Composition mapping in InGaN by scanning transmission electron microscopy.

Nanoposts of 10?40?nm top diameter on an InGaN/GaN quantum well structure were fabricated using electron-beam lithography and inductively coupled plasma reactive ion etching. Significant blue shifts up to 130?meV in the photoluminescence (PL) spectrum were observed. The blue-shift range increases with decreasing post diameter. For nanoposts with significant strain relaxation, the PL spectral peak position becomes less sensitive to carrier screening. On the basis of the temperature-dependent PL and time-resolved PL measurements and a numerical calculation of the effect of quantum confinement, we conclude that the optical behaviours of the nanoposts are mainly controlled by the combined effect of 3D quantum confinement and strain relaxation.

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Strain relaxation and quantum confinement in InGaN/GaN nanoposts

The dynamics of photoexcited excitons in thin $\mathrm{In}\mathrm{Ga}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ multiple quantum wells (QW's) with different In contents was studied by comparing the experimental data obtained by photoluminescence (PL), PL excitation, and photoreflectance spectroscopy techniques with the results of Monte Carlo simulations of exciton hopping. The temperature dependence of the PL linewidth was demonstrated to be in a fair agreement with the model of phonon-assisted exciton in-plane hopping within In-rich regions with inhomogeneous broadening taken into account. The band potential fluctuations, which scale the dispersion of localized states the excitons are hopping over, were attributed to compositional disorder inside the In-rich regions. Meanwhile, the inhomogeneous broadening was explained by variation in mean exciton energy among the individual In-rich regions. For typical $2.5\text{\ensuremath{-}}\mathrm{nm}$-thick ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ $(x\ensuremath{\approx}0.22)$ QW's, the simulation revealed fluctuations of the band potential $(31\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$ with additional inhomogeneous broadening $(29\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$ and a crossover from a nonthermalized to thermalized exciton energy distribution at about $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Both the fluctuations and inhomogeneous broadening showed an enhancement with increasing of In content. Simultaneously, a Bose-Einstein-like temperature dependence of the exciton energy in the wells was extracted using data on the PL peak position. The dependence exhibited a fair conformity with the photoreflectance data. Moreover, the density of localized states used in the simulation was found to be consistent with the PL excitation spectrum.

Exciton hopping in In x Ga 1 − x N multiple quantum wells

The authors demonstrate the spectral redshift of the quantum wells (QWs) designated for green emission into the orange range in a light-emitting diode by adding a violet-emitting QW at the bottom in metal-organic chemical vapor deposition An electroluminescence redshift of 53nm was obtained The cathodoluminescence spectra indicated that the long-wavelength QWs close to the violet one were strongly influenced by this added QW and mainly emitted the orange photons Those near the top were less affected This influence is supposed to originate from the prestrained effect in the barrier layer right above the violet QW Such a prestrained effect is expected to be more effective when the underlying QW is well shaped and the heterojunction strain is strong, like the case of the violet QW This effect is weak between the high-indium QWs, in which the formation of indium-rich clusters releases the strain

Prestrained effect on the emission properties of InGaN∕GaN quantum-well structures

Magnetic nanoparticles are of immense current interest because of their possible use in biomedical and technological applications. Here we demonstrate that the large magnetic anisotropy of FePt nanoparticles can be significantly modified by surface design. We employ X-ray absorption spectroscopy offering an element-specific approach to magnetocrystalline anisotropy and the orbital magnetism. Experimental results on oxide-free FePt nanoparticles embedded in Al are compared with large-scale density functional theory calculations of the geometric- and spin-resolved electronic structure, which only recently have become possible on world-leading supercomputer architectures. The combination of both approaches yields a more detailed understanding that may open new ways for a microscopic design of magnetic nanoparticles and allows us to present three rules to achieve desired magnetic properties. In addition, concrete suggestions of capping materials for FePt nanoparticles are given for tailoring both magnetocrystalline anisotropy and magnetic moments.

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A guideline for atomistic design and understanding of ultrahard nanomagnets

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 (PL) measurements showed quite consistent results. Blueshift of the PL peak position and narrowing of the PL spectral width after thermal annealing were observed.

Quasiregular quantum-dot-like structure formation with postgrowth thermal annealing of InGaN/GaN quantum wells

Optical measurements of temperature-dependent photoluminescence (PL) 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 (QW) 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 nm) structure led to regularly distributed quantum dots (QDs) and improved optical quality. However, thermal annealing at the same temperature of a sample of larger QW width (4 nm) did not show QD formation. In this situation, even higher local strains around QWs were speculated. Also, degraded optical quality was observed.

Quantum-well-width dependencies of postgrowth thermal annealing effects of InGaN/GaN quantum wells

Material and optical analyses of three InGaN/GaN quantum well (QW) samples with different silicon doping conditions were conducted. Quantum dot (QD) structures were observed in samples of silicon doping either in barriers or wells. Optical characterization indicated that quantum-confined Stark effect (QCSE) has been tremendously reduced, particularly in the barrier-doped sample. However, contrary to the interpretation in the past, the major mechanism for QCSE reduction is due to strain relaxation, instead of carrier screening, in conjunction with the formation of QD structures. Such a conclusion is supported by the smaller change of Stokes shift upon silicon doping in wells. In the well-doped sample, besides strain relaxation, enhanced carrier localization might represent another important mechanism for photon emission improvement.

Quantum dot formation in InGaN/GaN quantum well structures with silicon doping and the mechanisms for radiative efficiency improvement

Summary form only given. In this paper, we report the formation of quasiregular QDs from randomly distributed indium aggregated clusters after post-growth thermal annealing (PGTA) of an InGaN/GaN QW sample. The optical measurements showed quite consistent trends of blue shift and spectral width variation.

http://ieeexplore.ieee.org/iel5/8035/22193/01034445.pdf

Cheng Hsu

Papers

Quasiregular quantum-dot-like structure formation with postgrowth thermal annealing of InGaN/GaN quantum wells

Quantum-well-width dependencies of postgrowth thermal annealing effects of InGaN/GaN quantum wells

Quantum dot formation in InGaN/GaN quantum well structures with silicon doping and the mechanisms for radiative efficiency improvement

Formation of quasi-regular quantum dots with post-growth thermal annealing and their optical characteristics in InGaN/GaN quantum wells