M. Ajaka

Characterization of the state of a droplet on a micro-textured silicon wafer using ultrasound

In this work, we propose acoustic characterization as a new method to probe wetting states on a superhydrophobic surface. The analysis of the multiple reflections of a longitudinal acoustic wave from solid-liquid and solid-vapor interfaces enables to distinguish between the two well known Cassie-Baxter and Wenzel wetting configurations. The phenomenon is investigated experimentally on silicon micro-pillars superhydrophobic surfaces and numerically using a finite difference time domain method. Numerical calculations of reflection coefficients show a good agreement with experimental measurements, and the method appears as a promising alternative to optical measurement methods.

New modelling method for forward junction I-V analysis

This work presents a theoretical calculation of the I-V characteristics of an n-p junction. The total current across the n-p junction is presented as the superposition of currents due to each region of the junction. It results in ideality factor values between 1 and 2. Threshold voltage values, related to the low and high level injection in each region, are introduced. The theoretical model is used to describe the experimental emitter-base I-V characteristics of a bipolar transistor. The effects of the electrical ageing are taken into account as a creation of a defect layer in the base, near the junction. A maximum error of 6% between the theoretical model and experiment is obtained.

Activation and diffusion during rapid thermal annealing of arsenic and boron implanted silicon

The redistribution of arsenic and boron implanted respectively in single crystal and in preamorphized silicon wafers is investigated after rapid thermal annealing (RTA) at temperatures ranging from 1000 up to 1150°C. SIMS dopant profiles are compared to calculated distributions for which the diffusion equation is solved with a diffusivity depending on temperature, on impurity concentration and, eventually, on time. For both ions, an additional diffusion has to be considered. This excess diffusivity is shown to be dependent on RTA duration. Using channeling-backscattering measurements, this behaviour is clearly related to the coarsening or to the annealing of residual extended defects. Thus, the enhanced diffusion is explained by an excess point defects surviving SPE regrowth, which can be trapped or released on dislocation arrays. The electrical characteristics of junctions prepared by implantation and RTA are presented.

Carriers temperature for an operating silicon p-n junction

An analytic expression is introduced to describe the dependence of the carriers temperature with the lattice temperature. An experimental study of the temperature dependence of silicon junction characteristics leads to an experimental determination of the carriers temperature. An average error of 2.5% between the experimental values and the calculated values is obtained. The introduction of the carriers temperature is used to study the temperature dependence of the energy gap and to determine the value of the energy bandgap at 0K. The carriers temperature is efficient to use in studying optoelectronic properties of silicon devices.

Electrical and optical characteristics of NAPS solar cells of Si (PiN) structure

The commercial solar cells of NAPS of structure (Si) PiN SnO/sub 2/ having a new geometry are fabricated with a conversion efficiency more than 5%. The I/V measurements allow determination of the conduction mechanisms. The spectral response of the cells is also studied and the adaptation to terrestrial utilisation is shown.

Characterization of the state of a droplet at a micro-textured silicon wafer using a finite difference time-domain (FDTD) modeling method

In this study, we introduce a finite difference time domain method to study the propagation and reflection of an acoustic wave on smooth and micro-textured silicon surfaces in interaction with droplets in different states. This will enable numerical investigations of interfaces composed of periodically distributed well-defined pillars. One type of transducer was modeled generating longitudinal wave. Three configurations were studied: the Cassie state, the Wenzel state and a composite state for which the droplet collapsed into the middle height of the pillars. After analysis of the displacement along y direction in the silicon wafer, we were able to show that a longitudinal wave is sensitive to the detection of the state of the droplet. The first experimental results made it possible to show a good agreement between modeling and experiments.

A study of a solar cell with an implanted defect layer

This work presents a theoretical calculation of the potential barrier due to a low-high interface for low doping, enabling evaluation of the degradation of irradiated Si solar cells. The effects of the radiation environment are taken into account as a reduction of doping level and carrier lifetime. Afterwards the theoretical results are compared with those obtained by simulation. The expressions found can be used to calculate the carrier concentration in each region (n and p) of an n-p junction. But the main feature lies in modelling a defect layer implanted in the bulk of an n-p junction. Results of majority carrier density, minority carrier density, and the low-high barrier height between the defect layer (DL) and the p region base, are shown for different doping values of DL.