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Morphology Evolution in Self-Healing Liquid-Gallium-Based Mg-Ion Battery Anode
Conventional solid battery anodes suffer from dendrite growth and volume changes, resulting in poor cycling performances. Self-healing electrodes made of liquid metals and alloys can overcome these issues by reversibly undergoing liquid–solid phase transformations during (dis)charging. Just recently, we achieved 1000 cycles in Mg-ion battery anodes using liquid Ga as the active material, which reversibly transformed into solid Mg 2 Ga 5 during (de)magnesiation. However, the fundamental mechanism behind this liquid–solid phase transformation remains unclear. In this follow-up work, cryogenic focused ion beam electron microscopy is used to elucidate the morphology evolution when liquid Ga embedded in a carbon-binder matrix transforms into solid Mg 2 Ga 5 during cycling. We found that liquid Ga gradually diffuses into the surrounding matrix, increasing the contact area between active material and electrolyte, facilitating the charge transfer process. These new insights will help understand the performance of alkali- and alkaline-earth-metal batteries based on liquid anodes.
14.8% Quantum Efficient Gallium Phosphide Photocatalyst for Hydrogen Evolution
Not Available
Controlling the propagation asymmetry of hyperbolic shear polaritons in beta-gallium oxide
Structural anisotropy in crystals is crucial for controlling light propagation, particularly in the infrared spectral regime where optical frequencies overlap with crystalline lattice resonances, enabling light-matter coupled quasiparticles called phonon polaritons (PhPs). Exploring PhPs in anisotropic materials like hBN and MoO 3 has led to advancements in light confinement and manipulation. In a recent study, PhPs in the monoclinic crystal β-Ga 2 O 3 (bGO) were shown to exhibit strongly asymmetric propagation with a frequency dispersive optical axis. Here, using scanning near-field optical microscopy (s-SNOM), we directly image the symmetry-broken propagation of hyperbolic shear polaritons in bGO. Further, we demonstrate the control and enhancement of shear-induced propagation asymmetry by varying the incident laser orientation and polariton momentum using different sizes of nano-antennas. Finally, we observe significant rotation of the hyperbola axis by changing the frequency of incident light. Our findings lay the groundwork for the widespread utilization and implementation of polaritons in low-symmetry crystals.
Polytypism in few-layer gallium selenide
Different stacking sequences due to polytypism in 2-dimensional few-layer GaSe were identified by Raman spectroscopy, HR-S/TEM, and theoretical calculations.
Enhancing interfacial charge transfer in a WO 3 /BiVO 4 photoanode heterojunction through gallium and tungsten co-doping and a sulfur modified Bi 2 O 3 interfacial layer
Photoanodes containing a WO 3 /BiVO 4 heterojunction have demonstrated promising photoelectrochemical water splitting performance, but the ability to effectively passivate the WO 3 /BiVO 4 interface has limited charge transport and collection.
Recovery from plasma etching-induced nitrogen vacancies in p-type gallium nitride using UV/O 3 treatments
Plasma etching of p-type GaN creates n-type nitrogen vacancy (V N ) defects at the etched surface, which can be detrimental to device performance. In mesa isolated diodes, etch damage on the sidewalls degrades the ideality factor and leakage current. Here a treatment was developed to recover both the ideality factor and leakage current, which uses UV/O 3 treatment to oxidize the damaged layers followed by HF etching to remove them. The temperature dependent I–V measurement shows that the reverse leakage transport mechanism is dominated by Poole–Frenkel emission at room temperature through the etch-induced V N defect. Depth resolved cathodoluminescence confirms that the damage is limited to first several nanometers and is consistent with the V N defect.
Temperature-dependent growth of hexagonal and monoclinic gallium sulfide films by pulsed-laser deposition
We demonstrate the selective, pulsed-laser deposition of hexagonal GaS and monoclinic Ga2S3 films on sapphire substrates from a single Ga 2 S 3 target in high-vacuum conditions. Growth at substrate temperatures below 550 °C causes GaS film formation, which indicates non-stoichiometric transfer from the target to the film. Surprisingly, stoichiometric transfer occurs at substrate temperatures above 650 °C with monoclinic Ga 2 S 3 as the preferred, higher S-content phase. Through a series of growth and annealing experiments, we show that GaS nucleation under S-deficient conditions leads to the preferred growth of this layered, hexagonal phase below 550 °C. Furthermore, GaS films annealed above 650 °C under high vacuum are transformed to Ga 2 S 3 , reflecting the greater stability of the monoclinic phase. By first growing Ga 2 S 3 at a higher temperature and subsequently growing GaS at a lower temperature, we can fabricate GaS/Ga 2 S 3 heterostructures in a single growth process.
External charge compensation in etched gallium nitride measured by x-ray photoelectron spectroscopy
Electronic states at GaN surfaces and at regrowth and heteroepitaxy interfaces inhibit electronic device performance. Understanding electronic state configuration at the GaN surface is, therefore, crucial for the development of GaN-based devices, which are currently of considerable interest in power electronic applications. GaN and other wurtzite III-nitrides possess large spontaneous polarization along the c-axis, producing a bound sheet charge at the surface, which affects the electronic state configuration through the formation of internal and external compensation charges. Defects induced by conventional plasma-based dry etching methods may inhibit the internal screening of this bound charge and thus increase the concentration of external charged states. The surface band bending of n-type Ga-face GaN (0001) was measured with x-ray photoelectron spectroscopy after inductively coupled plasma etching to investigate the impact of dry etching on external charge compensation. GaN samples were etched using inductively coupled plasma with varying rf power and a novel plasma-enhanced atomic layer etching method using an oxidation, fluorination, and ligand-exchange mechanism. The band bending varied from 0.0 to 0.8 ± 0.1 eV for the samples measured.
Role of defects in ultra-high gain in fast planar tin gallium oxide UV-C photodetector by MBE
Here we report ultra-high responsivity of epitaxial (Sn x Ga 1–x ) 2 O 3 (TGO) Schottky UV-C photodetectors and experimentally identified the source of gain as deep-level defects, supported by first principles calculations. Epitaxial TGO films were grown by plasma-assisted molecular beam epitaxy on (–201) oriented n-type β-Ga 2 O 3 substrates. Fabricated vertical Schottky devices exhibited peak responsivities as high as 3.5 ×10 4 A/W at –5 V applied bias under 250 nm illumination with sharp cutoff shorter than 280 nm and fast rise/fall time in milliseconds order. Hyperspectral imaging cathodoluminescence (CL) spectra were examined to find the mid-bandgap defects, the source of this high gain. Irrespective of different tin mole fractions, the TGO epilayer exhibited extra CL peaks at the green band (~2.20 eV) not seen in β-Ga 2 O 3 along with enhancement of the blue emission-band (~2.64 eV) and suppression of the UV emission-band. Based on hybrid functional calculations of the optical emission expected for defects involving Sn in β-Ga 2 O 3 , V Ga –Sn complexes are proposed as potential defect origins of the observed green and blue emission-bands. Such complexes behave as acceptors that can efficiently trap photogenerated holes and are predicted to be predominantly responsible for the ultra-high photoconductive gain in the Sn-alloyed Ga 2 O 3 devices by means of thermionic emission and electron tunneling. Regenerating the V Ga –Sn defect complexes by optimizing the growth techniques, we have demonstrated a planar Schottky UV-C photodetector of the highest peak responsivity.
Selective incorporation of antimony into gallium nitride
Dilute concentrations of antimony (Sb) incorporation into GaN induce strong bandgap bowing and tunable room-temperature photoluminescence from the UV to the green spectral regions. However, the atomistic details of the incorporation of Sb into the GaN host remain unclear. In this work, we use first-principles calculations to understand the thermodynamics of Sb substitution into GaN and its effect on the optical and Raman spectra. Although it is empirically considered that Sb is preferentially incorporated as an anion (Sb3−) into the N sublattice, we demonstrate that Sb can also be incorporated as a cation (Sb3+, Sb5+) into the metal sublattice. Our thermodynamic analysis demonstrates that SbN0, SbGa2+, and SbGa0 can co-exist under Ga-rich conditions in n-type samples. We further confirm the dual incorporation of Sb by calculating the vibrational frequencies of different anionic and cationic substitutions to explain the origins of experimentally observed additional Raman peaks of Sb-doped GaN. Moreover, the calculated band structures of different Sb substitutions into GaN explain the experimental photoluminescence and optical absorption spectra. Overall, our analysis suggests that the coexistence of Sb3−, Sb3+, and Sb5+ substitutions into GaN explains the totality of experimental measurements. Our results demonstrate that the selective incorporation of Sb into GaN (and potentially other group-V elements such as As, P, or Bi) by tuning the growth conditions can drastically modify the electronic properties, for applications in visible light emitters and photocatalysis.
All-MOCVD-grown gallium nitride diodes with ultra-low resistance tunnel junctions
In this work, we carefully investigate three important effects including postgrowth activation annealing, delta (δ) dose and magnesium (Mg) buildup delay as well as experimentally demonstrate their influence on the electrical properties of GaN homojunction p–n diodes with a tunnel junction (TJ). The diodes were monolithically grown by metalorganic chemical vapor deposition (MOCVD) in a single growth step. By optimizing the annealing parameters for Mg activation, δ-dose for both donors and acceptors at TJ interfaces, and p + -GaN layer thickness, a significant improvement in tunneling properties is achieved. For the TJs embedded within the continuously-grown, all-MOCVD GaN diode structures, ultra-low voltage penalties of 158 mV and 490 mV are obtained at current densities of 20 A cm –2 and 100 A cm –2 , respectively. The diodes with the engineered TJs show a record-low differential resistivity of 1.6 × 10 –4 Ω cm 2 at 5 kA cm –2 .
Theory of magnetic 3 d transition metal dopants in gallium nitride
Not Available
Mass measurements of neutron-rich gallium isotopes refine production of nuclei of the first r -process abundance peak in neutron-star merger calculations
We report mass measurements of neutron-rich Ga isotopes 80–85 Ga with TRIUMF's Ion Trap for Atomic and Nuclear science. The measurements determine the masses of 80–83 Ga in good agreement with previous measurements. Here, the masses of 84 Ga and 85 Ga were measured for the first time. Uncertainties between 25 and 48 keV were reached. The new mass values reduce the nuclear uncertainties associated with the production of A ≈ 84 isotopes by the r-process for astrophysical conditions that might be consistent with a binary neutron star (BNS) merger producing a blue kilonova. Our nucleosynthesis simulations confirm that BNS merger may contribute to the first abundance peak under moderate neutron-rich conditions with electron fractions Y e = 0.35–0.38.
Low-Temperature Geiger-Mode Characterization of a Gallium Nitride p-i-N Avalanche Photodiode
Not provided.
Temperature-Agnostic Pt/Au Ohmic Contacts on n -Type Gallium Nitride for Self-Aligned MOSFETs
Five alternative metals are investigated as ohmic contacts to n-GaN including Cr/Au, Mo/Au, Pt/Au, Pd/Au, and Ge/Au. Ti-based contacts are traditionally used for ohmic contacts on n-GaN. However, conventional Ti/Al/Ni/Au metallization is found to be incompatible with a self-aligned process for GaN trench MOSFETs due to wet etch restrictions. Therefore, an alternative metallization is needed that is unreactive to the etch chemistry used in the self-aligned process. Additionally, the contact should remain ohmic following anneal at 900 °C so that contact formation can precede the anneal required for p-dopant activation. Here, in the present work, an n-GaN bilayer, consisting of a thin heavily doped contact layer (n 0 = 1 × 10 20 cm −3 ) atop a thick lesser doped layer, is used to demonstrate ohmic contacts of alternative metals with low specific contact resistance and extended thermal budget. Cr/Au ohmic contacts are demonstrated up to anneal temperatures of 800 °C, an increase of 200 °C compared to the highest known reports for Cr/Au contacts on n-GaN. Pt/Au metallization is demonstrated as an ohmic contact to n-GaN for the first time and exhibits true temperature-agnostic behavior up to anneal temperatures of 900 °C with specific contact resistance that is near parity with Ti/Al/Ni/Au. The temperature-agnostic behavior of Pt/Au ohmic contacts on the n-GaN bilayer, in addition to chemical compatibility with the self-aligned process, positions Pt/Au contacts as a key enabling element for self-aligned trench MOSFETs on GaN.