EP2516692A1 - Method for coating a substrate with aluminum-doped zinc oxide - Google Patents

Abstract

The invention relates to a method for coating a substrate with aluminum-doped zinc oxide, comprising the steps of generating a nucleation coating between 5 nm and 400 nm thick, comprising zinc oxide or doped, in particular aluminum-doped zinc oxide, on the surface of the substrate by atomizing a solid target; generating a quasi-epitaxially propagating top coating comprising aluminum-doped zinc oxide on the nucleation coating; and wet chemically etching the top coating.

Description

 description

 Process for coating a substrate with aluminum-doped zinc oxide

 The present invention relates to a method of coating a substrate with aluminum-doped zinc oxide.

 From the prior art it is known that

 Silicon thin-film solar cells, in a so-called

 In order to achieve this, TCO layers must have low transparent resistances with a high transparency in the visible spectral range (400 to 800 nm) nm) for amorphous silicon solar cells (a-Si: H) and up to 1100 nm for microcrystalline silicon solar cells

 (C-Si: H). In addition, a suitable one

 Surface structure and the lateral structure size - in particular from the aspect of surface roughness - required to effectively couple light by scattering in the solar cell and thereby achieve greater absorption in the silicon layers.

 For the production of TCO layers in particular so-called sputtering (synonymously also referred to as sputtering) can be used. When atomizing, atoms become one

 Solid-body target by bombardment with high-energy noble gas ions dissolved out and thereby transferred to the gas phase. Near the solid-state target from which the atoms are extracted is a

 Substrate provided on which the atoms can condense, so that they form a layer on the surface of the substrate.

Layers of aluminum-doped zinc oxide (ZnO: Al layers) are particularly suitable for use in silicon thin-film solar cells. The ZnO: Al layers produced with the aid of sputtering processes are generally relatively smooth. This means that their roughness is only a few nanometers. By means of a wet-chemical etching step, these layers can be roughened so that crater-like structures having a relatively broad spectrum of structural sizes are formed (see: J. Müller, G. Schöpe, O. Kluth, B. Rech, V. Sittinger, B. Szyszka, R Geyer, P. Lechner, H. Schade, M. Ruske, G. Dittmar, H.-P. Bochem, in: Thin Solid Films 442 (2003), p. 158; J. Müller, B. Rech, J. Springer, M. Vanecek: "TCO and Light Trapping in Silicon Thin Film Solar Cells" in: Solar Energy 77 (2004), p.

917-930; Siekmann, B. Rech, T. Rebmann, W. Appenzeller, B. Sehrbrock: "Process for the treatment of substrates with prestructured zinc oxide layer", German patent DE 10 2004 017 680 B4) mean roughness (English: root mean square roughness;

hereinafter RMS roughness) can be increased to about 200 nm. Such surface-textured layers have very good light-scattering properties and, in particular with the aid of high-frequency magnetron sputtering methods (in short:

HF magnetron sputtering method) (see B. Rech, O. Kluth, T. Repmann, T. Roschek, J. Springer, J. Müller, F. Finger, H. Stiebig and H. Wagner, in: Sol., Energy Mater, Sol., Cells 74, 439 (2002); O. Kluth, G. Schöpe, J. Hüpkes, C. Agashe, J. Müller, B. Rech, in Thin Solid Films 442 (2003) page 80 -85). In M. Breedon et al: "ZnO Nanostructured Arrays Grown from Aqueous Solutions on Different Substrates" in "Conference Proceedings,

International Conference on Nanoscience and Nanotechnology ", ICONN 2008, pp. 9 to 12 discloses various substrates with ZnO layers, which are produced from aqueous solution and are produced on a 1, 2 μm thick, by high-frequency magnetron sputtering

ZnO nucleation layer can be applied. It explicitly deals with the production of so-called "nanorods" (nanorods) .The ZnO layer is used in this document, the orientation and

To support uniformity of nanorods.

Basically, it is advantageous to pass a substrate through

To coat high-frequency magnetron sputtering with aluminum-doped zinc oxide in order to obtain suitable layer properties. However, the high-frequency magnetron sputtering is a relatively slower

Sputtering process compared to DC magnetron sputtering, so that the production of aluminum-doped zinc oxide layers on a substrate can take a long time. It has also been found that the process conditions during atomization, the resulting optical and electrical

 Determine the material properties of the ZnO layers significantly. The surface structures which can be produced by the wet-chemical etching are mainly determined by the process parameters temperature and deposition pressure and by the selected

 Substrate material influenced. Another important parameter is the doping of the solid-state target with aluminum. Thus, it is possible, depending on the doping concentration and temperature, to find an optimum "coating window" for layers produced by RF magnetron sputtering methods, which have an optimized optical waveguide structure after the wet-chemical etching step (see M. Berginski, B. Rech, J. Hüpkes, H. Stiebig, M. Wuttig: "Design of ZnO: AI Films with Optimized Surface Texture for Silicon Thin-film Solar Cells" in: SPIE 6197 (2006), pp. 61970Y 1-10, M. Berginski, J. Hüpkes, M. Schulte, G. Schöpe, H. Stiebig, B. Rech: "The effect of front ZnO: AI surface texture and optical

 Transparency on efficient light trapping in Silicon thin-film solar cells "in: Journal of Applied Physics 101, pp. 74903 (2007).) The optimal design of the interface has a considerable influence on the

 Efficiency of the solar cell. Important in this context is the optimization of the roughness with regard to the lateral and vertical

 Dimensions. It has proven to be advantageous if the lateral dimensions in the order of the wavelength of the light to be scattered and thus in the μηη range for solar cells

 microcrystalline silicon (c-Si: H) or so-called tandem cells (a-Si: H / c-Si: H) and an average roughness of about 100 nm to about 200 nm is achieved.

The texture etching of ZnO: Al layer systems exploits the anisotropy of the etch rate of crystalline ZnO layers to convert conventionally smooth deposited layers with a columnar growth (lateral dimension about 50 to 100 nm) into a rough interface whose lateral Dimensions with optimized process conditions in the μηη range. When texture etching, it is mainly of interest that the usually difficult production of large crystallites is avoided. The method is based on etching the ZnO: Al layers in dilute acid (for example, 0.5% HCl). The etching takes place anisotropically, so that the O-terminated, deposited in c-axis orientation crystallites an order of magnitude faster than the corresponding

 Etched Zn-terminated crystallites. Orthogonal to this, even an increase in the etching rate can be observed by a factor of 40 (compare F.S. Hickernell: "The microstructural properties of sputtered zinc oxide SAW transducers." In: Review Phys., Appl., 20: 319-324 (1985)).

In principle, it is also possible with DC magnetron sputtering of a ceramic solid state targets and by means of reactive

 Medium frequency sputtering (MF sputtering) of a metallic one

 To find solid-state targets comparable in etch morphology as in optimized high-frequency sputtering conditions (see B. Rech, T. Repmann, J. Hüpkes, M. Berginski, H. Stiebig, W. Beyer, V. Sittinger, F. Ruske: "Recent progress in amorphous and microcrystalline silicon based solar cell technology ", in: Proceedings of 20th European Photovoltaic Solar Energy Conference, (Barcelona) (2005), pp. 1481-1486; J. Hüpkes, B. Rech, O. Kluth, T. Repmann, B. Zwaygardt, J Muller, R Drese, M. Wuttig: "Surface-textured MF-sputtered ZnO films for microcrystalline silicon-based thin-film solar cells" in: Solar Energy Materials and Solar Cells 90 (2006), p. 3054 -3060). However, it has been shown that this Ätzmorphologie is very rarely reproducible and thus can be implemented on relatively large areas only with great difficulty.

 In the case of reactive medium-frequency (MF) magnetron sputtering of ZnO: Al, the desired etch morphology can be set by the process control (see Szyszka, B.: "Magnetron sputtering of ZnO films").

 Transparent Conductive Zinc Oxide: Basics and Applications in: Thin Film Solar Cells. Ellmer, K .; Rech, B .; Klein, A. (ed.). Springer Series in Materials Science, 2007, pp. 187-229). It is known that the

 desired Zn termination of the ZnO crystallites by a

Operation in metallic mode at high substrate temperature can be achieved when excess zinc desorbs from the surface due to the high vapor pressure. High substrate temperatures are generally advantageous in this context. At a high oxygen partial pressure, rough, fissured structures with a small lateral dimension result. The etching images show deep holes. It can be assumed that O-terminated crystallites were etched here at a high etch rate, whereas the etching attack over the flanks of the surrounding grains apparently does not occur. A possible

 The explanation for this is the thermodynamically favorable segregation of aluminum at the grain boundaries, which leads to the formation of an etch-resistant A ^ Oß enrichment there. At a low

 Oxygen partial pressure results in flat structures, indicating a uniform Zn termination. Furthermore, it turns out that a repeated overflow in front of a cathode is necessary in order to suppress the throughput of defects.

The growth and thus the termination of the layer by the different energy inputs (in particular by the

 Substrate temperature, neutral particle energies, ion energies), ion current measurements in the production of aluminum-doped zinc oxide show the different ion energy contribution depending on

 Plasma excitation. In order to achieve an etching structure suitable for solar cells, it is therefore important to influence the layer growth in such a way that a predominantly Zn-terminated surface with little O-terminated crystallites is present.

 DE 10 2004 048 378 A1 discloses zinc oxide thin films consisting of a substrate of monocrystalline sapphire (Al 2 O 3) with a or

 c-cut orientation and a ZnO layer with epitaxial

 Consist of crystal structure. These zinc oxide thin films allow a particularly intense and fast light emission (luminescence) in the ultraviolet spectral range at room temperature. These

 Zinc oxide thin films are produced laser-based by laser plasma deposition.

In JT Chen et al .: The effect of AI doping on the morphology and optical Property of ZnO nanostructures prepared by hydrothermal process "(Applied Surface Science 255 (2009) pp. 3959-3964) 200 nm thick nucleation layers of ZnO on an indium-tin oxide substrate (ITO substrate) are used, which from aqueous solution by

 Rotational coating can be produced.

 The present invention is based on the object to provide a method for coating a substrate with aluminum-doped zinc oxide available, by means of which ZnO: Al layers with improved layer properties, high process reliability and high deposition rate can be generated.

 This object is achieved by a method having the features of

 Claim 1 solved. The subclaims relate to advantageous

 Further developments of the invention.

 An inventive method for coating a substrate with aluminum-doped zinc oxide comprises the steps

 - Generating a between 5 nm and 400 nm thick nucleation layer containing zinc oxide or doped, in particular aluminum-doped zinc oxide, on the surface of the substrate by sputtering a

 Solid targets;

 - Generating a quasi-epitaxial on the nucleation layer

 further growing topcoat containing aluminum-doped zinc oxide; and

 wet-chemical etching of the cover layer.

 It has been found that the prepared by the method according to the invention on the substrate ZnO: Al layers advantageous

 Have Lichtleitstrukturen, so that they in particular as

 Front contact suitable for silicon thin-film solar cells. According to the invention, the nucleation layer, which contains zinc oxide or doped, in particular aluminum-doped, zinc oxide, by sputtering a

 Solid state targets produced. The doped zinc oxide may in principle have any dopants. In addition to aluminum are here

in particular to mention dopants with gallium, indium or boron. This nucleation layer provides optimized conditions for the cover layer, which also contains aluminum-doped zinc oxide, can continue to grow quasi-epitaxially on the nucleation layer. In particular glass, plastic, metals or ceramics can be used as substrate materials. Wet chemical etching of the

 Cover layer, which structures this, is preferably carried out with dilute hydrochloric acid. The nucleation layer may advantageously have a thickness which is <300 nm. The nucleation layer serves primarily to positively influence the electrical properties of the later-growing layer, which contains ZnO: Al, as well as its etching behavior. The nucleation layer can be used in particular on amorphous substrates such as glass. Since it is furthermore a polycrystalline layer and not a monocrystalline layer, there is no epitaxy but only quasi-epitaxy.

In a particularly advantageous embodiment, it is proposed that the nucleation layer is produced with a thickness between 5 nm and 30 nm on the substrate. It has surprisingly been found that even relatively thin nucleation layers (in particular about 5 to about 30 nm thick nucleation layers) are sufficient to increase the quasi-epitaxial growth of the cover layer on the nucleation layer

 favor.

 In order to obtain an optimized growth of the cover layer on the nucleation layer, it is proposed in a particularly preferred embodiment that the nucleation layer passes through

 High-frequency magnetron sputtering of a ceramic solid-state target, the ZnO and a content of Al2O3 and / or any other

 Dopings is generated, which in particular maintains the lattice structure or at least almost maintained (and thus only slightly changed). It could be determined that such a nucleation layer produced by high-frequency magnetron sputtering during the subsequent deposition of the ZnO: Al layer, which is advantageous for

 Example by DC magnetron sputtering or

Medium-frequency magnetron sputtering can take place, its predominant Zn termination quasi-epitaxy can continue. Such a made After the wet-chemical etching step, which can be carried out in particular with dilute hydrochloric acid, the covering layer has an improved light-guiding trap structure. This is characterized in particular by the fact that the crater width predominantly in the area of the incident

 Light wavelength in the near infrared spectral range (about 1 μηη) is. Further, it has been found that the depth of the craters can be varied to some extent by the etch time.

 In an advantageous embodiment, it is proposed that

 Producing the nucleation layer is a ceramic solid-state target is used, the ZnO and has a content of Al2O3, which is greater than 0 wt.% And less than 1 wt.%, And by

 High-frequency magnetron sputtering at a temperature T> 300 ° C is atomized. By setting the content of Al 2 O 3 (greater than 0% by weight and less than 1% by weight) at a temperature T> 300 ° C., it was possible to determine an optimized "coating window" for the sputtering of the ceramic solid-state target for the production of the Nucleation layer can be obtained.

 In an alternative embodiment, there is also the possibility that a ceramic solid-state target is used to produce the nucleation, the ZnO and a content of Al2O3 between 1 and 2 wt.% Has and by high-frequency magnetron sputtering at a temperature T <300 ° C is atomized. It has been found that by setting the content of Al 2 O 3 between 1 and 2% by weight at a temperature T <300 ° C., a further optimized "coating window" for sputtering of the ceramic solid-state target for producing the nucleation layer can be obtained.

 In the present case, it is a dynamic coating process in which the substrate during sputtering at a certain speed on the solid-state target, from which the atoms

 be removed, passed by. To the growth of

 Nucleation layer on the substrate and the quality of the

 Improving nucleation layer is one in particular

advantageous embodiment provided that the deposition rate, with the nucleation layer is applied to the substrate is less than 20 nm m / min.

 In a further alternative embodiment, the

 Possibility that a ceramic solid-state target is used to produce the nucleation layer, which has ZnO and a content of Al2O3 and / or any other dopants and by

 DC magnetron sputtering, wherein the deposition rate at which the nucleation layer is deposited on the substrate is less than 20 nm m / min. There is thus advantageously also the possibility that the nucleation layer by DC magnetron sputtering a

 ceramic solid state targets is generated. The must

 Deposition rate be adjusted so that it is less than 20 nm m / min, so that the nucleation layer has a corresponding nature, so that the cover layer quasi-epitaxially on the

 Nucleation layer can continue to grow.

 In an advantageous embodiment, it is proposed that the covering layer further growing on the nucleation layer by sputtering a ceramic solid-state target containing ZnO and a content of Al2O 3, by DC magnetron sputtering or

 DC pulse magnetron sputtering is generated. DC magnetron sputtering or DC pulse magnetron sputtering of a ceramic solid state target allows rapid growth of the capping layer on the nucleation layer. In addition, these sputtering processes are very robust from the process engineering point of view.

 In an alternative advantageous embodiment, it is proposed that the covering layer which continues to grow on the nucleation layer is produced by sputtering a metallic solid-state target, which

 aluminum-doped zinc oxide (Zn: Al), is produced in the reactive gas process by DC magnetron sputtering or medium frequency magnetron sputtering. These methods also allow a quick

 Layer growth and are characterized by their robustness

 correspondingly fast oxygen partial pressure control.

The cover layer which continues to grow on the nucleation layer can be alternatively also by

 - hollow cathode gas flow sputtering; or

 - vapor deposition; or

 - wet chemical deposition; or

 atmospheric chemical vapor deposition (CVD); or

 Low pressure CVD (LP-CVD); or

 - atmospheric plasma enhanced chemical vapor deposition (PECVD); or

 - Low pressure PECVD

 be generated.

 The method described here represents a novel approach to

 Available to produce zinc oxide layers with good etching properties and excellent electrical mobility. In this case, the deposition rate of the total layer can advantageously be greatly increased, since the slowly grown nucleation layer the

 Growth determined.

 Further features and advantages of the present invention will be

 clearly with reference to the following description

 Embodiments.

 Embodiment 1

 In the first embodiment, several samples were examined, in which by high-frequency magnetron sputtering

 (RF magnetron sputtering) produced seed layer was gradually reduced from 390 nm to 25 nm. On the

 Nucleation layer was deposited in each case a cover layer of ZnO: Al by DC magnetron sputtering, wherein the total thickness was about 1 μηη. All layers deposited in this manner were etched with 0.5% hydrochloric acid (HCl).

 The etching morphology of the samples was then by means of

 Scanning electron microscopy (SEM) examined. It could be determined that all cover layers were independent of the thickness of the

Nucleation layer have similar Ätzmorphologien. All SEM images showed a similar etch structure with crater widths of about 1 μηη. The etch structures are comparable to the cover layers, which are produced purely by means of RF magnetron sputtering.

 By applying a relatively thin nucleation layer, the growth of the layer subsequently produced by DC magnetron sputtering can thus be sustainably influenced. The nucleation layer initially applied to the substrate evidently provides a quasi-epitaxial growth of the further growing ZnO: Al layer.

 Furthermore, it was found that the so produced

 ZnO: Al layers have excellent resistivity between 286 and 338 ohmcm. This is also due to the quasi-epitaxial growth of the ZnO: Al layer on the

 Due to nucleation layer.

 Embodiment 2

 Two layers with different thickness of the nucleation layer

 (Seedlayerdicke) were removed from the sputtering and exposed to normal atmosphere. The layers were then fed into the sputtering device together with an uncoated glass plate for the production of the ZnO: Al cover layer by DC magnetron sputtering. These experiments served as a test for a possible etch structure change due to vacuum fracture (build-up of moisture and so on on the layer). In addition, the different etching structure in pure DC deposition was compared to that by

 RF magnetron sputtering produced nucleation layers verified.

 SEM investigations showed that the etching morphology of the pure

 DC layer shows much smaller structure sizes of the etching trenches.

 In contrast, showed with the by

 High-frequency magnetron sputtering nucleation-provided substrates clearly marked etching craters, wherein the layers have somewhat flatter structures at the same etching depth compared to the non-atmospheric samples. These structures can be optimized by adjusting the etching time.

[0036] Sample characterization By atomic force microscopy, the average roughness (RMS roughnesses) of several layers, which were produced with the aid of the method presented here, were determined in Table 1. In this way, the structures shown in the SEM images could also be quantitatively recorded.

 Table 1:

 [0039]

 Table 1

 The samples with a nucleation layer without vacuum fracture (samples Nos. 2 to 5) showed, independent of the thickness of the nucleation layer, a mean roughness of the cover layers (on average -150 nm), the

 comparable to that purely by

 High-frequency magnetron sputtering generated layer (Sample No. 1). The cover layers of Sample Nos. 7 and 8, which underwent vacuum fracture, showed improved roughness compared to the pure DC layer (Sample No. 6). However, the roughness is about 50 nm lower compared with the layers without vacuum break at about 100 nm. In the AFM images, as in the SEM images, the lateral extent of each crater can be seen. Here could

Comparable lateral structure sizes as they can be achieved in pure HF layers are observed. In addition, the layer showed that without nucleation layer under the same conditions (parallel coating), a much smaller lateral structure size.

 An additional possibility of characterizing the samples is the angle-resolved scattered light measurement, which determines the proportion of in

 different angular ranges of scattered light reflects. A morphology optimized for the application should scatter as much of the red and near infrared light as possible into large angles.

 Experimentally, the light scattering of the etched ZnO: Al layers on different thickness nucleation layers (25 nm, 80 nm, 155 nm and 390 nm) was studied at a wavelength of 700 nm. The samples were illuminated from the slab side under normal incidence while the detector intercepted the transmitted light at the various angles. The investigations showed that all samples scatter the light essentially very well. Both the form and the intensity are similar to those of a pure

 High frequency magnetron sputter deposition can be obtained.

Claims

claims  A method of coating a substrate with aluminum-doped zinc oxide, comprising the steps  - Generating a between 5 nm and 400 nm thick nucleation layer containing zinc oxide or doped, in particular aluminum-doped zinc oxide, on the surface of the substrate by sputtering a solid state target;  - Generating a quasi-epitaxial on the nucleation layer  further growing topcoat containing aluminum-doped zinc oxide; and wet-chemical etching of the cover layer.  2. The method according to claim 1, characterized in that the  Nucleation layer with a thickness between 5 nm and 30 nm on the  Substrate is generated.  3. The method according to any one of claims 1 or 2, characterized in that the nucleation layer is generated by high-frequency magnetron sputtering of a ceramic solid-state target containing ZnO and a content of Al2O3 and / or any other dopants, which retains the lattice structure or at least almost maintains.  4. The method according to claim 3, characterized in that for the production of the nucleation layer, a ceramic solid-state target is used which has ZnO and a content of Al2O3, which is greater than 0 wt.% And less than 1 wt.%, And is sputtered by high frequency magnetron sputtering at a temperature T> 300 ° C.  5. The method according to claim 3, characterized in that for the production of nucleation a ceramic solid-state target is used, the ZnO and a content of Al2O3 between 1 and 2 wt.% And atomized by high-frequency magnetron sputtering at a temperature T <300 ° C. becomes.  6. The method according to any one of claims 1 to 5, characterized in that the deposition rate at which the nucleation layer is applied to the substrate is less than 20 nm m / min. 7. The method according to any one of claims 1 or 2, characterized in that a ceramic solid-state target is used to produce the nucleation, the ZnO and a content of Al2O3 and / or arbitrary has other dopants and is sputtered by DC magnetron sputtering, wherein the deposition rate at which the nucleation layer is deposited on the substrate, less than 20 nm m / min.  8. The method according to any one of claims 1 to 7, characterized in that on the nucleation layer further growing cover layer  Sputtering a ceramic solid state target containing ZnO and containing Al 2 O 3 by DC magnetron sputtering or  DC pulse magnetron sputtering is obtained.  9. The method according to any one of claims 1 to 7, characterized in that on the nucleation layer further growing cover layer  Atomizing a metal solid-state target containing aluminum-doped zinc oxide generated in the reactive gas process by DC magnetron sputtering or medium frequency magnetron sputtering.  10. The method according to any one of claims 1 to 7, characterized in that on the nucleation layer further growing cover layer  - hollow cathode gas flow sputtering; or  - vapor deposition; or  - wet chemical deposition; or  atmospheric chemical vapor deposition (CVD); or  Low pressure CVD (LP-CVD); or  - atmospheric plasma enhanced chemical vapor deposition (PECVD); or  - low pressure PECVD  is produced.  1 1. Use according to one of claims 1 to 10 with  aluminum doped zinc oxide coated substrate as a front contact of a silicon thin film solar cell.