JP3817074B2 - Manufacturing method of distributed feedback semiconductor laser - Google Patents

Description

[0001]
[Explanation of terms]
In the present invention, the “tilted diffraction grating forming layer” has a film thickness at the end in the longitudinal direction of the resonator in the distributed feedback semiconductor laser that is thicker than the film thickness at the center, and Means a layer for forming a diffraction grating having a thickness gradient distribution in which the film thickness gradually decreases from the center to the center, and the "tilted diffraction grating" refers to the tilted diffraction grating forming layer. A diffraction grating having a film thickness distribution as described above obtained by processing.
In the present invention, the “selective growth mask” is used as a patterning mask when the tilted diffraction grating forming layer is formed by a crystal growth method such as MOCVD with a desired thickness gradient distribution. In this case, it is a kind of insulating film patterned in advance at the position where the inclined diffraction grating forming layer is formed. In this selective growth mask, the “mask pattern width” means the pattern width of the mask covered in the direction perpendicular to the length direction of the resonator, and the “mask opening width” means the length of the resonator between the mask patterns. The distance in the direction perpendicular to the vertical direction.
[0002]
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing a distributed feedback semiconductor laser (hereinafter referred to as a DFB laser) with small optical modulation distortion.
[0003]
[Prior art]
In recent years, in the field of optical communication using an optical fiber, since the minimum loss wavelength band of the optical fiber is a 1.5 μm band, a semiconductor laser diode of a 1.5 μm band is mainly used. In particular, in an optical communication system using an analog modulation method, a DFB laser having high linearity of optical output / current characteristics (so-called LI characteristics) and low optical modulation distortion is required as a light source.
[0004]
For example, a conventionally known DFB laser 200 shown in FIGS. 16 and 17 incorporates a diffraction grating 10 having a uniform film thickness formed by a normal crystal growth method or the like. In the semiconductor laser having the above structure, the inversion distribution in the laser medium is spatially modulated by the standing wave generated in the resonator including the diffraction grating 10, and the electric field is the length of the resonator in the semiconductor laser. Due to the non-uniform distribution in the direction (usually having a distribution in which the electric field is low at the end of the resonator and is extremely high at the center), a phenomenon in which oscillation of the next mode is promoted may occur. Such a phenomenon is generally known as an axial hole burning effect, and due to this phenomenon, wavelength fluctuation occurs in the semiconductor laser. That is, the optical gain in the laser becomes non-uniform, and the optical output with respect to the injected current increases nonlinearly. As a result, there arises a problem that optical modulation distortion increases.
[0005]
As a means to solve the above problem, we have found that the occurrence of the above phenomenon can be eliminated by changing the coupling constant corresponding to the optical coupling strength of the distributed feedback semiconductor laser (Japanese Patent Application No. Hei9-19553). Specifically, the invention described in the above-mentioned patent application is directed to a guide layer (ie, a diffraction grating) in a distributed feedback semiconductor laser so as to uniformize a non-uniform electric field distribution that would be generated by the above phenomenon. The tilted diffraction grating is formed by changing the composition of the semiconductor crystal constituting the formation layer.
[0006]
[Problems to be solved by the invention]
However, although the invention described in the above-mentioned patent application (Japanese Patent Application No. Hei 9-19553) describes a diffraction grating having a thickness gradient distribution, it relates to a process for producing a layer for forming the diffraction grating. Details are not listed.
Therefore, the present invention aims to further improve the effect of the invention of the above-mentioned application, and after forming a tilted diffraction grating forming layer by a crystal growth method using a selective growth mask having a specific mask pattern. The diffraction grating forming layer is processed by a conventional method using photolithography, etching technology, etc., thereby achieving formation of a tilted diffraction grating having a film thickness distribution.
[0007]
[Means for Solving the Problems]
A first aspect of the present invention is a method for producing a DFB laser having a high linearity of optical output / current characteristics and a small optical modulation distortion, which includes a step of using a selective growth mask having a specific pattern. It is.
Specifically, in the first embodiment of the present invention, the mask pattern width in the direction perpendicular to the length direction of the resonator is the maximum and the mask opening width is the minimum at the end portion of the resonator length direction of the DFB laser. A semiconductor crystal is selectively grown using a selective growth mask having a pair of mask patterns in a mirror-symmetry form in which the mask pattern width gradually decreases from the end toward the center and the mask opening width increases. An inclination having a film thickness gradient distribution in which the film thickness at the end in the longitudinal direction of the resonator is thicker than the film thickness at the center and the film thickness gradually decreases from the end toward the center. The present invention relates to a method for manufacturing a DFB laser including a step of forming a diffraction grating forming layer.
The tilted diffraction grating forming layer obtained by the above method has a thickness gradient distribution suitable for uniformizing the electric field distributed in the length direction of the resonator in the manufactured DFB laser. Is characterized in that the film thickness ratio between the lengthwise end and the center of the resonator is at least 3: 1.
[0008]
The present invention also provides a DFB laser having a tilted diffraction grating obtained by processing a tilted diffraction grating forming layer manufactured by the above method. The DFB laser of the present invention is characterized in that the axial hole burning effect does not occur, so that the linearity of the light output / current characteristics is excellent and the light modulation distortion is small.
[0009]
A second aspect of the present invention is a selective growth mask for use in the above method,
At the end of the distributed feedback semiconductor laser in the length direction of the resonator, the mask pattern width in the direction perpendicular to the length direction of the resonator is the maximum and the mask opening width is the minimum, and from the end to the center A selective growth mask characterized by having a pair of mask patterns in a mirror-symmetrical form with a gradually decreasing mask pattern width and a wider mask opening width.
[0010]
The mask pattern of the selective growth mask is from an end portion in the length direction of the cavity of the distributed feedback semiconductor laser toward the central portion.
(i) a region where the mask pattern width in the direction perpendicular to the length direction of the resonator is the maximum and the mask opening width is the minimum,
(ii) the mask pattern width is constant and the area where the mask opening width gradually increases, and
(iii) The different mask pattern width reduction rates A and B include two portions satisfying the formula: A> B, and at least three regions having a constant mask opening width.
[0011]
In the present invention, the mask pattern is outside the end portion in the longitudinal direction of the resonator of the DFB laser,
(iv) The mask pattern width and the mask opening width may further include a region equivalent to the region (i). In this case, the end face of the DFB laser can be formed by cleaving the semiconductor substrate at a portion corresponding to the end portion in the length direction of the resonator and removing the portion outside the end portion.
Due to the presence of the region (iv), the film thickness ratio between the end portion in the longitudinal direction of the resonator and the center portion in the inclined diffraction grating formation layer to be formed can be increased.
[0012]
The mask pattern of the selective growth mask of the present invention is generally formed from a SiO ₂ insulating film.
[0013]
The selective growth mask of the present invention may include a region having no mask pattern at the center in the length direction of the laser resonator. That is, the tilted diffraction grating forming layer formed using the selective growth mask of the present invention may have a flat portion having no film thickness distribution at the center thereof.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in more detail with reference to FIGS.
[0015]
In the first embodiment of the present invention, the mask pattern width in the direction perpendicular to the length direction of the resonator is the largest and the mask opening width is the smallest at the end of the distributed feedback semiconductor laser in the length direction of the resonator. Then, a semiconductor crystal is selectively grown using a selective growth mask having a pair of mask patterns having a mirror-symmetrical shape in which the mask pattern width gradually decreases from the end toward the center and the mask opening width increases. The film thickness gradient distribution is such that the film thickness at the end in the longitudinal direction of the resonator is thicker than the film thickness at the central part and the film thickness gradually decreases from the end part toward the central part. This is a method of manufacturing a distributed feedback semiconductor laser including a step of forming an inclined diffraction grating formation layer.
[0016]
Process sectional drawing showing the one aspect | mode of the 1st form of this invention is shown in FIGS.
I) First, a P-type InP clad layer 22, an InGaAsP active layer 23, and an N-type InP clad layer 24 are sequentially grown on the P-type InP semiconductor substrate 21 by MOCVD (FIG. 1).
II) Next, the inclined diffraction grating forming layer 12 is formed on the cladding layer 24 by MOCVD using the selective growth mask of the first embodiment of the present invention described above (FIG. 2).
[0017]
III) After forming the SiO ₂ insulating film 34 on the formed tilted diffraction grating forming layer 12, electron beam exposure is performed to form an insulating film pattern 34 as shown in FIG. That is, in the method of the present invention, a SiO ₂ insulating film formed by a conventional method can be used as a selective growth mask.
IV) Using the insulating film pattern 34 as a mask, the inclined diffraction grating forming layer 12 is dry-etched under known conditions to form the inclined diffraction grating 11 (FIG. 4). In the thickness gradient distribution of the tilted diffraction grating forming layer formed by the method of the present invention, the film thickness ratio between the end portion and the center portion in the length direction of the resonator is at least 3: 1.
V) Further, InP is grown on the clad layer 24 having the inclined diffraction grating 11 while maintaining the shape of the diffraction grating, thereby forming an N-type InP layer 28 (FIG. 5).
[0018]
VI) Next, an SiO ₂ insulating film 33 is formed on the N-type InP layer 28 and then patterned.
VII) A mesa structure having a mesa width of about 1 μm is formed by dry etching the multilayer film described above using the insulating film pattern 33 as a mask (FIG. 6).
VIII) As shown in FIG. 7, the following layers are formed in order by MOCVD:
-P-type InP layer 25;
An N-type InP current blocking layer 26; and a P-type InP layer 27.
[0019]
IX) After removing the insulating film 33, an N-type InP layer 28 'and an N-type InGaAs contact layer 29 are formed on the P-type InP layer 27 and the N-type InP layer 28 by crystal growth by MOCVD (see FIG. FIG. 8).
[0020]
X) Finally, the SiO ₂ insulating film 30, the N-type electrode (Au / Ge / Ni / Au) 31 and the P-type electrode (Ti / Pt / Au) 32 are formed, respectively. A DFB laser having a tilted diffraction grating in the form 2 is obtained (FIGS. 9 and 10).
[0021]
FIG. 10 is a sectional view in the length direction of the resonator of the DFB laser manufactured by the above method. As is clear from this, the DFB laser produced by the method of the present invention is characterized in that an inclined diffraction grating having a film thickness distribution in the cavity direction is formed. By possessing such a diffraction grating, it is possible to achieve high linearity and reduced optical modulation distortion of optical output / current characteristics conventionally required for DFB lasers.
[0022]
Next, a selective growth mask having a specific pattern according to the second embodiment of the present invention will be described. FIG. 11A shows a mask pattern plan view of a selective growth mask having a specific pattern according to the second embodiment of the present invention. FIG. 11 (2) is a sectional view in the resonator length direction of the tilted diffraction grating forming layer 12 selectively grown using the selective growth mask, that is, a line bb ′ of the selective growth mask. FIG.
[0023]
Here, the selective growth mask of the present invention forms a SiO ₂ insulating film in a desired pattern form on a silicon wafer commonly used in the field by a combination of known means such as photolithography, vapor deposition and etching. It is a thing.
[0024]
In FIG. 11 (1), both ends in the horizontal direction correspond to end portions in the length direction of the resonator of the DFB laser. In the selective growth mask, the left and right mask patterns 1 have a mirror-symmetric shape, and the mask pattern width in the direction perpendicular to the length direction of the resonator is the maximum and the mask opening width is the minimum at both ends. The mask pattern width gradually decreases from the end toward the center, and the mask opening width increases.
[0025]
A feature of the selective growth mask of the present invention is that, as shown in FIG. 11 (1), the mask pattern extends from the end portion in the length direction of the resonator of the distributed feedback semiconductor laser toward the central portion.
(i) a region where the mask pattern width in the direction perpendicular to the length direction of the resonator is the maximum and the mask opening width is the minimum,
(ii) the mask pattern width is constant and the area where the mask opening width gradually increases, and
(iii) The different mask pattern width reduction rates A and B include two portions satisfying the formula: A> B, and at least three regions having a constant mask opening width. Here, the selective growth mask may include a region having no mask pattern in the central portion, if necessary.
[0026]
Specific examples of the first mode of the second mode include mask patterns (1 ′ and 1 ″) as shown in FIGS. 13 (1) and 13 (2). Mask whose mask pattern width is large and the mask opening width is narrow in the region corresponding to the cavity end of the laser, and that the mask pattern width gradually decreases and the mask opening width is widened toward the region corresponding to the center of the resonator Has a pattern.
FIGS. 14 (1) and 14 (2) show the film thickness distributions of the tilted diffraction grating forming layer grown using these selective growth masks. In FIG. 14, the actually measured film thickness value is a dot, the computer simulation value of the film thickness value that will be obtained by each mask pattern is a solid line, and the target film thickness gradient at the time of creating each mask pattern in FIG. Each distribution profile is represented by a dotted line.
In FIG. 14 (1), it can be seen that the thickness distribution of the obtained tilted diffraction grating forming layer is almost as simulated. Furthermore, it is also shown that the relative film thickness decreases in a linear shape from the region corresponding to the end portion of the resonance portion toward the region corresponding to the central portion. The film thickness ratio between the center and the resonator end is about 3: 1. Alternatively, in FIG. 14 (2), it can be seen that the relative film thickness decreases in a parabolic manner from the region corresponding to the end of the resonance portion toward the region corresponding to the center portion. Also in this case, as in the former case, the film thickness ratio of the resonator center portion and the resonator end portion in the formed inclined diffraction grating formation layer is about 3: 1.
[0027]
Alternatively, in the selective growth mask of the present invention, in order to increase the film thickness ratio between the end portion and the center portion in the length direction of the resonator in the formed inclined diffraction grating formation layer, the length direction of the resonator On the outside of the end of
(iv) It may further have a mask pattern region in which the mask pattern width and the mask opening width are equivalent to the region (i) (FIG. 15 (1)).
[0028]
Here, the film thickness distribution of the inclined diffraction grating forming layer can be adjusted by arbitrarily setting the mask pattern width and mask opening width of the selective growth mask to be used as described above. Here, the phenomenon in which the film thickness distribution is formed in the inclined diffraction grating forming layer will be briefly described with reference to FIG. FIG. 12 is a cross-sectional view corresponding to the aa ′ line of the mask pattern 1 during crystal growth using the selective growth mask of FIG. During the crystal growth of the material for the inclined diffraction grating formation layer by MOCVD or the like, the material does not grow on the mask pattern 1, so that the material gas concentration above the mask pattern 1 becomes high. On the other hand, since the material for the tilted diffraction grating formation layer grows in a portion where the mask pattern 1 does not exist, the material gas concentration in the upper portion of the portion is lower than that in the upper portion of the mask pattern 1. As a result, the material gas diffuses from a portion having a high material gas concentration to a portion having a low material gas concentration, and the material gas concentration becomes relatively high above the portion sandwiched between the mask patterns 1. As a result, the thickness of the tilted diffraction grating forming layer corresponding to the portion where the mask pattern width of the selective growth mask is wide becomes thicker than the thickness of the layer corresponding to the portion where the mask pattern width is narrow.
[0029]
In correspondence with the mask pattern of the selective growth mask, the formed tilted diffraction grating forming layer 12 has a film thickness that is thick at the resonator end and gradually decreases from the resonator end toward the center. It has a uniform film thickness distribution (FIG. 11 (2)). When a diffraction grating pattern is subsequently formed on the tilted diffraction grating forming layer having the film thickness distribution as described above by etching, for example, a tilted diffraction grating 11 as shown in FIG. 4 is obtained. In the DFB laser having the tilted diffraction grating 11, the electric field distributed in the length direction of the resonator can be made uniform, so that the axial spatial hole burning effect is eliminated, and the light modulation distortion is reduced. .
[0030]
As described above, the tilted diffraction grating whose film thickness is gradually reduced from the resonator end equivalent region to the center equivalent region is required for a laser with a waveguide lens, for example.
[0031]
Another embodiment of such a selective growth mask is a mask as shown in FIG. In FIG. 15 (1), regions of 150 μm from both ends of the selective growth mask are portions that are not used for laser fabrication. In this portion, the mask pattern width and the mask opening width are both constant.
In the tilted diffraction grating formation layer formed using the selective growth mask, the result of evaluating the relative film thickness distribution of the layer corresponding to the region extending from the mask end portion to the central portion in the same manner as in FIG. As shown in FIG. In FIG. 15 (2), it is apparent that the thickness distribution of the obtained tilted diffraction grating forming layer is almost as simulated.
In FIG. 15 (2), the region to the left of the broken line is a portion that is not used for laser fabrication as described above. From FIG. 15 (2), it can be seen that the thickness of the formed inclined diffraction grating formation layer decreases in a downward convex curve from the resonator end portion to the center portion indicated by the broken line. I understand. Here, the film thickness ratio between the resonator end indicated by a broken line in the drawing and the resonator end at the right end of the graph is about 5: 1.
[0032]
When the selective growth mask of another embodiment is used, the tilted diffraction grating forming layer formed as described above is cleaved by a broken line (that is, a portion corresponding to the resonator end) in FIGS. 15 (1) and 15 (2). By doing so, a laser end face can be formed.
[0033]
【The invention's effect】
According to the present invention, in the length direction of the resonator, the end portion is thick and the central portion is thin, and the slope has a thickness distribution with a slope in which the thickness gradually changes from the end portion to the central portion. A type diffraction grating layer can be formed. A distributed feedback semiconductor laser having a tilted diffraction grating fabricated using such a tilted diffraction grating layer prevents the axial hole burning effect from occurring, and has high linearity in optical output and current characteristics. Light modulation distortion is small.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a step (I) of a method for producing a DFB laser according to a first embodiment of the present invention.
FIG. 2 is a cross-sectional view showing a step (II) in the method for manufacturing the DFB laser according to the first embodiment of the present invention.
FIG. 3 is a cross-sectional view showing a step (III) of a method for manufacturing a DFB laser according to the first embodiment of the present invention.
FIG. 4 is a cross-sectional view showing a step (IV) in the method for manufacturing the DFB laser according to the first embodiment of the present invention.
FIG. 5 is a cross-sectional view showing a step (V) of the method for manufacturing the DFB laser according to the first embodiment of the present invention.
6 is a cross-sectional view showing one step (VI to VII) of the method for manufacturing the DFB laser according to the first embodiment of the present invention. FIG.
FIG. 7 is a cross-sectional view showing a step (VIII) of the method for manufacturing the DFB laser according to the first embodiment of the present invention.
FIG. 8 is a cross-sectional view showing a step (IX) of the method for manufacturing the DFB laser according to the first embodiment of the present invention.
FIG. 9 is a cross-sectional view of a DFB laser fabricated by the method of the present invention.
FIG. 10 is a longitudinal sectional view of a resonator of a DFB laser produced by the method of the present invention.
FIG. 11 is a mask pattern plan view (1) of a selective growth mask representing a second embodiment of the present invention, and a film of a tilted diffraction grating forming layer formed using the mask in the longitudinal direction of the resonator. Represent thickness distribution (2), respectively.
FIG. 12 is a cross-sectional view showing a state where a tilted diffraction grating forming layer is formed using the selective growth mask of the second embodiment of the present invention.
FIG. 13 is a plan view showing a mask pattern of a selective growth mask according to the first aspect of the second mode of the present invention.
FIG. 14 is a graph showing a relative film thickness of a tilted diffraction grating forming layer formed using the selective growth mask according to the first aspect of the second mode of the present invention.
FIG. 15 shows a mask pattern plan view (1) of a selective growth mask according to another aspect of the second embodiment of the present invention, and the relative film thickness of the inclined diffraction grating formation layer formed using the selective growth mask. It is a graph (2).
FIG. 16 is a cross-sectional view of a conventionally known DFB laser.
FIG. 17 is a longitudinal sectional view of a resonator of a conventionally known DFB laser.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1, 1 ', 1 ", 1''' ... Mask pattern of the selective growth mask of this invention, 2 ... Mask pattern width, 2 '... Tapered mask pattern part, 3 ... Mask opening part, 5 ... Inclined type Flow of material gas for diffraction grating forming layer, 10 ... conventional diffraction grating, 11 ... tilted diffraction grating, 12 ... tilted diffraction grating forming layer, 21 ... P-type InP semiconductor substrate, 22 ... P-type InP clad layer, 23 ... InGaAsP active layer, 24 ... N-type InP clad layer, 25 ... P-type InP layer, 26 ... N-type InP current blocking layer, 27 ... P-type InP layer, 28 ... N-type InP layer, 29 ... N-type InGaAs contact layer , 30, 33, 34... SiO ₂ insulating film, 31... N-type electrode, 32... P-type electrode, 100 ... DFB laser manufactured according to the present invention, 200.

Claims (5)

In a method of manufacturing a distributed feedback semiconductor laser having an inclined diffraction grating,
At the end of the distributed feedback semiconductor laser in the length direction of the resonator, the mask pattern width in the direction perpendicular to the length direction of the resonator is the maximum and the mask opening width is the minimum, and from the end to the center A semiconductor crystal is selectively grown using a selective growth mask having a pair of mask patterns in a mirror-symmetry form in which the mask pattern width gradually decreases and the mask opening width increases, and the length direction of the resonator An inclined diffraction grating forming layer having a thickness gradient distribution in which the film thickness at the end of the film is thicker than the film thickness at the central part and the film thickness gradually decreases from the end part toward the central part is formed. A method of manufacturing a distributed feedback semiconductor laser including the step of 2. The film thickness gradient distribution of the tilted diffraction grating forming layer, wherein a film thickness ratio between an end in the length direction of the resonator and the central portion is at least 3: 1. A manufacturing method of a distributed feedback semiconductor laser. The mask pattern extends from an end portion in the longitudinal direction of the cavity of the distributed feedback semiconductor laser toward the central portion.
(i) a region where the mask pattern width in the direction perpendicular to the length direction of the resonator is the maximum and the mask opening width is the minimum,
(ii) the mask pattern width is constant and the area where the mask opening width gradually increases, and
(iii) different mask pattern width decrease rate A and B has the formula: Claim comprises two portions which satisfies A> B, and the mask opening width, characterized in that it has at least three regions of the constant region 1 A manufacturing method of the distributed feedback semiconductor laser described. The mask pattern is on the outside of the lengthwise end of the cavity of the distributed feedback semiconductor laser,
4. The method of manufacturing a distributed feedback semiconductor laser according to claim 3 , further comprising a region in which the mask pattern width and the mask opening width are equal to the region (i). The selective growth mask, the manufacture of distributed feedback semiconductor laser according to claim 1, comprising a region having no mask pattern in the center of the length direction of the resonator of the distributed feedback semiconductor laser Way .

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