JP7109079B2 - Nitride semiconductor multilayer reflector - Google Patents

Description

The present invention relates to a nitride semiconductor multilayer reflector.

In surface-emitting lasers using nitride semiconductors, a structure using a semiconductor multilayer reflector as a reflector has been proposed. Non-Patent Documents 1 and 2 disclose that an AlN/GaN multilayer film or an AlInN/GaN multilayer film is used as a semiconductor multilayer reflector.
However, when an AlN/GaN multilayer film is used as a multilayer reflector, there is a problem that cracks occur due to lattice mismatch between AlN and GaN. When an AlInN/GaN multilayer film is used as a multilayer reflector, AlInN and GaN can be epitaxially grown with lattice matching. Since the difference in index is small, the number of pairs of multilayer films required to obtain a reflecting film with high reflectance in the long wavelength region is increased, and the growth time of the multilayer film reflector is lengthened.
For example, when the emission wavelength at the center of reflection is 400 nm, a multilayer reflector with 40 pairs and a layer thickness of 3.7 μm is required for VCSEL. In order to further increase the emission wavelength of the reflection center, the number of pairs increases due to the increase in layer thickness and the decrease in the difference in refractive index due to the increase in the emission wavelength of the reflection center. It becomes thicker, and the burden and cost in crystal growth increase. In order to solve these problems, it is conceivable to use an AlInN/GaInN multilayer film, which can increase the refractive index difference by pseudolattice matching, as a multilayer reflector.

Daiji Kasahara, Daisuke Morita, Takao Kosugi, Kyosuke Nakagawa, Jun Kawamata, Yu Higuchi, Hiroaki Matsumura, Takashi Mukai"Demonstration of Blue and Green GaN-Based Vertical-Cavity Surface-Emitting Lasers by Current Injection at Room Temperature", Applied Physics Express , (USA), 2011, Vol. 4, Number 7, P.072103 Takashi Furuta, Kenjo Matsui, Kosuke Horikawa, Kazuki Ikeyama, Yugo Kozuka, Shotaro Yoshida, Takanobu Akagi, Tetsuya Takeuchi, Satoshi Kamiyama, Motoaki Iwaya, Isamu Akasaki"Room-temperature CW operation of a nitride-based vertical-cavity surface-emitting laser." using thick GaInN quantum wells", Japanese Journal of Applied Physics, (Japan), 2016, Vol. 55, No. 5S, P.S. 05FJ11

For example, instead of the AlInN/GaN multi-layered multilayer reflector, a pseudo-lattice-matched AlInN/GaInN multilayer reflector that cancels out the tensile strain of AlInN and the compressive strain of GaInN can be used. It is possible to fabricate a multilayer reflector in which the difference in refractive index between materials (AlInN and GaInN) is increased.
However, since both materials (AlInN and GaInN) are grown at a relatively low temperature (approximately 870° C.), the crystallinity tends to deteriorate, making it difficult to obtain a multilayer reflector with good crystallinity. Therefore, there is a problem that the reflectance expected for a multilayer film reflector cannot be obtained.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a nitride semiconductor multilayer reflector that can function satisfactorily in the long wavelength region.

In the nitride semiconductor multilayer film reflector of the present invention,
a plurality of In-containing layers having different In compositions;
a non-In-containing layer that does not contain In;
A nitride semiconductor multilayer reflector comprising
One layer of the non-In-containing layer is stacked every time one to three layers of the In-containing layer are stacked in succession ,
The AlInN layer and the GaInN layer, which are the In-containing layers, are repeatedly laminated, and at least one layer of either the GaN layer or the AlGaN layer, which is the non-In-containing layer, is laminated between the AlInN layer and the GaInN layer. characterized by being

This nitride semiconductor multilayer reflector can restore the crystallinity that has been degraded by stacking a plurality of In-containing layers by stacking non-In-containing layers, and achieve high reflectance. In addition, by using the AlInN layer and the GaInN layer, light can be efficiently reflected, and by laminating the In-free GaN layer or AlGaN layer, the crystallinity is reduced by laminating the AlInN layer and the GaInN layer. sexuality can be restored.

Therefore, the nitride semiconductor multilayer film reflector of the present invention can function satisfactorily in the long wavelength region.

1 is a schematic diagram showing the structure of a nitride semiconductor multilayer reflector of Example 1. FIG. FIG. 10 is a schematic diagram showing the structure of a nitride semiconductor multilayer reflector according to another embodiment in which one GaN layer is laminated and crystal-grown every time two GaInN layers and AlInN layers are successively laminated in this order. FIG. 5 is a schematic diagram showing the structure of a nitride semiconductor multilayer reflector according to another embodiment in which one GaN layer is laminated for every two consecutively laminated AlInN layers and GaInN layers in this order for crystal growth. Another embodiment in which, in a multi-layered film in which a GaInN layer and an AlInN layer are repeatedly laminated in order for crystal growth, a GaN layer is laminated and crystal-grown every time three layers of a GaInN layer and an AlInN layer are consecutively laminated. 1 is a schematic diagram showing the structure of a nitride semiconductor multilayer reflector. 5 is a graph showing the insertion limit of the thickness of the GaN layer at a plurality of mutually different reflection center emission wavelengths with respect to the mole fraction of InN in the GaInN layer. FIG. 10 is a schematic diagram showing the structure of a nitride semiconductor multilayer reflector according to another embodiment in which one AlGaN layer is laminated for each GaInN layer and then an AlInN layer for crystal growth.

A preferred embodiment of the present invention will be described.

In the nitride semiconductor multilayer reflector of the present invention, a GaN layer can be laminated between the AlInN layer and the GaInN layer. In this case, since the GaN layer can use the material of the nitride semiconductor multilayer reflector, the manufacturing process can be simplified.

The maximum thickness of the GaN layer of the nitride semiconductor multilayer reflector of the present invention is a(λ) in Formula 2, b(λ) in Formula 3, and b(λ) in Formula 4, which are determined by the reflection center emission wavelength λ (nm). f(x) of Equation 1 determined by c(λ) and the mole fraction x of InN in the GaInN layer.
f(x)=a(λ)×10000x ² +b(λ)×100x+c(λ) Equation 1
a(λ)=2.6×10 ⁻⁶ ×λ ² −0.0034×λ+0.66 Equation 2
b(λ)=-3.3× ^10-5 ×λ2+0.051×λ ^- 9.8 Equation 3
c(λ)=-6.2× ^10-5 ×λ2+0.1×λ ^- 16 Equation 4
In this case, by using these formulas, the maximum thickness of the laminated GaN layer can be calculated only by determining the reflection center emission wavelength λ (nm) and the mole fraction x of InN in the GaInN layer.

Next, Example 1, which embodies the nitride semiconductor multilayer film reflector of the present invention, will be described with reference to the drawings.

<Example 1>
As a result of intensive studies by the inventors, a GaN layer, which is a third layer, was newly used in addition to the AlInN layer and the GaInN layer in order to improve the crystallinity in the nitride semiconductor multilayer reflector. . Specifically, by appropriately adjusting the position of the GaN layer and the thickness of the GaN layer in the nitride semiconductor multilayer reflector, both high crystallinity and high reflectance can be achieved in the nitride semiconductor multilayer reflector. I found that it can be done. The nitride semiconductor multilayer reflector of the present invention is used, for example, when reflecting light emitted from the active layer of a VCSEL to obtain laser light.

The nitride semiconductor multilayer reflector 1 of Example 1, as shown in FIG. An AlInN layer 13 is provided. The GaInN layer 11 has an InN molar fraction of 0.04 and a GaN molar fraction of 0.96. The AlInN layer 13 has an InN molar fraction of 0.144 and an AlN molar fraction of 0.856. The GaInN layer 11 and the AlInN layer 13 have different In compositions. The nitride semiconductor multilayer reflector 1 is laminated and crystal-grown on the surface of a substrate (not shown) made of GaN or the like, for example, by MOCVD (metal organic chemical vapor deposition) or the like. In the nitride semiconductor multilayer reflector 1, a GaInN layer 11 and an AlInN layer 13 are repeatedly laminated, and one GaN layer 12 is laminated for each GaInN layer 11 and AlInN layer 13 laminated one by one.

The GaInN layer 11 is first crystal-grown when forming the nitride semiconductor multilayer reflector 1 . The temperature of the substrate during crystal growth of the GaInN layer 11 is about 870.degree.
The GaN layer 12 is laminated and crystal-grown on the surface of the GaInN layer 11 (front and back sides are upper and lower sides in FIG. 1). The temperature of the substrate during crystal growth of the GaN layer 12 is about 1100.degree. One GaN layer 12 is laminated between the GaInN layer 11 and the AlInN layer 13 .
The AlInN layer 13 is layered on the surface of the GaN layer 12 and crystal-grown. The temperature of the substrate during crystal growth of the AlInN layer 13 is about 870.degree. The substrate temperature (870° C.) during the crystal growth of the GaInN layer 11 and the AlInN layer 13 is lower than the substrate temperature during the crystal growth of the GaN layer 12 . Therefore, it is difficult for the GaInN layer 11 and the AlInN layer 13 to grow crystals while maintaining good crystallinity compared to the GaN layer 12 .

When the GaInN layers 11 and the AlInN layers 13 are alternately stacked and crystal-grown, the compressive strain of the GaInN layers 11 and the tensile strain of the AlInN layers 13 cancel each other out, and the crystals can grow with pseudo lattice matching.
A multilayer film reflector is constructed by repeatedly stacking a plurality of pairs of laminated layers of two materials. ), and the thickness of one material is generally set to a thickness corresponding to a quarter of the optical wavelength at the reflection central emission wavelength λ. Here, the optical wavelength is a value obtained by dividing the reflection central emission wavelength λ by the inherent refractive index of each layer. If the intrinsic refractive index of each layer is different from each other, the optical wavelength in each layer will also be different from each other.
The GaN layer 12 is inserted at a position including the interface between the two materials (the GaInN layer 11 and the AlInN layer 13) laminated and crystal-grown to form a multilayer reflector. However, the insertion position of the GaN layer 12 is not limited to this. It may be biased toward the GaInN layer 11 side. When the GaN layer 12 is biased toward the AlInN layer 13 side, the crystallinity is improved, but the reflectance tends to decrease slightly. Also, if the GaN layer 12 is biased toward the GaInN layer 11 side, the reflectance is maintained, but the crystallinity is degraded.

In the nitride semiconductor multilayer reflector 1 , the GaInN layer 11 and the AlInN layer 13 are pseudomorphically matched, and the GaN layer 12 is inserted between the GaInN layer 11 and the AlInN layer 13 .
The substrate temperature during crystal growth of the GaN layer 12 is higher than the substrate temperature during crystal growth of the GaInN layer 11 and the AlInN layer 13 . Therefore, in the nitride semiconductor multilayer reflector 1, the crystallinity (surface smoothness) that has deteriorated due to the crystal growth of the GaInN layer 11 and the AlInN layer 13 can be replaced by crystal growth by laminating the GaN layer 12. can be improved (smoothed surface) by As a result, when a resonator or the like is laminated on the surface of the nitride semiconductor multilayer reflector 1 and crystal growth is performed, the influence of the crystallinity of the nitride semiconductor multilayer reflector 1 on the resonator or the like can be suppressed.
However, if the thickness of the GaN layer 12 is made too thick, the effective refractive index of the nitride semiconductor multilayer film reflector 1 is lowered. , and the AlInN layer 13 is increased.
Therefore, as a result of simulation, when the reflection center emission wavelength λ is 580 nm, the nitride semiconductor multilayer film reflector 1 has the same number of pairs as the conventional AlInN/GaN multilayer film reflector. It was found that the maximum thickness of the GaN layer 12 that can achieve the highest reflectance is 47.1 nm.

In the nitride semiconductor multilayer reflector 1, when the GaN layer 12 is laminated and the crystal is grown, the total thickness of the GaN layer 12, the GaInN layer 11, and the AlInN layer 13 is the optical wavelength at the reflection center emission wavelength λ. It should be half as thick.
Specifically, the thickness of 47.1 nm in the GaN layer 12 corresponds to 0.190 times the optical wavelength when the central reflection wavelength λ is 580 nm. Therefore, the thickness of each of the GaInN layer 11 and the AlInN layer 13 must be set to a thickness corresponding to 0.155 times the optical wavelength when the reflection center emission wavelength λ is 580 nm.
AlInN has a refractive index of 2.151 and GaInN has a refractive index of 2.356 at a reflection central emission wavelength λ of 580 nm. As a result, the thickness of the AlInN layer 13 corresponding to 0.155 times the optical wavelength when the reflection center emission wavelength λ is 580 nm is 41.8 nm, and the thickness of the GaInN layer 11 is 38.2 nm.
In this way, the total thickness of the GaInN layer 11, the GaN layer 12, and the AlInN layer 13 can be set to a thickness corresponding to half the optical wavelength when the reflection center emission wavelength λ is 580 nm, and the nitride semiconductor multilayer reflection In the mirror 1, the thickness of the GaN layer 12 is set to 47.1 nm (thickness equivalent to 0.190 times the optical wavelength when the reflection center emission wavelength λ is 580 nm), which is the same as the conventional AlInN/GaN multilayer reflector. With the number of pairs, it is possible to achieve a higher reflectance than the conventional AlInN/GaN multilayer reflector.

Next, in the nitride semiconductor multilayer reflector 1 in which the GaN layer 12 is inserted between the GaInN layer 11 and the AlInN layer 13 that are pseudo-lattice-matched, the conventional AlInN/GaN multilayer film Table 1 shows the result obtained by simulation of the insertion limit amount of the thickness of the GaN layer 12 that can improve the crystallinity while suppressing the increase in the number of pairs compared to the reflector.

The InN mole fraction in Table 1 is the InN mole fraction in the GaInN layer 11 . Then, the conditions under which the cumulative strain of the GaInN layers 11 and the AlInN layers 13 is zero (that is, the molar fraction of InN in the GaInN layers 11 that can be alternately laminated and can be pseudomorphic, and the molar fraction of InN in the AlInN layers 13 Mole fraction combinations) are shown in Table 2.

The values shown in Table 2 were obtained by setting the lattice constants of AlN, GaN, and InN in the a-axis direction to 3.112 Å, 3.189 Å, and 3.548 Å, respectively. As shown in Table 2, the condition under which the cumulative strain of the GaInN layer 11 and the AlInN layer 13 is zero is that as the mole fraction of InN in the GaInN layer 11 increases, the mole fraction of InN in the AlInN layer 13 decreases. Become.

FIG. 5 shows a graph of the insertion limit of the thickness of the GaN layer 12 at a plurality of mutually different reflection center emission wavelengths λ with respect to the molar fraction x of InN in the GaInN layer 11 .
As shown in FIG. 5, the insertion limit of the thickness of the GaN layer 12 increases as the mole fraction of InN in the GaInN layer 11 increases at any reflection center emission wavelength λ.

A formula obtained by fitting the results in FIG. 5 with a quadratic function is shown below.
f(x)=a(λ)×10000x ² +b(λ)×100x+c(λ) Equation 1
a(λ)=2.6×10 ⁻⁶ ×λ ² −0.0034×λ+0.66 Equation 2
b(λ)=-3.3× ^10-5 ×λ2+0.051×λ ^- 9.8 Equation 3
c(λ)=-6.2× ^10-5 ×λ2+0.1×λ ^- 16 Equation 4
f(x) is the maximum reflection center emission wavelength λ (design wavelength) [nm] of the GaN layer 12 in one period when the molar fraction of InN in the GaInN layer 11 is x. thickness [nm].
x is the molar fraction of InN in the GaInN layer 11 (0.01≤x≤0.1).
λ is the reflection center emission wavelength (design wavelength) [nm] (400≦λ≦650).
As a result, the maximum thickness f(x) of the GaN layer 12 is determined by the reflection central emission wavelength λ (nm), a(λ) in Equation 2, b(λ) in Equation 3, and c(λ ) and the molar fraction x of InN in the GaInN layer 11 . That is, by determining the reflection center emission wavelength λ (design wavelength) [nm] and the molar fraction x of InN in the GaInN layer 11 from Equations 1 to 4, the maximum GaN layer 12 that can be inserted is The thickness f(x) [nm] can be calculated.

As described above, the nitride semiconductor multilayer reflector 1 restores the crystallinity that has been deteriorated by laminating the plurality of GaInN layers 11 and the AlInN layers 13 by laminating the GaN layers 12, and achieves a high reflectance. can be realized.

Therefore, the nitride semiconductor multilayer film reflector 1 of the present invention can function satisfactorily in the long wavelength region.

In the nitride semiconductor multilayer reflector 1, AlInN layers 13 and GaInN layers 11, which are In-containing layers, are repeatedly stacked, and between the AlInN layers 13 and GaInN layers 11, a GaN layer 12, which is a non-In-containing layer, is provided. One layer is laminated. Therefore, by using the AlInN layer 13 and the GaInN layer 11, light can be efficiently reflected, and by laminating the GaN layer 12 containing no In, by laminating the AlInN layer 13 and the GaInN layer 11, Decreased crystallinity can be restored.

The nitride semiconductor multilayer reflector 1 has a GaN layer 12 laminated between an AlInN layer 13 and a GaInN layer 11 . Therefore, since the GaN layer 12 can use the material of the nitride semiconductor multilayer reflector 1, the manufacturing process can be simplified.

The maximum thickness of the GaN layer 12 of the nitride semiconductor multilayer reflector 1 is a(λ) in Equation 2, b(λ) in Equation 3, and c in Equation 4 determined by the reflection center emission wavelength λ (nm). (λ) and the molar fraction x of InN in the GaInN layer 11, f(x) in Equation 1.
f(x)=a(λ)×10000x ² +b(λ)×100x+c(λ) Equation 1
a(λ)=2.6×10 ⁻⁶ ×λ ² −0.0034×λ+0.66 Equation 2
b(λ)=-3.3× ^10-5 ×λ2+0.051×λ ^- 9.8 Equation 3
c(λ)=-6.2× ^10-5 ×λ2+0.1×λ ^- 16 Equation 4
Therefore, by using these formulas, the maximum thickness of the laminated GaN layer 12 can be calculated only by determining the reflection central emission wavelength λ (nm) and the mole fraction x of InN in the GaInN layer 11. can.

The present invention is not limited to the first embodiment explained by the above description and drawings, and the following embodiments are also included in the technical scope of the present invention.
(1) In Example 1, the GaN layers in the nitride semiconductor multilayer reflector are arranged at both interfaces (that is, one GaN layer is laminated for each GaInN layer and AlInN layer laminated). However, as shown in FIGS. 2 and 3, the GaN layers 112 and 212 are arranged only on one interface (that is, one GaN layer 112 is provided for each successive stack of two GaInN layers 111 and AlInN layers 113). layer stacking, or stacking one layer of GaN layer 212 each time two layers of GaInN layer 211 and AlInN layer 213 are successively stacked), and as shown in FIG. One GaN layer 312 may be laminated every time three AlInN layers 313 are successively laminated.
(2) In Example 1, the GaN layer is inserted between the GaInN layer and the AlInN layer, but as shown in FIG. An AlGaN layer 14, which is an inclusion layer, may be inserted. That is, the AlInN layer 13 and the GaInN layer 11 are laminated repeatedly, and one AlGaN layer 14 which is a non-In-containing layer is laminated between the AlInN layer 13 and the GaInN layer 11 .

11, 111, 211, 311...GaInN layers (In-containing layers)
12, 112, 212, 312...GaN layer (non-In containing layer)
13, 113, 213, 313... AlInN layers (In-containing layers)
14... AlGaN layer (non-In containing layer)

Claims (3)

a plurality of In-containing layers having different In compositions;
a non-In-containing layer that does not contain In;
A nitride semiconductor multilayer reflector comprising
One layer of the non-In-containing layer is stacked every time one to three layers of the In-containing layer are stacked in succession ,
The AlInN layer and the GaInN layer, which are the In-containing layers, are repeatedly laminated, and at least one layer of either the GaN layer or the AlGaN layer, which is the non-In-containing layer, is laminated between the AlInN layer and the GaInN layer. A nitride semiconductor multilayer film reflector characterized by : 2. The nitride semiconductor multilayer reflector according to claim 1 , wherein said GaN layer is laminated between said AlInN layer and said GaInN layer . The maximum thickness of the GaN layer is
a(λ) in Equation 2, b(λ) in Equation 3, and c(λ) in Equation 4, determined by the reflected central emission wavelength λ (nm);
InN mole fraction x in the GaInN layer;
3. The nitride semiconductor multilayer reflector according to claim 2, wherein f(x) in Equation 1 is determined by:
f(x)=a(λ)×10000x ² +b(λ)×100x+c(λ) Equation 1
a(λ)=2.6×10 ⁻⁶ ×λ ² −0.0034×λ+0.66 Equation 2
b(λ)=-3.3× ^10-5 ×λ2 +0.051×λ ^- 9.8 Equation 3
c(λ)=-6.2× ^10-5 ×λ2 +0.1×λ ^- 16 Equation 4

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