JPH04398B2 - - Google Patents

Description

[Detailed Description of the Invention] [Summary] Consisting of a stack of multiple semiconductor layers with different forbidden band widths, the optical confinement ability in the vertical direction (layer thickness direction) is determined by the difference in refractive index based on the difference in the forbidden band widths. Regarding the improvement of making the light emitting spot of a double heterojunction semiconductor laser nearly circular, the optical confinement ability in the lateral direction (layer direction) is achieved by using the difference in refractive index based on the difference in impurity concentration. Semiconductor light emitting devices, especially double heterojunction type semiconductor lasers, in which the lateral light emission angle is approximately the same as the longitudinal light emission angle, the light emitting spot is close to a circle, and the threshold current is small, especially semiconductor light emitting devices The object of the present invention is to provide a pedal heterojunction type semiconductor laser, which is composed of an active layer made of a semiconductor layer having a first conductivity type, and a semiconductor layer having the first conductivity type disposed with the active layer sandwiched therebetween. The first clad layer and the second
a cladding layer, and either between the first cladding layer and the active layer, or between the second cladding layer and the active layer, the first cladding layer or the active layer is provided. The light-emitting stripe extends from the second cladding layer into the active layer to form a substantially symmetrical curved shape to the left and right of the light-emitting stripe, and its tip is located at the interface between the active layer and the second cladding layer or at the active layer. A semiconductor light emitting device, in particular a double heterojunction type semiconductor laser, has a region having a second conductivity type in contact with the interface between the first cladding layer and the first cladding layer.

[Industrial application field]

The present invention relates to a semiconductor light emitting device. In particular, it consists of a stack of multiple semiconductor layers with different forbidden band widths, and the optical confinement ability in the vertical direction (layer thickness direction) is achieved by utilizing the difference in refractive index based on the difference in the forbidden band widths.
The optical confinement ability in the lateral direction (layer direction) relates to an improvement in which the light emitting spot of a double heterojunction semiconductor laser is made closer to a circular shape by utilizing a difference in refractive index based on a difference in impurity concentration.

[Conventional technology]

In semiconductor light emitting devices, particularly semiconductor lasers, it is desirable that the light emission pattern, that is, the light emitting spot, be approximately circular. This is because if the light emitting spot is circular, the light emitted from the semiconductor laser can be efficiently coupled to the optical fiber. However, in a conventional double heterojunction type semiconductor laser, the ratio of the vertical radiation angle to the horizontal radiation angle is 3 to 5, and the light radiation pattern is a vertically long ellipse.

The reason why the light radiation pattern is a vertically elongated oval is because
It is as follows. In other words, in a double heterojunction type semiconductor laser, the optical confinement ability in the vertical direction (vertical direction) is achieved by utilizing the difference in refractive index based on the difference in forbidden band width, and by using the extremely thin layer sandwiched between the cladding layers. It attempts to confine light within the active layer (0.1 to 0.2 μm), and the lateral (layer direction) optical confinement ability is achieved by utilizing the difference in refractive index based on the difference in impurity concentration within the stripe region. However, because the thickness of the active layer is extremely thin, it is difficult to effectively prevent the light from spilling out in the vertical direction (up and down), and the light is trapped in the vertical direction. Although the radiation output angle is large, the lateral width of the active layer cannot be made that small; it needs to be on the order of several μm; therefore, the ``overflow'' of light in the lateral direction (layer direction) is generally effectively prevented. This is because the light emission angle in the lateral direction becomes smaller.

In order to make the light radiation pattern nearly circular, it is necessary to make the light confinement effect in the lateral direction approximately the same as the light confinement effect in the vertical direction.

In addition, increasing the confinement effect between the carrier and the light also reduces the threshold current, which is also effective for improving the luminous efficiency.

Although it is known that forming a heterojunction on the side surface of the light emitting region is one effective means to improve the lateral confinement ability, it is difficult to minimize the lateral width of the light emitting region of the active layer. Since it is not easy to reduce the size, generally, as described above, the transverse mode guide method that utilizes the carrier and light confinement ability based on the difference in impurity concentration is widely used, and the present invention also uses this configuration. Assuming that.

FIG. 1 shows an example of the layer structure of a double heterojunction type semiconductor laser that uses a double heterostructure for longitudinal optical confinement and uses a difference in impurity concentration as a transverse mode guide. In the figure, 1 is a substrate made of n-type gallium arsenide (n-GaAs), and 2 is a substrate made of n-type gallium aluminum arsenide (n-GaAs).
The first clad layer (lower clad layer) is made of n-type gallium aluminum arsenide (n-Ga 1 -zAlzAs), and 4 is the active layer made of n-type gallium aluminum arsenide (n _- Ga ₁ -zAlzAs).
-Ga ₁ -yAlyAs), 5 is a p-type layer made of p-type gallium aluminum arsenide (p-Ga ₁ -xAlxAs), and 6 is n-type gallium arsenide (n −GaAs)
The contact layer consists of: 7 is a PN junction surface, and a region 8 surrounded by this PN junction surface 7 is a p-type region. The mixed crystal ratio z is selected depending on the emission frequency, and in order to obtain an oscillation wavelength in the 0.8 μm band, z is usually selected.
= about 0.05 is widely used, and the mixed crystal ratio x and y
and z must have the relationship x=y>z.

In a double heterojunction type semiconductor laser with a double heterojunction structure having such a mixed crystal ratio, the ability to confine the carrier and light in the vertical direction depends on the refractive index difference based on the difference in forbidden band width.
Transverse mode guidance relies on refractive index differences based on density differences. That is, as shown in FIG. 2, the light confinement effect in the lateral direction is caused by the fact that the active layer 3 is laterally divided into three regions having different refractive indexes, so that the light is focused in the center.

[Problem to be solved by the invention]

However, in a double heterojunction type semiconductor laser having the above layer structure, when the stripe width is 6 cm, the light emission angle is about 7°, and when the stripe width is 4 μm, the light emission angle is 8.5°. When the stripe width is 3 μm, the light emission angle is about 11°. That is, as the stripe width becomes narrower, the light emission angle tends to become larger. In order to make the light emission angle 10 degrees or more, the stripe width needs to be about 3 μm or less.

On the other hand, since the thickness of the active layer 3 is about 0.1 μm, the light radiation angle in the vertical direction is extremely large, and the light radiation pattern becomes a vertically elongated ellipse as shown in FIGS. 8a and 8b. FIG. 8a shows the layer structure of the active layer 3 and the upper and lower cladding layers 2 and 4, and FIG. 8b shows the layer structure of the active layer 3 and the upper and lower cladding layers 2 and 4.
This figure shows a light emission pattern in roughly the same area as in Figure a.

Therefore, in order to make the light emitting pattern circular, it is desirable that the horizontal light emission angle be as close as possible to the vertical light emission angle, and be at least 10° or more.
However, in order to increase the lateral light emission angle to 10° or more, it is necessary to reduce the stripe width to about 3 μm or less, but it is not necessarily easy to reduce the stripe width to about 3 μm or less.

The object of the present invention is to provide a semiconductor light emitting device, in particular, the longitudinal optical confinement effect depends on the refractive index difference based on the difference in forbidden band width caused by a double heterojunction, and the transverse mode guide depends on the refractive index difference based on the impurity concentration difference. To provide an improvement in a double heterojunction type semiconductor laser which depends on the difference, by making the longitudinal light emission angle approximately the same as the lateral light emission angle, making the light emitting spot nearly circular, and reducing the threshold current. It is in.

[Means to solve the problem]

The above object is to form an active layer 13 made of a semiconductor layer having a first conductivity type, a first cladding layer 12 made of a semiconductor layer having a first conductivity type disposed with the active layer 13 in between, and a 2 cladding layer 14, this first cladding layer 12 and the active layer 1
3 or between the second cladding layer 14 and the active layer 13, the first cladding layer 12 or the second cladding layer 14 to the active layer 13 It extends inwards and forms a curved shape that is substantially symmetrical to the left and right sides of the light-emitting stripe, and its tip is at the interface between the active layer 13 and the second cladding layer 14 or at the interface between the active layer 13 and the first cladding layer 14. This is achieved by a semiconductor laser in which a region 22 of the second conductivity type in contact with the interface with the cladding layer 12 is formed.

[Effect]

As described above, the semiconductor light emitting device according to the present invention, particularly the double heterojunction type semiconductor laser, has a PN formed in the light emitting layer 13 as shown in FIG. Junction 17,
A drive-in diffusion method or the like is used to form a curved shape with a narrow width and a downward convexity, and the lower end thereof is brought into contact with the lower surface of the active layer 13 in the lower region of the active layer 13. has a very small lateral dimension, and this lateral dimension increases as it moves toward the upper region of the active layer 13,
As a result, the lateral light emission angle increases in the lower region of the active layer 13, decreases as it moves toward the upper region of the active layer 13, and in the upper region of the active layer 13, By reducing the horizontal light emission angle to the same extent as in the case of the prior art (shown in Figure 8b), and making active use of this gradually changing light emission angle, we can reduce the vertical light emission angle as shown in Figure 9b. The directional light emission angle is made to be about the same size as the lateral light emission angle,
A circular light emission pattern is realized.

Furthermore, as shown in FIG. 5, the difference in refractive index in the lateral direction in the light emitting region (p-type region 22) in the light emitting layer 13 gradually slopes (as shown in FIG. 5, it becomes large at the center in the lateral direction). In particular, the light confinement effect at the left and right ends becomes smaller and the lateral light emission angle increases, and as a result, the vertical light emission angle becomes the same as the lateral light emission angle. It has a circular luminescence emission pattern.

The semiconductor light emitting device according to the present invention, particularly the double heterojunction type semiconductor laser, takes advantage of these effects to make the vertical light emission angle approximately the same as the horizontal light emission angle, and to make the light emitting spot nearly circular. , which reduces the threshold current.

What is particularly important here is that the p-type region of the active layer 13 is in contact with the lower cladding layer 12. Otherwise, the active layer 13 would also have an n-type region below it, and this n-type region would also emit some light, but the light emission in this n-type region would be guided (confined). Since there is no function, the light emitting spot in this n-type region becomes large.On the other hand, the laser light emission is a combination of the above-mentioned light emission in the p-type region and this n-type region.
This is because the light emission pattern of the entire laser is not circular because the light emission in the mold region is superimposed.

〔Example〕

Hereinafter, a manufacturing process of a semiconductor laser according to an embodiment of the present invention will be described with reference to the drawings.

Refer to Figure 3. Using a liquid phase epitaxial growth method, a 1.5 cm thick film is grown on an n-type gallium arsenide (n-GaAs) substrate 11.
~2μm n-type gallium aluminum arsenide (n-
A first cladding layer (lower cladding layer) 12 made of Ga ₁ -yAlyAs), an active layer 13 made of n-type gallium aluminum arsenide (n-Ga _{1 -zAlzAs) with a thickness of 0.15 μm, and an active layer 13 made of n-type gallium aluminum arsenide (n-Ga 1} -zAlzAs) with a thickness of 0.5 μm. a second cladding layer (upper cladding layer) 14 made of n-type gallium aluminum arsenide (n-Ga ₁ -yAlyAs);
1 μm thick p-type gallium aluminum arsenide (p
-Ga ₁ -xAlxAs) and a p-type layer 15 with a thickness of
A contact layer 16 of 1 μm of n-type gallium arsenide (n-GaAs) is then formed. At this time, the n-type impurity concentration of the light emitting layer 13 is 3×10 ¹⁸ /cm ³
level or higher. In addition, it is essential that there is a relationship x=y>z among the mixed crystal ratios x, y, and z, and z
It is well known that the value of is determined depending on the wavelength of the light to be emitted.

Refer to FIG. 4. A diffusion mask 19 having a slit-shaped opening in a region corresponding to a light emitting region is formed on the contact layer 16. This mask 19 is made of a double layer of silicon dioxide glass and silicon nitride glass, and the width of the slit-shaped opening is suitably 1 to 1.5 μm.

Next, using this diffusion mask 19, diffusion is performed for about 30 minutes at a temperature of 700°C using zinc arsenide (ZnAs ₂ ) as a source, and the p-type impurity concentration is 1×.
A region of 10 ²¹ /cm ³ is formed to a depth of about 1.5 μm.
After removing the diffusion source, drive-in diffusion is performed at a temperature of 900° C. for about 3 hours. As a result, a downwardly convex curved PN joint surface 17 is formed as shown in the figure, and its cross-sectional shape is nearly semicircular as shown in the figure.
A mold region 22 is formed. p of this p-type region 22
The type impurity concentration in the light emitting layer 13 is 0.5×
It will be about 10 ¹⁹ /cm ³ . Furthermore, the lowest end of the PN junction surface 17 is located at the first cladding layer (lower cladding layer) 12.
is formed so as to be in contact with the interface between and the active layer 13,
The slope of the PN junction surface 17 in the active layer 13 is generally appropriate, and the width of the p-type region 22 of the active layer 13 is about 3 μm.

Refer to FIG. 5 As shown in FIG.
In the type impurity region 22, it is large at the center in the lateral direction, but gradually becomes smaller at the left and right ends. Therefore, the light confinement effect also
Although it is large at the center in the lateral direction, it gradually becomes smaller at the left and right ends, so that the light emission angle becomes particularly large at the left and right ends.

At the same time, as mentioned in the [effect] section,
In the lower region of the active layer 13 the lateral dimensions are very small;
As a result, the lateral light emission angle increases as it moves to the upper region of the active layer 13, and as a result, the lateral light emission angle increases as it moves to the upper region of the active layer 13. In the upper region of the active layer 13, the lateral light emission angle is reduced to the same extent as in the prior art case (shown in FIG. 8b), and due to the effect of this gradually changing light emission angle, the vertical light emission angle is The light emission angle and the lateral light emission angle are about the same size, and as a result, the light emission spot or light emission pattern is such that the longitudinal light emission angle is equal to the lateral light emission angle, as shown in FIG. 9b. They are of similar size and realize a circular light emission pattern.

Refer to Figure 6 Diffusion mask 1 using phosphoric acid etc. with hydrofluoric acid etc.
After dissolving and removing 9, a positive electrode 20 is deposited on the contact layer 16, and a negative electrode 21 is deposited on the lower surface of the substrate 11.

As mentioned above, in the double heterojunction type semiconductor laser manufactured through the above process, the lateral light emission angle is extremely large at the bottom of the active layer 13, and this light emission angle gradually increases toward the top. Since it is smaller, the lateral light emission angle of the laser as a whole becomes larger, and the longitudinal and lateral light emission angles of the laser as a whole become approximately the same.

According to the experimental results, when the stripe width is 6 cm, the lateral light radiation angle is 8 degrees, and the stripe width is 6 cm.
In the case of 4 μm, the lateral light radiation angle is 10°, and when the stripe width is 3 μm, the lateral light radiation angle is 14°, which is 20 ~
It has been made 30% larger. As an example, FIG. 7 shows a far-field pattern when the stripe width is 3 μm and the output is 5 mW. It is clear that the characteristic B of the semiconductor laser according to the present invention, which is shown by a solid line, has a wider lateral light emission angle and is significantly improved compared to the characteristic A of the conventional semiconductor laser, which is shown by a broken line. It is.

Furthermore, in the semiconductor light emitting device according to the present invention, particularly in the double heterojunction type semiconductor laser, the cross-sectional area of the light emitting region is smaller than that in the conventional double heterojunction type semiconductor laser. The effect of reducing the threshold current is also significant.

In the above explanation, gallium arsenide (GaAs)
This article describes a semiconductor light-emitting device configured with a combination of gallium aluminum arsenide (GaAlAs), but other semiconductors that can be crystal matched, have an appropriate basic absorption edge wavelength, and have an appropriate forbidden band width are also available. Needless to say, it is also possible to configure a combination of the following. For example, a combination of indium phosphide (InP) and indium gallium arsenide phosphide (InGaAsP) is used.

〔Effect of the invention〕

As explained above, the semiconductor light emitting device according to the present invention, in particular, is made of a laminate of a plurality of semiconductor layers having different forbidden band widths, and the optical confinement ability in the vertical direction (layer thickness direction) depends on the difference in the forbidden band widths. In the case of a double heterojunction type semiconductor laser, the optical confinement ability in the lateral direction (layer direction) is achieved by utilizing the difference in refractive index based on the difference in impurity concentration. an active layer made of a semiconductor layer having a conductivity type of Either between the cladding layer and the active layer, or between the second cladding layer and the active layer, the active layer is formed from the first cladding layer or the second cladding layer. It extends into the layer and has a substantially symmetrical curved shape on the left and right sides of the light emitting stripe, and its tip is at the interface between the active layer and the second cladding layer or between the active layer and the first cladding layer. Since a region having the second conductivity type is formed which is in contact with the interface of As the lateral dimension increases as we move to The effect of this gradually changing light emission angle is that the longitudinal light emission angle and the lateral light emission angle are of similar magnitude, resulting in a circular light emission pattern. In addition, the light emitting region (p-type region) within the active layer
Since the refractive index difference in the lateral direction gradually slopes (as shown in FIG. 5, it becomes large at the center in the lateral direction and gradually decreases at the left and right ends),
Particularly, the light confinement effect at the left and right ends is reduced, the lateral light emission angle is increased, the vertical light emission angle is made approximately the same as the lateral light emission angle, and a circular light emission pattern is realized.

As a result, it is possible to provide a semiconductor light emitting device, particularly a double heterojunction type semiconductor laser, in which the light emitting spot is nearly circular and the threshold current is small.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a substrate of a semiconductor light emitting device according to the prior art. FIG. 2 is a graph showing changes in the lateral refractive index difference within the active layer in the layer configuration shown in FIG. 3, 4, and 6 are cross-sectional views of a substrate after completion of the main steps in a method for manufacturing a semiconductor light emitting device according to an embodiment of the present invention. Fifth
The figure is a graph showing changes in the lateral refractive index difference within the active layer in the layer configuration shown in FIG. 4. FIG. 7 is a graph comparing a far-field pattern of a semiconductor light-emitting device according to an embodiment of the present invention with a far-field pattern of a semiconductor light-emitting device according to the prior art. FIG. 8a is a diagram showing the layer structure of a light emitting layer and upper and lower cladding layers of a double heterojunction type semiconductor laser according to the prior art. FIG. 8b is a diagram showing a light emission pattern in a region generally the same as the region shown in FIG. 8a of a double heterojunction type semiconductor laser according to the prior art. FIG. 9a is a diagram showing the layer structure of a light emitting layer and upper and lower cladding layers of a double heterojunction type semiconductor laser according to the present invention. FIG. 9b is a diagram showing a light emission pattern in a region generally the same as the region shown in FIG. 9a, of a double heterojunction type semiconductor laser according to the present invention. 1, 11... Substrate (n-GaAs), 2, 12... First cladding layer (n-Ga ₁ -yAlyAs), 3, 13...
Active layer (n-Sa ₁ -zAlzAs), 4, 14... second cladding layer (n-Ga ₁ -yAlyAs), 5, 15... p-type layer (p-Ga ₁ -xAlxAs), 6, 16... contact layer (n-GaAs), 7, 17...PN junction surface, 8, 1
8...p region of the double heterojunction type semiconductor laser according to the prior art, 19...diffusion mask, 20...positive electrode, 21...negative electrode, 22...p type region of the double heterojunction type semiconductor laser according to the present invention.

Claims (1)

[Claims] 1. An active layer 13 made of a semiconductor layer having a first conductivity type; and a first cladding layer 12 made of a semiconductor layer having a first conductivity type disposed with the active layer 13 sandwiched therebetween. and a second cladding layer 14, between the first cladding layer 12 and the active layer 13, or between the second cladding layer 14 and the active layer 13, the first cladding layer 12. Alternatively, it extends from the second cladding layer 14 into the active layer 13 to form a curved shape that is substantially symmetrical to the left and right of the light-emitting stripe, and its tip ends at the interface between the active layer 13 and the second cladding layer 14 or the active layer 1
3. A semiconductor light emitting device characterized in that a region 22 of a second conductivity type is formed in contact with the interface between the first cladding layer 12 and the first cladding layer 12, and the light emitting spot is approximately circular.