Gas Sheet Research Articles

Enhanced ocean heat storage efficiency during the last deglaciation

Proxy reconstructions suggest that increasing global mean sea surface temperature (GMSST) during the last deglaciation was accompanied by a comparable or greater increase in global mean ocean temperature (GMOT), corresponding to a large heat storage efficiency (HSE; ∆GMOT/∆GMSST). An increased GMOT is commonly attributed to surface warming at sites of deepwater formation, but winter sea ice covered much of these source areas during the last deglaciation, which would imply an HSE much less than 1. Here, we use climate model simulations and proxy-based reconstructions of ocean temperature changes to show that an increased deglacial HSE is achieved by warming of intermediate-depth waters forced by mid-latitude surface warming in response to greenhouse gas and ice sheet forcing as well as by reduced Atlantic meridional overturning circulation associated with meltwater forcing. These results, which highlight the role of surface warming and oceanic circulation changes, have implications for our understanding of long-term ocean heat storage change.

Focal cone high harmonic generation driven by a 400 TW laser

The generation of self-focusing beams of extreme ultraviolet (XUV) radiation using the focal cone high harmonic generation (FCHHG) technique is examined for high energy lasers. The FCHHG geometry is created by passing a focusing laser beam through a gas sheet prior to reaching focus and thus creating a converging beam of high harmonic radiation. This leads to a larger interaction area that increases the total area of XUV emission while not exceeding the saturation intensity of the target atoms or increasing the density of the atoms. Such a method allows for scaling of HHG to any incident laser power. An experiment was conducted demonstrating such scaling to incident 400 TW pulses, showing both the expected spectral signature of HHG and the converging cone of XUV radiation. It was found that this technique is very sensitive to spatial non-uniformity in the driving laser, which has become more prevalent in high energy laser systems.

Temperature‐Dependent Characteristics of AlN/Al0.5Ga0.5N High Electron Mobility Transistors with Highly Degenerate n‐Type GaN Regrown Ohmic Contacts

Herein, the temperature‐dependent characteristics of AlN/Al0.5Ga0.5N high electron mobility transistors (HEMTs) with highly degenerate n‐type GaN (d‐GaN) ohmic contacts fabricated via pulsed sputtering deposition (PSD) are investigated. The ohmic contacts formed at the source and drain regions demonstrate low resistivity within the temperature range of 77–473 K. It is found that the temperature dependence of the contact resistance (RC) for the AlN/Al0.5Ga0.5N HEMTs with regrowth d‐GaN contacts is attributable to the energy barrier induced by the Fermi level difference between the d‐GaN contacts and Al0.5Ga0.5N channel layer. Notably, the RC between the d‐GaN contacts and the channel layer is 1.4 and 0.15 Ω mm at 77 and 473 K, respectively. Temperature‐dependent device on‐resistance coincides with the temperature variation of the two‐dimensional electron gas sheet resistance because the contact resistance is negligibly low in the studied temperature range. Moreover, AlN/Al0.5Ga0.5N HEMTs with d‐GaN contacts exhibit excellent switching characteristics, achieving an ION/IOFF ratio of ≈106, even at high temperatures up to 473 K. The results indicate that the PSD d‐GaN contact technique has potential to overcome the limitations of ohmic contacts for high Al‐composition AlGaN electronic devices, especially for next‐generation high‐power‐density radio frequency transistors operating at high temperatures.

Observation of Beam Emittance Reduction due to Gas Sheet Injection for Beam Profile Measurement

In a high intensity ion accelerator, a non-destructive beam monitor is required to realize a continuous measurement of the beam for improving the beam quality in operation for users. We have developed a non-destructive 2-D beam profile monitor detecting photons produced by interaction between a beam and a sheet-shaped gas. Though the developed monitor is a non-destructive type, the injected gas sheet should induce scattering of the beam particles which makes the beam emittance large. We have measured a beam current and a phase space distribution of the J-PARC 3 MeV, 60 mA H− beam with change of the gas sheet flux. It was found that the beam current reduction was in linear relation and the root mean square emittance was constant or decreased by up to 4.5% in y-y′ plane and did not change in x-x′ plane against a rise in the gas sheet flux. These result indicate that the developed monitor can be utilized as a non-destructive one depending on the gas sheet flux condition.

Effect of SiO2Surface Passivation on the Performance of GaN Polarization Superjunction Heterojunction Field Effect Transistors

In this article, the effects of the SiO2surface passivation layer are reported on normally‐on 1.2 kV GaN polarization superjunction (PSJ) heterojunction field effect transistors (HFETs) by comparing the electrical performances of PSJ HFETs with and without SiO2surface passivation. A slight recovery of the 2D electron gas sheet density is observed in the slight negative shift ofVthafter SiO2surface passivation. Passivation also increases the breakdown voltage. This improvement may result from removing positive surface charges in defects along the P‐GaN gate sidewall and top u‐GaN layer after the specifically designed SiO2surface passivation. Furthermore, the SiO2surface passivation can also effectively suppress the surface gate leakage currents in the PSJ HFETs by eliminating the conductive channel created by the positive surface charges in defects.

Gas sheet beam induced fluorescence based beam profile measurement for high intensity proton accelerators

The paper describes the design simulations, developmental details and characterization results of a dual gas sheet generator system for non-invasive beam profile measurement of high-intensity proton beams. The system is being developed for the Low Energy High Intensity Proton Accelerator facility at Bhabha Atomic Research Centre, India. To our knowledge, this is the first time an intense low energy proton beam was used to directly view the beam induced fluorescence of a gas sheet. The system works by generating two thin nitrogen gas sheets which produces fluorescence when a charged particle beam passes through them, thereby providing a visual representation of the beam's intensity profile. The gas sheets are generated at 45° to the beam axis, which allows the fluorescence created by the interaction to be viewed directly through a viewport or camera capture. The beam profile measurements were carried out with a typically 20 keV, 10 mA pulsed proton beam from an ECR ion source under different operating parameters like gas sheet pressure, beam current, beam energy and focusing magnetic field. The Geant4 simulation has been performed to study the scintillation process and optical transport. It may become useful to study methods of reconstructing the beam profile from image obtained.

Electron transport mechanism in AlN/β-Ga<sub>2</sub>O<sub>3</sub> heterostructures

The β-Ga<sub>2</sub>O<sub>3</sub> has received much attention in the field of power and radio frequency electronics, due to an ultrawide bandgap energy of ~4.9 eV and a high breakdown field strength of ~8 MV/cm (Poncé et al. <ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1103/PhysRevResearch.2.033102">2020 <i>Phys. Rev. Res.</i> <b>2</b> 033102</ext-link>). The in-plane lattice mismatch of 2.4% between the (<inline-formula><tex-math id="Z-20230109105419-1">\begin{document}$ \bar 201 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20221545_Z-20230109105419-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20221545_Z-20230109105419-1.png"/></alternatives></inline-formula>) plane of β-Ga<sub>2</sub>O<sub>3</sub> and the (0002) plane of wurtzite AlN is beneficial to the formation of an AlN/β-Ga<sub>2</sub>O<sub>3</sub> heterostructure (Sun et al. <ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1063/1.5003930">2017<i> Appl. Phys. Lett.</i> <b>111</b> 162105</ext-link>), which is a potential candidate for β-Ga<sub>2</sub>O<sub>3</sub>-based high electron mobility transistors (HEMTs). In this study, the Schrödinger-Poisson equations are solved to calculate the AlN/β-Ga<sub>2</sub>O<sub>3</sub> conduction band profile and the two-dimensional electron gas(2DEG) sheet density, based on the supposition that the 2DEG originates from door-like surface states distributed evenly below the AlN conduction band. The main scattering mechanisms in AlN/β-Ga<sub>2</sub>O<sub>3</sub> heterostructures, i.e. the ionized impurity scattering, interface roughness scattering, acoustic deformation-potential scattering, and polar optical phonon scattering, are investigated by using the Boltzmann transport theory. Besides, the relative importance of different scattering mechanisms is evaluated. The results show that at room temperature, the 2DEG sheet density increases with the augment of AlN thickness, and reaches 1.0×10<sup>13</sup> cm<sup>–2</sup> at an AlN thickness of 6 nm. With the increase of the 2DEG sheet density, the ionized impurity scattering limited mobility increases, but other scattering mechanisms limited mobilities decrease. The interface roughness scattering dominates the mobility at low temperature and moderate temperature (<i>T </i>< 148 K), and the polar optical phonon scattering dominates the mobility at temperatures above 148 K. The room-temperature mobility is 368.6 cm<sup>2</sup>/(V·s) for the AlN/β-Ga<sub>2</sub>O<sub>3</sub> heterostructure with an AlN thickness of 6 nm.

H2S Gas Sensitivity of Doped SbP Monolayer：First Principle Study

The sensing behaviors of monolayer antimonide phosphorus (SbP) for hydrogen sulfide (H2S) are investigated by means of the density functional theory. In this paper, we calculated the best adsorption configuration, charge transfer, adsorption distance, band gap, electronic structure and recovery time of H2S on the SbP monolayer and mental-doped SbP (X-SbP). The calculated results indicate that Al atom replaces Sb atom of SbP (Al-Sb-SbP), adsorption capacity was greatly increased, but the covalent bond formed between the gas molecules and the substrate was not suitable for sensing materials. And though Co or Ni atom could improve the interactions between H2S gas and SbP sheets, the recovery time was too long. It was also not suitable for the sensor material of H2S gas. However, for Pd doped SbP, Pd-Sb and Pd-P doping all exhibit excellent gas sensing performance for H2S gas with the adsorption energy of -0.677eV and -0.520eV, the charge transfer 0.1113e and 0.0930e, the recovery time 0.19s and 5.30and#215;10-4 s, respectively. These characters made Pd-SbP suitable for H2S gas sensing material. Which further analysis we knew that these changes were mainly due to the orbital hybridization between the s, p orbitals of Pd atom and the p orbitals of S atom. Theoretical studies show that Pd-doped SbP is a promising H2S gas sensing material

Development of gas sheet stamping technique of parts corrugationless

An overview and analysis of the main methods of pressing the stamped workpiece is made. Experimental studies are carried out and the technological capabilities of two-chamber gas sheet stamping are tested. The optimal clamp pressure for the workpiece is selected, which produces parts without corrugations. Advanced designs of two-chamber devices for gas sheet stamping with one and two combustion chambers are presented. Their advantages are described.

Atomic-column resolution quantitative composition analysis of AlN interlayer in MOCVD-grown AlGaN/AlN/GaN heterostructure using HAADF-STEM

Quantitative compositional analysis using high-angle annular dark-field scanning transmission electron microscopy was performed for a metal-organic chemical vapor deposition-grown AlGaN/AlN/GaN heterostructure at atomic-column resolution. In addition, the effects that different concentrations of Al in the AlN interlayer had on the electronic properties were evaluated. The maximum Al concentration in the AlN interlayer was 0.41, and the compositional grading at the upper and lower AlN interfaces resulted in an asymmetric broadening of the interface. Calculations show that increasing the Al concentration in the AlN interlayer leads to an approximately linear increase in the two-dimensional electron gas sheet density. The alloy scattering is also effectively suppressed when the Al concentration is greater than 0.4.

(Invited) Micromechanical Aspects of GaN Hemt Performance and Reliability

Performance and reliability of microelectronic devices are governed by the mechanical strain. Typically, strain engineering implies uniformly distributed strain. However, we offer a different perspective by hypothesizing that very small but localized strain (or stress) may have significant impact on the overall behavior of AlGaN/GaN high electron mobility transistors (HEMTs). Micro to nanoscale confined mechanical stress fields may develop unavoidably and are ignored because their spatial average is insignificant. We exploit high resolution techniques such as transmission electron microscopy (TEM) and micro-Raman spectroscopy to spatially resolve the stress field in GaN HEMTs to demonstrate the stress localization and then radiation effects.To study stress localization effects on the global electrical characteristics, we introduced a highly localized strain relief by milling a 20×30 μm2 micro trench about 70 μm deep on the backside of an 800×840 μm2 size HEMT die. The resulting local relaxation of in-plane residual strain was mapped using micro-Raman technique. Our results show that a decrease of only 0.02% strain can decrease the overall output saturation current up to ~20 %. The drop of output current is attributed to reduced two-dimensional electron gas (2DEG) sheet carrier density and electron mobility due to the strain relief in the device layers. However, the mechanistic process of strain relief also causes defect generation at the interfaces, which increases leakage current. Our technique for localized strain re-distribution could be an effective tool to surrogate the influence of inherent localized strain build-up across the channel of electronic devices.To study radiation effects, we exposed the HEMTs to gamma rays (up to 10 megaRads dose at 180 kiloRads per hour). We report the thermal and mechanical responses in terms of the changes in lattice strain and temperature, which were simultaneously characterized by changes in phonon frequency of E2 (high) and A1 (LO) from on- state and unpowered/pinched off reference states. Lower doses of radiation improved the electrical properties, however degradation initiated at about 1 megaRads. We observed about 16% decrease in saturation current and 6% decrease in transconductance at the highest dose. However, leakage current increase by 3 orders of magnitude was the most notable radiation effect. We observed temperature increase by 40% and mechanical stress increase by a factor of three at dose of 10 megaRads compared to the pristine devices. Spatial mapping of mechanical stress along the channel identifies the gate region as mechanically affected area, whereas the thermal degradation was mostly uniform. Transmission electron microscopy showed contrast changes reflecting high vacancy concentration in the gate region. These findings suggest that localized stress (mechanical hotspots) may increases vulnerability to radiation damage by accommodating higher concentration of defects that promote leakage current.Finally, we present experimental results on a novel concept of electron wind force driven room temperature annealing of GaN and SiC devices. This technique was applied on Gamma irradiated HEMT and SiC diodes. We demonstrate full recovery of the HEMTs irradiated to 10 mega-Rads gamma radiation. About 200% performance improvement was recorded for the SiC diodes. Our hypothesis is that mechanical stress waves are instrumental in eliminating interfacial defects.

Local strain modification effects on global properties of AlGaN/GaN high electron mobility transistors

Performance and reliability of microelectronic devices are governed by the mechanical strain within the device layers. Typically, this is studied with uniformly distributed strain applied externally or internally. The focus of this study is on AlGaN/GaN high electron mobility transistors (HEMTs), which is expected to be more sensitive to strain due to its piezo-resistive and piezo-electric nature. Accordingly, we hypothesize that even small but localized strain may have significant impact on the overall behavior of a HEMT. To investigate this hypothesis, we introduce highly localized strain relief by milling a 20 × 30 μm2 micro trench about 70 μm deep on the backside of an 800 × 840 μm2 size HEMT die. The resulting local relaxation of in-plane residual strain was mapped using micro-Raman technique. Our results show that a decrease of only 0.02% strain can decrease the overall output saturation current up to ~20%. The drop of output current is attributed to reduced two-dimensional electron gas (2DEG) sheet carrier density and electron mobility due to the strain relief in the device layers. However, the mechanistic process of strain relief also causes defect generation at the interfaces, which increases leakage current. Our technique for localized strain re-distribution could be an effective tool to surrogate the influence of inherent localized strain build-up across the channel of electronic devices.

Characterization of trap states in AlN/GaN superlattice channel high electron mobility transistors under total-ionizing-dose with 60Co γ-irradiation

In this Letter, the effects of trap states in AlN/GaN superlattice channel HEMTs (high electron mobility transistors) under total ionizing dose with γ-irradiation have been systematically investigated. After 1 Mrad γ-irradiation with a dose rate of 50 rad/s, negative drifts in threshold voltage and C–V characteristics are observed. Simultaneously, the two-dimensional electron gas sheet density of the upper channel increases from 5.09 × 1012 to 5.47 × 1012 cm−2, while that of the lower channel decreases from 4.41 × 1012 to 3.86 × 1012 cm−2, respectively. Furthermore, frequency-dependent capacitance and conductance measurements are adopted to investigate the evolution of trap states in an electron channel. The trap state density (DT = 0.21–0.88 × 1013 cm−2 eV−1) is over the ET range from 0.314 to 0.329 eV after irradiation for the upper channel, while the trap state in the lower channel decreases from 4.54 × 1011 cm−2 eV−1 at ET = 0.230 eV to 2.38 × 1011 cm−2 eV−1 at ET = 0.278 eV. The density (1.39–1.54 × 1011 cm−2 eV−1) of trap states with faster τT (0.033–0.037 μs) generated in a lower channel is located at shallower ET between 0.227 and 0.230 eV. The results reveal the mechanism of trap states in the channel, affecting the performance of HEMTs, which can provide a valuable understanding for hardening in space radiation.

Non-destructive beam spatial profile measurement using a gas sheet monitor for a high-intensity ion beam

A non-destructive beam spatial profile monitor system using a gas sheet has been developed. The 4 mm thick gas sheet of over 40 mm width enables increase of the local gas pressure high enough to record a two-dimensional beam profile. A CCD camera coupled to an image intensifier detects photons produced by beam-gas interaction with the signal intensity that makes the time evolution study of the beam possible. We demonstrated that the J-PARC 3MeV, 60 mA H- beam profile which is measured with intensity error of 8.4-12% and spatial resolution of 1 mm changed depending on the controls of the acceleration cavity in the beam pules of 50 μs.

Characterization of AlXGa1−XN/GaN High Electron Mobility Transistor Structures with Mercury Probe Capacitance–Voltage and Current–Voltage

Characterization of high electron mobility transistor (HEMT) structures is important for predicting device behavior, monitoring and developing processes, and improving performance. Multifrequency capacitance–voltage (CV) and direct current–voltage (IV) methods are applied with a mercury probe to assess the HEMT with and without capping layers to monitor critical parameters such as pinch‐off voltage, two‐dimensional electron gas (2DEG) sheet charge, GaN carrier density profile, AlGaN thickness, and dielectric constant and properties of the top capping layer.

Method for noncontact measurement of 2DEG mobility and carrier density in AlGaN/GaN on semi-insulating wafers

We present a charge-assisted sheet resistance technique for noncontact wafer level determination of two-dimensional electron gas (2DEG) mobility versus sheet carrier density without any test structures or gates. Instead, the electrical biasing of the 2DEG is provided by surface charge deposition, using a corona charging method. Analysis of the sheet resistance versus deposited charge identifies the 2DEG full depletion condition and enables calculation of the 2DEG sheet carrier density required for the mobility. Results for AlGaN/GaN heterostructures on semi-insulating SiC and sapphire substrates show good agreement with Hall results at a zero-bias condition.

Influence of the GaN cap layer thickness on the two- dimensional electron gas (2-DEG) sheet charge density of GaN/AlInN/GaN HEMTs with polarization effect

In this study, GaN/AlInN/GaN high electron mobility transistor (HEMT) with various GaN cap layer thickness and with heavily n-doping GaN cap layer were presented. In order to investigate the effects of a GaN capping layer on performance of the (GaN/AlInN/GaN) heterostructures, a simple analytical model for the threshold voltage of GaN/AlInN/GaN high electron mobility transistor (HEMT) is proposed by solving one dimensional (1 D) Poisson equation, leads to find the relation between the two dimensional electron gas (2DEG) and the control voltage. Spontaneous and piezoelectric polarizations at AlInN/GaN and GaN/AlInN interfaces have been incorporated in the analysis. Our simulations indicate that the GaN cap layer reduces the sheet density of the two dimensional electron gas (2DEG), leading to a decrease of the drain current, and that n+ doped GaN cap layer provides a higher sheet density than undoped one.

Strain-engineering in AlGaN/GaN HEMTs: impact of silicon nitride passivation layer on electrical performance

In this work, we present a physics-based analysis of two-dimensional electron gas (2DEG) sheet carrier density and other microwave characteristics such as transconductance and cutoff frequency of AlxGa1−xN/GaN high electron mobility transistors (HEMT). An accurate polarization-dependent charge control-based analysis is performed for microwave performance assessment in terms of current, transconductance, gate capacitances, and cutoff frequency of lattice-mismatched AlGaN/GaN HEMTs. The influence of stress on spontaneous and piezoelectric polarization is included in the simulation of an AlGaN/GaN HEMT. We have shown the change in threshold voltage (Vt) due to tensile and compressive strain with different gate lengths. Also, the influence of stress due to the change in nitride thickness is presented. Our simulation results for drain current, transconductance, and current-gain cutoff frequency for various gate length devices are calibrated and verified with experimental data over a wide range of gate and drain applied voltages, which are expected to be useful for microwave circuit design. The predicted transconductance, drain conductance, and operation frequency are quite close to the experimental data. The AlGaN/GaN heterostructure HEMTs with nitride passivation layers show great promise as a candidate in future high speed and high power applications.

AlGaN/GaN Heterostructures Electrical Performance by Altering GaN/Sapphire Buffers Growth Pressure and Low‐Temperature GaN Interlayers Application

AbstractIn this work, the possibility of improving 2D electron gas mobility and sheet carrier concentration in AlGaN/AlN/GaN heterostructures by altering GaN/sapphire buffer growth pressure and application of low‐temperature GaN interlayers in GaN buffer is investigated. Obtained results show some improvements in 2DEG resistivity reduction after the introduction of two GaN buffer sublayers grown at different pressures. It is observed that the MOVPE process pressure influences the size of GaN buffer block size visible on the scanning electron microscope images of the AlGaN/AlN/GaN surface. These data are complemented by the results of high‐resolution X‐ray diffraction, photoluminescence, and impedance spectroscopy of the investigated heterostructures.

Measurement and Reconstruction of a Beam Profile Using a Gas Sheet Monitor by Beam-Induced Fluorescence Detection in J-PARC

Measurement and Reconstruction of a Beam Profile Using a Gas Sheet Monitor by Beam-Induced Fluorescence Detection in J-PARC