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Understanding the relation between phase morphology and physical processes in polymer blends is the key to the fabrication of reproducible and reliable polymer optoelectronic devices. In this work, taking the advantage of low-temperature spectroscopy, we have observed the on-site generation of excitons and long-lived charges in different phase morphology polymer/fullerene blends. Probing at 10K, the photo-generated species are localized to where they are generated. We found that the generation of excitons and long-lived charges is highly influenced by the local molecular phase morphology. We further demonstrated that although the influence of phase morphology is localized to the place that excitons and long-lived charges are generated, this influence can persist over sub-millisecond timescales. Thus, we believe that the fate of excitons and long-lived charges is determined by the location at which they are generated, which can in turn be controlled precisely by molecular phase morphology.

Second-order Raman scattering has been extensively studied in carbon-based nanomaterials, for example, nanotube and graphene, because it activates normally forbidden Raman modes that are sensitive to crystal disorder, such as defects, dopants, strain, and so forth. The sp2-hybridized carbon systems are, however, the exception among nanomaterials, where first-order Raman processes usually dominate. Here we report the identification of four second-order Raman modes, named D1, D1′, D2 and D2′, in exfoliated black phosphorus (P(black)), an elemental direct-gap semiconductor exhibiting strong mechanical and electronic anisotropies. Located in close proximity to the Ag1 and Ag2 modes, these new modes dominate at an excitation wavelength of 633 nm. Their evolutions as a function of sample thickness, excitation wavelength, and defect density indicate that they are defect-activated and involve high-momentum phonons in a doubly resonant Raman process. Ab initio simulations of a monolayer reveal that the D′ and D mo...

Owing to its crystallographic structure, black phosphorus is one of the few 2D materials expressing strongly anisotropic optical, transport, and mechanical properties. We report on the anisotropy of electron–phonon interactions through a polarization-resolved Raman study of the four vibrational modes of atomically thin black phosphorus (2D phosphane): the three bulk-like modes Ag1, B2g, and Ag2 and the Davydov-induced mode labeled Ag(B2u). The complex Raman tensor elements reveal that the relative variation in permittivity of all Ag modes is irrespective of the atomic motion involved lowest along the zigzag direction, the basal anisotropy of these variations is most pronounced for Ag2 and Ag(B2u), and interlayer interactions in multilayer samples lead to reduced anisotropy. The bulk-forbidden Ag(B2u) mode appears for n ≥ 2 and quickly subsides in thicker layers. It is assigned to a Davydov-induced IR to Raman conversion of the bulk IR mode B2u and exhibits characteristics similar to Ag2. Although this mod...

Ever since their dawn in 2009 1, hybrid organic–inorganic perovskite materials have been experiencing an ever increase in research investment to finally emerge as a strong alternative for high power conversion efficiency (PCE) photovoltaic devices at low costs. One of the main challenges that is currently impeding the upscaling of these solar cells is the phenomenon of hysteresis on their j–V response that has been correlated with slow ion migration which, in turn, affects their photovoltaic properties. Although hysteresis has been described to some extent of detail, the precise role of the external contacts remains under debate. Furthermore, there has been a great deal of focus on materials and interface design while less attention has been given to optical aspects of these cells. Our work highlights the role of electron transport layer (ETL) of different compositions and synthesized through various routes on the hysteresis response of these systems. We show that there’s a clear correlation between thickness, morphology and composition of the ETL on the photovoltaic response these systems. To better elucidate such a response, we have utilized photoluminescence, electroluminescence, absorption and kelvin probe force microscopy to get a substantial grasp of the optoelectronic phenomena occurring at the interfaces in perovskite solar cells. Acknowledgments: This research was supported by a Hydro-Quebec/NSERC CRD grant and the McGill Sustainability Systems Initiative (MSSI). 1- Kojima, A., Teshima, K., Shirai, Y. and Miyasaka, T., 2009. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. Journal of the American Chemical Society, 131(17), pp.6050-6051.

We measure the homogeneous excitation linewidth of regioregular poly(3-hexylthiophene), a model semicrystalline polymeric semiconductor, by means of two-dimensional coherent photoluminescence excitation spectroscopy. At a temperature of 8\,K, we find a linewidth that is always $\gtrsim 110$\,meV full-width-at-half-maximum, which is a significant fraction of the total linewidth. It displays a spectral dependence and is minimum near the 0--0 origin peak. We interpret this spectral dependence of the homogeneous excitation linewidth within the context of a weakly coupled aggregate model., 13 pages, 4 figures, Supplementary Material

With strongly bound and stable excitons at room temperature, single-layer, two-dimensional organic-inorganic hybrid perovskites are viable semiconductors for light-emitting quantum optoelectronics applications. In such a technological context, it is imperative to comprehensively explore all the factors --- chemical, electronic and structural --- that govern strong multi-exciton correlations. Here, by means of two-dimensional coherent spectroscopy, we examine excitonic many-body effects in pure, single-layer (PEA)$_2$PbI$_4$ (PEA = phenylethylammonium). We determine the binding energy of biexcitons --- correlated two-electron, two-hole quasiparticles --- to be $44 \pm 5$\,meV at room temperature. The extraordinarily high values are similar to those reported in other strongly excitonic two-dimensional materials such as transition-metal dichalchogenides. Importantly, we show that this binding energy increases by $\sim25$\% upon cooling to 5\,K. Our work highlights the importance of multi-exciton correlations in this class of technologically promising, solution-processable materials, in spite of the strong effects of lattice fluctuations and dynamic disorder., Comment: 37 pages including 4 figures and supplemental information

We present theoretical and experimental results showing the effects of incoherent population mixing on two-dimensional (2D) coherent excitation spectra that are measured via a time-integrated population and phase-sensitive detection. The technique uses four collinear ultrashort pulses and phase modulation to acquire two-dimensional spectra by isolating specific nonlinear contributions to the photoluminescence or photocurrent excitation signal. We demonstrate that an incoherent contribution to the measured lineshape, arising from nonlinear population dynamics over the entire photoexcitation lifetime, generates a similar lineshape to the expected 2D coherent spectra in condensed-phase systems. In those systems, photoexcitations are mobile such that inter-particle interactions are important on any timescale, including those long compared to the 2D coherent experiment. Measurements on a semicrystalline polymeric semiconductor film at low temperature show that, in some conditions in which multi-exciton interactions are suppressed, the technique predominantly detects coherent signal and can be used, in our example, to extract homogeneous linewidths. The same method used on a lead-halide perovskite photovoltaic cell shows that incoherent population mixing of mobile photocarriers can dominate the measured signal since carrier-carrier bimolecular scattering is active even at low excitation densities, which hides the coherent contribution to the spectral lineshape., Comment: 22 pages, 6 figures

Gallium selenide (GaSe) is a 2D material with a thickness-dependent gap, strong non-linear optical coefficients and uncommon interband optical selection rules, making it interesting for optoelectronic and spintronic applications. In this work, we monitor the oxidation dynamics of GaSe with thicknesses ranging from 10 to 200 nm using Raman spectroscopy. In ambient temperature and humidity conditions, the intensity of all Raman modes and the luminescence decrease rapidly with moderate exposure to above-gap illumination. Concurrently, several oxidation products appear in the Raman spectra: Ga$_2$Se$_3$, Ga$_2$O$_3$ and amorphous and crystalline selenium. We find that no safe measurement power exists for optical measurements on ultrathin GaSe in ambient conditions. We demonstrate that the simultaneous presence of oxygen, humidity, and above-gap illumination is required to activate this photo-oxidation process, which is attributed to the transfer of photo-generated charge carriers towards aqueous oxygen at the sample surface, generating highly reactive superoxide anions that rapidly degrade the sample and quench the optical response of the material.

Liquid phase encapsulation of α-sexithiophene (6T) molecules inside individualized single-walled carbon nanotubes (SWCNTs) is investigated using Ramanimaging and spectroscopy. By taking advantage of the strong Raman response of this system, we probe the encapsulation isotherms at 30°C and 115°C using a statistical ensemble of SWCNT deposited on a Si/SiO2 substrate. Two distinct and sequential stages ofencapsulation are observed: Stage 1 is a one-dimensional (1D) aggregation of 6T alignedhead-to-tail inside the nanotube and stage 2 proceeds with the assembly of a second row, giving pairs of aligned 6Ts stacked together side-by-side. The experimental data are fitted using both Langmuir (type VI) and Ising models, in which the single-aggregate (stage 1) forms spontaneously whereas the pair-aggregate (stage 2) is endothermic in toluene with formation enthalpy of 8Hpair = 260±20 meV. Tunable Raman spectroscopy for each stage reveals a bathochromic shift of the molecular resonance of the pair-aggregate, which is consistent with strong inter-molecular coupling and suggestive of J-type aggregation. This quantitative Raman approach is sensitive to roughly 10 molecules per nanotube andprovides direct evidence of molecular entry from the nanotube ends. These insights into the encapsulation process guide the preparation of well-defined 1D molecular crystals having tailored optical properties.

We explore charge recombination dynamics at electron donor-acceptor heterojunctions, formed between a semiconductor polymer (PCDTBT) and a fullerene derivative (PC70BM), by means of combined time-resolved photoluminescence and transient absorption spectroscopies. Following prompt exciton dissociation across the heterojunction, a subset of bound electron-hole pairs recombines with a temperature-independent rate distribution spanning submicrosecond timescales to produce luminescent charge-transfer excitons (CTX). At 14 K, this slow mechanism is the dominant geminate charge recombination pathway, whereas we also observe CTX emission on subnanosecond timescales at 293 K. We thus find that at these temperatures, a fraction of the initial charge-pair population is trapped deeply such that they only recombine slowly over a broad distribution of timescales by quantum tunneling. We identify geminate polaron pairs (GPP) as a reservoir of long-lived localized states that repopulate the CTX up to microsecond timescales. The observation of such distributed geminate-charge recombination highlights the importance of the molecular nature of specific donor–acceptor electronic interactions in defining the relaxation pathways of trapped GPP. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

We address the binding energy of charge-transfer excitons at organic semiconductor heterojunctions by investigating a polymer blend where the energy of the intramolecular singlet exciton is just su...

We have studied the optical properties of ultrathin InAs/InP quantum wells and Stranski?Krastanov nanostructures using photoluminescence and photoluminescence excitation experiments. For InAs epilayers thinner than 2.4 monolayers, the emission spectrum consists of a single peak and the ground-state exciton energy is in good agreement with predictions based on the tight-binding method for ultrathin quantum wells. Beyond this thickness, the photoluminescence spectra evolve to a multimodal emission indicative of the presence of families of quantum dots with small heights. The emission of these quantum dots is blue-shifted significantly (~100 meV) from the predicted values. The discrepancy is explained by As/P intermixing that occurs during quantum dot formation.

Pulsed laser ablation (PLA) from a Si target in an inert He ambient has been used to produce Si/SiOx nanostructured thin films on Si substrates. After the film exposition to atmospheric air, they exhibited photoluminescence (PL) signals with peak energy between 1.58 and 2.15 eV. It was found that both natural and thermal oxidation of the films can cause dramatic changes in PL properties giving rise to the appearance or considerable enhancement of fixed PL peaks around 1.6–1.65 and 2.2–2.25 eV. Quite different time-dependent PL degradation behavior under continuous laser irradiation gives an evidence for different mechanisms responsible for the PL peaks. Possible origins of PL are discussed.

We have performed detailed optical measurements of ultrathin InAs/InP quantum wells grown by metalorganic vapor phase epitaxy. Absorption and photoluminescence excitation spectra reveal the excitonic resonances associated with two- and three-monolayer-thick InAs layers. Photoluminescence spectra also show an emission band at intermediate photon energies which is associated with excitons localized in thin InAs quantum dots. Polarization-dependent measurements clearly show the heavy-hole or light-hole nature of the quantum well resonances. Such an identification of both type of transitions provides a test for electronic structure models. We find that the energy positions of the excitonic transitions in ultrathin InAs/InP quantum wells are not consistent with calculations based on the envelope function model.

We have investigated the effect of the misorientation (001) InP substrates on the optical properties of submonolayers of InAs in InP grown by metalorganic chemical vapor deposition. InAs submonolayers were systematically studied using low temperature photoluminescence (PL), photoluminescence excitation spectroscopy and temperature-dependent, excitation density PL. For submonolayer samples with oriented substrates, the observed PL linewidths and energies are satisfactorily explained within a two-dimensional (2D) quantum well picture. The formation of InAs isolated quantum dots which is found in the submonolayer samples with misoriented substrates towards (110) orientations, however, results in 0D exciton localization.

We present investigations of the temperature evolution of the photoluminescence (PL) and optical absorption spectra of a series of tensile-strained InGaP/InP multiple quantum wells (MQW) samples. As in previous work on compressive-strained InAsP/InP MQW, the results support the assignment of the low temperature PL transitions to recombination from excitonic band-tail states. The energy of such transition is redshifted with respect to the free exciton recombination energy. This results in a large apparent energy difference between PL and optical absorption peaks which is, at 6 K, about 5 meV in the coherently strained samples and less than 1 meV in the partially relaxed ones. The analysis of the low temperature PL line shape which we present enables the determination of the excitonic band-gap energy for all the samples. The difference between the energy of the optical absorption transition and the excitonic band-gap energy thus determined gives a measure of the Stokes shift arising from thermalization effects alone. The values of the Stokes shift thus obtained are in better agreement with the sharpness of the optical absorption transitions which indicate samples of high crystalline quality.

We have investigated the effect of the shape of a band of localized deep donors on the electronic transport of three high-purity n-type InP epilayers. A model accounting for a broad asymmetric band of localized deep donors or complexes of unknown origin, centered 167 meV below the conduction band edge with a width of about 245 meV, provided a remarkable description of both the Hall mobility and Hall electronic concentration in the temperature range of 4.2–300 K.

We report on optical absorption of the interband transitions in zero-net strained multiple quantum wells (MQW) grown by low-pressure metalorganic vapour phase epitaxy (LP-MOVPE), using tertiarybutylarsine as a group V source. Sharp interfaces are obtained using a growth interruption procedure. Analysis of this procedure with different interruption times leads to the same optimal times as those obtained for InP/InAsP superlattices grown in the same reactor. We have achieved the growth of modulation-free strain-balanced heterostructures, as indicated by cross-sectional transmission electron microscopy. High-resolution x-ray diffraction and optical absorption analysis demonstrate the high crystallographic and optical quality of these structures. The absorption spectrum of an x = 0.06, y = 0.14 sample was accurately fitted using the Bastard/Marzin model, and a strained conduction band offset of was deduced. This corresponds to about of the total strained bandgap difference.

In this paper we derived the selection rules for band-to-band transitions in low-symmetry crystalline GaTe and explained the low-temperature (2 K) absorption spectra at two different polarization configurations for this material. It is found that the top two valence bands of crystalline GaTe at the high symmetry Γ-point have one-dimensional degeneracy and are weakly separated (

Heteroepitaxial InP layers were grown on Si(111) by metalorganic vapor phase epitaxy using thermal cycle growth. The best crystallographic and optical quality was obtained when thermal cycle growth was begun after only a thin InP layer had been deposited. High resolution x‐ray diffraction rocking curves of 4.8 μm thick InP layers yield full widths at half‐maximum as low as 76 arc s and show that epilayers have a positive tilt with respect to the substrate. Cross‐section transmission electron microscopy observations and Rutherford backscattering measurements show that thermal cycling induces a net reduction of defect density in the interfacial region. Photoluminescence (PL) measurements performed on the best quality thermal cycle grown sample show a thermal strain induced energy splitting of 3.8 meV between the free exciton emissions associated with heavy and light holes. Two other peaks in the PL spectra correspond to acceptor‐bound (A0,X)mj=±3/2 and (A0,X)mj=±1/2 excitonic transitions, as confirmed by ph...

A combination of transmission electron microscopy and high‐resolution x‐ray diffraction analyses has been used to determine the exact strain in each layer of InAsP/InP multiple‐quantum‐well structures grown by metalorganic vapor phase epitaxy on InP(001) using trimethylindium, tertiarybutylarsine, and phosphine as precursors. The strain‐relaxed structures are characterized by misfit dislocations located exclusively at (i) the interface between the buffer layer and the multilayer, and (ii) the interface between the multilayer and the cap layer. The low‐temperature optical absorption spectra show well resolved excitonic transitions that are significantly shifted by strain relaxation. The spectra are analyzed with a solution to the Schrodinger equation in the envelope function formalism using the Bastard–Marzin model. The energies for the major transitions involving light‐ and heavy‐holes are predicted accurately for all samples, allowing the determination of the heterojunction band offset. The heavy‐ and li...

Free exciton photoluminescence (PL) spectra of crystalline GaTe has been studied as a function of continuous wave (cw) argon laser excitation intensity at T=6 K. When the excitation intensities are low (P4 W ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$), the integrated PL intensities of free excitons for both singlet and triplet states increase approximately linearly with increasing excitation intensity. From about 4 to 9 W ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$, the peak energy redshift, the linewidth broadening, and the sharp increase of the integrated PL intensity of the exciton peaks are observed. These results are explained tentatively as large coherent emissions from excitons of GaTe. \textcopyright{} 1996 The American Physical Society.

We have carried out a detailed structural and optical characterization of Ga0.47In0.53As/InP multiple quantum wells grown by chemical beam epitaxy using a well-defined sequence of growth interruption times between successive layers. These growth interruption times result in the formation of interfacial layers which drastically alter the structural properties of Ga0.47In0.53As/InP multiple quantum wells. An analysis of double-crystal x-ray diffraction data reveals that exposure of InP to arsine for 2 s is sufficient to create approximately 3 monolayers of InAs0.55P0.45 ternary under biaxial compressive strain at the InP/Ga047In0.53As interface. Moreover, exposure of Ga0.47In0.53As to phosphine for 2 s results in the formation of approximately 2 monolayers of Ga0.48In0.52As0.21P0.79 quaternary under biaxial tensile strain at the Ga0.47In0.53As/InP interface. We find that long exposures to hydrides (over 5 s) rather than short ones give rise to interfacial layers with less compositional disorder and/or thickness fluctuation. Moreover, photoluminescence and absorption spectroscopy data reveal the negligible effect of InAsxP1−x and GaxIn1−xAsyP1−y interfacial layers on the emission and optical absorption properties of Ga0.47In0.53As/InP multiple quantum wells with sufficiently thick Ga0.47In0.53As layers

Low- and room-temperature optical absorption spectra are presented for a series of $\mathrm{In}{\mathrm{As}}_{x}{\mathrm{P}}_{1\ensuremath{-}x}/\mathrm{InP}$ strainedlayer multiple quantum well structures ($0.11l~xl~0.35$) grown by low-pressure metal-organic vapor phase epitaxy using trimethylindium, tertiarybutylarsine, and phosphine as precursors. The well widths and compositions in these structures are exactly determined from the use of both high-resolution x-ray diffraction and transmission electron microscopy on the same samples. The absorption spectra are then analyzed by selfconsistently fitting, for the five samples, the excitonic peak energy positions with transition energies determined from a solution to the Schr\"odinger equation in the envelope function formalism using the well-known Bastard/Marzin model [J. Y. Marzin et al., in Semiconductors and Semimetals, edited by Thomas P. Pearsall, (Academic, New York, 1990), Vol. 32, p. 56]. From these self-consistent fits, both the bowing parameter of bulk unstrained $\mathrm{In}{\mathrm{As}}_{x}{\mathrm{P}}_{1\ensuremath{-}x}$ and the band offsets of the heterostructures are deduced self-consistently. The conduction-band offsets thus determined represent 75%\ifmmode\pm\else\textpm\fi{}3% of the total strained band-gap differences at both low (liquid He) and room temperatures. These values of the band offsets are consistent with the predictions of the quantum dipole model [J. Tersoff, Phys. Rev. B 30, 4874 (1984)]. The values determined for the bowing parameters are found to differ slightly between 0.10\ifmmode\pm\else\textpm\fi{}0.01 eV at low temperature and 0.12\ifmmode\pm\else\textpm\fi{}0.01 eV at room temperature.

Temperature-dependent photoluminescence measurements have been performed to study the linewidth of the ground and first excited states of the neutral shallow-donor bound-exciton transition in a high-purity n-type InP epilayer. For temperatures below 20 K, a theoretical model developed by Chou and Neumark to take into account the electron-hole correlation and the usual electron-phonon coupling is found to be in acceptable agreement with the linewidth data which follows a linear dependence upon temperature.

A detailed investigation of the structural and optoelectronic properties of thick GaInP epilayers on sulfur‐doped InP substrates is reported. Significant variations of the optical absorption and photoluminescence transition energies from light‐ and heavy‐hole states are observed as a function of the epilayer composition as well as of the degree of relaxation of the misfit strain. High‐resolution x‐ray measurements were used to determine the Ga concentrations and the strains and indicate significant anisotropic relaxation in several films. Even small relaxations result in a significant increase in the optical linewidths and a rapid drop in the transition intensities. A model with no free parameters based on the strain Hamiltonian of Pikus and Bir provides excellent agreement with the transition energies and serves to identify unambiguously the transitions observed in the optical spectra. Within this model, isotropic in‐plane relaxation produces a shift of both light‐ and heavy‐hole energies whereas anisotr...

Low temperature photoluminescence measurements of GaxIn1−xAsyP1−y alloys nearly lattice matched to InP to study the line broadening of the observed band to band and near band gap transitions in these materials were performed. We find that the dominant broadening mechanism is alloy broadening that originates from the spatial fluctuations of the band gap energy due to random anion and cation distribution. A model that assumes that occupation of the group‐III sites by Ga and In atoms and of the group‐V sites by As and P atoms occurs randomly, is fitted to the photoluminescence spectra of our samples. This provides an excellent description of the experimental results.

Ultrathin InAs/InP single quantum wells were grown by low-pressure metalorganic vapor phase epitaxy using tertiarybutylphosphine and tertiarybutylarsine, with nominal thicknesses between 2 and 3 ML. Characterization of a large number of samples by high-resolution x-ray diffraction (HR-XRD) and photoluminescence (PL) indicates an average InAs thickness very close to the nominal one. Photoluminescence excitation and absorption spectra of selected samples contain excitonic resonances associated with electron-hole transitions in 2 or 3 ML of InAs buried in InP. Only one set of these resonances appears in a given sample, thereby corroborating HR-XRD and PL measurements. Thus, our measurements indicate an absence of significant As–P intermixing.

In organic photovoltaic diodes, singlet intrachain excitons dissociate into geminate polaron pairs (GPP) at the heterojonction, which further dissociate into photocarriers or relax into charge transfer exciton (CTX) states. Our temperature-dependent, time-resolved spectroscopic approach unravels the dynamics of those species in films of polycarbazole/fullerene derivative blend. We find that GPP act as a dark reservoir that feeds the CTX. At low temperature, the GPP are trapped and feed the CTX via tunneling, but not the free photocarriers. At room temperature, some of the GPP can overcome the Coulomb barrier and feed the CTX promptly, or be deeply trapped and feed the CTX on several timescales. We find that at room temperature, 16% of the geminate recombination is accounted for by trapped geminate polaron pairs.

We have studied the temperature dependence (\ensuremath{\sim}10--293 K) of the photoluminescence (PL) from InAs/InP(001) quantum dot (QD) multilayers with thin spacer layers (\ensuremath{\sim}5 nm) emitting in the 0.6--0.8 eV spectral region. The QD emission redshifts less than the InAs bulk material band gap with increasing temperature. This behavior is accompanied by an important rise of the relative PL intensity of the higher-energy contributions to the spectra. The room-temperature emission is rather strong---over 20% of the low-temperature value---since deep confinement effects prevent the thermal escape of the carriers out of these relatively large QD's. In addition, the carrier transfer from the wetting layers to the QD's increases with the number of layers at low temperatures. A dot-size-dependent analysis of the carrier dynamics using a rate-equation model leads to the following interpretation of our experimental results: (i) the radiative emission intensity from thicker dots quenches at lower temperature through thermalization to excited nearly dark states, and (ii) carriers initially captured by the wetting layer are preferentially transferred to thinner QD's whose emission energy is higher than \ensuremath{\sim}0.7 eV. In multilayers, the experimental observations can be explained without involving electronic coupling between QD's of different layers, even though the distance between the vertically aligned nanostructures is small.

We present a photoluminescence study of the free-exciton singlet and triplet ground states in GaTe as a function of temperature from 5 to 40 K. Over the range 5-25 K, the temperature variation of excitonic-emission linewidth and peak energy comes mainly from exciton-acoustic-phonon interactions via the deformation potential. The contribution from exciton scattering by optical phonons of energy 14 meV begins at temperatures above ∼25 K. The exciton-optical-phonon interaction is the dominant scattering process above 30 K and effectively dissociates excitons since the energy of the optical phonon mode is close to the exciton binding energy. The luminescence thus decreases rapidly above 30 K and disappears at about 40 K

We examine the relation between surface morphology and lattice distortions of a number of GaxIn1-xP epilayers (x ≈ 0.04) grown on InP (001) substrates using atomic force microscopy (AFM) and high resolution X-ray diffractometry (HRXRD). The heteroepitaxial layers (thickness ≈ μm) were grown by low pressure metalorganic vapour phase epitaxy (MOVPE) at temperatures from 600 to 660°C. AFM images over a scale of 75 μm by 75 μm show no surface corrugations for samples grown at 640°C. For other growth temperatures between 600 and 660°C, unidirectional corrugations aligned along the [1 1 0] direction or a cross-hatched pattern can be seen on the sample surfaces with a typical interline separation of 10 μm. Another feature revealed by AFM is the presence, for some samples, of weak parallel corrugations along the direction making a 54° angle with the [1 1 0] direction. HRXRD spectra recorded first with the [110] and then with the [1 1 0] direction in the plane of incidence reveal an asymmetric relaxation in the sample plane accompanied by a broadening of the epilayer X-ray peak for diffraction with the [110] direction in the plane of incidence. The lattice distortion changes from tetragonal to orthorhombic as the surface corrugations appear along the [1 1 0] direction. Proton microprobe scans indicate that these samples are compositionally uniform so that the asymmetry is not the result of lateral variations of the alloy composition. Finally, low temperature photoluminescence from these samples shows good agreement between observed and calculated peak positions on the basis of the strains obtained from HRXRD measurements.

We have mesured the carrier recombination dynamics in InGaN/GaN multiple quantum wells over an unprecedented range in intensity. We find that at times shorter than 30\,ns, they follow an exponential form, and a power law at times longer than 1\,$\mu$s. To explain these biphasic dynamics, we propose a simple three-level model where a charge-separated state interplays with the radiative state through charge transfer following a tunneling mechanism. We show how the distribution of distances in charge-separated states controls the dynamics at long time. Our results imply that charge recombination happens on nearly-isolated clusters of localization centers.

We report low-temperature resonant Raman scattering from excitons in single-crystal ${\mathrm{TiO}}_{2}$. From the selection rules, and from the position of the maximum of the scattering efficiency with respect to luminescence and absorption peaks, the resonance is attributed to the 2${\mathit{p}}_{\mathit{x}\mathit{y}}$ exciton. These results support the symmetry band assignment of ${\mathrm{\ensuremath{\Gamma}}}_{1}$ and ${\mathrm{\ensuremath{\Gamma}}}_{3}$ to the conduction and valence bands, respectively, and confirm that the band gap of ${\mathrm{TiO}}_{2}$ is direct forbidden.

We report a low-temperature, time-resolved photoluminescence study near the band gap of TiO2 single crystals. From the decay times and polarisation dependence of the emission intensity, we show that the luminescence spectrum consists of 2px,y exciton emission followed by phonon replicas of the ls exciton state. These results indicate that TiO2 is a material with a forbidden direct band gap where, in spite of strong electron-phonon coupling and high static dielectric constant, Wannier excitons are stable.

The growth of heteroepitaxial GaxIn1−xP on InP for 0

A strained GaInAs/AlInAs saturable Bragg reflector (SBR) was fabricated from layers grown by molecular-beam epitaxy. The device consisted of nominally 25 periods of GaAs/AlAs Bragg structures incorporating double GaInAs/AlInAs quantum wells latticed matched to InP. The SBR was measured to be more than 99.5% reflective from 1410 to 1525 nm. The structural parameters for the sample were obtained from high-resolution x-ray diffraction rocking curves. The quantum wells exhibited partially relaxed interfaces where the relaxation was characterized by misfit dislocations. The photoluminescence measurements showed a lack of well-defined band edges and exciton structure at room temperature. Luminescence peaks were obtained at liquid-helium temperature, corresponding to heavy-hole and light-hole related exciton levels. The fast recovery of the SBR nonlinear response was explained by the misfit dislocations, which act as nonradiative recombination centers.

The changes in refractive index, optical absorption, and volume of synthetic fused silica resulting from the implantation of germanium and silicon ions at energies of 3 and 5 MeV are reported. Implantation changes the density and generates ultraviolet color centers in the silica, which increases the refractive index at visible wavelengths by ~1%. Irradiation of the implanted samples with 249-nm light from a KrF excimer laser photobleaches the color centers and reduces the index by more than 0.1%. Photobleaching is used to write a 4.3-microm pitch diffraction grating in the implanted silica.

We have performed detailed optical measurements of ultrathin InAs/InP quantum wells grown by metal organic vapor phase epitaxy. Photoluminescence excitation spectra reveal the excitonic resonances associated with two- and three-monolayer thick InAs layers while polarization-dependent measurements clearly show the heavy- or light-hole nature of the resonances. These resonances, together with their emission bands, can be detected on the same sample, indicating the presence of well defined regions of different InAs layer thickness. We find that the energy position of the excitonic resonances cannot be reproduced by effective mass calculations based on the envelope function approximation.

The contribution of the fundamental gap E_ as well as those of the E_ + Delta(so) and E+ transitions to the dielectric function of GaAs1-xNx alloys near the band edge were determined from variable-angle spectroscopic ellipsometry and modulated photoreflectance spectroscopy analyses. The oscillator strength of the E_ optical transition increases weakly with nitrogen incorporation. The two experimental techniques independently reveal that not only the oscillator strength of the E+ transition but also that of E_ + Delta(so) become larger compared to that of the fundamental gap as the N content increases. Since the same conduction band is involved in both the E_ transition and its split-off replica, these results reveal that adding nitrogen in GaAs1-xNx alloys affects not only the conduction but also the valence bands., Comment: 32 pages, 8 figures, 3 tables. Submitted to Physical Review B on April 18th 2008

Thin layers of black phosphorus have recently raised interest for their two-dimensional (2D) semiconducting properties, such as tunable bandgap with layer thickness and high carrier mobilities. This lamellar crystal of P atoms can be exfoliated down to monolayer 2D-phosphane (also called phosphorene) using procedures similar to that for monolayer graphene. The devices are however challenging to fabricate due to fast degradation of the thin layers upon exposure to light in air. We investigated this degradation process using in-situ Raman and transmission electron spectroscopies and reported on a thickness dependent reactivity of the layers. Moreover, the degradation process was identified to be due to an ubiquitous photo-induced oxidation of the layers by adsorbed oxygen in water. Optimum experimental conditions to prepare n-layer 2D-phosphane in their pristine states were applied to determine the Raman signatures of degradation. Here, we report on the kinetics of the photo-oxidation and provide sigantures in the Raman spectra that could be used to assess the crystal quality.

A self-starting passive mode-locked Cr4+:yttrium–aluminum–garnet laser that uses a saturable Bragg reflector with a single prism for dispersion compensation instead of the standard prism pair is demonstrated. We observed over 230 mW average output power with a pulse width of 400 fs at center wavelengths shorter than 1.5 μm. The tuning range for cw laser operation extends from 1345 to 1557 nm. The mode-locked pulses are self-starting and tunable from 1420 to 1510 nm by means of lossless prism tuning without aperture.

We present a joint experimental and theoretical study of the optical properties of $\mathrm{GaAsN}$ alloys. The ${E}_{+}$ optical transition is identified by variable-angle ellipsometry measurements. It induces an edge characteristic of a ${M}_{0}$ critical point in the imaginary part of the dielectric function of the alloys. The experimental results are modeled by ab initio calculations of the dielectric function of ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ in the nitrogen concentration range $x=0.009--0.037$. The origin of the ${E}_{+}$ transition is analyzed by calculating contributions from different conduction states of ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ to the dielectric function. Our study confirms that optical transitions from the valence band to electronic states of ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ derived from $L$ states of the conduction band of $\mathrm{GaAs}$ are at the origin of the ${E}_{+}$ edge.

By means of Raman scattering, photoluminescence, and x-ray diffraction experiments, we have studied a series of ultrathin $\mathrm{In}\mathrm{As}∕\mathrm{In}\mathrm{P}$ single quantum wells grown with nominal well thickness varying between one and four monolayers. The observed Raman peaks are attributed to the first InAs-confined longitudinal optical phonon. When the quantum well is thinner than three monolayers, the measured frequencies of the confined modes depart from the predictions of the standard confined-phonon model but are in good agreement with the results of ab initio calculations based on density-functional theory. This indicates that there is a significant penetration of the InAs layer vibrations in the InP barriers and that thickness fluctuations occur at a scale short enough to further shift the phonon frequencies. When the wells contain more than three monolayers, a doublet Raman structure is observed which parallels the evolution of the photoluminescence spectra from single- to multiple-peak emission. We associate this simultaneous change in Raman and photoluminescence spectra with the two- to three-dimensional growth-mode transition.

We have measured the near-band-gap absorption of ${\text{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ thin films with $xl0.012$. The spectra were analyzed with a model which allows a precise determination of the band gap and of the width of the optical transitions; the latter is found to increase, for $xg0.002$, well beyond what is expected in a III-V alloy. Ab initio calculations were performed within the generalized-gradient approximation with the exchange-correlation functional of Engel and Vosko [Phys. Rev. B 47, 13164 (1993)]. They reveal that the band gap depends markedly on nitrogen atomic configuration. The anomalous broadening of the intrinsic optical transitions thus gives strong evidence for configuration-induced band-gap fluctuations.

We report low temperature optical absorption measurements on GaxIn1−xP/InP (x