Laser-zone growth in a ribbon-to-ribbon (RTR) process silicon sheet growth development for the large area silicon sheet task of the low-cost solar array project. Annual report, October 1, 1977--September 30, 1978

Abstract

A new calculation is included of the effects of thermal stresses during growth on silicon ribbon quality. Thermal stress distributions are computed for ribbon growth under a variety of temperature profiles. It is shown that the width dependence is not as large as had previously been assumed. Under practical growth conditions, the stresses are actually often below the yield point. It is also shown that, for the thermal profiles presently in use, buckling is probable for widths greater than 4 - 6 cm. Two ideal temperature profiles are described which could result in ribbons grown without either high dislocation generation or buckling. A growth rate of 55 cm/sup 2//min with a single ribbon was achieved. The growth of RTR ribbon with a fairly uniform parallel dendritic structure was also demonstrated. Encouraging results were obtained with two approaches for reducing the Mo impurity level in polycrystalline feedstock. Neutron Activation Analysis has shown that the Mo level can be reduced to below the level of detection by etching the polycrystalline ribbon surface. Coating the Mo substrate with Si/sub 3/N/sub 4/ does not affect thermal shear separation of the polyribbon (Si/sub 3/N/sub 4/ is a well-known diffusion barrier); this process shows promise of improving cell efficiencies and also increasing the useful life of the molybdenum substrate. A number of solar cells have been fabricated on RTR silicon grownfrom CVD feedstock.