Method for connecting two sic-containing bodies to form a composite body having a p-i-n junction, and composite body

Abstract

The invention relates to a method for connecting two SiC-containing bodies (2, 3) to form a composite body (1) having a p-i-n junction. The method comprises: forming an arrangement (5) having a first SiC-containing, n-doped body (2), a second SiC-containing, p-doped body (3), and a layer (4) of an undoped SiC precursor composition arranged between the first body (2) and the second body (3), and heat-treating the arrangement (5) in such a way that the SiC precursor composition transitions into a gas phase and reacts with surfaces (7, 8) of the first body (2) and the second body (3) in order to connect the first body (2) to the second body (3), wherein a connecting layer (6) comprising undoped SiC remains between the first body (2) and the second body (3). The invention further relates to a composite body (1) having a p-i-n junction, comprising a first SiC-containing, n-doped body (2), a second SiC-containing, p-doped body (3), and a connecting layer (6) comprising undoped SiC arranged between the first SiC-containing body (2) and the second SiC-containing body (3).

Description

[0001] Method for joining two SiC-containing bodies to form a composite body with a pin junction and composite body

[0002] The invention relates to a method for joining two SiC-containing bodies to form a composite body with a pin junction, and to a composite body with a pin junction.

[0003] The permanent joining of macroscopic and microscopic silicon carbide bodies presents numerous challenges. AA van Veggel, D. van den Ende, J. Bogenstahl, S. Rowan, W. Cunningham, GHM Gubbels, H. Nijmeijer, Journal of the European Ceramic Society 28 (2008) 303-310, Hydroxide catalysis bonding of silicon carbide, and WO 2005 / 097 709 A1 describe a method for joining two silicon carbide (SiC) bodies. This involves first oxidizing the SiC surfaces to silica and then chemically crosslinking the silica surfaces. However, this process results in a loss of strength as well as optical and electrical properties at the joints. The differing coefficients of thermal expansion of SiC and silica further weaken this bond. Since surface oxidation requires a total thickness of 0.5 pm, this method is also not suitable for microscopic SiC bodies.

[0004] Furthermore, the use of components with a pin junction is known in photodetectors, solar cells, and high-frequency circuits. A pin junction means that the component has the following material sequence: p-doped material – undoped material or material with intrinsic conductivity – n-doped material. For example, the use of semi-transparent silicon-rich silicon carbide solar cells is described in Cheng et al. RSC Adv., 2015, 5, 36262 and Cheng et al. RSC Adv., 2014, 4, 18397, where the fabrication is carried out by layer deposition using PECVD followed by doping. The described process is complex and therefore not particularly suitable for large-scale production.

[0005] Against this background, the object of the invention is to provide a method for joining two SiC-containing bodies to form a composite body with a pin junction, as well as a composite body with a pin junction, with which the aforementioned disadvantages can be avoided as far as possible.

[0006] This problem is solved by the subject matter of the independent claims. The dependent claims relate to embodiments. A first aspect of the invention relates to a method for joining two bodies containing SiC to form a composite body with a pin junction. The two bodies can have SiC on their surface, at least in a region to be joined, or consist entirely of SiC. The bodies can be, for example, components such as electrodes and can have microscopic or macroscopic dimensions.

[0007] Microscopic dimensions here mean that the dimensions in at least one spatial direction, e.g., in two or all three spatial directions, are smaller than 5 mm, e.g., smaller than 1 mm, smaller than 100 pm, or smaller than 10 pm. Macroscopic dimensions are those that are not microscopic. For example, electronic components made of SiC, which are intended to be "glued" to an undoped SiC heat sink or can be used in other ways, can have microscopic dimensions.

[0008] Composite body means that its components, i.e. the first SiC-containing, n-doped body, the second SiC-containing, p-doped body and the compound layer, are firmly connected to each other and, for example, form a single component or are part of a single component.

[0009] In a first process step, an arrangement is formed with a first SiC-containing, n-doped body, a second SiC-containing, p-doped body and a layer of undoped SiC precursor composition arranged between the first body and the second body.

[0010] The SiC precursor composition includes a carbon source and a silicon source, e.g., in the form of free and / or chemically bound carbon or silicon, and is capable of forming SiC based on a carbothermal reaction. For example, the precursor composition may include atomic carbon and SiO₂ and / or SiO₂. The carbothermal reaction may involve the reduction of SiO₂ and / or SiO₂ by carbon at elevated temperature.

[0011] To form the assembly, for example, a surface of the first body in the area to be joined can be coated with the precursor composition using conventional coating processes. The second body can then be placed on top of the deposited layer of precursor composition or otherwise positioned on the layer. In a second process step, the assembled assembly is heat-treated. The heat treatment is carried out in such a way that the SiC precursor composition transitions into a gas phase and reacts with the surfaces of the first and second bodies to bond them together. The heat treatment is controlled so that a bonding layer of undoped SiC remains between the first and second bodies.

[0012] The mode of energy input is essentially irrelevant, as long as the temperature is locally high enough, i.e., within the compound region, to allow a carbothermal reaction and thus bonding to occur. However, the properties of the compound region can be varied by the mode of energy input, as described below.

[0013] Due to the temperature increase associated with heat treatment, the precursor composition partially transitions into the gas phase and reacts with the SiC on the surface of the two bodies to be joined. Since the precursor composition is not completely transferred into the gas phase, the compound layer remains between the two doped SiC-containing bodies in the resulting composite. Advantageously, the chemical reaction of the precursor composition with the two bodies to be joined can form a strong bond, which can correspond to a crystalline SiC bond.

[0014] For example, for n-type or p-type doping, a dopant can be selected from a group including boron, iron, chromium, vanadium, aluminum, and nitrogen. In other words, doping can be achieved, for example, with one or more elements selected from a group including boron, iron, chromium, vanadium, aluminum, and nitrogen. n-type doping can be achieved, for example, with aluminum or boron as the dopant, and n-type doping with nitrogen as the dopant.

[0015] The proposed method enables the creation of a composite body with a pin interface, which is also suitable for large-scale production. Furthermore, a large-area pin interface can be generated. Advantageously, the two bodies to be joined can be prefabricated, and their properties, e.g., with regard to dopant, degree of doping, porosity, etc., can be individually adjusted.

[0016] When a voltage is applied to the pin junction, electrons and holes can migrate through the intrinsically conductive junction layer, resulting in a current flow. The pin junction enables efficient charge carrier separation, making it particularly useful for optoelectronic applications. The structure and function of the pin junction are explained in more detail below:

[0017] The p-layer contains a high concentration of holes, which can be created, for example, by doping with elements such as aluminum or boron. Holes are the main charge carriers in p-doped materials. The undoped and intrinsically conductive i-layer is adjacent to the p-layer. This layer has a very low number of free electrons and holes. It acts as a kind of "buffer" between the p- and n-layers. The n-layer follows the i-layer and, due to doping, for example with nitrogen, has an excess of electrons; in the n-layer, the electrons are the main charge carriers.

[0018] Thermal excitation can induce charge carrier movement. At room temperature, some electrons in the intrinsic layer can be excited into the conduction band by thermal energy, thus becoming free electrons. This results in a very low number of free charge carriers in the i-layer.

[0019] If the pin junction is not supplied with an external voltage, electrons and holes recombine at the interface between the p- and n-layers. This leads to the formation of a depletion region where there are hardly any free charge carriers. This region acts as a barrier to current flow.

[0020] When a positive voltage is applied to the p-layer and a negative voltage to the n-layer, the depletion region narrows. Holes from the p-layer and electrons from the n-layer move into the i-layer, resulting in current flow (forward bias). When the polarity is reversed, the depletion region widens, and the current flow is greatly restricted. In this state, only a very small leakage current flows (backward bias).

[0021] Depending on the design, heat treatment can be carried out by applying energy using a laser and / or microwaves, or by heat treatment in an oven.

[0022] Energy input via laser has the advantage that a high temperature can be achieved with spatial resolution within a very short time, e.g., within a period of less than 1 second. According to further implementation variants, heat treatment can take place at a temperature in the range between 1200 °C and 1800 °C.

[0023] For example, heat treatment can be carried out at a maximum temperature of 1800 °C for a duration of 10 minutes or less. Alternatively, a temperature in the range of 1500 °C to 1700 °C, e.g., 1600 °C, can be selected. Preferably, the temperature can be chosen such that the SiC sublimation on the surface of the bodies to be joined is low, but sufficient SiO₂ gas and CO₂ are still formed from the precursor composition to enable the joining of the two SiC bodies.

[0024] Heat treatment can also be carried out in multiple stages, e.g., in two stages. For example, a lower temperature can be chosen in a first stage and a higher temperature in a second stage.

[0025] The temperature can also be selected depending on the desired crystal structure of the SiC forming in the compound layer. If a hexagonal polytype of SiC is desired in the compound layer, higher crystallization temperatures are necessary. At temperatures above 2100 °C, the hexagonal SiC polytypes 4H-SiC and 6H-SiC are preferentially formed.

[0026] According to further design variants, heat treatment can be carried out at reduced ambient pressure, e.g. in a vacuum.

[0027] According to further design variations, heat treatment can be carried out under isostatic or uniaxial pressure. Alternatively, heat treatment can also be carried out at ambient pressure.

[0028] According to further design variants, heat treatment can take place in a protective gas atmosphere, e.g. under argon or nitrogen.

[0029] According to further embodiment variants, the layer thickness of the undoped precursor composition before heat treatment can be less than 10 pm, for example less than 5 pm or less than 1 pm.

[0030] According to further embodiments, SiC can be at least one of the two SiC-containing bodies and / or SiC of the precursor composition 3C-SiC. Silicon carbide with a cubic crystal structure is referred to as "3C-SiC" or "β-SiC".

[0031] 3C-SiC is the most chemically stable of all semiconductor materials and, with a band gap of 2.35 eV, is best suited for use in solar molecular splitting.

[0032] According to further implementation variants, the SiC precursor composition can be a sol.

[0033] For example, the sol can contain silicon and carbon in a ratio of 1:4. The sol can contain, for example, an alcohol, such as ethanol or tetraethyl orthosilicate, a sugar, such as sucrose, and an acid, such as hydrochloric acid.

[0034] Using a sol has the advantage that the precursor composition can be easily positioned between the two bodies, for example by spin coating or dip coating. The layer thickness can be influenced, for example, by choosing the viscosity of the sol.

[0035] If the total thickness of the sol remains below 10 pm, then a single-crystal compound layer can be expected, provided that the surfaces also belong to single-crystal regions of the bodies to be joined.

[0036] According to further embodiment variants, the first SiC-containing body and / or the second SiC-containing body can be amorphous or crystalline, at least in the surface region.

[0037] According to further embodiment variants, the first SiC-containing body and / or the second SiC-containing body can be porous and / or a foam.

[0038] For example, porosity can range between 20% and 60%. As a dimensionless quantity, porosity is a measure of the ratio of cavity volume to the total volume of the body.

[0039] Another aspect of the invention relates to a composite body comprising a first n-doped body containing SiC, a second p-doped body containing SiC, and a compound layer of undoped SiC arranged between the first n-doped body containing SiC and the second p-doped body containing SiC. The compound layer may also consist of undoped SiC. The first and second bodies may consist of appropriately doped SiC.

[0040] The composite body can, for example, be manufactured using a process as described above. Therefore, the above explanations and advantages also apply to the composite body.

[0041] The composite body can, for example, be designed as a large-area 3C-SiC photocell with very effective charge separation through the intrinsic bonding layer.

[0042] For example, the first and / or the second SiC-containing body can be formed from sintered SiC powder.

[0043] Depending on the design, the first SiC-containing body and / or the second SiC-containing body can be porous and / or a foam.

[0044] For example, the first and / or the second SiC-containing body can have a porosity in a range between 20% and 60%.

[0045] The proposed composite body advantageously exhibits a large surface area due to its porosity. This allows, for example, electrolysis reactions to be carried out efficiently with the composite body.

[0046] According to further embodiment variants, SiC can be at least one of the two SiC-containing bodies and / or SiC of the 3C-SiC compound layer.

[0047] According to further embodiments, the first SiC-containing body and / or the second SiC-containing body can be at least partially amorphous or crystalline.

[0048] According to further design variants, the thickness of the compound layer can range between 5 nm and 100 nm.

[0049] A layer thickness in this area can have a beneficial effect on the properties of the pin junction.

[0050] Another aspect of the invention relates to the use of a composite body according to the above description as a photocell or for solar molecular splitting.

[0051] For example, the composite material can be used as or in a photodetector. Here, the pin junction can be used to convert light into electrical signals. Furthermore, the composite material can be used for photovoltaic applications, e.g., as or in a solar cell. In this case, the intrinsic layer can be used to convert light energy, e.g., energy from solar radiation, into electrical energy.

[0052] Alternatively, light energy can also be used directly, for example, for solar molecular splitting or photoelectrochemical molecular splitting (PEC). For instance, water can be electrolyzed into hydrogen and oxygen. Other molecules can also be split by solar molecular splitting or PEC, such as CO2 into carbon monoxide and oxygen. If a mixture of water and CO2 is split, further or different carbon and hydrogen-containing fission products, such as methane, can be formed. Furthermore, it is also possible to directly produce synthesis gas, i.e., a mixture of hydrogen and carbon monoxide, which can be used in a variety of applications in the chemical industry, for example.

[0053] Furthermore, there are application possibilities in the field of high-frequency circuits, where the fast response of the pin transition to changes in the electrical voltage can be exploited.

[0054] The invention is explained in more detail below with reference to exemplary embodiments and the accompanying figures. These show:

[0055] Fig. 1 shows a flowchart of an exemplary process for joining two SiC-containing bodies to form a composite body with a pin junction.

[0056] Figure 1 shows an exemplary arrangement 5, which is formed by a first SiC-containing body 2, which is n-doped, a second SiC-containing body 3, which is p-doped, and a layer 4 of an undoped SiC precursor composition arranged between the first body 2 and the second body 3.

[0057] The first body 2 consists of a 3C-SiC plate, where the 3C-SiC is n-doped. The second body 3 also consists of a 3C-SiC plate, but this time the 3C-SiC is p-doped. Both bodies 2 and 3 are formed from appropriately doped SiC powder, which has been sintered and is therefore porous. The porosity can, for example, range between 20% and 60%. The dimensions of the two bodies 2 and 3, e.g., their thickness, can be selected as required. For example, the thickness of the two bodies 2 and 3 can be the same or different.

[0058] In the exemplary embodiment, layer 4 consists of a SiC precursor sol containing silicon and carbon in a ratio of 1:4 and which can be formed, for example, from ethanol, tetraethyl orthosilicate, sucrose and hydrochloric acid.

[0059] To form the arrangement 5, the surface 7 of the first body 2 is wetted with the sol of the SiC precursor composition, so that layer 4 is created. The second body 3 is placed on this layer 4 with its surface 8.

[0060] The thickness d of the wetting layer or layer 4 can be used to compensate for surface inaccuracies. If the total layer thickness d of the sol remains below 10 pm, a single-crystal layer can be expected, provided that surfaces 7 and 8 also belong to single-crystal regions. In principle, however, any SiC surfaces, crystalline or amorphous, can be bonded together. Coarse and fine adjustments are easily possible before the subsequent heat treatment. For this purpose, for example, the bodies to be bonded can be adjusted relative to each other, i.e., shifted, after coating and before heat treatment.

[0061] To obtain the composite body 1, the arrangement is subjected to heat treatment, which is indicated in Figure 1 by a vertical arrow pointing from the arrangement 5 to the composite body 1.

[0062] In the exemplary embodiment, the heat treatment is carried out in two stages. The assembly 5 is first heated to 1050 °C for 2 minutes and then to a maximum of 1800 °C for 30 minutes, each time in an argon atmosphere. This heat treatment forms a single-crystal SiC compound layer 6. If a hexagonal polytype of SiC is desired in the compound layer 6, higher crystallization temperatures are necessary. At temperatures above 2100 °C, the hexagonal SiC polytypes 4H-SiC and 6H-SiC are preferably formed.

[0063] The result of the heat treatment is the composite body 1, in which the first, n-doped SiC-containing body 2 is firmly bonded to the second, p-doped SiC-containing body 3 via the undoped compound layer 6, thus creating a pin junction. The thickness d of the compound layer 6 is in the range between 5 nm and 100 nm. The composite body 1 can be used, for example, as a photovoltaic cell.

[0064] 1 composite body

[0065] 2 first SiC containing body

[0066] 3 second SiC containing body

[0067] 4 Layer of the SiC precursor composition 5 Arrangement

[0068] 6. Compound layer

[0069] 7 Surface area of ​​the first body

[0070] 8 Surface area of ​​the second body

[0071] d layer thickness

Claims

Patent claims 1. Method for joining two SiC-containing bodies (2,3) to form a composite body (1) with a pin junction, comprising the method: Forming an arrangement (5) comprising a first SiC-containing, n-doped body (2), a second SiC-containing, p-doped body (3) and a layer (4) of an undoped SiC precursor composition arranged between the first body (2) and the second body (3) and Heat-treating the arrangement (5) such that the SiC precursor composition transitions into a gas phase and reacts with surfaces (7,8) of the first body (2) and the second body (3) to connect the first body (2) with the second body (3), leaving a compound layer (6) of undoped SiC between the first body (2) and the second body (3).

2. The method according to claim 1, wherein the heat treatment is carried out at a temperature in a range between 1200 °C and 1800 °C.

3. Method according to one of the preceding claims, wherein the heat treatment is carried out under isostatic or uniaxial pressure.

4. Method according to any of the preceding claims, wherein the layer thickness (d) of the layer (4) is less than 10 pm, for example less than 5 pm or less than 1 pm.

5. Method according to one of the preceding claims, wherein SiC is at least one of the two SiC-containing bodies (2,3) and / or SiC of the precursor composition 3C-SiC.

6. Method according to any of the preceding claims, wherein the SiC precursor composition is a sol.

7. The method of claim 6, wherein the sol comprises silicon and carbon in a ratio of 1:

4.

8. The method of any one of the preceding claims, wherein the first body (2) and / or the second body (3) is porous and / or a foam.

9. Composite body (1 ) with a pin junction, the composite body (1 ) comprising: a first SiC containing, n-doped body (2), a second SiC-containing, p-doped body (3), and a Interconnect layer (6) with undoped SiC arranged between the first SiC-containing, n-doped body (2) and the second SiC-containing, p-doped body (3).

10. Composite body (1) according to claim 9, wherein SiC is at least one of the two SiC-containing bodies (2,3) and / or SiC of the compound layer (6) 3C-SiC.

11. Composite body (1) according to claim 9 or 10, wherein a layer thickness (d) of the bonding layer (6) is between 5 nm and 100 nm.

12. Use of a composite body (1) according to any one of claims 9 to 11 as a photocell.

13. Use of a composite body (1) according to any one of claims 9 to 11 for solar molecular splitting.

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