WO2010081857A2

WO2010081857A2 - Method for producing a semiconductor component, in particular a solar cell, based on a germanium thin film

Info

Publication number: WO2010081857A2
Application number: PCT/EP2010/050416
Authority: WO
Inventors: Enrique Garralaga Rojas; Barbara Terheiden; Jan Hensen; Heiko Plagwitz
Original assignee: Institut Für Solarenergieforschung Gmbh
Priority date: 2009-01-14
Filing date: 2010-01-14
Publication date: 2010-07-22
Also published as: WO2010081857A3; DE102009004560B3

Abstract

The invention relates to a method for producing a semiconductor component, in particular a solar cell, based on a germanium thin film. The method comprises the following steps: providing a germanium substrate (1); forming a porous film (3) on a surface of the germanium substrate (1) by electrochemically etching the germanium substrate (1) in an etching solution (7); depositing a semiconductor thin film (5) on the porous film (3); and separating the semiconductor thin film (5) from the germanium substrate (1), wherein the porous film (3) is used as a predetermined breaking point. The polarity of a voltage applied between the germanium substrate (1) and an external electrode (11) is reversed multiple times during the electrochemical etching of the germanium substrate (1). In this way, a porous film can be produced in a germanium substrate, wherein the porous film enables the semiconductor thin film deposited thereon to be subsequently separated as part of a film transfer method.

Description

Method for producing a semiconductor component, in particular a solar cell, on

Base of a germanium thin film

FIELD OF THE INVENTION

The invention relates to a method for producing a semiconductor component, in particular a solar cell, based on a germanium thin film. In particular, the invention relates to a production process for a germanium solar cell with a layer transfer process by means of which a thin germanium layer can be detached from a germanium substrate.

BACKGROUND OF THE INVENTION

Special high-efficiency solar cells designed for use in space, for example, are currently mostly manufactured on the basis of compound semiconductors such as gallium arsenide (GaAs). As a rule, a compound semiconductor thin film is deposited on a substrate. Due to a good agreement with regard to the lattice constants, the substrate used today is usually a germanium wafer. In addition, an additional germanium layer remaining on the finished solar cell can contribute to the current generation of the solar cell, for example by forming a hetero-pn junction. RAK: sis To reduce the cost of a space mission, the solar cells used should be as light as possible. Therefore, so far the germanium wafer, which serves as a carrier substrate for the space solar cell and on which the individual layers of a multi-junction or single-junction solar cell are applied, after the formation of the actual thin-film solar cell is largely removed. This can be done chemically, for example by etching away the germanium substrate, or mechanically, for example by grinding away or polishing away the substrate. In this case, the germanium wafer originally serving as carrier substrate is thus sacrificed.

On the one hand, additional work for etching or grinding is necessary for removing the germanium wafer. On the other hand, the subsequent sacrifice of the germanium wafer means a useless for the subsequent function of the solar cell cost. The germanium wafer is only needed during the manufacture of the solar cell. Subsequently, however, the example, about 150 microns thick germanium wafer is consuming removed. There are thus costs for both the germanium wafer and for the subsequent removal of the germanium wafer.

The prior art discloses methods for producing crystalline silicon-based solar cells in which a porous silicon layer is first produced on a silicon substrate and then a further layer of silicon is deposited over the porous silicon layer, for example epitaxially. This further layer can then be separated from the silicon substrate, wherein the previously produced porous layer serves as a predetermined breaking point.

The separated further layer can be formed, for example, with a thickness of a few micrometers and then serve as a thin-film substrate for a solar cell, wherein in the subsequent steps essential components of Solar cell, such as their emitter and / or their contact metallization, can be formed.

Such a so-called layer transfer method is described, for example, in an article by R. Brendel in Solar Energy, 77, 2004, 969-982 and in DE 197 30 975 A1 and US Pat. No. 6,645,833. It takes advantage of the fact that the silicon thin film deposited on the porous layer preferably grows with the same crystal structure as the silicon substrate adjacent thereto. For example, when a high-quality single-crystalline wafer is used as the silicon substrate, high-quality silicon thin films which can be used as substrates for high-efficiency solar cells can be produced. The silicon substrate is scarcely consumed except for slight losses by the generation of the porous layer and can be reused several times.

Although it has already been theorized to apply this layer transfer process, known from silicon technology, also to the use of germanium substrates, this has hitherto always failed in attempting to actually implement real-world problems.

SUMMARY OF THE INVENTION

There may therefore be a need for a method for producing a semiconductor component, in particular a solar cell, in which the above-mentioned problems are at least partially overcome. In particular, there may be a need for a method for producing a semiconductor component, in particular a solar cell, in which a thin germanium layer can be detached from a germanium substrate and subsequently serve as a substrate for the semiconductor component based on a semiconductor thin film. According to one aspect of the present invention, a method for producing a semiconductor component, in particular a solar cell, based on a germanium thin film is proposed. The method has the following process steps: provision of a germanium substrate; Forming a porous layer on a surface of the germanium substrate by electrochemically etching the germanium substrate in an etching solution; Depositing a semiconductor thin film on the porous layer; and separating the germanium thin film from the silicon substrate, wherein the porous layer serves as a predetermined breaking point. During the electrochemical etching of the germanium substrate, a voltage applied between the germanium substrate and an external electrode is reversed several times.

The present invention can be considered to be based on the following finding:

So far, layer transfer processes for the formation of semiconductor thin films could be carried out successfully only with silicon substrates. Attempts to use layer transfer processes even with germanium substrates have hitherto failed due to problems occurring during the practical implementation. As such a problem encountered, it has been recognized by the inventors of the present invention that the methods for forming a porous layer by electrochemical etching, as known in the production of a porous layer in silicon and successful in the realization of layer transfer processes, are the use of Germanium substrates do not lead to satisfactory results.

In electrochemical etching, the surface of a substrate to be etched is brought into contact with an etching solution. Between an electrode in contact with the substrate and an external electrode in contact with the etching solution is then an electric voltage is applied, which allows a so-called etching current to flow. Due to electrochemical reactions, the surface of the substrate may be oxidized and then the etched-on etching solution may be etched away. Since this process is generally not homogeneous, but focuses on nucleation nuclei, an inhomogeneous etching of the substrate surface occurs, which may result in a porous surface layer.

For silicon substrates, it has been observed that by appropriately adjusting the voltage between the silicon substrate and the external electrode, the resulting etching current can be adjusted so that a desired porosity of the generated porous layer can be affected. Among other things, the porosity results from the number or density as well as the size of the pores produced. By successively changing the etching voltage used or the etching current used, a so-called porous double-layer structure can be produced in which a lower porosity is generated directly at the surface of the substrate to be etched than deeper inside the substrate. The area of the deeper, higher porosity can later serve as a predetermined breaking point when separating the overlying layer with low porosity.

The production of such an advantageous double-layer structure with correspondingly arranged regions of different porosity has hitherto not been successful with germanium substrates. As a possible explanation for this, the inventors of the present invention have recognized that during the electrochemical etching of silicon, the etching process primarily leads to a depression of the pores, but the surface areas next to the pores are apparently hardly attacked during etching. By correspondingly varying the etching current, the pore size can be adjusted in the interior of the substrate where etching is taking place.

This does not seem to be readily possible with germanium substrates. It has been observed by the inventors of the present invention that although the depth of the pores is successively increased in the electrochemical etching of germanium, at the same time apparently the adjacent regions on the surface of the germanium substrate are also etched away. The etching rate, with which the depth of the pores increases, seems to be at most slightly higher than the etching rate at the substrate surface.

As a result, it can be explained that if a porous layer of sufficient thickness is to be produced, the substrate is also etched back strongly at the same time, that is to say that its total thickness decreases. The presumed fact that germanium is etched during the electrochemical etching not only at the depth of the pores, but also at the surface, thus leads to the fact that a considerable total loss of germanium must be accepted for forming a sufficiently thick porous layer.

In addition, due to the presumed fact that the surface is always etched as the pores deepen, it is not possible to produce a suitable bilayer structure with large pores in the depth and small pores on the surface.

In addition, it has been observed that under certain etching conditions, an uncontrolled self-detachment of a germanium surface layer may occur which, due to its lack of mechanical stability, can no longer serve as a substrate for the epitaxial formation of a thin-film solar cell.

The inventors of the present invention have now recognized that the etching of the substrate surface occurring simultaneously during the electrochemical etching of the pores can be at least temporarily prevented or inhibited by the fact that during the electrochemical etching, a voltage applied between the germanium substrate and an external electrode is repeated briefly is reversed. It was observed that by such temporary, multiple reversal of the applied voltage during the electrochemical etching, an advantageous embodiment of the porous layer produced can be achieved.

A possible explanation of this observed effect developed by the inventors of the present invention is as follows: During an etching phase in which a so-called anodic bias between the germanium substrate and the external electrode prevails, so that an etching current flows, the germanium becomes due to electrochemical Processes etched. This corresponds to conventional electrochemical etching. When the applied voltage is reversed, that is to say under cathodic bias, in which the applied voltage is selected in such a way that no substantial etching current flows, there is no significant etching of the germanium. Accordingly, growth of the pores is stopped. At the same time, however, it appears that the germanium surface is passivated by hydrogen ions (H ⁺ ). This passivation of the germanium surface appears to temporarily inhibit the etching of the surface in a subsequent etching step in which anodic prestressing is again set. One possible cause of this could be leakage currents. By reversely polarizing the applied voltage, during the anodic bias phases, on the one hand, the size and depth of the generated pores can be influenced by appropriate selection of the anodic bias, and on the other hand, during the cathodic bias phases, passivation of the substrate surface can be achieved Etching the substrate surface during a subsequent phase of anodic bias. In this way, porous layers of suitable pore size and geometry suitable for use in a layer transfer process for producing a semiconductor device can be produced.

The production method presented here makes it possible, on the one hand, to produce, for example, thin-film solar cells, for example, for use as space solar cells or concentrator solar cells, on the other hand, this can be used in the production Reuse semiconductor substrate after detachment of the thin film deposited thereon advantageously, whereby a considerable material savings is possible.

Possible features and advantages of embodiments of the inventive manufacturing method will be described in more detail below.

The germanium substrate provided in the context of the production method according to the invention can have any desired structure and geometry. Preferably, a germanium wafer of high quality, for example of monocrystalline germanium, is used as the germanium substrate. , The germanium substrate may have planar or textured surfaces. In particular, in the production of solar cells, it may be advantageous if the substrate surface, which later forms the solar radiation directed side of the solar cell, has a surface texturing.

The porous layer may be formed on the surface of the germanium substrate by electrochemical etching by bringing the surface of the germanium substrate into contact with an etching solution and simultaneously applying an electric voltage between the substrate surface and the etching solution. In other words, the surface of the germanium substrate and the etching solution are at different electrical potentials. With a suitable polarity of the applied voltage, an electrochemical reaction can occur which can lead to an etching of the substrate surface, in particular locally at nucleation centers.

During the formation of the porous layer, the applied voltage is reversed several times to temporarily apply an anodic bias to the actual etch and, at times, a cathodic bias to passivate the exposed germanium substrate surface. The voltage can be reversed abruptly or continuously, for example, from an anodic bias of a first Voltage value to a cathodic bias of a second voltage value with opposite sign to the first voltage value. Depending on the size of the substrate, the voltage values can range from below 1 V, for example 0.001 V, to many volts, for example 1000 V. The first and second voltage values may vary in magnitude, that is, for example, the amount of negative anodic bias may be greater than the amount of positive cathodic bias. The time periods between the voltage reversals can range from a few seconds or even less than a second to a few minutes. For example, a phase in which a certain bias is applied may be shorter than 10 minutes. Furthermore, the durations of the phases of anodic bias may differ from those of cathodic bias.

The semiconductor thin film deposited on the porous germanium layer may be formed by various epitaxial methods. , Only a homogeneous layer or alternatively a plurality of layers stacked on top of one another can be deposited. The term "thin film" can be understood in this case such that the deposited layer consists of one or more sub-layers, each sub-layer alone having a small thickness compared to semiconductor wafers, for example less than 50 μm, preferably less than 10 μm chemical vapor deposition (CVD), physical vapor deposition (PVD) and liquid phase epitaxy (LPE) deposition, and the semiconductor thin film can be deposited directly on and in mechanical contact with the germanium surface The germanium surface and the semiconductor thin layer may also be formed as electrically conductive intermediate layers, for example of TCO (transparent conductive oxides) or buffer layers termaterialien be preferred, which have a similar lattice constant as germanium. Besides germanium itself this can also be compound semiconductors such as gallium arsenide (GaAs). The semiconductor thin film can be deposited with a thickness of a few 100 nm up to more than 100 μm, for example between 500 nm and 100 μm, preferably between 10 μm and 30 μm.

In order to separate the semiconductor thin film from the germanium substrate, for example, a mechanical force may be applied to the semiconductor thin film. For example, the semiconductor thin film may be adhered / bonded to a supporting substrate, such as glass. For this purpose, a method such as used in module encapsulation or a sol-gel method can be used. With the aid of the carrier substrate, the semiconductor thin film can then be lifted off the germanium substrate, wherein the previously produced porous layer can serve as a predetermined breaking point, in particular in the areas with the highest porosity, along which the separation process takes place.

Already before the separation of the semiconductor thin layer or alternatively after the separation, further process steps can be carried out on the semiconductor thin layer in order to form components which may be necessary or helpful for the function as a semiconductor component, in particular as a solar cell. For example, doped regions which form an emitter or a BSF (Back Surface Field) can be produced in the semiconductor thin film. The doped regions can be produced, for example, by diffusion of dopants. Alternatively, doped regions may be formed by epitaxially depositing doped semiconductor layers so that heterostructures can be formed in which, for example, the emitter may be formed by a layer of a first semiconductor material and the base by a layer of a second, different semiconductor material. Furthermore, electrical contacts can be formed, for example, in the form of metallizations or by transparent conductive oxides (TCO) on the surfaces of the semiconductor thin film. In addition, dielectric layers can be formed on the surface which can serve as surface passivation, antireflective layer or back mirror.

According to an embodiment of the present invention, the voltage applied during the electrochemical etching of the germanium substrate between the germanium substrate and the external electrode is periodically reversed. In other words, within a cyclically repeating period, the voltage may be applied as an anodic bias for a first period of time and then as a cathodic bias for a second period, with the respective polarities repeating periodically. Such a periodically repeating voltage scheme can be generated technologically simple. The voltage can be reversed abruptly or continuously from an anodic bias to a cathodic bias.

According to another embodiment of the present invention, during the electrochemical etching, a first period of time in which an anodic bias prevails, that is, during which the voltage applied between the germanium substrate and the external electrode is selected such that an etching current flows, becomes longer be selected as a subsequent second period of time during which a cathodic bias prevails, that is, during which the applied voltage is selected such that substantially no etching current flows. Observations have shown that it is sufficient to apply a cathodic bias only for a short time, in order to achieve the desired passivation of the germanium Substratoberfiäche. Thus, the cathodic bias necessary for passivation can be applied shorter in time than the anodic bias used for the actual etching, so that the total etching time can be kept as short as possible.

According to a further embodiment of the present invention, the etching solution has a proportion of at least 10% by volume of hydrofluoric acid (HF). Observations have shown that both the etching rate and the total etching result can depend strongly on the concentration of the etching solution. It was observed that with decreasing concentration, the etching rate can not only decrease correspondingly, but that below a certain minimum concentration, possibly no significant etching can be observed at all. A solution of hydrofluoric acid (HF) in water having a certain minimum concentration of at least 10% by volume of HF, preferably at least 20% by volume, and more preferably at between 30 and 50% by volume, has been identified as a suitable etching solution.

According to another embodiment of the present invention, the provided germanium substrate has a resistivity of less than 1 ohm-cm (ohm-centimeter). It has been observed that the result of the electrochemical etching depends strongly on the charge carrier density in the germanium substrate used and improves with increasing carrier density, ie with decreasing resistivity of the substrate. In particular, it was observed that with germanium substrates with too low a charge carrier density, ie a too high specific resistance, no satisfactory etching result could be achieved. Preferably, the resistivity of the germanium substrate should be less than 100mθhm-cm, more preferably less than 50mθhm-cm, and even more preferably between 10 and 35mOhm-cm. Furthermore, a specific crystal orientation of the germanium substrate can lead to an advantageous etching result. Germanium wafers with a 100 orientation were found to be advantageous. Furthermore, a polishing of the surface can act advantageously.

According to another embodiment of the present invention, the provided germanium substrate is of p-type semiconductor. It has been observed that better etch results can be achieved with p-type germanium substrates than with n-type germanium substrates. In order to be able to achieve satisfactory etching results on n-type germanium substrates, additional measures may be advantageous. For example by illumination with light additional charge carriers are generated in the substrate, whereby the etching process can be assisted.

According to a further embodiment of the present invention, influencing parameters during the electrochemical etching are selected such that the porous layer is formed as a microporous or mesoporous layer. Under a microporous layer is understood according to IUPAC (International Union of Pure and Applied Chemistry), a layer having an average pore size of less than 10 nm. For a mesoporous layer, the pore size is between 10 and 50 nm. Such porous layers, which have a small pore size compared to macroporous layers, may be advantageous for use in layer transfer processes. Influencing parameters which may influence the type and speed of the etching process and the resulting pores include the etching current caused by the applied anodic bias, the temperature, the concentration of the etching solution and the doping concentration of the germanium substrate on the surface to be etched , Another influencing parameter is the duration of the respective phases with an anodic or cathodic bias, that is, the phase durations between the switching operations of the applied etching voltage.

In accordance with a further embodiment of the present invention, influencing parameters during the electrochemical etching are selected such that a second low-porous layer is formed outside a first highly porous layer. The term "outside" can be interpreted as being "closer to the surface of the germanium substrate". In other words, the influencing parameters should be chosen such that the electrochemical etching results in a double layer in which a smaller pore size or pore density is produced directly on the surface of the germanium substrate than deeper inside the germanium substrate. For example, the porous layer may be microporous directly at the surface of the substrate, while below it may be mesoporous. An important and easy to influence Influencing parameters here are the voltage applied during the etching and the durations of the individual etching or passivation phases.

The highly porous layer may, for example, have a porosity of between 20% and 60%, preferably between 30% and 50%. Experiments have shown that the porous layer can serve poorly as a predetermined breaking point in the subsequent separation process at too low a porosity. If the porosity is too high, problems can occur in forming the semiconductor thin film on the porous layer, because the epitaxially deposited semiconductor material may no longer form a closed semiconductor thin film due to excessively large pores or craters within the porous layer.

According to another embodiment of the present invention, different etching solutions are used during the etching in chronological order. The etching solutions may differ both with regard to the etching substances contained in the solutions and with regard to the concentration of these etching substances. For example, HF solutions of various concentrations can be used. It has been observed that the use of different etching solutions can lead to different etch results, especially with regard to the porosity produced. Therefore, it may be advantageous to begin the etching process first with an etching solution, which leads to a low-pore layer on the surface of the germanium substrate, and then continue the etching process with a different etching solution deeper in the interior of the germanium substrate to a stronger Etching activity and thus leads to a higher porosity. In this way, a desired porous double layer structure can be achieved.

According to another embodiment of the present invention, a wetting agent is added to the etching solution used for electrochemical etching. This wetting agent can cause the actual etching substances of the etching solution to uniformly wet the surface of the germanium substrate during the etching process can. In addition, due to the wetting agent, gas bubbles can easily detach from the surface of the germanium substrate. As the wetting agent, for example, ethanol (C ₂ H ₆ O) or acetic acid (C ₂ H ₄ O ₂ ) may be used.

According to another embodiment of the present invention, the porous layer is subjected to annealing. Under "annealing" while an additional high-temperature step at temperatures of, for example, above 450 ⁰ C, preferably to be understood above 550 ⁰ C, but below the melting temperature of germanium (938 ⁰ C). It was observed that the porous surface is of the Germanium substrate may close due to deformation processes, so that the germanium substrate after annealing from the outside has a closed, smooth, preferably crystalline surface and only below it extends a porous layer Depending on the temperature and gas phase used during the annealing, for example nitrogen, nitrogen-hydrogen mixtures or argon, in addition to the transformation of the porous germanium surface, formation of germanium oxides may occur sweise be carried out together with another subsequent processing operation such as the epitaxial deposition of another layer.

According to another embodiment of the present invention, the porous layer is annealed in a reducing gas atmosphere such as a 100% hydrogen atmosphere. Such a reducing gas atmosphere can dissolve previously formed germanium oxides and contribute to the formation of a closed germanium surface.

According to a further embodiment of the present invention, the porous layer is annealed in a protective gas atmosphere, for example in an atmosphere of argon (Ar). The annealing in a protective gas atmosphere allows a reshaping of the porous germanium substrate surface without any annoying oxide formation.

It is noted that the embodiments, features, and advantages of the invention have been described in part with respect to the semiconductor device manufacturing method of the present invention and partially with respect to the semiconductor device fabricatable by such method. One skilled in the art will recognize, however, that unless otherwise stated, the embodiments and features of the invention may be analogously applied to the semiconductor device / process of the invention, and vice versa. In particular, a person skilled in the art will recognize that features of the various embodiments can also be combined with one another in any desired manner.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will become apparent to those skilled in the art from the following description of exemplary embodiments, which should not be construed as limiting the invention, and with reference to the accompanying drawings.

FIG. 1 shows a semiconductor thin film deposited on a germanium substrate according to an embodiment of the manufacturing method of the present invention.

Fig. 2 shows an arrangement with which the manufacturing method according to an embodiment of the invention can be performed.

Fig. 3 shows an alternative arrangement with which the manufacturing method according to an embodiment of the invention can be carried out. FIGS. 4a and 4b are graphs showing the timing of an etching potential and an etching current, respectively, in a method according to an embodiment of the present invention.

The drawings are only schematic and not to scale. Like reference numerals designate like or similar elements throughout the figures.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following, a basic principle and specific embodiments of the production method according to the invention will be described with reference to the semiconductor layer structure shown in FIG. 1 and the test arrangement shown in FIGS. 2 and 3.

A porous layer 3 is produced by electrochemical etching on a single-crystal p-type germanium wafer serving as germanium substrate 1 with a specific resistance of between 10 and 35 milliohm centimeters. For this purpose, the germanium wafer is contacted with a first electrode 9 and the wafer surface to be etched is wetted with an etching solution 7 containing at least 30% by volume of HF. A second electrode 11 is in electrical contact with the etching solution 7. With the aid of an external voltage source 13, an electrical voltage between the two electrodes 9, 11 is applied, wherein the voltage is reversed at short time intervals. The applied voltages and optionally the etching solutions used are selected such that a porous double layer is formed in which only a small porosity is formed directly on the surface of the germanium substrate 1, whereas deeper inside the germanium substrate 1, for example in FIG a depth of about 1 micron, a higher porosity is generated.

After such a porous double-layer structure has been produced during the etching process, The germanium substrate 1 is separated from the etching solution and cleaned with deionized water and then blown dry.

Subsequently, the germanium substrate 1 with the porous layer 3 thereon is subjected to a high-temperature step at about 600 ^° C. for a few minutes in a 100% hydrogen atmosphere. In this case, the porous layer 3 is partially formed and preferably forms on its surface facing outward a closed germanium layer, which can serve as a starting layer for a semiconductor thin film 5 to be subsequently deposited. The semiconductor thin film 5 may subsequently be separated from the germanium substrate and further processed to a desired semiconductor device such as a thin film solar cell.

In the device shown in FIG. 2, a germanium substrate 1 is supported horizontally on an electrode 9. In a vessel 15 which is open at the top and bottom, a 30% HF etching solution 7 is introduced. By a sealing O-ring 17, which is arranged between the bottom of the vessel 15 and the germanium substrate 1, an escape of the etching solution 7 is prevented. In a Teflon block 19 is a branching platinum wire 21 is inserted, which serves as a further electrode 11. By immersing the Teflon block 19, the further electrode 11 comes into contact with the etching solution 7. The two electrodes 9, 11 are connected to a voltage source 13, which is designed to umzupolen the voltage applied between the two electrodes 9, 11 at certain intervals.

In the device shown in FIG. 3, a HF-containing etching solution 7 is contained in a vessel 15 '. A germanium substrate 1 is supported vertically at a first electrode 9. Both the first electrode 9 and another platinum electrode 11 are immersed in the etching solution. Both electrodes 9, 11 are in turn connected to a voltage source 13. A tunnel 21 serves to homogenize the electric field. In the Fign. 4a and 4b, the course of the etching current or of the etching voltage as a function of time controlled by the voltage source 13 is shown.

Finally, it is pointed out that the terms "comprise", "exhibit" etc. do not exclude the presence of further elements. The term "a" also does not exclude the presence of a plurality of articles The reference signs in the claims are for convenience of reference only and are not intended to limit the scope of the claims in any way.

Claims

claims

A method of manufacturing a semiconductor device, in particular a solar cell, based on a germanium thin film, the method comprising: providing a germanium substrate (1);

Forming a porous layer (3) on a surface of the germanium substrate (1) by electrochemically etching the germanium substrate (1) in an etching solution; Depositing a semiconductor thin film (5) on the porous layer (3); and separating the semiconductor thin film (5) from the germanium substrate (1), wherein the porous layer (3) serves as a predetermined breaking point, wherein between the germanium substrate (1) and an external electrode (11) during the electrochemical etching of the germanium substrate (1) Voltage is reversed several times.

2. The method according to claim 1, wherein the voltage applied during the electrochemical etching of the germanium substrate (1) is periodically reversed.

3. The method of claim 1 or 2, wherein during the electrochemical etching, a first time period during which the voltage applied between the germanium substrate (1) and the external electrode (11) is selected such that an etching current flows is longer than a subsequent one second time period during which the applied voltage is selected such that no etching current flows.

4. The method according to any one of claims 1 to 3, wherein the etching solution (7) has a proportion of at least 10 vol% HF.

5. The method according to any one of claims 1 to 4, wherein the provided germanium substrate (1) has a resistivity of less than 1 ohm-centimeter.

6. The method according to any one of claims 1 to 4, wherein the provided germanium substrate (1) of the p-type semiconductor.

7. The method according to any one of claims 1 to 6, wherein influencing parameters during the electrochemical etching are selected such that the porous layer (3) is formed as a microporous or mesoporous layer.

8. The method according to any one of claims 1 to 7, wherein influencing parameters during the electrochemical etching are selected such that outside a first highly porous layer, a second low-porous layer is formed.

9. The method according to any one of claims 1 to 8, wherein during the etching in time sequence different etching solutions (7) are used.

10. The method according to any one of claims 1 to 9, wherein the etching solution is added to a wetting agent.

11. The method according to any one of claims 1 to 10, wherein the porous layer is annealed.

12. The method of claim 11, wherein the porous layer is annealed in a reducing gas atmosphere.

13. The method of claim 11, wherein the porous layer is annealed in a protective gas atmosphere.