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Vertically aligned ZnO nanorods (NRs) with a diameter in the range of 160-200 nm were grown on p-GaN/sapphire substrates by aqueous chemical growth technique and white light emitting I diodes (LEDs ...

The position-controlled growth and structural and optical characteristics of ZnO nanotubes and their coaxial heterostructures are reported. To control both the shape and position of ZnO nanotubes, hole-patterned SiO2 growth-mask layers on Si(111) substrates with GaN/AlN intermediate layers using conventional lithography are prepared. ZnO nanotubes are grown only on the hole patterns at 600 °C by catalyst-free metal–organic vapor-phase epitaxy. Furthermore, the position-controlled nanotube growth method allows the fabrication of artificial arrays of ZnO-based coaxial nanotube single-quantum-well structures (SQWs) on Si substrates. In situ heteroepitaxial growth of ZnO and Zn0.8Mg0.2O layers along the circumference of the ZnO nanotube enable an artificial formation of quantum-well arrays in a designed fashion. The structural and optical characteristics of the ZnO nanotubes and SQW arrays are also investigated using synchrotron radiation X-ray diffractometry and photoluminescence and cathodoluminescence spectroscopy.

Optical properties of ZnO-based single nanorods are probed by cathodoluminescence (CL) measurements at T = 5 K. We observe a variation of the ZnO near band edge CL by three orders of magnitude along the nanorod axis, accompanied by a spectral blueshift of 10-30 meV. This indicates a rather poor structural quality of the nanorod bottom part, close to the substrate. ZnO/ZnMgO quantum wells grown on top of ZnO nanorods are found to exhibit much stronger confinement effects as compared to their two-dimensional counterparts, suggesting a reduced spontaneous and piezoelectric polarization effects.

We report on the self organized growth of GaN quantum dots deposited on Al x Ga 1-x N layer by plasma-assisted molecular beam epitaxy. It is found that the relaxation of Al x Ga 1-x N layer on AlN depends on Al composition and thickness. The measurement of the variation of lattice parameter indicated that GaN quantum dots do not relax completely on Al x Ga 1-x N layer. Optical properties of undoped and Eu-doped quantum dots on AlGaN layer were studied. Visible light emission from Eu 3+ ions has been observed and showed strong thermal stability.

We study the self organized growth of GaN quantum dots deposited by molecular beam epitaxy on Al x Ga 1-x N, with x varying between 1 and 0.21. As a result of the specific strain relaxation mechanism of Al x Ga 1-x N on AlN, it is found that dots can be formed for an Al content as low as about 30%. Monopolar devices have been realized by depositing Schottky contacts on n-type doped Al 0.5 Ga 0.5 N. Finally, electroluminescence of Eu-doped GaN quantum dots embedded in n-type Al 0.5 Ga 0.5 N has been demonstrated, emphasizing the potentialities of such an active layer for room temperature visible light emission.

We report on the growth of Tb-doped GaN quantum dots deposited by plasma-assisted molecular beam epitaxy. It was found that Tb ions enhance the desorption process of Ga ad-atom on the growing surface. This effect becomes more important with increasing growth temperature, and above a certain amount of Tb, the QDs formation is completely inhibited. Transitions from the 5D4 excited state were found intense and temperature stable, while the transitions from 5D3 excited state were only observed at low temperature. A comparative optical study with Tb-doped AlN thick layer indicates that Tb ions are incorporated in GaN QDs. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of stacking on the optical properties of self-organized GaN/AlN quantum dots (QDs) grown by plasma-assisted molecular beam epitaxy was investigated by photoluminescence (PL), PL excitation (PLE), and cathodoluminescence (CL) spectroscopic techniques. With an increase in the number of the GaN QDs stacking layer, we observed a red shift in the main QD PL peaks. A Stokes-like shift was observed between PL emission and PLE absorption edge for all the samples. From the depth-resolved CL of a 200-period GaN QD sample, we observed that the main QD CL emission peak energy position does not change much, while the higher-energy side CL peak around ∼3.7 eV shows a variation of its peak position with the accelerating voltage probably due to the change in either distribution of QD size/shape or wetting layer properties during the growth of multiple stacking of GaN QDs. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on a comparative optical study of molecular beam epitaxy grown GaN:Eu quantum dots (QDs) and GaN:Eu layer. In the GaN:Eu layer the narrow emission lines of the 5D0-7F2 transition allow to identify two different Eu locations corresponding to near the sample surface and lying deeper in the bulk. Eu3+ emission from QDs has been found to be at the same spectral position than that assigned to Eu3+ ions located in the bulk of the GaN:Eu layer. Eu emission in the temperature range 4-300 K is thermally stable in doped QDs, but exhibits a strong thermal quenching in the doped layer, particularly for Eu3+ ions located near the surface. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this work we discuss the optical properties of a GaN/Al 0.11 Ga 0.89 N multiple-quantum-well structure grown on 4H-SiC by plasma-assisted molecular-beam epitaxy. Photoluminescence lines related to the fundamental and excited electronic levels are identified, in good agreement with simulations of the electronic structure. Temperature dependence studies reveal an anomalous behaviour with quenching of the e 1 -hh 1 while the e 2 -hh 1 line becomes dominant at room temperature. This behaviour could be explained by the increasing thermal population of the e 2 state and the higher radiative efficiency of the e 2 -hh 1 transition compared to the e 1 -hh 1 transition due to the intense electric field in the well.

We report on the controlled growth of Si doped GaN/AIN quantum dot (QD) superlattices, in order to tailor their intersubband absorption within the 1.3-1.5 μm telecommunication wavelengths. The QD size is tuned by modifying the amount of GaN in the QDs and the growth temperature. Silicon can be incorporated in the QDs to populate the first electronic level, without significant perturbation of the QD morphology. As a proof of the capability of these structures for infrared detection, a quantum-dot intersubband photodetector at 1.38 μm with lateral carrier transport is demonstrated.

We have studied CdTe/CdMgTe microcavity samples containing 4 quantum wells and characterized by a vacuum field Rabi-splitting of 13.2 meV. Electron-hole pairs were excited at 7.3K by linearly-polarized light at 1.74 eV provided by a Ti:sapphire laser operating in the pulsed mode (at 80 MHz rate). Spectroscopic imaging in reciprocal space as well as near-field spectroscopy show that, above a critical stimulation threshold, polaritons in our microcavity undergo a spontaneous transition to a condensed phase characterized by the massive occupation of the polariton ground state. The coherence of this quantum state is evidenced by the spectral and angular narrowing of its emission. We point out the conditions which should be fulfilled to achieve BEC of microcavity polaritons. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the MBE growth of GaN and InGaN quantum dots (QDs) doped with rare earth ions, namely Eu, Tm and Tb exhibiting red, blue and green luminescence, respectively. Intense photoluminescence/cathodoluminescence is observed, resulting from the spatial localization of rare earth ions in dots combined with the confinement properties of the carriers. White light emission has been produced by combining the three rare earths in a multilayer sample of stacked GaN QD planes. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have performed a two-beam experiment on a II–VI microcavity in the strong coupling regime. A pulsed non-resonant excitation creates a transient population of excitons of large in-plane wave-vector and a very weak polariton population at k‖ = 0, center of the Brillouin zone. A cw laser beam resonantly injects polaritons at the inflexion point of the lower polariton branch and efficiently populates the k‖ = 0 via polariton-polariton scattering. In the presence of the two laser beams, we observe a strong increase of the transient emission at k‖ = 0 associated to a drastic modification of the emission dynamics. The signal gain can reach 200. We show that the key parameter controlling the gain is the cw polariton population at k‖ = 0, which stimulates the polariton relaxation from large k states toward k‖ = 0. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We demonstrate Eu, Tm and Er doping of GaN quantum dots. In Er-doped samples, two very sharp blue lines were observed. These lines were dominant over the green lines reported in the case of Er-doped GaN thick layers. However, intensities were very low in comparison with Eu-doped and Tm-doped GaN QDs. Intense blue emission from Tm-doped GaN QDs was observed. Temperature-dependent photoluminescence was studied, and the intensity only decreased to 27% between room temperature and 10 K. Structural studies on Tm-doped samples showed that the kinetic role of Tm atoms was different from that of Eu atoms.

We present detailed analysis of inter- and intraband transitions in GaN/AlN self-organized quantum dots grown on sapphire, silicon (111) and 6H-SiC substrates. Samples containing quantum dots of different size were characterized by means of transmission electron microscopy, photoluminescence and photoinduced absorption spectroscopy. Interlevel transitions in the conduction band were identified in the 0.52-0.98 eV range covering the telecommunication wavelength band. The s-p(z), transition ranges from 0.52 eV to 0.8 eV for dots with height of 6 down to 1.5 nm, respectively. Experimental results are compared with theoretical calculations showing that in larger dots the transition energy is governed by the value of the internal field.

In this paper we present experimental and theoretical study of intersubband transitions at telecommunication wavelengths in GaN/AlN quantum wells grown by molecular beam epitaxy on sapphire substrates. Crossed Rutherford back-scattering and photoluminescence experiments show that strong in-plane carrier localization occurs due to thickness fluctuations at GaN/AlN interfaces. Fourier transform infrared spectroscopy and photo-induced absorption spectroscopy performed on doped and undoped samples reveal a systematic blue-shift of the e1–e2 transition due to many body interactions. A good agreement is achieved between experiment and self-consistent Schrodinger–Poisson calculations. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We demonstrate the growth of GaN quantum dots doped with Eu by plasma-assisted MBE. Intense emission from the intra-atomic 4f transition of Eu3+ was observed at 622 nm. GaN quantum dots (QDs) doped with 1% Eu showed the maximum emission. The full-width at half maximum (FWHM) of the CL spectrum of Eu-doped GaN QDs was 2.1 nm at room temperature. Eu doping increased the size of QDs by 4 times and decreased the density of QDs by one order of magnitude. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present the growth of AlN films on vicinal substrates of SiC(0001). The layers were grown by plasma-assisted molecular beam epitaxy (PAMBE) and examined in-situ using reflection high-energy electron diffraction (RHEED). Morphological and structural properties were characterized by atomic force microscopy (AFM) and High Resolution X-Ray Diffraction (HRXRD), respectively. The obtained results by AFM show two main differences with AlN films grown on nominally flat SiC substrates: a) the absence of spiral growth hillocks, and b) the formation of predominantly straight steps. HRXRD measurements on misoriented substrate rocking curves have revealed good structural quality of AlN films (tilt and twist), as well as a disorientation of the c-planes of the AlN with respect to the substrate.

We used angle-resolved time-integrated four-wave mixing to study the pump dynamics in a II–VI semiconductor microcavity polariton parametric amplifier. The low-energy polariton dephasing time decreases at large angles when the excitonic reservoir becomes accessible to polariton–polariton scattering. Additionally, in the parametric amplification regime, with phase matching for the polariton–polariton scattering at the magic angle, we observe a strong reduction of the FWM decay time. Numerical analysis of the polariton coherent dynamics in this regime allows to interpret the observed features in terms of time dependent pump depletion.

After a short review of the present stage of knowledge of the Si[100] and β-SiC surface reconstructions and Si carbonisation mechanism, we discuss the possible origin for the change in sign of the interfacial strain recently observed in this strongly mismatched cubic hetero-epitaxial system as a function of the initial carbonisation temperature. Both the interfacial and anisotropic character of this strain are confirmed by micro-Raman analysis of cleaved CVD-grown samples, which reveal also how extended defects relieve the residual strain after the growth of a few micrometers of 3C-SiC. Such pseudo-substrates have been used as a cubic template for the MBE growth of cubic nitrides and Raman scattering results on β-AlN materials and cubic GaN/AlN stacked quantum boxes are presented. Finally, the potential and limitations of such 3C-SiC pseudo-substrates for nitride growth are discussed.

We have fabricated CdMgTe/CdMnTe based pillar microcavities with diameters down to 2 μm by optical lithography and reactive ion etching. Three-dimensional optical confinement is demonstrated by the occurrence of higher lateral modes. In the high excitation regime, the emission linewidth in pillar microcavities is found to be significantly reduced as compared to planar microcavities.

We propose a method to determine the intrinsic non-linearities in semiconductor microcavities in the non-perturbative regime. The polariton coherent dynamics is experimentally studied using time-integrated four-wave mixing and analyzed by numerical resolution of the coupled Bloch-Maxwell equations. The relative weight of phase-space filling to Coulomb non-linearities is measured as a function of excitation density and detuning. It is shown that depending on the excitation conditions, phase-space filling can be the dominant non-linearity at carrier densities as low as one order of magnitude below the saturation of the strong coupling regime.

We investigate the growth modes for GaN homoepitaxy. Several parameters have been taken into account: the growth rate, the metal/N ratio value and the substrate temperature. The observation of time variation of the RHEED diffraction pattern and the study of thick samples has permitted the determination of a phase diagram of the Ga surface coverage during GaN homoepitaxy as a function of III/V ratio and substrate temperature. Furthermore, we address the different GaN surface morphologies obtained in different regions of the phase diagram.

We studied and improved gallium nitride (GaN) nanowire (NW) based light emitting diodes (LEDs). PIN nanodiodes with and without InGaN/GaN multiple quantum wells (MQWs) were grown by molecular beam epitaxy (MBE) under N-rich conditions on n-doped Si(111) substrates. Thanks to the coalescence of the p-type region of the NWs grown at low temperature, an autoplanarization process has been performed to obtain LEDs. Ni/Au top contacts have been deposited and patterned in order to bias the devices. A multiple-scale characterization approach has been carried out through the comparison of localized cathodoluminescence (CL) and macroscopic electroluminescence (EL) spectra. It shows that the EL emission of PIN-based LED at room temperature is related to defects in the p-type region of the NWs. In order to enhance the radiative recombinations of NW-based LEDs, we have first added InGaN/GaN MQWs, and secondly an electron blocking layer (EBL) has been inserted between the MQWs and the p-type zone of the NWs. The LED with EBL exhibited an emission band at 420 nm. The blue-shift of this emission band with increasing injected current is attributed to quantum confined Stark effect (QCSE) and evidences the radiative emission of InGaN/GaN MQWs. At 50 mA dc current, this improved NW-based LED emits about 500 times more light than the heterostructure without EBL. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In this work we report on the fabrication and characterization of a n-ZnO/p-GaN heterojunction LED. The p-GaN layer was fabricated using MOCVD on Al2O3 with Mg as the acceptor whereas the ZnO nanostructures were grown in a very simple vapor transport system without any additionally doping. Room temperature electroluminescence (EL) measurements show green deep band emission centered at 2.3 eV which is clearly visible with the naked eye when the structure is forward biased. Cathodoluminescence mapping was performed to explain the absence of the band edge emission in the EL spectrum. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the growth, structural and optical properties of GaN/AlN free standing NWs. The NW lateral growth rate was investigated using thin AlN marker layers. The lateral growth is found to be homogeneous along the NW axis. Its rate can be tuned from ∼1% to ∼10% of the axial growth rate by changing III/V flux ratio. The resulting axial and lateral heterostructures were studied by micro-photoluminescence and micro-cathodoluminescence, which revealed sharp (several meV broad) emission lines. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)