J.M. Serra

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.

Thin-film characterization for high-temperature applications

Most thin films produced by a wide variety of methods, either physical or chemical (PVD, CVD, sputtering, etc.) for temperature sensor applications, can be used only in very narrow ranges of temperatures, where their components are not subjected to differential thermal expansions, recrystallizations, and grain size modifications. This paper reports the production and characterization of thin films of platinum and titanium in ceramic substrates by one of the physical vapor deposition techniques, the e-gun evaporation. The choice of materials and the determination of film thickness, density, electrical resistivity, surface roughness, and structural characterization (X-ray, SEM, and AES) are studied. Special emphasis is given to the thermal and electrical behavior of these films between room temperature and 1000°C.

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.

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.

New Stress Activation Method for Kerfless Silicon Wafering Using Ag/Al and Epoxy Stress-Inducing Layers

The SLIM-cut technique provides a way to obtain thin silicon foils without a standard sawing step, thus avoiding kerf losses. This process consists of three steps: depositing a stress-inducing layer on top of the silicon surface; stress activation by heating and cooling, resulting in crack propagation in the silicon and detachment of a thin silicon layer; and a chemical cleaning to remove the stress-inducing layer. This paper describes a new stress activation method using Ag/Al and epoxy stress-inducing layers. The crack propagation is controlled along the sample length in order to avoid unwanted additional crack formation and interaction with other crack fronts. Silicon foils with thickness ranging between 50 and 130 μm were obtained with effective lifetimes between 1 and 81 μs.

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.

First Solar Cells on Exfoliated Silicon Foils Obtained at Room Temperature by the SLIM-Cut Technique Using an Epoxy Layer

In this paper, we report on the first solar cells fabricated on silicon foils employing the stress-induced liftoff methodcut technique and using an epoxy stress-inducing layer. The latter is a 900-μm-thick epoxy layer, which was manually dispensed on the surface of a monocrystalline silicon sample and cured at 150 °C for 1 h. The crack propagation is then activated by cooling down the sample to room temperature on an aluminum plate. The structural and electrical properties of the resulting silicon foils are presented. The thickness of the silicon foil and the maximum roughness height after this process are found to be around 130 and 37.4 μm, respectively. Tensile and compressive regions present in the foil were detected by the birefringence technique and indicate the formation of structural defects such as dislocations during the process. Average minority carrier lifetimes around 28 μs were measured on n-type silicon foils after surface passivation by an iodine-ethanol solution. Photon conversion efficiencies of 12.6% and 13.4% are measured using conventional sun simulator (1-sun AM1.5G) and SINTON Suns-Voc setup systems, respectively.

Room temperature thin foil SLIM-cut using an epoxy paste: experimental versus theoretical results

The stress induced lift-off method (SLIM) -cut technique allows the detachment of thin silicon foils using a stress inducing layer. In this work, results of SLIM-cut foils obtained using an epoxy stress inducing layer at room temperature are presented. Numerical analyses were performed in order to study and ascertain the important experimental parameters. The experimental and simulation results are in good agreement. Indeed, large area (5 × 5 cm2) foils were successfully detached at room temperature using an epoxy thickness of 900 μm and a curing temperature of 150 °C. Moreover, three foils (5 × 3 cm2) with thickness 135, 121 and 110 μm were detached from the same monocrystalline substrate. Effective minority carrier lifetimes of 46, 25 and 20 μs were measured using quasi-steady-state photoconductance technique in these foils after iodine ethanol surface passivation.