J.P. Kalejs

Gettering and hydrogen passivation of edge‐defined film‐fed grown multicrystalline silicon solar cells by Al diffusion and forming gas anneal

This study shows for the first time that a combination of Al treatment on the back, oxide passivation on the front, and 400 °C forming gas anneal in the presence of Al, raised the double‐layer antireflection‐coated edge‐defined film‐fed grown (EFG) silicon cell efficiency from 7.8% to 14.1%. Front oxide passivation contributed an ∼0.8% increase in absolute cell efficiency, Al diffusion on the back increased the efficiency by 1.4% (absolute), and the forming gas anneal (FGA) after the Al diffusion improved the cell efficiency by an additional 4.1% (absolute). A combination of the above three steps improved the EFG cell efficiency by 6.3%, indicating that the above three effects are complimentary. Oxide passivation reduced front surface recombination velocity and Al diffusion, while FGA improved diffusion length via gettering. We propose that the large increase in cell efficiency produced by the forming gas anneal results from bulk defect passivation by atomic hydrogen generated in the processing.

Analysis of operating limits in edge-defined film-fed crystal growth

Abstract The limits on growth rate and sheet thickness attainable in edge-defined film-fed growth depend on the interaction of heat transfer in the melt, crystal and die with the shape of the molten zone. The temperature fields in the three phases and the shapes of the melt/solid and melt/gas interfaces are calculated by a new finite-element algorithm for solving simultaneously two-dimensional heat transfer models in each phase and the Young-Laplace equation for meniscus shape. The dependenct of crystal thickness on growth rate and die temperature is analyzed for a particular die geometry and the operating limits for these parameters are predicted. Depending on the thermal operating strategy, the mode of failure at high growth rates may be either by thermal supercooling of the melt or by the decrease of the crystal thickness to an unreasonably small size. The sensitivities of the form of the operating region to changes in the height of the die and the contact angle at the growing crystal are discussed. Melt/solid interface shapes are found to vary from concave to convex, depending on the growth rate and the heat input into the die.

High speed EFG of wide silicon ribbon

Abstract A system for high speed EFG of silicon ribbon is described which is capable of growing ribbon up to 7.5 cm in width and 7.5 cm/min in speed. A technique for achieving constant width growth has been developed through the utilization of temperature gradients along the ribbon width to stabilize and control the ribbon edge. Problems which have to be solved with respect to maintaining growth stability and eliminating buckling of the ribbon at higher growth speeds are examined in detail.

Self‐interstitial enhanced carbon diffusion in silicon

Out‐diffusion of carbon from silicon wafers during annealing at 900 °C is observed to be enhanced by the in‐diffusion of phosphorus as compared to annealing in a nitrogen ambient. This diffusion enhancement is suggested to be due to phosphorus‐induced silicon self‐interstitials and an appreciable diffusion component of carbon in silicon involving self‐interstitials.

Impurity redistribution in EFG

Abstract Numerical solutions of convective and diffusive transport equations in the melt contained by the EFG capillary die are presented for a two-dimensional model of EFG of silicon ribbon. Die geometry is shown to influence convective impurity transport in melt supplying the interface region during growth. Nonuniformity in the component of melt velocity parallel to the growth interface gives rise to impurity redistribution across the width of the ribbon. Enhancement of impurity levels above those of the bulk melt is associated with regions of low velocity, depletion of levels with regions of high velocity. The degree of redistribution varies with growth speed and the interface segregation and liquid diffusion coefficients of the impurity species. The close relation between redistribution patterns and die geometry suggests levels of certain impurities in preselected regions of the ribbon may be controlled by die design.

Plastic deformation influence on stress generated during silicon sheet growth at high speeds

Abstract Plastic deformation processes have been studied in high speed silicon sheet growth using finite element analysis. Stress and strain rate distributions are calculated for steady-state growth of thin sheet under plane stress conditions. Predictions of the model are used to examine factors affecting residual stress and buckle formation for growth of silicon ribbon by the EFG method.

Defect passivation in multicrystalline‐Si materials by plasma‐enhanced chemical vapor deposition of SiO2/SiN coatings

It is shown for the first time that plasma‐enhanced chemical vapor deposition (PECVD) passivation which involves low temperature PECVD of ∼100 A SiO2 and ∼600 A SiN followed by photoassisted anneal is very effective for both surface and bulk defect passivation in multicrystalline‐Si materials. It is found that the effective recombination lifetime increased by a factor of 2–10 depending upon the multicrystalline material. Some solar cells were fabricated using a three‐layer PECVD coating (100 A SiO2/600 A SiN/950 A SiO2), the bottom two for passivation and the top two for antireflection coating. The bulk and surface passivation effects were quantified and decoupled by a combination of internal quantum efficiency measurements and computer modeling. It was found that the PECVD passivated solar cells increased bulk lifetime from 10 to 20 μs, and decreased the surface recombination velocity from 2×105 to 5×104 cm/s.

Interface shape studies for silicon ribbon growth by the EFG technique: I. Transport phenomena modeling

Abstract The influence of process parameters on the interface shape of silicon ribbon grown by the edge-defined film-fed growth (EFG) technique has been studied with the help of finite element solutions of the heat and mass transport equations. The interface shape is calculated to be convex toward the melt for a wide range of growth parameters. Significant inhomogeneities of segregated impurities through the ribbon thickness are associated with this interface shape for impurities with segregation coefficients much less than unity. The influence of asymmetry in die geometry and temperature fields on interface configuration and solute segregation is examined. Interface tilt produced by this asymmetry is shown to lead to deviations from normal segregation behavior at large tilt angles.

Model for diffusion and trapping of hydrogen in crystalline silicon

A simplified analysis is presented to describe diffusion and trapping of atomic hydrogen in crystalline silicon. Predictions of the rate of advancement of the trapped hydrogen front as a function of process and material variables are obtained from a numerical solution of the transport equations. We demonstrate that the solutions are compatible with data on boron passivation at 150 °C only when the diffusivities obtained by extrapolation from the high‐temperature results of Van Wieringen and Warmoltz [Physica 22, 849 (1956)] are used to represent atomic hydrogen diffusion in the absence of trapping.

Comparison of growth characteristics of sapphire and silicon ribbon produced by EFG

Abstract Finite element analysis of heat transfer and meniscus shape is used to compare the operating conditions for Edge-defined Film-fed Growth (EFG) of silicon and sapphire (Al2O3) sheets. The relationships between sheet thickness and growth velocity are predicted for both systems with varying ambient thermal conditions. More effective radiative heat transfer between the melt and surroundings makes the Al2O3 growth less sensitive to growth rate variations and leads to more comvex (with respect to the melt) melt/solid interface shapes than for Si sheets grown from a die with the same dimensions. Considering capillarity alone would lead to the opposite conclusion for the sensitivity of sheet thickness to growth rate.