JP3106852B2 - Distributed feedback semiconductor laser and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a distributed feedback semiconductor laser (DFB-LD) having excellent single-mode characteristics, and more particularly to an improvement in the manufacturing accuracy of an oscillation wavelength.

[0002]

2. Description of the Related Art In a conventional semiconductor laser (LD), a so-called buried type in which a low threshold value is easily obtained is obtained by etching an active layer into a stripe shape and filling the periphery with a current constriction layer having a low refractive index. The structure is widely adopted.
The same applies to the DFB-LD. In particular, most of the InP-based materials, which have a small reliability problem even when the active layer is processed, are buried or improved types.

[0003]

However, as in the case of using a DFB-LD as a light source for frequency-stabilized light sources or a light source for wavelength multiplexing (WDM), which is expected as a next-generation optical communication system, the accuracy of the oscillation wavelength is reduced. It was clarified that there were the following problems when strictly required.

That is, the oscillation wavelength λ of the DFB-LD is: λ = 2 · Λ · n _eff where Λ is the pitch of the diffraction grating n _eff is represented by the equivalent refractive index, but since n _eff varies, the oscillation wavelength λ Accuracy cannot be increased. This is because the conventional embedded type has a large difference in the refractive index in the horizontal direction, and even a slight change in the width of the active layer affects the entire refractive index n _eff .

[0005] In order to solve this, the difference in refractive index may be reduced. The so-called ridge type, in which a groove is formed near the active layer while the active layer is kept flat, has the advantages that the refractive index difference can be reduced by the distance between the groove and the active layer, and that the number of times of crystal growth can be reduced. However, in such a ridge type, it is necessary to cut a deep groove of about 1 μm, and furthermore, to control the distance to the active layer to about 0.2 μm.

On the other hand, a so-called rib type in which a part of the surface of a light confinement layer (SCH layer) close to a flat active layer as shown in FIG. Since the etching is easy and the etching process is easy, the reproducibility of the transverse mode confinement is high. However, since current confinement is performed by separately forming a groove, a mask alignment step is required, and it is difficult to control leakage current to areas other than the active layer immediately below the stripe. For this reason,
Variations occur in threshold values and operating currents, which also affects wavelength accuracy.

In order to determine the oscillation wavelength with high accuracy, it is necessary to improve the controllability of the equivalent refractive index and improve the reproducibility of the leakage current. Factors that determine the equivalent refractive index are the shape of the waveguide and the refractive index of each layer, but there are few problems because the control of the film thickness and the refractive index of each layer is determined by crystal growth. On the other hand, the accuracy of manufacturing stripes for confining the transverse mode is determined by lithography, and thus tends to cause a problem.
In particular, by cutting the active layer, which is generally used in InP
In the method in which P is filled, the difference in the refractive index of the transverse mode confinement is large, so that the slight variation in the stripe width greatly changes the equivalent refractive index. A solution to this problem is to reduce the difference in the refractive index of the transverse mode confinement as far as possible without impairing the stability of the transverse mode, and to increase the etching accuracy.

Although it is widely known that there are many advantages to using a quantum well structure for an active layer, when a quantum well is used, a gain peak occurs during a process after the growth of the quantum well active layer. It is desired that no shift occurs.
In a structure in which the active layer is cut and embedded, the impurity used for the embedded layer is diffused from the side surface of the quantum well active layer and the quantum well is mixed, so that the gain peak often does not meet the design. Since the degree of occurrence of defects and recombination levels is also likely to depend on the process, it is desirable to avoid processing the active layer also from this point.

On the other hand, in order to eliminate the leakage current or reproduce the leakage ratio, it is desired that the transverse mode confinement and the current confinement are simultaneously produced and self-aligned. The buried type is ideal in this respect, but in a structure in which the active layer is not processed such as a rib type, it is a normal process to align the lateral mode control confinement and current confinement by mask alignment. However, it is difficult to perform positioning with high accuracy and high reproducibility. As a method of self-alignment, a method of selectively growing a current confinement is effective. Recently good reproducibility selective growth by metal organic chemical vapor deposition, i.e. crystal method where you do not want to grow to only the crystal growth where no mask to form a mask such as S _i O ₂ film, is capable of It is attracting attention as a method for making various structures in a plane.

In view of the foregoing, the present invention provides a semiconductor laser and a method of manufacturing the same, which can utilize a selective growth, have a flat active layer, can control a small difference in refractive index, and can perform self-alignment. The purpose is to provide.

[0011]

In order to achieve the above object, a semiconductor laser according to the present invention comprises a diffraction grating for obtaining at least a distributed feedback, a laterally flat active layer, An optical confinement layer provided in contact with the active layer or with another semiconductor layer interposed therebetween, and formed in a stripe shape for lateral mode confinement provided in contact with this optical confinement layer or interposed with another semiconductor layer. A ribbed layer, a low-refractive-index cladding layer for vertically confining light, and a current confinement layer having the same refractive index as the cladding layer and formed on the side surface of the rib layer and having a (111) side surface. The refractive index of the rib layer is set higher than that of the clad layer, and the etching depth of the rib layer is smaller than the lateral width of the rib layer.

Further, according to the method of manufacturing a semiconductor laser of the present invention, a diffraction grating for obtaining at least distributed feedback, an active layer flat in the lateral direction, and another semiconductor in contact with the active layer are provided on the substrate. Forming a light confinement layer provided with a layer interposed therebetween, forming a stripe on the light confinement layer, partially etching the surface of the light confinement layer using the stripe as a mask, and etching depth thereof. Forming a rib layer so that is smaller than the width of the stripe, and forming a current confinement layer such that a crystal is selectively grown using the stripe as a mask and a (111) low growth rate surface appears on a side surface. And forming a cladding layer on the rib layer and the current confinement layer after removing the stripe.

[0013]

A distributed feedback semiconductor laser having at least a diffraction grating for obtaining distributed feedback, an active layer, a light confinement layer, a rib layer, and a low refractive index clad layer for vertically confining light on a substrate. In the above, the etching depth required for processing the rib layer into a stripe shape is made smaller than the lateral width of the rib layer. For example, it is set to 1/10 or less. A current confinement layer having the same refractive index as the cladding layer is provided on the side surface of the rib layer. The current confinement layer is formed by selectively growing a crystal using a mask used for processing the rib layer into a stripe shape so that a (111) low growth rate surface appears on the side surface of the current confinement layer. This makes it possible to manufacture a rib-type semiconductor laser in which the active layer is flat and the refractive index difference can be controlled to be small.

[0014]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
FIG. 1 is a perspective view showing one embodiment of a semiconductor laser according to the present invention, and FIG. 2 is a cross-sectional view showing each step of a method for manufacturing a semiconductor laser according to the present invention. Table 1 shows an example of the thickness and composition of each layer in the GaInAsP / InP material system.

(1 line blank)

On a conductive substrate 1, a cladding layer 2 (first
And a layer for forming a diffraction grating are formed by crystal growth, and a diffraction grating 3 is formed on this surface by electron beam exposure (or interference exposure) and reactive ion etching (or chemical etching). On this, the cladding layer 4 (second cladding layer), the SCH layer 5 (first light confinement layer), the active layer 6, and the SCH layer 7 (second light confinement layer)
Is grown. The structure and manufacturing method up to this point are the same as those of a general DFB-LD. The diffraction grating may include a phase shift.

Next, plasma CVD (Chemical Vapor Dep.)
An SiO ₂ film is deposited by an osition method, and a stripe 8 as shown in FIG. 2B is formed by photolithography. Using this stripe 8 as a mask, the surface of the SCH layer 7 is partially etched as shown in FIG. The step acts as a rib layer. Note that, when etching is performed in the depth direction, side etching simultaneously proceeds in the horizontal direction, so that the lateral width of the rib changes by an amount corresponding to the side etching. In order to reduce this effect, it is effective to increase the rib width and to reduce the side etching by making the etching shallow. The etching depth is shallower than the rib width, but it is desirable that the etching depth is less than one tenth of the rib width.

The difference in the refractive index is almost proportional to the etching amount.
If only a very shallow part of the surface is etched slightly, the controllability of the etching depth is good, and it is possible to monitor precisely with a stylus type step meter, etc., so that the reproducibility of the refractive index difference should be improved. Easy. In the example of Table 1, when the etching amount is 70 nm, the difference in the refractive index is 0.02.

Next, as shown in FIG. 2D, the current confining layer 9 (9a, 9a,
9b) is grown. Table 2 shows an example of conditions for selective growth of the current confinement layer.

[0020]

Under this condition, nothing is attached on the stripe 8 used as the mask, and the current confinement layer is automatically arranged on the side surface of the rib layer. Moreover, (11)
1) Since a surface appears and the growth on this surface has stopped or the growth rate is very slow, even if a thick current confinement layer is grown, the mask will not be covered.

Next, after removing the stripe 8 used as a mask, the cladding layer 12 (third cladding layer) and the contact layer 13 are grown, as shown in FIG. If the electrodes 14 and 15 are formed on the lower surface and separated into elements, the semiconductor laser shown in FIG. 1 is completed. It is to be noted that, if necessary, a treatment such as an anti-reflection coating on the end face is performed similarly to a general DFB-LD.

The present invention is not limited to the embodiment. For example, a separation layer 10 and a rib layer 11 are further grown on the SCH layer 7 in FIG. 2A, and a stripe 8 is formed thereon as shown in FIG. May be etched. The subsequent steps are the same as in the above embodiment. As a specific example of the composition and the film thickness in this case, the separation layer 10 p-InP 50 nm and the rib layer 11 p-GaInAsP (λ _g = 1.3 μm) 100 nm are added to those in Table 1. FIG. 3 shows the structure of FIG.
It is sectional drawing corresponding to (d). This structure has an advantage that the refractive index difference can be made the same as long as the end point of the etching is between the upper surface and the lower surface of the separation layer 10.

Further, the current confinement layer 9 is made to have a high resistance (the current confinement layers 9a and 9b are made of, for example, Fe-doped InP), and all others are the first embodiment (FIGS. 1 and 2) or the second embodiment (FIG. It may be the same as 3).

Furthermore, by dividing the upper electrode and adjusting the amount of current injection, the oscillation wavelength can be adjusted with higher accuracy. The diffraction grating may be a gain-coupled DFB having gain or absorption.

In the semiconductor laser formed by the above-described method, since the variation of the equivalent refractive index due to the stripe width is small, the oscillation wavelength can be accurately determined by the pitch of the diffraction grating. FIG. 4 shows a calculation example of the change of the equivalent refractive index with respect to the stripe width of the conventional embedded structure (dotted line) and the structure of the present invention (when the refractive index difference Δn = 0.01 and 0.02). In FIG. 4, the range indicated by the broken line is a range corresponding to a wavelength change of ± 0.6 nm in the 1.55 μm band, and the manufacturing accuracy of the stripe width required to adjust the oscillation wavelength within this range is 1 ± 1. 0.03nm to 2
It can be seen that it is greatly relaxed to ± 0.3 nm (Δn = 0.02).

In addition, since a mask alignment step is not required in the process and etching can be easily performed with high accuracy, the reproducibility of the process is high. Therefore, the characteristics as designed can be obtained with good reproducibility. Further, even if a quantum well is used for the active layer, the gain peak does not change during the process. Therefore, the frequency stabilizing light source L for which the oscillation wavelength range is strictly specified
Multi-wavelength integrated LD for WDM requiring accuracy of D and wavelength interval
It is most suitable for use in such applications.

Another feature is that the difference in the refractive index of the transverse mode confinement can be freely given by the thickness and the refractive index of the rib. For example, in the conventional embedded type, the refractive index difference is too large and it is difficult to make the higher-order transverse mode cut-off with the active layer width that can be actually produced. A mode cutoff can be designed, and a stable transverse single mode can be easily obtained. If you increase the cutoff width,
The light density at the end face can be reduced, which is effective for increasing the output. The fact that the difference in the refractive index is small and the amount of etching can be reduced has the effect of reducing the loss due to the scattering of the waveguide, and is also effective in obtaining a narrow spectral line width.

[0029]

As described above, according to the present invention, since the variation of the equivalent refractive index due to the stripe width is small, the oscillation wavelength can be determined accurately by the pitch of the diffraction grating. In addition, since the current constriction is self-aligned with the transverse mode confinement, the reproducibility of the leakage current can be improved. Further, the refractive index difference of the transverse mode confinement is freely given by the thickness and the refractive index of the rib. The fact that the difference in refractive index can be reduced and the amount of etching can be reduced has the effects of reducing the loss due to scattering of the waveguide and obtaining a narrow spectral line width. Such a semiconductor laser of the present invention is particularly effective when used as a light source for communication or measurement using long wavelength materials (GaInAsP / InP).

[Brief description of the drawings]

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

FIG. 2 is a cross-sectional view illustrating each step of a method for manufacturing a semiconductor laser according to the present invention.

FIG. 3 is a perspective view showing another embodiment of the semiconductor laser of the present invention.

FIG. 4 is a diagram illustrating a calculation example of a change in an equivalent refractive index with respect to a stripe width.

FIG. 5 is a perspective view showing an example of a conventional semiconductor laser.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 Cladding layer 3 Diffraction grating 4 Cladding layer 5 Optical confinement layer 6 Active layer 7 Optical confinement layer 8 Stripe 9a, 9b Current confinement layer 10 Separation layer 11 Rib layer 12 Clad layer 13 Contact layer 14, 15 Electrode

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-62-296588 (JP, A) JP-A-5-275801 (JP, A) JP-A-56-18484 (JP, A) JP-A-57-1984 66685 (JP, A) JP-A-60-45084 (JP, A) JP-A-3-229481 (JP, A) JP-A-5-37079 (JP, A) JP-A-4-6889 (JP, A) Yokogawa Technical Report Vol. 38, No. 3, pp. 117-120 (58) Field surveyed (Int. Cl. ⁷ , DB name) H01S 5/00-5/50 JICST file (JOIS)

Claims (10)

(57) [Claims] A diffraction grating for obtaining at least distributed feedback, an active layer flat in a lateral direction, and a light confinement layer provided on or in contact with this active layer or with another semiconductor layer interposed therebetween. A rib layer provided in contact with the light confinement layer or sandwiching another semiconductor layer and formed in a stripe shape for lateral mode confinement, a low refractive index cladding layer for vertically confining light, A current constriction layer having the same refractive index as the cladding layer and being formed on the side surface of the rib layer and having a side surface of a (111) plane; A distributed feedback semiconductor laser, wherein the etching depth of the rib layer is smaller than the lateral width of the rib layer. 2. The distributed feedback semiconductor laser according to claim 1, wherein an etching depth of said rib layer is set to be 1/10 or less of a lateral width of said rib layer. 3. The distributed feedback semiconductor laser according to claim 1, wherein a semiconductor layer below said rib layer has the same refractive index as said current confinement layer. 4. The distributed feedback semiconductor laser according to claim 1, wherein said current confinement layer is a combination of a p-type layer and an n-type layer. 5. The distributed feedback semiconductor laser according to claim 1, wherein said current confinement layer is a high resistance layer. 6. The distributed feedback semiconductor laser according to claim 1, wherein a material of said current confinement layer is InP. 7. The distributed feedback semiconductor laser according to claim 1, wherein said active layer has a quantum well structure. 8. A substrate, comprising: a diffraction grating for obtaining at least distributed feedback; an active layer which is flat in the lateral direction; and a light confinement layer provided in contact with the active layer or with another semiconductor layer interposed therebetween. Forming, forming a stripe on the light confinement layer, and partially etching the surface of the light confinement layer using the stripe as a mask, so that the etching depth is smaller than the width of the stripe. Forming a rib layer, and selectively growing crystals using the stripe as a mask,
Forming a current confinement layer such that a (111) low growth rate surface appears on the side surface, and forming a cladding layer on the rib layer and the current confinement layer after removing the stripe. Of manufacturing a distributed feedback semiconductor laser. 9. The distributed feedback semiconductor laser according to claim 8, wherein, in the step of forming the rib layer, the etching is performed so that an etching depth is one tenth or less of a width of the rib layer. Manufacturing method. 10. The method of manufacturing a distributed feedback semiconductor laser according to claim 8, wherein in the step of forming the current confinement layer, metal organic chemical vapor deposition is used for selective growth of the current confinement layer. .