JP2014116549A - Multiple quantum well semiconductor light-emitting element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-performance semiconductor light-emitting element with high optical output power, which improves a light extraction efficiency.SOLUTION: The semiconductor light-emitting element comprises: a first semiconductor layer 11 formed on a reflection layer 17; a light-emitting layer 12 including a first quantum well structure formed on the first semiconductor layer and a second quantum well structure formed on the first quantum well structure; and a second semiconductor layer 13 having a conductivity type opposite to that of the first semiconductor layer, formed on the light-emitting layer and defining the surface thereof as a light extraction plane 13A. A band gap of the first quantum well structure is smaller than that of the second quantum well structure, one well layer in the first quantum well structure is defined as a reference well layer of a phase matching position, and an interval between the reference well layer and a reflection plane is determined so as to match phases in the case where directly emitted light from the reference well layer and reflection light emitted from the reference well layer and reflected by the reflection plane are advanced towards the light extraction plane.

Description

  The present invention relates to a semiconductor light emitting device having a light emitting layer having a multiple quantum well (MQW) structure, and more particularly to a light emitting diode (LED) having an MQW light emitting layer.

  In order to improve the light emission efficiency and performance of semiconductor light emitting devices such as light emitting diodes (LEDs) and semiconductor lasers, it is known to employ a multiple quantum well (MQW) structure in the light emitting layer. .

  In addition, a semiconductor light-emitting laminated body grown on a growth substrate (or a temporary substrate) by a vapor phase growth method such as MOCVD (Metal-Organic Chemical Vapor Deposition) is used as a conductive support substrate (or a permanent substrate) via a reflection mirror. An LED (also referred to as a metal bonding (MB) structure or a bonded structure) configured by bonding and then removing the growth substrate is disclosed (for example, Patent Document 1). In such a light emitting element, a reflection mirror is provided on the side opposite to the light extraction surface to improve the output light output to the outside. In addition, it is disclosed that the light output is improved by optimizing the interval between the quantum well (QW) and the reflection mirror (for example, Non-Patent Document 1).

JP 2009-10359 A

Appl. Phys. Lett., Vol. 82, No. 14, pp.2221-2223, 2003

  In the present invention, when a light emitting layer having an MQW structure having a plurality of quantum wells (QW) is used, the distance between each quantum well (QW) and the reflecting mirror is different, so that the light directly emitted from the well layer and the reflected light are reflected. The condition that the phase of the reflected light from the mirror is matched was made paying attention to the point that it differs depending on each quantum well layer.

  In the present invention, even when the MQW light emitting layer is used, the phase of the light directly emitted from the quantum well (QW) and the reflected light from the reflection mirror are matched (that is, strengthened), and the light extraction efficiency is improved. An object of the present invention is to provide a high-performance semiconductor light emitting device having a high optical output and a high optical output.

The light emitting device of the present invention is
A reflective layer having a reflective surface;
A first semiconductor layer formed on the reflective layer;
A first quantum well structure formed on the first semiconductor layer and including a single well layer or a plurality of well layers having the same band gap, and a single quantum well structure formed on the first quantum well structure. A light emitting layer having a multiple quantum well structure having a well layer or a second quantum well structure including a plurality of well layers having the same band gap;
A second semiconductor layer having a conductivity type opposite to that of the first semiconductor layer, the second semiconductor layer being formed on the light emitting layer and having the surface as a light extraction surface;
The band gap of the first quantum well structure is smaller than the band gap of the second quantum well structure, and one well layer of the first quantum well structure is used as a reference well layer at a phase matching position. The distance from the reflection surface is determined so that the phase of the direct emission light from the reference well layer and the reflection light emitted from the reference well layer and reflected by the reflection surface are aligned when traveling toward the light extraction surface. It is characterized by that.

It is sectional drawing which illustrates typically the structure of the light emitting diode (LED) which is embodiment of this invention, and the principle of light extraction efficiency improvement. (A), (b) is sectional drawing which shows the structure of LED which is Example 1, and a band structure, respectively. (A), (b) is sectional drawing which shows the structure and band structure of LED which are Example 2, respectively. It is sectional drawing which shows typically the structure of LED which is Example 4. FIG. FIG. 10 is a cross-sectional view schematically showing a configuration of an LED that is a modification example of Example 4.

  A semiconductor light emitting diode (LED) having a light emitting layer having a multiple quantum well (MQW) structure according to the present invention will be described below. Moreover, although the preferable Example of this invention is described, these may be changed suitably and may be combined. In the following description and the accompanying drawings, substantially the same or equivalent parts will be described with the same reference numerals.

  In the case of a light emitting device that uses an MQW structure layer as a light emitting layer and extracts light by reflecting light emitted from the MQW structure layer, the distance between each well layer and the reflecting surface is different depending on each quantum well layer. The phases of light in the light extraction direction (forward direction) and reflected light are out of phase, canceling each other, and reducing the light intensity. The inventor of the present application has noticed such a point, and the present invention is made by paying attention to the point that the condition that the phase of the light directly emitted from the well layer and the reflected light from the reflection mirror is matched differs depending on each quantum well layer. It is a thing. FIG. 1 is a cross-sectional view schematically illustrating the configuration of a semiconductor light emitting element (light emitting diode: LED) 10 according to an embodiment of the present invention and the principle of improving light extraction efficiency. As shown in FIG. 1, a first semiconductor layer 11, a light emitting layer having a multiple quantum well (MQW) structure (hereinafter referred to as MQW light emitting layer) 12, and a second semiconductor having a conductivity type opposite to that of the first semiconductor layer. The layer 13 is sequentially laminated in this order to form the semiconductor structure layer 15. Further, on the first semiconductor layer 11, a light reflection layer (hereinafter simply referred to as a reflection layer) 17 that reflects light emitted from the MQW light emission layer is provided, and the light emitted from the LED 10 is emitted from the second semiconductor layer 13. Extracted from the surface (also referred to as a light extraction surface) 13A. A first electrode 21 and a second electrode 22 are provided on the first semiconductor layer 11 and the second semiconductor layer 13, respectively.

  The MQW light-emitting layer 12 may generally be composed of a plurality of quantum well (QW) layers, but in the following, it is composed of five quantum well layers (hereinafter also simply referred to as well layers or well layers). Explained as an example. More specifically, the MQW light-emitting layer 12 includes a first quantum well structure including the first well layer group W1 and a second quantum layer including the second well layer group W2 in order from the first semiconductor layer 11 side. Well structures are sequentially stacked. Further, the first well layer group W1 includes a first well layer W11 and a second well layer W12 in order from the first semiconductor layer 11 side, and the second well layer group W2 includes the first semiconductor layer W1. The first well layer W21, the second well layer W22, and the third well layer W23 are sequentially formed from the layer 11 side. A barrier layer (hereinafter also referred to as a barrier layer) BL is provided between the well layers W11, W12, W21, W22, and W23. A barrier layer BL (not shown) is also provided between the first semiconductor layer 11 and the first well layer group W1 and between the second well layer group W2 and the second semiconductor layer 13. However, for ease of explanation and understanding, the case where the first semiconductor layer 11, the second semiconductor layer 13, and the barrier layer BL are made of semiconductor crystals having the same composition will be described. In the following, the first well layer group W1 and the first well layer group W1 including the well layers and the barrier layers BL at both ends are referred to as the first quantum well structure 12A, and the second well layer group W2 and the second well layer group W2 are referred to as a second quantum well structure 12B including the well layers and the barrier layers BL at both ends. For ease of explanation and understanding, a case where the barrier layer BL is formed of a semiconductor layer having the same composition as the first semiconductor layer 11 and the second semiconductor layer 13 will be described as an example. As the barrier layer of the MQW light emitting layer 12, a semiconductor layer having a composition different from that of the first semiconductor layer 11 and the second semiconductor layer 13 can be used.

  The well layers W11 and W12 of the first well layer group W1 are configured to emit light at the same emission wavelength. In other words, the energy (band gap) Eg1 between the hole and electron quantum levels in the well layers W11 and W12 is the same. Similarly, the well layers W21, W22, W23 of the second well layer group W2 are configured to emit light at the same emission wavelength. In other words, the energy between the quantum levels in the well layers W21, W22, and W23 (hereinafter referred to as the quantum well band gap) Eg2 is configured to be the same. The band gap Eg2 of the well layers W21, W22, W23 of the second well layer group W2 is larger than the band gap Eg1 of the well layers W11, W12 of the first well layer group W1 (Eg1 <Eg2). It is configured. That is, the emission wavelengths of the well layers W21, W22, W23 of the second well layer group W2 are shorter than the emission wavelengths of the well layers W11, W12 of the first well layer group W1 (λ1> λ2). It is configured.

[Emission light and phase from each well layer]
As shown in FIG. 1, the band gaps (Eg1) of the well layers W11 and W12 in the direction from the well layers W11 and W12 of the first well layer group W1 to the light extraction surface 13A (hereinafter also referred to as forward direction), respectively. Light (wavelength λ1) L11 and L12 having a wavelength corresponding to is emitted.

  On the other hand, emitted light E2 of wavelength λ2 emitted in the direction from the well layers W21, W22, W23 of the second well layer group W2 toward the reflective layer 17 (hereinafter also referred to as the reverse direction) (that is, E21, E22, respectively). , E23) are partly absorbed in the well layer W12 of the first well layer group W1 and excite the well layer W12. In addition, a part of the light emitted in the reverse direction from the well layers W21, W22, and W23 that is not absorbed by the well layer W12 is absorbed by the well layer W11 and excites the well layer W11. . That is, light from the well layer of the second well layer group W2 is absorbed by the well layer of the first well layer group W1, and the first well layer group W1 is excited to excite the first well layer group W1. Light having the same wavelength as that of the well layer of the layer group W1 is emitted in the same phase. Therefore, as will be described later, even in the light emitting layer having the MQW structure, the phase of light in the forward direction can be matched, and the light extraction efficiency can be increased.

  First, light (wavelength λ1) having a wavelength corresponding to the band gap (Eg1) of the well layer W11 is emitted from the well layer W11. The emitted light (denoted as E11 in the figure) emitted from the well layer W11 toward the reflective layer 17 is reflected by the reflective layer 17 and travels toward the light extraction surface 13A (reflected light R11). The phase of the reflected light R11 is changed by π with respect to the emitted light E11 due to reflection by the reflective layer 17. Further, light (wavelength λ1) having a wavelength corresponding to the band gap (Eg1) of the well layer W12 is emitted from the well layer W12. Emission light in the reverse direction (denoted as E12 in the figure) emitted from the well layer W12 toward the reflection layer 17 is reflected by the reflection layer 17 and travels toward the light extraction surface 13A (reflection light R12). The phase of the reflected light R12 is changed by π with respect to the emitted light E12 due to reflection by the reflective layer 17.

[Distance between the reflective layer and the well layer of the first well layer group W1]
The distance D between the reflective layer 17 and the well layer of the first well layer group W1 is the order of direct emission from one well layer (hereinafter also referred to as a reference well layer) of the first well layer group W1. The direction light and the reflected light emitted from the reference well layer and reflected by the reflection surface (the surface of the reflection layer 17) are determined so as to be strengthened (that is, the phases are aligned) when traveling toward the light extraction surface. Is done.

  As described above, since the reflected light from the reflective layer 17 is shifted in phase by π with respect to the incident light to the reflective layer 17, the forward direct emission light from the well layer of the first well layer group W1. And the condition where the reflected light emitted from the well layer and reflected by the reflective layer 17 is intensified is expressed by the following equation.

2D · n _d = λ (m + 1/2) (1)
Here, _nd is the refractive index of the substance between the reflective layer 17 and the well layer, and m is an integer.

  For example, referring to FIG. 1, the well layer W11 is used as a reference well layer so that the directly emitted light L11 from the well layer W11 and the reflected light R11 from the reflective layer 17 are intensified, or the well layer W12. Can be determined so that the directly emitted light L12 from the well layer W12 and the reflected light R12 from the reflective layer 17 intensify each other.

  The reference well layer can be any one well layer of the first well layer group W1, but the well layer W11 closest to the first semiconductor layer 11 (that is, the reflective layer 17). It is preferable that In the following description, it is assumed that the reflective layer 17 and the well layer W11 are arranged at a distance D that satisfies the above formula (1). More specifically, the interval (distance) D may be an interval based on a specific position in the reference well layer. For example, the interface between the reference well layer and the semiconductor layer adjacent to the reference well layer and the surface of the reflective layer The distance to the (reflecting surface).

Specifically, in the present embodiment, for ease of explanation and understanding, when the barrier layer of the first well layer group W1 is made of a semiconductor having the same composition as the first semiconductor layer 11, that is, the first well. Although the case where the first semiconductor layer 11 having the same composition as that of the barrier layer of the layer group W1 also serves as the barrier layer will be described, the present invention is not limited thereto. That is, the first semiconductor layer 11 may include a barrier layer provided between the well layer W11 and a semiconductor layer having a composition different from that of the barrier layer. Further, the first semiconductor layer 11 may be composed of a plurality of semiconductor layers having different compositions. In such cases, the equivalent refractive index for light emission from the well layer W11 of the plurality of semiconductor layers provided between the well layer W11 and the reflective layer 17 (that is, the refractive index nd weighted by the layer thickness of each layer _). ) Or the optical path length can be obtained, and the conditions under which the light emitted from the well layer W11 and the light reflected by the reflective layer 17 are intensified can be determined. Even when the other well layer (for example, W12) of the first well layer group W1 is used as the reference well layer and the phase matching condition is obtained for the light emitted from the reference well layer, the reference well layer is similarly used. The phase matching condition can be determined by obtaining the optical path length between the reflecting layer and the reflective layer.

  Alternatively, the reflective layer 17 includes a reflective metal layer and a transparent conductive layer provided between the reflective metal layer and the first semiconductor layer, and the surface of the reflective metal layer (that is, the interface between the reflective metal layer and the transparent conductive layer). May be a reflective surface. In this case, in addition to the first semiconductor layer, the light path length in the transparent conductor layer can be taken into consideration to determine the conditions for strengthening the light emitted from the well layer W11 and the light reflected by the reflective metal layer.

[Improvement of light extraction efficiency]
The mechanism for improving the light extraction efficiency of the present invention will be described below. The emission wavelength of the semiconductor light emitting element is determined by the band gap of the well layer used for the light emitting layer. Since the light emitted from the well layer is emitted in all directions, light other than the direction of the light extraction surface is reflected and absorbed. At this time, part of the reflected light is directed in the light extraction direction (forward direction), but the phase of the light in the forward direction and the reflected light changes depending on the distance from the reflection surface. When the phases are aligned, the light is intensified, but when the phases are shifted, the lights cancel each other and the light intensity decreases.

  In the present invention, the distance between the reference well layer and the reflecting surface, that is, the optical path length for the light emitted from the reference well layer is determined so that the phases of the forward light and the reflected light emitted from the reference well layer are aligned. . At this time, with respect to the light emitted from the well layers other than the reference well layer, the phase between the forward light and the reflected light is attributed to the fact that the interval between the well layer and the reflecting surface is different from that of the reference well layer. The light intensity is weakened because it is a condition that shifts and cancels. Therefore, the band gap of the well layer group (that is, the second well layer group W2) on the side far from the reflecting surface is increased, that is, light having a short wavelength is generated, and the reference well layer (or the first well layer) is generated. Absorb and excite the group W1). The light generated thereby is strengthened because the phase is the same as that of the reflected light, and as a result, the light intensity increases. When the wavelength is extremely short, there is a problem that the full width at half maximum of the light is widened. Therefore, the wavelength is preferably in the range of −10 nm from the wavelength of the light from the reference well layer.

  As described above, according to the present invention, the emission wavelength of the well layer group (second quantum well structure) on the side far from the reflecting surface is shortened, and the well on the side close to the reflecting surface by the short wavelength light. By exciting the well layers of the layer group (first quantum well structure), the MQW light emitting layer emits light having a uniform phase toward the reflecting surface. Therefore, it is possible to provide a high-performance semiconductor light emitting device with high light output efficiency, in which the phases of the light traveling in the light extraction direction and the reflected light from the reflecting surface are intensified.

  Example 1 of the present invention will be described below. 2A and 2B are cross-sectional views showing the configuration and band structure of a light-emitting diode (LED) 10 that is Example 1. FIG. The case where the LED 10 is made of a GaN (gallium nitride) based semiconductor will be described, but the crystal system is not limited to this.

  As shown in FIG. 2A, the first semiconductor layer 11 includes a p-GaN layer 11A (layer thickness, 75 nm) and a p-AlGaN layer 11B (layer thickness, 40 nm) formed on the p-GaN layer 11A. ). An MQW light emitting layer 12 is provided on the p-AlGaN layer 11B. As the MQW light emitting layer 12, on the p-AlGaN layer 11B, a first quantum well structure 12A composed of the first well layer group W1 and the well layers of the first well layer group W1 and the barrier layers BL at both ends, The first quantum well structure 12B composed of the well layers of the second well layer group W2 and the second well layer group W2 and the barrier layers BL at both ends are sequentially stacked. The first well layer group W1 is composed of a single well layer W11, the second well layer group W2 is composed of four quantum well layers, and the first, second, It consists of third and fourth well layers W21, W22, W23 and W24. On the MQW light emitting layer 12, an n-GaN layer (layer thickness: 4 μm) 13 is provided as a second semiconductor layer. Further, barrier layers BL are provided between the well layers W11, W21, W22, W23, W24, between the well layer W11 and the p-AlGaN layer 11B, and between the well layer W24 and the n-GaN layer 13. ing.

The MQW light emitting layer 12 has an InGaN / GaN quantum well structure. FIG. 2B is a band diagram of the MQW light-emitting layer 12, in which CB indicates a conduction band and VB indicates a valence band. As shown in FIG. 2B, the well layer W11 is an In _x Ga _1-x N layer having a layer thickness (Lz) of about 4.0 nm, and the first to fourth well layers W21 to W24 are This is an In _y Ga _1-y N layer of about 4.0 nm having the same layer thickness (Lz) as the well layer W11. The In composition (y) of the well layers W21 to W24 is smaller than the In composition (x) of the well layer W11 (that is, y <x). Moreover, all the barrier layers BL in the first quantum well structure 12A and the second quantum well structure 12B have the same composition. That is, the well layers W21 to W24 (second quantum well structure) have the same structure (composition and layer thickness) and the same band gap (Eg2). The band gap (Eg2) of the well layers W21 to W24 is larger than the band gap (Eg1) of the well layer W11 (first quantum well structure). In other words, the light emission wavelength λ2 of the well layers W21 to W24 is configured to be shorter than the light emission wavelength λ1 of the well layer W11 (λ2 <λ1). The barrier layer BL is a GaN layer having a layer thickness of about 5.0 nm, for example. The emission wavelength (λ1) of the well layer W11 is, for example, 445 nm, and the emission wavelength λ2 of the well layers W21 to W24 is, for example, 10 nm shorter.

  That is, the crystal composition of the well layer W11 having the first quantum well structure and the crystal composition of the well layers W21 to W24 having the second quantum well structure are made different from each other, so that the band gap (Eg2) of the well layers W21 to W24 is different. The well layer W11 is configured to be larger than the band gap (Eg1). In other words, the light emission wavelength λ2 of the well layers W21 to W24 is configured to be shorter than the light emission wavelength λ1 of the well layer W11 (λ2 <λ1).

  In addition, a reflective layer 17 that reflects light emitted from the MQW light emitting layer 12 is provided on the p-GaN layer 11A, and light emitted from the LED 10 is extracted from the surface (light extraction surface) 13A of the n-GaN layer 13. The reflective layer 17 includes a transparent conductor (ITO: Indium Tin Oxide) layer 17A formed on the p-GaN layer 11A and a metal (Ag) layer 17B formed on the ITO layer 17A. A p-electrode 21 is formed on the metal (Ag) layer 17B. An n electrode 22 is partially provided on the n-GaN layer 13.

In this case, the thickness (distance) from the well layer W11 of the first quantum well structure to the metal (Ag) layer 17B is a material having a refractive index based on the thickness of the layers included between these layers. Formula (1) can be applied by converting to a combined one, adding these, and calculating the converted D. Specifically, in the case of Example 1, the barrier layers BL, p-AlGaN layer 11B, p-GaN layer 11A, and ITO layer 17A, which are conversion target layers, are made of materials having a reference refractive index. D can be calculated by converting the thickness. In order to convert the layer thickness of the target layer into the thickness of the substance having the reference refractive index, the following formula can be used. That is, when the layer thickness of the target layer is t, the reference refractive index is n _d , and the refractive index of the target layer is n _t , the converted layer thickness D _t of the target layer is expressed by the following equation. .
D _t = t × (n _t / n _d )... Formula (1 ′)

  Note that the first semiconductor layer and the second semiconductor layer may include various semiconductor layers having different compositions and carrier concentrations, such as a carrier block layer and a current diffusion layer, or an undoped layer. Moreover, LED10 of a present Example has what is called a thin film structure (or bonding structure). Accordingly, a support substrate (not shown) may be provided on the metal (Ag) layer 17B side via the bonding metal layer.

  Example 2 of the present invention will be described below. 3A and 3B are cross-sectional views showing the configuration and band structure of a light-emitting diode (LED) 10 that is Example 2. FIG. This embodiment has the same structure as that of the first embodiment except that the first embodiment is different from the first embodiment in the following points.

The light emitting layer 12 has an InGaN / GaN quantum well structure. FIG. 2B is a band diagram of the MQW light emitting layer 12. Specifically, the single well layer W11 of the first quantum well structure 12A is an In _y Ga _1-y N layer having a layer thickness (Lz1) of about 4.0 nm, and the second quantum well structure 12B. The first to fourth well layers W21 to W24 are InGaN layers having the same In composition (x) as the well layer W11, but the layer thickness (Lz2) is about 3.5 nm, which is compared with the well layer W11. (Ie, Lz2 <Lz1).

  That is, the layer thickness of the well layer W11 having the first quantum well structure and the layer thickness of the well layers W21 to W24 having the second quantum well structure are made different from each other, so that the band gap (Eg2) of the well layers W21 to W24 is The well layer W11 is configured to be larger than the band gap (Eg1). In other words, the light emission wavelength λ2 of the well layers W21 to W24 is configured to be shorter than the light emission wavelength λ1 of the well layer W11 (λ2 <λ1).

  Example 3 of the present invention will be described below. As described above, in Example 1, the crystal compositions of the well layer W11 and the well layers W21 to W24 are made different. In Example 2, the thicknesses of the well layer W11 and the well layers W21 to W24 are made different. Thus, the light emission wavelength λ2 of the well layers W21 to W24 is configured to be shorter than the light emission wavelength λ1 of the well layer W11 (λ2 <λ1).

  However, the first embodiment and the second embodiment may be combined. That is, the crystal composition of the well layer of the first quantum well structure and the crystal composition of the well layer of the second quantum well structure are different from each other and / or the layer thickness of the well layer of the first quantum well structure And the thickness of the well layer of the second quantum well structure may be different from each other so that the band gap of the first quantum well structure is smaller than the band gap of the second quantum well structure.

  For example, the well layer W11 and the well layers W21 to W24 are made to have different crystal compositions and layer thicknesses, and the emission wavelength λ2 of the well layers W21 to W24 is shorter than the emission wavelength λ1 of the well layer W11 ( (λ2 <λ1) may be configured.

  FIG. 4 is a cross-sectional view schematically showing a configuration of a light emitting diode (LED) 10 according to the fourth embodiment. Note that the description of the first electrode 21 and the second electrode 22 is omitted.

  As shown in FIG. 4, a barrier layer BL, a first well layer group W <b> 1, and a second well layer group W <b> 2 are sequentially stacked on the p-GaN layer 11 as the MQW light emitting layer 12. As in the case of the first embodiment, the first well layer group W1 includes a single well layer W11, the second well layer group W2 includes four quantum well layers, and the first semiconductor layer 11 includes The first, second, third, and fourth well layers W21, W22, W23, and W24 are sequentially formed from the side. An n-GaN layer 13 is provided on the MQW light emitting layer 12. Further, a barrier layer BL made of GaN is formed between each well layer W11, W21, W22, W23, W24, between the well layer W11 and the p-AlGaN layer 11, and between the well layer W24 and the n-GaN layer 13. Is provided.

The MQW light emitting layer 12 has an InGaN / GaN quantum well structure. More specifically, the well layer W11 is an In _x Ga _1-x N layer, and the first to fourth well layers W21 to W24 are In _y Ga _1-y N having the same layer thickness as the well layer W11. Is a layer. Here, the In composition (y) of the well layers W21 to W24 is smaller than the In composition (x) of the well layer W11 (y <x). That is, the band gap (Eg2) of the well layers W21 to W24 is larger than the band gap (Eg1) of the well layer W11. In other words, the light emission wavelength λ2 of the well layers W21 to W24 is configured to be shorter than the light emission wavelength λ1 of the well layer W11 (λ2 <λ1). In this embodiment, the In _z Ga _1-z N having an intermediate In composition between the barrier layer BL (GaN layer) and the In composition of the well layer W11 is formed on both sides of the well layer W11 which is the reference well layer. The reflection relaxation layers (that is, 0 <z <x) 31 and 32 are provided.

  More specifically, InGaN used for the well layer has a higher refractive index than GaN, and light is reflected at the InGaN / GaN interface. By providing an InGaN layer having an intermediate composition between the well layer and the barrier layer as in this embodiment, reflection at the interface between the well layer and the barrier layer can be reduced.

Specifically, assuming that the refractive index of GaN is n1, the refractive index of InGaN of the well layer is n2, and the refractive index of the reflection relaxation layer is n3, these relationships are n1 <n3 <n2. The reflectance R is
R = (n1−n2) ² / (n1 + n2) ² ... Formula (2)
Can be expressed as

  Further, assuming that the reflectance between the well layer and the barrier layer is R1 (%), the reflectance between the well layer and the reflection relaxation layer is R2 (%), and the reflectance between the reflection relaxation layer and the barrier layer is R3 (%), The relationship is R2 <R1, R3 <R1. The light extraction when the reflection relaxation layer is not provided is (100−R1) [%], and the light extraction when the reflection relaxation layer is provided is (100−R2) × (100−R3) [%]. Can be expressed as

Taking these differences,
(100-R2) × (100-R3)-(100-R1)
= R1− (R2 + R3) + R2 · R3 / 100> 0 Formula (3)
It becomes. Here, in the calculation range from the refractive indexes of GaN and InGaN, R1> R2 + R3. Therefore, it was confirmed that the light extraction efficiency is increased by providing the reflection relaxation layer as in this example.

  That is, since reflection at the interface from the well layers W21 to W24 of the second well layer group is reduced, incident light (excitation light) from the second well layer group to the first well layer group is increased. Can be made. Further, the light extraction efficiency in the forward direction (light extraction direction) from the first well layer group and the light extraction efficiency of the reflected light from the reflection layer can be increased. When the first well layer group is composed of a plurality of well layers, it is preferable to provide a reflection relaxation layer on all of the plurality of well layers and on both sides of each layer. Alternatively, at least one of them may be provided. In this case, the band gap (emission wavelength) is made the same as the other well layers of the first well layer group.

  As shown in FIG. 5, as a modified example of the fourth embodiment, even if the reflection relaxation layers 33 and 34 are provided on all the well layers W21 to W24 of the second well layer group and on both sides of each layer, the reflection reduction effect is obtained. is there. Or you may make it provide regarding at least 1 well layer among these. In this case, as described above, the band gap (emission wavelength) is made the same as the other well layers of the second well layer group. Further, Example 4 and the above modification examples may be combined.

  When the difference in band gap (emission wavelength) between the first well layer group and the second well layer group is obtained by the difference in In composition, that is, the In composition (x) of the well layer W11 is the well layer W21- In the case where it is configured to be larger than the In composition (y) of W24 (y <x), the reflection at the first well layer group (well layer W11) is more than the reflection at the second well layer group. Therefore, it is more effective to provide a reflection relaxation layer in the first well layer group (well layer W11).

  As described above, when the conventional MQW light emitting layer is used, light in the light extraction direction (forward direction) is caused by the fact that the interval between each well layer and the reflecting surface varies depending on each quantum well layer. The reflected light is out of phase and cancels each other, reducing the light intensity. However, according to the present invention, the well layer emission wavelength of the well layer group on the side far from the reflecting surface is shortened, and the well layer of the well layer group on the side close to the reflecting surface is excited by the light having the short wavelength. As a result, emitted light having a uniform phase is emitted from the MQW light emitting layer toward the reflecting surface. Therefore, it is possible to provide a high-performance semiconductor light emitting device with high light output efficiency, in which the phases of the light traveling in the light extraction direction and the reflected light from the reflecting surface are intensified.

  The light emitting diode (LED) has been described as an example, but can be applied to other semiconductor light emitting elements. That is, the present invention can also be applied to a light emitting element configured to have a light emitting layer composed of a multiple quantum well layer and a reflecting surface facing the quantum well layer.

  In addition, the numerical values and materials in the above-described examples, for example, the composition and layer thickness of the semiconductor layer, the number of well layers of the MQW light emitting layer, the emission wavelength, the material of the conductor and the metal layer, the layer thickness, etc. are merely examples, It can be modified appropriately. Further, the above-described embodiments may be appropriately modified and combined.

DESCRIPTION OF SYMBOLS 10 Semiconductor light emitting element 11 1st semiconductor layer 12 Light emitting layer of multiple quantum well (MQW) structure 12A 1st quantum well structure 12B 2nd quantum well structure 13 2nd semiconductor layer 13A Light extraction surface 15 Semiconductor structure layer 17 Light reflecting layer W1 First well layer group W2 Second well layer group

Claims (8)

A reflective layer having a reflective surface;
A first semiconductor layer formed on the reflective layer;
A first quantum well structure formed on the first semiconductor layer and including a single well layer or a plurality of well layers having the same band gap; and a single quantum well structure formed on the first quantum well structure. A light emitting layer having a multiple quantum well structure having one well layer or a second quantum well structure including a plurality of well layers having the same band gap;
A second semiconductor layer having a conductivity type opposite to that of the first semiconductor layer, the second semiconductor layer being formed on the light emitting layer and having the surface as a light extraction surface;
The band gap of the first quantum well structure is smaller than the band gap of the second quantum well structure,
One well layer of the first quantum well structure is used as a reference well layer at a phase matching position, and the distance between the reference well layer and the reflecting surface is determined by direct emission light from the reference well layer and the reference well. A light emitting device characterized in that the phase of the reflected light emitted from the layer and reflected by the reflecting surface is aligned when traveling toward the light extraction surface.   The crystal composition of the well layer of the first quantum well structure and the crystal composition of the well layer of the second quantum well structure are different from each other, and / or the layer thickness of the well layer of the first quantum well structure And the thickness of the well layer of the second quantum well structure are different from each other, whereby the band gap of the first quantum well structure is made smaller than the band gap of the second quantum well structure. The light emitting device according to claim 1, wherein   The first quantum well structure includes a plurality of well layers, and the reference well layer is a well layer closest to the first semiconductor layer among the plurality of well layers. Light emitting element.   The light emitting device according to claim 1, wherein the first quantum well structure includes a single well layer.   5. The semiconductor layer according to claim 1, wherein a semiconductor layer having an intermediate crystal composition of the well layer and the barrier layer is provided between the well layer and the barrier layer of the first quantum well structure. The light emitting element as described in.   6. The semiconductor layer according to claim 1, wherein a semiconductor layer having an intermediate crystal composition of the well layer and the barrier layer is provided between the well layer and the barrier layer of the second quantum well structure. The light emitting element as described in.   The light emitting device according to claim 1, wherein the first quantum well structure and the second quantum well structure are InGaN / GaN quantum well structures.   The reflective layer includes a reflective metal layer and a transparent conductor layer provided between the reflective metal layer and the first semiconductor layer, and the reflective surface is an interface between the reflective metal layer and the transparent conductor layer. The light-emitting element according to claim 1, wherein the light-emitting element is a light-emitting element.

Images