Raymond G. Seidensticker

SiC boule growth by sublimation vapor transport

Abstract Silicon carbide is an attractive candidate for high power and high temperature electronics due to its inherent high thermal conductivity, large saturated drift velocity, high breakdown strength and large bandgap. A review of the material properties which influence semiconductor device characteristics is presented, and recent advances in crystal growth technology leading to the preparation of 25 mm and larger wafers for “silicon-like” device fabrication processes are reviewed. A sublimation vapor transport system is described and preliminary results on growth of 6H-SiC boules are presented.

Silicon ribbon growth by the dendritic web process

Abstract Silicon dendritic web is a unique mode of ribbon growth in which crystallographic and surface tension forces, rather than shaping dies, are used to control crystal form. The single crystal webs, typically 2–4 cm wide, have been made into solar cells which exhibit AM1 conversion efficiencies as high as 15.5%. During crystallization, silicon webs effectively segregate metal impurities to the melt ( k eff ≈ 10 −5 ) so that the use of cheaper, less pure silicon as feedstock for crystal growth appears feasible. Recent studies described here indicate that higher growth output rates can be achieved by control of the thermal profiles in the web itself and in the melt from which the crystal grows. The improvements stem from an enhancement in the dissipation of latent heat and a reduction in stress within the crystals. To sustain high output rates for prolonged periods will require melt replenishment during growth.

Crystal growth considerations in the use of “solar grade” silicon

Abstract The use of lower purity, lower cost, “solar grade” silicon for crystal growth feed stock has been proposed as one means to reduce the cost of solar cells. The metallic impurities in this material have a deleterious effect on cell efficiency; however, the maximum tolerable impurity concentrations depend not only on the acceptable cell efficiency, but also on the details of the crystal growth process. We wuantitatively analyze the problem using data on relative solar cell efficiency as a function of metallic impurity concentration and the effective segregation coefficients for Czochralski growth. We find, for example, that solar cell performance rather than interface breakdown during crystal growth will most likely determine the permissable impurity content in solar grade silicon. We also show that continuous rather than sequential melt replenishment is advantageous for Czochralski growth.

Modeling thermal stress effects in silicon web growth

Abstract If stresses generated by the temperature profile in a growing ribbon crystal exceed critical values, they may cause plastic deformation or buckling which limit ribbon width and, therefore, the throughput rate of the growth process. We outline here a methodology for computing the widths and thickness of silicon web ribbons which define the transition from flat to buckled growth under a given set of thermal conditions. The approach employs the iterative application of three models (web thermal profile, thermally generated stress and buckling calculations) and has considerably reduced the experimental time required to evolve low stress growth configurations. Flat web crystals up to 5.5 cm wide have been grown.

Solute partitioning during silicon dendritic web growth

Abstract Measurements of the impurity content of dendritic webs grown from melts containing aluminium, gallium and indium indicate effective solute partition coefficients essentially the same as the interface partition coefficients reported in the literature. This is consistent with the predictions of a simplified model of the partitioning process which indicates that the two segregation coefficients may be of similar magnitude. Web growth is thus almost as effective as Czochralski pulling in its ability to reject impurities to the liquid from the growing crystal.

Advancements in silicon web technology

Abstract Low defect density silicon web crystals up to 7 cm wide are produced from systems whose thermal environments are designed for low stress conditions using computer techniques. During growth, the average silicon melt temperature, the lateral melt temperature distribution and the melt level are each controlled by digital closed loop systems to maintain thermal steady state and to minimize the labor content of the process. Web solar cell efficiencies of 17.2% AM1 have been obtained in the laboratory while 15% efficiencies are common in pilot production.

Analysis of plastic deformation in silicon web crystals

Abstract Numerical calculation of {111}〈110〉 slip activity in silicon web crystals generated by thermal stresses is in good agreement with etch pit patterns and X-ray topographic data. The data suggest that stress redistribution effects are small and that a model, similar to that proposed by Penning and Jordan but modified to account for dislocation annihilation and egress, can be used to describe plastic flow effects during silicon web growth.

Dendritic Web Growth of Silicon

The dendritic web growth process is a crystal growth technique which produces thin, long, ribbons of essentially single-crystalline material. The ribbon morphology results from an interaction of crystallographic and surface tension forces so that possible contamination from shaping dies is avoided. Growth is from a melt so that the usual solutes can be used to “dope” the crystals to the desired conductivity type and resistivity. Rejected solutes readily diffuse away from the growth front so that impurities segregate very efficiently as in Czochralski growth. The structural quality of the material is excellent so that all usual semiconductor processing techniques compatible with a (111) orientation are applicable to dendritic web material.