Photoluminescence Wavelength Research Articles

Process for Preparing Macroscopic Quantities of Brightly Photoluminescent Silicon Nanoparticles with Emission Spanning the Visible Spectrum

Silicon nanoparticles with bright visible photoluminescence have been prepared by a new combined vapor phase and solution phase process, using only inexpensive commodity chemicals. CO2 laser induced pyrolysis of silane was used to produce Si nanoparticles at high rates (20−200 mg/h). Particles with an average diameter as small as 5 nm were prepared directly by this vapor phase (aerosol) synthesis. Etching these particles with mixtures of hydrofluoric acid (HF) and nitric acid (HNO3) reduced the size and passivated the surface of these particles such that after etching they exhibited bright visible luminescence at room temperature. The wavelength of maximum photoluminescence (PL) intensity was controlled from above 800 nm to below 500 nm by controlling the etching time and conditions. Particles with blue emission (maximum PL intensity at 420 nm) were prepared by rapid thermal oxidation of orange-emitting particles. The particle synthesis methods; steady-state photoluminescence spectra; results of their characterization using TEM, XRD, FTIR absorption spectroscopy, and XPS; and preliminary assessments of the stability of the photoluminescence properties with time are presented here. Preparation of macroscopic quantities by the methods described here opens the door to chemical studies of free silicon nanoparticles that could previously be carried out only on porous silicon wafers, as well as to potential commercial applications of silicon nanoparticles.

Studies of water-borne based PPV composites

The properties of poly( p-phenylenevinylene) (PPV) composite films, with PU and PMMA, obtained from water-borne systems were investigated. UV-visible (UV-Vis) as well as photoluminescence (PL) spectroscopy and dynamic mechanical analysis (DMA) results and comparison between the characteristics of pure PPV and its composites are reported. It was concluded from UV-Vis absorption spectra that the conjugation length of PPV in these composites, and of pure PPV were alike. The PL wavelength are independent on the matrix and on the composition of the composites but the type of matrix influences the intensity of the peaks. Usually PPV forms rigid films, but composite films cast from water suspension are flexible. It is confirmed by the lower value of the storage modulus E′ and the amplitude of tan δ of the composites compared to the PPV one.

Amorphous silica nanowires grown by the vapor–solid mechanism

Silica nanowires were synthesized by using silica nanoparticles as a growth catalyst using a gas composed of CH 4 and H 2 at 1050 °C. Silica nanoparticles provide silicon and oxygen atoms for the formation of the nanowires, as well acting as a growth site. The nanowires nucleated on graphitic carbon layers formed around the seed particles, indicating that the nanowires grow by the vapor–solid mechanism. Photoluminescence spectra of the nanowires normally showed strong blue emission peaked at 3.1 and 2.8 eV under 3.8 eV laser excitation. Post-hydrogen annealing resulted in the appearance of longer wavelength photoluminescence band.

Multiwafer gas source MBE development for InGaAsP/InP laser production

We studied in details the interplay between various key elements required for multiwafer gas source molecular beam epitaxy (GSMBE) such as gas cell cracking efficiency, group-III and -V uniformity, pumping system capacity and pyrometer control of growth temperature. We demonstrate the compatibility of the accurate and reproducible control of Ga x In 1− x As y P 1− y alloys of GSMBE process with large-scale 1.55 μm telecom laser production. Excellent photoluminescence (PL) wavelength uniformity is obtained in 4×2 in configuration. Standard deviation of PL wavelength is smaller than 0.5 nm over the entire surface of the four wafers. Extension of multiwafer growth to heterostructures lasing at shorter wavelength is also discussed.

Synthesis and characterization of novel soluble alternating copoly(phenylene vinylene) derivative for light‐emitting electrochemical cell

AbstractA novel alternating copolymer, poly{[2,5‐di(2‐(2‐ethoxy ethoxy)ethoxy)‐1,4‐phenylene vinylene]‐alt‐1,4‐[phenylene vinylene]}, has been synthesized through the Wittig condensation as electroluminescent material. In this copolymer, one component is phenylene vinylene with flexible oligo(ethylene oxide) side chain that facilitates ion transportation and phase miscibility between nonpolar and polar part of composite luminescent layer, and another is a rigid phenylene vinylene moiety to improve luminescent quantum efficiency and tune color. The copolymer shows good solubility and thermal stability for device fabrication compared to poly(phpeylene vinylene)(PPV). The band gap value of copolymer is between those of corresponding homopolymers, which indicates that alternating copolymerization is a suitable way to obtain luminescent polymer with desired band gap. The maximum wavelength of photoluminescence of copolymer is 539 nm (yellowish‐green). The HOMO and LUMO energy levels obtained by cyclic voltammetry measurement indicate that the electron injection ability of copolymer has been greatly improved compared with that of the PPV. A more balanced carrier injection and higher quantum efficiency are proved by electroluminescent properties of corresponding light‐emitting devices. The turn‐on voltage of LEC device (ITO/copolymer + PEO + LiClO4/Al) is found to be 2.3 V, with current comparative to LED (ITO/copolymer/Al) at 9.5 V. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1350–1356, 2003

Effects of hydrogenation on photoluminescence of Si nanoparticles formed by pulsed laser ablation

We report a photoluminescence (PL) study of hydrogenated Si nanoparticles formed by pulsed laser ablation. Two kinds of visible luminescence centered at 480 and 750 nm were observed by varying hydrogen gas pressure. The 480 and 750 nm emissions were obtained from the sample formed in higher- and lower-pressure region, respectively. The PL wavelength did not depend on the particle size. From a comparison of luminescence spectra and surface environment of the particle, the 480 nm emission attributed to defects in native oxide layer and the 750 nm emission appeared as the result of hydrogen passivating interface states at the Si/SiO 2. The result indicates that the surface environment plays an important role for PL characteristics of Si nanoparticle.

Highly strained very high-power laser diodes with InGaAs QWs

With the aim of realizing laser diodes in the wavelength range beyond 1100 nm on GaAs, we have studied the indium incorporation behaviour into pseudomorphic InGaAs-quantum wells with extremely high indium content grown by metalorganic vapour phase epitaxy. A wide growth temperature range between 490°C and 770°C as well as the dependence on V/III-ratio and strain compensation has been studied. At the maximum In-content of 41%, a photoluminescence wavelength of 1238 nm at room temperature is obtained. Laser diodes with an emission wavelength up to 1206 nm were processed. Structures with a slightly reduced In-content, emitting at 1120 nm, were processed to broad-area devices (100 μm×1000 μm) and show output powers up to 12 W, which corresponds to a record high internal power density of 23 MW/cm 2, with a good reliability. LOC structures with a reduced far field below 30° and a higher indium content emit 8.5 W at 1170 nm.

Wavelength extension of GaInAs/GaIn(N)As quantum dot structures grown on GaAs

Self-assembled GaInAs quantum dots (QDs) embedded in GaIn(N)As were grown by atmospheric pressure metalorganic vapor phase epitaxy. The dependence of the photoluminescence (PL) properties on the material composition of the barrier layer was investigated. The emission wavelength and intensity of the QDs could be tuned by controlling the indium and nitrogen compositions in the barrier layer. By using a Ga 0.8In 0.2As barrier layer, the room-temperature PL wavelength of the QDs was extended up to 1.42 μm and the PL intensity was increased by a factor of three compared to the conventional GaInAs/GaAs QD structure. Preliminary results show that by using N-containing Ga 0.85In 0.15NAs as a barrier layer instead of Ga 0.85In 0.15As an increase in the PL wavelength and intensity in the 1.3 μm wavelength range can be obtained.

Room-temperature photoluminescence at 1.55 μm from heterostructures with InAs/InGaAsN quantum dots on GaAs substrates

Room-temperature photoluminescence (PL) at 1.55 μm from heterostructures with InAs/InGaAsN quantum dots (QDs) grown by MBE on GaAs substrates is demonstrated for the first time. The effect of nitrogen incorporated into InAs/InGaAsN QDs on the PL wavelength and intensity was studied. The integral intensity of PL from the new structure with InAs/(In)GaAsN QDs is comparable to that from a structure with InGaAsN quantum wells emitting at 1.3 μm.

Stable light emission from cadmium sulphide quantum dots in N, N′-dimethylformamide

Quantum dots (nanoparticles) of cadmium sulphide have been synthesized in N, N′-dimethylformamide (DMF). The preparation shows that particles are air stable for quite some time. Freshly prepared colorless solutions containing CdS nanoparticles show blue shifts of >100 nm in their UV–visible absorption spectra. Bright yellow light emission from the samples was observed when illuminated with 365-nm ultraviolet light. The photoluminescence (PL) measurements showed that the emission wavelengths are at about 530–545 nm; despite small shifts in absorption spectra over a period of time, the PL wavelengths are unchanged. The absorption spectra indicate the particles to be in the range 3–5 nm. It is noted that in some cases, the absorption bands are red shifted in the range of about 10–15 nm over a period 72–96 h. The CdS nanoparticles in DMF show so small a change in absorption wavelength with respect to time that they can be considered highly stable for useful applications.

Room-temperature photoluminescence in amorphous SrTiO 3 - the influence of acceptor-type dopants

Room-temperature photoluminescence (PL) was observed in undoped and 2 mol % Cr-, Al- and Y-doped amorphous SrTiO3 thin films. Doping increased the PL, and in the case of Cr significantly reduced the associated PL wavelength. The optical bandgaps, calculated by means of UV–vis absorption spectra, increased with crystallinity and decreased with the doping level. It was considered that yttrium and aluminum substituted Sr2+, whereas chromium replaced Ti4+. It is believed that luminescence centers are oxygen-deficient BO6 complexes, or the same centers with some other defects, such as oxygen or strontium vacancies, or BO6 complexes with some other defects placed in their neighborhood. The character of excitation and the competition for negatively charged non-bridging oxygen (NBO) among numerous types of BO6 defect complexes in doped SrTiO3 results in various broadband luminescence peak positions. The results herein reported are an indicative that amorphous titanates are sensitive to doping, which is important for the control of the electro-optic properties of these materials. The probable incorporation of Cr into the Ti site suggests that the existence of a double network former can lead to materials displaying a more intense photoluminescence.

Synthesis and LED device properties of carbazole and naphthalene contained conjugated polymers

Two new fully conjugated alternating copolymers containing both carbazole and naphthalene units were prepared through the Wittig condensation polymerization (carbazole units were linked with napthalene units by 1,4- and 2,6-linkages). The maximum photoluminescence wavelengths of 2,6-PNCPV and 1,4-PNCPV were found at 498 and 545 nm, which correspond to the greenish–blue and the yellow emission region, respectively, depending on the napthalene linkage. The turn-on voltages of 2,6-PNCPV and 1,4-PNCPV were 10.4 and 9.6 V, respectively, when the single-layer light-emitting diodes of Al/2,6-PNCPV or 1,4-PNCPV/ITO glass were fabricated. The electroluminescence spectrum of 1,4-PNCPV exhibited EL emissive band at 553 nm (yellow emission). Interestingly, the EL spectrum of 2,6-PNCPV was tailed to the longer wavelength region from the EL maximum point of 496 nm, which may be caused by formation of the intermolecular excimers. The emission color of 2,6-PNCPV was almost white to the naked eyes due to the tailing of 2,6-PNCPV emission and the weak interchain excimer peak at 750 nm.

Photoluminescence from photochemically etched silicon

Abstract The photoluminescence (PL) of photochemically etched silicon is studied. In the photochemical etching process, an n-type silicon wafer is immersed in an etchant solution of hydrofluoric acid (HF) and H 2 O 2 . A low-power visible laser (typically He–Ne) is used to illuminate the samples. The etching process occurs through the photogeneration of carriers. Although no electrodes are used in this etching method, the final samples show PL similar to electrochemically etched porous silicon. The samples were prepared using (1 0 0) n-type silicon with a resistivity of 1.0–5.5 Ω cm. An He–Ne laser with 20 mW of maximum power output was used and the spot radius (on the samples) was varied from 1 to 4 mm. A strong emission in the red-yellow optical region can be present in the final samples depending on the HF:H 2 O 2 concentration ratio, etching time and laser intensity of the etching process. The PL spectra excited with the monochromated output of an Xe light source as excitation is studied. The peak wavelength of the PL intensity shifts to the blue region of the spectrum when increasing the laser intensity. Quantum confinement can explain this blue shifting if smaller silicon nanocrystallites are formed with higher laser intensity. The peak PL intensity also decreases when increasing the laser intensity, although a “threshold” condition must be reached to have measurable PL. Each sample also exhibits a shifting in peak PL wavelength when varying the PL excitation wavelength. The corresponding dependence and the variations of the PL intensity are studied. Other experimental conditions are discussed.

Growth of highly strained GaInAs-GaAs quantum wells on patterned substrate and its application for multiple-wavelength vertical-cavity surface-emitting laser array

We carried out the growth of highly strained GaInAs-GaAs quantum wells (QWs) on a patterned substrate for extending emission wavelength on a GaAs substrate. We examined the shift of photoluminescence wavelength of the QWs and showed a large wavelength shift due to the spatial modulation in well thickness and indium composition. We demonstrated a single-mode multiple-wavelength vertical-cavity surface-emitting laser (VCSEL) array on a patterned GaAs substrate covering a new wavelength window of 1.1-1.2 /spl mu/m. By optimizing pattern shape, we achieved multiple-wavelength operation with widely and precisely controlled lasing wavelengths. The maximum lasing span is as large as 77 nm. We carried out a data transmission experiment through 5-km of single-mode fiber with a 2.5 Gb/s/channel. The total throughput reaches 10 Gb/s. The VCSEL-based wavelength-division-multiplexing (WDM) source would be a good candidate for WDM-LAN beyond 10 Gb/s.

Composition Control of InGaAsP Laser Structures Using In-Situ Emissivity Corrected Pyrometry During Epitaxial Growth

Wafer temperature during metal-organic chemical vapor deposition growth strongly influences the composition, and therefore, photoluminescence wavelength of InGaAsP layers when using AsH3 and PH3 precursors. Emissivity corrected pyrometry was used to measure and control the wafer temperature during epitaxial growth of 1.5 µm InGaAsP laser structures. Control of laser structure wavelength was demonstrated using an Emcore D180 LDM reactor. Average peak wavelength for five, six wafer runs had a standard deviation of 2.1 nm indicating excellent repeatability as long as wafer temperature was identical from run to run. Composition control was further demonstrated by intentionally reducing the wafer temperature during two successive growth runs. Average peak PL wavelength increased from 1522 nm to 1530 nm by reducing RealTemp wafer temperature 2°C from 607.7°C to 605.7°C during active region growth, corresponding to a 4 nm shift of wavelength for a 1°C change.

Electron Microprobe and Photoluminescence Analysis of Europium-Doped Gallium Nitride Light Emitters

ABSTRACTRare-earth doped GaN structures offer potential for optical devices emitting in the visible region [1,2]. We describe a study of MOVPE grown GaN-on-sapphire epilayers implanted with Europium ions, producing characteristic red emission lines between 540 and 680 nm due to intra-4f(n) electron transitions. As-implanted and subsequently annealed samples are investigated using a combination of wavelength dispersive x-ray analysis (WDX), electron microscopy, cathodoluminescence (CL) and photoluminescence (PL). WDX is shown to be a powerful technique for quantifying rare-earth concentrations in GaN, with varying electron beam voltages allowing a degree of depth profiling, further enhanced by the simultaneous collection of room temperature luminescence (CL) from the analysed region [3]. The intensities of the sharp lines observed in the luminescence spectrum are compared to the doping density (between 1014 – 1015 cm−2) and the Eu content measured by WDX, using a Eu-doped glass standard. Differences observed in the luminescence spectra produced by laser and electron beam excitation will be discussed along with the importance of the annealing conditions, which “heal” defects visible in the electron micrographs.

1.5 μm Range Self-Organized In0.65Ga0.35As/In0.52Al0.48As Quantum Wire Structures Grown on (775)B-Oriented InP Substrates by Molecular Beam Epitaxy

ABSTRACTHigh quality quantum wire (QWR) structures with an emitting wavelength in the 1.5-μm range were self-organized in an In0.65Ga0.35As/In0.52Al0.48As quantum well layer grown on a (775)B-oriented InP substrate by molecular beam epitaxy. Photoluminescence (PL) from the (775)B In0.65Ga0.35As/In0.52Al0.48As QWRs with a nominal well width of 4.8 nm was observed at 1.43 μm at 12 K, which corresponds to a PL wavelength of about 1.5 μm at room temperature. The PL peak was considerably polarized along the wire direction with a polarization degree of P [= (I∥ - I⊥) / (I∥ + I] ⊥)] = 0.14, indicating its good one-dimensionality. The FWHM of the PL peak was as small as 17 meV, which is the best value for InGaAs QWRs on InP substrates.

The growth of infrared antimonide-based semiconductor lasers by metal-organic chemical vapor deposition

We describe the metal-organic chemical vapor deposition (MOCVD) growth of InAsSb/InAs and GaAsSb/GaAs(P) multiple quantum well (MQW) and InAsSb/InAsP and InAsSb/InPSb strained-layer superlattice (SLS) active regions for use in mid-infrared emitters. We also describe the growth and initial characterization of GaAsSbN/GaAs MQW structures. By changing the layer thickness and composition of the InAsSb SLSs and MQWs, we have prepared structures with low temperature (<20 K) photoluminescence wavelengths ranging from 3.2 to 6.0μ m. We have made gain-guided, injection lasers using undoped, p-type AlAs0.16Sb0.84 for optical confinement and both strained InAsSb/InAs MQW and InAsSb/InAsP and InPSb SLS active regions. The lasers and LEDs utilize the semi-metal properties of a p-GaAsSb/n-InAs heterojunction as a source for electrons injected into the active regions. Cascaded, semi-metal, mid-infrared, injection lasers with pseudomorphic InAsSb multiple quantum well active region lasers and LEDs are reported. We also report on GaAsSb/GaAs(P) lasers and LEDs emitting at 1.1 to 1.2 μm grown on GaAs substrates and using AlGaAs layers for confinement.

Morphology and strongly enhanced photoluminescence of porous GaAs layers made by anodic etching

Porous GaAs layers obtained by electrochemical etching were investigated using SEM, TEM and optical methods. The morphology, chemical composition and photoluminescence of porous layers were investigated. It was shown that porous layer consists of Ga 2O 3, As 2O 3, As 2O 5 and GaAs submicron grains. Photoluminescence spectra of investigated porous layers consist of “infrared” and “green” spectral structures. The observed short wavelength photoluminescence (PL) at 590–778 nm of the porous layer is explained by the quantum size effect in the GaAs nanocrystals.

Strain dependence on barrier material and its effect on the temperature stability of photoluminescence wavelength in self-assembled GaInAs quantum wires

The strain-induced lateral-layer ordering process was used to fabricate GaInAs quantum wire (QWR) heterostructures on InP whose QWR layers are separated by different barrier materials. Using cross-sectional transmission electron microscopy the microstructure of the different QWR samples was studied. It was found that GaInAs QWR heterostructures with nominally lattice matched AlGaInAs barriers resulted in heavily strained QWR regions relative to the same structure with nominally lattice matched AlInAs or InP barriers. Furthermore, photoluminescence studies demonstrate that these heavily strained QWR samples with AlGaInAs barriers have a wavelength-shift rate of 0.2 to 1.0 Å/°C in the 293–360 K range. This is in comparison to GaInAs QWR samples with ternary or binary barriers and a GaInAs quantum well control sample that have wavelength-shift rates of ∼2.5 and ∼6.0 Å/°C, respectively. It is shown empirically that a minimum amount of strain in the GaInAs QWR/AlGaInAs barrier region is required to achieve wavelength-shift rates below 1 Å/°C.