In order to check the Prokowski-Talapov (P-T) universality class behaviour, the profiles of equilibrated silicon crystals have been studied in the vicinity of a (111) facet at 900 degrees C. The equilibrium shapes of three-dimensional (3D) or two-dimensional (2D) crystals have been produced by thermally equilibrating either an array of small silicon columns on a silicon substrate or a grating of silicon ridges. The samples have been observed either ex situ, by high-resolution SEM, or in situ by TEM and REM. The profiles have been measured along a (high symmetry) <110> zone, toward (110), within the angular domain 3-17 degrees. For smaller angles (0-1.5 degrees away from [111]) the profile has been reconstructed via the visualisation of steps by REM.The theoretical prediction of a 3/2 power law for the profile equation has been checked. The results are at variance with those obtained for metals. Within the angular domain 3-17 degrees, the profile is compatible with a 3/2 power law. From the profile equation, the step interaction constant can be determined (0.36+/-0.09 J m(-2) at 900 degrees C), in good agreement with previous experiments on the terrace width distributions. No difference can be detected between 3D and 2D crystals. All this is the signature of a P-K behaviour, only involving repulsive 1/x(2) step interactions, again in agreement with previous studies. However, at small angles (0-1.5 degrees), no physically reasonable law can be assigned to the profile. This behaviour is not understood at present. (C) 1998 Elsevier Science B.V. All rights reserved.

Multicrystalline-Si ingots were prepared by a cold crucible continuous pulling technique and 230 μm thick wafers were cut using a wire saw. By means of mapping techniques such as light beam induced current, it is shown that the electrical properties of the wafers are homogeneous. Impurities have a tendency to aggregate in the form of quasi neutral precipitates and/or inclusions. Minority carrier diffusion lengths are relatively high in the grains: around 90 μm due to a low recombination strength of extended defects. Conversely the recombination strength of grain boundaries is high. Solar cells made with this material lead to conversion efficiencies higher than 12%

F.Z. silicon wafers were submitted to high temperature annealings during long times in oxygen and in nitrogen atmosphere in order to reproduce the same treatments which are necessary to develop power and high voltage transistors or diodes. It is shown by electrical techniques (microwave detected photoconductivity decay and surface photovoltage) and by revelation techniques (scanning infrared microscope, X-ray topography, Fourier transformed infrared spectroscopy, chemical etchings) that annealings in nitrogen added to annealings in oxygen have a deleterious effect on the lifetime of minority carriers and can create dislocations and precipitates.

In order to obtain the equilibrium shape of silicon, small monocrystalline silicon columns (diameter ∼ 1–5 μm, height ∼ 10 μm) have been formed on a silicon (111) substrate by photolithography. Indashsitu observation, in a UHV transmission electron microscope, of the shape changes of these columns upon heating them shows that the equilibrium shape of clean silicon can be installed over the apices, once the columns have become bulbous by evaporation and surface diffusion. Equilibrium profiles, along the 〈110〉 and 〈112〉 zones, have been visualised indashsitu at 1323 K, recorded and analysed. Well-characterised {111} and {113} facets exist on the equilibrium shape at this temperature. They are separated by rounded regions that display a tangential merging into the facets. Hence, all orientations belong to the equilibrium shape at 1323 K. The γ-plot has been constructed for the 〈110〉 zone. It shows cusps at 〈111〉 and 〈113〉 and rather broad minima at 〈110〉 and 〈100〉. However, within the resolution of our micrographs, it is not possible to decide whether flat facets exist at 〈110〉 and 〈100〉 or whether the crystal surface is merely slightly rounded. The anisotropy of the surface specific free energy is found surprisingly weak (∼ 4% maximum). Within the experimental accuracy (∼ 1%), the hierarchy is γ111 ≥ γ110 > γ113 > γ100, the relative anisotropies with respect to {111} being 0.99, 0.98 and 0.97 for {110}, {113} and {100} respectively. An order of magnitude for the step-free energy β has been obtained (β111 ≈ 3 × 10−11J m−1, β113 ≈ 1 × 10−11J m−1). Our results are compared to those obtained by other authors by using voids in silicon.

External gettering by phosphorus diffusion is used to improve the minority carrier diffusion lengths L/sub n/ in multicrystalline silicon solar cell wafers obtained by different growth techniques. Polix, Silso, Semix and Eurosolare materials were characterized. It is shown that gettering efficiency depends strongly on the origin of the material. For Polix and Eurosolare wafers, the optimal diffusion temperature is around 900/spl deg/C; L/sub n/ increases with diffusion time and tends to saturate, after 8 hours, at about 300 to 500 /spl mu/m. However the presence of a too large density of defects, or (and) a high concentration of dissolved oxygen atoms (i.e 10/sup 18/ cm/sup -3/) limits the external gettering efficiency, probably by an internal gettering of impurities. Competition between external and internal gettering seems to occur.<>

In thin multicrystalline silicon cells (d < 200 mm) previously improved by phosphorus gettering a back surface field (BSF) is necessary. The BSF could be obtained by aluminium alloying and diffusion. Longly gettered samples are markedy improved, during the annealing of the Si-AI structure at 800°C for 30 mn, and Ln overpass the thickness of the samples. Although the influence of an additional gettering effect is not precluded, it is emphasized that the aluminium treatment results of the hydrogenation of the wafer.

In thin multicrystalline silicon cells (d<200 mm), previously improved by phosphorus gettering, a back surface field (BSF) is necessary. The BSF could be obtained by aluminum alloying and diffusion. Samples gettered for a long time are markedly improved during the annealing of the Si-Al structure at 800 degrees C for 30 min, and L/sub n/ overpass the thickness of the samples. Although the influence of an additional gettering effect is not precluded, it is emphasized that the aluminum treatment results in the hydrogenation of the wafer.< >

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Interdigitated back contact (IBC) silicon solar cells with 25.6% efficiency at 10 W/cm/sup 2/ and 24.4% at 30 W/cm/sup 2/ were fabricated. The authors report on the technological process, which produces a high effective carrier lifetime in the bulk (780 mu s), and on the characterization of the cells. The front side of these cells is textured and has a very efficient polka-dot floating tandem junction. IBC and point-contact (PC) cells are fabricated on the same substrate and their efficiencies are compared. The possibility of reaching 29% efficiency at 300X is shown.

In order to show the effect of reducing the size of diffused regions for lowering their saturation current, two kinds of Interdigitated Back Contact (IBC) silicon solar cells have been manufactured: Linear Contact (LC) cell, and Point Contact (PC) cell. Isc — Voc measurements allow to determinate the saturation currents under different working conditions. It is shown that LC cell present better VQC than PC cell under low illumination levels (<500 kW/m2 AM 1.5) but under higher levels, the effect of emitters size reducing leads to an increased Voc. However, the Fill Factor and the efficiency can be limited by the simultaneous increase of the cell intrinsic series resistance. Efficiencies over 24% have been obtained under 300 kW/m2 with LC cells.

An autonomous photovoltaic converter with a linear focusing concentrator allowing an effective concentration gain of 20 was designed, realized and tested. The photocells are special silicon cells; their cooling system is passive. The concentrator is a Fresnel-type segmented mirror. Part of the electrical power supplied by the device is used in its own sun-tracking system, the other part charges a 12 V, 30 A h, lead battery and the energy so stored during the day is used at night to light a room by a neon tube.

The electron work function variation of tungsten samples has been experimentally studied in a strontium vapor environment: single crystal sample in a thermoelectronic emission microscope, polycrystalline sample in a strontium filled cylindrical diode. The data show the emission current density versus the reciprocal sample temperature T-1, and work function variation versus the ratio T/TSr, TSr being the strontium reservoir temperature. Comparison of these results obtained by two methods in different strontium pressure ranges and the study of oxygen influence are of interest for thermionic conversion.