Newsfeeds

Journal of Physics D: Applied Physics - latest papers

Latest articles for Journal of Physics D: Applied Physics

IOPscience

  • GaN-based double-heterojunction bipolar transistors with regrown p++-InGaN contact and etching damage recovery
    GaN-based heterojunction bipolar transistors (HBTs) have great potential in the fields of radio frequency power amplifier and power switching applications. However, the surface etching damage of p-type base layer has posed significant challenges to implementing ohmic contact and low resistance, which is the major bottleneck in improving the current drive capability of GaN-based HBTs. In this work, n-AlGaN/p-InGaN/n-GaN double HBTs (DHBTs) featured with ultralow specific on-resistance and high output current density were successfully fabricated. To reduce the base resistance and improve the ohmic contact of the etched p-InGaN base, a maskless regrowth of heavily Mg-doped p++-InGaN contact layer by metal–organic chemical vapor deposition (MOCVD) was exploited. MOCVD thermal treatment was further adopted to recover the etching damages of the base layer by filling the nitrogen vacancies with active nitrogen atoms. The base resistance and surface recombination current of the p-InGaN base were both substantially reduced. As a result, the maximum output current density of the DHBTs with an emitter area of 40 × 40 μm2 was increased from 0.2 to 14 kA cm−2, and the specific on-resistance was decreased from 38 to 0.45 mΩ·cm2. This work paves the way for the practical application of GaN-based DHBTs.

  • Diagnostic study on spatial distribution of electron temperature and density in gliding arc air plasma
    Gliding arc air plasma exhibits significant application potential in energy, environmental protection, and other fields due to its non-equilibrium characteristics (high electron temperature and high concentration of reactive species). However, the spatial distribution characteristics of its electron temperature and electron density play a crucial role in determining plasma chemical efficiency. In this study, a multi-spectral imaging method combined with a collisional-radiative model was employed to conduct high-resolution spatial distribution diagnosis of the high-energy electron temperature and electron density of air gliding arc plasma, and the influence law of gas flow rate on plasma parameters was analyzed. The experimental results show that the electron temperature in the plasma core region can reach 4 eV, and the electron density is on the order of 1014–1015 cm−3, with significant spatial inhomogeneity. By adjusting the gas flow rate of the gliding arc generator, the distribution law of electron parameters in the gliding arc plasma generator can be significantly changed. The research results can provide important data support for optimizing the design of gliding arc plasma reactors and promoting their industrial applications, meanwhile, this diagnostic method may achieve effective parameter distribution monitoring in the industrial application of gliding arc plasma.

  • Nitric oxide decomposition in a helium radio frequency atmospheric pressure plasma: kinetic and transport processes
    Plasma can remediate nitrogen oxides (NOx) emission from combustion processes. Nitric oxide (NO) oxidation reactions have been studied extensively. However, NO decomposition by plasma in a non-oxidizing environment, the timescales for NO dissociation reactions and their coupling with transport processes, the focus of this work, remains relatively unexplored. We report on axially resolved laser-induced fluorescence measurements of NO densities in a plasma in helium (He) with small admixtures of NO generated in a capillary tube by two outer ring electrodes, where one ring is powered by radio frequency (rf) high voltage. A limited number of chemical reactions describe the He/NO model system, and this description of the plasma chemistry allows for the quantification of the kinetic and transport mechanisms associated with the plasma-mediated NO decomposition. A 1D plug flow model shows the dominant role of electrons in addition to helium metastable species, and atomic nitrogen for NO decomposition, while the effect of metastables is largely counteracted by the recombination at the capillary wall, yielding a significant source of NO. A 2D reaction transport model assuming a given distribution of short-lived species in the plasma zone is also able to describe the NO recovery experimentally found in the plasma effluent. The observed NO recovery as a result of inhomogeneous NO decomposition by short-lived species and the resulting radial transport underlines the limitations of the widely used plug flow approximation for such systems.

  • Understanding the efficient microwave absorption of NiCo2O4-wrapped flaky FeCo composites after long-term elevated temperature (773 K) treatment
    High Curie temperature (Tc) flaky FeCo alloys with high dielectric and magnetic losses have great potential applications for preparing high-temperature (<773 K) absorbing materials, except for the disadvantages of easy to be oxidized and excessive complex permittivity. In this work, flaky FeCo was wrapped in NiCo2O4 nanoparticles (NiCo2O4@flaky FeCo) via a hydrothermal method and subsequently calcination process. The NiCo2O4 layer formed on the surface effectively prevents the oxidation of flaky FeCo and decreases the complex permittivity. NiCo2O4@flaky FeCo treated at elevated temperature (773 K) for 24 h (NiCo2O4@flaky FeCo-773 K) maintains moderated complex permittivity and desirable higher intricate permeability. As a result, remarkable microwave absorption performance can be achieved in NiCo2O4@flaky FeCo-773 K. In particular, the optimum reflection loss (RLmin) value reaches −47.78 dB at 8.0 GHz, and the effective absorption bandwidth (RL<−10 dB) is up to 7.52 GHz with a matching thickness of 1.9 mm, covering the whole Ku band. The corresponding maximum radar cross-section reduction value of 17.09 dBm2 is also realized. The designed NiCo2O4@flaky FeCo is expected to be a valuable absorber, which has the potential to be applied in elevated temperature environments.

  • Switchable multichannel vortex beam generation via temperature response and polarization multiplexing
    Vortex beams (VBs) hold substantial importance within the realms of optics and photonics. Currently, while numerous studies have focused on generating VBs using metasurfaces, most schemes are restricted to static modulation. Vanadium dioxide (VO2) exhibits significant insulator–metal transition properties, and its physical properties change under specific temperature or external stimuli. VO2 is an ideal candidate for designing dynamic metasurfaces and is expected to break through the static limitations of existing VB generation techniques. In this study, we propose an innovative scheme to generate polymorphic VBs using VO2 metasurface. In this scheme, meta-atoms are especially designed to exhibit different optical responses at different temperatures. When VO2 is metallic, multiple VBs with different topological charges (TCs) in the far field are generated in three polarization channels. When VO2 undergoes insulation, focused VBs with specific TCs in the near field are generated in three different channels. Therefore, multichannel VBs can be switched by temperature response and polarization multiplexing. Our scheme provides a novel way to dynamically generate and manage VBs.