InN/InGaN multiple quantum wells emitting at 1.5 mu m grown by molecular beam epitaxy

This work reports on the growth by molecular beam epitaxy and characterization of InN/InGaN multiple quantum wells (MQWs) emitting at 1.5 μm. X-ray diffraction (XRD) spectra show satellite peaks up to the second order. Estimated values of well (3 nm) and barrier (9 nm) thicknesses were derived from transmission electron microscopy and the fit between experimental data and simulated XRD spectra. Transmission electron microscopy and XRD simulations also confirmed that the InGaN barriers are relaxed with respect to the GaN template, while the InN MQWs grew under biaxial compression on the InGaN barriers. Low temperature (14 K) photoluminescence measurements reveal an emission from the InN MQWs at 1.5 μm. Measurements as a function of temperature indicate the existence of localized states, probably due to InN quantum wells’ thickness fluctuations as observed by transmission electron microscopy.

Internal quantum efficiency of III-nitride quantum dot superlattices grown by plasma-assisted molecular-beam epitaxy

We present a study of the optical properties of GaN/AlN and InGaN/GaN quantum dot (QD) superlattices grown via plasma-assisted molecular-beam epitaxy, as compared to their quantum well (QW) counterparts. The three-dimensional/two-dimensional nature of the structures has been verified using atomic force microscopy and transmission electron microscopy. The QD superlattices present higher internal quantum efficiency as compared to the respective QWs as a result of the three-dimensional carrier localization in the islands. In the QW samples, photoluminescence (PL) measurements point out a certain degree of carrier localization due to structural defects or thickness fluctuations, which is more pronounced in InGaN/GaN QWs due to alloy inhomogeneity. In the case of the QD stacks, carrier localization on potential fluctuations with a spatial extension smaller than the QD size is observed only for the InGaN QD-sample with the highest In content (peak emission around 2.76 eV). These results confirm the efficiency of the QD three-dimensional confinement in circumventing the potential fluctuations related to structural defects or alloy inhomogeneity. PL excitation measurements demonstrate efficient carrier transfer from the wetting layer to the QDs in the GaN/AlN system, even for low QD densities (~1010 cm-3). In the case of InGaN/GaN QDs, transport losses in the GaN barriers cannot be discarded, but an upper limit to these losses of 15% is deduced from PL measurements as a function of the excitation wavelength.

O 1s excitation and ionization processes in the CO2 molecule studied via detection of low-energy fluorescence emission

Oxygen 1s excitation and ionization processes in the CO2 molecule have been studied with dispersed and non-dispersed fluorescence spectroscopy as well as with the vacuum ultraviolet (VUV) photon?photoion coincidence technique. The intensity of the neutral O emission line at 845 nm shows particular sensitivity to core-to-Rydberg excitations and core?valence double excitations, while shape resonances are suppressed. In contrast, the partial fluorescence yield in the wavelength window 300?650 nm and the excitation functions of selected O+ and C+ emission lines in the wavelength range 400?500 nm display all of the absorption features. The relative intensity of ionic emission in the visible range increases towards higher photon energies, which is attributed to O 1s shake-off photoionization. VUV photon?photoion coincidence spectra reveal major contributions from the C+ and O+ ions and a minor contribution from C2+. No conclusive changes in the intensity ratios among the different ions are observed above the O 1s threshold. The line shape of the VUV?O+ coincidence peak in the mass spectrum carries some information on the initial core excitation

The structure of lean hydrogen-air flame balls

An analysis of the structure of flame balls encountered under microgravity conditions, which are stable due to radiant energy losses from H₂O, is carried out for fuel-lean hydrogen-air mixtures. It is seen that, because of radiation losses, in stable flame balls the maximum flame temperature remains close to the crossover temperature, at which the rate of the branching step H + O₂ -> OH + O equals that of the recombination step H + O₂ + M -> HO₂ + M. Under those conditions, all chemical intermediates have very small concentrations and follow the steady-state approximation, while the main species react according to the overall step 2H₂ + O₂-> 2H₂O; so that a one-step chemical-kinetic description, recently derived by asymptotic analysis for near-limit fuel-lean deflagrations, can be used with excellent accuracy to describe the whole branch of stable flame balls. Besides molecular diffusion in a binary-diffusion approximation, Soret diffusion is included, since this exerts a nonnegligible effect to extend the flammability range. When the large value of the activation energy of the overall reaction is taken into account, the leading-order analysis in the reaction-sheet approximation is seen to determine the flame ball radius as that required for radiant heat losses to remove enough of the heat released by chemical reaction at the flame to keep the flame temperature at a value close to crossover. The results are relevant to burning velocities at lean equivalent ratios and may influence fire-safety issues associated with hydrogen utilization.