Fabrication and measurement of low-stress membrane mirrors for adaptive optics

We have fabricated low-stress membranes from single-crystal silicon for use as deformable mirrors in adaptive optics. These membranes have lower stress than membranes made from silicon nitride or other materials and therefore are capable of greater deformation than previously used membrane mirrors. Membranes were assembled into devices by flip chip bonding to electrode chips with either 256 or 1024 electrodes. We have characterized devices with static and dynamic tests and compared their performance with an analytical model. We tracked the evolution of strain in the membrane during the device's fabrication and assembly and identified sources of stress and strain in this process. We identified boron dopant concentration as a critical determinant of intrinsic stress in the membrane.     

Distribution of Te inclusions in a CdZnTe wafer and their effects on the electrical properties of fabricated devices

We quantified the size and concentration of Te inclusions along the lateral- and the growth-directions of a ∼6 mm-thick wafer cut axially along the center of a CdZnTe ingot. We fabricated devices, selecting samples from the center slice outward in both directions, and then tested their response to incident X-rays. We employed, in concert, an automated IR transmission microscopic system and a highly collimated synchrotron X-ray source that allowed us to acquire and correlate comprehensive information on Te inclusions and other defects to assess the material factors limiting the performance of CdZnTe detectors.     

Examination of the structural and optical failure of ultra-bright LEDs under varying degrees of electrical stress using synchrotron X-ray topography and optical emission spectroscopy

Failure analysis of ultra-bright LED arrays under varying degrees of electrical stress was performed. Green (565 nm), red (660 nm) and infrared (890 nm) LEDs were subjected to currents between 0 and 1 A and voltages between 0 and 7 V. Using white beam synchrotron X-ray topography (SXRT) in back reflection large-area and section modes, the failure modes of the devices were observed. As the power to each device was increased, a reduction in the definition of device lattice structure due to increased thermal stressing was observed. An increase in strain is witnessed in the devices as they are stressed to the point of near failure. It was noted that at or near failure, the strain fields in the ball-bonded regions of the device become anomalously large (0.08% for the red LED, 0.19% for the green LED and 0.27% for the infrared LED), as observed via orientational contrast on the topographs. This is most likely due to thermally induced damage. The onset of failure took place when the power supplied to each individual LED exceeded 600 mW in the case of the green LEDs, 500 mW for the red LEDs and 745 mW for the infrared LEDs. Surprisingly, this is approximately 25 times greater than the nominal recommended supply for each LED array. This was confirmed by studying the I –V characteristics of the devices in conjunction with their emission spectra. As the power supplied to the devices was increased a narrowing of the radiative bandgap was witnessed in the red and green LEDs; whereas a broadening occurred in the infrared LED. When complete failure occurred in the samples, it was observed that large lattice deformations of the original device structure took place. Optical micrographs indicated that the structure of the devices remains spatially unaltered although the gold bond wire became detached. This confirmed that the distortion observed in the X-ray topographs is mainly due to severe thermally induced lattice distortion. The induced lattice distortion is greatest for the red LEDs; the sample appears to possess distinct and completely misorientated sub-grains.     

Correlative High‐Resolution Mapping of Strain and Charge Density in a Strained Piezoelectric Multilayer

A key to strain engineering of piezoelectric semiconductor devices is the quantitative assessment of the strain‐charge relationship. This is particularly demanding in current InGaN/GaN‐based light‐emitting diode (LED) designs as piezoelectric effects are known to degrade the device performance. Using the state‐of‐the‐art inline electron holography, we have obtained fully quantitative maps of the two‐dimensional strain tensor and total charge density in conventional blue LEDs and correlated these with sub‐nanometer spatial resolution. We show that the In_0.15Ga_0.85N quantum wells are compressively strained and elongated along the polar growth direction, exerting compressive stress/strain on the GaN quantum barriers. Interface sheet charges arising from a polarization gradient are obtained directly from the strain data and compared with the total charge density map, quantitatively verifying only 60% of the polarization charges are screened by electrons, leaving a substantial piezoelectric field in each In_0.15Ga_0.85N quantum well. The demonstrated capability of inline electron holography provides a technical breakthrough for future strain engineering of piezoelectric optoelectronic devices.     

Nanometer scale assessment of mechanical strain induced in silicon by a periodic line array

Measuring stress and strain, induced by nanostructures, at the nanometer scale is still a challenge. In this work, we investigate the strain induced by sub-micrometric periodic line arrays deposited on single crystal (001) Si substrate. We study the influence of the lines width and the spacing between the lines for two sets of samples: a silicon nitride lines array and a poly-silicon line array capped with a Si3N4 stressor layer. The periodic strain field in mono-crystalline silicon is investigated by High Resolution X-ray Diffraction which is very sensitive to local strain (< 10(-4)), has the required resolution, and is non-destructive. X-ray reciprocal space maps (RSM) are measured on a 4 circles goniometer with a laboratory source. The line arrays induce a periodic strain field in silicon, which gives rise to distinct satellites in reciprocal space. The intensity envelope of these satellites is related to the strain field in one cell. In order to assess this strain field in silicon, mechanical modeling is necessary. Elastic calculations are performed with a Finite Element Modeling (FEM) code in order to extract the displacement field that is used for structure factor calculations within kinematical approximation. The calculated reciprocal space map is compared to the experimental results in order to validate the strain field. We show that for capped poly arrays, the diffracted intensity envelope is influenced by the spacing between the lines. This area is filled with silicon nitride which induces a noticeable change in displacement and strain field. While for bare stressor arrays the nitride line width is responsible of change in displacement field and thus on the RSM intensity envelope.     

Multiferroics: progress and prospects in thin films

Multiferroic materials, which show simultaneous ferroelectric and magnetic ordering, exhibit unusual physical properties - and in turn promise new device applications - as a result of the coupling between their dual order parameters. We review recent progress in the growth, characterization and understanding of thin-film multiferroics. The availability of high-quality thin-film multiferroics makes it easier to tailor their properties through epitaxial strain, atomic-level engineering of chemistry and interfacial coupling, and is a prerequisite for their incorporation into practical devices. We discuss novel device paradigms based on magnetoelectric coupling, and outline the key scientific challenges in the field.     

On the use of the non direct tensile loading on a classical split Hopkinson bar apparatus dedicated to sheet metal specimen characterisation

The facilities used to determine behaviour laws under dynamic loading are classified into two categories: impulsive and impact loading. For the first category, the properties of the industrial materials and their evolution at moderate strain rates range is generally obtained using split Hopkinson bars devices. Shear, tensile or compression strain modes, observed on crashed frameworks, are then applied on specimens to establish classical plastic stress/strain relations. Tensile testing using a non direct loading configuration raise the problem of the set-up and the holding of sheet specimens on the split Hopkinson bars devices, which generally induces impedance mismatches and perturbs the elastic pulses during their run through the specimen assemblies. This paper presents an original tensile testing configuration required bonded sheet metal specimens on the external part of threaded sleeves. A test programme is carried out on two metallic alloys (XES low carbon steel and 2024 T3 aluminium) at plastic strain rates between 180 and 440 s⁻¹. All results are compared with others experimental raw databases and validate this first stage of tensile loading.     

Strain-driven electronic band structure modulation of si nanowires

One of the major challenges toward Si nanowire (SiNW) based photonic devices is controlling the electronic band structure of the Si nanowire to obtain a direct band gap. Here, we present a new strategy for controlling the electronic band structure of Si nanowires. Our method is attributed to the band structure modulation driven by uniaxial strain. We show that the band structure modulation with lattice strain is strongly dependent on the crystal orientation and diameter of SiNWs. In the case of [100] and [111] SiNWs, tensile strain enhances the direct band gap characteristic, whereas compressive strain attenuates it. [110] SiNWs have a different strain dependence in that both compressive and tensile strain make SiNWs exhibit an indirect band gap. We discuss the origin of this strain dependence based on the band features of bulk silicon and the wave functions of SiNWs. These results could be helpful for band structure engineering and analysis of SiNWs in nanoscale devices.     

Evolution and control of oxygen order in a cuprate superconductor

The disposition of defects in metal oxides is a key attribute exploited for applications from fuel cells and catalysts to superconducting devices and memristors. The most typical defects are mobile excess oxygens and oxygen vacancies, which can be manipulated by a variety of thermal protocols as well as optical and d.c. electric fields. Here we report the X-ray writing of high-quality superconducting regions, derived from defect ordering, in the superoxygenated layered cuprate, La?CuO(4+y). Irradiation of a poor superconductor prepared by rapid thermal quenching results first in the growth of ordered regions, with an enhancement of superconductivity becoming visible only after a waiting time, as is characteristic of other systems such as ferroelectrics, where strain must be accommodated for order to become extended. However, in La?CuO(4+y), we are able to resolve all aspects of the growth of (oxygen) intercalant order, including an extraordinary excursion from low to high and back to low anisotropy of the ordered regions. We can also clearly associate the onset of high-quality superconductivity with defect ordering in two dimensions. Additional experiments with small beams demonstrate a photoresist-free, single-step strategy for writing functional materials.     

Electromechanical response of single-walled carbon nanotubes to torsional strain in a self-contained device

Nanoscale electronics seeks to decrease the critical dimension of devices in order to improve performance while reducing power consumption. Single-walled carbon nanotubes fit well with this strategy because, in addition to their molecular size, they demonstrate a number of unique electronic, mechanical and electromechanical properties. In particular, theory predicts that strain can have a large effect on the band structure of a nanotube, which, in turn, has an influence on its electron transport properties. This has been demonstrated in experiments where axial strain was applied by a scanning probe. Theory also predicts that torsional strain can influence transport properties, which was observed recently in multiwalled nanotubes. Here we present the first experimental evidence of an electromechanical effect from torsional strain in single-walled nanotubes, and also the first measurements of piezoresistive response in a self-contained nanotube-based nanoelectromechanical structure.     

Structural and optical properties of vacuum evaporated CdxZn1−xS thin films

II–VI semiconductors are of great importance due to their applications in various electro-optic devices. Sulphides of zinc and cadmium have been utilized effectively in various opto-electronic devices. We have prepared vacuumed CdZnS films by the vacuum evaporation method. Wide band gap binary films have wide application in solar cells. The structural and optical properties of these films have been studied. The band gap of these films is studied by absorption spectra in the wavelength range 400–650 nm. The films have a direct band gap, which varies from 3.50 eV for zinc sulphide to 2.44 eV for cadmium sulphide. The X-ray diffraction pattern of these films for structural analysis is also reported.     

Self-transducing silicon nanowire electromechanical systems at room temperature

Electronic readout of the motions of genuinely nanoscale mechanical devices at room temperature imposes an important challenge for the integration and application of nanoelectromechanical systems (NEMS). Here, we report the first experiments on piezoresistively transduced very high frequency Si nanowire (SiNW) resonators with on-chip electronic actuation at room temperature. We have demonstrated that, for very thin (~90 nm down to ~30 nm) SiNWs, their time-varying strain can be exploited for self-transducing the devices' resonant motions at frequencies as high as approximately 100 MHz. The strain of wire elongation, which is only second-order in doubly clamped structures, enables efficient displacement transducer because of the enhanced piezoresistance effect in these SiNWs. This intrinsically integrated transducer is uniquely suited for a class of very thin wires and beams where metallization and multilayer complex patterning on devices become impractical. The 30 nm thin SiNW NEMS offer exceptional mass sensitivities in the subzeptogram range. This demonstration makes it promising to advance toward NEMS sensors based on ultrathin and even molecular-scale SiNWs, and their monolithic integration with microelectronics on the same chip.     

Electronic--mechanical coupling in graphene from in situ nanoindentation experiments and multiscale atomistic simulations

We present the in situ nanoindentation experiments performed on suspended graphene devices to introduce homogeneous tensile strain, while simultaneously carrying out electrical measurements. We find that the electrical resistance shows only a marginal change even under severe strain, and the electronic transport measurement confirms that there is no band gap opening for graphene under moderate uniform strain, which is consistent with our results from the first-principles informed molecular dynamics simulation.     

Emerging X-ray imaging technologies for energy materials

With the growing need for sustainable energy technologies, advanced characterization methods become more and more critical for optimizing energy materials and understanding their operation mechanisms. In this review, we focus on the synchrotron-based X-ray imaging technologies and the associated applications in gaining fundamental insights into the physical/chemical properties and reaction mechanisms of energy materials. We will discuss a few major X-ray imaging technologies, including X-ray projection imaging, transmission X-ray microscopy, scanning transmission X-ray microscopy, tender and soft X-ray imaging, and coherent diffraction imaging. Researchers can choose from various X-ray imaging techniques with different working principles based on research goals and sample specifications. With the X-ray imaging techniques, we can obtain the morphology, phase, lattice and strain information of energy materials in both 2D and 3D in an intuitive way. In addition, with the high-penetration X-rays and the high-brilliance synchrotron sources, operando/in-situ experiments can be designed to track the qualitative and quantitative changes of the samples during operation. We expect this review can broaden readers’ view on X-ray imaging techniques and inspire new ideas and possibilities in energy materials research.     

Structural characterization of luminescent Ca0.95Cd0.05S

Cadmium-doped calcium sulphide has great potential as a broadband light source in both powder and thin film electroluminescent devices. We now report the first structural investigation of polycrystalline cadmium-doped calcium sulphide using a combination of transmission electron microscopy (TEM) with microanalysis, and X-ray diffraction. We show that the crystals formed by direct reaction of CaS and CdS under excess sulphur do not yield an even distribution of cadmium. We also show that the crystals are heavily dislocated with defect structures typical of rock-salt structures. We show that TEM can be successfully applied to the study of these moisture-sensitive materials without the formation of artefacts and that TEM should be a valuable tool for studying their thin film structures.     

Experimental Design for a Class of Accelerated Degradation Tests

Traditionally, reliability assessment of new devices has been based on accelerated life tests. This approach is not practical for highly reliable devices, such as lasers, which are not likely to fail in experiments of reasonable length. An alternative approach is to monitor the devices for a period of time and assess their reliability from the changes in performance (degradation) observed during the experiment. In this article, we propose a methodology for designing experiments for degradation processes in which the amount of degradation over time levels off toward a plateau (maximum degradation) that is a function of stress. We provide (a) the stress levels for the experiment, (b) the proportion of devices to test at each stress level, (c) the times at which to measure the devices, and (d) the total number of devices to test. We apply the proposed methodology to an actual example.     

Formation of uniaxially strained SiGe by selective ion implantation technique

Uniaxially strained SiGe layers were fabricated with a newly developed selective-ion-implantation technique. The SiGe layer was grown on the Si substrate, into which laterally selective ion-implantation with stripe pattern was carried out prior to the SiGe growth. A strain-relaxation of the SiGe layer was largely enhanced due to ion-implantation-induced defects selectively in the ion-implanted area while it was hardly enhanced in the neighboring unimplanted area. However, micro-Raman mapping and X-ray diffraction reciprocal space mapping measurements obviously revealed that the relaxed SiGe in the implanted area remarkably influenced a strain state of the neighboring strained SiGe in the unimplanted area, that is, the strain along the stripe line direction was highly relieved due to the stress caused by the neighboring relaxed SiGe while the strain in the direction perpendicular to the line was well maintained. As a result, highly asymmetric strain state, that is, uniaxial strain was realized, where 4 times different relaxation ratios in the two directions were observed. These results indicate that the selective-ion-implantation technique developed in this study has a high potential to realize uniaxially strained Si/Ge channel devices with high mobility.     

High Performance Flexible Strain Sensors Based On Silver Nanowires/thermoplastic Polyurethane Composites for Wearable Devices

Applied Composite Materials - Flexible strain sensors have attracted numerous attentions due to their application in wearable devices. However, it is still a significant challenge to fabricate...     

Bending-mode vibration of a suspended nanotube resonator

We have used a suspended carbon nanotube as a frequency mixer to detect its own mechanical motion. A single gate-dependent resonance is observed, which we attribute to the fundamental bending mode vibration of the suspended carbon nanotubes. A continuum model is used to fit the gate dependence of the resonance frequency, from which we obtain values for the fundamental frequency, the residual and gate-induced tension in the nanotube. This analysis shows that the nanotubes in our devices have no slack and that, by applying a gate voltage, the nanotube can be tuned from a regime without strain to a regime where it behaves as a vibrating string under tension.     

A system for ultrasonic beacon-guidance of catheters and other minimally-invasive medical devices

Catheters and other interventional medical devices are presently guided by X-ray imaging, despite the advantages of ultrasound imaging over X-ray imaging in cost, safety, and availability. X-ray imaging is used because ultrasound reflects specularly from catheters and similar devices; their visibility is highly angle-dependent. With an omni-directional receiver mounted on a device, the receiver's location in the ultrasound image can be deduced from knowing which acoustic ray struck the receiver and the time from transmission of the imaging pulse to its reception by the receiver. This information is independent of specular reflection. The location of the device can then be indicated in the ultrasound image by an arrow pointing to the sensor, making possible ultrasound guidance of these devices. This paper describes the technical and practical considerations in the design and construction of the device-mounted receiver and associated electronics, and describes some clinical uses.