WO2003040426A1

WO2003040426A1 - Zinc oxide layer and method for the production thereof

Info

Publication number: WO2003040426A1
Application number: PCT/EP2002/012589
Authority: WO
Inventors: Xin Jiang; Chunlin Jia; Bernd Szyszka
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.; Forschungszentrum Jülich GmbH
Priority date: 2001-11-09
Filing date: 2002-11-11
Publication date: 2003-05-15
Also published as: KR100957892B1; DE10155712A1; DE10155712B4; KR20040083414A

Abstract

The invention relates to a doped or undoped zinc oxide layer (9a, 9b, 9c) and to a method for the production thereof by chemical vapor deposition onto a substrate (10a, 10b, 10c). One of the surfaces (atomic level), which has the lowest surface energy, is grown in a manner that is surface parallel with regard to the geometry of the substrate when the substrate has a higher surface energy than that of the ZnO (0001) crystal surface and/or when the process parameters are set so that a 2D lateral layer growth can be realized.

Description

Zinc oxide layer and process for its production

The present invention relates to doped and undoped zinc oxide layers and to a method for the production thereof.

ZnO has a Wurtz crystal structure. The doping with etallele ducks such as aluminum enables multifunctional materials to be created that combine both high transparency in the visible spectral range and metal-like electrical conductivity. These oxides are of crucial importance for a large number of technical applications. Examples are transparent and conductive electrodes in solar cells, flat screens and electrically switchable glazing. A particularly interesting application is due to their piezoelectric properties for electronic SAW (Surface Acoustic Wave) signal filters in mobile radio for the high-frequency range (> 2GHz). So far, both (PVD, Physical Vapor Deposition) and CVD (Chemical Vapor Deposition) have been established as gas separation processes for the production of ZnO layers. One advantage of layer growth from the gas phase is that layers which grow up can be mixed with other elements in situ and made conductive. The best results are achieved with sputtered ZnO: Al layers.

A large number of "technical applications of the ZnO layers depend on the microstructure of the layers, such as the texture and surface roughness. In order to optimize these structural properties, deposition processes have been tested by adapting the deposition parameters.

The heteroepitaxial growth of ZnO on a substrate such as sapphire is state of the art.

The ZnO layer structure, in particular the grain size and the crystal orientation, which is very important for the applications, can only be achieved in limited cases, for example by heteroepitaxial growth. A location-dependent selective orientation of the layers has so far not been possible. According to the state of the art, heteroepita- xy is suitable for producing layers of high quality, but it is cost-intensive and only possible with certain documents, the lattice parameters of which correspond to that of ZnO. On the other hand, the heteroepitaxially deposited layers have a certain orientation relation to the substrate and therefore their orientation can not be chosen freely. The restrictions are the main obstacle to further development in the science and technology of the TCO layers (Transparent Conductive Oxide).

On the basis of this prior art, the object of the present invention is to provide a method or a zinc oxide layer which is inexpensive on the one hand and which enables a greater variety of geometric structures of the zinc oxide layers on the other hand.

This object is achieved in relation to the method according to the invention by a method according to patent claim 1 and in relation to a layer according to the invention by the subject matter of patent claim 9.

This object is achieved in that a low-energy surface of a layer crystal grows parallel to the surface of the substrate so that the layer structure is produced in a defined manner by means of the geometric structuring of the substrate. This ensures that the production of zinc oxide layers, the layer structure / texture and surface roughness of which can be defined or ^' controlled by structuring (patterning) the substrate, is made possible in a gas deposition process. A surface-parallel growth of the zinc oxide (0001) layer structure on the substrate is achieved. This is practically independent of epitaxial influences. The surface growth (nucleation) depends solely on the substrate geometry, the surface energy of the substrate and the process parameters, especially the supersaturation of the reactive species in the gas phase. The nucleation mode (two- or three-dimensional) depends on the supersaturation. The supersaturation depends on the gas pressure and the substrate temperature. According to the invention, the layer structure (C texture) is realized by "self-texturing". According to the prior art, the texture formation of thin layers is due to a so-called Van der Drift evolution during growth, in addition to the particle or ion flow factor. That is, the germs that are initially statistically oriented cannot all survive. Only those whose fastest crystal growth direction is parallel to the growth direction of the layer will grow out. All others are buried little by little and the remaining ones form a texture. According to the prior art, this type of texture formation is independent of the substrate geometry and can only be achieved with a large layer thickness.

In the layer growth according to the invention, the crystallites grown on the substrate all orient themselves and form an almost perfectly textured column structure already at the interface (nucleation sites). The process is influenced by the surface or Interface energy of the layer / substrate material composite and the process parameters controlled.

If the crystal material forms the nucleus on a substrate in thermodynamic equilibrium, the shape of the growing crystal depends on the surface energy and the Wulff theorem applies here,

^ E = _Z ± = q ± = - ■ - = σ _a -ß

where Δμ is the chemical potential variation of the phase change, v the particle volume, σ_. the free energies of the surface and ß mean the separation work of the crystal from the substrate.

These relationships are illustrated by figures la and lb explained in more detail.

Figure la shows the case of a three-dimensional

Layer growth. Here ß is smaller than σ _a (i.e. the surface energy of the [0001] -

Area of the zinc oxide structure.

Figure lb shows the case in which ß is greater than 2σ _a . There is a high level of interaction between the substrate and the crystal growing on it. This leads to a two-dimensional growth of the crystal structures which is (0001) parallel to the surface of the substrate. The speed of the lateral growth parallel to the substrate surface is thus significantly higher than the growth speed perpendicular to it. This in turn leads to the (0001) zinc oxide layer structures growing essentially parallel to the surface of the substrate.

According to the Wulff theorem, the area with the lowest energy will try to grow parallel to the substrate surface. This tendency increases with increasing separation work from crystal to substrate. Since the (0001) surface of ZnO has the lowest free energy, this surface will try to be parallel to the substrate surface if it is possible to set the process parameters so that the process runs close to the thermodynamic equilibrium.

Advantageous further developments of the present invention are specified in the dependent claims. A particularly advantageous embodiment provides that the substrate may be amorphous. This makes it possible to provide a particularly cost-effective substrate which can also be microstructured on its surface at low cost. Depending on this microstructuring, crystal columns, for example, will then grow on the substrate perpendicular to the surface. Of course, it is also possible to use other materials (crystalline materials are also possible). However, care should be taken to ensure that the influence of the higher surface energy described above is stronger than any epitaxial influences. It is then also possible according to the invention that the substrate has a higher surface energy than the non-energy surface of the zinc oxide (0001) crystal surface and / or the process parameters are set such that two-dimensional layer growth is realized.

In particular, glass, thermally oxidized silicon or silicon can be considered as substrates for low-cost production.

Depending on the training, the substrate can also be flat in addition to a micro-structured shape. Fields with pyramid-shaped trenches are particularly suitable as microstructuring. In cross section, these have sawtooth structures or sawtooth structures displaced in parallel, which then each produce different designs of crystal columns growing thereon.

Any gas phase deposition method known in the prior art can be used in the various developments. For us, these are in particular PVD or CVD. It is particularly advisable to use an RF (radio frequency) or a medium-frequency sputtering source with an operating frequency of 10 kHz to 100 kHz (preferably 40 kHz) for sputtering.

Depending on the area of application, the zinc oxide layer can be doped with various metals. The doping is e.g. in that the target is enriched with a corresponding doping material during sputtering.

The layers according to the invention have potential for several areas of application:

a) As transparent conductive electrodes for the solar cells. The surface roughness of the ZnO layers is important for increasing the efficiency through light trapping.

b) C-axis textured layers with defined ^• axis orientation for use in microsystem technology and electronic components, such as SAW electronic signal filters and LEDs, in combination of the good piezo-electrical properties of the materials zinc oxide with the unsurpassed acoustic ZnO / Diamant / Si appear to be the promising candidates for SAW devices up to frequencies of 9 GHz.

Further advantageous developments of the present invention are described in the remaining dependent claims.

Show it: Figures la and lb theoretical foundations for the growth of zinc oxide crystals (this has already been discussed above),

FIGS. 2a to 2c the growth of zinc oxide layers according to the invention on substrates with different geometries,

FIG. 3 shows a lattice image of an interface between a zinc oxide layer doped with aluminum and a substrate,

FIGS. 4a to 4c show different views of a layer according to the invention which has been deposited on a structured silicon surface,

Figure 5 shows the basic structure of a coating system according to the invention.

With regard to Figures la and lb, reference is made to what has already been said above.

FIG. 2a shows a zinc oxide layer 9a growing on a flat substrate surface 10a. This consists of juxtaposed crystal columns 11a, which grow in the direction perpendicular to the plane of the substrate surface 10a. The layer thus grows in the c-axis, which is perpendicular to the (0001) surfaces of a zinc oxide crystal.

Such a layer is produced by sputtering in a medium-frequency magnetron sputtering system, for example. According to the invention, it should be noted here that for clean growth in the direction perpendicular to the substrate surface, this by determining process parameters is set so that 2D growth occurs and that the substrate surface has a higher surface energy than the [0001] crystal surfaces of the zinc oxide.

Glass (also quartz), polymer / plastic, thermally oxidized silicon or also silicon come into consideration as substrate materials. It should be emphasized that the substrate does not have to be selected on the basis of epitaxy, and amorphous substrates can also be selected. According to the invention, only the surface energy of the substrate and its microstructurable surface are decisive for the structure. The order of magnitude for the microstructures in the substrate surface is in the [nm] to [μm] range.

In principle, the zinc oxide layers growing on according to the invention can be doped or undoped. Whether or how much doping depends on the requirement profile for the layer to be deposited.

While zinc oxide in undoped form is an electrical insulator, e.g. zinc oxide doped with aluminum is an excellent electrical conductor.

The layer 9a shown in FIG. 2a can be used as a transparent conductive electrode for solar cells. Here, the surface roughness of the zinc oxide layer 9a on the upper side 12a thereof is used to increase the efficiency by light traping.

FIGS. 2b and 2c show further embodiments of layers according to the invention, the substrate surface having a different geometry than in FIG. 2a. It should be emphasized again that compared to the methods according to the prior art for the generation of a layer texture in a gas deposition process, the texture formation of the zinc oxide layers according to the invention does not depend on the particle or ion flow factor and is also not attributable to the so-called van der drift evolution during layer growth. It is independent of the crystallographic structure of the substrate, it is determined solely by the process control and the geometry of the substrate or by the surface energy of the layer-substrate composite. This so-called self-extraction enables the layer structure to be designed and manufactured as required by the application, which is particularly important for the specific electronic and optical application. By using a specially structured substrate, the grain size and the surface morphology and the roughness of the deposited layers can be controlled according to the invention. The layer crystallites oriented in the C-axis can be produced on localized areas on the micrometer scale.

In the following, Figures 2b and 2c will be discussed. Unless expressly stated otherwise, what has been said for FIG. 2a applies in full.

In contrast to the substrate 10a, the surface structure of the substrate 10b in FIG. 2b is not flat, but rather shows spaced pyramid trenches and thus a sawtooth structure in section AA, the individual essentially triangular teeth being connected by flat intermediate pieces 13. This leads to the fact that the surfaces on the surface of the substrate 10b grow in a total of three directions. On the one hand in the directions Cl and C2, which are perpendicular to the flanks of the teeth of the substrate surface. Also in the direction C3, which is perpendicular to the flat intermediate sections 13. This results in a zinc oxide layer 9b with the column geometry shown in FIG. 2b.

FIG. 2c shows a further embodiment of a substrate 10c. This has directly adjacent pyramid trenches and thus, in section A'-A ', a pure sawtooth structure, there being no longer a parallel replacement between the essential triangular teeth. Here too, crystal growth always takes place in the direction perpendicular to the (0001) faces of the zinc oxide crystals, namely in the directions C1 and C2, which are each arranged perpendicular to the flanks of the teeth. This results in a layer 9c which has a different orientation than the layers 9a and 9b. The ZnO (0001) axis is perpendicular to the flanks. The structure of the layer can thus be adjusted by varying the flank angle.

As FIGS. 2a to 2c show, the self-texturing process provides the possibility of specifically designing and producing the layer structure, which is of crucial importance for the specific electronic and optical application. With a specially structured substrate surface, the grain size and surface morphology as well as the roughness of the layers for the photovoltaic technology can be deposited in a defined manner. The orientation (c-axis) of the layer crystallites can be generated in a controlled manner on a localized area in a microscale, which has an application potential due to the piezoelectric properties of ZnO in microsystem technology. FIG. 3 shows a grating image of the interface between a zinc oxide layer doped with aluminum and the underlying silicon substrate. The image was taken along the [110] zone axis of the silicon crystal. The arrangement of the zinc oxide layer corresponds to that of FIG. 2a. The deposited ZnO: Al layers and the interface structures between the ZnO layer and the substrate were examined using high-resolution transmission electron microscopy (HRTM). In order to differentiate or avoid the influence of the epitaxial effect, oxidized Si was used as the substrate. If you look at the image along the interface, it is clear to see that the crystallites grown on the amorphous Si0 ₂ layer are all oriented with their (0001) plane parallel to the substrate surface and form an almost perfect c-axis textured columns Structure at the interface. No secondary phase such as Al ₂ 0 ₃ and amorphous ZnO can be observed at the grain boundary. The column diameter is approx. 30 n.

This phenomenon provides a strong indication that precisely c-axis textured ZnO crystals can be deposited on an amorphous substrate without the influence of epitaxy. This also indicates that the orientation of the crystal is controlled by minimizing the surface or interface energies. Because of the low free surface energy, the ZnO (1.6 J / m ² ) will preferably grow parallel to the substrate surface.

FIGS. 4a to 4c show a further zinc oxide layer in various sizes, which is applied to a silicon substrate. The substrate has one Structure similar to the structure shown in Figure 2b, but with the tips of the teeth cut off. The transition from a flat section to a flank section of the substrate is shown in FIG. 4a.

A microscopy image is shown in FIG. 4b, a 50 nm comparison line is drawn in. Below the white line running through the middle of the image is silicon, above is the zinc oxide layer.

A further enlargement is shown in FIG. 4c, the alignment of the crystal lattices being recognizable. A 3 nm comparison line is shown in the lower right edge of the image. The directions of the [0001] faces are also shown. These are essentially horizontal in the area of the oblique flanks parallel to the flank surface and in the right-hand area of the image. For comparison, the (111) and (001) faces of the silicon running parallel to this are also shown.

In the arrangement in FIGS. 4a to 4c, the silicon surface was structured in advance by wet chemical etching and the layer grown on it was examined and photographed. It is particularly noteworthy that the zinc oxide (0001) surface actually runs over the edge parallel to the substrate surface; it is thus shown that the layer microstructure can be manipulated by specifying the substrate surface.

Finally, a coating system according to the invention shown in FIG. 5 is presented.

The ZnO layers were quenz (MF) magnetron sputtering produced in a laboratory sputtering system 15. The basic structure of this sputtering system is shown in FIG. 5:

The sputtering system is based on a commercial one

High vacuum coating system (PLS 580, company Pfeiffer). The system has a coolable and heatable recipient 1, which is evacuated via a turbopump system. A base pressure of p ₀ <4xl0 ^{~ 6} mbar was guaranteed in all coating experiments.

The MF sputter source 2 (TwinMag®, Leybold Systems) is made up of two conventional PK 500 planar magnetron cathodes (target format: 488 x 88 mm ² ), which are connected to an MF generator via a special adaptation unit. The generator delivers a harmonic output signal with a frequency of 40 kHz. The maximum output power is 10 kW, which corresponds to a maximum power density of P / A = 11.5 W / cm ² . Sputter gas and reactive gas are supplied separately via two independent gas inlet systems 3.

All gas flows are supplied via a high-purity gas system and controlled via mass flow controllers. The gas purity is better than 4.8. For the work presented here, the system was provided with a on a controllable rotary drive 4 with a first shutter 5 and a rotatable substrate support platform 6 with a rotatable substrate holder 7, which accommodates a total of four substrates in the format 50 × 50 mm ² . Such a substrate holder also accommodates the three substrate materials glass / quartz (20 x 20 mm ² ) thermally oxidized Si (20 x 20 mm ² ) and non-oxidized Si (5 x 40 mm ² ). The substrate temperature can be controlled by a boron carbide radiant heater 8 (with a second shutter) be specified. For glass substrates, reference measurements are available to determine the substrate temperature depending on the heating temperature.

In the following, examples are given for the equilibrium conditions according to the invention during deposition:

Table 1: Process parameters for reactive MF (mid-frequency) magnetron sputtering (RMFMS) from ZNO: Al layers in metallic and transition mode.

According to the Wulff theorem mentioned above, the dominant crystal surfaces are those with the smallest surface energy. These areas tend to grow parallel to the substrate. This tendency increases as the binding energy of the layer onto the substrate increases, as has been shown schematically in FIG. 1. Since the (0001) areas of ZnO have the lowest free energy [1.6J / m ² for (0001), 2 _f 0J / m ² for (1120) and 3.4J / m ² for (1010)], they will try to wake up parallel to substrate surfaces (> 2J / m ² ) If the process parameters are set so that the process runs close to the thermodynamic equilibrium. If the substrate surface now has an energy level of, for example, 1.8 J / m ² , provided that there is a thermodynamic equilibrium, ZnO (0001) nuclei alone or primarily will form, and a corresponding layer growth according to the invention is ensured.

The structure of the layers is heavily dependent on the so-called process mode. The c-axis oriented ZnO layers were obtained in metallic mode, which takes place in an equilibrium or near-equilibrium state. The values shown in Table 1 for this purpose are only given as examples for the person skilled in the art; Of course, other pairs of values are also possible in which the deposition proceeds in thermodynamic equilibrium (zinc with a 1.5 percent by weight aluminum was used as the target material for the conventional planar magnetron cathodes). If the partial pressure of oxygen is reduced and the ion energy density to the ZnO layer surface is adjusted accordingly, a transition mode is also realized (Table 1). The HRTEM investigations showed that, in contrast to the c-axis textured layers in metallic mode, the layers which were produced under a transition mode are statistically oriented and have no column structure according to the invention. The ZnO layers produced according to the invention are also e.g. usable as waveguide.

Claims

claims

1. Process for the production of doped and undoped zinc oxide layers (9a, 9b, 9c)

L0 vapor deposition on a substrate (10a, 10b,

10c), characterized in that to form the undoped and doped zinc oxide layers, a low-energy surface of a layer crystal grows parallel to the surface of the substrate 5, so that the layer structure is produced in a defined manner by means of the geometric structuring of the substrate.

2. The method according to claim 1, characterized in that the substrate has a higher surface energy than the low energy surface of the zinc oxide (0001) crystal surface and / or the process parameters are set such that a two-dimensional layer growth is realized.

5 3. The method according to claim 2, characterized in that the substrate can be any amorphous or crystalline solid, in particular glass, quartz or thermally oxidized silicon and polymer / plastic.

4. The method according to any one of the preceding claims, characterized in that the substrate is level (10a) or microstructured (10b, 10c).

5. The method according to claim 4, characterized in that the microstructuring in a cross section has a sawtooth structure (10c) or a sawtooth structure (10b) displaced in parallel.

6. The method according to any one of the preceding claims, characterized in that the gas phase deposition is carried out by PVD or CVD.

7. The method according to claim 6, characterized in that an RF or medium-frequency sputtering source (2) with a frequency of 10 to 100, preferably 40 kHz is used for sputtering.

8. The method according to any one of the preceding claims, characterized in that the zinc oxide layer is doped with aluminum.

9. Doped or undoped zinc oxide layer, characterized in that it is produced by a method according to claims 1-8.

10. Layer according to claim 9, characterized in that it is a transparent conductive electrode for solar cells, flat screens and electrical glazing or a waveguide.

11. Layer according to claim 9, characterized in that these for use in electronics and microsystem technology such as can be used as a SAW signal filter for mobile radio devices.