JPS6334293Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to the structure of a semiconductor laser that oscillates in a fundamental mode. A semiconductor laser that oscillates in a fundamental mode with transverse mode control is PCW (Plano−
A structure called a convex waveguide type semiconductor laser has been proposed by the inventors of the present invention (Japanese Patent Application No. 33742/1983). In this structure, a plano-convex optical guide layer is provided below the active layer to form a refractive index distribution in the lateral direction of the junction, giving the active layer a strong optical waveguide effect. First, the structure, functions, etc. of this type will be briefly explained using drawings, and what problems should be solved by the present invention. Figure 1 shows the conventional
1 is a cross-sectional view showing a schematic diagram of a PCW type semiconductor laser.

For example, on the (100) plane n-type InP substrate 1, <011>
Rectangular grooves 2 are formed in stripes parallel to the direction. On this InP substrate 1, an n-type InGaAsP light guide layer 3,
An InGaAsP active layer 4 and a P-type InP cladding layer 5 are sequentially grown, a SiO ₂ film 6 is formed on the P-type InP cladding layer 5, and a SiO _{2 window is formed so that current flows into the active layer region directly above the trench 2} . and attach the electrodes 7 and 8.
A PCW type semiconductor laser is manufactured. According to this structure, the refractive index of the light guide layer 3 and the active layer 4 is designed to be close to each other, and that of the light guide layer 3 is slightly smaller. In such a structure, when the active layer thickness is constant, laser light leaks from the active layer 4 to the light guide layer 3 side, and the extent to which it leaks varies depending on the difference in the light guide layer thickness. Therefore, the amount of laser light seeping into the light guide layer 3 differs between the convex region of the light guide layer 3 and both sides thereof, which causes an effective refractive index distribution between both regions. This effective refractive index distribution brings about an optical waveguide effect that confines the laser light in the lateral direction parallel to the cemented surface, thereby stabilizing the transverse mode of laser oscillation. Also, guide layer 3
Effective refractive index difference between the thick convex part and the outside △
In order to maintain the fundamental transverse mode stably up to high output, n needs to form an order of 10 ^-2 . As the value of Δn increases, higher-order transverse mode oscillation generally becomes easier, and conversely, as the value of Δn decreases, a stable optical waveguide effect cannot be obtained.

When designing, it is necessary to set Δn with sufficient consideration to such matters. For example, when the guide layer 3 has a wavelength (λG) of 1.1 μm, and the active layer 4 has λA = 1.3 μm and layer thickness d = 0.2 μm, the thickness of the convex portion of the guide layer 3 is 0.5 μm, By setting the outside to 0.3 μm (groove depth t = 0.2 μm), △n is approximately 1×10 ^-2 and fundamental transverse mode oscillation is maintained up to a current value that is more than twice the oscillation threshold. .

However, in a conventional PCW type semiconductor laser, when the compositions of the guide layer 3 and the active layer 4 are similar, that is, the larger the refractive index of the guide layer 3, the more light seeps into the guide layer, and △n It is sensitive to changes in layer thickness, for example, the guide layer 3 is made with λG = 1.03 μm, and the active layer 4 is made with λA
= 1.2 μm, layer thickness d = 0.2 μm, and if the thickness of the convex portion of the guide layer 3 is 0.5 μm and the outside is 0.3 μm (groove depth t = 0.2 μm), then △n is approximately 8× 10 ^-3 , and a stable optical waveguide effect cannot be obtained.
It becomes difficult to maintain the fundamental transverse mode stably up to high output.

In order to form a stable optical waveguide, the surface of the optical guide layer must be flat, and an active layer of uniform thickness must be grown on the surface.
In the groove 2 used in the PCW type semiconductor laser, the thickness of the layer grown outside the groove must be made large relative to the groove depth t, otherwise the groove will not be filled reliably.
In order to form a Δn of 10 ⁻² , the thickness of the layer grown outside the groove 2 must be equal to or less than the groove depth t. In the conventional PCW type semiconductor laser,
The grooves 2 are not completely filled with the guide layer 3, causing unevenness in the active layer 4, making it difficult to obtain stable fundamental transverse mode oscillation, which is an inherent feature of PCW type semiconductor lasers, and also increasing the oscillation threshold value.

The purpose of the present invention is to eliminate the above-mentioned drawbacks of conventional semiconductor lasers, and to achieve a sufficient refractive index difference even when using a guide layer that has a relatively large refractive index and has a composition close to that of the active layer. A semiconductor laser structure that maintains fundamental transverse mode oscillation over a wide operating current range, has a small oscillation threshold, is capable of high output, is easy to manufacture, and is suitable for high-yield mass production. The goal is to provide the following.

The semiconductor laser of the present invention includes at least a light guide layer having a bandgap narrower and a refractive index smaller than that of the semiconductor substrate on a semiconductor substrate having a groove with a rectangular cross section having both sides of the (211) A plane. , an active layer having a bandgap width smaller than that of the light guide layer and a larger refractive index, and a refractive index larger than the bandgap width of both the light guide layer and the active layer and smaller than the refractive index of both. The active layer has a structure in which a cladding layer is sequentially formed and a predetermined current is injected into the active layer to excite it.

According to research by the present inventors, by making the rectangular groove that constitutes a conventional PCW type semiconductor laser into a groove whose both sides have a (211)A-plane crystal orientation, a layer grows outside the groove. It has been experimentally found that even if the thickness is less than the groove depth t, the groove is reliably filled and the surface becomes flat. Figure 2 shows groove depth t = 0.2μm.
The relationship between the constant groove width W and the thickness TG of the guide layer grown outside the groove, which is necessary to fill the groove portion flatly, is shown. The groove used in the experiment was {100} plane n-type
These are striped rectangular grooves parallel to the <011> direction on the InP substrate, and the black circles in the figure indicate the groove depth t, which is the guide layer thickness TG required to fill the conventional groove flatly, as described above. This shows that the groove can be reliably filled if it is made larger than that. On the other hand, the white circle indicates the guide layer thickness required to flatly embed a groove with a rectangular cross section with (211) A side as both sides.
TG, indicating that the groove can be reliably filled with a guide layer thickness TG that is approximately 1/2 of the groove depth t, indicating that a stable optical waveguide can be formed.

The details of this invention are explained below with reference to the drawings.
Regarding the case of using InGaAsP as a semiconductor layer,
An example of this will be described.

FIG. 3 is a main cross-sectional view perpendicular to the laser beam, showing a typical example of a semiconductor laser in which the present invention is implemented.

First, the width parallel to the <011> direction was measured using a photoresist film as a mask on the (100) plane n-type InP basic 9 surface.
A long and narrow etching window of 2.0 μm is formed. Next, selective etching is performed with a mixture of hydrochloric acid and phosphoric acid, and the width is
A striped rectangular groove 10 of 2.3 μm and a depth of 0.2 μm is formed. At this time, both sides are (211)A
A groove 10 having a surface is formed. The remaining photoresist film is removed from the surface of the InP substrate 9, and this
The following layers are successively grown on the InP substrate by liquid phase epitaxial growth. First _{, the n-type In 0.92 Ga 0.08 As 0.19 P 0.81 optical guide layer 11 of} the first semiconductor layer _.
grow. This growth continues until the rectangular groove 10 having both sides of the (211) A plane is completely filled and the entire top surface is flat. Next _, a P-type _{In0.81Ga0.19As0.43P0.57} layer _{12 , which} is the _active layer _of the second semiconductor layer _,
A third semiconductor layer, the P-type InP layer 13, is then grown.

Typical layer thicknesses are respectively 0.3 .mu.m for the light guide layer 11, 0.2 .mu.m for the active layer 12 and 2 .mu.m for the carrier confinement layer 13 in the region of the grooves 10.

Finally, the P-type electrode 14 is connected via the SiO ₂ film 15,
Further, the n-type electrodes 16 are formed on the back side of the InP substrate 9 to complete the process.

Note that the guide layer 11 has a wavelength λG = 1.03 μm,
The active layer 12 has λA = 1.2 μm, and even though the guide layer 11 has a relatively large refractive index (n≒3.35), that is, even if the light seeps into the guide layer 11 is large, in this example, the semiconductor The experimental value of △n obtained by measuring the light emission angle in the direction parallel to the P-N junction of the laser is approximately 2 × 10 ^-2 , and the fundamental transverse mode oscillation is close to the oscillation threshold. The current value was maintained to more than double. This means that in the example, the thickness of the optical guide layer 11 grown outside the groove 10 is 0.1 μm for the depth t of the groove 10 = 0.2 μm, and the thickness is uniform (0.2 μm). This is because the active layer 12 is formed.

Furthermore, according to the present invention, since the chemical etching speed of the (211) A plane, which is the side surface of the groove 10, is slow in manufacturing, it is possible to form an optical waveguide with a narrow groove width, so that the fundamental transverse mode is stabilized. It has the advantage that the oscillation threshold is small and high output is possible.
In addition, using a guide layer with λG = 1.1 μm,
Also when the active layer is λA = 1.3 μm,
According to the present invention, it goes without saying that effects similar to those described above can be obtained.

In the above experimental example, the liquid phase epitaxial growth method was applied as the crystal growth method, but other growth methods such as the vapor phase epitaxial growth method and the molecular beam epitaxial growth method could also be applied to the present invention. Exactly the same effect can be obtained by implementing.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view schematically showing a conventional PCW type semiconductor laser, and Figure 2 is a liquid phase epitaxy of the optical guide layer grown outside the groove, which is necessary for flatly filling the groove part of the PCW type semiconductor laser. FIG. 3 is a cross-sectional view schematically showing a PCW type semiconductor laser according to an embodiment of the present invention. In the figure, 1, 9... semiconductor substrate, 2... rectangular groove, 3, 11... optical guide layer, 4, 12... active layer, 5, 13... cladding layer, 6, 15...
SiO ₂ film, 7, 14...P-type electrode, 8, 16...
N-type electrode, 10... (211) A rectangular groove with both sides facing A-plane is shown.

Claims (1)

[Scope of utility model registration request] On a semiconductor substrate having a groove having a rectangular cross section, at least a light guide layer having a narrower bandgap width and a lower refractive index than the semiconductor substrate; and a light guide layer having a smaller bandgap width and a larger refractive index than the light guide layer. An active layer and a cladding layer having a refractive index larger than the forbidden band width of both the optical guide layer and the active layer and smaller than the refractive index of both are sequentially formed, and a predetermined current is injected into the active layer. What is claimed is: 1. A semiconductor laser having a structure in which the cross section is rectangular and both side surfaces thereof are (211)A planes.