PRODUCING A MONO-CRYSTALLINE SHEET OF SEMICONDUCTOR MATERIAL

Abstract

A method for producing a mono-crystalline sheet includes providing at least two aperture elements forming a gap in between; providing a molten alloy including silicon in the gap; providing a gaseous precursor medium comprising silicon in the vicinity of the molten alloy; providing a silicon nucleation crystal in the vicinity of the molten alloy; and bringing in contact said silicon nucleation crystal and the molten alloy. A device for producing a mono-crystalline sheet includes at least two aperture elements at a predetermined distance from each other, thereby forming a gap, and being adapted to be heated for holding a molten alloy including silicon by surface tension in the gap between the aperture elements; a precursor gas supply supplies a gaseous precursor medium comprising silicon in the vicinity of the molten alloy; and a positioning device for holding and moving a nucleation crystal in the vicinity of the molten alloy.

Claims

1. A mono-crystalline silicon sheet of a semiconductor material produced by a method of: providing at least two aperture elements forming a gap in between; providing a molten alloy comprising a mono-crystalline sheet of semiconductor material in the gap between said at least two aperture elements, whereby the molten alloy is held between the gaps by surface tension below a horizontal arrangement of the at least two aperture elements; providing a gaseous precursor medium delivering the semiconductor material in the vicinity of the molten alloy; providing a nucleation crystal below the molten alloy; bringing in contact said nucleation crystal and the molten alloy; and retracting the mono-crystalline sheet of semiconductor material that is continuously growing below the molten alloy.

2. A solar cell arrangement comprising a mono-crystalline sheet of a semiconductor material produced by a method of: providing at least two aperture elements forming a gap in between; providing a molten alloy comprising a mono-crystalline sheet of semiconductor material in the gap between said at least two aperture elements, whereby the molten alloy is held between the gaps by surface tension below a horizontal arrangement of the at least two aperture elements; providing a gaseous precursor medium delivering the semiconductor material in the vicinity of the molten alloy; providing a nucleation crystal below the molten alloy; bringing in contact said nucleation crystal and the molten alloy; and retracting the mono-crystalline sheet of semiconductor material that is continuously growing below the molten alloy.

3. A device for producing a mono-crystalline semiconductor sheet comprising: at least two aperture elements at a predetermined distance from each other thereby forming a gap, and being adapted to be heated for holding a molten alloy comprising a semiconductor material by surface tension in the gap between the aperture elements below a horizontal arrangement of the at least two aperture elements; a precursor gas supply for delivering a gaseous precursor medium comprising the semiconductor material in the vicinity of the molten alloy; and a positioning device for holding and moving a nucleation crystal below of the molten alloy that is continuously growing below the molten alloy.

4. The device of claim 3, wherein the two aperture elements have sections that are one of v-shaped and grooved.

5. The device of claim 4, wherein the two aperture elements have a coating comprising one of metal oxides and a carbon coating.

6. The device of claim 5, wherein the predetermined distance is less than 200 micrometers.

7. The device of claim 6, wherein the device is adapted to: produce a mono-crystalline sheet of a semiconductor material comprising: provide at least two aperture elements forming a gap in between; provide a molten alloy comprising the semiconductor material in the gap between said at least two aperture elements, whereby the molten alloy is held between the aperture by surface tension; provide a gaseous precursor medium delivering the semiconductor material in the vicinity of the molten alloy; provide a nucleation crystal in the vicinity of the molten alloy; and bring in contact said nucleation crystal and the molten alloy.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] FIG. 1 shows a block flow diagram of an embodiment of a method for producing a mono-crystalline silicon sheet;

[0048] FIGS. 2-7 show illustrations of method steps and device arrangements for implementing an embodiment of a method for producing a mono-crystalline silicon sheet;

[0049] FIG. 8 shows a schematic drawing of an embodiment of a device for producing a mono-crystalline silicon sheet;

[0050] FIG. 9 shows a schematic drawing of a mono-crystalline silicon sheet; and

[0051] FIG. 10 shows a schematic drawing of an embodiment of a solar cell based on a mono-crystalline silicon sheet.

DETAILED DESCRIPTION

[0052] Like or functionally like elements in the drawings have been allotted the same reference characters if not otherwise indicated.

[0053] As used herein, bringing into contact may refer to the act of touching, making contact or immediate proximity, in particular between the nucleation crystal and the molten alloy. Bringing into contact comprises the relative movement of the elements towards each other to be brought into contact.

[0054] As used herein, molten may refer to a substance that is melted, wherein melting is the process of heating a solid substance to the melting point where it turns into a liquid. Molten alloy may therefore imply that an appropriate temperature for creating the liquid phase of the alloy is chosen and applied.

[0055] An alloy may refer to a homogeneous mixture of two or more elements, at least one of which is a metal. As for example, a gold silicon alloy can be employed according to an aspect of a method or device for producing a mono-crystalline silicon sheet.

[0056] The term eutectic means that the alloy comprising a combination of materials melts at a temperature lower than that of the melting temperature of either of its components. Gold-silicon eutectic alloy may melt at about 700 C. lower than either of its components, at about 360 C.

[0057] As used herein the gap may also be called an aperture, slot, trench or vent. The gap may be a space between two aperture elements, wherein a width of the aperture is smaller than the length of the aperture. Preferably, a ratio between the length and the width is at least two, wherein the width corresponds to a distance between the aperture elements. The gap can be implemented as a slit.

[0058] The term mono-crystalline silicon sheet corresponds to mono-crystalline silicon without a supporting substrate. The sheet may also be called a silicon film, layer, membrane or foil.

[0059] The term mono-crystalline may refer to a solid material wherein the crystal lattice of the entire sheet is continuous and unbroken to the edges of the sheet. The produced mono-crystalline sheet is in contrast to amorphous materials where there is no long-range order of the positions of the material atoms. Typically amorphous phases are produced by depositing a layer of the material on a substrate. Mono-crystalline materials may have advantages over poly-crystalline materials that are composed of many crystallites or grains of varying size and orientation. Solar cells based on amorphous silicon substrates (aSi) may show efficiencies in terms of produced electricity per solar power unit of less than 10%, based on multi- or poly-crystalline silicon (mc- or poly-Si) show around 15% efficiency, and solar cells employing single- or mono-crystalline silicon (c-Si) may arrive at 22% efficiency.

[0060] Referring now to FIG. 1 showing a block flow diagram of one embodiment of a method for producing a mono-crystalline silicon sheet, and referring to FIGS. 2-7 illustrating potential method steps certain aspects of such a method for producing silicon sheets are explained. In particular, FIGS. 2-7 also show schematically features and elements of an apparatus or a device suitable for performing a method for producing a mono-crystalline silicon sheet.

[0061] In a first method step S1, as shown in FIG. 1, two aperture elements as shown in FIG. 2A and FIG. 2B are provided. The aperture elements 1, 2 may each have a section 1A, 2A, having a groove-like or v-like shape. The two groove-like sections 1A and 2A are arranged oppositely from each other such that between the two aperture elements 1, 2 a gap 3 occurs. FIG. 2A shows a cross-sectional view, wherein in the orientation of FIG. 2A a top side T and a bottom side B is defined by the two horizontally arranged aperture elements 1, 2.

[0062] FIG. 2B shows a top view of the two aperture elements 1, 2. The two aperture elements are arranged at a predetermined distance D. The gap 3, as can be seen from the top view in FIG. 2B, has the form of an elongated slit of a length L. Preferably, the length L exceeds the distance D significantly. For example, the aperture or gap width D can be defined by a distance D smaller than 200 m. However, in certain embodiments, the length of the slit or gap 3 may be 10 cm or longer. The slit or gap 3 may be straight, i.e. the two groove-like aperture sections 1A, 2A are arranged in a parallel fashion. The aperture elements are preferably micro-machined from silicon and can be coated by silicon dioxide or carbon. However, other materials as a protective cover can be contemplated.

[0063] In the next step S2 a molten alloy comprising silicon is placed into the space of the gap 3 between the two aperture elements 1, 2. FIGS. 3A and 3B show, for example, a molten alloy 4 fitting into the gap along the length of the aperture or gap. The alloy, for example comprising a gold-silicon (AuSi) alloy is eutectic and has a temperature of about 360 C. This is illustrated as the dotted area 4.

[0064] In the next process step S3, a precursor medium in its gaseous phase is brought to the vicinity of the molten alloy 4. This is shown in FIG. 4. The precursor medium or precursor gas is shown as a schematic cloud 5. For example, a gas supply device supplies silicon chloride SiCl.sub.4 or silane SiH.sub.4 or another metal-organic compound from silicon to the molten alloy 4. The precursor containing silicon releases silicon into the gold-silicon alloy which is illustrated by the arrows A. After a while the alloy 4 becomes supersaturated.

[0065] The temperature of the aperture and the precursor gas is chosen according to the used precursor material and alloy material. Temperatures may be arranged between 350 C. and 850 C. However, also higher temperatures, for example if silicon chloride is used, may be employed.

[0066] Next, in step S4, as illustrated in FIG. 5, a mono-crystalline silicon crystal employed as a nucleation crystal 6, is brought from below into the vicinity of the molten supersaturated alloy 4. FIG. 5 illustrates the nucleation crystal 6, which is preferably cut along the <111> growth direction and positioned with its tip 7 close to the molten alloy with supersaturated silicon.

[0067] FIG. 6 shows a perspective view of the two elongated aperture elements forming an elongated slit or gap in between where the molten alloy 4 is situated. This molten alloy 4 may be held between the two aperture elements 1, 2 by surface tension only. Below the horizontal arrangement of the first aperture element 1, the molten alloy 4 in the gap and the second aperture element 2, the nucleation crystal 6 made of silicon is positioned. The arrangement as shown in FIG. 6 corresponds to one embodiment of a device or apparatus for producing mono-crystalline silicon sheets.

[0068] As the nucleation crystal is brought into contact with the supersaturated molten alloy (step S4), which is shown in FIG. 7, a mono-crystalline silicon sheet evolves. As the molten alloy, for example comprising an eutectic mixture of gold and silicon, continuously remains supersaturated by the influence of the precursor gas 5, silicon 8 crystallizes at the tip 7 of the nucleation crystal 6 being in close proximity to the molten alloy 4. One may continuously retract the nucleation crystal 6 with the growing mono-crystalline sheet 8 as indicated by the arrow M in FIG. 7. The retraction speed of the nucleation crystal from the alloy 4 is adapted to the growth rate of the sheet 8. During the fabrication process the supersaturated alloy continuously dissolves silicon onto the nucleation crystal. The speed can be controlled as to guarantee a constant shape or cross-sectional shape of the molten alloy material 4 between the aperture elements 1, 2. The retracting step S5 can be controlled by a control device like a microprocessor or appropriately programmed computer controlling the entire device for producing the mono-crystalline silicon layer.

[0069] The growth rate or retraction speed of the grown mono-crystalline silicon sheet may vary according to preferred embodiments of the invention between 0.1 mm-3 mm per minute. However, also higher growth rates may be obtained.

[0070] FIG. 8 shows another embodiment of a device for producing mono-crystalline silicon sheets and thereby implementing another embodiment of the before depicted method. FIG. 8 shows the production device 20 having two aperture elements forming a gap in between that is suitable to be heated such that a molten alloy 4 is retained between the aperture elements by surface tension. A precursor gas supply 15, also denoted as means for delivering a gaseous precursor medium, provides a precursor medium 5 comprising silicon. By appropriately adapting the temperature of the molten alloy, as for example gold and silicon, and of the precursor medium 5, the alloy 4 becomes supersaturated by silicon. This is indicated by the arrows A.

[0071] Further, a positioning or pulling device 16, also denoted as positioning means, holds the nucleation crystal 6 and gradually retracts the continuously growing mono-crystalline sheet in the growth region 8 from the molten alloy 4. For example, the grown mono-crystalline silicon sheet 11 can be winded on a roll 9 for further processing.

[0072] The embodiment of a device 20 for fabricating or producing a mono-crystalline silicon thin sheet also comprises a control device 17 producing appropriate control signals CT to the precursor gas supply 15, the heated aperture elements 1, 2 and the positioning device 16. The entire arrangement 20 can be capped in a protective atmosphere using an inert gas as for example argon.

[0073] One may also contemplate of adding doping gases into the process for doping the semiconductor sheet during its growth. For example silicon, phosphor, boron or carbon can be used as doping materials. One may further contemplate of adding etching gases as cleaning agents for the aperture elements. Appropriate etching gases may comprise molecules containing chlorine, fluorine or bromine. As mentioned before, the aperture elements 1, 2 can be made from silicon, and coated by an oxide layer or carbon.

[0074] Due to the geometric implementation of the parallel aperture elements, thereby realizing an elongated slit or gap, rectangular shapes of the silicon sheet that may be rolled for further processing are obtained. FIG. 9 shows a schematic illustration of such a mono-crystalline silicon sheet produced by the presented method or device.

[0075] The single crystalline sheet material of silicon may be applied for realizing solar cells. In particular, single crystalline materials may allow for a higher performance or efficiencies in photovoltaic applications. Since the thin sheets may be directly fabricated no small material waste occurs due to conventionally necessary sawing or cutting. This may result in reduced material and process costs. The entire process may occur at relatively low temperatures. This may provide for an energy efficient production method.

[0076] FIG. 10 shows an exemplary embodiment of a solar cell comprising a thin mono-crystalline silicon sheet produced for example according to the method explained with respect of FIGS. 1-7. The silicon sheet is doped as to provide for a p-n junction across its thickness. The solar cell 12 comprises the doped silicon sheet 11 with electric contacts 13 on the top side and additional contacts 14 on the bottom side. Such solar cells may be employed in solar panels or photovoltaic modules comprising a plurality of such solar cells used to generate electricity.

[0077] Although the proposed method and device is explained in terms of embodiments for mono-crystalline silicon sheet production, also other mono-crystalline semiconductor sheets can be produced. For example, instead of a gold silicon alloy, metallic alloys comprising germanium, indium or gallium can be used as an alloy. Then, appropriate precursor gases containing germanium, indium or gallium would be employed instead of a silicon-based precursor medium.

[0078] For realizing alternative solar cell arrangements the silicon foil may be placed onto a transparent support or substrate and provided with wiring. Further to photovoltaic applications the thin mono-crystalline silicon film may be used in all other electronic devices that employ semiconducting substrates, as for example sensors, nano-technological systems, displays and the like.