517 research outputs found
Optical and Structural Studies of High-quality Bulk-like GaN Grown by HVPE on a MOVPE AlN Buffer Layer
No full textHigh-quality 400 μm thick GaN has been grown by hydride vapour phase epitaxy (HVPE) on (0 0 0 1) sapphire with a 2 μm thick AlN buffer layer. The material’s crystalline quality and homogeneity was verified by x-ray diffraction (XRD), low-temperature photoluminesence (LT-PL) and LT cathodoluminescence. Plan-view transmission electron microscopy images reveal a low dislocation density of ?1.25 × 10^7 cm^(?2). The residual stress of the material was studied by two complementary techniques. LT-PL spectra show the main neutral donor bound exciton line at 3.4720 eV. This line position suggests virtually strain-free material with a high crystalline quality as indicated by the small full width at half maximum value of 0.78 meV. The presence of well resolved A- and B-free excitons in the LT-PL spectra and the absence of a yellow luminescence band prove the high quality of the HVPE-GaN in terms of purity and crystallinity. These findings are consistent with the XRD results, implying the high crystalline quality of the material grown. Hence, the material studied is well suited as a lattice parameter and thermal-expansion-coefficient matched substrate for further homoepitaxy, as needed for high-quality III-nitride device applications. Strain-free homoepitaxy on native substrates is needed to decrease considerably the defect density and in that way an improvement of the device’s performance and lifetime can be achieved
Delay and distortion of slow light pulses by excitons in ZnO
No full textistortion of light pulses in ZnO caused by both bound and free excitons is demonstrated by time-of-flight spectroscopy. Numerous lines of bound excitons dissect the pulse spectrum and induce slowdown of light propagation around the dips. Exciton-polariton resonances determine the overall pulse delay, which approaches 1.6 ns at 3.374 eV for a 0.3 mm propagation length, as well as the pulse curvature in the time-energy plane and its attenuation. Analysis of cw and time-resolved data yields the excitonic parameters inherent for bulk ZnO. A discrepancy is found between these bulk parameters and those given by surface-probing techniques.Original Publication:T V Shubina, M M Glazov, N A Gippius, A A Toropov, D Lagarde, P Disseix, J Leymarie, B Gil, Galia Pozina, J Peder Bergman and Bo Monemar, Delay and distortion of slow light pulses by excitons in ZnO, 2011, Physical Review B. Condensed Matter and Materials Physics, (84), 7, 075202.http://dx.doi.org/10.1103/PhysRevB.84.075202Copyright: American Physical Societyhttp://www.aps.org
Optical and structural studies of high-quality bulk-like GaN grown by HVPE on a MOVPE AIN buffer layer
No full textHigh-quality 400 mu m thick GaN has been grown by hydride vapour phase epitaxy (HVPE) on (0 0 0 1) sapphire with a 2 Am thick AIN buffer layer. The material's crystalline quality and homogeneity was verified by x-ray diffraction (XRD), low-temperature photoluminesence (LT-PL) and LT cathodoluminescence. Plan-view transmission electron microscopy images reveal a low dislocation density of similar to 1.25 x 10(7) cm(-2). The residual stress of the material was studied by two complementary techniques. LT-PL spectra show the main neutral donor bound exciton line at 3.4720 eV. This line position suggests virtually strain-free material with a high crystalline quality as indicated by the small full width at half maximum value of 0.78 mev The presence of well resolved A- and B-free excitons in the LT-PL spectra and the absence of a yellow luminescence band prove the high quality of the HVPE-GaN in terms of purity and crystallinity. These findings are consistent with the XRD results, implying the high crystalline quality of the material grown. Hence, the material studied is well suited as a lattice parameter and thermal-expansion-coefficient matched substrate for further homoepitaxy, as needed for high-quality Ill-nitride device applications. Strain-free homoepitaxy on native substrates is needed to decrease considerably the defect density and in that way an improvement of the device's performance and lifetime can be achieved
Internal structure of acceptor-bound excitons in wide-band-gap wurtzite semiconductors
No full textWe describe the internal structure of acceptor-bound excitons in wurtzite semiconductors. Our approach consists in first constructing, in the context of angular momentum algebra, the wave functions of the two-hole system that fulfill Paulis exclusions principle. Second, we construct the acceptor-bound exciton states by adding the electron states in a similar manner that two-hole states are constructed. We discuss the optical selection rules for the acceptor-bound exciton recombination. Finally, we compare our theory with experimental data for CdS and GaN. In the specific case of CdS for which much experimental information is available, we demonstrate that, compared with cubic semiconductors, the sign of the short-range hole-exchange interaction is reversed and more than one order of magnitude larger. The whole set of data is interpreted in the context of a large value of the short-range hole-exchange interaction Xi(0)=3.4 +/- 0.2 meV. This value dictates the splitting between the ground-state line I-1 and the other transitions. The values we find for the electron-hole spin-exchange interaction and of the crystal-field splitting of the two-hole state are, respectively, -0.4 +/- 0.1 and 0.2 +/- 0.1 meV. In the case of GaN, the experimental data for the acceptor-bound excitons in the case of Mg and Zn acceptors, show more than one bound-exciton line. We discuss a possible assignment of these states.Original Publication:Bernard Gil, Pierre Bigenwald, Plamen Paskov and Bo Monemar, Internal structure of acceptor-bound excitons in wide-band-gap wurtzite semiconductors, 2010, PHYSICAL REVIEW B, (81), 8, 085211.http://dx.doi.org/10.1103/PhysRevB.81.085211Copyright: American Physical Societyhttp://www.aps.org
HVPE GaN substrates: growth and characterization
No full textGaN substrates with low dislocation densities were prepared by halide vapor-phase epitaxy (HVPE) on c-plane sapphire and by means of a post-growth laser-induced lift-off or natural stress-induced (self-) separation process. The HVPE growth on InGaN/GaN buffer layers and subsequent self-separation method was seen as advantageous, in comparison with the laser-induced lift-off one, in terms of lower cost and better crystalline quality of the GaN material obtained. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)</p
Effect of annealing on metastable shallow acceptors in Mg-doped GaN layers grown on GaN substrates
No full textMg-doped GaN layers grown by metal-organic vapor phase epitaxy on GaN substrates produced by the halide vapor phase technique demonstrate metastability of the near-band-gap photoluminescence (PL). The acceptor bound exciton (ABE) line possibly related to the C acceptor vanishes in as-grown samples within a few minutes under UV laser illumination. Annealing activates the more stable Mg acceptors and passivates C acceptors. Consequently, only the ABE line related to Mg is dominant in PL spectra for the annealed samples. The temporal changes in PL are permanent at low temperatures; however, they can be recovered after heating to 100 K or higher.Original publication: Pozina, G., Hemmingsson, C., Paskov, P.P., Bergman, J.P., Monemar, B., Kawashima, T., Amano, H., Akasaki, I. and Usui, A., Effect of annealing on metastable shallow acceptors in Mg-doped GaN layers grown on GaN substrates, 2008, Applied Physics Letters, (92), 15, 151904. http://dx.doi.org/10.1063/1.2909541. Copyright: American Institute of Physics, http://apl.aip.org/apl/top.js
Transient photoluminescence of shallow donor bound excitons in GaN
No full textWe present a detailed study of photoluminescence transients for neutral donor bound excitons (DBEs) in GaN, notably the ON donor DBE at 3.4714 eV and the SiGa DBE at 3.4723 eV. The studied samples are thick strain free nominally undoped bulk GaN samples, with a spectroscopic linewidth <0.5 meV at 2 K. The photoluminescence (PL) decay curves for these no-phonon (NP) lines are strongly nonexponential, and do not allow a proper assessment of the characteristic BE decay time. The decay of the LO-phonon replicas as well as the so-called two-electron transitions (TETs) at lower energies show a nicely exponential behavior, and allow extraction of DBE decay times of about 1.1 ns for the Si DBE and 1.8 ns for the O DBE, respectively. The initial nonexponential decay behavior of the NP lines has been studied in both the common front surface excitation-detection mode and with detection in transmission through the sample. This initial decay is explained as related to scattering processes in the near surface region, involving the DBEs and free excitons (FEs). Light scattering processes may also contribute to this complex decay shape. The DBE-LO-phonon decay does not discriminate between the O and Si DBEs because of spectral overlap involving different LO modes. The TET decays at 2 K are very different for transitions related to the DBE ground state and DBE excited states (going to p-like donor final states), for T>10 K thermalization between the DBE ground state and DBE excited states produces a common decay time. Thermalization between free and bound excitons appears to occur above about 20 K, when the DBE decay follows the FE decay. A simple two-level modeling of exciton capture and recombination for the PL decay curves of the FE and the DBEs, as commonly used in the literature, is shown to be generally inadequate. A broad PL background in the TET spectral region is suggested to be related to a radiative Auger process, where the DBEs recombine while leaving the donors ionized.Original Publication: Bo Monemar, Plamen Paskov, Peder Bergman, Galia Pozina, A.A. Toropov, T.V. Shubina, T. Malinauskas and A. Usui, Transient photoluminescence of shallow donor bound excitons in GaN, 2010, Physical Review B Condensed Matter, (82), , 235202. http://dx.doi.org/10.1103/PhysRevB.82.235202 Copyright: American Physical Society http://www.aps.org
Defect States in Red-Emitting InxAl1-xAs Quantum Dots
No full textCopyright 2002 The American Physical Society. Link to the original site http://journals.aps.org/prb/abstract/10.1103/PhysRevB.66.08533
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