GB2198966A - Method of growing silicon dendritic-web crystals - Google Patents

Abstract

Method of growing silicon dendritic-web crystals using magnetic fields to suppress destabilizing boundary-driven convection in silicon melts to enable growth of long lengths of silicon dendritic-web crystals from large and deep melts. Current practices rely on the use of very shallow melt configurations and complex melt replenishment procedures.

Description

METHOD OF GROWING SILICON DENDRITIC-WEB CRYSTALS This invention relates to a method of growing silicon dendritic-web crystals from deep melts.

Silicon dendritic-web crystals are long, thin ribbons of single crystalline material of high structural quality which can be grown in the (111) orientation. The current impetus for developing silicon dendritic-web is its application to the production of low-cost, highly efficient solar cells for direct conversion of sunlight to electrical energy. The thin ribbon form of the crystal requires little additional processing prior to device fabrication, in contrast to wafer substrates from the more traditional Czochralski crystal which must be sliced, lapped and polished prior to use, a costly process even though large volume economies are practiced. Additionally, the rectangular shape of the silicon ribbon leads to efficient packing of individual cells into large modules and arrays of solar cells.

Recent developments have resulted in the growth of long lengths of flat web crystals up to 5.5 centimeters in width, from which highly efficient solar cells have been produced, exhibiting over 16% AMI conversion efficiency.

The critical factor in achieving controlled dendritic-web crystal growth as in any solidification process is the dissipation of the latent heat released on solidification. In web growth, in contrast to -the more conventional Czochralski growth of larger, round cross-section crystals, small temperature excursions in the melt perturb the growth interface, while larger temperature fluctuations result in loss of crystal integrity due to either loss of the dendrite when the melt is too hot, or growth of extraneous "third" dendrites when the melt is too cold. The following problems, addressed by this disclosure, occur in present-day silicon dendrite web growth, due to convective flows and temperature fluctuations.

The growth of long lengths of silicon web crystals for low-cost solar cell arrays requires melt replenishment during growth from shallow melts. While melt replenishment techniques have been developed and long lengths of web have been grown, the ability to grow from large deep melts will reduce significantly the growth complexity and result in a more economical growth technique.

A balanced heat flow from the growth interface to the solidification web and undercooled melt must be maintained to achieve a flat interface temperature profile and avoid residual stress in the web crystal. This balance is primarily established by the spacing between the melt surface and the radiation shield positioned over the melt surface. This melt to shield distance must then be maintained during crystal growth.

While large turbulent convective flow and temperature excursions result in catastrophic interruption of growth, smaller temperature oscillations near one of the supporting dendrites will affect dendrite penetration into the melt and lead to increased dislocation generation. The majority of dislocations in web crystals are generated in bursts at melt entrapment centers on the dendrite surface, which may slip into the web sheet and then turn and propagate in the growth direction. These melt entrapment centers may be avoided by control of dendrite penetration and width.

Local temperature oscillations at the normally planar web sheet growth interface may result in dopant or impurity striations due to change in the instantaneous growth rate at the interface, in a manner analogous to the effects observed in Czochralski growth.

Turbulent convective flow in the silicon melt sweeps the surface of the quartz crucible, and readily transports oxygen and crucible impurities to the growth interface, where these impurities may be incorporated non-uniformly into the crystal. An attempt is now made to solve the above problems by providing large deep melts which are thermally quiescent, in which the thermal stability of the growth interface is maintained through the utilization of a magnetic field impressed across the melt volume.

Accordingly, the present invention resides in a method of growing silicon dendritic-web crystals which comprises charging a crucible with polycrystalline silicon; melting the silicon; applying a magnetic field transversely across the silicon melt of strength suitable to control temperature oscillations in the melt; positioning a radiation shield a spaced distance above the melt appropriate to provide a heat flow balance between the melt and a dendritic web during growth of such a web; and growing a dendritic web from the melt through a slot in the radiation shield.

The application of a magnetic field of sufficient strength across a silicon melt, or any melt in which the melt is conductive, imposes an inductive drag on fluid motion within the melt and inhibits turbulent convective fluid flow. The magnetic field will inhibit convectioninduced temperature fluctuations in the vicinity of the dendritic-web growth interface with the result that web crystals can be grown -from deep melts without catastrophic disruptions. In addition, the magnetic field inductive drag on melt motion will reduce local melt temperature oscillations at the dendrite and web interface, and lead to control of dendrite width and dislocation, reduction in dopant and impurity striations, and to a reduction of oxygen and impurity contamination arising from crucible dissolution.

The present invention allows for successful growth of web crystals from 30-millimeter deep melts by application of a transverse magnetic field to the melt. A mechanical arrangement of melt-containing crucible and an independently suspended radiation shield maintains a constant melt surface-to-shield distance, and leads to balanced heat flow from the growth interface, as well as controlled growth of silicon dendritic-web crystals.

In order that the invention can be more clearly understood, convenient embodiments thereof will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic drawing of a growing web crystal indicating seed dendrite, button, and solidification interfaces of dendrite and web, Figure 2 is a schematic drawing showing the silicon melt and dendritic-web crystal within the transverse magnetic field, Figure 3 is a graph showing the change in temperature oscillations in accordance with the present invention of the melt as measured by a thermocouple probe positioned two millimeters into the melt, Figure 4 is a graph showing the relative difference in temperature oscillations as measured by a thermocouple probe positioned two millimeters into the melt as a function of applied magnetic field, and Figure 5 is a graph showing temperature profiles through a 27 millimeter deep melt comparing a melt without a magnetic field and a melt with a 1730 gauss field.

Referring to Figure 1, dendritic-web crystal growth is a process whereby a pair of coplanar dendrites 24 growing from an undercooled region of melt 26 frame a thin sheet or "web" of crystal 25. A temperature gradient is imposed across the melt to obtain the undercooled region necessary for growth, a gradient which results in buoyancy effects in bulk fluid motion, commonly termed natural or "free" convection in the melt. As the melt depth is increased, the increased buoyancy effect results in turbulent convective flow and finally in the formation of convection cells within the melt. Prior to the present invention, silicon dendritic-web crystal growth was always accomplished from shallow melts generally seven to ten millimeters deep to avoid turbulent convective effects.In melts deeper than ten millimeters, buoyancy driven temperature fluctuations generally resulted in the loss of crystal integrity.

Referring to Figure 2, the application of a horizontal magnetic field to silicon dendritic-web growth was accomplished using a NRC Czochralski puller 10 modified to a suitable configuration. A VARIAN 12-inch pole electromagnet 11 with the puller chamber 12 positioned between the pole faces 13 is shown in Figure 2. In this orientation, a large magnetic flux return circuit surrounds the apparatus. Preferably, a web pulling reel is positioned above the puller 10, in line with an extended slot 16 in the top flange 17 of the puller chamber 12. Preferably, a temperature controller using thermo-radiation sensing from the base 19 of the crucible 31, a pull speed control, and a magnetic field power supply are also used.A series of movable guides 22 beneath the top flange 17 of the puller serve to position the dendrite 24 in crystal 25 within the undercooled region 26 of the melt during seeding and pulling operations. The cylindrical graphite resistance heater 28 is heated by direct current to avoid flexing and vibration which occurs when using low frequency alternating currents created by the electromagnet 11. The undercooled region of the melt 26 was defined by a molybdenum radiation shield containing a "dog-bone" shaped slot. The melt was contained in an 80 millimeter diameter quartz crucible 31, standard in silicon Czochralski growth. As shown in Figure 2, the puller chamber 12 is surrounded by thermal insulation 33 and a water-cooled vessel 34.

The important melt surface-to-radiation shield spacing was achieved by suspending the shield independently from the crucible 31, such that the crucible 31 could be controllably raised as the melt volume is depleted. This arrangement for crucible lift is common in conventional silicon Czochralski crystal growth.

In a typical growth, a polycrystalline charge of approximately 500 grams was placed within the quartz crucible 31 along with appropriate doping for solar cell application. The loaded crucible was placed within the hot zone 32 of the puller, and raised until the radiation shield extended inside the top of the quartz crucible 31.

After suitable purging with high purity argon gas, the silicon was melted under a flowing argon stream, and the crucible 31 raised to give the desired melt-to-shield spacing of about seven millimeters. Typical melt depths using this arrangement were 30-40 millimeters, a depth considerably in excess of the seven to ten millimeters considered maximum in normal growth of silicon web.

While this melt and crucible arrangement was used here to reduce this concept to practice, there is no a-priori reason that very large diameters and deep melts could not be used with magnetic field coupling. In a different application, a large Czochralski puller with an axial magnetic field supplied by a superconducting magnet has been used to obtain thermally quiescent 12-inch diameter, 10-inch deep, 20-kilogram silicon melts. The application of silicon dendritic-web growth to such a large system could result in preparation of very long lengths of silicon web without need for melt replenishment.

In a typical growth, a horizontal magnetic field of 1160 to 2330 gauss was applied across the melt before initiating growth. Seeding and growth were accomplished in a manner completely analogous to growth from shallow non-magnetic field growth. The dendritic-web crystals grown from a 30-millimeter deep melt were in all respects similar to web crystals grown from seven to ten millimeter deep non-magnetic field metals, even though no attempt was made to optimize the growth configurations.

The effect of magnetic fields in reducing the temperature turbulence was quantitatively evaluated by using a thermocouple probe within the melt. A fine type B thermocouple (Pt 6% Rh, Pt 20% Rh) sheathed in thin-walled alumina was used for this measurement. A measurement sensitivity of one microvolt was displayed on a recording chart using a KEITHLEY 155 null detector/microvolt meter and EUROTHERM millivolt bucking source. With the thermocouple immersed two millimeters into the melt, positioned within the center of the undercooled region, temperature oscillations of 0.60C were measured.As shown in Figure 3, the application of a 1740 gauss magnetic field reduced the melt temperature oscillations to less than 0'. 10C. The reduction in melt temperature by 2.30C with application of magnetic field is illustrative of reduced temperaturedriven convective fluid flow in the melt.

Figure 4 compares the temperature oscillations in the melt at several magnetic field strengths. Normal temperature oscillations of 0.6 C are shown to be increased at 600 gauss, but reduced to less than 0.10C at 1160 gauss and above. Therefore, only moderate fields are required to achieve thermally quiescent melts within the melt geometry used in this experiment.

Temperature profiles were measured in the residual melt following the growth of web crystals by immersing the thermocouple to the bottom of the melt and slowly pulling the thermocouple (attached to the pulling apparatus) up through the melt. The profiles and thermo-oscillations are shown in Figure 5 without magnetic field and with 730 gauss field through the 27 millimeter deep residual melts. Note that the temperature deep into the melt is reduced almost 30C by the magnetic field, decreasing as the thermocouple is pulled into the steep gradient region near the undercooled melt surface.

These measurements are representative, and illustrate the effect of magnetic fields in reducing thermal oscillations in the melt geometry described.

Similar reductions can be expected in large and deeper melts allowing dendritic-web growth from large melts with controlled heat flow geometry at the growth interface.

Claims (6)

CLAIMS:

1. A method of growing silicon dendritic-web crystals which comprises charging a crucible with polycrystalline silicon; melting the silicon; applying a magnetic field transversely across the silicon melt of strength suitable to control temperature oscillations in the melt; positioning. a radiation shield a spaced distance above the melt-appropriate to provide a heat flow balance between the melt and a dendritic web during growth of such a web; and growing a dendritic web from the melt through a slot in the radiation shield.

2. A method according to claim 1, wherein the silicon melt has a depth of at least 10 millimeters.

3. A method according to claim 1 or 2, wherein the magnetic field applied across the silicon melt is greater than about 1160 gauss.

4. A method according to claim 1, 2 or 3, wherein the silicon is melted using a cylindrical graphite resistance heater heated by direct current.

5. A method according to any of claims 1 to 4, wherein the radiation shield is independently spaced above the melt such that the crucible can be controllably raised as melt volume is depleted.

6. A method of growing silicon dendritic-web crystals as claimed in claim 1 and substantially as described herein with particular reference to Figures 2 to 5 of the accompanying drawings.