Photoluminescence Wavelength Research Articles

Effect of temperature on InGaAsP alloy composition

The influence of growth temperature on the composition of InGaAsP films grown by low pressure metalorganic vapor phase epitaxy (MOVPE) is reported for quaternary (Q) alloys having bandgap wavelengths of λg = 1.1, 1.3, and 1.5 μn. Films with these different Q-compositions were deposited lattice matched to InP at a growth temperature of 675°C. Subsequent growth experiments were then performed for each Q-composition in which the input gas flow rates were kept the same and only the temperature changed in 25°C decrements down to 600°C. Photoluminescence (PL) and lattice mismatch (LMM) measurements of the resulting films were used to determine the effect of growth temperature on film composition. The PL data indicate a temperature shift in the PL wavelength of −1.8 nm/ °C for the 1.5Q composition, −2.9 nm/°C for 1.3Q, and −4.3 nm/°C for 1.1Q. Negative shifts were also observed in LMM of −80 ppm/°C for 1.5Q, −150 ppm/°C for 1.3Q, and −250 ppm/°C for 1.1Q. The Ga/In and P/As ratios of the Q-filmswere measured by secondary ion mass spectroscopy and correlated with full-wafer maps of the PL wavelength and lattice mismatch to gain insight into the processes responsible for wafer nonuniformity in MOVPE.

Third-order nonlinear optical susceptibility and photoluminescence in porous silicon.

Third-order nonlinear-optical-susceptibility spectra \ensuremath{\Vert}${\mathrm{\ensuremath{\chi}}}^{(3)}$(-3\ensuremath{\omega};\ensuremath{\omega},\ensuremath{\omega},\ensuremath{\omega})\ensuremath{\Vert} have been obtained for free-standing porous silicon films by third-harmonic-generation (THG) measurements in a third-harmonic photon energy region from 540 to 720 nm, which covers the visible photoluminescence (PL) wavelength region. In the THG wavelength region between 540 and 720 nm, the \ensuremath{\Vert}${\mathrm{\ensuremath{\chi}}}^{(3)}$\ensuremath{\Vert} spectrum is not sensitive to the excitation laser wavelength and shows about 0.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}12}$ esu. This suggests that there is no excitonic resonance enhancement of the \ensuremath{\Vert}${\mathrm{\ensuremath{\chi}}}^{(3)}$\ensuremath{\Vert} spectrum in the PL wavelength region. The optical absorption and luminescence properties of porous silicon are discussed.

Strain effects in InGaSb/AlGaSb quantum wells grown by molecular beam epitaxy

The dependence of photoluminescence spectra and surface morphology on misfit strain are investigated for highly strained InxGa1−xSb/Al0.35Ga0.65Sb quantum wells grown on GaSb(001) by molecular beam epitaxy. Infrared photoluminescence with a wavelength as long as 1.95 μm is observed at room temperature from a multiple-quantum well. The measured photoluminescence wavelength is in good agreement with that calculated by the theory in which strain effects are taken into account for a single-strained quantum well condition. The photoluminescence intensity decreases remarkably when width of strained quantum well exceeds the critical layer thickness (CLT) that depends on the misfit, i.e., indium content of the well layer. The mechanical-equilibrium theory of the generation of misfit dislocations predicts the CLT observed successfully.

Concentration quenching of photoluminescent Ru(bpy) dispersed in a polysiloxane film containing 2,2′‐bipyridine pendant groups in dependence of molecular distribution

AbstractThe photoluminescent Ru(bpy) complex was dispersed in a polysiloxane film containing 2,2′‐bipyridine (bpy) pendant groups. The unusually long photoluminescence lifetime of the Ru(bpy) (1,94 μs at 25°C) and the blue‐shifted photoluminescent wavelength suggest a rigid polymer matrix. The fluorescence yield becomes lower with higher probe concentration, indicating concentration quenching. According to the analysis based on Stern‐Volmer plots, the quenching obeys a mechanism composed of both static and dynamic processes. A statistical intermolecular distance distribution between the probes was used to interpret the results in terms of static and dynamic quenching. It is shown that in the present system the dispersed complexes diffuse slightly during the excited state.

Correlation of surface morphology and optical properties of GaN by conventional and selective-area MOCVD

ABSTRACTWe have studied GaN films grown by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) on sapphire substrates using different buffer layer structures. Surface morphology was characterized by scanning electron microscopy (SEM). Optical properties were measured using photoluminescence (PL), cathodoluminescence (CL) spectroscopy and catho-luminescence wavelength imaging (CLWI) method. It is found that the hexagonal pit-like defects in morphology are associated with the D-A/e-A transition band in the PL and CL spectra. The same correlation of morphology with optical properties is observed for the GaN films grown by selective area epitaxy (SAE). In addition, the possibility of improving optical quality by SAE is investigated. The SAE depth profile is simulated for the first time, and satisfactory results are obtained.

A new lattice relaxation mode in InGaAs on GaAs

A systematic X-ray and photoluminescence (PL) investigation of lattice relaxation in InxGa1 − xAs (0 < x < 0.7) thin layers grown on GaAs substrates revealed a new relaxation mode. This relaxation causes a blue shift of the PL wavelength away from the value for coherent growth. The relaxation also reduces the vertical lattice constant more than predicted by previous misfit dislocation theories. This new mode appears in a middle strain region between the relaxed region with misfit dislocations and the relaxed region with three-dimensional (3D) growth. The critical layer thickness of the InxGa1 − xAs layers drops quickly due to this new mode when the strain is more than 1.8%. When the strain exceeds twice the critical strain of this new mode, the 3D growth region appears. The X-ray results suggest that the InxGa1 − xAs layers interact with GaAs in this new region and that the interaction is a precursor phenomenon of the 3D growth.

Synthesis of Thin Films of Size Selected Silicon Nanocrystallites Using Pulsed Laser Ablation Supersonic

ABSTRACTThin films composed of size selected silicon nanocrystallites have been fabricated and their photoluminescence behavior has been studied. The silicon nanocrystallites were produced using a pulsed laser ablation supersonic expansion source. This source typically generates silicon clusters of many different sizes, which leads to photoluminescence spectra with a large fiill width at half maximum. To best utilize our gas phase synthesis technique, we have performed size selection of the silicon clusters using a mechanical velocity selection apparatus. Films of these size selected nanocrystallites exhibited photoluminescence spectra which had a smaller full width at half maximum and which were peaked to the blue of the spectra of samples with a broader size distribution. These results suggest that the photoluminescence wavelength of these nanocrystallite thin films is determined by quantum confinement of the electrons and holes in the crystallites.

Low threshold λ = 1.3 μm multi-quantum well laser diodes grown by metalorganic vapor phase epitaxy using tertiarybutylarsine and tertiarybutylphosphine precursors

Low threshold λ = 1.3 μm InGaAsP multi-quantum well (MQW) lasers have been realized by metalorganic vapor phase epitaxy (MOVPE) using tertiarybutylarsine (TBA) and tertiarybutylphosphine (TBP). Quaternary InGaAsP growth with TBA and TBP have demonstrated an improved group-V compositional controllability, compared to that with the conventional hydrides AsH 3 and PH 3. As a result, the base InGaAsP MQW wafers have exhibited an excellent photoluminescence (PL) wavelength uniformity with the standard deviation of 2.6 nm over a 2-inch wafer, and the PL full width at half maximum (FWHM) of 8.3 meV at 4.2 K. Laser characteristics, such as threshold and efficiency, were comparable to those grown by hydrides. The threshold currents for 70%/95% coated 170 μm long devices were as low as 9 and 22 mA at 20 and 85°C, respectively. Thus, TBA and TBP are applicable for long wavelength lasers as substitutes for AsH 3 and PH 3.

Design and operating characteristics of a metalorganic vapor phase epitaxy production scale, vertical, high speed, rotating disk reactor

The optoelectronic and digital compound semiconductor electronics industry is a critical area of development for the 90's and into the next century. Optoelectronic devices are used in all aspects of communications (lasers and detectors) and displays (LEDs). High speed electronic devices find applications in wireless communications, microwave systems, and advanced computers. As production levels of these devices increase, so too does the need for economical and high yield production equipment. Production features must emphasize repeatability, improved safety, environmental compatability, waste reduction, reliability (maximum MTBF), serviceability (minimum MTTR), flexible manufacturing, integrated manufacturing, and economics. In this paper, we report on the development and characteristics of such a large area MOVPE production Rotating Disk Reactor system (300 mm diameter deposition platform). This new reactor has been used to produce multiple 4 inch GaAs/AlAs Bragg reflectors with <1.0% variation in peak reflectivity wavelength and to simultaneously demonstrate multiple 2 inch InGaP films with better than ±1.5 nm photoluminescence wavelength uniformity.

Optical study of II-VI semiconductor nanostructures

In this paper we review some recent optical studies on CdTe-based nanostructures fabricated either by etching two-dimensional structures or by direct growth by molecular beam epitaxy. Photoluminescence can be observed on as-etched wires for sizes as small as 40 nm, due to the combined effect of exciton trapping on radiative centers and low surface damage. It is found that electrodynamic effects are very strong in gratings of wires for grating periods of about the photoluminescence wavelength.

Bright visible photoluminescence of spark-processed Ge, GaAs, and Si

High-frequency spark discharges were applied to single-crystalline wafers of Ge, GaAs, and Si. The spark-processed (sp-) samples were characterized by photoluminescence (PL) and Raman measurements. Strong and stable luminescence with wavelengths centered at 416 and 525 nm was observed in sp-Ge and sp-Si layers, respectively, when excited with a 325 nm laser beam. A considerable blue shift of the PL (compared to the unsparked specimen) was also detected for sp-GaAs with an average peak wavelength around 500 nm. The Raman shifts of the spark-processed materials indicate that nanocrystals were formed, having diameters of 3.5–4 nm for Si and about 6 nm for Ge. A correlation between the PL wavelengths, the nanocrystal sizes, and the different semiconductor materials has been established based on the effective-mass approximation. Making use of this model the nanocrystallite sizes have been found to range between ∼3 nm for Si and ∼5 nm for Ge. The related wavelengths for optical transitions confirm the PL results. The findings support the quantum confinement model.

Lateral band-gap control of InGaAsP multiple quantum wells by laser-assisted metalorganic molecular beam epitaxy for a multiwavelength laser array

We report a new growth technique for laterally changing the band gap of InGaAsP multiple quantum wells by laser-assisted epitaxy. Laser irradiation during InGaAsP well growth can be used to fabricate asymmetric quantum wells consisting of InGaAsP and InAsP layers. The photoluminescence (PL) wavelength of multiple asymmetric quantum wells (MAQWs) shows a continuous gradual shift as the delay in starting time of laser irradiation during the well growth is changed while keeping the supplying materials constant. Using this laser-assisted growth, four MAQWs with different PL wavelengths were grown on different areas of a substrate. This result was applied in the fabrication of a four-wave laser array operating at the wavelength between 1.3 and 1.51 μm by a single-step growth.

Photo-assisted tuning of luminescence from porous silicon

A method is proposed to control the wavelength of visible photoluminescence (PL) from porous silicon (PS). This is based on the post-anodization illumination technique which employs a filtered light with various maximum cutoff photon energies. As the illumination time increases, the PL spectra shift toward the higher energy side. This blue shift tends to stop at the time when the PL peak energy becomes close to the cutoff photon energy. The emission wavelength can be controllably tuned in a wide spectral range by changing the cutoff filter. These results give us a strong indication that the visible PL of PS is closely related to interband excitation in Si nanocrystallites, not to some surface chemical compounds.

Comparison of room temperature photoluminescence decays in anodically oxidized crystalline and X-ray-amorphous porous silicon

The photoluminescence (PL) decay after pulsed YAG:Nd excitation at 353 nm of anodized p-type porous silicon layers (either PECVD-deposited X-ray-amorphous or bulk crystalline) has been studied as a function of PL wavelength in the ns to ms time range. The decays are similar for both materials, being strongly non-exponential with a slow (typically 10–100 μs) and a fast (<50 ns) component, the latter being stronger in X-ray-amorphous films. The only major difference occurs at large wavelengths (above 750 nm), where the slow component of the decay in the amorphous layer becomes quite different from the “universal” [1] line shape observed in its crystalline counterpart, which is shown here to be that of a stretched exponential. These observations are borne out by delayed PL spectra obtained with a boxcar integrator in the same time range. We discuss various decay analysis procedures leading to an effective recombination rate distribution and compare the results derived for both types of material.

Depth profiles of strain in In0.10Ga0.90As/GaAs multiquantum well structures obtained by variable-pump wavelength photoluminescence

We have performed photoluminescence measurements probing the strain in a set of In0.10Ga0.90As/GaAs multiquantum well (MQW) samples. By using different continuous wave pump laser wavelengths we obtain a depth profile of strain in individual quantum wells within the MQW structure. We also present evidence for the existence of an equilibrium strain between In0.10Ga0.90As quantum well layers and GaAs barrier layers for thick strain-relaxed MQW structures.

Visible photoluminescence of porous Si1−xGex obtained by stain etching

We have investigated visible photoluminescence (PL) from thin porous Si1−xGex alloy layers prepared by stain etching of molecular-beam-epitaxy-grown material. Seven samples with nominal Ge fraction x varying from 0.04 to 0.41 were studied at room temperature and 80 K. Samples of bulk stain etched Si and Ge were also investigated. The composition of the porous material was determined using x-ray photoemission spectroscopy and Rutherford backscattering techniques to be considerably more Ge rich than the starting epitaxial layers. While the luminescence intensity drops significantly with the increasing Ge fraction, we observe no significant variation in the PL wavelength at room temperature. This is clearly in contradiction to the popular model based on quantum confinement in crystalline silicon which predicts that the PL energy should follow the band-gap variation of the starting material. However, our data are consistent with small active units containing only a few Si atoms that are responsible for the light emission. Such units are present in many compounds proposed in the literature as the cause of the visible PL in porous Si.

The effects of probe power on the spatial variation of the room temperature photoluminescence wavelength of an InGaAsP epitaxial structure

Photoluminescence mapping is widely employed as a non-destructive characterization technique for epitaxial device structures, yielding measurements that may be correlated with final device performance. In order to obtain useful correlations, however, a number of experimental factors must be considered. We discuss herein the effect of probe laser power on the results of mapping experiments and demonstrate how increased probe power can result in underestimation of both the average properties of an epitaxial layer and its inhomogeneity.

Relation between photoluminescence wavelength and lattice mismatch in metalorganic vapor-phase epitaxy InGaAs/InP

The photoluminescence (PL) wavelength of high-purity 0.5-μm-thick metalorganic vapor-phase epitaxy In1−xGaxAs/InP was measured at room temperature (RT) as a function of composition. The relaxed mismatch of these coherently strained layers was between −0.25% and +0.13%, corresponding to x=0.503 – 0.448. The composition was determined by x-ray diffraction (XRD) using a four-crystal monochromator. Extremely narrow XRD linewidths as a function of layer thickness are reported. The PL peak wavelength λp at RT of exactly lattice-matched InGaAs is 1668 nm, corresponding to a band gap Eg=731 meV. The bowing parameter of the strain-free Eg(x) relation is c=−0.494. The measured λp vs x relation agrees well with the strain-free Eg(x) corrected by the hydrostatic part of the strain energy. From line-shape analysis of RT and 77 K PL spectra it is found that the spectra of strained layers contain two lines in accordance with the valence band splitting under uniaxial strain. The conditions for a reliable determination of the compositional homogeneity of large InGaAs wafers by a mapping of λp are given; e.g., a variation Δx=8.7×10−3 over a 2 in. wafer is found.

Correlation between Optical Properties and Crystallite Size in Porous Silicon

Recently a quantum size effect was proposed to be responsible for the blue shift of optical absorption edge and photoluminescence peak wavelength as well as for the porous silicon (PS) formation itself. In the debate about the mechanism of light emission from PS a correlation between particle size and luminescence peak position would be a key test of the confinement approach. In this letter X-ray diffraction results of as-etched PS samples will be presented that indicate a decrease of particle size and an increase of stress in conjunction with a blueshift of photoluminescence wavelength and absorption edge.

A theoretical study of various effects on the wavelength of photoluminescence in Si nanostructures

The photoluminescence in Si nanostructures is studied theoretically through the effects of size, shape and condition of the cluster surface on the energy gap of the Si cluster. Si clusters with a spherical shape or columnar shape are considered. The effect of the condition of the cluster surface is investigated through the chemisorption of hydrogen atoms at the surface. It is found that the energy gap of a cluster decreases with increase in cluster size. For the cluster to simulate an Si column, the calculated energy gap can fall into the energy region of visible-light luminescence with decrease in the cluster size. In the size range (less than 20 AA) considered, the energy gap of the cluster is strongly dependent on the shape, and the surface condition might be important for the determination of the electronic structures of Si nanostructures. These results might be useful in the understanding of photoluminescence in porous Si. The electronic structures of clusters are also investigated in detail.