Roberto Gamboa

Linear electric molten zone in semiconductors

This letter describes how the temperature dependence of the electrical conductivity in semiconductors may be used to produce a linear floating molten zone which is intrinsically stable and uniform along its length. An analytical model and an experimental demonstration of such electric molten zone are both presented. This effect may be of particular interest for crystal growth and semiconductor recrystallization.

Optical absorption coefficient of polycrystalline silicon with very high oxygen content

Abstract We report measurements of the room temperature optical absorption coefficient α(λ) of polycrystalline silicon ribbon material with high oxygen concentration (1.7 × 1018 at. cm−3) in the spectral range from 800 to 1200 nm. Between 800 and 1000 nm, absolute error is estimated as ≈2%. Sample to sample variations in this range are ±2%, and differences from Czochralski control samples are also within 2%. Within the experimental error, the α(λ) for our ribbon samples is indistinguishable from that of monocrystalline silicon between 800 and 1000 nm. Our data are well fitted in this region by α(λ) = (85.6/λ − 77.7)2, with λ in μm and α in cm−1. Although the experimental error rises sharply at longer wavelengths due to maximum ribbon thickness of only 450 μm, we can state that our results are consistent with the data compiled by Green [1] between 800 and 1200 nm for pure monocrystalline silicon.

Silicon tubes by a closed molten zone: a characterisation study

We report on the characterisation of silicon tubes recrystallised by closed molten zone, a technique developed as a step to a possible process for thin silicon sheet production. The tube faces are quite flat and have a smooth surface. For the electrical characterisation, samples were cut from the tube faces and simple photovoltaic solar cells were formed. The average diffusion length of minority carriers was found, from spectral response, to be around 100 μm. Low-resolution LBIC measurements showed lower diffusion lengths (around 40 μm) in the regions close to the tube edges. This behaviour was correlated to measurements of residual thermal stresses using infrared photoelasticity. Measurements of the changes of spectral response with increasing bias light intensity reveal an increase in the effective diffusion length, a known effect that is interpreted in terms of a density of trapping states.

Patterned surfaces in p-type silicon by photodefined etching

We present a simple technique for photodefined etching of p-type silicon. The technique involves wet photoselective corrosion of silicon by an aqueous HF solution containing a strong oxidising agent (bromine). The etching process can be largely inhibited by light, allowing the shaping of photodefined 3-D structures on the surface of p-type silicon samples. These structures are not dependent on crystalline orientation, which is an important advantage in polycrystalline silicon texturing. The fundamentals behind the process, involving electrochemistry of semiconductors, are presented along with a discussion of the process parameters. The process was studied both in open circuit conditions, which have the advantage of requiring no electrical contacts, and with an imposed current, which should allow better etching contrast at any illuminated to dark area ratios. The results clearly demonstrate that it is possible to obtain effective etching inhibition in illuminated regions of the semiconductor, while high etch rates are retained in dark areas.

MarinEye — A tool for marine monitoring

This work presents an autonomous system for marine integrated physical-chemical and biological monitoring - the MarinEye system. It comprises a set of sensors providing diverse and relevant information for oceanic environment characterization and marine biology studies. It is constituted by a physical-chemical water properties sensor suite, a water filtration and sampling system for DNA collection, a plankton imaging system and biomass assessment acoustic system. The MarinEye system has onboard computational and logging capabilities allowing it either for autonomous operation or for integration in other marine observing systems (such as Observatories or robotic vehicles. It was designed in order to collect integrated multi-trophic monitoring data. The validation in operational environment on 3 marine observatories: RAIA, BerlengasWatch and Cascais on the coast of Portugal is also discussed.

Photodefined etching of n+ layers diffused on p-type silicon substrates

Abstract Non uniform n+ diffused layers on p-silicon are usually obtained by a uniform doping process followed by etch back in areas defined by a photolitographic mask applied onto the sample. We present a study on an alternative technique to obtain nonuniform n+ doped layers using a photoselective etching process, thus avoiding the photolitographic mask. The electrochemical behaviour of n+ silicon (as obtained by phosphorus diffusion into p-type samples) in HF solutions was studied, in order to evaluate the sensitivity of anodic current density to illumination, and thus predict the possibility of photocontrolled etching. Current density vs. potential ( j – V ) curves were measured as etching proceeded, and deeper, less doped layers, were exposed to the electrolyte. Two different regimes are suggested by these measurements: at high current densities, the etch rate in the dark regions should be higher than in the illuminated regions, while at low currents the opposite should occur. The high current regime was explored in etching experiments in which several samples were subjected to a dark/illuminated pattern. The prediction of higher etch rates in the dark areas was confirmed; however, total etched depth contrast was experimentally found to be much higher than predicted from the j – V curves. Contrasts (as measured by sheet resistance) of 59 Ω/□ in the dark against 34 Ω/□ in the illuminated regions were obtained.