JPS5940317B2 - Rib guide stripe type semiconductor multilayer thin film optical waveguide and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a striped semiconductor multilayer thin film optical waveguide structure and a method for manufacturing the same.

Generally, when manufacturing a thin film optical waveguide using the multilayer epitaxial growth method, the structure is uniform in the plane direction of the thin film, and in order to have an optical waveguide structure with a constant width in a constant direction within the plane, it is necessary to use a striped mesa type. Methods such as etching and ion implantation have been used. However, the mesa structure has drawbacks such as the side surfaces of the optical waveguide being in direct contact with the air and creating a large difference in refractive index, which tends to result in a multi-mode waveguide, and the exposed side surfaces causing deterioration. Further, even when using ion implantation, there are various damages associated with the implantation, and good results have not been obtained. This problem also applies to a heterostructure semiconductor laser having a multilayer thin film structure. In order to control the transverse mode of a heterostructure semiconductor laser and improve its characteristics, it is necessary to provide a striped waveguide structure with a constant direction and appropriate width within the heteroplane. The object of the present invention is to provide such a striped optical waveguide (in the present invention, the term "optical waveguide" includes not only a simple optical waveguide but also the active layer of a semiconductor laser).

) is automatically formed into a multilayer thin film grown on a substrate in one epitaxial growth by subjecting the substrate crystal to a certain processing, and a structure suitable for the crystal growth method is provided. Striped optical waveguide according to this invention &
Unlike ζ mesa type optical waveguides, etc., it is easy to manufacture because it can be manufactured by one-time multilayer epitaxial growth, there is no risk of deterioration, and the effective refractive index difference between the inside and outside of the stripe can be adjusted by selecting the manufacturing parameters. Since it can be set to an arbitrary value, light propagation in a low-order mode becomes possible, and at the same time, it has many advantages, such as low scattering loss at stripe edges.

Now, let's start by considering the conventional example below. It can be said that the semiconductor laser is faced with the most severe requirements as an optical waveguide, so the following explanation will use the semiconductor laser as an example (this does not limit the present invention, but the transverse mode of the semiconductor laser) Control was first realized when an attempt was made to confine the current injected into the active layer to a specific striped region, that is, when an electrode stripe type was proposed.

Since then, various striped structures have been developed and are still available today, but all of them have their own drawbacks and are unsatisfactory in terms of characteristics. For example, when only the current distribution is specified, such as with an electrode stripe type, the laser light is mainly guided by the force in the stripe direction due to the gain distribution (the waveguide effect due to this gain is also unstable, and easily leads to higher-order transverse modes). This causes oscillation and multi-mode oscillation, which often distorts the current-optical output characteristics.

Therefore, attempts have been made to correct these drawbacks by structurally creating a waveguide effect inside the laser element. The so-called embedded stripe laser can also be understood as one such attempt. In this type, a striped active region is surrounded by a semiconductor with a high bandgap and low refractive index, giving it a strong optical waveguide effect. Although the results were quite good, the refractive index difference was unnecessarily large;
Although fundamental mode oscillation occurs when the stripe width is less than 1 μm, when the stripe width becomes larger than that, it becomes difficult to avoid mixing of higher-order modes. Furthermore, if the stripe width is narrow, it is unavoidable that the optical output is limited to several MW or less, but the output is still too small. Furthermore, in order to create this structure, epitaxial growth must be performed twice with mesa etching in between, which is a major drawback in terms of manufacturing technology. A rib guide stripe type semiconductor laser has been proposed for the purpose of improving the various drawbacks and problems of low output of the buried type.

That is, in this structure, as described below, by making the striped region of the active layer thicker than the outside, it has a weaker waveguide effect than the buried type, and fundamental mode oscillation occurs even with a wider stripe width. Therefore, an attempt is made to obtain a larger optical output. This discussion can be applied in exactly the same way to the case of a simple optical waveguide. This rib guide stripe type semiconductor laser should be mentioned as the prior art prior to the present invention, and below, we will first discuss the manufacturing method and structure of this type, and explain what needs to be solved by the present invention using drawings. Explain briefly.

FIG. 1 is a sketch showing the outline thereof. For example, {1
00}-plane n-type GaAs substrate 2 using a liquid phase epitaxial method. 7A'0.3As layer 3
, p-type GaAs active layer 4 is grown in sequence.

At this point, the growth is temporarily stopped, and a photoresist film extending in a stripe shape in the [011] axis direction is provided on the surface of the p-type GaAs active layer 4, and this is used as a mask for selective etching to cover the active layer portions where no photoresist film is attached. The active layer is etched so that it is slightly thinner than the thickness of the grown active layer. In this way, striped mesa-shaped convex portions are formed in the active layer itself. Next, the p-type GaAs active layer 4
On top of the p-type GaO. 7A'0. , As layer 5, and p-type GaAs layer 6 are grown. Next, a SiO film 7 is applied, a SiO2 window is provided in the area corresponding to the striped current region 9 so that the current flows uniformly through the mesa-shaped convex part of the active layer 4, and the electrodes 1 and 8 are attached. A rib guide stripe type semiconductor laser (FIG. 1) is manufactured by forming the surfaces 10 and 11 with open arms. The SiO2 film 7 and electrode 8 provided with windows in this way are
A stripe-shaped current region 9 extending in a direction perpendicular to the laser reflecting mirror surfaces 10 and 11 is formed, and an n-type GaO.
7A'0.3 Injected current is supplied in the form of a stripe to the zero junction formed by the AS layer 3 and the p-type GaAs active layer. When this semiconductor laser is forward biased, that is, a negative voltage is applied to the electrode 1 and a positive voltage is applied to the electrode 2VC, a current flows through the striped current region 9 and the active layer 4 therebelow causes laser oscillation. The active layer thickness of this oscillation part is thick at the center and slightly thinner at the outer part.
The crystal layers sandwiching this active layer from above and below are made of GaO. This is the 7A'03AS layer. Since the thickness of the active layer differs in the width direction, light in the active layer is transmitted to the third layer of GaO. It becomes more susceptible to the influence of the 7A'0.3AS layer. This means that the refractive index of the thin part of the active layer is
This is equivalent to the fact that the refractive index is effectively smaller than the refractive index of the thicker part. That is, even if this is just an optical waveguide, a striped waveguide structure having a certain direction and an appropriate width is provided within the plane of the active layer. For example, if we consider only the rib guide stripe type semiconductor laser, it has a transverse mode control function and does not depend on the gain distribution like the ζ electrode stripe type, and as a result, it has stable fundamental mode oscillation over a wide range of injection current levels and is large. Light output can be easily obtained, and the drawbacks of the conventional electrode stripe type are greatly improved. In this respect, this rib guide stripe type can be said to be an innovative proposal. However, this rib guide stripe type has problems in that it is very complicated to manufacture and requires new technology.

In particular, recent advanced experimental and theoretical verification has revealed that the fact that the crystal growth process has to be carried out in two steps significantly impairs reliability. That is, during the second growth, the active layer grown in the first time is
Before starting the second growth layer after the third layer, the surface layer is exposed to a hydrogen atmosphere at a high temperature for a long time, so the surface layer becomes extremely disturbed due to evaporation of As due to thermal decomposition. state. Since the third and subsequent growth layers are grown on this disordered crystal surface, the growth layers inevitably grow while introducing a large number of lattice defects.

What is worse is that it has been confirmed that lattice defects also occur in the already grown active layer at this time. Therefore, in recent years, a means for removing the disordered crystal surface portion, that is, a melt-back process, has generally been carried out immediately before such a growth process. However, in manufacturing the rib guide stripe type that we are currently discussing. Unfortunately, this melt-back process cannot be carried out because it is necessary to seed the crystal layers while faithfully maintaining the shape of the convex portions formed in the active layer without disturbing it. In other words, with this conventional rib guide stripe type manufacturing method, it is extremely difficult to prevent lattice defects from occurring.
This excellent structural merit could not be utilized. The purpose of the present invention is to eliminate the drawbacks of the conventional rib guide stripe type semiconductor laser and its manufacturing method, and to create a structure of a semiconductor laser or optical waveguide that is highly reliable and easily realized, and that is most suitable for the structure. The purpose of this invention is to provide a method for growing crystals.

This invention relates to a semiconductor having a structure in which the thickness of the active layer having a flat surface is distributed in the width direction of the oscillation region, and the layer thickness of the oscillation region is larger than the thickness of the outer part of the oscillation region. In order to obtain a laser or an optical waveguide, the crystal is grown after a certain processing is performed on the substrate crystal, and the difference in growth rate depending on the crystal orientation is skillfully combined, sometimes including the meltback phenomenon.

As a result, if the present invention is understood from the characteristics of the structure of the optical waveguide or semiconductor laser as a product, it is as follows.

That is, when viewed from the optical waveguide or the active layer of the semiconductor laser, it exhibits the characteristic that concave grooves extending in a stripe form remain on the substrate crystal surface, which is at least a crystal interface that is further away than the adjacent layers sandwiching the active layer. It has a rib guide stripe type structure. Some embodiments of the present invention will be described below with reference to the drawings.

FIGS. 2 and 3 are for explaining the concept of the present invention, and are diagrams showing the concepts of a substrate crystal and a multilayer growth layer, respectively.

FIG. 4 explains an embodiment closest to the concept explained using FIGS. 2 and 3, and is a diagram schematically showing a rib guide stripe type semiconductor laser made in this way. For example, as shown in FIG. 2, the surface 14 is (100)
A striped recess 1 with a width of 10 μm and a depth of about 2 μm parallel to the 010 direction is formed on an n-type GaAs substrate 2 that is
2 is formed by selective etching.

Using a liquid phase epitaxial method on this substrate crystal 2,
As shown in Figures 3 and 4, the thickness is 3μm (7) n-type G
aO. 7A'0.3As layer 3, thickness 1μm (7) p-type G
aAs layer 4, 1 μm thick p-type GaO. 7A'0.3A
S layer 5 and finally p-type GaAs layer 6 with a thickness of 0.8 μm (7)
are formed sequentially. Next, the p-type GaAs layer 6 is covered with a SiO2 film 7.
A stripe-like window with a width of 10 μm is provided by selective etching to form a stripe-like current region 9. At this time, this striped window is formed by the striped recess 12.
It extends directly above and parallel to its longitudinal direction. A rib guide stripe type semiconductor laser unique to the crystal growth method of the present invention is realized by vapor-depositing electrodes 10 and 8 on the entire surface, and forming reflective surfaces 10 and 11 perpendicular to the longitudinal direction of the striped window by cracking. The form is completed.

Even in the case of a simple optical waveguide, electrodes and the like are not required, and the shapes of the active layer and substrate are exactly the same. The principle of the present invention will be explained below by taking the above embodiment as an example.

The first stripe-shaped recesses 12 to be formed in the GaAs substrate 2 shown in FIG. It is preferable to perform selective etching. Then, {111}A planes appear on both side surfaces 13 of the striped recess 12, forming a mesa-shaped recess. In this way, by using an etching solution that selectively exhibits a strong etching effect depending on the crystal orientation, striped recesses can be easily formed with fine and highly accurate surface orientation. Simply obtained. Furthermore, when crystal growth is performed using a substrate crystal processed into a shape in which each constituent plane has a specific plane orientation, the growth rate becomes strongly dependent on the crystal orientation. It can be said that the present invention skillfully utilizes this phenomenon. Generally, the orientation dependence of the growth rate in the liquid phase epi-mexial method is {111}A-plane〉{1
00} plane>{111}B plane. Therefore, the second
When grown on a substrate like the one shown in the figure, Figures 3 and 4
As shown in FIG.
Influenced by the fact that it is the A-plane, it becomes effectively thicker (faster) than the growth thickness b (which may be described as the growth rate) of the {100} substrate surface 14, and as the growth progresses and becomes thicker, the width of the recess increases. The difference in height gradually narrows as the area gradually narrows.

When a certain growth thickness is reached, the recess is completely filled.
Its surface also becomes flat. Such a phenomenon is a characteristic property often observed in liquid phase epitaxial growth, and the reason for this is explained by the above-mentioned theory. The present invention has solved the problem by proactively and skillfully applying this phenomenon.
That is, the growth of the first layer is stopped at a time when traces of the recesses initially formed in the substrate remain on the surface of the substrate, and then the active layer is grown so that the surface becomes flat.
In this way, the resulting active layer is slightly thicker in the region directly above the substrate recess and thinner in its outer regions. The structure of this active layer itself is basically the same as that of a conventional rib-guide stripe type semiconductor laser, but the features of the present invention appear in the peripheral structure that accompanies the shape of the active layer. Of course, the same applies to a simple optical waveguide. According to the present invention, there is no need to divide the liquid phase epitaxial growth process into two as in the conventional manufacturing method, and a rib guide stripe type structure can be obtained with one continuous growth, further simplifying the manufacturing process. Naturally, during crystal growth and processes, there is less chance of inducing defects that cause deterioration.

The result is longer service life, improved reliability and reproducibility. The present invention has been described in detail above using the striped portion of a rib-guided semiconductor laser and its manufacturing method as an example.The crystal growth method according to the present invention is useful for manufacturing a striped optical waveguide in a general semiconductor crystal multilayer thin film.

According to this method, the ratio of the thickness of the optical waveguide layer inside and outside the stripe can be adjusted arbitrarily, and thereby the difference in effective refractive index inside and outside the stripe can be selected to a value that allows only the desired low-order transverse mode to propagate. I can do it. The present invention is particularly suitable when using a compound semiconductor such as a (GaA')As multilayer thin film, and can achieve this purpose by skillfully utilizing selective etching depending on the crystal direction and differences in crystal growth rate. In particular, according to the present invention, a major advantage is that if the substrate is processed in advance, the optical waveguide structure can be incorporated into the multilayer thin film while the multilayer epitaxial growth itself is almost the same as the usual method. It is. That is 6
Expected to be easy to manufacture and highly reliable. Furthermore, the present invention can be applied not only to the liquid phase epitaxial method but also to the gas phase epitaxial method, the molecular beam epitaxial method, etc., and the same effects can be obtained.

[Brief explanation of the drawing]

Fig. 1 is a schematic perspective view of a conventional rib guide stripe type semiconductor laser, Fig. 2 is a schematic perspective view showing the substrate crystal size u used for crystal growth of the present invention, and Fig. 3 is a schematic perspective view of a conventional rib guide stripe type semiconductor laser. A schematic perspective view showing an example of a grown layer obtained when
FIG. 4 shows a schematic perspective view of a rib guide stripe type semiconductor laser according to an embodiment of the present invention. In the drawings, 1, 8... Denka 2... n-type GaAs
Substrate, 3... n-type GaA'As layer, 4°・GaAs
Active layer, 5... p-type GaA'As layer, 6... p-type G
aAs layer, 7... SlO2 film, 9... Striped current area, 10, 11... Laser reflecting mirror surface, 12...
Striped convex portions in the [011] direction, 13...(11
1) A plane, 14... (100) plane of substrate crystal, a...
・Growth layer thickness of striped recesses in [01'T) direction, b
...The growth layer thickness of the (100) plane is shown.

Claims (1)

[Claims] 1. A rib-guided stripe type semiconductor multilayer in which the thickness of the striped region constituting the waveguide is thicker than the thickness of the outer part of the striped region in an active layer having a flat surface. A thin film optical waveguide, characterized in that a concave portion extending in a stripe shape is provided at a substrate crystal interface, which is a crystal interface farther from an active layer than at least a crystal layer sandwiching the active layer. Rib guide stripe type semiconductor multilayer thin film optical waveguide. 2 The base material crystal has a zincblende crystal structure, and the {100} plane is selected as the principal plane curve that appears as the substrate crystal surface, and <
011> The rib guide stripe type semiconductor multilayer according to claim 1, characterized in that {111}A plane or {111}B plane appears as both side surfaces of the stripe-shaped recess extending in the axial direction. Thin film optical waveguide. 3. A method for manufacturing a rib-guide stripe type semiconductor multilayer thin film optical waveguide, in which the thickness of the stripe region constituting the optical waveguide in the active layer having a flat surface is made thicker than the thickness of the outer part of the stripe region. A substrate crystal whose surface is preliminarily provided with concave portions extending in a stripe shape is prepared, and at least a first crystal layer sandwiching an active layer, an active layer, and an active layer are formed on the substrate crystal. Manufacture of a rib guide stripe type semiconductor multilayer thin film optical waveguide, characterized in that the configuration of the active layer is obtained by sequentially stacking intervening second crystal layers, and at this time, the dependence of crystal growth rate on crystal orientation is obtained. Method. 4 Adjust the main plane of a semiconductor substrate having a zincblende crystal structure to be a {100} plane, and
11> Extends in the axial direction, the bottom is {100} plane, and both sides are {100} plane.
When stripe-shaped recesses are formed by selective etching so as to be on the {111}A plane or the {111}B plane, and multilayer epitaxial stacking is further repeated, the crystal growth rate on the {111}A plane becomes higher than that on the {100} plane. 4. A method for manufacturing a rib guide stripe type semiconductor multilayer thin film optical waveguide according to claim 3, which utilizes the fact that the rate is higher than the crystal growth rate.