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Optical and electrical properties of Hf- and Zn-doped β-Ga₂O₃ samples, which are n-type and insulating, respectively, were altered via high-energy electron irradiation at 2.5 or 0.5 MeV. The β-Ga₂O₃:Hf samples irradiated with 2.5 MeV electrons experienced a color change from blue to yellow and a large drop in conductivity, attributed to the creation of gallium vacancies, which compensate donors. This irradiation resulted in the absence of free carrier absorption and the presence of Cr³⁺ photoluminescence (PL). PL mapping prior to irradiation revealed optically active ZnO precipitates that formed during the growth of β-Ga₂O₃:Zn. These precipitates have a 384 nm (3.23more » eV) stacking fault emission in the core; in the outer shell of the precipitate, the PL blue-shifts to 377 nm (3.29 eV) and a broad defect band is observed. After 0.5 MeV electron irradiation, the defect band broadened and increased in intensity. The blue PL band (435 nm) of β-Ga₂O₃ was enhanced for both Hf- and Zn-doped samples irradiated with 0.5 MeV. This enhancement is correlated with an increase in oxygen vacancies.« less

Polyolefin separators are the most common separators used in rechargeable lithium (Li)-ion batteries. However, the influence of different polyolefin separators on the performance of Li metal batteries (LMBs) has not been well studied. By performing particle injection simulations on the reconstructed three-dimensional pores of different polyethylene separators, it is revealed that the pore structure of the separator has a significant impact on the ion flux distribution, the Li deposition behavior, and consequently, the cycle life of LMBs. It is also discovered that the homogeneity factor of Li-ion toward Li metal electrode is positively correlated to the longevity and reproducibility ofmore » LMBs. This work not only emphasizes the importance of the pore structure of polyolefin separators but also provides an economic and effective method to screen favorable separators for LMBs.« less

Cations with suitable sizes to occupy an interstitial site of perovskite crystals have been widely used to inhibit ion migration and promote the performance and stability of perovskite optoelectronics. However, such interstitial doping inevitably leads to lattice microstrain that impairs the long-range ordering and stability of the crystals, causing a sacrificial trade-off. Here, we unravel the evident influence of the valence states of the interstitial cations on their efficacy to suppress the ion migration. Incorporation of a trivalent neodymium cation (Nd³⁺) effectively mitigates the ion migration in the perovskite lattice with a reduced dosage (0.08%) compared to a widely usedmore » monovalent cation dopant (Na⁺, 0.45%). As a result, the photovoltaic performances and operational stability of the prototypical perovskite solar cells are enhanced with a trace amount of Nd³⁺ doping while minimizing the sacrificial trade-off.« less

Amorphous silicon films prepared by electron-beam evaporation have systematically and substantially greater atomic density for higher thickness, higher growth temperature, and slower deposition rate, reaching the density of crystalline Si when films of thickness greater than ~300 nm are grown at 425 °C and at <1 Å/s. Here, a combination of spectroscopic techniques provide insight into atomic disorder, local strains, dangling bonds, and nanovoids. Electron diffraction shows that the short-range order of the amorphous silicon is similar at all growth temperatures, but fluctuation electron microscopy shows that films grown above room temperature show a form of medium-range order not previouslymore » observed in amorphous silicon. Atomic disorder and local strain obtained from Raman spectroscopy are reduced with increasing growth temperature and show a nonmonotonic dependence on thickness. Dangling bond density decreases with increasing growth temperature and is only mildly dependent on thickness. Positron annihilation Doppler broadening spectroscopy and electron energy loss spectroscopy show that nanovoids, and not density variations within the network, are responsible for reduced atomic density. Specific heat and mechanical loss measurements, which quantify the density of tunneling two-level systems, in combination with the structural data, suggest that two-level systems in amorphous silicon films are associated with nanovoids and their surroundings, which are in essence loosely bonded regions where atoms are less constrained.« less

Wide-bandgap (WBG) mixed-halide perovskites as the front cell absorber are accomplishing perovskite-based tandem solar cells with over 29% power conversion efficiency. However, their large voltage deficits limit their ultimate performance. Only a handful of studies probe the fundamental mechanisms underlying the voltage deficits, which remain an unsolved challenge in the field. In this study, we investigate the formation dynamics and defect physics of WBG mixed-halide perovskites in contrast with their corresponding triiodide-based perovskites. Our results show that the inclusion of bromide introduced a halide homogenization process that occurs during the perovskite growth stage from an initial bromide-rich phase toward themore » final target stoichiometry. We further elucidated a physical model that correlates the role of bromide with the formation dynamics, defect physics, and eventual optoelectronic properties of the film. This work provides a fundamental and unique perspective toward understanding the performance-limiting factors affecting WBG mixed-halide perovskites.« less

Here, the formation and character of gallium vacancies (V_Ga) and their complexes in near surface and bulk regions of single crystal β-Ga₂O₃ were explored using unintentionally doped single crystals grown by the Czochralski method. As-grown and O₂ annealed (up to 1550 °C) samples were investigated using positron annihilation spectroscopy (PAS) to study the top 0.05–6 μm, and also current–voltage measurements and infrared (IR) spectroscopy, with hydrogenated samples to probe V_Ga, to study the bulk. After annealing in O₂ > 1000 °C, the β-Ga₂O₃ resistivity begins increasing, up to ~10⁹ Ω cm for 1550 °C treatment, with the top 0.5 mmmore » being many orders of magnitude more resistive. PAS measurements of the top 6 μm (S values) and very near surface 200 nm (diffusion length, L) indicate differential behavior as a function of peak annealing temperature. At least four temperature regimes of behavior are described. V_Ga are present in the bulk after growth, but considerable changes occur upon annealing at a temperature ≈1000 °C, where L and S decrease simultaneously, suggesting an increasing defect concentration (L) but a decreasing defect volume (S). Annealing at a temperature ≈1400 °C increases S again, showing an increasing volume concentration of V_Ga, with IR absorption showing a large signature of V_Ga-2H, indicative of increased V_Ga formation that was not present when annealing at a temperature ≈1000 °C. These results suggest that defect changes from annealing in oxygen are depth dependent, and that V_Ga configuration may not be the same near the oxygen-exposed surface of the sample and in the bulk.« less