In Press
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• ## Recent progress of SiC UV single photon counting avalanche photodiodes

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4H-SiC single photon counting avalanche photodiodes (SPADs) are prior devices for weak ultraviolet (UV) signal detection with the advantages of small size, low leakage current, high avalanche multiplication gain, and high quantum efficiency, which benefit from the large bandgap energy, high carrier drift velocity and excellent physical stability of 4H-SiC semiconductor material. UV detectors are widely used in many key applications, such as missile plume detection, corona discharge, UV astronomy, and biological and chemical agent detection. In this paper, we will describe basic concepts and review recent results on device design, process development, and basic characterizations of 4H-SiC avalanche photodiodes. Several promising device structures and uniformity of avalanche multiplication are discussed, which are important for achieving high performance of 4H-SiC UV SPADs.

• ## Electrospun flexible sensor

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Flexible sensors have received wide attention because of their ability to adapt to a variety of complex environments. Electrospinning technology has significant advantages in the preparation of flexible sensors. This paper summarizes the progress in the preparation of flexible sensors by electrospinning. Sensors that respond to light, stress, and gas are presented separately. Finally, some directions for electrospinning and flexible sensors are discussed.

• ## Influence of self-heating on the millimeter-wave and terahertz performance of MBE grown silicon IMPATT diodes

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The influence of self-heating on the millimeter-wave (mm-wave) and terahertz (THz) performance of double-drift region (DDR) impact avalanche transit time (IMPATT) sources based on silicon (Si) has been investigated in this paper. The dependences of static and large-signal parameters on junction temperature are estimated using a non-sinusoidal voltage excited (NSVE) large-signal simulation technique developed by the authors, which is based on the quantum-corrected drift-diffusion (QCDD) model. Linear variations of static parameters and non-linear variations of large-signal parameters with temperature have been observed. Analytical expressions representing the temperature dependences of static and large-signal parameters of the diodes are developed using linear and 2nd degree polynomial curve fitting techniques, which will be highly useful for optimizing the thermal design of the oscillators. Finally, the simulated results are found to be in close agreement with the experimentally measured data.

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Mobile robots behaving as humans should possess multifunctional flexible sensing systems including vision, hearing, touch, smell, and taste. A gas sensor array (GSA), also known as electronic nose, is a possible solution for a robotic olfactory system that can detect and discriminate a wide variety of gas molecules. Artificial Intelligence (AI) applied to an electronic nose involves a diverse set of machine learning algorithms which can generate a smell print by analyzing the signal pattern from the GSA. A combination of GSA and AI algorithms can empower intelligent robots with great capabilities in many areas such as environmental monitoring, gas leakage detection, food and beverage production and storage, and especially disease diagnosis through detection of different types and concentrations of target gases with the advantages of portability, lower-power-consumption and ease-of-operation. It is exciting to envisage robots equipped with a "nose" acting as family doctor who will guard every family member's health and keep their home safe. In this review, we give a summary of the state-of the-art research progress in the fabrication techniques for GSAs and typical algorithms employed in artificial olfactory systems, exploring their potential applications in disease diagnosis, environmental monitoring, and explosive detection. We also discuss the key limitations of gas sensor units and their possible solutions. Finally, we present the outlook of GSAs over the horizon of smart homes and cities.

• ## Advances in flexible and wearable pH sensors for wound healing monitoring

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Wound healing has been recognized as a complex and dynamic regeneration process and attracted increasing interests on its management. For effective wound healing management, a continuous monitoring on the wound healing based on sensors is essential. Since pH has been found to play an important role on wound healing process, a variety of pH sensors systems for wound healing monitoring have been greatly developed in recent years. Among these pH sensors, flexible and wearable pH sensors which can be incorporated with wound dressing have gained much attention. In this review, the recent advances in the development of flexible and wearable pH sensors for wound healing monitoring have been comprehensive summarized from the range of optical and electrochemical bases.

• ## One-pot preparation and applications ofself-healing, self-adhesive PAA-PDMS elastomers

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A new family of transparent, biocompatible, self-adhesive, and self-healing elastomer has been developed by a convenient and efficient one-pot reaction between poly(acrylic acid) (PAA) and hydroxyl-terminated polydimethylsiloxane (PDMS-OH). The condensation reaction between PAA and PDMS-OH has been confirmed by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra. The prepared PAA-PDMS elastomers possess robust mechanical strength and strong adhesiveness to human skin, and they have fast self-healing ability at room temperature (in ~10 s with the efficiency of 98%). Specifically, strain sensors were fabricated by assembling PAA-PDMS as packaging layers and polyetherimide-reduced graphene oxide (PEI-rGO) as strain-sensing layers. The PAA-PDMS/PEI-rGO sensors are stably and reliably responsive to slight physical deformations, and they can be attached onto skin directly to monitor the body’s motions. Meanwhile, strain sensors can self-heal quickly and completely, and they can be reused for the motion detecting after shallowly scratching the surface. This work provides new opportunities to manufacture high performance self-adhesive and self-healing materials.

• ## Improvement of tunnel compensated quantum well infrared detector

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To reduce the difficulty of the epitaxy caused by multiple quantum well infrared photodetector (QWIP) with tunnel compensation structure, an improved structure is proposed. In the new structure, the superlattices are located between the tunnel junction and the barrier as the infrared absorption region, eliminating the effect of doping concentration on the well width in the original structure. Theoretical analysis and experimental verification of the new structure are carried out. The experimental sample is a two-cycle device, each cycle contains a tunnel junction, a superlattice infrared absorption region and a thick barrier. The photosurface of the detector is 200 × 200 μm2 and the light is optically coupled by 45° oblique incidence. The results show that the optimal operating voltage of the sample is –1.1 V, the dark current is 2.99 × 10–8 A, and the blackbody detectivity is 1.352 × 108 cm·Hz1/2·W–1 at 77 K. Our experiments show that the new structure can work normally.

• ## A contrivance of 277 nm DUV LD with B0.313Ga0.687N/B0.40Ga0.60N QWs and AlxGa1–xN heterojunction grown on AlN substrate

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In this paper, an ultraviolet C-band laser diode lasing at 277 nm composed of B0.313Ga0.687N/B0.40Ga0.60N QW/QB heterostructure on Mg and Si-doped AlxGa1–xN layers was designed, as well as a lowest reported substitutional accepter and donor concentration up to NA = 5.0 × 1017 cm–3 and ND = 9.0 × 1016 cm–3 for deep ultraviolet lasing was achieved. The structure was assumed to be grown over bulk AlN substrate and operate under a continuous wave at room temperature. Although there is an emphasizing of the suitability for using boron nitride wide band gap in the deep ultraviolet region, there is still a shortage of investigation about the ternary BGaN in aluminum-rich AlGaN alloys. Based on the simulation, an average local gain in quantum wells of 1946 cm–1, the maximum emitted power of 2.4 W, the threshold current of 500 mA, a slope efficiency of 1.91 W/A as well as an average DC resistance for the VI curve of (0.336 Ω) had been observed. Along with an investigation regarding different EBL, designs were included with tapered and inverse tapered structure. Therefore, it had been found a good agreement with the published results for tapered EBL design, with an overweighting for a proposed inverse tapered EBL design.

• ## Improved efficiency and photo-stability of methylamine-free perovskite solar cells via cadmium doping

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Although perovskite solar cells containing methylamine cation can show high power conversion efficiency, stability is a concern. Here, methylamine-free perovskite material CsxFA1–xPbI3 was synthesized by a one-step method. In addition, we incorporated smaller cadmium ions into mixed perovskite lattice to partially replace Pb ions to address the excessive internal strain in perovskite structure. We have found that the introduction of Cd can improve the crystallinity and the charge carrier lifetime of perovskite films. Consequently, a power conversion efficiency as high as 20.59% was achieved. More importantly, the devices retained 94% of their initial efficiency under 1200 h of continuous illumination.

• ## Contact etch process optimization for RF process wafer edge yield improvement

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Radio-frequency (RF) process products suffer from a wafer edge low yield issue, which is induced by contact opening. A failure mechanism has been proposed that is based on the characteristics of a wafer edge film stack. The large step height at the wafer’s edge leads to worse planarization for the sparse poly-pattern region during the inter-layer dielectric (ILD) chemical mechanical polishing (CMP) process. A thicker bottom anti-reflect coating (BARC) layer was introduced for a sparse poly-pattern at the wafer edge region. The contact open issue was solved by increasing the break through (BT) time to get a large enough window. Well profile and resistance uniformity were obtained by contact etch recipe optimization.

• ## Simulation and application of external quantum efficiency of solar cells based on spectroscopy

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In this study, a method for optical simulation of external quantum efficiency (EQE) spectra of solar cells based on spectroscopy is proposed, which is based on the tested transmittance and reflectance spectra. First, to obtain a more accurate information of n, k values, we modified the reported optical constants from the measured reflectance and transmittance spectra. The obtained optical constants of each layer were then collected to simulate the EQE spectra of the device. This method provides a simple, accurate and versatile way to obtain the actual optical constants of different layers. The EQE simulation approach was applied to the flat and textured heterojunctions with intrinsic layers (HIT) solar cells, respectively, which showed a perfect matching between the calculation results and the experimental data. Furthermore, the specific optical losses in different devices were analyzed.

• ## A compact two-dimensional analytical model of the electrical characteristics of a triple-material double-gate tunneling FET structure

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This paper presents a compact two-dimensional analytical device model of surface potential, in addition to electric field of triple-material double-gate (TMDG) tunnel FET. The TMDG TFET device model is developed using a parabolic approximation method in the channel depletion space and a boundary state of affairs across the drain and source. The TMDG TFET device is used to analyze the electrical performance of the TMDG structure in terms of changes in potential voltage, lateral and vertical electric field. Because the TMDG TFET has a simple compact structure, the surface potential is computationally efficient and, therefore, may be utilized to analyze and characterize the gate-controlled devices. Furthermore, using Kane's model, the current across the drain can be modeled. The graph results achieved from this device model are close to the data collected from the technology computer aided design (TCAD) simulation.

• ## Analytical model for the effects of the variation of ferrolectric material parameters on the minimum subthreshold swing in negative capacitance capacitor

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In this paper, we analytically study the relationship between the coercive field, remnant polarization and the thickness of a ferroelectric material, required for the minimum subthreshold swing in a negative capacitance capacitor. The interdependence of the ferroelectric material properties shown in this study is defined by the capacitance matching conditions in the subthreshold region in an NC capacitor. In this paper, we propose an analytical model to find the optimal ferroelectric thickness and channel doping to achieve a minimum subthreshold swing, due to a particular ferroelectric material. Our results have been validated against the numerical and experimental results already available in the literature. Furthermore, we obtain the minimum possible subthreshold swing for different ferroelectric materials used in the gate stack of an NC-FET in the context of a manufacturable semiconductor technology. Our results are presented in the form of a table, which shows the calculated channel doping, ferroelectric thickness and minimum subthreshold for five different ferroelectric materials.

• ## Recent advances in lithographic fabrication of micro-/nanostructured polydimethylsiloxanes and their soft electronic applications

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The intensive development of micro-/nanotechnologies offers a new route to construct sophisticated architectures of emerging soft electronics. Among the many classes of stretchable materials, micro-/nanostructured poly(dimethylsiloxane) (PDMS) has emerged as a vital building block based on its merits of flexibility, stretchability, simple processing, and, more importantly, high degrees of freedom of incorporation with other functional materials, including metals and semiconductors. The artificially designed geometries play important roles in achieving the desired mechanical and electrical performances of devices and thus show great potential for applications in the fields of stretchable displays, sensors and actuators as well as in health-monitoring device platforms. Meanwhile, novel lithographic methods to produce stretchable platforms with superb reliability have recently attracted research interest. The aim of this review is to comprehensively summarize the progress regarding micro-/nanostructured PDMS and their promising soft electronic applications. This review is concluded with a brief outlook and further research directions.

• ## Deep-ultraviolet integrated photonic and optoelectronic devices: A prospect of the hybridization of group III-nitrides, III-oxides, and two-dimensional materials

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Progress in the design and fabrication of ultraviolet and deep-ultraviolet group III–nitride optoelectronic devices, based on aluminum gallium nitride and boron nitride and their alloys, and the heterogeneous integration with two-dimensional and oxide-based materials is reviewed. We emphasize wide-bandgap nitride compound semiconductors (i.e., (B, Al, Ga)N) as the deep-ultraviolet materials of interest, and two-dimensional materials, namely graphene, two-dimensional boron nitride, and two-dimensional transition metal dichalcogenides, along with gallium oxide, as the hybrid integrated materials. We examine their crystallographic properties and elaborate on the challenges that hinder the realization of efficient and reliable ultraviolet and deep-ultraviolet devices. In this article we provide an overview of aluminum nitride, sapphire, and gallium oxide as platforms for deep-ultraviolet optoelectronic devices, in which we criticize the status of sapphire as a platform for efficient deep-ultraviolet devices and detail advancements in device growth and fabrication on aluminum nitride and gallium oxide substrates. A critical review of the current status of deep-ultraviolet light emission and detection materials and devices is provided.

• ## Study of the morphology evolution of AlN grown on nano-patterned sapphire substrate

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This study focused on the evolution of growth front about AlN growth on nano-patterned sapphire substrate by metal-organic chemical vapor deposition. The substrate with concave cones was fabricated by nano-imprint lithography and wet etching. Two samples with different epitaxy procedures were fabricated, manifesting as two-dimensional growth mode and three-dimensional growth mode, respectively. The results showed that growth temperature deeply influenced the growth modes and thus played a critical role in the coalescence of AlN. At a relatively high temperature, the AlN epilayer was progressively coalescence and the growth mode was two-dimensional. In this case, we found that the inclined semi-polar facets arising in the process of coalescence were \begin{document}$\left\{ {11\bar 21} \right\}$\end{document} type. But when decreasing the temperature, the \begin{document}$\left\{ {11\bar 22} \right\}$\end{document} semi-polar facets arose, leading to inverse pyramid morphology and obtaining the three-dimensional growth mode. The 3D inverse pyramid AlN structure could be used for realizing 3D semi-polar UV-LED or facet-controlled epitaxial lateral overgrowth of AlN.

• ## Hot electron effects on the operation of potential well barrier diodes

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A study has just been carried out on hot electron effects in GaAs/Al0.3Ga0.7As potential well barrier (PWB) diodes using both Monte Carlo (MC) and drift-diffusion (DD) models of charge transport. We show the operation and behaviour of the diode in terms of electric field, mean electron velocity and potential, mean energy of electrons and Γ-valley population. The MC model predicts lower currents flowing through the diode due to back scattering at anode (collector) and carrier heating at higher bias. At a bias of 1.0 V, the current density obtained from experimental result, MC and DD simulation models are 1.35, 1.12 and 1.77 μA/μm2 respectively. The reduction in current over conventional model, is compensated to a certain extent because less charge settles in the potential well and so the barrier is slightly reduced. The DD model results in higher currents under the same bias and conditions. However, at very low bias specifically, up to 0.3 V without any carrier heating effects, the DD and MC models look pretty similar as experimental results. The significant differences observed in the I–V characteristics of the DD and MC models at higher biases confirm the importance of energy transport when considering these devices.

• ## Growth properties of gallium oxide on sapphire substrate by plasma-assisted pulsed laser deposition

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Gallium oxide was deposited on a c-plane sapphire substrate by oxygen plasma-assisted pulsed laser deposition (PLD). An oxygen radical was generated by an inductive coupled plasma source and the effect of radio frequency (RF) power on growth rate was investigated. A film grown with plasma assistance showed 2.7 times faster growth rate. X-ray diffraction and Raman spectroscopy analysis showed β-Ga2O3 films grown with plasma assistance at 500 °C. The roughness of the films decreased when the RF power of plasma treatment increased. Transmittance of these films was at least 80% and showed sharp absorption edge at 250 nm which was consistent with data previously reported.

• ## Optimization of erase time degradation in 65 nm NOR flash memory chips

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Reliability issues of flash memory are becoming increasingly significant with the shrinking of technology nodes. Among them, erase time degradation is an issue that draws the attention of academic and industry researchers. In this paper, causes of the " erase time degradation” are exhaustively analyzed, with proposals for its improvement presented, including a low stress program/erase scheme with a staircase pulse and disturb-immune array bias condition. Implementation of the optimized circuit structure is verified in a 128 Mb SPI NOR Flash memory chip, which is fabricated on a SMIC 65 nm ETOX process platform. Testing results indicate a degradation of the sector erase time from 10.67 to 104.9 ms after 105 program/erase cycles, which exhibits an improvement of approximately 100 ms over conventional schemes.