We have studied the microstructure of InGaN layers grown on two different GaN substrates: a standard GaN film on sapphire and an epitaxial lateral overgrown GaN (ELOG) structure. These two materials exhibit two distinct mechanisms of strain relaxation. InGaN epilayers on GaN are typically pseudomorphic and undergo elastic relaxation by the opening of threading dislocations into pyramidal pits. A different behavior occurs in the case of epitaxy on ELOG where, in the absence of threading dislocations, slip occurs with the formation of periodic arrays of misfit dislocations. Potential slip systems responsible for this behavior have been analyzed using the Matthews-Blakeslee model and taking into account the Peierls forces. This letter presents a comprehensive analysis of slip systems in the wurtzite structure and considers the role of threading dislocations in strain relaxation in InGaN alloys.

The In x Ga 1-x N system has electronic band gaps extending from under 1eV to 3.4 eV, and as such they are used as the active layer in commercially available visible-light emitting devices. There are many interesting features that make these nitride semiconductor alloys especially useful for efficient light emitters. It has been conjectured that the combination of piezoelectric fields and local composition inhomogeneities may be responsible for the observed high emission efficiencies, in spite of their characteristic high dislocation densities. But it is very difficult to grow In x Ga 1-x N layers with high indium composition. This paper presents an overview of the properties of In x Ga 1-x N epilayers based on a systematic study of thick layers and of quantum well structures. We find that the microstructure of thick films varies significantly with indium composition. For x < 0.08, the composition is uniform and unperturbed by dislocations. For 0.10 < x < 0.20, secondary phases nucleate at threading dislocations. For x > 0.20, spontaneous phase separation occurs resulting in a polycrystalline, inhomogeneous layer. A correlation between optical properties and microstructure is presented. It is observed that the misfit strain is affected by threading dislocations. Mechanisms of misfit strain relaxation are presented for In x Ga 1-x N layers grown on standard GaN on sapphire and on epitaxial-lateral-overgrowth GaN layers. In addition, we have studied the properties of quantum well structures using several novel techniques. The electrostatic fields across the wells have been profiled using electron holography in the TEM. The effect of well thickness on the strength of the fields is reported. The effects of localization by compositional fluctuations and of internal field screening have been studied using time-resolved cathodoluminescence spectroscopy. In spite of significant progress that has been made in the last ten years, much work remains ahead in order to master the science and technology of these alloys.

Al x Ga 1−x N layers with 0.05⩽x⩽0.25 were studied using spectrally and time resolved cathodoluminescence (CL). Continuous wave spectra were taken at temperatures ranging from 5 to 300 K. The near-band-edge peak emission energy exhibits an s-shaped temperature dependence characteristic of disordered systems. This effect is quantitatively explained within a model of potential fluctuations caused by alloy disorder. An s-shape temperature dependence has been observed in other alloy systems including InGaN, however, no systematic study exists for AlGaN. In this work, the s-shape temperature dependence is systematically analyzed as a function of aluminum content and quantitatively correlated with a model of alloy disorder. The shift in the luminescence peak position with respect to the usual temperature dependence of the band gap has been quantified by −σE2/kBT, where σE is the standard deviation of the potential fluctuations. Its dependence on aluminum concentration, x, was found to systematically increase from 7 meV at x=0.05 to 21 meV at x=0.25, following the theory for alloy disorder. The recombination and relaxation kinetics investigated using time-resolved CL are fully consistent with our potential fluctuation model. At 5 K, when the excitons are strongly localized, the exciton lifetime increases monotonically with aluminum content. At elevated temperatures, when the excitons are delocalized, the decay is significantly faster and preferentially nonradiative, regardless of the aluminum content.

An environmentally benign synthesis of allyl ethers has been developed which can be applied to a highly chemoselective protection of hydroxyl groups. A highly efficient [CpRu]–2‐quinolinecarboxylic acid (L) catalytic system was successfully employed for the dehydrative allylation of various alcohols without additional activators and solvent (see scheme; R=alkyl, aryl, multifunctional alkyl, Cp=C5H5).

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ABA triblock copolymers comprised of poly(β-benzyl-l-aspartate) (PBLA) as an A segment and poly(ethylene oxide) (PEO) as the B segment were precisely synthesized by a ring-opening polymerization of the N-carboxyanhydride of BLA initiated from amines at both terminals of the PEO chain. The copolymers have a number-average molecular weight (M n) of 1.5 × 104−3.6 × 104 and a monodisperse molecular weight distribution in the range 1.04−1.07. The copolymer films casted from a dichloromethane solution were flexible and elastic despite their low M n. The film formed a hierarchical structure, i.e., the chains adopted an α-helix−72 helix−α-helix conformation and was highly crystallized, forming a long-range ordering structure which resulted in a rodlike construct arranged like ripples, as confirmed by Fourier transformed infrared spectroscopy, X-ray diffraction, and atomic force microscopy. Differential scanning calorimetry confirmed that the film showed a melting endotherm of the PEO segment at 313−326 K depending on the PBLA content. The film became very soft and behaved like rubber above the melting temperature. When the film was successively cooled, the crystalline diffraction became less intense, especially in terms of the PBLA segments whose conformation partially transformed from α-helices to β-sheets, and the film showed enhanced strength and drastically increased deformation, with a strain of more than 500% accompanied by a necking phenomenon. The relationship between the mechanical properties and the structure is also discussed.

A cationic CpRu(II) complex in combination with quinaldic acid shows high reactivity and chemoselectivity for the catalytic deprotection of hydroxyl groups protected as allyl ethers. The catalyst operates in alcoholic solvents without the need for any additional nucleophiles, satisfying the practical requirements of operational simplicity, safety, and environmental friendliness. The wide applicability of this deprotection strategy to a variety of multifunctional molecules, including peptides and nucleosides, may provide new opportunities in protective group chemistry. [structure: see text]

The optical properties of thick InxGa1−xN layers have been studied using optical absorption and cathodoluminescence techniques. The indium composition x of the layers ranged from 0.03 to 0.17 as determined by Rutherford backscattering measurements. The difference between the band gap and the peak emission energy (Stokes shift) was found to be considerably smaller than reported in the past for these alloys. Monochromatic images show that light emission from most of the film is homogeneous and is associated with a low Stokes shift. A second emission band at longer wavelengths is observed for x⩾0.08. This band originates from indium-rich regions in the vicinity of extended defects, and exhibits a larger Stokes shift. Our observations indicate that it is possible to grow InGaN epilayers with high indium composition, high homogeneity, and lower Stokes shift.