AU1006202A - Columnar-grained polycrystalline sheet - Google Patents

Description

AUSTRALIA

Patents Act COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: Name of Applicant: AstroPower Inc.

Actual Inventor(s): Robert B Hall, Allen M Barnett, Sandra R Collins, Christopher L Kendall, James A Rand, Chad B Moore Joseph C Checchi, David H Ford, Address for Service: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title: COLUMNAR-GRAINED POLYCRYSTALLINE SHEET Our Ref: 659967 POF Code: 872/454462 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): -1- 1A COLUMNAR-GRAINED POLYCRYSTALLINE SHEET FIELD OF THE INVENTION This invention relates generally to an improved columnar-grained polycrystalline sheet, and in particular a columnar-grained silicon sheet for forming a substrate for a solar cell, as well as a solar cell having the sheet substrate.

BACKGROUND OF THE INVENTION Photovoltaic solar cells and semiconductor devices which convert sunlight into electricity. Solar cells based on crystalline silicon offer the advantage of high performance and stability. The principal barrier to expanded utilization of silicon solar cells for electric power generation is the present high cost of the solar cells.

In conventional solar cells based on single crystal or large grain polycrystalline silicon ingot processes, the major cost factor is determined by the requirement of sawing ingots into wafers. Sawing is an expensive processing step, and furthermore results in the loss of approximately half the costly ingot material as o 20 silicon 9 Itll* *11 W:\kate\SPECI\Divo739048.doc dust. The problem to be solved requires the development of a low-cost process, that efficiently employs low-cost materials while maintaining solar cell performance.

The solutionl to the problem requires the achievement of a process that is controllable, has high areal throughput, and generates material with adequate crystalline morphology. Prior art includes several processes which either effectively achieve controlled growth, or high areal throughput of silicon sheer or ribbons- All these approaches eliminate the costly process of sawing large areas to create wafers from ingots. For example, publications by Hiopkcins (WEB), Zttoufley, et al. (F.PG) Gutler (RTR) and Eyer, et al. (SSP) describe processes that achieve controlled polycrystallinle growth of grains greater than 1 mm in size at low linear speeds (and consequently *low areal generation rates). Common to these sheermanufacturinlg processes is the fact that the 5heet pulling direction and the direction of grain growth are collinear.

Alli of these processes employ a large temperature gradient (>500 degrees Centigrade per centimeter) along the sheet pulling~ direction- This gradient is necessary to achieve the practical linear sheet pulling velocity (typi.cally les than 2 cm/utif), but also introduces large thermal- induced stresses- In many cases these stresses limit the achievable practical sheet width by causing sheet deformations which make solar cell fabrication untenable.

Thermal stresses can also create crystalline defects which limit solar cell performance. Each of these processes attempts to achieve grain sizes that are as large as possible in order to avoid the deleterious effects of grain boundaries on solar cell performance.

Another set of processes has been developed that can achieve high areal throughput rates. For example, publications by Bates, et al. (LASS), Helmreich, et al.

(RAFT), Falckenberg, et al. (S-Web), Hide, et al. (CRP?) Lange, et al. (RGS) a-nd Hall et al. (SF) describe that achieve polycrystallinle sheet growth with grain sizes in the 10 microns to 3 =m range at high linear *rates (10 to 1800 em/mn) Typically, these processes have difficulty maintainling geometric control (width and thickness) (eog. LASS, FAET, P.GS), and/or experience difficulty with contamination of the silicon by the contactinfg materials RA S-Web, CRP) The process of Hall, et al-. S. Patents -5,336,335 and 5,496,416) effects geometric control and minimizes the contact of the growing silicon with deleterious materials. Common to these sheet growth processes is the fact that the sheet pulling di.rection and the direction of crystalline grain growth are nearly perpendicular. It is this critical feature of these processes that allows the simultaneous achievement of high linear sheet pulling veloci.ties and reduced crystal growth velocities. Reduced crystal growth .000 velocities are necessary for the achievement of materials with high crystalli.ne quality.

The prior art regarding the fabrication of solar cells from polycrystalinle silicon materials requires that the grain si.ze be greater than 1.0 mm. This requirement on grain size was necessitated by the need to mi.nimize the deleteri.ous effects of grain boundaries evident in prior art materials. Historically, small-grained polycrystalline silicon (grain size less than 1.0 mm) has not been a candidate for photovoltaic material due to grain boundary effects. Grain boundary recombination led to degradation of voltage, current and fill factors in the solar cell. Previous models, for example Ghosh (1980) and Possum (1980), based on recombination at active grain boundaries correctly predicted performance of historical materials. These models teach that if active grain boundaries are present, they prohibit the utilization of small grained materials in high performance solar cells.

SUMMARY OF THE INVENTION It is the object of this invention to provide low-cost columnar-grained sheets that can be employed in high performance solar cells.

A further object of this invention is to provide columnar-grained polycrystalline silicon sheets for use as a substrate in solar cells, which overcomes one or more disadvantages of the prior art.

In one broad aspect, the present invention provides a sheet of silicon having a pair of free surfaces, the columnar grains extending axially through the sheet from one free surface to the other free surface, the sheet having an electrical resistivity in the range of 0.1 to 10 ohm-cm.

i l** W:kate\SPEClDivof739048.doc The sheet can be formed by using a columnar growth technique that manages the details of nucleation, growth, and heat flow to control the material quality and decouple the grain growth velocity from the linear sheet pulling velocity of the polycrystalline material. The process begins with granular silicon that is applied to a setter material; the setter and silicon are then subjected to a designed thermal sequence which results in the formation of a columnargrained polycrystalline silicon sheet at high areal throughput rates.

The equipment employed to accomplish the process can include a distributed source of energy application, such as by graphite-based infrared heating. The sheet may also be formed with a process which includes a line source, such as by optical focusing.

15 BRIEF DESCRIPTION OF THE DRAWINGS fee.

FIGURE 1 illustrates a perspective view showing the sequence for fabricating low stress, columnar-grained silicon sheets usable as solar cells substrates; S.o o.oeoo W:AkateSPECDivof739048.doc FIGURE 2 illustrates a perspective view showing the sequence of nucleation and growth of the silicon sheet; and FIGURE 3 illustrates the manifold of process steps leading to the manufacture of the same.

DETAILED DESCRIPTION The present invention is directed to improved columnar grain polycrystalline sheets which are particularly adaptable for use as substrates or wafers in solar cells. The techniques for making those sheets are based upon the techniques in U.S. Patents 5,336,335 and 5,496,446, all of the details of which are incorporated herein by reference thereto. The ability to use the sheet as a solar :cell substrate makes possible the provision of a solar cell consisting entirely of 15 silicon material where the sheet would function as a substrate made of silicon and the remaining layers of the solar cell would also be made of silicon.

oo.oo The desired properties of the columnar-grained silicon sheet or substrate fabricated with the teaching of this invention are: flatness, a smooth surface, 20 minority WAkateSPEC\Divof73948.doc carrier diffusion length greater than 40 microns, minimum grain dimension at least two ti.mes the minority carrier diffusion length, low residual stress and relatively inactive grain boundari.es. Since the minimum grain dimension of the columnar grain silicon sheet is at least two times the minority carrier diffusion length which in turn is greater than 40 microns, the columnar grains would have a grain size greater than 80 microns. Grain sizes down to 10 microns can be employed with minority carrier diffusion length greater than 10 microns, and will lead to solar cells having lower currents, and lower power. The desired properties of a process for fabricating columnargrain silicon material appropriate for inclusion in a lowcost solar cell in accordance with the teachings of this invention are; low thermal stress procedure, controlled nucleation, high areal throughput, and simple process control.

The criteria for the columnar-grain silicon material product of flatness and smoothness are required to make solar cell fabrication tenable. The requirements on diffusion length and grain size are to minimize recombinatini losses in the bulk and at grain surfaces grain boundaries), respectively. The requirement of relatively inactive grain boundaries is to effect the minimization of grain boundary recombination. The requirement of low residual stress is to minimize mechanical breakage and to maintain high minority carrier diffusion lengths.

The criteria for the columnar-grained silicon process of a low thermal stress procedure is to effect minimization of bulk crystalline defects. The requirement of controlled nucleation is to affect the achievement of the required grain morphology and size. The criteria for high areal throughput and simple process control are to achieve lowcost and muanufacturability.

Figure I is a perspective view illustrating the sequence for fabricating low stress, columnar-grained silicon sheets. The process as depicted moves from left to right.

In general, a setter material 100, which serves as a mechanical support, is coated with a granular silicon layer 200, and is passed through a prescribed thermal profile.

The prescribed thermal profile first creates a melt region 300 at the top of the granular silicon 200, and then creates a nucleation and growth region 400 where both liquid and a growing layer of polycrystalline layer coexist. Finally, there is an annealing region 500 where the temperature of the polycrystalline silicon sheet layer 600 is reduced in a prescribed manner to effect stress relief. Any or all of the preheat, melting, growth and anneal thermal profiles for the granular powder and resultant sheet could be achieved by graphite-based heater technology.

The setter material 100 is selected based on the following requirements. It must: maintain its shape during the sheet formation thermal processing; not chemically interact with, or adhere to, the silicon material; and possess the proper thermal characteristi.cs to effect the required sheet growth and annealing. The form of the setter material may either be as a rigid board or as a flexible thin belt.

Several materials including, but not limited to, quartz, refractory boards silica and/or alumina) graphite, silicon nitride and silicon carbide have been employed and mai.ntained the proper geometric shape during thermal processing.

To assure that the setter 100 does not adhere to the final polycrystallile silicon sheet 600, a release agent coating 110 is applied to the setter. Either, or a comnbinati.on of, silicon ni.tride, silicon oxynitride, silica, powdered sili.con, al.umin-fa, silicon carbide or carbon in any form have been employed as this agent. A low-cost method for applying this coating i.s to form a liquid slurry that is painted or sprayed on the bare setter, and subsequently dried before use. The release agent coating may also be applied by the method of chemical vapor deposition. The release agent facilitates separati.on of the sheet and permits reuse of the setter materi~al.

in the process design the thermal characteristics of the setter 100 play a kcey role in managing the melt and growth processes. In the melt region 300 It is preferred that the thermal conductivity of the setter be high to assure the efficient deploym~ent of the energy being used to melt the -11granular sil.icon 200.

in a preferred embodiment the setter material is graphi-te.

The setter preparation i-s completed by coati.ng the top surface with a release agent 110. This is accomplished using an aqu.eous colloidal soluti.on of silicon nitride that i-s painted on the top surface and baked to form a nonwetting, non-adhering oxynitride layer, before the initial" application of granular silicon.

The granular silicon 200 must be properly sized and be of adequate purity. The range of proper sizes for the *granular silicon 200 employed in the process is between and 1000 micrometers. The upper limit is determined by the design thickness for the silicon sheet material.

Preferably the silicon powder is comprised of sizes less 0: than 500 microns and the sheet i-s formed at a sheer pulli.ng of greater than 20 cm/min. As a rule the dimensions of the sili-con particles should be equal to or less than the desi-red thickness of the silicon sheet material. The lower size limit of the particle distribution is dependent on the dynamics of the melting process, and the need to limit the -12amount of silJicon oxide. The siliconl oxi~de is a source of sheet conltamuinationh and naturally occurs at all silicon surfaces- There are several techniques for applying the silicon to the setter that include, but are not li.mited to, doctor blading, plasma-arc spraying and tape casting.

The purity level necessary in the zheet sili.con ia determined by the requirements of the specific application .:of the sheet. Whereas the employment of low-processed .00.metallurgical grade silicon is not adequate for the efficient operation of a solar cell devi.ce, utilization of highly processed semi~conduictor grade silicon is not necessary. In practice, for direct solar cell applications the preferred process can be executed with off-grade semiconductor grade sili.con.

The additi~on of a separate constituent i.n, or with, the granular silicon may be employed to effect the optical bandgap of the sheet. Additions of carbon, and in particular germanium, can increase (carbon) or reduce -13- (germanium) the optical bandgap. Such changes in the optical bandgap of the sheet material are desirable depending on the spectral output of the inci.dent radiation being employed with a solar cell design. in the case of germanium combinations of sil.icon and germanium can be used where either can be from 0 to 100% (by mass)- The addition of a separate constituent in, or with, the granular silicon may be employed to effect an electrical resi.stivity in the range of 0.1 to 10 ohm-am~ in the sheet material. Typically, for p-type conductivity in the sheet material the preferred elements are boron, aluminum, or indium. As an example of the preferred embodiment, the addition of powdered boron silicide followed by mechanical mixing of the granular silicon provides for the accomplishment of the required p-type resistiv~ity in the subsequently grown silicon sheet.

The properly doped p-type granular silicon 200 is uniformly layered on the coated setter 100. For example, in a preferred emibodimenlt this process9 can be effectively accomplished by using a doctor blade. The spacing between -14the edge of the doctor blade and the setter surface needs to be at leas5t two times the dimension of the largest parta.cle in the granular si.liconl size distribution.

Furthermore, the thickness of the final silicon sheet 600 can be the dimension of the largest particle in the granular size distribution.

The silicon-coated setter is transported into an environmental chamb~er with an argon or nitrogen overpressure. In a preferred embodimient a mixture of argon and hydrogen gas is employed to effectively limit the amount of siliconl oxide that is formed during the growth process. The percent of hydrogen employed is determined by *the water vapor content in the chasuber. The ratio of hydrogen to water vapor controls the magnitude of si.licon oxide formation. Preferably 5-100% by volume hydrogen gas 1s used to reduce the silicon oxide. The chamber may ainclude a pre-heat zone employed to raise the temperature to 1.100* to 14000 which in combination with the hydrogen has the effect of reducing the nati.ve oxide of silicon that exists on the granular silicoA. In another preferred embodiment a combination of nitrogen and argon -Is- (other non-reacting gases such as helium, neon and krypton will also work) in the envir-oiental chamber is employed where either can be from 0 to 100% (by volume) After the granular si~licon 200 has been pre-heated it is then brought into a thermal zone 300 where the top portion of the granular sili.con layer 200 is melted. The depth of the granLular silicon that is melted depends on the intensity of the input energy from thermal zone 300, the thickness of the granular silicon layer, the li.near speed of the granular silicon coated setter through thermal zone 300, and the details of heat transfer between the granular silicon 200 and the setter 100. Between 25 and 90% (and preferably between 50% and 90%) of rhe granular si.licon depth. is melted, primarily f rom the top. The materi.al at the bottom of the granular layer is partially melted by liquid silicon penetrating from the molten silicon layer above. This partially melted layer of silicon forms a net 220. The net 220 is responible for two process features.

First, because it is wetted by the molten silicon above, this layer stabilizes the melt and growth zones by defeating the surface tension of the molten silicon over- -16layer- This allows the production of wide sheets, with smooth surfaces. Sheets widths of up to 38 cm have been manufactured. Second, this layer can serve as a plane to nucleate subsequent growth (as described in U. S. Patent 5,496,416).

Figure 2 is a perspective view showing the sequence of nucleation and growth of the silicon sheet. A thin-film capping layer is formed on the top surface of the liquid silicon 301 while it is in the melt zone 300. The role of the capping layer (nucleation layer 305 in Figure 2) is to effect the mechanism of heterogeneous nucleation. In this process (see Chalmers, Principles of 5oidification, John Wiley SonS, New York, 1964) nucleation is controlled by the formation of nuclei of critical size catalyzed by a suitable surface in contact with the liquid. The Anucleation catalyst" or "nucleant" may be either a solid particle suspended in the liquid, a liquid containing surface, or a solid film, such as an oxide or nitride. In this invention the "nucleant" may be applied as a coating to the granular silicon before the introduction into the environmental chamber or applied in situ.

-17in the grain growth process described by the present invrentionl the rate of grai.n growth is determined by the details of heat extraction from the melt, and the grain size i.s determined by the nucleation density. By employing a nucleation layer 305 the nucleation occurs in a preferred manner at the nucleation- Iayer /molten- siliconl interface, and the nucleation density is actually reduced compared to a free molten liquid surface. This allows for the achievement of controlled growth and increased grain sizes in the manufactured sheet.

one method to effect a "nceat layer is to apply it to the granular silicon prior to introduction into the environmental chamber. The coating materials in this manner to effect nucleation of the silicon growth include, but are not limnited to, the carbides, nitrides, oxides and oxynitrides of silicon, the oxides and nitrides of boron, and aluminum oxide. The selected materials were chosen based on the need to maintain required purity requirements 0900 of the resultant silicon sheet.

In another method the coating 305 may be formed in-situ on -U8the free surface at the top of the liquid 301 in the melt zone 300. The coating materials employed on the free surface of the liquid 301 to effect nucleation of the silicon growth include, but are not limited to, the carbides, nitrides, oxides and oxynitrides of silicon, the oxides and nitrides of boron, and aluminum oxide. The selected materials were chosen based on the need to maintain required puri.ty requiremenlts of the resultant =iilicaa sheet. In a preferred em~bodi.men~t the carbides, nitrides, oxides and oxynitride employed as coatings 305 on the free surface of the melt 300 may be formed by the utilizati.on of carbon, oxygen, and/or nitrogen containing gases as the process gas i.n the environmental chamnber.

In the preferred embodiment the nucleation layer 305 is formed by the reaction of nitrogen in the process gas with th e free li.quid silicon surface 301. For examiple, an effective nucleation layer is formed in a combination of nitrogen and argon (comibined total 100%) when the nitrogen is 10% or greater (by volume). The reaction of the nitrogen gas and the free surface of the liquid silicon 301 forms a layer of nitrided silicon. It is a further -19embodiment of the present inventi.on that a small amounit of oxygen gas (10 to 1000 ppm by volume) can be added to the nitrogen-argonl combination to improve the nucleation properties of the nucleation layer 305.

After leaving the melt creation zone 300 of the thermal profile, the melt pool 301 with a nucleation layer 305 and the parti.ally melted silicon net 220 moves into the nucleation and growth zone. Figure 2 also illustrates the process occurri.ng in the nucleation and growth zone 400.

Figure 2 indicates schematically the means of controlling the process of nucleation and growth in zone 400 by managing the application and removal of heat. This is depicted in the figure by Heat Management from the top 450, and Heat Management from the bottom 460.

Generally, nucleation and growth proceed as follows- A series of preferred nucleation sites 405 are formed in zone 400 at the interface between the liquid silicon 301 and the nu cleation layer 305. The details of this formation process are effected by the means of heat management from the top 450 and the bottom 460. After formation of the -2Gpreferred nucleation sites 405, grain growth 410 on these sites is effected by modifying the heat management 450 and 460. The direction 470 of the grain growth front i.s approximately perpendicular to the plane of the setter., and perpendicular to the direction 610 in which the sheet i~s being pulled. The length of the growth zone along the direction of setter motion is from 10 centimeters to a maximum related to the sheet thickcness, and the magnitude of tile sheet pulling velocity and the grain growth velocity, The length of the growth zone is determined by controllinlg the rate of loss of heat (and therefore growth rate) attending the solidification process. A.s a consequence of the growth processo the grains that are grown are columnar in nature. Typically, individual columnar grains 605 in the resulting sheet 600 have their axial direction extend from the top surface to the bottom, and are at least as wide as they are high. Sheet thicknesses in the range of 400 to 800 microns can be achieved at sheet pulling speeds in excess of 120 cm/min.

~'**After leaving the nucleation and growth zone 400 of the thermal profile, the sheet 600 moves into the annealing -21zone 5oo of the thermal profile, in this zone the grown sheet, still at approximately 1400* C, is subjected to a linear temperature gradient along the direction of setter motion. The linear temperature profile eliminates buckling and cracking of the as-grown sheet, and minimizes the generation of dislocations. The grown sheet may have a thickness between 50 microns and 2 mm. The thickness of the grown sheet is in the range of 350 to 1000 microns in the preferred process. Because the thickness of the final 0 grown sheet 600 is determined by the precise application of granular silicon 200 to the setter 100, exceptional sheet control and process stability are achieved in 0 0 0 6 comparison to sheet technologies pulled from a melt, where thickness is controlled by the melt meniscus. After coal 00 0 down, the sheet is removed from the setter, and 0*0*04 appropriately sized by sawing or scribing, for fabrication 0 0 into solar cells. The setter is reused for making further columnar- grained polycrystalline sheets.

Figure 3 depicts the process steps that can be employed by the invention to achieve the silicon sheet at high sheet pulling velocities. There are several nucleation and -22growth reg-imes that have been successfully employed. Below are several examples derived from Figure 3.

The properties of the sheet material fabricated with the above process are quite amenable to the fabrication of effici.ent solar cells. This process generates material that has un~ique propeti~es of size and character. Although the grains are columnlar, and have average sizes in the range of 0.002 to 1,000 CM in entent, solar cells fabricated on material in the range of 0.01 to 0.10 cm. may achieve voltages in excess of 560 mV, and fill factors in 0@Oe excess of 0.72. The achievement of these values on such 0ee0 small grained material indicate that this material is not being limited by recomubinationl at grain boundaries as had been previously predicted by Ghosh. Previously, columnar grains that extend from surface to surface of the sheet wjere dismissed as being ineffective since columnar grains were always small, and small grains were thought not to work. The process herein described achieves columnar 0840 grains that yield material with relati-vely benign grai~n boundaries with the result that efficient, low-cost solar cells can be manufactured.

-23- It is a unique feature of the top-down grain growth process described above that the device-active region at the top of the sheet is crystallized from the silicon melt while the bottom Of the sheet next to the setter is still solid, thus minimizing any contamination of the top of the sheet by the setter. It is a further advantage of the top-down grain growth process that purification of the device-active region is effected by fractional solidification (see; Zief and Wilcox, Frgrtionial Sqol eficati.on, Marcel Dekker-, Neu York, 1-967). By this method the most pure grown material is at the top of the sheet where the initial grain growth occurs. The subsequ~ent grain growth process has the effect of sweeping impurities to the bottom of the solidified sheet away from the device-active region (similar to zone refining), The process herein described can be carried out in a continuous manner, resulting in continuous sheets that can be appropriately sized using an in line scribe or a saw.

impurity content in the melt and grown sheet quickcly reaches steady-state; it does not increase during continuous processing. Since al.l embodiments include -24application of granular silicon to the setter, and since materiaBl enters the melt creation zone in this form, melt replenishmenlt is not a problem, unlike sheet technologies pulled from a melt pool. After being properly si.zed, the sheets function as a substrate by having the remaining layers formed thereon to produce solar cells. Where the remaining layers are of silicon (or an alloy to tailor the optical bandgaP, such as carbon or germanium), a complete solar cell results. Tris sheet materlal can alzo be the -substrate for other solar cell fabrication processes including those employing cadmium telluride and copperindi.um selenide. Thus an advantage of the invention is that it lends itself to mass production. The invention results in a free standing grown continuous sheet of silicon which could then be cut to individual sizes in accordance with its ende use.

The present invention anticipates the forward integration of the silicon sheet product in subsequent processing into solar cells and Modules. It is expected that the solar cell fabrication steps, including, but not limited to, surface preparation, junction formation, electrical contacts and anti-ref lection coatings, can all be accomplished wi.th the product in its sheet form, Such continuous processing will have the advantage of signifticantly reducing manufacturing costs.

The mechanism of spontaneous nucleation (see Figure 3) i~s operative when the li.quid silicon 302. has become supercooled, and in the absence of any uniftorm extraction of heat in the nucl.eation and growth zone 400, results in a solid-liquid interface that, is inherently unstable. This instability leads to dendritic growth which is typi.cally equiaxed in geometry. Such grain growth may or :may not extend from one sheet surface to the other.

in another variation of the process (see Figure 3) the silicon net 220 serves as the source of nucleation of grain growth from the liquid silicon 301. in this case the heat is most effectively extracted from the bottom 460. In .26order to reduce the rate of grain growth it may be desirable to apply some heat from the top 450. Heat extraction from the bottom can be effected by means of radiation plates. Heat addition at the top is effected by means of planar heaters. For example, to effect a grain growth rate of 0-15 cm/min for a 700 micrometer thick sheet moving at sheet speed of 100 cm/min requires a net heat removal rare from the bottom heat extracted from bottom minus heat added to top) of approximately watts/cm2, and results in a nucleation and growth zone length of 46 cm. In this process regime the solid-liquid eoooeo interface is stable leading to the growth of columnar grains that extend from the net at the bottom of the sheet up to the free surface of the sheet. This example is similar to the description in U.S. Patent 5,496,416, indicating that the silicon net may serve as a plane to nucleate subsequent growth.

on another variation of the process (see Figure 3 formation of, and growth from, a nucleation layer) granular silicon in the size range of 75 to 350 micron is doctorbladed onto a graphite setter coated by a mold release -27coating consisting of a mixture of the oxides and nitrides of silicon. The granular silicon and setter are then transported at a continuous velocity through a graphitebased heating chamber under a 100% nitrogen gas atmosphere bringing the silicon and setter to a temperature close to 1400-C. During this time the majority of silicon oxide that existed on the granular silicon surfaces is volatilized and subsequently removed from the heated .chamber. The silicon and setter continues in the chamber *to a zone where the silicon is melted, primarily from the top, leaving a silicon net at the bottom. During this time a thin-film capping layer of silicon nitride is formed on the free surface of the silicon. The thin-film of silicon nitride plays the role of a nucleation layer in the next zone of the chamber where minrimal. external heat is applied and the nucleation sites are preferentially created at the thin-film layer/molten silicon interface. The layer combination of setter, silicon net-, liquid silicon and thin layer of silicon nitride with an array of nucleation sites continues into a zone where heat is preferentially removed from the top 450. In this case, the grains grow down from the nucleation sites occurring at the thin-film/molten silicon interface and terminate at the silicon net at the bottom. The grain growth rate is determined by the details of heat extraction 450. in thi.s process regime the soli.dliquid interface is stable leading to the growth of columnar grains that extend from the nucleation layer at the top of the sheet to the net at the bottom of the sheet.

Same as Example #3 except af ter an iLnitial top-down grain growth from the nucleation layer through most of the molten thickcness, the sheet moves into a subsequent zone that adds heat from the bottom 460, such that the net layer and some portion of the previously top-down grown grains are melted.

The sheet then continues to a zone where the heat is again extracted from the top 450. The growth now resumes from the solid-liquid interface of the initially grown grains and continues all the way through the sheet to the bottom of the sheet. In this process regime the solid-liquid interface is stable and results in the growth of columnar grains that extend from the nuclear-ion layer at the top of the sheet to the bottom of the sheet. This process also ,29has the capability of sweeping out impurities to the back of the sheet by a process similar to zone refining.

These examples are for illustrative purposes and are not intended to represent all the processes for making the sheet of the present invention.

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Claims (4)

1. A sheet of silicon having a pair of free surfaces, the columnar grains extending axially through the sheet from one free surface to the other free surface, the sheet having an electrical resistivity in the range of 0.1 to ohm-cm.

2. A sheet as claimed in claim 1, wherein the sheet includes at least some amount of nitrogen.

3. A sheet as claimed in claim 1 or 2, wherein the sheet functions as a substrate, and the sheet is in combination with photovoltaic layers to comprise a got# solar cell. DATED:

4 January 2001 PHILLIPS ORMONDE FITZPATRICK Attorneys for: ASTROPOWER INC. o0000n<ojc c D 0 0o W:1kate\SPECI\Divof739048.doc