JP2012129245A - Vertical-resonator surface-emission laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a vertical-resonator surface-emission laser that includes an electrode layer having an intra-cavity structure and is capable of operating at a low threshold value.SOLUTION: A resonator has therein three or more odd number of spacer layers 503, 505, and 507, and they are portioned by two or more intermediate mirrors 504 and 506. When an optical distance between an arbitrary number (2N'-1) (N' represents an integer equal to or greater than 1) of mutually adjacent mirrors is represented by d, and phase skips of rays of light reflected by mutually adjacent intermediate mirrors are represented by Φ, and Φ, respectively, a relationship expressed by the following equation (1): 2d*2π/λ+Φ+Φ=2N*π(1) is satisfied. When an optical distance between 2N' mutually adjacent mirrors is represented by d', a relationship expressed by the following equation (2): 2d'*2π/λ+Φ+Φ=(2N-1)*π (2) (where N represents an integer equal to or greater than 1) is satisfied. An electrode layer is provided in an even-numbered spacer layer counting from a lower mirror or in its adjacent portion.

Description

  The present invention relates to a vertical cavity surface emitting laser.

In recent years, surface emitting laser elements have been actively studied. A surface emitting laser has advantages such as a low threshold, low power consumption, easy integration into an array, and low cost, and is expected to be applied in fields such as communication, electrophotography, and sensing. In particular, it has already been put into practical use in the communication field such as infrared short-range communication.
Surface emitting lasers can be classified into several types, and one of them is a vertical cavity surface emitting laser (VCSEL).
This is a laser element in which an active layer is sandwiched between upper and lower DBRs (Distributed Bragg Reflectors) to form a resonator, and light is resonated in a direction perpendicular to the substrate to emit light.

A problem of the vertical cavity surface emitting laser is a problem of current injection.
That is, in an element manufactured using a semiconductor material having high electrical resistance (particularly a p-type semiconductor), current injection through a mirror or current injection through a ring electrode becomes difficult due to the influence of the high resistance.
For example, in a blue region semiconductor laser, a gallium nitride (GaN) -based material is mainly used. When a vertical cavity surface emitting laser is manufactured using this, the p-side DBR has a high electric resistance, Current injection through the mirror is very difficult.
Furthermore, the electric resistance of the p-type semiconductor is high and current does not spread, and it is difficult to inject current from an oblique direction using a ring electrode.
In addition, in GaN-based VCSELs, it is difficult to produce a current confinement structure, which is one of the factors that make current injection difficult from an oblique direction.

To deal with these problems, intracavity structures are often used.
This is because the electrode layer is installed at the boundary between the mirror and the clad layer (that is, inside the resonator), and the current is injected directly above the active layer, so that the resistance value is reduced and the current is efficiently injected into the active layer. It was made to do.
The intracavity structure in the GaN system often uses a structure in which a dielectric DBR is used as a p-side mirror and current is injected with an electrode layer sandwiched between the mirror and the resonator. At this time, a measure is taken to reduce light absorption by the electrode layer, usually by placing the electrode layer at the node of the standing wave of the laser.
Non-Patent Document 1 discloses a structure of a vertical cavity surface emitting laser using such an intracavity structure.

Applied Physics Letters 90, 181128 (2007)

  However, in the vertical cavity surface emitting laser using such an intracavity structure, the electrode layer is disposed in the resonator sandwiched between the DBRs. Even so, there is a problem that light absorption is still large.

  In view of the above problems, an object of the present invention is to provide a vertical cavity surface emitting laser that can operate at a lower threshold by further reducing light absorption by an electrode layer having an intracavity structure.

The vertical cavity surface emitting laser of the present invention has a vertical oscillation wavelength λ in which a resonator is formed by a semiconductor layer including a lower mirror, an upper mirror, and an active layer disposed therebetween, which are stacked on a substrate. A cavity surface emitting laser,
The resonator includes three or more odd number of spacer layers therein, and the three or more odd number of spacer layers are partitioned by two or more intermediate mirrors,
The optical distance between any (2N′−1) adjacent mirrors in the surface-emitting laser is d (N ′: an integer of 1 or more), and the phase jump of the reflected light from the adjacent intermediate mirrors, respectively. When Φ ₁ and Φ ₂ are satisfied, the following equation (1) is satisfied, and when the optical distance between 2N ′ adjacent mirrors in the adjacent intermediate mirror is d ′, the following equation ( Satisfy the relationship of 2)
An electrode layer for current injection is provided in an even-numbered spacer layer counted from the lower mirror or in an adjacent portion thereof.
2d · 2π / λ + Φ ₁ + Φ ₂ = 2N · π Formula (1)

2d ′ · 2π / λ + Φ ₁ + Φ ₂ = (2N−1) · π Formula (2)

N: an integer greater than or equal to 1

In the vertical cavity surface emitting laser of the present invention, a plurality of semiconductor layers including a lower mirror, an upper mirror, and an active layer disposed therebetween are stacked on a substrate, thereby forming a resonator. Vertical cavity surface emitting laser,
The resonator has three or more odd spacer layers therein, and the three or more odd spacer layers are partitioned by an intermediate mirror,
An electrode layer for current injection is introduced into the spacer layer at a portion where the envelope of the light intensity of the standing wave of light oscillated by the resonator is minimized, or in the adjacent portion thereof To do.

  According to the present invention, it is possible to realize a vertical cavity surface emitting laser that can operate at a lower threshold by further reducing light absorption by an electrode layer having an intracavity structure.

The schematic diagram explaining the structural example of the vertical cavity surface emitting laser in Embodiment 1 of this invention. The schematic diagram for demonstrating the relationship between the envelope intensity of the standing wave of light of the surface emitting laser in Embodiment 1 of this invention, and the intensity | strength of the light in an electrode layer. The schematic diagram explaining the structural example of the vertical cavity surface emitting laser in Embodiment 2 of this invention. The schematic diagram explaining the structural example of the vertical cavity surface emitting laser in Embodiment 3 of this invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic cross-sectional view illustrating a configuration example of a vertical cavity surface emitting laser according to Embodiment 1 of the present invention. The figure which calculated intensity distribution of the standing wave of the light inside the resonator of the vertical cavity surface emitting laser in Example 1 of this invention. FIG. 6 is a schematic cross-sectional view illustrating a configuration example of a vertical cavity surface emitting laser in Example 2 of the present invention. The figure which calculated intensity distribution of the standing wave of the light in the resonator of the vertical cavity surface emitting laser in Example 2 of this invention. The figure which calculated intensity distribution of the standing wave of the light inside the resonator of the vertical cavity surface emitting laser in Example 3 of this invention. The figure which calculated intensity distribution of the standing wave of the light in the resonator of the vertical cavity surface emitting laser in Example 3 of this invention. The figure which plotted the value of the light intensity (amplitude) of the spacer layer part of the vertical cavity surface emitting laser in Embodiment 3 of this invention.

Hereinafter, a configuration example of a vertical cavity surface emitting laser according to an embodiment of the present invention will be described.
[Embodiment 1]
A configuration example of the vertical cavity surface emitting laser according to the first embodiment of the present invention will be described with reference to FIG.
In FIG. 1, reference numeral 101 denotes a substrate, on which a lower mirror 102 is provided. Further thereon, 103 spacer layers 1 (first spacer layer), 104 intermediate mirror 1 (first intermediate mirror), 105 spacer layer 2 (second spacer layer), 106 intermediate mirror 2 (first spacer layer). 2 intermediate mirrors) and 107 spacer layers 3 (third spacer layers). And the upper mirror 108 is provided on it and is laminated | stacked in these order.
The upper electrode layer 109 is provided in the spacer layer 2 105 (in the second spacer layer).
A lower electrode layer 110 is provided on the back surface of the substrate 101.
111 is an envelope of the light intensity of the standing wave of the resonant light (light that resonates in the resonator) in this element (the light intensity at the antinode of the standing wave is connected), 112 is the standing wave This is a schematic representation of a part of the light intensity.

Next, the arrangement of the spacer layer and the light intensity control will be described.
As shown in FIG. 1, by introducing three spacer layers between two mirrors and partitioning them with a mirror, the envelope distribution of the light intensity of the standing wave can be controlled.
Since the envelope is a line connecting the vertices of the antinodes of the light intensity of the standing wave, it represents the light intensity distribution in the antinodes.
That is, controlling the envelope distribution is equivalent to controlling the light intensity distribution.
Specifically, as shown by reference numeral 111 in FIG. 1, the light intensity distribution of the outer two spacer layers can be increased, and the light intensity distribution of the central spacer layer can be decreased.
By introducing the electrode layer into the portion where the light intensity distribution is reduced, light absorption by the electrode layer can be reduced. Even when the electrode is placed at the node of the standing wave, since the light intensity distribution itself is small, the effect of further reducing the absorption can be expected.

Here, the light intensity distribution control method will be described qualitatively.
First, the positional relationship of the mirrors that partition the spacer layer in this embodiment will be described.
In the vertical cavity surface emitting laser of this embodiment, it is necessary to design the distance between the mirrors appropriately.
The distance between mirrors is the distance between the mirror and the boundary between the spacer layers in each mirror. There can be a case where there are two boundary portions for each mirror. For example, since the 104 intermediate mirror 1 has a boundary between the 103 spacer layer 1 and the 105 spacer layer 2, there are two boundaries. In such a case, the boundary part used to define the distance between the mirrors is a boundary part on the side close to another mirror. For example, in the case of 104 intermediate mirror 1 and 106 intermediate mirror 2, the boundary portion uses the boundary portion between 104 intermediate mirror 1 and 105 spacer layer 2, and the boundary portion between 106 intermediate mirror 2 and spacer layer 2. Accordingly, the distance between the 104 intermediate mirror 1 and the 106 intermediate mirror 2 is equal to the distance between the boundary portions, that is, the thickness of the 105 spacer layer 2. The optical distance between the mirrors is obtained by multiplying the distance between the mirrors by the refractive index of the substance existing between the mirrors. When there are a plurality of substances, the optical distance is calculated for each substance according to the distance and refractive index occupied by the substance, and each is added.
The design guideline for the distance between the mirrors is specifically as follows.
The optical distance between any adjacent mirrors is d, the phase jumps of the reflected waves by the two mirrors are Φ ₁ and Φ ₂ , respectively, and the resonance wavelength is λ (oscillation wavelength λ).
Regarding the distance between the mirrors, for example, the distance between the 104 intermediate mirror 1 and the 106 intermediate mirror 2 in the present embodiment, the following equation (1) may be satisfied.

2d · 2π / λ + Φ ₁ + Φ ₂ = 2N · π Formula (1)
(However, N is an integer of 1 or more)

In the above description, the intermediate mirror 1 of 104 and the intermediate mirror 2 of 106 are taken as an example, but this relationship holds in all adjacent mirrors.

Further, for the distance between two adjacent mirrors, in this embodiment, the optical distance d ′ between the intermediate mirror 1 104 and the upper mirror 108, the following equation (2) may be satisfied.

2d ′ · 2π / λ + Φ ₁ + Φ ₂ = (2N−1) · π Formula (2)

In the above description, the intermediate mirror 1 and the upper mirror 108 are taken as an example, but this relationship is established in all the two adjacent mirrors.
Finally, regarding the distance between three adjacent mirrors, in this embodiment, the distance between the lower mirror 102 and the upper mirror 108, the relationship of the above equation (1) may be established.
In the resonator designed in this way, the light intensity distribution becomes small at the position of the spacer layer 2 105 as shown in FIG.

To generalize the above, for a vertical cavity surface emitting laser element having three or more odd spacer layers in the resonator and divided by two or more even mirror layers, respectively. It is necessary to satisfy the following conditions.
That is, N ′ must be an integer greater than or equal to 1, and (2N′−1) adjacent mirrors must satisfy the relationship of the above equation (1), and 2N ′ adjacent mirrors must satisfy the relationship of the above equation (2). There is.
The light intensity distribution of the even-numbered spacer layers counted from the lower mirror is minimal.
Therefore, light absorption by the electrode layer can be reduced by providing the electrode layer at the position of the spacer layer where the light intensity distribution is minimized.
However, since errors occur in the phase of the reflected wave from the mirror for various reasons, the distance between the mirrors has a width, and N is not always an integer.
Therefore, it is necessary to adjust the value of N in light of the resonance wavelength to be obtained in an actual design.
However, the smaller the range of the value N deviates from the integer, the better, preferably ± 1/8, more preferably ± 1/16.
Accordingly, the corresponding deviation in the distance between the mirrors is preferably ± λ / 8 or less, more preferably ± λ / 16 or less.
The above is the basic design guideline, but since the value of the phase skip Φ at the boundary portion of the spacer layer differs depending on the type of mirror, each typical case will be described below.

First, the type of mirror will be described.
The mirror in the vertical cavity surface emitting laser of this embodiment refers to a multilayer mirror (including a single layer film), a metal mirror, or a composite mirror of both.
In the case of an ordinary standard multilayer mirror, the thickness d of each layer constituting the multilayer mirror satisfies the relationship of the above formula (2), where Φ is the phase of the reflected light from the adjacent layer. Yes.
In such a case, the distance between the mirrors is determined by the magnitude relationship between the refractive index of the layer constituting the spacer layer and the layer constituting the mirror at the boundary between the mirror and the spacer layer.
Hereinafter, a layer constituting the spacer layer in the boundary portion is referred to as a boundary spacer layer, and a layer constituting the mirror is referred to as a boundary mirror layer. Here, all the boundary spacer layers are assumed to be dielectrics or semiconductors.
When the refractive indexes of the boundary spacer layer and the boundary mirror layer are nd and nm, respectively, Φ = π when the relationship of the following expression (3) is satisfied at the boundary, and Φ = 0 when the relationship of the expression (4) is satisfied. It becomes.

nd <nm Formula (3)

nd> nm Formula (4)

In addition, when nd = nm, light is not reflected there, so that it is not regarded as a boundary in the present invention.

Here, considering the case of the mirrors adjacent to each other, when the above formula (3) or the above formula (4) is obtained at the boundary between both mirrors (when the refractive index relationship is the same), the above formula ( In order to satisfy the relationship 1), the distance between the mirrors is λN / 2.
When the refractive index relationship is different from each other at the boundary between both mirrors, that is, when one is the above equation (3) and the other is the above equation (4), the distance between the mirrors is λ (2N−1) / 4. Become.
The same applies to the relationship between two adjacent mirrors and between three adjacent mirrors.
When the refractive index relationship is the same at both mirror boundaries, the distance between the mirrors is λ (2N-1) / 4 and λN / 2, respectively, and when they are different, λN / 2 and λ (2N-1) / 4, respectively. It becomes.

Next, a case where there is an error in the thickness of the multilayer mirror will be described.
In any of the above cases, the phase jump Φ by the multilayer mirror varies depending on the error of the film thickness of each layer constituting the mirror.
For example, if there is a random error in each layer thickness, the layer thickness error accumulates, and the phase jump Φ of the entire mirror deviates to some extent from the above conditions.
The phase jump Φ is strictly in the above relationship only when the above expression (2) is strictly satisfied in each layer of the multilayer mirror. Therefore, in such a case, N is not completely an integer.

Next, the case of an irregular multilayer mirror or the case of a metal mirror will be described. If the irregular multilayer mirror, that is, the mirror as a whole functions as a mirror but the conditions of the individual layers do not necessarily satisfy the relationship of the above formula (2), the value of the phase jump Φ is still a mirror. It changes as a whole.
For this reason, the distance between the mirrors cannot be designed based on the refractive index relationship at the boundary.
For example, when most of the layers satisfy the relationship of the above equation (2) and only a few layers deviate significantly from the relationship of the above equation (2), or layers with different thicknesses and refractive indexes overlap. When the λ / 4 period is formed as a whole, the distance between the mirrors cannot be designed by, for example.
Also, when the mirror is a metal mirror, the magnitude of the phase jump Φ varies depending on the material, and the thickness between the mirrors varies accordingly.
For example, for light having a resonance wavelength of 589 nm, the phase jump Φ of the reflected light on the silver mirror surface is approximately π5 / 6.
Therefore, in the above case, the Φ of each mirror is individually determined, and the distance between the mirrors is individually set so that the relationship of the above formula (1) and the above formula (2) is satisfied for each numerical value. It is necessary to decide.

Next, the reason why the light intensity in the portion 105 of the spacer layer 2 can be weakened will be described.
The reflected lights from the mirrors that satisfy the condition of the above formula (1) weaken each other due to interference.
Accordingly, when focusing on the spacer layer 2 of 105, the reflected light from the intermediate mirror 2 of the upper mirrors 108 and 106 satisfying the relationship of the above formula (1) weakens each other, and the lower mirrors 102 and 104 below are weakened. The reflected light from the intermediate mirror 1 is also weakened.
The mirrors in which the reflected lights weaken each other have the effect of reducing light confinement in the resonator by the mutual mirrors.
Therefore, the optical confinement of 105 in the spacer layer 2 is weakened by the intermediate mirror 1 of 106 and the intermediate mirror 1 of 104, respectively, rather than the case of confining only by the upper mirror 108 and the lower mirror 102. The light intensity of In practice, the strength of confinement is determined by the mirror with the lower reflectivity.

On the other hand, when focusing on the spacer layer 103, the reflected light from the intermediate mirror 2 of the upper mirrors 108 and 106 satisfying the relationship of the above formula (1) weakens each other.
However, since the reflected light from the mirror satisfying the relationship of the above formula (2) strengthens each other due to interference, in the intermediate mirror 1 of the upper mirrors 108 and 104 satisfying the relationship of the above formula (2), the reflected light of the upper mirror Interferes with the reflected light of the intermediate mirror and is strengthened.
Therefore, in the spacer layer 1 of 103, the light confinement by the upper mirror 108 is weakened by the intermediate mirror 2 of 106, but the amount enhanced by the intermediate mirror 1 of 104 comes out. Become stronger.
Regarding the lower lower mirror 102 as well, since the spacer layer 1 of 103 is confined only by the lower mirror 102, the confinement of the lower mirror 102 is stronger than the spacer layer 2 of 105 which is weakened by the intermediate mirror 1 of 104. .
The same can be said for the 107 spacer layer 3 because it is symmetrical.

As described above, the light intensity of the standing wave is weak in the 105 spacer layer 2 having a weak confinement, and the light intensity is strong in the spacer layer 1 103 and the spacer layer 3 107 having a strong confinement.
Here, the reflectance of the upper and lower mirrors is greater than the reflectance of the intermediate mirror.
However, only one of the three resonance modes has such a light distribution.
There are three resonance modes in the coupling mode by the three spacer layers, and each mode is a different mode.
It has been found by calculation that the mode as in the present embodiment is the central mode of the three on the wavelength axis.
In this embodiment, it is possible to oscillate in a desired mode by matching the gain peak of the active layer and its resonance wavelength.
Also, the resonance modes on both sides may disappear when the reflection band of the mirror is narrowed.

Below, the light absorption in the electrode layer in this embodiment is demonstrated.
FIG. 2 is a diagram showing the relationship between the position of the electrode layer for the standing wave of light and the intensity of the standing wave.
In FIG. 2, 201 indicates the light intensity of a standing wave having a low intensity, and 202 indicates the light intensity of a standing wave having a high intensity. 203 is an envelope of light intensity of a standing wave having a low intensity, 204 is an envelope of light intensity of a standing wave having a high intensity, and 205 is an electrode layer.
A portion surrounded by a dotted-line square in the drawing is an enlarged view of a standing wave node provided with an electrode layer.
Although the electrode layer in the figure is placed at the node of the standing wave of light, the electrode layer 205 has a finite thickness, so even when it is placed at the node part (obviously at other locations), the light envelope The loss due to absorption can be reduced by placing the wire at a small portion.
In FIG. 2, it is clear that the strength of the standing wave envelope is stronger and the area of the standing wave that overlaps the electrode layer 205 is smaller than the weaker one.

In the present embodiment, in order to reduce absorption, it is preferable to introduce the electrode layer into the node portion of the standing wave.
Further, although the upper electrode layer 109 in FIG. 1 is introduced into the spacer layer 105, it may be introduced at the boundary between the spacer layer 2 and the intermediate mirror 2.
Moreover, it is also possible to introduce into the adjacent part of the spacer layer 2 (in this case, the electrode layer can also be regarded as a part of the intermediate mirror 2).

In the present embodiment, since the lower electrode layer 110 is provided on the back surface of the substrate, considering current injection, the lower side from the upper electrode layer 109 is preferably made of a semiconductor material.
Since the upper side has to form a layer on the electrode layer, it is preferable to use a dielectric material, but it is also possible to use a semiconductor material.
Further, in this embodiment, the upper electrode layer to the substrate are all made of a semiconductor material, and the lower electrode layer 110 is provided on the back surface of the substrate. However, the substrate can be made of a dielectric such as sapphire.
In such a case, the lower electrode layer is usually not provided on the back surface of the substrate, but is provided on a portion of the resonator formed on the substrate, such as a cladding layer.

As a dielectric material that can be used in the present embodiment, oxides such as SiO ₂ , Al ₂ O ₃ , HfO ₂ , ZrO ₂ , Ta ₂ O ₅ , TiO ₂ , and Y ₂ O ₃ can be used.
As the semiconductor material, carrier-doped or non-doped GaAs, AlGaAs, InP, GaAsInP, AlGaInP, GaN, InGaN, AlGaN, AlN, InN, and other III-V group compound semiconductors and arbitrary mixed crystals thereof are used. be able to.
Moreover, II-VI group compound semiconductors, such as ZnSe, CdS, ZnO, and those arbitrary mixed crystals, IV group semiconductors, such as Si and SiGe, and those arbitrary mixed crystals, etc. can be used. Furthermore, various organic semiconductors can be used.
In addition, as a material of the metal mirror, metal materials such as Au, Al, Ti, Ni, Pd, Zn, Pt, and Ag, and arbitrary alloys thereof can be used.
Furthermore, as an electrode material to be used, metal materials such as Au, Al, Ti, Ni, Pd, Zn, Pt, Ag, and Ge, and their arbitrary alloys can be used.
These are techniques that are frequently used in electrical devices such as laser elements, because the type and composition of a suitable alloy can be selected depending on the semiconductor and dielectric materials used.
For example, materials such as Au—Ge—Ni and Au—Ge—Pt are usually used for the n-electrode layer of GaAs, and Ag—Zn and Au—Zn are usually used for the p-electrode layer.
In addition, a transparent conductive oxide such as ITO (Indium Tin Oxide), SnO ₂ , or In ₂ O ₃ can be used as the electrode material.

In this embodiment, the active layer of the laser is introduced into the 103 spacer layer 1 (not shown).
Further, the active layer is introduced into the standing wave peak portion in the spacer layer 103 of 103, so that the optical confinement coefficient is increased and the gain is increased.
The active layer is preferably provided on 103 spacer layers 1 where the light intensity distribution is strong, but may be provided on 105 spacer layers 2 or both.
As the structure used for the active layer, a quantum well structure, a strained quantum well structure, a quantum dot structure, or the like can be used.
The upper and lower layers of the active layer 103 of the spacer layer 1 become an upper and lower cladding layer, respectively.
Although not depicted in the present embodiment, it is also possible to provide a current confinement layer due to water vapor oxidation or ion implantation and a carrier block layer for suppressing overflow from the active layer of carriers at any part of the resonator. It is.

The vertical cavity surface emitting laser according to this embodiment can be driven by injecting current from the electrodes, using the upper electrode layer 109 and the lower electrode layer 110 as a p-electrode layer and an n-electrode layer, respectively. At that time, the semiconductor layer above the active layer is p-type and the lower semiconductor layer is n-type doped.
Of course, the polarity of the electrode can be changed, and in that case, the doping of the semiconductor layer also needs to be changed.
In this embodiment, the active layer is provided on the spacer layer 3 107, but it can also be provided on the spacer layer 2 105.

[Embodiment 2]
A configuration example of the vertical cavity surface emitting laser according to the second embodiment of the present invention will be described with reference to FIG.
In FIG. 3, reference numeral 301 denotes a substrate, on which a lower mirror 302 is provided.
Further thereon, a spacer layer 1 303 (first spacer layer), an intermediate mirror 1 304 (first intermediate mirror), a spacer layer 2 305 (second spacer layer), an intermediate mirror 2 306 (first spacer layer). 2 intermediate mirrors) are provided.
Furthermore, the spacer layer 3 (third spacer layer) 307, the intermediate mirror 3 (third intermediate mirror) 308, the spacer layer 4 (fourth spacer layer) 309, the intermediate mirror 4 (third) 4 intermediate mirrors) and 311 spacer layers 5 (fifth spacer layers) are provided.
And the upper mirror 312 is provided on it and is laminated | stacked in these order. In the present embodiment, the lower electrode layer 314 and the upper electrode layer 313 are introduced into the spacer layer 2 of 305 and the spacer layer 4 of the 309 (in the fourth spacer layer), respectively.
Reference numeral 315 schematically represents an envelope of the light intensity of a standing wave that resonates in the element.

In the present embodiment, the light intensity distribution can be controlled as indicated by 315 by increasing the number of spacer layers to five. Therefore, both the upper electrode layer 313 and the lower electrode layer 314 are introduced into two spacer layers having low light intensity (the spacer layer 4 of 309 and the spacer layer 2 of 305, respectively).
Of course, as in the first embodiment, it can be provided not only in the spacer layer, but also at the boundary portion with the adjacent mirrors or at the adjacent portion.
Furthermore, in the case of this embodiment, the active layer is introduced into the spacer layer 3 307.
In the present embodiment, the relationship between the adjacent mirrors to the three adjacent mirrors is all the same as that of the first embodiment.
Further, the distance between the four adjacent mirrors and the five adjacent mirrors satisfies the relationship of the above formula (2) and the above formula (1) of the first embodiment, and takes the same value for the error between the mirrors. .

In the present embodiment, a dielectric material is preferably used above the upper electrode layer and below the lower electrode layer, but a semiconductor material can also be used.
A semiconductor material is preferably used below the upper electrode layer 313 and above the lower electrode layer 314.
Materials that can be used are the same as those described in the first embodiment.
The substrate material may be a dielectric material such as sapphire or glass, or a flexible substrate such as polyimide, but the same semiconductor material as in the first embodiment can also be used. Materials such as electrodes are the same as those in the first embodiment.
The configuration of the active layer, the clad layer, etc., the arrangement of the p electrode layer, the n electrode layer, etc. are all the same as in the first embodiment.

[Embodiment 3]
A configuration example of the vertical cavity surface emitting laser according to the third embodiment of the present invention will be described with reference to FIG. The element configuration in FIG. 4 is exactly the same as in FIG.
4 differs from FIG. 1 in that the light intensity distribution is larger in the spacer layer 1 of 403 and smaller in the spacer layer 2 of 405 than the distribution of the first embodiment.
In order to obtain such a light intensity distribution, the reflectance of the intermediate mirror 1 404 should be increased, and the reflectance of the intermediate mirror 2 406 should be decreased.
The reason can be explained from the principle of the light intensity distribution control described above.
First, in order to increase the light intensity of the spacer layer 1 at 403, the reflectance of the intermediate mirror 1 at 404 for increasing confinement with respect to the upper mirror 408 is increased, or at 406 for reducing confinement with respect to the upper mirror 408. It is necessary to reduce the reflectivity of the intermediate mirror 2.
Here, it is confirmed that the reflectance of the intermediate mirror 2 406 does not become 0 no matter how small.
Next, in order to reduce the light intensity of the spacer layer 2 405, it is necessary to increase the reflectivity of the intermediate mirror 1 404 and the intermediate mirror 2 406.
However, when the reflectance of the intermediate mirror 2 at 406 is increased, confinement of the spacer layer 1 at 403 is weakened.
Therefore, in order to increase the light intensity of the spacer layer 1 at 403 and decrease the light intensity of the spacer layer 2 at 405, the reflectance of the intermediate mirror 1 at 404 is increased and the reflectance of the intermediate mirror 2 at 406 is decreased. Is good.
Of course, only increasing the reflectance of the intermediate mirror 1 404 is also effective.

In this embodiment, an active layer is introduced into the spacer layer 1 403, and an upper electrode layer 409 is introduced into the spacer layer 2 405.
Therefore, by increasing the light intensity of the spacer layer 1 403 and decreasing the light intensity of the spacer layer 2 405 as described above, the gain in the active layer can be further increased and the absorption loss due to the electrode layer can be further reduced. The configuration is suitable for laser applications. Preferably, the reflectance of the intermediate mirror 1 of 404 is larger than the reflectance of the intermediate mirror 2 of 406, more preferably 15% or more, and further preferably 30% or more.
The reflectance of the mirror as described above can be controlled by adjusting the material and configuration of the mirror.
When the mirror is a single layer film or a multilayer film mirror, the mirror can be controlled by adjusting the refractive index difference between layers, the number of layers, and the layer thickness.
The difference in refractive index between layers here means the difference in refractive index between layers constituting the multilayer mirror, the layer at the end of the multilayer mirror (boundary mirror layer as defined above) and the adjacent layer in the spacer layer portion (boundary spacer layer). ) Includes both refractive index differences.

In FIG. 11, when the intermediate mirror 1 of 404 has a thickness λ / 4 layer 1 layer, when the refractive index is lowered (that is, when the refractive index difference with the boundary spacer layer is increased), The graph showing the mode of change of light intensity distribution is shown.
The graph is a plot of the light intensity (envelope) values of the portions 403 of the spacer layer 1 and 405 of the spacer layer 2.
As the refractive index of the intermediate mirror 1 404 decreases (the difference in refractive index from the boundary spacer layer increases), the reflectivity of the mirror increases.
Therefore, it can be seen that the light intensity in the spacer layer 1 of 403 increases and the light intensity in the spacer layer 2 of 405 decreases.
In this embodiment, the configuration and materials other than the reflectance of the mirror are all the same as those in the first embodiment.
However, since the effect cannot be obtained unless the spacer layer 1 of 403 has an active layer, it is necessary that at least the spacer layer 1 has an active layer introduced therein.

[Example 1]
As Example 1, a configuration example of a vertical cavity surface emitting laser to which the present invention is applied will be described with reference to FIG.
In FIG. 5, reference numeral 501 denotes a substrate, and a lower mirror 502 is laminated on the substrate.
The lower mirror 502 is composed of DBR. Further, 503 spacer layers 1 are laminated thereon, and an active layer 511 is introduced therein.
Of the spacer layer 1 503, a layer below the active layer 511 is a lower cladding layer 512, and an upper layer is an upper cladding layer 513.
On top of that, the intermediate mirror 1 of 504, the spacer layer 2 of 505, the intermediate mirror 2 of 506, and the spacer layer 3 of 507 are laminated in this order, and the DBR of the upper mirror 508 is laminated on the top.
The electrode is provided with a p-electrode layer 510 as a part of the intermediate mirror 2 of the n-electrode layers 509 and 506 on the back surface of the substrate. The drawings are schematic diagrams, and the scales and the like do not reflect actual objects.
The dimensions and materials of each component are as follows.
The substrate 501 is a GaN substrate and has a thickness of 430 μm.
The lower mirror 502 is a DBR of GaN / Al _0.85 In _0.15 N, and each pair of layers has a thickness of 44 nm and 48 nm (˜λ / 4), and a total of 30.5 pairs are stacked.
λ / 4 is a value converted into an optical distance, and “˜” is an approximate meaning. Hereinafter, the distance in wavelength units refers to the optical distance.
In FIG. 5, the number of pairs is drawn less than the actual number for convenience of description. The order of the layers is AlInN and GaN from the substrate.
The spacer layer 1 of 503 is GaN, the thickness is 176 nm (˜λ), and the thicknesses of the lower cladding layer 512 and the upper cladding layer 513 are each 76 nm.
The active layer 511 has a multiple quantum well structure of In _0.2 Ga _0.8 N / GaN, and each layer thickness of 3 nm / 7.5 nm is composed of three layers / two layers.
Further, the intermediate mirror 1 of 504 is composed of one λ / 4 layer, and the material is Al _0.85 In _0.15 N and the thickness is 48 nm.
The spacer layer 2 of 505 is GaN, the thickness is 85 nm (˜λ / 2), the intermediate mirror 2 of 506 is HfO _{2 as} the material of the dielectric portion, and the thickness is 42 nm. The intermediate mirror 2 is ˜λ / 4 together with the p electrode layer 510, and the thickness of the p electrode layer 510 is 16 nm in this embodiment.
Further, the spacer layer 3 of 507 is ZrO ₂ and has a thickness of 86 nm (˜λ / 2), and the upper mirror 508 is DBR of ZrO ₂ and HfO ₂ and has a thickness of 43 nm (˜ λ / 4), 50 nm (˜λ / 4), and the number of stacked pairs is 20 pairs.
The electrodes are Ti / Al and Ni / Au, respectively, for the n electrode layer 509 and the p electrode layer 510.
The substrate 501, the lower mirror 502, and the lower cladding layer 512 are doped n-type, and the intermediate mirrors 1 and 505 of the upper cladding layers 513 and 504 are doped p-type.
The element is a circular mesa having a diameter of 20 μm, and the height is 1.8 μm.

FIG. 6 shows the result of calculating the intensity distribution of the standing wave in the resonator in the element of this example.
Reference numeral 601 denotes the light intensity of the standing wave, 602 denotes the dielectric constant for each layer, and 603 denotes the envelope of the light intensity of the standing wave.
This calculation shows the light distribution in the vicinity of the spacer layer portion of the element (region surrounded by the dotted line in the upper right figure). However, the number of DBR layers in the upper right figure is different from the actual one.
In each figure, the dielectric constant distribution and the element name of the element corresponding to the portion are shown.
As can be seen from FIG. 6, the envelope of the spacer layer 2 portion is lowered, so that the light intensity of this portion is weakened. Therefore, as described in the embodiment of the present invention, the light intensity existing in the electrode layer introduced into the node of the standing wave in this portion is also reduced, and the absorption can be reduced.
According to the calculation, in this embodiment, the intensity of light located in the electrode layer can be reduced by about 20% compared to a resonator having a single spacer layer in a normal VCSEL.

The element in this embodiment operates near a wavelength of 410 nm by current driving. In this embodiment, the p-electrode layer 510 and one dielectric layer on the p-electrode layer 510 are combined to form the intermediate mirror 2 506.
The total thickness of the two layers is λ / 4, and the thickness of the p-electrode 510 is very thin at about λ / 10, so that it hardly affects the reflectivity of the mirror and the phase jump Φ with respect to the reflected wave. In designing, the influence of the electrodes is excluded.
Originally, the phase jump Φ of the reflected wave by the intermediate mirror 2 of 506 does not become 0 because of the presence of the electrode layer, but is regarded as almost 0 here (that is, it is assumed that there is no p electrode layer), and therefore the spacer Layer 2 is ˜λ / 2.
The fact that the light intensity distribution of the spacer layer 2 at 505 is still small suggests that the thin p-electrode layer 510 has little influence on the characteristics of the mirror.

When the p electrode layer 510 is thick, it is necessary to consider Φ including the p electrode layer.
Still, since absorption increases, it is preferable that the thickness of the p-electrode layer 510 is thin, and the optical distance is preferably λ / 4 or less. More preferably, it is λ / 8 or less.
However, if it is too thin, the resistance will increase, so it is preferable to have λ / 50 or more.
Note that, when ITO or the like having lower conductivity is used, the electrode layer 510 is preferably thicker, more preferably λ / 8 or more and λ / 2 or less.
In this embodiment, it is considered that the p-electrode layer 510 is placed adjacent to the spacer layer 2 505, and is therefore regarded as a part of the intermediate mirror 2 506.
However, it can also be considered that it functions as a spacer layer including the electrode part from a different viewpoint.
In this case, it is necessary to consider the thickness of the spacer layer 2 505 including the thickness of the p electrode layer 510. However, as described above, since the p electrode layer is thin in this embodiment, the thickness of the spacer layer is the electrode layer. Designed except for.
When the p electrode layer 510 is thick, it is necessary to design the thickness of the spacer layer 2 of 505 including the thickness of the p electrode.

Next, a method for manufacturing a laser element in this example will be described.
The laser element of this embodiment can be manufactured using a crystal growth process, a film forming process such as sputtering, an photolithography, a lift-off process, an etching process such as wet / dry etching, and an electrode forming process such as vapor deposition.
First, the spacer layer 1 (lower cladding layer 512, active layer 511, upper cladding layer 513) of the lower mirrors 502 and 503, and the spacer layer 2 of the intermediate mirrors 1 and 505 of the 504 are fabricated by crystal growth on the substrate. A p-electrode layer 510 (Ni / Au) is deposited thereon and annealed in an oxygen atmosphere.
The intermediate mirror 2 of 506, the spacer layer 3 of 507, and the upper mirror 508 are formed thereon by sputtering, and finally an n-electrode layer 509 is formed on the back surface of the substrate 501. Thereafter, dry etching is performed along the patterning of photolithography around the element to form a mesa.

In this embodiment, the lower mirror 502 is made of a semiconductor material, but a dielectric material can also be used. In that case, a mode having five spacer layers as described in the embodiment of the present invention is preferable.
Further, GaN-based materials are used as the semiconductor material, and ZrO ₂ and HfO ₂ are used as the dielectric material. However, all the materials described in the embodiment of the present invention can be used. The same applies to the material of the electrode layer.
Further, the dielectric material of the upper mirror 508 is the same material as that of the spacer layer 3 of the intermediate mirrors 2 and 507 of the 506, but it is of course possible to make it different.
In particular, when the intermediate mirror 2 of 506 is a DBR, the number of layers of the upper mirror can be reduced by making the refractive index difference between the layers of the upper DBR larger than the refractive index difference between the layers of the intermediate mirror 2 of 506. .
The definition of the refractive index difference between the layers here is as described above. However, when the value is not constant (the refractive index difference between the layers constituting the mirror and the refractive index difference between the mirror and the adjacent material). Are different from each other), the average value is used.
The upper mirror 508 and the lower mirror 502 also use DBR in this embodiment, but it is also possible to use a grating mirror or a photonic crystal mirror that reflects incident light in the vertical direction of the periodic structure.
Similarly, as described in the embodiment of the present invention, a current confinement layer can be introduced or a sapphire substrate can be used.
Moreover, p and n can also be exchanged for the kind of electrode. At this time, the crystal doping is reversed from that of this embodiment. Furthermore, the upper and lower sides of the element can be switched. In Examples 2 and 3 described below, the semiconductor / dielectric / electrode materials that can be used, the introduction of a current confinement layer, the point that a grating mirror / sapphire substrate can be used, and the replacement of p and n are described below. Are all possible as well.

[Example 2]
As a second embodiment, a configuration example of a vertical cavity surface emitting laser having a different form from the first embodiment will be described with reference to FIG.
In FIG. 7, an intermediate mirror 704 is a multilayer mirror, and 2.5 pairs of Al _0.85 In _0.15 and GaN are stacked in this order from the bottom of the element. The thickness of each layer is the same as in Example 1.
The configuration, dimensions, and materials other than the intermediate mirror 1 of 704 in this embodiment are all the same as those in the first embodiment. Hereinafter, the difference will be described.
In this embodiment, since the intermediate mirror 1 of 704 is a multilayer mirror and the number of layers is increased from that of the first embodiment, the reflectance of the intermediate mirror 1 of 704 is larger than the reflectance of the intermediate mirror 2 of 706. ing.
Therefore, the light intensity of the spacer layer 1 703 increases and the light intensity of the spacer layer 2 705 decreases for the reason described in the embodiment of the present invention.

FIG. 8 shows the result of calculating the intensity distribution of the standing wave in the resonator in the laser device of this example.
In FIG. 8, the light intensity (envelope) value of 705 in the spacer layer 2 is 7.9% smaller than that in Example 1, and the value in the spacer layer 1 of 703 is 25.7% larger.
Therefore, since the light intensity in the portion where the electrode layer is provided decreases and the absorption decreases, and the light intensity in the active layer portion increases and the gain increases, this is a more advantageous condition for the vertical cavity laser. .
However, increasing the number of layers of the intermediate mirror 1 of 704 increases the electrical resistance of the element itself. Therefore, it is not preferable that the number of mirror layers is as large as possible, and there is an optimum value.
In this embodiment, the number of mirror layers is preferably 1.5 to 5.5 pairs, and more preferably 2.5 to 4.5 pairs.
The manufacturing method of the element in this example is the same as that of Example 1 except that the intermediate mirror 1 of 704 is a multilayer film.

[Example 3]
As Example 3, a configuration example of a vertical cavity surface emitting laser having a different form from each of the above examples will be described with reference to FIG.
Regarding FIG. 9, the element names of the elements are exactly the same as those in FIG. Only the differences from the first embodiment will be described below.
In this embodiment, the refractive index of the intermediate mirror 1 904 is low, and the refractive index difference from the adjacent layer is large.
Accordingly, the reflectance of the intermediate mirror 1 at 904 is increased as compared with the first embodiment, and is larger than that of the intermediate mirror 2 at 906.
Therefore, for the reason described in the embodiment of the present invention, the light intensity of the spacer layer 1 903 is increased and the light intensity of the spacer layer 2 905 is decreased.

FIG. 10 shows the result of calculating the intensity distribution of the standing wave in the resonator in the laser device of this example.
Also in FIG. 10, the value of the light intensity (envelope) in 905 spacer layer 2 is 28.3% lower than that in Example 1, and the value in 903 spacer layer 1 is 15.4% higher.
Therefore, also in this embodiment, the light intensity in the electrode layer portion decreases, the absorption decreases, and the light intensity in the active layer portion increases to increase the gain. It becomes.
In the laser element of the present embodiment, the difference in refractive index between the layers of the intermediate mirror 1 at 904 is preferably larger than the difference in refractive index between the layers of the intermediate mirror 2 at 906.
In this embodiment, since the p electrode layer 910 is included, the difference in refractive index between the layers constituting the intermediate mirror 2 of 906 is smaller than that of the intermediate mirror 1 of 904, but the electrode layer is extremely thin and therefore hardly contributes to the reflection characteristics.
Therefore, the refractive index difference is substantially determined by the intermediate mirror 2 of 906, the spacer layer 2 of 905 on both sides thereof, and the spacer layer 3 of 907.
In this embodiment, the intermediate mirror 1 904 is made of porous GaN, and the average refractive index is 1.86.
The device fabrication method of this example is the same as that of Example 1 except that the step of introducing a porous GaN layer is included in Example 1.
Examples 1 to 3 described above are exemplary, and various conditions such as a structural material, a size, and a shape of the laser element used in the present invention are not limited by the above examples.

501: Substrate 502: Lower mirror 503: Spacer layer 1
504: Intermediate mirror 1
505: Spacer layer 2
506: Intermediate mirror 2
507: Spacer layer 3
508: Upper mirror 509: n electrode layer 510: p electrode layer 511: active layer 512: lower clad layer 513: upper clad layer

Claims (10)

A vertical cavity surface emitting laser having an oscillation wavelength λ in which a resonator is formed by a semiconductor layer including a lower mirror, an upper mirror, and an active layer disposed between the lower mirror and the upper mirror, which are stacked on a substrate,
The resonator includes three or more odd number of spacer layers therein, and the three or more odd number of spacer layers are partitioned by two or more intermediate mirrors,
The optical distance between any (2N′−1) adjacent mirrors in the surface-emitting laser is d (N ′: an integer of 1 or more), and the phase jump of the reflected light from the adjacent intermediate mirrors, respectively. When Φ ₁ and Φ ₂ are satisfied, the following equation (1) is satisfied:
When the optical distance between 2N ′ adjacent mirrors in the adjacent intermediate mirror is d ′, the relationship of the following formula (2) is satisfied:
A vertical cavity surface emitting laser characterized in that an electrode layer for current injection is provided in an even-numbered spacer layer counted from the lower mirror or in an adjacent portion thereof.
2d · 2π / λ + Φ ₁ + Φ ₂ = 2N · π Formula (1)

2d ′ · 2π / λ + Φ ₁ + Φ ₂ = (2N−1) · π Formula (2)

N: an integer greater than or equal to 1
The spacer layer has five spacer layers of the first spacer layer to the fifth spacer layer,
On the lower mirror, a first spacer layer, a first intermediate mirror, a second spacer layer, a second intermediate mirror, a third spacer layer, a third intermediate mirror, a fourth spacer layer, a first spacer layer, 4 intermediate mirrors and a fifth spacer layer are stacked in this order.
2. The vertical resonator type according to claim 1, wherein two electrodes for current injection are provided in the second spacer layer, the fourth spacer layer, or their adjacent portions, respectively. Surface emitting laser. Of the two electrode layers, the upper side of the upper electrode layer and the lower side of the lower electrode layer are constituted by a dielectric,
3. The vertical cavity surface emitting laser according to claim 2, wherein a gap between the two electrode layers is constituted by a semiconductor. The spacer layer has three spacer layers of a first spacer layer to a third spacer layer,
On the lower mirror, a first spacer layer, a first intermediate mirror, a second spacer layer, a second intermediate mirror, and a third spacer layer are stacked in this order.
2. The vertical cavity surface emitting laser according to claim 1, wherein an electrode layer for current injection is provided on the second spacer layer or on an adjacent portion thereof. The upper side of the electrode layer is made of a dielectric,
5. The vertical cavity surface emitting laser according to claim 4, wherein the lower side of the electrode layer is made of a semiconductor.   6. The vertical cavity surface emitting laser according to claim 4, wherein the reflectivity of the first intermediate mirror is larger than the reflectivity of the second intermediate mirror. The first intermediate mirror and the second intermediate mirror are single layer films or multilayer film mirrors;
The vertical cavity surface emitting laser according to claim 6, wherein the number of layers of the first intermediate mirror is larger than the number of layers of the second intermediate mirror. The upper mirror and the second intermediate mirror are single layer films or multilayer film mirrors;
8. The vertical cavity surface emitting laser according to claim 6, wherein a difference in refractive index between layers of the upper mirror is larger than a difference in refractive index between layers of the second intermediate mirror. A vertical cavity surface emitting laser in which a semiconductor layer including a lower mirror, an upper mirror, and an active layer disposed therebetween is stacked on a substrate, and a resonator is formed by these layers,
The resonator has three or more odd spacer layers therein, and the three or more odd spacer layers are partitioned by an intermediate mirror,
An electrode layer for current injection is introduced into the spacer layer at a portion where the envelope of the light intensity of the standing wave of light resonating in the resonator is minimized or adjacent thereto. Vertical cavity surface emitting laser.   The vertical resonator according to any one of claims 1 to 9, wherein the electrode layer for current injection is provided at a node of a standing wave of light resonating in the resonator. Type surface emitting laser.

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