Semiconductor light emitting element array illuminator using the same

A semiconductor light emitting element array includes a substrate made of SiC and having a first surface and a second surface opposite to the first surface. The array also includes a plurality of semiconductor light emitting elements supported by the first surface of the substrate. Each of the light emitting elements includes an n-type semiconductor layer, an active layer, and a p-type semiconductor layer. The second surface of the substrate serves as a light emitting surface, from which light produced by the light emitting elements is emitted out.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor light emitting element array including a plurality of semiconductor light emitting elements such as LEDs, and also to an illuminator using such an array.

2. Description of the Related Art

FIG. 4shows a conventional semiconductor light emitting element array disclosed by JP-A-2005-79202. In the illustrated semiconductor light emitting element array X, a plurality of semiconductor light emitting elements Ed are provided on a substrate91. Each of the semiconductor light emitting elements Ed has a multilayer structure consisting of an n-GaN layer92, an active layer93and a p-GaN layer94. Electrons injected from the n-GaN layer92and holes injected from the p-GaN layer94recombine in the active layer93, whereby blue light is emitted. The blue light emitted from the active layer93travels through a p-side electrode95, which is transparent, and then enters a fluorescent layer96. The fluorescent layer96contains fluorescent particles, which convert part of the blue light into yellow light. The yellow light obtained by the conversion mixes with the remaining blue light to change into whitish light, which is emitted from the upper surface of the fluorescent layer96.

The conventional array, however, suffers the following problem when its brightness is to be increased.

The fluorescent layer96is made of a resin material mixed with fluorescent particles. The refractive index of the resin material greatly differs from that of GaN (which is suitable for emitting blue light). Accordingly, when the blue light travels from the p-GaN layer94to the fluorescent layer96, a large part of the blue light is subjected to total internal reflection. As a result, the emission efficiency, i.e., the ratio of the amount of light emitted from the fluorescent layer96to the amount of light produced at the active layer93is not satisfactorily high.

Further, when a current is applied to energize the semiconductor light emitting elements Ed, the n-GaN layer92, the active layer93and the p-GaN layer94are heated. Since most part of the semiconductor light emitting elements Ed is covered by the fluorescent layer96mainly composed of resin, heat is unlikely to escape. Heat is also generated by the color conversion of the blue light at the fluorescent layer96. This heat is trapped in the fluorescent layer96. As the amount of current to be applied to the semiconductor light emitting element array X increases, more heat is produced. In light of these, it has been desired to enhance the heat dissipation of the array X, so that the array X can be more bright.

SUMMARY OF THE INVENTION

The present invention has been proposed under the circumstances described above. It is an object of the present invention to provide a semiconductor light emitting element array suitable for increasing the luminance, and to provide an illuminator using such a semiconductor light emitting element array.

According to a first aspect of the present invention, there is provided a semiconductor light emitting element array comprising: a substrate made of SiC and including a first surface and a second surface opposite to the first surface; and a plurality of semiconductor light emitting elements supported by the first surface of the substrate, where each of the light emitting elements includes an n-type semiconductor layer, an active layer and a p-type semiconductor layer. Light produced by the light emitting elements is emitted out from the second surface of the substrate.

With the above structure, since SiC has high heat conductivity, the heat conductivity of the substrate is high. Therefore, heat generated from the semiconductor light emitting elements can be dissipated through the substrate. Further, since light is emitted from an light emitting surface (the second surface) positioned on the opposite side of the semiconductor light emitting elements, the semiconductor light emitting elements do not need to be covered by a color-conversion layer made of e.g. resin. Thus, the semiconductor light emitting element array can properly dissipate the heat produced in light emission, so that the current can be increased to increase the luminance. Further, the refractive index of SiC does not differ greatly from that of GaN, which is a typical semiconductor material. Therefore, the light emitted from the active layer is less likely to be totally reflected inward by the substrate.

In a preferred embodiment of the present invention, a color conversion layer containing SiC is provided between the substrate and the semiconductor light emitting elements. With this structure, the heat from the semiconductor light emitting elements can be transferred to the substrate through the color conversion layer. Further, when SiC containing donor and acceptor is used, blue light emitted from the semiconductor light emitting elements can be converted into white light with high conversion efficiency.

In a preferred embodiment of the present invention, in each of the semiconductor light emitting elements, the n-type semiconductor layer, the active layer and the p-type semiconductor layer are stacked in the mentioned order from the substrate side, and adjacent ones of the semiconductor light emitting elements are separated from each other by a groove extending toward the substrate beyond the n-type semiconductor layers. This structure electrically separates the semiconductor light emitting elements from each other. Therefore, the generation of leakage current between the semiconductor light emitting elements can be properly prevented, which is advantageous for increasing the current for the semiconductor light emitting element array.

In a preferred embodiment of the present invention, an additional p-type semiconductor layer is interposed between the substrate and the n-type semiconductor layers of the respective light emitting elements. With this structure, there exists a boundary surface having a high resistance between the n-type semiconductor layer and the additional p-type semiconductor layer. This is advantageous for suppressing leakage current between the semiconductor light emitting elements.

In a preferred embodiment of the present invention, the semiconductor light emitting element array further comprises metal inter-element wiring for electrically connecting the light emitting elements to each other, where the inter-element wiring is arranged to overlap at least part of the active layer of each light emitting element as viewed in the thickness direction of the substrate. With this structure, the light traveling from the active layer is reflected by the inter-element wiring and sent back toward the substrate. This is advantageous for improving the luminance of the semiconductor light emitting element array.

In a preferred embodiment of the present invention, the semiconductor light emitting element array further comprises a boundary surface which is positioned between the active layer and the substrate. Such a boundary surface may be made between an SiC-containing layer and a buffer layer having a refractive index different from that of SiC. In this embodiment, the distance t between the boundary surface and the active layer of each light emitting element is chosen to satisfy a formula: t=a×(λ/2n)×(1±x), where λ is the wavelength of light emitted from the light emitting elements, a is an integer, n is the refractive index of each n-type semiconductor layer, and 0≦x≦10% (i.e. 0.1). With this arrangement, light from the active layer can be amplified between the active layer and the boundary surface. Accordingly, the luminance of the semiconductor light emitting element array can be further increased.

In a preferred embodiment of the present invention, the semiconductor light emitting element array further comprises a pair of terminals for supplying power to the semiconductor light emitting elements. In this embodiment, all the light emitting elements may be divided into two groups, that is, a first group and a second group. In each group, the semiconductor light emitting elements are connected in series between the two terminals. The forward direction of the light emitting elements belonging to the first group is opposite to that of the light emitting elements belonging to the second group. With this arrangement, it is possible to cause the light emitting element array to produce light continuously (to human eyes) by utilizing an AC power source.

According to a second aspect of the present invention, there is provided an illuminator comprising a semiconductor light emitting element array according to the first aspect of the present invention. Such an illuminator may further comprise a metal supporting member held in contact with the substrate of the light emitting element array, and a fixing base connected to the metal supporting member. The fixing base may be a screw-in base, for example, and configured to receive power from an external power source. The received power is supplied to the light emitting element array through the metal supporting member.

With the above arrangement, heat from the semiconductor light emitting element array can be properly dissipated through the metal supporting member.

Other features and advantages of the present invention will become more apparent from detailed description given below with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2show an example of a semiconductor light emitting element array according to the present invention. The illustrated array A includes a substrate1and a plurality of semiconductor elements Ed which are provided on the substrate1and configured to emit light. As shown inFIG. 1, light is emitted downward from the substrate1of the array A. As viewed in plan, the array A may be square, having a side of about 0.4 to 1.5 mm, for example.

The substrate1, made of SiC, permits the passage of visible light such as red, green, blue and white light, while selectively absorbing ultraviolet rays. In the illustrated example, the substrate1has a thickness of about 200 μm. The lower surface of the substrate1serves as a light emitting surface1athrough which light is to be emitted. In the illustrated example, the light emitting surface1ais a flat, smooth surface. Alternatively, the light emitting surface1amay be made as an irregular surface suitable for enhancing the light emission efficiency. On the substrate1, an SiC color conversion layer11, an n-SiC layer12, a buffer layer13and a p-GaN layer14are stacked.

The SiC color conversion layer11serves to convert blue light emitted from the light emitting elements Ed into white light. The SiC color conversion layer11is mainly composed of SiC, in which donors and acceptors are contained. In the SiC color conversion layer11, the donor serves as an electron provider, whereas the acceptor serves to a hole provider. When the SiC color conversion layer is irradiated with blue light, a radiative recombination occurs between a donor and an acceptor, thereby emitting light. For converting blue light into white light, preferably the donor in the color conversion layer11may be N and the acceptor may be B or Al. The concentration of the donor and the acceptor may preferably be about 1.0×1015to 1.0×1020atoms/cm3. The thickness of the SiC color conversion layer11may be about 20 to 200 μm.

The n-SiC layer12is an n-type semiconductor layer obtained by doping e.g. N into SiC. Though the crystal structure of the SiC color conversion layer11tends to deteriorate due to the inclusion of the donors and the acceptors, the defect can be compensated by the provision of the n-SiC layer12having a better crystal structure than the SiC color conversion layer11. In the illustrated example, the n-SiC layer12may have a thickness of about 2 to 4 μm.

The buffer layer13, for example made of AlGaN, is provided for alleviating lattice defects of SiC and GaN, such as lattice strain and lattice mismatch. Due to the provision of the buffer layer13, the p-GaN layer14, the n-GaN layer2, the active layer3and the p-GaN layer4can be stable on the n-SiC layer12. In the illustrated example, the buffer layer13may have a thickness of about 20 to 200 nm.

The refractive index of AlGaN (used for making the buffer layer13) is about 2.5, whereas the refractive index of SiC (used for making the n-SiC layer12) is about 2.6. The boundary surface12abetween the n-SiC layer12and the AlGaN layer13(having different refractive indices) is more likely to reflect light than a boundary surface between layers of the same material. The distance t between the boundary surface12aand the active layer3is chosen to satisfy the formula t=a×(λ/2n)×(1±x), where λ is the wavelength of the light emitted from the active layer3, a is a positive integer, n is the refractive index of the n-GaN layer2, and x is 0 or a positive number which is no greater than 0.1 (≦10%). In the illustrated example, the wavelength λ is about 460 nm, and the distance t is about 0.92 to 1.84 μm.

The p-GaN layer14is made of a p-type semiconductor obtained by doping e.g. Mg into GaN. In the illustrated example, the p-GaN layer14has a thickness of about 300 nm. The light emitting elements Ed are arranged on the p-GaN layer14.

Each of the light emitting elements Ed is made up of the n-GaN layer2, the active layer3and the p-GaN layer4. About 5 to 50 semiconductor light emitting elements Ed maybe arranged in a matrix.

The n-GaN layer2is made of an n-type semiconductor obtained by doping e.g. Si into GaN. In the illustrated example, the n-GaN layer2includes a thicker portion having a thickness of about 0.6 to 1.34 μm and a thinner portion having a thickness of about 0.3 to 0.67 μm. The thinner portion is provided with an n-side electrode21. The n-side electrode21may be formed by stacking Ti and Al.

The active layer3has a multiple quantum well (MQW) structure including e.g. InGaN, and serves to amplify the light emitted by the recombination of electrons and holes. The active layer3may include a plurality of InGaN layers and a plurality of GaN layers which are alternately stacked. Each InGaN layer may contain about 17% of In and has a band gap which is smaller than that of the n-GaN layer2. Thus, the InGaN layers constitute well layers in the active layer3. The GaN layers, on the other hand, constitute barrier layers in the active layer3. In the illustrated example, the InGaN layers (each having a thickness of about 1.5 to 4.0 nm) and the GaN layers (each having a thickness of about 6 to 20 nm) are stacked so that the overall thickness of the active layer3becomes about 100 nm. To alleviate the lattice defect, a superlattice layer in which InGaN and GaN are alternately stacked for every atom may be provided between the n-GaN layer2and the active layer3.

The p-GaN layer4is made of a p-type semiconductor obtained by doping e.g. Mg into GaN. In the illustrated example, the p-GaN layer4has a thickness of about 50 to 200 nm. On the p-GaN layer4, a p-side electrode41is provided. The p-side electrode41is made of Ni, for example, and covers a right-side portion of the upper surface of the p-GaN layer4, as shown in the figure. It is to be noted that a GaN layer or an InGaN layer containing about 0.1% of In may be provided between the active layer3and the p-GaN layer4.

The p-side electrode41of each semiconductor light emitting element Ed is connected to the n-side electrode21of the adjacent semiconductor light emitting element Ed via an inter-element wiring5. The inter-element wiring5is made of Al or Pt, for example, and has relatively high reflectivity. The inter-element wiring5is formed to bridge between the p-side electrode41, which is formed on the right-side portion of the upper surface of each semiconductor light emitting element Ed, and the n-side electrode21of the light emitting element Ed on the left side of that semiconductor light emitting element Ed. Thus, the portion of the p-GaN layer4, which is not covered by the p-side electrode41, is covered by the inter-element wiring5. The illustrated three semiconductor light emitting elements Ed are connected in series to each other by the inter-element wiring5.

Grooves6are formed between respective adjacent semiconductor light emitting elements Ed. The grooves6are provided for electrically separating the adjacent semiconductor light emitting elements Ed. The bottom6aof each groove6is at a position which is closer to the substrate1beyond the n-GaN layer2. In the illustrated example, each groove6extends downward through the n-GaN layer2and the p-GaN layer14until the bottom6areaches the buffer layer13. The grooves6may be formed by etching.

An insulating film71is provided in each groove6, at the region between each semiconductor light emitting element Ed and the inter-element wiring5and at part of the obverse surface of the light emitting element Ed. The insulating film71is made of SiO2, for example, and transparent to the visible light.

FIG. 2is a schematic view showing the array A. As shown in the figure, the light emitting elements Ed of the array A are divided into two groups Ge1and Ge2. In each of the groups Ge1and Ge2, the light emitting elements Ed are connected in series to each other by the above-described inter-element wiring5. Each group Ge1, Ge2of the light emitting elements is connected to a pair of terminals15provided on the substrate1. The terminals15are utilized for connecting e.g. an AC power source P to the array A. The light emitting elements Ed of the group Ge1are so connected that the forward direction thereof is from the terminal15on the left side to the terminal15on the right side in the figure. On the other hand, the light emitting elements Ed of the group Ge2are so connected that the forward direction thereof is from the terminal15on the right side to the terminal15on the left side in the figure. With this arrangement, the light emitting elements Ed of the group Ge1and those of the group Ge2are alternately turned on when AC voltage is applied from the AC power source P.

FIG. 3shows an example of illuminator using the array A described above. The illustrated electric lamp B includes a screw-in base81, a glass bulb82, metal members83and a semiconductor light emitting element array A.

The base81is utilized for supplying power to the array A and connecting the electric lamp B to a socket, for example. The base81is generally cylindrical and formed with a helical projection. The base81may be of a type that conforms to Japanese Industrial Standards (JIS) E17 or E26.

The glass bulb82is made of glass and permits the passage of light emitted from the array A. As required, the glass bulb82may be colored for adjusting the tone of the light from the array A.

The metal members83are provided for fixing the array A to the base81. The metal members83are made of Cu, for example, and bonded to the substrate1of the array A. The metal members83electrically connect the array A and the base81to each other. The metal members83are connected to the paired terminals15shown inFIG. 2by e.g. a wire (not shown).

In the electric lamp B, the light emitting surface1aof the substrate1is directed toward the top of the glass bulb82. Thus, the light emitted from the surface1apasses through the spherical portion of the glass bulb82as it spreads out.

The function and advantageous features of the array A and the electric lamp B will be described below.

According to the present invention, the light emitted from the light emitting elements Ed is subjected to color conversion at the SiC color conversion layer11held in contact with the substrate1(seeFIG. 1). The heat generated upon the color conversion is readily transmitted to the substrate1which is made of SiC having relatively high heat conductivity. Further, unlike the conventional structure shown inFIG. 4, the light emitting elements Ed are not covered with a thick fluorescent resin layer. Thus, the heat from the light emitting elements Ed is not unduly trapped, which permits application of strong electric current to the array A to increase the luminance. Further, in the electric lamp B, the metal members83are bonded to the substrate1, whereby the heat generated at the array A can be transmitted from the substrate1to the metal members83. This ensures a prolonged life of the electric lamp B even with strong current applied to the lamp.

By the color conversion using the SiC color conversion layer11containing donors and acceptors, the blue light emitted from the light emitting elements Ed can be converted into white or whitish light. The conversion efficiency of this process is considerably higher than that of the conventional color conversion using a fluorescent material. Thus, the array A can emit strong white light. Further, SiC forming the substrate1absorbs ultraviolet rays. Precisely, the substrate1absorbs ultraviolet light emitted from the elements Ed, while allowing the passage of other light such as visible light. (In other words, the substrate1selectively absorbs ultraviolet light.) In this manner, the array A can suppress effects of ultraviolet rays on the human body and is hence suitable for the electric lamp B.

The light emitting elements Ed are separated from each other by the grooves6. The bottom6aof each groove6is positioned closer to the substrate1beyond the n-GaN layer2. With this structure, respective n-GaN layers2of the light emitting elements Ed are completely separated from each other. Further, by the provision of the p-GaN layer14under the n-GaN layer2, a boundary surface having a high resistance is formed between the n-GaN layer2and the p-GaN layer14. With the structure, the light emitting elements Ed are not electrically connected to each other, so that the generation of leakage current can be properly prevented. This is advantageous for increasing the current for the array A.

The light traveling from the active layer3upward in the figure passes through the transparent insulating film71and is reflected downward by the inter-element wiring5. Since the inter-element wiring5has relatively high reflectivity, the attenuation of light due to the reflection can be prevented. This is advantageous for increasing the luminance of the array A. The p-side electrode41, made of e.g. Ni, ensures good ohmic contact with the p-GaN layer4, but its contact surface with the p-GaN layer4tends to darken. In the illustrated example, however, the p-side electrode41is formed on only a limited part of the p-GaN layer4. Therefore, the absorption of light by the darkened contact surface can be suppressed.

By setting the distance t between the active layer3and the boundary surface12ato a value which satisfies the equation described above, the light having a wavelength λ emitted from the active layer3can be amplified by the repetitive reflection in this region having the distance t. This light amplification effect is obtained with respect to the thickness direction of the substrate1, while the light traveling in the in-plane direction of the substrate1is not amplified. Therefore, the light traveling in the thickness direction of the substrate1becomes dominant, and the brightness of the light traveling in the in-plane direction of the substrate1is negligibly small as compared with that of the light in the thickness direction. Therefore, while increasing the luminance of the light emitted from the light emitting surface1a, it is possible to suppress the leakage of light from portions other than the light emitting surface1a.The light amplification effect can be properly exhibited by setting the distance t within +10% or 0.1 of an integer multiple of λ×½n.

By dividing the light emitting elements Ed into two groups Ge1and Ge2whose respective forward directions are opposite from each other, the light emitting elements Ed of the group Ge1and those of the group Ge2can be turned on alternately by the alternating current from the AC power source P. To the naked eye, the light emitting elements Ed of the group Ge1and those of the group. Ge2appear to be turned on simultaneously when the frequency of the AC power source P is 50 Hz or 60 Hz, for example. Therefore, the array A can uniformly illuminate a relatively wide area by utilizing the power supply from the household power source. Further, the electric lamp8is provided with the base81conforming to JIS E17 and E26. Thus, the electric lamp8can be widely used as a replacement for a conventional lamp such as a incandescent lamp.

The semiconductor light emitting element array and the illuminator of the present invention is not limited to the foregoing embodiment. The specific structure of each part of the semiconductor light emitting element array and the illuminator can be varied in various ways.

The n-type semiconductor layer, the active layer, the p-type semiconductor layer and the additional p-type semiconductor layer in the present invention are not limited to those mainly composed of GaN and may be made of other material which is capable of properly emitting light by the recombination of electrons and holes. The color conversion layer containing SiC is suitable for converting light into white light with high conversion efficiency and absorbing ultraviolet rays. However, in the semiconductor light emitting element array according to the present invention, a different kind of color conversion layer may be used or the light from the active layer may be emitted directly without using a color conversion layer. Although light emission by utilizing an AC power source becomes possible by dividing the light emitting elements into two groups of different forward directions, the present invention is not limited to this. The light emitting elements may not be divided into a plurality of groups, and the light emission may be performed by utilizing a DC power source.

The illuminator according to the present invention is not limited to the above-described electric lamp. The illuminator may have a structure suitable for use as a replacement for a general bar-shaped fluorescent lamp. Such a structure can be provided by appropriately changing the shape of the connection portion, for example. Further, the illuminator according to the present invention is not limited to one usable as a replacement for a conventional standardized illuminator. The illuminator according to the present invention may be structured as a special illuminator including a semiconductor light emitting element array of a size capable of covering the entire surface of the illuminator.