JP2018125453A - Vertical resonator type light-emitting element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a vertical resonator type light-emitting element capable of highly efficiently recoupling carriers in an active layer and having a low oscillation threshold value.SOLUTION: A vertical resonator type light-emitting element comprises a semiconductor structure layer 13 including a first semiconductor layer 13A having a first conductivity type, an active layer 13B, and a second semiconductor layer 13D having a second conductivity type opposed to the first conductivity type, and a first reflecting mirror and a second reflecting mirror facing to each other via the semiconductor structure layer 13. The active layer 13B has a multiquantum well structure composed of a plurality of well layers W1 to W4 and a plurality of barrier layers B1 to B4. At least one barrier layer among the plurality of barrier layers B1 to B4 comprises a first sub-barrier layer BA, a second sub-barrier layer BB formed on the first sub-barrier layer and having a smaller bandgap than the first sub-barrier layer BA, and a third sub-barrier layer BC formed on the second sub-barrier layer BB and having a larger bandgap than the second sub-barrier layer BB.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a vertical cavity surface emitting laser such as a vertical cavity surface emitting laser (VCSEL).

  A vertical cavity surface emitting laser (hereinafter simply referred to as a surface emitting laser) is a semiconductor laser having a structure in which light is resonated perpendicularly to a substrate surface and light is emitted in a direction perpendicular to the substrate surface. . For example, Patent Document 1 discloses a surface emitting laser having a nitride semiconductor layer and a reflecting mirror made of a dielectric material.

Japanese Patent No. 5707742

  For example, a vertical cavity light emitting element such as a surface emitting laser has reflecting mirrors that face each other with an active layer interposed therebetween, and the reflecting mirror constitutes a resonator. Then, the light emitted from the active layer is resonated (laser oscillation) in the resonator, and the resonated light is extracted outside. In order to lower the oscillation threshold of the surface emitting laser, it is preferable that the carriers injected into the element are recombined with high efficiency in the active layer.

  The present invention has been made in view of the above points, and an object of the present invention is to provide a vertical cavity light emitting device having a low oscillation threshold in which carriers are recombined with high efficiency in an active layer.

  A vertical cavity light emitting device according to the present invention includes a first semiconductor layer having a first conductivity type, an active layer, and a second semiconductor layer having a second conductivity type opposite to the first conductivity type. A multi-quantum well structure comprising a plurality of well layers and a plurality of barrier layers, wherein the active layer includes a plurality of well layers and a plurality of barrier layers. And at least one of the plurality of barrier layers has a first sub-barrier layer and a bandgap formed on the first sub-barrier layer and smaller than the first sub-barrier layer. It is characterized by having a second sub-barrier layer and a third sub-barrier layer formed on the second sub-barrier layer and having a larger band gap than the second sub-barrier layer.

1 is a cross-sectional view of a surface emitting laser according to Example 1. FIG. (A) is sectional drawing of the active layer of the surface emitting laser which concerns on Example 1, (b) is a band figure of the semiconductor structure layer of the surface emitting laser which concerns on Example 1. FIG. FIG. 3 is a diagram showing light output from each well layer of the surface emitting laser according to Example 1. 6 is a cross-sectional view of a surface emitting laser according to Example 2. FIG. (A) is sectional drawing of the active layer of the surface emitting laser which concerns on Example 2, (b) is a band figure of the semiconductor structure layer of the surface emitting laser which concerns on Example 2. FIG.

  Examples of the present invention will be described in detail below. In the following examples, a surface emitting laser (semiconductor laser) will be described. However, the present invention can be applied not only to a surface emitting laser but also to a vertical cavity light emitting element.

  FIG. 1 is a vertical cavity surface emitting laser (VCSEL: Vertical Emitting Laser, hereinafter referred to as a surface emitting laser) according to a first embodiment. The surface emitting laser 10 includes first and second reflecting mirrors 12 and 14 that are disposed to face each other with a semiconductor structure layer 13 including an active layer 13B interposed therebetween.

  The surface emitting laser 10 has a structure in which a first reflecting mirror 12, a semiconductor structure layer 13, and a second reflecting mirror 14 are laminated on a substrate 11. Specifically, a base layer (not shown) is formed on the substrate 11, and the first reflecting mirror 12 is formed on the base layer. A semiconductor structure layer 13 is formed on the first reflecting mirror 12, and a second reflecting mirror 14 is formed on the semiconductor structure layer 13. In this embodiment, the substrate 11 is a GaN substrate. The underlayer has a GaN composition.

  First, the first and second reflecting mirrors 12 and 14 will be described. In this embodiment, the first reflecting mirror 12 is a semiconductor multilayer in which a low refractive index semiconductor layer L1 and a high refractive index semiconductor layer H1 having a refractive index larger than that of the low refractive index semiconductor layer L1 are alternately stacked a plurality of times. It is a film reflector. In this embodiment, the low refractive index semiconductor layer L1 is an InAlN layer. The high refractive index semiconductor layer H1 has a structure in which a GaN layer, an AlGaN layer, and a GaN layer are stacked.

In the present embodiment, the second reflecting mirror 14 is formed by alternately laminating a low refractive dielectric layer L2 and a high refractive dielectric layer H2 having a refractive index larger than that of the low refractive dielectric layer L2. This is a dielectric multilayer film reflecting mirror. In this embodiment, the low refractive index dielectric layer L2 is made of an SiO ₂ layer, and the high refractive index dielectric layer H2 is made of an Nb ₂ O ₅ layer.

  In other words, in the present embodiment, the first reflecting mirror 12 is a distributed Bragg reflector (DBR) made of a semiconductor material, and the second reflecting mirror 14 is a distributed Bragg reflection made of a dielectric material. It is a vessel.

Next, the semiconductor structure layer 13 will be described. In this embodiment, the semiconductor structure layer 13 includes an n-type semiconductor layer (first semiconductor layer having a first conductivity type) 13A, an active layer 13B, an electron block layer 13C, and a p-type semiconductor layer (first semiconductor layer). And a second semiconductor layer 13D having a second conductivity type opposite to the first conductivity type) 13D. In the present embodiment, the semiconductor structure layer 13 has a composition of Al _x In _y Ga _1-xy N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1).

  The surface-emitting laser 10 includes an n-electrode (first electrode) 15 connected to the n-type semiconductor layer 13A of the semiconductor structure layer 13, and a p-electrode (second electrode) 16 connected to the p-type semiconductor layer 13D. Have The n electrode 16 is formed on the n-type semiconductor layer 13A. The p electrode 16 is formed on the p-type semiconductor layer 13D.

  Specifically, in this embodiment, the semiconductor structure layer 13 has a portion from which the p-type semiconductor layer 13D, the electron block layer 13C, and the active layer 13B are partially removed, and n exposed at the removal portion. An n-electrode 15 is formed on the upper surface of the type semiconductor layer 13A.

  Further, an insulating film 17 is formed on the semiconductor structure layer 13 so as to cover the side surface and top surface of the semiconductor structure layer 13 and to have an opening (first opening) exposing a part of the p-type semiconductor layer 13D. Yes. The insulating film 17 functions as a current confinement layer.

  The p-electrode 16 is formed on the insulating film 17 by filling the opening of the insulating film 17, and has a translucent electrode 16 </ b> A that is in contact with the p-type semiconductor layer 13 </ b> D exposed from the opening. The p-electrode 16 has a connection electrode 16B formed on the translucent electrode 16A and having an opening (second opening) in a region corresponding to the opening (first opening) of the insulating film 17. .

  In the present embodiment, the second reflecting mirror 14 is formed in a region on the opening (second opening) of the connection electrode 16B on the translucent electrode 16A of the p-electrode 16. Accordingly, the connection electrode 16B is formed so as to surround the second reflecting mirror 14 on the translucent electrode 16A. The second reflecting mirror 14 faces the first reflecting mirror 12 with the translucent electrode 16A and the semiconductor structure layer 13 interposed therebetween.

  An outline of the light emission operation of the surface emitting laser 10 will be described with reference to FIG. First, in the surface emitting laser 10, the first and second reflecting mirrors 12 and 14 facing each other constitute a resonator. The light emitted from the semiconductor structure layer 13 (active layer 13B) is repeatedly reflected between the first and second reflecting mirrors 12 and 14 to reach a resonance state (laser oscillation is performed). Further, a part of the resonance light passes through the second reflecting mirror 14 and is extracted outside. In this way, the surface emitting laser 10 emits light in a direction perpendicular to the substrate 11.

  FIG. 2A is a cross-sectional view showing the structure of the active layer 13 </ b> B of the surface emitting laser 10. The active layer 13B includes a plurality of quantum well layers (hereinafter simply referred to as well layers) W1 to W4 and a plurality of barrier layers B1 to B5 having a larger band gap than the well layers W1 to W4.

  In this embodiment, barrier layers B1 to B4 and well layers W1 to W4 are alternately stacked on the n-type semiconductor layer 13A, and a barrier layer B5 is formed on the well layer W4. An electron block layer 13C is formed on the barrier layer B5. In other words, in the present embodiment, the active layer 13B has a multiple quantum well (MQW) structure having four well layers W1 to W4.

  In the present embodiment, each of the barrier layers B1 to B4 has a first sub-barrier layer BA and a band gap that is formed on the first barrier layer BA and is smaller than the first barrier layer BA. A second sub-barrier layer BB and a third sub-barrier layer BC formed on the second sub-barrier layer BB and having a larger band gap than the second sub-barrier layer BB are included. The first sub-barrier layer BA is a sub-barrier layer closer to the n-type semiconductor layer 13A than the second sub-barrier layer BB, and the third sub-barrier layer BC is an electron blocking layer than the second sub-barrier layer BB. It is a sub-barrier layer on the 13C side.

  FIG. 2B is a band diagram of the semiconductor structure layer 13. In this embodiment, the n-type semiconductor layer 13A and the p-type semiconductor layer 13D are composed of GaN layers. The electron block layer 13C is made of an AlGaN layer.

  In this embodiment, each of the well layers W1 to W4 is composed of an InGaN layer. The first and third sub-barrier layers BA and BC are made of GaN layers. The second sub-barrier layer BB is an InGaN layer having a smaller In composition than the well layers W1 to W4. The barrier layer B5 is made of a GaN layer.

  Therefore, each layer of the semiconductor structure layer 13 has a band gap as shown in FIG. In the present embodiment, the first and third sub-barrier layers BA and BC have a layer thickness of 1 nm. The second sub-barrier layer BB has a layer thickness of 4 nm. Each of the well layers W1 to W4 has a layer thickness of 3 nm.

  Here, the barrier layers B1 to B5 will be described. Since the active layer 13B has the barrier layers B1 to B5, the Fermi level between the well layers W1 to W4 in the active layer 13B is made uniform. Thereby, the output of light emitted from each of the well layers W1 to W4 is made uniform. In addition, carriers (electrons and holes) injected into the well layers W1 to W4 are recombined with high efficiency, and light is emitted from each with high efficiency.

  Specifically, each of the barrier layers B1 to B4 includes the first and third sub-barrier layers BA and BC in which the second sub-barrier layer BB having a relatively small band gap has a larger band gap. It has a sandwiched structure. Therefore, a certain amount of carriers (electrons or holes) are trapped in the second sub-barrier layer BB. As a result, there is a high possibility that carriers remain in the active layer 13B.

  In addition, each of the barrier layers B1 to B4 (first to third sub-barrier layers BA to BC) functions as a barrier layer (tunnel barrier layer) that causes a tunnel effect to carriers injected into the active layer 13B. . That is, some of the carriers injected (reached) into each of the well layers W1 to W4 can reach the other well layers beyond the barrier layers B1 to B4. Therefore, the degree of recombination of carriers (light intensity) is made uniform between the well layers W1 to W4.

  FIG. 3 is a diagram showing the light output from the active layer 13B and the well layers W1 to W4. The horizontal axis in FIG. 3 indicates the wavelength, and the vertical axis indicates the optical output. In FIG. 3, four curves C1a, C1b, C1c, and C1d indicate the light outputs (optical gain) from each of the well layers W1 to W4. Curve C2 represents the light output (optical gain) from the entire active layer 13B. A curve C3 indicates the light output from the surface emitting laser 10.

  As shown by the curves C1a to C1d in FIG. 3, the intensity and wavelength band of emitted light from each of the well layers W1 to W4 are substantially the same. This is considered due to the fact that the Fermi levels of the well layers W1 to W4 are made uniform by the barrier layers B1 to B4.

  Therefore, as shown by the curve C2, the light emitted from the entire active layer 13B is maximized in a predetermined wavelength band (in the range of about 445 nm to 450 nm in the example of FIG. 3). Since the surface emitting laser 10 has the active layer 13B, most of the emitted light oscillates. Therefore, the oscillation threshold can be reduced. Therefore, it is possible to provide the surface emitting laser 10 having carriers that recombine with high efficiency in the active layer 13B and have a low oscillation threshold.

  The composition of each layer of the surface emitting laser 10 described above is only an example. For example, in the present embodiment, the case where the first and third sub-barrier layers BA and BC are made of GaN layers and have the same band gap is described. However, the first and third sub-barrier layers BA and BC need only have a larger band gap than the second sub-barrier layer BB. For example, the first sub-barrier layer BA or the third sub-barrier layer BC may be composed of an InGaN layer having an In composition smaller than that of the second sub-barrier layer BB. Further, the first and third sub-barrier layers BA and BC may have different band gaps.

  In the present embodiment, the case where the active layer 13B has four well layers W1 to W4 has been described. However, the number of well layers is not limited thereto. Further, the composition of the well layers W1 to W4 is not limited to InGaN. The well layers W1 to W4 may have different band gaps and compositions.

  Further, the case where the active layer 13B has the barrier layer (final barrier layer) B5 on the well layer (final well layer) W4 positioned closest to the p-type semiconductor layer 13D, and the barrier layer B5 is composed of a GaN layer has been described. . That is, in this embodiment, the final barrier layer B5 located closest to the p-type semiconductor layer 13D among the barrier layers B1 to B5 has a single-layer structure, and other barrier layers B1 to B1 other than the final barrier layer B5 Each of B4 has first to third sub-barrier layers BA to BC.

  However, the final barrier layer B5 is not limited to having a single layer structure, and each of the other barrier layers B1 to B4 is not limited to having the first to third sub-barrier layers BA to BC. In the active layer 13, it is only necessary that at least one of the plurality of barrier layers B1 to B5 includes the first to third sub-barrier layers BA to BC.

  In the present embodiment, the case where the semiconductor structure layer 13 has the electron blocking layer 13C between the final barrier layer B5 and the p-type semiconductor layer 13D has been described. However, the semiconductor structure layer 13 may not have the electron block layer 13C.

  In consideration of stable and uniform light output between the well layers W1 to W4, each of the barrier layers B1 to B4 preferably includes the first to third sub-barrier layers BA to BC. . When the electron blocking layer 13C (AlGaN layer) is provided on the well layer W4 (InGaN layer), it is preferable to provide a GaN layer as the final barrier layer B5 in order to protect the well layer W4. That is, it is preferable that only the final barrier layer B5 closest to the p-type semiconductor layer 13D has a single-layer structure among the barrier layers B1 to B4 sandwiching the well layers W1 to W4.

  As described above, in this embodiment, the surface emitting laser 10 includes the semiconductor structure layer 13 having the active layer 13 </ b> B and the first and second reflecting mirrors 12 and 14 facing each other with the semiconductor structure layer 13 interposed therebetween. Have. The active layer 13B has a multiple quantum well structure including a plurality of well layers W1 to W4 and barrier layers B1 to B4.

  In addition, at least one of the barrier layers B1 to B4 has a first sub-barrier layer BA and a band gap formed on the first barrier layer BA and smaller than the first barrier layer BA. A second sub-barrier layer BB and a third sub-barrier layer BC formed on the second sub-barrier layer BB and having a larger band gap than the second sub-barrier layer BB are included. Therefore, it is possible to provide the surface emitting laser 10 having carriers that recombine with high efficiency in the active layer 13B and have a low oscillation threshold.

  FIG. 4 is a cross-sectional view of the surface emitting laser 20 according to the second embodiment. The surface emitting laser 20 has the same configuration as that of the surface emitting laser 10 except for the configuration of the semiconductor structure layer 21. The semiconductor structure layer 21 has the same configuration as the semiconductor structure layer 13 except for the structure of the active layer 21B.

  FIG. 5A is a cross-sectional view showing the structure of the active layer 21 </ b> B of the surface emitting laser 20. The active layer 21B has a multiple quantum well structure including well layers W1 to W4 and barrier layers B11 to B5 having a larger band gap than the well layers W1 to W4. In this embodiment, barrier layers B11 to B41 and well layers W1 to W4 are alternately stacked on the n-type semiconductor layer 13A, and a barrier layer B5 is formed on the well layer W4.

  This example corresponds to an example in which the barrier layers B1, B2, B3, and B4 of the active layer 13B in Example 1 are replaced with barrier layers B11, B21, B31, and B41, respectively. In the present embodiment, each of the barrier layers B11 to B41 has the same configuration as the barrier layers B1 to B4 except for the configuration of the third sub-barrier layer BD.

  FIG. 5B is a band diagram of the semiconductor structure layer 21B. The barrier layers B11 to B41 will be described with reference to FIG. Similar to the third sub-barrier layer BC in the active layer 13B, the third sub-barrier layer BD is formed on the second sub-barrier layer BB and has a larger band gap than the second sub-barrier layer BB. .

  In this embodiment, the third sub-barrier layer BD has a larger band gap than the first sub-barrier layer BA. That is, the third sub-barrier layer BD closer to the p-type semiconductor layer 13D than the second sub-barrier layer BB having a relatively small band gap has a larger band than the first and second sub-barrier layers BA and BB. Has a gap.

  In the present embodiment, the third sub-barrier layer BD has a larger band gap than the n-type semiconductor layer 13A. In the present embodiment, the third sub-barrier layer BD is composed of an AlGaN layer having an Al composition smaller than that of the electron block layer 13C. Therefore, each layer of the semiconductor structure layer 21 has a band gap as shown in FIG.

  Next, the barrier layers B11 to B41 according to the present embodiment will be described. In this embodiment, each of the barrier layers B11 to B41 is provided with first and third sub-barrier layers BA and BD having a larger band gap with the second sub-barrier layer BB interposed therebetween. Have Further, the third sub-barrier layer BD has a larger band gap than the first sub-barrier layer BA.

  The first and third sub-barrier layers BA and BD have a preferable band gap relationship in consideration of the difference in mobility (ease of movement) of electrons and holes. Specifically, when the element is driven, electrons move from the n-type semiconductor layer 13A and holes move from the p-type semiconductor layer 13D toward the active layer 21B. Here, since electrons have a higher mobility than holes, the amount of electrons and the amount of holes in the well layers W1 to W4 tend to be biased.

  In the barrier layers B11 to B41, the third sub-barrier layer BD has an action of restricting the movement of electrons and retaining the electrons in the active layer 21B. On the other hand, the first sub-barrier layer BA has a function of limiting the movement of holes and staying in the active layer 21B. Then, by making the band gap of the third sub-barrier layer BD larger than the band gap of the first sub-barrier layer BA, the limiting force of electron movement is made larger than the limiting force of holes. Therefore, the amount of electrons and holes transferred in the active layer 21B can be matched, and the amount of injection into each of the well layers W1 to W4 can be made uniform.

  Accordingly, it is possible to obtain the emitted light that is uniformized in each of the well layers W1 to W4, and to oscillate the laser with a low threshold. Therefore, it is possible to provide the surface emitting laser 20 having carriers that recombine with high efficiency in the active layer 21B and that has a low oscillation threshold.

  In the present embodiment, the third sub-barrier layer BD has a larger band gap than the n-type semiconductor layer 13A. Accordingly, high luminous efficiency can be obtained when driving at a large current or driving at a high temperature.

  Specifically, when a large current is applied, the mobility of electrons increases, and the possibility that electrons injected from the n-type semiconductor layer 13A pass through the active layer 21B increases. Accordingly, there is a possibility that the light emission efficiency is lowered. In contrast, in the present embodiment, the third sub-barrier layer BD having a larger band gap than the n-type semiconductor layer 13A is provided in the active layer 21B. Electron movement can be relatively limited, and the electrons can be retained in the active layer 21B. Therefore, for example, even when driving at a high current and at a high temperature, it is possible to provide the surface emitting laser 20 having carriers that recombine with high efficiency in the active layer 21B and having a low oscillation threshold.

  Further, in consideration of obtaining high luminous efficiency at the time of driving with a large current, the electronic block layer in which the semiconductor structure layer 21 has a larger band gap than the p-type semiconductor layer between the active layer 21B and the p-type semiconductor layer 13D. It is preferable to have 13C.

  In the present embodiment, the case where the third sub-barrier layer BD has a larger band gap than the n-type semiconductor layer 13A has been described. However, the configuration of the third sub-barrier layer BD is not limited to this. The compositions of the first and third sub-barrier layers BA and BD are not limited to the above. The third sub-barrier layer BD only needs to have a larger band gap than the first sub-barrier layer BA. For example, the first sub-barrier layer BA may be an InGaN layer, and the third sub-barrier layer BD may be a GaN layer.

  As described above, in this embodiment, the semiconductor structure layer 21 of the surface emitting laser 20 has the active layer 21B having a multiple quantum well structure in which the well layers W1 to W4 and the barrier layers B11 to B41 are stacked. Further, each of the barrier layers B11 to B41 is formed on the first and second sub-barrier layers BA and BB and the second sub-barrier layer BB and has a larger band gap than the first sub-barrier layer BA. A third sub-barrier layer BD. The semiconductor structure layer 21 includes an n-type semiconductor layer 13A and a p-type semiconductor layer 13D, and the third sub-barrier layer BD is located closer to the p-type semiconductor layer 13D than the second sub-barrier layer BB. Therefore, it is possible to provide the surface emitting laser 20 having carriers that recombine with high efficiency in the active layer 21B and that has a low oscillation threshold.

10, 20 Semiconductor laser (vertical resonator type light emitting element)
13B Active layers W1 to W4 Well layers B1 to B4, B11 to B41 Barrier layer BA First sub-barrier layer BB Second sub-barrier layer BC, BD Third sub-barrier layer

Claims (7)

A semiconductor structure layer including a first semiconductor layer having a first conductivity type, an active layer, and a second semiconductor layer having a second conductivity type opposite to the first conductivity type;
First and second reflecting mirrors facing each other through the semiconductor structure layer,
The active layer has a multiple quantum well structure including a plurality of well layers and a plurality of barrier layers,
At least one of the plurality of barrier layers includes a first sub-barrier layer and a first bandgap formed on the first sub-barrier layer and having a smaller band gap than the first sub-barrier layer. Vertical resonance comprising: a second sub-barrier layer; and a third sub-barrier layer formed on the second sub-barrier layer and having a larger band gap than the second sub-barrier layer. Type light emitting device. The first and second semiconductor layers are an n-type semiconductor layer and a p-type semiconductor layer, respectively.
The third sub-barrier layer is positioned closer to the p-type semiconductor layer than the second sub-barrier layer;
The vertical resonator type light emitting device according to claim 1, wherein the third sub-barrier layer has a larger band gap than the first sub-barrier layer.   The vertical cavity light emitting device according to claim 2, wherein the third sub-barrier layer has a larger band gap than the n-type semiconductor layer.   The vertical structure according to claim 2, wherein the semiconductor structure layer includes an electron block layer having a larger band gap than the p-type semiconductor layer between the active layer and the p-type semiconductor layer. Resonator type light emitting device. The n-type semiconductor layer and the p-type semiconductor layer are composed of a GaN layer,
Each of the plurality of well layers comprises an InGaN layer,
The second sub-barrier layer is composed of an InGaN layer having an In composition smaller than each of the plurality of well layers,
2. The vertical cavity light emitting device according to claim 1, wherein each of the first and third sub-barrier layers comprises a GaN layer. The n-type semiconductor layer and the p-type semiconductor layer are composed of a GaN layer,
Each of the plurality of well layers comprises an InGaN layer,
The second sub-barrier layer is composed of an InGaN layer having an In composition smaller than each of the plurality of well layers,
The vertical resonator type light emitting device according to any one of claims 2 to 4, wherein the third sub-barrier layer is formed of an AlGaN layer. Of the plurality of barrier layers, a final barrier layer located closest to the second semiconductor layer has a single layer structure,
7. Each of the barrier layers other than the final barrier layer among the plurality of barrier layers includes the first, second, and third sub-barrier layers. The vertical resonator type light emitting device described in 1.

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