TRANSPARENT CONDUCTING FILM BASED ON ZINC OXIDE

Abstract

A transparent conducting film including a nominally undoped conducting ZnO base layer covered with a ZnO cover. The ZnO base layer has a preferred crystallographic orientation, whereas the ZnO cover includes one or more ZnO sublayers, of which at least one has a crystallographically randomly oriented or amorphous structure or a preferred crystallographic orientation different from the preferred crystallographic orientation of the base layer. The invention further relates to a process for the manufacture of such a transparent conducting film.

Claims

1-23. (canceled)

24. A transparent conducting film comprising a nominally undoped conducting ZnO base layer, the ZnO base layer having a preferred crystallographic orientation and being covered with a ZnO cover, the ZnO cover comprising one or more ZnO sublayers, of which at least one has a crystallographically randomly oriented or amorphous structure or has a preferred crystallographic orientation different from the preferred crystallographic orientation of the base layer.

25. The transparent conducting film as claimed in claim 24, wherein the ZnO cover consists of a single ZnO sublayer having a crystallographically randomly oriented or amorphous structure.

26. The transparent conducting film as claimed in claim 24, wherein the ZnO cover consists of a single ZnO sublayer having a single preferred crystallographic orientation.

27. The transparent conducting film as claimed in claim 24, wherein the ZnO cover is a multilayer cover comprising plural ZnO sublayers, and wherein the ZnO sublayers of the ZnO cover have different preferred crystallographic orientations.

28. The transparent conducting film as claimed in claim 27, wherein at least one of the ZnO sublayers of the ZnO cover has a crystallographically randomly oriented or amorphous structure.

29. The transparent conducting film as claimed in claim 27, wherein at least one of the plural ZnO sublayers of the ZnO cover has a thickness comprised in the range from 2 nm to 40 nm and the ZnO cover has an overall thickness comprised in the range from 10 nm to 200 nm.

30. The transparent conducting film as claimed in claim 24, wherein the ZnO cover is arranged in direct contact with the ZnO base layer.

31. The transparent conducting film as claimed in claim 24, wherein the base layer surface has a normal, wherein the preferred crystallographic orientation of the ZnO base is (001) with respect to the normal.

32. The transparent conducting film as claimed in claim 24, wherein the base layer surface has a normal, wherein the preferred crystallographic orientation of at least one of the ZnO sublayers of the ZnO cover is (110) or (101) with respect to the normal.

33. The transparent conducting film as claimed in claim 24, comprising a flexible substrate carrying the ZnO base layer and the ZnO cover applied thereon.

34. A semiconductor device comprising a transparent conducting film as claimed in claim 24.

35. The semiconductor device as claimed in claim 34, wherein the semiconductor device comprises one or more of a transparent transistor, an array of transparent transistors, a flat panel display, an RFID chip, a photovoltaic cell and a capacitive sensor.

36. The semiconductor device as claimed in claim 34, comprising a thin-film solar cell.

37. A method of manufacturing a transparent conducting film comprising a nominally undoped conducting ZnO base layer, the ZnO base layer having a preferred crystallographic orientation and being covered with a ZnO cover, the ZnO cover comprising one or more ZnO sublayers, of which at least one has a crystallographically randomly oriented or amorphous structure or has a preferred crystallographic orientation different from the preferred crystallographic orientation of the base layer, the method comprising depositing a nominally undoped conducting ZnO base layer and a nominally undoped ZnO cover on the ZnO base by sputtering from a ZnO target in an inert gas atmosphere or from a Zn target in a mixed oxygen and inert gas atmosphere onto a substrate while maintaining a plasma close to the substrate; wherein the deposition of the ZnO base layer and the deposition of the ZnO cover are carried out with different densities of the plasma close to the substrate.

38. The method as claimed in claim 37, wherein the plasma densities close to the substrate are chosen such that the ZnO base layer is deposited with a preferred crystallographic orientation and that the ZnO cover is deposited with a crystallographically randomly oriented or amorphous structure or with a preferred crystallographic orientation different from the preferred crystallographic orientation of the ZnO base layer.

39. The method as claimed in claim 37, wherein the plasma density close to the substrate is maintained constant during the deposition of the ZnO base layer.

40. The method as claimed in claim 37, wherein the plasma density close to the substrate is maintained constant during the deposition of the ZnO cover.

41. The method as claimed in claim 37, wherein the plasma density close to the substrate is varied during the deposition of the ZnO cover.

42. The method as claimed in claim 41, wherein the plasma density close to the substrate is maintained constant after one or more of the variations in such a way as to achieve a multilayer cover comprising plural ZnO sublayers with different crystallographic properties.

43. The method as claimed in claim 41, wherein at least part of the variations of the plasma density close to the substrate are carried out with an amplitude and frequency such that a single preferred crystallographic orientation or a crystallographically randomly oriented or amorphous structure of the deposited ZnO results.

44. The method as claimed in claim 37, wherein the plasma at the substrate is generated and maintained by RF biasing the substrate, and wherein the different plasma densities are obtained by modifying RF power density applied to the substrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] By way of example, preferred, non-limiting embodiments of the invention will now be described in detail with reference to the accompanying drawings, in which:

[0035] FIG. 1: is a schematic transversal cross-sectional view of a transparent conducting film;

[0036] FIG. 2: is a schematic transversal cross-sectional view of further transparent conducting film;

[0037] FIG. 3: is a schematic transversal cross-sectional view of yet another transparent conducting film;

[0038] FIG. 4: is a schematic drawing of a deposition apparatus,

[0039] FIG. 5: is a power delivery scheme for the deposition of a transparent conducting ZnO film;

[0040] FIG. 6: is a power delivery scheme for the deposition of another transparent conducting ZnO film;

[0041] FIG. 7: is a transversal SEM image of an example of a transparent conducting ZnO film according to the invention;

[0042] FIG. 8: is a graph illustrating the transmittance of the ZnO film depicted in FIG. 7;

[0043] FIG. 9: is a diagram of the evolution of the resistivity of a ZnO film similar to that in FIG. 7 monitored over several thousands of hours of heating in comparison with other ZnO films.

DETAILED DESCRIPTION PREFERRED EMBODIMENTS

[0044] Preferred embodiments of the present invention relate to a nominally undoped ZnO layer structure that is highly conducting, has a high VIS and NIR transparency (with respect to ZnO:Al films), but which possesses a significantly improved environmental stability (with respect to conventional conducting ZnO films).

[0045] The ZnO transparent conducting film comprises of two principal parts: [0046] i) a thicker base layer made of a nominally undoped and crystallographically highly ordered ZnO layer, and [0047] ii) a thinner cover structure consisting of a single ZnO sublayer or a multilayered ZnO stack.

[0048] In the cover, at least one of the ZnO sublayers has a crystallographically randomly oriented or amorphous structure or a preferred crystallographic orientation different from the preferred crystallographic orientation of the base layer.

[0049] While the thicker base layer assures the optoelectrical performance (e.g., high conductivity and high VIS and NIR transparency), the primary function of the ZnO cover is to slow down or inhibit penetration of corroding agents towards and within the underlying base layer.

[0050] A scheme of a first example of TCO film 10 using the suggested ZnO is offered in FIG. 1. The ZnO base layer 12 consists of a comparatively thick layer of pure ZnO exhibiting the commonly observed columnar microstructure featuring numerous columns that enlarge upwards in width during film growth from underlying substrate (not shown). The column boundaries that span through the height (thickness) of the ZnO base layer 12 represent possible routes for corroding agents, which may at least partly be responsible for degradation of the conductivity when the ZnO base is exposed to harsh environmental conditions, alongside the grain boundaries and other imperfections exposed at the film's surface. The cover layer 14 consists of a stack of several thinner ZnO sublayers 14a, 14b, 14c, a4d, 14e from which at least one has a different preferred crystallographic orientation than the base layer 12. In the example of FIG. 1, the preferred crystallographic orientations of the individual sublayers 14a, 14b, 14c, a4d, 14e are all different from that of the ZnO base layer 12, but that is not a requirement. The ZnO cover 14 provides a diffusion barrier to water and other corroding agents. Indeed, the chemically active substances have to travel through complex (tortuous) paths in order to reach and react with the underlying base layer 12.

[0051] FIG. 2 schematically shows a second example of a TCO film 10, wherein the ZnO base layer 12 is the same as in FIG. 1 but the ZnO cover 14 is provided by a monolayer having a preferred crystallographic orientation that is different from the one of the base layer 12. (This does not result in a different orientation of the columnar structures.)

[0052] FIG. 3 schematically shows a third example of a TCO film 10. The ZnO base layer 12 is again the same as in FIG. 1 but the ZnO cover 14 is provided by a monolayer having a crystallographically randomly oriented or amorphous structure.

[0053] The transparent conducting film structure illustrated in FIGS. 1 to 3 can be prepared in an RF magnetron sputtering process with an additional RF discharge to increase and maintain an elevated plasma density above the substrate holder onto which the sputtered vapours condensate. A schematic illustration of a deposition apparatus is provided in FIG. 4. FIG. 4 illustrates an interior of a vacuum vessel (or chambernot shown) that features a magnetron 16 with a highly pure ceramic ZnO target 18 and an RF-power biasable and rotatable sample holder 20 into which the substrate 22 can be mounted. The purity of the ZnO target preferably amounts to 99.99 at-% (atomic percent). The deposition apparatus further comprises a pumping system (including, e.g., a turbomolecular pump, not shown) capable of generating high vacuum (pressures below about 10.sup.3 Pa) and a controllable working gas inlet (not shown), a pressure control system, and two independent RF generators 24, 26 for power delivery to the sputtering target and the substrate holder, respectively. As an alternative to using two RF generators, one could use a single RF generator capable of delivering power independently on (at least) two separate channels. The working gas may be an inert gas (usually Ar of high-grade purity, e.g. 99.999 at-%). Instead of sputtering from a ZnO target, one could also use a Zn target but in this case the deposition has to take place in a reactive atmosphere providing oxygen atoms.

[0054] Prior to starting the deposition, the pressure within the vessel should be low enough to eliminate any unwanted impurities, e.g. lower than 10.sup.3 Pa. During the deposition of the film, Ar working gas is leaked into the vacuum vessel in a controllable fashion, e.g. at the rate of 25 sccm (standard cubic centimetres per minute). During the entire deposition process, pressure inside the vessel is kept at about 10.sup.1 Pa by balancing the inflow of Ar gas and the pumping speed of the pump.

[0055] During deposition, the substrate is rotated around the central axis of the substrate holder so as to improve the condensing layer's homogeneity. Deposition is carried out at ambient temperature (about 18 C. to 25 C.). The substrate is not heated otherwise than by the incoming plasma-originated radiation (which somewhat raises temperature on the substrate surface above the room temperature, e.g. by about 10-20 C.). The substrate and the substrate holder are electrically isolated from the remaining chamber, in order to permit their biasing by the second RF power generator 26.

[0056] During the deposition process, the ZnO target is bombarded by Ar ions generated within the dense plasma 28, magnetically confined close to the target, excited by the primary discharge. This discharge is powered by the first RF generator 24. In a test example, a power of 140 W was applied to a 7.5 cm diameter target during the entire deposition process, which corresponded to an average target power density of approximately 2.75 Wcm.sup.2. The ZnO film forms by condensation of the sputtered species onto the substrate 22 placed atop the sample holder 20 that is itself positioned facing the sputtering target at about 13 cm distance. ZnO films grown in these conditions are highly resistive (p>10.sup.3 0 cm) unless they are exposed to an additional RF power-driven secondary plasma discharge 30 maintained above the substrate 22. The optimal power density of the secondary discharge, P.sub.b, that should be delivered to the substrate in order to obtain ZnO films with resistivity values around or below 10.sup.3 0 cm is about P.sub.b=15.Math.10.sup.3 Wcm.sup.2. This capacitively-coupled RF discharge causes a self-induced negative DC voltage (bias), U.sub.b, at the substrate holder. In the test example, at P.sub.b=15.Math.10.sup.3 Wcm.sup.2, this bias was measured to be approximately U.sub.b=25 V.

[0057] During the first processing step (corresponding to the deposition of the ZnO base layer) the RF power delivery to both sputtered target (primary discharge) and to the substrate holder (secondary discharge) are kept constant during a period of time sufficient to obtain a ZnO layer of the desired thickness (e.g. in the range from 300 to 1500 nm). ZnO films obtained under the above biasing conditions (P.sub.b=15.Math.10.sup.3 Wcm.sup.2) possesses a highly ordered hexagonal structure (wurtzite) that typically exhibits a (001) texture (i.e., the c-axis of hexagonal crystals is oriented perpendicular to the substrate plane), independently on the type of substrate (crystalline or amorphous).

[0058] In the second processing step (employed for formation of the ZnO cover), the discharge close to the sputtering target is kept constant (using the same parameters as in the first processing step), but the driving power of the secondary discharge maintained above the sample holder is varied in a repetitive manner in between two extremal values. In the test example, the extremal values were P.sub.b=15.Math.10.sup.3 Wcm.sup.2 (resulting in U.sub.b=25 V) and P.sub.b=65.10.sup.3 Wcm.sup.2 (resulting in Ub=100 V). The particular selection of these two biasing power values is related to the preferred crystallographic orientation (texture) of the growing ZnO sublayers: at 15.Math.10.sup.3 Wcm.sup.2, a strong ZnO (001) texture can be achieved, while at P.sub.b=65.Math.10.sup.3 Wcm.sup.2 a ZnO (110) texture is obtained. At higher P.sub.b values (e.g., P.sub.b=12.10.sup.2 Wcm.sup.2) a ZnO (101) texture can also be obtained. The textures of the prepared layers can be verified, e.g., by -2 X-ray diffraction (XRD) analysis.

[0059] In the first fabrication step, in which the base layer is formed, the density of the plasma above the growing ZnO film is maintained at a level such that a highly conducting ZnO film with a preferred crystallographic orientation is obtained. In the second fabrication step, in which the ZnO cover is formed, the density of the plasma above the growing ZnO film is altered in such a way as to induce a distinct modification in the crystallographic order of the film under preparation, with respect to the crystallographic order of the base layer. The density of the (secondary) plasma can be modified abruptly or in a gradual manner, depending on the targeted ZnO cover microstructure. With abrupt (stepwise) modulation of the secondary RF power between substantial intervals of constant applied power, distinct ZnO sublayers with different textures can be obtained. By stepwise or abrupt modulation it is meant that P.sub.b is altered from one extremal value to another within a time less than 1 to 2 minutes. With gradual modulation of the secondary RF power, inner boundaries of the ZnO cover may become less distinguishable or disappear completely.

[0060] The frequency of gradual or stepwise P.sub.b modulations between two extremal values also has an impact on the microstructure: when the frequency of the modulation increases, the height (thickness) of ZnO sublayers deposited under the same conditions decreases. At higher frequencies of the modulation of the secondary RF power, the biasing conditions may average out, so that a single preferred crystallographic orientation develops. Nevertheless, it is also possible that a crystallographically randomly oriented or amorphous structure can be obtained.

[0061] The crystallographic state of the resulting film is thus affected by i) the selected extremal values of P.sub.b, ii) the rate of P.sub.b alteration, and iii) the time(s) during which the secondary discharge is kept steadyi.e., by the time(s) during which the growing ZnO films can develop their proper textures. The ZnO cover can thus consist of multiple thin ZnO films of altered preferred crystallographic orientations only if the power delivery to the secondary discharge remains substantially constant in-between the alterations for sufficiently long time(s), e.g., several minutes or more. It is of interest to keep the thickness of the individual sublayers of the ZnO cover relatively thin (e.g., thinner than 20 nm), in order to permit a larger amount of preferred crystallographic orientation alterations over the entire height of the cover. The interface in between the two adjacent sublayers is defined by the rate of P.sub.b alteration. A neat interface is obtained if the modification in power amplitude abrupt. A gradual or smeared-out interface is obtained if the transition in power amplitude is slow (e.g., if P.sub.b is modified on a timescale of several minutes).

[0062] FIG. 5 shows a power delivery scheme for both target and substrate discharges that can be used to fabricate a transparent conducting film with a (001)-oriented ZnO base layer topped by a multilayer ZnO cover. In FIG. 5, during the deposition of the ZnO cover, the power density of the secondary discharge is ramped from P.sub.b=15.10.sup.3 Wcm.sup.2 up to P.sub.b=65.10.sup.3 Wcm.sup.2 and then back to P.sub.b=1510.sup.3 Wcm.sup.2, with plateaux of 8 minutes in duration at each of these extremal values. The resulting sublayers of the cover are expected to have different textures, e.g., alternating (110) and (001) textures.

[0063] FIG. 6 shows another power delivery scheme for both target and substrate discharges that can be used to fabricate a transparent conducting film with a (001)-oriented ZnO base layer topped by a ZnO cover, wherein the secondary RF discharge operated above the substrate is varied periodically and in a continuous manner without plateaux at the selected extremal values. In other words, the ramp-up and ramp-down phases follow one another directly. In FIG. 6, during the deposition of the ZnO cover the power density of the secondary discharge is ramped from P.sub.b=15.10.sup.3 Wcm.sup.2 up to P.sub.b=65.Math.10.sup.3 Wcm.sup.2 and then immediately back to P.sub.b=15.10.sup.3 Wcm.sup.2. The resulting ZnO cover microstructure has a unique (110) texture with an abrupt interface with the underlying ZnO base layer.

Example

[0064] A prototype of a transparent conducting film was produced under the deposition conditions described hereinbefore, using the two RF power delivery scheme presented in FIG. 6.

[0065] During the fabrication of the respective film structure, the primary RF discharge at the target was operated in pure Ar at 1.3.10.sup.1 Pa working pressure, the power delivery being kept at 2.75 Wcm.sup.2. In the first processing step the secondary discharge excited above the substrates by means of the second RF power supply was also kept constant at P.sub.b=15.10.sup.3 Wcm.sup.2. The deposition was carried out on a soda lime glass substrate. The resulting ZnO base layer had a thickness of 770 nm and a resistivity of 1.4.Math.10.sup.3 cm. It is to be noted that this resistivity is identical to that of the AZO film of comparable thickness and prepared in the same conditions. However, a lower resistivity values can be reached in the optimized deposition conditions (e.g. a higher substrate temperature). The plasma wavelength deduced from absorbance analyses performed on ZnO films prepared in identical conditions is close to 2500 nm which assures good NIR transparency (in contrast to the AZO film of comparable thickness, which has its plasma wavelength at 2200 nm), as also depicted in FIG. 8. -2 XRD analyses also performed on the ZnO films prepared in identical conditions suggest a strong (001) texture, as indicated by very pronounced (002) and (103) diffractogram reflection peaks that are both fingerprints of (001) texture.

[0066] In the second processing step, the secondary RF discharge above the substrate holder was altered in a continuous manner in between the two extremal values at P.sub.b=15.10.sup.3 Wcm.sup.2 and P.sub.b=65.10.sup.3 Wcm.sup.2, with a 1-minute-long ramping intervals as shown in FIG. 6. The total number of cycles (ramping up-ramping down) was 14.

[0067] The deposited cover layer had a thickness of 75 nm. Its microstructure possesses a unique (110) crystallographic texture (sublayers were not discernible using scanning electron microscope (SEM) imaging). The microstructure was checked by -2 XRD analysis on another thicker sample of the cover layer applied directly on the substrate in a separate experiment using identical deposition conditions except of the larger amount of ramping up-ramping down cycles (70 cycles).

[0068] FIG. 7 shows a cross-sectional SEM (scanning electron microscopy) image of the transparent conducting film of the example. The resistivity of this transparent conducting film as a whole is approximately equal to that of the ZnO base layer on its own: 1.4.Math.10.sup.3 cm.

[0069] FIG. 8 shows a transmittance analysis obtained by UV-VIS-NIR spectrophotometry for the ZnO transparent conducting film depicted in FIG. 7, in comparison with a) a single layer ZnO film having (001) crystallographic texture (ZnO (001)), and b) a standard ZnO:Al (AZO) film (AZO (001)). All samples had about the same thickness as the ZnO+14ML film.

[0070] It can be observed that both single layer ZnO (001) film and multilayer ZnO+14ML film have substantially higher transmittance in the NIR spectral region than the AZO (001) film. This is their principal asset of commercial interest.

[0071] The environmental stability of another prototype ZnO structure was tested in the ambient air and also by the annealing in the environmental chamber that was filled with hot air at 105 C. The only difference between the tested prototype and that of FIG. 7 is that the thickness of the base layer is 275 nm rather than 770 nm. FIG. 9 depicts the resistivity rise monitored during several thousands of hours of heating for the ZnO transparent conducting film similar to the above example (ZnO+14ML), in comparison with a) a single layer ZnO film having (001) crystallographic texture (ZnO (001)), b) a single layer ZnO film having (110) crystallographic texture (ZnO (110)), and c) a standard ZnO:Al (AZO) film (AZO (001)). All samples had about the same thickness as the base layer of the ZnO+14ML film.

[0072] It can be observed that the transparent conducting film ZnO+14ML exhibits a much lower resistivity rise related to heat-induced degradation than its single layer ZnO counterparts. For instance, after the first 3000 hours the resistivity of the ZnO+14ML film increased 12 times while the resistivity of the ZnO (001) film and the ZnO (110) film increased 45 times and 28 times, respectively. This indicates a significantly improved environmental stability due to the presence of the ZnO cover. Nevertheless, the resistivity rise of the AZO film was lower (2.4 times).

[0073] It was furthermore verified over a period of six months that the transparent conducting film of the example has excellent stability in ambient air.

[0074] Last but not least, ageing of a transparent conducting film according to the invention in damp heat (DH) environment will prove its enhanced stability in harsh environment combining both elevated temperature and high humidity.

[0075] While specific embodiments have been described herein in detail, those skilled in the art will appreciate that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims and any and all equivalents thereof.