IP, Reports & Roadmaps | Aug 20, 2007

Nitride based LED with a p-type injection region

An LED chip (2) is composed of a p-GaN layer (10), an n-GaN layer (14), and an MQW emission layer (12) that is sandwiched between the GaN layers (10 and 14). Each layer is made of a GaN semiconductor. Light exits the LED chip (2) through the n-GaN layer (14). A p-electrode (16) of the LED chip (2) has a surface profile (24B) defined by a plurality of columnar projections (24A) formed in a uniformly distributed relation on the surface facing toward the p-GaN layer (10). The p-electrode (16) is in contact with the p-GaN layer (10) at the top surface of each projection (24A).

Background of the invention

TECHNICAL FIELD

[0001] The present invention relates to a semiconductor light emitting device, a lighting module, a lighting device, a surface mounting device, and a display device. Especially, the present invention relates to a nitride semiconductor light emitting device having a quantum well emission layer.

BACKGROUND ART

[0002] Gallium nitride (GaN) semiconductors are III-V nitride semiconductors represented by a general formula B.sub.zAl.sub.xGa.sub.1-x-y-zIn.sub.yN.sub.1-v-wAs.sub.vP.sub.w, where 0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, 0.ltoreq.z.ltoreq.1, 0.ltoreq.x+y+z.ltoreq.1, 0.ltoreq.v.ltoreq.1, 0.ltoreq.w.ltoreq.1, 0.ltoreq.v+w.ltoreq.1 (generally denoted as BA1GaInNAsP). A light emitting diode (hereinafter "LED") is one known semiconductor light emitting device having a semiconductor multilayer structure each layer of which is made of a GaN semiconductor material. Such an LED emits light at a wide wavelength region from 200 nm to 1700 nm (from ultra-violet to infra-red), depending on the compositional ratios noted above. Especially, LEDs emitting blue light in a shorter wavelength range than blue-green light are now coming into wide use.

[0003] Ever increasing number of LEDs emitting blue light (blue LEDs) are widely used in electronic devices typified by mobile phones, in addition to white LEDs manufactured with blue LEDs in combination with phosphors. Furthermore, vigorous researches have been underway to use white LEDs for illumination purpose in view of its longevity superior to incandescent and halogen lamps. Currently, white LEDs are promising replacements for existing illumination sources.

[0004] In order for LEDs to be useable for a general illumination purpose, it is essential that the luminous efficiency be further improved. Generally, the luminous efficiency of LED is described by the internal quantum efficiency and the external quantum efficiency. The internal quantum efficiency is the ratio between the electric current injected into an emission layer and the amount of light produced within the emission layer. The internal quantum efficiency is proportional to the ratio of radiative recombination of electrons and positive holes. On the other hand, the external quantum efficiency is the ratio between the injection current and the amount of light extracted from the LED chip. In other words, the external quantum efficiency is the product of the internal quantum efficiency and the ratio of light emitted by the emission layer to light extracted from the LED chip (light extraction efficiency).

[0005] One basic LED has a junction structure of a p-type semiconductor layer, an emission layer, and an n-type semiconductor layer laminated in the stated order. The emission layer emits light in response to a current supplied from an n-electrode and a p-electrode formed on the respective semiconductor layers. It is important that the electrode provided on a light extraction surface does not obstruct light escaping from the LED. For example, when the p-semiconductor layer constitutes the light extraction surface, it is desirable that the p-electrode is provided at a corner of the main surface of the p-semiconductor layer in a manner of occupying a smallest possible area.

[0006] In the case of GaN semiconductor materials, it is generally difficult to manufacture a p-semiconductor layer having low resistance. With the electrodes provided as above are in sufficient to uniformly supply an electric current throughout the entire emission layer. As a result, the light emission takes place in the limited regions of the emission layer, such as directly under and in the vicinity of the electrodes. To address the above problem, one conventional technique provides a layer of transparent electrode on the entire surface of the p-semiconductor layer, and then provides a p-electrode on the transparent electrode (See JP Patent Application Publication No. 2003-110138). By the presence of the transparent electrode, an electric current supplied from the p-electrode spreads throughout the p-semiconductor layer and reaches the emission layer from the entire contacting surface. As a result, the luminance efficiency improves.

[0007] In another attempt made to improve the luminous efficiency, there is disclosed a quantum well structure, i.e. an emission layer that is made as thin as the wavelength of electron wave (See JP Patent Application Publication No. 11-330552). By employing a quantum well structure, the ratio of recombination of electrons and positive holes (radiative recombination) increases, and thus the luminous efficiency further improves.

[0008] Unfortunately, however, GaN based LEDs have the following problem, although LEDs employing a quantum well structure exhibit improved luminous efficiency than that would otherwise be.

[0009] Existing GaN semiconductor materials suffer from piezoelectric effects generated under stress induced due to the property inherent in the materials. The piezoelectric effects obstruct radiative recombination of electrons and holes, thereby decreasing the internal quantum efficiency. The mechanism of decrease will be briefly described below.

[0010] A quantum well structure improves the ratio of radiative recombination within the emission layer by confinement of electrons and positive holes (i.e. carriers) with an energy barrier. The existence probability of carriers in the well layer is obtained by a wave distribution function. The spatial overlap between electrons and positive holes (the probability existence of electrons and positive holes at the same locations) is proportional to the ratio of radiative recombination.

[0011] However, the electric field created by the piezoelectric effect scatters electrons and positive holes away toward mutually opposite ends of the well layer, thereby reducing the spatial overlap between the electrons and positive holes. This spatial separation of electrons and positive holes reduces the ratio of radiative recombination, thereby decreasing the luminous efficiency.

[0012] The piezoelectric effect can be canceled by increasing the carrier density in the well so as to cause the screening effect which compensates the internal electric field. Consequently, the spatial overlap between electrons and positive holes increases, and thus the ratio of radiative recombination increases. As a result, the internal quantum efficiency improves.

[0013] The carrier density increases with the increase of current injected to the emission layer. With the increase of current, however, it is inevitable that more heat is generated to elevate the temperature of LED chip. As a result, various problems are caused, such as deterioration of property of the LED chip itself or of resin normally provided to cover the LED chip.

[0014] In view of the above problems, the present invention aims to provide a semiconductor light emitting device with improved luminous efficiency, while maintaining the injected current within a permissible range. The present invention also aims to provide a lighting module, a lighting device, a surface mounting device, and a display device all of which employs the above semiconductor light emitting device.

 

Summary of the invention

DISCLOSURE OF THE INVENTION

[0015] A semiconductor light emitting device according to the present invention includes: a semiconductor multilayer structure composed of a p-semiconductor layer, a quantum well emission layer, and an n-semiconductor layer each made of a nitride semiconductor and laminated in the stated order, light from the emission layer exiting through the n-semiconductor layer; and a p-electrode facing and in ohmic contact with the p-semiconductor layer. The p-semiconductor layer has an intensive-injection region into which an electric current from the p-electrode is injected more intensively than another region, and the intensive-injection region spans substantially across an entire surface of the p-semiconductor layer. With the stated structure, the electric current from the p-electrode is intensively injected into the p-semiconductor layer. That is to say, the current supplied to the p-electrode is injected to the p-semiconductor layer and then to the quantum well emission layer, with increased density (current density). Accordingly, the current density (carrier density) in the emission layer is increased to cause the screening effect, which cancels out the piezoelectric effect. As a result, the ratio of electron-hole recombination increases, and thus emission light increases. In addition, since the intensive-injection region spans substantially across the entire surface of the p-semiconductor layer, emission light increases substantially uniformly throughout the emission layer. Thus, light emitted by the overall emission layer increases. As a result, the luminous efficiency improves without requiring an increase of the drive current (the total amount of current injected to the emission layer).

[0016] Here, the intensive-injection region may be realized by a contact structure of the p-electrode with the semiconductor layer. In this case, the p-electrode may have, on a surface facing toward the p-semiconductor layer, a plurality of projections or depressions that are distributed substantially uniformly, and the p-electrode may be in contact with the p-semiconductor layer at a top surface thereof. With the stated structure, on supply of a drive current to the p-electrode, the current concentrates at the top surface of the p-electrode, thereby increasing its density (current density). With the increased density, the current is injected to the p-semiconductor layer and sequentially to the emission layer. Consequently, the current density (carrier density) in the emission layer is higher in a region corresponding laterally to the top surface of the p-electrode. In the corresponding region of the emission layer, the screening effect is caused to cancel the piezoelectric effect, thereby increasing the radiative recombination ratio. Since the top surface of the p-electrode spans substantially across the entire surface facing toward the p-semiconductor layer, emission light increases substantially uniformly throughout the emission layer. Thus, light emitted by the overall emission layer increases. Consequently, the luminous efficiency improves without requiring an increase of the drive current (the amount of current injected to the emission layer).

[0017] Alternatively, the intensive-injection region may be realized by a contact structure of the p-semiconductor layer with the p-electrode. In this case, the p-semiconductor layer may have, on a surface facing toward the p-electrode, a plurality of projections or depressions that are distributed uniformly, and the semiconductor multilayer structure may be in contact with the p-electrode at a top surface of the p-semiconductor layer. With the stated structure, on supply of a drive current to the p-electrode, the current is injected from the p-electrode to the semiconductor multilayer structure through the top surface of the p-semiconductor layer. Thus, the current is made to converge at the top surface of the p-semiconductor layer, thereby increasing its density (current density). With the increased density, the injected current is successively injected into the emission layer. As a result, the current density (carrier density) in the emission layer is higher in a region corresponding laterally to the top surface of the p-semiconductor layer. In the region of the emission layer, the screening effect is caused to cancel the piezoelectric effect, thereby increasing the radiative recombination ratio, and thus increasing emission light. Since the top surface of the p-semiconductor layer spans substantially across its entire surface, emission light increases substantially uniformly throughout the emission layer. Thus, light emitted by the overall emission layer increases. Consequently, the luminous efficiency improves without requiring an increase of the drive current (the amount of current injected to the emission layer).

[0018] A lighting module, a lighting device, a surface mounting device, and a display device according to the present invention each employ semiconductor light emitting devices as stated above having high luminous efficiency. Consequently, improved luminous efficiency and/or downsize of the respective modules and devices are achieved.

[0019] Furthermore, owing to its higher light efficiency, the semiconductor light emitting device produces the same level of light output with significantly less heat, in comparison with a conventional device. Consequently, the longevity of the semiconductor light emitting device is increased. Furthermore, since it is possibly to further simplify a heat dissipation mechanism, the lighting device, the surface mounting device, and the display device can be reduced both in size (thickness) and manufacturing cost.

 

Claims

1. A semiconductor light emitting device, comprising: a semiconductor multilayer structure composed of a p-semiconductor layer, a quantum well emission layer, and an n-semiconductor layer each made of a nitride semiconductor and laminated in the stated order, light from the emission layer exiting through the n-semiconductor layer; and a p-electrode facing and in ohmic contact with the p-semiconductor layer, wherein the p-semiconductor layer has an intensive-injection region into which an electric current from the p-electrode is injected more intensively than another region, the intensive-injection region spanning substantially across an entire surface of the p-semiconductor layer.

2. The semiconductor light emitting device according to claim 1, wherein the intensive-injection region is realized by a contact structure of the p-electrode with the p-semiconductor layer.

3. The semiconductor light emitting device according to claim 2, wherein the p-electrode has, on a surface facing toward the p-semiconductor layer, a plurality of projections or depressions that are distributed substantially uniformly, and the p-electrode is in contact with the p-semiconductor layer at a top surface thereof.

4. The semiconductor light emitting device according to claim 3, wherein the p-electrode is made of a metal that reflects light from the emission layer toward the n-semiconductor layer.

5. The semiconductor light emitting device according to claim 4, further comprising an insulator disposed on a recessed surface of the p-electrode to fill a space between the recessed surface and the p-semiconductor layer.

6. The semiconductor light emitting device according to claim 5, wherein the insulator is made of a material transparent to light emitted by the emission layer.

7. The semiconductor light emitting device according to claim 5, wherein the insulator has a substantially same refractive index as a refractive index of the nitride semiconductor forming the p-semiconductor layer.

8. The semiconductor light emitting device according to claim 3, wherein a drive current for driving the semiconductor light emitting device is maintained within such a range that results in an average current density not exceeding 50 A/cm.sup.2, the average current density being calculated by dividing the drive current by an area of a main surface of the emission layer, the p-electrode faces substantially entirely of the main surface of the emission layer, and a ratio between the top and recessed surfaces of the p-electrode is determined so that an electric current flowing through the top surface of the p-electrode measures at least 100 A/cm.sup.2 in current density.

9. The semiconductor light emitting device according to claim 3, wherein the p-semiconductor layer has, on a surface facing toward the p-electrode, a high-defect region in which lattice defects are localized and a low-defect region formed adjacent to the high-defect region, and the p-electrode is in contact with the low-defect region of the p-semiconductor layer.

10. The semiconductor light emitting device according to claim 1, wherein the intensive-injection region is realized by a contact structure of the p-semiconductor layer with the p-electrode.

11. The semiconductor light emitting device according to claim 10, wherein the semiconductor multilayer structure has, on a surface facing toward the p-electrode, a plurality of projections or depressions that are distributed substantially uniformly, and the semiconductor multilayer structure is in contact with the p-electrode at a top surface of the p-semiconductor layer.

12. The semiconductor light emitting device according to claim 11, wherein the p-electrode is made of a metal that reflects light from the emission layer toward the n-semiconductor layer.

13. The semiconductor light emitting device according to claim 11, wherein a recessed surface of the semiconductor multilayer structure is present in the n-semiconductor layer.

14. The semiconductor light emitting device according to claim 11, wherein the semiconductor multilayer structure has, on the surface facing toward the p-electrode, a high-defect region in which lattice defects are localized and a low-defect region formed adjacent to the high-defect region, and the low-defect region is present at the top surface of the semiconductor multilayer structure.

15. The semiconductor light emitting device according to claim 1, further comprising: a base substrate supporting the semiconductor multilayer structure from a direction of the p-semiconductor layer; and a phosphor film disposed on a main surface of the semiconductor multilayer structure facing away from the base substrate, the phosphor film extending across a side surface of the semiconductor multilayer structure to the base substrate.

16. A lighting module comprising: a mounting substrate; and the semiconductor light emitting device as defined in claim 1.

17. A lighting device comprising, as a light source, the lighting module as defined in claim 16.

18. A surface mounting device comprising: a substrate; a semiconductor light emitting device as defined in claim 1, and mounted on the substrate; and a resin molding the semiconductor device.

19. A dot-matrix display device comprising: semiconductor light emitting devices as defined in claim 1 and are arranged in a matrix.

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