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1. Enhancement of 2D topological semimetal transport properties by current annealing

   https://doi.org/10.1063/5.0102933  

   Fereidouni, A.; Doha, M. H.; Pandey, K.; Basnet, R.; Hu, J.; Churchill, H. O. (September 2022, Applied Physics Letters)

   Observation of intrinsic quantum transport properties of two-dimensional (2D) topological semimetals can be challenging due to suppression of high mobility caused by extrinsic factors introduced during fabrication. We demonstrate current annealing as a method to substantially improve electronic transport properties of 2D topological semimetal flakes. Contact resistance and resistivity were improved by factors up to [Formula: see text] and [Formula: see text], respectively, in devices based on exfoliated flakes of two topological semimetals, ZrSiSe and BaMnSb 2 . Using this method, carrier mobility in ZrSiSe was improved by a factor of 3800, resulting in observation of record-high mobility for exfoliated ZrSiSe. Quantum oscillations in annealed ZrSiSe appeared at magnetic fields as low as 5 T, and magnetoresistance increased by a factor of 10 4 . We argue that a thermal process underlies this improvement. Finally, Raman spectroscopy and analysis of quantum oscillations in ZrSiSe indicate that the phonon modes and Fermi surface area are unchanged by current annealing. 

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2. Weak magnetic field-dependent photoluminescence properties of lead bromide perovskites

   https://doi.org/10.1063/5.0085947  

   Butler, Rory; Burns, Randy; Babaian, Dallar; Anderson, Matthew J.; Ullrich, Carsten A.; Morrell, Maria V.; Xing, Yangchuan; Lee, Jaewon; Yu, Ping; Guha, Suchismita (March 2022, Journal of Applied Physics)

   The strong spin–orbit coupling (SOC) in lead halide perovskites, when inversion symmetry is lifted, has provided opportunities for investigating the Rashba effect in these systems. Moreover, the strong orbital moment, which, in turn, impacts the spin-pair in singlet and triplet electronic states, plays a significant role in enhancing the optoelectronic properties in the presence of external magnetic fields in lead halide perovskites. Here, we investigate the effect of weak magnetic fields (<1 T) on the photoluminescence (PL) properties of [Formula: see text] nanocrystals with and without Ruddlesden–Popper (RP) faults and single crystals of [Formula: see text]. Along with an enhancement in the PL intensity as a function of an external magnetic field, which is observed in both lead bromide perovskites, the PL emission red-shifts in [Formula: see text] nanocrystals. Density-functional theory calculations of the electronic band-edge in [Formula: see text] show almost no change in the energy gap as a function of the external magnetic field. The experimental results, thus, suggest the role of mixing of the triplet and singlet excitonic states under weak magnetic fields. This is further deduced from an enhancement in PL lifetimes as a function of the field in [Formula: see text]. In [Formula: see text], an increase in PL intensity is observed under weak magnetic fields; however, no changes in the peak energy or PL lifetimes are observed. The internal magnetic fields due to SOC are characterized for all three samples and found to be the highest for [Formula: see text] nanocrystals with RP faults. 

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3. Transport properties of polarization-induced 2D electron gases in epitaxial AlScN/GaN heterojunctions

   https://doi.org/10.1063/5.0108475  

   Casamento, Joseph; Nguyen, Thai-Son; Cho, Yongjin; Savant, Chandrashekhar; Vasen, Timothy; Afroz, Shamima; Hannan, Daniel; Xing, Huili; Jena, Debdeep (November 2022, Applied Physics Letters)

   AlScN is attractive as a lattice-matched epitaxial barrier layer for incorporation in GaN high electron mobility transistors due to its large dielectric constant and polarization. The transport properties of polarization-induced two-dimensional (2D) electron gas of densities of [Formula: see text]/cm 2 formed at the AlScN–GaN interface is studied by Hall-effect measurements down to cryogenic temperatures. The 2D electron gas densities exhibit mobilities limited to ∼300 cm 2 /V s down to 10 K at AlScN/GaN heterojunctions. The insertion of a ∼2 nm AlN interlayer boosts the room temperature mobility by more than five times from ∼300 cm 2 /V s to [Formula: see text] cm 2 /V s, and the 10 K mobility by more than 20 times to ∼6980 cm 2 /V s at 10 K. These measurements provide guidelines to the limits of electron conductivities of these highly polar heterostructures. 

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4. Severe Electron Environment for Surface Charging at Geostationary and Medium Earth Orbits

   https://doi.org/10.2514/1.A36011  

   Ganushkina, Natalia Yu; Matéo-Vélez, Jean-Charles; Dubyagin, Stepan (November 2024, Journal of Spacecraft and Rockets)

   Electron radiation environment with keV energies on medium earth orbit (MEO) is investigated around the times when worst-case severe environments for surface charging were detected at Los Alamos National Laboratory (LANL) satellites on geostationary earth orbit (GEO). The Inner Magnetosphere Particle Transport and Acceleration Model (IMPTAM) is employed to model 1–100 keV electron fluxes in the inner Earth’s magnetosphere (inside [Formula: see text]) during four representative events selected out of 400 LANL worst-case severe environment events. Modeled fluxes are compared to the observed ones on GEO, and the peak electron fluxes are then extracted at [Formula: see text] which is considered as approximate MEO environment following the LANL events. IMPTAM generally reproduces the 10–50 keV electron fluxes on GEO but underestimates (of factor of 2 to 5) at energies above. It is found that the maximum electron flux on MEO is reached in about 1.5–2 h after the worst-case environment for surface charging was detected at GEO. Whereas the magnetic local time (MLT) location of the worst-case severe environments on GEO varies from 23 to 05, the maximum electron flux on MEO is located on 6–7.8 MLT. The maximum electron flux values on MEO can be 2 to 10 times higher than those observed on GEO and those estimated using European Cooperation for Space Standardization and National Aeronautics and Space Administration guidelines, which can be a significant increase of the risks in terms of surface charging. 

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5. Non-destructive depth-resolved characterization of residual strain fields in high electron mobility transistors using differential aperture x-ray microscopy

   https://doi.org/10.1063/5.0109606  

   Pagan, Darren C.; Rasel, Md A.; Lim, Rachel E.; Sheyfer, Dina; Liu, Wenjun; Haque, Aman (October 2022, Journal of Applied Physics)

   Localized residual stress and elastic strain concentrations in microelectronic devices often affect the electronic performance, resistance to thermomechanical damage, and, likely, radiation tolerance. A primary challenge for the characterization of these concentrations is that they exist over sub-[Formula: see text]m length-scales, precluding their characterization by more traditional residual stress measurement techniques. Here, we demonstrate the use of synchrotron x-ray-based differential aperture x-ray microscopy (DAXM) as a viable, non-destructive means to characterize these stress and strain concentrations in a depth-resolved manner. DAXM is used to map two-dimensional strain fields between the source and the drain in a gallium nitride (GaN) layer within high electron mobility transistors (HEMTs) with sub-[Formula: see text]m spatial resolution. Strain fields at various positions in both pristine and irradiated HEMT specimens are presented in addition to a preliminary stress analysis to estimate the distribution of various stress components within the GaN layer. [Formula: see text]-irradiation is found to significantly reduce the lattice plane spacing in the GaN along the sample normal direction, which is attributed to radiation damage in transistor components bonded to the GaN during irradiation. 

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