Fritz Wald

Natural and forced convection during solution growth of CdTe by the Traveling Heater Method (THM)

Abstract Solution growth by the “traveling heater method” (THM) employs a thermal arrangement where the highest temperature occurs near the middle of the solvent zone. One of the consequences for crystal growth in the generation of convection currents which can aid material transport in the molten zone. Since the magnitude of the convection depends on the properties of solvent and crystal, it is suggested that forced convection using “acceleration rotation” (AR) might be employed because it is reproducible and amenable to control. To observe the effect of rotation speed, cycle time, growth rate and temperature on crystallinity and numbers of voids or inclusions, a series of THM experiments was performed on the crystal growth of CdTe from Te solution. These showed that AR in this system is of only modest effect (it can increase the growth rate by a factor of 2), because the natural convection currents generated under the particular experimental conditions are already very strong indeed.

Imperfections and impurities in EFG silicon ribbons

Abstract The structure, morphology, and distribution of crystallographic defects in silicon ribbons grown by the edge defined film-fed growth (EFG) process are found to be functions of crystal growth rate. Two distinct types of defect distributions are observed in these crystals when grown at different rates. At growth rates below about 2.5 cm/min, the structure consists of parallel arrays of dislocations, twins and stacking faults parallel to the ribbon edges and extending through the thickness of the crystal. At growth rates in excess of 2.5 cm/min, a subsurface structure composed of large angle grain boundaries and, sometimes, equiaxed grains develops. An examination of the structure of these crystals is followed by the presentation of a model to account for the particular defect structure developed. The model is based on the proposition that defect generation is a consequence of the association of native point defects in the form of silicon self-interstitials and dissolved carbon which is a major impurity in EFG silicon. The electrical consequences of the development of the particular defect structure are examined, and it is postulated that imperfections in shaped crystals can function as intrinsic gettering agents for impurities, thus causing an improvement in the electrical properties of the crystals.

Long nonagons — An approach to high productivity silicon sheet using the EFG method

Abstract In this paper we describe the development work that led to the capability of growing closed shapes of silicon, i.e., nonagons of 5 cm face width, in lengths up to 6 m. Results of initial growth experiments and data on solar cells prepared from the material grown are also presented.

Aluminum redistribution in EFG of silicon ribbon

Abstract Impurity transport in EFG of silicon ribbon has been studied with the help of spreading resistance measurements and infrared transmission spectroscopy taken on ribbon grown from aluminum-doped melts. Redistribution of aluminum do pant is observed both in the ribbon thickness and width dimensions. The redistribution pattern across the ribbon width can be explained with the help of a two-dimensional model of impurity transport in the melt ahead of the growth interface. The model predictions have been used to obtain a value for the diffusion coefficient of aluminum in liquid silicon.

Multiple EFG Silicon Ribbon Technology as the Basis for Manufacturing Low-Cost Terrestrial Solar Cells

The development of a technology for production of low-cost silicon sheet substrates for solar cells based on the EFG process has been extended to growth of 10 cm wide material. Simultaneous growth of three 10 cm wide ribbons from a multiple ribbon furnace with continuous melt replenishment has been demonstrated. Run lengths were from six to eight hours, at growth speeds of ∼3 cm/minute. A system for automatic control of the ribbon width has been developed and implemented to permit constant width growth over periods of up to three hours without surveillance by an operator. In single cartridge operation, 10 cm wide ribbon has been grown at speeds up to 4.2 cm/minute. Stress levels and buckling for 200 μm thick ribbon grown at high speeds have been low. Efficiencies for large area (∼50 cm2) solar cells prepared from this 10 cm wide ribbon have been shown to reach ∼10% (AM1 and AR coated).

Crystal Growth of Silicon Ribbons

The growth of shaped silicon crystals has been attractive for a number of years and several methods to obtain various shapes, in particular flat ribbons, have been developed. Recently, however, the problem has been much more sharply focused due to the desire for the preparation of very low cost solar cells for terrestrial power generation, and a number of articles in the present volume address this problem.