JPH037153B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor laser and a method for manufacturing the same.
More specifically, the present invention relates to a buried hetero (hereinafter abbreviated as BH) structure laser having a distributed feedback mechanism to enable single-axis mode oscillation, and a method for manufacturing the same.

As a semiconductor laser capable of stable single transverse mode and single axial mode oscillation, a strip-embedded hetero (Strip) as shown in the perspective view in Figure 1 is used.
Buried Hetero (hereinafter abbreviated as SBH) - Distributed Feed Back (hereinafter abbreviated as DFB) lasers have been proposed in the past. (Applied
Physics Letters, 1979, Vol. 34, No. 11, p.
752-755).

The n-type GaAs active layer 4 has a lower refractive index than the GaAs active layer 4 provided below, which extends in a stripe shape in the direction of emission of this SBH-DFB laser beam.
Al _0.15 Ga _0.85 As Feedback layer 2 and n-type Al _0.36
In the so-called SBH structure sandwiched between the Ga _0.64 As cladding layer 3, the P-type Al _0.36 Ga _0.64 As cladding layer 5 and the p-type Al _0.36 Ga _0.64 As buried layer 7, the n-type
Al _0.15 Ga _0.85 As feedback layer 2 and p-type Al _0.36
Periodic unevenness 11 at the interface with Ga _0.64 As buried layer 7
was formed.

In this structure, the laser beam oscillated in the GaAs active layer 4 leaks into the n-type Al _0.15 Ga _0.35 As feedback layer 2 and is guided, so that the GaAs active layer 4 has an effective refractive index. A gentle refractive index distribution is formed in the lateral direction. As a result, it can operate in a stable fundamental transverse mode even at high outputs.

On the other hand, n-type Al _0.15 Ga _0.85 As feedback layer 2
The light seeped out interacts with the refractive index distribution in the laser beam propagation direction due to the periodic irregularities described above, is subjected to optical feedback, and light of a specific wavelength that satisfies the bragging condition of the periodicity of the irregularities is selected. . For this reason, it is reported that the oscillation wavelength does not change and oscillation occurs in a stable single-axis mode up to a drive current that is about three times the oscillation threshold current.

However, the conventional SBC-DFB laser has a problem in that its manufacturing method is complicated and difficult. SBH−
DFB lasers are manufactured through the steps outlined below.
That is, first, n-type Al _0.36 is deposited on the n-type GaAs substrate 1.
Ga _0.64 As cladding layer 3, n-type Al _0.15 Ga _0.35 As feedback layer 2 GaAs active layer 4, p-type Al _0.36 Ga _0.64
A substrate crystal is formed by a first liquid phase epitaxial growth step in which an As cladding layer 5 and a p-type GaAs contact layer 6 are laminated. Next, by selective etching, the p-type GaAs contact layer 6 and the p-type Al _0.36 Ga _0.64
The As clad layer 5 and the GaAs active layer 4 are removed leaving stripes to form a mesa structure. Next, exposed n-type Al _0.15 Ga _0.35 As feedback layer 2
Periodic irregularities 11 are formed on the surface by a photoresist mask and ion milling method. After that, p-type Al _0.36
A Ga _0.64 As buried layer 7 and an n-type Al _0.36 Ga _0.64 As buried layer 8 are grown to cover the n-type Al _0.15 Ga _0.85 As feedback layer 2 and the exposed mesa sides.

Electrodes 9 and 10 are attached to the crystal wafer manufactured as described above, and the crystal is cut out to form a laser beam emitting end face, thereby completing an SBH-DFB laser.

Now, in the above process, the crystal layer containing Al is exposed by the etching process, but since Al is easily oxidized, the crystal surface of the growth melt is exposed during the second liquid phase epitaxial growth process. There is a problem in that the wetting is poor, and uniform epitaxial growth is not performed, resulting in poor yield.

Furthermore, since there is a step of holding the substrate crystal in a high temperature hydrogen atmosphere during the liquid phase epitaxial growth step, constituent elements of the crystal are thermally dissociated and crystallinity is impaired. As a result of these problems, the surface of the substrate crystal and the p-type Al _0.36
There are disadvantages in that reliability is poor, such as crystal defects being introduced near the interface with the Ga _0.64 As buried layer 7 and the n-type Al _0.36 Ga _0.64 As buried layer 8, resulting in a shortened lifetime as a semiconductor laser. As a means to solve these problems, in the second liquid phase epitaxial process, immediately before crystal growth, the substrate crystal is washed with an unsaturated melt, and a melt back process is performed in which the crystal surface is etched to expose a new surface. is common.

However, in the case of the conventional SBH-DFB laser,
This measure cannot be taken because the shape of the unevenness formed in the n-type Al _0.15 Ga _0.85 As feedback layer 2 will be destroyed by the meltback, and the wavelength selection effect will be degraded.

An object of the present invention is to solve the drawbacks of the conventional BH laser and obtain stable and single axial mode oscillation.
Another object of the present invention is to provide a semiconductor laser having a novel structure with a low oscillation threshold and a method for manufacturing the same.

According to the present invention, a first semiconductor layer 2 of a second conductivity type whose thickness periodically changes in one direction is formed on a semiconductor substrate 1 of a first conductivity type;
The first layer is applied to the substrate crystal on which the semiconductor layer 12 is formed.
A groove extending in a direction parallel to the direction of periodic change in the thickness of the semiconductor layer 2 is provided at a depth reaching the semiconductor substrate 1, and at least a third crystal of the first conductivity type is formed on the substrate crystal including the groove. A semiconductor layer 3, an active layer 4, and a fourth semiconductor layer 5 of a second conductivity type are formed, and the third semiconductor layer 3 and the active layer 4 are formed inside the trench, separated from the outside of the trench, and the active layer 4 has a structure in which it is in contact with the first semiconductor layer 2 inside the groove, and the first
the refractive index of the semiconductor layer 2 is lower than that of the active layer 4;
A semiconductor laser formed to be larger than any of the second semiconductor layer 12, third semiconductor layer 3, and fourth semiconductor layer 5 is obtained.

Further, in the semiconductor laser of the present invention, after forming the periodic unevenness 11 on the semiconductor substrate 1 of the first conductivity type, the first semiconductor layer 2 of the second conductivity type having a lower refractive index than the active layer 4 is formed. A first epitaxial growth step in which a second semiconductor layer 12 of a first conductivity type having a refractive index lower than that of the first semiconductor layer 2 is sequentially grown to form a substrate crystal; an etching step of forming grooves extending in parallel and deep enough to reach the semiconductor substrate 1; and etching a first conductivity type groove having a refractive index lower than that of at least the first semiconductor layer 2 on the substrate crystal including the groove. 3 semiconductor layer 3, active layer 4, and fourth semiconductor layer 5 of a second conductivity type having a lower refractive index than the first semiconductor layer 2. The third semiconductor layer 3 and the active layer 4 are arranged inside the groove and outside the groove. The active layer 4 is separated into two parts and the active layer 4 is separated into
and a second semiconductor layer formed so that the first semiconductor layer 2 and the first semiconductor layer 2 are in contact with each other.
and an epitaxial growth step of the present invention.

Hereinafter, the present invention will be described along with examples using drawings.

FIG. 2 is a perspective view of a semiconductor laser showing an embodiment of the present invention.

FIGS. 3 to 7 are process diagrams showing one embodiment of the method for manufacturing a semiconductor laser of the present invention.

As an example, a semiconductor laser with an oscillation wavelength of 1.3 μm, in which the substrate 1 is made of n-type InP and the active layer 4 is made of InGaAsP, is used, but a semiconductor laser with another oscillation wavelength may be used depending on the composition of the active layer 4. This can also be realized by the present invention. In the semiconductor laser according to the embodiment of the present invention shown in FIG.
is formed, a p-type InGaAsP feedback layer 2 (first semiconductor layer 2) is formed thereon so that the surface thereof is flat, and an n-type InP layer 12 is further formed thereon.
(Second semiconductor layer 12) A groove having a V-shaped cross section (hereinafter abbreviated as V-groove) is provided in a part of the substrate crystal formed.

The V-groove reaches the InP substrate 1, and its extending direction is the n
It is parallel to the repeating direction of the period of the type InP substrate 1.

An n-type InP cladding layer 3 (third semiconductor layer 3) and an InGaAsP active layer 4 are embedded in the V-groove, and an n-type InP layer 12 outside the V-groove is provided with an n-type n-type InP at the same time as InP cladding layer 3
A cladding layer 3' is formed. A p-type InP cladding layer 5 (fourth semiconductor layer 5) is formed on the surfaces of the InGaAsP active layer 4, the n-type InP layer 12, and the n-type InP cladding layer 3'.
type InGaAsP contact layer 6, SiO ₂ insulating film 13 for sandwiching the injection current width, and this SiO ₂ insulating film 1
p-type through the striped window opened in 3.
The p-side electrode 9 in contact with the InGaAsP contact layer 6 is formed in sequence. An n-side electrode 10 is formed on the back surface of the n-type InP substrate 1 . The InGaAsP active layer 4 has a higher refractive index than the p-type InGaAsP feedback layer 2, is formed only within the V-groove, and is in contact with the p-type InGaAsP feedback layer 2 at the side surface of the V-groove.

Note that the typical thickness of each layer is a minimum of 0.55 μm for the p-type InGaAsP feedback layer 2 and a minimum thickness of 0.55 μm for the n-type InP layer 12.
is 0.4μm. The thickest part of the n-type InP cladding layer 3 is 1.5 μm, the InGaAsP active layer 4 is 0.15 μm, the p-type InP cladding layer 5 is 1.2 to 1.5 μm, and the p-type InGaAsP contact layer 6 is 1.0 μm, which are formed outside the V groove. n-type
The InP cladding layer 3' has a thickness of 0.3 μm. V-groove is approximately deep
2 μm, width approximately 3 μm. Furthermore, in order to ensure wavelength selectivity, the laser beam emitting end is not a cleavage plane, but a rough surface formed by cutting out the crystal.

As can be seen from the above explanation, in the semiconductor laser of the present invention, the InGaAsP active layer 4 is a p-type layer with a refractive index smaller than itself and a large forbidden band width.
It is completely surrounded by an InGaAsP feedback layer 2, an n-type InP cladding layer 3, and a p-type InP cladding layer 5 except in the direction in which the laser beam is emitted. Furthermore, p-type InGaAsP feedback layers 2 are present on both sides of the V-groove.
The conductivity type of this part is p-p-n-n-p- from the top.
It has a structure called n that makes it difficult for current to flow. Therefore, in the semiconductor laser of the present invention, the p-side electrode 8 to n
When a forward current is passed to the side electrode 9, current and light are concentrated in the InGaAsP active layer, which oscillates with a low threshold current of 100 mA or less, allowing highly efficient operation.

On the other hand, the axial mode of the semiconductor laser of the present invention is p
As a result of the novel structure introducing the type InGaAsP feedback layer 2, it is extremely stable. That is, a part of the oscillated laser light is transferred from the InGaAsP active layer 4 to the p-type InGaAsP feedback layer 2 with a slightly lower refractive index.
Under the influence of the periodicity of the layer thickness of the p-type InGaAsP feedback layer 2, the laser beam with a wavelength that satisfies the bragging condition of the period is intensified when passing between the two emission end faces. The light propagates and produces light with a specific oscillation wavelength. In this example, p
A composition with a light wavelength of 1.05 μm was used as the InGaAsP feedback layer 2, and a secondary lattice with a period of 3920 Å was used, resulting in oscillation at a wavelength of about 1.30 μm. Although the oscillation transverse mode was of course a single mode, the effect of the present invention also made the axial mode a single mode, and its temperature stability was about 1 Å/°C, which is higher than the conventional one.

Next, the manufacturing method of the present invention will be explained based on the same embodiment as described above. A structural feature of the semiconductor laser of the present invention is that the thickness of the first semiconductor layer 2 of the second conductivity type changes periodically and functions as a feedback layer. As a means of realizing such a structure, a p-type InGaAsP feedback layer 2 is formed on an n-type InP substrate 1 having a flat surface, and then the surface is roughened by etching to form a substrate crystal. is possible.

However, this method has the disadvantage that the irregularities are deformed during the next crystal growth step, disrupting the periodicity and reducing the effect of wavelength selection. In order to solve this problem, in the manufacturing method of the present invention, first, irregularities are formed on the n-type InP substrate 1, and then the p-type InGaAsP feedback layer 2 is formed thereon.

Hereinafter, the manufacturing method of the present invention will be explained step by step.

First, the n
A photoresist film is applied to the surface of the type InP substrate 1.

Next, an interference fringe pattern of He--Cd laser light is created using a photogram and exposure is performed. After development, it is etched with an etching solution containing hydrochloric acid as a main component to form periodic irregularities 11 on the surface (FIG. 4). The depth of the unevenness was 500 Å.

After thoroughly cleaning the n-type InP substrate 1 prepared as described above, the p-type is grown by liquid phase epitaxial growth.
An InGaAsP feedback layer 2 is grown so that its surface is flat, and an n-type InP layer 12 is grown thereon (FIG. 5). The closer the refractive index of the p-type InGaAsP feedback layer 2 is to the refractive index of the InGaAsP active layer, the more light leaks out and the axial mode stability increases. However, the amount of light that leaks out of the active region is wasted and the oscillation threshold current increases. Therefore, p-type
The composition of the InGaAsP feedback layer is determined with these considerations in mind. For example, the oscillation wavelength is 1.3μm
In this case, it is appropriate that the wavelength of light corresponding to the forbidden band width is about 1.0 to 1.2 μm. When the p-type InGaAsP feedback layer 2 grows to a thickness of about 0.2 μm, it becomes uneven 1.
Although the surface can be made flat by completely filling the layer 1, the layer is grown to a thickness of 0.5 to 0.6 μm in consideration of controllability of the position of the active layer in the second liquid phase epitaxial growth step. Next, on the n-type InP layer 12 by CVD method etc.
A SiO ₂ film is attached, and a striped SiO ₂ film is formed in the [01 1] direction using a normal photoresist method. Using this SiO ₂ film as a mask, etching is performed with a mixture of hydrochloric acid and phosphoric acid to form a V-groove. The depth of the V-groove is controlled so that it reaches the n-type InP substrate 1.
Next, after removing the SiO ₂ mask (FIG. 6) and thoroughly cleaning the substrate crystal, the following four layers are grown by a second liquid phase epitaxial process. That is, an n-type InP cladding layer 3, an InGaAsP active layer 4, a p-type InP cladding layer 5, and a p-type InGaAsP contact layer 6 are grown in sequence. At this time, the n-type InP cladding layer 3 also grows on the surface of the n-type InP layer 10 outside the V-groove. the
However, the InGaAsP active layer 4 can be grown only inside the V-groove. The InGaAsP active layer 4 is located at the p-type InGaAsP feedback layer 2 as described above.
Grow in a controlled manner so that it is in contact with. Also, p-type
The InP cladding layer 5 fills the V-groove and grows until the surface of the substrate crystal becomes almost flat.

In the manner described above, a crystal having a structure as shown in FIG. 7 is obtained. Next, a SiO ₂ film 7 is deposited on the surface of the p-type InGaAsP contact layer 6 by a CVD method or the like, and a striped window is opened just above the active layer by a photoresist method. Depositing metal on top of it
A side electrode 8 is obtained. On the other hand, metal is also deposited on the back surface of the n-type InP substrate 1 to form the n-side electrode 9. The semiconductor laser of the present invention is manufactured as described above (second
figure).

As described above, according to the present invention, a BH laser with a low oscillation threshold current, high efficiency, and stable oscillation in a single transverse mode and a single axis mode can be obtained.

As described above, the mechanism for stabilizing and unifying the axial mode is similar to the conventional SBH-DFB laser and the so-called distributed feedback laser. However, unlike in the case of the conventional example, since the active layer is not directly formed with unevenness, the crystallinity is not impaired and there is an advantage that a highly reliable semiconductor laser can be easily obtained.

As described above, unlike the conventional example, the BH laser of the present invention grows the active layer 4 in the second epitaxial growth process, so there is no problem of thermal dissociation and the laser has excellent crystallinity. Furthermore, when crystals are grown in the V-groove as in the example, no problem was found that the crystal growth melt does not get wet.

However, in the case of the BH laser of the present invention, even if meltback is performed in the second epitaxial growth step, the periodic asperities 11 are not exposed, so there is an advantage that the shape of the asperities is not affected.
Therefore, according to the BH laser of the present invention, the conventional example
It is possible to obtain devices with higher reliability than with the SBH-DFB laser at a higher yield.

In the semiconductor laser of the present invention, the n-type InP layer 12 is provided during the first epitaxial growth step, but this layer not only serves to protect the surface of the P-type InGaAsP feedback layer 2; This makes it possible to reliably form a p-n-p-n reverse junction in a portion outside the V-groove. That is, the former is intended to prevent the p-type InGaAsP feedback layer 2, which is a quaternary mixed crystal, from being easily melted back in the second liquid phase epitaxial step, resulting in deterioration of crystallinity. Moreover, if the latter does not have the n-type InP layer 12, the n-type InP cladding layer 3 will grow during the second liquid phase epitaxial process.
This solves the problem that the surface of the InGaAsP feedback layer 2 is thermally damaged, resulting in poor crystallinity at the interface, making it impossible to form an ideal n-p reverse junction.

Note that the above embodiments are based on an oscillation wavelength of 1.3 μm.
Although the InGaAsP semiconductor laser has been described, the wavelength may be different as long as the requirements of the present invention are met.
It goes without saying that the same effect can be obtained even if the material is (AlGa)As or the like. Furthermore, it is clear that the first and second epitaxial growth methods may be not only liquid phase but also vapor phase or molecular beam epitaxy. The same effect as in this embodiment can be obtained with grooves having other cross-sectional shapes instead of V-grooves.

[Brief explanation of drawings]

Figure 1 is a perspective view of a conventional SBH-DFB laser.
FIG. 2 is a perspective view of a buried hetero semiconductor laser having a distributed feedback section according to the present invention. 3 to 7 are schematic process diagrams illustrating the method for manufacturing a semiconductor laser of the present invention. In each figure, 1...semiconductor substrate (n-type InP substrate, n
type GaAs substrate), 2...first semiconductor layer (p-type
InGaAsP feedback layer, n-type Al _0.15 Ga _0.85 As
feedback layer), 3...third semiconductor layer (n-type
InP clad layer, n-type Al _0.36 Ga _0.64 As clad layer),
4...Active layer (InGaAsP active layer, GaAs active layer),
5...Fourth semiconductor layer (p-type InP clad layer, p-type
Al _0.36 Ga _0.64 As cladding layer), 6... contact layer, 7... p-type Al _0.36 Ga _0.64 As buried layer, 8...
...n-type Al _0.36 Ga _0.64 As buried layer, 9... p-side electrode, 10... n-side electrode, 11... unevenness, 12...
n-type InP layer, 13...SiO ₂ insulating film. However, the numbers in parentheses are in the order of (Figure 2, Figure 1).

Claims (1)

[Claims] 1. At least a second semiconductor substrate on a first conductivity type semiconductor substrate.
A first conductivity type semiconductor layer and a first conductivity type second semiconductor layer are formed, and at least the first conductivity type is formed on a substrate crystal provided with a band-shaped groove having a depth reaching from the second semiconductor layer to the semiconductor substrate. a third semiconductor layer, an active layer, and a fourth semiconductor layer of a second conductivity type; the third semiconductor layer and the active layer are discontinuously formed inside and outside the groove; The first semiconductor layer is in contact with a first semiconductor layer, and is formed such that the first semiconductor layer has a refractive index smaller than that of the active layer and larger than any of the second semiconductor layer, the third semiconductor layer, and the fourth semiconductor layer. 1. A semiconductor laser having a structure in which the first semiconductor layer has a thickness that changes periodically in a direction in which the groove band extends. 2 After forming periodic irregularities on a semiconductor substrate of a first conductivity type, a first semiconductor layer of a second conductivity type having a refractive index lower than that of the active layer and a first conductivity layer having a refractive index lower than that of the first semiconductor layer are formed. a first epitaxial growth step of sequentially growing a second semiconductor layer of the same type to form a substrate crystal; an etching step of forming a band-shaped groove having a groove; and etching at least the first
a third semiconductor layer of a first conductivity type having a lower refractive index than the semiconductor layer; an active layer having a higher refractive index and a narrower forbidden band width than the first semiconductor layer; and a third semiconductor layer of a second conductivity type having a lower refractive index than the first semiconductor layer. A fourth semiconductor layer is formed such that the third semiconductor layer and the active layer are separated into the inside and outside of the trench, and the active layer and the first semiconductor layer 2 are in contact with each other inside the trench.
and an epitaxial growth step.