Semiconductor light-emitting element

The present invention relates to a semiconductor light-emitting element, such as a light-emitting diode (LED).

A semiconductor light-emitting element is usually manufactured by growing, on a growth substrate, a semiconductor structure layer composed of an n-type semiconductor layer, an active layer, and a p-type semiconductor layer and forming an n-electrode and a p-electrode that apply voltage to the n-type semiconductor layer and the p-type semiconductor layer, respectively.

Patent Literature 1 discloses a semiconductor light-emitting element including an active layer that is laminated on a substrate and has a surface containing a portion whose inclination angle smoothly changes with respect to the substrate as well as a method of manufacturing the semiconductor light-emitting element. Non-patent Literature 1 discloses a light-emitting diode including an active layer of a multiple quantum well structure where an InGaN layer is laminated on another InGaN layer of a nanostructure with a high indium composition.

Patent Literature

Patent Literature 1: Japanese Patent No. 4984119

Non-Patent Literature

Non-Patent Literature 1: Applied Physics Letters 92, 261909 (2008)

Technical Problem

Emission of light by a semiconductor light-emitting element is induced when binding (recombination) of an electron and a hole injected into the element through electrodes occurs in the active layer of the element. The wavelength of the light emitted from the active layer (i.e. light-emitting color) differs depending on the band gap of the semiconductor materials that make up the active layer. For example, a light-emitting element using a nitrite-based semiconductor emits blue light from its active layer.

A light source is required to have color rendering properties for, for example, lighting applications. A light source with high color rendering properties is a type of light source that emits near natural light. To achieve high color rendering properties, it is preferable that light emitted from a light source have a wavelength that substantially covers the entire wavelength of the visible region. For example, light extracted from a light source with high color rendering properties is observed as white light.

In this regard, various methods of using a semiconductor light-emitting element to extract white light have been proposed. In one example method of manufacturing a light-emitting device, a wavelength conversion component, such as a phosphor, is mixed into a sealing resin to seal the element by the sealing resin. For example, in the case of a semiconductor light-emitting element using an active layer that emits blue light, a part of blue light from the active layer is converted into yellow light by a phosphor, and the two types of light are mixed and emitted to the outside. The emitted light as a whole is thus observed as white light. Patent Literature 1 proposes a method of widening the light-emitting wavelength without using a phosphor by forming a protrusion-recess pattern using a processing method, such as etching, prior to active layer growth and forming an inclined surface on the active layer.

However, a light-emitting device manufactured using the aforementioned methods has problems relating to the uniformity of the light-emitting wavelength within the device, the complexity of the manufacturing process, and light-emitting intensity. Possible reasons include: addition of a phosphor mixing step; change of the wavelength conversion efficiency of a phosphor over time, addition of a processing step of a semiconductor layer, and deterioration of crystallinity due to processing of a semiconductor layer.

The present invention has been made in view of the aforementioned problems. An object of the present invention is to provide a semiconductor light-emitting element that does not require a wavelength conversion component, such as a phosphor, and that has a light-emitting wavelength band (spectral width) over a wide range of the visible region and has high color rendering properties and a high light-emitting intensity.

Solution to Problem

A semiconductor light-emitting element according to the present invention includes: a first semiconductor layer of a first conductivity type; a first light-emitting layer formed on the first semiconductor layer; a second light-emitting layer formed on the first light-emitting layer; and a second semiconductor layer that is formed on the second light-emitting layer and is of a conductivity type opposite to the conductivity type of the first semiconductor layer. The first light-emitting layer has: a base layer that has a composition subject to stress strain from the first semiconductor layer and has a plurality of base segments partitioned into a random net shape; and a first quantum well structure layer composed of at least one quantum well layer and at least one barrier layer and formed on the base layer, the first quantum well structure layer retaining segment shapes of the plurality of base segments. The second light-emitting layer has a second quantum well structure layer composed of a plurality of barrier layers that have different compositions from that of the at least one barrier layer of the first quantum well structure layer, and at least one quantum well layer, and the second light-emitting layer has a groove, which retains the segment shape, in a surface of an end barrier layer located closest to the first light-emitting layer among the plurality of barrier layers.

FIG. 1( a ) is a cross-sectional view illustrating a structure of a semiconductor light-emitting element according to a first embodiment, and FIG. 1( b ) is a schematic top plan view of a base layer of a first light-emitting layer.

FIG. 2 is a cross-sectional view illustrating a structure of a light-emitting layer of the semiconductor light-emitting element according to the first embodiment.

FIG. 3 is a cross-sectional view illustrating a structure of a semiconductor light-emitting element according to a first modified example of the first embodiment.

FIG. 4 is a cross-sectional view illustrating a structure of a semiconductor light-emitting element according to a second modified example of the first embodiment.

FIGS. 5( a ) to ( c ) are spectrum diagrams of the semiconductor light-emitting element according to the second modified example of the first embodiment and a semiconductor light-emitting element according to a comparative example.

Embodiments of the present invention will now be described in detail hereinbelow. In this specification, an identical reference numeral is assigned to like constituent elements.

First Embodiment

FIG. 1( a ) is a cross-sectional view illustrating a structure of a semiconductor light-emitting element (hereinafter, simply referred to as “light-emitting element” or “element” in some cases) 10 according to a first embodiment. In the structure of the semiconductor light-emitting element 10 , a semiconductor structure layer SL is formed on a mounting substrate (hereinafter, simply referred to as “substrate” in some cases) 11 . The semiconductor structure layer SL includes an n-type semiconductor layer (first semiconductor layer) 12 formed on the mounting substrate 11 , a light-emitting functional layer 13 formed on the n-type semiconductor layer 12 , an electron blocking layer 14 formed on the light-emitting functional layer 13 , and a p-type semiconductor layer (second semiconductor layer whose conductivity type is opposite to the conductivity type of the first semiconductor layer 12 ) 15 formed on the electron blocking layer 14 .

According to the embodiment, the mounting substrate 11 is composed of a growth substrate used for growth of, for example, the semiconductor structure layer SL, and is made of sapphire, for example. In addition, the semiconductor structure layer SL is composed of a nitride-based semiconductor. The semiconductor light-emitting element 10 can be manufactured by growing the semiconductor structure layer SL on the sapphire substrate, for example, whose C-plane surface serves as a crystal growth surface, using the Metal Organic Chemical Vapor Deposition (MOCVD) method. Although not illustrated in the drawings, the light-emitting element 10 has an n-electrode and a p-electrode that apply voltage to the n-type semiconductor layer 12 and the p-type semiconductor layer 15 , respectively.

The embodiment is illustrated with reference to the structure of the light-emitting element 10 where the semiconductor structure layer SL is formed on a growth substrate serving as the mounting substrate 11 . However, the embodiment is not limited to the structure where the mounting substrate 11 is the growth substrate. For example, the semiconductor light-emitting element 10 may have a structure obtained by first growing the semiconductor structure layer SL on a growth substrate, bonding the semiconductor structure layer SL to another substrate, and removing the growth substrate. In this case, the other substrate thus bonded is formed on the p-type semiconductor layer 15 . The aforementioned bonded substrate may use materials with a high degree of heat dissipation, such as, Si, AlN, Mo, W, and CuW.

Although not illustrated in the drawings, a buffer layer (underlayer) may be provided between the mounting substrate 11 and the n-type semiconductor layer 12 . The buffer layer is, for example, provided for the purpose of mitigating the strain possibly generated on the interface between the growth substrate and the semiconductor structure layer SL as well as on the interface between layers of the semiconductor structure layer SL. In this embodiment, after a GaN layer is grown on a sapphire substrate (mounting substrate 11 ) as a buffer layer, the n-type semiconductor layer 12 is laminated.

The n-type semiconductor layer 12 is composed of, for example, a GaN layer containing an n-type dopant (for example, Si). The electron blocking layer 14 is composed of, for example, an AlGaN layer. The p-type semiconductor layer 15 is composed of, for example, a GaN layer containing a p-type dopant (for example, Mg). The n-type semiconductor layer 12 may include a plurality of n-type semiconductor layers with different dopant concentrations. The electron blocking layer 14 may contain a p-type dopant.

In this embodiment, the first and second n-type semiconductor layers (not illustrated) are formed as the n-type semiconductor layer 12 . Specifically, the first n-type semiconductor layer is formed on the substrate 11 , and the second n-type semiconductor layer whose dopant concentration is smaller than that of the first n-type semiconductor layer is formed on the first n-type semiconductor layer. As the electron blocking layer 14 , an AlGaN layer containing a p-type dopant is formed.

The light-emitting functional layer 13 has first and second light-emitting layers 13 A and 13 B. The first light-emitting layer 13 A is formed on the n-type semiconductor layer 12 , and the second light-emitting layer 13 B is formed on the side of the p-type semiconductor layer 15 (i.e., on the first light-emitting layer 13 A in this embodiment) with respect to the first light-emitting layer 13 A. The electron blocking layer 14 is formed on the second light-emitting layer 13 B. Each of the first and second light-emitting layers 13 A and 13 B has a quantum well (QW) structure.

The first light-emitting layer 13 A has a base layer BL that has a different composition from that of the n-type semiconductor layer 12 . The base layer BL has a groove (hereinafter referred to as “first groove”) GR 1 formed to have a random net shape due to stress strain from the n-type semiconductor layer 12 . The first groove GR 1 has a mesh shape formed by a plurality of combined grooves created by stress (strain) generated in the base layer BL due to the composition difference between the n-type semiconductor layer 12 and the base layer BL. The stress strain generated in the base layer BL is strain of the crystal structure of the base layer BL caused by the lattice constant difference between the n-type semiconductor layer 12 and the base layer BL.

The first light-emitting layer 13 A has a quantum well structure layer (hereinafter referred to as “first quantum well structure layer”) QW 1 composed of a first quantum well layer WA formed on the base layer BL and a first barrier layer BA. The first quantum well layer WA is formed on the base layer BL, and the first barrier layer BA is formed on the first quantum well layer WA. The base layer BL serves as a barrier layer for the first quantum well layer WA. The first quantum well layer WA is formed as a strained quantum well layer.

A description will now be made for the base layer BL with reference to FIG. 1( b ) . FIG. 1( b ) is a diagram schematically illustrating the upper surface of the base layer BL. The base layer BL is partitioned by the first groove GR 1 and has many fine base segments BS formed to have random sizes. Each of the base segments BS is partitioned in a random net shape since the base layer BL has a composition subject to stress strain from the n-type semiconductor layer 12 .

The first groove GR 1 is composed of groove portions with random and different lengths and shapes. The first groove GR 1 is formed in a net (mesh) shape all over the surface of the base layer BL. Each of the base segments BS is a portion (segment) that is randomly partitioned and formed by the first groove GR 1 within the base layer BL. Furthermore, the upper surface of each of the second base segments BS 2 may have a various shape, such as a substantially round shape, a substantially oval shape, and a polygonal shape.

The first groove GR 1 has, for example, a V-shaped cross section ( FIG. 1( a ) ). Also, as illustrated in FIG. 1( b ) , the first groove GR 1 has a line-like bottom portion BP. In this embodiment, an end portion of each of the base segments BS is the bottom portion BP of the first groove GR 1 . Each of the base segments BS adjoins another base segment BS at the bottom portion BP.

Further, the base layer BL has a flat portion (hereinafter referred to as “first flat portion”) FL 1 , which corresponds to each of the base segments BS. The surface of the base layer BL is composed of the first flat portion FL 1 and the inner wall surface of the first groove GR 1 . Each of the first flat portions FL 1 is partitioned by the first groove GR 1 for each base segment BS. The base segment BS has an upper surface composed of the first flat portion FL 1 and side surfaces composed of the inner wall surfaces of the first groove GR 1 .

In other words, the first flat portion FL 1 constitutes the upper surface of each of the base segments BS, and the inner wall surface of the first groove GR 1 constitutes the side surface of the base segment BS. Each of the base segments BS thus has an inclined side surface and has a cross section with, for example, a substantially trapezoidal shape.

With reference to FIG. 1( a ) again, the first light-emitting layer 13 A has, on its surface, a groove (hereinafter referred to as “second groove”) GR 2 that is formed to inherit (i.e. maintain) various the shape of the first groove GR 1 and has the same mesh shape as that of the first groove GR 1 . Specifically, as illustrated in FIG. 1( a ) , the first quantum well layer WA and the first barrier layer BA are formed on the base layer BL with the segmental shape of the base segment BS remained. Thus, the first quantum well layer WA and the first barrier layer BA have a groove at a location corresponding to each groove portion of the first groove GR 1 of the base layer BL. A groove formed in the first barrier layer BA, which is the nearest layer to the p-type semiconductor layer 15 , is the second groove GR 2 .

The portion of the surface of the first light-emitting layer 13 A, i.e. the surface of the first barrier layer BA, other than the second groove GR 2 is formed as a flat portion (hereinafter, referred to as “second flat portion”) FL 2 . Each of the second flat portions FL 2 is formed so that its location and shape correspond to those of each of the first flat portions FL 1 .

In other words, the first light-emitting layer 13 A has the second flat portion FL 2 and the second groove GR 2 on its surface. The second groove GR 2 is formed so as to partition the first light-emitting layer 13 A into a plurality of island-shaped light-emitting segments ES. Each of the light-emitting segments ES is formed so as to correspond to each of the base segments BS. The first light-emitting layer 13 A thus has a plurality of light-emitting segments ES partitioned into a random net shape. The sizes and shapes of the light-emitting segments ES are randomly varied and distributed and randomly disposed (arranged) on the surface of the first light-emitting layer 13 A.

The second light-emitting layer 13 B is formed on the first light-emitting layer 13 A. The second light-emitting layer 13 B has a quantum well structure layer (hereinafter referred to as “second quantum well structure layer”) QW 2 composed of two barrier layers (hereinafter referred to as “second barrier layer”) BB 1 and BB 2 and a quantum well layer (hereinafter referred to as “second quantum well layer”) WB. Each of the second barrier layers BB 1 and BB 2 has a different composition from those of the base layer BL of the first light-emitting layer 13 A and the first barrier layer BA of the first quantum well structure layer QW 1 . In this embodiment, the second barrier layers BB 1 and BB 2 have the same composition as those of the n-type semiconductor layer 12 and the p-type semiconductor layer 15 . The second quantum well layer WB is formed as a strained quantum well layer.

Among the second barrier layers BB 1 and BB 2 , the barrier layer of one end (hereinafter referred to as “first end barrier layer”) BB 1 which is located closest to the first light-emitting layer 13 A has a groove (hereinafter referred to as “third groove”) GR 3 formed to inherit (or maintain) the shape of the second groove GR 2 . The third groove GR 3 has a smaller and deeper groove portion than those of the first and second grooves GR 1 and GR 2 . In other words, on the surface of the first end barrier layer BB 1 , the third groove GR 3 that retains the segment shape of the base segment BS is formed. The first end barrier layer BB 1 has a flat portion (hereinafter referred to as “third flat portion”) FL 3 corresponding to each of the base segments BS.

The second light-emitting layer 13 B has, on its surface, a groove (hereinafter referred to as “fourth groove”) GR 4 formed to inherit the shape of the third groove GR 3 . In other words, the second barrier layer BB 2 and the second quantum well layer WB of the second light-emitting layer 13 B are formed to inherit the shape of the first end barrier layer BB 1 . The fourth groove GR 4 is smaller-sized and shallower than the third groove GR 3 . The portion of the surface of the second light-emitting layer 13 B, i.e. the surface of the second barrier layer BB 2 , other than the fourth groove GR 4 is formed as a flat portion (hereinafter, referred to as “fourth flat portion”) FL 4 .

As described above, in a top plan view of each of the first to fourth grooves GR 1 to GR 4 (in other words, view perpendicular to the crystal growth surface), the grooves have their respective bottom portions at the same location. The segment shape of the light-emitting segment ES formed on the first light-emitting layer 13 A is substantially inherited by the second light-emitting layer 13 B. Thus, in this embodiment, the light-emitting functional layer 13 is entirely partitioned into a plurality of light-emitting segments ES by the first to fourth grooves GR 1 to GR 4 .

FIG. 2 is a cross-sectional view illustrating the structure of the light-emitting functional layer 13 . With reference to FIG. 2 , a description will be given in detail of the first and second light-emitting layers 13 A and 13 B of the light-emitting functional layer 13 . In the first light-emitting layer 13 A, the base layer BL and the first barrier layer BA have a composition of AlN or AlGaN. In this embodiment, an AlN layer is formed as the base layer BL and the first barrier layer BA. The first quantum well layer WA of the first light-emitting layer 13 A has a composition of InGaN. The base layer BL serves as a barrier layer of the first light-emitting layer 13 A.

In the second light-emitting layer 13 B, each of the second barrier layers BB 1 and BB 2 has a composition of GaN. The second quantum well layer WB has a composition of InGaN. In other words, the first and second light-emitting layers 13 A and 13 B, each of which has a quantum well structure, have barrier layers with different compositions. Therefore, the band gap of the second quantum well layer WB is different from that of the first light-emitting layer WA. Here, the band gap of a quantum well structure denotes the energy between the quantum levels of the quantum well layer.

A layer thickness T 1 of the first end barrier layer BB 1 is smaller than the layer thickness of another end barrier layer (hereinafter referred to as “second end barrier layer”) BB 2 , which is located closest to the p-type semiconductor layer 15 of the second barrier layers BB 1 and BB 2 . Specifically, the layer thickness T 1 of the first end barrier layer BB 1 is smaller than a layer thickness T 2 of the second end barrier layer BB 2 . The third groove GR 3 is smaller-sized and shallower than the first and second grooves GR 1 and GR 2 . Within the same light-emitting segment ES, the third flat portion FL 3 is thus larger-sized than the second flat portion FL 2 .

A description will now be made for the first light-emitting layer 13 A. In this embodiment, the base layer BL is composed of an AlN layer. The base segment BS (i.e., the first groove GR 1 ) of the base layer BL can be formed, for example, by growing an AlN layer serving as the base layer BL on the n-type semiconductor layer 12 at a relatively low temperature.

More specifically, when a base layer BL different from the n-type semiconductor layer 12 in terms of crystal composition is grown on the n-type semiconductor layer 12 , stress (strain) is generated in the base layer BL. For example, the base layer BL has a smaller lattice constant than that of the n-type semiconductor layer 12 . For example, when an AlN layer serving as the base layer BL is formed on a GaN layer serving as the n-type semiconductor layer 12 , tensile strain is generated in the AlN layer due to the GaN layer. Thus, tensile stress is generated in the AlN layer during its growth. When the AlN layer is grown on the GaN layer, a groove is formed in the AlN layer at the beginning of or during its growth, and the AlN layer grows in a three-dimensional manner. Accordingly, the AlN layer is grown in a stereoscopic manner, and a plurality of fine protrusions and recesses are formed. The starting point of formation of this groove is the bottom portion BP of the first groove GR 1 .

When the AlN layer is grown on the GaN layer at a low temperature, three-dimensional growth of the AlN layer is promoted. Thus, a large number of groove portions are formed (the first groove GR 1 is formed) on the surface of the AlN layer while being combined with each other, whereby the surface of the AlN layer is partitioned into a plurality of segments. The base layer BL with the plurality of base segments BS can thus be formed. In this embodiment, the AlN layer is formed as the base layer BL at a growth temperature of 780° C.

When an InGaN layer serving as the first quantum well layer WA is formed on the base layer BL, the first quantum well layer WA is formed as a strained quantum well layer. Also, a certain distribution of the In content is generated within the first quantum well layer WA. In other words, the first quantum well layer WA is formed such that the region on the first flat portion FL 1 differs from the region on the first groove GR 1 in terms of In composition. The layer thickness of the first quantum well layer WA on the upper surface of the base segment BS is different from that on the side surface of the base segment BS. Thus, the band gap is not constant within the layer of the first quantum well layer WA.

By forming the first barrier layer BA (AlN layer) that has the second groove GR 2 so as to maintain the shape of the first groove GR 1 , the first light-emitting layer 13 A is formed. Accordingly, due to its random shape and band gap structure, the light-emitting segments ES partitioned in the first light-emitting layer 13 A emit light of various wavelengths. In this embodiment, the first light-emitting layer 13 A emits light whose wavelength widely covers a near green region. As the first light-emitting layer 13 A has fine island-shaped protrusions and recesses, the first light-emitting layer 13 A emits various colors of light. The inventors confirmed that the first light-emitting layer 13 A according to this embodiment emits light whose spectral bandwidth covers a wide wavelength range of approximately 450 to 650 nm.

In this embodiment, the base layer BL has the first flat portion FL 1 , and the surface of the first light-emitting layer 13 A has the second flat portions FL 2 . The first light-emitting layer 13 A has the second flat portion FL 2 on a region of the first flat portion FL 1 , whereby a preferable level of crystallinity is ensured within the first light-emitting layer 13 A.

In this embodiment, the case where the surfaces of the base layer BL and the first light-emitting layer 13 A are composed of the flat portion and the groove was described. However, the surface configurations of the base layer BL and the first light-emitting layer 13 A are not limited to the aforementioned case. For example, the base layer BL may have a curved surface portion on the upper surface of the base segment BS.

As the size of the base segment BS decreases, the amount of In introduced into the quantum well layer WA increases, and the light-emitting wavelength shifts toward the longer wavelength side. Specifically, when an InGaN layer serving as the quantum well layer WA is formed on an AlN layer serving as the base layer BL, the AlN layer exerts compressive stress (compressive strain) on the InGaN layer. When the InGaN layer is subject to compressive strain, In is readily introduced into the first quantum well layer WA. The band gap of the InGaN layer, i.e., the energy between quantum levels, thereby decreases. The first quantum well layer WA emits light with a light-emitting wavelength of the longer wavelength side.

Also, the present inventors examined formation of not a light-emitting layer like the first light-emitting layer 13 A, but of a multiple quantum well structure having a plurality of quantum well layers which have one flat surface and in which the In composition is changed differently from one another. However, the range of the In composition that can be formed is limited. In the case of a light-emitting element having a light-emitting layer of a multiple quantum well structure with In compositions changed, the spectrum with a wavelength band as wide as that of the light-emitting element 10 of this embodiment was not possible. Specifically, light with a constant wavelength in a wide range and a certain level of intensity was not acquired.

Accordingly, light with high color rendering properties could not be acquired by simply increasing the In composition. When a quantum well layer with an excessively large In composition was formed to change the In composition in a wide range, segregation of In was noticeable, and In precipitated and blackened. Also, a portion that does not function as a light-emitting layer was formed. Thus, it can be said that there is a limit in the effort to simultaneously achieve a wider light-emitting spectrum and a higher light-emitting intensity based on the In composition.

In another example examination, the present inventors manufactured a light-emitting element by laminating light-emitting layers that are formed of different materials and have different band gaps. Simply laminating materials of different types to produce a light-emitting layer only produced light whose peak wavelength corresponds to the band gap, and the spectral intensity between the peaks was small. Acquisition of white light was difficult since colors were mixed in an unbalanced and unstable manner. A step of forming light-emitting layers containing different types of materials was added, and the resulting product did not have a preferable level of crystallinity. In this embodiment, on the other hand, by forming a light-emitting functional layer 13 having a first quantum well layer WA of a microstructure, light that has a light-emitting wavelength band (half-value width) over a wide range of the visible region was readily and certainly achieved.

A description will next be made for the second light-emitting layer 13 B. The second light-emitting layer 13 B is formed on the first light-emitting layer 13 A and inherits the shape of the first light-emitting layer 13 A. Specifically, the layer thickness T 1 of the first end barrier layer BB 1 is smaller than a typical layer thickness of the barrier layer (for example, the layer thickness T 2 of the second end barrier layer BB 2 ). Accordingly, the third groove GR 3 corresponding to the second groove GR 2 is formed in the surface of the first end barrier layer BB 1 . Also, the fourth groove GR 4 is formed in the surface of the second light-emitting layer 13 B.

In other words, the second light-emitting layer 13 B inherits the segment shape of the light-emitting segment ES corresponding to the base segment BS. The second light-emitting layer 13 B thus has randomly formed grooves in the surface. Therefore, the spectral width in this case is wider than that in the case of simply forming a flat light-emitting layer.

According to the embodiment, the second barrier layers BB 1 and BB 2 have the same composition as that of the n-type semiconductor layer 12 and have a composition different from that of the first barrier layer BA of the first light-emitting layer 13 A. The tensile strain from the n-type semiconductor layer 12 to the first light-emitting layer 13 A is mitigated by the second light-emitting layer 13 B. The first end barrier layer BB 1 of the second light-emitting layer 13 B inherits the shape of the second groove GR 2 formed in the first barrier layer BA and is formed to partially embed the second GR 2 . The third groove GR 3 formed in the surface of the first end barrier layer BB 1 is formed so as to be shallower and smaller than the second groove GR 2 . By forming the second light-emitting layer 13 B on the first light-emitting layer 13 A, the effect of strain exerted by the n-type semiconductor layer 12 on the entire light-emitting functional layer 13 can be suppressed.

The second barrier layers BB 1 and BB 2 have a different composition from that of the first barrier layer BA of the first light-emitting layer 13 A. Thus, with respect to the first light-emitting layer 13 A and the second light-emitting layer 13 B, the band gap, i.e., the energy between quantum levels, differs between the quantum well layers WA and WB thereof. It is thereby possible to emit light with different light-emitting intensity peaks and a wide spectral width near the peaks. For example, according to this embodiment, the first light-emitting layer 13 A emits light in a wide wavelength bandwidth range in the green region, and the second light-emitting layer 13 B emits light in a wide wavelength bandwidth range in the blue region.

According to the embodiment, each of the second barrier layers BB 1 and BB 2 has a GaN composition, and the second quantum well layer WB has an InGaN composition. The present inventors confirmed that the second light-emitting layer 13 B thus configured emits light whose spectral width ranges from 420 to 450 nm. On the other hand, when a flat light-emitting layer was simply formed, the light-emitting layer emitted light with an intensity peak in the blue region (around 420 nm) and emitted light with an extremely small intensity in the neighborhood of the aforementioned wavelength region.

According to the embodiment, the case where the fourth groove GR 4 is formed in the surface of the second light-emitting layer 13 B is described. However, the embodiment is not limited to the case where the second light-emitting layer 13 B has, on the surface thereof, the fourth groove GR 4 . In the first end barrier layer BB 1 of the second light-emitting layer 13 , a groove that retains the segment shape of the base segment BS (third groove GR 3 ) is desirably formed. For example, the second quantum well layer WB and the second end barrier layer BB 2 may have a flat-shaped surface. For example, a flat surface can be formed by, for example, increasing the layer thicknesses of the second quantum well layer WB and the second end barrier layer BB 2 .

As one example embodiment, the present inventors formed a light-emitting functional layer 13 with the following layer thicknesses: The base layer BL has a layer thickness of 4 nm. The first quantum well layer WA has a thickness of 3 nm. The first barrier layer BA has a layer thickness of 4 nm. The first end barrier layer BB 1 has the layer thickness T 1 of 3 nm or 6 nm. The second quantum well layer WB has a layer thickness of 3 nm. The second end barrier layer BB 2 has a layer thickness of 12 nm. The size of the base segment BS, i.e., the size of the light-emitting segment ES in the in-plane direction, ranges from several tens nm to several μm.

In the embodiment, an n-GaN layer serving as the n-type semiconductor layer 12 was formed at a growth temperature of 1130° C. Also, the first light-emitting layer 13 A was formed at a growth temperature of 780° C., and the second light-emitting layer 13 B was formed at a growth temperature of 780° C. Further, an AlGaN layer serving as the electron blocking layer 14 was formed at a growth temperature of 980° C., and a p-GaN layer serving as the p-type semiconductor layer 15 was formed at a growth temperature of 1020° C.

In general, electrons and holes recombine in an area of the light-emitting functional layer 13 close to the p-type semiconductor layer 15 . Specifically, since the mobility of an electron is higher than that of a hole, the electrons and the holes are more likely to recombine on the side where the holes are injected, i.e., on the side closer to the p-type semiconductor layer 15 . Thus, light with a relatively high light-emitting intensity can be obtained from the second light-emitting layer 13 B, which is close to the p-type semiconductor layer 15 . A lower light-emitting intensity of light from the first light-emitting layer 13 A is of concern.

However, in the embodiment, the aforementioned concern can be addressed by decreasing the layer thickness T 1 of the first end barrier layer BB 1 to a relatively small value. Specifically, by decreasing the layer thickness T 1 , the likelihood increases that holes are injected to the first light-emitting layer 13 A through the first end barrier layer BB 1 . Accordingly, the first light-emitting layer 13 A can emit light with a high intensity.

First Modified Example

FIG. 3 is a cross-sectional view illustrating a structure of a semiconductor light-emitting element 30 according to a first modified example of the first embodiment. Except for the structure of a light-emitting functional layer 33 , the configuration of the light-emitting element 30 is the same as that of the light-emitting element 10 . The light-emitting functional layer 33 of the light-emitting element 30 has first and second light-emitting layers 33 A and 33 B which have a multiple quantum well (MQW) structure. The first light-emitting layer 33 A has, for example, two first quantum well layer WA, and the second light-emitting layer 33 B has two second quantum well layers WB.

Specifically, on the base layer BL, the first light-emitting layer 33 A has a quantum well structure layer QW 1 of a multiple quantum well structure where two first quantum well layers WA and two first barrier layers BA are alternately laminated. The second light-emitting layer 33 B has a quantum well structure layer QW 2 of a multiple quantum well structure where two second quantum well layers WB are formed so as to be held between three second barrier layers BB 1 , BB 2 , and BB 3 . The configuration of the second barrier layer BB 3 is the same as that of the second barrier layer BB 2 .

In this modified example, the second barrier layer BB 1 , which is located closest to the n-type semiconductor layer 12 (the first light-emitting layer 33 A) among the second barrier layers BB 1 to BB 3 of the second light-emitting layer 33 B, is a first end barrier layer. The second barrier layer BB 3 , which is located closest to the p-type semiconductor layer 15 , is a second end barrier layer.

In this modified example, the first and second light-emitting layers 33 A and 33 B have a multiple quantum well structure. The wavelength bandwidth of light emitted from the light-emitting layer 33 is thus widened. More specifically, the number of spectral peaks of the wavelength of light emitted from the light-emitting functional layer 33 increases. In this modified example, a spectrum with four peaks can be achieved. Accordingly, the advantageous effect of widening of the light-emitting wavelength range and an increase in the intensity thereof is further enhanced.

In this modified example, the case where both the first and second light-emitting layers 33 A and 33 B have a multiple quantum well structure has been illustrated. However, both the first and second light-emitting layers 33 A and 33 B do not have to have a multiple quantum well structure. For example, the first light-emitting layer may have a multiple quantum well structure as in the case of the first light-emitting layer 33 A of this modified example, and the second light-emitting layer may have a single quantum well structure as in the case of the first light-emitting layer 13 B of the first embodiment. Further, the number of layers of each quantum well layer may be three or greater.

In other words, the first light-emitting layer may have a quantum well structure where at least one first quantum well layer and at least one first barrier layer are laminated on the base layer BL. In addition, the second light-emitting layer may have a quantum well structure where at least one second quantum well layer is laminated so as to be held between a plurality of second barrier layers. Of the plurality of second barrier layers of the second light-emitting layer, the first end barrier layer BB 1 , which is located closest to the n-type semiconductor layer 12 , have the third groove GR 3 corresponding to the first and second grooves GR 1 and GR 2 of the first light-emitting layer.

As illustrated in FIG. 3 , in this modified example, a groove is not formed in the surface of the second light-emitting layer 33 B. The entire surface is configured to be the flat portion FL 4 . In other words, the second light-emitting layer 33 B is formed by embedding, in the surface thereof, the first and second grooves GR 1 and GR 2 formed in the first light-emitting layer 33 A. The reason is that among the second barrier layers BB 1 , BB 2 , and BB 3 of the second light-emitting layer 33 B, the second barrier layers BB 2 and BB 3 other than the first end barrier layer BB 1 have a relatively large layer thickness.

Second Modified Example

FIG. 4 is a cross-sectional view illustrating a structure of a semiconductor light-emitting element 50 according to a second modified example of the first embodiment. Except for the structure of the light-emitting functional layer 53 , the configuration of the light-emitting element 50 is the same as that of the light-emitting element 10 or 30 . Between the n-type semiconductor layer 12 and the first light-emitting layer 13 A of the light-emitting element 10 , the light-emitting functional layer 53 of the light-emitting element 50 has a third light-emitting layer 53 A having a third quantum well structure layer QW 3 of a multiple quantum well structure composed of at least one third quantum well layer WC and a plurality of third barrier layers BC.

In this modified example, in the structure of the third light-emitting layer 53 A, two third quantum well layers WC and three third barrier layers BC are alternately laminated on the n-type semiconductor layer 13 . On the third barrier layer BC, which is closest to the p-type semiconductor layer 15 , the first light-emitting layer 13 A (base layer BL) is formed. The composition of each of the third quantum well layers WC is the same as that of the second quantum well layer WB, for example, an InGaN composition. The composition of each of the third barrier layers BC has the same as those of the second barrier layers BB 1 to BB 3 , for example, a GaN composition. In other words, the third light-emitting layer 53 A has the same composition as that of, for example, the second light-emitting layer 13 B or 33 B.

In the configuration of this modified example, the third quantum well structure layer QW 3 is added to the side of the n-type semiconductor layer 12 of the light-emitting functional layer 13 of the light-emitting element 10 of the first embodiment. Compared with the first embodiment, additional light with a light-emitting wavelength peak in a pure blue region can be readily emitted. The configuration according to this modified example is advantageous for, for example, increasing the light intensity in the blue region.

FIGS. 5( a ) to 5( c ) illustrate a spectrum obtained from the light-emitting element 50 according to this modified example. FIGS. 5( a ) to 5( c ) illustrate the spectra obtained from the element 50 of the second modified example of the first embodiment and the spectra of a comparative example. For the comparative example, a light-emitting element was prepared so that this light-emitting element differs from the light-emitting element 50 in that the former does not have the second light-emitting layer 13 B. Schematically speaking, in this modified example, blue light is emitted from both the second light-emitting layer 13 B and the third light-emitting layer 53 A. In the comparative example, blue light is emitted only from the third light-emitting layer 53 A.

FIG. 5( a ) is a view of spectra standardized by the intensity of emitted blue light. The compositions are the same in this modified example, but the spectrum of the blue region is shifted toward the longer wavelength side. FIG. 5( b ) is a view of spectra standardized by the light-emitting intensity of the element of the comparative example. Comparison of the light-emitting intensity of the comparative example and this modified example illustrates that the intensity of the blue component of this modified example is higher. FIG. 5( c ) is a view of spectra which are standardized by the light-emitting intensity of the element of the comparative example and whose peak locations are identical. As illustrated in FIG. 5( c ) , the half-value width of this modified example is wider than that of the comparative example. It was confirmed on the basis of the foregoing results that the color rendering properties of the white color element of this modified example become greater than the color rendering properties of the comparative example.

This modified example can be combined with the first modified example. More specifically, the third light-emitting layer 53 A can be further added to the light-emitting element 30 , which has the first and second light-emitting layers 33 A and 33 B of a multiple quantum well structure. By combining the first and second modified examples, the spectrum can be adjusted at a high degree of freedom. An element applicable to various uses can be produced.

The case of forming the electron blocking layer 14 between the light-emitting functional layer 13 (or 33 and 53 ) and the p-type semiconductor layer 15 has been illustrated in this embodiment and the modified examples thereof. However, this embodiment and the modified examples thereof are not limited to the case of providing the electron blocking layer 14 . For example, the p-type semiconductor layer 15 may be formed on the light-emitting functional layer 13 . Also, the electron blocking layer 14 has a larger band gap than those of the n-type semiconductor layer 12 , the light-emitting functional layer 13 , and the p-type semiconductor layer 15 . Accordingly, electrons can be prevented from overflowing to the side of the p-type semiconductor layer 15 through the light-emitting functional layer 13 . Thus, it is preferable to dispose the electron blocking layer 14 for large-current driving and high-temperature operation.

In this embodiment and the modified examples thereof, the first light-emitting layer 13 A has: the base layer BL that has a composition subject to stress strain from the n-type semiconductor layer 12 and has a plurality of base segments BS that are partitioned into a random net shape; and the first quantum well structure layer QW 1 composed of at least one quantum well layer WA formed on the base layer BL and at least one barrier layer BA, wherein the at least one quantum well layer WA retains the segment shapes of the plurality of base segments BS. The second light-emitting layer 13 B has the second quantum well structure layer QW 2 composed of a plurality of barrier layers BB 1 and BB 2 whose compositions differ from that of the barrier layer BA of the first quantum well structure layer QW 1 , and at least one quantum well layer WB. In the surface of the end barrier layer BB 1 , which is located closest to the first light-emitting layer 13 A among the plurality of barrier layers BB 1 and BB 2 , the second light-emitting layer 13 B has the groove GR 3 that retains the segment shape. Accordingly, it is possible to provide a light-emitting element that can emit light with a high light-emitting intensity over a wide range of the visible region.

In this embodiment, the case where the first conductivity type is an n-conductivity type and the second conductivity type is a p-conductivity type, which is opposite to the n-conductivity type. However, the first conductivity type may be a p-type, and the second conductivity type may be an n-type.

10 , 30 , 50 semiconductor light-emitting element

12 n-type semiconductor layer (first semiconductor layer)

13 , 33 , 53 light-emitting functional layer

13 A, 33 A first light-emitting layer

13 B, 33 B second light-emitting layer

53 A third light-emitting layer

QW 1 , QW 2 , QW 3 quantum well structure layer

14 electron blocking layer

15 p-type semiconductor layer (second semiconductor layer)

BL base layer

BS base segment

ES light-emitting segment

GR 1 to GR 4 first to fourth grooves