Francesca Cavallo

Semiconductors turn soft: inorganic nanomembranes

The distinction between hard and soft materials is most frequently described at the microscopic level as a difference in the nature of the bonding and molecular assembly, but concepts of “soft” and “hard” are more intuitively associated with macroscopic mechanical properties, such as stiffness or hardness. The behavior of a hard material may resemble that of a soft material in many aspects when a structural dimension is reduced to the nanoscale, even if elastic constants and moduli do not change. When semiconductor crystals, strongly covalently bonded and brittle, are made into membranes as thin as a few nanometres, they can behave like traditional soft matter, while maintaining their single-crystal structure without defects even after substantial deformation. This review presents single-crystal Si in a new fashion, i.e., as a form of soft matter, if fabricated as flexible nanomembranes, and suggests a plethora of interesting science and many applications that may benefit from the use of Si in this “soft matter” form. We suggest that the nanomembrane form gives Si an additional, new life as a soft material.

Strained-Germanium Nanostructures for Infrared Photonics

The controlled application of strain in crystalline semiconductors can be used to modify their basic physical properties to enhance performance in electronic and photonic device applications. In germanium, tensile strain can even be used to change the nature of the fundamental energy band gap from indirect to direct, thereby dramatically increasing the interband radiative efficiency and allowing population inversion and optical gain. For biaxial tension, the required strain levels (around 2%) are physically accessible but necessitate the use of very thin crystals. A particularly promising materials platform in this respect is provided by Ge nanomembranes, that is, single-crystal sheets with nanoscale thicknesses that are either completely released from or partially suspended over their native substrates. Using this approach, Ge tensilely strained beyond the expected threshold for direct-band gap behavior has recently been demonstrated, together with strong strain-enhanced photoluminescence and evidence of population inversion. We review the basic properties, state of the art, and prospects of tensilely strained Ge for infrared photonic applications.

Exceptional Charge Transport Properties of Graphene on Germanium

The excellent charge transport properties of graphene suggest a wide range of application in analog electronics. While most practical devices will require that graphene be bonded to a substrate, such bonding generally degrades these transport properties. In contrast, when graphene is transferred to Ge(001) its conductivity is extremely high and the charge carrier mobility derived from the relevant transport measurements is, under some circumstances, higher than that of freestanding, edge-supported graphene. We measure a mobility of ∼ 5 × 10(5) cm(2) V(-1) s(-1) at 20 K, and ∼ 10(3) cm(2) V(-1) s(-1) at 300 K. These values are close to the theoretical limit for doped graphene. Carrier densities in the graphene are as high as 10(14) cm(-2) at 300 K.

Soft Si: effective stiffness of supported crystalline nanomembranes.

We investigate the effective mechanical response of a layered system consisting of a thin crystalline sheet (nanomembrane) on a bulk substrate, with a high elastic mismatch (in the range of 5 to 9 orders of magnitude) between the stiff sheet and the compliant substrate. Using finite-element mechanics models and indentation experiments ranging from micro to nano, we show that the mismatch between the sheet and substrate elastic moduli, the length scale of deformation, and the sheet thickness all play a significant role in defining the effective stiffness of the layered system. For a wide range of indenter sizes, the mechanical response of the composite system is indistinguishable from that of the compliant substrate. In particular, at large indenter sizes, the mechanical response of the layered system is dominated by that of the compliant substrate. For decreasing indenter sizes, the effective stiffness of the layered structure reaches a finite value different from either the one expected for the compliant substrate or for a bulk crystal of the same material as the stiff top membrane.

Neurite Guidance and Three-Dimensional Confinement via Compliant Semiconductor Scaffolds

Neurons are often cultured in vitro on a flat, open, and rigid substrate, a platform that does not reflect well the native microenvironment of the brain. To address this concern, we have developed a culturing platform containing arrays of microchannels, formed in a crystalline-silicon nanomembrane (NM) resting on polydimethylsiloxane; this platform will additionally enable active sensing and stimulation at the local scale, via devices fabricated in the silicon. The mechanical properties of the composite Si/compliant substrate nanomaterial approximate those of neural tissue. The microchannels, created in the NM by strain engineering, demonstrate strong guidance of neurite outgrowth. Using plasma techniques, we developed a means to coat just the inside surface of these channels with an adhesion promoter (poly-d-lysine). For NM channels with openings larger than the cross-sectional area of a single axon, strong physical confinement and guidance of axons through the channels are observed. Imaging of axons that grow in channels with openings that approximate the size of an axon suggests that a tight seal exists between the cell membrane and the inner surface of the channel, mimicking a myelin sheath. Such a tight seal of the cell membrane with the channel surface would make this platform an attractive candidate for future neuronal repair. Results of measurements of impedance and photoluminescence of bare NM channels are comparable to those on a flat NM, demonstrating electrical and optical modalities of our platform and suggesting that this scaffold can be expanded for active sensing and monitoring of neuron cellular processes in conditions in which they exist naturally.

Semiconductor nanomembranes: a platform for new properties via strain engineering

New phenomena arise in single-crystal semiconductors when these are fabricated in very thin sheets, with thickness at the nanometer scale. We review recent research on Si and Ge nanomembranes, including the use of elastic strain sharing, layer release, and transfer, that demonstrate new science and enable the fabrication of materials with unique properties. Strain engineering produces new strained forms of Si or Ge not possible in nature, new layered structures, defect-free SiGe sheets, and new electronic band structure and photonic properties. Through-membrane elastic interactions cause the double-sided ordering of epitaxially grown nanostressors on Si nanomembranes, resulting in a spatially and periodically varying strain field in the thin crystalline semiconductor sheet. The inherent influence of strain on the band structure creates band gap modulation, thereby creating effectively a single-element electronic superlattice. Conversely, large-enough externally applied strain can make Ge a direct-band gap semiconductor, giving promise for Group IV element light sources.

Semiconductor nanomembranes: a platform for new science and technology

Semiconductor nanomembranes, extremely thin (<10 to ~1000 nm) single-crystal sheets, promise considerable new science and technology. They are flexible, they are readily transferable to other hosts and conform and bond easily, and they can take on a large range of shapes (tubes, spirals, ribbons, wires) via appropriate strain engineering and patterning. The ready ability to stack membranes allows the integration of the properties of different materials and/or orientations. A brief review of nanomembrane fabrication and manipulation with a view toward different types of applications is provided.

Group IV nanomembranes and nanoepitaxy: new properties via local and global strain engineering

Semiconductor nanomembranes, single-crystal sheets as thin as ten nanometers, offer many opportunities for novel devices and new science. The most interesting involve epitaxy to introduce strain at both local and global levels. Coming into play are membrane thinness, access to both sides of a sheet, transferability, and enhanced compliancy. Advances in Group IV optoelectronics, thermoelectrics, and photonics may be achievable by combining epitaxy with Si and Ge nanomembranes. Nanoepitaxy allows formation of new strained materials, periodic strain lattices, and mix and match membranes with hybrid orientations or compositions.