JP3615499B2 - Semiconductor laser - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a buried ridge type semiconductor laser used for a light source of an optical disk system.
[0002]
[Prior art]
Conventionally, a high-power embedded ridge type semiconductor laser has been actively developed as a light source for an optical disk system such as a recording / reproducing DVD or a high-speed CD-R / RW.
[0003]
FIG. 4 shows the general structure. A buffer layer 202, a lower cladding layer 203, an active layer 204, a first upper cladding layer 205, and an etching stop layer 206 are formed in this order on the substrate 201, and the etching is stopped. A ridge-type second upper clad layer 207 and an intermediate layer contact 208 are formed on the layer 206, a current blocking layer 209 is formed on the side surface and the bottom surface of the ridge, and a GaAs contact is formed on the top surface of the ridge and the current blocking layer 209. Layer 210 is formed. When this semiconductor laser device is adapted to high output, the light that is spilled from the active layer is reduced by causing the light to ooze out from the active layer. When absorbed by the GaAs contact layer 210, the operating current increases and the laser characteristics deteriorate. Therefore, by increasing the film thickness of the ridge, the distance from the active layer to the GaAs contact layer 210 is increased to prevent light from reaching the GaAs contact layer.
[0004]
However, this structure has the following problems. In the case of a semiconductor laser that requires high output operation, the kink generated light output is required to be sufficiently high, and therefore the width of the ridge is usually limited to several μm or less. Therefore, when the ridge is thick, the resistance becomes high and the operating voltage of the element increases. In particular, when the ridge is a forward mesa (a trapezoid with a wide base), the upper end width (current path width) of the ridge becomes narrower as the ridge thickness is increased, so that the increase in operating voltage due to an increase in the series resistance of the element becomes significant.
[0005]
FIG. 5 is a structural diagram of a conventional buried ridge type semiconductor laser proposed in Japanese Patent Laid-Open No. 2000-31585 which avoids this problem. Compared with FIG. 4, which is a general structure, the only difference is that a third upper cladding layer 311 is formed between the ridge portion (307) and the current blocking layer and the GaAs contact layer 313.
[0006]
The manufacturing method of the conventional example will be described with reference to FIGS. First, an n-type GaAs buffer layer 302 on an n-type GaAs substrate 301 and a lower clad layer 303 (thickness 1.3˜ 1.8 μm), an active layer 304 (film thickness 50 to 100 nm) made of an AlGaInP layer and a GaInP layer, a first upper cladding layer 305 (film thickness 220 to 400 nm) made of p-type AlGaInP, and an etching made of p-type GaInP. Stop layer 306 (film thickness 10 to 30 nm), second upper cladding layer 307 (film thickness 0.6 to 1.5 μm) made of p-type AlGaInP, intermediate contact layer 308 (film thickness 10 to 10 μm) made of p-type GaInP 30 nm) (FIG. 6A). Then, a striped SiO ₂ mask 309 is formed on the intermediate contact layer 308 by photolithography, and the intermediate contact layer 308 and the second upper cladding layer 307 are etched to the etching stop layer 306 using the mask 309. A ridge is formed (FIG. 6B). Next, the current block layer 310 made of n-type GaAs is selectively grown on both sides of the ridge by the second MOCVD growth (FIG. 6C). After removing the SiO ₂ mask 309, the third upper cladding layer 311 made of p-type AlGaInP (film thickness: 0.3 to 1.0 μm) is formed on the ridge portion and the current blocking layer 310 by the third MOCVD growth. An intermediate band gap layer 312 (thickness 30 to 100 nm) made of type GaInP and a contact layer 313 (thickness 0.5 to 3.0 μm) made of p-type GaAs are grown (FIG. 6D). Since the third upper clad layer 311 is formed, a sufficiently long distance between the active layer and the GaAs contact layer can be ensured. Therefore, as shown in the general structural diagram of FIG. There is no need to increase the ridge thickness in order to prevent light absorption. Accordingly, it has been reported that the narrowing of the ridge top width is avoided, so that the operating voltage is not increased due to the increase in series resistance.
[0007]
[Problems to be solved by the invention]
However, the conventional techniques have the following problems. That is, the third upper cladding layer is not formed by the first growth on a clean substrate, but is regrown on the ridge upper surface and the current blocking layer after passing through etching and selective growth processes. It is formed by. Therefore, the crystallinity is affected by the crystallinity of the ridge upper surface and the current blocking layer. The refractive index of the third upper cladding layer is preferably about the same as that of the lower cladding layer and the second upper cladding layer in order to make the light distribution approximately symmetrical about the active layer, and the Al composition of the third upper cladding layer Is approximately the same as the lower cladding layer and the second upper cladding layer. Therefore, since the third upper cladding layer has a high Al composition, its crystallinity is considered to be more susceptible to the crystallinity of the ridge upper surface and the current blocking layer than in the case of the GaAs contact layer.
[0008]
Actually, in the conventional example, n-type GaAs is used as the current blocking layer, but the present inventors have activated the band gap to reduce the waveguide loss and further reduce the operating current as the current blocking layer. When n-type AlInP larger than the layer was used, there was a problem that the operating voltage was abnormally increased and laser oscillation did not occur. As a result of transmission electron microscope (TEM) analysis for investigating the cause, as shown in FIG. 7 (a), dislocations are present in the region growing on the ridge side in the current blocking layer made of n-type AlInP. It was found that dislocations occurred mainly in two directions from the region toward the third upper cladding layer made of p-type AlGaInP. That is, the direction toward the inside of the region that becomes the current path above the upper end surface of the ridge and the direction toward the outside of the region. Since dislocations occurred in the former direction, the activation rate of the p-type carrier in the third upper cladding layer in that region was greatly reduced and the resistance increased, which is considered to be the cause of no laser oscillation. In the current blocking layer made of n-type AlInP, dislocation occurs in the region growing on the side surface of the ridge because the proportion of Al and In taken in on the side surface of the ridge is different from the bottom surface of the ridge. This is because AlInP formed on the side surface of the ridge does not lattice match with GaAs and has a large strain due to lattice mismatch.
[0009]
On the other hand, the operating voltage was normal when the GaAs contact layer was formed instead of the third upper cladding layer made of p-type AlGaInP on the ridge upper surface and the current blocking layer just like the conventional general structure.
[0010]
In the conventional example, a semiconductor laser made of AlGaInP is described. However, the present inventors have described a lower cladding layer, an active layer, a first upper cladding layer, a second upper cladding layer, a current blocking layer, and a third upper cladding layer. When a semiconductor laser made of AlGaAs was fabricated and operated, the same problem that laser oscillation occurred but the operating voltage increased was caused. As a result of the TEM analysis for investigating the cause, as shown in FIG. 7B, no dislocation occurs in the current blocking layer made of n-type AlGaAs, but the current blocking layer and the p-type AlGaAs first. Dislocations occurred in the direction from the vicinity of the interface between the three upper cladding layers toward the region of the current path above the upper surface of the ridge in the third upper cladding layer. It is considered that the increase in operating voltage is caused by the decrease in the activation rate of the p-type carrier in the third upper cladding layer in that region and the increase in resistance. The reason why dislocation occurs near the interface between the current blocking layer and the third upper cladding layer made of p-type AlGaAs is considered to be because the current blocking layer having a high Al composition is oxidized.
[0011]
On the other hand, the operating voltage was normal when the GaAs contact layer was formed instead of the third upper cladding layer made of p-type AlGaAs on the ridge upper surface and the current blocking layer just like the conventional general structure.
[0012]
As described above, when the current blocking layer contains Al, there is a problem that the crystallinity of the third upper cladding layer is deteriorated and the operating voltage is increased.
[0013]
In order to solve the above-described conventional problems, an object of the present invention is to provide a buried ridge type semiconductor laser having a low operating voltage and high efficiency.
[0014]
[Means for Solving the Problems]
To achieve the above object, the semiconductor lasers of the present invention includes a first conductive type lower cladding layer of the first conductivity type semiconductor substrate, an active layer and a first upper cladding layer of a second conductivity type, A second conductivity type second upper cladding layer having a ridge shape is formed, and a current blocking layer having a refractive index lower than that of the second upper cladding layer and containing Al is formed so as to sandwich the ridge, and the ridge and the current are formed. a semiconductor laser third upper cladding layer of a second conductivity type having a larger band gap than the active layer is formed on the blocking layer, Ri said ridge inverted mesa cross section der, the current blocking layer Is thinner than the ridge and covers the side surface of the ridge, and the end of the current blocking layer and the third upper cladding layer formed on the side surface of the ridge Interface, characterized that you have been formed so as not to face the said ridge upper face an upper region serving as a current path of the third upper cladding layer.
[0015]
In the semiconductor laser, the reverse mesa cross-sectional shape is preferably in a range in which an angle on the ridge side of a surface of the ridge contacting the second upper cladding layer with respect to the substrate is greater than 90 ° and not greater than 135 °.
[0016]
The current blocking layer is preferably formed closer to the first conductivity type semiconductor substrate than the upper surface of the ridge.
[0017]
The second upper cladding layer comprises a AlGaIn P, the current blocking layer comprises AlGaInP or AlIn P, the third or upper cladding layer comprises AlGaInP or AlIn P, or, wherein the second upper cladding layer It is preferable that AlGaAs is included, the current blocking layer includes AlGaAs or AlAs, and the third upper cladding layer includes AlGaAs or AlAs .
Preferably, the second upper cladding layer includes AlGaAs, the current blocking layer includes AlGaAs or AlAs, and the third upper cladding layer includes AlGaAs or AlAs.
[0018]
【Example】
Embodiments of the present invention will be described below with reference to the drawings.
[0019]
(Example 1)
1 and 2A to 2D are a schematic sectional view of a semiconductor laser according to an embodiment of the present invention and an explanatory view showing a manufacturing method. By the first MOCVD growth, a GaAs buffer layer 102 made of n-type GaAs and a lower cladding layer 103 made of n-type Al _0.5 Ga _0.5 As (n = 1 ××) on an n-type GaAs substrate 101. 10 ¹⁸ cm ⁻³ , film thickness 2.5 μm), and Al _0.3 Ga _0.7 As layer (film thickness 5 nm) and Al _0.1 Ga _0.9 As layer (film thickness 8 nm) quantum well structure An active layer 104, a first upper cladding layer 105 (p = 7 × 10 ¹⁷ cm ⁻³ , film thickness 0.1 μm) made of p-type Al _0.5 Ga _0.5 As, and p-type Al _{0. .2} Etching stopper layer 106 (p = 1 × 10 ¹⁸ cm ⁻³ , film thickness 10 nm) made of Ga _0.8 As layer and second upper cladding layer made of p-type Al _0.5 Ga _0.5 As 107 (p = 1 × 10 ¹⁸ cm ⁻³ And an intermediate contact layer 108 (p = 1 × 10 ¹⁸ cm ⁻³ , film thickness 5 nm) made of a p-type Al _0.2 Ga _0.8 As layer (FIG. 5). 2 (a)).
[0020]
Thereafter, a striped SiN mask 109 is formed by photolithography in a direction in which a ridge stripe having a reverse mesa shape (trapezoid with a short base) can be formed, for example, the [011] direction, and the etching stopper layer 106 is formed using a hydrofluoric acid-based etchant. Etching was performed to form a reverse mesa-shaped ridge (FIG. 2B). For the inverted mesa shape, the range in which the ridge angle of the surface where the ridge is in contact with the second upper cladding layer with respect to the substrate is greater than 90 ° and not greater than 135 ° is preferable.
[0021]
In the second MOCVD growth, a current blocking layer 110 (n = 1 × 10 ¹⁸ cm ⁻³ , film thickness 0.7 μm) made of n-type Al _0.56 Ga _0.44 As and an n-type GaAs cap layer 111 ( n = 1 × 10 ¹⁸ cm ⁻³ , film thickness of 0.1 μm) was selectively grown (FIG. 2C).
[0022]
Next, after removing the SiN mask 109 by etching, the third upper cladding layer 112 (p = 1 × 10 ¹⁸ cm ⁻³ , film made of p-type Al _0.5 Ga _0.5 As is formed by the third MOCVD growth. The p-type GaAs contact layer 113 was grown (thickness: 1.6 μm) (p = 1 × 10 ¹⁹ cm ⁻³ , film thickness: 3.0 μm) (FIG. 2D).
[0023]
FIG. 3 shows a TEM analysis result in the vicinity of the ridge in this example. It can be seen that the dislocation caused by the current blocking layer 110 is directed out of the region serving as a current path above the upper end surface of the ridge. This is because the ridge has an inverted mesa shape and the crystal orientation of the current blocking layer 110 grown on the side surface of the ridge is different from that in the normal mesa. Therefore, since the activation rate of the p-type carrier in the region of the third upper cladding layer that becomes the current path above the upper end surface of the ridge is improved as compared with the forward mesa, the operating voltage can be reduced. Actually, the operating voltage of this element was 2.5 V or more at an optical output of 90 mW when the ridge has a forward mesa shape, but in this example, the operating voltage could be greatly reduced to 1.9 V. As a result, heat generation in the vicinity of the active layer could be reduced. As a result, heat saturation of the laser due to heat generation was improved, and the optical output could be increased from 90 mW to 120 mW. Further, in order to further suppress dislocations in the region of the third upper cladding layer that becomes the current path above the ridge upper end surface, it is desirable that the current blocking layer be formed only on the substrate side from the ridge upper end surface. . This is because when the current blocking layer is also formed on the p-type GaAs contact layer side from the top surface of the ridge, the dislocation starts from that region into the region that becomes the current path above the top surface of the ridge in the third upper cladding layer This is because of this.
[0024]
【The invention's effect】
As described above, according to the present invention, since the ridge has an inverted mesa shape, the direction of dislocation generated in the third upper cladding layer can be directed to the outside of the ridge. Accordingly, it is possible to improve the crystallinity of a region of the third upper cladding layer which becomes a current path above the upper end surface of the ridge. Therefore, a buried ridge type semiconductor laser having a low operating voltage and high efficiency can be realized.
[Brief description of the drawings]
1 is a schematic cross-sectional view of a semiconductor laser according to an embodiment of the present invention. FIGS. 2A to 2D are process cross-sectional views illustrating a method of manufacturing the semiconductor laser shown in FIG. FIG. 4 is a schematic cross-sectional view of a conventional general semiconductor laser. FIG. 5 is a schematic cross-sectional view of a conventional semiconductor laser. FIG. 7D is a process cross-sectional view showing the method of manufacturing the semiconductor laser shown in FIG. 5. FIG. 7 is an explanatory cross-sectional view showing the direction of dislocation generation in the conventional example.
101, 301 n-type GaAs substrate 102, 302 n-type GaAs buffer layer 103 n-type AlGaAs lower cladding layer 104, 204, 304 active layer 105 p-type AlGaAs first upper cladding layer 106, 206, 306 etching stop layer 107 p-type AlGaAs Second upper cladding layer 108 p-type AlGaAs intermediate contact layer 109 SiN mask 110 n-type AlGaAs current blocking layer 111 n-type GaAs cap layer 112 p-type AlGaAs third upper cladding layer 113 p-type GaAs contact layer 201 substrate 202 buffer layer 203 lower part Cladding layer 205 First upper cladding layer 207 Second upper cladding layer 208 Intermediate contact layer 209 Current blocking layer 210 p-type GaAs contact layer 303 n-type AlGaInP lower cladding layer 305 p AlGaInP first upper cladding layer 307 p-type AlGaInP second upper cladding layer 308 p-type GaInP intermediate contact layer 309 SiO ₂ mask 310 n-type GaAs current blocking layer 311 p-type AlGaInP third upper cladding layer 312 p-type GaInP intermediate band gap layer

Claims (5)

A first conductivity type lower cladding layer, an active layer, a second conductivity type first upper cladding layer, and a second conductivity type second upper cladding layer having a ridge shape are formed on a first conductivity type semiconductor substrate. A current blocking layer having a refractive index lower than that of the second upper cladding layer and containing Al is formed so as to sandwich the ridge, and a band gap larger than that of the active layer is formed on the ridge and the current blocking layer. A semiconductor laser in which a third upper cladding layer of a second conductivity type is formed,
Wherein Ri ridge inverted mesa cross section der,
The current blocking layer is thinner than the height of the ridge and is formed so as to cover the side surface of the ridge;
An interface between the end portion of the current blocking layer formed on the side surface of the ridge and the third upper cladding layer does not face a region serving as a current path above the upper end surface of the ridge in the third upper cladding layer. a semiconductor laser which is characterized that you have been formed. 2. The semiconductor laser according to claim 1, wherein the reverse mesa cross-sectional shape is such that an angle on the ridge side of a surface of the ridge in contact with the second upper cladding layer with respect to the substrate is greater than 90 ° and not more than 135 °. . 3. The semiconductor laser according to claim 1, wherein the current blocking layer is formed closer to the semiconductor substrate of the first conductivity type than an upper surface of the ridge. Wherein the second upper cladding layer is AlGaIn P, wherein the current blocking layer is AlGaInP or AlIn P, the semiconductor laser according to any one the third upper cladding layer of claim 1 comprising AlGaInP or AlIn P . 4. The semiconductor laser according to claim 1, wherein the second upper cladding layer includes AlGaAs, the current blocking layer includes AlGaAs or AlAs, and the third upper cladding layer includes AlGaAs or AlAs.

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