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We present a vertically aligned multiwall carbon nanotube (VA-MWCNT) based humidity sensor with low cost and simple fabrication method compatible with mass production. The VA-MWCNT is well aligned and possesses an interconnect topography. Initially, the VA-MWCNT showed superhydrophobic characteristics, which are not suitable for humidity sensing. Therefore, to protect the topography of the material and make the surface more hydrophilic, a low cost and easy handling treatment method was adopted with O2 plasma surface functionalization. The fabricated device showed good linearity at the range of 40% RH–90% RH, low hysteresis ( H = 1.5%RH), fast response (3.3 s), high stability ( Cv Rr _max 0.6% ), demonstrating excellent performance in both static and dynamic measurements. The high performance and low-cost fabrication methods make the VA-MWCNT-based sensor a promising candidate for the application of humidity sensing. Moreover, benefiting from the simple fabrication process, it can be easily extended to resistive sensing fusion with the integration of the temperature sensor and flow sensor for more complex applications.

In this work, we report on the fabrication and characterization of a miniature GaN chip with a size of $1.1\times 1.1\times0.21$ mm3 for surface roughness measurement. Unlike conventional optical metrology that requires external components for the optical coupling between light source and detector, a monolithic GaN-on-sapphire chip fabricated through wafer-scale processes provides light emission and photodetection functions and is capable of sensing a range of average surface roughness from 0.025 to $1.6~\mu \text{m}$ . The sensing performances are compared with fiber-optic metrology, and the ability to real-time monitor the roughness of the moving surface is demonstrated. The novel chip offers the advantage of non-contact and non-destructive, compact structure, simple operation, and in situ measurement capability, which is highly valuable for various practical and industrial surface metrology applications.

This paper presented the principle, design, fabrication, and characterization of Molecular Electronic Transducer (MET) single-axis tilt sensors. The proposed sensor has a circular channel deposited with symmetric concentric electrode pairs and half-filled with a liquid electrolyte. Such inclination-dependent liquid level is detected by the differential current from the electrochemical reaction at the two symmetric concentric electrode pairs. In order to reduce the form factor, an organic solvent is added to the aqueous electrolyte to lower the surface tension force. Limited by the liquid motion and reaction rate, the sensors are capable to measure tilt angle under static conditions only. From the tilt angle stepping experiments, the performance of the proposed tilt sensors is characterized and compared. The sensors’ response time is dominated by the electrochemical equilibrium arrival time. They can achieve a low discrimination threshold of 0.016° and a short time constant of 0584s within the ±85° measurement range with a power consumption of less than 0.1mW. Compared to previously reported MET tilt sensors with a parallel swirl electrode configuration, the signal-to-noise ratio and response time are improved significantly.

In this work, we report on the monolithic integrated photodetectors (PDs) in InGaN/GaN light-emitting diodes (LEDs) to monitor the fluctuation of LED intensity over time. The effect of PD position on the light-receiving performance is studied through ray-trace simulation. Three LED-PD devices with different PD positions are fabricated via standard microfabrication processes, and a series of experimental measurements are performed to examine their performances. The design of the PD located in the center of the LED exhibits a 58% increase in the detected photocurrent and has less influence on the emission profile of the LED, compared with the PD formed at the edge. The white light emission is demonstrated by encapsulating the LED-PD device with yellow phosphor and the on-chip PD shows highly similar sensing behaviors as the external Si-based photodiode. Compared with the traditional off-chip detection scheme, the optimal monolithic PD design can provide comparable sensing capabilities, higher compactness, insensitivity to ambient lighting, and less influence on the LED emission profile, making it highly suitable for real-time monitoring of LED intensity variations in various practical and scientific lighting applications.

This article reports on a design journey for Sc x Al1 – N -based dual-mode hybrid quasi-surface and bulk acoustic wave (quasi-SAW/BAW) resonators. Four types of acoustic excitation configurations are proposed in these designs based on Sc x Al1– x N/6H-SiC stack architectures. The influence of Sc x Al1– x N materials, film thickness, and device configurations on the performance is investigated. After design optimization, the final dual-mode hybrid quasi-SAW/BAW resonators comprise of top electrode, partially etched Sc x Al1– x N pillars, and bottom electrode, which are stacked on a 6H-SiC substrate. The coupled quasi-SAW and BAW excited in the hybrid resonators enhance the resonance for both quasi-Rayleigh and quasi-Sezawa modes which results in a high effective coupling coefficient ( K eff2) and phase velocity ( ${v}$ ). Simulation results show that the optimized hybrid quasi-SAW/BAW resonator based on Mo/Sc0.4 Al0.6N/Mo/6H-SiC configuration with SiO2 filling the grooves has a remarkable K eff2 value of 14.55% and a high ${v}$ above 7500 m/s, which make this kind of dual-mode hybrid quasi-SAW/BAW resonators have great potential in wideband and high-frequency applications.

As an optical material, Y2O3 transparent ceramics are desirable for application as laser host materials. However, it is difficult to sinter and dense of Y2O3 hinders the preparation of high-quality optical ceramics via traditional processes. In this work, we use La2O3 as a sintering aid for fabricating high-transparency Y2O3 ceramics using a vacuum sintering process. It is demonstrated that the in-line optical transmittance of 15.0 at% La-doped Y2O3 at a wavelength of 1100 nm achieves a transmittance of 81.2%. A sintering kinetics analysis reveals that a grain-boundary-diffusion-controlled mechanism dominates the faster densification at high La3+ concentrations. It is also shown that both the mechanical and thermal properties of Y2O3 transparent ceramics are significantly improved upon the increase of La2O3 sintering additives. The results indicate that a La-doped Y2O3 transparent ceramic is a promising candidate for a laser host material.

Up-conversion (UC) luminescence properties of Nd:Y2O3transparent ceramics are investigated. It is shown that, under a laser excitation of 808 nm, the samples exhibit significant UC emission. With an increase in the Nd3+ concentration, the intensity of green emission decreases, while the intensity of red emission first increases and then decreases. In particular, both green and red UC emissions are found to be two-photon processes, but with different quantum transition mechanisms. In addition, the dependence of UC emission intensity on the pumping power is studied. Our studies indicate that Nd:Y2O3 transparent ceramics are promising candidates as short-wavelength UC laser material.

Harvesting the potential performance of GaN-based devices in terms of the areal power density and reliability, relies on the efficiency of their thermal management. Integration of extremely high thermal conductivity Single-crystalline CVD-diamond serves as an efficient solution to their strict thermal requirements. However, the major challenge lies in the Thermal Boundary Resistance (TBR) at the interface of GaN/Diamond or SiC/Diamond. Junction temperature of the device shows a sensitivity of 1.28°C for every unit of TBR for GaN-on-Diamond compared to 0.43°C for every 10 units of TBR for GaN/SiC-on-Diamond. Finite Volume Thermal Analysis has shown a limit of around 22 m2K/GW beyond which the merit of proximity to the heat-source for GaN-on-Diamond can no more outperform GaN/SiC-on-Diamond. Besides, due to the temperature dependency of the thermal conductivity K, an increase in the temperature causes an increase in the thermal resistivity of the device which is more significant in high power operations. Simplified assumption of constant K overestimates the device performance by resulting in 17.4°C lower junction temperature for the areal power density of 10W/mm. Other part of the project regarding the in-house growth of CVD-diamond to be bonded to the GaN device has been simultaneously in progress.

Recess processes for the fabrication of normally-off GaN HEMTs generally compromise devices’ on-state performance. In this work, recess-free quasi-normally-off GaN HEMTs with a threshold voltage of 0.24 V is realized by local control of two-dimensional electron gas (2DEG) density. The devices feature a $0.1~{\mu }\text{m}$ gate length, SiNx stress liner, and comb gate. SiNx liner can provide significant stress to AlGaN/GaN heterostructure in the scaled gate region. The additional stress translates to the additional electric field and depletes the 2DEG in the gate region. As a result, the quasi-normally-off operation is achieved. Furthermore, the comb gate structure is introduced to suppress the short channel effects, supported by TCAD simulation. The quasi-normally-off devices’ excellent on-state performances are benchmarked against the normally-off devices reported recently and a p-GaN HEMT purchased from a commercial foundry. The results support strain engineering as a promising technique to pursue the normally-off operation of GaN HEMTs.

This paper demonstrates a new fabrication process for gallium nitride high-electron mobility transistors (HEMTs) free of plasma damages in the sub-100 nm T-shape gate area. The common peeling-off problems of electron beam resists during gate metal deposition process were solved by introducing a fluorine plasma treatment process before gate metal deposition. By combining dry and wet etching processes appropriately, an on/off ratio of 107 at a drain-to-source voltage of 1 V was achieved. This work also investigated the short channel effect in devices with gate lengths from 70 to 440 nm. Reducing gate length results in decrease of threshold voltage due to the drain-induced barrier-lowering effect. Current gain cut-off frequency fT and maximum oscillation frequency fmax increase while gate length reduces till 250 nm. However, below 250 nm, fT and fmax no longer increase while gate length reduces till sub-100 nm, which reflect the short channel effect.

Hybrid heterojunctions based on two-dimensional (2D) and conventional three-dimensional (3D) materials provide a promising way toward nanoelectronic devices with engineered features. In this work, we investigated the band alignment of a mixed-dimensional heterojunction composed of transferred MoS2 on β-Ga2O3($$ 2- $$2-01) with and without nitridation. The conduction and valence band offsets for unnitrided 2D-MoS2/3D-β-Ga2O3 heterojunction were determined to be respectively 0.43 ± 0.1 and 2.87 ± 0.1 eV. For the nitrided heterojunction, the conduction and valence band offsets were deduced to 0.68 ± 0.1 and 2.62 ± 0.1 eV, respectively. The modified band alignment could result from the dipole formed by charge transfer across the heterojunction interface. The effect of nitridation on the band alignments between group III oxides and transition metal dichalcogenides will supply feasible technical routes for designing their heterojunction-based electronic and optoelectronic devices.

In this work, a thin-film transistor gas sensor based on the p-N heterojunction is fabricated by stacking chemical vapor deposition-grown tungsten disulfide (WS2) with a sputtered indium–gallium–zi...

Thermoelectric materials can effectively convert waste heat into electrical energy and receive more and more attention. Zintl phase Mg3Sb2 is a typical layered thermoelectric material and the anisotropy in crystalline structure induces strong anisotropy in its thermoelectric properties. As a result, polycrystalline Mg3Sb2 usually can not achieve the maximum thermoelectric performance. Our research proves that the dimensionality reduction in material structure can not only avoid certain crystalline direction's poor thermoelectric properties but also effectively reduce the lattice thermal conductivity. Within the framework of the density functional theory, the thermoelectric performance of two-dimensional Mg3Sb2 mono layer is accurately investigated. The ZT value of the Mg3Sb2 nano-thin film can reach 2.5 at 900 K, which is larger than that of the bulk. The theoretical results indicate that the process of nano-engineered structure can significantly enhance the energy conversion efficiency of thermoelectrics.

Transparent ceramic is considered to be a fascinating material with high photonic quality for a diverse range of applications. Here, we fabricated series of highly transparent Yb:Y2O3 (Yb:1.0, 3.0, 5.0, 7.0 and 9.0 at.%) ceramics via a vacuum solid-state reactive sintering. The room temperature up-conversion spectra for blue emissions are observed under the laser excitation of 980 nm. It is shown that the content of Yb3+ plays an important role in emission efficiency. Especially, the sample containing 7.0 at.% Yb3+ achieves the strongest cooperative luminescence. Analysis reveals that Yb3+-Yb3+ cluster cooperating in Y2O3 transparent ceramics takes place via phonon-assist energy transfer mechanism. Furthermore, the blue emission spectroscopy and decay lifetime of 7.0 at.% Yb:Y2O3 are also discussed.

The humidity sensor is an essential sensing node in medical diagnosis and industrial processing control. To date, most of the reported relative humidity sensors have a long response time of several seconds or even hundreds of seconds, which would limit their real application for certain critical areas with fast-varying signals. In this paper, we propose a flexible and low-cost humidity sensor using vertically aligned carbon nanotubes (VACNTs) as electrodes, a PDMS-Parylene C double layer as the flexible substrate, and graphene oxide as the sensing material. The humidity sensor has an ultrafast response of ~20 ms, which is more than two orders faster than most of the previously reported flexible humidity sensors. Moreover, the sensor has a high sensitivity (16.7 pF/% RH), low hysteresis (

Stretchable strain sensors are of great importance in developing intelligent robotics, virtual reality, and e-skins. Here, we propose a capacitive stretchable strain sensor based on wavy structured interdigital metal electrodes encapsulated in elastomers, where wavy design not only endows metal electrodes with stretchability, ensuring long-term stability, but also provides sidewalls to form 3D nonplanar capacitor. Distances between electrodes change with the elongation when stretching, resulting in capacitance variation with high sensitivity. The devices are fabricated, and the sensing properties are characterized. The simulation on device deformation, stress distribution, and sensitivity is also performed for validation and comparison. The presented stretchable strain sensor is reliable, suitable with high-volume manufacturing, and promising in various applications.

The bilayer Al 2 O 3 /in-situ SiN x gate dielectric stacks were applied to fabricate high-performance AlGaN/GaN MIS-HEMTs. Both the normally-off and normally-on operations were realized on the ultrathin-barrier Al 0.05 Ga 0.95 N/GaN heterojunctions. Moreover, the low $V_{th}$ hysteresis of 50 mV and subthreshold swing of 70 mv/dec were achieved based on the in-situ SiN x /Al 0.05 Ga 0.95 N interface. The combination of Al 2 O 3 layer and in-situ SiN x layer can also increase the output current and suppress the gate current leakage.

The performance of perovskite photovoltaics is fundamentally impeded by the presence of undesirable defects that contribute to non-radiative losses within the devices. Although mitigating these losses has been extensively reported by numerous passivation strategies, a detailed understanding of loss origins within the devices remains elusive. Here, we demonstrate that the defect capturing probability estimated by the capture cross-section is decreased by varying the dielectric response, producing the dielectric screening effect in the perovskite. The resulting perovskites also show reduced surface recombination and a weaker electron-phonon coupling. All of these boost the power conversion efficiency to 22.3% for an inverted perovskite photovoltaic device with a high open-circuit voltage of 1.25 V and a low voltage deficit of 0.37 V (a bandgap ~1.62 eV). Our results provide not only an in-depth understanding of the carrier capture processes in perovskites, but also a promising pathway for realizing highly efficient devices via dielectric regulation., Performance of perovskite photovoltaics is greatly affected by undesirable defects that contribute to non-radiative losses. Here, the authors mitigate these losses by doping perovskite with KI to alter the dielectric response, thus defect capturing probability, resulting in inverted device with PCE of 22.3% and low voltage loss.

We report a dramatic current reduction, or a resistance increase, by a few orders of magnitude of two common-anode Au/Ti/pGaN Schottky junctions annealed within a certain annealing condition window (600 - 700 oC, 1 - 4 min). Results from similar common-anode Schottky junctions made of Au/p-GaN, Al/Ti/p-GaN and Au/Ti/graphene/p-GaN junctions demonstrated that all the three layers (Au, Ti and p-GaN) are essential for the increased resistance. Raman characterization of the p-GaN showed a decrease of the Mg-N bonding, i.e., the deactivation of Mg, which is consistent with the Hall measurement results. Moreover, this high-resistance junction structure was employed in p-GaN gate AlGaN/GaN HEMTs. It was shown to be an effective gate technology that was capable to boost the gate breakdown voltage from 9.9 V to 13.8 V with a negligible effect on the threshold voltage or the sub-threshold slope.

A well-developed platform of normally-off p-GaN gate HEMT was reported in this article. The threshold voltage attained 2.4 V, and the off-state breakdown voltage attained over 600 V. The different types of field plate structure were also implemented and studied. The devices equipped with the dual field plates demonstrated better dynamic performance. Some preliminary developments of the recess-free normally-off AIGaN/GaN HEMTs using strain engineering were also demonstrated. The normally-off strained devices featured the optimized SiN x stressor and the comb-gate structure.

The dynamic ON-resistance (Ron) of p-GaN HEMTs was reduced using the dual field plate (FP) structure combined with the source field plate (S-FP) and gate field plate (G-FP). The electric field redistribution at the gate edge reduces the electron trapping during the high voltage blocking, avoiding the Ron increase after the device switching on. The devices with the dual FPs attained the breakdown voltage over 600 V, and the Ron increased only 50% after 200 V blocking, outperforming the devices with single FP or without FPs. Moreover, a plasma surface treatment process was introduced to further improve dynamic Ron performance, which is a promising method to alleviate the current collapse effect. The electric field distribution was also simulated using TCAD to support the experimental results. The results support the dual field plate design a useful structure for high-voltage GaN based HEMTs.

MOSFET power device with beta-Ga 2 O 3 substrate are simulated via TCAD and discussed in this paper. Material models are established and calibrated based on the existing experimental results. Device design specifications like channel doping profile, gate-source distance, selective doping, field plate enhancement, and ion-implantation profile are evaluated and discussed. The results serve as a guide and reference for the beta-Ga 2 O 3 , MOSFET device design.

The quasi-normally-off operation of AlGaN/GaN HEMTs was realized using strain engineering. The dual-layer silicon nitride (SiNx) stressors applied compression to the gate region in devices, and the introduced compression translated to piezoelectric polarization charges and hence, increased the threshold voltage of devices. The devices with dual-layer stressors showed quasi-normally-off characteristics and the threshold voltage of 0.17 V. Besides, the comb-gate structure was introduced to suppress the short channel effects. The on-state performance of the comb-gate devices, including on-resistance $(\pmb{R}_{\pmb{on}})$ and saturation current $(\pmb{I}_{\pmb{d},\pmb{sat}})$ , was benchmarked against that of p-GaN gate HEMTs from a commercial foundry. The dual-layer stressor and comb-gate structure further improved the feasibility of strain engineering as a practical approach in the pursuit of the normally-off operation of AIGaN/GaN HEMTs in power electronics applications.

In this work, a charge storage based enhancement mode (E-mode) AlGaN/GaN high electron mobility transistor (HEMT) is proposed and studied. A stacked gate dielectrics, consisting of a tunnel oxide, a charge trap layer and a blocking oxide are applied in the HEMT structure. The E-mode can be realized by negative charge storage within the charge trap layer during the programming process. The impact of the programming condition and the thickness of the dielectrics on the threshold voltage (Vth) are simulated systematically. It is found that the Vth increases with the increasing programming voltage and time due to the increase of the storage charge. Under proper programming condition, the Vth can be increased to more than 2 V. Moreover, It is also found that the Vth increases with the decrease of the thickness of the dielectrics. In addition, it is found that the breakdown voltage of such HEMT can be adjusted by varying the gate dielectric stacks.

Flexible thermoelectric generators (f‐TEGs) have demonstrated great potential in wearable self‐powered health monitoring devices. However, the existing wearable f‐TEGs are neither flexible enough to bend and stretch while maintaining the device's integrity with a good TE performance nor directly compatible with clothes materials. Here, ultraflexible fabric‐based thermoelectric generators (uf‐TEGs) are proposed with conductive cloth electrodes and elastic fabric substrate. The patterned elastic fabric substrate fits the rigid cuboids well, together with serpentine structured cloth electrodes, rendering uf‐TEG with excellent integrity and flexibility, thereby achieving a highly functional TE performance when strain reaches 30% or on arbitrarily shaped heat sources. The uf‐TEGs show a large peak power of 64.10 μW for a temperature difference of 33.24 K with a high voltage output of 111.49 mV, which is superior compared to previously reported fabric‐based TEG devices, and it is still functional after the water immersion test. Besides the energy harvesting function, with both the temperature sensing ability and the touch perception, this uf‐TEG is demonstrated as the electrical skin when mounted on a robot. Moreover, due to the wind‐sensitive performance and self‐power ability, the uf‐TEGs are assembled on cloth as wearable health and motion monitoring devices., An ultraflexible whole‐fabric assisted thermoelectric generator (uf‐TEG) with thermoelectric cuboids, elastic fabric as assembling material, and conductive flexible cloth‐based electrodes is presented here. The uf‐TEG can be applied in different applications such as electrical skin (temperature sensing and touch perception) and human health and motion monitoring (respiration rate, finger gesture, and steps counting).

Understanding the individual contributions of the factors that influence the electrical properties of a material is important for controlling these properties. In this study, the effects of multiple factors on the electrical properties of thin-barrier AlGaN/GaN heterojunctions with in-situ SiNx caps were investigated using step etching and annealing treatments. The sheet density of the two-dimensional electron gas decreased by 5.9% and 29.5% due to the decrease in the piezoelectric polarization of the AlGaN barrier and the variations in interface charges during SiNx removal, respectively, and it recovered greatly by 72% after annealing-induced surface reconstruction. Capacitance-voltage results clearly revealed few interface traps on the heterojunction with the in-situ SiNx cap. Moreover, a maximum output current of 705 mA/mm and threshold voltage hysteresis below 50 mV were achieved by optimizing the in-situ SiNx interlayer on a thin-barrier metal-insulator-semiconductor structure. This study presents an efficient method for modulating the properties of two-dimensional electron gas and provides insights into the functionality of in-situ SiNx passivation on thin-barrier AlGaN/GaN heterojunctions.

In this work, the charge storage based split floating gates (FGs) enhancement mode (E-mode) AlGaN/GaN high electron mobility transistors (HEMTs) are studied. The simulation results reveal that under certain density of two dimensional electron gas, the variation tendency of the threshold voltage (Vth) with the variation of the blocking dielectric thickness depends on the FG charge density. It is found that when the length sum and isolating spacing sum of the FGs both remain unchanged, the Vth shall decrease with the increasing FGs number but maintaining the device as E-mode. It is also reported that for the FGs HEMT, the failure of a FG will lead to the decrease of Vth as well as the increase of drain current, and the failure probability can be improved significantly with the increase of FGs number.

The present work reports on the hydrogen gas detection properties of Pt-AlGaN/GaN high electron mobility transistor (HEMT) sensors with recessed gate structure. Devices with gate recess depths from 5 to 15 nm were fabricated using a precision cyclic etching method, examined with AFM, STEM and EDS, and tested towards H2 response at high temperature. With increasing recess depth, the threshold voltage ( ${V}_{\textit {TH}}$ ) shifted from −1.57 to 1.49 V. A shallow recess (5 nm) resulted in a 1.03 mA increase in signal variation ( $\Delta {I}_{\textit {DS}}$ ), while a deep recess (15 nm) resulted in the highest sensing response ( ${S}$ ) of 145.8% towards 300 ppm H2 as compared to reference sensors without gate recess. Transient measurements demonstrated reversible H2 response for all tested devices. The response and recovery time towards 250 ppm gradually decreased from 7.3 to 2.5 min and from 29.2 to 8.85 min going from 0 nm to 15 nm recess depth. The power consumption of the sensors reduced with increasing recess depth from 146.6 to 2.95 mW.

This paper introduces a tilting sensor based on Molecular Electronic Transducer (MET) technology. By applying electrical potential between two electrodes covered by lithium iodide electrolyte, the output current from the sensor reflects the covered area of electrodes. The continuously change of electrode area with tilting angle is achieved by patterning metal traces on top of a SiO 2 encapsulated silicon wafer. Polydimethylsiloxane (PDMS) is used as the electrolyte chamber which has high bond strength with SiO 2 after the surface treatment with O 2 plasma. The MET tilting sensor achieves a promising performance with a linear range of ± 90°, resolution of 0.184° and time constant of 40s.

Digital etching is an effective method to lower dry etch damages in A1GaN/GaN HEMTs. This work systematically investigated O 2 -plasma-based digital etching of AlGaN and p-GaN. AlN layers were used as the etch stop layers in the AlGaN etch. Important process aspects such as the use of the AlN layers, the RF power, the oxygen flow rate, the oxidation time and the resulting roughness were studied. These are technically relevant to obtain controllable, uniform etch surfaces with low surface damages for better HEMT performance.

This paper reports the successful atomic layer patterning of 2-dimensional Black Phosphorus (BP) and the simulation of the etching process by Density Functional Theory (DFT) method. The wet etching process can etch selected regions of few-layer black phosphorous with an atomic layer accuracy, which provides a feasible patterning approach for large-scale manufacturing of few-layer BP materials and devices. Absorption energies of iodine atoms/molecules at different location of BP layer edge were also calculated by DFT method, shown a vertical etching direction preference which was important for achieving high quality patterns.

Due to the piezoelectric nature of GaN, the two-dimensional electron gas (2DEG) in AlGaN/GaN high electron mobility transistor (HEMT) could be engineered by strain. In this work, SiN x deposited using dual-frequency plasma-enhanced chemical vapor deposition (PECVD) was used as a stressor. The output performance of the devices was dominated by the surface passivation instead of the strain effect. However, the threshold voltage was increased by the liner-induced strain, supporting strain engineering as an effective approach to pursue the normally-off operation of AlGaN/GaN HEMTs.

The concentration of the two-dimensional electron gas (2DEG) at the AlGaN/GaN heterojunction is dominated by the stress developed in AlGaN layer. In this work, the output performance of AlGaN/GaN high electron mobility transistors (HEMTs) was improved by depositing a stressed SiN x as a passivation layer, and the external strain applied onto AlGaN was measured using Raman spectroscopy. As a result, an improvement in on-state resistance and drive current of the device was attained due to the extra 2DEG concentration induced by the external strain. Also, to determine the strain-induced threshold-voltage shift, the stress distribution over the gate region was analyzed.

Surfaces and heterojunction interfaces, where defects and energy levels dictate charge-carrier dynamics in optoelectronic devices, are critical for unlocking the full potential of perovskite semiconductors. In this progress report, chemical structures of perovskite surfaces are discussed and basic physical rules for the band alignment are summarized at various perovskite interfaces. Common perovskite surfaces are typically decorated by various compositional and structural defects such as residual surface reactants, discrete nanoclusters, reactions by products, vacancies, interstitials, antisites, etc. Some of these surface species induce deep-level defect states in the forbidden band forming very harmful charge-carrier traps and affect negatively the interface band alignments for achieving optimal device performance. Herein, an overview of research progresses on surface and interface engineering is provided to minimize deep-level defect states. The reviewed subjects include selection of interface and substrate buffer layers for growing better crystals, materials and processing methods for surface passivation, the surface catalyst for microstructure transformations, organic semiconductors for charge extraction or injection, heterojunctions with wide bandgap perovskites or nanocrystals for mitigating defects, and electrode interlayer for preventing interdiffusion and reactions. These surface and interface engineering strategies are shown to be critical in boosting device performance for both solar cells and light-emitting diodes.

In this work, we report the performance study of InGaN-based green light-emitting diodes (LEDs) with on-chip photodetectors (PDs) based on wire-bonding and flip-chip configurations. Compared with a conventional wire-bonding design, the LED-PD device, which incorporates a flip-chip design, can offer superior optical and thermal performances and, under an LED current of 200 mA, its light output and detected photocurrent increase by 37.7% and 14.7%, respectively. The different extents of enhancement in both light output and photocurrent are also studied by analyzing their optical, electrical, and thermal properties under varying LED currents. The results provide important guidance for the design of LED-PD integrated systems operating at different current densities.

With technology computer-aided design (TCAD) simulation software, we design a new structure of gallium oxide on gallium–nitride Schottky barrier diode (SBD). The parameters of gallium oxide are defined as new material parameters in the material library, and the SBD turn-on and breakdown behavior are simulated. The simulation results reveal that this new structure has a larger turn-on current than Ga2O3 SBD and a larger breakdown voltage than GaN SBD. Also, to solve the lattice mismatch problem in the real epitaxy, we add a ZnO layer as a transition layer. The simulations show that the device still has good properties after adding this layer.

Stretchable and twistable sensors attract much attention due to their excellent flexibility, stability, and sensitivity for most wearable devices. Star nasal moles in nature possess a flexible and highly sensitive nose that is composed of dozens of epidermal appendages or rays densely covered with small organs, which provides us inspiration for fabricating highly sensitive sensors. Herein, we present a strategy to build a star-nose-inspired sensor with stretchable and twistable structure, where the nose’s ray-like parallel polyimide films were fabricated by electrospinning and chemical silver coating. The sensor is finally encapsulated in PDMS and its sensitivity is significantly improved due to the design of bio-inspired parallel films. The bio-inspired sensor finally exhibits ultra-light, ultra-thin, and high sensitivity (Gauge Factor (GF) ~ 4000) characteristics, ensuring a perfect fit for human skin to achieve stretch, twist, and pressure detection functions. At last, an anisotropic sensor for omnidirectionally sensing human motions was built by crosswise assembling two of functional layers. This study highlights interesting possibilities for bioinspired sensors, with integrated light, thin, stretchable, twistable, and anisotropic features for wearable devices for monitoring human movement and personal health.

Beta phase gallium oxide (β-Ga2O3) has attracted wide attention due to its unique material property; however, the lack of p-type doping hinders its practical application. Here, we report a 2H-MoTe2/β-Ga2O3 ( 2 - 01 ) vertical p-n heterojunction diode via chemical vapor deposition, which exhibited excellent rectification characteristics with a rectifying ratio of 105, and an on-current density of up to 1 mA/cm2. The depletion comes from strain-free interface of the 2H-MoTe2 and β-Ga2O3 ( 2 - 01 ) by van der Waals force. The band offsets and the interface element distribution of the heterojunction were investigated by the X-ray photoelectron spectroscopy and transmission electron microscope. The valence and conduction band offsets between the 2H-MoTe2 and β-Ga2O3 were consequently determined to be 0.16 eV and 3.56 eV, with a nested gap (type I) band alignment. Moreover, an abnormal build-in potential was observed in the 2H-MoTe2/β-Ga2O3 p-n heterojunction diode which could be attributed to the formation of an interfacial layer of MoOX. These observations in the 2H-MoTe2/β-Ga2O3 heterojunction show the great potential in optoelectronic devices.

A novel O2 plasma-based digital etching technology for p-GaN/AlGaN structures without any etch-stop layer was investigated using an inductively coupled plasma (ICP) etcher, with 100 W ICP power and 40 W rf bias power. Under 40 sccm O2 flow and 3 min oxidation time, the p-GaN etch depth was 3.62 nm per circle. The surface roughness improved from 0.499 to 0.452 nm after digital etching, meaning that no observable damages were caused by this process. Compared to the dry etch only methods with Cl2/Ar/O2 or BCl3/SF6 plasma, this technique smoothed the surface and could efficiently control the etch depth due to its self-limiting characteristic. Furthermore, compared to other digital etching processes with an etch-stop layer, this approach was performed using ICP etcher and less demanding on the epitaxial growth. It was proved to be effective in precisely controlling p-GaN etch depth and surface damages required for high performance p-GaN gate high electron mobility transistors.

In this paper, a method to extend the detection range of hydrogen sulfide (H2S) gas sensor is demonstrated. The sensor is based on AlGaN/GaN high electron mobility transistors (HEMTs) with Pt gate. It is observed that the as-fabricated devices exhibited sensing signal saturation at 30 ppm H2S exposure in dry air. A pre-treatment using H2 pulses in dry air ambient at 250 °C was applied to extend the detection range of the sensor. The H2 treated H2S gas sensor was able to detect a higher H2S concentration up to 90 ppm at 250 °C without complete saturation.

Author(s): Luo, D; Zou, T; Yang, W; Xiang, B; Yang, X; Wang, Y; Su, R; Zhao, L; Zhu, R; Zhou, H; Russell, TP; Yu, H; Lu, ZH | Abstract: Controlling defects and energy-band alignments are of paramount importance to the development of high-performance perovskite-based photodiodes. Yet, concurrent improvements in interfacial contacts and defect reduction simply by tailoring bottom contacts have not been investigated. An effective strategy is reported that can simultaneously improve energy-band alignments and structural defects by introducing low-dimensional contact (LDC) layers at the bottom interface. It is found that LDC-based perovskites considerably suppress undesirable structural defects induced by microstrains, resulting in reduced nonradiative recombination centers and improved carrier lifetimes. Additionally, the resulting LDC-based interface structures help block minority carrier injection from the electrodes by forming built-in electric fields. As a consequence, LDC-based perovskite photodiodes showed improved light detection capabilities. The result opens an avenue to yield highly efficient photodiodes.

In this work, AlGaN/GaN HEMTs with dual-layer SiNx stressors (composed of a low-stress layer and a high-stress layer) were investigated. The low-stress padding layer solved the surface damage problem which was caused during the deposition of the high-stress SiNx and provided a good passivated interface. The HEMTs with the dual-layer stressors showed a 1 V increase in the threshold voltage (Vth) with comparable DC and RF amplification performance to the baseline devices. Moreover, the off-current (Ioff) was shown to be reduced by one to three orders of magnitude in the strained devices. The reduction in the off-currents was a result of the lower electric field in AlGaN, which suppressed the gate injection current. These improvements using the dual-layer stressor scheme supports strain engineering as an effective approach in the pursuit of the normally-off operation of AlGaN/GaN HEMTs.

Ohmic contacts to AlGaN/GaN with different metal stacks on Si or Sapphire substrate are fabricated and compared in this paper. For Au-capped ohmic contacts, the lowest contact resistances of 0.7 Ω·mm and 1.3 Ω·mm are achieved by Ti/Al/Ti/Au (20/110/40/50 nm) and Ti/Al/Ni/Au (20/110/40/50 nm) stacks, respectively. It also shows that the substrate material and epitaxial structure play an important role in ohmic contact engineering. For CMOS compatible Au-free structures, the Ti/Al/W (20/100/30 nm), Ti/Al/Ni/W (20/100/20/10 nm) and (20/100/10/20 nm) are demonstrated with the minimum contact resistance values of 0.45, 1.3, and 1.6 Ω·mm, respectively. The three metal stacks of Au-free ohmic contact are compared and obtained results are explained.

This paper discussed a top–down integration method to achieve the three-dimensional (3-D) microscale conformal transferring of functional devices on flexible elastomeric substrates at ambient conditions. By the tunable laser-induced pressure, the functional device inherits the microscale wrinkle-like patterns, without compromising functions. The functional materials are encapsulated in the biocompatible parylene layers to avoid the drastic plastic deformations in functional layers. The electrical resistivity of functional device increases marginally with the applied laser intensity, aspect ratios of microscale features, and overall tensile strain applied to the whole flexible assembly. The stretchability of the transferred functional devices was studied by measuring the electrical property as function of bending and tensile strains. It shows that the device can sustain more than 40% strain in the stretchable substrate. It is demonstrated that the process can achieve the flexible and stretchable functional integration conformal to 3-D micrometer-patterns in a fast and scalable way. [2013-0365]

AlGaN/GaN high electron mobility transistor (HEMT)-based sensors with catalytic platinum gate were micro-fabricated on commercially available epitaxial wafers and extensively characterized for ppm level hydrogen sulfide (H2S) detection for industrial safety applications. High operating temperature above 150 °C enabled large signal variation (ΔIDS) of 2.17 and sensing response of 112% for 90 ppm H2S in dry air as well as high stability across a wide range of biasing conditions. Transient response measurements demonstrated stable operation, superb response and recovery, with good repeatability. The measured sensing signal rise (fall) times reduced from 476 (1316) s to 219 (507) s when the temperature was increased from 200 °C to 250 °C. The response to 90 ppm H2S was 4.5x larger than to H2 and the device showed stable operation over an extended time period.

In this paper, we sums up our resent progress on the enhancement-mode (E-mode) AlGaN/GaN high electron mobility transistors (HEMTs) which are based on charge storage, including the split floating gates (FGs) HEMT and the one with oxide-SiN-oxide (ONO) gate dielectrics stack. For the FGs HEMT, the V th variation with the polarization charge density, the density of two dimensional electron gas, the tunnel dielectric thickness, the blocking dielectric thickness, the length and isolating spacing of FGs are presented. The reliability of the FGs HEMT are also revealed. For the HEMT with ONO gate dielectrics stack, the E-mode is realized by negative charge storage within the SiN trap layer. The V th variation with the programming condition and the thickness of the dielectrics are presented. Under proper programming voltage and time, the V th can be increased to more than 2V. Moreover, it is found that the breakdown voltage of such HEMT can be adjusted by varying the gate dielectric stacks.

The surface polarization of Ga-face gallium nitride (GaN) (2 nm)/AlGaN (22 nm)/GaN channel (150 nm)/buffer/Si with Al2O3 capping layer is investigated by angle-resolved X-ray photoelectron spectroscopy (ARXPS). It is found that the energy band varies from upward bending to downward bending in the interface region, which is believed to be corresponding to the polarization variation. An interfacial layer is formed between top GaN and Al2O3 due to the occurrence of Ga–N bond break and Ga–O bond forming during Al2O3 deposition via the atomic layer deposition (ALD). This interfacial layer is believed to eliminate the GaN polarization, thus reducing the polarization-induced negative charges. Furthermore, this interfacial layer plays a key role for the introduction of the positive charges which lead the energy band downward. Finally, a N2 annealing at 400 °C is observed to enhance the interfacial layer growth thus increasing the density of positive charges.

AlGaN/GaN high electron mobility transistor (HEMT)-sensor with a catalytic Pt-gate is fabricated and tested for toxic H2S gas detection. AlGaN/GaN was chosen to extend the sensor detection range and to be able to operate at temperatures beyond those allowed by state-of-art Si-FET sensors. Testing was performed using a gas mixing apparatus in dry synthetic air ambient. High sensitivity, ΔI/I0, 8% for 80 ppm and 0.23% for 0.5 ppm H2S/air, is achieved at a temperature of 250 °C, with a corresponding ΔI of 617 μA and 18 μA, respectively, indicating suitability of the proposed sensor for industrial gas safety detectors.