J. Maia Alves

Measurement of residual stress in multicrystalline silicon ribbons by a self-calibrating infrared photoelastic method

This article reports on a method for the measurement of residual stress in multicrystalline silicon ribbons, based on the infrared photoelastic technique. This self-calibrating method allows the in situ determination of the photoelastic coefficients and can thus be used for any crystal orientation. The method was validated by the experimental determination of the photoelastic coefficient of monocrystalline (100) silicon wafers and by comparison with strain measurements using asymmetrical x-ray diffraction. The distribution of residual stress in multicrystalline silicon ribbons was also measured. The results showed strong evidence for tensile stress in the central region and compressive stress near the edges of the ribbons. Both the measured residual stress and the photoelastic coefficient distributions are correlated to grain boundaries.

Residual stress and dislocations density in silicon ribbons grown via optical zone melting

We investigate the relationships between growth rate, time-temperature profile, residual stress, dislocation density, and electrical performance of silicon ribbons grown via optical zone melting. The time-temperature profiles of ribbons grown at different velocities were investigated using direct measurements and computational fluid dynamics (CFD) modeling. Residual stresses up to 20 MPa were measured using infrared birefringence imaging. The effect of crystallization speed on dislocation density and residual stress is discussed from the context of thermal stresses during growth. More broadly, we demonstrate the usefulness of combining spatially resolved stress and microstructure measurements with CFD simulations toward optimizing kerfless silicon wafer quality.

The silicon on dust substrate path to make solar cells directly from a gaseous feedstock

In this paper, we present a silicon on dust substrate (SDS) process, a new method for the growth of silicon ribbons. As a demonstration of the concept, we also present results on solar cells made of these new silicon ribbons. SDS ribbons were obtained directly from a gaseous feedstock by a fast CVD step using silane. The resulting self-supported intrinsic ribbons were microcrystalline and porous. To make these ribbon films suitable for photovoltaic applications, a novel recrystallization with an in situ doping step was developed. To this purpose, the ribbons were sprayed with boric acid and then recrystallized by float zone melting. Simple solar cells were prepared by employing: aluminium back contacts, Ti/Pd/Ag front grid contacts, with no anti-reflective coating, doping optimization, passivation or gettering. The 1-sun I–V characteristics of the cells were: Voc ∼ 530 mV and Jsc ∼ 24 mA cm −2 .T he minority carrier diffusion length obtained from a spectral response at long wavelengths gave values of Ln ∼ 70 μm. (Some figures in this article are in colour only in the electronic version)

A differential mechanical profilometer for thickness measurement

A low cost differential profilometer based on standard commercial displacement transducers is fully described. Unlike most common profilometers this device can be used to measure the thickness profile of samples having both surfaces irregular. A sensitivity of about 0.2 μm, independent of the sample thickness is achieved.

A new concept in underwater high fidelity low frequency sound generation.

This article reports on a new type of system for high fidelity underwater sound generation (patent pending PT105474). The system includes an underwater sound actuator and the corresponding electronic driver. The sound is generated by a rigid plate that is actuated (both for positioning/dumping and excitation) using purely electromagnetic forces, thus, avoiding the use of any elastic membrane. Since there is no compressible air inside the device, which is flooded by water, the operation of this device is independent from depth, broadening its applications to any water pressure. Characterization of the frequency response, the radiation characteristics, and the dynamic range of this new device for underwater sound generation is presented.

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.

Electric molten zone crystallization of silicon wafers

A new method for molten zone crystallization is presented. The method is based on the formation of a molten capillary by applying an electric current. Since the power is delivered directly to the liquid, the technique has the potential for low energy budget. On the other hand, being a floating molten zone method, the liquid silicon never contacts foreign materials and therefore is essentially contamination free. Experimental results show that the crystallized samples feature relatively low minority carrier lifetimes which are correlated to relatively high dislocation densities, associated with the sample temperature profile.

A wave lab inside a coaxial cable

The study of electromagnetic wave propagation in a coaxial cable can be a powerful approach to the study of waves at an undergraduate level. This study can explore different experimental situations, going from those where the finite velocity of propagation must be considered (distributed or transmission line behaviour), to those where this velocity may be considered infinite (lumped behaviour). We believe that the student observation of the existence of these two regimes can be important for the understanding of wave phenomena in general. In this work we show that this can be achieved using low-cost equipment and a set of quite simple experiments, such as the measurement of wave propagation velocity or the study of standing waves and resonance. The results obtained in a coherent set of selected experiments are discussed.

Preparation of superconductors of the BiSrCaCuO system by glass crystallization

Abstract The preparation of high-temperature superconductors of the Bi-Sr-Ca-Cu-O system by glass crystallization is reported. Glasses with nominal compositions Bi2Sr2Ca2Cu3Ox are obtained by quenching of the molten oxides. These glasses are crystallized by heat treatment in the range 600–850 °C for 76 h in air, producing a high-density multiphase polycrystalline material. Samples heat treated at 800–835 °C have zero resistivity at 87 K, and those heat treated above or below this range have transport properties dominated by semiconducting phases.

High temperature sound velocity measurement with piezoelectric transducers

A technique for high temperature sound velocity measurement using piezoelectric transducers, that avoids the existence of the usual high temperature bond between the sample and the buffer, was developed. This bond is frequently responsible for a large attenuation and distortion of the sound field, with a consequent inaccuracy in the experimental results. This technique was successfully used by the authors to make good quality measurements of sound velocity in iron–silicon single crystals from room temperature to T≈1000 °C, unlike other authors who report strong attenuation in different regions of this temperature range for similar samples. The furnace, which is only 35 mm wide, allows work under magnetic field and a controlled atmosphere up to 1300 °C.