Composite wavelength converter

Abstract

The invention refers to a composite wavelength converter (1) for an LED (100), comprising a substrate (10) and an epitaxial film (20) formed by liquid phase epitaxy on the top and bottom of the substrate (10). Furthermore, the invention refers to a method of preparation of a composite wavelength converter (1) for an LED (100). Furthermore, the invention refers to a white LED light source comprising an LED (100) and an inventive composite wavelength converter (1) mounted on a light emitting surface of the LED (100).

Claims

1. A composite wavelength converter for an LED, comprising a substrate (10) formed by a first layer of single crystalline garnet phosphor presenting first emission and excitation spectra, having a cubic crystal structure, a first lattice parameter and oriented crystal planes, wherein the substrate (10) contains a first activator, and an epitaxial film (20) formed by liquid phase epitaxy on the top and bottom of the substrate (10) as a second layer of single crystalline garnet phosphor with the first activator, presenting second emission and excitation spectra, having a cubic crystal structure and a second lattice parameter, wherein the epitaxial film (20) is arranged directly on the top and bottom of the substrate (10) on the oriented crystalline planes of the substrate (10), wherein the second emission and excitation spectra are different from the first emission and excitation spectra, wherein the difference between the first lattice parameter and the second lattice parameter results in a lattice mismatch within a range of 2.0 to +2.0%, and wherein the thickness of the epitaxial film (20) on the top of the substrate (10) is at least 2 m and the thickness of the epitaxial film (20) on the bottom of the substrate (10) is at least 2 m.

2. The converter according to claim 1, wherein the converter (1) comprises one or two additional epitaxial films, namely a first (30) and optionally a second (40) additional epitaxial film, wherein the first additional epitaxial film (30) is formed by liquid phase epitaxy as a third layer of single crystalline garnet phosphor with the first activator, arranged directly on the epitaxial film (20) on the top and bottom side of the substrate (10), presenting third emission and excitation spectra, having a cubic crystal structure and a third lattice parameter, wherein the third emission and excitation spectra are different from the first emission and excitation spectra, wherein the difference between the third lattice parameter and the second lattice parameter results in a lattice mismatch within a range of 2.0 to +2.0%, and wherein the thickness of the first additional epitaxial film (30) on the top side of the substrate (10) is at least 2 m and the thickness of the first additional epitaxial film (30) on the bottom side of the substrate (10) is at least 2 m, wherein the second additional epitaxial film (40) is formed by liquid phase epitaxy as a fourth layer of single crystalline garnet phosphor with the first activator, arranged directly on the first additional epitaxial film (30) on the top and bottom side of the substrate (10), presenting fourth emission and excitation spectra, having a cubic crystal structure and a fourth lattice parameter, wherein the fourth emission and excitation spectra are different from the first emission and excitation spectra, and wherein the difference between the fourth lattice parameter and the third lattice parameter results in a lattice mismatch within a range of 2.0 to +2.0%, and wherein the thickness of the second additional epitaxial film (40) on the top side of the substrate (10) is at least 2 m and the thickness of the second additional epitaxial film (40) on the bottom side of the substrate (10) is at least 2 m.

3. The converter according to claim 1, wherein the first activator is Ce.sup.3+ ions.

4. The converter according to claim 1, wherein the second activator is one from Eu.sup.3+, Pr.sup.3+, Tb.sup.3+ and Mn.sup.2+ ions.

5. The converter according to claim 4, wherein the epitaxial film (20), first additional epitaxial film (30) and/or second additional epitaxial film (40) is/are formed by a mixed garnet compound having a composition represented by a formula X=A.sup.1.sub.3B.sup.1.sub.2C.sup.1.sub.3O.sub.12:Ce,R, wherein A.sup.1 is Ca.sup.2+ ions; B.sup.1 is Sc.sup.3+ or Al.sup.3+ or Ga.sup.3+ ions, C.sup.1 is Si.sup.4+ or Ge.sup.4+ ions and R is the second dopant, or represented by a formula Y=A.sup.1.sub.3-xA.sup.2.sub.xB.sup.1.sub.2-yB.sup.2.sub.yC.sup.1.sub.3-zC.sup.2.sub.zO.sub.12:Ce,R, wherein A.sup.1 is Ca.sup.2+ ions; A.sup.2 is Y.sup.3+, Lu.sup.3+, La.sup.3+, Tb.sup.3+ and Gd.sup.3+ ions; B.sup.1 is Mg.sup.2+ ions, B.sup.2 is Sc.sup.3+ or Al.sup.3+ or Ga.sup.3+ ions, C.sup.1 is Si.sup.4+ ions, C.sup.2 is Ge.sup.4+ ions and R is the second dopant, wherein 0<x<1.0, 0<y<2, and 0<z<3.

6. The converter according to claim 1, wherein the substrate (10) is Y.sub.3Al.sub.5O.sub.12:Ce garnet, Lu.sub.3Al.sub.5-sGa.sub.sO.sub.12:Ce garnet or Gd.sub.3Al.sub.5-sGa.sub.sO.sub.12:Ce garnet, wherein s is between 2.5 and 3.

7. The converter according to claim 1, wherein the thickness of the substrate (10) is between 0.3 and 1 mm.

8. The converter according to claim 1, wherein the thickness of the epitaxial film (20) on the top side of the substrate (10) is at most 200 m, and/or the thickness of the epitaxial film (20) on the on the bottom side of the substrate (10) is at most 200 m, wherein the thickness of the first additional epitaxial film (30) on the top side of the substrate (10) is at most 200 m, and/or the thickness of the epitaxial film (30) on the bottom side of the substrate (10) is at most 200 m, and/or wherein the thickness of the second additional epitaxial film (40) on the top side of the substrate (10) is at most 200 m, and/or the thickness of the second additional epitaxial film (40) on the bottom side of the substrate (10) is at most 200 m.

9. A white LED light source comprising an LED (100) and a composite wavelength converter (1) mounted on a light emitting surface of the LED (100), the converter (1) converting at least a portion of light having a wavelength lying within a first range emitted by the LED (100) into light having a wavelength lying in a second range, wherein the wavelength lying within the second range is higher than the wavelength lying within the first range the composite wavelength converter comprising a substrate (10) formed by a first layer of single crystalline garnet phosphor presenting first emission and excitation spectra, having a cubic crystal structure, a first lattice parameter and oriented crystal planes, wherein the substrate (10) contains a first activator, and an epitaxial film (20) formed by liquid phase epitaxy on the top and bottom of the substrate (10) as a second layer of single crystalline garnet phosphor with the first activator, presenting second emission and excitation spectra, having a cubic crystal structure and a second lattice parameter, wherein the epitaxial film (20) is arranged directly on the top and bottom of the substrate (10) on the oriented crystalline planes of the substrate (10), wherein the second emission and excitation spectra are different from the first emission and excitation spectra, wherein the difference between the first lattice parameter and the second lattice parameter results in a lattice mismatch within a range of 2.0 to +2.0%, and wherein the thickness of the epitaxial film (20) on the top of the substrate (10) is at least 2 m, and the thickness of the epitaxial film (20) on the bottom of the substrate (10) is at least 2 m.

10. The white LED light source according to claim 9, wherein the wavelength lying within the first range belongs to the blue wavelength range, and wherein the converter (1) is adapted to convert an appropriate portion of the light having the wavelength lying within the first range into light having the wavelength lying within the second range for creating a white light spectrum comprising the remaining portion of the light having the wavelength lying within the first range and the portion converted into light having the wavelength lying within the second range, or wherein the wavelength lying within the first range belongs to the UV wavelength range, wherein the wavelength lying within the second range belongs to the visible wavelength range, and wherein the converter (1) is adapted to convert essentially 100% of the light having the wavelength lying within the first range into light having the wavelength lying within the second range.

Description

(1) The invention is now explained in more detail with reference to embodiment examples. There are shown in

(2) FIG. 1 a schematic structure of a white LED light source,

(3) FIG. 2 examples of a schematic structure of the composite wavelength converter,

(4) FIG. 3A XRD pattern of an example of an CSSG:Ce epitaxial film grown onto a GAGG:Ce substrate,

(5) FIG. 3B XRD pattern of an example of a CYMSSG:Ce epitaxial film grown onto a YAG:Ce substrate forming an inventive composite wavelength converter,

(6) FIG. 4A excitation spectra of Ce.sup.3+ luminescence in three different substrates,

(7) FIG. 4B emission spectra of Ce.sup.3+ luminescence in the same three different substrates,

(8) FIG. 5 a comparison of excitation (1, 2) and emission (3, 4) photoluminescence spectra of CYMSSG:Ce epitaxial film (1, 3) with YAG:Ce epitaxial film (2, 4),

(9) FIG. 6 a comparison of excitation (1, 2) and emission (3, 4) photoluminescence spectra of a composite wavelength converter formed by a of CYMSSG:Ce epitaxial film together with a YAG:Ce substrate (1, 3) in comparison with a YAG:Ce substrate alone (2, 4),

(10) FIG. 7 a comparison of excitation (1, 2) and emission (3, 4) photoluminescence spectra of a composite wavelength converter formed by a Mn.sup.2+ co-doped CYMSSG:Ce epitaxial film together with a YAG:Ce substrate (1, 3) in comparison with a YAG:Ce substrate alone (2, 4),

(11) FIG. 8A chromaticity color coordinates of a YAG:Ce substrate alone, of a composite wavelength converter formed by a of CYMSSG:Ce epitaxial film grown on a YAG:Ce substrate (A), and of a composite wavelength converter formed by a Mn.sup.2+ co-doped CYMSSG:Ce epitaxial film grown on a YAG:Ce substrate (B),

(12) FIG. 8B color temperatures of a YAG:Ce substrate alone, of a composite wavelength converter formed by a of CYMSSG:Ce epitaxial film grown on a YAG:Ce substrate (A), and of a composite wavelength converter formed by a Mn.sup.2+ co-doped CYMSSG:Ce epitaxial film grown on a YAG:Ce substrate (B), and

(13) FIG. 9 a flow diagram illustrating the inventive method.

(14) The FIG. 1 shows a schematic structure of a white light emitting diode (white LED or WLED) light source. A composite wavelength converter 1 is mounted on a light emitting diode (LED) 100. The LED 100 comprises a carrier 101 formed by sapphire (Al.sub.2O.sub.3). The carrier 101 is adapted to mount the composite wavelength converter 1 on the further layers of the LED 100. The LED 100 further comprises a n-doped layer 102 of GaN, an active layer 103 of (AlGaIn)N, a p-doped layer 104 of GaN, an adhesive and mirror layer 105, and an electroplating metal layer 106. Furthermore, the LED is provided with an ohmic contact 107.

(15) The active layer 103 preferably emits blue light. In this case, the composite wavelength converter 1 is adapted to convert a portion (preferably about 60%) of the blue light into green, yellow, orange and red light such that, as a result of mixing the light emitted by the LED 100 and the light converted the composite wavelength converter 1, a spectrum of white light is emitted by the WLED.

(16) Alternatively, the active layer 103 emits UV-light. In this case, the composite wavelength converter 1 is adapted to convert essentially 100% of the UV-light into visible light such that, as a result, a spectrum of white light is emitted by the WLED.

(17) The FIG. 2 shows examples of a schematic structure of the composite wavelength converter 1. The inventive composite wavelength converters 1 are produced by liquid-phase epitaxy (LPE) by depositing one or more layers of single crystalline garnet phosphor onto a substrate 1 formed by a first layer of single crystalline garnet phosphor.

(18) The FIG. 2A shows the schematic structure of a composite wavelength converter 1 having three layers. The substrate 10 is situated in the center of the three layers. The substrate 10 is enclosed by an epitaxial film 20 (second layer of single crystalline garnet phosphor) at its top side and at its bottom side. Due to the preparation by LPE, the epitaxial film 20 is identical at the top side and at the bottom side of the substrate 10.

EXAMPLE 1

(19) TABLE-US-00001 Ca.sub.3Sc.sub.2Si.sub.3O.sub.12:Ce, R epitaxial film 20 Gd.sub.3Al.sub.2.5Ga.sub.2.5O.sub.12:Ce (GAGG:Ce) substrate 10 Ca.sub.3Sc.sub.2Si.sub.3O.sub.12:Ce, R epitaxial film 20 R=Eu.sup.3+, Pr.sup.3+, Tb.sup.3+ or Mn.sup.2+ lattice parameter of the substrate a=12.228

EXAMPLE 2

(20) TABLE-US-00002 Ca.sub.2YMgScSi.sub.3O.sub.12:Ce, R (CYMSSG:Ce, R) epitaxial film 20 Y.sub.3Al.sub.5O.sub.12:Ce (YAG:Ce) substrate 10 Ca.sub.2YMgScSi.sub.3O.sub.12:Ce, R (CYMSSG:Ce, R) epitaxial film 20 R=Eu.sup.3+, Pr.sup.3+, Tb.sup.3+ or Mn.sup.2+ lattice parameter of the substrate a=12.0005

EXAMPLE 3

(21) TABLE-US-00003 Ca.sub.2YMgScSi.sub.3O.sub.12:Ce, R (CYMSSG:Ce, R) epitaxial film 20 Lu.sub.3Al.sub.2Ga.sub.3O.sub.12:Ce (LuAG:Ce) substrate 10 Ca.sub.2YMgScSi.sub.3O.sub.12:Ce, R (CYMSSG:Ce, R) epitaxial film 20 R=Eu.sup.3+, Pr.sup.3+, Tb.sup.3+ or Mn.sup.2+ lattice parameter of the substrate a=12.088

(22) The FIG. 2B shows the schematic structure of a composite wavelength converter 1 having five layers. Again, the substrate 10 is situated in the center. The substrate 10 is enclosed by the epitaxial film 20 at its top side and at its bottom side. Onto the epitaxial film 20, a first additional epitaxial film 30 (third layer of single crystalline garnet phosphor) is deposited, both, at the top side and at the bottom side of the substrate 10. Due to the preparation by liquid-phase epitaxy (LPE), the first additional epitaxial film 30, as well, is identical at the top side and at the bottom side of the substrate 10.

EXAMPLE 4

(23) TABLE-US-00004 Ca.sub.2YMgScSi.sub.3O.sub.12:Ce first additional epitaxial film 30 Tb.sub.3Al.sub.5O.sub.12:Ce epitaxial film 20 Y.sub.3Al.sub.5O.sub.12:Ce (YAG:Ce) substrate 10 Tb.sub.3Al.sub.5O.sub.12:Ce epitaxial film 20 Ca.sub.2YMgScSi.sub.3O.sub.12:Ce first additional epitaxial film 30

(24) In this example, the epitaxial film 20 at the top side of the substrate 10 and at the bottom side of the substrate 10 can alternatively be Tb.sub.3Al.sub.5O.sub.12:Ce, R, wherein R=Eu.sup.3+, Pr.sup.3+ or Mn.sup.2+.

EXAMPLE 5

(25) TABLE-US-00005 Tb.sub.3Al.sub.5O.sub.12:Ce first additional epitaxial film 30 Ca.sub.2YMgScSi.sub.3O.sub.12:Ce (CYMSSG:Ce) epitaxial film 20 Y.sub.3Al.sub.5O.sub.12:Ce (YAG:Ce) substrate 10 Ca.sub.2YMgScSi.sub.3O.sub.12:Ce (CYMSSG:Ce) epitaxial film 20 Tb.sub.3Al.sub.5O.sub.12:Ce first additional epitaxial film 30

(26) In this example, the first additional epitaxial film 30 at both sides can alternatively be Tb.sub.3Al.sub.5O.sub.12:Ce, R, wherein R=Eu.sup.3+, Pr.sup.3+ or Mn.sup.2+.

EXAMPLE 6

(27) TABLE-US-00006 Tb.sub.2GdGa.sub.2Al.sub.3O.sub.12:Ce first additional epitaxial film 30 Ca.sub.2YMgScSi.sub.3O.sub.12:Ce (CYMSSG:Ce) epitaxial film 20 Y.sub.3Al.sub.5O.sub.12:Ce (YAG:Ce) substrate 10 Ca.sub.2YMgScSi.sub.3O.sub.12:Ce (CYMSSG:Ce) epitaxial film 20 Tb.sub.2GdGa.sub.2Al.sub.3O.sub.12:Ce first additional epitaxial film 30

(28) In this example, the first additional epitaxial film 30 at both sides can alternatively be Tb.sub.2GdGa.sub.2Al.sub.3O.sub.12:Ce, R, wherein R=Eu.sup.3+, Pr.sup.3+ or Mn.sup.2+.

(29) The FIG. 2C shows the schematic structure of a composite wavelength converter 1 having seven layers. The structure of substrate 10, epitaxial film 20 and first additional epitaxial film 30 corresponds to the structure shown in FIG. 2B. This structure is enclosed by a second additional epitaxial film 40 (fourth layer of single crystalline garnet phosphor) at its top side and at its bottom side. Due to the preparation by liquid-phase epitaxy (LPE), the second additional epitaxial film 40, as well, is identical at the top side and at the bottom side of the substrate 10.

(30) The WLED of FIG. 1 is shown comprising a composite wavelength converter 1 having three layers according to the FIG. 2A. The inventive WLED, of course, can comprise any inventive composite wavelength converter 1 according to any one of the FIG. 2A, 2B or 2C.

(31) The FIGS. 3A and 3B show XRD patterns of a first and second example of an epitaxial film grown onto a substrate forming an inventive composite wavelength converter. The XRD patterns show pronounced peaks, the so-called characteristic lines. Each characteristic line is denominated according to the so-called Siegbahn notation. Accordingly, K-alpha 1 (K1) indicates the characteristic line of an electron transition from the highest 2p orbital of the L-shell to the innermost K-shell. K-alpha 2 (K2) indicates the characteristic line of an electron transition from the second 2p orbital of the L-shell to the innermost K-shell.

(32) In the first example (FIG. 3A), a single crystalline film of Ca.sub.3Sc.sub.2Si.sub.3O.sub.12:Ce (CSSG:Ce) with thickness 10 m was grown as an epitaxial film by LPE from a PbO based flux onto a single crystalline Gd.sub.3Al.sub.2.5Ga.sub.2.5O.sub.12:Ce (GAGG:Ce) garnet substrate (Example 1 from FIG. 2A). The XRD pattern of the first example refers to the (1200) plane. The lattice parameter of the epitaxial film results as a=12.2586 , the lattice parameter of the substrate results as a=12.228 . Accordingly, the lattice mismatch a/a accounts to 0.25%.

(33) In the second example (FIG. 3B), a single crystalline film of Ca.sub.2YMgScSi.sub.3O.sub.12:Ce (CYMSSG:Ce) was grown as an epitaxial film by LPE from a PbO based flux onto a single crystalline Y.sub.3Al.sub.5O.sub.12:Ce (YAG:Ce) substrate (Example 2 from FIG. 2A). The XRD pattern of the second example refers to the (1042) plane. The lattice parameter of the epitaxial film results as a=12,1639 , the lattice parameter of the substrate results as a=12,0054 . Accordingly, the lattice mismatch a/a accounts to 1.32%.

(34) The FIGS. 4A and 4B show excitation (FIG. 4A) and emission (FIG. 4B) spectra of the Ce.sup.3+ luminescence in three different substrates, namely: (1): Y.sub.3A.sub.15O.sub.12:Ce substrate (YAG:Ce), (2): Lu.sub.3Al.sub.2Ga.sub.3O.sub.12:Ce substrate (LuAG:Ce) and (3): Gd.sub.3Al.sub.2.5Ga.sub.2.5O.sub.12:Ce substrate (GAGG:Ce).

(35) According to the excitation spectra (FIG. 4A), each of the YAG:Ce substrate (1) and the GAGG:Ce substrate (3) show a peak in the range of an excitation wavelength of about 440-465 nm which corresponds to the wavelength range of a blue light emitting LED. The LuAG:Ce substrate (2) shows a peak in the range of an excitation wavelength of about 340-360 nm which corresponds to the wavelength range of a UV-light emitting LED.

(36) According to the emission spectra (FIG. 4B), the YAG:Ce substrate (1) shows an emission peak of 535 nm (yellowish-green) with a broad shoulder to the yellow and orange wavelength range, the LuAG:Ce substrate (2) shows an emission peak of 483 nm (greenish-blue), and the GAGG:Ce substrate (3) shows a broad emission peak at 563 nm (yellow-green) which extends to the yellow and orange wavelength range.

(37) The FIG. 5 shows a comparison of photoluminescence (PL) excitation (1, 2) and emission (3, 4) spectra of a single crystalline epitaxial film of Ca.sub.2YMgScSi.sub.3O.sub.12:Ce (CYMSSG:Ce) and a single crystalline Y.sub.3A.sub.15O.sub.12:Ce (YAG:Ce) substrate. FIG. 5 combines the respective excitation spectra (1, 2) (identified also by Ce.sup.3+.sub.ex) and the respective emission spectra (3, 4) (identified also by Ce.sup.3+.sub.em) in one plot. The spectra have been normalized. The respective spectra are as follows: (1) excitation spectrum of CYMSSG:Ce, (2) excitation spectrum of YAG:Ce, (3) emission spectrum of CYMSSG:Ce, (4) emission spectrum of YAG:Ce.

(38) The excitation spectrum of CYMSSG:Ce (1) has been detected by a respective emission at 550 nm. The emission spectrum of CYMSSG:Ce (3) has been excited by a respective excitation at 445 nm. The PL measurements were performed at room temperature.

(39) It is observed that the excitation band (1) of CYMSSG:Ce is broader than the excitation band (2) of YAG:Ce. Furthermore, it is observed that the emission band (3) of CYMSSG:Ce is broader than the emission band (4) of YAG:Ce.

(40) The reason for the broader bands of CYMSSG:Ce is the formation of Ce.sup.3+ multicenters in CYMSSG:Ce. Accordingly, a Ceca-center is formed in the position of Ca.sup.2+, and a Ce.sub.Y-center is formed in the position of Y.sup.3+. The formation of these centers causes an elongation of the PL emission spectrum into the orange and red wavelength range.

(41) Furthermore, in the FIG. 5, the denominations E1, E2, E3 correspond to the absorption bands of cerium ions in the valence state 3+. The denomination Pb.sup.2+ corresponds to the absorption bands of lead ions in the valence state 2+.

(42) The FIG. 6 shows a comparison of photoluminescence (PL) excitation (1, 2) and emission (3,4) spectra of a composite wavelength converter formed by a single crystalline epitaxial film of Ca.sub.2YMgScSi.sub.3O.sub.12:Ce (CYMSSG:Ce) together with a single crystalline Y.sub.3A.sub.15O.sub.12:Ce (YAG:Ce) substrate and a single crystalline Y.sub.3A.sub.15O.sub.12:Ce (YAG:Ce) substrate alone. The FIG. 6, again, combines the respective excitation spectra and the respective emission spectra in one plot. The spectra have been normalized. The respective spectra are as follows: (1) excitation spectrum of the CYMSSG:Ce+YAG:Ce composite wavelength converter, (2) excitation spectrum of the YAG:Ce substrate alone, (3) emission spectrum of the CYMSSG:Ce+YAG:Ce composite wavelength converter, (4) emission spectrum of the YAG:Ce substrate alone.

(43) The excitation spectra (1, 2) are detected by a respective emission at 580 nm. The emission spectra (3, 4) are excited by a respective excitation at 450 nm. Accordingly, a blue LED is used for exciting (see Blue LED). The PL measurements were performed at room temperature.

(44) Again, the emission band (3) of the CYMSSG:Ce+YAG:Ce composite wavelength converter is broader than the emission band (4) of the YAG:Ce substrate due to the formation of Ce.sup.3+ multicenters in CYMSSG:Ce. Accordingly, a Ceca-center is formed in the position of Ca.sup.2+, and a Ce.sub.Y-center is formed in the position of Y.sup.3+. The formation of these centers causes an elongation of the PL emission spectrum into the orange and red wavelength range.

(45) The FIG. 7 shows a comparison of photoluminescence (PL) excitation (1, 2) and emission (3,4) spectra of a composite wavelength converter formed by a single crystalline epitaxial film of Ca.sub.2YMgScSi.sub.3O.sub.12:Ce, Mn (CYMSSG:Ce, Mn) together with a single crystalline Y.sub.3A.sub.15O.sub.12:Ce (YAG:Ce) substrate and a single crystalline Y.sub.3A.sub.15O.sub.12:Ce (YAG:Ce) substrate alone. The FIG. 7, again, combines the respective excitation spectra and the respective emission spectra in one plot. The spectra have been normalized. The respective spectra are as follows: (1) excitation spectrum of the CYMSSG:Ce, Mn+YAG:Ce composite wavelength converter, (2) excitation spectrum of the YAG:Ce substrate alone, (3) emission spectrum of the CYMSSG:Ce, Mn+YAG:Ce composite wavelength converter, (4) emission spectrum of the YAG:Ce substrate alone.

(46) The excitation spectra (1, 2) have been detected by a respective emission at 580 nm. The emission spectra (3, 4) have been excited by a respective excitation at 450 nm. Accordingly, a blue LED is used for exciting (see Blue LED). The PL measurements were performed at room temperature.

(47) The comparison of FIG. 7 differs from the comparison of FIG. 6 in that the epitaxial film of composite wavelength converter is co-doped with Mn.sup.2+ ions. Thus, in FIG. 7, the single crystalline epitaxial film of Ca.sub.2YMgScSi.sub.3O.sub.12:Ce, Mn (CYMSSG:Ce, Mn) provides a second activator.

(48) Again, the emission band (3) of the CYMSSG:Ce, Mn+YAG:Ce composite wavelength converter is broader than the emission band (4) of the YAG:Ce substrate due to the formation of Ce.sup.3+ multicenters in CYMSSG:Ce. Accordingly, a Ceca-center is formed in the position of Ca.sup.2+, and a Ce.sub.Y-center is formed in the position of Y.sup.3+. The formation of these centers causes an elongation of the emission spectrum into the orange and red wavelength range.

(49) Furthermore, an effective energy transfer from the Ce.sup.3+ ions to the Mn.sup.2+ ions causes a further elongation of the emission spectrum (3) of the CYMSSG:Ce, Mn+YAG:Ce composite wavelength converter into the orange and partly red wavelength range.

(50) Thus, advantageously, Ce.sup.3+ doped and Eu.sup.3+, Pr.sup.3+, Tb.sup.3+ and Mn.sup.2+ co-doped Ca.sub.2YMgScSi.sub.3O.sub.12 and Ca.sub.3Sc.sub.2Si.sub.3O.sub.12 SCFs are grown using the LPE method onto Ce.sup.3+ doped YAG:Ce or LuAG:Ce and GAGG:Ce substrates, respectively.

(51) Advantageously, the substrate emits mainly in the yellow range, and the Ce.sup.3+ doped and rare-earth or transition metal doped Ca.sub.2YMgScSi.sub.3O.sub.12 and Ca.sub.3Sc.sub.2Si.sub.3O.sub.12 epitaxial films emit in the orange-red ranges. Accordingly, a high-power white LED can be realized.

(52) By the mixing of the emissions coming from the substrate and the epitaxial film, the CRI coordinates and the color temperature resulting by the inventive composite wavelength converter and/or the CRI coordinates and the color temperature of the inventive WLED can be easily tuned.

(53) The FIGS. 8A and 8B show chromaticity color coordinates (FIG. 8A) and color temperatures (FIG. 8B) of the following converters: YAG:Ce: light converting substrate with thickness h=550 m; A: single crystalline epitaxial film of Ca.sub.2YMgScSi.sub.3O.sub.12:Ce (CYMSSG:Ce) with thickness h=15 m grown onto both side of YAG:Ce light converting substrate with thickness h=550 m and B: single crystalline epitaxial film of Ca.sub.2YMgScSi.sub.3O.sub.12:Ce,Mn (CYMSSG:Ce,Mn) with thickness of h=52 m grown onto both side of YAG:Ce light converting substrate with thickness h=550 m.

(54) A 464 nm LED emitting light in the blue wavelength range (see Diode in FIG. 8A) has been used for the excitation of the respective converters.

(55) It is observed from the FIG. 8 that in the succession YAG:Ce-A-B, an increasingly warmer white impression is realized.

(56) The FIG. 9 shows a flow diagram illustrating the inventive method of preparation of a composite wavelength converter for an LED.

(57) In a first step S1, a substrate 10 is provided. For example, the substrate is Y.sub.3Al.sub.5O.sub.12:Ce garnet (YAG:Ce), Lu.sub.3Al.sub.2Ga.sub.3O.sub.12:Ce garnet (LuAG:Ce) or Gd.sub.3Al.sub.2.5Ga.sub.2.5O.sub.12:Ce garnet (GAGG:Ce) with Ce.sup.3+ ions being the first activator. In particular, the Ce.sup.3+ ions are provided as the first dopant. The Ce.sup.3+ ions have a concentration of for example 0.1 mole %.

(58) In a step S2, an epitaxial film 20 is deposited onto both sides of the oriented planes of the substrate 10 using the liquid phase epitaxy method. Accordingly, the substrate 10 is embedded in a melt-solution containing flux oxides and garnet crystal forming components. The melt-solution, for example, contains PbO and B.sub.2O.sub.3 as flux oxides. The thickness of the epitaxial film 20 on each side of the substrate 10 is for example 100 m. The epitaxial film 20 is provided with Ce.sup.3+ ions as first activator (first dopant). The Ce.sup.3+ ions have a concentration of for example 0.1 mole %. Furthermore, the epitaxial film 20 is provided with a second activator selected from one of Eu.sup.3+, Pr.sup.3+, Tb.sup.3+ and Mn.sup.2+ ions. The second activator can be a second dopant or a substituent (garnet constituent). The second activator has a concentration for example 2% of Eu.sup.3+, Pr.sup.3+ or Tb.sup.3+ ions per mole, or 0.7% of Mn.sup.2+ ions per mole. The epitaxial film 20, for example, could have one of the following compositions: Tb.sub.3Al.sub.5O.sub.12:Ce,Eu, Tb.sub.3Al.sub.5O.sub.12:Ce,Pr or Tb.sub.3Al.sub.5O.sub.12:Ce,Mn; Ca.sub.2YMgScSi.sub.3O.sub.12:Ce,Mn, Ca.sub.2YMgScSi.sub.3O.sub.12:Ce,Eu or Ca.sub.3Sc.sub.2Si.sub.3O.sub.12:Ce,Pr.

(59) In an optional step S3, a first additional epitaxial film 30 is deposited directly onto the epitaxial film 20 on both sides of the substrate 10 using the liquid phase epitaxy method. The thickness of the first additional epitaxial film 30 on each side of the substrate is for example 100 m. Further optionally, a second additional epitaxial film 40 can be deposited directly onto the first additional epitaxial film 30 on both sides of the substrate 10 using the liquid phase epitaxy method. The thickness of the second additional epitaxial film 40 on each side of the substrate 10 is for example 100 m. Advantageously, the first additional epitaxial film 30 (and optionally the second additional epitaxial film 40) is provided with Ce.sup.3+ ions as first activator (first dopant). The Ce.sup.3+ ions have a concentration of for example 0.1 mole %. Furthermore, the first additional epitaxial film 30 is provided with a second activator selected from one of Eu.sup.3+ Pr.sup.3+, Tb.sup.3+ and Mn.sup.2+ ions. Optionally, the second additional epitaxial film 40 is provided with a second activator selected from one of Eu.sup.3+, Pr.sup.3+, Tb.sup.3+ and Mn.sup.2+ ions. The second activator can be a second dopant or a substituent (garnet constituent). The second activator has a concentration for example 2% of Eu.sup.3+ Pr.sup.3+ or Tb.sup.3+ ions per mole, or 0.7% of Mn.sup.2+ ions per mole. The first additional epitaxial film 30, for example, could have one of the following compositions: Tb.sub.3Al.sub.5O.sub.12:Ce,Eu, Tb.sub.3Al.sub.5O.sub.12:Ce,Pr or Tb.sub.3Al.sub.5O.sub.12:Ce,Mn; Ca.sub.2YMgScSi.sub.3O.sub.12:Ce,Mn, Ca.sub.2YMgScSi.sub.3O.sub.12:Ce,Eu or Ca.sub.3Sc.sub.2Si.sub.3O.sub.12:Ce,Pr. Optionally, the second additional epitaxial film 40 could have one of the following compositions: Tb.sub.3Al.sub.5O.sub.12:Ce,Eu, Tb.sub.3Al.sub.5O.sub.12:Ce,Pr or Tb.sub.3Al.sub.5O.sub.12:Ce,Mn, Ca.sub.2YMgScSi.sub.3O.sub.12:Ce,Mn, Ca.sub.2YMgScSi.sub.3O.sub.12:Ce,Eu or Ca.sub.3Sc.sub.2Si.sub.3O.sub.12:Ce,Pr.

(60) The step S2 (and optionally S3), i.e. the deposition of the epitaxial film 20, first additional epitaxial film 30 and/or second additional epitaxial film 40, occurs at growth temperatures of about 1000 C., at an overcooling rate of the melt-solution of about 20 C., and at a growth rate of about 0.3 m/min.

(61) The substrate 10 having the epitaxial film 20, first additional epitaxial film 30 and/or second additional epitaxial film 40 deposited thereon forms an epitaxial structure. The epitaxial structure is for example a wafer of a diameter of 3 inch. The epitaxy is performed in a crucible having a diameter of about 6 inch.

(62) In a step S4, the epitaxial structure is cut in a plurality of individual samples, each having identical optical properties. Preferably, the epitaxial structure is cut in 4000 individual samples being rectangular cuboids, each having a square base plane of 2 mm*2 mm. The cutting is for example performed by a laser device.

REFERENCE SIGNS

(63) 1 composite wavelength converter 10 substrate 20 epitaxial film 30 first additional epitaxial film 40 second additional epitaxial film 100 LED 101 carrier 102 n-doped layer of GaN 103 active layer of (AlGaIn)N 104 p-doped layer of GaN 105 adhesive and mirror layer 106 electroplating metal layer 107 ohmic contact