White light emitting element and white light source

A white light source has an excitation light source and a white light emitting element provided at a position which allows the transmission of light from the excitation light source to generate white light through irradiation with the light from the excitation light source. The white light emitting element has a sapphire substrate made of sapphire or the like which transmits visible light, an InGaAlN semiconductor layer formed on a surface of the sapphire substrate to emit red light through irradiation with visible light, and a fluorescent layer formed on the surface opposite to the surface provided with the semiconductor layer to emit yellow light or green light through irradiation with visible light.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 on patent application Ser. No. 2004-43561 filed in Japan on Feb. 19, 2004, the entire contents of which are hereby incorporated by reference. The entire contents of patent application Ser. No. 2005-040853 filed in Japan on Feb. 17, 2005 are also incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a white light emitting element and a white light source comprising the white light emitting element which are applicable to, e.g., a white light emitting diode.

In a GaN-based III-V compound semiconductor (hereinafter referred to as “GaN-based semiconductor”) represented by AlxGayIn(1-x-y)N (0≦x≦1, 0≦y≦1, 0≦1-x-y<1), the forbidden band width of GaN at a room temperature is 3.4 eV, which indicates that the GaN-based semiconductor is a material capable of implementing a high-output light emitting element which emits blue or green light. The GaN-based semiconductor has already been commercialized as a blue/green light emitting diode in various displays including a large-scale display and a traffic signal. Further, the use of the GaN-based semiconductor for the light emitting element of a white light source has also been examined.

If such a white light source can be realized, semiconductor illumination as a replacement for currently prevailing fluorescent and incandescent lamps can be implemented. Accordingly, vigorous research and development has been promoted with the aim of enhancing brightness and improving a light emitting efficiency. For example, a white light source in which a blue light emitting diode (hereinafter referred to as “GaN-based blue light emitting diode”) using a nitride semiconductor as a light emitting element is used for an excitation light source and a YAG fluorescent material is used for a white light emitting element is disclosed in, e.g., S. Nakamura et al., “The Blue Laser Diode” Springer-Verlag Berlin Heidelberg New York. In the white light source, the YAG fluorescent material is irradiated with blue light (light with a peak wavelength of about 470 nm) from the blue light emitting diode so that a portion of the blue light is absorbed by the YAG fluorescent material. As a result, the YAG fluorescent material emits yellow light, while the remaining portion of the blue light passes through the YAG fluorescent material without being absorbed thereby. Thus, the irradiation of the YAG fluorescent material with blue light causes the emission of white light composed of blue light and yellow light, as is disclosed in the foregoing document. The white light source has already been commercialized and used for various displays and the like.

To use the white light source for illumination, however, the improvement of the manner in which the white light source used for illumination appears, i.e., a color rendering property is important in addition to the enhancement of brightness and the improvement of the light emitting efficiency. The emission spectrum of the white light source is composed only of yellow light and blue light and therefore the ratio of the red component to the other color components is low. As a result, the light emitted from the white light source has a poor color rendering property as white light.

SUMMARY OF THE INVENTION

In view of the technological problems described above, it is therefore an object of the present invention to provide a white light emitting element which emits white light featuring a high color rendering property and a white light source in which the white light emitting element and an excitation light source have been integrated.

A first white light emitting element according to the present invention is a white light emitting element for generating white light through irradiation with visible light, comprising: a substrate which transmits the visible light; a semiconductor layer formed on a surface of the substrate; and a fluorescent layer formed on a surface of the substrate opposite to the surface thereof provided with the semiconductor layer or on a surface of the semiconductor layer, wherein the fluorescent layer emits yellow light or green light through irradiation with the visible light and the semiconductor layer emits red light through irradiation with the visible light.

A second white light emitting element according to the present invention is a white light emitting element for generating white light through irradiation with ultraviolet light, comprising: a substrate which transmits the ultraviolet light; a semiconductor layer formed on a surface of the substrate; and a fluorescent layer formed on a surface of the substrate opposite to the surface thereof provided with the semiconductor layer or on a surface of the semiconductor layer, wherein the fluorescent layer emits yellow light or green light and blue light through irradiation with the ultraviolet light and the semiconductor layer emits red light through irradiation with the ultraviolet light.

In the second white light emitting element, the fluorescent layer preferably contains a plurality of fluorescent materials and the fluorescent materials preferably emit light in different colors through irradiation with the ultraviolet light.

In each of the first and second white light emitting elements, the fluorescent layer for emitting the yellow light or the green light may contain a YAG fluorescent material.

A third white light emitting element according to the present invention is a white light emitting element for generating white light through irradiation with ultraviolet light, comprising: a substrate which transmits the ultraviolet light; and a semiconductor layer formed on a surface of the substrate, wherein the semiconductor layer emits red light, yellow light or green light, and blue light through irradiation with the ultraviolet light.

Each of the first, second, and third white light emitting elements preferably emits white light composed of the red light, the yellow light or the green light, and the blue light through irradiation with the visible light or with the ultraviolet light.

A fourth white light emitting element according to the present invention is a white light emitting element for generating white light through irradiation with ultraviolet light or visible light, comprising: a substrate which transmits the ultraviolet light or the visible light; and a semiconductor layer formed on a surface of the substrate, wherein the semiconductor layer is composed of a plurality of compound semiconductor layers having different element composition ratios and each of the compound semiconductor layers is formed to have a forbidden band width the value of which monotonously increases or decreases in a direction from the surface of the substrate toward a surface of the semiconductor layer.

In the fourth white light emitting element, a lattice constant in a crystal structure of each of compound semiconductors composing the individual compound semiconductor layers preferably has a constant value.

In the fourth white light emitting element, light emitted from individual compound semiconductors preferably has different wavelengths due to the different element composition ratios of the individual compound semiconductors and the fourth white light emitting element preferably emits white light composed of the plurality of light components emitted from the individual compound semiconductors.

A fifth white light emitting element according to the present invention is a white light emitting element for generating white light through irradiation with ultraviolet light or visible light, comprising: a substrate which transmits the ultraviolet light or the visible light; and a semiconductor layer formed on a surface of the substrate, wherein the semiconductor layer is composed of compound semiconductors and element composition ratios of the compound semiconductors continuously vary in a direction from the surface of the substrate toward a surface of the semiconductor layer.

In the fifth white light emitting element, a lattice constant in a crystal structure of each of the compound semiconductors composing the semiconductor layer preferably has a constant value in the direction from the surface of the substrate toward the surface of the semiconductor layer.

In the fifth white light emitting element, light components emitted from the individual compound semiconductors preferably have different wavelengths due to the different element composition ratios of the compound semiconductors composing the semiconductor layer and the fifth white light emitting element preferably emits white light composed of the plurality of light components emitted from the individual compound semiconductors.

In each of the first, second, third, fourth, and fifth white light emitting elements, the semiconductor layer preferably contains impurities and the semiconductor layer preferably emits light in a visible region through recombination between an electron and a hole via an energy level resulting from the impurities through irradiation with the visible light or with the ultraviolet light. The semiconductor layer may contain any one or two or all of Si, Mg, and Zn each as an impurity.

In each of the first, second, third, fourth and fifth white light emitting elements, it is preferable that the semiconductor layer contains impurities and the semiconductor layer emits light in a visible region via inner shell transition of the impurity through irradiation with the visible light or the ultraviolet light. Further, the semiconductor layer may contain any one or two or all of Eu, Sm and Yb each as the impurity. Also, the impurity is preferable to be introduced to the semiconductor layer by ion implantation.

In each of the first, second, third, fourth, and fifth white light emitting elements, the semiconductor layer may be made of AlxGayIn(1-x-y)N (0≦x≦1, 0≦y≦1, 0≦1-x-y≦1).

In each of the first, second, third, fourth, and fifth white light emitting elements, the substrate which transmits the visible light or the ultraviolet light may be made of sapphire.

A first white light source according to the present invention is a white light source comprising: an excitation light source; and a white light emitting element for generating white light through irradiation with light from the excitation light source, wherein the excitation light source emits visible light, the white light emitting element comprises: a substrate which transmits the light from the excitation light source; a semiconductor layer formed on a surface of the substrate; and a fluorescent layer formed on a surface of the substrate opposite to the surface thereof provided with the semiconductor layer or on a surface of the semiconductor layer, the fluorescent layer emits yellow light or green light through irradiation with the visible light, and the semiconductor layer emits red light through irradiation with the visible light.

A second white light source according to the present invention is a white light source comprising: an excitation light source; and a white light emitting element for generating white light through irradiation with light from the excitation light source, wherein the excitation light source emits ultraviolet light, the white light emitting element comprises: a substrate which transmits the light from the excitation light source; a semiconductor layer formed on a surface of the substrate; and a fluorescent layer formed on a surface of the substrate opposite to the surface thereof provided with the semiconductor layer or on a surface of the semiconductor layer, the fluorescent layer emits yellow light or green light and blue light through irradiation with the ultraviolet light, and the semiconductor layer emits red light through irradiation with the ultraviolet light.

In the second white light source according to the present invention, the fluorescent layer preferably contains a plurality of fluorescent materials and the fluorescent materials preferably emit light in different colors through irradiation with the ultraviolet light.

In each of the first and second white light sources, the fluorescent layer for emitting the yellow light or the green light may contain a YAG fluorescent material.

A third white light source according to the present invention is a white light source comprising: an excitation light source; and a white light emitting element for generating white light through irradiation with light from the excitation light source, wherein the excitation light source emits ultraviolet light, the white light emitting element comprises: a substrate which transmits the light from the excitation light source; and a semiconductor layer formed on a surface of the substrate, and the semiconductor layer emits red light, yellow light or green light, and blue light through irradiation with the ultraviolet light.

The third white light source preferably emits white light composed of the red light, the yellow light or the green light, and the blue light through irradiation with the visible light or with the ultraviolet light.

A fourth white light source according to the present invention is a white light source comprising: an excitation light source; and a white light emitting element for generating white light through irradiation with light from the excitation light source, wherein the excitation light source emits ultraviolet light or visible light, the white light emitting element comprises: a substrate which transmits the light from the excitation light source; and a semiconductor layer formed on a surface of the substrate, the semiconductor layer is composed of a plurality of compound semiconductor layers having different element composition ratios, and each of the compound semiconductor layers is formed to have a forbidden band width the value of which monotonously increases or decreases in a direction from the surface of the substrate toward a surface of the semiconductor layer.

In the fourth white light source, a lattice constant in a crystal structure of each of compound semiconductors composing the individual compound semiconductor layers preferably has a constant value.

In the fourth white light source, light components emitted from individual compound semiconductors preferably have different wavelengths due to the different element composition ratios of the individual compound semiconductors and the fourth white light emitting element preferably emits white light composed of the plurality of light components emitted from the individual compound semiconductors.

A fifth white light source according to the present invention is a white light source comprising: an excitation light source; and a white light emitting element for generating white light through irradiation with light from the excitation light source, wherein the excitation light source emits ultraviolet light or visible light, the white light emitting element comprises: a substrate which transmits the light from the excitation light source; and a semiconductor layer formed on a surface of the substrate, the semiconductor layer is composed of compound semiconductors, and element composition ratios of the compound semiconductors continuously vary in a direction from the surface of the substrate toward a surface of the semiconductor layer.

In the fifth white light source, a lattice constant in a crystal structure of each of the compound semiconductors composing the semiconductor layer preferably has a constant value in the direction from the surface of the substrate toward the surface of the semiconductor layer.

In the fifth white light source, light components emitted from the individual compound semiconductors preferably have different wavelengths due to the different element composition ratios of the compound semiconductors composing the semiconductor layer and the fifth white light source preferably emits white light composed of the plurality of light components emitted from the individual compound semiconductors.

In each of the first, second, third, fourth, and fifth white light sources, the semiconductor layer preferably contains impurities and the semiconductor layer preferably emits light in a visible region through recombination between an electron and a hole via an energy level resulting from the impurities through irradiation with the visible light or with the ultraviolet light. The semiconductor layer may contain any one or two or all of Si, Mg, and Zn each as an impurity.

In each of the first, second, third, fourth and fifth white light sources, it is preferable that the semiconductor layer contains impurities and the semiconductor layer emits light in a visible region via inner shell transition of the impurities through irradiation with the visible light or the ultraviolet light. Further, the semiconductor layer may contain any one or two or all of Eu, Sm and Yb each as the impurity. Also, the impurity is preferable to be introduced to the semiconductor layer by ion implantation.

In each of the first, second, third, fourth, and fifth white light sources, the semiconductor layer may be made of AlxGayIn(1-x-y)N (0≦x≦1, 0≦y≦1, 0 ≦1-x-y ≦1).

In each of the first, second, third, fourth, and fifth white light sources, the substrate which transmits the visible light or the ultraviolet light may be made of sapphire.

Thus, the white light emitting element according to the present invention comprises not only the semiconductor layer for emitting blue light and the semiconductor or fluorescent layer for emitting yellow or green light but also the semiconductor layer for emitting red light. Consequently, the white light emitted from the white light emitting element according to the present invention is composed of blue light, yellow or green light, and red light. Accordingly, the white light emitting element according to the present invention emits white light which is higher in color rendering property than white light emitted from a conventional white light source which excites a YAG fluorescent material with light from a blue light emitting diode.

Since the white light source according to the present invention comprises the above-mentioned white light emitting element according to the present invention, it can emit white light having a high color rendering property.

This renders the present invention useful as a technology for improving the color rendering property of white light emitted from a white light source such as a white light emitting diode (LED).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before describing the individual embodiments of the present invention, a description will be given first to a structure of a white light source7as a comparative embodiment, which uses a GaN-based blue light emitting diode as an excitation light source and uses a YAG fluorescent material as a white light emitting element, and to the mechanism of the emission of white light from the white light source7with reference toFIGS. 17 and 18, of whichFIG. 17is a cross-sectional view showing the structure of the white light source7according to the comparative embodiment andFIG. 18is a spectrum chart of light from the white light source7shown inFIG. 17.

First, the structure of the white light source7will be described.

As shown inFIG. 17, the white light source7comprises: a substrate72formed on a surface of a package71; a semiconductor layer73formed on a surface of the substrate72; electrodes76,77, and78formed on a surface of the semiconductor layer73; and a fluorescent layer79formed to cover the semiconductor layer73and the electrodes76,77, and78. The substrate72is made of sapphire and transmits visible light. The semiconductor layer73is composed of three compound semiconductor layers which are an n-type InGaAlN layer73a, an InGaAlN active layer73b, and a p-type InGaAlN layer73cstacked in this order. The InGaAlN active layer73band the p-type InGaAlN layer73chave been partly removed so that the n-type InGaAlN layer73acomposing the semiconductor layer73has been partly exposed. The Ni/Au transparent electrode76is formed on a surface of the p-type InGaAlN layer73c. The Au electrode77has been formed on a portion of a surface of the Ni/Au transparent electrode76. The Ti/Au electrode78has been formed on the exposed surface of the n-type InGaAlN layer73a.

Next, a method for fabricating the white light source7will be shown.

First, the n-type InGaAlN layer73a, the InGaAlN active layer73b, and the p-type InGaAlN layer73care formed in this order by, e.g. MOCVD (Metal Organic Chemical Vapor Deposition) on the substrate72made of sapphire.

Next, dry etching using, e.g., a Cl2gas is performed with respect to the InGaAlN active layer73band the p-type InGaAlN layer73c. Then, the Ti/Au electrode78is formed on the surface (the surface of the n-type InGaAlN layer73a) to which dry etching has been performed. This allows the extraction of an electrode onto the n-type semiconductor layer via the Ti/Au electrode78and allows the formation of a bonding pad on the n-type InGaAlN layer73a. On the other hand, the Ni/Au transparent electrode76is formed on the surface portion of the p-type InGaAlN layer73cthat has not undergone dry etching and the Au electrode77is formed on a portion of a surface of the Ni/Au transparent electrode76. At this time, the Ni/Au electrode76can be formed as the transparent electrode by adjusting the film thickness of the Ni/Au electrode76to 10 nm or less.

Subsequently, the semiconductor layer with the electrodes fabricated by the foregoing process steps is divided into light emitting diode chips each configured as a square with 300-μm sides. Each of the light emitting diode chips is mounted on the surface of the package71and subjected to wire bonding.

Thereafter, a YAG fluorescent material is applied dropwise onto the package71and hardened to form the fluorescent layer79covering the semiconductor layer73and the electrodes76,77, and78, whereby the fabrication of the white light source7is complete.

Next, the mechanism of the emission of white light from the white light source7will be described.

When an external voltage is applied to the Au electrode77and to the Ti/Au electrode78in the white light source7, a current flows in the white light source7. In response, the InGaAlN active layer73bemits blue light with a peak wavelength of 470 nm. Since the Ni/Au electrode76is a transparent electrode, the blue light passes through the Ni/Au electrode76and excites the fluorescent layer79. On absorbing the blue light, the YAG fluorescent material emits yellow light with a peak wavelength of about 550 nm. Accordingly, the blue light absorbed by the YAG fluorescent material becomes the yellow light, which is emitted from the fluorescent layer79, while the portion of the blue light that has not been absorbed by the fluorescent layer79passes through the fluorescent layer79. Thus, as shown inFIG. 21, the light emitted from the white light source7is white light composed of the yellow light (the peak on the right side ofFIG. 18) emitted from the fluorescent layer79and the portion of the blue excitation light (the peak on the left side ofFIG. 18) that has not been absorbed by the fluorescent layer79. As a result, the light emitted from the white light source7becomes the white light composed of the blue light and the yellow light.

However, since the white light emitted from the white light source7is composed of two colors, the ratio of the red component in the spectrum of the white color to the other color components is low so that the color rendering property of the white light is poor. The poor color rendering property of the white light (the low color rendering property of the white light) indicates that a target object appears differently when it is illuminated with the white light and with natural light. Conversely, the excellent color rendering property of the white light (the high color rendering property of the white light) indicates that the target object appears substantially the same when it is illuminated with the white light and with natural light. To use white light for illumination, therefore, it is preferable to use white light having an excellent color rendering property. In view of the foregoing, the present invention which improves the color rendering property of white light emitted from a white light source has been achieved by using a white light emitting element comprising not only a semiconductor layer which emits blue light and a semiconductor or fluorescent layer which emits yellow light but also a semiconductor or fluorescent layer which emits red light as the white light emitting element of a white light source. The following is the individual embodiments of the present invention, which are exemplary and not restrictive of the invention.

Referring toFIGS. 1 through 3, a first embodiment of the present invention will be described herein below.

In the present embodiment, a description will be given to a structure of a white light source1, a method for fabricating the white light source1, and the mechanism of the emission of white light from the white light source1.FIG. 1is a schematic diagram of the white light source1in the present embodiment.FIGS. 2A through 2Care views illustrating the method for fabricating the white light source1in the present embodiment.FIG. 3is a spectrum chart of light emitted from the white light source1in the present embodiment.

First, the structure of the white light source1will be described.

As shown inFIG. 1, the white light source1comprises: an excitation light source19; and a white light emitting element10provided at a position which allows the transmission of light emitted from the excitation light source19. The white light emitting element10comprises: a sapphire substrate (substrate)11which transmits light emitted from the excitation light source19; a semiconductor layer12epitaxially grown on a surface of the sapphire substrate11; and a fluorescent layer13formed on the surface of the sapphire substrate11opposite to the surface thereof provided with the semiconductor layer12. As can be seen, for example, inFIG. 1, the excitation light source19can be placed apart from the light emitting element.

The excitation light source19is a blue light emitting diode which emits blue light with a peak wavelength of 470 nm. The semiconductor layer12is made of AlxGayIn(1-x-y)N (0≦x≦1, 0≦y≦1, 0≦1-x-y≦1) such as, e.g., In0.4Ga0.6N. The forbidden band width of the semiconductor layer12is 1.9 eV. Accordingly, the semiconductor layer12emits red light with a peak wavelength of 650 nm when it is irradiated with visible light (e.g., blue light) or with ultraviolet light. The semiconductor layer12emits red light with a peak wavelength of 650 nm. The fluorescent layer13is made of a YAG fluorescent material. Accordingly, the fluorescent layer13emits yellow light with a peak wavelength of 550 nm when it is irradiated with visible light (e.g., blue light) or with ultraviolet light.

Next, a method for fabricating a white light source1′ with the white light emitting element10integrated therein will be described.

First, an In0.4Ga0.6N layer serving as the semiconductor layer12is formed by MOCVD on the surface of the sapphire substrate11. As a result, the semiconductor layer12is grown epitaxially on the sapphire substrate11. At this time, the surface of the sapphire substrate11is the (0001) plane of sapphire.

Next, as shown inFIG. 2B, a YAG fluorescent layer serving as the fluorescent layer13is formed on the surface of the sapphire substrate11opposite to the surface with the epitaxially grown semiconductor layer12. Thus, the semiconductor layer12and the fluorescent layer13are formed individually on the both surfaces of the sapphire substrate11, whereby the white light emitting element10shown inFIG. 1is formed. It is also possible to polish the sapphire substrate11and reduce the thickness thereof to, e.g., 100 μm or less before forming the fluorescent layer13.

Then, the white light emitting element10is diced into squares with, e.g., 1-mm sides. Thereafter, a blue light emitting diode chip19′ serving as the excitation light source19of the white light source1is provided on a package16and the white light emitting element10is adhered to the package16by using an adhesive agent17as shown inFIG. 2C, whereby the white light source1′ miniaturized for commercial use is fabricated.

Subsequently, the mechanism of the emission of the white light from the white light source1will be described with reference toFIG. 1. The arrows shown inFIG. 1represent light components emitted from the semiconductor layer12, the fluorescent layer13, and the excitation light source19.

In general, when the In0.4Ga0.6N layer is irradiated with blue light with a peak wavelength of 470 nm, the blue light is absorbed and red light is emitted. When the YAG fluorescent layer is irradiated with blue light with a peak wavelength of 470 nm, the blue light is absorbed and yellow light is emitted. The present embodiment has used a blue light emitting diode as the excitation light source19and provided the white light emitting element10at a position which allows the transmission of light from the excitation light source19.

When the blue light emitted from the excitation light source19has reached the white light emitting element10, a portion of the blue light is absorbed by the semiconductor layer12. Consequently, the semiconductor layer12emits red light with a peak wavelength of 650 nm. The red light passes through the sapphire substrate11and the fluorescent layer13to be radiated to the outside of the white light emitting element10.

The portion of the blue light that has not been absorbed by the semiconductor layer12passes through the sapphire substrate11and reaches the fluorescent layer13so that the portion of the blue light is partly absorbed by the fluorescent layer13. Consequently, the fluorescent layer13emits yellow light with a peak wavelength of 550 nm, which is radiated to the outside of the white light emitting element10.

The portion of the blue light that has not been absorbed by the semiconductor layer12and the fluorescent layer13passes through the semiconductor layer12, the sapphire substrate11, and the fluorescent layer13to be radiated to the outside of the white light emitting element10.

Thus, when the white light emitting element10is irradiated with the blue light emitted from the excitation light source19, the white light emitting element10emits the white light composed of the red light emitted from the semiconductor layer12, the yellow light emitted from the fluorescent layer13, and the blue light emitted from the excitation light source19and passing through the white light emitting element10without being absorbed. Specifically, as shown inFIG. 3, the white light source1emits the white light composed of the red light with a peak wavelength of 650 nm (the peak on the right side), the yellow light with a peak wavelength of 550 nm (the peak at the center), and the blue light with a peak wavelength of 470 nm (the peak on the left side). In the comparative embodiment, the white light emitted from the white light source7is composed of the blue light and the yellow light, while the white light emitted from the white light source1contains the red light in addition to the blue light and the yellow light. Accordingly, the color rendering property of the white light emitted from the white light source1is higher than that of the white light emitted from the white light source7in the comparative embodiment.

The effects achieved by the white light source1and the white light emitting element10in the present embodiment will be shown herein below.

The white light emitted from the white light source7in the comparative embodiment has been composed of the portion of the blue light emitted from the excitation light source and having passed through the semiconductor layer and the fluorescent layer and the yellow light emitted through the absorption of the portion of the blue light by the fluorescent layer. On the other hand, the white light emitting element10comprises the semiconductor layer12which emits the red light with a peak wavelength of 650 nm and the fluorescent layer13which emits the yellow light with a peak wavelength of 550 nm when they are irradiated with the blue light with a peak wavelength of 470 nm. Of the blue light emitted from the excitation light source19, the portion that has not been absorbed by the semiconductor layer12and the fluorescent layer13passes through the white light emitting element10. Consequently, the white light emitted from the white light source1is composed of the red light, the yellow light, and the blue light. Since the red light is contained additionally in the white light emitted from the white light source according to the comparative embodiment, the color rendering property of the white light emitted from the white light source1is improved compared with that of the white light emitted from the white light source7in the comparative embodiment. By merely providing the semiconductor layer12which emits red light in the white light emitting element in the comparative embodiment, the color rendering property of white light can be enhanced.

In the present embodiment, the surface of the sapphire substrate11is not limited to the (0001) plane of sapphire and any surface orientation may be assumed. For example, a surface orientation at an off-angle from a representative surface, such as the (0001) plane of sapphire, may also be assumed.

Although the present embodiment has used the sapphire substrate11as the substrate, the substrate is not limited thereto. Any substrate may be used provided that it transmits visible light. For example, a substrate made of SiC, Si, or GaAs may also be used. In the case of using the substrate made of Si or GaAs, it is necessary to cause the crystal growth of a semiconductor and then separate and remove the semiconductor.

A method for crystal growth used to form the semiconductor layer12on the sapphire substrate11is not limited to MOCVD. The crystal growth may also be caused by, e.g., MBE (Molecular Beam Epitaxy) or HVPE (Hydride Vapor Phase Epitaxy).

The semiconductor layer12may contain a group V element such as As or P or a group III element such as B as a constituent element. The semiconductor layer12may also contain Zn, Mg, or Si as an impurity. In the case where Zn, Mg, or Si is contained as an impurity, the semiconductor layer12emits red light due to the recombination between electrons and holes via the energy level of the contained impurity.

Alternatively, a GaN underlie layer or a thin-film buffer layer made of GaN or AlN may also be formed between the sapphire substrate11and the semiconductor layer12. The semiconductor layer12may also be constituted to contain a multiple quantum well structure for improved brightness or composed of InxGa(1-x)N (0≦x≦1) of which the In composition need not be uniform in the plane thereof.

Further, the fluorescent layer13may also be formed on the surface provided with the semiconductor layer12.

In the excitation light source19, the element which emits the blue light may be a semiconductor layer made of AlxGayIn(1-x-y)N (0≦x≦1, 0≦y≦1, 0≦1-x-y≦1) or a semiconductor layer represented by a compositional formula other than the above.

Referring toFIGS. 4 and 5, a second embodiment of the present invention will be described herein below.

In the present embodiment, a description will be given to a structure of a white light source2, a method for fabricating the white light source2, and the mechanism of the emission of white light from the white light source2.FIG. 4is a schematic diagram of the white light source2according to the present embodiment.FIG. 5is a spectrum chart of light emitted from the white light source2in the present embodiment. InFIG. 4, portions indicating the same materials and functions as shown inFIG. 1are designated by the same reference numerals as used inFIG. 1.

The white light source2according to the present embodiment is different from the white light source1according to the first embodiment described above in that light emitted from an excitation light source29is ultraviolet light and a fluorescent layer23is composed of two layers.

First, the structure of the white light source2will be described.

As shown inFIG. 4, the white light source2comprises: the excitation light source29; and the white light emitting element20provided at a position which allows the transmission of light from the excitation light source29. The white light emitting element20comprises: a sapphire substrate11which transmits light from the excitation light source29; a semiconductor layer12epitaxially grown on a surface of the sapphire substrate11; and the fluorescent layer23formed on the surface opposite to the surface provided with the semiconductor layer12.

The excitation light source29is an ultraviolet light emitting diode which emits ultraviolet light with a peak wavelength of 340 nm. The first embodiment described above has used the blue light emitting diode as the excitation light source19so that the blue light composing the white light emitted from the white light source1is the light emitted from the excitation light source19. By contrast, the present embodiment uses the ultraviolet light emitting diode as the excitation light source29. Accordingly, the blue light composing the white light emitted from the white light source2is not the light emitted from the excitation light source29but the light emitted from the fluorescent layer23through the absorption of the ultraviolet light from the excitation light source29thereby. In other words, the white light emitted from the while light source2is determined only by the light emitted through the absorption of the ultraviolet light from the excitation light source29. This allows the color rendering property of the white light emitted from the white light source2to be controlled more easily than that of the white light emitted from the white light source1according to the first embodiment described above and allows the emission of white light having excellent reproducibility and an excellent color rendering property.

The semiconductor layer12is made of In0.4Ga0.6N and the forbidden band width thereof is 1.9 eV. Accordingly, the semiconductor layer12emits red light with a peak wavelength of 650 nm when it is irradiated with visible light or ultraviolet light. The fluorescent layer23is composed of: a fluorescent layer23awhich emits green light through irradiation with visible light or ultraviolet light (hereinafter referred to as “green light emitting fluorescent layer”); and a fluorescent layer23bwhich emits blue light through irradiation with visible light or ultraviolet light (hereinafter referred to as “blue light emitting fluorescent layer”).

Next, the method for fabricating a white light source (not shown) with the white light emitting element20integrated therein will be described.

In the present embodiment, the green light emitting fluorescent layer23aand the blue light emitting fluorescent layer23bmay be provided appropriately instead of providing the YAG fluorescent layer in the step (the step shown inFIG. 2B) of forming the YAG fluorescent layer serving as the fluorescent layer13on the surface of the sapphire substrate opposite to the surface thereof provided with the semiconductor layer12according to the first embodiment. Otherwise, the fabrication method according to the present embodiment is the same as the method for fabricating the white light source1′ with the white light emitting element10integrated therein according to the first embodiment described above.

Subsequently, the mechanism of the emission of white light from the white light source2will be described with reference toFIG. 4. The arrows shown inFIG. 4represent light components emitted from the semiconductor layer12, the fluorescent layer23, and the excitation light source29.

In the white light source2, the ultraviolet light emitting diode is used as the excitation light source29and the white light emitting element20is provided at a position which allows the transmission of light from the excitation light source29.

When the ultraviolet light emitted from the excitation light source29has reached the white light emitting element20, a portion of the ultraviolet light is absorbed by the semiconductor layer12. Consequently, the semiconductor layer12emits red light with a peak wavelength of 650 nm. The red light passes through the sapphire substrate11and the fluorescent layer23to be radiated to the outside of the white light emitting element20.

The portion of the ultraviolet light that has not been absorbed by the semiconductor layer12passes through the sapphire substrate11and reaches the green light emitting fluorescent layer23aso that the portion of the ultraviolet light is partly absorbed by the green light emitting fluorescent layer23a. Accordingly, the green light emitting fluorescent layer23aemits green light with a peak wavelength of 550 nm, which passes through the blue light emitting fluorescent layer23bto be radiated to the outside of the white light emitting element20.

The portion of the ultraviolet light that has not been absorbed by the semiconductor layer12and the green light emitting fluorescent layer23ais partly absorbed by the blue light emitting fluorescent layer23b. Accordingly, the blue light emitting fluorescent layer23bemits blue light with a peak wavelength of 470 nm, which is radiated to the outside of the white light emitting element20.

Thus, when the white light emitting element20is irradiated with the ultraviolet light emitted from the excitation light source29, the white light emitting element20emits the white light composed of the red light emitted from the semiconductor layer12, the green light emitted from the green light emitting fluorescent layer23aof the fluorescent layer23, and the blue light emitted from the blue light emitting fluorescent layer23bof the fluorescent layer23. Specifically, as shown inFIG. 5, the white light source2emits the white light composed of the red light with a peak wavelength of 650 nm (the peak on the right side), the green light with a peak wavelength of 550 nm (the peak at the center), and the blue light with a peak wavelength of 470 nm (the peak on the left side). Accordingly, the white light emitted from the white light source2is composed of the white light emitted from the white light source7in the comparative embodiment and the red light added thereto so that the color rendering property of the white light emitted from the white light source2is higher than that of the white light emitted from the white light source7.

It is to be noted that the peak observed in the vicinity of 340 nm in the spectrum shown inFIG. 5has resulted from the radiation of the portion of the ultraviolet light that has not been absorbed by the semiconductor layer12and the fluorescent layer23to the outside of the white light emitting element20.

The effects achieved by the white light source2and the white light emitting element20in the present embodiment will be shown herein below. The fluorescent material used in the present embodiment has been used widely for fluorescent lamps so that the brightness thereof is sufficiently high. Accordingly, the color rendering property of the white light emitted from the white light source2is improved.

Although it is assumed in the present embodiment that the fluorescent layer23is composed of the green light emitting fluorescent layer23aand the blue light emitting fluorescent layer23b, the fluorescent layer23need not have a layered structure. It is sufficient for the fluorescent layer23to merely contain a fluorescent material which emits green light and a fluorescent material which emits blue light.

Referring toFIGS. 6 and 7B, a third embodiment of the present invention will be described herein below.

In the present embodiment, a description will be given to a structure of a white light source3, a method for fabricating the white light source3, and the mechanism of the emission of white light from the white light source3.FIG. 6is a schematic diagram of the white light source3in the present embodiment.FIGS. 7A and 7Bare views illustrating the method for fabricating the white light source3in the present embodiment. InFIG. 6, portions indicating the same materials and functions as shown inFIG. 1are designated by the same reference numerals as used inFIG. 1.

The white light emitting element30according to the present embodiment is different from the white light emitting element10according to the first embodiment described above in that it does not comprise a fluorescent layer and comprises a semiconductor layer32composed of three layers.

First, the structure of the white light source3will be described.

As shown inFIG. 6, the white light source3comprises: the excitation light source29; and the white light emitting element30provided at a position which allows the transmission of light from the excitation light source29. The white light emitting element30comprises: a sapphire substrate11which transmits light from the excitation light source29; and the semiconductor layer32epitaxially grown on a surface of the sapphire substrate11.

The excitation light source29is an ultraviolet light emitting diode which emits ultraviolet light with a peak wavelength of 340 nm. The semiconductor layer32is composed of: an InGaAlN layer32awhich emits blue light through irradiation with visible light or ultraviolet light (hereinafter referred to as “blue light emitting InGaAlN layer”); an InGaAlN layer32bwhich emits green light through irradiation with visible light or ultraviolet light (hereinafter referred to as “green light emitting InGaAlN layer”); and an InGaAlN layer32cwhich emits red light through irradiation with visible light or ultraviolet light (hereinafter referred to as “red light emitting InGaAlN layer”), which are stacked in this order. The blue light emitting InGaAlN layer32ais made of, e.g., In0.2Ga0.8N, the forbidden band width thereof is 2.6 eV, and the peak wavelength of the light emitted therefrom is 470 nm. The green light emitting InGaAlN layer32bis made of, e.g., In0.3Ga0.7N, the forbidden band width thereof is 2.3 eV, and the peak wavelength of the light emitted therefrom is 550 nm. The red light emitting InGaAlN layer32cis made of, e.g., In0.4Ga0.6N, the forbidden band width thereof is 1.9 eV, and the peak wavelength of the light emitted therefrom is 650 nm. It is to be noted that InGaAlN is a simplified representation of In1-x-yGaxAlyN (0≦x≦1, 0≦y≦1, 0≦1-x-y≦1).

Next, a method for fabricating a white light source3′ with the white light emitting element30integrated therein will be described.

First, a semiconductor layer serving as the blue light emitting InGaAlN layer32ais formed by MOCVD on the surface of the sapphire substrate11, a semiconductor layer serving as the green light emitting InGaAlN layer32bis formed by MOCVD on a surface of the blue light emitting InGaAlN layer32a, and then a semiconductor layer serving as the red light emitting InGaAlN layer32cis formed by MOCVD on a surface of the green light emitting InGaAlN layer32b, whereby the white light emitting element30shown inFIG. 7Ais formed.

Next, the white light emitting element30is diced into squares with, e.g., 1-mm sides. Thereafter, an ultraviolet light emitting diode chip29′ serving as the excitation light source29of the white light source3is provided on a package16and the white light emitting element30is adhered to the package16by using an adhesive agent17as shown inFIG. 7B, whereby the white light source3′ miniaturized for commercial use is fabricated.

Subsequently, the mechanism of the emission of white light from the white light source3will be described with reference toFIG. 6.

When the ultraviolet light emitted from the excitation light source29has reached the white light emitting element30, a portion of the ultraviolet light is absorbed by the red light emitting InGaAlN layer32c. Accordingly, the red light emitting InGaAlN layer32cemits red light with a peak wavelength of 650 nm. The red light passes through the green light emitting InGaAlN layer32b, the blue light emitting InGaAlN layer32a, and the sapphire substrate11to be radiated to the outside of the white light emitting element30.

The portion of the ultraviolet light that has not been absorbed by the red light emitting InGaAlN layer32cis partly absorbed by the green light emitting InGaAlN layer32b. Accordingly, the green light emitting InGaAlN layer32bemits green light with a peak wavelength of 550 nm, which passes through the blue light emitting InGaAlN layer32aand the sapphire substrate11to be radiated to the outside of the white light emitting element30.

The portion of the ultraviolet light that has not been absorbed by the red light emitting InGaAlN layer32cand the green light emitting InGaAlN layer32bis partly absorbed by the blue light emitting InGaAlN layer32a. Accordingly, the blue light emitting InGaAlN layer32aemits blue light with a peak wavelength of 470 nm, which passes through the sapphire substrate11to be radiated to the outside of the white light emitting element30.

Thus, when the white light emitting element30is irradiated with the ultraviolet light emitted from the excitation light source29, the white light emitting element30emits the white light composed of the blue light emitted from the blue light emitting InGaAlN layer32a, the green light emitted from the green light emitting InGaAlN layer32b, and the red light emitted from the red light emitting InGaAlN layer32c. Accordingly, the white light emitted from the white light source3is composed of the white light emitted from the white light source7in the comparative embodiment and the red light added thereto so that the color rendering property of the white light emitted from the white light source3is higher than that of the white light emitted from the white light source7.

The effects achieved by the white light source3and the white light emitting element30in the present embodiment will be shown herein below.

In the present embodiment, the semiconductor layer32emits the red light, the green light, and the blue light. This obviates the necessity to provide a fluorescent layer in the white light emitting element30and allows the white light emitting element30to be fabricated more easily than the white light emitting elements10and20according to the first and second embodiments. In addition, the white light source3and the white light emitting element30also achieve the effects described above in the first and second embodiments.

Although the semiconductor layer32is composed of the blue light emitting InGaAlN layer32a, the green light emitting InGaAlN layer32b, and the red light emitting InGaAlN layer32cin the present embodiment, the semiconductor layer32need not have a layered structure. It is sufficient for the semiconductor layer32to merely contain InGaAlN which emits blue light, InGaAlN which emits green light, and InGaAlN which emits red light.

Referring toFIGS. 8 through 10, a fourth embodiment of the present invention will be described herein below.

In the present embodiment, a description will be given to a structure of a white light source4, a method for fabricating the white light source4, and the mechanism of the emission of white light from the white light source4.FIG. 8is a schematic diagram of the white light source4according to the present embodiment.FIG. 9is a view showing a forbidden band width (band gap) relative to the direction of height of a white light emitting element40according to the present embodiment.FIG. 10is a spectrum chart of light emitted from the white light source4in the present embodiment. InFIG. 8, portions indicating the same materials and functions as shown inFIG. 1are designated by the same reference numerals as used inFIG. 1.

The white light emitting element40according to the present embodiment is different from the white light emitting element10according to the first embodiment described above in that it semiconductor layers44and45which are irrelevant to light emission are provided.

First, the structure of the white light source4will be described.

As shown inFIG. 8, the white light source4comprises: the excitation light source29; and the white light emitting element40provided at a position which allows the transmission of light from the excitation light source29. The white light emitting element40comprises: a sapphire substrate11which transmits light from the excitation light source29; the AlN buffer layer44formed on a surface of the sapphire substrate11; a GaN layer45formed on a surface of the AlN buffer layer44; and a semiconductor layer42formed on a surface of the GaN layer45. The semiconductor layer42is composed of a blue light emitting InGaAlN layer42a, a green light emitting InGaAlN layer42b, and a red light emitting InGaAlN layer42c, which are stacked in this order.

The excitation light source29is an ultraviolet light emitting diode which emits ultraviolet light with a peak wavelength of 340 nm. The blue light emitting InGaAlN layer42a, the green light emitting InGaAlN layer42b, and the red light emitting InGaAlN layer42ccontain Si and Zn and emit blue light, green light, and red light, respectively, due to recombination via the energy levels of Si and Zn. The blue light emitting InGaAlN layer42ais made of In0.1Ga0.9N, the green light emitting InGaAlN layer42bis made of In0.2Ga0.8N, and the red light emitting InGaAlN layer42cis made of In0.3Ga0.7N. Since the white light emitting element40comprises the AlN buffer layer44and the GaN layer45, the crystal defect density of the semiconductor layer42can be reduced. This reduces nonradiative recombination in the white light emitting element40and improves brightness. Each of the AlN buffer layer44and the GaN layer45has a thickness of about 1 μm.

Next, a method for fabricating a white light source (not shown) with the white light emitting element40integrated therein will be described.

First, an AlN layer serving as the AlN buffer layer44is formed by MOCVD on the surface of the sapphire substrate11and the GaN layer45is formed by MOCVD on the surface of the AlN layer. Thereafter, the In0.1Ga0.9N layer serving as the blue light emitting InGaAlN layer42a, the In0.2Ga0.8N layer serving as the green light emitting InGaAlN layer42b, and the In0.3Ga0.7N layer serving as the red light emitting InGaAlN layer42care formed by MOCVD in this order, whereby the white light emitting element40shown inFIG. 8is formed. By this time, the In0.1Ga0.9N layer, the In0.2Ga0.8N layer, and the In0.3Ga0.7N layer have been doped with Si and Zn.

Next, the white light emitting element40is diced into squares with, e.g., 1-mm sides. Thereafter, an ultraviolet light emitting diode chip serving as the excitation light source29of the white light source4is provided on the package described above in each of the first and third embodiments and the white light emitting element40is adhered to the package by using an adhesive agent, whereby the white light source with the white light emitting element40integrated therein is fabricated.

The mechanism of the emission of white light from the white light source4is substantially the same as the emission of white light from the white light source3according to the third embodiment described above. Consequently, as shown inFIG. 10, the white light source4emits the white light composed of the red light with a peak wavelength of 650 nm (the peak on the right side), the green light with a peak wavelength of 550 nm (the peak at the center), and the blue light with a peak wavelength of 470 nm (the peak on the left side). Accordingly, the white light emitted from the white light source4is composed of the white light emitted from the white light source7in the comparative embodiment and the red light added thereto so that the color rendering property of the white light emitted from the white light source4is higher than that of the white light emitted from the white light source7.

Although the semiconductor layer42has a structure in which the In0.1Ga0.9N layer, the In0.2Ga0.8N layer, and the In0.3Ga0.7N layer are stacked in the present embodiment, it is not limited thereto. However, if consideration is given to the phenomenon in which the lattice constant of the semiconductor increases with an increase in In composition and to the possibility that the resulting lattice mismatch may cause a crystal defect, it is preferable to control the In, Ga, and Al compositions of InGaAlN, while maintaining the same lattice constant of 3.19 Å as the lattice constant of the a-axis of hexagonal GaN, in terms of suppressing the occurrence of a crystal defect resulting from the lattice mismatch. Even under such control, white light including blue light, green light, and red light, which is similar to the white light emitted from the white light emitting element40, can be emitted.

Although it is assumed in the present embodiment that the semiconductor layer42is composed of the blue light emitting InGaAlN layer42a, the green light emitting InGaAlN layer42b, and the red light emitting InGaAlN layer42cwhich are stacked in this order, the InGaAlN layers may also be stacked appropriately in the order of the red light emitting InGaAlN layer42c, the green light emitting InGaAlN layer42b, and the blue light emitting InGaAlN layer42a. The blue light emitting InGaAlN layer42a, the green light emitting InGaAlN layer42b, and the red light emitting InGaAlN layer42cneed not be doped appropriately with Si and Zn. In that case, the composition ratio of In in each of the InGaAlN layers may be increased appropriately. If the blue light emitting InGaAlN layer42ais composed of, e.g., In0.2Ga0.8N, the same effects as obtained in the present embodiment are obtainable.

Referring toFIGS. 11 through 13, a fifth embodiment of the present invention will be described herein below.

In the present embodiment, a description will be given to a structure of a white light source5, a method for fabricating the white light source5, and the mechanism of the emission of white light from the white light source5.FIG. 11is a schematic diagram of the white light source5in the present embodiment.FIG. 12is a view showing a forbidden band width (band gap) relative to the direction of height of the white light emitting element50in the present embodiment.FIG. 13is a spectrum chart of light emitted from the white light source5in the present embodiment. InFIG. 11, portions indicating the same materials and functions as shown inFIG. 1are designated by the same reference numerals as used inFIG. 1.

In the white light emitting element50according to the present embodiment, a fluorescent layer is not formed and only one semiconductor layer52is formed.

First, the structure of the white light source5will be described.

As shown inFIG. 11, the white light source5comprises: the excitation light source29; and the white light emitting element50provided at a position which allows the transmission of light from the excitation light source29. The white light emitting element50comprises: a sapphire substrate11which transmits light from the excitation light source29; the AlN buffer layer44formed on a surface of the sapphire substrate11; a GaN layer45formed on a surface of the AlN buffer layer44; and the semiconductor layer52formed on a surface of the GaN layer45.

The excitation light source29is an ultraviolet light emitting diode which emits ultraviolet light with a peak wavelength of 340 nm. Each of the AlN buffer layer44and the GaN layer45is excellent in crystal properties and low in defect density. The element composition of the semiconductor layer52is In0.38Ga0.62N in the surface thereof and is InN in the surface of the white light emitting element50. The element composition ratio changes continuously in a direction from the surface of the GaN layer45toward the surface of the white light emitting element50.

Next, a method for fabricating a white light source (not shown) with the white light emitting element50integrated therein will be described.

First, an AlN layer serving as the AlN buffer layer44is formed by MOCVD on the surface of the sapphire substrate11and the GaN layer45is formed by MOCVD on the surface of the AlN layer. Thereafter, the semiconductor layer52is formed such that the element composition of the semiconductor is In0.38Ga0.62N in the surface of the GaN layer45, the composition of Ga decreases and the composition ratio of In increases as more layers are stacked, and the element composition of the semiconductor is InN in the surface of the semiconductor layer52, whereby the white light emitting element50shown inFIG. 11is formed.

Next, the white light emitting element50is diced into squares with, e.g., 1-mm sides. Thereafter, an ultraviolet light emitting diode chip serving as the excitation light source29of the white light source5is provided on the package described above in each of the first and third embodiments and the white light emitting element50is adhered to the package by using an adhesive agent, whereby the white light source with the white light emitting element50integrated therein is fabricated.

Subsequently, a description will be given to the mechanism of the emission of light from each of the compound semiconductors and to the mechanism of the emission of white light from the white light source5with reference toFIG. 12.

The present embodiment has also utilized the phenomenon in which the value of the forbidden band width of a semiconductor layer varies due to different composition ratios between In and Ga, similarly to the fourth embodiment described above. In the present embodiment, the element composition ratio in the semiconductor layer52continuously changes in the direction from the surface of the GaN layer45toward the surface of the white light emitting element50so that the peak wavelength of light emitted from the white light source5does not have discrete values such as 470 nm, 550 nm, and 650 nm, but has indiscrete values such as, e.g., 469 nm, 471 nm, and 472 nm. Accordingly, white light152emitted from the white light source5includes an infinite number of light components so that the white light emitting element50emits light having a wide band as shown inFIG. 13. Consequently, the white light emitted from the white light emitting element50is extremely similar to natural light and the color rendering property thereof is extremely higher than that of white light emitted from each of the white light sources in the first to fourth embodiments described above.

Referring toFIGS. 14 through 16, a sixth embodiment of the present invention will be described herein below.

In the present embodiment, a description will be given to a structure of a white light source6, a method for fabricating the white light source6, and the mechanism of the emission of white light from the white light source6.FIG. 14is a schematic diagram of the white light source6in the present embodiment.FIGS. 15A through 15Dare views illustrating the method for fabricating the white light source6in the present embodiment.FIG. 16is a spectrum chart of light emitted from the white light source6in the present embodiment.

In the white light emitting element60in the present embodiment, an rare earth element is doped in the semiconductor layer62. Wherein, the detailed description of the overlapping portion with the first embodiment is omitted.

First, the structure of the white light source6will be described.

As shown inFIG. 14A, the white light source6comprises: an excitation light source19; and a white light emitting element60provided at a position which allows the transmission of light emitted from the excitation light source19. The white light emitting element60comprises: a sapphire substrate (substrate)11which transmits light emitted from the excitation light source19; a semiconductor layer62epitaxially grown on a surface of the sapphire substrate11; and a fluorescent layer13formed on the surface of the sapphire substrate11opposite to the surface thereof provided with the semiconductor layer62.

The excitation light source19is a blue light emitting diode which emits blue light with a peak wavelength of 470 nm.

The semiconductor layer62is made of Eu doped Al0.5Ga0.5N layer formed by doping Eu+to an undoped Al0.5Ga0.5N layer162. It is to be noted that the undoped Al0.5Ga0.5N layer162may be a GaN layer or has a quantum well structure such as an In0.02Ga0.98N/Al0.4Ga0.6N multi quantum well. Further, Eu+may be added at crystal growth of the undoped layer162.

The dose amount of Eu+is preferably set in the range between 1×1013cm−3and 1 ×1016cm−3, both inclusive, and more preferably set to 1×1015cm−3. The acceleration voltage of Eu+is preferable set in the range between 100 keV and 500 keV, both inclusive, and more preferable set to 200 keV. As shown inFIG. 14B, the concentration distribution of the Eu+in the depth direction of the semiconductor layer62has a peak at the depth around 75 nm, and the concentration of the Eu+at the peak is approximately 1×1020cm−3. Wherein, the concentration distribution of the Eu+varies depending on the dose amount and the acceleration voltage. Further, as shown inFIG. 15B, Eu+is added to only the surface portion of the semiconductor layer62. Irradiation of visible light or ultraviolet light causes the inner shell electrons of the Eu to be excited. When the electrons return to the base level, the semiconductor layer62emits red light of 622 nm. The light intensity of the red light is increased by increasing the dose amount of Eu+.

Next, a method for fabricating a white light source6′ with the white light emitting element60integrated therein will be described.

First, as shown inFIG. 15A, the undoped Al0.5Ga0.5N layer162having a thickness of about 1 μm is formed on the surface of a sapphire substrate11by MOCVD.

Next, as shown inFIG. 15B, while the sapphire substrate11is heated by about 500° C., Eu+of 1×1015cm−3is implanted from the surface of the undoped Al0.5Ga0.5N layer162at the acceleration voltage of 200 keV. Whereby, a layer162ain which Eu+is doped is formed on the surface of the semiconductor layer62, as shown in the enlarged view ofFIG. 15B. The concentration distribution of Eu+after the implantation has a peak at the depth around 75 nm. Then, the sapphire substrate11is annealed at about 1000° C. in N2atmosphere after the ion implantation for ion activation of the Eu+, so that the undoped Al0.5Ga0.5N layer162is changed to the Eu doped Al0.5Ga0.5N layer (semiconductor layer)62.

Subsequently, as shown inFIG. 15C, a YAG fluorescent layer serving as the fluorescent layer13is formed on the surface of the sapphire substrate11opposite to the surface where the semiconductor layer62is formed. It is also possible to polish the sapphire substrate11and reduce the thickness thereof to, e.g., 100 μm or less before forming the YAG fluorescent layer13. Whereby, the white light emitting element60shown inFIG. 14is obtained.

Then, the white light emitting element60is diced into squares with, e.g., 1-mm sides. Thereafter, a blue light emitting diode chip19′ is provided on a package16and the white light emitting element60is adhered to the package16by using an adhesive agent17as shown inFIG. 15D, whereby the white light source6′ miniaturized for commercial use is fabricated.

The mechanism of the white light source6for emitting white light is as described in the first embodiment. Specifically, when the white light emitting element60is irradiated with the blue light emitted from the excitation light source19, the white light emitting element60emits the white light composed of the red light emitted from the semiconductor layer62(peak wavelength of 622 nm), the yellow light emitted from the fluorescent layer13(peak wavelength of 550 nm), and the blue light emitted from the excitation light source19and passing through the white light emitting element60without being absorbed (peak wavelength of 470 nm), as shown inFIG. 16. Thus, the white light source6has more light components of the red light than that of the white light source7in the comparative embodiment and emits white light excellent in color rendering. As a result, a white light emitting diode excellent in color rendering is realized.

It is to be noted that Eu+is doped in the semiconductor layer62in the present embodiment, but Sm+or Yb+may be doped.