Search

Time-resolved luminescence spectra due to an electron-hole plasma (EHP emission) and an exciton-exciton scattering process (P emission) were measured in ZnO epitaxial thin films under band-to-band excitation by the optical Kerr gate method. We find that both the EHP and P emissions emerge with a long time delay of a few picoseconds. The time delay observed decreases quadratically with the excitation density below the critical Mott density. These results imply that the P (EHP) emission emerges after completing thermal cooling of excitons (electrons and holes) toward the exciton band bottom (renormalized band gap) through multiple LO-phonon emissions. As the excitation density increases, the exciton-exciton scattering process also plays an important role on cooling and relaxation of high-density excitons. (c) 2006 Elsevier B.V. All rights reserved.

Using deep-level-transient spectroscopy, we have investigated deep levels in heavily Al-doped ZnSe layers grown by molecular-beam epitaxy. The Al concentration of the ZnSe layers lies in the range of 5×1018–9×1018cm−3. The ZnSe:Al layers exhibit two electron-trap centers with the thermal activation energies of 0.16eV (ND1) and 0.80eV (ND2). ND2 is a dominant trap center with a trap density of 3×1016cm−3, while the trap density of ND1 is estimated to be 2×1015cm−3. However, ND2 shows anomalous behaviors, different from isolated point defects, in the following points: (1) the emission peak of ND2 moves to the low temperature side with increasing filling pulse duration; (2) the emission peak of ND2 is broader than theoretically calculated one for an isolated point defect; and (3) the capacitance-transient curve is nonexponential. It is observed by high-resolution x-ray diffraction that heavy Al doping results in the relaxation and plastic deformation of the ZnSe lattice. These behaviors can be ascribed to ex...

We have performed in situ and ex situ measurements of reflectance difference (RD) spectra for Cl-doped n-type ZnSe grown by molecular beam epitaxy. The linear electro-optic (LEO) effect (or Pockels effect) in the depletion layer induces characteristic RD features near the E1 and E1+Δ1 transition energy. The intensity of these features is correlated with the carrier concentration determined from capacitance–voltage measurements. A quantitative model is presented to explain the observed dependence of the LEO signal on the carrier concentration. This model quantitatively considers the effect of the finite surface state density. A good fit with the experimental results was obtained by assuming the surface state densities of 3.8×1013 and 2.6×1013 cm−2 eV−1, respectively, for in situ measurement of a Se-terminated surface at 300 °C and for ex situ measurement of an oxide-covered surface at room temperature.

Vertical light-emitting diodes (LEDs) were successfully fabricated by a chemical lift-off process using a selectively etchable CrN buffer layer. The novel CrN metallic layer worked well as a buffer layer for growth of the GaN LED and was etched out clearly during selective chemical etching. The vertical LED by chemical lift-off showed very good current-voltage performance with low series resistance of 0.65 Omega and low operated voltage of 3.11 V at 350 mA. Also, this device could be operated at a much higher injection forward current (1118 mA at 3.70 V) by thermally conductive metal substrate which enabled the high current operation with excellent heat dissipation.

A laser diode which includes II–VI device structure directly grown on III–V device structure is proposed. This idea makes possible one-chip-multiple-wavelength operation in the visible wavelength region by the vertical integration of individual light emitters, which is demonstrated by a ZnCdSe/ZnSe/ZnMgBeSe quantum-well structure for a blue-green light emitter grown by molecular beam epitaxy directly on a metalorganic chemical vapor deposition grown InGaP/InGaAlP device wafer for a red light emitting device. The feasibility of the II–VI/III–V complex light emitter is demonstrated by optical-pumping experiments. Optically pumped lasing at 504 and 664 nm is achieved from II–VI and III–VI laser structures on one chip simultaneously at room temperature with a threshold power of 115 and 84 kW/cm2, respectively. The present results clearly show the feasibility of epitaxial integration of II–VI and III–V laser structures, which will stimulate the investigation of multifunctional optical devices.

N-type ZnTe layers with high electron concentration are grown by molecular-beam epitaxy using aluminum as the donor species. The ZnTe:Al layers show a high structural quality with a narrow x-ray diffraction linewidth (24 arcsec) and a high carrier concentration up to n=4×1018 cm−3 with low resistivity (ρ=0.017 Ω cm). The dependence of the electron mobility on the carrier concentration suggests that the dominant scattering mechanisms in the ZnTe:Al layers are ionized impurity scattering and polar optical phonon scattering. The photoluminescence spectrum of moderately doped ZnTe layers shows strong Al–donor-related bound exciton lines: I2 (2.378 eV) and donor–acceptor pair emission (zero phonon energy=2.324 eV) with a weak deep-level emission (2.19 eV). Highly Al-doped layers show an increase in the deep-level emission intensity and a decrease in carrier mobility, which are interpreted in terms of the increase in the carrier compensation.

Adsorption of Zn on the GaAs(001)-(2×4) surface has been studied by reflection high-energy electron diffraction (RHEED) and total-reflection-angle x-ray spectroscopy in real time. Adsorption sites of Zn atoms on the (2×4) surface have been determined using rocking-curve analysis of RHEED. The exposure of the (2×4) surface to a Zn beam results in the preferred adsorption of Zn at Ga-vacancy sites in the missing dimer trenches of the (2×4) unit cell. The amount of adsorbed Zn atoms under a Zn flux has been estimated to be ∼0.50 and ∼0.20 monolayers at 200 and 250 °C, respectively.

Eu 3+ - and Ce 3+ -doped yttrium silicate, as well as Eu 2+ -doped zinc silicate nanoparticles, were grown in a porous SiO 2 matrix using an impregnation method. For Y 2 Si 2 O 7 :Eu 3+ , particles of about 50 nm size were obtained that exhibited several photoluminescence (PL) peaks in red. Different peaks showed slightly different decay times; however, their excitation mechanism was found the same. Increasing the Eu concentration increased the PL intensity while reducing the decay time. Y 2 Si 2 O 7 :Ce 3+ nanoparticles in the porous matrix showed bright blue emission, consisting of two peaks at 358 nm and 378 nm. Re-impregnation process was found effective in changing the relative intensity of the two peaks. Zn 2 SiO 4 :Eu 2+ nanoparticles in porous glass consisted of amorphous particles of about 20 nm size inside the porous matrix. The luminescence was a broad peak centered at 418 nm. These phosphor systems, together with our previously reported Zn 2 SiO 4 :Mn 2+ in porous SiO 2 structure, comprise a red–green–blue system that can be used in display applications.

International audience; Monodispersed Co clusters with mean cluster diameter d=13 nm have been deposited on a stepped graphite surface and a lithography-patterned Si wafer using a plasma-gas-condensation cluster beam deposition apparatus. High-resolution scanning electron microscope observation indicates that 1) cluster aggregation is much more limited at the steps than on the flat terrace regions of the graphite surface, and 2) cluster density is much higher in the grooves than on the flat top of the lithography-patterned Si wafers. These results suggest the possibility of the regular arrangement of monodispersed Co clusters if the pattern size (the width of grooves and tops) is comparable with the cluster size.

International audience; Porous silicon layers were activated by the addition of Zn2SiO4:Mn2+ which resulted in green or yellow luminescent samples depending on the treatment conditions. The yellow peak occurs at 575 nm and has a considerably longer decay time than the green peak, which occurs at 525 nm. It is demonstrated that the yellow luminescence comes from small particles, typically 30 nm in size, which have been crystallized in the β-phase, while the green luminescence comes from the usual α-phase, which occurs with larger particles. β-Phase appears to be the dominant phase for small particles and any condition that results in the formation of small particles gives rise to yellow emission.

A new method is proposed to activate porous silicon layers with luminescent phosphor particles. It is based on chemical impregnation of porous silicon followed by controlled heat treatment. Porous silicon is used as one of the starting materials and as a template for the synthesis of silicate phosphor particles. In this paper, we present the results for the case of Zn2SiO4 : Mn2+ into mesoporous silicon layers. Samples show strong photoluminescence peaking at about 525 nm. The resulting layer is a porous SiO2 body in which Zn2SiO4 : Mn2+ particles with typical size of 150 nm are incorporated. This method offers wide flexibility, in terms of the morphology of the matrix and the possibility to synthesize various silicates with different activators to cover the whole range of visible spectrum.

Zn2SiO4:Mn2+ nanocrystals were grown in an oxidized porous silicon layer using a chemical impregnation method. Apparently two classes of samples have been obtained. One is characterized by the formation of α-phase zinc silicate crystalline particles, which show green luminescence, and the other one is characterized by β-phase particles, showing yellow luminescence. It was found that in general prolonged annealing, as well as a high degree of impregnation leads to the formation of green-luminescent samples. The decay time of both yellow and green luminescence decreases with the concentration of Mn activator, while generally the decay time of yellow luminescence is considerably larger than that of green luminescence.

Optical properties of wurtzite ZnO bulk single crystals in which an arbitrary number (typically 109–1010 cm−2) of fresh dislocations were introduced intentionally by the plastic deformation at elevated temperatures (923–1073 K) were examined. Deformed specimens showed excitonic light emission with photon energies of 3.100 and 3.345 eV, as well as their LO phonon replicas at 11 K. The light intensities increased with increasing dislocation density. The activation energy for a thermal quenching of the 3.100 or 3.345 eV emission band, which corresponds to the depth of the localized energy level associated with the emission band, was estimated to be 0.3±0.1 or 0.05±0.01 eV, respectively. The origin of the energy levels was proposed as point defect complexes involving dislocations. The introduction of the dislocations at the elevated temperatures above 923 K did not influence the intensities of the emission bands except the dislocation-related emission bands.

We have investigated electron-trap centers in ZnO layers grown under different Zn∕O flux ratios by molecular-beam epitaxy. Frequency-dependent capacitance measurements show that ZnO layers grown under Zn-rich and stoichiometric flux conditions suffer from larger dispersion than a ZnO layer grown under an O-rich flux condition. Temperature-dependent capacitance measurements reveal that all the ZnO layers have shallow electron-trap centers ET1 and deep electron-trap centers ET2, while the Zn-rich ZnO layer has another shallow electron-trap center ET3 besides ET1 and ET2: the thermal activation energies of ET1, ET2, and ET3 are estimated to be 0.033–0.046, 0.12–0.15, and 0.065 eV, respectively. Moreover, it is exhibited that the trap density of ET2 is larger than those of ET1 or ET3 in all the cases and increases as the Zn∕O flux ratio increases. Consequently, it is suggested that the large dispersion effect observed in the Zn-rich and stoichiometric ZnO layers is ascribed to the large density of deep electr...

We have investigated the electrical properties of ZnO∕GaN heterostructures by capacitance-voltage (C-V) measurements. ZnO∕GaN heterostructures are fabricated on Ga-polar GaN templates by plasma-assisted molecular-beam epitaxy. The ZnO∕GaN heterostructures exhibit a plateau region of 6.5V in the C-V curves measured at 10kHz and room temperature. Moreover, it is found that a large electron density is accumulated at the interface of ZnO∕GaN, where the concentration approaches ∼1018cm−3. The distinct C-V characteristics are ascribed to large conduction-band discontinuity at the ZnO∕GaN heterointerface. It is suggested that the ZnO∕GaN heterostructure is a very promising material for the application to heterojunction transistors.

We report on the structure study of MgxZn1−xO films and, in particular, we will focus on MgxZn1−xO layers with x=0.28 and 0.41 MgxZn1−xO layers with different crystal structures of cubic and wurtzite that have been grown by plasma-assisted molecular-beam epitaxy on MgO∕c-sapphire with Mg∕Zn flux ratio control. The MgxZn1−xO films have been characterized by high-resolution transmission electron microscopy (HRTEM) and high-resolution x-ray diffraction. The dependence of the cation-anion bond length to Mg content has been studied. A virtual crystal model of MgZnO has been applied to interpret the bond-length variation. HRTEM results indicate that the initial stage of the MgZnO growth on a MgO buffer layer starts with a cubic structure even in the case of a wurtzite structure at the end of growth.