching process. In order to explore statistical studies of dynamic formation of microtubes, we fabricated arrays of square bilayers. Due to the dynamic change in curvature of the bilayers, and hence the underlying etch channels, the etching process deviates from a transport-controlled regime to one of kinetic controlled. We identified two distinct modes of etching. A slow etching mode is associated with symmetric surface deformation in which the bilayers mostly retain their initial pattern. In the fast mode, bilayers are asymmetrically deformed while large etch channels are induced and etching becomes kinetically controlled. Etch rate dispersion is directly related to the degree of asymmetry in surface deformation. When the dimensions of the bilayers become significantly larger than the curvature radius, kinetic etching dominates. During the formation of tubes, SiGe/Si bilayers strongly interact with the liquid environment of etchant and solvent. Assisted by the surface tension of evaporating liquids, the tubes are drawn near the substrate and eventually fixed to it because of van der Waals forces. Our study illuminates the dynamic etching and curling processes involved with and provides insight on how a uniform etch rate and consistent curling directions can be maintained.

Flexible thin-film transistors on biaxial-and uniaxial-strained Si and SiGe membranes

We report flexible thin-film transistors (TFTs) fabricated on single-crystal Si-based semiconductor membranes using Schottky source/drain contacts. Unstrained-Si, strained-Si/SiGe/Si and unstrained-Si0.8Ge0.2 alloy membranes were integrated on plastic substrates via transferring the top template layers from silicon-on-insulator (SOI) and silicon–germanium-on-insulator (SGOI) substrates. Biaxially tensile-strained Si/SiGe/Si is realized by allowing elastic strain sharing between Si and SiGe alloy. High current drive capability and high electron mobility are demonstrated on these membranes. Further enhancement of the source-to-drain current is exhibited by using mechanically introduced uniaxial strain to the flexible TFTs. We propose that the enhancement of current drive capability is attributed to both carrier mobility enhancement and Schottky barrier height modification due to strain.

Germanium hut nanostressors on freestanding thin silicon membranes

The heteroepitaxial growth of Ge on thin Si membranes can lead to significant bending under self-assembled Ge hut nanostructures. Undercut silicon-on-insulator mesas approximate a Si freestanding membrane and serve as a crystalline substrate for the growth of Ge huts. Synchrotron x-ray microdiffraction shows a local curvature on the lateral scale of the size of the hut and an overall bending of the freestanding region. In comparison with conventional mechanically rigid substrates, the freestanding film can bend significantly. We have found a local radius of curvature of 6μm beneath huts on 30-nm-thick Si membranes.

Silicon-based nanomembrane materials: the ultimate in strain engineering

The lattice-mismatch-induced strain in growth of Ge on Si produces a host of exciting scientific and technological consequences, both in 3D nanostructure formation and, when silicon-on-insulator (SOI) is used as a substrate, in 2D membrane fabrication. One can use the ideas of strain sharing and critical thickness, combined with the ability to release the top layers of SOI, to create freestanding, dislocation-free, elastically strain relieved flexible Si/Ge membranes with nanometer scale thickness, which we call NanoFLEXSi or Si nanomembranes (SiNMs). The membranes can be transferred to new substrates, producing the potential for novel heterogeneous integration. The very interesting, and in some cases surprising, structural and electronic properties of these very thin membranes have been revealed using STM, X-ray diffraction, and electronic transport measurements. For example, STM shows that conduction in very thin Si layers on SOI with bulk-Si mobilities is possible even though the membrane is bulk depleted. Using the effect of elastic strain, we have fabricated two-dimensional electron gases (2DEGs) in membrane structures; we support the transport measurements with calculations suggesting that we are observing a single bound state in the well. We have fabricated thin-film transistors (TFTs) that we have transferred to flexible-polymer hosts that show a very high saturation current and transconductance. Thus very highspeed flexible electronics over large areas become possible

N-type Thin-film Transistors Fabricated on Transferred, Elastically Strain-Shared Si/SiGe/Si Membranes

Strained silicon is known for both electron and hole mobility enhancement as stated in J. L. Hoyt et al. (2002) and C. W. Leitz et al. (2002). However, the dislocations could be detrimental to the device performance without carefully engineering the relaxation according to K. Ismail et al. (1994). To overcome these limitations, compliant substrate and Si/SiGe/Si sandwich structure that utilized elastic strain-sharing have been proposed according to Y. H. Luo et al. (2001) and G. M. Cohen et al. (2005). In this paper, we report N-type thin-film transistors (TFTs) fabricated on these dislocation-free, stain sharing Si/SiGe/Si membranes

Flexible Thin-film Transistors on Strained Si/SiGe Membranes

Electronics built on flexible polymer substrates have great potential for a number of applications. The largest potential lies in active-matrix flat-panel display applications, because of the continually increasing demand for light weight and robustness from wireless technologies. For the back-plane circuitry of active-matrix organic light emitting diodes (AMOLEDs), high transconductance (gm ) and high current drive capability are critical requirements to reduce the gate overdrive voltage and to provide higher brightness. These requirements pose great challenges to the currently widely investigated channel materials, such as amorphous Si, poly-Si, and organic semiconductors. Single-crystal Si, on the contrary, offers excellent performance in comparison with the materials mentioned above. Quite recently single-crystal Si has been transferred onto plastic substrate to create thin-film transistors (TFTs). We report here a novel membrane fabrication method that allows us to create strained-Si TFTs on plastic substrate.

Direct Synchrotron X-Ray Microdiffraction Measurements of Strain and Bending in Micromachined Silicon Devices

The manipulation of strain in micromachined silicon structures is an important aspect of the design of emerging mechanical and electronic devices. Strain also has a fundamental role in the formation of devices through its effects on surface processes in epitaxial growth including diffusion and can be an important tool for studying these processes. Microfabricated silicon structures offer the opportunity to control the strain at length scales of less than one micron to several hundred microns. Synchrotron x-ray microdiffraction allows simultaneous independent measurements of the strain and bending in these structures. Microdiffraction measurements show that bending is the dominant source of strain in a prototypical microfabricated Si bridge loaded at its ends by silicon nitride thin films. The total strain difference between the top and bottom of the bent bridge exceeds 0.1% in our prototype structures and can potentially be increased in optimized devices.Copyright