Abstract:
A white light source is described and has a UV-/blue-emitting semiconductor LED and an embedding compound provided with phosphor particles. The LED is provided with a plurality of light-emitting zones that are applied within a layer structure on a common substrate. An emission maxima of the light-emitting zones are energetically detuned relative to one another by different choice of the composition or of the layer thickness of the semiconductor material.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of copending International Application No. PCT/DE00/03520, filed Oct. 6, 2000, which designated the United States and was not published in English. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a light emitting diode (LED) white light source. In particular, the present invention relates to an LED white light source having a semiconductor LED based on GaN or InGaN, which is at least partly surrounded by an encapsulation made of a transparent material which contains a converter substance for at least partial wavelength conversion of the light emitted by the LED. The LED has a plurality of light-emitting zones by which a relatively broadband light emission spectrum is generated energetically above the emission spectrum of the converter substance. 
     A component of this type is disclosed for example in Published, German Patent Application DE 38 04 293 A1, which describes a configuration having an electroluminescent or laser diode in which the emission spectrum irradiated by the diode is shifted toward longer wavelengths by a plastic element treated with a phosphorescent, light-converting organic dye. The light radiated by the configuration consequently has a different color from the light emitted by the light-emitting diode. Depending on the nature of the dye added to the plastic, light-emitting diode configurations that emit light in different colors can be produced using one and the same type of light-emitting diode. 
     In many potential areas of application for light-emitting diodes, such as, for example, in display elements in motor vehicle dashboards, lighting in aircraft and automobiles, and in full-color LED displays, there is increasingly a demand for light-emitting diode configurations with which polychromatic light in particular white light, can be produced. International Patent Disclosure WO 98/12757 describes a wavelength-converting potting compound for an electroluminescent component having a body which emits ultraviolet, blue or green light, based on a transparent epoxy resin treated with a luminescent material, in particular with an inorganic luminescent material pigment powder with luminescent material pigments from the group of phosphors. As a preferred embodiment, a description is given of a white light source in which a radiation-emitting semiconductor LED based on GaN, GaInN, GaAlN or GaInAlN is described with an emission maximum of between 420 nm and 460 nm and a luminescent material which is chosen such that a blue radiation emitted by the semiconductor body is converted into complimentary wavelength ranges, in particular blue and yellow, or to form additive color triads, e.g. blue, green and red. In this case, the yellow or the green and red light is generated by the luminescent materials. The hue (color locus in the CIE chromaticity diagram) of the white light generated in this way can be varied in this case by suitable choice of the luminescent material or materials with regard to mixture and concentration. 
     Likewise, International Patent Disclosure WO 98/54929 discloses a visible-light-emitting semiconductor component having a UV/blue LED which is disposed in a depression of a carrier body, whose surface has a light-reflection layer and is filled with a transparent material which surrounds the LED at its light exit sides. In order to improve the coupling-out of light, the transparent material has a refractive index that is lower than the refractive index of the light-active region of the LED. 
     U.S. Pat. No. 5,851,905 and Japanese Patent JP 0100022525 in each case describe an LED chip with stacked quantum wells that have such different emission wavelengths that the chip emits white light. 
     The previous known white light sources of the type described have the disadvantage, however, that the spectral light emission curve of the white light sources is still not optimal, so that the physiological-optical impression of a white light source is in many cases not provided to a sufficient extent. This is due not to the luminescent materials used, for instance, but rather to the fact that, during the wavelength conversion, the energy gap between absorbed photon and emitted photon cannot be arbitrarily reduced. For this reason, a spectral hole is produced in the emission curve. 
     This problem could be solved by disposing an additional LED component with an emission maximum in the blue wavelength range. However, the solution is unsatisfactory since it is associated with a considerable additional outlay on material and manufacturing time, since not only must a further semiconductor component be fabricated but it must be specifically contact-connected and wired in the white light source to be manufactured. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide an LED white light source with broadband excitation which overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which the light spectrum emitted by it is improved in the sense of an improved optical-physiological white light impression. In particular, it is an object of the present invention to specify an improved white light source in which the emitted light spectrum has an intensity profile that is as uniform as possible. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a white light source. The white light source contains a semiconductor light emitting diode having a light exit side and a main radiating direction, and the semiconductor LED emits a light. An encapsulation at least partly surrounds the semiconductor LED and is made of a transparent material containing a converter substance for at least partial wavelength conversion of the light emitted by the semiconductor LED. The semiconductor LED has at least two light-emitting zones embodied such that an maxima of their emission spectra are energetically detuned relative to one another and lie above an emission spectrum of the converter substance. The light-emitting zones are disposed one behind another in the main radiating direction of the semiconductor LED such that an energy of an emission maximum increases in a direction of the light exit side of the semiconductor LED. 
     Accordingly, the present invention describes a white light source having the semiconductor LED and the encapsulation, which at least partly surrounds the LED and is made of a transparent material containing the converter substance for at least partial wavelength conversion of the light emitted by the LED. In which case the LED has at least two light-emitting zones which are formed in such a way that the maxima of their emission spectra are energetically detuned relative to one another and lie above the emission spectrum of the converter substance, and which are furthermore disposed one behind the other in a main radiating direction of the LED in such a way that the photon energy of the emission maximum increases in the direction of the light exit side of the LED. 
     This order of the light-emitting zones prevents the longer-wave photons from being absorbed again in the zones with short-wave emission. Consequently, the spectral hole that is present in the case of white light sources according to the prior art is filled by virtue of the invention. This can be brought about by a single additional light-emitting zone or else by a larger number of additional light-emitting zones that energetically adjoin the first emission maximum of the converter substance above the maximum. The light-emitting zones are disposed on a common substrate and between two outer electrical contact layers, so that they can be connected to a common voltage source. 
     In a first embodiment, the LED has exactly one pn junction and the light-emitting zones are formed by a corresponding number of single or multiple quantum well layers of different thickness and/or of different material composition. In this embodiment, the energetic displacement between the emission maxima results from the displacement of the bottom most conduction band and the top most valance band in the case of variation of the thickness and/or of the material composition in the quantum well layers. In the simplest conceivable exemplary embodiment, two light-emitting zones are formed by two single quantum well layers made of InGaN of different thickness and/or different Indium concentration in each case being embedded and disposed one behind the other between two layers with a larger band gap. 
     In a second embodiment, the light-emitting zones of the LED are formed by a corresponding number of pn junctions. In this case, the pn junctions may be formed from a bulk material of different material composition, i.e. for example different proportion of indium in the material combination InGaN. However, for their part the pn junctions may also in each case contain a single or multiple quantum well layer and the quantum well layers of the different pn junctions may in this case have different thicknesses and/or material compositions. Respectively adjacent pn junctions can be electrically connected to one another in a particularly simple manner by a metallic compact layer, such as a solder layer. However, the adjacent pn junctions can also be monolithically integrated by the junctions being isolated from one another by extremely low-resistance np tunnel junctions which contain an n + -doped layer and a directly adjoining p + -doped layer. The n + -doped layer adjoins the n-type region of one pn junction and the p + -doped layer adjoins the p-type region of the other pn junction, and the n + -type and p + -type doping concentration, respectively, being chosen in such a way as to produce a relatively low electrical resistance of the tunnel junction during operation. This type of connection of two pn junctions is known per se in the prior art (e.g. van der Ziel, et al., “Appl. Phys. Lett.” 41, p. 500, 1982) and will not, therefore, be discussed any further at this point. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in an LED white light source with broadband excitation, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic, cross-sectional view of a white light source in accordance with a first embodiment with a semiconductor layer construction shown enlarged according to the invention; 
     FIG. 2 is a cross-sectional view of the white light source according to a second embodiment with the semiconductor layer construction shown enlarged; 
     FIG. 3 is a graph showing an emission spectrum of a conventional, commercially available white light source; and 
     FIG. 4 is a graph of an emission spectrum of the white light source according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawing in detail and first, particularly, to FIG. 3 thereof, there is shown a spectral emission curve of a white light source that is commercially available from Hewlett-Packard and bears the product designation HLMP-CW15/16, which uses an InGaN LED and a potting compound with red and green phosphor particles. In the emission curve, an emission maximum of the LED is designated by A, while an emission maxima of the luminescent material are designated by B 1  and B 2 . Such an emission spectrum regularly arises by virtue of the fact that only a proportion of the light radiation emitted by the LED is ever absorbed in the conversion material and converted into light of a longer wavelength. The physically governed energy gap between A and B1 gives rise to a spectral hole, which significantly reduces the blue component of the spectrum. 
     The problem can be solved by disposing an additional LED component with an emission maximum in the blue wavelength range. However, the solution is unsatisfactory since it is associated with a considerable additional outlay on material and manufacturing time, since not only must a further semiconductor component be fabricated but it must be specifically contact-connected and wired in the white light source to be manufactured. 
     In FIG. 1, there is shown a white light source according to the invention. A UV-blue-emitting semiconductor LED  1  is fixed by its rear side contact on a first electrical connection  2  by an electrically conductive bonding method, e.g. a metallic solder or a conductive adhesive such as a conductive silver. A front side contact is connected to a second electrical connection  3  by a bonding wire  9 . 
     The free surfaces of the semiconductor LED  1  and partial regions of the electrical connections  2  and  3  are directly enclosed by a cured, wavelength-converting potting compound  5 . The compound  5  may contain for example 80-90% by weight of epoxy resin and ≦15% by weight of luminescent material particles  6  containing YAG:Ce, further constituents such as adhesion promoters, processing aids, hydrophobizing agents, mineral diffusers and also thixotropic agents being contained for the rest. 
     The right-hand part of the drawing in FIG. 1 shows the semiconductor layer construction of the LED  1  enlarged and in detail. On an n-doped GaN substrate  10 , by a growth method such as metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE), semiconductor layers made of InGaN with a changing proportion of indium are grown with the aim of fabricating two single quantum well layers. A band gap of the material In x Ga 1-x N decreases as the Indium proportion X increases. 
     First, a nominally undoped InGaN barrier layer  11  with a relatively small indium proportion x is grown. An InGaN quantum well layer  12  with a relatively large indium proportion x and a thickness d 1  is applied thereon. The quantum well layer  12  is followed by a further InGaN barrier layer  13 . Consequently, the quantum well layer  12  forms a first light-emitting zone whose emission maximum is determined both by its thickness and by its indium proportion and also the indium proportion of the barrier layers. A further InGaN quantum well layer  14  with a relatively low Indium proportion x and a thickness d 2 &lt;d 1  is then applied to the barrier layer  13 . The quantum well layer has grown on it once again an InGaN barrier layer  15  with a relatively large indium proportion x, whereon a p-doped GaN contact layer  16  terminates the semiconductor layer sequence. 
     Consequently, a second light-emitting zone is formed by the InGaN quantum well layer  14 . The quantum well layers  12  and  14  may have the same indium proportion. In this case, as a result of the larger gap between the bottom most conduction band and the topmost valence band, the upper quantum well layer  14  has the energetically higher emission maximum than the quantum well layer  12 . For the fine tuning of the energetic displacement between the quantum well layers  12  and  14 , however, it is also possible for the indium proportion additionally to be varied. Thus, by way of example, at the other extreme, it is also for the thickness of the two quantum well layers  12  and  14  to be identical, but for the energetic detuning to be brought out solely by the different indium concentration. The layers  11  to  15  forming the light-active section of the layer structure are nominally undoped. 
     By virtue of the fact that the quantum well layer  12  with the smaller photon energy of the band gap is disposed at the bottom, the light emitted by it passes through the overlying layers—having a higher band gap—virtually without any losses into the conversion compound  5  surrounding the LED  1 . 
     FIG. 4 illustrates, by way of example, an emission spectrum of the kind that can be obtained by a white light source in accordance with FIG.  1 . In the emission spectrum, the light radiation emitted by the quantum well layer  12  appears as a further line A 2 . This portion of the emission spectrum is formed by radiation of the quantum well layer  12  that has passed through the conversion material  5  without being converted in the phosphor particles  6 . The line A 2  thus closes the spectral hole in the emission spectrum, thereby bringing about a more uniform intensity distribution of the emission spectrum. 
     In this way, it is also possible to dispose more than two quantum well layers one above the other, in which case care should always be taken to ensure that the light radiation of a lower quantum well layer is not absorbed by the overlying semiconductor material. The band gap of the quantum well layers must thus continuously increase in the growth direction of the semiconductor layer structure, which results in that the layer thickness must decrease and/or the indium proportion must decrease. 
     It is also possible for the single quantum well layers  12  and  14  in FIG. 1 to be replaced in each case by multiple quantum well layers within which the layer thickness and the indium proportion remains constant. Accordingly, it is then also possible to dispose more than two multiple quantum well layers. 
     An example of a second embodiment of the white light source according to the invention is illustrated in cross section in FIG.  2 . In this embodiment, by way of example, two pn junctions  21  and  26  are stacked vertically one above the other and electrically contact-connected to one another by an n + p +  tunnel junction  25 . The tunnel junction  25  contains two highly doped n + -type and p + -type layers (10 20  cm −3 ), of which the n + -type layer adjoins the n-type region of one adjacent pn junction and the p + -type layer adjoins the p-type region of the other adjacent pn junction. Each pn junction has an active, light-emitting and intrinsic layer  23  and  28 , respectively. 
     In detail, on an n-doped GaN substrate  20 , there are grown an n-doped InGaN layer  22 , a p-doped InGaN layer  24 , the n + p +  tunnel junction  25 , an n-doped InGaN layer  27  and finally a p-doped InGaN layer  29 . Situated between the layers  22  and  24 , and  27  and  29 , are the light-active zones  23  and  28 , respectively, which may be formed either by the space charge zones between the n-type and p-type layers when bulk pn junctions are used, or by specially applied single or multiple quantum well layers. If the pn junctions  21  and  26  are formed from bulk material, the energetic detuning between the light-active zones  23  and  28  must be set by way of the indium proportion. Therefore, the light-active zone  28  has a lower indium proportion. In the case where bulk material is used, the respectively adjoining layers  22 ,  24  and  27 ,  29  may also have the same indium proportion as the light-active zones  23  and  28 , respectively. In the case where single or multiple quantum well layers are used, reference is made to the explanations with regard to the first embodiment. 
     Care should be taken to ensure that the n + p +  tunnel junction  25  is chosen from a material with a sufficiently high band gap, e.g. GaN, so that absorption of the light radiation of the light-active zone  23  does not take place. 
     If desired, it is also possible for more than two pn junctions to be stacked one above the other and be electrically contact-connected to one another in each case by n + p +  tunnel junctions. 
     The advantage of using highly doped tunnel junctions is that the entire semiconductor LED  1  can thus be monolithically fabricated in accordance with the second embodiment and can thus be fabricated in one growth pass. As an alternative to this, however, it may also be provided that the pn junctions are soldered areally to one another or electrically contact-connected to one another in another way by a metallic contact layer  25 . 
     An emission spectrum in accordance with FIG. 4 can also be brought about with an embodiment in accordance with FIG.  2 . 
     The invention has been described in accordance with FIGS. 1 and 2 on the basis of a surface mounted design (SMD) construction, but it can equally be realized in a so-called radial diode.