JP2940106B2 - Manufacturing method of semiconductor laser - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for manufacturing a semiconductor laser, particularly a distributed feedback semiconductor laser (DFB).

[Summary of the Invention]

The present invention relates to a method for manufacturing a semiconductor laser, and relates to a semiconductor substrate having a (100) crystal plane on which stripe-shaped mesa projections are formed along a direction where an angle θ with respect to a crystal axis direction is 0 ° <θ <45 °. On the main surface of the above, at least the first conductivity type clad layer, the active layer, and the second conductivity type clad layer are sequentially separated from the active layer on the mesa projection by the active layer on the other part.
The epitaxial growth is performed so as to be arranged between the first and second conductivity type cladding layers, and a diffraction grating is formed on the side surface of the epitaxial growth layer, thereby improving the reliability and productivity by simplifying the manufacturing method. Measure.

[Conventional technology]

As a semiconductor laser that can realize single wavelength oscillation, DF
There is a B laser.

In this DFB laser, a diffraction grating, that is, a grating is formed near an active layer constituting a light emitting operation region. Therefore, in general, in manufacturing this type of DFB semiconductor laser, the epitaxial growth of each semiconductor layer is divided before and after the etching operation for forming the grating.

On the other hand, in the case of a semiconductor laser having a low threshold current I _th, even in the case of forming a buried structure on the stripe to form a light emitting operation region of the active layer, the buried structure or optical and carrier Is generally divided into two epitaxial growth operations with an etching operation for forming a groove for obtaining a structure for confining the trench.

However, when there is an interface divided by two epitaxial growths near the active layer forming portion, the surface of the previous epitaxial growth is naturally oxidized between the two epitaxial growths, resulting in deterioration of characteristics. Inconveniences such as occur.

On the other hand, as a semiconductor laser having a low threshold current I _th , an SDH (Separated Double Hetero Junction) which can be formed by a single epitaxial growth operation.
ion) semiconductor laser is disclosed in, for example,
-183987.

On the other hand, the present applicant has previously disclosed in Japanese Patent Application Laid-Open No.
An H-type semiconductor laser was proposed. As shown in a schematic enlarged cross-sectional view of an example of this SDH type semiconductor laser in FIG. 2, first, for example, a GaAs compound semiconductor substrate having a first conductivity type, for example, n-type and one principal surface having a (100) crystal plane. A stripe-shaped mesa projection (12) extending in the <011> crystal axis direction perpendicular to the plane of the paper in FIG. 2 is formed on one main surface of (1), and a substrate (1) having the projection (12) is formed. The first conductivity type, eg, n, is successively formed on one principal surface by a normal MOCVD (chemical vapor deposition using an organic metal) method, that is, a methyl MOCVD method.
Type cladding layer (3), a low impurity concentration or undoped active layer (4), a second conductivity type such as a p-type first cladding layer (5), and a first conductivity type such as an n-type current blocking layer ( 6), a second conductive type, for example, a p-type second cladding layer (7), and a second conductive type cap layer (8) are formed by a single epitaxial growth.

Here, the first conductivity type cladding layer (3), the second conductivity type first and second cladding layers (5) and (7), and the first conductivity type current blocking layer (6) are active. It is made of a material having a larger band gap, that is, a smaller refractive index than the layer (4).

In this case, the crystal orientation of the substrate (1) and the stripe-shaped mesa projections (12), the width and height of the projections (12), ie, the depths of the mesa grooves on both sides thereof, and the cladding layer of the first conductivity type (3) By selecting the thickness of the active layer (4), the second conductive type first clad layer (5), etc., the first conductive type clad layer (3) is formed on the mesa protrusion (12). The active layer (4) and the first cladding layer (5) of the second conductivity type are separated from those on the mesa trench by forming slopes by the slopes (9A) and (9B). (9
The epitaxial growth layer (30) in the form of stripes separated by B) is formed in the shape of a mesa protrusion (12).

This is based on the fact that in the case of the normal MOCVD method, that is, the MOCVD method using a methyl-based organic metal as a source gas, once a (111) B crystal plane is formed, epitaxial growth is unlikely to occur on this plane. Thus, a stripe-shaped epitaxial growth layer (30) is formed. In this case, the current block layer (6) is divided on both sides of the stripe-shaped epitaxial growth layer (30) by the stripe-shaped epitaxial growth layer (30). The divided striped active layer (4) is brought into contact with both end faces, that is, end faces facing the slopes (9A) and (9B).

In this way, the active layer (4) in the stripe-shaped epitaxial growth layer (30) on the mesa protrusion (12) is formed so as to be sandwiched by the current blocking layer (6) having a smaller refractive index than the active layer (4). The confinement is performed to form a light emitting operation region, and the presence of the current blocking layer (6) provides a second cladding layer (7) of the second conductivity type on both sides of the striped epitaxial growth layer (30). The block layer (6), the second conductive type first clad layer (5), and the first conductive type clad layer (3) form a pnpn thyristor, The current here is blocked, so that the current is concentrated on the active layer (4) of the stripe-shaped epitaxial growth layer (30) on the mesa protrusion (12), so that the threshold current can be reduced. It has been made to work.

[Problems to be solved by the invention]

Further, regarding such an SDH type semiconductor laser, the present inventors have previously disclosed in Japanese Patent Application No. 2-20086 a schematic perspective view and a manufacturing process diagram of an example thereof in FIGS. 3 and 4. As shown in the figure, the upper surface or side surface of the stripe-shaped mesa protrusion (2) formed along the <011> crystal axis direction on the principal surface (11A) of the (100) crystal plane of the compound semiconductor (11), or A main surface (11A) having a diffraction grating (13) formed by arranging a number of parallel grooves extending on the upper surface and side surfaces, in this case, the direction perpendicular to the direction in which the stripes extend, and having the stripe-shaped mesa projections (2). A first conductivity type clad layer (3), an active layer (4), and a second conductivity type clad layer (5) are sequentially epitaxially grown, and the first conductivity type clad layer (5) is formed on the stripe-shaped mesa protrusion (2). A cladding layer (3);
There has been proposed a semiconductor laser in which an active layer (4) and a cladding layer (5) of the second conductivity type are separated from other portions to form a stripe-shaped epitaxial growth layer (30).
In the case of such a structure, the active layer (4) has the advantage of having a low threshold current I _th because of adopting the SDH structure.
The existence of the diffraction grating (13) near
A semiconductor laser emitting a single wavelength by the B operation can be configured.

After the formation of the stripe-shaped mesa projections (2) and the formation of the diffraction grating (13), the semiconductor layers (3) and (4) are formed.
Since (5) can be formed by a series of epitaxial growth, a stable semiconductor laser having excellent characteristics can be obtained.

For the formation of the diffraction grating (13) composed of parallel grooves on such a stripe-shaped mesa projection (2), a known interference exposure method using two light beams can be applied. That is, for example, after forming the mesa protrusion (2), a photoresist layer is formed on the entire surface, and one coherent, for example, a semiconductor laser beam is divided into two light fluxes, and this is again divided into two layers. Exposure is performed using the interference fringes concentrated on a predetermined portion.
In this case, the resist is patterned by selecting the wavelength of light, and then the compound semiconductor (11) is etched by using the resist as an etching mask to form grooves arranged in parallel, thereby forming the diffraction grating (13), that is, the grating. Can be formed.

However, when a diffraction grating is formed by such a method, a complicated apparatus for interference exposure is required,
There is a risk of increasing the number of steps.

The method for manufacturing a semiconductor laser according to the present invention aims to improve reliability and productivity by simplifying the manufacturing apparatus and reducing the number of steps as described above.

[Means for solving the problem]

1A to 1C show an example of a method for manufacturing a semiconductor laser according to the present invention.
D is shown in the manufacturing process diagram.

As shown in FIG.
1> A main surface (1A) of a semiconductor substrate (1) having a (100) crystal plane on which stripe-shaped mesa protrusions (2) are formed along a direction in which an angle θ with respect to a crystal axis direction is 0 ° <θ <45 °. )
As shown in the schematic cross-sectional view of FIG. 1D, a cladding layer (3) of the first conductivity type, an active layer (4), and a cladding layer (5) of the second conductivity type are sequentially formed thereon. ), The active layer (4) on the mesa protrusion (2) is separated from the active layer (5) on the other part, and is disposed between the first and second conductivity type cladding layers (3) and (5). Epitaxial growth as
The diffraction grating (1
3) Form.

[Action]

As described above, according to the manufacturing method of the semiconductor laser of the present invention, the angle θ with respect to the <011> crystal axis direction is set to 0 ° <θ <4.
A semiconductor substrate (1) having a (100) crystal plane on which stripe-shaped mesa protrusions (2) are formed along a direction of 5 ° is prepared, and a stripe-shaped epitaxial growth layer (30) is sequentially formed on the main surface (1A). At this time, the stripe-shaped epitaxial growth layer (30) is made of a normal MOCV
Method D, that is, M performed using a methyl-based organic metal as a source gas
In the case of the OCVD method, the direction in which the stripe-shaped mesa protrusions (2) extend is inclined by θ degrees from the <011> crystal axis direction, so that the first (111) B crystal plane is generated spontaneously. FIG. B shows a perspective view of one of the manufacturing steps, as shown in FIG.
0) is formed. Therefore, the active layer (4) also has a stepped or saw-toothed side surface, and a diffraction grating (13) is formed. As described above, the generation of the step-like or saw-tooth-like slope (10) is achieved by setting the above-mentioned angle θ to 0 ° <θ <45.
When it was selected as ゜, it was confirmed that it occurred favorably.

The pitch of the diffraction grating (13) can be controlled by changing the angle θ of the stripe-shaped mesa projections (2) from the <011> crystal axis direction.

According to such a method, the diffraction grating (13) can be obtained only by forming the stripe-shaped mesa protrusion (2) inclined at an angle of θ from the <011> crystal axis direction. A DFB laser can be obtained without increasing the number of steps, and reliability and productivity can be improved.

〔Example〕

Hereinafter, the production method of the present invention will be described in detail with reference to FIGS.

In this case, in the case of obtaining an AlGaAs III-V compound semiconductor laser, first, a semiconductor substrate (1) made of a first conductivity type, for example, an n-type GaAs substrate is prepared. This semiconductor substrate (1) has one main surface (1A) having a (100) crystal plane, and a stripe-shaped etching mask layer (not shown) having a required width W on the main surface (1A). <011> An inclination in the range of 0 ° <θ <45 ° from the crystal axis direction, for example, θ = 5
沿 っ is formed along the direction of ゜.
A) From the side, for example, H ₂ SO ₄ , H ₂ O ₂ and H ₂
Crystallographic etching is performed with an etchant in which O is mixed at a ratio of 3: 1: 1. In this case, etching proceeds from a portion not covered by the mask layer, and the first
As shown in FIG. A, a mesa groove (22) is formed along a direction inclined by 5 ° from the <011> crystal axis direction, whereby the ridge line (2A) of the stripe-shaped mesa protrusion (2) is formed by the <011> crystal. It is formed in a direction inclined by 5 ° from the axial direction.

Next, after removing the etching mask, as shown in a schematic perspective view in FIG. 1B, the first conductivity type, for example, n-type, is obtained by a normal MOCVD method, that is, a MOCVD method using a methyl-based organic metal as a source gas. The Al _x Ga _1-x As clad layer (3) is epitaxially grown. In this case, the epitaxial growth is performed in the mesa groove (22) and on the mesa protrusion (2). When the epitaxial growth proceeds, the angle of the upper surface of the mesa protrusion (2) with respect to the (100) crystal plane is about 55.
A slope (10) composed of a (111) B crystal plane forming 生 じ naturally occurs. However, the ridge line (2A) of the mesa protrusion (2) is on a line where the (111) B crystal plane and the (100) crystal plane formed by the main surface (1A) intersect, that is, <011> is inclined by 5 ° from the crystal axis direction. Therefore, the slope (10) is formed in a stepped or sawtooth shape with small irregularities, whereby a diffraction grating (13) can be formed.

In this case, the degree of this step, that is, the pitch of the diffraction grating, can be changed in proportion to the angle θ between the mesa projection (2) and the <011> crystal axis direction. Therefore, by controlling the angle θ, the pitch of the diffraction grating (13) is changed according to each transmission wavelength, and the DF that forms good single-wavelength oscillation is formed.
B laser can be obtained.

Then, while forming such a step-like slope (10), as shown in the schematic perspective view of FIG.
For example, an active layer (4) made of undoped Al _y Ga _1-y As is epitaxially grown. In this case, since the epitaxial growth layer by the methyl MOCVD method is hardly generated on the (111) B crystal plane formed by the slope (10), the active layer (4) is hardly grown on the slope (10). , On the mesa protrusion (2) and the cladding layer (3) on the mesa groove (22) on both sides thereof. Following this, for example, p-type Al _x Ga _{1 -x} As of the second conductivity type
Is continuously epitaxially grown by the methyl-based MOCVD method. In this case, the growth of the p-type cladding layer (5) proceeds to a thickness at which the slopes (10) on both sides of the mesa projection (2) intersect.

On the other hand, FIGS. 1B and 1C are perspective views excluding the epitaxially grown layers other than the stripe-shaped epitaxially grown layers on the mesa projections (2).
In FIG. 1D, as shown in a schematic cross-sectional view of FIG. 1D, the p-type clad layer ( 5) The growth in the mesa groove (22) is performed.

Next, a current block layer (6) made of, for example, n-type Al _x Ga _{1 -x} As having the same composition as the n-type, ie, first conductivity type cladding layer (3), is similarly epitaxially grown by a methyl MOCVD method. In this way, little growth occurs on the slope (10) formed by the (111) B plane,
A current block layer (6) is formed only on the mesa groove (22), and a stripe-shaped epitaxial growth layer (30) is formed on the mesa protrusion (2). The current blocking layer (6) can be formed so that the opposing end face abuts on the end face of 4).

Further, the second conductive type clad layer (5) and the second conductive type clad layer (7) made of, for example, p-type Al _x Ga _1-x As having the same composition and a high conductive layer made of GaAs of the same conductive type. The cap layer (8) having the impurity concentration is sequentially formed with a methyl-based or ethyl-based MO.
Epitaxial growth by CVD method. In this case, when the methyl-based MOCVD method is used, the cladding layer (7) of the second conductivity type hardly grows on the slope (10) formed by the (111) B crystal plane at the initial stage, but it grows on the mesa groove (22). As the growth progresses, when a crystal plane other than the (111) B crystal plane is generated at the abutting portion of both slopes (10),
The cladding layer (7) and the cap layer (8) of the second conductivity type are entirely formed across the stripe-shaped epitaxial growth layer (30). Then, although not shown, opposing electrodes are ohmically deposited on the cap layer (8) and the back surface of the semiconductor substrate (1), respectively, to obtain a desired DF.
A B semiconductor laser is obtained.

In this case, each of the layers (3) to (8) can be continuously formed only by changing the type of the impurity source gas, the group III-V source gas and the ratio thereof in each MOCVD method. Here, trimethyl aluminum and trimethyl gallium as a raw material gas in the methyl system MOCVD method, it is possible to use arsine AsH _3, also ethyl system MO
It can be performed triethyl aluminum, triethyl gallium, by arsine AsH ₃ in a CVD method.

In addition, the composition Al _x Ga _{1 -x} As of the cladding layers (3), (5) and the current blocking layer (6) is x> with respect to the composition Al _y Ga _{1 -y} As of the active layer (4). The band gap of each of the layers (3), (5), (6) and (7) is selected to be larger than the band gap of the active layer (4).

According to the DFB type and SDH type semiconductor laser having the above configuration, the epitaxial growth layer (30) on the mesa protrusion (2)
Is surrounded by a cladding layer (3) (5) and a current blocking layer (6) having a bandgap larger than that of the active layer (4) on the upper, lower, and both end faces thereof. , Light and carriers are confined, and the threshold current I _th is reduced by current concentration by the current blocking layer (6), and a diffraction grating ( 13) to form this diffraction grating (1
A single wavelength launch can be realized by Bragg reflection according to 3).

In the example described above, the semiconductor substrate (1) is Ga
In the case of As, single wavelength oscillation is performed by absorption coupling. However, the substrate (1) is not limited to this GaAs-based material, and may have a configuration in which, for example, an InP substrate is used. The operation is performed as a rate couple.

〔The invention's effect〕

According to the manufacturing method of the present invention described above, the stripe-shaped mesa projections (2) are formed along the direction where the angle θ with respect to the <011> crystal axis direction is 0 ° <θ <45 °, for example, several degrees (100).
The stripe-shaped epitaxial growth layer (30) is spontaneously formed on the main surface (1A) of the semiconductor substrate (1) having a crystal plane simply by MOCVD using a methyl-based organic metal as a source gas. A slope (10) having a diffraction grating (13) can be formed stepwise or in a sawtooth shape by the generated (111) B crystal plane.

That is, according to the method of the present invention, the diffraction grating (13) is formed by utilizing the peculiarity of crystal growth, so that the diffraction grating (13) having a predetermined pitch can be easily and reliably formed into the stripe-shaped mesa projection (2).
<011> can be controlled by changing the angle θ from the crystal axis direction.
That can oscillate the required single wavelength by Bragg reflection
The B laser can be obtained reliably.

In addition, according to such a method, since it is only necessary to form the diffraction grating (13) and then perform one continuous epitaxial growth, a stable DFB without changing the film quality and without increasing the number of steps. A laser can be easily obtained, and reliability, productivity, and yield can be improved.

[Brief description of the drawings]

1A to 1D are manufacturing process diagrams showing an example of a method for manufacturing a semiconductor laser according to the present invention, FIG. 2 is a sectional view showing a conventional semiconductor laser, FIG. 3 is a perspective view showing an example of a conventional semiconductor laser,
FIG. 4 is a perspective view showing one manufacturing process of a conventional semiconductor laser. (1) is a semiconductor substrate, (1A) is a main surface, (2) is a mesa projection, (22) is a mesa groove, (2A) is a ridgeline, (3) is a first conductivity type cladding layer, and (4) is a An active layer, (5) a cladding layer of the second conductivity type, (6) a current blocking layer, and (7) a second
A conductive clad layer, (8) is a cap layer, (10) is a slope, (13) is a diffraction grating, and (30) is an epitaxial growth layer.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H01S 3/18 H01L 21/205

Claims (1)

(57) [Claims] <011> On a main surface of a semiconductor substrate having a (100) crystal plane on which stripe-shaped mesa projections are formed along a direction in which an angle θ with respect to a crystal axis direction is 0 ° <θ <45 °. The active layer on the mesa protrusion is separated from the active layer on the other part by sequentially separating at least the first conductive type clad layer, the active layer, and the second conductive type clad layer. A method of manufacturing a semiconductor laser, comprising: epitaxially growing so as to be arranged between two conductive type cladding layers; and forming a diffraction grating on a side surface of the epitaxial growth layer.