Semiconductor Membrane Research Articles

Strain Engineered Silicon Nanomembranes

Very thin, crystalline semiconductor membranes represent a new nanosystem with very high potential for both technology and novel science. This article reviews aspects of the fabrication and properties of silicon-based nanomembranes and nanowires, including processing, strain engineering, and electronic transport. Particular emphasis is on elastic strain sharing, in which significantly strained single-crystal sheets or novel nanoshapes can be made.

Porous-Semiconductor-Based Hydrogen-Permeable Membrane

A technology for the fabrication of multilayer semiconductor membranes based on pore formation in Si, GaP, and InP substrates by electrolytic etching in acidic and alkaline solutions has been developed. Studies of the membranes by X-ray diffraction and transmission and scanning electron microscopy showed that solid ultrathin (down to 10 nm) and porous (1-200-μm-thick) layers have essentially single-crystal structure with a degree of structural perfection not worse than that of the starting substrate. The thickness of the damaged layer of the Si membrane pore walls is not over 1 nm. A special technology permitting fabrication of cellular Si membranes with a critical breaking stress of 61.3 ± 5 kPa, which exceeds by more than a factor 12 the critical breaking stresses of a completely porous silicon substrate of the same area and thickness, has been developed. It is shown that the membranes are highly permeable only for gases of light atoms (hydrogen, helium), in particular, with a H2/O 2 selectivity of 10 3 . The behavior of their permeance with temperature was found to be of an activated nature (Arrhenius law). This also implies the absence of microdefects in the central ultrathin solid layer, which accounts for the high selectivity of the membrane as a whole. The permeance of a multilayer cellular Si membrane for hydrogen was found to be 10 -7 mol m -2 s - 1 Pa - 1 at 60 °C. Calculations suggest that this value can be improved 4-fold by optimizing the technology employed in the fabrication of the thin solid layer. The Si membrane was found to be capable of providing a hydrogen flux necessary to supply a hydrogen fuel cell of a specific power of 300 mW/cm 2 . A Si membrane 1 cm 2 in area fabricated by this technology was used simultaneously for purification and support of the anode catalyst in an actual 100-mW hydrogen fuel cell.

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.

Improved photocatalytic deposition of palladium membranes

In order to enhance ultra-thin palladium (Pd) membranes integrity, an improved photocatalytic deposition method (PCD) was developed for the Pd/TiO 2 composite membranes. The PCD reactions for Pd membranes occurred in the solution previously and the mechanism of the improved PCD method is interpreted by the photocatalytic reaction at the TiO 2 semiconductor/Pd(II) liquid film interface by directly irradiating. The reaction processes conducted in the liquid film could effectively avoid the homogeneous reaction and increase the irradiation efficiency. Ultra-thin tubular Pd membranes with the thickness of about 0.4 μm on TiO 2 semiconductor membranes were fabricated by the method. X-ray diffraction (XRD) and SEM analysis of the Pd/TiO 2 composite membranes indicated that the metal existed in the form of pure Pd element and Pd membrane surface appeared homogeneous. A gas permeation experiment was conducted under temperature of 612–704 K and pressure of 0.05–0.12 MPa. Hydrogen permeance and selectivity of hydrogen/nitrogen (H 2/N 2) were 4.8 × 10 −6 mol m −2 s −1 Pa −1 and 120 at 704 K, respectively.

Spontaneous emission rates of dipoles in photonic crystal membranes

We show theoretically that two-dimensional (2D) photonic crystals in semiconductor membranes strongly modify the radiative decay of dipole emitters. Three-dimensional finite-difference time-domain calculations show over 7 times inhibition and 15 times enhancement of the emission rate compared with vacuum for judiciously oriented and positioned dipoles. Emission rate modifications inside the membrane mimic the local mode density in a simple 2D model. The inhibition of emission saturates with crystal size around the source, with a 1/e size that scales as the inverse gap bandwidth. Owing to the vertically guided mode structure, inhibition occurs only near the slab center, but enhanced emission persists also outside the membrane. We find that emission changes can even be observed in experiments with ensembles of randomly oriented dipoles.

Inside Front Cover: Freely Suspended Layer-by-Layer Nanomembranes: Testing Micromechanical Properties (Adv. Funct. Mater. 5/2005)

A study of the micromechanical properties of layer-by-layer nanomembranes composed of a center layer of gold nanoparticles is reported by Tsukruk and co-workers on p. 771. The micro- and nanomechanical properties of these membranes are measured using a combination of resonance-frequency tests, bulging tests, and point-load nanodeflection experiments. These freely suspended nanomembranes (right) with an elastic modulus of 5–10 GPa are very robust and can sustain multiple significant deformations (left, image obtained by B. Rybak and P. Kladitis). They are sensitive to variations in pressure and therefore have potential applications in pressure and acoustic sensors. Freely suspended nanocomposite layer-by-layer (LbL) nanomembranes composed of a central layer of gold nanoparticles sandwiched between polyelectrolyte multilayers are fabricated via spin-assisted LbL assembly. The diameter of the circular membranes is varied from 150 to 600 μm and the thickness is kept within the range of 25–70 nm. The micro- and nanomechanical properties of these membranes are studied using a combination of resonance-frequency and bulging tests, and point-load nanodeflection experiments. Our results suggest that these freely suspended nanomembranes, with a Young's modulus of 5–10 GPa are very robust and can sustain multiple significant deformations. They are very sensitive to minor variations in pressure, surpassing ordinary semiconductor and metal membranes by three to four orders of magnitude and therefore have potential applications as pressure and acoustic microsensors.

Freely Suspended Layer-by-Layer Nanomembranes: Testing Micromechanical Properties

Freely suspended nanocomposite layer-by-layer (LbL) nanomembranes composed of a central layer of gold nanoparticles sandwiched between polyelectrolyte multilayers are fabricated via spin-assisted LbL assembly. The diameter of the circular membranes is varied from 150 to 600 μm and the thickness is kept within the range of 25–70 nm. The micro- and nanomechanical properties of these membranes are studied using a combination of resonance-frequency and bulging tests, and point-load nanodeflection experiments. Our results suggest that these freely suspended nanomembranes, with a Young's modulus of 5–10 GPa are very robust and can sustain multiple significant deformations. They are very sensitive to minor variations in pressure, surpassing ordinary semiconductor and metal membranes by three to four orders of magnitude and therefore have potential applications as pressure and acoustic microsensors.

InP bonded membrane photonics components and circuits: toward 2.5 dimensional micro-nano-photonics

The general objective of this presentation is to demonstrate the great potential of III-V semiconductor -membrane photonic devices, with a special emphasis on InP and related materials in the prospect of new developments in the field of micro-nano-photonics. Various classes devices will be presented, which will have the communality of being based on the use of high index contrast structuration of semiconductor materials. The structuration is achieved vertically for the first class, by forming thin semiconductor membranes surrounded by low optical index material, or laterally for the second class via a two-dimensional (2-D) lateral structuration of the membranes (thus, resulting in 2-D photonic crystal (PC) structures); both structurations are also combined, according to a "2.5-dimensional" approach, which should broaden considerably the combinations of functionality beyond those presently contemplated with the two first classes. The general technological scheme of the membrane approach is fully compatible with planar technology which is widely in use in the world of silicon microelectronics and with heterogeneous integration of III-V active microphotonic devices with silicon microphotonics and microelectronics (e.g., molecular bonding of InP active membranes on silica on silicon substrate). A variety of devices will be presented, featuring micro-lasers based on 2-D PC micro-cavities as well as on 2-D Bloch modes (2-D distributed-feed-back micro-laser) for in plane and surface emission.

Hybridization of III-V semiconductor membranes onto ion-exchanged waveguides

In this paper, we present an original method for the realization of hybrid photonic integrated circuits (PICs) based on ion-exchanged (IE) waveguides and III-V semiconductor materials. In our approach, we propose to bond thin semiconductor membranes at designated locations of the PIC where active functions are required. The advantage of very thin membranes is the possibility to keep the optical pathway defined by the IE waveguides even if the semiconductor refractive index is much higher than the glass refractive index. Modal analysis and beam propagation method simulations have been carried out in order to predict the modal structure of the propagated light into the membrane loaded IE waveguide and the transition from the IE waveguide itself through adiabatic tapers.

Scanning near field thermal microscopy on a micromachined thin membrane

A resistive probe based scanning thermal microscope (SThM) has been used to perform complementary near field thermal measurements on the surface of a thin semiconductor membrane. This thin structure is part of a micromachined thermal rf power sensor and includes an integrated resistive heater used as an absorbing element for the input power. The resulting 2D surface temperature distribution and the 2D thermal wave propagation characteristic were determined. Considering the thermal wave behaviour at near field conditions, the local thermal conductivity of the thin membrane and the surrounding bulk material was investigated by usage of the 3 ω -method.

Scanning near field thermal microscopy on a micromachined thin membrane

A resistive probe based scanning thermal microscope (SThM) has been used to perform complementary near field thermal measurements on the surface of a thin semiconductor membrane. This thin structure is part of a micromachined thermal rf power sensor and includes an integrated resistive heater used as an absorbing element for the input power. The resulting 2D surface temperature distribution and the 2D thermal wave propagation characteristic were determined. Considering the thermal wave behaviour at near field conditions, the local thermal conductivity of the thin membrane and the surrounding bulk material was investigated by usage of the 3 ω-method.

3D structuring of multilayer suspended membranes including 2D photonic crystal structures

New types of switching devices with spatial and spectral resolution combining photonic crystal (PC) and micro-opto-electro-mechanical structures (MOEMS) are proposed. They are based on multilayer suspended high refractive index III–V-based semiconductor membranes, some of these membranes being laterally structured to form a bidimensional PC. We present a method for fabricating these devices implying an original approach that combines micro- and nanotechnologies. The PC is patterned by using electron beam lithography and reactive ion etching. The MOEMS structure is an InP-air gaps multilayer realized by a combination of classical processing of epitaxially grown layers with a selective postprocess micromachining of sacrificial epilayers. Primary demonstrator acting as a wavelength selective and “switchable” coupler including a triangular lattice of PC holes on top of a InP–air gap pair-based Bragg reflector has been realized. We present the preliminary results on the technological fabrication and optical characterizations.

Evaluation of the Textile Goods in Connection with Improving Blood Flow : The Efficacy of Semiconductor Membrane for the Treatment of Pain

Evaluation of the Textile Goods in Connection with Improving Blood Flow : The Efficacy of Semiconductor Membrane for the Treatment of Pain

Photonic band structure of dielectric membranes periodically textured in two dimensions

The real and imaginary photonic band structure of modes attached to two-dimensionally textured semiconductor membranes is determined experimentally and theoretically. These porous waveguides exhibit large (1000 ${\mathrm{cm}}^{\ensuremath{-}1}$ at 9500 ${\mathrm{cm}}^{\ensuremath{-}1})$ second-order optical gaps, highly dispersive lifetimes, and bands with well-defined polarization along directions of high symmetry.

EPR spin-trapping of hydroxyl radicals onto photocatalytic membranes immobilizing titanium dioxide, and spin adduct competition, as a probe of reactivity with aqueous organic micropollutants

Spin-trapping of photogenerated •OH radicals, with electron paramagnetic resonance (EPR) detection of spin adducts, has been evidenced on to photocatalytic membranes, prepared by photografting, and immobilizing 30±3 wt.% TiO 2 semiconductor. By employing the known rate constant for the reaction of •OH radicals with the spin-trap (DMPO), rate constants for the reaction of •OH radicals with a given substrate could be obtained by a competition kinetic scheme, using the initial rates of the first step of photodegradation of the substrate. This method used, for a parallel series of runs, also membranes, immobilizing 7 wt.% of trialkyl vanadates as photocatalytic promoters together with the semiconductor, able to yield unitary quantum yields for the photogeneration of •OH radicals from semiconductor. Quantum yields for the non-photocatalytically promoted semiconductor membrane could thus be obtained. This method has been validated with phenol, 4-chlorophenol, 2,4-dichlorophenol, atrazine, propazine, prometryn, ametryn, and formate ion, and compared with a similar technique, set up recently in the literature, based on production of •OH by homogeneous photolysis of H 2O 2 and direct EPR measurements of the rate of spin adduct formation.

A transparent nanostructured semiconductor membrane modified by an adsorbed electron donor and electron acceptor

Unsupported transparent nanoporous-nanocrystalline TiO2 membranes are prepared which are 20 µm thick anatase structures and have an associated surface roughness of ca. 1100. Visible-light induced charge injection by the adsorbed ruthenium complex, [bis(4,4′-dicarboxy-2,2′-bipyridine)(4,4′-dimethyl-2,2++′-bipyridine)ruthenium(ii)] dichloride 1, has been demonstrated for these membranes. Also demonstrated has been blue-light induced reduction of the adsorbed viologen, 1-ethyl-1′-[4-carboxy-3-hydroxyphenyl)methyl]-4,4′-bipyridini++um perchlorate 2.

Visible-Light-Induced and Long-Lived Charge Separation in a Transparent Nanostructured Semiconductor Membrane Modified by an Adsorbed Electron Donor and Electron Acceptor

The preparation of transparent nanostructured TiO2 (anatase) membranes is described. Detailed characterization shows these membranes to be 50 μm thick nanoporous-nanocrystalline structures with associated values for porosity and surface roughness of 50% and 5000, respectively. Modification of these membranes by coadsorption of a ruthenium complex, bis[(4,4‘-dicarboxy-2,2‘-bipyridine)(4,4‘-dimethyl-2,2‘-bipyridine)ruthenium(II)] dichloride (I), and of a viologen, 1-ethyl-1‘-[(4-carboxy-3-hydroxyphenyl)methyl]-4,4‘-bipyridinium perchlorate (II), is also described. Detailed studies show that visible-light-induced electron transfer by electronically excited I to the conduction band of the nanostructured TiO2 membrane is followed by membrane mediated electron transfer to coadsorbed II. Detailed studies also show that, as a consequence of the rectifying properties of the semiconducting membrane, charge separation is long-lived. The possible significance of these findings for the development of a practical water splitting device is considered.

Phonon Deformation Potentials from Raman Measurements on Semiconductor Membranes

Phonon Deformation Potentials from Raman Measurements on Semiconductor Membranes

Novel technique for fabrication of one- and two-dimensional systems

A new technique for fabrication of nano- and atomic-scale device elements and quantum systems which cannot be obtained by any other technology is proposed. The technique is based on the experimentally established possibility of creating atomically sharp-edged cracks of desired length and direction in a given layer of a heterostructure. Ultra-narrow (1 nm) and perfectly straight window-slits in the GaAs layer-mask of semiconductor structures and ultra-thin (2 nm) semiconductor membranes have been fabricated for the first time.

Dynamic model for pseudomorphic structures grown on compliant substrates: An approach to extend the critical thickness

We use a dynamic model to analyze the critical thickness for pseudomorphic structures grown on thin membranes considering the strain relaxation process. Our results show that a pseudomorphic structure of arbitrary thickness can be achieved on a semiconductor membrane which works as a compliant substrate. The analysis on GaAs/InGaAs material system shows that the membrane should be thinner than 1200 Å for 1% strain and 1 μm for 0.5% strain. Such thickness can be achieved by existing technology. Using the proposed method, semiconductor lasers of yellow and green colors, among many new devices, can be fabricated with strained InGaP/InAlGaP active and cladding layers on GaAs substrates.