Synthesis, assembly and applications of semiconductor nanomembranes
(01 September 2011)
31 August 2011
Research in electronic nanomaterials, historically dominated by studies of nanocrystals/fullerenes and nanowires/nanotubes, now incorporates a growing focus on sheets with nanoscale thicknesses, referred to as nanomembranes. Such materials have practical appeal because their two-dimensional geometries facilitate integration into devices, with realistic pathways to manufacturing. Recent advances in synthesis provide access to nanomembranes with extraordinary properties in a variety of configurations, some of which exploit quantum and other size-dependent effects. This progress, together with emerging methods for deterministic assembly, leads to compelling opportunities for research, from basic studies of two-dimensional physics to the development of applications of heterogeneous electronics.
Figures at a glance
Figure 1: Unique physical properties in NMs.
a, NMs have exceptionally high degrees of bendability, as illustrated in the scanning electron microscope (SEM) image. The flexural rigidity of a 2-nm-thick, silicon NM is ~10 15 times smaller than that of its bulk wafer counterpart (200 μm thick), as illustrated in the red curve of the graph (dashed line at 2 nm). Related mechanics allows bonding of NMs to nearly any surface. Here energy release rates associated with opening of interfaces between NMs and supporting substrates decrease linearly with thickness. The blue line represents calculations for silicon NMs bonded to sheets of polyimide at room temperature, and then heated to 300 °C. b, Electronic confinement effects in silicon NMs lead to splitting of the conduction band valleys ( ) for the (001) orientation (left) with representative 1-s.d. error bars. Here the surface roughness ( Δ ) strongly affects the 2D density of states δ (DOS; right). a.u., arbitrary units. 24 c, Phonon confinement in NMs offers opportunities for manipulating heat flow, to optimize figures of merit in thermoelectrics. The image shows a suspended silicon NM (22 nm thick; red arrow) perforated with arrays of nanoholes (diameter, ~10–15 nm; period, ~35 nm) that scatter phonons, thereby frustrating thermal transport . The data compare such structures (NM) with arrays of nanowires (NWA; 28 26 nm wide, 20 nm thick), coarsely patterned NMs (EBM; square mesh with period of 385 nm, 22 nm thick) and uniform NMs (TF; 25 nm thick), with representative 1-s.d. error bars . 26 d, Photon confinement in NMs allows for low-threshold lasers. The SEM image shows a photonic crystal that consists of an array of nanoholes (period, ~500 nm) in a GaInAsP NM (245 nm thick), which is designed to suppress rates of spontaneous emission and, simultaneously, to direct light into vertical modes . The graph shows measurements 28 (symbols) of emission efficiency, normalized to the case without nanoholes, as a function of the ratio of the period of the array ( 28 a) to the emission wavelength ( λ). The results indicate enhancements for a range of a/ λ values. Calculations (solid lines) with various surface-recombination velocities ( v s) capture the trends . The blue region corresponds to the location of the photonic bandgap. 28
Figure 2: Representative routes for synthesizing inorganic monocrystalline semiconductor NMs.
a, Atomic structure of MoS 2, showing its layered configuration . Chemical or mechanical exfoliation of this material yields single-layer NMs (0.65 31 nm thick), as shown in the transmission electron microscope image on the right . 30 b, Process for generating multilayer stacks of silicon NMs from a bulk wafer by anisotropic etching. Patterned features of etch resist (gold) on the structured sidewalls of vertically etched trenches allow access of an anisotropic wet chemical etchant only to certain regions of the silicon. Etching releases silicon NMs (~100 nm thick), as shown at two intermediate times in the cross-sectional SEM images on the right . 35 c, Epitaxial multilayer assembly of GaAs and aluminium arsenide (AlAs) grown on a GaAs wafer . Etching vertically through the thickness of the stack and then immersing the structure in hydrofluoric acid leads to the selective removal of the AlAs layers. Complete undercut etching releases large numbers of GaAs NMs. The SEM image shows a collection of GaAs membranes formed using this process 36 . 36 d, Release of a silicon NM from a SOI wafer. Etching vertically through the top silicon layer exposes the underlying SiO 2 layer, allowing its removal by etching in hydrofluoric acid. The optical image on the right shows a wrinkled, but completely single-crystal, silicon NM (~50 nm thick) formed in this manner that can then be transferred to a new host, where it will flatten and bond . 10
Figure 3: Inorganic monocrystalline semiconductor NMs in non-planar configurations.
a, Rolling and curling in a strained bilayer NM, illustrating the geometric parameters that determine the morphology: L 0 = 2 π R 0 is the circumference of a tube that may form; L and W are the length and width of the strip, respectively; and t is its thickness. The critical angle for coil formation over tube formation is θ c. The arrows indicate the folding direction . 14 b, SEM image of a collection of GaAs NMs with embedded quantum well structures. The tubular shapes form on release from the substrate, owing to strain in the epitaxial layers . 63 c, SEM image of an array of partly released spiral structures formed by SiGe (10 nm)/Si (7 nm)/Cr (20 nm) NMs attached at their centres to a silicon wafer . 64
Figure 4: NMs of conjugated carbon and their synthesis using interfacial methods.
a, Approach to synthesis based on chemical crosslinking of a SAM . 70 b, Optical micrographs of a highly conjugated carbon NM synthesized by crosslinking a 2,5-substituted dialkynylbenzene SAM by alkyne metathesis, resting on a SiO 2/Si substrate . A wrinkled region of the NM appears in magnified view on the top right. The chemical structure of the monomer appears on the bottom left. 70 c, Fluorescence resonance energy transfer image of a ~1-nm-thick ‘Janus’ NM (blue) suspended over a supporting, hexagonal grid structure (black). This NM, which has some tears and other defects, was formed by exposing a 4′-nitro-1,1′-biphenyl-4-thiol SAM to electrons at 100 71 eV and 50 mC cm −2. d, Chemical synthesis of chevron-shaped graphene nanoribbon structures on Au(111), formed by thermolytic condensation and cyclodehydrogenetic conversion of the molecular precursor 6,11-dibromo-1,2,3,4-tetraphenyltriphenylene . 74 e, High-magnification (inset) and low-magnification (main image) scanning tunnelling microscope images of straight and chevron-shaped graphene nanoribbons on Au(111) synthesized in the manner illustrated in d . 74
Figure 5: Operation of elastomeric stamps for deterministic assembly of NMs, with examples of printed sparse arrays and multilayer assemblies.
a, Colourized SEM images of a single post in an elastomeric stamp (blue) that uses soft, pyramidal relief features to provide strong adhesion in a collapsed state (ON) and weak adhesion in a retracted state (OFF). Control of the applied pressure allows reversible switching between these two states. 10 b, Measured adhesion strength in the ON and OFF states, as a function of peeling rate . Viscoelastic effects in the elastomer lead to monotonic increases in adhesion with rate, with pronounced effects observable in the ON state. The dashed lines are guides for the eye. 10 c, Sparse array of GaAs membranes (small black squares) assembled by printing onto a plate of glass (main image) and a bent sheet of plastic (inset) . 40 d, Cross-sectional SEM of an eight-layer stack of silicon NMs (each ~340 nm thick) separated by transparent layers of polymer. Inset, schematic; the red box outlines the cross-section shown in the main image.
Figure 6: NMs as active materials in unusual electronic and optoelectronic devices.
a, Bio-integrated electronics for high-resolution mapping of cardiac electrophysiology in a porcine animal model, with applicability in humans . The device consists of nearly three hundred independent measurement locations, with local amplifier circuits and multiplexers that collectively use more than 2,000 silicon NM transistors in a waterproof construction on a thin sheet of polyimide. The colour inset provides a representative map collected using this device. 21 b, Small-signal current gain ( H 21) and power gain ( G max) as functions of frequency for high-speed silicon NM transistors on a plastic substrate . The results show limiting frequencies of 3.8 18 GHz for current gain ( f T) and 12 GHz for power gain ( f max). For power gain, the solid and dashed lines correspond to measurement and simulation, respectively. c, Cross-sectional transmission electron microscope images of a transistor that uses an InAs NM heterogeneously integrated on an oxidized silicon wafer, at moderate (top) and high (bottom) magnifications . 96 d, Measured (solid) and simulated (dashed) width-normalized drain–source currents ( I DS) as functions of gate–source voltage ( V GS) for transistors as in c with InAs NM thicknesses of 8, 13, 18 and 48 nm (ref. 96). A cross-sectional schematic of the transistor is shown at top. e, SEM images of a cylindrical microcavity laser formed with a NM (~50 nm thick, with two InGaAs/GaAs quantum dot layers and a pseudomorphic In 0.18Ga 0.82As quantum well), at low (top) and high (bottom) magnifications . 97 f, Integrated output intensity as a function of excitation power (HeNe laser emission at 632.8 nm) for emission in an optically resonant mode with a wavelength of 1,240.7 nm (ref. 97).