JPS6234473Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a semiconductor junction laser, and particularly to a structure for fundamental mode oscillation.

In order to operate a semiconductor laser continuously at high temperatures, optical energy and injection must be directed to specific areas that provide the best thermal path to remove heat from the junction, while minimizing light loss and wasteful recombination. Structural dimensions must be designed to confine the current.

Therefore, the electrodes of the semiconductor laser are made into striped electrodes to confine the current flowing through the active layer and at the same time confine the optical energy. A so-called electrode stripe type semiconductor laser has appeared.

However, even if the stripe electrode width is narrowed beyond a certain level, the current in the active layer spreads much more than the electrode width, making the current confinement effect insufficient.
As a result, there was a drawback that sufficient mode control could not be performed. Furthermore, a semiconductor laser was proposed that compensated for the drawbacks of this electrode stripe type. This is the so-called embedded stripe type.

FIG. 1 is a schematic diagram of an example of a buried stripe type semiconductor laser. The structure of this type will be briefly explained below.

For example, an n-type Ga _{0.7 Al 0.3 As} layer ₃ , a GaAs active layer 4, and a p-type Ga _{0.7 Al 0.3 As} layer ₅ are formed on an n-type GaAs substrate 2 by liquid phase growth. grow sequentially. In order to form striped current regions 11 from _the surface of the p-type Ga _0.7 Al _0.3 As layer ₅ by selective etching,
Until the substrate 2 is reached, only the part immediately below the striped current area 11 is left and the other parts are removed. Next, an n-type _{Ga 0.7 Al 0.3} As layer 8 is grown again on the etched portion by _the liquid phase growth method. Subsequently, a SiO ₂ film 6 is applied, a window is provided in a portion of the SiO ₂ film 6 corresponding to the striped current area 11, electrodes 1 and 7 are attached, and reflective surfaces 9 and 10 are formed by cleavage. A buried stripe diameter semiconductor laser is fabricated.

SiO ₂ film 6 and electrode 7 provided with windows in this way
is a striped current region 11 extending in a direction perpendicular to the laser reflecting surfaces 9 and 10, and a striped current region 11 extending in a direction perpendicular to the laser reflecting surfaces 9 and 10, as well as providing an injected current in a striped form to the double heterojunction consisting of layers 3 and 5. It constitutes a current area forming section.

When a forward bias is applied to this semiconductor laser, that is, a negative voltage is applied to the electrode 1 and a positive voltage is applied to the electrode 7, a current flows through the p-type injection region 11, and only the active layer 4 below this oscillates.

Furthermore, since the active layer is surrounded by a GaAlAs layer, the active layer serves as a complete optical waveguide. Therefore, a sufficient light confinement effect was obtained, and the drawbacks of the electrode stripe type were greatly improved.

However, the manufacturing process for this element is extremely complicated.
It has drawbacks that require new technology. In particular, crystal growth occurring twice tends to significantly deteriorate the reliability of this semiconductor laser.

In other words, once the surface of a GaAlAs crystal is exposed to air, a strong oxide film is formed, and it is generally impossible to deposit another crystal on top of it. Therefore, during the second growth, because this condition is met, that is, there are exposed parts of the GaAlAs layer on both walls of the mesa type, and it is not clear how this part actually grows. It is naturally understood that the conditions are such that lattice defects are likely to occur. Such drawbacks must be eliminated in order to improve reliability.

Furthermore, in a semiconductor laser having this structure, the refractive index step for confining light becomes larger than necessary, reducing the spot size of the fundamental transverse mode. Therefore, even if the optical power is small, the optical density at the end face of the active layer is high, and the end face can easily be destroyed at a few milliwatts.

The purpose of this invention is to eliminate the drawbacks of the conventional buried stripe type semiconductor laser and to provide a highly reliable semiconductor junction laser capable of fundamental mode oscillation. Instead of burying both sides of the active layer with AlGaAs layers, liquid phase epitaxial method is used to form the active layer under the striped current injection region on the stepped side surface parallel to the optical axis direction of the laser beam (in this specification). The stepped side surface refers to the portion indicated by 27 in FIG. 3A.

Examples of this invention will be described below with reference to the drawings.

FIG. 2 is a schematic diagram of a semiconductor junction laser in which the present invention is implemented. For example, an inclined surface parallel to the [011] direction is formed on the n-type GaAs substrate 13, which is a (100) plane, by selective etching. continue,
On this substrate crystal 13, an n-GaAlAs layer 14 and a GaAs-active layer 1 are formed using a liquid phase epitaxial method.
5. The p-GaAlAs layer 16 and the p-GaAs layer 17 are successively grown. Next, a SiO ₂ film 18 is applied to the p-type GaAs layer 17, and striped windows are formed by selective etching to form striped current regions 20.

At this time, the striped windows are provided so as to extend in parallel directly above the active layer 24 which is bent in an inclined manner. By depositing electrodes 19 and 12 on the entire surface and forming end faces 21 and 22 perpendicular to the longitudinal direction of striped current region 20 by cleaving, a semiconductor junction laser embodying the present invention is completed.

Positive voltage of electrode 19 in this example. When a negative voltage is applied to the electrode 12, current is injected from the stripe-shaped current region 20, and oscillation light is generated in the active layer 24 directly below it. According to this embodiment, the active layer 24 directly below the striped current region 20 has a curved and inclined cross section. This inclined active layer 24
is surrounded by a layer with a smaller refractive index than the active layer 15.
It has the same effect as being surrounded by GaAlAs layers 14 and 16.

That is, when there is a boundary surface with a change in refractive index, light is reflected and confined to the side with a larger refractive index. Therefore, the structure of the present invention is equivalent to providing an optical waveguide based on the properties of light in the oscillation region 24 of the active layer 15.

Therefore, complete confinement of light is obtained, point-like light emission is possible, and mode control is facilitated.

Next, the specific manufacturing method of this example will be explained based on the drawings.

(100) plane 2 of GaAs substrate 13 shown in FIG. 3A
5, first, the n-type GaAs substrate 13 is made to a thickness of 400 μm.
Polished to a depth of m and chemically etched to remove the destructive layer on the surface and create a mirror surface. Apply an ordinary photoresist film to the mirror-finished (100) surface 25, and
1] A portion parallel to the direction to which the photoresist film is not attached is formed with a width of 100 .mu.m and a pitch of 500 .mu.m to form the substrate structure shown in FIG. 3A. It is sufficient that the width of the groove 26 is smaller than its pitch. Further, the depth of the groove 26 may be 1 to 5 μm or more.

Grooves 26 are selectively formed by conventional chemical etching. That is, in the case of GaAs, NH ₄ OH ₁ :
Preferably, the reaction is carried out with a mixture of 10 ml of H ₂ O ₂ and maintained at room temperature with stirring.

The etched groove 26 constitutes an inclined side wall 27 (stepped side surface), and the inclined wall becomes a {111}A plane. This {111} A plane has an angle of exactly 54°44' with the substrate surface (100). Following the formation of trench 26, the remaining photoresist film is removed and layers 14, 15, 16, and 17 are successively grown by conventional liquid phase epitaxial growth to form the layers of FIG. 3B. form a structure.

Layer 14 must be grown in a manner that maintains the configuration formed on substrate 13.

The configuration of the bent portion 24 of the active layer 15 is governed by the growth shape of the layer 14.

In fact, the basis for growing the layers 14, 15, and 24 is the depth of the groove 26, the width of the groove 26, the crystallographic direction of the substrate crystal 13, the growth time, the growth temperature, the solubility saturation,
It depends on factors such as.

In particular, when growing in a shape where the substrate crystal has constituent planes having a specific plane direction, the growth layer is strongly influenced by the direction dependence of the growth rate.

The relationship between growth rate and crystal orientation is {111} A side > {100} side > {111} B side.

Therefore, when grown on the n-type substrate 13 as shown in FIG. 3A, since the sloped sidewall 27 of the groove 26 is the {111}A plane, the growth layer thickness on the {100} flat surface of the groove 26 is: It is effectively thicker than the thickness of the substrate surface 25, which is the same {100} plane, and as the growth progresses and the layer becomes thicker, the groove width becomes narrower and the groove depth gradually decreases. Slanted side wall 2 of groove 26
The slope of 7 gradually changes gradually. When the thickness of the grown layer exceeds a certain level, the groove portion becomes completely flat.

If we actively apply this phenomenon to decay,
The active layer 15 has a bent portion 24, allowing the layers 16 and 17 to be flat growth layers.

At this time, the thickness of the grown layer in the bent portion 24 of the active layer 15 is generally thicker than the layer thickness on the {100} flat surface.

When the width of the groove 26 is 100 μm and the depth is 1 μm, the optimal thickness of each layer on the {100} plane of the substrate surface 25 is as follows.

The thickness of layer 14 is 0.2 μm, and the thickness of layer 15 is 0.1 μm.
m, the thickness of layer 16 is 2.0 μm, and the thickness of layer 17 is 2.0 μm.
Let it be μm. At that time, the thickness of the bent portion 24 of the layer 15 is 0.2 μm. The concentration of aluminum in layers 14 and 16 is typically 0.3, although ranges from 0.15 to 0.7 are acceptable. Layer 14, 15, 1
The doping levels of 6 and 17 are approximately 5×10 ¹⁷ respectively.
electrons/cm ² , approximately 2×10 ¹⁷ vacancies/cm ² , approximately 7×10 ¹⁷ vacancies/cm ² , and approximately 1×10 ¹⁸ vacancies/cm ² . A crystal having the layer structure shown in FIG. 3B is cut from the center of its groove 26 to form a crystal shown in FIG. 3C, and electrodes are attached to this to form a semiconductor junction laser.

The electrode 19 forms a striped current region 20 through the SiO ₂ film 18 so that a uniform injection current is applied only to the inclined bent portion 24 of the active layer 15 .

For efficient operation of this embodiment, it is desirable that the surface of layer 17 be flat. This is because heat dissipation from the active layer, electrode attachment process, etc. are advantageous.

Furthermore, forming electrodes on the flat surface of the crystal layer tends to reduce the occurrence of lattice defects that impair reliability during electrode formation, which greatly contributes to longer life.

Furthermore, by controlling the layer thickness of the first layer 14, the inclination angle of the active layer can be formed optimally.

As explained above, the laser of the present invention oscillates in the fundamental mode, and has a low threshold and high reliability. This operation is possible because the central portion 24 of the active layer 15 is bent and almost completely surrounded by the layers 14 and 16 of low index material.

Furthermore, since the thickness of the active layer in the bent portion 24 is greater than the layer thickness in the flat portion thereof, the refractive index of the bent portion region is greater than the refractive index of the flat regions on both sides thereof. That is, the bent portion of the active layer acts as an optical waveguide mechanism.

Furthermore, although the present invention appears to be similar at first glance to a conventional semiconductor laser (Japanese Unexamined Patent Publication No. 51-114887) in which the active layer region is bent vertically, the feature of the present invention is that the active layer region is bent in an inclined manner. be. That is, this difference in structure brings about the following advantages. In the semiconductor laser of the present invention, compared to a conventional semiconductor laser in which the active layer is vertically bent, the refractive index difference that occurs in the active layer region is smaller if the dimensions of the bent step portion are the same. Therefore, the height of the stepped portion required for fundamental transverse mode oscillation can be increased with the present invention. This leads to the provision of a semiconductor laser that can maintain fundamental transverse mode oscillation over a wider range of operating currents.

For example, in the case of a conventional structure (Japanese Patent Application Laid-Open No. 51-114887), when the active layer of the semiconductor laser is 0.2 μm thick, the refractive index difference is about 10%. This difference in refractive index cuts off the first-order transverse mode, and the width of the active layer in which only the fundamental transverse mode oscillates, that is, the size of the step, is about 1.0 μm. If the above-mentioned oscillation conditions are to be obtained by making the step larger to facilitate manufacturing, it is naturally necessary to reduce the refractive index difference. In conventional structures, this can be achieved by reducing the thickness of the active layer or by using a crystal with a small refractive index in the core region. However, either method results in the loss of the basic characteristics of the laser. That is, if the thickness of the active layer is reduced, the oscillation threshold increases, and if the refractive index of the core region is reduced, the temperature characteristics deteriorate, making high-temperature operation difficult. On the other hand, the structure of the present invention has the advantage that the refractive index can be reduced without causing the above-mentioned drawbacks. That is, by appropriately changing the angle of the slope region of the active layer, the required magnitude of the refractive index difference can be easily controlled. This angle control of the slope region is possible because a convex portion is formed on the substrate crystal, the side surface of which is a crystal plane that has a different crystal growth rate than other regions. In the conventional case, the refractive index difference is inevitably determined when the material is determined, but in the present invention, even if the material is determined, the refractive index difference is not determined unless the angle of the inclined bent portion is determined. Therefore, an optimal refractive index difference can be obtained by appropriately selecting the inclination angle. As described above, the present invention has one more design parameter than the conventional one, and provides a wide degree of freedom in optimal design, which is a very important factor in device manufacturing. In this invention, by changing the inclination angle and reducing the refractive index difference from 10% to 1%, the active layer width at which only fundamental transverse mode oscillation can be obtained can be increased to 2 to 3 μm, making it easier to fabricate the device. The operating current range for fundamental transverse mode oscillation also becomes wider.

On the other hand, even if the active layer width of the conventional device (Japanese Patent Application Laid-Open No. 114887/1987) is set to 2 to 3 μm as described above, the operating current range of fundamental transverse mode oscillation does not widen, and primary transverse mode oscillation occurs at several mW. As described above, the present invention not only allows fundamental transverse mode oscillation to be easily obtained even if the length of the sloped portion of the active layer is increased, but also has the advantage of widening the operating current range. In addition, since there are many design parameters in manufacturing the device, its shape can be easily controlled, and it has excellent reliability and mass productivity.

In an embodiment of the present invention, the n-
The layer thickness of the AlGaAs layer 14 on the substrate surface (100) is made thin enough that the laser light seeps into the GaAs substrate side.
For example, if the thickness is 0.4 μm or less, it is possible to obtain a semiconductor laser having a structure in which a loss-type optical waveguide effect due to light absorption is provided.

So far, we have only explained semiconductor lasers made of GaAs-AlGaAs materials, but for example,
A semiconductor laser having the same effect can be manufactured even when applied to InP-InGaAsP material. The substrate 13 is an n-InP crystal, the first crystal layer 14 is an n-InP layer, and the active layer 15 is an InGaAsP layer. , the second crystal layer 16 is a P-InP layer, and the electrode forming layer 17 is a P-InP layer.
By growing the InGaAsP layer, the oscillation wavelength can be increased from 1.0 to 1.5.
A μm semiconductor laser can be obtained.

As an example of the implementation of the present invention, only the case where the striped current region forming portion for applying an injection current to the double heterojunction is of the electrode stripe type has been described above. The same effect can be obtained whether the current forming section is applied to a planar or stripe type semiconductor laser, or to other stripe type semiconductor lasers.

Moreover, 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 perspective view of a conventional buried stripe-type semiconductor laser, FIG. 2 is a perspective view of a stripe-type semiconductor junction laser that is an embodiment of the present invention, and FIGS. 3A, B, and C are the lasers shown in FIG. FIG. In the drawings, 2, 13... n-type GaAs substrate,
3, 8, 14... n-type GaAlAs layer, 4, 15, 2
4...GaAs active layer, 16,5...p-type GaAlAs
Layer, 17... p-type GaAs layer, 6, 18... SiO ₂
Membrane, 1, 10, 12, 19... Electrode, 9, 10,
21, 22... end face, 11, 20... striped current area, 26... groove in [011] direction, 27...
(111) A-plane inclined side wall (step side) is shown.

Claims (1)

[Scope of utility model claims] In a semiconductor laser with a double heterostructure, in which two crystal layers with different conductivity types, each having a wider band gap than the active layer crystal, are joined to sandwich the active layer crystal, the crystal growth rate differs from that on the substrate crystal surface. An active layer crystal having an inclined bent part is formed on a substrate crystal having a convex part whose slope is a surface where the crystal grows at a high speed, the inclined bent part is used as a light emitting region, and the inclined step is formed. 1. A semiconductor junction laser, characterized in that the side surfaces are parallel to the optical axis direction of the laser beam, a striped current injection region is formed directly above the light emitting region, and further has a flat electrode forming layer.