JPS63185887A - Method of growing dendrite web crystal of silicon - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for growing dendrite web crystals of silicon from a deep melt.

BACKGROUND OF THE INVENTION Dendrite web crystals of silicon are long, thin ribbons of high structural quality single crystal material that can be grown in the (zi) orientation. A major impetus for research and development of silicon dendrite web crystals is that these forms of silicon crystals can be used in low-cost, high-efficiency solar cells that directly convert sunlight into electrical energy. It is. Unlike wafer-like substrates made from conventional Czochralski crystals, which must be sliced, lapped, and polished prior to use, which are costly steps even when carried out on a large scale, thin ribbon-like crystals Little pretreatment is required prior to manufacturing the device. Furthermore, the rectangular shape of the silicon ribbon allows for efficient packing of individual cells into large solar cells consisting of multiple modules and rows.

As a result of recent technological advances, long flat web crystals up to 5.5 cm wide can be grown and 1
A highly efficient solar cell with a conversion efficiency of 696 AMI has been created.

Regardless of the solidification process, an important factor controlling the growth of dendrite web crystals is the dispersion of the latent heat released during solidification. Unlike traditional Czochralski growth, which grows crystals with larger round cross-sections, small fluctuations in the temperature of the melt during web growth can disrupt the growth interface, and large temperature fluctuations can cause the melt to melt. Crystal integrity is compromised when the melt temperature is too high due to the disappearance of dendrites, and conversely when the melt temperature is too low due to the growth of extraneous third dendrites. The following problems that the invention overcomes are some of the problems currently encountered during silicon dendrite web growth due to convection and temperature fluctuations.

Creating long silicon web crystals for use in low cost solar cell arrays requires melt replenishment during growth from a shallow melt. Techniques for melt replenishment have been developed that allow the growth of long webs, but the ability to grow from large amounts of deep melt considerably reduces the complexity of the growth method and improves its economics. It seems that it will.

In order to obtain a flat boundary temperature profile and to prevent residual stress from remaining in the web crystal, the heat flow from the growth boundary to the solidified web and the supercooled melt must be balanced and maintained. There must be. The distance between the surface of the melt and the thermal radiation shield located above the surface of the melt determines this balance. This melt-to-shield distance must remain the same during subsequent crystal growth.

The growth process determines that convection and temperature fluctuations due to large turbulence occur, and when small temperature fluctuations occur near one supporting dendrite, the manner in which the dendrite penetrates into the melt changes, causing a fault to occur. This results in an increase in Most of the faults in web crystals occur suddenly at melt trapping centers on the dendrite surface, and after penetrating into the web sheet, they turn and propagate in the crystal direction.

By controlling the degree of penetration and width of the dendrites, the formation of melt trapping centers as described above can be avoided.

Similar to what is observed in the case of Czochralski growth, local variations in temperature at the growth interface of a normally planar web sheet result in the formation of dopant or impurity lines due to variations in the instantaneous growth rate at the interface. Articles may also occur.

When turbulent convection occurs in the silicon melt, this turbulent flow propagates across the entire surface of the quartz crucible and carries oxygen and impurities from the crucible constituents to the growth interface, causing the impurities to crystallize unevenly. It may get inside.

<Problems to be Solved by the Invention> According to the present invention, a thermally immobile, large-capacity, deep melt that maintains the thermal stability of the growth interface by using a magnetic field that crosses the melt Attempts have been made to solve the above-mentioned problems by

<Means for Solving the Problems> Accordingly, the present invention provides a method for growing silicon dendrite web crystals, in which polycrystalline silicon is charged into a crucible, the silicon is melted, and the temperature inside the melt is increased. A magnetic field of a strength suitable for controlling fluctuations is applied across the silicon melt, and a radiation shield is placed at a distance above the melt to control the heat flow between the melt and the dendrite web during web growth. and growing a dendrite web from the melt through a slot in a radiation shield.

Effects and Effects When a sufficiently strong magnetic field is applied to a silicon melt, or any conductive melt, the induced drag on the movement of the fluid within the melt prevents the generation of turbulent convection. The magnetic field counteracts convection-induced temperature fluctuations near the dendrite-web growth interface, so that web crystals can be grown from deep melts without irreversible interruptions. In addition, the magnetic field-induced drag on the melt movement acts to reduce the local temperature fluctuations of the melt at the dendrite-web interface, allowing control of the dendrite width and faults, as well as melting of the crucible. Contamination due to oxygen and impurities is also reduced.

According to the present invention, web crystals have been successfully grown from a melt 30 mm deep by applying a magnetic field across the melt. The mechanical configuration of the melt-containing crucible and the separately suspended radiation shield maintain a constant distance between the melt surface and the shield, balancing heat flow from the growth interface and directing the silicon dendrite web. Crystal growth is controlled.

<Examples> In order to understand the present invention more clearly, preferred embodiments of the present invention will be illustrated below with reference to the accompanying drawings, but the following examples are given for illustrative purposes only and do not constitute the present invention. It does not limit the invention.

Beginning with reference to FIG. 1, the growth of dendrite web crystals occurs in the undercooled region of the melt
A process in which a pair of coplanar dendrites 24 growing from a cooled region forms a thin sheet or "web" of crystals 25. A temperature gradient is applied to the melt to obtain the region of supercooling necessary for growth, and this gradient creates a buoyant force on the movement of the fluid in the melt, commonly referred to as natural or "free" convection. As the depth of the melt increases, the effect of the correspondingly increasing buoyancy forces causes turbulent convection, which eventually forms convection cells in the melt. Prior to the proposal of the present invention, it was customary to grow silicon dendrite web crystals from shallow melts, typically 7-10 mm deep, to avoid the effects of turbulent convection. For melts deeper than 10 mm, temperature fluctuations caused by buoyancy forces usually compromised crystal integrity.

In Figure 2, the NRC Czochralski lifting device (NRCCzoch
A horizontal magnetic field is applied to a silicon dendrite web crystal using a Ralski Puller 10. VAR with a pulling chamber 12 in the middle of both polar surfaces 13
A 12-inch plate electromagnet 11 manufactured by IAN is shown in FIG.

In this configuration, a large flux return circuit surrounds the device. A web pulling reel is positioned above the pulling device 10, preferably in line with the slot 16 in the upper flange 17 of the pulling chamber 12. Preferably, a temperature controller using detection of thermal radiation from the base 19 of the crucible 31, a pulling speed controller, and a magnetic field generation power supply means are used. A series of eight movable guide members 22 below the upper flange 17 of the pulling device serve to position the dendrites 24 of the crystal 25 within the subcooled region 26 during seeding and pulling operations. The cylindrical graphite resistive thermal heater 28 is heated with direct current to prevent bending and vibrations that occur when using the low frequency alternating current generated by the electromagnet 11. The subcooled region of the melt 26 is defined by a molybdenum radiation shield with a "dog bone" shaped slot. The melt was placed in a quartz crucible 31 with a diameter of 80 mm, which is a standard device used for Czochralski growth of silicon. As shown in FIG. 2, the pulling chamber 12 is
It is surrounded by a thermal insulator 33 and a water-cooled container 34.

The critical factor of spacing between the melt surface and the radiation shield is maintained by independently suspending or suspending the shield to allow controlled elevation of the crucible 31 as the melt volume decreases. The equipment for raising the crucible is common to conventional Czochralski crystal growth.

In a typical growth method, approximately 500 grams of polycrystalline charge material was placed in a quartz crucible 31 along with an appropriate doping agent for the solar cell application. The crucible containing the charge was placed in the hot area 32 of the pulling device and the crucible was raised until the radiation shield was in a position extending inside the top of the quartz crucible 31. After appropriately scavenging with high-purity argon gas, the silicon is melted under the flowing argon stream, and the crucible 31
Raise the desired distance between the melt and the shield to approximately 7
The distance was set to mm. Typical melt depths utilizing this configuration are 30-40 mm, which is significantly greater than the 7-10 mm considered maximum for conventional methods of growing silicon webs.

In the above embodiment, the idea of the present invention was put into practice by adopting the configuration of the melt and the crucible as described above, but it is not possible to use the melt with an extremely large diameter and depth in combination with a magnetic field. There is no a priori reason for this. In another example, a thermally stationary 12 inch (30.5 cm) diameter and 10 inch (25.4 cm) depth of silicon was removed using a large Chorillalski-type puller with an axial magnetic field created by a superconducting magnet. Melt 20k
I got g. Utilizing such large equipment for the growth of silicon dendrite webs allows the preparation of extremely long silicon webs without melt replenishment.

In a typical growing case, prior to initiating growth, 11
A horizontal magnetic field of 60-2330 Gauss was applied. Seed crystal formation and growth were carried out in exactly the same manner as in the case of shallow growth without the application of a magnetic field. Even though no measures were taken to optimize the growth morphology, the dendrite web crystals grown from the melt at a depth of 3f1 mm were in all respects the same as those from the melt at a depth of 7-8 mm with no applied magnetic field. It was similar to the grown web-like crystals.

A thermocouple measuring needle was placed inside the melt to quantitatively evaluate the effect of reducing temperature disturbances in the magnetic field. For measurements, a thin B-type thermocouple (P
t 6°6Rh.

Pt 2096Rh) was used. KEITHLEY 1
55 zero detector/microvolt meter and El]RO
THERM millivolt backing source (mill
1 using ivolt bucking 5source)
The measurement sensitivity of microvolts was displayed on the recording chart.

'J! ! i 2 mm deep into the melt in the center of the cooling zone
When measured using a thermocouple immersed in water, the temperature fluctuation range was 0.6°C. As shown in Figure 3, 174
Applying a magnetic field of 0 Gauss reduced the melt temperature fluctuation range to less than 0.1°C. The reduction in melt temperature by a few degrees Celsius with the application of the magnetic field indicates that the heat flow due to temperature-induced convection in the melt has been reduced.

FIG. 4 shows a comparison of the temperature fluctuations in the melt in several cases where the magnetic field strength is varied. The temperature fluctuation of 0.6°C in the conventional method is amplified at 600 Gauss, but it becomes 0.1°C at magnetic field strength of 1160 Gauss or higher.
has decreased to Therefore, the melt geometry used in this example would require only a moderate strength magnetic field to produce a thermally stationary melt.

The temperature profile was measured by dipping a thermocouple into the bottom of the residual melt after the growth of the web crystals and then gradually pulling up the thermocouple (fixed to the pulling device). FIG. 5 shows the temperature profile and thermal oscillations in the residual melt at a depth of 27 mm when the magnetic field is zero and when a magnetic field of 730 Gauss is applied. Note that the temperature deep in the melt is reduced by as much as 3° C. due to the magnetic field, and the difference decreases as the thermocouple is drawn into a region of high temperature gradient near the supercooled melt surface.

The above measurement is a typical example, and the dimensions and dimensions of the melt mentioned above are
2 is an example showing the effect of magnetic fields on reducing thermal oscillations in the case of shapes. A similar reduction in temperature fluctuation can be expected when growing a dendrite web by controlling the heat flow conditions at the growth boundary in a large and deep melt.

[Brief explanation of the drawing]

FIG. 1 is a schematic explanatory diagram of a growing web crystal, and the diagram shows a dendrite seed crystal, a button, a solidified dendrite interface, and a web. FIG. 2 is a schematic diagram showing a silicon melt and dendrite web crystals placed inside a transverse magnetic field. FIG. 3 is a graph showing changes in temperature fluctuation according to the present invention, measured by inserting a measuring needle of a thermocouple 2 mm into the melt. FIG. 4 is a graph showing the relative value of the difference in temperature fluctuation measured by inserting a measuring needle of a thermocouple 2 mm into the melt as a function of the applied magnetic field. FIG. 5 is a graph showing the temperature profile of the melt at a depth of 27 mm, comparing the melt with no magnetic field and the melt with a magnetic field of 1730 Gauss applied. 10... Pulling device 11... Electromagnet 24... Dendrite 25... Crystal 26... Supercooling area of molten material 31... Crucible 32... Heat radiation area FIG, 3 FIG, 4

Claims (1)

[Claims] 1. A method for growing a silicon dendrite web crystal, in which a crucible is charged with polycrystalline silicon, the silicon is melted, and the silicon has a strength suitable for controlling temperature fluctuations inside the melt. A magnetic field is applied across the silicon melt, a radiation shield is placed at a distance above the melt to balance the heat flow between the melt and the dendrite web during web growth, and the slots in the radiation shield are A method characterized in that a dendrite web is grown from a melt through a process. 2. Process according to claim 1, characterized in that the depth of the silicon melt is at least 10 mm. 3. The method of claim 1 or 2, wherein the strength of the magnetic field applied across the silicon weldment is greater than or equal to about 1160 Gauss. 4. The method according to claim 1, 2 or 3, characterized in that the silicon is melted using a cylindrical graphite resistor heated by direct current. 5. Claims 1 to 5, characterized in that the radiation shield is arranged independently and spaced apart above the melt, and the crucible is controlled to rise when the volume of the melt decreases. The method described in any of paragraph 4.