JP3455404B2 - Semiconductor optical device 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 structure and a manufacturing method of a semiconductor laser operating in a longitudinal single mode, and more specifically, a high performance distributed feedback resonator laser,
Further, the present invention relates to a structure of a distributed Bragg reflection type resonator laser and a method for easily manufacturing this structure.

[0002]

2. Description of the Related Art In an optical transmission system, a semiconductor laser operating in a vertical single mode is used in order to prevent deterioration of an optical signal to be transmitted and to obtain high quality optical transmission. Conventionally,
A distributed feedback resonator (DFB) laser or a distributed Bragg reflector resonator (DBR) laser is known as a semiconductor laser operating in the longitudinal single mode. The schematic structure of these DFB / DBR lasers is shown in FIG. DF
In the B / DBR laser, a refractive index periodic structure having a relationship determined by the following equation is formed in the optical waveguide structure in order to make the oscillation wavelength a longitudinal single mode. Λ = λ / (2ne) (1) In formula (1), Λ is the repeating size of the periodic structure, λ is the wavelength of the laser, and ne is the average refractive index of the optical waveguide structure. This refractive index periodic structure brings about a reflection effect on the guided light. Normally, in a semiconductor laser used for optical communication, λ = 1.55 μm and ne = 3.2, so
Is approximately 240 nm.

In manufacturing the laser structure shown in FIG. 8, first, as shown in FIG. 8A, an active layer 32 is grown on a semiconductor substrate 31, and subsequently, a guide layer 33 for efficiently guiding light. Grow. Further, in order to obtain the above-mentioned periodic structure of refractive index, as shown in FIG.
An etching mask pattern 34 having a period Λ is formed on the guide layer 33. Then, by etching the substrate having the mask pattern 34, the guide layer 33 is processed into a shape having a periodic uneven structure as shown in FIG. Note that electron beam exposure was used to form the etching mask pattern 34, and a groove having a depth of about 30 nm was etched using a mixed solution of sulfuric acid-hydrogen peroxide solution-water to etch the guide layer 33. .

Then, as shown in FIG. 8D, a cladding layer 35 is grown to form a laser substrate. In this case, in the step of forming the concavo-convex structure, since the semiconductor substrate is once taken out of the crystal growth reaction furnace, impurities are mixed into the substrate, and the clad layer 3 to be regrown on the substrate is produced.
5 caused deterioration of the quality, and eventually, deterioration of the laser characteristics. In order to use this substrate as a semiconductor laser device, electrodes 36 are formed on the upper and lower sides of the substrate, and then the substrate is cleaved at predetermined cleavage positions 37 and 38. The cleavage positions 37 and 38 are separated by approximately 400 μm, and the laser cavity is determined by the distance between these two points. The period of the concavo-convex structure formed in the guide layer 33 is 240 nm, which is extremely fine, and it is impossible to strictly determine the mutual positional relationship between the concavo-convex structure and the cleavage positions 37 and 38.
The relative position of each element was different.

[0005]

In the DFB / DBR semiconductor laser, the oscillation threshold value and the periodical structure of the refractive index provided in the guide layer 33 and the standing wave of light existing inside the laser resonator are related to each other. Important laser characteristics such as spectral linewidth are determined. The standing wave of light existing inside the laser resonator is restricted by the position of the end face of the laser resonator. However, when the end face of the laser resonator is formed by cleavage as in the prior art, since the refractive index periodic structure has a size of nm and a fine structure, the mutual positions of the refractive index periodic structure and the cleavage positions 37 and 38 are present. It was impossible to define the relationship exactly. For this reason, this mutual positional relationship differs for each element, and when the laser light is reflected by the end face and returns to the laser resonator, the mutual relationship between the return light and the standing wave of the light generated inside the laser resonator is It will be different for each element. As a result, there is a problem in that the characteristics of the semiconductor laser device vary. Further, in order to avoid such a problem, as shown in FIG. 8E, the step of coating the antireflection film 39 on the cleaved surface was indispensable. For this reason, there is a problem that the price of the element becomes extremely high. The present invention has been made in order to solve the above-mentioned problems, and it is possible to strictly define the mutual positional relationship between the end face of the laser resonator and the refractive index periodic structure in the resonator,
An object of the present invention is to provide a semiconductor optical device which does not require a coating process of a non-reflective film and a manufacturing method thereof.

[0006]

According to the present invention, as described in claim 1, an optical waveguide is formed on a semiconductor substrate having an active layer and cladding layers having different conductivity types above and below the active layer. A method of manufacturing a semiconductor optical device, comprising: forming a stripe-shaped etching mask perpendicular to the optical axis direction and having a width a in the optical axis direction on a region having a length L along the optical axis direction of the optical waveguide on the substrate. Forming an etching mask window region having a length c in the optical axis direction on one side or both sides of the end of the etching mask region along with the period b along the optical axis direction; At least a step of etching a semiconductor substrate provided with a mask window region to form a groove having a width b-a in the optical axis direction and a period b, and a window region having a length c in the optical axis direction. Including It is obtained by way. Thus, by forming a stripe-shaped etching mask and an etching mask window region on the semiconductor substrate and performing etching on this semiconductor substrate, the groove that gives the periodic structure of the refractive index and the resonance that defines the standing wave in laser oscillation are formed. Since the device end face (waveguide end face) can be simultaneously formed by etching, the mutual positional relationship between the end face of the laser resonator and the refractive index periodic structure in the resonator can be strictly defined. Further, as described in claim 2, the length c of the etching mask window region is set to be 10 times or more larger than the period b of the etching mask, and the groove and the window region are formed by using the difference in etching rate due to the microloading effect. By etching to different depths, the width in the optical axis direction is ba
The groove having a period of b and the window region having a length of c in the optical axis direction are formed in the same etching step.

Further, as described in claim 3, a striped etching mask which is perpendicular to the optical axis direction and has a width 2a in the optical axis direction is arranged at one end of the etching mask region having the length L. It is something that is done. in this way,
By arranging an etching mask having a width of 2a at one end of the etching mask region, it is possible to specify that the phase of light on the end face of the laser cavity becomes a node. Further, as described in claim 4, in the optical waveguide, a convex portion having a width a in the optical axis direction and a groove having a width ba are arranged in the optical waveguide at a cycle b. a semiconductor optical device having a periodic structure of rates, the positional relationship between the refractive index periodic structure with the end face of the optical waveguide have a certain relationship, the upper Symbol refractive index periodic structure, the width a at both ends The optical waveguide has a convex portion and a convex portion having a width 2a, and the position of one end face of the optical waveguide coincides with the position of the outermost end of the convex portion having the width a and the position of the other end face of the optical waveguide has a width 2a. It corresponds to the position of the outermost end of the convex portion.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION 1. First Embodiment Next, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a perspective view showing a schematic configuration of a semiconductor laser according to a first embodiment of the present invention. The semiconductor laser according to the present embodiment includes an n-type InP substrate (first clad layer) 1, InG.
It is formed on a semiconductor substrate composed of an active layer 2 made of aAsP (bandgap wavelength = 1.5 μm) and a second cladding layer 3 made of p ⁻ -type InP. The clad layer 3 has a width of 120 nm in the optical axis direction (horizontal direction in FIG. 1),
Grooves 4 having a period of 240 nm are formed. The bottom of the groove 4 is located 400 nm above the surface of the active layer 2. Reference numeral 5 denotes a processed side wall which is formed at the same time as the etching process for forming the groove 4 and constitutes an end face of the laser resonator. Reference numeral 6 is a ridge-type waveguide having a height of 1.1 μm. An electrode 7 for current injection is provided on the top of the ridge type waveguide 6 and the lower part of the substrate 1.

Next, a method of manufacturing the semiconductor laser of FIG. 1 will be described with reference to FIGS. First, as shown in FIG. 2A, an active layer 2 made of InGaAsP (bandgap wavelength = 1.5 μm) and a cladding layer made of p ⁻ -type InP having a thickness of 1.2 μm are formed on an n-type InP substrate 1. 3 are sequentially formed by a metal organic chemical vapor deposition method.

Subsequently, as shown in FIG. 2B, a SiO ₂ film 11 having a thickness of 0.2 μm is deposited. The length L on the SiO ₂ film 11 (left-right direction in FIG. 2) is 600 μm.
A line-space pattern 12 having a width a of 120 nm in the optical axis direction (horizontal direction in FIG. 2) and a repeating cycle b of 240 nm is formed by an electron beam exposure method in a region of m and a width (perpendicular to the paper surface) of 500 μm. . At this time, there is no pattern on both sides in the length direction of the region, and the length c is 3
A 0 μm window region 13 was formed.

Then, as shown in FIG. 2C, the SiO ₂ film 11 is formed by using the line-space pattern 12 as a mask.
Then, reactive ion etching with C ₂ F ₆ gas is performed to transfer the line-space pattern 12 to the SiO ₂ film 11. In this way, a plurality of etching masks 14 each having a width a in the optical axis direction which is perpendicular to the optical axis direction of the optical waveguide and has a period b is formed on the semiconductor substrate. At the same time, the length in the optical axis direction is c on both sides of the region where the etching mask 14 is formed.
Etching mask window region 15 is formed.

Using the thus patterned etching mask 14 and etching mask window region 15, the semiconductor substrate is subjected to reactive beam etching using a bromine-nitrogen mixed gas to form the groove 4. At this time, the region of the groove 4 is the etching mask window region 1 having a large pattern size.
Since it is finer than that of 5, the micro-loading effect, which is one of the features of beam etching, is exhibited, and the etching rate is lower than that of the window region 15. This microloading effect becomes remarkable when the pattern size ratio is about 10 times.

In this embodiment, the etching conditions are bromine gas pressure 0.04 mTorr and nitrogen gas pressure 0.24 mTor.
rr, microwave discharge power 40 W, beam acceleration voltage 35
It was set to 0V. At this time, the etching rate of the region of the groove 4 was about 1/3 of the etching rate of the window region 15.
By using such characteristics, when etching is performed so that the bottom of the groove 4 is 400 nm above the surface of the active layer 2 (that is, the depth of the groove 4 is 800 nm), the etching depth in the window region 15 is increased. Is 2.4 μm.

Thus, as shown in FIG. 2D, the groove 4
During the etching process for forming the, the processed side wall 5 is formed by the etching action of the etching mask window region 15, and the processed side wall 5 forms the end face of the laser resonator. The bottom of the groove 4 is 40
The position above 0 nm is set because the groove 4 is deep enough to exert a periodic refractive index change on the light guided in the active layer 2.

Next, after the etching mask 14 is removed, as shown in FIG. 3 (a), a stripe having a width (depth direction in FIG. 3) of 2 μm perpendicular to the groove 4 is formed on the region where the groove 4 is formed. The pattern 16 is formed using photolithography. Then, using the stripe pattern 16 as a mask, the cladding layer 3 is etched to a depth of 1.1 μm. Thus, the width in the optical axis direction is 120 nm and the depth is 8
A ridge type waveguide 6 having a height of 1.1 μm and having a groove 4 having a period of 00 nm and a period of 240 nm is formed.

A pattern (not shown) is formed on the portion of the substrate 1 corresponding to the window region 15 when the stripe pattern 16 is formed, and this pattern serves as a mask. Therefore, the substrate 1 is not etched in the etching process for forming the ridge type waveguide 6. Next, after removing the stripe pattern 16, gold electrodes 7 are formed on the ridge type waveguide 6 and below the substrate 1, as shown in FIG. Finally, the laser element of FIG. 1 is completed by cutting out the element from the semiconductor substrate at the portion corresponding to the window region 15 described above.

In this embodiment, the etching mask 14 is used.
At the right end of the area for forming the mask, the position where no mask originally exists is used as the mask. That is, FIG. 2 and FIG.
As shown in (a), the rightmost etching mask 14a
Is the width a of the ordinary etching mask 14 = 120 n
2a = 240 nm, which is twice as large as m.

As shown in FIG. 5 (c), when the right end of the above region is also the normal mask 14 like the left end,
The standing wave 20 of light that can exist in the laser resonator after fabrication has two states, as shown in FIGS. 5D and 5E. Therefore, even during laser oscillation, the laser oscillation light changes between these two states with a slight disturbance,
The laser operation becomes unstable.

On the other hand, if the structure as in this embodiment is adopted, the standing wave 20 of the light in the laser resonator after fabrication can take a state as shown in FIG. 5B. The only condition is that it will become a node of a standing wave at the end face.
This provides a very stable laser operation. The semiconductor laser manufactured as described above has a threshold value of 10 mA.
, And the oscillation spectrum obtained with this semiconductor laser showed stable longitudinal single-mode operation.

2 of the embodiment. FIG. 6 is a perspective view showing a schematic configuration of a semiconductor laser according to another embodiment of the present invention. In the semiconductor laser shown in FIG. 6, the repeating periodic structure of the groove 4 is divided into two regions shown by a right region 17 and a left region 18 in the ridge type waveguide 6a, and the phases of the mutual periods are shifted by a half period. It has a so-called λ / 4 structure, and its manufacturing method is similar to that of the first embodiment. The oscillation characteristics of this semiconductor laser showed stable single-mode oscillation characteristics without causing mode hopping even in high-speed modulation operation exceeding 20 GHz.

3. of the embodiment. FIG. 7 is a perspective view showing a schematic configuration of a semiconductor laser according to another embodiment of the present invention. In the semiconductor laser of FIG. 7, an active layer 2 is formed in a stripe shape along the optical axis direction, and a semi-insulating Fe-doped InP crystal 10 is grown on both sides of the stripe, with respect to a semiconductor substrate. The same manufacturing method as in No. 1 is used. In this semiconductor laser, current trapping in the active layer 2 is efficiently performed, so that the oscillation threshold value is as low as 4 mA.

In the above embodiment, the laser using InP as the substrate has been described, but the present invention is not limited to this, and it goes without saying that the present invention is also applicable to a laser using GaAs as the substrate.

[0023]

According to the present invention, since the mutual positional relationship between the end face of the laser cavity and the refractive index periodic structure in the cavity can be strictly defined, a semiconductor laser having uniform characteristics can be obtained. it can. In addition, there is no need for a so-called re-growth step that has been conventionally used, that is, after the active layer is epitaxially grown, the growth is temporarily stopped, the active layer is provided with the periodic structure of the refractive index, and the cladding layer and the contact layer are grown again. Therefore, it is possible to greatly simplify the device fabrication. Further, since it is not necessary to apply the antireflection coating to the end face of the resonator, a high performance semiconductor laser can be obtained at a very low cost.

Further, as described in claim 3, an etching mask having a width of 2a is arranged at one end of the etching mask region, or the other end surface of the optical waveguide is defined as in claim 5. By aligning the position of with the position of the outermost end of the convex portion of the width 2a, it is possible to specify that the node of the standing wave of light in the laser resonator is located on the end face of the resonator, which is very stable. It is possible to obtain the desired laser operation.

[Brief description of drawings]

FIG. 1 is a perspective view showing a schematic configuration of a semiconductor laser according to a first embodiment of the present invention.

2A to 2D are process cross-sectional views for explaining a method for manufacturing the semiconductor laser of FIG.

3A and 3B are process perspective views for explaining a method for manufacturing the semiconductor laser of FIG.

FIG. 4 is a process perspective view for explaining a method for manufacturing the semiconductor laser of FIG.

FIG. 5 is a diagram showing a state of a standing wave of light inside a laser resonator.

FIG. 6 is a perspective view showing a schematic configuration of a semiconductor laser according to another embodiment of the present invention.

FIG. 7 is a perspective view showing a schematic configuration of a semiconductor laser according to another embodiment of the present invention.

8A to 8D are process cross-sectional views for explaining a conventional method for manufacturing a semiconductor laser.

[Explanation of symbols]

1 ... n-type InP substrate, 2 ... active layer, 3 ... cladding layer, 4
... grooves, 5 ... processed side wall, 6 ... ridge type waveguide, 7 ... electrode,
14 ... Etching mask, 15 ... Etching mask window region.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Etsuo Noguchi 3-19-2 Nishishinjuku, Shinjuku-ku, Tokyo Nihon Telegraph and Telephone Corporation (56) References Japanese Patent Laid-Open No. 61-216383 (JP, A) Kaihei 6-148413 (JP, A) JP-A-7-122814 (JP, A) JP-A-8-255948 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01S 5 / 00-5/50

Claims (4)

(57) [Claims] 1. A method of manufacturing a semiconductor optical device, comprising forming an optical waveguide on a semiconductor substrate having an active layer and cladding layers having different conductivity types above and below the active layer, wherein the optical waveguide is formed on the substrate. Length L along the optical axis
In the region of (1), a stripe-shaped etching mask perpendicular to the optical axis direction and having a width a in the optical axis direction is formed at a period b along the optical axis direction, and the optical axis is formed on one side or both sides of the end of the etching mask region. A step of forming an etching mask window region having a length c in the direction, and etching the semiconductor substrate provided with the etching mask region and the etching mask window region so that the width in the optical axis direction is ba and the cycle is A method for manufacturing a semiconductor optical device, comprising at least a step of forming a groove of b and a window region having a length of c in the optical axis direction. 2. The groove c and the window region are etched to different depths by making the length c of the etching mask window region 10 times or more larger than the period b of the etching mask and using the difference in etching rate due to the microloading effect. By doing so, the width in the optical axis direction is b-a and the cycle is b.
2. The method of manufacturing a semiconductor optical device according to claim 1, wherein the groove and the window region having a length c in the optical axis direction are formed in the same etching step. 3. A stripe-shaped etching mask having a length 2a in the optical axis direction and having a width 2a in the optical axis direction is arranged at one end of the etching mask region having the length L. A method for manufacturing a semiconductor optical device as described above. 4. The optical waveguide has a periodic structure of a refractive index in which a convex portion having a width a in the optical axis direction and a groove having a width ba are arranged at a period b along the optical axis direction. a semiconductor optical device, the mutual positional relationship of the refractive index periodic structure with the end face of the optical waveguide have a constant relation, the refractive index periodic structure, at both ends
The optical waveguide has a convex portion with a width a and a convex portion with a width 2a.
The position of the end face on one side coincides with the position of the outermost end of the convex portion having the width a,
The position of the other end face of the optical waveguide is the outermost end of the convex portion of width 2a.
A semiconductor optical device characterized by being aligned with a position .