Luminescence temperature and pressure studies of Zn2SiO4 phosphors doped with Mn2+ and Eu3+ ions

Zn2SiO4:Mn2+, Zn2SiO4:Eu3+ and Zn2SiO4:Mn2+ Eu3+ phosphors were prepared by a sol-gel process and their luminescence spectra were investigated. The emission bands from intra-ion transitions of Mn2+ and Eu3+ samples were studied as a function of pressure. The pressure coefficient of Mn2+ emission was found to be -25.3 +/- 0.5 and -28.5 +/- 0.9 meV/GPa for Zn2SiO4:Mn2+ and Zn2SiO4:Mn2+ Eu3+, respectively. The Eu3+ emission shows only weak pressure dependence. The pressure dependences of the Mn2+ and Eu3+ emissions in Zn2SiO4:Mn2+ Eu3+ are slightly different from that in Zn2SiO4:Mn2+ and Zn2SiO4:Eu3+ samples, which can be attributed to the co-doping of Mn2+ and Eu3+ ions. The Mn2+ emission in the two samples, however, exhibits analogous temperature dependence and similar luminescence lifetimes, indicating no energy transfer from Mn2+ to Eu3+ occurs. (c) 2005 Elsevier B.V. All rights reserved.

Thermal quenching of luminescence from buried and surface InGaAs self-assembled quantum dots with high sheet density

Variable-temperature photoluminescence (PL) spectra of Si-doped self-assembled InGaAs quantum dots (QDs) with and without GaAs cap layers were measured. Narrow and strong emission peak at 1075 nm and broad and weak peak at 1310 nm were observed for the buried and surface QDs at low temperature, respectively. As large as 210 meV redshift of the PL peak of the surface QDs with respect to that of the buried QDs is mainly due to the change of the strain around QDs before and after growth of the GaAs cap layer. Using the developed localized-state luminescence model, we quantitatively calculate the temperature dependence of PL peaks and integrated intensities of the two samples. The results reveal that there exists a large difference in microscopic mechanisms of PL thermal quenching between two samples. (c) 2005 American Institute of Physics.

Synthesis and luminescence of ZnMgS : Mn2+ nanoparticles

Efficient green emission from ZnMgS:Mn2+ nanoparticles prepared by co-doping Mg2+ and Mn2+ ions into ZnS lattices has been observed. The synthesis is carried out in aqueous solution, followed by a post-annealing process, thus showing the features of less complexity, low cost, and easy incorporation of dopants. In comparison with the emission of ZnS:Mn2+ nanoparticles, which is located generally around 590 nm, the photoluminescence of ZnMgS:Mn2+ nanoparticles is blue-shifted by 14 nm in wavelength, leading to the enhanced green emission. The X-ray diffraction, electron spin resonance, and pressure dependent photoluminescence measurements suggest that the change of the crystal field caused by Mg2+ ionic doping and the lower symmetry in the nanoparticles may account for the blue-shift of the photoluminescence. The ZnMgS:Mn2+ nanoparticles with 1% Mn2+ doping exhibit the strongest luminescence, which could potentially meet the requirements for the construction of green light emitting diodes.

Luminescence properties of multi-layer InGaN quantum dots grown on C- and R-plane sapphire substrates

In this study, we report comparative luminescence properties of multi-layer InGaN quantum dots grown on C- and R-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). High-density InGaN quantum dots (QDs) are formed on GaN templates by decreasing the growth temperature and increasing the adatom hopping-barrier through surface passivation. Atomic force microscopy (AFM) has been employed to estimate the size and height of these dots. Photoluminescence (PL) spectra recorded from (1120) InGaN QDs/(1102) sapphire show much stronger emission intensity compared to spectra recorded from (0001) InGaN QDs/(0001) sapphire. Due to the absence of strong spontaneous polarization and piezoelectric field, such (1150) InGaN QDs in the active layers would lead to high efficiency light emitting devices. (c) 2005 Elsevier B.V. All rights reserved.

Near infrared photoluminescence from Yb,Al Co-implanted SiO2 films on silicon

Intense room-temperature near infrared (NIR) photoluminescence (980 nm and 1032 nm) is observed from Yb,Al co-implanted SiO2 films on silicon. The optical transitions occur between the F-2(5/2) and F-2(7/2) levels of Yb3+ in SiO2. The additional Al-implantation into SiO2 films can effectively improve the concentration quenching effect of Yb3+ in SiO2. Photoluminescence exitation sprectroscopy shows that the NIR photoluminescence is due to the non-radiative energy transfer from Al-implantation-induced non-bridging oxygen hole defects in SiO2 to Yb3+ in the Yb-related luminescent complexes. It is believed that the defect-mediated luminscence of rare-earth ions in SiO2 is very effective.

Structure and visible luminescence of ZnO nanoparticles

ZnO nanoparticles were synthesized in ethanolic solution using a sol-gel method. The structural and optical properties were investigated by X-ray diffraction, transmission electron microscopy, UV absorption, and photoluminescence. After annealing at 200 degrees C, the particle size is increased and the peak of defect luminescence in the visible region is changed. A yellow emission was observed in the as-prepared sample and a green emission in the annealed sample. The change of the visible emission is related to oxygen defects. Annealing in the absence of oxygen would increase oxygen vacancies. (c) 2006 Elsevier Ltd. All rights reserved.

Defect influence on luminescence efficiency of GaN-based LEDs

Wafers with normal light-emitting diode structure were grown by metal organic chemical vapor deposition system. The pressure and temperature were varied during growth of buffer layer in order to grow different types of epilayers. The cathodoluminescence results show that the interface distortion of quantum well plays an important role in radiant efficiency. The electroluminescence detections indicate that the dislocations also influence the external quantum efficiency by lowering the electron injection efficiency. (c) 2006 Elsevier Ltd. All rights reserved.

Role of edge dislocations in enhancing the yellow luminescence of n-type GaN

We investigate the origin of yellow luminescence in n-type GaN. It is found that the relative intensity of yellow luminescence increases as the full width at half maximum of the x-ray diffraction rocking curve at the (102) plane increases. This indicates that the yellow luminescence is related to the edge dislocation density. In addition, the relative intensity of yellow luminescence is confirmed to increase with increasing Si doping for the high quality GaN we have obtained. We propose that the yellow luminescence is effectively enhanced by the transition from donor impurities such as Si to acceptors around the edge dislocations in n-type GaN. (c) 2006 American Institute of Physics.

Magnetism and luminescence evolution due to nitrogen doping in manganese-gallium oxide nanowires

We report a new method for large-scale production of GaMnN nanowires, by annealing manganese-gallium oxide nanowires in flowing ammonia at high temperature. Microstructure analysis indicates that the GaMnN nanowires have wurtzite GaN structure without Mn precipitates or Mn-related second phases. Magnetism evolution due to nitrogen doping in manganese-gallium oxide nanowires was evaluated by magnetic measurements. Magnetic measurement reveals that the magnetization increases with the increase of nitrogen concentration. Ferromagnetic ordering exists in the GaMnN nanowires, whose Curie temperature is above room temperature. Luminescence evolution was investigated by the cathodoluminesence measurement for a single nanowire and photoluminescence measurement in a temperature range between 10 and 300 K. Experimental results indicate that optical properties can be modulated by nitrogen doping in manganese-gallium oxide nanowires. (c) 2005 Elsevier B.V. All rights reserved.

Pressure dependence of Mn2+ luminescence in differently sized ZnS : Mn nanoparticles

The pressure behavior of Mn2+ emission in the 10-, 4.5-, 3.5-, 3-, and 1-nm-sized ZnS:Mn2+ nanoparticles is investigated. The emission shifts to lower energies with increasing pressure, and the shift rate (the absolute value of the pressure coefficient) is larger in the ZnS:Mn2+ nanoparticles than in bulk. The pressure coefficient increases with the decrease in particle size with the 1-nm-sized particles as an exception. Pressure coefficient calculations based on the crystal field theory are in agreement with the experimental results. The pressure dependence of the emission intensity is also size dependent. For nanoparticles 1 and 3 nm in size, the luminescence intensity of Mn2+ decreases dramatically with increasing pressure, while, for bulk and particles with average sizes of 3.5, 4.5, and 10 nm, the luminescence intensity of Mn2+ is virtually unchanged at different pressures. The bandwidth increases faster with increasing pressure for smaller particles. This is perhaps due to the fact that there are more Mn2+ ions at the near-surface sites and because the phonon frequency is greater for smaller particles. These new phenomena provide some insight into the luminescence behavior of Mn2+ in ZnS:Mn2+ nanoparticles.

The effect of inserting strain-compensated GaNAs layers on the luminescence properties of GaInNAs/GaAs quantum well

Photoluminescence (PL) properties of GaInNAs/GaAs quantum wells (QWs) with strain-compensated GaNAs layers grown by molecular beam epitaxy are investigated. The temperature-dependent PL spectra of GaInNAs/GaAs QW with and without GaNAs layers are compared and carefully studied. It is shown that the introduction of GaNAs layers between well and barrier can effectively extend the emission wavelength, mainly due to the reduction of the barrier potential. The PL peak position up to 1.41 mum is observed at the room temperature. After adding the GaNAs layers into QW structures, there is no essential deterioration of luminescence efficiency. N-induced localization states are also not remarkably influenced. It implies that with optimized growth condition, high-quality GaInNAs/GaAs QWs with strain-compensated GaNAs layers can be achieved. (C) 2003 Elsevier Science B.V. All rights reserved.

Luminescence study of (11(2)over-bar0) GaN film grown by metalorganic chemical-vapor deposition

Investigations on photoluminescence properties of (11 (2) over bar0) GaN grown on (1 (1) over bar 02) Al2O3 substrate by metalorganic chemical-vapor deposition are reported. Several emission lines not reported before are observed at low temperature. The sharp peak at 3.359 eV is attributed to the exciton bound to the neutral acceptor. Another peak at 3.310 eV represents a free-to-bound, probably a free electron-to-acceptor, transition. The 3.241 and 3.170 eV lines are interpreted as phonon replica lines of the 3.310 eV line. The phonon energy is 70 meV, consistent with the energy of transverse optical E-1 phonon. The optical properties of the lines are analyzed. (C) 2003 American Institute of Physics.

Comparative study on the broadening of exciton luminescence linewidth due to phonon in zinc-blende and wurtzite GaN epilayers

Linewidth broadening of exciton luminescence in wurtzite and zinc-blende GaN epilayers was investigated as a function of temperature with photoluminescence. A widely accepted theoretical model was used to fit the experimental data, so that the coupling parameters between exciton and acoustic and longitudinal optical phonons were obtained for both structures. It was found that the coupling constants of both exciton-acoustic optical phonon coupling and exciton-longitudinal optical phonon coupling for zinc-blende GaN are almost twice as much as the corresponding values of wurtzite GaN. These results show that the relatively strong exciton-phonon scattering seems to be characteristic to zinc-blende GaN film. (C) 2002 American Institute of Physics.

Photovoltage and luminescence study of stacked InAs/GaAs self-organized quantum dots

The ground and excited state excitonic transitions of stacked InAs self-organized quantum dots (QDs) in a laser diode structure are studied. The interband absorption transitions of QDs are investigated by non-destructive PV spectra, indicating that the strongest absorption is related to the excited states with a high density and coincides with the photon energy of lasing emission. The temperature and excitation (electric injection) intensity dependences of photoluminescence and electroluminescence indicate the influence of state filling effect on the luminescence of threefold stacked QDs. The results indicate that different coupling channels exist between electronic states in both vertical and lateral directions.

The mechanism of blueshift in excitation-intensity-dependent photo luminescence spectrum of nitride multiple quantum wells

(1 1 (2) over bar 0) GaN/InGaN multiple quantum wells (MQWs) were grown on (1 (2) over bar 0 2) sapphire by metal-organic vapor phase epitaxy. The excitation-intensity-dependent photoluminescence (PL) spectrum of these samples was measured, and no peak shift was observed. This phenomenon was attributed to the absence of piezoelectric field (PEF) along the growth orientation of the (1 1 (2) over bar 0) face MQWs. Our experimental results showed that PEF was the main reason causing peak blueshift in excitation-intensity-dependent PL spectrum of (0 0 0 1) InGaN/GaN NIQWs. It was expected that fabricating (1 1 (2) over bar 0) face nitride device should be a method to avoid PEF and get low-threshold, high-quantum-efficiency and stable-emission-wavelength light-emission devices. (C) 2002 Elsevier Science B.V. All rights reserved.