JPH0770792B2 - Surface emitting laser and manufacturing method thereof - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface emitting laser, and more particularly to a microcavity laser capable of obtaining a highly efficient surface emitting output and a method for manufacturing the same.

[0002]

2. Description of the Related Art In the fields of optical switching, optical computers, optical information processing, etc., surface emitting lasers capable of two-dimensional integration are required and are being actively researched and developed. One example is R. S.
Geels and L.L. A. Coldren et al., Applied Physics Letters (Applied, Phys.
ics, Letters) 57: 1605-1607 (1990). In this paper,
R. S. Geels et al. Reported that a 7 μm square surface emitting laser oscillated with a threshold current of 0.7 mA.

[0003]

However, in order to integrate a large number of surface emitting lasers of 1000 or more in future optical integrated circuits, it is necessary to further reduce the threshold value. In response to this demand, a concept called a microcavity laser has recently been proposed and studied. For example, in the article of the Journal of Applied Physics, Vol. 61, No. 9, 890-901 (1992), Yokoyama describes that the light in the active layer, which is the light emitting layer, has a size of 1 in the vertical, horizontal, and height directions. The possibility of laser operation at about μA in a laser with a structure confined to the wavelength is described. In this laser, by confining light in a very small active layer, the mode density of the light is extremely reduced, and the coupling efficiency of spontaneous emission light to the cavity mode is extremely increased (~
By doing 1), an optical output equivalent to that of laser light is obtained with an extremely small threshold current.

However, when manufacturing such a laser (herein called a microcavity laser),
There is a problem that it is difficult to trap light in a small active layer. For example, in the thickness direction of the active layer, a DBR reflective film having a high reflectance can be formed by alternately laminating two semiconductor layers having different refractive indexes. Therefore, light confinement in the vertical direction (layer thickness direction) can be realized relatively easily. However, confinement of light in the lateral direction of the active layer is difficult to realize. For example, even when forming a gold reflective film having a reflectance of about 90%, there is a problem that the reflectance is not sufficiently high. Also,
Even when the DBR reflective film is formed in the lateral direction, there is a problem that it is difficult to accurately control the thickness of the film formed on the side surface.

Therefore, an object of the present invention is to confine strong light not only in the vertical direction (layer thickness direction) but also in the horizontal direction, so that it is possible to operate as a microcavity laser with a low current. And a method of manufacturing the same.

[0006]

In a surface emitting laser according to the present invention, a multi-layer structure including a first DBR reflective film formed on a semiconductor substrate, a PN junction formed on the first DBR reflective film, and an active layer. A second DBR reflective film formed on the multilayer structure, a semiconductor pillar composed of the first DBR reflective film, the multilayer structure and the second DBR reflective film, and a semiconductor pillar formed to laterally surround the semiconductor pillar. When at least one semiconductor wall is provided and the wavelength in the medium of the light emitted from the active layer is λ, the thickness of the semiconductor wall and the distance between the semiconductor pillar and the semiconductor are approximately λ / 4 +, respectively.
(Integral multiple of λ / 2), and when there are two or more semiconductor walls, the interval is approximately λ / 4 + (an integral multiple of λ / 2).

In the method of manufacturing a surface emitting laser according to the present invention, a step of forming a first DBR reflective film by alternately stacking semiconductor layers having different refractive indexes on a semiconductor substrate, and a PN on the first DBR reflective film. A step of forming a multilayer structure including a junction and an active layer, a step of forming a second DBR reflective film by alternately laminating semiconductor layers having different refractive indexes on the multilayer structure, the first DBR reflective film and the multilayer A step of forming a semiconductor pillar composed of the structure and the second DBR reflective film by dry etching; and a step of forming at least one semiconductor wall laterally surrounding the semiconductor pillar at the same time as the semiconductor pillar by the dry etching When the in-medium wavelength of the light emitted from the active layer is λ, the thickness of the semiconductor wall, between the semiconductor pillar and the semiconductor wall Almost septum each λ / 4 + (λ / 2
Of the semiconductor wall, and when there are two semiconductor walls above, the interval is approximately λ / 4 + (an integer multiple of λ / 2).

[0008]

According to the present invention, in order to confine light in the thickness direction of the active layer, a DBR reflective film formed by alternately stacking two semiconductors having different refractive indexes is formed on both sides of the active layer as in the conventional surface emitting laser. And then confine the light.

On the other hand, for confining light in the lateral direction of the active layer, the walls of the semiconductor multilayer film are regularly arranged to form an effective DBR reflective film between the semiconductor multilayer film and the air, and the light is blocked. I'm trapping. In this case, the refractive index of the semiconductor is usually around 3, and the refractive index of air is 1
Therefore, the difference in refractive index is larger than that of the semiconductor DBR reflective film, and the DBR reflective film having high reflectance can be easily formed. For example, when the effective refractive index of the semiconductor wall is set to 3.25, the reflectance R is estimated to be 89% even when there is one semiconductor, 98.9% when there are two semiconductor walls, and 99.99 when there are three semiconductor walls.
9% DBR with extremely high reflectance
You can see that it can be obtained with a reflective film.

Further, in the production of this DBR reflection film, the semiconductor crystal including the active layer is etched by a method using dry etching, not by a film formation method like the conventional DBR reflection film, to thereby form a light emitting layer. That is, the semiconductor pillar to be formed and the semiconductor wall on which the DBR reflective film is formed are collectively formed. In the conventional method of forming a film, it is difficult to precisely control the film forming rate in the lateral direction, and thus it is difficult to form a DBR reflective film having a sufficiently controlled film thickness. However, according to the method of manufacturing the DBR reflective film of the present invention, the semiconductor pillars that will be the light emitting portions and the semiconductor walls that will be the DBR reflective film can be formed with good controllability by using dry etching excellent in vertical etching. I can.

[0011]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 shows the structure of a surface emitting laser according to the first embodiment of the present invention. FIG. 1A is a plan view and FIG.
(B) shows a sectional view. In the figure, 1 is an n-type GaAs substrate,
2 is an n-type DBR reflection film (n-GaAs / n-AlAs multilayer film, thickness is λ / 4, λ is the medium wavelength of the oscillation light. For example, when the design vacuum wavelength is 980 nm, d _{G a As} = 69. ．
It becomes 53 nm and d _{A 1 A s} = 82.94 nm. The reflectance increases as the number of cycles increases, but typically 15 to 3
0 period, 3 is an n-type confinement layer (n-AlGaAs, A
1 composition is 0-1, preferably 0.2-0.5), 4, 4
a is an active layer (InGaAs single quantum well, In composition =
0.1-0.3, typically 0.2 and 10 nm thick) where 4a represents the active layer of the surface emitting laser body, and 5 is a p-type confinement layer (p-AlGaAs, Al composition is 0-). 1, preferably 0.2-0.5) and 6 are p-type DBR reflective films (p
-GaAs / p-AlAs multilayer film, the thickness is λ / 4, and λ is the medium wavelength of oscillation light. For example, in the case of the design vacuum wavelength of 980 nm, d _{G A A s} = 69.53 nm, d _{A I A s} = 8
It becomes 2.94 nm. The reflectance increases as the number of cycles increases, but typically 15 to 30 cycles), 7 is a p-type electrode,
Reference numeral 8 is an n-type electrode, 9 is a surface emitting laser body (width is 10 μm or less, preferably 0.25 to 2 μm), 10 is a semiconductor wall (here, width is 3λ / 4, for example, vacuum design wavelength 980n
m, 229 nm in case of effective refractive index 3.21), 11
Is a groove (here, width ~ λ / 4, for example, vacuum design wavelength 980
In the case of nm, it is 245 nm), 12 is the lateral DB
It is an R reflector. Here, the n-type confinement layer 3 and the active layer 4
The total thickness of the p-type confinement layer 5 and the p-type confinement layer 5 is an integral multiple of λ / 2. For example, these total thickness lambda, 980 nm design vacuum wavelength, an In ₀ of the active layer 4 to 10 _{thick. 2} Ga _{0. 8}
When the As _{is, Al 0. 25 Ga 0.} 7 5 n -type confinement layer 3 and p-type confinement layer 5 consisting of As, the respective 140.5Nm. Here, the surface emitting laser body 9 and the semiconductor wall 10 are rectangular in shape.

In the laser structure of the first embodiment, the light generated from the active layer 4a of the surface-emitting laser body 9 is the n-type DBR reflection film 2 in the vertical direction (thickness direction).
The p-type DBR reflecting film 6 and the lateral DBR reflecting mirror 12 confine the light in the lateral direction. In the DBR reflective film in the vertical direction, the reflectance differs depending on the doping concentration and the number of cycles. As an example, the number of cycles is 20,
Electron concentration 10 ^{1 8} cm ^{- 3} of n-type GaAs / AlAsD
For the BR reflective film, the calculated reflectance is 99.86%. Also, p with a period number of 20 and a hole concentration of 10 ¹⁸ cm ⁻³
In the case of the type GaAs / AlAsDBR reflective film, the calculated reflectance is 99.77%.

On the other hand, in the case of the DBR reflecting mirror 12 in the lateral direction of 3 periods as shown in FIG. 1, the calculated reflectance is 9
It becomes 9.90%. Moreover, when the DBR reflecting mirror 12 in the lateral direction has two cycles, a reflectance of 98.9% is expected. As described above, in the lateral DBR reflecting mirror 12, since the difference in refractive index between air and the semiconductor is large, a large reflectance can be obtained with a small number of cycles.

As described above, the surface emitting laser of this embodiment is surrounded by the DBR reflecting mirror having a reflectance of 99% or more in both the vertical and horizontal directions.
3 is efficiently confined as shown by the arrow in FIG. Due to such good light confinement, the modal density of light is reduced, and due to the microcavity effect,
In the low current injection region of A or less, output light 14 having a sharp wavelength spectrum and radiation directivity similar to laser light can be obtained.

Particularly, the width of the surface emitting laser body 9 is set to λ / 2.
If it is set to a non-integer multiple of, for light of wavelength λ,
Since there are few modes that emit light in the lateral direction, spontaneous emission in the lateral direction is suppressed. For portrait mode,
By setting the total thickness of the n-type confinement layer 3, the active layer 4, and the p-type confinement layer 5 to be an integral multiple of λ / 2, the light emission in the vertical direction becomes a resonance state and is accelerated. Therefore, since the light output of the spontaneous emission light is concentrated only in the vertical direction, unlike the conventional LED light emission, the extraction efficiency of the output light 14 is expected to be extremely high, that is, 10% or more. On the other hand, in the conventional LED, since the light from the active layer is isotropically radiated, the light extraction efficiency was as low as about 1% at most.

In addition to improving the light confinement as described above, it is possible to simultaneously reduce the respiratory loss with respect to the laser internal light 13 and sufficiently reduce the non-radiative recombination of carriers injected into the active layer 4a. If possible, even in an extremely low current region of the order of μA, the spontaneous emission light is concentrated in the vertical direction, and the output light 14 having a sharp wavelength spectrum and radiation directivity similar to laser light can be efficiently obtained. . Further, in the high current injection region where the current injection is further increased, normal laser oscillation can be obtained.

Next, a method of manufacturing the surface emitting laser of the first embodiment will be described. First, the n-type DBR reflection film 2, n is formed on the n-type GaAs substrate 1 by MBE growth.
Type confinement layer 3, active layer 4, p type confinement layer 5, p type D
The BR reflective film 6 is sequentially grown. Next, the p-type DBR reflective film 6
A p-type electrode 7 is formed on top. Next, the surface emitting laser body 9 and the semiconductor wall 10 are separated by forming a groove 11 by using a dry etching method, and the lateral DBR reflecting mirror 1 is formed.
Form 2. For dry etching in this case,
EC of chlorine with chlorine radical and chlorine ion as reactive species
A reactive ion beam etching method (RIBE method) using R plasma, a chemical assisted ion beam etching method (CAIBE method) using argon ions or chlorine gas as a reactive species, and the like can be used. In particular, CA
When IBE method is used for dry etching, Ni / A
The p-type electrode 7 is formed by patterning the u film by the lift-off technique.
Then, the NiAu film of the p-type electrode 7 is formed by CAIB.
Since it can be used as a mask material in dry etching by the E method, a fine pattern can be easily formed. Next, the n-type electrode 8 is formed. Finally, n-type GaA
s Substrate 1 is thinned to a desired thickness by mechanical polishing.

FIG. 2 is a sectional view of the surface emitting laser of the second embodiment. The laser structure of this embodiment is almost the first
Is the same as the embodiment described above. The difference is that polyimide 20
Is the point where the groove 11 is filled. In the second embodiment, since the p-side surface is flattened by the polyimide 20, there is an advantage that the lead electrode 21 can be easily formed. Further, in order to set the width of the groove 11 to λ / 4,
The width of the groove 11 in the first embodiment needs to be (1 / refractive index of polyimide). The manufacturing method of the second embodiment is almost the same as that of the first embodiment. The difference is that after the dry etching, the coating of the polyimide 20 and the patterning for forming the opening of the extraction electrode 21 and the opening of the n-type electrode 8 are performed.

In the above embodiments, the active layer is a single quantum well, but the present invention is not limited to this, and a multiple quantum well or a thick active layer of 20 nm or more may be used. Although the total thickness of the n-type confinement layer, the p-type confinement layer, and the active layer is set to λ, the thickness is not limited to this, and the thickness can be changed if it is an integral multiple of λ / 2. Further, as a material, GaAs / AlAs
Although the system material is used, the present invention is not limited to this, and other semiconductor materials such as InP / InGaAs system material can be used. In the second embodiment, polyimide is used to fill the groove, but the present invention is not limited to this, and another dielectric material such as SiO ₂ may be used. Further, in the plan view of the embodiment, the DBR reflecting mirror in the lateral direction is formed in a quadrangle, but the present invention is not limited to this and may be a circle or the like.

In the embodiment, the semiconductor wall thickness is 3λ / 4.
However, the thickness is not limited to this, and may be any thickness such as λ / 4 and 5λ / 4, which is generally expressed by λ / 4 + (an integral multiple of λ / 2). However, if the thickness is λ / 4, the thickness is about 60 nm, and the semiconductor wall is likely to collapse. If there is such a problem, as shown in the plan view of FIG. 3, the reinforcing semiconductor wall 30 for connecting the semiconductor walls to each other is used.
Should be provided. Further, in the embodiment, the width of the groove 11 is λ /
However, the width is not limited to this, and may be any width that is generally written as λ / 4 + (an integral multiple of λ / 2).

[0022]

According to the present invention, since light can be efficiently confined not only in the vertical direction but also in the horizontal direction, the optical mode density can be reduced, and the microcavity effect allows laser light to be emitted in a low current region. It is possible to realize a surface emitting laser capable of obtaining output light having such a sharp wavelength spectrum and directivity.

[Brief description of drawings]

FIG. 1 is a plan view (A) and a sectional view (B) of a surface emitting laser according to a first embodiment of the present invention.

FIG. 2 is a sectional view of a surface emitting laser according to a second embodiment of the present invention.

FIG. 3 is a plan view of another embodiment of the present invention.

[Explanation of symbols]

 1 n-type GaAs substrate 2 n-type DBR reflection film 3 n-type confinement layer 4 active layer 5 p-type confinement layer 6 p-type DBR reflection film 7 p-type electrode 8 n-type electrode 9 surface-emitting laser body 10 semiconductor wall 11 groove 12 lateral Direction DBR reflector 13 Internal laser light 14 Output light 20 Polyimide 21 Extraction electrode 30 Reinforced semiconductor wall

Claims (2)

[Claims] 1. A first DBR reflection film formed on a semiconductor substrate, a multi-layer structure including a PN junction and an active layer formed on the first DBR reflection film, and a second DBR reflection film formed on the multi-layer structure. A film, a semiconductor pillar composed of the first DBR reflection film, the multilayer structure and the second DBR reflection film, and at least one semiconductor wall formed so as to laterally surround the semiconductor pillar. When the in-medium wavelength of the emitted light is λ, the thickness of the semiconductor wall and the distance between the semiconductor pillar and the semiconductor wall are approximately λ /
4+ (an integer multiple of λ / 2), and when there are two or more semiconductor walls, the interval is approximately λ / 4 + (λ /
The surface emitting laser is characterized by being an integral multiple of 2. 2. A step of forming a first DBR reflective film by alternately stacking semiconductor layers having different refractive indexes on a semiconductor substrate, and a multilayer structure including a PN junction and an active layer on the first DBR reflective film. A step of forming the second DBR reflective film by alternately stacking semiconductor layers having different refractive indexes on the multilayer structure; and the first DBR.
A semi-conducting pillar composed of a reflective film, the multilayer structure, and the second DBR reflective film is formed by dry etching. At least one semiconductor wall that laterally surrounds the semiconductor pillar at the same time as the semiconductor pillar is formed by the dry etching. And the wavelength in the medium of the light emitted from the active layer is λ, the thickness of the semiconductor wall and the distance between the semiconductor pillar and the semiconductor wall are approximately λ /
4+ (integer multiple of λ / 2), and when there are two or more semiconductor walls, the interval is approximately λ / 4 + (integer multiple of λ / 2). Production method.