Luminescence mechanism and energy level structure of Eu-doped GaN powders investigated by cathodoluminescence spectroscopy

1.5at% Eu-doped GaN powders were prepared by a co-precipitation method.Powder X-ray diffraction(XRD)results shows that there is only the wurtzite phase.Cathodoluminescence spectra were measured at room temperature and liquid nitrogen temperature,respectively.The band-to-band luminescence of GaN was shifted from 373 nm to 368 nm with the temperature decreasing from room temperature to liquid nitrogen temperature.The luminescence peaks at 537,557,579,590,597,614,653 and 701 nm are attributed to the Eu ions related transitions in the host of GaN powders and the peak positions were not influenced by the variation of temperature.With the increase of accelerating voltage,the intensity of all luminescence peaks was increased.The strongest luminescence peak at 614 nm shows non-symmetrical shape and is composed of 612,615 and 621 nm through Lorentzian fitting,which indicates there are oxygen and nitrogen environments of the Eu3+ions in the Eu-doped GaN powders.     

Modified Photoluminescence by Silicon-Based One-Dimensional Photonic Crystal Microcavities

photoluminescence (PL) lrom one-dimensional photonic band structures is investigated. The doped photonic crystal with microcavitles are fabricated by using alternating hydrogenated amorphous silicon nitride (a-SiNx:H/a-SiNy:H) layers in a plasma enhanced chemical vapour deposition (PECVD) chamber. It is observed that microcavities strongly modify the PL spectra from active hydrogenated amorphous silicon nitride (a-SiNx:H) thin film. By comparison, the wide emission band width 208nm is strongly narrowed to 11 nm, and the resonant enhancement of the peak PL intensity is about two orders of magnitude with respect to the emission of the λ/2-thick layer of a-SiNx:H. A linewidth of Δλ=11 nm and a quality factor of Q=69 are achieved in our one-dimensional a-SiNz photonic crystal microcavities. Measurements of transmittance spectra of the as-grown samples show that the transmittance resonant peak of a cavity mode at 710nm is introduced into the band gap of one-dimensional photonic crystal distributed Bragg reflector (DBR), which further verifies the microcavity effects.     

Investigation of the mode splitting induced by electro-optic birefringence in a vertical-cavity surface-emitting laser by polarized electroluminescence

The mode splitting induced by electro-optic birefringence in a P-I-N InGaAs/GaAs/A1GaAs vertical-cavity surface- emitting laser （VCSEL） has been studied by polarized electroluminescence （EL） at room temperature. The polarized EL spectra with E｜｜[110] and E ｜｜ [150] directions, are extracted for different injected currents. The mode splitting of the two orthogonal polarized modes for a VCSEL device is determined, and its value increases linearly with the increasing injected current due to electro-optic birefringence; This article demonstrates that the polarized EL is a powerful tool to study the mode splitting and polarization anisotropy of a VCSEL device.     

High-strain InGaAs/GaAs quantum well grown by MOCVD

High-strain InGaAs/GaAs quantum wells （QWs） are grown by low-pressure metal-organic chemical vapor deposition （LP-MOCVD）. Photoluminescence （PL） at room temperature is applied for evaluation of the optical property. The influence of growth temperature, V/III ratio, and growth rate on PL characteristic are investigated. It is found that the growth temperature and V/III ratio have strong effects on the peak wavelength and PL intensity. The full-width at half-maximum （FWHM） of PL peak increases with higher growth rate of InGaAs layer. The FWHM of the PL peak located at 1039 nm is 20.1 meV, which grows at 600 ℃ with V/ III ratio of 42.7 and growth rate of 0.96 μm/h.     

Improving nonlinearity tolerance of 112-Gb/s PDM OFDM systems with coherent detection using BLAST algorithm

A novel long wavelength photodetector with dual-wavelength spectral response is designed and fabricated using a step-shaped Fabry-Prot (F-P) filter structure.The step-shaped GaAs/AlGaAs distributed Bragg reflectors and the InP PIN photodetector are grown on a GaAs substrate using low pressure metal organic chemical vapor deposition.High quality GaAs/InP heteroepitaxy is realized by employing a thin low temperature buffer layer.The photodetector structure is optimized by theoretical simulation.This device has a dual-peak distance of 19 nm (1 558 and 1 577 nm).The 3-dB bandwidth of 16 GHz is simultaneously obtained with peak quantum efficiencies of 8.5% and 8.6% around 1 558 and 1 577 nm,respectively.     

Electroluminescence and Photoluminescence from Scored Si—Rich SiO2 Film／p—Si Structure

Electroluminescence(EL)is observed from the Au/Si-rich SiO2 film/p-Si diodes,in which the Si-rich SiO2 films are scroed deliberately by a diamond tip.The EL intensity of the scroed diode annelaed at 800℃ is about 6 times of that of the unscored counterpart,The EL sectrum of the usscored diode could be decomposed into two Gaussian luminescence bands with peaks at about 1.83 and 2.23eV,while for the EL spectrum of the scored diode,an additional Gaussian band at about 3.0eV appears,and the 1.83-eV peak increases significantly in intensity,The photoluminescence(PL) spectrum of an unscored Si-rich SiO2 film has only one band peaking at about 1.48eV,whereas the Pl spectrum of the scored one has two bands at about 1.48 and 1.97eV.We consider that the high-density defect regions produced by the scoring provide new luminescence centres and become some types of nonradiative centres in the Si oxide layer,which thus result in changes of the EL and PL spectra.     

Temperature dependence of photoluminescence property in BaIn2O4

The temperature-dependent photoluminescence(PL) spectra of BaIn2O4,prepared by coprecipitation,are measured and discussed.Aside from the reported 3.02-eV violet emission,the 1.81-eV yellow emission involved with oxygen vacancy is also observed at room temperature wherein the deep donor level is at 1.2 eV.With the temperature increasing,the peak energies for both emissions show a red shift.Moreover,the yellow emission intensity decreases while the violet emission intensity increases.The temperature dependence of the yellow emission intensity fits very well into the one-step quenching process equation,indicating a fitted activation energy at 19.2 meV.     

Structure and photoluminescence properties of Er3+-doped TiO2-SiO2 powders prepared by sol-gel method

Er 3+-doped TiO 2-SiO 2 powders are prepared by the sol-gel method,and they are characterized by high resolution transmission electron microscopy (HR-TEM),X-ray diffraction (XRD) spectra,and Raman spectra of the samples.It is shown that the TiO 2 nanocrystals are surrounded by an SiO 2 glass matrix.The photoluminescence (PL) spectra are recorded at room temperature.A strong green luminescence and less intense red emission are observed in the samples when they are excited at 325 nm.The intensity of the emission,which is related to the defect states,is strongest at the annealing temperature of 800 C.The PL intensity of Er 3+ ions increases with increasing Ti/Si ratio due to energy transfer between nano-TiO 2 particles and Er 3+ ions.     

UV Light-Emitting Diodes at 340nm Fabricated on a Bulk GaN Substrate

We demonstrate an ultra-violet light-emitting diode （UV-LED） fabricated on a bulk GaN substrate with elec- troluminescence （EL） emission centered at about 340 nm. The UV-LED exhibits low reverse leakage current on the order of 10^-9 A under -5 V at room temperature, which can be explained by the low defect density in the epi-structure. The evolution of EL spectra as a function of injection current levels reveals the improved heat dissipation of the LEDs with vertical geometry on the bulk GaN substrate. The unusual increase of EL intensity at elevated temperatures can be explained by thermally assisted p-dopant ionization.     

Atmospheric pressure plasma jet utilizing Ar and Ar/H_2O mixtures and its applications to bacteria inactivation

An atmospheric pressure plasma jet generated with Ar with H2O vapor is characterized and applied to inactivation of Bacillus subtilis spores. The emission spectra obtained from Ar/H2O plasma shows a higher intensity of OH radicals compared to pure argon at a specified H2O concentration. The gas temperature is estimated by comparing the simulated spectra of the OH band with experimental spectra. The excitation electron temperature is determined from the Boltzmann’s plots and Stark broadening of the hydrogen Balmer Hβline is applied to measure the electron density. The gas temperature, excitation electron temperature, and electron density of the plasma jet decrease with the increase of water vapor concentration at a fixed input voltage. The bacteria inactivation rate increases with the increase of OH generation reaching a maximum reduction at 2.6%(v/v) water vapor. Our results also show that the OH radicals generated by the Ar/H2O plasma jet only makes a limited contribution to spore inactivation and the shape change of the spores before and after plasma irradiation is discussed.     

The photoluminescence of ZnSe bulk single crystals excited by femtosecond pulse

This paper reports on the photoluminescence spectra of ZnSe single crystal with trace chlorine excited by the femtosecond laser pulse. Three emission bands, including second-harmonic-generation, two-photon-excited peak and a broad band at 500--700nm, were detected. The thermal strain induced by femtosecond pulse strongly influences the photoluminescence of ZnSe crystal. The corresponding strain \va in ZnSe crystal is estimated to be about 8.8 \ti10^-3 at room temperature. The zinc-vacancy, as the main point defect induced by femtosecond pulse, is successfully used to interpret the broad emission at 500--700nm. The research shows that self-activated luminescence possesses the recombination mechanism of donor--vacancy pair, and it is also influenced by a few selenium defects and the temperature. The rapid decrease in photoluminescence intensity of two-photon-excited fluorescence and second-harmonic generation emission at lower temperature is attributed to the fact that more point defects result in the thermal activation of the two-photo-absorption energy converting to the stronger recombination emission of chlorine--zinc vacancy in 500--700nm. The experimental results indicate that the femtosecond exciting photoluminescence shows a completely different emission mechanism to that of He--Cd exciting luminescence in ZnSe single crystal. The femtosecond laser exhibits a higher sensitive to the impurity in crystal materials, which can be recommended as an efficient way to estimate the trace impurity in high quality crystals.     

Tunable Ba0.5Sr0.5TiO3 film bulk acoustic resonators using SiO2/Mo Bragg reflectors

Tunable and switchable Ba 0.5 Sr 0.5 TiO 3 film bulk acoustic resonators（FBARs） based on SiO 2 /Mo Bragg reflectors are explored,which can withstand high temperature for the deposition of Ba x Sr 1 x TiO 3（BST） films at 800 C.The dc bias-dependent resonance may be attributed to the piezoelectricity of the BST film induced by an electrostrictive effect.The series resonant frequency is strongly dc bias-dependent and shifts downwards with dc bias increasing,while the parallel resonant frequency is only weakly dc bias-dependent and slightly shifts upwards at low dc bias（ 45 V） while downwards at higher dc bias.The calculated relative tunability of shifts at series resonance frequency is around 2.3% and the electromechanical coupling coefficient is up to approximately 8.09% at 60-V dc bias,which can be comparable to AlN FBARs.This suggests that a high-quality tunable BST FBAR device can be achieved through the use of molybdenum（Mo） as the high acoustic impedance layer in a Bragg reflector,which not only provides excellent acoustic isolation from the substrate,but also improves the crystallinity of BST films withstanding higher deposition temperature.     

Effect of sample temperature on laser-induced semiconductor plasma spectroscopy

We investigate the temperature dependence of the emission spectrum of a laser-induced semiconductor(Ge and Si) plasma. The change in spectral intensity with the sample temperature indicates the change of the laser ablation mass. The reflectivity of the target surface is reduced as the sample is heated, which leads to an increase in the laser energy coupled to the surface of the sample and eventually produces a higher spectral intensity.The spectral intensities are enhanced by a few times at high temperatures compared with the cases at low temperatures. The spectral intensity of Ge is enhanced by 1.5 times at 422.66 nm, and 3 times at589.33 nm when the sample temperature increases from 50°C to 300°C. We can obtain the same emission intensity by a more powerful laser or by less pulse energy with a higher sample temperature. Based on experimental observations we conclude that the preheated sample can improve the emission intensity of laser-induced semiconductor plasma spectroscopy.     

Optoelectronic characterization of ZnS/PS systems

ZnS thin films are deposited on porous silicon （PS） substrates with different porosities by pulsed laser deposition （PLD）. The photoluminescence （PL） spectra of the samples are measured at room temperature. The results show that the PL intensity of PS after deposition of ZnS increases and is associated with a blue shift. With the increase of PS porosity, a green emission at about 550 nm is observed in the PL spectra of ZnS/PS systems, which may be ascribed to the defect-center luminescence of ZnS films. Junction current- voltage （I-V） characteristics were studied. The rectifying behavior of I-V characteristics indicates the formation of ZnS/PS heterojunctions, and the forward current is seen to increase when the PS porosity is increased.     

Tunable Ba_0.5 Sr_0.5 TiO₃ film bulk acoustic resonators using SiO₂ /Mo Bragg reflectors

Tunable and switchable Ba 0.5 Sr 0.5 TiO 3 film bulk acoustic resonators(FBARs) based on SiO 2 /Mo Bragg reflectors are explored,which can withstand high temperature for the deposition of Ba x Sr 1 x TiO 3(BST) films at 800 C.The dc bias-dependent resonance may be attributed to the piezoelectricity of the BST film induced by an electrostrictive effect.The series resonant frequency is strongly dc bias-dependent and shifts downwards with dc bias increasing,while the parallel resonant frequency is only weakly dc bias-dependent and slightly shifts upwards at low dc bias(< 45 V) while downwards at higher dc bias.The calculated relative tunability of shifts at series resonance frequency is around 2.3% and the electromechanical coupling coefficient is up to approximately 8.09% at 60-V dc bias,which can be comparable to AlN FBARs.This suggests that a high-quality tunable BST FBAR device can be achieved through the use of molybdenum(Mo) as the high acoustic impedance layer in a Bragg reflector,which not only provides excellent acoustic isolation from the substrate,but also improves the crystallinity of BST films withstanding higher deposition temperature.     

Three—Colour SIngle—Mode Electroluminescence from Alq3 Tuned by microcavities

Organic metal microcavities were fabricated by using full-reflectivity aluminium film and semi-transparent silver film as cavity mirrors.Unlike conventional organic microcavities,such as the typical structure of glass/DBR/ITO/-organic layers /metal mirror,a microcavity with a shorter cavity length was obtained by using two metal mirrors,where DBR is the distributed Bragg reflector consisting of alternate quarter-wave layers of high and low refractive index materials.It is realized that red,green and blue single-mode electroluminescence(EL) from the microcavities with the structure,glass/Ag/TPD/Alq3/Al,are electrically-driven when the thickness of the Alq3 layer changes,Compared to a non-cavity reference sample whose EL spectrum peak is located at 520nm with a full width at half maximum(FWHM)of 93nm,the microcavity devices show apparent cavity effects.The EL spectra of red,green and bule microcavities are peaked at 604nm,540nm and 491nm,with FWHM of 43nm,38nm and 47nm respectively.     

Photoluminescence and Optically Pumped Ultraviolet Lasing from Nanocrystalline ZnO Thin Films Prepared by Thermal Oxidation of High—Quality ZnS Thin Films

We present a simple and useful method for preparing high-quality nanocrystalline ZnO thin films,i.e.the thermal oxidation of high-quality ZnS films prepared by the low-pressure metal-organic chemical vapour deposition technique.The x-ray diffraction measurements reveal that the nanocrystalline ZnO has a hexagonal wurtzite structure.Raman spectra show that the longitudinal optical phonon with the E1-mode appears at 578 cm^-1.The multiple phonon scattering process is also observed,indicating the formation of a high-quality nanocrystalline ZnO thin film.The photoluminescence spectrum has a single emission peak at 3.264eV from the free-exciton mission,under the condition of low excitation power at room temperature.However,when excitation intensities exceed the threshold of 150kW/cm^2,a new and narrow peak emerges at lower energies,which are attributed to exciton-exciton collisions,and is called the P line.The intensity of this peak increases superlinearly with the pumping power over a threshold value.This supplies strong evidence of stimulated emission.The multiple longitudinal cavity modes observed in the stimulated emission spectrum indicate the successful realization of optically pumped lasing from nanocrystalline ZnO films at room temperature.     

Effect of annealing on structural, optical and electrical properties of ZnS/porous silicon composites

ZnS films were prepared by pulsed laser deposition (PLD) on porous silicon (PS) substrates. This paper investigates the effect of annealing temperature on the structural, morphological, optical and electrical properties of ZnS/PS composites by x-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) and I--V characteristics. It is found that the ZnS films deposited on PS substrates were grown in preferred orientation along β-ZnS (111) direction, and the intensity of diffraction peak increases with increasing annealing temperature, which is attributed to the grain growth and the enhancement of crystallinity of ZnS films. The smooth and uniform surface of the as-prepared ZnS/PS composite becomes rougher through annealing treatment, which is related to grain growth at the higher annealing temperature. With the increase of annealing temperature, the intensity of self-activated luminescence of ZnS increases, while the luminescence intensity of PS decreases, and a new green emission located around 550~nm appeared in the PL spectra of ZnS/PS composites which is ascribed to the defect-center luminescence of ZnS. The I--V characteristics of ZnS/PS heterojunctions exhibited rectifying behavior, and the forward current increases with increasing annealing temperature.     

Structural Characterization and Photoluminescent Properties of Zn1—xMgxO Films on Silicon

Zn1-xMgxO films have been grown on silicon at various substrate temperatures by pulsed laser deposition.The structural and photoluminescent properties of films as a function of substrate temperature have been studied.The optimized substrate temperature is 650℃.The x-ray diffraction spectra indicate that the films are highly C-axis oriented,and no phase separation is observed.The crystal grain size of the films is about 100nm as examined by atomic force microscopy.The cross-sectional transmission electron microscopy verified the C-axis orientation of the Zn1-xMgxO.Thesr films showed ultraviolet photoluminescence at room temperature.The near-band-edge emission peak of the Zn1-xMgxO film deposited at 600℃ has a blueshift (0.40eV) larger than that of the film deposited at 500℃ (0.33eV).The ratio of the near-band-edge to defect level peak intensity is as large as 159.     

Preparation and Properties of GaN Films on GaAs Substrates

Polycrystalline gallium nitride films with hexagonal structure were prepared by a post-nitridation technique. A strong blue photoluminescence located at 458 nm and a UV photoluminescence located at 368 nm were observed at room temperature. The 368 nm peak is PL from band-edge emission. The blue luminescence is attributed to the transition from deep donor level to the valence band.