The metal-semiconductor-metal transition in dense lithium is considered as an archetype of interplay between interstitial electron localization and delocalization induced by compression, which leads to exotic electride phases. In this work, the dynamic dielectric response and optical properties of the high-pressure electride phases of cI16, oC40 and oC24 in lithium spanning a wide pressure range from 40 to 200 GPa by first-principles calculations are reported. Both interband and intraband contribution to the dielectric function are deliberately treated with the linear response theory. One intraband and two interband plasmons in cI16 at 70 GPa induced by a structural distortion at 2.1, 4.1, and 7.7 eV are discovered, which make the reflectivity of this weak metallic phase abnormally lower than the insulating phase oC40 at the corresponding frequencies. More strikingly, oC24 as a reentrant metallic phase with higher conductivity becomes more transparent than oC40 in infrared and visible light range due to its unique electronic structure around Fermi surface. An intriguing reflectivity anisotropy in both oC40 and oC24 is predicted, with the former being strong enough for experimental detection within the spectrum up to 10 eV. The important role of interstitial localized electrons is highlighted, revealing diversity and rich physics in electrides.
We report interface engineering in ZnO epitaxy to grow high-quality layers by plasma-assisted molecular beam epitaxy. Through interface engineering, we have succeeded in two-dimensional layer-by-layer growth of ZnO both on sapphire and GaN, and control of lattice polarity of ZnO films on a Ga-polar GaN template. MgO buffer has been used to convert the growth mode from a three-dimensional to a two-dimensional mode in ZnO epitaxy on sapphire. O-plasma pre-exposure on Ga-polar GaN templates has been employed to form a monoclinic Ga 2 O 3 interface layer at the ZnO/GaN heterointerface, while Zn pre-exposure prevents oxidation of the GaN surface resulting in a ZnO/GaN heterointerface without an interface layer. The lattice polarity of ZnO films on Ga-polar GaN templates with and without a Ga 2 O 3 interface layer with inversion symmetry have been revealed as Zn-and O-polar, respectively, by coaxial impact collision ion scattering spectroscopy. Structural properties of ZnO films grown with MgO buffer on sapphire or with Zn pre-exposure on GaN templates are better than those grown without MgO buffer or with an O-plasma preexposure, respectively. A buffer mechanism of MgO is discussed based on reflection high-energy electron diffraction and high-resolution X-ray diffraction analyses, while the mechanism for controlling the polarity in ZnO epitaxy is discussed using an interface layer with inversion symmetry. It is suggested that the present method offers a general approach to control the crystal polarity.
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We present our first results for the growth of ZnO layers by Molecular Beam Epitaxy on commercial {0001} ZnO substrates of different polarities. On O-face oriented substrates, a two-dimensional growth mode was achieved as verified by the presence of a streaky Reflection High-Energy Electron Diffraction (RHEED) pattern and a faint (3x3) reconstructed surface. Moreover, a distinct specular spot and reproducible RHEED oscillations were found. On the other hand, the RHEED pattern for the growth on Zn face ZnO substrates was spotty indicating three-dimensional growth, and a lateral coherence length of the layer of only 88 nm was estimated by Williamson-Hall plots. Bound exciton lines in low temperature photoluminescence for a layer on an O-face ZnO substrate were narrow (FWHM: 1.8 meV) compared to the substrate material, while the root mean square roughness measured by Atomic Force Microscopy was 20 nm.
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