M. Geddo

Effect of temperature on the optical properties of (InGa)(AsN)/GaAs single quantum wells

InxGa1−xAs1−yNy/GaAs single quantum wells emitting at room temperature in the wavelength range λ=(1.3–1.55) μm have been studied by photoluminescence (PL). By increasing temperature, we find that samples containing nitrogen have a luminescence thermal stability and a room temperature emission efficiency higher than that of the corresponding N-free heterostructures. The temperature dependence of the PL line shape shows a progressive carrier detrapping from localized to extended states as T is increased. Finally, the extent of the thermal shift of the free exciton energy for different y indicates that the electron band edge has a localized character which increases with nitrogen content.InxGa1−xAs1−yNy/GaAs single quantum wells emitting at room temperature in the wavelength range λ=(1.3–1.55) μm have been studied by photoluminescence (PL). By increasing temperature, we find that samples containing nitrogen have a luminescence thermal stability and a room temperature emission efficiency higher than that of the corresponding N-free heterostructures. The temperature dependence of the PL line shape shows a progressive carrier detrapping from localized to extended states as T is increased. Finally, the extent of the thermal shift of the free exciton energy for different y indicates that the electron band edge has a localized character which increases with nitrogen content.

Quantum dot strain engineering of InAs/InGaAs nanostructures

We present a complete study both by experiments and by model calculations of quantum dot strain engineering, by which a few optical properties of quantum dot nanostructures can be tailored using the strain of quantum dots as a parameter. This approach can be used to redshift beyond 1.31μm and, possibly, towards 1.55μm the room-temperature light emission of InAs quantum dots embedded in InGaAs confining layers grown on GaAs substrates. We show that by controlling simultaneously the lower confining layer thickness and the confining layers’ composition, the energy gap of the quantum dot material and the band discontinuities in the quantum dot nanostructure can be predetermined and then the light emission can be tuned in the spectral region of interest. The availability of two degrees of freedom allows for the control of two parameters, which are the emission energy and the emission efficiency at room temperature. The InAs∕InGaAs structures were grown by the combined use of molecular beam epitaxy and atomic l...

The role of wetting layer states on the emission efficiency of InAs/InGaAs metamorphic quantum dot nanostructures

We report on a photoluminescence and photoreflectance study of metamorphic InAs/InGaAs quantum dot strain-engineered structures with and without additional InAlAs barriers intended to limit the carrier escape from the embedded quantum dots. From: (1) the substantial correspondence of the activation energies for thermal quenching of photoluminescence and the differences between wetting layer and quantum dot transition energies and (2) the unique capability of photoreflectance of assessing the confined nature of the escape states, we confidently identify the wetting layer states as the final ones of the process of carrier thermal escape from quantum dots, which is responsible for the photoluminescence quenching. Consistently, by studying structures with additional InAlAs barriers, we show that a significant reduction of the photoluminescence quenching can be obtained by the increase of the energy separation between wetting layers and quantum dot states that results from the insertion of enhanced barriers. These results provide useful indications on the light emission quenching in metamorphic quantum dot strain-engineered structures; such indications allow us to obtain light emission at room temperature in the 1.55 microm range and beyond by quantum dot nanostructures grown on GaAs substrates.

Metamorphic buffers and optical measurement of residual strain

We show that the residual strain occurring in constant-composition metamorphic buffer layers of III–V heterostructures can be accurately predicted by the suitable design of the epitaxial structures and measured all optically by means of photoreflectance spectroscopy. This result allows one to single out the nonequilibrium models among those that have been proposed to predict strain relaxation. The resulting ∝t−1∕2 dependence of the residual in-plane strain on buffer thickness t can be used to design metamorphic buffers not only for 1.3–1.55μm emitting quantum dot structures, but also for sophisticated graded-composition metamorphic structures for different classes of devices.

Photoreflectance and reflectance investigation of deuterium-irradiated GaAsN

The effect of deuterium irradiation on the optical and strain properties of GaAsN∕GaAs heterostructures was investigated by photoreflectance and reflectance techniques. The strain occurring in as-grown and deuterated GaAsN layers is monitored and measured by means of photoreflectance spectroscopy, highlighting the strain inversion after irradiation. By combining static and modulated reflectance results, evidence is given that the deuterium-induced recovery of the GaAs band gap as well as the strain inversion in GaAsN layers are accompanied by a 0.4%–0.8% reduction of the refractive index in the 1.31 and 1.55μm spectral windows of interest for fiber optic communications. These results anticipate a single step process to an in-plane confinement of carriers and photons.

Photoreflectance study of growth mode in InAs–GaAs quasimonolayer single quantum wells

Photoreflectance measurements have been performed in a number of InAs/GaAs single-quantum wells with nominal thickness L ranging from 0.6 to 2.0 ML. The InAs growth mode was investigated by analyzing the evolution, with increasing coverage, of the optical response associated with the InAs layer. For L⩽1.6 ML, the experimentally derived energies for the optical transition originating in the InAs are consistent with those evaluated in a simple square-well envelope-function scheme. The dependence of the photoreflectance line shape broadening on L is well described up to L=1.4 ML in terms of a disordered InAs/GaAs interface made by interconnected InAs and GaAs islands with a typical size of order 2 nm. For L=1.6 ML, the quantum well spectral features broaden abruptly and vanish for L=2 ML, suggesting the disappearance of the InAs 2D layer in favor of a predominant nucleation of large quantum dots.

A New Computer-Aided Apparatus for Simultaneous Measurements of Water Uptake and Swelling Force in Tablets

AbstractWater uptake and disintegrating force development have frequently been related to tablet disintegration properties.Water penetration into compressed tablets has been studied by many authors using modified Enslin apparatus. Meanwhile, in previous papers by our group, a great deal of attention has been paid to the measurements of disintegrating force and to the kinetics of force development.Given the fact that water penetration and swelling force development are related to each other, a new apparatus was set up which allows simultaneous measurements of water penetration and force development. It consists of a modified apparatus for force measurements, integrated with a modified Enslin apparatus. Both force and water uptake data were collected by a computer and stored for subsequent analysis.Fitting of both water penetration and force development curves was performed with a commercially available software package for non-linear regression analysis.This enables an examination of the relationships betw...

Photoreflectance evidence of the N-induced increase of the exciton binding energy in an InxGa1−xAs1−yNy alloy

The binding energy of the heavy-hole ground-state exciton in In0.25Ga0.75As1−yNy/GaAs single quantum wells (y=0, 0.011) was experimentally derived by photoreflectance measurements. We measured a binding energy of 6.6 and 8.5 meV for the N-free and the N-containing sample, respectively. The observed increase of the exciton binding energy can be accounted for by an increase of the exciton reduced mass of about 30% upon N introduction into the InxGa1−xAs lattice, consistently with recent experimental results and in agreement with earlier theoretical predictions.

Infrared study of oxygen precipitates in Czochralski grown silicon

Oxygen precipitation in the bulk of silicon wafers was investigated by using micro‐Fourier transform infrared spectroscopy. It was found that even at 1100 °C annealing (in single step) SiO2 precipitates are formed in platelet shape, in the bulk, giving rise to characteristic absorption peak in the infrared spectrum at 1230 cm−1. Complete mapping of wafer cross section demonstrated that these precipitates are not distributed homogeneously but are agglomerated in irregularly shaped clusters and are easily detectable up to distance of about 100 μm from the back surface and from the epi‐substrate interface.

Quantitative determination of high‐temperature oxygen microprecipitates in Czochralski silicon by micro‐Fourier transform infrared spectroscopy

Oxygen content in the bulk of Czochralski silicon was analyzed by using micro‐Fourier transform infrared spectroscopy in a transversal wafer cross‐section configuration. This technique locally distinguishes between interstitial oxygen and oxygen precipitates in wafers used as substrates for epitaxial layer growth. Systematic measurements performed in the 5000–700 cm−1 wavenumber range clearly indicate the presence of oxygen microprecipitates in the bulk of the processed silicon wafers. Quantitative determination of oxygen precipitate density is reported and compared with the measured interstitial oxygen concentration.