J.P. Roger

A simple model for the prediction of thermal conductivity in pure and doped insulating crystals

We present a simplified approach to evaluate the thermal conductivity of insulating crystals, taking into account chemical and structural properties. We complete the thermal conductivity evaluation in the case of point defects encountered in luminescent materials. Thermal conductivity coefficients have been determined by photothermal experiments and are in good agreement with the predicted values in Yb:Y3Al5O12 and Yb:Gd3Ga5O12 solid-state lasers.

Temperature field determination of InGaAsP/InP lasers by photothermal microscopy: Evidence for weak nonradiative processes at the facets

Photothermal microscopy was used for absolute temperature determination of InGaAsP/InP lasers. High modulation frequencies were employed in order to improve spatial contrast. The strong thermal mismatch between InGaAsP and InP induces a heat confinement in the active region as shown experimentally and confirmed by finite element calculations. Facet temperature was found to be 10 K/mW for a 2‐μm‐wide active layer, which is low compared to those reported for GaAlAs/GaAs and GaInP. Moreover, measurements along the cavity do not show differences in temperature when approaching the facets. Both results indicate a weak nonradiative recombination process at the facets, in agreement with the higher mirror reliability of this type of laser.

Thermal diffusivity measurement of micron-thick semiconductor films by mirage detection

Abstract We describe a method using the mirage detection technique for thermal diffusivity measurements of thin films. High frequency measurements are required for such materials. By determining the difference between two phase measurements, we have been able to avoid probe beam size effects which strongly affect the high frequency responses. The thermal diffusivity of a polycrystalline semiconductor thin film has been measured by using optical heating and compared with the corresponding single-crystal diffusivity. The thermal diffusivity of polymer semiconductor thin films has been obtained for various dopings by using electrical heating.

Microscopic thermal characterization of composites

The microscopic thermoelastic properties of matrix and fibers of composites are often not known before and after the process of preparation. To master the final properties of the composite, it is essential to characterize these components in order to supply the code with realistic material parameters. This article illustrates how the photoreflectance microscopy is able to measure the thermal properties, at a microscopic scale.

Photothermal microscopy of silicon epitaxial layer on silicon substrate with depletion region at the interface

Photothermal microscopy has been used for investigating semiconductor materials to evaluate carrier diffusivity, lifetime, and surface recombination velocity. The effects of the depletion region between epitaxial and substrate Si with different conduction types are studied. Although the observed curves are well explained by the theoretical predictions for surface reflectivity, no drastic change is observed for the different structures. This may be due to the fact that the thickness of the epitaxial layer is too large to reveal clearly the effect of the depletion region at the PN junction. However, the result for low-frequency modulation at 10 kHz may indicate this effect.

Beam size and collimation effects in spectroscopic ellipsometry of transparent films with optical thickness inhomogeneity

Abstract In spectroscopic ellipsometry (SE) measurements of thin films with inhomogeneous optical thickness, the spot size of the probe beam on the sample surface is a highly critical parameter in the treatment of the experimental data. SE measurements were performed on transparent samples prepared by various techniques: plasma enhanced chemical vapor deposition of diamond film, spin-coated PMMA films and thermally grown SiO 2 film. The first two deposition techniques can produce films with inhomogeneous optical thickness. By comparing SE measurements using different set-up configurations, we demonstrate that a combination of lenses and small diaphragms is most suitable for minimizing the effects related to the optical thickness distribution and light beam divergence. In this way mistakes are avoided in extracting physical parameters from SE optical modeling of these films.

Thermal diffusivity of amorphous semiconductor superlattice films

Abstract The thermal diffusivity of a series of amorphous semiconductor superlattice films ( a-Si:H a-SiN x : H , where a-Si:H is hydrogenated amorphous silicon and a-SiNx:H is hydrogenated amorphous SiNx) has been investigated both experimentally and theoretically. The mirage effect detection technique has been used to determine the difference between two phase measurements of photothermal signals, where the heating beam illuminates the front and the rear surface of the sample successively. Using a one-dimensional model and an approximate theoretical calculation, we have been able to determine the thermal diffusivity of the superlattices by the experimental measurement of the phase lag. These results are in good quantitative agreement with a model based on the effective medium approximation.

Columnar aggregates of crown ether substituted phthalocyanines perpendicularly anchored on a surface via a selective binding site

The purpose of this work is to anchor perpendicularly to a surface the aggregates obtained by complexation of crown-ether substituted lutetium bisphthalocyanine, [(15C5)4Pc]2Lu, in the presence of KSCN. In order to orientate these aggregates perpendicularly to the substrate, silica surface is grafted with an unsymmetrical lutetium bisphthalocyanine substituted, on one macrocycle by four crown ether subunits, and on the other one with four lateral chains terminated by a carboxylic group. The latter compound is used as a selective binding site to anchor pillar-like aggregates formed in solution. The aggregates were characterized in a mixture of chloroformic based solution by UV-visible and light scattering experiments and gave evidence for the formation of rod-like particles in the presence of excess of KSCN. Then, the aggregates were deposited on surfaces and their morphologies were studied by atomic force microscopy (AFM). In the case of substrates having non-specific binding sites such as silica and functionalized silica with 3-trimethoxy-propylaminosilane, rods were observed lying parallel on the surface. In the case of the substrate grafted with the selective binding site, single columns of these supramolecular assemblies perpendicular to the surface have been observed by AFM.

Ex-situ spectroscopic ellipsometry studies of micron thick CVD diamond films

Abstract Micron thick diamond films have been studied by spectroscopic ellipsometry (SE). The films were grown, on previously prepared Si(100) substrates, by the plasma enhanced chemical vapor deposition (PECVD) technique. Ex situ SE measurements were carried out on samples grown under different conditions, such as substrate temperature and methane fraction in the gas mixture. An optical model consisting of five layers was constructed in order to explain the SE spectra and to provide the optical and structural parameters of the films. This model was deduced from results of various measurements performed by other characterization techniques (Raman spectroscopy, scanning electron microscopy, atomic force microscopy and positron annihilation spectroscopy) which have revealed the optical and structural parameters of the samples. Its sensitivity to the surface and interface roughness as well as to the absorption of the nondiamond phase of the film is demonstrated. Several values of the percentage of the nondiamond phase can be obtained, with the same fit quality, however, depending on the amorphous carbon reference used in the model. These references were obtained by performing SE measurements on various amorphous carbon films. Finally, our SE analysis has allowed us to monitor the lateral homogeneity of the thickness, surface and interface roughness and nondiamond phase concentration over the diamond film.

Micro thermal characterization of carbon/carbon composite matrix by the photoreflectance method

Microscopic thermal characterization measurements are performed on carbon/carbon composite matrix using photoreflectance method. In our experiment, the sample is periodically heated by a laser focused by a microscope and the thermally induced variations of the refractive index are detected by the reflection of a second laser beam. A model that calculates the periodic temperature variations in an orthotropic medium allows to determine the thermal diffusivity and its anisotropy. Experimental results in terms of thermal diffusivity and thermal anisotropy are presented showing the influence of process parameters on the local thermal properties of the composite matrix.