COATING FOR OPTICAL AND ELECTRONIC APPLICATIONS

Abstract

Single- or multilayered coating, such as a selective solar absorber coating or a coating being part of an integrated electronic circuit, comprising one or more layers containing germanium (Ge) doped VO.sub.2+x, where 0.1x0.1.

Claims

1. Single- or multilayered coating, such as a selective solar absorber coating or a coating being part of an integrated electronic circuit, comprising at least one layer containing VO.sub.2+x, with 0.1x0.1, doped with one or several elements and wherein one of those elements is germanium (Ge).

2. Coating according to claim 1 for use as a solar absorber wherein the temperature of the thermochromic transition in the said layer is above 75 C.

3. Coating according to claim 1 where the total cumulated layer thickness of the Ge doped VO.sub.2+x (0.1x0.1) containing layer is in the range from 70 nm to 330 nm.

4. Coating according to claim 1 with an atomic concentration of germanium in the VO.sub.2+x (0.1x0.1) containing layer in the range from 0.01 at. % and 7 at. %.

5. Coating according to claim 1 where a highly infrared reflective substrate such as Al, Cu, stainless steel is used.

6. Coating according to claim 1 comprising a diffusion that contains AlO.sub.x, SiO.sub.x, metal nitrides or ternary compounds such as TiSi.sub.xN.sub.y, CrSi.sub.xN.sub.y etc. . . . and wherein the thickness of said barrier is between 20 and 90 nm.

7. Coating according to claim 1 where one or more layers of solar absorbing layers are used, such as e.g. TiAl.sub.xO.sub.yN.sub.z, TiSi.sub.xO.sub.yN.sub.z, CrAl.sub.xO.sub.yN.sub.z, CrSi.sub.xO.sub.yN.sub.z, a-C:H/Me, a-Si:C:H/Me, TiAl.sub.xN.sub.y, NbTiXO.sub.yN.sub.z, SiO.sub.xN.sub.y, where x, y, z0.

8. Coating according to claim 1 where a top coating is used as anti-reflection layer with a thickness between 20 and 150 nm and wherein the real part of the refractive index of this top coating is in the range from 1.4 to 1.8 at a wavelength of 550 nm.

9. Coating according to claim 8 where the top coating contains SiO.sub.x or AlO.sub.x.

10. Coating according to claim 1 where the layer containing Ge doped VO.sub.2+x (0.1x0.1) is deposited by reactive magnetron sputtering using a pure target, a composite target, an alloy target or several targets containing vanadium or germanium.

11. Coating according to claim 10 where the power density on the sputtering target is between 2 W/cm.sup.2 and 50 W/cm.sup.2 for a target containing vanadium and/or germanium.

12. Coating according to claim 10 where the power density on the sputtering target is between 0.05 W/cm.sup.2 and 10 W/cm.sup.2 for the germanium containing target used in cosputtering.

13. Coating according to claim 1 where the layer containing Ge doped VO.sub.2+x (0.1x0.1) is deposited with a substrate temperature in the range from 400 C. to 650 C.

14. Coating according to claim 1 where the layer containing Ge doped VO.sub.2+x (0.1x0.1) is deposited with stationary substrate, rotating substrate at a speed in the range from 1 to 50 rotations/min, or translational displacement with a speed in the range of 0.05 m/min and 4 m/min.

15. Coating according to claim 1 where the layer containing Ge doped VO.sub.2+, (0.1x0.1) is deposited at a total pressure in the range from 5.Math.10.sup.4 mbar to 5.Math.10.sup.2 mbar.

16. Coating according to claim 1 where the layer containing Ge doped VO.sub.2+x (0.1x0.1) is deposited at an oxygen partial pressure in the range from 5.Math.10.sup.5 mbar to 5.Math.10.sup.3 mbar.

17. Coating according to claim 1 where the target substrate distance is in the range from 2 cm to 15 cm.

18. Thermal solar collector containing a coating according to claim 1, said coating being used as selective solar absorber.

19. Thermal solar collector according to claim 18 comprising a thermochromic or thermotropic glazing.

20. Thermal solar energy system containing a solar collector according to claim 18.

21. Electronic circuit containing a coating according to claim 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0081] The invention will be better understood in the present chapter, in association with the following figures (some of them being already presented in the previous chapters):

[0082] FIG. 1: The characterization of radiance usually is made in solid angles. Direction is measured by the zenith and azimuthal angles and . =cos . This is an example of a hemisphere.sup.[1]

[0083] FIG. 2: Solar spectrum at AM 1.5 G, the normalized emission spectrum of a blackbody at 100 C. and the reflectance curve of an ideal solar absorber.sup.[1]

[0084] FIG. 3: A complete drainback thermal solar system. Credits: Home Power Inc.

[0085] FIG. 4: Schematic of a possible layer stack for a switchable selective absorber coating

[0086] FIG. 5: Simulation of the thermal emittance switch with increasing thickness for pure VO.sub.2.sup.[16]

[0087] FIG. 6: Spectral absorptance of a design containing: 250 nm VO.sub.2 and 80 nm SiO.sub.2 on Al below T.sub.C

[0088] FIG. 7: Spectral absorptance of a multilayer design containing: 250 nm VO.sub.2/180 nm a-C:H/Ti 37%/100 nm a-C:H/Ti 11%/70 nm a-C:H/Ti 0.12%/80 nm SiO.sub.2 on Al

[0089] FIG. 8: Resistivity measurements of Ge doped samples. Effect of doping on the transition temperature

[0090] FIG. 9: Experimental data (points) and simulated RBS spectra (solid line) of such a Ge doped VO.sub.2+x (0.1x0.1) based film on Si (100) substrate. The result of the simulation agrees well with the experimental RBS spectrum and the Ge concentration was determined to be 5.9 at. %.

[0091] FIG. 10: XRD spectra of a Ge doped VO.sub.2+x (0.1x0.1) based film on Si (100) substrate. All diffraction lines were assigned to the stoichiometric VO.sub.2 monoclinic phase according to [Rakotoniaina, J. C. et al., J. Solid State Chem. 103, 81-94 (1993)].

[0092] The single- or multilayerd material according to the invention may be associated with the following features, the list being not exhaustive. [0093] a highly infrared reflective substrate such as Al, Cu, stainless steel or any other mechanically stable substrate covered with a highly reflective thin film. [0094] a diffusion barrier coating which would prevent the diffusion of elements from the substrate into the thermochromic film and could improve the adherence of said film onto the substrate. The diffusion barrier could be an AlO.sub.x, SiO.sub.x, metal nitrides or ternary compounds such as TiSi.sub.xN.sub.y, CrSi.sub.xN.sub.y etc. The thickness of the diffusion barrier is preferably between 20 and 90 nm. [0095] a thermochromic layer containing Ge doped VO.sub.2+x (0.1x0.1). [0096] a selective absorber coating such as TiAl.sub.xO.sub.yN.sub.z, TiSi.sub.xO.sub.yN.sub.z, CrAl.sub.xO.sub.yN.sub.z, CrSi.sub.xO.sub.yN.sub.z, a-C:H/Me, a-Si:C:H/Me, TiAl.sub.xN.sub.y, NbTi.sub.xO.sub.yN.sub.z/SiO.sub.xN.sub.y etc., where x, y, z0. [0097] a top coating serving as both an anti-reflection layer and as oxidation barrier with a preferred thickness between 20 and 150 nm. e.g.: SiO.sub.x, AlO.sub.x etc.

[0098] One variant of stacking the layers is shown in FIG. 4. Other multilayered structures, containing all or several of the above mentioned layers, are possible and can be imagined.

[0099] The key element is the Ge doped VO.sub.2+x (0.1x0.1) containing layer which may be obtained by a very strictly controlled reactive magnetron co-sputtering process. The substrate temperature during the deposition is a critical parameter in order to obtain highly crystalline thin films. Amorphous films do not exhibit optical switching, therefore a high enough temperature is required. It was determined that, depending on the substrate holder used in the process, a temperature between 400 C. and 650 C. is necessary to obtain crystalline and, therefore, switching doped VO.sub.2+x (0.1x0.1) films.

[0100] Furthermore, it is preferred that the doping is kept between certain limits as a strong doping leads to the loss of the switching character of the film. The preferred range of the Ge atomic concentration is between 0.01 at % and 7 at %. Ge increases the insulating character of the films and at higher concentrations of Ge than the one set as the upper limit of doping, the switching of the doped film from semiconducting to metallic state is lost.

[0101] The deposted thermochromic layer can then contain a mixture of one or more dopant elements, one of which is Ge, one or more metal oxides coming from said doping elements (e.g. GeO.sub.x) and at least VO.sub.2+x (0.1x0.1), however not exclusively, as small or large amounts of other vanadium oxides can be present.

[0102] Computer simulation has been carried out and the thermal emittance was calculated using Planck's law. The thickness of a VO.sub.2 based film is critical with regard to the thermal emittance switch. FIG. 5 clearly shows that the VO.sub.2 based film becomes more and more emissive in the semiconducting state by increasing the film thickness. For thin VO.sub.2 based films, at low temperature the thermal emittance is prevalently due to the substrate and at T>T.sub.t to the VO.sub.2 based film. The thickness of the thermochromic layer is suggested to preferably be between 70 and 330 nm.

[0103] The thickness of the thermochromic layer is critical regarding the selective coating efficiency. A 5 to 25% thermal emittance switch of the thermochromic selective coating has been simulated in order to get the efficiency of the whole system. A solar absorptance of about 85% is obtained by using an antireflective SiO.sub.2 layer on VO.sub.2 (see FIG. 6). This coating already behaves as an efficient selective surface.

[0104] FIG. 7. shows that a solar absorptance efficiency up to 97.3% is obtained for the full solar spectrum using a selective stack of five layers. The solar absorptance below T.sub.C is not affected by the thermochromic layer of optimum thickness (needed for emittance switch).

[0105] The results are therefore promising for VO.sub.2 based thermochromic layers. However, in a solar thermal system high temperatures occur and the switching of pure VO.sub.2 at 68 C. is not sufficient. A solar thermal system with a thermochromic layer switching at higher than 68 C. critical temperatures leads to higher quantities of absorbed energy, therefore, higher efficiencies. For solar thermal collectors a suitable switching temperature of the thermochromic layer is in the range of 80 C. and 100 C.

[0106] The base pressure in the deposition chamber is in the range of 3.Math.10.sup.8 mbar. The temperature is between 400 C. and 650 C., depending on the sample holder. Ar is used as process gas. The O.sub.2 partial pressure is precisely controlled with the help of a PID feedback control which keeps the O.sub.2 partial pressure constant in the chamber by regulating the oxygen valve. During co-sputtering, the presence of a second plasma coming from the doping element introduces perturbations in the deposition chamber. The oxygen flow has to be adjusted in function of target depletion. An eroded target is sputtered more efficiently as the magnets are closer to its surface and the magnetic field is more intense. Therefore, the oxygen content has to be adjusted in function of how used the target is. The optimal deposition parameters for doped VO.sub.2+x (0.1x0.1) based thermochromic films were inferred. The process parameters were kept in the following ranges:

[0107] Target substrate distance: 2-15 cm,

[0108] Rotation speed of the substrate: 1-50 rot/min,

[0109] Speed of substrate displacement: 0.05-4 m/min,

[0110] Process pressure: 5.Math.10.sup.4 mbar to 5.Math.10.sup.2 mbar,

[0111] Oxygen partial pressure: 5.Math.10.sup.5 mbar to 5.Math.10.sup.3 mbar.

[0112] The deposition can be done using pure or composite or alloy targets containing germanium and/or vanadium during the co-sputtering process.

[0113] The RBS (Rutherford Backscattering Spectrometry) and X-ray diffraction spectra of a such deposited Ge doped VO.sub.2+x (0.1x0.1) based thin film is shown in FIGS. 9 and 10 respectively.

[0114] As already mentioned the single- or multilayered material according to the invention is primarily intended for solar thermal applications. It may however be used in other applications such as solid state storage applications, reconfigurable microelectronics, steep-slope devices, RF switches, capacitors with variable capacitance, PV technology or chip technology. For these applications, high-temperature switching VO.sub.2 films are required and highly seeked.

BIBLIOGRAPHY

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