G. Landa

OPTICAL DETERMINATION OF STRAINS IN HETEROSTRUCTURES - GAAS/SI AS AN EXAMPLE

Raman and photoluminescence spectroscopies are used to characterize crystalline quality and interfacial strain in heterostructures. The effect of a biaxial stress on electronic and vibronic energies is reviewed and then applied to the case of a GaAs layer. Measurements on GaAs grown on Si(100) by molecular‐beam epitaxy are made over a wide temperature range (4→700 K). The evolution of the strain is deduced from the shift of both the energy‐band gaps and the long‐wavelength transverse and longitudinal‐optical‐phonon frequencies. The sensitivity of the Raman probe is dramatically enhanced by excitation under resonant conditions at the E1 edge of GaAs. The measurements confirm the anisotropy of the strain and demonstrate that both its sign and value at room temperature result from a balance between two reverse phenomena: the thermal expansion and the lack of complete relaxation of the lattice mismatch during growth.

Characterization of implantation and annealing of Zn‐implanted InP by Raman spectrometry

First‐ and second‐order Raman scattering by Zn‐implanted InP is investigated in order to determine critical fluences needed for a complete disturbance of the lattice and recovery temperature during thermal annealing. Disorder‐activated first‐order acoustical scattering and second‐order optical scattering are shown to be highly sensitive probes.

Tensile and compressive strain relief in InxGa1−xAs epilayers grown on InP probed by Raman scattering

Strain relaxation has been investigated by means of Raman scattering in strained InxGa1−xAs layers (with x ranging from 0 to 1) grown on In0.53Ga0.47As/InP (001). The epilayers are either under tensile (x 0.53) strain. Relaxation coefficients have been deduced from the frequency shifts of the GaAs-like optical phonons. A marked dissymmetry in strain relief is found over the whole composition range between equivalent tensile and compressive misfits. Disorder activated Raman scattering features have been analyzed and correlated to the structural defects resulting from the strain relief in the two and three-dimensional growth modes. Strain inhomogeneities resulting from surface corrugation are evidenced by micro-Raman measurements on layers with tensile misfits.

Raman study of longitudinal optical phonon‐plasmon coupling and disorder effects in heavily Be‐doped GaAs

Raman spectroscopy measurements have been performed on GaAs:Be samples with high crystalline quality and exceptional heavy doping level ranging from 1019 to 1.4×1021 cm−3. The recorded spectra show a structure we assigned to a coupled LO phonon‐damped plasmon mode. A theoretical expression for the Raman scattering rate by this mode has been derived from a dielectric model and compared to the experimental data. Using a fitting procedure the doping level of the samples has been estimated in agreement with Hall measurements. Moreover, the study of the Raman intensity evolution of both unscreened‐LO and coupled phonon‐plasmon structures, provided a convenient and rapid method to determine the activated carrier density in p‐doped polar semiconductors. Disorder effects due to the dopant impurities have been also observed and analyzed using a spatial correlation model description.

RAMAN CHARACTERIZATION OF TWINNING IN HETEROEPITAXIAL SEMICONDUCTOR LAYERS - GAAS/(CA,SR)F2

Detailed analysis of Raman spectra recorded from (100)‐oriented GaAs layers grown by molecular‐beam epitaxy on the lattice‐matched insulator (Ca,Sr)F2 gives evidence of internal misorientation effects (twins). This analysis accounts for the various phenomena (doping, disorder, electron‐phonon coupling) likely to modify the scattering efficiency. Calculations are performed in order to obtain quantitative evaluations of the misoriented volume amount.

Spectroscopic detection of carbon nanotube interaction with amphiphilic molecules in epoxy resin composites

Incorporation of carbon nanotubes into epoxy resin composites has the effect of increasing electrical conductivity at low percolation levels. An amphiphilic molecule such as palmitic acid has been used to increase the surface contact area and to improve the dispersion of the carbon nanotube bundles in the prepolymer. The chemical environment of the dispersed nanotubes has been probed using vibrational Raman spectroscopy. Spectroscopic Raman maps on sample surfaces (60×60μm2) with ratios of nanotubes to palmitic acid varying from 1:2 to 2:1 by weight, have been recorded to test the uniformity of the dispersion. Substantial spatial inhomogeneities have been observed in the G-band shift and an additional spectral band at 1450cm−1. The 1450cm−1 band has been attributed to the CH3 group of the amphiphilic molecules adsorbed onto the nanotube surface. The maps are correlated with the measured electrical conductivity values. The highest conductivity has been observed for the best dispersed nanotubes and nanotube...

RAMAN-STUDY UNDER RESONANT CONDITIONS OF DEFECTS NEAR THE INTERFACE IN A GAAS-SI HETEROSTRUCTURE

A Raman study has been performed, under resonant conditions, on a GaAs bevelled‐edge layer grown on a Si substrate to characterize the optical and crystalline properties of the epilayer near the interface. According to the geometrical characteristics of the sample, a theoretical expression for the Raman intensities profile has been established and compared to the experimental data. This fitting procedure enables us to investigate the absorption coefficient of the GaAs layer due to the disorder‐induced softening of the E1 edge. A quantitative analysis of the lattice disorder has been carried out on both longitudinal and transverse optical modes by studying the Raman line‐shape evolution versus the laser spot position on the bevel edge. From this study, we have followed the recovery of the crystalline quality of the epilayer while going away from the interface, and evaluated the ‘‘Raman thickness’’ of the dislocated layer. Using the spatial correlation model as a relationship between the disorder amount and...

Raman determination of the composition in semiconductor ternary solid solutions

The use of Raman spectrometry for the precise determination of the mole fraction in semiconductor solid solutions is presented. It is shown that if the frequency difference between the modes related to the two binaries involved is used, the scattering of the results due to various experimental conditions (temperature, spectrometer calibration, ...) is avoided. In addition, the method is rapid, nondestructive, reliable, and allows to probe the composition at different depths into the layer.

A computational chemist approach to gas sensors: Modeling the response of SnO2 to CO, O2, and H2O Gases

A general bottom‐up modeling strategy for gas sensor response to CO, O2, H2O, and related mixtures exposure is demonstrated. In a first stage, we present first principles calculations that aimed at giving an unprecedented review of basic chemical mechanisms taking place at the sensor surface. Then, simulations of an operating gas sensor are performed via a mesoscopic model derived from calculated density functional theory data into a set of differential equations. Significant presence of catalytic oxidation reaction is highlighted.

BEYOND THE SOLID ON SOLID MODEL : AN ATOMIC DISLOCATION FORMATION MECHANISM

We investigate the growth of mismatched thin films by a kinetic Monte Carlo computer simulation. The strain is introduced through an elastic energy term based on a valence force field approximation and stress is relaxed along “atomic chains” at each step of the simulation. The calculations use a set of elementary atomic processes including, besides well-known standard processes, the collective incorporation of atoms. This leads us to introduce a new “hanging” position with only one bond created toward the substrate contrary to solid on solid models. This position plays a role of defects initiation, and thus an atomic dislocation nucleation mechanism is described. Finally, we present the influence of a step in the dislocations creation.