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We describe the application of Terahertz pulse spectroscopy to polymorph identification. The particular compounds investigated were the different crystalline Forms 1 and 2 of ranitidine hydrochloride, both in the pure form and also obtained as a marketed pharmaceutical product. Identification was clear. The technique has advantages that excitation is not via a powerful laser source, as used in Raman spectroscopy, so phase changes or photochemical reactions in polymorphs do not occur. Terahertz absorption spectral interpretation and instrumentation are similar to basic Fourier transform infrared (FTIR) spectroscopy and therefore easy to understand. The sample preparation techniques used are the same as those used in FTIR and Raman spectroscopies. The data obtained is complementary to Raman Spectroscopy. As the selection rules are different between the two techniques, we are able to obtain new data set directly related to crystalline structure adding to that obtained by Raman spectroscopy. Terahertz pulse spectroscopy provides information on low-frequency intermolecular vibrational modes; these are difficult to assess in Raman spectroscopy due to the proximity of the laser exciting line. It is concluded that the method has a wide range of applications in pharmaceutical science including formulation, high throughput screening, and inspection in storage. more...

Type-II CdSZnSe strained layer superlattice (SLS) grown on a CdS buffer on (1 1 1)A GaAs provide evidence for the intrinsic Stark effect at visible wavelengths. Shifts in photoluminescence (PL) peak energies are expected when photoexcited carriers screen the internally generated piezoelectric fields. Characteristic PL blue shifts under increasing excitation for CdSZnSe samples are observed. Transmission electron micrographs show that a superlattice, grown at 400°C, is mostly hexagonal in structure but a similar sample, grown at 300°C, adopts a cubic habit. However, both samples show similar shift factors, 70 meV per decade of excitation intensity, in the range of power densities from 50 mW cm −2 to 20 kW cm −2 . These shift factors are only slightly smaller than that found in CdSCdSe hexagonal superlattices of similar period. This result is not expected since the bulk piezoelectric coefficients of cubic ZnSe are thought to be much smaller than those for wurtzite phase II–VI semiconductors. A resolution of these apparent contradictions may be found by involving the recently reported nonlinearity of cubic piezoelectric coefficients as a function of strain by Andre et al. [Phys. Rev. B 53 (1996) 6951]. more...

The low-temperature photoluminescence (PL) spectra of a range of CdS-CdSe and CdS-ZnSe type-II superlattices was measured under both cw and quasi-cw excitation over the excitation range from mW cm −2 to MW cm −2 . In all cases, a single PL band due to the recombination of separated electrons and holes in the type-II bandstructure was observed. As the excitation density increases from a low value, the PL peak shifts logarithmically towards the blue. Total shifts of up to 200 meV for a CdS-CdSe sample of 10 nm period are observed, with a corresponding shift-factor of 30 meV per decade of excitation density. The shift factor increases in a regular fashion with the superlattice period. A comparison with the literature shows that a logarithmic dependance of the blue shift with increasing excitation density appears to be a universal feature of type-II SL, even in the absence of strain. more...

We report a survey of samples containing ultrathin layers of CdSe embedded in ZnSe or ZnS0.06Se0.94 matrices. CdSe single quantum wells (SQW) with well widths in the range 0.3–3 ML show several bands in the photon energy range from 1.7 to 2.4 eV. Similar bands have been attributed recently to self-organised quantum dots. Temperature-dependent photoluminescence (PL) shows that the bands in our samples vanish, without significant peak shift, at moderately high temperatures. Preliminary time-resolved PL shows that they also have relatively long lifetimes (ca. 30−250 ns). In all samples, the SQW exciton band lying at higher energies shows line narrowing with increasing temperature. Variations due to quantum-well thickness induce a correlation between the luminescence peak energy and the optical bandwidth which may be used to systematise our observations. more...

The first evidence of the intrinsic Stark effect at visible wavelengths (∼ 600 nm) is reported for a CdSZnSe strained layer superlattice (SLS) grown on a (111)A GaAs substrate. At the same time, smaller but significant exciton peak shifts with excitation power occur for similar samples that were grown on (100)GaAs substrates and thus predicted not to exhibit the effect. We attribute these shifts to band filling in a relaxed type-II SLS. We also report the largest light-induced exciton peak shift ever observed, 600 meV, in a CdSCdSe piezoelectric type-II strained layer superlattice. A shift of 150 meV per decade of excitation power occurs in a superlattice of period 16 nm under both pulsed and CW excitation at low temperature. Finally we mention the prospect of producing optically activated modulators that work in the visible and near infrared spectral region (500–1100 nm) from such structures. more...

We have investigated the effect of intermixing on the excitonic reflectivity spectra of a 100 Å thick InGaAs/GaAs quantum well (QW). We could resolve both the El - Hl and E2 - H2 excitonic features, up to LD = 80 Å. We show that both energy shifts agree well with a simple mode1 which assumes a Fick's law diffision, with a diffusion coefficient that is independent of the concentration of the cationic species. Next, considering the change (improvement) in signal to : noise ratio versus thermal annealing, we suggest that, in our samples, we not only get a broadening of the quantum well as the diffision proceeds but, also, a diffision of the lattice constituents in the plane of the well. This in-plane diffision smoothes the interfaces and reduces the monolayer well width fluctuations. more...

The effects of silicon and beryllium at doping levels of up to 1019 cm−3 on the interdiffusion of GaAs/AlxGa1−xAs and InxGa1−xAs/GaAs quantum wells after annealing have been studied using photoluminescence. It was found that for beryllium concentrations up to 2.5 ×1019 cm−3 and for silicon doping concentrations up to 1018 cm−3, no change in the interdiffusion coefficients could be measured. For a silicon doping concentration of 6×1018 cm−3 a dramatic degradation of the material quality was observed following annealing at 750 °C for 15 s. This resulted in the luminescence from the well disappearing and the appearance of deep level luminescence related to donor‐gallium vacancy complexes and arsenic antisite defects. From these results we suggest that the position of the Fermi level plays no role in the intermixing of III‐V heterostructures and that most of the enhanced intermixing observed in silicon‐doped GaAs/AlxGa1−xAs structures is related to silicon relocation at very high doping levels. more...

Photoluminescence coupled with repetitive thermal annealing has been used to determine the diffusion coefficients for intermixing in InxGa1−xAs/GaAs quantum wells and to study the subsequent effects of ion implantation on the intermixing. It is shown that following ion implantation there is a very fast interdiffusion process, which is independent of the implanted ion and that is thought to be due to the rapid diffusion of interstitials created during the implantation. Following this rapid process, it was found that neither gallium nor krypton ions had any effect on the subsequent interdiffusion coefficient. Following arsenic implantation in addition to the initial damage related process, an enhanced region of interdiffusion was observed with a diffusion coefficient that was an order of magnitude greater than that of an unimplanted control wafer. This enhanced process is thought to be due to the creation of group III vacancies by the arsenic atoms moving onto group V lattice sites. This fast process was pr... more...

Implants of arsenic and gallium ions have been made into InGaAs/GaAs and GaAs/AlGaAs single quantum wells to a dose of 10 13 atoms/cm 2 . In each case the implantation energies are varied so as to get the peak of the implantation profile at the centre of the well. Using repetitive anneals and photoluminescence from the n = 1 electron to heavy hole transition, it has been possible to follow the evolution of the diffusion coefficients for the interdiffusion with anneal time. Following gallium implantation no effect was observed on the diffusion coefficient for intermixing in either material system. However, a region of very rapid intermixing which occurred either during implantation or the first anneal was seen in both materials. This is attributed to either radiation enhanced diffusion or the movement of interstitials created by the ion beam. Following arsenic implantation, the GaAs/AlGaAs material behaved identically to the gallium implanted samples. The InGaAs/GaAs materials, however, showed a region of intermixing where the diffusion coefficient was enhanced by an order of magnitude before returning to approximately the unimplanted control value. more...

We report on the use of plasma hydrogenation of Si doped, p‐type GaAs crystalline samples to form infrared waveguides through acceptor passivation. Epilayers grown by liquid phase epitaxy were exposed to a deuterium plasma for ninety minutes at three different temperatures. Secondary‐ion mass spectrometry (SIMS) analysis indicated that the deuterium concentrations in the crystals after plasma exposure were nearly equal to the acceptor level and extended to depths between 2.0 and 4.0 μm. Reflectivity measurements showed that the epilayers had passivated regions whose thicknesses corresponded to those determined by SIMS analysis. Laser coupling experiments at 1.15 μm showed optical waveguiding in each sample and lowest propagation losses were on the order of 35 dB/cm. At a wavelength of 1.523 μm, only the sample processed at the highest temperature exhibited laser guiding and losses were considerably higher. more...

We report on the use of plasma hydrogenation of silicon doped p-type GaAs epilayer samples to form infrared waveguides through acceptor passivation. Secondary-ion mass spectroscopy (SIMS) analysis indicated that the deuterium concentrations in the epilayers after plasma exposure were nearly equal to the acceptor concentration and extended to depths between 2.0 and 4.0 μm. Reflectivity measurements showed that the epilayers had passivated regions whose thicknesses corresponded to those determined by SIMS analysis. Laser-coupling experiments at 1.15 μm showed optical waveguiding in each sample and the lowest waveguide losses were on the order of 35 dB cm−1. more...

The operation of the intrinsic Stark effect at visible wavelengths is reported, for both cubic and wurtzite type-II CdS–ZnSe strained layer superlattices grown on CdS buffer layers on (111)A GaAs. An observed increase of the effective piezoelectric coefficient from the measured cubic bulk value is attributed to a nonlinear effect, as previously reported for CdTe quantum wells [R. Andre, J. Cibert, Le Si Dang, J. Zeman, and M. Zigone, Phys. Rev. B 53, 6951 (1996)]. Exciton peak shifts of 70 meV per decade change of excitation intensity are similar in magnitude to those found in CdS–CdSe superlattices of similar period. These shifts are attributed to screening of the internal piezoelectric fields by photoexcited carriers. Exciton peak shifts that are smaller by an order of magnitude appear in CdS–ZnSe samples, grown on (100) ZnSe buffers and therefore predicted to exhibit no piezoelectric effects of any order. These small shifts, which we attribute to space charge buildup, are a characteristic feature of ty... more...

Visible photoluminescence (PL), optical transmission, and Raman scattering spectra of Si nanocrystals embedded in SiO2 matrix have been studied. Films were deposited by rf magnetron sputtering of compound SiO2/Si targets. Films containing silicon in the form of nanocrystals clusters show intense luminescence at room temperature. Films containing only amorphous Si clusters show little or no PL. Annealing in vacuum or in specific atmosphere leads to strong change of the PL spectrum and its intensity. Results are explained by strong influence of the Si/SiO2 interface state on integral PL emission. more...

The diffusion coefficient for arsenic and phosphorous interdiffusion has been measured as a function of temperature in In/sub 0.66/Ga/sub 0.33/As/ In/sub 0.66/Ga/sub 0.33/As/sub 0.7/P/sub 0.3/ strained quantum wells, using the photoluminescence from the n = 1 electron to heavy hole transition. The authors have used the shift in the photoluminescence from a 100/spl Aring/ single quantum well of In/sub 0.66/Ga/sub 0.33/As in In/sub 0.66/Ga/sub 0.33/As/sub 0.7/P/sub 0.3/ barriers with annealing to monitor the interdiffusion. In this system there is no concentration gradient to drive group III interdiffusion and therefore the variation in the photoluminescence transition energy was modeled using only phosphorous/arsenic interdiffusion in the model. By performing several anneals to a single sample and measuring the shift in the photoluminescence after each anneal the interdiffusion was monitored as a function of time. Between temperatures of 900/spl deg/C and 700/spl deg/C the diffusion coefficient, D, can be described using the relationship D = D/sub o/exp(-E/sub A//kT) where D/sub o/ = 507 cm/sup 2//s and E/sub A/ = 3.9 eV. > more...

The interaction between strong THz radiation from a free electron laser and an electron sheet in a high-mobility, low-density, GaAs/AlGaAs structure has been investigated in magnetotransport experiments over a wide range of wavelengths, intensities, magnetic fields, and temperatures. Photovoltage and photocurrent effects are evident in both longitudinal and transverse potential differences. Broad cyclotron resonance is observed to high temperature and connected with a decrease in electron density. The change in electron temperature under THz radiation is estimated from changes in the magnitude of the magnetoresistivity oscillations at low lattice temperature. The effect of the magnetic field is to suppress the rise in electron temperature relative to the zero field case. more...

The afterglow effect of slowly decaying photoluminescence, well known for phosphor materials, is identified in silicon. A component with a lifetime of up to 100 ms is observed in kinetics of the $\ensuremath{\lambda}\ensuremath{\approx}1.5 \ensuremath{\mu}\mathrm{m}$ emission of Er-doped crystalline silicon. This is measured at $T\ensuremath{\approx}4.2\char21{}40 \mathrm{K}$ under pulsed band-to-band excitation with a Nd:YAG laser (532 nm). The slow component is found to be superimposed upon the Er-related emission characterized by the commonly reported lifetime of approximately 1 ms. Thermalization and subsequent recombination of nonequilibrium carriers trapped at shallow levels after the excitation pulse is proposed as the microscopic mechanism responsible for this slow emission. Using time-resolved two-color spectroscopy with a free-electron laser, the earlier reported mid-infrared induced enhancement of Er emission is related to the presently observed afterglow. A simple kinetic model is developed and shown to successfully link the amplitude and temporal characteristics of both effects. Measurements at higher temperatures support the proposed interpretation. more...

We report time-resolved (ps) studies of the dynamics of intersubband transitions in p-Si/SiGe multi-quantum well structures in the FIR regime, ℏω

We present far-infrared (FIR) modulated photo luminescence measurements of self-assembled InAs/GaAs quantum dots. The sample was annealed to remove any indium profile roughness from within the quantum dots and thus from the inhomogeneous broadening. The annealed sample shows resonant FIR excitations just as the unannealed sample. but exhibits spectral hole burning unlike the unannealed sample. The results indicate that indium roughness is one of the primary inhomogeneous broadening mechanisms in self-assembled quantum dots. (C) 2002 Elsevier Science B.V. All rights reserved. more...

We report on verification of the proposed energy transfer mechanism responsible for photoluminescence of rare earth (RE) ions in semiconductors. Using two-color spectroscopy in the visible and the midinfrared regions (with a free-electron laser) we demonstrate reversal of the most important step in the excitation process. In that way, formation of the intermediate state bridging atomic states of the RE ion core and extended orbitals of a semiconducting host is explicitly confirmed and its characteristic energy spectroscopically determined. The study is performed for InP:Yb. It is argued, however, that the conclusions are valid for all semiconductor:RE systems, including the notorious Si:Er. more...

Electron intersubband lifetimes of 1.8 ps and 2.5 ps were measured in GaAs/AlGaAs asymmetric stepped quantum well samples by using a pump–probe three-beam technique at 4 K to room temperature. At high pump intensities, an absorptive signal of sub-nanosecond lifetime was detected following a normal positive transmissive signal of picosecond lifetime. This absorptive signal is believed to be caused by the absorption from the free electrons pumped from the ground subband to the continuum conduction band in the barrier layers associated with incoherent multi-photon process. The intersubband lifetimes and free carrier effects play an important role in optically pumped multiple-quantum-well far infrared lasers. A three-level electron transition model is built to simulate the process, which agrees very well with the experimental data. more...

Lyman transitions of the Be acceptor in GaAs have been investigated under intense radiation generated by a free electron laser. Photoconductivity was found to be a more sensitive method than transmission for detecting the transitions. Three regimes of photoconductive response were distinguished: at high intensity, the signal saturates; at intermediate intensity, it is proportional to intensity; at low intensity, it is background limited. Spectra were obtained by wavelength scanning and showed excellent reproducibility from run to run. With increasing radiation intensity, the D and C lines, being transitions from the 1s3/2(Γ8+) ground state to the 2p5/2(Γ8−) and 2p5/2(Γ7−) excited states, respectively, exhibit a pronounced splitting and broadening of components. This behavior is attributed to thermal stress induced by localized heating. more...

Interdiffusion of both the group III and group V sublattices has been studied in the InGaAs/GaAs and InGaAs/InGaAsP systems respectively using photoluminescence and consecutive annealing. By measuring the diffusion coefficients between the temperatures of 1050°C to 600°C we have been able to determine the activation energies for diffusion in both sublattices and have found them to be equal (for temperatures above 650°C) at 3.7 eV. From the effects of gallium and arsenic implantation on the interdiffusion of InGaAs/GaAs we have established that the diffusion is controlled by vacancies on both sublattices, and that except in the case of the group V diffusion below 650°C the controlling vacancies are thermally created. more...

In this letter we present the results of a photoluminescence study of the interdiffusion of arsenic and phosphorous in the In0.66Ga0.33As/In0.66Ga0.33As0.7P0.3 system over the temperature range 950–600 °C. We have shown that the diffusion is Fickian with no dependence of the diffusion coefficient on the substrate doping type or etch pit density. For both tin‐ and sulfur‐doped substrates the diffusion can be described by a diffusion coefficient D, which is given by D=D0 exp(−EA/kT), where D0=23 cm2/s and EA=3.7 eV for temperatures greater than 675 °C. This activation energy is the same as that determined for the group III interdiffusion in In0.2Ga0.8As/GaAs. Below this temperature a lower activation energy process takes over with D0=5×10−10 cm2/s and EA=1.7 eV. more...

We report the photoluminescence study of low‐energy, low‐dose oxygen implantation induced compositional disordering of AlGaAs/GaAs quantum well structures. Significant disordering of both single and multiple AlGaAs/GaAs quantum wells has been achieved using low‐energy (155 keV) oxygen ion implantation with doses as small as 5 × 1013 cm−2 after a moderate annealing step. These doses are significantly lower than those reported previously (500 keV and 1016 cm−2). more...

The intermixing of InGaAs/GaAs strained quantum wells following gallium ion implantation was characterised using photoluminescence. It was found that the implanted atoms had no effect of the diffusion coefficient for intermixing when compared to an unimplanted control sample. However, it was seen that the well was broadened by a diffusion length of approximately 37A during the first anneal, before a photoluminescence measurement could be made. A simulation of the implantation process shows that cascade mixing is not sufficient to account for this and it is suggested that we are observing the diffusion of interstitials created by the implantation. more...