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Gas diffusion layers (GDL) have become a critical component in electrochemical CO2 reduction (CO2R) systems because they can enable high current densities needed for industrially relevant productivity. Besides this function,...

It has been long known that low molecular weight resists can achieve a very high resolution, theoretically close to the probe diameter of the electron beam lithography (EBL) system. Despite technological improvements in EBL systems, the advances in resists have lagged behind. Here we demonstrate that a low-molecular-mass single-source precursor resist (based on cadmium(II) ethylxanthate complexed with pyridine) is capable of a achieving resolution (4 nm) that closely matches the measured probe diameter (∼3.8 nm). Energetic electrons enable the top-down radiolysis of the resist, while they provide the energy to construct the functional material from the bottom-up─unit cell by unit cell. Since this occurs only within the volume of resist exposed to primary electrons, the minimum size of the patterned features is close to the beam diameter. We speculate that angstrom-scale patterning of functional materials is possible with single-source precursor resists using an aberration-corrected electron beam writer with a spot size of ∼1 Å.

C-MOSHEMT (Circular-Metal Oxide Semiconductor High Electron Mobility Transistor) has been modeled, fabricated and sensitivity analysis has been done for pH detection application for the first time. Prototype model of the sensor has been developed considering a ring gate structure. The perimeter of the gate inner ring is taken as the gate length and a gate width of $100~\mu \text{m}$ has been used to plot normalized device characteristics. Advantage of C-MOSHEMT over L-MOSHEMT (linear MOSHEMT) is that the area available for sensing increases and process cost and complexity involved remains low from the fabrication point of view. The overall fabrication cost and process overheads reduce with elimination of the mesa isolation etch. ALD (Atomic Layer Deposition) deposited Al2O3 used as the gate oxide and passivation scheme in the device design ensures higher sensitivities along with current collapse free device operation. Analytical model along with experimental analysis have been done using 3 standard buffer pH samples, pH=4, 7, and 9.2. The devices fabricated showcase a maximum sensitivity of 1.74 mA/pH and 1.86 mA/pH when gate lengths ( $\text{L}_{G}$ ) of $3~\mu \text{m}$ and $5\mu \text{m}$ are considered. Further, the model and the physical detection results are compared and contrasted to study the strengths and weaknesses of the model developed. It has been observed that as sensing area is doubled, the sensitivity increases by 41% which can be attributed to the discreteness of charge distribution on the sensing surface.

Molybdenum disulfide (MoS2) is traditionally grown at a high temperature and subsequently patterned to study its electronic properties or make devices. This method imposes severe limitations on the...

We report sensitive and selective AlGaN/GaN High Electron Mobility Transistor (HEMT)-based sensor for Lead ion (Pb2+) detection. The gate region of the HEMT was functionalized by 2,5-dimercapto-1,3,4-thiadiazole (DMTD). The response of the sensor is observed by monitoring drain to source current ( $\text{I}_{\textsf {DS}}$ ) for different concentrations of Pb2+ ions at a fixed drain to source voltage ( $\text{V}_{\textsf {DS}}$ ). Our sensor reaches the lower detection limit of 0.018 ppb, which is much lower than the standard detection limit recommended by the World Health Organization (WHO) for drinking water. Furthermore, the sensor exhibited a rapid response time of ~4 seconds and high sensitivity of $0.607~\mu \text{A}$ /ppb. Moreover, the selectivity analysis was performed and found that the sensor was highly selective towards Pb2+ ions. The change in electron concentration at 2-dimensional electron gas (2DEG) upon the capture of Pb2+ ions at gate region by DMTD, causes a change in the $\text{I}_{\textsf {DS}}$ , which showed excellent sensing response towards Pb2+ ions. The highly sensitive, selective, and rapid detection of Pb2+ ions paves the way for stable sensing performance based on DMTD functionalized AlGaN/GaN HEMT sensor.

We report significantly enhanced sensitivity of AlGaN/GaN-based high electron mobility transistor (HEMT) sensor by the targeted synthesis of IT and 2H coexisting phase MoS

This paper demonstrates a novel AlGaN/GaN high-electron mobility transistor (HEMT)-based cadmium ion (Cd2+) sensor with mercaptopropionic acid (MPA) and glutathione (GSH) functionalization. The sensing response of the sensor was analyzed by detecting Cd2+ ions at different concentrations. The AlGaN/GaN HEMT sensor exhibits excellent response with the sensitivity of $0.241~\mu \text{A}$ /ppb, a fast response time of ~ 3 s, and a lower detection limit of 0.255 ppb. The observed lower detection limit is significantly lower than the World Health Organization (WHO) standard recommended limit for Cd2+ ions in drinking water. Furthermore, the sensor showed good selectivity of Cd2+ ions toward other heavy metal ions. The results indicate that the binding properties of GSH to Cd and the sensitivity of 2-D electron gas toward the variation of charges at the gate region make the device highly sensitive with rapid detection of Cd2+ ions.

In this article, sensitivity enhancement of undoped AlGaN/AlN/GaN HEMT for pH detection by using dielectric (10 nm Al 2 O 3 )-based MOS-gated structure is demonstrated. Linear and circular MOS-HEMT (L-MOSHEMT and C-MOSHEMT, respectively) with similar dimensions are fabricated on Si substrate. Novel sensing metric gd/IDS (drain conductance to current ratio) is introduced, and C-MOSHEMT attains the highest sensitivity of 1.74 mA/pH and 58 mV -1 /pH when change in drain current (IDS) and gd/IDS are taken as the sensing metrics, respectively.

We report on the demonstration and investigation of Ta2O5 as high-k dielectric for InAlN/GaN-MOS high-electron mobility transistor (HEMT)-on-Si. Ta2O5 of thickness 24 nm and dielectric constant ~30 was sputter deposited on InAlN/GaN HEMT and was investigated for different post deposition anneal (PDA) conditions. The gate leakage was 16 nA/mm at −15 V which was ~five orders of magnitude lower compared to reference HEMT. The 2-D electron gas (2-DEG) density was found to vary with annealing temperature suggesting the presence of net charge at the Ta2O5/InAlN interface. Dispersion in the capacitance–voltage ( ${C}$ – ${V}$ ) characteristics was used to estimate the frequency-dependent interface charge, while energy band diagrams under flat band conditions were investigated to estimate fixed charge. The optimum anneal condition was found to be 500 °C which has resulted into a flat band voltage spread ( $\Delta {V}_{\text {FB}}$ ) of 0.4 V and interface fix charge ( ${Q} _{\text {f}}$ ) of ${3.98} \times {10}^{{13}}$ cm $^{-2}$ . X-ray photoelectron spectroscopy spectra of as-deposited and annealed Ta2O5film were analyzed for Ta and O compositions in the film. The sample annealed at 500 °C has shown Ta:O ratio of 0.41. X-ray diffraction analysis was done to check the crystallization of amorphous Ta2O5 film at higher annealing temperatures.

We report significantly enhanced sensitivity of AlGaN/GaN-based high electron mobility transistor (HEMT) sensor by the targeted synthesis of IT and 2H coexisting phase MoS2 and applying the gate bias voltage. The HEMT structures on Si (111) substrates were used for the detection of Hg2+ ions. The optimum sensitive regime in terms of V GS and V DS of the sensor was investigated by keeping the drain source voltage V DS constant at 2 V and by only varying the gate bias voltage V GS from 0 to 3 V. The strongest sensing response obtained from the device was around 0.547 mA ppb−1 at V GS = 3 V, which is 63.7% higher in comparison to the response achieved at 0 V which shows a sensing response of around 0.334 mA ppb−1. The current response depicts that the fabricated device is very sensitive and selective towards Hg2+ ions. Moreover, the detection limit of our sensor at 3 V was calculated around 6.21 ppt, which attributes to the strong field created between the gate electrode and the HEMT channel due to the presence of 1T metallic phase in synthesized MoS2, indicating that the lower detection limits are achievable in adequate strong fields.

Molybdenum disulfide (MoS

We report on the band offset and interfacial electronic properties of e-beam evaporated HfO2 gate dielectrics on III-nitride device stacks on Si. A conduction band offset of 1.9 eV is extracted for HfO2/GaN along with a very low density of fixed bulk and interfacial charges for optimally annealed samples. Normally-ON HfO2/AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors exhibit negligible shifts in threshold voltage, transconductances of 120 mS/mm for 3- $\mu \text{m}$ gate length devices, and three-terminal off-state gate leakage currents of 55 nA/mm at a ${V}_{D}$ of 100 V. Dynamic capacitance dispersion measurements in the temperature range of 25 °C–200 °C show two peaks at the AlGaN/GaN interface corresponding to slow and fast interface traps with a peak ${D}_{\text {it}}$ of ${5.5}\times {10}^{13}$ and ${1.5} \times {10}^{13}$ eV−1cm−2 respectively as a function of Fermi level position above ${E}_{C}$ . The HfO2/AlGaN interface exhibits a reasonably constant peak ${D}_{\text {it}}$ of ${2} \times {10}^{13}$ – ${4.4} \times {10}^{13}$ eV−1cm−2 at trap levels of 0.42–0.72 eV below ${E}_{\text {C}}$ . Hysteretic pulsed $I_{D}$ – $V_{G}$ measurements revealed a negative shift in threshold voltages indicative of unoccupied donor-like trap states at the HfO2/AlGaN interface and comparable ${D}_{\text {it}}$ to that inferred from conductance measurements.

Coherently strained pseudo-superlattices (PSLs) of 20-period GaAs/Ge have been epitaxially grown on [001]-oriented Ge and GaAs substrates by metalorganic chemical vapor deposition. High-resolution X-ray diffraction and dynamic simulation revealed an increase in the growth rate of Ge (RGe) and a decrease in that of GaAs (RGaAs) when GaAs is used, instead of Ge, as the substrate under otherwise the same growth conditions. They also revealed an increase in RGe while a decrease in RGaAs when the growth temperature is increased. These changes are related to atomic intermixing at the interfaces that resulted in interfacial (GaAs)1−xGex alloys with lattice constants larger than that calculated assuming Vegard's law. Cross-sectional transmission-electron microscopy revealed regular in-plane undulations of the PSLs and energy-dispersive X-ray spectroscopy provided evidence that the Ge sublayers were conformally grown on GaAs while the GaAs sublayers were grown in isolated islands that initiated in the valleys of the Ge sublayers. The growth mechanism was interpreted based on surface chemical potentials that depend on the balance between the surface-energy and the local strain-relaxation energy. The three-dimensional periodic island-in-network nanostructures, as well as their optical and lattice dynamical properties, are of great importance not only for fundamental studies but also for fabricating thermoelectric devices.

In-situ X-ray diffraction (XRD), with the sample temperature stepwise increased up to 900 °C, together with ex-situ high-resolution XRD (HRXRD), has been used to study the lattice expansion along the surface normal direction of GaN-on-Si heterostructures grown by metalorganic chemical vapor deposition (MOCVD). The thermal stabilities induced by step-graded (SG) AlxGa1−xN/AlN and multiple low-temperature (MLT) AlN buffer layers have been addressed for the heterostructures grown on 700 and 1500 µm thick Si (111) wafers, respectively. It reveals that the thermal expansion of the GaN epilayer is slightly larger than that of the Si substrate and the nitride growth tends to decrease the thermal expansion of the Si (111) wafer at lower temperatures. Onset of drop in the lattice expansions has been observed when the temperature is increased to a transition point, Ttr, and the Ttr of the MLT-AlN buffered GaN on the 1500 µm thick Si is ~500 °C, which is apparently lower than that of the SG-AlxGa1−xN/AlN buffered GaN on the 700 µm thick Si. These observations have been interpreted and attributed to convolutions among the residual lattice strains, their relaxations, and the thermal expansion coefficient mismatch induced wafer curvatures (opposite to that during MOCVD growth) at elevated temperatures.

We report on the characterization of the interfaces of Al2O3/InAlN/GaN HEMT structure grown on 200 mm diameter silicon using conductance dispersion technique. Irreversible threshold voltage (VTH) shift of up to +similar to 2.5 V was observed due to the gate stress induced activation of acceptor states. Further, frequency dependent VTH shift during capacitance voltage measurements were also recorded due to the presence of slow traps at InAlN/GaN interface. The conductance dispersion indicated the presence of acceptor traps of the order of similar to 4 x 10(12) to 7 x 10(13) cm(-2) eV(-1) with a time constant of similar to 10 to 350 ls at the InAlN/GaN interface. Trap density at the Al2O3/InAlN was found to be in similar range but with a time constant of similar to 2 ms. The presence of high density of traps at InAlN/GaN interface is attributed to the unavoidable growth interruption before the start of InAlN growth. (C) 2017 Elsevier Ltd. All rights reserved.

This work presents a procedure for large-area patterning of designed plasmon resonators that are much smaller than possible with conventional lithography techniques. Fused Colloidal Nanopatterning combines directed self-assembly and controlled fusing of spherical colloidal nanoparticles. The two-step approach first patterns a surface covered with hydrogen silsesquioxane, an electron beam resist, forming traps into which the colloidal gold nanoparticles self-assemble. Second, the patterned nanoparticles are controllably fused to form plasmon resonators of any 2D designed shape. The heights and widths of the plasmon resonators are determined by the diameter of the nanoparticle building blocks, which can be well below 10 nm. By performing the fusing step with UV ozone and heat exposure, we demonstrate that the process is easily scalable to cover large areas on silicon wafers with designed gold nanostructures. The procedure neither requires adhesion layers nor a lift-off process, making it ideally suited for plasmonics, in comparison with regular electron beam lithography. We use monochromated electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy and boundary element method simulations to demonstrate that the designed plasmon resonators are directly tunable via the pattern design. We foresee future expansion of this approach for applications such as plasmon-enhanced photocatalysis and for large-scale patterning where chemical, optical, or confinement properties require sub-10 nm metal lines.

Compared with bulk-Si wafer, Al x Ga1– x N/gallium nitride (GaN) heterostructures grown on a 150-mm silicon-on-insulator (SOI) substrate with a 35-nm-thick Si overlayer are shown to have $\sim 50$ % less wafer bowing. As a result, the 2-D electron gas mobility and the sheet-resistivity uniformity on SOI are improved due to a lower defect density. In terms of device performance, high-electron-mobility transistors (HEMTs) fabricated on the Al x Ga1– x N/GaN-on-SOI exhibit $\sim 20.5$ % higher saturation drain current as compared with the bulk-Si counterparts. However, due to the poorer conductivity of the buried oxide layer, the Al x Ga1– x N/GaN-on-SOI HEMT suffers greater self-heating, with $\sim 50$ K higher channel temperature. With mitigation of self-heating, the Al x Ga1– x N/GaN-on-SOI, in view of its more superior structural and thermal stability, should offer an attractive alternative for integration of the GaN technology with the Si CMOS platform.

A sensor for highly sensitive, selective, and rapid determination of the trace amount of toxic Hg2+ ions is developed for the first-time using molybdenum disulfide (MoS2) functionalized AlGaN/GaN high electron mobility transistor (HEMT). The vertically aligned, flower-like MoS2 structures are synthesized through a simple hydrothermal route and applied on the gate region of AlGaN/GaN HEMT. The scanning electron microscopy, Raman spectroscopy, and X-ray diffraction are performed for structural characterization of MoS2. Further, the sensing of Hg2+ ions is performed by electrical characterizations of MoS2 functionalized AlGaN/GaN HEMT. The sensor showed an excellent sensitivity of 0.64 μA/ppb and detection limit of 0.01152 ppb with the rapid response time of 1.8 s. The sensor exhibits the linear range of detection from 0.1 ppb to 100 ppb and highly selective behavior towards Hg2+ ions. The results demonstrated that the MoS2 possess excellent Hg2+ ions capture property, that could be attributed to the complexation of Hg2+ ions with sulfur and the electrostatic interaction between MoS2 and Hg2+ ions alters the drain to source current (IDS) of the HEMT at a constant drain to source voltage (VDS). Therefore, the MoS2 based AlGaN/ GaN HEMT devices have a huge potential for next-generation ion sensing applications.

In this letter, we report on a unique device design strategy for increasing the breakdown voltage and hence Baliga Figure of Merit (BFOM) of III-nitride HEMTs by engineering the gate edge towards the drain. The breakdown of such devices with meandering gate-drain access region (M-HEMT) are found to be 62% more compared to that of conventional HEMT while the ON resistance suffers by 76%, leading to an overall improvement in the BFOM for by 28%. 3D-TCAD simulations show that the decrease in the peak electric field at the gate edge was responsible for increased breakdown voltage.

We demonstrate an InAlN/GaN-on-Si HEMT based UV detector with photo to dark current ratio > 107. Ti/Al/Ni/Au metal stack was evaporated and rapid thermal annealed for Ohmic contacts to the 2D electron gas (2DEG) at the InAlN/GaN interface while the channel + barrier was recess etched to a depth of 20 nm to pinch-off the 2DEG between Source-Drain pads. Spectral responsivity (SR) of 34 A/W at 367 nm was measured at 5 V in conjunction with very high photo to dark current ratio of > 10^7. The photo to dark current ratio at a fixed bias was found to be decreasing with increase in recess length of the PD. The fabricated devices were found to exhibit a UV-to-visible rejection ratio of >103 with a low dark current < 32 pA at 5 V. Transient measurements showed rise and fall times in the range of 3-4 ms. The gain mechanism was investigated and carrier lifetimes were estimated which matched well with those reported elsewhere., Comment: 12 pages

In this study we report the existence of novel ultraviolet (UV) and blue emission in rare-earth based perovskite NdGaO3 (NGO) and the systematic quench of the NGO photoluminescence (PL) by Ce doping. Study of room temperature PL was performed in both single-crystal and polycrystalline NGO (substrates and pellets) respectively. Several NGO pellets were prepared with varying Ce concentration and their room temperature PL was studied using 325 nm laser. It was found that the PL intensity shows a systematic quench with increasing Ce concentration. XPS measurements indicated that nearly 50% of Ce atoms are in the 4+ state. The PL quench was attributed to the novel concept of super hydrogenic dopant (SHD)”, where each Ce4+ ion contributes an electron which forms a super hydrogenic atom with an enhanced Bohr radius, due to the large dielectric constant of the host. Based on the critical Ce concentration for complete quenching this SHD radius was estimated to be within a range of 0.85 nm and 1.15 nm whereas the predicted theoretical value of SHD radius for NdGaO3 is ~1.01 nm.

The authors report on the fabrication and characterization of low-temperature processed gold-free Ohmic contacts for AlxGa1−xN/GaN high electron mobility transistors (HEMTs). The HEMT structure grown on a 200-mm diameter Si(111) substrate is used in this study. Using the Ti/Al/NiV metal stack scheme, the source/drain Ohmic contact optimization is accomplished through the variation of Ti/Al thickness ratio and thermal annealing conditions. For an optimized Ti/Al stack thickness (20/200 nm) annealed at 500 °C for 30 s with smooth contact surface morphology, a specific contact resistivity of ∼6.3 × 10−6 Ω cm2 is achieved. Furthermore, with gold-free Ni/Al gates, the fabricated HEMTs exhibit ION/IOFF ratio of ∼109 and a subthreshold swing of ∼71 mV/dec. The demonstrated gold-free contact schemes thus provide a solution toward the implementation of GaN-based HEMT process on a Si foundry platform. ASTAR (Agency for Sci., Tech. and Research, S’pore) Published version

The temperature profiles of InxAl1−xN/GaN high electron mobility transistors (HEMTs) were investigated using nondestructive optical spectroscopic techniques. In this study, HEMT structures were epitaxially grown on a Si(111) substrate with a diameter of 200 mm. In particular, the channel temperature underneath the gate was able to be accurately probed by using a RuOx-based semitransparent Schottky contact in the ultraviolet photoluminescence (PL) and visible Raman excitation modes. A maximum channel temperature as high as ∼475 K was probed near the gate edge using the PL technique at a power dissipation of ∼11.6 W/mm, thus leading to a minimum thermal conductance of about 64.7 Wm−1K−1 in such a HEMT structure. Furthermore, the temperature profiles at the GaN buffer and AlN/Si(111) interface were determined using micro-Raman measurements.

We have demonstrated 0.17-µm gate-length In0.17Al0.83N/GaN high-electron-mobility transistors (HEMTs) on Si(111) substrates using a non-gold metal stack (Ta/Si/Ti/Al/Ni/Ta) with a record-low ohmic contact resistance (Rc) of 0.36 Ω mm. This contact resistance is comparable to the conventional gold-based (Ti/Al/Ni/Au) ohmic contact resistance (Rc = 0.33 Ω mm). A non-gold ohmic contact exhibited a smooth surface morphology with a root mean square surface roughness of ~2.1 nm (scan area of 5 × 5 µm2). The HEMTs exhibited a maximum drain current density of 1110 mA/mm, a maximum extrinsic transconductance of 353 mS/mm, a unity current gain cutoff frequency of 48 GHz, and a maximum oscillation frequency of 66 GHz. These devices exhibited a very small (