JP3242958B2 - Optical semiconductor device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical semiconductor device having a buried quantum well structure, and more particularly to an optical semiconductor device having improved characteristics such as a laser having a multiple quantum well structure.

[0002]

2. Description of the Related Art In recent years, a so-called quantum well semiconductor having at least one quantum well structure in which a well layer having a layer thickness less than the de Broglie wavelength of electrons is sandwiched between barrier layers having a wider bandgap than the well layer. Lasers are being developed. This quantum well semiconductor laser can lower the oscillation threshold value, extend the modulation band, sandwich the oscillation spectrum width, and improve the temperature characteristics as compared with a double heterostructure semiconductor laser having no quantum well in the active layer. It has various advantages such as high output.

Recently, by using a material that does not lattice-match with the substrate for the quantum well, it has become possible to increase the degree of freedom of the oscillation wavelength and further improve the characteristics. Generally, if there is a material near the active layer that does not lattice match with the substrate, a large amount of crystal defects occur, and the characteristics and reliability of the laser deteriorate significantly. However, even if a material that does not lattice-match with the substrate is used for the active layer, if the layer thickness is smaller than a certain critical thickness, the active layer is elastically distorted, and no crystal defects that deteriorate the characteristics and reliability of the laser are generated. . As a measure of the critical thickness, a calculation result by the Matthews and Blakeslee model can be used.

[0004] 0.98 employing such a strained quantum well structure
As an InGaAs / AlGaAs semiconductor laser on a μm-band GaAs substrate, a highly reliable high-output laser, an ultra-low threshold laser, and the like have been realized. In optical semiconductor devices other than semiconductor lasers, such as semiconductor laser amplifiers, optical switches, and optical modulators, the introduction of a strained optical waveguide layer allows TE
Various possibilities can be considered, such as changing the / TM mode characteristics, changing the switching characteristics and the absorption characteristics, and the like.

In the case of an InGaAs / InGaAsP semiconductor laser on an InP substrate mainly used for optical fiber communication, in order to obtain a low threshold single transverse mode oscillation, generally, the left and right sides of the active layer are separated from the active layer. A so-called buried structure in which a material having a large forbidden band width is buried is often used. In this case, since the strained quantum well layer has a strained heterointerface on the side surface, large strain and stress are generated in the vicinity thereof.

[0006] will be specifically described below as an example In _0.7 Ga _0.3 As strained quantum well structure laser on (001) InP substrate shown in FIG. 15. This semiconductor laser has an n-type
4.2n thickness on nP substrate 1 (also serving as cladding layer)
m undoped In _0.7 Ga _0.3 As well layer 2
An active layer (optical waveguide layer) 4 composed of four strained quantum wells sandwiched between undoped InGaAsP (1.2 μm composition) barrier layers 3 lattice-matched with a p-type InP cladding layer 5 having a thickness of 1.5 μm and a thickness of 0.8 μm P-type InGaAsP (1.2 μm
Composition) A laminated structure composed of a contact layer 6 is grown, and the laminated structure is embedded with an Fe-doped semi-insulating InP layer 7.

[0007] The barrier layers 3 are stacked between the well layers 2 by 12 nm, 20 nm above and below the well layers 2 at both ends, and a total thickness of 76 nm. Therefore, the total layer thickness of the active layer 4 is 92.8 nm. The width of the buried active layer 4 is 2 μm. Note that electrodes 11 and 12 for current injection are formed above and below the element substrate. Further, a laser resonator having a length of 1 mm is formed by cleavage.

The lattice constant of the InP substrate is 0.58688 nm, whereas the lattice constant of In _0.7 Ga _0.3 As is 0.593.
81 nm. Here, assuming an infinite plane without side surfaces, the well layer 2 is elastically distorted because it is as thin as 4.2 nm. The magnitude of the strain is ε _xx = ε _yy = −0.01167, ε _zz = −2 (C ₁₂
/ C ₁₁ ) ε _xx = 0.011974, ε _yz = ε _zx = ε _xy = 0. However, C ₁₂ / C ₁₁ = 0.504 is In _0.7 Ga _0.3 As
Is the Poisson's ratio. As a result, the distorted In _0.7 Ga
The lattice constant of the _0.3 As well layer 2 is equal to InP in the xy plane, and the lattice constant in the z direction is 0.60092 nm.

However, in practice, the active layer 4 is buried in a stripe having a width of 2 μm by the Fe-doped InP burying layer 7. Therefore, a lattice mismatch occurs at the boundary between the well layer 2 and the buried layer 7. Well layer 2 is thick
Since it is 4.2 nm, it will consist of about 6.99 unit cell layers on average. In the face-centered cubic zinc blende structure, <0
In the <01> direction, there are four atomic layers whose positions are shifted between lattice intervals a. Here, each of the four aligned layers (lattice constant a) is counted as a unit lattice layer.

On the other hand, when this thickness is divided by the lattice constant of InP, it consists of 7.16 unit lattice layers. Well layer 2 is 4
Since there are layers, lattice mismatch of a total of 0.68 unit lattice layers occurs on the side surface of the active layer 4. Since the width of the active layer 4 is as thick as 2 μm, the entire active layer is not elastically compressed. As a result, lattice defects such as dislocations are likely to occur near the side surface of the active layer 4.

Since the lattice mismatch on the side surface of the upper strained quantum well layer is larger than 0.5 unit lattice layer, it is better to connect the lattice surface of the strained quantum well layer to the lattice surface immediately above the buried layer on the side surface. Close, the probability of dislocation is high. When such lattice defects are present, segregation of impurities is likely to occur. Also,
Such defects cause deterioration of laser characteristics such as a decrease in luminous efficiency, an increase in oscillation threshold, and a decrease in differential gain, or a significant decrease in reliability.

In optical semiconductor devices other than semiconductor lasers, various disadvantages arise because defects due to lattice mismatch occur on the side surfaces of the buried strain quantum well optical waveguide layer. For example, in a buried strain quantum well optical waveguide,
This defect causes an increase in absorption coefficient and an increase in light scattering. Also, in a photodetector having a buried strain quantum well light absorption layer, the recombination dark current generated from the defect on this side surface increases, the internal electric field becomes nonuniform due to impurities segregated in the defect, and the quantum recombination due to the defect causes a defect. Various problems such as a decrease in efficiency and a decrease in reliability occur.

In semiconductor devices other than the optical semiconductor device, for example, a strain channel layer of a pseudomorphic HEMT, a strained In (Ga) As ohmic contact layer, etc.
A strained semiconductor layer is used for improving characteristics. When this strained semiconductor layer is embedded in a part of the substrate instead of the entire surface, the carrier lifetime is reduced due to lattice defects and segregation of impurities caused by lattice mismatch on the side surface, and the electron transport characteristics are increased due to an increase in scattering centers. Various problems such as deterioration, increase in noise, and decrease in reliability occur.

On the other hand, InG widely used in optical communications
In a quantum well laser having a well layer of aAsP or InGaAs, since the well barrier in the conduction band is low and the well barrier in the valence band is high, overflow to the electron barrier layer and the optical waveguide layer is likely to occur, and holes There is a problem that the efficiency of injection into the well layer is low. For this reason, the carrier utilization efficiency is poor, and the effect of reducing the oscillation threshold and expanding the modulation band is not remarkable as compared with a bulk type double hetero semiconductor laser having the same oscillation wavelength.

Further, when stimulated emission becomes large and carriers are insufficient, the efficiency of hole injection into the well layer is low, so that the hole density differs between wells, and the gain saturation ε for light also decreases. There was also the problem of being large. This means that, despite the increase in differential gain due to the quantum well effect, damping increases and the frequency band does not extend. Although a high-power laser can provide good characteristics when the number of wells is large in a long cavity, the number of wells cannot be increased so much due to such a problem.

In general, in a quantum well laser, the saturation of the gain G with respect to the carrier is remarkable. Therefore, when the oscillation threshold carrier density increases, the differential gain rapidly deteriorates. Therefore,
From the viewpoint of high-speed response characteristics, it is important to reduce the oscillation threshold. Since the oscillation condition is represented by ΔG = α, in order to lower the oscillation threshold, it is necessary to reduce the optical total loss α to lower the gain required for oscillation.

In a quantum well laser, particularly in a strained quantum well laser in which strain is introduced into a well layer, the waveguide loss in the active layer can be extremely reduced, but the loss increases when the acceptor density of the p-cladding layer is high. The quantum well laser has an optical confinement coefficient Γ
Is small, the influence of the absorption of the cladding layer is extremely large. Therefore, in order to obtain a low threshold quantum well laser, it is necessary to lower the acceptor density of the p-clad layer.

However, in this case, a large band barrier is formed at the heterojunction between the p-cladding layer and the active layer, which causes a problem that the efficiency of injecting holes into the quantum well active layer is reduced. This means that it is difficult to produce a quantum well laser with a low threshold value and a fast response. In order to solve the problem of the barrier at the hetero interface, adoption of a GRIN (graded index) structure has been proposed. However, in this case, precise control is required for crystal growth, and there is a problem that the process becomes complicated.

In addition, the buried-structure strained quantum well laser has a problem that dislocation defects are likely to occur due to lattice mismatch on the side surface of the active layer. In particular, when there is a layer doped with a transition metal such as Fe on the side surface, there has been a concern that a composite defect of a transition metal and a dislocation, which is an impurity, deteriorates laser characteristics.

[0020]

As described above, in a conventional semiconductor device having a strained semiconductor layer having a buried structure, defects such as lattice defects and impurity segregation occur due to lattice mismatch on the side surface of the strained semiconductor layer. There has been a problem that characteristics and reliability are significantly deteriorated.

The present invention has been made in view of the above circumstances, and has as its object to reduce the lattice mismatch on the side surface of a strained semiconductor layer to thereby provide a buried strained quantum well having excellent characteristics and reliability. An optical semiconductor device having a structure is provided.

[0022]

SUMMARY OF THE INVENTION The present invention provides a strained quantum well.
It is intended to reduce lattice mismatch on the side surface of the layer .

In the optical semiconductor device according to the first aspect of the present invention, the average lattice constant of the semiconductor layer in a portion of the clad layer in a plane parallel to the substrate extending to the left and right of the strained quantum well layer has an average lattice constant between the substrate and the strained quantum well layer. Materials with intermediate values are used. The average lattice constant here corresponds to the thickness of the active layer when a multilayer structure is used for a part of the semiconductor layer in a plane parallel to the substrate extending to the left and right of the strained quantum well layer in the cladding layer. It means the value obtained by taking the average of the lattice constant over the multilayer structure.

In the optical semiconductor device according to the second aspect of the present invention, the c-axis of the clad layer in a part of the semiconductor layer in a plane parallel to the substrate extending to the left and right of the strained quantum well layer has its c-axis pointing in the direction normal to the substrate. An oriented tetragonal semiconductor layer is used, and its a-axis lattice constant is smaller than that of the semiconductor substrate.

In any of these inventions, the stripe-shaped optical waveguide layer (corresponding to an active layer in a semiconductor laser and a light absorption layer in a photodetector) has a bandgap at the top, bottom, left and right sides of the barrier layer of the strained quantum well. Is surrounded by a wide cladding layer. Here, the side of the strained quantum well layer is
Be tapered at least 45 degrees to the substrate normal direction
It is desirable to be constituted. The gist of the third invention is the second invention.
Is to reduce the lattice mismatch that occurs on the side surface of the strained semiconductor layer in the first region embedded in the region by providing a composition change region in which the lattice constant changes continuously to reduce the probability of dislocation occurrence. is there.

Here, the term “strained semiconductor layer” refers to a semiconductor layer in which a semiconductor layer having a composition different from the lattice constant is intentionally laminated to a thickness equal to or less than the critical thickness to elastically introduce strain. This is not an assumption of the strain introduced due to imperfect composition control of crystal growth. In general, in the case of a semiconductor layer grown on a GaAs or InP substrate, the distortion introduced due to imperfect control of the composition of crystal growth is 0.2% or less at the most, and a lattice mismatch larger than that is required. If there is a match, it can be considered as a strained semiconductor layer intentionally created.

In an optical semiconductor device using a buried optical waveguide having a strained semiconductor layer as a part thereof, a portion where the optical waveguide is formed is a first region, and a buried portion adjacent to a side surface of the optical waveguide is a second region. It can be considered as an area.

The composition change region can be formed, for example, by promoting the mutual diffusion of atoms at the boundary by diffusion of impurities. Also, when one of the first region and the second region is formed by selective growth, the fact that the crystal composition changes depending on the distance from a mask formed on the other region is also used. Can be formed.

The gist of the fourth invention is to reduce the probability of occurrence of dislocations by making the lattice mismatch on the side surface of the buried strained semiconductor layer smaller than half of the unit cell spacing. Here, the term “strained semiconductor layer” means that, as described above, a semiconductor layer having a composition different from the lattice constant is stacked with a thickness per layer of less than the critical thickness and elastically introduces strain. It is a semiconductor layer.

According to the present invention, in a semiconductor device in which a strained semiconductor layer is embedded in a first region, a coordinate system Z is set upward in a direction normal to the lower principal surface of the lowest strained semiconductor layer as a reference lattice plane. The coordinates Z1 (n) of the n-th unit lattice plane from the reference lattice plane of the first area when the first area is assumed to be infinitely wide, the second area adjacent to the first area Is assumed to extend infinitely, the coordinates of the n-th unit lattice plane from the reference lattice plane of the second area are Z2
(N), Z2 (n + 1) -Z2 (n) Z2 (n) -Z2 (n-1) Z1 (n + 1) -Z1 (n) Z1 (n) -Z1 (n-1) When the smallest one is δ _min (n), ABS {Z2 (n) −Z1 (n)} <δ _min (n) / 2 (1) is satisfied for an arbitrary n. Things. Where ABS (x)
Represents the absolute value of x. In other words,
That is, the lattice mismatch at the boundary between the first region and the second region is suppressed so as to be smaller than at most 単 位 of the unit lattice layer. Here, the term unit lattice layer is defined as the period of the crystal structure in the Z direction. For example, in the case of a zinc blende structure, when the Z direction is taken in the <001> direction, the lattice constant a becomes one unit lattice layer, and when the Z direction is taken in the <111> direction, 3 ^1/2 a becomes one unit lattice layer. Becomes When the reference plane is not a low index plane, for example, in the case of an off-substrate, the above discussion can be applied mutatis mutandis by setting the normal direction of the nearest low index plane to the Z direction.

As one mode for satisfying the expression (1), a first strained semiconductor layer having a larger lattice constant than the semiconductor substrate and a second strained semiconductor layer having a smaller lattice constant than the semiconductor substrate are respectively formed as one layer. A method is considered in which the layers are stacked as described above, and the lattice mismatch on one side is canceled by the lattice mismatch on the other side. This method is particularly effective when the total thickness of one strained semiconductor layer alone is too large to satisfy the expression (1).

In some cases, there are m homologous atomic planes in one unit cell in the Z direction. For example, III-V of sphalerite structure
Assuming that the reference lattice plane of Z = 0 in the <001> direction of the group III compound semiconductor is a group III atom plane, m = 2 since a / 2 also has a group III atom plane. III for (110) plane
Since the group atoms and the group V atoms coexist on the same plane, there is a homo atom plane of m = 4 in the unit lattice layer 2 ^{1 /} 2a. In such a case, there is a possibility that dislocations with different homologous atomic planes in the unit lattice layer may occur together with lateral displacement. In this case, it is preferable to satisfy ABS {Z2 (n) −Z1 (n)} <δ _min (n) / (2m) (2) for an arbitrary n.

In an optical semiconductor device using an embedded optical waveguide having a strained semiconductor layer as a part thereof, a portion where the optical waveguide is formed is defined as a first region, and an embedded portion adjacent to the optical waveguide is defined as a second region. You can think.

In the case of a strained quantum wire or a strained quantum box, a semiconductor forming a strained quantum wire or a strained quantum box may be considered as a first region, and a semiconductor forming a barrier layer on a side surface thereof may be considered as a second region. it can. Of course, the embodiments described above can be used in combination.

[0035]

In the optical semiconductor device according to the first aspect of the invention, the average lattice constant in a plane parallel to the substrate including the stripe-shaped optical waveguide layer has an intermediate value between the lattice constant of the substrate and the lattice constant of the strained quantum well layer. Therefore, the lattice mismatch on the side surface of the optical waveguide layer is reduced.
As a result, lattice defects such as dislocations and segregation of impurities can be prevented,
An optical semiconductor element having excellent characteristics and reliability can be realized.

In the optical semiconductor device of the second invention, a tetragonal material (chalcopyrite structure semiconductor) is used in a plane parallel to the substrate including the stripe-shaped optical waveguide layer, and the tetragonal material lattice is used. The constant a has a value smaller than the lattice constant of the substrate. C of this chalcopyrite structure semiconductor mixed crystal
The value c / 2 obtained by dividing the axial lattice constant by 2 is generally smaller than the lattice constant a in the a-axis direction. Therefore, by appropriately selecting the thickness, in an optical semiconductor device having a buried strain quantum well optical waveguide layer in which the lattice constant of the strain quantum well layer is smaller than the substrate, the average of the buried layer and the active layer in the normal direction of the substrate is obtained. The lattice constant can be roughly matched. Therefore, the lattice mismatch on the side surface of the optical waveguide layer can be reduced without causing the lattice mismatch with the semiconductor substrate. As a result, segregation of lattice defects such as dislocations and impurities can be prevented, and an optical semiconductor device having excellent characteristics and reliability can be realized.

In the third aspect , the stress generated by the lattice mismatch between the first region and the second region is dispersed throughout the composition change region formed therebetween. Therefore, generation of dislocation due to stress concentration on the boundary surface can be suppressed, and a semiconductor element without deterioration in characteristics and reliability due to the dislocation can be realized.

In the case where the second region is heavily doped with impurities, if impurity diffusion from the boundary to the first region occurs, mutual diffusion of base atoms is promoted in the diffusion region. Therefore, at the boundary between the first area and the second area,
In parallel with the disordering of the strained quantum well, interdiffusion of host atoms between regions proceeds, and a composition change region is formed.

When a semiconductor mixed crystal is selectively grown, the composition, film thickness, and strain amount of the mixed crystal depend on the distance from the mask, the aperture ratio, and the like. By utilizing this fact, a region where the lattice constant is changed can be created at the boundary. The above composition change region can be formed by setting the direction of this change so as to approach the average lattice constant of the other region as it approaches the other region.

According to the fourth aspect of the present invention, the lattice mismatch at the boundary between the first region and the second region is suppressed so as to be smaller than at most １ ／ of the unit lattice layer by satisfying the expression (1). Since the unit lattice plane that is not displaced is closer in distance than the unit lattice plane that is displaced by one, the occurrence of dislocation at the boundary between the first region and the second region can be efficiently prevented.

A first strained semiconductor layer having a larger lattice constant than the semiconductor substrate, and a second strained semiconductor layer having a smaller lattice constant than the semiconductor substrate.
By stacking one or more strained semiconductor layers with each other, the lattice mismatch on one side is canceled by the other, thereby increasing the thickness and the number of strained semiconductor layers satisfying the expression (1). Can be.

When m homologous atomic planes exist in one unit cell in the Z direction, the lattice mismatch in the Z direction at the boundary between the first region and the second region is satisfied so as to satisfy Expression (2). Is set to be at most smaller than half of the distance between the homologous atomic planes, so that the homologous atomic planes that are not shifted from the adjacent homologous atomic planes in the adjacent region can be closer in distance, and the lateral shift The generation of dislocations with the homologous atomic planes can also be efficiently prevented.

In any case, generation of lattice defects such as dislocation near the boundary between the first region and the second region can be prevented, and a semiconductor device having excellent characteristics and reliability can be realized.

[0044]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below with reference to the illustrated embodiments.

First, embodiments of the first and second inventions will be described.

FIG. 1 is a sectional view showing a schematic structure of a semiconductor laser according to a first embodiment of the present invention. n-type InP
N-type InP buffer layer (cladding layer) 1b on substrate 1a
Are formed. On the buffer layer 1b, InGa
A multiple quantum well active layer 4 formed by alternately stacking As strained quantum well layers 2 and InGaAsP barrier layers 3 is formed in a stripe shape. On the side surface of the active layer 4, p-type InAlAs
Embedded layer 8 in which layers 18 and p-type InP layers 17 are alternately stacked
Are formed. Then, on the active layer 4 and the buried layer 8, the p-type InP cladding layer 5 and the p-type InGaAs
A P contact layer 6 is formed. Then, a p-side electrode 11 is formed on the contact layer 6, and an n-side electrode 12 is formed on the lower surface of the substrate 1a.

This semiconductor laser is manufactured as follows. First, as shown in FIG. 2A, a 2 μm-thick n-type InP buffer layer 1b is grown on a (001) n-type InP semiconductor substrate 1a by a metal organic chemical vapor deposition (MOCVD) method. And undoped InGa
As / InGaAsP multiple quantum well active layer 4 and thickness 10
A thin undoped InP dummy layer 14 having a thickness of nm is grown.
The active layer 4 is made of undoped In _0.7 Ga having a thickness of 4.2 nm.
Four _0.3 As strained quantum well layers 2 and an undoped InGaAsP (wavelength: 1.2 μm) having a thickness of 12 nm (only 20 nm at both ends)
m composition) Five barrier layers 3 are alternately stacked.

An SiO ₂ film 15 is deposited on this laminated structure, and is patterned by a usual PEP technique.
It is removed by etching in units of μm. At this time, the thin InP
A hydrochloric acid-based etchant is used for etching the dummy layer 14, and an SH-based (a mixture of sulfuric acid, hydrogen peroxide, and water) is used for etching the active layer 4. The photoresist is removed after the etching of the SiO ₂ film 15. By SH-based etching using the SiO ₂ film 15 and the InP dummy layer 14 as a mask, a cross section of the active layer having a concave central portion as shown in FIG. 2A is obtained. The width of the tapered portion is 0.4 μm on one side, and the width of the active layer at the center is 1 μm.

Next, as shown in FIG.
_{With the two} films 15 remaining, the strain buried layers 8 are laminated on the portions 16 on both sides of the active layer 4 by MOCVD. The strain buried layer 8 is made of p-type In _0.7 A having a thickness of 4.2 nm.
1 _0.3 As layer 18 is laminated so as to be sandwiched between four p-type InP layers 17, and the total film thickness is the active layer thickness (92.8 n
m).

Next, as shown in FIG.
₂ After removing the film 15, a p-type InP cladding layer 5 having a thickness of 2 μm and a p-type InP cladding layer 5 having a thickness of 0.8 μm are formed thereon by MOCVD.
A type InGaAsP contact layer 6 is sequentially grown. Then, on the contact layer 6, a p-type ohmic electrode (Ti
/ Pt / Au) 11, and an n-type ohmic electrode (Au / Ge) 12 is formed below the substrate 1.

Next, the contact layer 6 and the p-type cladding layer 5 are formed into a 10 μm-wide mesa including the active layer 4 by selective etching. Further, by removing the outer active layer 4b with an SH-based etchant, a semiconductor laser having a mushroom-shaped cross-sectional shape as shown in FIG. 1 and having a self-aligned pinched mesa (SACM) structure is obtained. This wafer is cleaved into a bar shape with a length of 1 mm and then a width of 300 μm.
It is cut out into m chips and mounted on a module (not shown in the figure). The oscillation wavelength is approximately 1.55 μm.

In such a configuration, In _0.7 Al
_{The 0.3} As layer 18 and the InP layer 17 have wider forbidden bands than the active layer 4. The In _0.7 Al _0.3 As layer 18 is made of I
The lattice constant is substantially equal to that of the n _0.7 Ga _0.3 As strained quantum well layer 2. Therefore, the buried layer 8 has a structure in which the lattice constant is substantially equal to that of the active layer 4, and the average lattice constant (0.58813 n
m) is the lattice constant of InP substrate 1 and In _0.7 Ga _0.3 As
The value becomes an intermediate value between the lattice constant of the strained quantum well layer 2 and the lattice constant. Ideally, the positions of the strained quantum well layer 2 and the In _0.7 Al _0.3 As layer 18 are perfectly matched, but a shift may occur in crystal growth. However, the strained quantum well layer 2 and In _0.7 Al _0.3
Even if the As layer 18 is displaced, the influence of the lattice mismatch is dispersed to the tapered portion because the side surface of the active layer 4 is tapered, and generation of extremely large distortion and stress can be prevented. Therefore, lattice defects such as dislocations and segregation of impurities can be prevented, and a buried-structure strained quantum well laser without deterioration in characteristics and reliability can be realized.

By adopting this structure, the line width increase coefficient (α
Excellent characteristics of the strained quantum well having a parameter of ~ 2 are stably obtained, and as a result, chirping (-20 dB) at the time of 10 Gb / s modulation (threshold bias, modulation current of 40 mApp).
A semiconductor laser having excellent characteristics with an average spectral width of 0.3 nm or less can be realized. In addition, the semiconductor laser of the present invention, which has high reliability and low chirping, makes it possible to commercialize a 10 Gb / s direct intensity modulation- direct detection optical fiber transmission system of several hundred km without using an external modulator.

As a modification of this embodiment, the buried layer 8 may be formed of a single layer of In _0.667 Al _0.333 As instead of a multilayer. Also in this case, the average lattice constant (0.58813 nm) of the active layer 4 and the buried layer 8 is substantially equal, and the value is an intermediate value between the lattice constant of the substrate and the lattice constant of the In _0.7 Ga _0.3 As strained quantum well layer 2. Become. In the micro, there is a lattice mismatch on the side surface of the active layer 4, but since the side surface of the active layer is formed in a tapered shape, strain and stress are dispersed in the tapered portion, and lattice defects and impurity segregation as in the above example. Can be prevented. Even when the average lattice constant of the active layer 4 and the average lattice constant of the buried layer 8 do not completely coincide with each other, the value is taken as the InP substrate 1
And the strain quantum well layer 2 have an effect of reducing lattice mismatch generated on the side surface of the active layer.

FIG. 3 is a sectional view showing a schematic structure of a semiconductor laser according to a second embodiment of the present invention. The structure and manufacturing method of the semiconductor laser of this embodiment are almost the same as those of the semiconductor laser of the first embodiment, but the strained quantum well layer 2 is made of In _0.48 G
a _0.52 As and the InP buried layer 17 has In
The difference is that a buried layer 9 of Ag _0.65 Cu _0.35 GaSe _{2 having} a chalcopyrite structure (tetragonal system) with a thickness of 1.56 nm is inserted instead of the AlAs layer. The oscillation wavelength is 1.48 μm suitable for pumping the optical fiber amplifier.

The lattice constant of the In _0.48 Ga _0.52 As strained quantum well layer 2 is 0.58485 nm, which is smaller than the lattice constant of the InP substrate, 0.58688 nm, contrary to the previous embodiment. Assuming that the strained quantum well layer 2 extends infinitely, the magnitude of the strain in the strained quantum well layer 2 is ε _xx = ε _yy = 0.003471 , ε
_zz = −2 (C ₁₂ / C ₁₁ ) ε _xx = −0.003422 , ε _yx = ε _zx
= Ε _xy = 0. Here, the Poisson's ratio of In _0.48 Ga _0.52 As was assumed to be C ₁₂ / C ₁₁ = 0.493. The lattice spacing of the strained quantum well layer 2 in the normal direction of the substrate after the strain is 0.58284 n
m. On the other hand, the lattice constants of the Ag _0.65 Cu _0.35 GaSe ₂ layer 9 are a = 0.5845 nm and c = 1.092 nm, and when the thickness is 1.56 nm, the distortion in the distorted state when the distortion in the a-axis direction is neglected. Lattice matching in the normal direction of the substrate can be obtained between the strained quantum well active layer and the macro. Actually Ag _0.65 Cu
Since there is also distortion in the _0.35 GaSe ₂ layer 9, perfect matching cannot be achieved unless the composition and thickness are further fine-tuned.

Further, even if the lattice matching of the film thickness average is completely achieved, there is a lattice mismatch on the side surface of the active layer 4 microscopically.
However, even if the lattice matching is imperfect, the side surface of the active layer 4 is tapered similarly to the previous embodiment, so that the influence of the lattice mismatch is dispersed throughout the tapered portion, Defects and segregation of impurities can be prevented. Ag _0.65 C
The forbidden band width of u _0.35 GaSe ₂ layer 9 is about 1.77 eV and I
Since it has a larger value than the nP substrate, it works in a preferable direction for current constriction and light confinement. Since there are various other chalcopyrite materials, the degree of freedom of selection is great in terms of lattice constant, forbidden band width, and the like. Therefore, this method can realize a buried structure strained quantum well semiconductor laser with high output and high reliability.

The present invention is not limited to the above embodiment, but can be variously modified. The barrier layer does not necessarily need to have a constant composition, and may have a GRIN structure in which the refractive index decreases continuously or stepwise from the active layer side toward the cladding layer (the forbidden band width increases). The quantum well layer may also be a double quantum well or a double barrier structure. The active layer material is also InGaAs, InGaAsP, I
nGaAlAs, InGaAsSb, GaAlAsS
b, GaAlPSb, InAlSb, InGaAlSb
For example, various combinations are possible.

The lateral light (and current) confinement structure of the optical waveguide layer is, in addition to the above SACM structure, a buried heterostructure,
Various types such as a semi-insulating InP buried structure are possible. For a semiconductor laser, a distributed feedback (DFB) laser,
A distributed reflection (DBR) laser, a tunable laser having a plurality of electrodes, a composite resonator laser, a monitor integrated laser, an optical waveguide integrated laser, a bistable laser, or the like may be used. Of course, other semiconductor laser, a semiconductor optical waveguide, semiconductor photodetecting element (photo diode), semiconductor optical switch, semiconductor optical directional coupler, a semiconductor optical modulator, etc. These photonic IC obtained by integrating a variety of optical semiconductor The present invention can be applied to an element. In the case of a semiconductor optical waveguide,
The stripe is not limited to a straight line, but can be applied to various optical waveguides having side surfaces, such as a bent optical waveguide, a total reflection optical waveguide, a Y-branch, a cross (X), and a taper. The cross-sectional taper shape is not limited to one having a concave central portion.

Next, an embodiment of the third invention will be described.

FIG. 4 is a sectional view showing a schematic structure of a semiconductor laser according to a third embodiment of the present invention. FIG. 5 is a sectional view showing the manufacturing process. This semiconductor laser is manufactured as follows. First, as shown in FIG. 5A, a 2 .mu.m thick metal film is grown on an n-type InP semiconductor substrate 1a having a (001) main surface by metal organic chemical vapor deposition (MOCVD).
m n-type InP buffer layer (cladding) 1b,
An undoped InGaAs / InGaAsP multiple quantum well active layer 4 is deposited. As shown in FIG. 5B, the active layer 4 has seven undoped In _0.7 Ga _0.3 As strained quantum well layers 2 having a thickness of 3 nm and a thickness of 10 nm (however, the lower layer 3a is 30 nm, and the upper layer 3b is Undoped InGaAsP having a wavelength of 1.2 μm and lattice-matched to InP (100 nm)
It has a structure sandwiching the layer 3.

After forming a diffraction grating on the InGaAsP layer 3b at the upper end, as shown in FIG. 5C, an SiO ₂ film 15 is deposited thereon and patterned by a usual PEP technique. Both sides of the stripe having a width of 2 μm to be active regions are removed by etching at a width of 1 μm. At this time, if an SH-based etchant (a mixture of sulfuric acid, hydrogen peroxide and water) is used for etching the active layer 4, FIG.
As shown in (c), a tapered active layer cross section with a concave central portion is obtained. The width of the tapered portion is 0.4 μm on one side, and the width of the active layer at the center is 1 μm.

Next, as shown in FIG.
₂ After removing the film 15, a p-type InP having a thickness of 2 μm and an acceptor concentration of 2 × 10 ¹⁸ cm ^-3 is entirely formed by MOCVD.
The cladding layer 5 and a 0.8 μm thick p-type InGaAsP contact layer 6 are embedded. During the burying growth, zinc as a p-type dopant diffuses from the regrowth interface into the active layer 4. As is well known, diffusion of impurities such as zinc into a semiconductor superlattice promotes interdiffusion of group III atoms, resulting in disorder. As a result, the zinc diffusion region on the side surface of the active layer 4 is a region having an intermediate composition between the strained quantum well layer 2 and the unstrained barrier layer 3.

Although this mixed crystal formation also occurs between the p-type InP cladding layer 5 and the active layer 4, the region of the intermediate composition on the side surface of the active layer is affected by the tapered side surface. The more outward, the composition change region 27 closer to the composition of InP.
Since the lattice mismatch on the side surface is dispersed throughout the composition change region 27, extreme concentration of stress is reduced, and defects such as dislocations are prevented from occurring. The diffusion of zinc from above into the active layer 4 is prevented by the InGaAsP layer 3b.

Thereafter, a p-type ohmic electrode (Ti / Pt / Au) 11 is formed on the contact layer 6, and an n-type ohmic electrode (Au / Ge) 12 is formed below the substrate 1. The clad layer 5 is sequentially formed into a 10 μm wide mesa including the active layer 4 by selective etching. Further, by removing the outer active layer with an SH-based etchant, a semiconductor laser wafer having a self-aligned pinching mesa (SACM) structure having a mushroom-shaped cross-sectional shape as shown in FIG. 4 can be obtained.

This wafer is cleaved into a bar having a length of 1 mm, cut into chips having a width of 300 μm, and mounted on a module (not shown). The oscillation wavelength is about 1.
55 μm. As described above, the occurrence of dislocations is suppressed in spite of the lattice mismatch on the side of the active layer, so that a low threshold, high output, high speed, low chirp, narrow spectral line width and highly reliable strained quantum wells are used. A distributed feedback semiconductor laser is realized.

In this embodiment, the diffusion of zinc is carried out in the over-closing step, but the diffusion may be carried out as an independent step separate from the over-growth step. Further, the impurity to be diffused is not limited to zinc, but may be other impurities such as cadmium, magnesium, beryllium, carbon, and silicon.

Next, a method of manufacturing the semiconductor laser according to the fourth embodiment will be described with reference to FIG. The steps from the initial crystal growth to the patterning of the active layer are the same as those shown in FIGS. Then, as shown in FIG. 6 (a), while the SiO ₂ film 15 used for patterning the active layer is left, the portion where the active layer is removed has a multilayer structure of In _0.52 Al _0.48 As layer 26a and InP layer 26b. Buried layer 26 is selectively grown. Since the buried layer 26 has a larger forbidden band width than the active layer 4, the buried layer 26 has a role of preventing a current leak without passing through the active layer 4.

At this time, since the diffusion distance of In is longer than that of Al on the SiO ₂ film 15, In _0.52 Al _0.48 A
During the growth of the s layer 26a, the portion near the SiO ₂ film 15 is supplied with In more than Al. As a result, In
_The In / (In + Al) ratio of the _0.52 Al _0.48 As layer 26a in the portion 28 near the SiO ₂ film 15 is larger than 0.52. In / (In + Al) as approaching the active layer 4
Since the ratio becomes larger than 0.52 and the lattice constant also becomes larger,
The average lattice constant of the buried layer 26 approaches the average lattice constant of the active layer 4. That is, the composition change region 28 works in a direction to disperse the influence of the lattice mismatch on the side surface of the active layer, in addition to the effect that the side surface is tapered. As a result, the concentration of stress is reduced, and the occurrence of defects such as dislocations can be suppressed.
The average lattice constant of the buried layer 26 near the active layer 4 can be adjusted by the crystal growth conditions, the thickness of the In _0.52 Al _0.48 As layer 26a, the number of layers, and the like.

Thereafter, the SiO ₂ film 1 is formed in the same manner as in the previous embodiment.
5 to remove the p-InP cladding layer 5, p-InGaAs
By growing the sP contact layer 6 and forming an electrode and a mesa, a semiconductor laser as shown in FIG. 6B is completed. Also in this embodiment, since the generation of dislocations due to lattice mismatch on the side of the active layer is effectively suppressed, a semiconductor laser with high performance and high reliability is realized. Conversely, a combination in which the active layer is formed by selective growth after the buried layer is also conceivable.

The characteristics of the quantum well laser depend on the number of quantum well layers. FIG. 7 shows In _0.7 Ga _0.3 As / InGaAs.
4 shows a calculation example of the dependence of the differential gain of an sP-based strained quantum well laser on the number of well layers. To obtain good characteristics, a certain number of layers is required. However, unless measures are taken to alleviate the influence of the lattice mismatch on the side as in this embodiment, the number of layers is large (the total film thickness is large). In this case, serious dislocation occurs near the side surface. According to the method of this embodiment, it is possible to further increase the total film thickness without occurrence of dislocation on the side surface of the active layer. By adopting the structure of this embodiment, a highly reliable semiconductor laser having a low threshold value, a small line width increase coefficient (α parameter), a large differential gain, and a high reliability can be obtained. This strained quantum well semiconductor laser with excellent characteristics, for example, as a light source for ultra-high-speed optical communication and coherent optical communication,
Alternatively, it is used as a high-power light source for exciting an optical amplifier.

The present invention is not limited to the above embodiment, but can be variously modified. Each semiconductor layer does not necessarily have to have a fixed composition. For example, the optical waveguide layer has a GRIN structure in which the refractive index decreases continuously or stepwise from the active layer side toward the cladding layer (the forbidden band width increases). It may be. As the lateral light (and current) confinement structure of the optical waveguide layer, various types such as a buried hetero structure and a semi-insulating InP buried structure are possible in addition to the SACM structure described above.
The shape of the side surface is not limited to the constricted tapered shape of the central portion as in the above embodiment, and may be vertical, a trapezoid having a narrow upper portion, an inverted trapezoid having a narrow lower portion, or other shapes.

In the case of a semiconductor laser, a distributed feedback type (DF
B) In addition to lasers, Fabry-Perot (FP) lasers, distributed reflection (DBR) lasers, tunable lasers with multiple electrodes, composite resonator lasers, monitor integrated lasers, optical waveguide integrated lasers, bistable lasers, etc. You may. Of course, in addition to semiconductor lasers, semiconductor laser optical amplifiers, semiconductor optical waveguides, semiconductor photodetectors (photodiodes), semiconductor optical switches, semiconductor directional optical couplers, semiconductor optical modulators, photonic ICs integrating these, etc. The present invention can be applied to various optical semiconductor elements or electronic devices such as pseudomorphic HEMTs and heterojunction bipolar transistors.

The shape of the first region is not limited to a linear stripe, and may be, for example, a rectangle, a polygon, a circle, a comb,
The present invention can be applied to a region having an arbitrary shape such as a Y-branch, a cross (X), and a curve. Even when the first region is not completely surrounded by other regions, when the strained semiconductor layer also exists in the second region, or when the third region is present, each of the regions including the strained semiconductor layer is included. It will be apparent that the effect of the present invention can be obtained by forming the composition gradient region on the side surface of the region. The manufacturing method is not limited to the MOCVD method. For example, another growth method such as ALE (atomic layer epitaxy) may be used, and the formation order of each region may be any one.

The material is also InGaAs, GaAlAs, In
GaAsP, InGaAlAs, InGaAlP, In
GaAsSb, GaAlAsP, GaAlAsSb, G
aAlPSb, InAlPSb, InGaAlSb, A
In addition to III-V semiconductor mixed crystals such as lGaN, SiG
e, SiC, ZnSSe, CdSSe, HgCdTe,
Various material combinations such as chalcopyrite materials are possible.

Next, an embodiment of the fourth invention will be described.

FIG. 8 is a sectional view showing a schematic structure of a semiconductor laser according to a fifth embodiment of the present invention. This semiconductor laser is manufactured as follows. An undoped InGa film is formed on a (001) n-type InP semiconductor substrate 1a having a main surface by a metal organic chemical vapor deposition (MOCVD) method via an n-type InP buffer layer (cladding) 1b having a thickness of 2 μm.
An As / InGaAsP multiple quantum well active layer 4 is laminated.

The active layer 4 has a thickness of 4.206 as shown in FIG.
The upper and lower sides of the undoped In _0.7 Ga _0.3 As strained quantum well layer 2 with a thickness of 2.005 nm are undoped In _0.48 Ga
_The laminated structure sandwiched between the _0.52 As _0.78 P _0.22 strain barrier layers 3a is
The layer has a structure of being sandwiched between undoped InGaAsP layers 3b having a wavelength of 1.2 μm and lattice-matched to InP having a thickness of 10 nm (the lower layer is 30 nm and the upper layer is 70 nm). After forming a diffraction grating on the InGaAsP layer 3b at the upper end, an SiO ₂ film is deposited thereon, and a normal PEP is formed.
Patterned by technology, width 2μ to be active area
Both sides of the m-th stripe are removed by etching at a width of 1 μm. At this time, when an SH-based etchant (a mixture of sulfuric acid, hydrogen peroxide, and water) is used for etching the active layer 4, a tapered active layer cross section having a concave central portion as shown in FIG. 8 is obtained. Can be The width of the tapered portion is 0.4 μm on one side, and the width of the active layer at the center is 1 μm.

Next, after removing the SiO ₂ film, the MOC
A p-type InP cladding layer 5 having a thickness of 2 μm and a p-type InGaAsP contact layer 6 having a thickness of 0.8 μm by the VD method.
Embed with A p-type ohmic electrode (Ti / Pt / Au) 11 is formed on the contact layer 6, and an n-type ohmic electrode (Au / Ge) 12 is formed below the substrate 1. The contact layer 6 and the p-type cladding layer 5 are sequentially formed into a 10 μm-wide mesa including the active layer 4 by selective etching. Further, when the outer active layer is removed with an SH-based etchant, a semiconductor laser having a self-aligned pinching mesa (SACM) structure having a mushroom-shaped cross-sectional shape as shown in FIG. 8 is obtained. The wafer is cleaved into a bar having a length of 1 mm, cut into chips having a width of 300 μm, and mounted on a module (not shown). Oscillation wavelength is about 1.55μ
m.

While the lattice constant of the InP substrate 1 is 0.58688 nm, the In _0.7 Ga _0.3 As strained quantum well layer 2
Has a lattice constant of 0.59381 nm, In _0.48 Ga _0.52 As _0.78
The lattice constant of the P _0.22 strain barrier layer 3a is 0.57986 nm.
Here, assuming an infinite plane without side surfaces, the strained quantum well layer 2 and the strain barrier 3a are elastically distorted because they are as thin as 4.206 nm and 2.005 nm, respectively. The magnitude of the strain in the strained quantum well layer 2 is as follows: ε _xx = ε _yy = −0.01167, ε _zz = −2 (C ₁₂ / C ₁₁ )
ε _xx = 0.011974 and ε _yz = ε _zx = ε _xy = 0. Here, C ₁₂ / C ₁₁ = 0.504 is the Poisson's ratio of In _0.7 Ga _0.3 As.

On the other hand, the strain magnitude of the strain barrier layer 3a is ε
_xx = ε _yy = 0.012106 , ε _zz = −2 (C ₁₂ / C ₁₁ ) ε _xx
= −0.012106, ε _yz = ε _zx = ε _xy = 0. here,
Since the Poisson's ratio of In _0.7 Ga _0.52 As _0.78 P _0.22 has not been determined accurately, it was assumed that C ₁₂ / C ₁₁ = 0.5.
As a result, the strained In _0.7 Ga _0.3 As strained quantum well layer 2
Is equal to InP in the xy plane, and the lattice constant in the z direction is 0.60092 nm. The thickness 4.206nm is 7
It corresponds to a unit lattice layer. On the other hand, distorted In _0.48 Ga _0.52
The lattice constant of the As _0.78 P _0.22 strain barrier layer 3a is equal to InP in the xy plane, and the lattice constant in the z direction is 0.57284 nm.
Becomes A thickness of 2.005 nm corresponds to a 3.5 unit lattice layer.

FIG. 10 shows the magnitude of the shift of the lattice plane in the <001> direction (Z) direction on the side surface of the active layer 4 at this time. The maximum deviation of the lattice plane is 0.04914 nm, and δ _min = 0.57
It is sufficiently smaller than 0.14321 nm, which is 1/4 of 284 nm, and satisfies the expression (2). Therefore, it is possible to prevent the dislocation due to the displacement of the (001) homologous atomic plane accompanied by the lateral displacement. Of course, the normal M which is not the ALE (atomic layer epitaxy) mode
Since it is difficult to completely control the layer thickness and the composition by OCVD, it is conceivable that a slight shift occurs in the thickness of each strained semiconductor layer.

In some cases, the values of the lattice constants and C ₁₂ / C ₁₁ are not accurate. However, in any case, the magnitude of the deviation of the Z-direction lattice plane in the seven layers is 0.28642n.
If the control is performed so as not to exceed m, that is, to satisfy the expression (1), it is possible to prevent the occurrence of dislocations corresponding to the displacement of the unit lattice plane. Microscopically, a small lattice mismatch remains on the surface of the active layer 4, but since the side surface of the active layer is formed in a tapered shape, strain and stress are dispersed and relaxed in the tapered portion, and each semiconductor layer is connected to the boundary portion of the side surface. Can also be elastically distorted. Therefore, lattice defects such as dislocations and segregation of impurities can be prevented also on the side surfaces of the strained semiconductor layers 2 and 3a, and a buried structure strained quantum well laser without deterioration in characteristics and reliability can be realized.

The characteristics of the quantum well laser depend on the number of quantum well layers. FIG. 7 shows In _0.7 Ga _0.3 As / InGaAs.
4 shows a calculation example of the dependence of the differential gain of an sP-based strained quantum well laser on the number of well layers. To obtain good characteristics, a certain number of layers is required. However, unless measures are taken to alleviate the influence of the lattice mismatch on the side as in this embodiment, the number of layers is large (the total film thickness is large). In this case, serious dislocation occurs near the side surface. For example, in the example of FIG. 8, In _0.48 Ga _0.52 As
Assuming that there is no _0.78 P _0.22 strain barrier layer 3a, the calculated cumulative mismatch on the side surface is 0.68796 nm, which exceeds the size of one unit lattice layer, so that dislocations are surely generated on the side surface first. According to the method of the present invention, it is possible to further increase the total film thickness without occurrence of dislocation on the side of the active layer.

With the adoption of the structure of this embodiment, excellent characteristics of a strained quantum well having a lower threshold value and a line width enhancement factor (α parameter) of ２2 can be obtained stably, and as a result, 10 Gb /
Chirping (-20 dB average spectral width) during s modulation (threshold bias, modulation current 40 mApp) is 0.3 n
m can be realized. The semiconductor laser of the present invention, which has high reliability and low chirping, enables the practical application of a 10 Gb / s direct intensity modulation-direct detection type several hundred km optical fiber transmission system without using an external modulator.

As a modification of this embodiment, the barrier layer except for the upper and lower ends may be formed of a single-layer strained InGaAsP layer instead of the multilayer structure of the strained layer 3a and the non-strained layer 3b. Also in this case, the same effect can be obtained by setting the composition of the strained InGaAsP barrier layer and the thickness on the xy plane in a lattice-matched state with the substrate so as to satisfy the equations (1) and (2).

The present invention is not limited to the above embodiment, but
Various modifications as described in the modification of the third invention are possible. Further, when the first region is not completely surrounded by other regions, when the strained semiconductor layer also exists in the second region,
Even when there is a third region, the amount, thickness, and position of each layer are set so that the displacement of the lattice plane satisfies Expression (1) on the side surface of each region including the strained semiconductor layer. ,
It will be apparent that the operation and effect of the present invention can be obtained. In addition, it can be applied to alleviate the influence of lattice mismatch on the side of a quantum wire or quantum box containing strain.

FIG. 11 is a sectional view showing a schematic structure of a semiconductor laser according to a sixth embodiment of the present invention. This semiconductor laser is manufactured as described below. Undoped InGa is deposited on a (001) n-type InP semiconductor substrate 1a having a main surface by metal organic chemical vapor deposition (MOCVD) via an n-type InP buffer layer (cladding) 1b having a thickness of 2 μm.
AsP layer optical waveguide layer 3a, undoped InAs / InGa
AsP multiple quantum well active layer 4, undoped In Ga As
The P-layer optical waveguide layer 3b is sequentially laminated. Active layer 4, FIG. 12
As shown in FIG. 3, 12 undoped InAs strained quantum well layers 2 each having a thickness of 2 nm and undoped InGaAs having a thickness of 5 nm are provided.
The sP barrier layer 3 is interposed therebetween. InGaAs
After forming a diffraction grating on the sP layer optical waveguide layer 3b, an SiO ₂ film is deposited thereon and patterned by a usual PEP technique, and both sides of a stripe having a width of 2 μm to be an active region are each 1 μm wide. Remove by etching.

Next, after removing the SiO ₂ film, the MOC
A p-type InP cladding layer 5 having a thickness of 2 μm and a p-type InGaAsP contact layer 6 having a thickness of 0.8 μm by the VD method.
Embed with A p-type ohmic electrode (Ti / Pt / Au) 11 is formed on the contact layer 6, and an n-type ohmic electrode (Au / Ge) 12 is formed below the substrate 1. Then, the contact layer 6 and the p-type clad layer 5 are formed into a 10 μm-wide mesa including the active layer 4 by selective etching. Further, by removing the outer active layer with an SH-based etchant, a semiconductor laser having a self-aligned pinching mesa (SACM) structure having a mushroom-shaped cross section as shown in FIG. 11 is obtained. This wafer is cleaved into a bar with a length of 1 mm, and then cut into chips having a width of 300 μm.
Mounted on the module (omitted in the figure).

[0090] Here, the composition of InGaAsP barrier layer 3, as shown in FIG. 12, it is assumed that selected so that the forbidden band edge is generally coincident with the forbidden band edge E _LH for light holes distortion InAs layer 2 . Distortion still on the InP substrate at present _{In 1-u Ga u As v} P 1-v (0 ≦ x ≦ 1,0 ≦ y ≦
The band alignment of 1) is not completely known.
However, since the strain InAs layer sandwiched InP have been found to form a shallow quantum wells than for the In _0.53 Ga _0.47 As layer sandwiched InP against light holes,
In the above condition is satisfied unstrained _1-x Ga x As _y P
There is always a _1-y composition (0 <x <0.47, 0 <y <1).

In this way, since a barrier cannot be formed for light holes, the light holes can move freely in each well layer 2 and barrier layer 3. Moreover, light holes are partially coupled with heavy holes inside the well layer because the overlap integral of the wave function is not zero. Accordingly, holes can be efficiently injected into wells far from the p-cladding layer, and coupling between the wells is improved. As a result, a strained multiple quantum well laser having a low threshold and excellent high-speed characteristics is realized.

The fact that no barrier is formed for light holes can be confirmed by measurement of, for example, photoluminescence. That is, light emission due to heavy hole and electron recombination is allowed only in the polarization component parallel to the well layer, whereas light emission due to light hole and electron recombination is a component parallel to the component perpendicular to the well. Can be distinguished from each other, and it can be distinguished. By examining whether or not the light emission from the light hole is due to the quantized level, the valence band edge E _LH for the light hole is approximately one. You can decide whether you are doing it.

The present invention is not limited to the embodiment described above, and various modifications are possible. For example, InGaAs
The sP barrier layer 3 may be made of a material having a smaller lattice constant than InP. FIG. 13 is a conceptual diagram of the band structure in this case.
Shown in In this way, in the strained multiple quantum well laser in which the active layer is buried, the effect of the lattice mismatch on the side surface can be offset by positive and negative strains. Generation can be suppressed.

When the well layer is a semiconductor layer having a lattice constant smaller than that of the substrate, on the contrary, the valence band edge E _HH for heavy holes in the strained well layer and the barrier layer is substantially matched.
Similarly, for a quantum well for light holes, it is possible to improve the injection efficiency of holes and the coupling efficiency between wells. FIG. 14 shows a band structure diagram in this case.

The lateral light (and current) confinement structure of the optical waveguide layer may be a buried heterostructure,
Various types such as a semi-insulating InP embedded structure are possible. In addition to distributed feedback (DFB) lasers, Fabry-Perot (FP) lasers, distributed reflection (DBR) lasers, tunable lasers with multiple electrodes, composite resonator lasers, monitor integrated lasers, optical waveguide integrated lasers, bistable A laser or the like may be used. Of course, the present invention can be applied to a semiconductor laser optical amplifier, a carrier injection type optical switch, a semiconductor optical modulator, and a photonic IC in which these are integrated. Of course, the material is InAs or InGaAs.
It is not limited to P.

[0096]

As described above in detail, according to the present invention, since the lattice mismatch on the side surface of the buried strained semiconductor layer is reduced, the occurrence of lattice defects and segregation of impurities near the side surface of the strained semiconductor layer is reduced. Thus, an optical semiconductor device having excellent characteristics and reliability can be realized.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a schematic structure of a semiconductor laser according to a first embodiment;

FIG. 2 is a sectional view showing a manufacturing process of the laser according to the first embodiment;

FIG. 3 is a sectional view showing a schematic configuration of a semiconductor laser according to a second embodiment;

FIG. 4 is a sectional view showing a schematic configuration of a semiconductor laser according to a third embodiment;

FIG. 5 is a sectional view showing a manufacturing process of the laser according to the third embodiment;

FIG. 6 is a cross-sectional view illustrating a manufacturing process of a semiconductor semiconductor laser according to a fourth embodiment;

FIG. 7 is a characteristic diagram showing the dependence of the laser differential gain on the number of well layers in the fourth embodiment.

FIG. 8 is a sectional view showing a schematic structure of a semiconductor laser according to a fifth embodiment;

FIG. 9 is a cross-sectional view illustrating a configuration of a main part of a laser according to a fifth embodiment;

FIG. 10 is a schematic diagram showing a shift of the lattice plane in the Z direction on the side surface of the active layer in the fifth embodiment;

FIG. 11 is a schematic configuration of a semiconductor laser according to a sixth embodiment.
Sectional view showing the structure,

FIG. 12 shows a band structure of an active layer of a laser according to a sixth embodiment.
Schematic diagram,

FIG. 13 shows the band structure of the active layer in the sixth embodiment.
Schematic diagram showing another example,

FIG. 14 shows the band structure of the active layer in the sixth embodiment .
Schematic diagram showing another example,

FIG. 15 is a schematic configuration of a conventional multiple quantum well semiconductor laser.
FIG.

[Explanation of symbols]

1a ... n-type InP substrate, 1b ... n-type InP buffer layer (cladding layer), 2 ... InGaAs strained quantum well layer, 3 ...
InGaAsP barrier layers, 4... Multiple quantum well active layers, 5.
p-type InP cladding layer, 6: p-type InGaAsP contact layer, 8: buried layer, 27, 28: composition change region .

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-292789 (JP, A) JP-A-2-36585 (JP, A) IEEE J.I. Quantum Electron, Vol. QE-22 No. 6 (1986), p. 833-844 IEEE J.C. Quantum Electron, Vol. 25 No. 6 (1989), p. 1320-1323 (58) Field surveyed (Int.Cl. ⁷ , DB name) H01S 5/00-5/50 G02F 1/35-1/39

Claims (4)

(57) [Claims] A first semiconductor layer formed on a semiconductor substrate and having a layer thickness equal to or less than the electron de Broglie wavelength and having no lattice matching with the semiconductor substrate has a wider bandgap than the first semiconductor layer. A striped optical waveguide layer having at least one strained quantum well sandwiched between the second semiconductor layers; a cladding layer formed of a semiconductor having a wider bandgap than the second semiconductor layer around the optical waveguide layer; An optical semiconductor device having an enclosed structure, wherein the average lattice constant of a cladding region in the same substrate parallel plane as the optical waveguide layer has an intermediate size between the semiconductor substrate and the first semiconductor layer. Semiconductor device. 2. A semiconductor device comprising: a first semiconductor layer formed on a semiconductor substrate, having a layer thickness equal to or less than the de Broglie wavelength of electrons and having a smaller lattice constant than the semiconductor substrate, having a forbidden band width smaller than that of the first semiconductor layer. A striped optical waveguide layer having at least one strained quantum well sandwiched between wide second semiconductor layers, and a cladding layer formed of a semiconductor having a wider forbidden band width than the second semiconductor layer around the optical waveguide layer In the optical semiconductor device having a structure surrounded by, a semiconductor layer having a tetragonal structure in which the c-axis is oriented in the normal direction of the substrate is provided in a cladding region in the same plane as the optical waveguide layer. An optical semiconductor device, wherein the lattice constant of the a-axis of the semiconductor layer having a tetragonal structure is smaller than the lattice constant of the semiconductor substrate. 3. A first region in which a strained quantum well including at least one strained semiconductor layer having a different lattice constant from a semiconductor substrate is formed, and a portion outside the first region and adjacent to the strained quantum well is formed. In an optical semiconductor device including the strained quantum well and a second region on which a second semiconductor layer having a different average lattice constant is formed on the same semiconductor substrate, a boundary between the first region and the second region is provided. An optical semiconductor, wherein a composition change region in which an average lattice constant changes continuously or intermittently between that of the first semiconductor layer and that of the second semiconductor layer is formed in the portion element. 4. A first region in which a strained quantum well including at least one strained semiconductor layer having a different lattice constant from a semiconductor substrate is formed, and a portion outside the first region and adjacent to the strained quantum well is formed. In an optical semiconductor device including the strained quantum well and a second region on which a second semiconductor layer having a different average lattice constant is formed on the same semiconductor substrate, a lowermost portion of the strained quantum well in the first region is provided. When the lower principal surface of the strained semiconductor layer at the reference plane is taken as a reference plane, the coordinate system Z is taken upward in the normal direction of the low index plane closest to the reference plane, and the first region is assumed to be infinitely wide. Coordinates Z1 (n) of the n-th unit lattice plane from the reference plane of the first area, the reference plane of the second area when the second area adjacent to the first area is assumed to be infinitely wide Let Z2 (n) be the coordinate of the n-th unit cell plane from (N + 1) -Z2 (n) Z2 (n) -Z2 (n-1) Z1 (n + 1) -Z1 (n) Z1 (n) -Z1 those smallest of the four values of (n-1) δ An optical semiconductor device characterized by satisfying a relation of ABS {Z2 (n) -Z1 (n)} <δ _min (n) / 2, where _min (n) is an arbitrary n.