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Silicon Info: Ribbon and Sheet Growth

More than a dozen techniques have been introduced over the years for growing silicon ribbons or sheets. Only the ones that were in use for commercial PV substrates during the last several decades will be addressed here.  For a description of the others, see Ciszek (1984).  The four methods being developed commercially are dendritic web growth, growth from a capillary shaping die, growth with edge supports or "strings," and growth on a substrate. These methods can be placed in two categories: (I) those pulled perpendicular to a solid/liquid interface with the same shape as the ribbon cross section (web growth, capillary die growth, and edge-supported growth), and (II) those pulled at a large angle to a solid/liquid interface that is much greater in area than the cross section of the sheet (growth on a substrate).


Dendritic Web Ribbon Growth

Growth From a Capillary  Die

Growth with Edge Supports Photo of Edge-Supported Growth

              Horizontal Growth on a Substrate

There is a large difference in the limiting pulling rate v between type I and type II.  For type I growth, 

       (I)                       

where L is latent heat of fusion, rm is density at the melting temperature, s is the Stefan-Boltzmann constant, e is emissivity, Km is the thermal conductivity at the melting temperature Tm, W is the ribbon width, and t is the ribbon thickness (Ciszek, 1976).  For type II growth, 

              (II)                                          
  
                          

where a is the effective coefficient of heat transfer, s is the length of the solid/liquid interface (in the pulling direction), and DT is the temperature gradient between melt and substrate (Lange and Schwirtlich, 1990).  For the case of a 250-mm-thick ribbon, equation (I) predicts a maximum type I growth rate of ~8 cm/min.  Experimentally, rates closer to 2 cm/min are realized.  Equation (II) predicts a 6-m/min growth rate at DT = 160 oC, and experimental pulling speeds near that value were realized.  The indication is that type II growth speeds can be hundreds of times faster than type I vertical pulling approaches, especially if s and DT are maximized.

Dendritic web growth, the oldest Si ribbon growth method, was introduced by Dermatis and Faust (1963).  The technique arose from the observation that long, thin, flat dendrites with a (111) face and <2TT>  growth direction could be pulled form Ge and Si melts.  One such dendrite is used as a seed and a thermally defined "button" is grown laterally from it.  Then, upward pulling is commenced with appropriate melt-temperature adjustments such that a dendrite of the same orientation propagates from each end of the button.  A web of crystalline silicon solidifies between the dendrites.  It is a single crystal except for an odd number (1,3,5, etc.) of twin planes in the central region.  Web ribbons are currently grown at about 1.5 to 2 cm/min pulling rates, with a width of ~5 cm, a thickness of 100 mm, and in lengths up to 100 m with continuous melt replenishment (~0.25 g/min).  Furnace runs are typically one week in duration, and produce more than 1 m2/day.  Material properties do not degrade over 100-m lengths.  Dislocation etch pit densities are about 104/cm2, and t is on the order of 100 ms or less.  Growth is conducted from an 8-mm-deep melt contained in a shallow, rectangular quartz crucible.  Thermal control is very important, not just for initiating the web but also to maintain steady growth with proper dendrite propagation characteristics at the ribbon edges, low thermal stresses in the ribbon region, and continuous melt replenishment without disturbing the growing web ribbon.  Edge dendrite thickness stability is an excellent indicator of melt-temperature stability.  Both induction heating with molybdenum hot zones and resistance heating with graphite heaters and hot zones have been used. The electrical energy used for growth is about 200-300 kWh/m2. The thin material is particularly well-suited for PV applications that require some bending flexibility, or for bifacial solar cell applications.  Since the material is nearly single crystalline, relatively high cell efficiencies can be achieved.  The best reported value is 17.3% for a 4-cm2 cell.  Initial production cell efficiencies are expected to be ~13%.  One growth furnace can produce web for about 50 kWp/yr cell production.

The growth of crystals from the tips of capillary shaping dies was introduced for sapphire growth using molybdenum dies by LaBelle et al. (1971), and was first applied to silicon ribbons using graphite shaping dies (by Ciszek 1972) and later to silicon tubes (Ciszek, 1975).  Liquid Si rises by capillarity up a narrow channel in the shaping die and spreads across the die's top surface, which defines the base of the meniscus from which the shaped crystal solidifies.  The meniscus base is typically wider than the wall thickness of the crystal.  Commercial development first concentrated on flat ribbons as wide as 100 mm, but edge-stability issues led to a preference for the tubular geometry (i.e., edges are eliminated).  Octagonal tubes with 100-mm-wide flat faces are now used for production of PV substrates. Pulling rates are comparable to those used in web growth, but the 800-mm effective width increases the throughput to about 20 m2/day.  A graphite crucible and graphite shaping dies are used with induction heating.  The electrical energy consumption for this method is approximately 20 kWh/m2.   After growth, rectangular 100-mm-wide "wafers" are laser-cut from the tube faces.  They provide 275-mm-thick multicrystalline substrates with longitudinal grains that routinely make 14% efficient solar cells.  The best efficiency attained on a 10-cm2 cell is 15.5%.  The capillary die method is somewhat more susceptible to impurity effects from solar-grade feedstock than other methods, because the narrow channel impedes mixing of segregated impurities back into the melt and thus increases the effective segregation coefficient.

Edge-supported pulling of "string ribbons" was introduced by Ciszek and Hurd (1980).  This technique is similar to dendritic web growth with foreign filaments or strings replacing the edge-stabilizing role of the dendrites.  This greatly relaxes the temperature control requirements and makes the technique easier to carry out than dendritic web growth.  Simpler equipment can be used.  A variety of carbon- and oxide-based materials were investigated for use as the filaments, with carbon-based filaments generating a higher density of grains at the edges of the ribbons than oxide-based filaments, but having a better thermal expansion match to silicon.  The filaments are introduced through small holes in the bottom of either quartz or graphite crucibles.  Ribbons as wide as 8 cm have been grown, with the standard commercial size now being 5.6 cm wide x 300 mm thick.  The ribbons are grown at about 1-2 cm/min pulling rates, giving a throughput of about 1 m2/day, which is comparable to that obtained with web growth.  Furnaces can be kept in continuous operation for weeks at a time by replenishing the melt.  Ribbon sections of a desired length are removed by scribing while pulling is in progress.  Continuous growth of more than 100 m of ribbon has been achieved, and lengths greater than 300 m have been obtained from a single furnace run (with successive seed starts).  Dislocation densities are ~5 x 105/cm2 and t is in the range 5-10 ms.  The highest cell efficiency obtained is 16.3%, although production efficiencies are <13%.  The steady-state grain structure contains longitudinal grains of about 1 cm2 area, predominantly with coherent boundaries, in the central portion of the ribbons, and newly generated grains at the ribbon edges.  The electrical energy used is about 85 kWh/m2.

The first application of type II sheet growth to a semiconductor material was by Bleil (1969), who pulled ice and germanium sheet crystals horizontally from the free surface of melts in a brim-full crucible.  Many approaches have been considered for applying type II growth to PV silicon, including horizontal growth from the melt surface.  The ones currently under commercial development move a substrate through a hot zone tailored in such a way that a long region of molten silicon in contact with the upper surface of the substrate solidifies with a long wedge-shaped crystallization front.  The front grades from 0 thickness at the tip to the sheet thickness t (where the sheet leaves the melt) over a distance s.  As indicated in Eq. (II), the pulling speed is proportional to s/t and to DT.  It is feasible to make s very large, on the order of tens of centimeters.  Coupled with moderate DT values (160oC), 250-mm-thick sheets can then be grown with pulling speeds vs as high as 6 m/min as mentioned earlier (Lange and Schwirtlich, 1990).  If W is also tens of centimeters, extremely high throughputs can be achieved - in the vicinity of 1,500 m2/day.  Heat removal is facilitated by the fact that the surface in which heat of crystallization is generated is nearly parallel to, and in close proximity to, the surface from which it is to be removed.  The solid/liquid interface's growth direction vg is essentially perpendicular to the pulling direction vs.  So, as grains nucleate at the substrate surface, their growth is columnar across the thickness of the sheet. This is in contrast to longitudinal grains aligned along the pulling direction obtained in the type I techniques, in which vg and vs are 180o apart, pointing in opposite directions. The grains tend to be smaller in type II growth methods, but are on the order of t.  Production solar cell efficiencies as high as 12% are attainable at the present time, and the best small-cell efficiency is 16%.  The substrate does not have to remain with the grown sheet, and may be engineered for clean separation at some point after solidification.
____________________
Bleil, C.E. (1969) J. Crystal Growth 5, 99.
Ciszek, T.F. (1972) Mat. Res. Bull. 7, 73l.
Ciszek, T.F. (1975) Phys. Stat. Sol. (a) 32, 521.
Ciszek, T.F. (1976) J. Appl. Phys. 47, 440.
Ciszek, T.F. (1984) J. Crystal Growth 66, 655.
Ciszek, T.F., and Hurd, J.L. (1980) in: Proceedings of the Symposia on Electronic and Optical Properties of Polycrystalline or Impure Semiconductors and Novel Silicon Growth Methods (K.V. Ravi and B. O'Mara, ed.) p. 213. The Electrochemical Soc., Proceedings Volume 80-5, Pennington, NJ.
Dermatis, S.N., and Faust Jr., J.W. (1963) IEEE Trans. Commun. Electron. 82, 94.
LaBelle, H.E., Mlavsky, A.I., and Chalmers, B. (1971) Mater. Res. Bull. 6, 571, 581, 681.
Lange, H., and Schwirtlich, I.A. (1990) J. Crystal Growth 104, 108-112.
 


Siliconsultant, P.O. Box 1453, Evergreen, CO 80437 USA                                   e-mailted_ciszek@siliconsultant.com

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This page was last updated on June 19, 2016