Recent advances in soft materials and system integration technologies have provided a unique opportunity to design various types of wearable flexible hybrid electronics (WFHE) for advanced human healthcare and human-machine interfaces. The hybrid integration of soft and biocompatible materials with miniaturized wireless wearable systems is undoubtedly an attractive prospect in the sense that the successful device performance requires high degrees of mechanical flexibility, sensing capability, and user-friendly simplicity. Here, the most up-to-date materials, sensors, and system-packaging technologies to develop advanced WFHE are provided. Details of mechanical, electrical, physicochemical, and biocompatible properties are discussed with integrated sensor applications in healthcare, energy, and environment. In addition, limitations of the current materials are discussed, as well as key challenges and the future direction of WFHE. Collectively, an all-inclusive review of the newly developed WFHE along with a summary of imperative requirements of material properties, sensor capabilities, electronics performance, and skin integrations is provided.

Advanced Soft Materials, Sensor Integrations, and Applications of Wearable Flexible Hybrid Electronics in Healthcare, Energy, and Environment.

Flexible electronics has significantly advanced over the last few years, as devices and circuits from nanoscale structures to printed thin films have started to appear. Simultaneously, the demand for high-performance electronics has also increased because flexible and compact integrated circuits are needed to obtain fully flexible electronic systems. It is challenging to obtain flexible and compact integrated circuits as the silicon based CMOS electronics, which is currently the industry standard for high-performance, is planar and the brittle nature of silicon makes bendability difficult. For this reason, the ultra-thin chips from silicon is gaining interest. This review provides an in-depth analysis of various approaches for obtaining ultra-thin chips from rigid silicon wafer. The comprehensive study presented here includes analysis of ultra-thin chips properties such as the electrical, thermal, optical and mechanical properties, stress modelling, and packaging techniques. The underpinning advances in areas such as sensing, computing, data storage, and energy have been discussed along with several emerging applications (e.g., wearable systems, m-Health, smart cities and Internet of Things etc.) they will enable. This paper is targeted to the readers working in the field of integrated circuits on thin and bendable silicon; but it can be of broad interest to everyone working in the field of flexible electronics.

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Ultra-thin chips for high-performance flexible electronics

Engineered nanomembranes are of great interest not only for large-scale energy storage devices, but also for on-chip energy storage integrated microdevices (such as microbatteries, microsupercapacitors, on-chip capacitors, etc.) because of their large active surfaces for electrochemical reactions, shortened paths for fast ion diffusion, and easy engineering for microdevice applications. In addition, engineered nanomembranes provide a lab-on-chip electrochemical device platform for probing the correlations of electrode structure, electrical/ionic conductivity, and electrochemical kinetics with device performance. This review focuses on the recent progress in engineered nanomembranes including tubular nanomembranes and planar nanomembranes, with the aim to provide a systematic summary of their fabrication, modification, and energy storage applications in lithium-ion batteries, lithium–oxygen batteries, on-chip electrostatic capacitors and micro-supercapacitors. A comprehensive understanding of the relationship between engineered nanomembranes and electrochemical properties of lithium ion storage with engineered single-tube microbatteries is given, and the flexibility and transparency of micro-supercapacitors is also discussed. Remarks on challenges and perspectives related to engineered nanomembranes for the further development of energy storage applications conclude this review.

Engineered nanomembranes for smart energy storage devices

The recent observation of a fundamental direct bandgap for GeSn group IV alloys and the demonstration of low temperature lasing provide new perspectives on the fabrication of Si photonic circuits. This work addresses the progress in GeSn alloy epitaxy aiming at room temperature GeSn lasing. Chemical vapor deposition of direct bandgap GeSn alloys with a high Γ- to L-valley energy separation and large thicknesses for efficient optical mode confinement is presented and discussed. Up to 1 μm thick GeSn layers with Sn contents up to 14 at. % were grown on thick relaxed Ge buffers, using Ge2H6 and SnCl4 precursors. Strong strain relaxation (up to 81%) at 12.5 at. % Sn concentration, translating into an increased separation between Γ- and L-valleys of about 60 meV, have been obtained without crystalline structure degradation, as revealed by Rutherford backscattering spectroscopy/ion channeling and transmission electron microscopy. Room temperature reflectance and photoluminescence measurements were performed to ...

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Direct Bandgap Group IV Epitaxy on Si for Laser Applications

Conventional solid films on certain substrates play a crucial role in various applications, for example in flat panel displays, silicon technology, and protective coatings. Recently, tremendous attention has been directed toward the thinning and shaping o

Thinning and Shaping Solid Films into Functional and Integrative Nanomembranes

Young’s modulus is a fundamental physical parameter that determines not only the mechanic but also the electronic properties of a solid thin film. In here, we show that the elastic modulus is not a constant as that of conventional treatment but varies with film thickness. Scaling behavior is found based on the theoretical analysis of the free energy of surface-to-volume ratio of the film and results of the elastic modulus measurement. It has been shown that there exists some film thickness hb when the surface energy of the film comes into play. The hb is inverse proportional to the bulk Young’s modulus and depends strongly on the in-plain strain e0 as e0−2. For Si nanofilms, the variation of dimensionless elastic modulus Ψ=E/Ebulk with the dimensionless film thickness η=h/hb can be represented in the following form: Ψ=η0.226. The present investigation illustrates the importance of the effect of dimensionality on the basic parameter of a thin film as well as providing important implications for electronic ...

Thickness dependence of nanofilm elastic modulus

We investigate the effects of biaxial tensile stress and n-type doping on the direct-transition optical gain in germanium infrared lasers, in which three differently oriented systems of (0 0 1), (1 1 0) and (1 1 1) Ge are analysed. We show our theoretical model for the strain tensor, strained electronic band structure, carrier occupation, optical gain and free-carrier absorption to evaluate the net optical gain under different strain and n-doping levels. Our analysis shows that for a large strain, (0 0 1) Ge can produce a larger optical gain than the others due to its clear direct-bandgap behaviour for mid-infrared lasers. For a moderate strain combined with the use of n-type doping, (1 1 1) Ge can generate a larger net optical gain than the others for near-infrared lasers. Our modelling results provide an insight into the improvement of optical emission in Ge for lasers.

http://iopscience.iop.org/article/10.1088/0022-3727/46/6/065103/pdf

Optical gain of germanium infrared lasers on different crystal orientations

We report an investigation on low dimensional Ge1−xSnx/Ge heterostructures. A series of strained-layer Ge1−xSnx/Ge superlattices with various Sn contents up to a threshold value that affords a direct bandgap is achieved by the technique of low temperature growth using molecular beam epitaxy. The Sn composition, strain status, and crystallographic are systematically characterized by cross-sectional transmission electron microscope and x-ray diffraction. Optical absorption measurements were carried out at room temperature to determine the bandgap energies of the Ge1−xSnx/Ge superlattices. Analyzing the direct transition energies reveals the room-temperature quantum confinement in the Ge1−xSnx/Ge superlattices. Present investigation demonstrates the growth and the quantum confinement of Ge1−xSnx/Ge superlattices, moving an important step forward toward the development of high-performance photonic devices based on Sn-containing group-IV low-dimensional structures.

Structural and optical characteristics of Ge_1-xSnx/Ge superlattices grown on Ge-buffered Si(001) wafers

This study reports both experimental and theoretical investigation on the strain-induced wrinkling on Si1−xGex∕Si free standing film. Clear periodical pattern is observed and attributed to the strain relaxation of the SiGe film. With increasing lateral length of the film, both wavelength and amplitude increase. Nonlinear Von Karman plate theory is employed to model the structure. From the analysis, it shows that the formation of the wrinkling pattern is a trade-off between bending energy and stretching energy. Based on the modeling, it is found that wavelength decreased with increasing Ge content, while vice versa for the amplitude.

Strain-induced wrinkling on SiGe free standing film

We report the first observation of room-temperature quantum-confined photoluminescence (PL) from low-dimensional Ge1−xSnx/Ge superlattices (SLs) up to a high Sn content of 6.96%. Both direct and indirect emissions associated with the interband transitions between minibands in the conduction bands and valence band were observed at room temperature. As the Sn content is increased, the energy difference between the lowest direct and indirect transitions is reduced, indicating an effective modification of the band structure desired for optoelectronics. The integrated PL intensity ratio of direct to indirect recombinations is significantly enhanced with increasing Sn content due to the reduced Γ-L energy separation and quantum confinement effect. Those results suggest that Sn-based low-dimensional structures are promising material for efficient Si-based lasers.

Henry H. Cheng

Papers

Thickness dependence of nanofilm elastic modulus

Optical gain of germanium infrared lasers on different crystal orientations

Structural and optical characteristics of Ge_1-xSnx/Ge superlattices grown on Ge-buffered Si(001) wafers

Strain-induced wrinkling on SiGe free standing film

Quantum-confined photoluminescence from Ge(1-x)Sn(x)/Ge superlattices on Ge-buffered Si(001) substrates.