Relative Photoluminescence Intensity Research Articles

Effects of barrier growth temperature on the properties of InGaN/GaN multi-quantum wells

The effects of the growth temperature and ambient of GaN quantum barriers on the characteristics of InGaN/GaN multi-quantum wells (MQWs) grown by a thermally pre-cracked ion-supplied metalorganic chemical vapor deposition (TPIS-MOCVD) system were investigated. The improvement of optical, structural properties and surface morphology in the MQWs with increasing the growth temperature of quantum barriers was found. Without a GaN capping layer, there were many pits and the thickness of quantum pair reduced by the thermal etching during the temperature-ramping process. Photoluminescence (PL) peaks showed a blue-shift and double peaks, but relative PL intensity abruptly increased due to the suppression of deep level related defects and smooth surface morphology caused by the increased surface mobility of adatom in the high temperature region. By using a GaN capping layer on the InGaN well layer, the thermal decomposition of the InGaN well layer was suppressed and pits on the surface abruptly reduced. A hydrogen carrier gas for the GaN barrier growth also improved the optical and structural properties of MQWs.

Effect of “Ph–NH–CH 2–” group on the luminescence properties of sol–gel silica xerogel

The effect of Ph–NH–CH 2– group (Ph: phenyl) on the luminescence properties of silica xerogel by sol–gel method has been investigated. The absorption spectra, excitation spectra and emission spectra of Ph–NH–CH 2– group embedded in the network of silica xerogel have a red shift with increasing the ratios of Ph–NH–CH 2– group in the samples. The relative photoluminescence (PL) intensity of the hybrid silica xerogel sample decreases with increasing the ratios of Ph–NH–CH 2– group. From the complex luminescent centers, the novel luminescence properties have been observed from the hybrid silica xerogel sample containing Ph–NH–CH 2– group.

Thermal redistribution of photocarriers between bimodal quantum dots

We study the photoluminescence (PL) properties of InAs/GaAs self-assembled quantum dots (QDs) by varying excitation power and temperature. Excitation power-dependent PL shows that there exists bimodal size distribution in the QD ensemble. Thermal carrier redistribution between the two branches of dots is observed and investigated in terms of the temperature dependence of their relative PL intensity. Based on a model in which carrier transfer between dots is facilitated by the wetting layer, the experimental results are well explained.

Enhanced diffusion as a mechanism for ion-induced damage propagation in GaN

Although GaN is a chemically inert, thermally stable material, it has demonstrated sensitivity to ion damage generated by dry etch processes such as reacting ion etching and inductively coupled plasma etching. Recombination-enhanced diffusion is an important mechanism which has been observed in other III–V semiconductor systems. In this study we examine the possibility of enhanced diffusion in GaN using quantum well (QW) probe structures. The deeper QWs (750 and 1000 Å deep) showed a steady decrease in relative photoluminescence (PL) intensity with time, providing evidence of the cooperative effects of channeling and defect diffusion in deep etch damage propagation in GaN. In contrast, shallow QWs (150 and 250 Å from the surface) showed a slight decrease followed by a gradual increase in relative PL intensity with time which was explained by defect annihilation. Exposure to above band gap illumination, used to simulate and enhance carrier generation during etch, appears to speed defect annihilation in high defect concentration regions resulting in an increase in QW luminescence, where as in lower defect concentration areas, above band gap illumination does not appear to significantly alter QW luminescence. We attribute this difference in behavior to a difference in diffusion constant. The diffusion constant in less damaged regions may be much lower than that of the highly damaged material.

Synthesis and characterization of Y2O3:Eu3+ thin films on siliconsubstrate by pulsed laser ablation

Y2O3:Eu3+ thin films were prepared by pulsed laser ablation on silicon (100)substrate. The relative photoluminescence intensity dependence on annealing time was studied.The microstructures of annealing thin films were studied by atomic force microscopy. It wasobserved that the photoluminescence intensity at 611 nm from thin film annealed at1000 °C for 4 h tended to saturate, the average crystal size of the thin film was estimated to be71 nm by x-ray diffraction measurements.

Quantitative topographic assessment of Cu incorporation in GaAs

The modification of the bulk resistivity of semi-insulating GaAs caused by incorporation of copper has been studied. Wafers cut from an ingot annealed, intentionally Cu contaminated LEC grown crystal were investigated optically and electrically. A characteristic variation pattern of the Cu related photoluminescence intensity, gradually decreasing towards the center, is observed. The resistivity exhibits a quite similar radial dependence. The results convincingly show that Cu is diffusing across macroscopic distances into GaAs and is responsible for the radial resistivity variation. The topographic correlation allows to quantify the Cu acceptor content.

In situ optical assessment of semi-insulating iron doped InP grown by liquid encapsulated Czochralski process

In semi-insulating, Fe doped InP, we demonstrate that infrared absorption technique provides quantitative measurements of Fe concentrations. We show that the quantum efficiency is reduced more than 70% in Fe doped samples, in comparison to undoped samples. Relative near band edge photoluminescence (PL) intensity is antiproportional to Fe concentration. This can provide a calibration method to transform scanning PL images to quantitative Fe distributions. We demonstrate that infrared absorption mapping images Fe distribution in InP, in a similar way to EL2 imaging in GaAs. The images show that the slip-like defects decorated by Fe centers predominantly lie along [011] direction.

Surface State Behavior and Neutralization of Impurities in Sulfur-Treated, Hydrogenated, and Annealed GaAs

Surface state behavior and neutralization of impurities in sulfur-treated, hydrogenated, and annealed Si-doped bulk GaAs and nominally undoped GaAs layers have been studied. While the intensity of the photoluminescence (PL) spectra for the sulfur-treated GaAs dramatically increased in comparison to that for as-grown GaAs, the signal for the as-grown GaAs measured by photoreflectance (PR) vanished after sulfur treatment. After the GaAs was hydrogenated and annealed, the relative PL intensity ratio between the donor-bound exciton and the carbon acceptor increased, and a variation in the PR broadening parameter was induced by neutralization of the carbon acceptors. Variations of the surface states and neutralization of the carbon ions affect the intensity of the PL spectra and the intensity and the broadening parameter of the PR signal. These results indicate that the sulfur treatment removes the dangling bonds at the GaAs surface and that the thermal treatment breaks the bonds between hydrogen atoms and carbon ions in GaAs.

Formation and Luminescent Properties of Oxidized Porous Silicon Doped with Erbium by Electrochemical Procedure

AbstractThe present work is concerned with Er-doped oxidized porous silicon (PS). The characteristic feature of the work is that PS doping has been realized by an electrochemical procedure followed by a high temperature treatment. 5-μm thick PS layers were formed on p-type Si of 0.3-Ohm-cm resistivity. Er incorporation was performed by a cathodic polarization of PS in a 0.1 M Er(NO3)3 aqueous solution. A high temperature treatment in an oxidizing ambient at 500-1000°C was utilized to provide either partial or total oxidation of PS:Er layers. X-ray microanalysis was used to study chemical composition of the samples. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were investigated. After the partial oxidation (in the temperature range of 600-800°C), weak Er3+-related PL at 1.53 ptm was observed. A high temperature anneal in Ar atmosphere at the temperature of 1100°C caused a significant increase in the Er3+-related PL intensity. Resonant features were observed in PLE spectra of fully oxidized PS. Five peaks at 381, 492, 523, 654 and 980 nm were revealed. The strongest excitation occurred at 381 and 523 nm. The excitation of different Er3+ energy levels, cross-relaxation interactions and emission due to the 4I13/2→4I15/2 transitions were considered. Application of the Er-doped oxidized PS for integrated optical waveguides is presented.

A study of the relationship between the electroluminescence characteristics and compositions of PPV-PVA-based polymers

A series of poly( p-phenylene vinylene) (PPV)-poly(vinyl alcohol) (PVA)-based polymers were prepared from the PPV precursor and PVA aqueous solution mixtures. The luminescence characteristics of the PPV-PVA-based polymers were dependent on their composition and the following treating conditions. The emitted light of the PPV-PVA-based light-emitting diodes (LEDs) shifted from yellow-green (550 nm) to blue (485 nm) as the PVA component increased. The infra-red (i.r.) absorption spectra showed that some part of the PPV precursor could react with PVA to form COC linkages and interrupted the conjugation of PPV polymer chains. This resulted in decrease of the conjugated chain length, as evidenced in ultraviolet-visible u.v.-vis. absorption spectra. The relative photoluminescence (PL) intensity increases with decreasing the conjugated chain length. The relative electroluminescence (EL) decreases with the increase of the PVA component in the polymers. However, both PL and EL intensity normalized with PPV molar ratio of PPV-PVA-based polymers were higher than that of pure PPV. The relative low threshold voltage at 4–6 V was found in these PPV-PVA-based polymer LEDs with Al (negative) and indium-tin oxide ITO (positive) as the two electrodes.

Study of Interfacial Degradation and Morphological Change of the Vapor-Deposited Bilayer of Alq3/TPD for Organic Electroluminescent Devices by AFM and PL Technique

Abstract The morphological change of vapor-deposited films of TPD (N, N′-diphenyl-N, N′-bis-(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine) and Alq3 (Tris-(8-hydroxyquinoline) aluminum) and the interfacial degradation of Alq3/TPD bilayer were characterized by atomic force microscopy (AFM) and fluorescence spectroscopy, respectively, in the temperature range from room temperature to 80°C under an ambient atmosphere. The TPD and the Alq3 film are widely used as a hole transport and an emitter layer, respectively, in an organic electroluminescent device (an indium tin oxide (ITO) coated glass/TPD/Alq3/metal). As the substrate plate, an ITO coated glass or a slide glass was used in this work. The relative photoluminescence (PL) intensities of Alq3 to that of TPD in an Alq3/TPD and a TPD/Alq3 bilayer were increased with time. The changes were accelerated with increasing temperature and ascribed to quenching of photo-excited TPD by Alq3 diffusing into the TPD layer.

Effects of Sulfur Treatment and Hydrogen Plasma Treatment on GaAs

Photoluminescence (PL) and photoreflectance (PR) measurements have been carried out to investigate the surface state behavior for sulfur-treated GaAs as well as the neutralization effect on the carbon acceptors for hydrogen- or nitrogen-plasma-treated GaAs. Whereas the intensity of the PL signal for the sulfur-treated GaAs increased monotonically as the sulfur-treatment time increased, the intensity of the PR signal increased until the sulfur treatment time of 110 min, and thereafter decreased. These results indicate that a decrease in the number of the surface states and the formation of a quasi-flat band as a result of the sulfur treatment. After the GaAs was hydrogen-plasma-treated and annealed at 400° C, the relative PL intensity ratio between the donor-bound exciton and the carbon acceptor increased by a factor of 2, and a variation in the PR broadening parameter was induced by neutralization of the carbon acceptors due to the combination of the hydrogenic ions and the carbon ions in the GaAs. Variation in the surface states and neutralization of the ions affect the intensity of the PL spectra and the intensity and the broadening parameter of the PR signal.

Study of Interfacial Degradation of the Vapor-Deposited Bilayer of Alq3/TPD for Organic Electroluminescent (EL) Devices by Photoluminescence

Abstract The interfacial change of a vapor-deposited TPD (N,N′-diphenyl-N,N′-bis-(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine) and Alq3 (Tris-(8-hydroxyquinoline) aluminum) bilayer film was characterized by photoluminescence (PL) in the temperature range from room temperature to 80 °C under an ambient atmosphere. The TPD and the Alq3 film are widely used as a hole transport and an emitter layer, respectively, in organic electroluminescent (EL) devices (ITO/TPD/Alq3/metal). As the substrate plate, a slide glass was used in this work. In an Alq3/TPD bilayered structure, two PL bands from Alq3 and TPD were observed and the relative PL intensity of Alq3 to that of TPD was increased with time. The change was accelerated with increasing temperature. In case of a co-deposited single layer of TPD and Alq3, an Alq3 PL band predominated and, in addition, the PL intensity was not changed remarkably with increasing temperature. These results support that the PL change observed for the Alq3/TPD bilayered structure is ascribed to quenching of photo-excited TPD by Alq3 as a result of diffusion of the components into each other layer.

High-quality InGaAs/InP multiquantum-wellstructures on Si fabricated by direct bonding

High-quality InGaAs/InP multiquantum-well (MQW) structures have been successfully fabricated on Si substrates by direct bonding. These structures were first grown on InP substrates, then bonded at 700 degrees C on to Si substrates with buffer layers. The etch-pit densities of the InP surfaces are significantly low, -10 cm/sup -2/, the lowest values ever reported. Furthermore, strong relative photoluminescence intensity from the MQW structures, over 70% of that before bonding, is obtained.

Gamma -X mixing effects on pseudodirect exciton transitions in GaAs/AlAs type-II superlattices.

We report on an experimental study of \ensuremath{\Gamma}-X mixing effects on the optical transition of the no-phonon-assisted (pseudodirect) exciton consisting of the n=1 AlAs-${\mathit{X}}_{\mathit{z}}$ electron and the n=1 GaAs-\ensuremath{\Gamma} heavy hole in [001]-(GaAs${)}_{\mathit{m}}$/(AlAs${)}_{\mathit{m}}$ type-II superlattices (m=8--13 monolayers) by using cw- and time-resolved-photoluminescence spectroscopies and photoluminescence-excitation spectroscopy. We have estimated the relative oscillator strength of the pseudodirect transition to the direct transition between the n=1 \ensuremath{\Gamma} electron and \ensuremath{\Gamma} heavy-hole states from the relative photoluminescence-excitation intensity, the relative photoluminescence intensity, and the relative photoluminesence decay rate. The values of the relative oscillator strength estimated from these three kinds of experimental results, which are around 1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$, are mutually consistent, and they are almost independent of the layer thickness. We evaluate the \ensuremath{\Gamma}-X mixing factor from the relative oscillator strength, using a first-order perturbation theory, as a function of the layer thickness and show that the mixing factor is mainly determined by the overlap of the envelope functions of the \ensuremath{\Gamma} and ${\mathit{X}}_{\mathit{z}}$ electrons.

Γ-X mixing effects on photoluminescence intensity in GaAs/AlAs type-II superlattices

We have investigated Γ-X mixing effects on the no-phonon assisted (pseudodirect) transition between the n=1 X z-electron and the n=1 Γ-heavy-hole states in [001]-(GaAs) m/(AlAs) m type-II superlattices ( m=8−13 monolayers) by using photoluminescence (PL) and PL-excitation (PLE) spectroscopies. We have estimated the relative oscillator strength of the pseudodirect transition to the direct transition between the n=1 Γ-electron and Γ-heavy-hole states from the relative PLE intensity and the extrapolated relative PL intensity at infinite temperature. It is found that the relative oscillator strength is almost constant value of ∼3×10 −4 for each sample. According to a first-order perturbation theory, we evaluate a Γ-X mixing factor as a function of the layer thickness from the relative oscillator strength, and show that the mixing factor, which gradually increases from ∼0.8 to ∼1.9 meV with decreasing the layer thickness, is mainly determined by the envelope-function overlap of the Γ and X z electrons.

Cap and capless annealing of Fe-implanted InGaAs

The distribution of Fe implanted at medium (1–4×1014 cm−2) and low (2×1012 cm−2) doses into InGaAs and annealed with or without a cap is investigated and the degree of compensation of such implanted regions is assessed. Secondary ion mass spectrometry profiles of low dose implanted Fe reveal a substantial role of the capping layer. Fe concentrations below as well as above the estimated metal vacancy concentration produced by implantation are observed. The effect of the cap strongly depends on the wet chemical surface preparation before insulator deposition. A correlation of the magnitude of the Fe accumulation at the InGaAs surface with defect related photoluminescence intensity is established. On the basis of the substitutional-interstitial diffusion model the barrier effects of the various caps for host and dopant atoms are analyzed. The best semi-insulating properties were obtained for plasma enhanced chemical vapor deposition SiO2 caped samples using a H2SO4:H2O2:H2O=1:1:125 surface preparation before deposition resulting in a 53% incorporation of Fe. A high electrical activation is proved directly by capacity-voltage profiles.

Si-acceptor passivation on n-type (AlGa)As post-growth hydrogenation

We have studied AlGaAs:Si grown by molecular beam epitaxy before and after post-growth hydrogen plasma treatment by low temperature photoluminescence (PL) capacitance-voltage, and Hall effect. The PL strength increases after hydrogen plasma treatment. We observe, for the first time, that in n-type AlGaAs:Si not only the shallow donors but also the Si acceptor can be passivated by a long time (3 h) hydrogen diffusion, at 570 K. The change of the relative PL intensities associated with the Si and C acceptors demonstrates the dependence of the passivation effect on the chemical species.

Changing PL Intensity from GaAs/Al0.3Ga0.7As Heterostructures due to the Chemisorption of SO2: Effects of Heterostructure Geometry

ABSTRACTChemical adsorption of gases onto semiconductor surfaces results in a change in the density of electronic surface states. The corresponding change in the number of surface charges gives rise to a measurable change in the bulk charge distribution within the semiconductor. This phenomenon forms the basis of a novel solid-state chemical sensor device using the intensity of photoluminescence (PL) as a measurement technique. We have observed changes in the PL intensity from MOVPE-grown GaAs/Al0 3Ga0 7As heterostructures due to the chemisorption of SO2 onto “photowashed” GaAs surfaces. The photowashing technique is believed to reduce the density of preexisting surface states by removing As from the GaAs surface oxide. The PL signal-to-noise ratio and PL sensitivity characteristics of these chemical sensors may be dramatically improved by varying semiconductor structure. We have observed a range of PL enhancements upon exposure to 0.6% SO2 in flowing N2 from heterostructure layers with various thicknesses and doping densities. These results are explained by comparison with a FEM numerical model derived to correlate the relative PL intensity from various heterostructures to the surface charge density.

Implant activation and redistribution in AlxGa1−xAs

The electrical activation characteristics of implanted Be, Mg, Si, and S in AlxGa1−xAs (x=0–1) were investigated as a function of ion dose for rapid annealing in the range 600–950 °C. The apparent activation energy for electrical activity of these species increases with increasing AlAs mole fraction—for Be, the activation energy is 0.35 eV for GaAs, and 0.49 eV for Al0.54Ga0.46As. There is no evidence for pairing of Be and O in AlGaAs, in contrast to the situation for GaAs. Self-compensation is the predominant limiting mechanism for Si activation in AlGaAs as determined by the relative photoluminescence intensities of the SiGa–to–SiAs -related transitions. No significant redistribution of implanted Si is observed for any AlAs mole fraction for rapid annealing (5 s) up to 900 °C, whereas S shows motion into the AlGaAs and no tendency to outdiffuse. By contrast, both Be and Mg display loss of the dopant to the surface and little redistribution toward the bulk. Minimal damage is observed by transmission electron microscopy in as-implanted AlGaAs for Be or Si doses below the amorphization threshold. Upon annealing at the conditions for optimum activation, a high density of small dislocation loops is observed near the end of the ion range.