JP7614488B2 - Vertical cavity surface emitting laser device and its manufacturing method - Google Patents

Description

This disclosure relates to a vertical cavity surface emitting laser element and a method for manufacturing the same.

Research has been conducted on laser elements that function as vertical cavity surface emitting lasers using nitride semiconductors, and the use of semiconductor multilayer films as reflectors has been proposed. Patent Document 1 describes a light emitting device having a (20-21) GaN main surface. Patent Document 2 describes a semiconductor light emitting device having a reflector in which an AlInN layer and a GaN layer are stacked on a semipolar surface of a semiconductor substrate.

WO2018/116596 JP 2017-168560 A

However, depending on the crystal plane of the nitride semiconductor used as a semiconductor multilayer film formed on a substrate whose main surface is a semipolar plane or the like, the reflectivity of the light-reflecting film may not be as designed.

The present disclosure includes the following inventions.
[1] A first light reflecting film including a first nitride semiconductor layer and a second nitride semiconductor layer having a different composition from that of the first nitride semiconductor layer, the first nitride semiconductor layer having a plurality of refractive indexes different between a first polarization direction and a second polarization direction for light incident on a primary surface, the refractive index for the first polarization direction being _n1 , and having a thickness _L1 represented by the following formula (1), and the second nitride semiconductor layer having a plurality of refractive indexes different between the first polarization direction and the second polarization direction for light incident on a primary surface, the refractive index for the first polarization direction being _n2 , and having a thickness _L2 represented by the following formula (2);
a nitride semiconductor layer disposed on the first light reflecting film and having an active layer;
and a second optical reflection film disposed above the laminate.
L ₁ =λ _c /(4・n ₁ ) (1)
L ₂ =λ _c /(4・n ₂ ) (2)
[2] Growing a first light-reflecting film on a substrate;
growing a nitride semiconductor layer including an active layer on the first light reflecting film;
a second optical reflecting film is grown on the laminate,
The first light reflecting film,
The semiconductor device is formed by laminating two or more types of single crystal nitride semiconductor layers, including a first nitride semiconductor layer and a second nitride semiconductor layer having a different composition from the first nitride semiconductor layer;
the first nitride semiconductor layer has a plurality of refractive indexes that differ between a first polarization direction and a second polarization direction for light incident on a primary surface, the refractive index for the first polarization direction being _n1 , and has a thickness _L1 expressed by the following formula (1); and the second nitride semiconductor layer has a plurality of refractive indexes that differ between the first polarization direction and the second polarization direction for light incident on a primary surface, the refractive index for the first polarization direction being _n2 , and has a thickness _L2 expressed by the following formula (2).
L ₁ =λ _c /(4・n ₁ ) (1)
L ₂ =λ _c /(4・n ₂ ) (2)
(In the formula, λ _c represents the resonance wavelength.)

According to one aspect of the present disclosure, it is possible to provide a vertical cavity surface emitting laser element having a light reflecting film with improved reflectance, and a method for manufacturing the same.

2 is a schematic cross-sectional view of a main portion for explaining the structure of a first reflector in the vertical cavity surface emitting laser element of the embodiment of the present invention. FIG. 1 is a schematic cross-sectional view for explaining a layered structure of a vertical cavity surface emitting laser element according to one embodiment of the present invention. FIG. 11 is a schematic cross-sectional view showing a modified example of a vertical cavity surface emitting laser element. 1 is a graph showing the reflection spectra of [−1014] axial polarized light, [−12-10] axial polarized light, and unpolarized light of an AlInN/GaN multilayer film having a (20-21) plane as a principal surface. FIG. 3B is an enlarged view of the portion X in FIG. 3A.

Below, an embodiment of the present disclosure will be described with reference to the drawings as appropriate. However, the embodiment described below is intended to embody the technical ideas of the present disclosure, and unless otherwise specified, the present disclosure is not limited to the following. Furthermore, the content described in one embodiment or example can be applied to other embodiments or examples. The size, thickness, positional relationship, etc. of the components shown in each drawing may be exaggerated for clarity. When indicating a numerical range, the values on both sides of "~" are included.

[Vertical Cavity Surface Emitting Laser Element 8]
As shown in Figures 1A and 1B, a vertical cavity surface emitting laser element 8 according to an embodiment of the present invention includes a first light reflecting film 10, a nitride semiconductor stack 3 having an active layer, and a second light reflecting film 6. The first light reflecting film 10 is made of a nitride semiconductor multilayer film including a first nitride semiconductor layer 11 and a second nitride semiconductor layer 12 having a different composition from the first nitride semiconductor layer 11. The first nitride semiconductor layer 11 has a plurality of refractive indexes different between a first polarization direction and a second polarization direction for light incident on the main surface, the refractive index for the first polarization direction is _n1 , and the thickness _L1 is expressed by the following formula (1). The second nitride semiconductor layer 12 has a plurality of refractive indexes different between a first polarization direction and a second polarization direction for light incident on the main surface, the refractive index for the first polarization direction is _n2 , and the thickness _L2 is expressed by the following formula (2). Here, the light incident on the main surface specifically refers to light emitted from the active layer. The nitride semiconductor stack 3 is disposed on the first light reflecting film 10. The second light reflecting film 6 is disposed above the stack 3.
L ₁ =λ _c /(4・n ₁ ) (1)
L ₂ =λ _c /(4・n ₂ ) (2)
In a laser element having a nitride semiconductor layer as an active layer, if the growth surface is a polar surface, there is a concern that a piezoelectric field will be generated and the threshold current will increase. On the other hand, a nitride semiconductor stack formed on a substrate having a semi-polar or non-polar surface rather than a polar surface can suppress the generation of such a piezoelectric field and further reduce the threshold current.
However, in the nitride semiconductor multilayer film constituting the first light reflecting film, the refractive index is not anisotropic on the polar plane, but is anisotropic on the semipolar and nonpolar planes. Therefore, by designing the nitride semiconductor multilayer film used in the first light reflecting film using the refractive index for light of a specific polarization direction, the reflectance of the first light reflecting film can be improved. As a result, it becomes possible to reduce the threshold current. In addition, by designing the reflecting film according to the refractive index of the nitride semiconductor multilayer film, it becomes possible to widen the reflection band, that is, the range in which the reflectance is stable and high. As a result, it becomes possible to improve the tolerance for deviations during manufacturing, and to improve the yield of vertical cavity surface emitting laser elements.

(First light reflective film 10)
As shown in FIG. 1A, the first light reflecting film 10 is a nitride semiconductor multilayer film including a first nitride semiconductor layer 11 and a second nitride semiconductor layer 12 having a different composition from the first nitride semiconductor layer 11. The first and second nitride semiconductor layers are preferably single crystal nitride semiconductor layers. When the nitride semiconductor layer is single crystal, a first light reflecting film 10 is formed on the first nitride semiconductor layer. This tends to result in better quality crystals of the nitride semiconductor laminate 3 that is formed.
The first nitride semiconductor layer 11 has a plurality of refractive indices that differ between a first polarization direction and a second polarization direction for light incident on the primary surface 11a. , and has a thickness _L1 represented by the above-mentioned formula (1). In formula (1), the refractive index of the first nitride semiconductor layer 11 with respect to the first polarization direction is represented by _n1 .
The second nitride semiconductor layer 12 has a plurality of refractive indices that differ between a first polarization direction and a second polarization direction for light incident on the primary surface 12a. , and has a thickness _L2 represented by the above-mentioned formula (2). In formula (2), the refractive index of the second nitride semiconductor layer 12 with respect to the first polarization direction is represented by _n2 .
For example, the first nitride semiconductor layer 11 and the second nitride semiconductor layer 12 can be formed of GaN, AlInN, _AlGaN , etc. When an _AlInN layer is used for the first nitride semiconductor layer 11, N may be 0.1≦X1≦0.25, 0.75≦Y1≦0.9, and is preferably 0.14≦X1≦0.21, 0.79≦Y1≦0.86. When the composition of the AlInN layer is in the above-mentioned range, the occurrence of dislocations and cracks tends to be more effectively suppressed. In addition, from the viewpoint of refractive index difference and structural matching, it is preferable that the first nitride semiconductor It is preferable that the layer 11 is an AlInN layer, and the second nitride semiconductor layer 12 is a GaN layer. Since the nitride semiconductor layers have a plurality of refractive indices as described above, the primary surfaces of these nitride semiconductor layers, that is, the light incidence surfaces, The surface may be a semi-polar surface or a non-polar surface. Specifically, the {10-10} plane, the {10-12} plane, the {10-11} plane, the {11-20} plane, the {20-21} plane, the {11-22} plane, the {11- 23} plane, {11-26} plane, {10-1-1} plane, {30-31} plane, {30-3-1} plane, etc. The nitride semiconductor layer may have an off-angle of about 3°. In particular, the main surface of these nitride semiconductor layers is preferably a {20-21} plane. In the case of a nitride semiconductor layer having such a structure, crystal growth is easier than in the case of a nitride semiconductor layer having another semipolar plane as the main surface, and piezoelectric polarization in the light-emitting device tends to be more effectively suppressed.
In addition, the first polarization direction and the second polarization direction are not particularly limited, and it is sufficient that the first polarization direction and the second polarization direction are different. For example, the first polarization direction is the a-axis polarization direction in a hexagonal crystal, the second polarization direction can be the c-axis polarization direction.
Specifically, the first polarization direction and the second polarization direction are a [-12-10] axial direction, a [-11-10] axial direction, a [-1014] axial direction, and a [-1012] axial direction. , [-1011] axial direction, [-11-26] axial direction, [11-2-3] axial direction, [-11-22] axial direction, [0001] axial direction, [1-100] axial direction, etc. For example, the second polarization direction may be perpendicular to the first polarization direction. In particular, the primary surfaces of the first nitride semiconductor layer 11 and the second nitride semiconductor layer 12 are (10-10) planes (m-planes, non-polar planes), and the first polarization direction is the [1-210] axis. (a-axis) direction, or the principal surface is a (10-12) plane (r-plane, semi-polar plane) and the first polarization direction is the [-12-10] axis (a-axis) direction. , the principal surface is a (11-22) plane (semi-polar plane), and the first polarization direction is [1-1 00] axis (m-axis) direction, or the principal surface is a (10-11) plane (semi-polar plane) and the first polarization direction is the [-12-10] axis (a-axis) direction. , the principal surface is a (20-2-1) plane (semi-polar plane), the first polarization direction is the [-12-10] axis (a-axis) direction, or the principal surface is a (20-21) It is preferable that the first polarization direction is a [-12-10] direction.
When the above-mentioned combination of the primary surfaces and the first polarization direction of the first nitride semiconductor layer 11 and the second nitride semiconductor layer 12 is adopted, the light having the first polarization direction is scattered at the interface of the stacked body. Since there is a tendency for loss to be small, the threshold current can be reduced by designing the reflective film using the refractive index for the first polarization direction.
Specifically, it is preferable that the main surface is a (20-21) plane, the first polarization direction is a [-12-10] axial direction, and the second polarization direction is a [-1014] axial direction.

Here, for the crystal structure, individual axial directions are indicated with [ ], individual planes with ( ), and collective planes with { }, and a minus sign before an index indicates that the index is negative.

The first nitride semiconductor layer 11 has a plurality of refractive indices that differ between the first polarization direction and the second polarization direction for light incident on the primary surface 11a. In other words, the first nitride semiconductor layer 11 has a plurality of refractive indices for light incident on the primary surface 11a depending on the difference in the polarization direction. The second nitride semiconductor layer 12 has a plurality of refractive indices that differ between the first polarization direction and the second polarization direction for light incident on the primary surface 12a. In other words, the second nitride semiconductor layer 12 has a plurality of refractive indices for light incident on the primary surface 12a depending on the difference in the polarization direction.
For example, when the first nitride semiconductor layer 11 is an Al _0.8 In _0.2 N layer having a (20-21) plane as its principal surface and the first polarization direction is the [-12-10] axial direction, the refractive index for light in the first polarization direction is, for example, 2.56 to 2.32 at a wavelength of 400 nm to 800 nm.
Furthermore, when the second nitride semiconductor layer 12 is a GaN layer whose principal surface is the (20-21) plane and the first polarization direction is the [-12-10] axial direction, the refractive index for light in the first polarization direction is, for example, 2.26 to 2.16 at a wavelength of 400 nm to 800 nm.
The first nitride semiconductor layer 11 and the second nitride semiconductor layer 12 may be repeatedly laminated alternately to a constant thickness.
Here, the resonant wavelength λ _c of the laser element is a value calculated from the resonator length L _c by the following formula (3).
λ _c =2n _c・L _c /m (3)
In the formula, n is the effective refractive index of the resonator, and m is a numerical value representing how many wavelengths of light are contained in the resonator.
In this embodiment, the length from the end of the second light reflecting film on the stack side to the end of the first light reflecting film on the stack side is defined as the resonator length, and the resonator length _Lc is 840 nm. However, this resonator length is appropriately set depending on the thickness of the nitride semiconductor stack and the p-side electrode, which will be described later.

(Nitride Semiconductor Laminate 3)
The nitride semiconductor stack 3 is preferably disposed on the upper surface of the nitride semiconductor multilayer film constituting the first light reflecting film 10. The nitride semiconductor stack 3 also has an active layer 3a. Specifically, as shown in Fig. 1B, the nitride semiconductor stack 3 is preferably configured by stacking an n-side semiconductor layer 3n made of a Group III nitride semiconductor, an active layer 3a made of a Group III nitride semiconductor, and a p-side semiconductor layer 3p made of a Group III nitride semiconductor in this order from the first light reflecting film 10 side. Examples of Group III nitride semiconductors include AlN, InN, GaN, AlInN, AlGaN, InGaN, and AlInGaN.
Among these, it is particularly preferable that the active layer 3a contains an InGaN well layer. Since the gain of a semiconductor light emitting device having an InGaN well layer in the active layer tends to be higher in the proportion of the polarization component in the [-12-10] direction than in other polarization components, the reflectance can be increased and the threshold current can be reduced by designing the first light reflecting film 10 using the refractive index in the [-12-10] direction.
The n-side semiconductor layer 3n is preferably disposed between the active layer 3a and the first light reflecting film 10. The n-side semiconductor layer 3n is a single layer or a multilayer, and may have an undoped layer, and has one or more n-type layers doped with n-type impurities, for example, Si. The active layer 3a has a laminated structure in which quantum well layers made of InGaN and barrier layers made of GaN are alternately laminated. The number of layers can be appropriately set according to the desired characteristics. The p-side semiconductor layer 3p can have a p-side light spacer layer and a p-side contact layer disposed thereon. The p-side contact layer is a layer doped with p-type impurities, for example, Mg. The p-side light spacer layer can be an undoped layer or a layer doped with p-type impurities at a lower concentration than the p-side contact layer. In this case, the p-side contact layer is the top layer of the p-side semiconductor layer 3p.
The thicknesses of the n-side semiconductor layer 3n, the active layer 3a, and the p-side semiconductor layer 3p can be set appropriately. The total thickness from the upper surface of the first light reflecting film 10 to the lower surface of the second light reflecting film 6 described later is set to an integer multiple of λ _c /(2n) so that a standing wave is generated therebetween. The strongest part of the standing wave is located in the active layer 3a, and the weakest part of the standing wave is located in the light-transmitting p-side electrode 4 described later.
The p-side semiconductor layer 3p may have an upper surface in contact with the p-side electrode 4. In particular, the p-side semiconductor layer 3p is preferably disposed between the active layer 3a and a second light reflecting film 6 described later.

The nitride semiconductor layer stack 3 has a protrusion 3x on the upper surface of the p-side semiconductor layer 3p. The upper surface of this protrusion 3x functions as a current injection region. The area directly below the current injection region becomes the light emitting area. The shape of the protrusion 3x in plan view can be a circle, an ellipse, a polygon, or the like, with a circle being preferable. The size of the upper surface of the protrusion 3x can be, for example, a diameter or a side length of 2 μm to 12 μm.

In the nitride semiconductor layer stack 3, it is preferable that the p-side semiconductor layer 3p, the active layer 3a, and the n-side semiconductor layer 3n are partially removed in the thickness direction on the surface around the protrusion 3x, that is, on the side of the p-side semiconductor layer 3p, so that the n-side semiconductor layer 3n is partially exposed. This allows the p-side electrode 4 and the n-side electrode 5n that supply current to the laser element to be arranged on the same side of the stack 3.

(Second light reflecting film 6)
1B , in the case where a convex portion 3x is formed on the surface of the nitride semiconductor layer stack 3, the second light reflecting film 6 can be disposed on at least a part of the surface surrounding the convex portion 3x from the upper surface of the convex portion 3x. In addition, in the case where no convex portion is formed, an insulating film that exposes a part of the nitride semiconductor layer stack 3 may be formed, and the second light reflecting film 6 may be formed so as to cover the upper part of the exposed nitride semiconductor layer stack 3.
The second light reflecting film 6 may have the same configuration as the first light reflecting film 10 described above, or may have a configuration including, for example, a dielectric multilayer film. Examples of the dielectric multilayer film include SiO ₂ /Nb ₂ O ₅ , SiO ₂ /Ta ₂ O ₅ , and SiO ₂ /Al ₂ O _3. The thickness of each layer is λ _c /(4n). The number of layers and the thickness of the second light reflecting film 6 can be appropriately set depending on the combination of the dielectric multilayer films. Specifically, when the second light reflecting film 6 is made of SiO ₂ /Nb ₂ O ₅ or the like, each layer has a thickness of 40 nm to 100 nm. The number of layers of the laminated film can be 2 or more, for example, 5 to 20 layers. The total thickness of the second light reflecting film 6 can be, for example, 0.08 μm to 2.0 μm, and can be 0.6 μm to 1.7 μm.

The vertical cavity surface emitting laser element of the present disclosure may further include a substrate 16 , an underlayer 9 , a p-side electrode 4 , an n-side electrode 5 n , and an insulating film 7 .
The vertical cavity surface emitting laser element 8 is preferably bonded to a heat dissipation substrate 17 having a bonding layer 13 and a metal film 15, and configured so that the substrate 16 side serves as the light emitting surface. This may result in a higher optical output than when the second light reflecting film 6 side serves as the light emitting surface.

(Substrate 16 and Underlayer 9)
The first light reflecting film 10 may be provided above the substrate 16. In other words, the substrate 16 may be provided below the first light reflecting film 10. The first light reflecting film 10 is preferably formed on the substrate 16, optionally via an underlayer 9. By forming the first light reflecting film 10 via the underlayer 9, a flatter layer can be obtained. As the substrate 16, any substrate known in the art can be used as long as it is capable of forming a vertical cavity surface emitting laser element.
For example, a substrate for growing a semiconductor layer, specifically, a substrate such as a nitride semiconductor (GaN, etc.), sapphire, SiC, or Si, can be used. Among them, a nitride semiconductor is preferable, and a GaN substrate is more preferable. By using a GaN substrate as a substrate for growing a semiconductor, a semiconductor reflector with fewer dislocations can be obtained. In addition, the underlayer 9 can be selected in consideration of lattice matching with the substrate 16 and the first light reflecting film 10, and for example, a GaN layer is preferable.

The substrate 16 or the underlayer 9 may have a surface other than a polar surface on which the first nitride semiconductor layer 11 is grown, such as a {10-10} surface, a {10-12} surface, a {10-11} surface, a {11-22} surface, a {20-21} surface, a {20-2-1} surface, a {11-2-2} surface, a {11-23} surface, a {11-26} surface, a {10-1-1} surface, a {30-31} surface, a {30-3-1} surface, or the like. Of these, the substrate 16 or the underlayer 9 may preferably have a surface on which the first nitride semiconductor layer 11 is grown that is a {20-21} surface of GaN. The substrate 16 or the underlayer 9 may have an off-angle of about 0 to 3° with respect to these surfaces. By adopting a substrate having a {20-21} plane, the main surfaces of the first nitride semiconductor layer and the second nitride semiconductor layer grown thereon tend to be the {20-21} plane. In addition, in such a plane, for example, the refractive index is anisotropic, that is, the incident light has a plurality of different refractive indices in the first polarization direction and the second polarization direction, which are different from each other, so that the first light reflecting film can be designed to match any one of the refractive indices. This makes it possible to broaden the reflection band as shown in FIG. 3A and FIG. 3B. As a result, it is possible to improve the tolerance for deviations during manufacturing, and to improve the yield. In addition, it is possible to improve the reflectance of the first light reflecting film, and it is possible to reduce the threshold current.
The underlayer 9 can be set to any thickness, for example, 100 nm to 3000 nm. The underlayer 9 can be doped with an n-type impurity, but is preferably undoped in terms of crystallinity.

(p-side electrode 4)
The p-side electrode 4 is an electrode for injecting a current from the protrusion 3x of the p-side semiconductor layer 3p in the nitride semiconductor stack, and is an electrode in contact with at least the upper surface of the protrusion 3x. The p-side electrode 4 may extend to the side surface of the protrusion 3x, or may extend to the upper surface of the p-side semiconductor layer 3p around the protrusion 3x.
The p-side electrode 4 is a conductive member made of a material that is transparent to the wavelength region of laser oscillation. Examples of the transparent material include transparent conductive materials with a base material such as ITO (indium tin oxide) and IZO (indium zinc oxide). A specific example is ITO. A thinner thickness can reduce light absorption by the p-side electrode 4, but also increases resistance, so the thickness can be appropriately adjusted taking into account the balance between these two. The thickness of the p-side electrode 4 is, for example, 5 nm to 100 nm.

(Insulating film 7)
The insulating film 7 is disposed at a distance from the upper surface of the protruding portion 3x. The insulating film 7 covers at least a part of the surface of the p-side semiconductor layer 3p around the protruding portion 3x. The insulating film 7 may cover not only the p-side semiconductor layer 3p, but also the side surface of the active layer 3a and a part of the upper surface of the exposed n-side semiconductor layer 3n, and may further cover the side surface of the stack 3.
The insulating film 7 can be made of an inorganic material _such as _SiO2 , _{Ta2O5 , ZrO2} , _Al2O3 , _{or Ga2O3} .

(n-side electrode 5n, p-pad electrode 5p)
The vertical cavity surface emitting laser element 8 preferably further includes an n-side electrode 5n electrically connected to the exposed n-side semiconductor layer 3n.
In addition to the light-transmitting p-side electrode 4 formed on the p-side semiconductor layer 3p, a p-pad electrode 5p electrically connected to the p-side electrode 4 may be disposed.
The n-side electrode 5n and the p-pad electrode 5p may be made of any conductive material commonly used as an electrode in the art, such as Ti/Pt/Au or Ti/Rh/Au.
The n-side electrode 5n and the p-pad electrode 5p may be formed in a single layer structure using the same or different materials, may be formed in the same laminate structure using the same material, or may be formed in different laminate structures using different materials. When the n-side electrode 5n and the p-pad electrode 5p are formed in the same laminate structure using the same material, the n-side electrode 5n and the p-pad electrode 5p can be formed in the same process.

(Heat dissipation substrate 17)
The vertical cavity surface emitting laser element 8 may be bonded to a heat dissipation substrate 17 as shown in Fig. 2. Examples of the heat dissipation substrate 17 include ceramics such as AlN, semiconductor substrates including semiconductors such as SiC, metal substrates made of a single metal or a composite of two or more metals. For example, a substrate using insulating AlN ceramics as a base material and having a plurality of metal films 15 formed on the surface thereof can be used as the heat dissipation substrate 17. The metal films 15 are electrically connected to the p-pad electrode 5p and the n-side electrode 5n, respectively. The thickness of the heat dissipation substrate 17 is, for example, about 50 µm to 500 µm.
The heat dissipation substrate 17 can be formed by a method commonly used in the art.
The bonding layer 13 may be made of, for example, the same material as the p-pad electrode 5p and the n-side electrode 5n described above, or solder or the like.
The metal film 15 can be formed of the same material as the p-pad electrode 5p and the n-side electrode 5n described above.

(Anti-reflection film 14)
The vertical cavity surface emitting laser element 8 emits laser light from the first light reflecting film 10 side, but an anti-reflection film 14 may be disposed on the surface of the first light reflecting film 10 opposite to the laminate 3, that is, the surface of the substrate 16 opposite to the laminate 3. The anti-reflection film 14 may be made of the same material as the dielectric multilayer film exemplified in the second light reflecting film 6 described above. By making the number of layers and the thickness of each layer different from that of the light reflecting film, a film having an anti-reflection function can be formed. For example, SiO ₂ /Nb ₂ O ₅ , SiO ₂ /Ta ₂ O ₅ , SiO ₂ /Al ₂ O ₃ , etc. may be mentioned. The thickness may be, for example, 0.1 μm to 1 μm.

[Method of Manufacturing Vertical Cavity Surface Emitting Laser Device]
The method for manufacturing a vertical cavity surface emitting laser element according to the present disclosure includes growing a first optical reflecting film on a substrate, growing a nitride semiconductor stack including an active layer on the first optical reflecting film, and growing a second optical reflecting film on the stack.
(Growth of the First Light Reflecting Film 10)
The first light reflecting film 10 includes a step of growing a first nitride semiconductor layer 11 and a step of growing a second nitride semiconductor layer 12. The step of growing the first nitride semiconductor layer 11 and the step of growing the second nitride semiconductor layer 12 are alternately repeated. As described above, the first light reflecting film 10 may have either the first nitride semiconductor layer 11 or the second nitride semiconductor layer 12 formed directly on the substrate 16 or the underlayer 9. For example, when the first nitride semiconductor layer 11 is the bottom layer in the first light reflecting film 10, it is preferable to grow the first nitride semiconductor layer 11 on the substrate 16 or the underlayer 9, and grow the second nitride semiconductor layer 12 thereon. In addition, in the layer above that, it is preferable to grow the first nitride semiconductor layer 11 on the second nitride semiconductor layer 12 grown on the first nitride semiconductor layer 11. Alternatively, when the second nitride semiconductor layer 12 is the bottom layer in the first light reflecting film 10, it is preferable to grow the second nitride semiconductor layer 12 on the substrate 16 or the underlayer 9, and grow the first nitride semiconductor layer 11 thereon. In the layer above that, it is preferable to grow the second nitride semiconductor layer 12 on the first nitride semiconductor layer 11 grown on the second nitride semiconductor layer 12. The number of times that the step of growing the first nitride semiconductor layer 11 and the step of growing the second nitride semiconductor layer 12 are alternately repeated can be set arbitrarily depending on the material used, the desired performance, and the like. For example, the alternating repetition may be performed 5 to 100 times, preferably 20 to 80 times, and more preferably 30 to 70 times. In other words, the number of pairs of the first nitride semiconductor layer 11 and the second nitride semiconductor layer 12 is preferably 5 to 100 pairs, more preferably 20 to 80 pairs, and even more preferably 30 to 70 pairs. By setting the number of pairs within this range, it is possible to increase the reflectance while maintaining controllability. In this way, a nitride semiconductor multilayer film can be formed to form the first light reflecting film 10.

(Step of Growing First Nitride Semiconductor Layer 11)
The first nitride semiconductor layer 11 can be formed as a layer made of, for example, a Group III nitride semiconductor containing aluminum and indium. Examples of such a semiconductor include _{Al.sub.X2In.sub.Y2Ga.sub.1 -X2-Y2N (} 0<X2, 0<Y2, X2+Y2.ltoreq.1), and more specifically, AlInGaN, AlInN, etc. Among them, an AlInN layer is preferable.
The first nitride semiconductor layer 11 can be grown by a method known in the art, such as MOCVD (metal organic chemical vapor deposition), HVPE (hydride vapor phase epitaxy), MBE (molecular beam epitaxy), etc. Among these, the MOCVD method is preferred in terms of ease of thickness control and growth time. In any case where any method is used, the first nitride semiconductor layer 11 may be grown at normal pressure or at reduced pressure.
For example, the substrate 16 on which the underlayer 9 is formed is set in a reactor of an MOCVD apparatus, the substrate temperature is raised to, for example, 700° C. to 1050° C., and a carrier gas and a source gas are flowed into the reactor. This allows the formation of a first nitride semiconductor layer 11 made of a group III nitride semiconductor containing, for example, aluminum and indium. Examples of the carrier gas include hydrogen gas, nitrogen gas, and argon gas. Examples of the source gas include ammonia and nitrogen as a source gas for nitrogen. Examples of the aluminum source gas include trialkylaluminum such as TMA (trimethylaluminum), aluminum chloride, and the like, and examples of the indium source gas include trialkylindium such as TMI (trimethylindium). Examples of the nitrogen source gas include ammonia, nitrogen, and the like. Note that when growing AlInGaN, a gallium source gas may be introduced. In addition, in forming the first nitride semiconductor layer 11, the surface on which the first nitride semiconductor layer 11 is grown is preferably the (20-21) plane of the substrate or underlayer (for example, GaN).
The thickness _L1 of the first nitride semiconductor layer 11 can be calculated by the formula (1) as described above, and can be set based on the calculated thickness. For example, the thickness can be 30 nm to 80 nm. The first nitride semiconductor layer 11 can be doped with an n-type impurity, but is preferably undoped in terms of crystallinity and reflectance. For example, when the n-type impurity is Si, the n-type impurity concentration can be 0.5 to 10×10 ¹⁸ cm ⁻³ .

The first nitride semiconductor layer 11 may be formed on the substrate 16 on which the second nitride semiconductor layer 12 is formed.

(Step of Growing Second Nitride Semiconductor Layer 12)
The second nitride semiconductor layer 12 can be formed as, for example, a layer made of GaN. The step of growing the second nitride semiconductor layer 12 can be performed in the same manner as the step of growing the first nitride semiconductor layer 11.
For example, the substrate 16 on which the underlayer 9 is formed is set in the reactor of the MOCVD apparatus. The substrate 16 is preferably one on which the above-mentioned first nitride semiconductor layer 11 is formed on the underlayer 9. Thereafter, the substrate temperature is raised to, for example, 700° C. to 1050° C., and the carrier gas and the source gas (TEG and ammonia) are flowed into the reactor. Examples of the carrier gas include hydrogen gas, nitrogen gas, and argon gas. Examples of the source gas include trialkylgallium such as TMG (trimethylgallium) and TEG (triethylgallium) as a source gas for gallium. Examples of the source gas for nitrogen include ammonia and nitrogen. This allows the second nitride semiconductor layer 12 made of, for example, GaN to be formed. The growth rate of the second nitride semiconductor layer 12 may vary during the growth, but is preferably constant.
The thickness _L2 of the second nitride semiconductor layer 12 here can be calculated by the formula (2) as described above, and can be set based on it. For example, it can be 30 nm to 80 nm. Therefore, the total thickness of the nitride semiconductor multilayer film constituting the first light reflecting film 10 can be, for example, 500 nm to 10000 nm, preferably 2000 nm to 8000 nm, and more preferably 3000 nm to 7000 nm. In addition, the second nitride semiconductor layer 12 can be doped with an n-type impurity, but is preferably undoped. For example, when the n-type impurity is Si, the n-type impurity concentration can be 0.5 to 10×10 ¹⁸ cm ⁻³ .

(Growth of nitride semiconductor stack)
On the upper surface of the first light reflecting film 10, a laminate 3 of nitride semiconductors having an active layer 3a is formed.
In the nitride semiconductor layer stack 3, it is preferable to form the n-side semiconductor layer 3n, the active layer 3a, and the p-side semiconductor layer 3p in this order from the first light reflecting film 10 side. The nitride semiconductor layer stack 3 can be formed by utilizing a method known in the art.
The nitride semiconductor layer stack 3 may have a protrusion 3x formed on its surface, that is, the surface opposite to the side on which the first light reflecting film 10 is disposed. The protrusion 3x can be formed by using photolithography, etching, or the like.

(Formation of the second light reflecting film)
1B , in the case where a convex portion 3x is formed on the surface of the nitride semiconductor layer stack 3, the second light reflecting film 6 can be disposed on at least a part of the surface surrounding the convex portion 3x from the upper surface of the convex portion 3x. In the case where no convex portion 3x is formed, an insulating film that exposes a part of the nitride semiconductor layer stack 3 can be formed, and the second light reflecting film 6 can be formed so as to cover the upper part of the exposed nitride semiconductor layer stack 3.
The second light reflecting film 6 may have the same configuration as the first light reflecting film 10 described above, or may have a configuration including, for example, a dielectric multilayer film. Examples of the dielectric multilayer film include SiO ₂ /Nb ₂ O ₅ , SiO ₂ /Ta ₂ O ₅ , and SiO ₂ /Al ₂ O _3. The thickness of each layer is λ _c /4n. The number of layers and the thickness of the second light reflecting film 6 can be appropriately set depending on the combination of the dielectric multilayer films. Specifically, when the second light reflecting film 6 is made of SiO ₂ /Nb ₂ O ₅ or the like, each layer has a thickness of 40 nm to 100 nm. The number of layers of the laminated film can be 2 or more, for example, 5 to 20 layers. The total thickness of the second light reflecting film 6 can be, for example, 0.08 μm to 2.0 μm, and can be 0.6 μm to 1.7 μm.

Test Example 1
A GaN layer was formed as an underlayer 9 on a substrate 16 whose main surface was a (20-21) plane, and a first light reflecting film 10 was formed thereon.
The thickness of the first light reflecting film 10 was designed based on the above-mentioned formulas (1) and (2) for polarized light in the [-12-10] direction with a wavelength of 500 nm, and the first light reflecting film 10 was a multilayer film in which 50 pairs of _Al0.8In0.2N layers (thickness: _56.7 nm, _n1 = 2.2049) were stacked as the first nitride semiconductor layer 11 and 50 pairs of GaN layers (thickness: 52.1 nm, _n2 = 2.4015) were stacked as the second nitride semiconductor layer.
The reflection spectrum of the prepared first light reflecting film was measured for each polarized light using a spectrophotometer. The results are shown in Figures 3A and 3B. As shown in Figure 3B, it was confirmed that the first light reflecting film exhibited a reflectance of 98% or more for polarized light in the [-12-10] direction around 499 nm to 505 nm, and in particular, a reflectance of 99% or more for polarized light in the [-12-10] direction around 504 nm.

Description of the Reference Signs 3 Stacked body 3a Active layer 3n n-side semiconductor layer 3p p-side semiconductor layer 3x Convex portion 4 p-side electrode 5n n-side electrode 5p p-pad electrode 6 Second optical reflection film 7 Insulating film 8 Vertical cavity surface emitting laser element 9 Underlayer 10 First optical reflection film 11 First nitride semiconductor layer 11a Main surface 12 Second nitride semiconductor layer 12a Main surface 13 Bonding layer 14 Anti-reflection film 15 Metal film 16 Substrate 17 Heat dissipation substrate

Claims (8)

a first light reflecting film including a first nitride semiconductor layer and a second nitride semiconductor layer having a different composition from the first nitride semiconductor layer, the first nitride semiconductor layer having a plurality of refractive indexes that differ between a first polarization direction and a second polarization direction for light incident on a primary surface, the refractive index for the first polarization direction being _n1 , and having a thickness _L1 represented by the following formula (1); and the second nitride semiconductor layer having a plurality of refractive indexes that differ between the first polarization direction and the second polarization direction for light incident on a primary surface, the refractive index for the first polarization direction being _n2 , and having a thickness _L2 represented by the following formula (2);
a nitride semiconductor layer disposed on the first light reflecting film and having an active layer;
and a second optical reflection film disposed above the laminate.
L ₁ =λ _c /(4・n ₁ ) (1)
L ₂ =λ _c /(4・n ₂ ) (2)
(In the formula, λ _c represents the resonance wavelength.) the first nitride semiconductor layer and the second nitride semiconductor layer have a primary surface which is a (20-21) plane, the first polarization direction which is a [-12-10] direction, and the second polarization direction which is a polarization direction other than the [-12-10] direction;
2. The vertical cavity surface emitting laser device according to claim 1, further comprising a substrate provided below said first light reflecting film, said substrate having a main surface which is a (20-21) plane . The vertical cavity surface emitting laser element according to claim 1 or 2, wherein the first nitride semiconductor layer is an AlInN layer, and the second nitride semiconductor layer is a GaN layer. The vertical cavity surface emitting laser element according to claim 3, wherein the reflectance of the first light reflecting film for light in the first polarization direction at the resonant wavelength is 98% or more. The vertical cavity surface emitting laser element according to any one of claims 1 to 4, wherein the nitride semiconductor stack has an n-side semiconductor layer disposed between the active layer and the first optical reflection film, and a p-side semiconductor layer disposed between the active layer and the second optical reflection film. The vertical cavity surface emitting laser element according to any one of claims 1 to 5, wherein the second light reflecting film includes a dielectric multilayer film. The vertical cavity surface emitting laser element according to any one of claims 1 to 6, wherein the first nitride semiconductor layer and the second nitride semiconductor layer are alternately and repeatedly stacked, and the number of repetitions is 30 times or more and 70 times or less . growing a first light-reflecting film on the substrate;
growing a nitride semiconductor layer including an active layer on the first light reflecting film;
a second optical reflecting film is grown on the laminate,
The first light reflecting film,
The semiconductor device is formed by laminating two or more types of single crystal nitride semiconductor layers, including a first nitride semiconductor layer and a second nitride semiconductor layer having a different composition from the first nitride semiconductor layer;
the first nitride semiconductor layer has a plurality of refractive indexes that differ between a first polarization direction and a second polarization direction for light incident on a primary surface, the refractive index for the first polarization direction being _n1 , and has a thickness _L1 expressed by the following formula (1); and the second nitride semiconductor layer has a plurality of refractive indexes that differ between the first polarization direction and the second polarization direction for light incident on a primary surface, the refractive index for the first polarization direction being _n2 , and is a film having a thickness _L2 expressed by the following formula (2).
L ₁ =λ _c /(4・n ₁ ) (1)
L ₂ =λ _c /(4・n ₂ ) (2)
(In the formula, λ _c represents the resonance wavelength.)

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