Showing papers by "Michel Kazan published in 2008"

Mechanical and Thermal Properties of Metallic and Semiconductive Nanostructures

Abstract: Using a top-down approach, we report a theoretical investigation of the melting temperature at the nanoscale, Tm, for different shapes of “free-standing” nanostructures. To easily calculate the nanoscale melting temperature for a wide range of metals and semiconductors, a convenient shape parameter called αshape is defined. Considering this parameter, we argue why smaller size effects are observed in high bulk melting temperature materials. Using Tm, a phase transition stress model is proposed to evaluate the intrinsic strain and stress during the first steps of solidification. Then, the size effect on the Thornton & Hoffman's criterion at the nanoscale is discussed and the intrinsic residual stress determination in nanostructures is found to be essential for sizes below 100 nm. Furthermore, the inverse Hall-Petch effect, for sizes below ∼15 nm, can be understood by this model. Finally, the residual strain in hexagonal zinc oxide nanowires is calculated as a function of the wire dimensions.

Role of optical phonon in Ge thermal conductivity

Abstract: We report on the contribution of optical phonon decay into acoustic phonon to the thermal conductivity [k(T)] of semiconductors. We have considered this mechanism as an acoustic phonon generation rate within the full modified Callaway theory to accurately describe experimental data of k(T) for Ge as a function of isotopic composition. The proposed model requires only one adjustable parameter for each mode. Our approach to describe k(T) in semiconductors is found to provide much better agreement with the experimental results than previously reported models, which did not account for the role of optical phonon.

Contribution of the decay of optical phonons into acoustic phonons to the thermal conductivity of AlN

Abstract: The thermal conductivity along two high-symmetry directions of several aluminum nitride (AlN) single crystals differing in their mass-fluctuation phonon-scattering parameter is described accurately in the full temperature range by taking into account the contribution of optical phonon decay into acoustic phonon. The accuracy of the proposed model shows the fundamental role of the optical phonon in the thermal conductivity of semiconductor materials.

Studies of the photonic and optical-frequency phonon properties of arrays of selectively grown GaN micropyramids

Abstract: An array of GaN micropyramids containing a near-surface InxGa1−xN∕GaN single quantum well has been fabricated using selective area epitaxial overgrowth above a patterned silica mask. The pyramid array has been studied by means of angle-resolved reflection measurements using s- and p-polarized incident light in the near- and mid-infrared optical ranges. We have found that the periodic array of flat-topped pyramids shows marked resonances in the near-infrared optical range due to resonant Bloch modes within the extraction cone and that the angular dispersion of these modes exhibits strong photonic crystal characteristics. The experimental results are in good agreement with the photonic band structure calculated using a scattering matrix formalism. The mid-infrared optical anisotropy properties of the micropyramids were investigated to probe the infrared active phonons of the pyramid array. The A1(LO) phonon of the InxGa1−xN∕GaN single quantum well was identified and the InN mole fraction was estimated from ...

Effect of introducing gettering sites and subsequent Au diffusion on the thermal conductivity and the free carrier concentration in n-type 4H-SiC

Abstract: We report on the application of introducing gettering sites by helium implantation prior to an annealing and subsequent gold diffusion as an approach to control the thermal conductivity and the charge carrier concentration in n-type 4H-SiC. Rutherford backscattering spectroscopy showed high diffusion of gold impurities to the introduced gettering sites which implied a success in introducing gettering sites. Data obtained from the measurements of the specific heat, thermal diffusivity, and room temperature density were used to deduce the thermal conductivity of the samples investigated. The thermal conductivity modeling showed that introducing gettering sites can increase the thermal conductivity in n-type 4H-SiC due to the reduction of phonon-impurity scattering. Raman measurements showed the presence of the desired defects introduted by ion implantation at 20°C. The analysis of the Fourier transform infrared reflectivity showed that the gettering sites can act as majority carrier traps and reduce the rec...

Surface morphology of Ge‐modified 3C‐SiC/Si films

Abstract: The influence of Ge deposition prior to carbon interaction with 3° off-axis Si(111) substrates on the structural and morphological properties of the formed silicon carbide (SiC) layer is studied. In situ reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD) revealed the formation of the cubic silicon carbide (3C-SiC) modification. In situ spectroscopic ellipsometry measurements revealed a decreasing 3C-SiC thickness with increasing Ge predeposition. Atomic force microscopy (AFM) studies revealed that the surface overlayer morphology is mainly formed by periodic step arrangements whose relevant geometric parameters, i.e. lateral separation, height and terrace width, depend on the Ge content. Besides the changes of the step morphology, the surface roughness and the grain size and the strain of the formed 3C-SiC decreases with increasing germanium precoverage. Copyright © 2008 John Wiley & Sons, Ltd.

Improvement of the Thermal Conductivity in 4H-SiC Epitaxial Layer by Introducing Gettering Sites

Abstract: Metal impurities are known to degrade dramatically the performances of silicon-based devices, even at concentrations as low as 1012 cm-3. A specific process, named proximity gettering, has been optimised by some authors in order to reduce the influence of these impurities [1]. This process consists in the building of a favourable impurity trapping zone in a non-active area of the device, by introducing implantation defects. This paper reports on the application of introducing such gettering sites as an approach to control phonon properties in 4H-SiC epilayer, and increase the thermal conductivity.

SiC Polytype Stability Influenced by Ge Impurities

Abstract: In this paper we present a methodology to affect the stability of polytypes formation during heteroepitaxial growth of SiC on Si. This methodology is based on the investigation of growth related parameters. These parameters involve substrate temperature, effect of impurities on surface diffusion, strain, and super-saturation conditions as solved by using SSMBE growth (Solid Source molecular beam epitaxy).