Free-standing Nanomembranes Research Articles

Defect-Free Single-Crystal SiGe: A New Material from Nanomembrane Strain Engineering

Many important materials cannot be grown as single crystals in bulk form because strain destroys long-range crystallinity. Among them, alloys of group IV semiconductors, specifically SiGe alloys, have significant technological value. Using nanomembrane strain engineering methods, we demonstrate the fabrication of fully elastically relaxed Si(1-x)Ge(x) nanomembranes (NMs) for use as growth substrates for new materials. To do so, we grow defect-free, uniformly and elastically strained SiGe layers on Si substrates and release the SiGe layers to allow them to relax this strain completely as free-standing NMs. These SiGe NMs are transferred to new hosts and bonded there. We confirm the high structural quality of these new materials and demonstrate their use as substrates for technologically relevant epitaxial films by growing strained-Si layers and thick, lattice-matched SiGe alloy layers on them.

Fabrication of metal patterns on freestanding graphenoid nanomembranes

Metallic patterns on freestanding ultrathin supports are desirable for many applications in modern optics or nanomechanics. The authors present four fabrication paths to create gold patterns that are supported by 1 nm thick freestanding graphenoid nanomembranes from self-assembled monolayers. Two fabrication schemes apply resist based lithographic processes to define gold structures on nanosheets. The gold/nanomembrane hybrids are then released from their substrates to form a freestanding structure. In two further fabrication schemes, the direct metal deposition of metal onto a freestanding graphenoid is performed. All four schemes are capable of producing gold patterns on two-dimensional nanomaterials, thus resembling new paths for the routine fabrication of free-floating metallic structures.

Ordering of nanostressors on free-standing silicon nanomembranes and nanoribbons

Epitaxial growth of self-assembled quantum dots (QDs) on single-crystal nanomembranes yields organized arrays of QDs via a growth mode mediated by QD-induced strains in the membrane. A crucial aspect of this effect arises because epitaxial growth on thin Si sheets and nanostructures derived from them can occur simultaneously on two surfaces separated only by the 10-nm-scale thickness of the membrane. A QD on one surface of a free-standing membrane causes the nucleation of QDs in specific positions on the opposite surface. Control experiments using molecular beam epitaxy to deposit QDs on a single surface do not yield long-range order. Through-membrane elastic interactions consistent with predictions from finite-element-based mechanics models are observed using synchrotron x-ray microdiffraction. The role of crystallographic anisotropy is evident in finite-element predictions of the strains that bias the nucleation events. The scaling of the dot spacing with membrane thickness is consistent with the spacing of nucleation sites predicted using the mechanical model.

Size Effect on Failure of Pre-stretched Free-Standing Nanomembranes

Free-standing nanomembranes are two-dimensional materials with nanometer thickness but can have macroscopic lateral dimensions. We develop a fracture model to evaluate a pre-stretched free standing circular ultrathin nanomembrane and establish a relation between the energy release rate of a circumferential interface crack and the pre-strain in the membrane. Our results demonstrate that detachment cannot occur when the radius of the membrane is smaller than a critical size. This critical radius is inversely proportional to the Young’s modulus and square of the pre-strain of the membrane.

Three-dimensional surface sculpting of freestanding metal-composite nanomembranes

In this work we report structural functionalization of metal-composite nanomembranes through formation of micrometer-sized three-dimensional hollow self-supported shapes with tailorable ordering and dimensions. The basic idea is to use double-sided, two-stage etching of the sacrificial layer, first to define the preform for the nanomembrane sculpting, then to fully remove the sacrificial structure and release the nanomembrane. In our experiments we utilized 15–20 μm thick ‹100› Si diaphragms as sacrificial layers. Conventional KOH-based bulk micromachining was used to sculpt pyramidal holes within the diaphragms and modified radiofrequent sputtering to deposit a 5–20 nm Cr-based nanomembrane precursors. After the removal of sacrificial membranes the released freestanding nanomembranes had an aspect ratio up to 1000,000 and contained hollow micrometer-sized pyramidal shapes which do not collapse in spite of their large wall-to-side aspect ratio (>1000). Although the geometry of the sculpted shapes in our experiments is defined by the Si bulk micromachining, one may readily fabricate a number of other shapes, including hemispherical, ellipsoidal, etc., the sizes and shapes being limited by the particular microfabrication technique chosen for the diaphragm pre-forming. Some potential applications of 3D sculpted nanomembranes can be found in subwavelength diffractive optics, plasmonics, energy harvesting, microfluidics, biointerfaces, to name just a few.

Fabrication, Self-Assembly, and Properties of Ultrathin AlN/GaN Porous Crystalline Nanomembranes: Tubes, Spirals, and Curved Sheets

Ultrathin AlN/GaN crystalline porous freestanding nanomembranes are fabricated on Si(111) by selective silicon etching, and self-assembled into various geometries such as tubes, spirals, and curved sheets. Nanopores with sizes from several to tens of nanometers are produced in nanomembranes of 20-35 nm nominal thickness, caused by the island growth of AlN on Si(111). No crystal-orientation dependence is observed while releasing the AlN/GaN nanomembranes from the Si substrate indicating that the driving stress mainly originates from the zipping effect among islands during growth. Competition between different relaxation mechanisms is experimentally revealed for different nanomembrane geometries and well-described by numerical calculations. The cathodoluminescence emission from GaN nanomembranes reveals a weak peak close to the GaN bandgap, which is dramatically enhanced by electron irradiation.

Versatile Approach for Integrative and Functionalized Tubes by Strain Engineering of Nanomembranes on Polymers

Flexible electronics, extremely sensitive sensors, strained-silicon technology, andmacromolecule separation, are only a few examples stimulating increasing interest in free-standing nanomembranes, which can be fabricated out of thin solid films, particle nanocomposites, organic layers, organic/inorganic networks, and even graphene sheets. Strain engineering offers an advanced strategy to deterministically rearrange such nanomembranes into threedimensional micro-/nanostructures including tubes, helices, rings, wrinkles and other advanced microarchitectures, all of which serve for applications in electronics, mechanics, fluidics, and photonics. The fabrication often requires a selective underetching procedure to release the nanomembranes from their substrate, which heavily constraints the number of desirable materials for exciting applications in, e.g., metamaterials or biomedical research. The material choice is limited, because the selective underetching not only removes the underlying sacrificial layer but also in many cases dissolves the nanomembrane material itself. We circumvent this problem for a broad range of materials and material combinations by a new approach outlined in Figure 1a. A pre-stressed inorganic nanomembrane deposited at low temperatures onto a polymer sacrificial layer (here: photoresist) is released from the substrate surface by removing

Wrinkled-up Nanochannel Networks: Long-Range Ordering, Scalability, and X-ray Investigation

Highly ordered two-dimensional self-organized nanochannel networks as well as free-standing nanomembranes are produced on rigid substrates by means of III-V semiconductor compressively strained layers grown on top of an etchant-sensitive material. The releasing process is controlled by regularly spaced pits obtained from photolithography and a subsequent wet chemical etching. By tuning basic film parameters such as strain and thickness, one obtains periodic arrays of two-dimensional nanochannel networks with symmetries defined by the shape and periodicity of the photolithographic starting pits. Such nanochannel networks with a submicroscale lateral feature size exhibit a surprising flexibility with respect to the crystal lattice symmetry, retaining the original film crystalline quality as confirmed by X-ray grazing-incidence diffraction (GID) measurements. Finite element modeling helps in understanding the particular process of the cross-nanochannel formation.

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.