JPS59229891A - Manufacturing method of semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain the distributed feedback type laser of a large amount of photo feedback and less threshold current by a method wherein a resonator utilizing Bragg's reflection due to periodical unevenness is formed at the interface between a waveguide layer and an enclosed layer, so that the side surface of this periodical unevenness makes an accurate perpendicular surface to the direction of photo radiation. CONSTITUTION:The laminated body of the lower waveguide layer 12 of N-InGaAsP, an active layer 13 of InGaAsP, the upper waveguide layer 14' of P-InGaAsP, and an auxiliary layer 20' of P-InP is formed on a substrate also serving as the lower enclosed layer 11 of N-InP. Successively, a resist mask 21 having a plurality of stripe apertures 21' is formed, and next stripe apertures 20' are formed by etching the auxiliary layer 20', thus forming a mask 20 to form the periodical unevennes in the layer 14' serving as the waveguide layer. Using the mask 20, new apertures 20''' are formed by etching the layer 14' to a depth approx. half, and accordingly the upper waveguide layer 14 having periodical unevenness is formed in the upper layer part. Since the side surface of the periodical aperture formed in such a manner is preserved as a plane perpendicular to the direction of photo radiation, a sufficient amount of photo feedback is secured, while the threshold current is reduced.

Description

[Detailed Description of the Invention] (!) Technical Field of the Invention The present invention does not have a pair of arm openings that function as resonators, but has a reflective function as a resonator in a direction parallel to the light emission direction with a certain regularity. It relates to a method for manufacturing a distributed feedback semiconductor laser in which the above-mentioned geometrically distributed reflection functions are provided in a geometrical (two-distributed manner) at intervals between a waveguide layer and a confinement layer. This invention relates to an improvement in a method for manufacturing a distributed feedback semiconductor laser that relies on periodic changes in refractive index realized by periodic irregularities provided at an interface.

(2) Background of the technology Semiconductor lasers include so-called Fabry-Perot semiconductor lasers, which have a resonator formed by a pair of reflecting mirrors realized by an arm-opening surface, etc.; There is a multilayer semiconductor laser having an active layer, a waveguide layer, and a confinement layer sandwiching these layers. Among these multilayer structure semiconductor lasers, there is one called a distributed feedback semiconductor laser in which a reflective function is distributed in one or both of the interfaces between a waveguide layer and a confinement layer.

In this distributed feedback semiconductor laser, the reflection function is distributed at geometric intervals that have a certain numerical relationship with the frequency of light, and as this reflection function, (a) a change in gain is used, or (b) It is further classified into two types depending on whether the difference in refractive index is used or not.

The present invention is an improvement of the distributed feedback semiconductor laser by utilizing the above-mentioned means (b), that is, the difference in refractive index realized by changing the composition of the semiconductor.

(3) Conventional technology and problems Figure 1 shows, for example, indium gallium arsenide phosphide.
FIG. 2 is a cross-sectional view of a substrate showing an example of the basic structure of a distributed feedback semiconductor laser made of aAsp)/indium phosphide (nP). In the figure, l is n-type indium phosphide (n-type n
2 is a lower waveguide layer made of n-type indium gallium arsenide phosphide (n-type nGaAsP), and 3 is an active layer made of undoped indium gallium arsenide phosphide (n-type nGaAsP). layer〒
A11, 4 has an upper waveguide layer f made of p-type indium gallium arsenide phosphide (p-type nGaAsP), and 5 has a p-type indium gallium arsenide phosphide (p-type nGaAsP).
The upper confinement layer is made of p-type indium phosphide (p-InP), and 6 is p-type indium gallium arsenide phosphide (p-InP).
This is an electrode contact layer made of GaAsP).
．． 8, each has a positive electrode and a rectangular 4 carrier. As is clear from the above, periodic irregularities are formed at the interface between the upper waveguide layer 4 and the upper confinement layer 5 [), thereby realizing a periodic change in the refractive index. It goes without saying that in order to realize the reflective function distributed with this periodic unevenness, it is desirable that the side surface of the unevenness be a plane exactly perpendicular to the light emission direction.

By the way, in the prior art, the liquid phase epitaxial growth method (hereinafter referred to as LPE) is a method for realizing the above structure.
It's called law. ) is used to continuously form a stack up to the upper waveguide layer 4 on the substrate l, and after forming a resist film on the surface of the upper waveguide layer 4, a photolithography method is used to form a stack of layers up to the upper waveguide layer 4. A resist mask having grooves corresponding to the periodic unevenness is formed by turning the resist mask. Using this resist mask, a part of the upper part of the upper waveguide layer 4 is etched. After removing the resist, LPE is applied again. A method in which a confinement layer 5 and a contact layer 6 are sequentially formed using a method has been mainly used. However, [2. In this method, the etching resistance of the resist mask is low, and it is not easy to form the desired concave portion accurately in the upper waveguide layer 4, and the upper confinement) @5 During the growth of the waveguide layer 4, melt knocks occur, especially in the convex portion B, and the surface A is no longer perpendicular to the light emission direction, or in extreme cases, the periodic irregularities are flattened and the reflection function is reduced. be done.

Then, the mask material was replaced with a resist and the upper waveguide layer 4 was
A method is also used in which a semiconductor layer is used that has selective etching properties with respect to the semiconductor layer, and a mask is formed using this semiconductor layer, and this mask is used to form periodic irregularities in the waveguide layer 4. According to the method described above, the etching resistance is improved, but no effect is observed on the method described above and the method described by colleagues and melt nozzle. In other words, it is difficult for the side surface of the periodic unevenness to be a plane exactly perpendicular to the direction of light emission, and it does not have a satisfactory reflection function, making it difficult for the distributed feedback semiconductor laser to perform its intended function. However, as a result, there are drawbacks such as an increase in threshold current.

(4) Purpose of the Invention The purpose of the present invention is to eliminate this drawback, and without having a pair of arm openings that function as a resonator, the reflection function as a resonator is directed in a direction parallel to the light emission direction. The periodic refractive index is distributed geometrically at regular intervals on the waveguide layer, and this reflection function is realized by the periodic irregularities provided at the interface between the waveguide layer and the confinement layer. In the manufacturing method using the LPE method of a distributed feedback semiconductor laser that depends on the change in temperature, it is possible to prevent the above-mentioned periodic irregularities, especially the melting of the convex portions, and to provide a periodicity that has a satisfactory reflection function. It is an object of the present invention to provide a method for manufacturing a distributed feedback semiconductor laser, which can achieve the desired shape and size of unevenness, increase the amount of optical feedback, and achieve a low threshold current.

(5) Structure of the Invention The method for manufacturing a semiconductor laser according to the present invention comprises: an active layer, a waveguide layer sandwiching the active layer, and a confinement layer sandwiching the waveguide layer. In the method for manufacturing a distributed feedback semiconductor laser having periodic irregularities at the interface between the waveguide layer and the confinement layer, the confinement layer (the confinement layer) is provided on the surface of the waveguide layer (the confinement layer). An auxiliary layer made of the same semiconductor as the waveguide layer (layer) is formed, selectively formed with a periodic open pattern on the auxiliary layer, and then etched using the iR-turned auxiliary layer as an etching mask. selectively etching the confinement layer) to form periodic irregularities on the surface of the waveguide layer (the confinement layer);
This is realized by including the step of forming the confinement layer (the waveguide layer) on the waveguide layer (the confinement layer) of the auxiliary layer.

In the conventional technology, after forming periodic asperities on the upper surface of the waveguide layer, when forming the confinement layer, not only the resist mask used to form the periodic asperities described above but also a mask made of semiconductor is used. had been removed. However, this has caused a problem in that the waveguide layer is melt-knocked during growth of the confinement layer, resulting in irregular shapes of the periodic irregularities.

Therefore, the inventor of the present invention believes that in order to prevent such a phenomenon, it would be effective to adopt a method that does not require the above-mentioned convex portions to come into contact with the growth solution for forming the confinement layer. After etching the waveguide layer using a mask made of the same semiconductor as the confinement layer, we did not remove this mask, but rather used it to guide the waveguide. If the confinement layer is grown while protecting the convex portions of the wave layer from meltback, the mask will remain on the convex portions, so the convex portions will not come into contact with the growth solution, and therefore the melt-back will not occur. The present invention was completed by experimentally confirming that the scales formed the desired periodic irregularities without any occurrence of irregularities.

The above steps will be described in more detail below. First, a lower waveguide layer, an active layer, a semiconductor layer that will become an upper waveguide layer, and an auxiliary layer that is made of the same semiconductor as the confinement layer and functions as a mask are formed on the substrate using the LPE method in one step. After forming a resist film on this auxiliary layer, patterning it and using it to form periodic irregularities in the upper waveguide layer) forming a resist mask;
This resist mask is used to etch the auxiliary layer, and the same pattern as above, ie, periodic openings, is formed in the auxiliary layer, which serves as an etching mask. Using this etching mask, the upper layer of the upper waveguide layer is etched to about half the depth to form periodic openings. At this time, the side surfaces of the periodic apertures become planes perpendicular to the light emission direction.

Next, two effective methods for growing the confinement layer will be described. The first method is to grow the confinement layer again using the LPE method without removing the etching mask made of the auxiliary layer made of the same semiconductor as the confinement layer! In this case, since the material of the etching mask and the material of the confinement layer are the same materials, the fact that the etching mask remains without being removed is not a disadvantage to the formation of the confinement layer. In this process, the remaining etching mask functions as a protective film from melt knocks, so that the irregularities of the waveguide layer, especially the convex parts, are not melted back and the condition of periodic irregularities is not worsened. Since the side surfaces of this periodic unevenness are preserved as planes perpendicular to the light emission direction, a sufficient amount of light feedback is ensured and the threshold current is sufficiently reduced.

Next, in the second method, heat treatment is applied without removing the etching mask made of the auxiliary layer formed of the same semiconductor as the confinement layer, as in the first method. The heat treatment causes the convex portions of the auxiliary nozzle to be thermally deformed, making it possible to fill in the concave portions of the upper waveguide layer. The thermal deformation described above is a phenomenon that utilizes the vapor pressure difference of the semiconductor constituent elements between the convex portions and the concave portions, and the surface diffusion from the convex portions to the four portions. Therefore, thermal deformation is noticeable in the corners of the convex portions of the auxiliary layer, and by making the auxiliary layer thicker and taking a longer heat treatment time, it is possible to efficiently fill the four parts of the upper waveguide layer. The convex portion of the lower waveguide layer does not undergo thermal deformation due to the layer structure protected by the auxiliary layer, and the periodic unevenness does not deteriorate as in the first method, and a sufficient amount of optical feedback is obtained. is ensured, and the threshold current can be sufficiently reduced. In the above configuration, the auxiliary layer etching process uses an etchant that selectively etches only the auxiliary layer without etching the waveguide layer. Wet etching using an etchant that selectively etches only the waveguide layer without etching the auxiliary layer is suitable for the etching process of the waveguide layer. law is appropriate.

(6) Embodiments of the Invention A method for manufacturing a semiconductor laser according to an embodiment of the present invention will be described below with reference to the drawings, and the structure and unique effects of the present invention will be clarified.

- As an example, the active layer is indium gallium arsenide phosphide (InGaAsP) whose fundamental absorption edge wavelength is 1.3 [μm], and the active layer is indium gallium arsenide phosphide (InGaAsP) whose fundamental absorption edge wavelength is 1.15 [μm]. We will describe the manufacturing process of a distributed feedback semiconductor laser with an oxide film stripe structure, which uses indium phosphide (nP) as the upper and lower waveguide layers and the confinement layer.

See Figures 2 and 3 Substrate/lower confinement layer ll made of n-type indium phosphide (n-InP) containing n-type impurities to the order of 18 [true 1]
On top, using the LPB method, (a) n-type impurity is added to 101
7 [cm-3], and the fundamental absorption edge wavelength is 1.15 (
n-type indium gallium arsenide phosphide (n
- lower waveguide layer made of GaAsP) with a thickness of about 0.2 [μm], (b) fundamental absorption edge wavelength of 1.3
1) Made of undoped indium gallium arsenide phosphide (GaAsP) with a thickness of about 0.1 [μm].
m] active layer 13 of about t, (c) p-type impurity 101
7 [clrL-3], basic absorption edge wavelength is 1.1
About 5 [μm]! A certain p-type indium gallium arsenide phosphide (p-InGaAsP) has a thickness of 0.1 μm.
The layer 14', which serves as the upper waveguide layer, is made of p-type indium phosphide (p-nP) containing about 10" [cIIL-3] of p-type impurities, with a thickness of 0.1 [μ
A laminated body consisting of the auxiliary layer 20' having a thickness of about 100 m] is formed in one step.

Subsequently, a resist (e.g. AZ) is applied to a thickness of O, OS (μm
After forming a resist by coating the film to an extent of about 19.5 mm, a two-beam interference exposure method using a helium (He)-cadmium (Od) laser beam is used to form a resist. Interference fringes having a pinch of (μm) are formed, the pattern of the interference fringes is exposed, and the resist is developed to form a resist mask 21 having a plurality of striped openings 21'.

Next, using this resist mask 21, hydrobromic acid (
The auxiliary layer 4' is etched using a wet etching method using HBr) as an etching solution to form striped openings 20' as shown in FIG. An etching mask is formed on N14' to form periodic irregularities.

Refer to Figure 4. Using this etching mask 20, sulfuric acid (H2BO3
), hydrogen peroxide (H2O2), and water (azo) at a ratio of 3:1
: The layer 14 that will become the upper waveguide layer is formed using a wet etching method using a mixed solution having a ratio of 1:1.
' is etched to a depth of about a hand's length, that is, about O,OS (μm) to form a new opening.

Through this step, the upper waveguide layer 14 having periodic irregularities in the upper layer portion is formed. The (1111) plane of the periodic openings formed in this manner is a plane perpendicular to the light emission direction.

See FIGS. 5 and 6. Again, without removing the etching mask addition described above,
p-type impurity 101018 (C3) using LPI method
An upper confinement layer 15 made of p-type indium phosphide (pInP) with a thickness of about 1.5 [μm] is formed.

The growth temperature of this p-nP confinement layer 5 is 550
[°C]. Needless to say, etching mask 20
Since the material of the confinement layer 15 is the same, it is not disadvantageous that the etching mask 2o remains, but rather,
When forming the upper confinement layer 15, this etching mask functions as a protective film against meltback, so it effectively prevents the periodic irregularities from being melted back, especially in the convex portions of the upper waveguide layer 14. Since the side surfaces of the periodic irregularities are preserved as planes perpendicular to the direction of light emission, a sufficient amount of light feedback is ensured, while the threshold magnetic current is also reduced.

Subsequently, using the same LPB method, p-type impurities were added to 10
P-type indium gallium arsenide phosphide (p-type nGaA8P) containing about 19 (tx-3) has a thickness of 0.5 [μ
After forming the electrode contact layer 16 with a thickness of about .m], a silicon dioxide (S1O2) layer 22 is formed using the snow ξ ivy growth method.

FIG. 6 is a sectional view 1 taken along line a-a of the laminate shown in FIG. On the silicon dioxide (Sin2) layer 22 described above, a stripe-shaped opening with a width of about 5 [μm] reaching the electrode contact layer 16 is formed using a known method, and then an electron beam evaporation method is used thereon. and the thickness o and os (μm)/
Old [μm] / 1 (μm) titanium (T1 / platinum (
A positive electrode 17 made of a triple layer of pt)/gold (Au) is formed. Here, the substrate 11 is polished from the back side to a thickness of 100 mm.
about [μm], and the substrate 11 is formed using a resistance heating evaporation method.
Gold/germanium (A
u′Ge)/Ninkel(
A negative electrode 18 consisting of a double layer of N1) is formed. Finally, in order to suppress Fabry-Perot mode oscillation, the light emitting surface is left open and the other surface is made rough to complete the manufacturing process of this example.

Next, a method for manufacturing a semiconductor laser according to a second embodiment using the second effect (thermal deformation) of the present invention will be described. In this method, too, the etching mask 20 of the auxiliary layer is used, and the wet etching method is used. Until the step of etching the layer 14', which will become the upper waveguide layer, to a depth of about a hand's length to 0.05 C μm) to form periodic irregularities on the lower waveguide layer, the thickness of the auxiliary layer 20 is The structure is exactly the same as the first embodiment except that the thickness is increased to about 0.3 μm.

Refer to FIGS. 7 and 8. Next, without removing the etching mask, 650 (
Wait at a temperature of about 30°F (°C). In this heat treatment process, the rate at which P vaporizes into the heat treatment atmosphere in the convex portion ↑ is greater than in the concave portion, and due to the difference in equilibrium vapor pressure, In diffuses from the convex portion to the concave portion, and regrowth in the concave sf occurs. occurs. Finally, the auxiliary layer 20 becomes concave.

The area is completely filled and the waveguide layer 14 is protected.

In this process, the auxiliary layer interface in contact with the convex portion of the upper waveguide layer 14 is not affected at all, and the side surfaces of the periodic irregularities of the waveguide layer are also protected from thermal deformation. Next, as in the first example, the p-type impurity was added to 1011019 (1011019) using the LPB method.
3) An upper confinement layer 15 made of p-type indium phosphide (1p-InP) having a thickness of about 1.5 [μm] is formed. This upper confinement layer 15 is a thermally deformed auxiliary tvr.
Waveguide layer 1 made of the same material as i- and protected by an auxiliary layer
To fill in the unevenness of the auxiliary layer caused by thermal deformation and form a flat growth layer without adversely affecting the unevenness of 4. Then, using the same L'PK method, p-type impurities were added to 10110
p-type indium gallium arsenide phosphide (containing about 19(3)
After forming an electrode contact with a thickness of about 0.5 [μm] made of p-type nGaAaP)) Wt 16, silicon dioxide (S10
Form J@22. The following steps are similar to those in the first embodiment to complete the manufacturing process of this embodiment. In this embodiment, the upper waveguide layer 14 and upper confinement layer 1
Although the method of forming the unevenness at the interface with the lower confinement layer 11 and the lower waveguide 1-12 has been described, a similar method may be used to form the unevenness at the interface between the lower confinement layer 11 and the lower waveguide 1-12. In this case, the auxiliary layer is made of the same material as the lower waveguide layer 12, and the surface of the confinement layer 11 is etched away using the auxiliary layer having periodic irregularities as a mask.

Through the above process, a resonator is formed that utilizes Zorag reflection due to the periodic asperities formed at the interface between the waveguide layer and the confinement layer, and the side surfaces of the periodic asperities are exactly perpendicular to the light emission direction. Since the surface is flat, it is possible to realize a distributed feedback laser with a large amount of optical feedback and a small threshold current.

(7) As described in detail, according to the present invention, the reflection function as a resonator is directed in a direction parallel to the light emission direction with a certain regularity, without having a pair of valve opening surfaces that function as a resonator. The reflection function depends on the periodic change in the refractive index realized by the periodic irregularities provided at the interface between the waveguide layer and the confinement layer. In the method of manufacturing a distributed feedback laser using the LPFi method, the above-mentioned periodic asperities, especially the shape of the periodic asperities that prevent melt knocks at the convex portions and have a satisfactory reflection function. It is possible to provide a method for manufacturing a distributed feedback semi-integral laser, which enables the realization of dimensions and dimensions, an increase in the amount of optical feedback, and a low threshold current.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a substrate showing the basic structure of a distributed feedback semiconductor laser, and FIGS. 2 to 6 are distributed feedback of an oxide film stripe structure according to the present invention. Cross-sectional views of the substrate after completion of each main process in the method for manufacturing a type semiconductor laser 7 and 8 are cross-sectional views seen from a direction perpendicular to the light emission direction. 1.11...Substrate and lower confinement layer (n-type nP),
2.12--Lower waveguide layer (n-tech nGaAsP), 3
．． 13-=active no(InGaAaP), 4.14 ・
・Lower waveguide layer (p-nGaAeP), 14' ・・
- Upper waveguide no (p-InGaAsP), 5
．． 15-...Top confinement layer (p-nP), 6.16
... p-electrode contact layer (p-electrode nGaAaP), 7.
17-...Positive electrode (T i/p t/A u3!i7m
), 8.18... Negative electrode (Au'Ge/Ni double layer), Processing... Etching mask (p process nP), 20'
・・・Auxiliary layer (p-etching nP) serving as an etching mask
), 20'...Striped openings provided in the etching mask, 20'''...Striped openings provided in the layer that will become the upper waveguide layer, processed''--thermally deformed auxiliary layer (p Engineering nP)
, 21...resist mask, 21'W' on the river resist
I cut striped openings, 22...silicon dioxide (Si
O2) layer. Figure 8 22

Claims (1)

[Claims] It has an active layer, a waveguide layer sandwiching the active layer, and a confinement layer sandwiching the waveguide layer, the interface between the waveguide layer and the confinement layer being periodically In the method for manufacturing a distributed feedback semiconductor laser having irregularities, an auxiliary layer made of the same semiconductor as the confinement layer (liquid guide layer) is formed on the surface of the waveguide layer (the confinement layer) with periodic openings/e.
The patterned auxiliary layer is then used as an etching mask to form the waveguide layer (
The confinement layer (the confinement layer) is selectively etched to form periodic irregularities on the surface of the liquid guide layer (the confinement layer), and the confinement layer (the confinement layer) is etched on the waveguide layer (the confinement layer). Method 0 of manufacturing a semiconductor laser, characterized by including a step of forming a layer)