Beta-phase gallium oxide ([Formula: see text]-Ga 2 O 3 ) is a promising semiconductor for high frequency, high temperature, and high voltage applications. In addition to the [Formula: see text]-phase, numerous other polymorphs exist and understanding the competition between phases is critical to control practical devices. The phase formation sequence of Ga 2 O 3 , starting from amorphous thin films, was determined using lateral-gradient laser spike annealing at peak temperatures of 500–1400 °C on 400 μs to 10 ms timescales, with transformations characterized by optical microscopy, x-ray diffraction, and transmission electron microscopy (TEM). The resulting phase processing map showed the [Formula: see text]-phase, a defect-spinel structure, first nucleating under all annealing times for temperatures from 650 to 800 °C. The cross-sectional TEM at the onset of the [Formula: see text]-phase formation showed nucleation near the film center with no evidence of heterogeneous nucleation at the interfaces. For temperatures above 850 °C, the thermodynamically stable [Formula: see text]-phase was observed. For anneals of 1–4 ms and temperatures below 1200 °C, small randomly oriented grains were observed. Large grains were observed for anneals below 1 ms and above 1200 °C, with anneals above 4 ms and 1200 °C resulting in textured films. The formation of the [Formula: see text]-phase prior to [Formula: see text]-phase, coupled with the observed grain structure, suggests that the [Formula: see text]-phase is kinetically preferred during thermal annealing of amorphous films, with [Formula: see text]-phase subsequently forming by nucleation at higher temperatures. The low surface energy of the [Formula: see text]-phase implied by these results suggests an explanation for the widely observed [Formula: see text]-phase inclusions in [Formula: see text]-phase Ga 2 O 3 films grown by a variety of synthesis methods.
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A combined helium atom scattering and density-functional theory study of the Nb(100) surface oxide reconstruction: Phonon band structures and vibrational dynamics
Helium atom scattering and density-functional theory (DFT) are used to characterize the phonon band structure of the (3 × 1)-O surface reconstruction of Nb(100). Innovative DFT calculations comparing surface phonons of bare Nb(100) to those of the oxide surface show increased resonances for the oxide, especially at higher energies. Calculated dispersion curves align well with experimental results and yield atomic displacements to characterize polarizations. Inelastic helium time-of-flight measurements show phonons with mixed longitudinal and shear-vertical displacements along both the ⟨[Formula: see text]⟩, [Formula: see text] and ⟨[Formula: see text]⟩, [Formula: see text] symmetry axes over the entire first surface Brillouin zone. Force constants calculated for bulk Nb, Nb(100), and the (3 × 1)-O Nb(100) reconstruction indicate much stronger responses from the oxide surface, particularly for the top few layers of niobium and oxygen atoms. Many of the strengthened bonds at the surface create the characteristic ladder structure, which passivates and stabilizes the surface. These results represent, to our knowledge, the first phonon dispersion data for the oxide surface and the first ab initio calculation of the oxide’s surface phonons. This study supplies critical information for the further development of advanced materials for superconducting radiofrequency cavities.
more » « less- Award ID(s):
- 2011854
- NSF-PAR ID:
- 10364221
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 156
- Issue:
- 12
- ISSN:
- 0021-9606
- Page Range / eLocation ID:
- Article No. 124702
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Perovskite oxides (ternary chemical formula ABO3) are a diverse class of materials with applications including heterogeneous catalysis, solid-oxide fuel cells, thermochemical conversion, and oxygen transport membranes. However, their multicomponent (chemical formula
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In this work, we report a study of the temperature dependent pulsed current voltage and RF characterization of [Formula: see text]-(Al x Ga 1−x ) 2 O 3 /Ga 2 O 3 hetero-structure FETs (HFETs) before and after silicon nitride (Si 3 N 4 ) passivation. Under sub-microsecond pulsing, a moderate DC-RF dispersion (current collapse) is observed before passivation in gate lag measurements, while no current collapse is observed in the drain lag measurements. The dispersion in the gate lag is possibly attributed to interface traps in the gate–drain access region. DC-RF dispersion did not show any strong dependence on the pulse widths. Temperature dependent RF measurements up to 250 °C do not show degradation in the cutoff frequencies. After Si 3 N 4 deposition at 350 °C, a shift of the threshold voltage is observed which changed the DC characteristics. However, the current collapse is eliminated; at 200 ns pulse widths, a 50% higher current is observed compared to the DC at high drain voltages. No current collapse is observed even at higher temperatures. RF performance of the passivated devices does not show degradation. These results show that ex situ deposited Si 3 N 4 is a potential candidate for passivation of [Formula: see text]-(Al x Ga 1−x ) 2 O 3 /Ga 2 O 3 HFETs.more » « less
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Phase pure PbZr 0.52 Ti 0.48 O 3 (PZT) films with up to 13 mol. % Nb were prepared on Pt-coated Si substrates using chemical solution deposition; charge compensation for Nb was accomplished by reducing the concentration of lead in the film. For high Nb doping levels, (1) superoxidation of the PZT film surface makes the PZT/Pt interface more p-type and, hence reduces electron injection over the Schottky barrier, (2) the bulk charge transport mechanism changes from electron trapping by Ti 4+ to hole migration between lead vacancies, and (3) the ionic conductivity due to migration of oxygen vacancies decreases. For [Formula: see text] Nb, electrical degradation was controlled via field-induced accumulation of oxygen vacancies near the cathode, which, in turn, leads to Schottky barrier lowering and electron trapping by Ti 4+ . In phase pure 13 mol. % Nb doped PZT films, on the other hand, the increase in the leakage current during electrical degradation was dominated by hole migration between lead vacancies ([Formula: see text]. A much lower lifetime and drastic increase in the leakage current upon electrical degradation was observed in mixed phase PNZT films, which was attributed to (1) a more electrically conductive pyrochlore phase and (2) a high concentration of lead vacancies.more » « less
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We have conducted three-dimensional (3D) 0–7.5 Hz physics-based wave propagation simulations to model the seismic response of the Long Valley Dam (LVD), which has formed Lake Crowley in Central California, to estimate peak ground motions and settlement of the dam expected during maximum credible earthquake (MCE) scenarios on the nearby Hilton Creek Fault (HCF). We calibrated the velocity structure, anelastic attenuation model, and the overall elastic properties of the dam via linear simulations of a Mw3.7 event as well as the Mw6.2 Chalfant Valley earthquake of 1986, constrained by observed ground motions on and nearby the LVD. The Statewide California Earthquake Center (SCEC) Community Velocity Model CVM-S4.26.M01 superimposed with a geotechnical layer using [Formula: see text] information tapered from the surface to a 700-m depth was used in the simulations. We found optimal fit of simulated and observed ground motions at the LVD using frequency-independent attenuation of [Formula: see text] ([Formula: see text] in m/s). Using the calibrated model, we simulated 3D nonlinear ground motions at the LVD for Mw6.6 rupture scenarios on the HCF using an Iwan-type, multi-yield-surface technique. We use a two-step method where the computationally expensive nonlinear calculations were carried out in a small domain with the plane wave excitation along the bottom boundary obtained from a full-domain 3D linear finite-fault simulation. Our nonlinear MCE simulation results show that peak ground velocities (PGVs) and peak ground accelerations (PGAs) as high as 72 cm/s and 0.55 g, respectively, can be expected at the crest of the LVD. Compared with linear ground motion simulation results, our results show that Iwan nonlinear damping reduces PGAs on the dam crest by up to a factor of 8 and increasingly depletes the high-frequency content of the waves toward the dam crest. We find horizontal relative displacements of the material inside the dam of up to [Formula: see text] and up to [Formula: see text] of vertical subsidence, equivalent to 1% of the dam height.
