Search

Nanostructured films have been widely used for preparing various advanced thin-film devices because of their unique electrical, optical, and plasmonic characteristics associated with the nano-size effect. In situ, nondestructive and high-resolution characterization of nanostructured films is essential for optimizing thin-film device performance. In this work, such thin-film characterization was achieved using a hyperspectral dark-field microscope (HSDFM) that was constructed in our laboratory by integrating a hyperspectral imager with a commercial microscope. The HSDFM allows for high-resolution (Δλ = 0.4 nm) spectral analysis of nanostructured samples in the visible-near-infrared region with a spatial resolution as high as 45 nm × 45 nm (corresponding to a single pixel). Four typical samples were investigated with the HSDFM, including the gold nanoplate array, the self-assembled gold nanoparticle (GNP) sub-monolayer, the sol-gel nanoporous titanium dioxide (TiO2) film, and the layer-stacked molybdenum disulfide (MoS2) sheet. According to the experimental results, the plasmon resonance scattering bands for nanoplate clusters are identical with those for individual gold nanoplates, indicating that the gap between adjacent nanoplates is too large to allow plasmonic coupling between them. A different case was observed with the self-assembled GNP sub-monolayer in which the aggregated clusters with the internal plasmonic interaction show a considerable red-shift of the plasmon resonance band relative to the isolated single GNP. In addition, the protein adsorption on the nanoporous TiO2 film was observed to be inhomogeneous on the microscale, and the stepped boundaries of the MoS2 sheet were clearly observed. A quasi-linear dependence of the single-pixel light intensity on the step height was obtained by combining the HSDFM with atomic force microscopy. The minimum thickness detectable by the present HSDFM is 6.5 nm, corresponding to the 10-layer MoS2 film. The work demonstrated the outstanding applicability of the HSDFM for nanostructured film characterization.

Gallium nitride (GaN) field-effect transistors have low ON resistance and switching losses in high-frequency (>MHz) resonant wireless power transfer systems. Nevertheless, their performance in the system is determined by their characteristics and operation mode. A particular operating mode in a 6.78-MHz magnetic resonant wireless transfer system that employs class-D GaN power amplifiers in the zero-voltage switching mode is studied. Two operation modes, the forward mode and the reverse mode, are investigated. The nonideal effect under the device-level dynamic resistance and thermal effect are also analyzed. The dynamic resistance under different operation modes is demonstrated to have different generation mechanisms. Finally, the device characteristics with system operating conditions are combined, and the effects of temperature and dynamic resistance under different operating conditions are evaluated.

In this work, we report a novel surface-enhanced Raman scattering (SERS) substrate based on nanostructured diamond. Hemispheric nanocrystalline diamond grown by MPCVD was annealed in air to achieve hemispheric NCD with nano-thorn-like porous structure (HSDNT). Subsequently, thin Au film was coated onto the HSDNT surface by evaporation. The large specific surface area and “lightning rod” effect provided by HSDNT greatly improved the SERS effect. Detection limit low to 10−12 M and even 10−14 M after ultrasonic process was obtained experimentally on Au-HSDNT for the first time.

This paper reports an integrated bifunctional metasurface composed of metal-dielectric-metal (MDM) trapezoidal nanoantennas. Polarization, wavelength, and angle are all multiplexed to improve the integration and the performance of the metasurface. Under orthogonal-polarized incidence, phase and amplitude are tuned independently by gap surface plasmon (GSP) resonance, leading to the combination of beam deflection and structural color printing functions. Both functions work in broadband, namely, 360 nm to 540 nm for beam deflection function and 400 nm to 800 nm for structural color printing function. Angle multiplexing further improves the tunability of the two functions. This integrated bifunctional metasurface is very suitable to be used in ultra-compact optics systems, and may find more applications in optical and biological sensing areas.

Computational prediction of inhibition efficiency (IE) for inhibitor molecules is a crucial supplementary way to design novel molecules that can efficiently inhibit corrosion onto metallic surfaces. Here we are dedicated to developing a new machine learning-based predictor for the inhibition efficiency (IE) of benzimidazole derivatives. First, a comprehensively numerical representation was given on inhibitor molecules from all aspects of energy, electronic, topological, physicochemical and spatial properties based on 3-D structures and 150 valid structural descriptors were obtained. Then, a thorough investigation of these structural descriptors was implemented. The multicollinearity-based clustering analysis was performed to remove the linear correlated feature variables, so 47 feature clusters were produced. Meanwhile, Gini importance by random forest (RF) was used to further measure the contributions of the descriptors in each cluster and 47 non-linear descriptors were selected with the highest Gini importance score in the corresponding cluster. Further, considering the limited number of available inhibitors, different feature subsets were constructed according to the Gini importance score ranking list of 47 descriptors. Finally, support vector machine (SVM) models based on different feature subsets were tested by leave-one-out cross validation. Through comparisons, the optimal SVM model with the top 11 descriptors was achieved based on Poly kernel. This model yields a promising performance with the correlation coefficient (R) and root-mean-square error (RMSE) of 0.9589 and 4.45, respectively, which indicates that the method proposed by us gives the best performance for the current data. Based on our model, 6 new benzimidazole molecules were designed and their IE values predicted by this model indicate that two of them have high potential as outstanding corrosion inhibitors.

Amplitude based coupled dual-microcantilevers have showed great sensitivity improvement compared with traditionally investigated frequency-shift based single cantilever which is meaningful for highly sensitive bio/chemical detection. In this paper, weaker coupling structures and less mass difference between the two coupled cantilevers were found to contribute to a better minus mass detection sensitivity. Emergence of the coupled states for coupled dual-microcantilevers under piezo-actuation was discovered to be easier with smaller damping force, stronger coupling effect and greater mass difference between the two coupled beams. Besides, the amplitude-ratio was found to be a more significant sensing factor for suppressing the actuating contributed noise compared to traditional amplitude index.

In this letter, we demonstrate a novel PEALD-AlN/LPCVD-Si3N4 dual-gate dielectric employed in enhancement-mode GaN MISFETs, where the gate recess is fabricated based on our proposed self-terminating gate recess etching technique using a GaN cap layer as recess mask. By using LPCVD-Si3N4 and PEALD-AlN dual-gate dielectric layer, the devices exhibit a high-quality gate dielectric and a good GaN channel interface, yielding a high gate swing up to 18 V and a high channel effective mobility of 137 cm $^{\textsf {2}}/\textsf {V}\cdot \textsf {s}$ at such high gate bias. Thus, the fabricated devices feature a high maximum drain current density of 823mA/mm, a threshold voltage of 2.6 V, an on-resistance of $7.4~\Omega \cdot \textsf {mm}$ , and an ON/ OFF current ratio of 108 with gate–drain distance of $2~\mu \text{m}$ . Meanwhile, a high off-state breakdown voltage of 1290 V is achieved with 10- $\mu \text{m}$ gate–drain distance. The corresponding specific on-resistance is as low as 1.76 $\text{m}\Omega \cdot \textsf {cm}^{\textsf {2}}$ , leading to a high Baliga’s figure of merit of 945 MW/cm2.

Reverse conduction of commercial GaN high electron mobility transistors (HEMT) for power applications, is found to induce significant dynamic ON resistance in some particular conditions, even though the commercial HEMTs show stable performance in normal operation modes. A test method is set up to evaluate dynamic Ron in this special mode. The conditions to activate the behavior are discussed. The cause of dynamic resistance is possibly due to traps located in gate region.

Plasmonic structure color has significant potential for visual biochemical sensing by simple instrumentation or even naked eye detection. Herein, we present a visual and real-time sensing strategy for refraction index sensing and detection of the biotin-avidin system based on three-dimensional cavity-coupled metamaterials. These metamaterials composed of a top array of gold disks, aluminium pillars and a bottom reflection film of aluminium have structures similar to the metal-insulator-metal structure. The insulating layer comprises air-gap cavities that are easily filled with gaseous or liquid dielectrics. Therefore, analytes can permeate into the nano-scale cavities and produce strong light-matter interactions. The sensor shows that any tiny change in the refraction index will induce a significant color variation and the sensitivity reaches 683.5 nm per refraction index unit with a figure of merit of 3.5. The color of the metamaterials changes from rose-red to violet and then loden after a monomolecular layer of thiolated biotin and streptavidin bind to the surface of the nanostructure successively. This sensing strategy offers new opportunities for the convenient detection of proteins, nucleic acids, and lipids.

Due to their unique electromagnetic properties, meta-atom arrays have always been a hotspot to realize all kinds of particular functions, and the research on meta-atom structure has extended from two-dimensions (2D) to three-dimensions (3D) in recent years. With the continuous pursuit of complex 3D meta-atom arrays, the increasing demand for more efficient and more precise nanofabrication methods has encountered challenges. To explore better fabrication methods, we presented a programmable nanofabrication method for a complex 3D meta-atom array based on focused-ion-beam stress-induced deformation (FIB-SID) effect and designed a distinctive nanostructure array composed of periodic 3D meta-atoms to demonstrate the presented method. After successful fabrication of the designed 3D meta-atom arrays, measurements were conducted to investigate the electric/magnetic field properties and infrared spectral characteristics using scanning cathodoluminescence (CL) microscopic imaging and Fourier transform infrared (FTIR) spectroscopy, which revealed a certain excitation mode induced by polarized incident IR light near 8 &mu, m. Besides the programmability for complex 3D meta-atoms and wide applicability of materials, a more significant advantage of the method is that a large-scale array composed of complex 3D meta-atoms can be processed in a quasi-parallel way, which improves the processing efficiency and the consistency of unit cells dramatically.

As chiral structures, three-dimensional (3D) nanohelixes have the circular dichroism and optical rotation under circularly polarized light. It also has a good application prospect in optical modulators with high modulation efficiency. However, it is still a challenging task to fabricate 3D nanohelixes with high precision, controllable parameters and perfect morphology. In this study, we develop FIB (focused ion beam)-SID (stress induced deformation) by using HIB (helium ion beam) instead of GIB (gallium ion beam) for nanofabrication. This general 3D nanofabrication technique allows much stronger manufacturing control, which lead to a greater programmable and accurate bidirectional folding $(-160^{\circ}-+75^{\circ})$ of various metal and dielectric thin films. Higher manufacturing accuracy of HIB makes it possible to fabricate smaller nanostructures (characteristic size is about 100 nm), which provides strong support for 3D nanostructure fabricated in visible light wave band.

Metal-insulator-metal (MIM) multilayer structures as representative plasmonic systems have been widely studied. In such a system, the metal surface can sustain bound surface plasmon polaritons (SPPs) and couple the SPPs in a dielectric cavity. Herein, a novel fabrication process, based on electron beam lithography (EBL) followed by electroplating deposition of gold, has been developed to form longitudinal metal-air-metal structures. The structures consist of a suspended top film of gold supported by an array of gold pillars and a bottom ultrathin gold film. When the gap of the two gold films is comparable to or smaller than the decay length of the interface mode, the interaction between SPPs gives rise to coupled modes which can localize the light into the insulating layer for stronger field confine and light-matter interaction. This structure exhibits vibrant reflective color at normal incident light and significant color varying with different matter filling in the nano-cavity. The sensitivity can reach 269.8 nm/RIU which offers great potential for visual and real-time biochemical detection.

This paper reports an arrayed nanometer precision gap-tunable bowtie nanoantennas fabricated by focused helium ion beam (HIB) milling. Plasmonic resonance is excited and tightly localized inside the nanogaps of the bowtie nanoantennas. Both numerical simulations and cathodoluminescence (CL) measurements manifest that the plasmon coupling resonance is significantly enhanced by decreasing the nanoantennas' gap width down to sub-10 nm scale. The stronger plasmon resonance also leads to a remarkable improvement of the spectral sensitivity to the surrounding media changes, which can be used to detect streptavidin with the concentration down to 100 ng/mL by simply monitoring the spectral shift.

GaN field-effect transistors (FET) enables low conduction loss and low switching loss in high-frequency (>MHz) resonant wireless power transfer systems. In this work, a GaN magnetic resonant wireless power transfer (WPT) system with 75% efficiency was designed and built. Advantages of GaN magnetic resonant wireless power transfer technology are introduced and the principle of magnetic resonant wireless power transfer technology is explained. Then, LTspice is used to simulate the designed system. Moreover, a GaN based wireless power transfer testing system was built and tested, the results of which indicate that both the coil coupling coefficient k and the load of the system affect the transmission efficiency of the system. In addition, it is found that the increment of input and output power will also increase the efficiency of the system. The system is further improved according to the actual application, the system can provide power for both lamp and mobile phone.

In this paper, we report the device performance of a high-voltage enhancement-mode (E-mode) GaN MOSHEMT on silicon substrate. Normally off operation is realized by a self-terminated precision gate recess process on an optimized high-electron mobility transistor structure. The GaN MOSHEMT is fully pinched off at zero gate bias, suggesting a “true” normally off operation. The threshold voltage is 0.4 V with a drain current density of 1 $\mu \text{A}$ /mm as the criteria. The device with 15- $\mu \text{m}$ gate-drain distance and 100- $\mu \text{m}$ gate width exhibits a maximum drain current of 356 mA/mm at 8-V gate bias. The on/off current ratio of the device is larger than 1010 with a subthreshold slope of 80 mV/dec. The gate leakage current is below 10−7 mA/mm up to 9-V gate bias. The off-state breakdown voltage (BV) is as high as 1528 V (880 V) measured with floating (grounded) silicon substrate at a drain leakage current criterion of 5 $\mu \text{A}$ /mm. The specific on-resistance ( ${R}_{\text {ON,SP}}$ ) of the device is 2.79 $\text{m}\Omega \cdot$ cm2, and the power figure of merit (BV2/ ${R}_{ \text {ON,SP}}$ ) is 277 MW/cm2. High-voltage pulsed I–V measurement indicates that the dynamic on-resistance is only 1.6 times the static one with a pulsewidth of 10 $\mu \text{s}$ at 400-V off-state quiescent drain bias. The high performance of the normally off GaN MOSHEMT is supposed to benefit from the high quality low pressure chemical vapor deposition Si3N4 passivation layer and the advanced E-mode device fabrication process.

Based on the self-terminating gate recess technique, two different processes featuring gate-recess-first (GF) and ohmic-contact-first (OF) were proposed for E-mode Al2O3/GaN MOSFETs. Increased maximum drain current (Idmax) ∼30% (420 vs 325 mA/mm), field-effect mobility (μFEmax) ∼67% (150 vs 90 cm2/Vs) and reduced on-state resistance (Ron) ∼42% (9.7 vs 16.8 Ω·mm) were observed in the devices fabricated by GF process. Such significant performance difference of GF- and OF-devices resulted from the presence of border traps at Al2O3/GaN interface with a time constant ∼7 × 10−6 s. Experimental results indicated that: (1) the near interface border traps in Al2O3 dielectric significantly affect device channel mobility; (2) a high temperature post-deposition annealing process could effective suppress generation of border traps.

For devices with a 15 micron anode-to-cathode distance, nearly 1.5 times increase in the blocking (breakdown) voltage (from 692 to 1030 V) has been achieved by replacing the alloyed Ohmic contact at the anode electrode of the conventional MOS gated hybrid-anode lateral field-effect rectifier (CMLFER) with a low barrier Schottky contact. The new Schottky-MOS hybrid-anode lateral field-effect rectifier is found to offer comparable low onset voltage ( ${V}_{{\textbf {ON}}}$ of 0.68±0.13 versus 0.65±0.11 V for CMLFER) independent of the anode-to- cathode distance. The immunity of the punch through caused by drain induced barrier lowering effect is obtained through the low barrier Schottky contact in anode, which is believed to be responsible for the reduction in the leakage current, and the improvement of rectifier breakdown voltage.

In this letter, a gate recessed normally-off GaN metal–oxide–semiconductor high-electron-mobility transistor on silicon substrate is fabricated using AlN/Si3N4 as the passivation layer. The thin AlN layer serves the dual role of protecting the gate channel region from direct plasma bombardment during the RIE Si3N4 removal and passivating the surface states in the access region. As a result, the effective carrier mobility in the normally-off channel is found to improve from the 568 cm2/ $\text {V}\cdot \text {s}$ in conventional Si3N4 passivation process to a high value of 1154 cm2/ $\text {V}\cdot \text {s}$ . A saturated output current density of 603 mA/mm and an ON-resistance of $5.3~\Omega \cdot \text {mm}$ was obtained for devices with $\text{L}_{\text {G}}/\text{L}_{\text {GS}}/\text{L}_{\text {GD}}/\text{W}_{\text {G}} = 1.5$ /1.5/3/ $20~\mu \text{m}$ . Meanwhile, the degradation of dynamic ON-resistance is significantly suppressed due to the effective passivation of surface states by the AlN layer grown by plasma-enhanced atomic layer deposition.

Time-dependent OFF-state leakage behavior of AlGaN/GaN MISHEMTs on silicon substrate is investigated and a novel degradation mechanism is proposed in this paper. Under constant high voltage OFF-state stress, drain leakage current gradually increases with stress time and the behavior is gate bias and temperature-dependent. Consecutive OFF-state breakdown measurement with drain injection technique indicates that the negative shift of threshold voltage ( $V_{\mathrm {th}}$ ) is responsible for the increase of drain leakage current during stress measurement. It is proposed that the negative shift of $V_{\mathrm {th}}$ is mainly induced by the ionization of uncompensated donor like deep levels near the channel, which are most likely to be located in the 300-nm-thick unintentionally doped GaN layer above the carbon doped buffer layer.

A novel three-dimensional (3D) nanoantenna array, consisting of double-claw shape unit cell, is fabricated by focused ion beam stress-introduced deformation (FIB-SID) technique [1]-[3], and is employed as an ultrabroadband infrared (IR) optical modulator. The measured reflection spectra of the device show that an ultrabroadband infrared optical modulation can be achieved with tiny change of the double claw’s distance.The relative modulation value calculated from reflection spectra for the nanoantenna with lager unit size indicates that a broad IR modulation range of 3.39 μm (2.93 μm - 6.32 μm) and a maximum modulation value of 17.5% at 5.8 μm is realized as the double claw’s distance changes. Furthermore, the scaled down nanoantennas were also fabricated and measured, which covers a surprisingly ultrabroadband IR modulation range, from long-wave (9.93 μm) to short-wave (2.04 μm), and realizes a maximum modulation value of 71.7% at 3.29 μm as the double claw’s distance changes about 180nm.

Three-dimensional (3D) air-gap metal-coated nanocavities with tunable geometries, changeable heights, and improved smoothness are fabricated by combining electron beam lithography (EBL), ultra dilute hydrofluoric acid solution wet etching (UDHFE), and metal magnetron sputtering technologies. With different shapes, heights, and separations of the nanocavities, the strong electromagnetic resonances inside the nanocavities are changed in different extent, resulting in broad gamut and sophisticated plasmonic color generation. The nanocavities-based metasurface is also used to construct a real-time and label-free refractive index sensor with 372 nm/RIU sensitivity, which shows distinct colorimetric change between different mediums. This nanocavities may find extensive potential applications in high-fidelity color printing, high-density information storage, and on-chip colorimetric label-free biomedical sensing.

We report a new arrayed three-dimensional (3D) airgap nanocavities with multiple tunable geometrical parameters. Light is tightly confined into the nanocavities and strong surface-plasmon coupling is introduced, realizing narrow band resonance. Vivid plasmonic colors are generated, and can be tuned by multiple geometrical parameters of the 3D nanocavities, including shapes, separations, and heights. What's more, the surface-plasmon coupling resonance has different dependence on different variable geometrical parameters. So, multi-dimensional color tuning with different spectral sensitivities is realized by proper and precise structural design, leading to both broad gamut and sophisticated plasmonic color printing at the optical diffraction limit.

The thermal-oxidation/wet-etching gate-recess mask using low-pressure-chemical-vapor-deposition (LPCVD) SiN/atomic-layer-deposition (ALD) AlN combined with high-quality LPCVD-SiN/ALD-SiO2 gate dielectric has been developed for the fabrication of normally-off GaN MISFETs by the self-terminated gate recess etching technique. The experimental results showed that the SiN/AlN layer could effectively hinder the formation of surface oxide on GaN cap, and ALD grown SiO2 could effectively protect the GaN channel from high-temperature damages by the following LPCVD. As a result, the fabricated devices exhibit a small on-resistance degradation, 200 mV hysteresis @ Vth = 2.4 V

The current saturation in a gateless AlGaN/GaN heterostructure device is investigated in this letter. Analysis of the micro-Raman spectroscopy results indicated that the self-heating effect could be eliminated by the 10 μs pulse measurement. Based on the proposed multi-probe gateless AlGaN/GaN heterostructure device, the uniform channel depletion is experimentally verified even in the case of current saturation. The pre-stress measurement and transient current measurement results suggest that the surface traps with an extracted surface trap energy level of 0.13 eV are uniformly charged along the channel. Furthermore, a forward electron injection explanation is proposed and confirmed by TCAD simulation.

We theoretically investigate the hybridization of the elemental surface plasmons in umbrella-shape plasmonic nanostructures and experimentally demonstrate the implementation of plasmonic multicolor metasurfaces as well as their application in colorimetric sensing. The three-dimension metallic umbrella arrays consist of a periodic canopy-capped-nanopillars with metal-coated sidewall and a backplane metal-film to form vertical nanocavity of canopy and film. Plasmonic coupling and energy confinement in nanocavity induce a noticeably resonance narrowing of multispectral reflection. The metasurfaced nanostructures appeared in vibrant and tunable colors with broad gamut derived from color blending mechanism due to multiple, narrow-band resonances. Vivid colors varied from red, yellow, green, blue to violet are easily achieved. It is also shown that such plasmonic metasurfaces can work as the feasible and real-time colorimetric refractive index sensor by measuring the distinct color variation to glucose concentration changes. Our sensor scheme shows its spectral sensitivity in the periodic umbrella array with respect to the refractive index change to be 242.5 nm/RIU with a figure of merit of 7.3. Furthermore, a refractive index resolution of colorimetric sensing up to 0.025 RIU has been accomplished.

In this letter, a plasma-free etch stop structure is developed for GaN HEMT toward enhancement-mode operation. The self-terminated precision gate recess is realized by inserting a thin AlN/GaN bilayer in the AlGaN barrier layer. The gate recess is stopped automatically at the GaN insertion layer after high-temperature oxidation and wet etch, leaving a thin AlGaN barrier to maintain a quantum well channel that is normally pinched off. With addition of an Al2O3 gate dielectric, quasi normally OFF GaN MOSHEMTs have been fabricated with high threshold uniformity and low ON-resistance comparable with the normally ON devices on the same wafer. A high channel mobility of 1400 cm $^{2}/\textrm {V}\cdot \textrm {s}$ was obtained due to the preservation of the high electron mobility in the quantum-well channel under the gate.

Due to the outstanding performance of resistance random access memory (RRAM) in the memory field, the study of resistive switching (RS) phenomena has become extremely noticeable in the recent years. The mechanism of metal conductive filamentary RRAM is already clear, but the conditions of the RS are still unclear. Therefore, this paper aims to explore the conditions for the occurrence of resistive, using a new RS structure called Electrolyte-Oxide-Semiconductor (EOS). This structure is based on the formation of metal conductive filament and exhibits the unipolar switching characteristics. Due to the formation or rupture of the conductive filaments, this device exhibits different resistance states. A series model of electrolyte and conductive filaments is used to explain the IV curve of this device. Compared with the device using a metal active electrode, the active electrode of this device is originally ionized. Therefore, it would be a better tool to explore the mechanism of ion migration and the formation of conductive filaments. Materials screening of metal in RRAM would also be more efficient.

We propose a polarization-controlled bifunctional metasurface composed of arrayed trapezoidal nanoantennas. Under orthogonal-polarized incidence, different types of gap-surface plasmons are generated, regulating the intensity and phase, respectively. Thus, structural color printing and beam deflection functions are achieved on a miniaturized chip. The color printing function works from 400 to 800 nm, exhibiting a subwavelength-scale chromatic image with a broad gamut. The beam deflection function works from 360 to 540 nm, mapping light to the first diffraction order with the anomalous angle from 40.4° to 76.6°. The proposed bifunctional metasurface could serve as a key component in integrated optics systems and will find many other wide-ranging applications in optical and biological areas.

An Electrolyte-Oxide Semiconductor (EOS) structure employing electrolyte top electrode is presented in this paper. This structure is based on the metal conductive filament mechanism and exhibits bipolar resistive switching. Due to the formation or rupture of the conductive filaments, this device exhibits different resistance states. The operating voltage of EOS is in the range of −1V~1V with 0.1mol/L CuSO 4 solution, and the performance is relatively stable under repeated voltage cycles. This device embodies the series characteristics of the electrolyte and conductive filaments. A series model of electrolyte and conductive filaments is used to explain the current-voltage (I-V) relationship of this device. Comparing with the active electrode of metal, this device doesn’t require metal ionization processing. Therefore, it would be a better tool to explore the mechanism of ion migration and the formation of conductive filaments. The metal materials screening in RRAM would also be more efficient.

The temperature dependence of current collapse (CC) in AlGaN/GaN high-electron mobility transistors on silicon substrate is studied in this paper. Devices without and with Si3N4 passivation are used to investigate the behavior of surface- and buffer-induced CC, respectively. It is found that the degree of surface-induced CC in unpassivated devices has a weak temperature dependence, which is induced by the cancelling out between enhanced carrier injection based on surface hopping and enhanced emission when the temperature is increased. On the other hand, the degree of buffer-induced CC in the Si3N4 passivated devices is reduced at higher temperature since the energy of hot electrons is reduced due to the phonon scattering and the trapping of hot electrons in the buffer is mitigated. Temperature-dependent transient measurement is also carried out to investigate the recovery process for these two type of CC. Two types of trap levels are identified in the unpassivated and Si3N4 passivated devices, respectively. The trap level $E1$ with an activation energy of 0.08 eV is supposed to be related to the surface trapping, while $E2$ with an activation energy of 0.22 eV is located in the buffer layer.

We report a type of plasmonic palettes formed by an array of cross-shaped cavity nanostructures for multicolor image generation. And the nanocross-shaped antennas actually act as plasmonic cavity resonators and could realize surface plasmon excitation, resonating and coupling, which can impart intriguing optical properties. These features enable the nanotexted metasurfaces to implement full-gamut palettes through spatially color blending and continuously tunable coloration. We also employ the dark-field scattering and near-field scanning analysis to experimentally study the circularly nanoscale optical scattering in the new cavity-nanopin photonic metasurfaces resulting from aperiodic Fibonacci spiral arrangements.

This work reports a kilovolt and low current collapse normally-off GaN MOSHEMT on silicon substrate. The device with a drift length of 3 μm features a threshold voltage of 1.7 V and an output current of 430 mA/mm at 8 V gate bias. The off-state breakdown voltage (BV) is as high as 1021 V (800 V) defined at a drain leakage criterion of 10 μA/mm with floating (grounded) substrate. The corresponding breakdown electric field is 3.4 MV/cm and the Baliga's figure-of-merit (BFOM) is 1.6 GW/cm2. A small degradation of the dynamic on-resistance (R on, d ) about 30% is observed with a short pulse width of 500 ns and a quiescent drain bias of 60 V. The record value is supposed to benefit from the intrinsic step-graded field plate, high quality LPCVD SI3N4 passivation and material optimization of 4.5 μm thick epitaxial layer.

We present a novel stereo metamaterial composed of three-dimensional (3D) gold split ring resonators (SRRs) in array to realize considerable optical modulation. The metamaterial is fabricated by a general 3D nanofabrication technique of focused ion beam stress induced deformation (FIB-SID). Such technique is a simple-step process, allowing programmable and accurate nanoscale origami on various metal and dielectric thin films. Theoretical and experimental results manifest that the metamaterial device with the 3D SRRs as meta-atoms has reflection minimum and abrupt phase change at 5.3 μm. In addition, to demonstrate the power of FIB-SID, other 3D meta-atoms with complex geometry such as sine shape, fold-line, and swiss-roll are also produced, proving FIB-SID an ideal tool to construct various 3D optical nanostructures and devices, including plamonic antennas, metamaterials and metasurfaces.

A novel simple approach to extract parasitic source and drain resistances of high electron mobility transistors (HEMT) is presented. This method could obtain the parasitic resistances by determining the portion of channel resistance involved in the measured end-resistance based on the identification of the channel position corresponding to the measured floating-gate voltage with the floating-gate, drain- and source-current-injection configurations on a single device. The technique is demonstrated on AlGaN/GaN HEMTs. It is found that the ratio of the channel resistance involving in the end-resistance to the total channel resistance approaches to a constant independent on the gate length, which could simplify the practical application of this novel method. The experimental results show that the source and drain resistances extracted by this method coincide with series resistance extracted by traditional Transmission Line Model (TLM) measurement.

In this paper, we report the device performance of a high-voltage normally off ${\rm Al}_{2}{\rm O}_{3}/{\rm GaN}$ MOSFET on the Si substrate. Normally off operation is obtained by multiple cycles of ${\rm O}_{2}$ plasma oxidation and wet oxide-removal gate recess process. The recessed normally off GaN MOSFET with 3 $\mu{\rm m}$ gate-drain distance exhibits a maximum drain current of 585 mA/mm at 9 V gate bias. The threshold voltage of the MOSFET is 2.8 V with a standard derivation of 0.2 V on the sample with an area of 2 $\,\times\,$ 2 ${\rm cm}^{2}$ . The gate leakage current is below $10^{-6}~{\rm mA}/{\rm mm}$ during the whole gate swing up to 9 V and the ${\rm I}_{{\scriptstyle{\rm ON}}}/{\rm I}_{{\scriptstyle {\rm OFF}}}$ ratio is larger than $10^{9}$ , indicating the good quality of ${\rm Al}_{2}{\rm O}_{3}$ gate insulator. The MOSFET with 10 $\mu{\rm m}$ gate-drain distance shows a three terminal OFF-state breakdown voltage (BV) of 967 V at zero gate-source bias with a drain leakage current criterion of 1 $\mu{\rm A}/{\rm mm}$ . The specific ON-resistance $(R_{{{\scriptstyle {\rm ON}}},{\rm SP}})$ of the device is 1.6 ${\rm m}\Omega\,{\cdot}\,{\rm cm}^{2}$ and the power figure of merit $({\rm BV}^{2}/R_{{{\scriptstyle {\rm ON}}},{\rm SP}})$ is 584 ${\rm MW}/{\rm cm}^{2}$ .

3D structures with characteristic lengths ranging from nanometer to micrometer scale often exhibit extraordinary optical properties, and have been becoming an extensively explored field for building new generation nanophotonic devices. Albeit a few methods have been developed for fabricating 3D optical structures, constructing 3D structures with nanometer accuracy, diversified materials, and perfect morphology is an extremely challenging task. This study presents a general 3D nanofabrication technique, the focused ion beam stress induced deformation process, which allows a programmable and accurate bidirectional folding (-70°-+90°) of various metal and dielectric thin films. Using this method, 3D helical optical antennas with different handedness, improved surface smoothness, and tunable geometries are fabricated, and the strong optical rotation effects of single helical antennas are demonstrated.

In this paper, AlGaN/GaN HEMTs with different device dimensions were designed and fabricated to investigate the relationship between off-state breakdown voltage and gate-to-drain spacing. It is found that the off-state breakdown voltage increases almost linearly with gate-to-drain spacing with the slop of about 46.8V/gm. Sentaurus software was used to find the physical mechanism of this phenomenon. By comparing the simulation results of devices with and without deep-level acceptor traps in GaN buffer layer, it is found that the deep-level acceptor traps in GaN buffer layer underneath the gate-to-drain channel could be occupied by hot electrons created in channel, which would extend the channel depletion region and then alleviate the maximum channel electric field. The theoretical and simulation analysis show that there is a positive correlation between the depletion length and the off-state gate-drain breakdown voltage. The simulation results show that with gate-to-drain spacing increasing, the negatively charged buffer traps region spread wider and the depletion region length becomes longer, playing a key role in the linear dependence of off-state gate-drain breakdown voltage on gate-to-drain spacing.

The charging current of surface traps in AlGaN/GaN high electron mobility transistors with a slot gate structure was investigated. A slot was formed in the middle of the gate metal via the lift-off process. Even though the slot gate only partially controlled the channel, the transient drain current gradually decreased toward a saturated off-state value with increasing measurement time after application of an off-state gate voltage. This indicated that the slot gate was still capable of turning off the entire channel. Analysis of the experimental results indicated that electrons were injected from the gate and trapped in the slot region, resulting in the depletion of the slot channel. An equivalent charging current by the surface traps on AlGaN could be inferred via the charge conservation principle. Temperature-dependent measurements of the charging current showed that it could be well fitted by the Poole-Frenkel conduction mechanism, with an extracted trap energy level of 0.129 eV.

A self-terminating gate recess wet etching technique with thermal oxidation of the AlGaN/GaN layer followed by etching in potassium hydroxide (KOH) solution was recently proposed by the present authors for normally-off AlGaN/GaN metal–oxide semiconductor field effect transistors (MOSFETs). In this present reported work, the oxidation process inside the AlGaN/GaN heterostructure involved in this technique was analysed using several material characterisation methods. The measurement results show that the concentration and depth of the O element distribution increase with increased thermal oxidation temperature. It is worth noting that after 650°C oxidation almost no O element could be found in the GaN layer and the O element mainly locates in the AlGaN layer with an obvious correlation between the distribution of Al and O elements, where the Al(Ga)-oxide was detected by X-ray photoelectron spectroscopy, which could be etched by 70°C KOH. Thus, self-terminating wet etching on the AlGaN/GaN material is achieved.

Based on our proposed self-terminating gate recess etching technique, normally-off recess-gated AlGaN/GaN MOSFET has been demonstrated with a novel method using GaN cap layer (CL) as recess mask, which, as a result, simplifies the device fabrication process and lowers the fabrication cost. The GaN CL is capable of acting as an effective recess mask for the gate recess process, which includes a thermal oxidation for 45 min at 650 degrees C followed by 4-min etching in potassium hydroxide (KOH) at 70 degrees C. After gate recess process, no obvious change is observed in terms of the surface morphology of the GaN CL, the contact resistance of the Ohmic contact formed directly on the GaN CL as well as the sheet resistance of the two-dimensional electron gas (2-DEG) channel layer under the GaN CL. The fabricated device exhibits a threshold voltage (V-th) as high as 5 V, a maximum drain current (I-dmax) of similar to 200 mA/mm, a high ON/OFF current ratio of similar to 10(10) together with a low forward gate leakage current of similar to 10(-5) mA/mm. Meanwhile, the OFF-state breakdown voltage (V-br) of the device with gate-drain distance of 6 mu m is 450 V.

Postgate annealing (PGA) in N2/O2 atmosphere at 300 °C for various annealing time is performed on enhancement mode AlGaN/GaN MOSFET fabricated using a self-terminating gate recess etching technique. After 45-min annealing, the device OFF-state leakage current decreases by more than two orders of magnitude and thus a low OFF-state leakage current of $\sim 10^{-13}$ A/mm is obtained at room temperature, resulting in an excellent ON/OFF current ratio of $\sim 10^{12}$ . At 250 °C, the device still exhibits a low OFF-state leakage current of $\sim 10^{-9}$ A/mm and high ON/OFF current ratio of $\sim 10^{8}$ . Meanwhile, a strong correlation between the OFF-state leakage current and mesa isolation current is observed as we change the annealing time: 1) the lower the mesa isolation current and 2) the lower the OFF-state leakage current and thus the higher the ON/OFF current ratio. It is the suppression of the mesa isolation current owing to the passivation of atomic layer deposition Al2O3 that leads to the improvement of the OFF-state leakage current and ON/OFF current ratio after PGA. Besides, the device shows no obvious change in terms of its threshold voltage and maximum drain current after PGA.

In this letter, we investigated the behaviors of surface- and buffer-induced current collapse in AlGaN/GaN high-electron mobility transistors (HEMTs) using a soft-switched pulsed \(I-V\) measurement with different quiescent bias points. It is found that the surface- and buffer-related current collapse have different relationship with the gate and drain biases ( V \(_{\mathrm { {\!GS0,}}}\) V \(_{\mathrm { {\!DS0}}}\) ) during quiescent bias stress. The surface-induced current collapse in devices without passivation monotonically increases with the negative V \(_{\mathrm { {\!GS0}}}\) , suggesting that an electron injection to the surface from gate leakage is the dominant mechanism and the Si3N4 passivation could effectively eliminate such current collapse. The buffer-induced current collapse in devices with intentionally carbon-doped buffer layer exhibits a different relationship with V \(_{\mathrm { {\!GS0}}}\) after surface passivation. The buffer-related current collapse shows a bell-shaped behavior with V \(_{\mathrm { {\!GS0}}}\) , suggesting that a hot electron trapping in the buffer is the dominant mechanism. The soft-switched pulsed \(I-V\) measurement provides an effective method to distinguish between the surface- and buffer-related current collapse in group III-nitride HEMTs.

Recently we have demonstrated the thermal oxidized TiO2/Al2O3 as gate dielectric for AlGaN/GaN metal-oxide-semiconductor high-electron-mobility-transistors (MOS-HEMTs) with enhanced breakdown voltage of 490 V compared with 60-100 V breakdown voltage for normal HEMTs. The characteristics of GaN-based MOS-HEMTs with TiO2/Al2O3 gate dielectrics are measured and simulated using Sentaurus Device Software. The significant improvement of breakdown voltage of MOS-HEMTs with TiO2/Al2O3 gate oxide was analyzed. It is shown that the effect of TiO2 on surface electric potential distribution results in the improvement of breakdown voltage. This work will represent a solid basis for improvement of breakdown voltage and enhancement of the device reliability.

A 1 × 8 MEMS (micro-electronic mechanical system(-based flexible integrated optical device with optical switch and variable optical power attenuating functions has been proposed to optimally serve optical fiber-based networking applications.The device manipulates the light with binary-slope mirrors driven by compound electrostatic actuator. The optical models for switching and attenuating are investigated, respectively. Performances of the fabricated devices assembled with ball-lensed fibers are measured and discussed. Measurements reveal that the insertion loss of the device is less than 2.8 dB and the controllable attenuation range is up to 39 dB, respectively. Moreover, polarization-dependent loss is less than 0.8 dB in the whole attenuation range. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:231–236, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27293

In view of the toxic potential of a bioweapon threat, rapid visual recognition and sensing of ricin has been of considerable interest while remaining a challenging task up to date. In this study, a gold nanopin-based colorimetric sensor is developed realizing a multicolor variation for ricin qualitative recognition and analysis. It is revealed that such plasmonic metasurfaces based on nanopin-cavity resonator exhibit reflective color appearance, due to the excitation of standing-wave resonances of narrow bandwidth in visible region. This clear color variation is a consequence of the reflective color mixing defined by different resonant wavelengths. In addition, the colored metasurfaces appear sharp color difference in a narrow refractive index range, which makes them especially well-suited for sensing applications. Therefore, this antibody-functionalized nanopin-cavity biosensor features high sensitivity and fast response, allowing for visual quantitative ricin detection within the range of 10–120 ng mL−1 (0.15 × 10−9–1.8 × 10−9 m), a limit of detection of 10 ng mL−1, and the typical measurement time of less than 10 min. The on-chip integration of such nanopin metasurfaces to portable colorimetric microfluidic device may be envisaged for the quantitative studies of a variety of biochemical molecules.

We present an electromechanically controlled metasurface, consisting of an array of three-dimensional nano-split-rings. By applying voltage, its resonance can be tuned from 5.5 μm to 10μm, with 110% reflectivity change and enormous optical rotations.

We report a new arrayed plasmonic nanopixels capable of producing a continuum of colors. The pixels are nanohelm-shaped metal-dielectric-metal resonators and could efficiently cause surface-plasmon coupling, resonances and hybridization to propagate enhanced electromagnetic energy to far-field and thus achieve highly sophisticated color generation. Moreover, the surface-plasmon resonance frequency, intensity and related reflection spectrum line-width are significantly dependent upon incident angle of light, also resulting in a broad color tuning. Such plasmonic nanohelm pixels could map white light to splendid colors and be thus used to engineer vivid color display beyond the diffraction limit.

We design and fabricate a randomly controlled metamaterial modulator (RCMM) which can dynamically tune reflection in infrared region. The RCMM is a metamaterial-cell array, and each cell is comprised of a raw of S-shaped cantilevers called meta-molecules. The devices were produced by using focused-ion-beam etching and deposition on suspended metal-dielectric-metal membranes. Word/bit-line method is introduced to precisely control the thermoelectricity-caused bending of every individual meta-cell in the RCMM. A 2×2 RCMM exhibits 16 different states, which represents different operation conditions of the device, resulting in the shift of the reflection spectrum. The end portion of a meta-cell with 20 μm-long meta-molecules can be raised up to 2μm, and the corresponding optical reflection of the device will be modulated by up to 40%.

This paper presents a silicon cantilever sensor based on n-type metal-oxide-semiconductor transistor for chemical sensing and analysis using the chemisorption-induced surface stress sensing principle. The cantilever is along the crystal orientation of the (100) silicon, and the transistor channel is parallel to as well as located at the rear part of the cantilever to obtain high stress sensitivity. The gold film deposited on the bottom surfaces of cantilevers is chemically functionalized with a self-assembled monolayer of 4-mercaptobenzoic acid via the Au-SH covalent bonding. The vapor phase chemical sensing experiments with acetone, ethanol, nitroethane and water vapor as targets are performed. The observed response differentiation implies that the molecular interaction mechanisms between different chemical molecules are different.