Andrea Parisini

Arsenic implanted specimens after liquid phase epitaxy followed by annealing treatment were studied in the range 450° C-900° C. A detailed analysis of As and interstitial profiles reveals that arsenic diffuses, starting from 550° C, in two stages : as an interstitial-arsenic complex for 550° C ≤ T ≤ 650° C and independently of interstitials for T ≥ 750* C. It is shown that interstitials are created during the liquid to solid transformation prior to the annealing treatments.

In a previous work on As⁺-implanted and annealed silicon we reported on the discrepancy between the electrically inactive arsenic and the amount of As atoms in the precipitates, as deduced from weak-beam and HREM observations. In this paper we discuss the contrast given in computed HREM images by spherical SiAs particles of 2 nm diameter with a sphalerite structure and coherent with the Si matrix. Different specimen thicknesses and imaging modes have been employed in the simulations. It is found that in areas thicker than about 10 nm the particles are invisible.

In this work, we compare the results of different Cliff–Lorimer (Cliff & Lorimer 1975) based methods in the case of a quantitative energy dispersive spectrometry investigation of light elements in ternary C–O–Si thin films. To determine the Cliff–Lorimer (C–L) k-factors, we fabricated, by focused ion beam, a standard consisting of a wedge lamella with a truncated tip, composed of two parallel SiO₂ and 4H-SiC stripes. In 4H-SiC, it was not possible to obtain reliable k-factors from standard extrapolation methods owing to the strong C_K-photon absorption. To overcome this problem, an extrapolation method exploiting the shape of the truncated tip of the lamella is proposed herein. The k-factors thus determined, were then used in an application of the C–L quantification procedure to a defect found at the SiO₂/4H-SiC interface in the channel region of a metal-oxide field-effect-transistor device. As in this procedure, the sample thickness is required, a method to determine this quantity from the averaged and normalized scanning transmission electron microscopy intensity is also detailed. Monte Carlo simulations were used to investigate the discrepancy between experimental and theoretical k-factors and to bridge the gap between the k-factor and the Watanabe and Williams ζ-factor methods (Watanabe & Williams, 2006).

In this work, we report on the production of regular (SiGe/SiO ₂ ) ₂₀ multilayer structures by conventional RF-magnetron sputtering, at 350 °C. Transmission electron microscopy, scanning transmission electron microscopy, raman spectroscopy, and x-ray reflectometry measurements revealed that annealing at a temperature of 1000 °C leads to the formation of SiGe nanocrystals between SiO ₂ thin layers with good multilayer stability. Reducing the nominal SiGe layer thickness ( t _SiGe ) from 3.5–2 nm results in a transition from continuous SiGe crystalline layer ( t _SiGe ∼ 3.5 nm) to layers consisting of isolated nanocrystals ( t _SiGe ∼ 2 nm). Namely, in the latter case, the presence of SiGe nanocrystals ∼3–8 nm in size, is observed. Spectroscopic ellipsometry was applied to determine the evolution of the onset in the effective optical absorption, as well as the dielectric function, in SiGe multilayers as a function of the SiGe thickness. A clear blue-shift in the optical absorption is observed for t _SiGe ∼ 2 nm multilayer, as a consequence of the presence of isolated nanocrystals. Furthermore, the observed near infrared values of n = 2.8 and k = 1.5 are lower than those of bulk SiGe compounds, suggesting the presence of electronic confinement effects in the nanocrystals. The low temperature (70 K) photoluminescence measurements performed on annealed SiGe/SiO ₂ nanostructures show an emission band located between 0.7–0.9 eV associated with the development of interface states between the formed nanocrystals and surrounding amorphous matrix.

The hole transport properties of heavily doped 4H-SiC (Al) layers with Al implanted concentrations of 3 × 10²⁰ and 5 × 10²⁰ cm⁻³ and annealed in the temperature range 1950–2100 °C, have been analyzed to determine the main transport mechanisms. This study shows that the temperature dependence of the resistivity (conductivity) may be accounted for by a variable range hopping (VRH) transport into an impurity band. Depending on the concentration of the implanted impurities and the post-implantation annealing treatment, this VRH mechanism persists over different temperature ranges that may extend up to room temperature. In this framework, two different transport regimes are identified, having the characteristic of an isotropic 3D VRH and an anisotropic nearly 2D VRH. The latter conduction mechanism appears to take place in a rather thick layer (about 400 nm) that is too large to induce a confinement effect of the carrier hops. The possibility that an anisotropic transport may be induced by a structural modification of the implanted layer because of a high density of basal plane stacking faults (SF) in the implanted layers is considered. The interpretation of the conduction in the heaviest doped samples in terms of nearly 2D VRH is supported by the results of the transmission electron microscopy (TEM) investigation on one of the 5 × 10²⁰ cm⁻³ Al implanted samples of this study. In this context, the average separation between basal plane SFs, measured along the c-axis, which is orthogonal to the carrier transport during electrical characterization, appears to be in keeping with the estimated value of the optimal hopping length of the VRH theory. Conversely, no SFs are detected by TEM in a sample with an Al concentration of 1 × 10¹⁹ cm⁻³ where a 3D nearest neighbor hopping (NNH) transport is observed.

Dual emission in asymmetric “giant” PbS/CdS/CdS core/shell/shell quantum dots enables color monitoring/modulation and control in the visible/NIR region for advanced applications.