Postgrowth thermal annealing of an InGaN/GaN quantum-well sample with a medium level of nominal indium content ͑19%͒ was conducted. From the analyses of high-resolution transmission electron microscopy and energy filter transmission electron microscopy, it was found that thermal annealing at 900 °C led to a quasiregular quantum-dot-like structure. However, such a structure was destroyed when the annealing temperature was raised to 950 °C. Temperature-dependent photoluminescence ͑PL͒ measurements showed quite consistent results. Blueshift of the PL peak position and narrowing of the PL spectral width after thermal annealing were observed.
Polypropylene (PP) and polystyrene (PS) are immiscible and incompatible. Since both PP and PS components possess no reactive functional group, reactive compatibilization of a PP/PS blend is impossible unless certain reactive functional groups are imparted to either PP or PS. In this study we provide a simple approach to reactively compatibilize the nonreactive PP/PS blend system by physically functionalizing PP and PS with the addition of maleic anhydride grafted PP (PP-g-MA) and styrene maleic anhydride random copolymer (SMA), respectively. An epoxy monomer, serving as a coupler and possessing four epoxy groups able to react with the maleic anhydride of PP-g-MA and SMA, was then added during melt blending. Observations of the finer PS domain sizes and improved mechanical properties support the plausibility of reactive compatibilization of this nonreactive PP/PS blend by combining physically functionalized PP and PS with tetra-glycidyl ether of diphenyl diamino methane (TGDDM) in a one-step extrusion process.
Optical measurements of temperature-dependent photoluminescence ͑PL͒ spectral peak, integrated PL intensity and PL decay time, and microstructure analyses with high-resolution transmission electron microscopy showed the strong dependencies of thermal annealing effects on quantum well ͑QW͒ width in InGaN/GaN QW structures. With different QW widths, different levels of strain energy were built. Upon thermal annealing, energy relaxation resulted in the reshaping of quantum dots and hence the changes of optical properties. Thermal annealing at 800 °C of a narrow QW width ͑2 nm͒ structure led to regularly distributed quantum dots ͑QDs͒ and improved optical quality. However, thermal annealing at the same temperature of a sample of larger QW width ͑4 nm͒ did not show QD formation. In this situation, even higher local strains around QWs were speculated. Also, degraded optical quality was observed.
. Although optical properties of InGaN/GaN quantum well structures are being widely investigated, the emission mechanisms in this system are still not completely understood [1][2][3][4][5][6][7]. Localised excitons is the most probable mechanism responsible for emission from InGaN/GaN MQWs, however the emission efficiency is highly sensitive to impurity and interface states, inhomogeneities and compositional separation of the InGaN alloy as well as to built-in electric field [3][4][5][6][7].Here we report on a study of InGaN/GaN MQW structures with various well thickness, d, by temperature-dependent, excitation power-dependent and time-resolved photoluminescence techniques. The samples were grown on sapphire substrates by metalorganic chemical vapour deposition. The undoped MQW structures were grown at temperatures of 1020 and 720 C for GaN and InGaN, respectively, and consisted of five periods of 10 nm thick GaN barriers and In x Ga 1--x N (x 0.15) wells. A series of samples with d ¼ 2, 2.5, 3, 3.5, and 4 nm were investigated.Luminescence spectra of structures studied show two main bands originated from the QW and barrier/buffer layers (Fig. 1). These two bands dominate in the emission over a broad range of excitation intensities. The QW luminescence is Stokes shifted by about 220 meV in respect to the estimated bandgap. It is attributed to the localised excitons. The barrier/buffer related band is situated at the bandgap energy of GaN and is attributed to the free-carrier emission. The QW luminescence blueshifts with increasing exci-
This work investigates dislocation etch pits in epitaxial lateral overgrowth (ELO) GaN by wet chemical etching. A mixture of H2SO4 and H3PO4 was used as a dislocation etchant, and SEM and AFM were employed to observe the surface topography. For the as-grown sample, SEM images present the flat, smooth surface without any pits or hillocks. After the chemical etching, hexagonal shaped etch pits were observed at the edge of ELO GaN. AFM observation of etched ELO GaN displayed high densities of etch pits clustered in the “window” region and the coalescent line of two growing fronts. In contrast, the overgrowth region was nearly free of etch pits. Moreover, we observed that different sizes of etch pits dominated in “window” region and coalescent region. This implied different types dislocations dominated in these regions.
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