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The optical properties of macroscopic materials are described, basing only on classical arguments. Starting from Maxwell’s equations, the propagation of electromagnetic waves in isotropic, linear, and homogeneous materials is treated. To this aim, complex macroscopic quantities as the dielectric function, the optical conductivity, and the refractive index are introduced, together with the dispersion relations that they obey. The reflection and transmission properties at the interface between different materials are analyzed, and the appropriate functions are defined for both unpolarized and polarized light waves, with the addition of the ellipsometric functions that are particularly interesting from the application point of view.

The FA(1-x)MA(x)PbI(3) solid solution has been carefully investigated in terms of the MA/FA stoichiometry, crystal structure, and optical properties. This work allowed for the determination of reliable correlations between the amount of protonated amine and the physicochemical properties. The deleterious effect of aging on the mixed MA/FA. composition was observed by diffraction and optical measurements, showing progressive phase separation within the samples.

The GaAs-like longitudinal-optical (LO) phonon frequency in hydrogenated GaAsN (x = 0.01) layers - with different H doses and similar low-energy irradiation conditions - was investigated by micro-Raman measurements in different scattering geometries and compared with those of epitaxial GaAs and as-grown GaAsN reference samples. A relaxation of the GaAs selection rules was observed, to be explained mainly on the basis of the biaxial strain affecting the layers. The evolution of the LO phonon frequency with increasing hydrogen dose was found to heavily depend on light polarization, thus suggesting that a linear relation between strain and the frequency of the GaAs-like LO phonon mode should be applied with some caution. Moreover, photoreflectance measurements in fully passivated samples of identical N concentration show that the blueshift of the GaAs-like LO frequency, characteristic of the hydrogenated structures, is dose-dependent and strictly related to the strain induced by the specific type of the dominant N-H complexes. A comparison of photoreflectance results with the finite element method calculations confirms that this dependence on the H dose is due to the gradual replacement of the N-2H complexes responsible for the electronic passivation of N with N-3H complexes, which are well known to induce an additional and sizeable lattice expansion.

In this paper we report the synthesis, the crystal structure and the optical response of APbX3 (A=MA, DMA, and TMA; X=I, Br) hybrid organic-inorganic materials including some new phases. We observe that as the cation group increases in size, the optical absorption edge shifts to higher energies with energy steps which are systematic and independent on the anion. A linear correlation between the optical bad gap and the tolerance factor has been shown for the series of samples investigated.

We study surface-enhanced Raman scattering (SERS) in fully dielectric periodic structures supporting Bloch surface waves. We demonstrate a lower bound for the SERS enhancement of 50 when the optical pump is resonantly coupled to the Bloch surface wave supported by the structure. A corresponding photoluminescence experiment shows an emission enhancement of more than two orders of magnitude, suggesting that the actual SERS enhancement is ∼200. Analogous conclusions are obtained by means of a semiclassical theoretical model. These results suggest an alternative to plasmonic materials to enhance Raman scattering at a surface and are of interest to those working in the field of Raman as well as fluorescence spectroscopy.

APbX3 (A: MeNH3+, Me2NH2+, Me3NH+; X: I, Br) hybrid organic-inorganic compounds are prepared by dissolving appropriate amounts of Pb(O-Ac)2 in excess HX (100 °C) followed by the addition of the respective amine (25—40% in H2O) in stoichiometric quantities and subsequent cooling to 46 °C at a rate of 1 °C/min.

Energy harvesting from human motion is a research field under rapid development. In this tutorial review we address the main physical and physico-chemical processes which can lead to energy generation, including electromagnetism, piezoelectricity, and electrostatic generation. Emphasis is put on the relationships among material properties and device efficiency. Some new and relatively less known approaches, such as triboelectric nanogeneration (TENG) and reverse electrowetting (REWOD), are reported in more detail.

We report the synthesis and characterization of a MASnxPb1-xBr3 (MA = methylammonium; nominal x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1) solid solution. The original synthetic method developed allowed one to obtain single-phase materials with homogeneous Sn/Pb distribution. All of the samples prepared are cubic, and the unit cell linearly decreases with increasing x value. The optical response indicates a linear trend (Vegard's law) of the band gap with increasing Sn content from 2.20 eV (x = 0) to 1.33 eV (x = 1), thus extending light absorption into the near-IR.

We report the synthesis and characterization of a MASnxPb1-xBr3 (MA = methylammonium; nominal x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1) solid solution. The original synthetic method developed allowed one to obtain single-phase materials with homogeneous Sn/Pb distribution. All of the samples prepared are cubic, and the unit cell linearly decreases with increasing x value. The optical response indicates a linear trend (Vegard's law) of the band gap with increasing Sn content from 2.20 eV (x = 0) to 1.33 eV (x = 1), thus extending light absorption into the near-IR.

The dispersion of photonic modes in one-dimensional (1-D) and two-dimensional (2-D) patterned silicon-on-insulator (SOI) waveguides, also containing line defects, is fully investigated both above and below the light line. Quasi-guided (radiative), as well as truly guided modes are probed by means of angle- and polarization-resolved microreflectance and attenuated total reflectance measurements. For the 1-D case, the sharp resonances observed in reflectance spectra are analyzed in terms of the Airy-Fano model, and the measured linewidths are shown to be very close to theoretical predictions. In the 2-D lattices containing W1 line defects the presence of a supercell repetition leads to the simultaneous excitation of defect and bulk modes which are folded in a reduced Brillouin zone. The measured dispersion is in very good agreement with full three-dimensional calculations based on expansion on the waveguide modes, indicating that a deep understanding of the propagation properties of patterned SOI waveguides is achieved.

Recent experimental and theoretical work on two-dimensional (2D) and waveguide-embedded photonic crystals is reviewed. The investigated systems are 2D macroporous silicon and photonic crystal slabs based on silicon-on-insulator as well as GaAs/AlGaAs. In all these structures, reflectance at varying angles of incidence allows to determine the dispersion of photonic modes above the light line. For macroporous silicon, reflectance from the side yields a complementary measurement of the photonic gaps. In the GaAs-based system, second-harmonic generation in reflection shows a resonant enhancement when the pump beam is frequency- and momentum-matched to a photonic mode in the slab. A theory of photonic states in waveguide-embedded photonic crystals leads to a determination of mode dispersion and diffraction losses for leaky photonic modes.

Second-harmonic (SH) reflection and diffraction measurements are performed, at the wavelengths of a Ti:sapphire laser on a GaAs/AlGaAs photonic crystal waveguide patterned with a square lattice, the basis consisting of rings of air in the dielectric matrix. The measured angles of diffracted SH beams agree with those predicted from nonlinear diffraction conditions. Results for reflected and diffracted SH intensities as a function of incidence angle, polarization, and pump wavelength show that, due to the low air fraction of the photonic crystal, the reflected one is dominated by the crystalline symmetry of GaAs, while the diffracted one is related to the photonic crystal structure. The large diffraction-to-reflection ratio points to the importance of nonlinear diffraction in photonic crystals. Preliminary measurements in the 1500nm range reveal explicit features related to photonic modes.

One-dimensional photonic crystals made of (Si/SiO2)m multilayers with m=2,…8 have been grown on SiO2 4-in. wafers by repeated polysilicon low-pressure chemical vapor deposition, oxidation, and wet etching steps. The poly-Si and SiO2 layers were about 220 and 660 nm thick, respectively, thus realizing λ/4 distributed Bragg reflectors. Spectroscopic ellipsometry in the 1.4–5 eV range was used to determine the dielectric function of poly-Si and the actual layer thicknesses, as well as to check the structural and compositional homogeneity of the structures. In order to measure the photonic crystal properties, specular reflectance and transmittance measurements were performed from 0.2 to 6 eV at different angles of incidence θ⩽50° and for transverse electric and transverse magnetic polarizations. The stop-bands characteristic of Bragg reflector multilayers appear up to the fifth order and become more pronounced with increasing m, reaching almost complete rejection for m=4 periods. The experimental spectra were...

In0.49Ga0.51P films, both undoped and doped n- and p-type (up to 1018 cm-3), were grown lattice matched on GaAs substrates, with different miscut angles, by Metal-Organic Vapour Phase Epitaxy (MOVPE) at different temperatures. The shift of the fundamental gap E0, caused by “ordering effect” was measured as a function of temperature by photoluminescence. The complex refractive index = n + ik and the dielectric function = ɛ 1 + iɛ 2 at room temperature were determined from 0.01 to 5.5 eV by using complementary data from fast-Fourier-transform far-infrared (FFT-FIR), dispersive, and ellipsometric spectroscopies. The effect of the native oxide was accounted for and the self-consistency of the optical functions was checked in the framework of the Kramers-Kronig causality relations. In the restrahlen region the dielectric function was well fitted by classical Lorentz oscillators; in the transparent region below E0, the refractive index was modelled by a Sellmeier dispersion relation; in the interband region the dielectric function was well reproduced by analytical lineshapes associated to seven critical points. Thus parametrized analytical expressions were obtained for the optical functions all over the spectral range, without discontinuities, to be used in the modelling and characterization of multi-layer structures, also on opaque substrates.

We report on the strong coupling between the Bloch surface wave supported by an inorganic multilayer structure and J-aggregate excitons in an organic semiconductor. The dispersion curves of the resulting polariton modes are investigated by means of angle-resolved attenuated total reflectance, as well as photoluminescence experiments. The measured Rabi splitting is 290 meV. These results are in good agreement with those obtained from our theoretical model.

The realization of long-wavelength normal-incidence photodetectors relying on interconduction subband absorption in GaSb-based multi-quantum well (MQW) systems has been proven to be possible. High efficiencies can be achieved based on the size-induced Γ -to-L subband crossover in GaSb wells with an electron spill-over from the Γ to the ellipsoidal L conduction subbands. A systematic study of the intersubband absorption in n-GaSb/AlGaSb MQWs as a function of the n-doping and temperature is presented. In particular, medium-infrared transmittance measurements at normal-incidence evidenced sharp bands, corresponding to a peak fractional absorption per well which is higher than that reported in literature on similar systems.

GaAs Schottky diodes, made on semi-insulating liquid encapsulated Czochralski grown material with concentrations of acceptor dopants \(\) varying from 1014 to 1017 cm-3, were investigated as alpha particle detectors. The charge collection efficiency (CCE) was found to decrease dramatically with increasing \(\). Optical spectra in transmittance and reflectance were accurately measured to determine the concentrations of both neutral and ionised EL2 defects as a function of \(\). The concentration of ionised EL2+ centres was shown to increase with \(\), and to be quasi inversely proportional to the CCE values. This behaviour strongly supports the hypothesis that the EL2 defects play the main role in the compensation of the material and in limitation of the detection properties.

Reflectance measurements from 50 to 5000 cm −1 were made at room temperature on several GaSb layers grown by molecular beam epitaxy on GaAs substrates, and n -doped from ∼10 16 to more than 10 18 cm −3 . A two-layer model, joined with Drude-Lorentz dielectric functions accounting for phonons and plasmons, was applied to calculate the reflectance spectra, which fitted the experimental data very well. With the optical parameters extracted it was possible to obtain the carrier concentration and mobility to be compared with the Hall measurements. Both the optical and transport analysis require a two-valley conduction model, because of the presence in GaSb of Γ and L conduction valleys, separated by only 80 meV. Populations and free-carrier scattering times in each valley were derived as a function of the phenomenological plasma frequency. Some discrepancies between optical and transport results were found and are discussed.

We studied the optical properties of high quality GaSb layers, grown by molecular beam epitaxy, in the region of the fundamental gap E0 using thermoreflectance spectroscopy in the temperature range between 80 and 300 K. The experimental line-shapes were analyzed with a functional form model including excitonic effects. Taking advantage of the derivative-like nature of the thermoreflectance spectroscopy, an accurate determination of the temperature dependence of the energy gap E0(T) is obtained, which is well reproduced by the semi-empirical Varshni relation.

In this paper we show that the orthorhombic phase of ${\mathrm{FeSi}}_{2}$ (stable at room temperature) displays a sizable anisotropy in the infrared spectra, with minor effects in the Raman data too. This fact is not trivial at all, since the crystal structure corresponds to a moderate distortion of the fluorite symmetry. Our analysis is carried out on small single crystals grown by flux transport, through polarization-resolved far-infrared reflectivity and Raman measurements. Their interpretation has been obtained by means of the simulated spectra with tight-binding molecular dynamics.

In this work we study the Surface Enhanced Raman Scattering (SERS) via Bloch Surface Waves (BSWs) in a dielectric multi-layered structure. When the pump beam is resonantly coupled to the BSW, our results demonstrate a lower bound for the SERS-enhancement of about 50. A photo-luminescence (PL) experiment performed on the same structure shows an emission enhancement in excess of two orders of magnitude, suggesting an actual SERS-enhancement of 200, in agreement with theoretical calculations. These findings suggest that purely dielectric BSW-based structures can be a valid alternative to plasmonic substrates for enhanced Raman and photo-luminescence spectroscopy.

We accurately measured the complex refractive index and dielectric functions from 0.2 to 5 eV of high-quality β-FeSi2 epitaxial films, with thicknesses ranging from 100 A to 8000 A, grown on Si(001) and Si(111) using two different techniques. Reflectance, transmittance and spectroscopic ellipsometry were used and the spectra were analyzed within a multilayer model, by checking the Kramers-Kronig consistency of the derived optical functions. These functions, compared with previous results for polycrystalline samples, showed a very good agreement with those obtained for bulk samples and a significant difference to films. The nature (direct or indirect) of the lowest optical gap and the effects of the film thickness and the substrate orientation on the optical response were also investigated.

Photoreflectance (PR) at different temperatures and spectroellipsometry (SE) at room temperature were used to study, in a systematic and complementary way, the optical response of a series of strained and relaxed InxGa1−xAs (x

A weak but clear optical structure was detected at 329 cm−1 by both reflectance and transmittance measurements in the far infrared on a 430 A film of CoSi2 grown on Si(100). This is the first observation of the IR vibrational mode of the cubic structure of CoSi2 and the result is in very good agreement with theoretical calculations. In order to characterize the sample, the reflectance was extended up to 5.2 × 104 cm−1 and the refractive index was also directly obtained in a more limited spectral range by spectroscopic ellipsometry. The IR structure was then quantitatively analyzed by means of a fit procedure, obtaining the values of ω0 = 327 cm−1 for the phonon energy, of γ = 10.5 cm−1 for the damping parameter and of ∼ 0.006 electronic charges for the screened effective ionic charge.

Two sets of GaInAs/InP lattice-matched superlattices containing 30 periods, with a wide range of barrier and well thicknesses, were grown on (100) InP substrates by metal organic vapour phase epitaxy, using growth interruptions for interface optimization. Structural, compositional and optical characterization was performed by transmission electron macroscopy, high-resolution X-ray diffraction and spectroscopic ellipsometry. The results from these complementary techniques agree quantitatively and show the good crystal quality of the samples. The interfaces in the long-period structures appear flat and sharp, while in the short-period ones they show undulations and graded-composition transition layers. A possible explanation for these effects is proposed.

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.

We used far-IR reflectance spectra to characterize a series of Ga 0.53 In 0.47 As/InP multiple quantum well and superlattices grown by metal-organic vapour phase epitaxy on InP:S substrates, with well widths ranging from 10 to 92 A and barriers from 7 to 135 A. We obtained good fitting to the experimental spectra by taking into account the complex dielectric functions of the quantum structures, in the effective-medium approximation, and using the transfer matrix formalism. The thicknesses of wells and barriers were determined from the fit, and the values of energies and broadening of phonon modes.

Photoreflectance (PR) and optical absorption (OA) have been used to study the energy-gap dependence on temperature in ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As thin layers, metalorganic vapor-phase epitaxy grown on InP substrates under nearly lattice-matched conditions (\ensuremath{\Vert}\ensuremath{\Delta}a/${\mathit{a}}_{0}$\ensuremath{\Vert}\ensuremath{\le}${10}^{\mathrm{\ensuremath{-}}3}$). Both PR and OA spectra show a split structure in the band-gap energy region. Moreover, the energy-gap values at all temperatures, obtained by Franz-Keldysh-oscillations analysis of PR line shapes, are blueshifted with respect to the values of the perfectly lattice-matched alloy (x=0.472). The results are related to the valence-band splitting at k=0 generated by the biaxial strain in ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As, due to the small lattice mismatch. We calculated the splitting and shift values using the orbital-strain Hamiltonian and accounting for compositional effects. The values are in agreement with the experimental results giving optical evidence of coherent growth of the alloy film.

The optical properties of C 60 fcc single-crystal are investigated for the first time in the 1.5–5 eV energy range by spectroscopic ellipsometry. The good quality of the samples allow a careful determination of the optical band gap at 1.72 eV. Optical and dielectric functions are directly obtained from the experimental data and compared with the results for isolated clusters and for thin films. The contribution of the optical transitions forbidden in the C 60 cluster is discussed. Experimental findings are compared with available theoretical calculations.

The optical properties of C60 thin films are investigated in the energy range 0.4 to 32 eV using reflectance, transmittance, and ellipsometric spectroscopies. The complex dielectric constant and the associated functions are determined by a combination of photometric and ellipsometric data and by Kramers-Kronig transformations. Spectral features above the fundamental absorption edge (at ≈ 1.8eV) are related to both one-electron interband transitions and collective electron effects. A comparison is made with recent results of electron energy loss spectroscopy and of theoretical calculations on the f.c.c. fullerite structure. Die optischen Eigenschaften dunner Schichten von C60 werden im Energiebereich zwischen 0.4 und 32 eV mittels Reflexions- und Transmissionsspektroskopie und spektroskopischer Ellipsomctrie untersucht. Die Ergebnisse werden zusammengestellt und mit den K ramers-Kronig Beziehungen verknupft, um die komplexen dielektrischen Funktionen (und die anderen verbundenen Funktionen) berechnen zu konnen. Die Spektraleigenschaften oberhalb der fundamentalen Absorptionskante (um ≈ 1.8 eV) werden sowohl mit den Interbandubergangen als auch mit den kollektiven elektronischen Resonanzen in Beziehung gebracht. Die Daten werden mit neuen veroffentlichten Angaben aus der “energy loss”-Spektroskopie und mit theoretischen Bandstrukturberechnungen des f.c.c. C60 Kristalls verglichen.

We measured room‐temperature reflectivity in the far‐infrared region (100–700 cm−1) of In1−xGaxAsyP1−y films grown by metalorganic phase epitaxy on InP substrate. The actual As content of the quaternary alloy (ranging from y=0.29 to y=1) was derived by combining photoluminescence and high‐resolution x‐ray diffraction results. Over the whole compositional range four different vibrational branches, corresponding to InAs‐like, GaAs‐like, InP‐like, and GaP‐like mode, have been observed, thus confirming the attribution of ‘‘four‐mode behavior’’ to the quaternary alloy. The frequency position of the four main peaks has been calculated within a valence‐force‐field model. The frequency shift of these peaks in the alloy with respect to the bulk values has been explained taking into account anharmonicity and strain effects by means of a simplified elastic model.

A detailed study of SiH stretching in porous silicon was performed based on the induction model. Good agreement with experimental infrared absorption spectra was achieved considering the oxygen nearest-neighbors and next nearest-neighbors. Carbon presence in the samples was detected and its influence on SiH stretching was determined.

Using a photonic crystal cavity and a hydrogen plasma treatment, we enhance the photoluminescence (PL) from optically active defects in silicon by a factor of 3000 compared to the as-bought material at room temperature.

Second- and third-harmonic generation in silicon photonic crystal nanocavities using a low-power, continuous wave laser at telecommunication wavelengths is demonstrated. This is achieved by employing cavities that are optimized for high quality factor and efficient coupling to the incoming beam in the far field.

High-Q photonic crystal (PhC) nanocavities [1] have become very popular in recent years, as they offer a unique way to enhance radiation-matter interaction at a nanometer scale. In particular, these systems allow to achieve strong enhancement of the optical nonlinearities such as harmonics generation [2,3], thus yielding the exciting possibility to extend the coherent emission of telecom lasers to shorter wavelengths in the visible and near ultraviolet spectral windows. Even though such high-Q PhC cavities are very well suited for in-plane applications on photonic chips, a major issue might be represented by their off-plane radiation pattern, which makes vertical in- and out-coupling difficult. In this work, we investigate the possibility to modify the far-field radiation pattern of different high-Q PhC nanocavities, to achieve a simultaneously high quality factor and coupling efficiency to an external laser beam in a vertical-coupling configuration. As a very promising application of this concept, we then exploit the far-field optimized silicon PhC nanocavities [4] to obtain a simultaneous low-power continuous-wave harmonic generation in the visible spectrum.

The effect of hydrogen irradiation on the optical properties of GaAs1xNx/GaAs heterostructures was investigated using photoreflectance and reflectance techniques. Systematic measurements performed on both as-grown and hydrogenated samples for N-concentrations ranging from 0.0% to 3.5% and for H-implanted doses from 31018 to 61018 ions/cm2 have shown that (a) the H-induced widening of the energy gap is accompanied by a decrease of the refractive index of the H-treated samples with respect to the as-grown ones, resulting in an index mismatch that can be as large as 2% in the subgap spectral region; and (b) the presence of compressive strain in fully passivated GaAsN determines a decrease of the refractive index even below that of GaAs that can be eliminated via moderate thermal annealing. These findings are promising for the development of heterostructures with planar geometry, in which the simultaneous confinement of both carriers and photons, even on a nanometric scale, can be obtained in a single step process.

We show visible frequency conversion by simultaneous second- and third-harmonic generation in a silicon photonic crystal nanocavity optimized for efficient far-field in-coupling of a low-power continuous-wave optical excitation at 1.5 microns.

Using Photonic crystal nanocavities, we first dramatically enhance third harmonic generation from silicon. Then, by virtue of a strong Purcell factor, we significantly increase defect state photoluminescence and greatly suppress thermal quenching.

We report on Raman scattering experiments on InAs/Al(x)Ga(1-x)As quantum dot heterostructures with 0 0, we detected several lines originating from the Al(x)Ga(1-x)As alloy. These can be related to scattering from GaAs-like and AlAs-like phonons with q congruent to 0, and weaker scattering from disorder-activated phonons with q not equal 0. In particular, we identified a line at similar to 250 cm(-1) as due to disorder-activated longitudinal optical phonons in the alloy. This conclusion is different than the attribution of this line to scattering from dots and, consequently, we do not recognize the possibility of deriving any information about the actual composition of the dots from an analysis of this line as proposed by other authors.

Reflectance and transmittance spectra of p‐type InP:Zn samples were measured by Fourier transform infrared spectroscopy (FTIR) in the spectral range from 40 to 700 cm−1. Zn was diffused into InP by an open‐tube method, and a subsequent short annealing at different temperatures for the electrical activation of Zn diffused layers was performed. Free‐carrier effects on vibrational structures around the restrahlen peak were evidenced. Concentration of electrically active Zn and free‐hole damping constant were obtained by fitting reflectance spectra with a classical Drude–Lorentz dielectric function. The results confirmed the model for the electrical activation of the samples, based on outdiffusion of interstitial Zn by thermal annealing.

Electrical transport and optical properties of transition-metal silicides are reviewed. They are integrated with thermal properties of single-crystal silicides. Most of these compounds behave as metals while some of them behave as semiconductors. The former show an increasing electrical resistivity ρ with increasing temperature. Several of them show a non-classical deviation of ρ(T) from linearity in the high-temperature limit. This deviation, related to intrinsic properties of the compound, can be affected both in sign and in amount by the presence of foreign atoms (impurities) and structural defects. Moreover, defects dominate the electrical transport at low temperatures both in metallic and semiconducting compounds. Therefore, the interpretation of the electrical properties measured as a function of temperature may give a non-realistic description of silicide intrinsic properties. Since also other physical properties, like thermal and optical ones, can be strongly affected by impurities and defects, results about single-crystal silicides will be first illustrated. Single-crystal preparation and structural characterization are described in detail, with emphasis on crystalline quality in terms of residual resistivity ratio. The electrical quantities, resistivity and magnetoresistance, are measured as a function of temperature and along the main crystallographic directions. The effect of impurities and defects on the transport properties is then evaluated by examining the electrical transport of polycrystalline thin-film silicides. The different contributions to the total resistivity are measured by changing: (i) film stoichiometry, (ii) impurity concentration, (iii) texture growth and (iv) film thickness. Hall-coefficient measurements are briefly discussed with the main purpose to evidence that great caution is necessary when deducing mobility and charge-carrier density values from these data. The theoretical models currently used to interpret the low- and high-temperature resistivity behavior of the metallic silicides are presented and used to fit the experimental resistivity curves. The results of these studies reveal that in several cases there are well-defined temperature ranges in which a specific electron—phonon scattering mechanism dominates. This allows a more detailed study of the microscopic processes. The optical functions from the far-infrared to the vacuum ultraviolet, derived from Kramers—Kronig analysis of reflectance spectra or directly measured by spectroscopic ellipsometry, are presented and discussed for some significant metallic disilicides, both single crystals and polycrystalline films. Different physical phenomena are distinguished in the spectra: intraband transitions at the lowest photon energies, interband transitions at higher energies, and collective oscillations. In particular, the free-carrier response derived from this analysis is compared with the transport results. The interpretation of the experimental spectra is based on the calculated electronic structures or optical functions. Moreover, it is shown how the optical studies contribute to assess definitively the semiconducting character of some disilicides. Specific-heat measurements on single crystals between 0.1 and 8 K are reported. The Debye temperature and the density of electronics states at the Fermi surface are deduced from the lattice and electronic contributions, respectively. Some silicides have been found superconductors with small electron—phonon coupling constants. Emphasis is given to the comparison between the properties deduced from these studies and those obtained from the analysis of electrical transport data. The final part of this review is devoted to the calculation of some microscopic physical quantities, as for example the electron mean free path, the charge-carrier density, the Fermi velocity. The parameters of the best fit to the experimental resistivity curves, the free-carrier parameters obtained from infrared spectra and the density of electronic states at the Fermi surface determined from specific-heat measurements were used in such evaluations.

In this communication we report on experimental results obtained from the combined use of ellipsometry and photoreflectance to investigate the optical response of typical MQW structures. The motivation for this approach is related to the need for a correct analysis of the lineshape of photoreflectance spectra. Ellipsometric data yield two important contributions about the nature and the degree of localization of the spectral structures in the excitonic region: (a) experimental curves instead of model profiles as starting reflectivity functions for derivations, (b) possible values of oscillator strengths of the transitions investigated. The energies of the main spectral features are compared with the quantized electronic levels obtained in the framework of the effective mass approximation in the envelope function scheme and a discussion of data previously reported in the literature is performed with the help of results obtained by the coupled techniques.

We present the first demonstration of frequency conversion by simultaneous second- and third-harmonic generation in a silicon photonic crystal nanocavity using continuous-wave optical excitation. We observe a bright dual wavelength emission in the blue/green (450-525 nm) and red (675-790 nm) visible windows with pump powers as low as few microwatts in the telecom bands, with conversion efficiencies of ∼ 10 (-5) /W and ∼ 10/ W(2) for the second- and third-harmonic, respectively. Scaling behaviors as a function of pump power and cavity quality-factor are demonstrated for both second- and third order processes. Successful comparison of measured and calculated emission patterns indicates that third-harmonic is a bulk effect while second-harmonic is a surface-related effect at the sidewall holes boundaries. Our results are promising for obtaining practical low-power, continuous-wave and widely tunable multiple harmonic generation on a silicon chip.

WSi2 polycrystalline films of different thicknesses were prepared by low pressure chemical vapor deposition on silicon wafers, and their crystallization properties were studied as a function of the annealing temperature. Structural measurements were performed by X-ray diffraction, detailing for the first time the phase transition from the amorphous to the hexagonal structure at an annealing temperature 380° C and from hexagonal to tetragonal above 700° C. The electrical sheet resistance showed the same transition temperatures.

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.

We performed optical reflectivity measurements on the whole spectral range from 0.01 up to 6 eV and ellipsometric spectral measurements in the range 1.4–5 eV on the three isoelectronic and isocrystalline metal disilicides VSi 2 , NbSi 2 and TaSi 2 . We analyzed the optical response in order to obtain the dielectric functions and we correlated the results with the available information on the electronic states. Both intraband and interband properties have been discussed and the numerical values of the plasma frequency, the scattering time and the energy of interband transitions were reported. Evidence is given to some general trends for the electronic properties of disilicides in passing from 3d to 5d metals.

HfSi2 polycrystalline thin films, grown by coevaporation of Hf and Si and subsequently annealed at 850 °C, were studied by electrical resistivity measurements (from 10 to 900 K), Hall voltage (from 10 to 300 K), and optical reflectance (at room temperature) from 5 meV to 12 eV. Composition and structure of the films were investigated by Rutherford backscattering spectroscopy and x‐ray diffraction. HfSi2 is metallic with (i) a high residual resistivity, (ii) a phonon contribution to the resistivity showing a negative deviation from linearity, and (iii) low‐energy interband transitions. Transport measurements yielded a Debye temperature of 430 K, a free‐carrier concentration of ∼4×1021 cm−3, and a mean free path of 139 A. The reflectivity was Kramers–Kronig transformed to obtain the dielectric functions which, at low energies, are discussed in term of the Drude model. The optical parameters agree quite well with transport results, thus permitting one to obtain a reasonable value for the Fermi velocity.