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In this work, we report on ZnO crystalline growth by pulsed-laser deposition (PLD) on quartz, sapphire, and GaN template. 1 m films were grown under different substrate temperature and background oxygen conditions. X-ray diffraction analysis indicates preferential growth along the c-axis direction with a full-width at half maximum (FWHM) of the rocking curve smaller than 200”. Low-temperature photoluminescence showed A-excitonic emission near 3.36 eV and a FWHM of D0XA emission as small as 2.89 meV at 9 K. The results prove that PLD is a low-cost technique suitable to grow heteroepitaxial ZnO structures for emitting devices.

Recent work on polariton-polariton scattering in semiconductor microcavities under continuous wave excitation conditions is reviewed. For weak non-resonant laser excitation, a marked bottleneck in the polariton distribution is observed, but which is suppressed by polariton-polariton scattering as the laser intensity is increased. However, the high excitation conditions necessary to observe stimulated emission lead to loss of strong coupling and conventional lasing in the weak coupling regime, By contrast for resonant excitation, polaritons are created directly in the polariton trap formed by the microcavity dispersion curve. Stimulated scattering of the bosonic quasi-particles occurs to the emitting state at the centre of the Brillouin zone, and to a companion state at high wavevector. The stimulation phenomena lead to condensation of the bosonic quasi-particles to two specific regions of k-space, and to the formation of a new state with macroscopic coherence. The prospects to achieve a polariton laser under conditions of non-resonant excitation are discussed.

In this study we present the optical properties of nonpolar GaN/(AlGa)N single quantum wells (QWs) grown on either a or m plane GaN templates for Al contents set below 15. In order to reduce the density of extended defects the templates have been processed using the epitaxial lateral overgrowth technique. As expected for polarization free heterostructures the larger the QW width for a given Al content the narrower the QW emission line. In structures with an Al content set to 5 or 10 we also observe emission from excitons bound to the intersection of I 1 type basal plane stacking faults (BSFs) with the QW. Similarly to what is seen in bulk material the temperature dependence of BSF bound QW exciton luminescence reveals intra BSF localization. A qualitative model evidences the large spatial extension of the wavefunction of these BSF bound QW excitons making them extremely sensitive to potential fluctuations located in and away from BSF. Finally polarization dependent measurements show a strong emission anisotropy for BSF bound QW excitons which is related to their one dimensional character and that confirms that the intersection between a BSF and a GaN/(AlGa)N QW can be described as a quantum wire.

The electronic structure of Al1-xInxNd(10 (1) over bar0) surfaces is investigated by cross-sectional scanning tunneling spectroscopy and density functional theory calculations. The surface exhibits empty Al and/or In-derived dangling bond states, which are calculated to be within the fundamental bulk band gap for In compositions smaller than 60%. The energy of the lowest empty In-derived surface state is extracted from the tunnel spectra for lattice-matched Al1-xInxN with In compositions of x = 0.19 and x = 0.20 to be EC-1.8260.41 and EC-1.8060.56 eV, respectively, in good agreement with the calculated energies. Under growth conditions, the Fermi level is hence pinned (unpinned) for In compositions smaller (larger) than 60%. The analysis of the tunnel spectra suggests an electron affinity of-3.5 eV for nonpolar lattice-matched Al1-xInxN cleavage surfaces, which is large compared to linearly interpolated values of polar AlN and InN (0001) surfaces. Published by AIP Publishing.

Stimulated polariton scattering in semiconductor microcavities is reported. The phenomena arise from the bosonic character of the coupled exciton-photon modes of the system. The ability to manipulate and control the polariton dispersions in microcavities is shown to be a key factor underlying the new observations. The potential of such phenomena to form the basis of a new type of coherent light source ("polariton" lasers) and highly efficient optical parametric oscillators is discussed.

We report the observation of midinfrared intersubband (ISB) absorption in nearly lattice-matched AlInN/GaN multiple-quantum-wells. A clear absorption peak is observed around 3 mu m involving transitions from the conduction band ground state to the first excited state. In addition to ISB absorption, photoluminescence experiments were carried out on lattice- matched AlInN/GaN single quantum wells in order to determine the spontaneous polarization discontinuity between GaN and Al0.82In0.18N compounds. The experimental value is in good agreement with theoretical predictions. Our results demonstrate that the AlInN/GaN system is very promising to achieve crack-free and low dislocation density structures dedicated to intersubband devices in the 2-4 mu m wavelength range.

The authors report on the achievement of narrow ultraviolet emission using GaN/AlGaN quantum wells grown by metal organic vapor phase epitaxy. The origin of the inhomogeneous broadening of the emission is explained by means of micro-photoluminescence and atomic force microscopy measurements. The effect of the well/barrier interface roughness on the linewidth is found to be marginal, while the impact of Al composition fluctuations of the barriers is highly critical. Emission linewidths as small as 5 meV at 10 K are obtained by decreasing the aluminum concentration fluctuations in the AlGaN barrier. (c) 2007 American Institute of Physics.

We report on the direct observation of biexcitons in a III-nitride based multiple quantum well microcavity operating in the strong light-matter coupling regime by means of nonresonant continuous wave and time-resolved photoluminescence at low temperature. First, the biexciton dynamics is investigated for the bare active medium (multiple quantum wells alone) evidencing localization on potential fluctuations due to alloy disorder and thermalization between both localized and free excitonic and biexcitonic populations. Then, the role of biexcitons is considered for the full microcavity: in particular, we observe that for specific detunings the bottom of the lower polariton branch is directly fed by the radiative dissociation of either cavity biexcitons or excitons mediated by one LO-phonon. Accordingly, minimum polariton lasing thresholds are observed, when the bottom of the lower polariton branch corresponds in energy to the exciton or cavity biexciton first LO-phonon replica. This singular observation highlights the role of excitonic molecules in the polariton condensate formation process as being a more efficient relaxation channel when compared to the usually assumed acoustical phonon emission one.

We report the growth over 2 in. sapphire substrates of crack-free fully epitaxial nitride-based microcavities using two highly reflective lattice-matched AlInN/GaN distributed Bragg reflectors (DBRs). The optical cavity is formed by an empty 3lambda/2 GaN cavity surrounded by AlInN/GaN DBRs with reflectivities close to 99%. Reflectivity and transmission measurements were carried out on these structures, which exhibit a stopband of 28 nm. The cavity mode is clearly resolved with a linewidth of 2.3 nm. These results demonstrate that the AlInN/GaN system is very promising for the achievement of strong light-matter interaction and the fabrication of nitride-based vertical cavity surface emitting lasers. (C) 2005 American Institute of Physics. [DOI: 10.1063/1.1849851]

By performing pump-probe experiments, we study the relaxation dynamics of spin-polarized excitons in a wurtzite GaN/AlGaN quantum well (QW)grown by metal organic vapor phase epitaxy. The circular degree of polarization of the differential reflectivity signal reaches 55-60 % for the heavy-hole exciton when it is resonantly excited before decaying in a few picoseconds. This is an indication that, even if excitons are confined in the growth direction, their propagation in the well plane makes them very sensitive to scattering processes which are responsible for exciton dephasing and, therefore, for efficient spin relaxation. Above 80 K the thermal escape of excitons toward the AlGaN barrier increases the spin-relaxation rate. However, when compared to bulk GaN epilayers, where no spin polarization could be detected above 50 K, the optical orientation is preserved in those QWs up to about 100 K.

The authors report room temperature polariton lasing at lambda similar to 345 nm in a hybrid AlInN/AlGaN multiple quantum well microcavity (MQW-MC) containing a GaN/AlGaN MQW active region, i.e., the achievement under nonresonant optical excitation of coherent light emission of a macroscopic population of polaritons occupying the lowest energy state of the lower polariton branch. This was made possible by taking advantage of the efficient relaxation of polaritons in a MQW-MC exhibiting a large vacuum Rabi splitting Omega(VRS)=56 meV. (c) 2008 American Institute of Physics.

When silicon thin films are deposited by plasma enhanced chemical vapor deposition in a plasma regime close to the formation of powder, a new type of material, called hydrogenated polymorphous silicon (pm-Si:H) is obtained. This material has increased transport properties with respect to device-grade hydrogenated amorphous silicon (a-Si:H). To understand the origin of such improved transport properties, we made electrical measurements from which we deduced that the density of states at the Fermi level N(E-F) and the carrier capture cross-section, sigma(c), in pm-Si:H films are at least 10 times lower and 5 times lower, respectively, than in a-Si:H films. The crystallite sizes deduced from Raman spectra confirm high-resolution transmission electron microscopy measurements. The infrared stretching modes of pm-Si:H films have a band at similar to 2035 cm(-1) which is attributed to hydrogen platelets. The smaller density of states at the Fermi level N(E-F) is explained in terms of improved amorphous matrix as confirmed by optical measurements. We suggest that the low capture cross-section, sigma(c), observed in these films results from a preferential carrier recombination path at grain boundary dangling bonds as predicted by theoretical calculations. (C) 2000 Elsevier Science B.V. All rights reserved.

Recent work on polariton-polariton scattering in semiconductor microcavities under continuous wave excitation conditions is reviewed. For weak non-resonant laser excitation, a marked bottleneck in the polariton distribution is observed, but which is suppressed by polariton-polariton scattering as the laser intensity is increased. However, the high excitation conditions necessary to observe stimulated emission lead to loss of strong coupling and conventional lasing in the weak coupling regime, By contrast for resonant excitation, polaritons are created directly in the polariton trap formed by the microcavity dispersion curve. Stimulated scattering of the bosonic quasi-particles occurs to the emitting state at the centre of the Brillouin zone, and to a companion state at high wavevector. The stimulation phenomena lead to condensation of the bosonic quasi-particles to two specific regions of k-space, and to the formation of a new state with macroscopic coherence. The prospects to achieve a polariton laser under conditions of non-resonant excitation are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.

Nonpolar GaN based microcavity (MC) made of a bottom AlN/GaN distributed Bragg reflector (DBR) and a top dielectric SiO2/SiNx DBR has been fabricated on a-plane GaN template. The 13 pair AlN/GaN DBR, centered around 372 nm, exhibits a peak reflectivity of similar to 95% together with a flat stopband of 30 nm width. The cavity mode centered around 390 nm is characterized by a full width at half maximum of 4 nm. The optical properties of both the DBR and MC are well reproduced when accounting for linear birefringence effects. (c) 2008 American Institute of Physics.

We present a temperature dependent optical and quantum-optical characterization of close-to-ideal lasing in GaN-based nanobeam cavities. Measuring the photon statistics of emission allows us to prove high-beta lasing at room temperature, and thresholdless lasing at 156K. Thresholdless lasing is explained via temperature dependent carrier redistribution in the 0D/2D gain medium.

We report on the dependence on temperature of the homogeneous and inhomogeneous broadening of the fundamental transition of single polar GaN/AlN quantum dots (QDs). Stranski-Krastanov QDs have been grown by molecular beam epitaxy using NH3 as a nitrogen source, with a very low surface density. Low temperature (LT) microphotoluminescence measurements have been performed on 200 nm wide mesas in order to isolate the luminescence of single QDs. The linewidth is found to vary from 590 mu eV at 4 K up to 1350 mu eV at 65 K in a dot of 6 monolayer height. Though the LT linewidth is still dominated by spectral diffusion, the temperature dependent broadening up to 50 K is mainly accounted for by interactions between excitons and acoustic phonons through a coupling coefficient value nearly two orders of magnitude larger than its counterpart in InAs QDs. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We present a comprehensive study of the anisotropic optical properties of nonpolar GaN/AlGaN multiple quantum wells intentionally designed to act as an active region of a planar microcavity operating in the strong-coupling regime. The strain induced by the underlying AlGaN-based Bragg reflector leads to a redistribution of exciton oscillator strength as revealed by photoluminescence and reflectivity measurements. Complementary k . p calculations show an excellent agreement with experiments and emphasize the opportunity to tune the nature of the light-matter coupling in a microcavity by means of strain engineering. Finally, the validity of the developed model is proven by angle-resolved photoluminescence studies carried out on the complete microcavity structure. The recorded eigenmode spectra reveal the coexistence of the weak- and the strong-coupling regime along the two orthogonal polarization planes.

We present a systematic study of the local hydrogen bonding in hydrogenated polymorphous silicon thin films (pm-Si:H), a heterogeneous material deposited on the edge of crystallinity, by means of Fourier transform infrared spectroscopy. A vibrational mode at similar to2030 cm(-1) is reported and attributed to hydrogen atoms bonded in hydrogen-rich regions present at the interface between ordered regions and the amorphous matrix. This assignment is found to be in good agreement with previous spectroscopic ellipsometry and Raman spectroscopy studies. Combined with stress measurements we draw a picture of the nanostructure of pm-Si:H films, namely a two-domain material exhibiting: (i) large fluctuations of H content, (ii) a high mass density and (iii) the coexistence of highly strained crystalline phases with a relaxed amorphous matrix. Unexplained optoelectronic properties and metastability phenomena can also be accounted for by this picture.

We report a thorough study of the growth of self-assembled GaN/AlN quantum dots (QDs). These QDs were grown on sapphire substrates using the Stranski-Krastanov (SK) growth mode by means of low-pressure metal organic vapor phase epitaxy. The influence of the V/III ratio on the QD height, shape, and density is studied. Emissions above (3.8 eV) and below (2.8 eV) the band gap of GaN are observed for small dots (1.3 nm) and large ones (4.4 nm), respectively. Special emphasis was given to the SK growth mode transition and the parameters influencing the QD formation. The two-dimensional-three-dimensional transition is studied continuously by photoluminescence mapping showing its high sensitivity to the V/III ratio. (C) 2006 American Institute of Physics. (C) 2006 American Institute of Physics.

We report a full optical deep sub-wavelength imaging of the vectorial components of the electric local density of states for the confined modes of a modified GaN L3 photonic crystal nanocavity. The mode mapping is obtained with a scanning near-field optical microscope operating in a resonant forward scattering configuration, allowing the vectorial characterization of optical passive samples. The optical modes of the investigated cavity emerge as Fano resonances and can be probed without the need of embedded light emitters or evanescent light coupling into the nanocavity. The experimental maps, independently measured in the two in-plane polarizations, turn out to be in excellent agreement with numerical predictions. (C) 2015 AIP Publishing LLC.

We report the growth of highly reflective distributed Bragg reflectors (DBRs) in the UV region using the Al0.85In0.15N/Al0.2Ga0.8N lattice-matched system. The DBRs were deposited on nearly strain-free Al0.2Ga0.8N templates to avoid strain-induced structural degradations. The appearance of cracks was then completely suppressed. The DBRs exhibit a reflectivity higher than 99% at a wavelength as short as similar to 340 nm and a stop band width of 215 meV (20 nm). (c) 2006 American Institute of Physics.

The electrical properties and deep trap spectra were compared for near-UV GaN/InGaN quantum well (QW) structures grown on free-standing GaN substrates. The structures differed by the presence or absence of a thin (110 nm) InGaN layer inserted between the high temperature GaN buffer and the QW region. Capacitance-voltage profiling with monochromatic illumination showed that in the InGaN underlayer (UL), the density of deep traps with optical threshold near 1.5 eV was much higher than in the QW and higher than for structures without InGaN. Irradiation with 5 MeV electrons strongly increased the concentration of these 1.5 eV traps in the QWs, with the increase more pronounced for samples without InGaN ULs. The observations are interpreted using the earlier proposed model explaining the impact of In-containing underlayers by segregation of native defects formed during growth of GaN near the surface and trapping of these surface defects by In atoms of the InGaN UL, thus preventing them from infiltrating the InGaN QW region. Deep level transient spectroscopy (DLTS) also revealed major differences in deep trap spectra in the QWs and underlying layers of the samples with and without InGaN ULs. Specifically, the introduction of the InGaN UL stimulates changing the dominant type of deep traps. Irradiation increases the densities of these traps, with the increase being more pronounced for samples without the InGaN UL. It is argued that light emitting diodes (LEDs) with InGaN UL should demonstrate a higher radiation tolerance than LEDs without InGaN UL. (C) 2020 Elsevier B.V. All rights reserved.

Two types of near-UV light-emitting diodes (LEDs) with an InGaN/GaN single quantum well (QW) differing only in the presence or absence of an underlayer (UL) consisting of an InAlN/GaN superlattice (SL) were examined. The InAlN-based ULs were previously shown to dramatically improve internal quantum efficiency of near-UV LEDs, via a decrease in the density of deep traps responsible for nonradiative recombination in the QW region. The main differences between samples with and without UL were (a) a higher compensation of Mg acceptors in the p-GaN:Mg contact layer of the sample without UL, which correlates with the presence of traps with an activation energy of 0.06 eV in the QW region, (b) the presence of deep electron traps with levels 0.6 eV below the conduction band edge (E-c) (ET1) and at E-c 0.77 eV (ET2) in the n-GaN spacer underneath the QW, and the presence of hole traps (HT1) in the QW, 0.73 eV above the valence band edge in the sample without UL (no traps could be detected in the sample with UL), and (c) a high density of deep traps with optical ionization energy close to 1.5 eV for the LEDs without UL. Irradiation with 5 MeV electrons led to a strong decrease in the electroluminescence (EL) intensity in the LEDs without UL, while for the samples with UL, such irradiation had little effect on the EL signal at high driving current, although the level of driving currents necessary to have a measurable EL signal increased by about an order of magnitude. This is despite the 5 times higher starting EL signal of the sample with UL. Irradiation also led to the appearance in the LEDs with UL of the ET1 and HT1 deep traps, but with concentration much lower than without the UL, and to a considerable increase in the Mg compensation ratio.

The authors report on the achievement of optically pumped III-V nitride blue microdisk lasers operating at room temperature. Controlled wet chemical etching of an AlInN interlayer lattice matched to GaN allows forming inverted cone pedestals. Whispering gallery modes are observed in the photoluminescence spectra of InGaN/GaN quantum wells embedded in the GaN microdisks. Typical quality factors of several thousands are found (Q>4000). Laser action at similar to 420 nm is achieved under pulsed excitation at room temperature for a peak power density of 400 kW/cm(2). The lasing emission linewidth is down to 0.033 nm.

We present the numerical optimization of GaN-based embedded 2D-Photonic Crystal (PhC) LEDs based on a full 3D scattering matrix model pointing out the importance of a proper vertical design in embedded PhC structures. The growth of such structures following the numerical simulation trends is shown to be feasible. Experimental results of a bright embedded 2D-PhC LED (9mW at 20mA) under electrical injection are reported. Angular measurements are performed to understand the efficiency in light extraction of the PhCs and the mechanisms limiting its full potential are presented. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The electrical properties, electroluminescence (EL) power output and deep trap spectra were studied before and after 5 MeV electron irradiation of near-UV single-quantum-well (SQW) light-emitting diodes (LED) structures differing by the presence or absence of InAlN superlattice underlayers (InAlN SL UL). The presence of the underlayer is found to remarkably increase the EL output power and the radiation tolerance of LEDs, which correlates with a much lower and more slowly changing density of deep traps in the QW region with radiation dose, and the higher lifetime of charge carriers, manifested by higher short-circuit current and open-circuit voltage in current–voltage characteristics under illumination. The observed phenomena are explained by the capture of native defects segregated at the growing surface by In atoms in the underlayer which traps them in the underlayer and prevents their penetration into the QW region.