We investigate the structural and optical properties of M-plane GaN(11 00) films grown on LiAlO 2 (100) with nucleation layers grown at high and low temperatures. Samples with a high temperature nucleation layer are found to exhibit a highly anisotropic surface morphology with pronounced corrugation, which basically replicates the surface morphology of the substrate. Photoluminescence spectra of these layers are dominated by a transition at 3.356 eV, which is absent for samples with a low-temperature nucleation layer. In conjunction with scanning electron microscopy, cathodoluminescence maps reveal that this transition predominantly stems from regions below the trenches of the surface corrugation. Transmission electron microscopy shows an abundance of stacking faults within these regions. Excitation-dependent and time-resolved photoluminescence demonstrates the intrinsic character of the 3.356 eV emission, which is thus attributed to excitons bound to stacking faults acting as ultrathin vertical quantum wells in these samples. Low-temperature nucleation is imperative to avoid thermal roughening of the substrate and thus the formation of a high density of stacking faults.
We present a detailed study of the magnetic properties of ͑Ga,Mn͒N layers grown directly on 4H-SiC substrates by reactive molecular-beam epitaxy. X-ray diffraction and transmission electron microscopy demonstrates that homogeneous ͑Ga,Mn͒N alloys of high crystal quality can be synthesized by this growth method up to a Mn-content of 10-12 %. Using a variety of magnetization experiments ͑temperature-dependent dc magnetization, isothermal remanent magnetization, frequency and field dependent ac susceptibility͒, we demonstrate that insulating ͑Ga,Mn͒N alloys represent a Heisenberg spin-glass with a spin-freezing temperature around 4.5 K. We discuss the origins of this spin-glass characteristics in terms of the deep-acceptor nature of Mn in GaN and the resulting insulating character of this compound.
We present a simple but reliable method to determine the azimuthal orientational spread of imperfect epitaxial layers by x-ray diffraction. This method requires the measurement of ω-scans in skew geometry from reflections with increasing lattice plane inclination φ, and a fit of the data by a geometrical model that considers the simultaneous presence of polar and azimuthal orientational spread within the layer. The values thus obtained for various GaN layers grown on SiC are shown to be in good agreement with those measured directly by φ-scans of the (11̄00) reflection in grazing incidence, and with those deduced from the edge dislocation densities determined by plan-view transmission electron microscopy.
We investigate the polarization anisotropy of the photoluminescence of an M-plane (11̄00) In0.1Ga0.9N/GaN multiple quantum well grown on γ-LiAlO2 (100) by molecular-beam epitaxy. In contrast to C-plane (0001) structures, a strong in-plane optical anisotropy with an energy-dependent polarization degree of up to 96% is observed for this M-plane sample. An apparent spectral shift of the emission with polarization angle is attributed to the impact of exciton localization on the polarization degree. The presence of localized states manifests itself further in the anomalous temperature dependence of the photoluminescence linewidth.
Electrochromic devices with tunable infrared radiation can meet the steadily growing demands in energy saving and thermal camouflage applications. Here, a mid-infrared radiation modulator based on flexible multilayer graphene thin films gated by nonvolatile ionic liquid on both rigid and flexible substrates is designed. The thermal emissivity of the device decreases nearly 80% within 2 s with the accumulation of anions in the multilayer graphene. The effective reduction of the emissivity results from the dramatic decrease in film's intraband absorption of graphene according to the Drude model. It has been demonstrated that with electrical control the film's midinfrared radiation is capable of adapting to different backgrounds for thermal camouflage applications. Moreover, a sandwiched structure with stacked graphene films is designed to realize structural flexibility and double-sided radiation control for a wide range of potential applications, including energyefficient buildings, infrared sources, and electrochromic displays.
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