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We report high-performance homojunction GaN avalanche photodiodes (APDs) grown on a low-defect GaN substrate and fabricated with an ion-implantation isolation method. High-quality p-i-n GaN layers were grown using metalorganic chemical vapor deposition (MOCVD), and an effective device isolation method using optimized nitrogen ion implantation was developed to provide significant leakage current suppression on the etched mesa sidewalls. Fabricated GaN APDs showed an ultralow dark current density 106 at a reverse bias of 71.5 V under deep-ultraviolet (DUV) illumination at $\lambda =280$ nm. A further temperature-dependent study of the dark current indicated that a trap-assisted tunneling process predominates the leakage current component that also contributes to the carrier multiplication process near the avalanching breakdown.

Front-illuminated GaN-based p-i-n ultraviolet (UV) avalanche photodiodes (APDs) were grown by metalorganic chemical vapor deposition on a free-standing GaN substrate. X-ray diffraction measurements confirm the high crystal quality of the grown structure. Atomic force microscopy measurements show well-developed atomic step-flow morphologies with sub-nanometer root-mean-square surface roughness for scan sizes of 1 × 1 $${\mu m}^{2}$$ and 5 × 5 $${\mu m}^{2}$$ . Mesa etching is one of the major challenges in the fabrication of these devices. Inductively coupled plasma mesa etching creates damage which leads to an increase of the dark current, premature device breakdown, and noise. Front-illuminated GaN-based UV APDs were fabricated using ion-implantation isolation to reduce the sidewall leakage current and improve reliability. The devices show a very low reverse-bias low leakage current density of below 10−10 A/cm2 up to − 35 V. A maximum multiplication gain of 1 × 106 under λ = 355 nm illumination was demonstrated (device limited to a current density of 10 A/cm2). The spectral responsivity of an 82-μm-diameter APD shows a peak responsivity of 1.05 A/W under UV illumination at λ = 375 nm at the avalanche-break-down voltage.

We determined the optical and electrical characteristics of GaN-based, ultraviolet micro light emitting diodes (microLEDs). Such microLEDs are essential to next-generation high-resolution micro-displays. Square-shaped microLEDs of different sizes (side lengths: 5-50 μm) were designed. The peak emission wavelength of these devices shifted

Although recent studies have revealed the occurrence of dimethylated arsenicals, little is known about their behavior in environment. This study investigates the adsorption behavior of dimethylarsinic acid (DMA

Gallium-nitride and related materials have become suitable semiconductor platforms for next-generation power electronic devices. This materials system leverages unique properties of wider energy bandgap, higher carrier mobility, and ultrashort carrier recombination lifetime to achieve much improved bipolar switching characteristics when compared to Si- or SiC-based counterparts. In recent years, vertical GaN switches such as PIN rectifiers, junction barrier Schottky (JBS) diodes, and junction field-effect transistors (JFETs) have been actively sought in hopes to achieve low-loss device performance at kilo-volt (kV) operation range. Over the years, significant progresses in low-defect-density substrate preparation and high-quality epitaxial growth using metalorganic chemical vapor deposition (MOCVD) reactors, along with robust fabrication processing techniques helped achieve many impressive GaN-based power device demonstrations. As vertical GaN devices operate at the kV ranges, proper engineering of the electric field is a key to exploiting the ultimate device performance. To this end, several device-level design strategies such as field plates, beveled sidewalls, and junction termination extensions have been studied. In the context of realizing a planar device topology in GaN materials systems, ion implantation into GaN materials systems offer new opportunities to provide precise control of inhomogeneous carrier distribution for the creation of versatile doping profiles and geometries such as selectively doped regions or highly resistive regions at arbitrary locations in GaN devices. In this presentation, we will discuss a study of the floating guard rings (FGRs) designs that utilize a nitrogen-ion implantation isolation technique. The designs can effectively mitigate the electric field crowding effect and enable robust avalanching capability in vertical GaN PIN rectifiers. Using 1.2-kV GaN PIN rectifiers as design examples, a set of numerical simulation study was performed using a TCAD-Sentaurus software package. The results showed that a closely packed FGRs with a gap of adjacent FGRs in the range of 1-3 microns, a width of the FGRs in the range of 4 to 6 microns, along with a variable-width FGR design toward the outer rings can effectively suppress the peak electric field distribution at edge of the main junction and the surface of junction termination extensions. For example, consider a GaN PIN rectifier structure with a drift layer that consists of 6 microns of unintentionally doped (UID) GaN (n= 5×1015 cm-3) and a 4-micron-thick lightly doped GaN (n = 2×1016 cm-3). The FGR structures can be formed by creating highly resistive GaN regions in the epitaxially grown p-GaN layer using an ion implantation technique. In this design, five of the FGRs have 1-micron-wide undoped GaN and a 6-micron-wide p-GaN region in the p-GaN layer. The outermost FGR (the 6th FGR) is designed to have a 2-micron-wide undoped GaN and 5-micron-wide p-GaN extension. The resulting field distribution showed an effective suppression of the field crowding effect at the edge of the main junction (Figure 1(a)) and a smooth electric potential drop along the lateral direction can be achieved (Figure 1(b)). As shown in Figure 2, the electric field distribution at the vertical junction was plotted for two FGR designs with and without a metal plate on top of the p-GaN region. The electric field distribution of the main junction was also plotted for comparison. It is clearly shown that the peak electric field at the vertical junction is effectively reduced with the metal-plated FGRs by >10% when compared to a non-plated FGRs. Reduced peak electric field in typically high-electric-field regions can safeguard the device from experiencing premature breakdown for robust switching operations. We found that 6 FGRs with a total lateral extension of FGR designs were implemented in GaN PIN rectifiers for experimental validation. The fabrication processing of vertical GaN PIN rectifiers was grown on n-type free-standing or bulk GaN substrates using a MOCVD reactor. The cathode was formed by depositing Ti/Al-based ohmic contact on the backside of the wafer, and the anode electrode was formed on the topside p-type GaN layer. Nitrogen-ion implantation was employed to create FGR structures and served as a mean of inter-device isolation. Detailed design strategies of the FGRs, device processing and device characterizations will be discussed in the conference. Figure 1

This article reports a nanoporous silicon (Si) thin-film-based high-performance and low-power hydrogen (H2) sensor fabricated by metal-assisted chemical etching (MaCE). The nanoporous Si thin film ...

Developing high response hydrogen sensors manufacturable in a large scale is desirable in hydrogen industry. In this study, a chromium oxidation-based nanogap formation process was developed to fabricate a hydrogen switch with suspended palladium and gold films having a tens of nanometer-sized gap. The nanogap was formed by using oxidized chromium as a self-alignment shadow mask. The hydrogen switch operates by the principle of volume expansion of palladium upon exposure to the hydrogen gas and the current reading by closing of a nanogap formed between suspended palladium and gold films. Further improvement of the sensor performance was achieved by optimizing the design parameters such as suspended film lengths and thicknesses. The fabricated palladium nanogap hydrogen sensor showed an ultrahigh sensitivity of Δ

We report the development of high-performance GaN ultraviolet avalanche photodiode technology with uniform dark current characteristics. These devices exhibited high-avalanche-gain (>105) in linear-mode operation in 6x6 array form with pixel sizes up to 70x70 μm2.

Front-illuminated p-i-n GaN-based ultraviolet (UV) avalanche photodiodes (APDs) were grown by metalorganic chemical vapor deposition (MOCVD) on 25 mm dia. bulk Ammono® n-GaN substrate having a low etch pit density (EPD) less than 5 × 104 [cm-2] and processed into 6×6 APD arrays. The devices employed N-ion implantation to achieve sidewall passivation. Evaluation of these 6×6 arrays will help to confirm the uniformity of the epitaxial materials and device processing. The maximum avalanche gain reached ~ 3×105 at the breakdown (current limited). The dark current density was 10-9 A/cm2 at reverse bias up to -20 V and the APDs exhibited a reverse breakdown voltage of 81 ± 1 V for all 36 devices without any leaky devices, confirming a high uniformity of the growth and fabrication processes.

A top-illuminated deep-ultraviolet Al0.6Ga0.4N p-i-n avalanche photodiode (APD) structure was designed and grown by metalorganic chemical vapor deposition on an AlN bulk substrate and on two different quality AlN/sapphire templates, and APDs were fabricated and tested. The APD devices with a circular diameter of 20 μm have demonstrated a distinctive reverse-bias breakdown behavior. The reverse breakdown voltage of the APDs is approximately −140 V, which corresponds to a breakdown electric field of 6–6.2 MV/cm for the Al0.6Ga0.4N material as estimated by Silvaco TCAD simulation. The APDs grown on the AlN bulk substrate show the lowest leakage current density of −8 A/cm2 (at low reverse bias) compared to that of the devices grown on the AlN templates. From the photocurrent measurement, a maximum gain (current limited) of 1.2 × 104 is calculated. The average temperature coefficients of the breakdown voltage are negative for APD devices fabricated from both the AlN bulk substrate and the AlN templates, but these data show that the coefficient is the least negative for the APD devices grown on the low-dislocation-density AlN bulk substrate.

We report high-performance GaN avalanche photodiodes using a novel ion implanted isolation technique. The devices showed low dark current, an equivalent noise power of < 5×10−16 WHz−0.5 and avalanche gain of 7×105 at λ = 280 nm.

This paper reports zinc oxide (ZnO)-coated piezoelectret polypropylene (PP) microfibers with a structure of two opposite arc-shaped braces for enhanced mechanical energy harvesting. The ZnO film was coated onto PP microfibers via magnetron sputtering to form a ZnO/PP compound structure. Triboelectric Nanogenerator (TENG) based on ZnO/PP microfiber compound film was carefully designed with two opposite arc-shaped braces. The results of this study demonstrated that the mechanical energy collection efficiency of TENG based on piezoelectret PP microfiber was greatly enhanced by the coated ZnO and high-voltage corona charging method. We found that, with the step-increased distance of traveling for the movable carbon black electrode, an electrical power with an approximately quadratic function of distance was generated by this mechanical-electrical energy conversion, because more PP microfibers were connected to the electrode. Further, with a full contact condition, the peak of the generated voltage, current, and charges based on the ZnO/PP microfibers by this mechanical-electrical energy conversion with 1 m/s

In this paper, we report a flexible comb electrode triboelectric–electret coupling nanogenerator using a separated friction microfiber object for self-powered position, motion direction and acceleration tracking sensing and its energy harvesting. The power was generated from the coupling of the electrostatic and triboelectric effects among a separated triboelectric object, a polytetrafluoroethylene (PTFE) film and interdigital electrodes. Under an acceleration of 1 m s−2, we found that the corona charged PTFE film with a sliding motion reached ∼3 times more short-circuit current (Isc) and ∼6 times more open-circuit voltage (Voc) than the PTFE film without corona charges, respectively. The device can be a good self-powered acceleration tracking sensor, where the reasons were a stability voltage out during the process of separated components frictions and a numerical relationship of Isc with increased accelerations. Moreover, potential applications using several different separated materials such as a bulk of carbon microfibers, a finger sliding with a nylon glove, and water microdrops were used to show the energy harvesting of various friction materials and their effective contact area. It was found that a much larger contact area “finger” sliding showed a Voc of ∼45 V, whereas a smaller contact area “microdrop” sliding on the prototype presented a maximum peak Voc of ∼1.8 V.

Flexible gas sensing is attracting more attention with the development of machine learning and Internet of Things (IoT). Herein, we report flexible and foldable high-performance hydrogen (H2) sensor on all textiles substrate-fabricated by inkjet–printing of reduced graphene oxide (rGO) and its application to wearable environmental sensing. The inkjet-printing process provides the advantages of the compatibility with various substrates, the capability of non-contact patterning and cost-effectiveness. The sensing mechanism is based on the catalytic effect of palladium (Pd) nanoparticles (NPs) on the wide bandgap rGO, which allows facile adsorption and desorption of the nonpolar H2 molecules. The graphene textile gas sensor (GT-GS) demonstrates about six times higher sensing response than the graphene polyimide membrane gas sensor due to the large surface area of the textile substrate. An analysis of the temperature influence on the GT-GS shows better H2 gas response at room temperature than at high temperature (e.g., 120 °C). In addition, with the machine learning-enabled technology and triboelectric-textile to power IoT (temperature and humidity for gas calibration), H2 is well identified for wearable applications with a robust mechanical performance (e.g., flexibility and foldability).

Integration of heterogeneous sensing materials in microelectronic devices is essential to accomplish compact and highly integrated environmental sensors. For this purpose, a micro-patterning method of electrospun metal oxide nanofibers based on electrohydrodynamic (EHD) printing process was developed in this work. Several types of metal oxide (SnO_2, In_2O_3, WO_3 and NiO) nanofibers that were produced by electrospinning, fragmented into smaller pieces by ultrasonication, and dissolved in organic solvents were utilized as inks for the printing. Constant or pulsed wave bias consisting of base and jetting voltages were applied between a nozzle and a substrate to generate a jetting of nanofiber solutions. Several parameters for EHD printing such as pulse width, inner diameter of the nozzle, distance from the nozzle to the substrate, and stage speed, were optimized for accurate micro-patterning of electrospun nanofibers. By using optimized printing parameters, microscale patterns of electrospun nanofibers with a minimum diameter less than 50 μm could be realized. Gas sensors were fabricated by EHD printing on the microelectrodes and then used for the detection of toxic gases such as NO_2, CO and H_2S. Four kinds of metal oxides could detect down to 0.1 ppm of NO_2, 1 ppm of H_2S and 20 ppm of CO gases. Also, heterogeneous nanofiber gas sensor array was fabricated by the same printing method and could detect NO_2 using the sensor array platform with microheaters. Furthermore, microscale patterns of nanofibers by EHD printing could be applied to the suspended MEMS platform without any structural damage and this sensor array could detect NO_2 and H_2S gases with 20 mW power consumption.

Inspired by the turbinate structure in the olfaction system of a dog, a biomimetic artificial nose based on 3D porous laser-induced graphene (LIG) decorated with palladium (Pd) nanoparticles (NPs) has been developed for room-temperature hydrogen (H2) detection. A 3D porous biomimetic turbinate-like network of graphene was synthesized by simply irradiating an infrared laser beam onto a polyimide substrate, which could further be transferred onto another flexible substrate such as polyethylene terephthalate (PET) to broaden its application. The sensing mechanism is based on the catalytic effect of the Pd NPs on the crystal defect of the biomimetic LIG turbinate-like microstructure, which allows facile adsorption and desorption of the nonpolar H2 molecules. The sensor demonstrated an approximately linear sensing response to H2 concentration. Compared to chemical vapor-deposited (CVD) graphene-based gas sensors, the biomimetic turbinate-like microstructure LIG-gas sensor showed ∼1 time higher sensing performance with much simpler and lower-cost fabrication. Furthermore, to expand the potential applications of the biomimetic sensor, we modulated the resistance of the biomimetic LIG sensor by varying laser sweeping gaps and also demonstrated a well-transferred LIG layer onto transparent substrates. Moreover, the LIG sensor showed good mechanical flexibility and robustness for potential wearable and flexible device applications.

In this paper, we have developed photocatalytic metal oxide gas sensors integrated on µUV-LEDs for more efficient photon energy transfer. The sensing nanomaterials (ZnO nanowires or SnO 2 nanotubes) were synthesized on a transparent quartz substrate through sequential aqueous synthesis methods. The transparency of the quartz substrate allows the UV-LED to directly irradiate onto the sensing materials. Due to the proximity of the light source to the sensing materials, the irradiation efficiency was improved. Furthermore, by controlling each content of ZnO and SnO 2 , the absorption spectrum of the sensing nanomaterials was modulated for more efficient photon energy absorption of the gas sensing material. As a result, the sensing performance of SnO 2 -ZnO hybrid nanotubes for NO 2 gas showed higher sensitivity, fast response, and complete recovery in room temperature.

This paper reports half-pipe shaped palladium (Pd) nanotubes using nanofiber templates as hydrogen (H 2 ) sensor for the first time. Various methods to fabricate Pd nanostructure gas sensors were reported earlier, such as standard bottom-up method and lateral electrodeposition method. The realization of Pd nanostructure based on electrospun nanofibers offers a facile and cost effective way to fabricate high performance H 2 sensor. As a result, for 4nm thick half-pipe Pd nanotubes exhibited a high response (2.1%) to H 2 gas with 21s of response time (τ 0-80 ).

Intense efforts have been made to eliminate toxic volatile organic compounds (VOCs) in indoor environments, especially formaldehyde (FA). In this study, the removal performances of gaseous FA using two metal-organic frameworks, MOF-5 and UiO-66-NH2, and two covalent-organic polymers, CBAP-1 (EDA) and CBAP-1 (DETA), along with activated carbon as a conventional reference material, were evaluated. To assess the removal capacity of FA under near-ambient conditions, a series of adsorption experiments were conducted at its concentrations/partial pressures of both low (0.1–0.5 ppm/0.01–0.05 Pa) and high ranges (5–25 ppm/0.5–2.5 Pa). Among all tested materials at the high-pressure region ㅐ (e.g., at 2.5 ppm FA), a maximum adsorption capacity of 69.7 mg g−1 was recorded by UiO-66-NH2. Moreover, UiO-66-NH2 also displayed the best 10% breakthrough volume (BTV10) of 534 L g−1 (0.5 ppm FA) to 2963 L g−1 (0.1 ppm FA). In contrast, at the high concentration test (at 5, 10, and 25 ppm FA), the maximum BTV10 values were observed as: 137 (UiO-66-NH2), 144 (CBAP-1 (DETA)), and 36.8 L g−1 (CBAP-1 (EDA)), respectively. The Langmuir isotherm model was observed to be a better fit of the adsorption data than the Freundlich model under most of the tested conditions. The superiority of UiO-66-NH2 was attributed to the van der Waals interactions between the linkers (framework) and the hydrocarbon “tail” (FA) coupled with interactions between its open metal sites and the FA carbonyl groups. This study demonstrated the good potential of these advanced functional materials toward the practical removal of gaseous FA in indoor environments.

Recent light-enhanced metal oxide gas sensors are reviewed in this article. The basic mechanisms of a light-enhanced metal oxide gas sensor are discussed. Many literatures reveal that the standalone sensitivity and the response/recovery time enhancements enabled by the exposing light are not as high as the performance enhancement provided by external heating. Therefore, both optimal amount of external heating and exposed light intensity are necessary to increase the performance of these light-enhanced gas sensors. The development of highly light sensitive materials and structures is important to lower the overall power consumptions of the sensors.

High-performance and low-power flexible Schottky diode-based hydrogen sensor was developed. The sensor was fabricated by releasing Si nanomembrane (SiNM) and transferring onto a plastic substrate. After the transfer, palladium (Pd) and aluminum (Al) were selectively deposited as a sensing material and an electrode, respectively. The top-down fabrication process of flexible Pd/SiNM diode H2 sensor is facile compared to other existing bottom-up fabricated flexible gas sensors while showing excellent H2 sensitivity (ΔI/I0 > 700–0.5% H2 concentrations) and fast response time (τ10–90 = 22 s) at room temperature. In addition, selectivity, humidity, and mechanical tests verify that the sensor has excellent reliability and robustness under various environments. The operating power consumption of the sensor is only in the nanowatt range, which indicates its potential applications in low-power portable and wearable electronics.

A hydrogen (H2 ) gas sensor based on a silicon (Si) nanomesh structure decorated with palladium (Pd) nanoparticles is fabricated via polystyrene nanosphere lithography and top-down fabrication processes. The gas sensor shows dramatically improved H2 gas sensitivity compared with an Si thin film sensor without nanopatterns. Furthermore, a buffered oxide etchant treatment of the Si nanomesh structure results in an additional performance improvement. The final sensor device shows fast H2 response and high selectivity to H2 gas among other gases. The sensing performance is stable and shows repeatable responses in both dry and high humidity ambient environments. The sensor also shows high stability without noticeable performance degradation after one month. This approach allows the facile fabrication of high performance H2 sensors via a cost-effective, complementary metal-oxide-semiconductor (CMOS) compatible, and scalable nanopatterning method.

High reflection Si/SiO 2 quarter-wavelength Distributed Bragg Reflector (DBR) is reported. The structure is composed of alternate single crystal Si and thermally-grown SiO2. Si Nanomembrane (Si NM) transfer and subsequent thermal oxidation achieves high reflection throughout the broad IR spectrum.

In this paper, the authors report resonant cavity (RC) metal-semiconductor-metal (MSM) germanium nanomembrane (Ge NM) photodetectors via transfer printing. The dislocation-free Ge NM layer was transferred onto an ultrathin Si NM/SiO2 distributed Bragg reflector. As a result, a low dark current density of 1 × 10−9 A/μm2 and a quantum efficiency of 17.3% at 1.55 μm, which is twice larger than the quantum efficiency without a bottom mirror, were measured from the transferred RC MSM Ge photodetector. The enhancement of the quantum efficiency is verified by simulation.

A distributed Bragg reflector (DBR) consisting of a single-crystal Si nanomembrane (NM) layer formed by the transfer printing technique on top of an evaporated amorphous Si (a-Si)/SiO2 DBR structure was demonstrated. The reflectivity of different DBR structures/pairs is measured and verified it by the simulation. An improved surface roughness of the top layer by employing a Si NM suggests that the smoother single crystalline surface not only minimizes light scattering loss but also can be an epitaxial template layer for subsequent Si growth without contributing any strain. The results indicate a simple pathway toward achieving high performance Si/SiO2 DBRs employing Si NM as a top layer. This method could also lead to the fabrication of large-area, high performance NM based DBRs at low cost with high throughput.

Metal-oxide-semiconductor (MOS) device is the basic building block for field effect transistors (FET). The majority of thin-film transistors (TFTs) are FETs. When MOSFET are mechanically bent, the MOS structure will be inevitably subject to mechanical strain. In this paper, flexible MOS devices using single crystalline Silicon (Si) and Germanium (Ge) nanomembranes (NM) with SiO2, SiO, and Al2O3 dielectric layers are fabricated on a plastic substrate. The relationships between semiconductor nanomembranes and various oxide materials are carefully investigated under tensile/compressive strain. The flatband voltage, threshold voltage, and effective charge density in various MOS combinations revealed that Si NM−SiO2 configuration shows the best interface charge behavior, while Ge NM−Al2O3 shows the worst. This investigation of flexible MOS devices can help us understand the impact of charges in the active region of the flexible TFTs and capacitance changes under the tensile/compressive strains on the change in electrical characteristics in flexible NM based TFTs.

A method of creating tunable and programmable biaxial compressive strain in silicon nanomembranes (Si NMs) transferred onto a Kapton® HN polyimide film has been demonstrated. The programmable biaxial compressive strain (up to 0.54%) was generated utilizing a unique thermal property exhibited by the Kapton HN film, namely, it shrinks from its original size when exposed to elevated temperatures. The correlation between the strain and the annealing temperature was carefully investigated using Raman spectroscopy and high resolution X-ray diffraction. It was found that various amounts of compressive strains can be obtained by controlling the thermal annealing temperatures. In addition, a numerical model was used to evaluate the strain distribution in the Si NM. This technique provides a viable approach to forming in-plane compressive strain in NMs and offers a practical platform for further studies in strain engineering.