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We attain N-polar and Al-polar AlN thin films on Si(111) substrates by plasma-assisted molecular beam epitaxy. The polarity of AlN epilayers has been validated by wet chemical etching using tetramethylammonium hydroxide and by the direct cross-sectional observation of atomic stacking under high-angle annular dark-field scanning transmission electron microscopy. For the 290 nm-thick as-grown N-polar AlN epilayer, x-ray diffraction (XRD) (002) and (102) ω rocking curve peak full width half maximums (FWHMs) are 475 and 1177 arcsec, and the surface mean square roughness (RMS) is 0.30 nm. We flipped the polarity using the metal-flux-modulation-epitaxy (MME) strategy. The MME strategy promotes anti-phase boundaries (APBs) on the { 2 2 ¯ 01 } crystalline planes instead of commonly observed lateral planar APBs in AlN epilayers. Merging of the tilted APBs at ∼50 nm leads to a complete Al-polar surface. For the 180 nm-thick Al-polar AlN epilayer, XRD (002) and (102) peak FWHMs are 1505 and 2380 arcsec, and the surface RMS is 1.41 nm. Strain analysis by XRD and Raman spectroscopy indicates a uniform tensile strain of 0.160% across the N-polar AlN epilayer surface and a strain distribution of 0.113%–1.16% through the epilayer. In contrast, the Al-polar AlN epilayer exhibits a much broader tensile strain distribution of 0.482%–2.406% along the growth direction, potentially due to the interaction of polarity inversion and strain relaxation. [ABSTRACT FROM AUTHOR]

The bremsstrahlung is an important source term in the shielding design of the electron accelerator. In order to study the characteristics of the photon source term in the 90° direction and the influence of target radius and thickness on the bremsstrahlung in the 90° direction, the Monte Carlo program of MCNPX27 was used to calculate the radiation source term from the 15 MeV3 GeV electron beam incident on the iron target with different thicknesses and radii. The variation characteristics of radiation source items such as photon radiation dose rate and photon energy spectrum with target thickness and radius were obtained and comparatively analyzed. By comparing with the photon source term in the 0° direction and the energy deposition of cascade electron in the target, the main features of the photon source term in the 90° direction were obtained. Finally, the effects of beam power and beam intensity on radiation dose rates in the 90° and 0° directions were analyzed. The result shows that, due to the combined influence of electron energy deposition in the target and self-absorption of photons from the target, the radius and thickness of the target are expected as important factors which affect the photon source term in the 90° direction. The energy of the cascade electrons deposited in the target and the radiation dose rate in the 90° direction increases with the thickness. As the energy of the cascade electrons is completely deposited in the target, the radiation dose rate in the 90° direction will not change with the target thickness. The radiation dose rate in the 90° direction also increases with the target radius, but when the target radius is larger, the radiation dose rate in the 90° direction decreases due to the selfabsorption. Apart from that, the effect of incident electron energy on the photon source terms in the 90° direction and 0° direction is different. More than 99% of the photons in the 90° direction are below 10 MeV, and most of them are within 3 MeV. In the 90° direction, the photon energy spectrum shape has little relationship with the incident electron energy. However, the photon energy in the 0° direction can reach the incident electron energy, which is generally much larger than the photon energy in the 90° direction, and the shape of the energy spectrum is greatly affected by the incident electron. As a suggestion, the influence of the target size should be considered in the shielding design of the electron accelerator, and the design in the lateral shielding should be optimized according to the difference between the photon source term in the 90° direction and 0° direction.

Sc-alloyed AlN epilayers were grown on commercial GaN/sapphire templates by molecular beam epitaxy (MBE) to validate the Sc composition of 18% and in-plane lattice matching with GaN. As-grown Sc0.18Al0.82N/GaN heterostructure exhibits a two-dimensional electron gas (2DEG) density of 3.58 × 1013 (2.96 × 1013) cm−2 and a mobility of 241 (295) cm2⋅V−1⋅s−1 at 300 (77) K, demonstrating the feasibility of employing its high spontaneous polarization toward high 2DEG density and highlighting detrimental impurity scattering due to the regrowth interface. Implementation of AlN impurity blocking layers boosts 2DEG mobility to 1290 (8730) cm2 V−1⋅s−1at 300 (77) K. In addition, we have engineered a surface-treatment strategy to selectively decompose the 2.5-nm-thick GaN cap layer of commercial GaN(2.5 nm)/Al0.20Ga0.80N(22 nm)/GaN heterostructure epi-wafers at high temperature prior to MBE regrowth of Sc0.18Al0.82N to eliminate impurity incorporation at the regrowth interface. Regrowth of a Sc0.18Al0.82N layer on the pristine Al0.20Ga0.80N surface increases 2DEG density from 7.89 × 1012 to 9.57 × 1012 cm−2, together with a slight reduction in mobility from 2160 to 1970 cm2⋅V−1⋅s−1 at 300 K, reducing the sheet resistance by 10%. Our calculation implies that it is practical to boost 2DEG density to over 2.0 × 1013 cm−2 by thinning Al0.20Ga0.80N down to 4 nm. [ABSTRACT FROM AUTHOR]

We attained wurtzite ScxAl1−xN (0.16 ≤ x ≤ 0.37) thin films by varying the Sc and Al fluxes at a fixed active nitrogen flux during plasma-assisted molecular beam epitaxy. Atomic fluxes of Sc and Al sources via measured Sc percentage in as-grown ScxAl1−xN thin films were derived as the feedback for precise determination of the ScxAl1−xN growth diagram. We identified an optimal III/N atomic flux ratio of 0.78 for smooth Sc0.18Al0.82N thin films. Further increasing the III/N ratio led to phase separation under N-rich conditions, validated by the observation of high-Sc-content hillocks with energy-dispersive X-ray spectroscopy mapping. At the fixed III/N ratio of 0.78, we found that phase separation with high-Al-content hillocks occurs for x > 0.37, which is substantially lower than the thermodynamically dictated threshold Sc content of ~0.55 in wurtzite ScxAl1−xN. We postulate that these wurtzite-phase purity degradation scenarios are correlated with adatom diffusion and the competitive incorporation process of Sc and Al. Therefore, the ScxAl1−xN growth window is severely restricted by the adatom kinetics. We obtained single crystalline Sc0.37Al0.63N thin films with X-ray diffraction (002)/(102) ω rocking curve full-width at half-maximums of 2156 arcsec and 209 arcsec and surface roughness of 1.70 nm. Piezoelectric force microscopy probing of the Sc0.37Al0.63N epilayer validates unambiguous polarization flipping by 180°. [ABSTRACT FROM AUTHOR]

Efficient precooling treatment and cold chain distribution are key to ensuring the quality of chilled food during processing, storage, and transportation. However, traditional cooling methods have challenges such as high energy consumption, low efficiency, and short shelf life. Slurry ice as a new type of green cooling medium, which has advantages such as low precooling temperature and fast cooling rate. It has great potential for application in food preservation. On the basis of summarizing the flow characteristics, heat transfer characteristics, and mass transfer characteristics of slurry ice, this article elaborated on the application prospects of slurry ice precooling technology in chilled aquatic products and chilled meat preservation, and summarized the application prospects of slurry ice coupled to various pretreatment technology, including coupling ozone, acidic electrolyzed water, antioxidants, and edible coatings in meat preservation, and the future development of slurry ice pretreatment technology was also prospected, aiming to provide theoretical basis and technical reference for the high-quality preservation of chilled food. [ABSTRACT FROM AUTHOR]

We implemented invisibility criterion and black–white contrast orientation analysis into low-tilt electron channeling contrast imaging (ECCI) for dislocation-type discrimination in GaN and AlGaN layers grown on a Si(111) substrate. Our ECCI and x-ray diffraction (XRD) analysis attained consistent threading dislocation densities for GaN and AlGaN grown on Si, but demonstrated drastic discrepancy in the percentage of edge-type dislocations, potentially due to the lack of appropriate consideration of mixed-type (a → + c →) dislocations in XRD. Further ECCI analysis of GaN/AlGaN heterointerface revealed mixed-type (a → + c →) dislocation half-loops and dislocation bending due to compressive strain relaxation, validating that not all the dislocations originated from the mosaic or columnar structure. As a result, XRD analysis based on the mosaic block model does not give reliable edge-to-screw dislocation ratio. The observation of classic van der Merwe–Matthews-type dislocation half-loop nucleation and dislocation gliding could be associated with potential GaN/AlGaN optoelectronic device degradation issues. [ABSTRACT FROM AUTHOR]