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Notes: GaAs/In0.07Al0.93As tensile-strained quantum wells were grown on [001] GaAs substrates using molecular-beam epitaxy. The incorporation of tensile strain is made possible by preparing a 1-μm-thick In0.07Al0.93As relaxed buffer which is followed by the growth of quantum wells. The strain of the GaAs was measured using Raman spectroscopy and photoluminescence. The photoluminescence measurements from wells ranging in thickness from 25 to 100 A(ring) reveal that the observed optical transition originates from the electron–light hole recombination for a 100 A(ring) well and from the electron–heavy hole recombination if the well thickness is less than 40 A(ring). Therefore, a thick Al-rich InxAl1−xAs relaxed buffer on the GaAs substrate can be used to engineer the relative energy position of the light and heavy holes for GaAs-based quantum wells. © 1996 American Institute of Physics.

Notes: An iterative procedure which utilizes the type-I and type-II transitions of a GaAs/AlAs single quantum well is used to determine the GaAs/AlAs valence-band offset. The iteration is based on the fact that the confinement energy is not sensitive to the change of the barrier potential. It has the advantage that knowledge of the thickness of the quantum well is not necessary to determine the valence-band offset. Both the type-I and type-II transitions can be observed in the low-temperature photoluminescence spectrum if the GaAs is thin enough so that the lowest-energy level of the electron in the GaAs well is higher than the X-conduction-band minimum of the adjacent AlAs. We illustrate this procedure with a 35 A(ring) GaAs/AlAs single quantum well, and the valence-band offset is found to be 36% of the Γ-gap difference. © 1995 American Institute of Physics.

Notes: We report the fabrication, chemical, optical, and photoluminescence characterization of amorphous silicon-rich oxynitride (SiOxNy:H) thin films by plasma-enhanced chemical-vapor deposition. The film compositions were followed by changes in the refractive index. X-ray photoelectron and Fourier transform infrared spectroscopy indicate that the chemical composition is dominated by silicon suboxide bonding with N present as a significant impurity. A broad tunable photoluminescence (PL) emission is visible at room temperature with a quantum efficiency of 0.011% at peak energies to 3.15 eV. The radiative lifetimes are less than 10 ns, and there is nearly no temperature dependence of the PL intensity down to 80 K. Ex situ annealing at temperatures above 850 °C results in an increase in PL efficiency by nearly three orders of magnitude, and the PL intensity is independent of the annealing ambient. The PL results are remarkably similar to literature results in oxidized porous silicon and oxidized nanocrystalline Si thin films, and suggest that the radiative center is due to the defect structure in the silicon suboxide moiety. © 1995 American Institute of Physics.

Notes: In the optical analysis of collections of particles embedded in a matrix, the ultimate goal is to predict the optical properties of the final film, given the optical characteristics of the materials of which it is made, the particle size, and the fraction of the volume occupied by the particles. The first task in the pursuit of this goal is to make connections between the optical properties of the bulk materials of which the particles and medium are made and the spatial distribution of the scattered intensity from the individual particles. The second is to relate the calculated far-field scattering and absorption cross sections for a single particle to the measured optical characteristics of real films. Here we accomplish these tasks for a regime that is rarely considered in the literature but is quite important in applications. This regime is characterized by three conditions: (a) the extinction coefficient of the particle is significant; (b) the particle size is comparable to the wavelength in the medium; and (c) the optical density of the films is large enough that multiple scattering is significant, and yet direct transmission of collimated radiation through the film is not negligible. We have measured the visible diffuse reflectance and transmission of films of quinacridone particles in a transparent resin at various particle volume concentrations, and used the Kubelka–Munk formalism as a parametrization method to extract scattering and absorption parameters for this strongly absorbing, multiple-scattering system in the resonant regime. We have modeled the scattering parameter S* as a convolution of the angular distribution of the scattering from a single particle (derived from Mie theory) and a multiple-scattering function that characterizes the effects of the concentration and arrangement of the particles in the film. We find that the multiple-scattering function has an angular distribution that is strongly peaked in the backscattering direction. This accounts for the transformation of the collimated and strongly forward-scattered light in the top layers of the film into diffusely reflected light as the film is traversed. The multiple-scattering function also accounts for the differences in diffuse reflection observed for films of identical volume fraction occupied by particles (particle volume concentration) but different grinding time. This method of analysis begins to allow one to make an explicit connection between the fundamental optical properties of the particles and the experimentally accessible parameters. We have thus developed a way to begin to bridge the gap between the fundamental physics of the scattering of light from individual particles and the practical characterization of a film using the simple Kubelka–Munk analysis. © 2001 American Institute of Physics.

Notes: When film coatings are made of pigment particles embedded in a transparent resin, the optical characteristics of the resulting film are determined not only by the bulk optical properties of the constituent materials, but also by the spatial distribution of the light scattered from small particles. If the particles are separated by distances comparable to their diameter, as is the case for high particle concentrations or agglomerated systems, the near-field interactions between the radiation fields of the particles can strongly influence the resulting far-field intensity distribution. In this work we have used full-field finite-element solutions of Maxwell's equations to calculate the near- and far-field scattering patterns for single 500 nm quinacridone spheres and for pairs of particles. We find that the scattered intensity forms a forward-directed plume that extends far beyond the particle surface, especially at short wavelengths and where the absorption is large. This results in near-field interactions between pairs of particles that can either increase or decrease the scattering (both the total scattering and the fraction of the scattered light that is directed into the backward hemisphere), depending on the orientation of the particle pair relative to the direction of the incident light. In some cases, particularly if the particles are aligned along the incident direction, the two spheres can produce a far-field scattering distribution that is approximately that of a single, larger (sometimes much larger) sphere. If the particles are aligned perpendicular to the incident direction, the strength of the scattering per particle volume is roughly the same as for a single particle, but the scattering is more forward directed. These interaction effects occur even though the surface-to-surface separation of the particles is larger than the distance for which a single particle shows significant scattered intensity. These near-field and far-field phenomena are beyond the limitations of single-scattering and independent multiple-scattering approaches, and the near-field interactions can have a significant effect on the scattering of light from films containing such particles, especially if they tend to form oriented clusters. © 2001 American Institute of Physics.

Notes: We have studied the photoluminescence (PL) mechanism of photo- and electroluminescent amorphous silicon oxynitride films grown by plasma enhanced chemical vapor deposition. The composition of the films was determined by Rutherford backscattering spectrometry and monitored by the index of refraction with single-wavelength ellipsometry. Two sets of samples were grown, each with different reactant gas residence times in the deposition chamber. For samples grown with a residence time of about 5 s, the energy of the PL peak for 2.54 eV excitation is 2.3 eV for stoichiometric films and redshifts with increasing silicon content to 1.7 eV for the most silicon-rich films. The energy of the PL peak for 3.8 eV excitation is 2.8 eV for stoichiometric films and 2.1 eV for the most silicon-rich films. For stoichiometric films, the PL intensity is independent of temperature between 80 and 300 K using 2.54 eV excitation, but the PL intensity decreases by a factor of two over the same temperature range for 3.8 eV excitation. The authors interpret these aspects of the PL as consistent with tail-state recombination. Other results imply the PL is due to a specific luminescence center related to Si–Si or Si–H bonding. A 450 °C anneal reduces the paramagnetic defect density in the films, as detected by electron paramagnetic resonance, by an order of magnitude, but does not increase the PL intensity, while a 950 °C anneal increases both the defect density and the PL intensity. In addition, films in a second set of samples, grown with a residence time of 1.8 s, display very different PL behavior than samples in the first set with the same composition. Samples near stoichiometry in the second set have a PL peak at 2.06 eV and are 20 times less intense than stoichiometric samples in the first set. Optical absorption measurements indicate both types of samples contain Si–Si bonds, with the second set containing many more Si–Si bonds than the first. Fourier-transform infrared measurements indicate the presence of a Si–H bond that is stable at temperatures of 950 °C in the first set, but not in the second set. Thus, the study as a whole suggests a complete picture of luminescence in our silicon oxynitride films must incorporate elements of both tail-state and luminescence center models. The relation of the results to other PL studies in silicon alloys and porous silicon is discussed. © 1999 American Institute of Physics.

Notes: We report electroluminescence (EL) from 50 nm silicon oxynitride films on p-type crystalline silicon substrates in a Au/silicon oxynitride/Si structure. The EL intensity has a peak below 2.45 eV, and is consistent with radiative recombination of injected carriers. The EL is present only in annealed samples, and the emission is similar to the photoluminescence from the same samples. The current–voltage behavior is indicative of space charge-limited current. No polarity or field dependence of the EL peak energy is observed. This phenomenon is attributed to the relaxation of carriers down the band tails before recombination. © 1999 American Institute of Physics.

Notes: The temperature and excitation-intensity dependence of the ∼0.9 eV photoluminescence (PL) band has been studied in several microcrystalline silicon (μc-Si) films with varied crystallinity. When the measurement temperature is increased from 15 to 180 K, the PL peak energy redshifts from 1.0 to 0.83 eV. The PL quenching of the intensity follows a model of carrier thermalization in an exponential band tail with a width of ∼20 meV. The total PL intensity (IPL) as a function of excitation intensity (Iex) obeys a power law of IPL∝Iexγ, where γ(approximate)0.65 or 1 for high or low excitation intensity, respectively. The experimental results suggest that the ∼0.9 eV PL band originates from radiative tail-to-tail transitions in the grain-boundary region in μc-Si. © 2000 American Institute of Physics.

Notes: We report the effect of the rapid thermal annealing (RTA) of amorphous silicon oxynitride (a-SiOxNy:H) thin films on the room-temperature photoluminescence (PL). Samples were prepared by plasma enhanced chemical vapor deposition. The PL intensity increased by one order-of-magnitude after annealing for 20 min at 800 °C compared to that of the as-deposited sample. We have followed the changes in the chemical microstructure of the materials by in situ real-time ellipsometry, and suggest that the annealing is characterized by two distinct mechanisms. The first is that hydrogen effusion from clustered hydrogen and/or =N–H bond breaking occurs in the first 5 min resulting in increasing dangling bond density and no increase in the PL intensity. The second occurs after longer annealing times due to local reconstruction of Si–O and Si–N bonds, rather than diffusional rearrangement of the atomic species. This results in a more dense material with a higher refractive index, fewer nonradiative recombination centers, and more efficient PL. A model is presented to describe both the radiative and nonradiative recombination processes. © 1995 American Institute of Physics.