In this study, we investigated phase separation and long-range atomic ordering phenomena in InGaN alloys produced by molecular beam epitaxy. Films grown at substrate temperatures of 700-750°C with indium concentration higher than 35% showed phase separation, in good agreement with thermodynamic predictions for spinodal decomposition. Films grown at lower substrate temperatures ͑650-675°C͒ revealed compositional inhomogeneity when the indium content was larger than 25%. These films, upon annealing to 725°C, underwent phase separation, similar to those grown at the same temperature. The InGaN films also exhibited long-range atomic ordering. The ordering parameter was found to increase with the growth rate of the films, consistent with the notion that ordering is induced at the growth surface. The ordered phase was found to be stable up to annealing temperatures of 725°C. A competition between ordering and phase separation has been observed, suggesting that the driving force for both phenomena is lattice strain in the alloy.
Strontium-doped lanthanum cobalt ferrite (LSCF) is a widely used cathode material due to its high electronic and ionic conductivity, and reasonable oxygen surface exchange coefficient. However, LSCF can have long-term stability issues such as surface segregation of Sr during solid oxide fuel cell (SOFC) operation, which can adversely affect the electrochemical performance. Thus, understanding the nature of the Sr surface segregation phenomenon and how it is affected by the composition of LSCF and strain are critical. In this research, heteroepitaxial thin films of La SrCoFeO with varying Sr content (x = 0.4, 0.3, 0.2) were deposited by pulsed laser deposition (PLD) on single-crystal NdGaO, SrTiO, and GdScO substrates, leading to different levels of strain in the films. The extent of Sr segregation at the film surface was quantified using synchrotron-based total-reflection X-ray fluorescence (TXRF) and atomic force microscopy (AFM). The electronic structure of the Sr-rich phases formed on the surface was investigated by hard X-ray photoelectron spectroscopy (HAXPES). The extent of Sr segregation was found to be a function of the Sr content in bulk. Lowering the Sr content from 40% to 30% reduced the surface segregation, but further lowering the Sr content to 20% increased the segregation. The strain of LSCF thin films on various substrates was measured using high-resolution X-ray diffraction (HRXRD), and the Sr surface segregation was found to be reduced with compressive strain and enhanced with tensile strain present within the thin films. A model was developed correlating the Sr surface segregation with Sr content and strain effects to explain the experimental results.
Luminescent silicon-rich nitride/silicon superlattice structures ͑SRN/Si-SLs͒ with different silicon concentrations were fabricated by direct magnetron cosputtering deposition. Rapid thermal annealing at 700°C resulted in the nucleation of small amorphous Si clusters that emit at 800 nm under both optical and electrical excitations. The electrical transport mechanism and the electroluminescence ͑EL͒ of SRN/Si-SLs have been investigated. Devices with low turn-on voltage ͑6 V͒ have been demonstrated and the EL mechanism has been attributed to bipolar recombination of electron-hole pairs at Si nanoclusters. Our results demonstrate that amorphous Si clusters in SRN/Si-SLs provide a promising route for the fabrication of Si-compatible optical devices.
Er-doped amorphous silicon nitride films with various Si concentrations ͑Er: SiN x ͒ were fabricated by reactive magnetron cosputtering followed by thermal annealing. The effects of Si concentrations and annealing temperatures were investigated in relation to Er emission and excitation processes. Efficient excitation of Er ions was demonstrated within a broad energy spectrum and attributed to disorder-induced localized transitions in amorphous Er: SiN x. A systematic optimization of the 1.54 m emission was performed and a fundamental trade-off was discovered between Er excitation and emission efficiency due to excess Si incorporation. These results provide an alternative approach for the engineering of sensitized Si-based light sources and lasers.
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