Sensor Electronic Technology (United States)

Formation of a magnetic coherent structure via the merger of two plasmoids at solar supergranular junctions Abraham C.-L. Chian, Erico L. Rempel, Luis Bellot Rubio, Milan Gosic, and Yasuhito Narita We discuss the formation of a large magnetic coherent structure in a vortex expansion–contraction interval, resulting from the merger of two plasmoids driven by a supergranular vortex observed by Hinode in the quiet-Sun (Chian et al., MNRAS 535, 2436, 2024). Strong vortical flows at the interior of vortex boundary are detected by the localized regions of high values of the instantaneous vorticity deviation, and intense current sheets in the merging plasmoids are detected by the localized regions of high values of the local current deviation. The spatiotemporal evolution of the line-of-sight magnetic field, the horizontal electric current density, and the horizontal electromagnetic energy flux is investigated by elucidating the relation between velocity and magnetic fields in the photospheric plasma turbulence. A local and continuous amplification of magnetic field from 286 G to 591 G is detected at the centre of one merging plasmoid during the vortex expansion–contraction interval of 60 min. During the period of vortex contraction of 22.5 min, the line-of-sight magnetic field at the centre of plasmoid-1 (2) exhibits a steady decrease (increase), respectively, indicating a steady transfer of magnetic flux from plasmoid-1 to plasmoid-2. At the end of the vortex expansion–contraction interval, the two merging plasmoids reach an equipartition of electromagnetic energy flux, leading to the formation of an elongated magnetic coherent structure encircled by a shell of intense current sheets. Evidence of the disruption of a thin current sheet at the turbulent interface boundary layers of two merging plasmoids is presented. 

The interplay between nonradiative recombination and carrier localization in AlxGa1−xN epilayers (0.11 < x < 0.78) was studied using the photoluminescence (PL) and light-induced transient grating (LITG) techniques. The carrier localization conditions were estimated from the temperature dependence of the PL band peak position. The room-temperature carrier lifetimes estimated by LITG were limited by the competition between the carrier localization and nonradiative recombination. Strong carrier localization was shown to be insufficient to achieve high internal quantum efficiency in AlGaN epilayers.

The photoluminescence (PL) spectroscopy of AlGaN epilayers and heterostructures under quasi‐steady‐state conditions at different excitations in a wide temperature range from 8 to 300 K established the connection between the internal quantum efficiency, efficiency droop, and carrier localization. The carrier localization conditions were estimated from the temperature dependences of the PL band peak positions. The temperature dependences of the PL intensity were fitted using the Arrhenius‐type equation with two activation energies, which were attributed to the carrier redistribution within the localized states and carrier delocalization, respectively. The delocalization of the localized carriers affects the luminescence efficiency via the enhancement of the two mechanisms of opposite signs: the bimolecular radiative recombination and nonradiative recombination due to an increased probability to reach nonradiative recombination centers.

Scanning near-field photoluminescence (PL) spectroscopy was applied to study spatial variations of emission spectra of AlxGa1−xN epilayers with 0.6≤x≤0.7. PL spectra were found to be spatially uniform with peak wavelength standard deviations of only ∼2 meV and ratios between peak intensity standard deviations and average peak intensity values of 0.06. The observed absence of correlation between the PL peak wavelength and intensity shows that spatial distribution of nonradiative recombination centers is not related to band potential fluctuations. Our results demonstrate that the homogeneous broadening and the random cation distribution primarily determine PL linewidths for layers grown under optimized conditions.

The results on design, fabrication, and characterization of a hot-electron bolometer (HEB) based on the low-mobility two-dimensional electron gas (2DEG) in a AlGaN/GaN heterostructures show that our HEBs have high coupling to incident THz radiation due to the Drude absorption. Significant heating by THz radiation is realized due to the small value of the electron heat capacity. A low contact resistance achieved in our devices ensures that the THz radiation couples primarily to the 2DEG. Due to the small electron momentum relaxation time, the real part of the 2DEG sensor impedance is ~50–100 Ω, which provides a good impedance match between the sensor and antenna. Currently the room temperature responsivity of our devices reaches ~ 0.04 A/W at 2.55 THz along with a noise equivalent power ~ 5 nW/Hz1/2 at room temperature.

III-Nitride heterostructure field-effect transistors (HFETs) demonstrated a new paradigm in microwave switching and control applications due to unique combination of extremely low channel resistance (leading to low loss), very high RF power, low off-state capacitance, broad range of operating temperatures, chemical inertness and robustness. The paper reviews novel approaches and recent advances in III-Nitride technology for RF switching devices leading to higher operating frequencies and even lower insertion loss.

Optical and electronic properties of mid-ultraviolet light emitting diodes with a two-step p-AlGaN region are measured at temperatures from 8 to 300 K. While the electroluminescence intensity increases down to a temperature lower than that reported in InGaN light emitting diodes (LEDs), there is still a collapse at cryogenic temperatures. Capacitance-voltage measurements across the same range of temperatures reveal an increasing depletion region with decreasing temperature indicating that hole injection is a significant factor in the temperature dependent behavior, as well as the cause of electroluminescence collapse.