JP2685720B2 - Semiconductor laser and manufacturing method thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser and its manufacturing method, and more particularly to a semiconductor laser suitable for optical fiber communication and its manufacturing method.

[0002]

2. Description of the Related Art In order to improve the manufacturing yield of semiconductor lasers, studies have been conducted to grow not only an active layer but also a current block portion on a substrate by using a MOVPE (Metal Organic Vapor Phase Epitaxial Growth) method. . All the semiconductor layers required for the semiconductor laser are I
A semiconductor laser grown on an nP crystal substrate has been reported. [Journal of Crystal Growth 93 (1988) 792
AWNelson etc, Journal of Crystal Growth 93 (1988) 792
Nelson et al.] The conventional semiconductor laser will be described with reference to FIG. In FIG. 18, 50 is an InP substrate, 52 is an active layer, 53 is a p-InP first current blocking layer, and 54 is an n-.
InP second current block layer, 55 is a p-InP third current block portion, 57 is a p-side electrode, and 58 is an n-side electrode.
In this semiconductor laser device, the current injected from the p-side electrode 58 is narrowed by the second current block layer 54,
It is injected into the active layer 52.

In order to improve the performance of semiconductor lasers,
Single quantum well (SQW) structure and multiple quantum well (MQW)
Research and development of semiconductor lasers having a structure have been actively conducted. A semiconductor laser having a quantum well active layer can exhibit superior characteristics to a semiconductor laser having a bulk active layer due to the quantum size effect of the active layer. For example, by increasing the differential gain and reducing TM light emission, it is possible to oscillate at a low threshold and obtain a high optical output with high efficiency. In addition, the increase in the relaxation oscillation frequency and the decrease in the line width increase coefficient increase the response speed and reduce the chirping. In order to further increase the differential coefficient, a “modulation doped structure” has been proposed in which a part of the barrier layer is doped with p-type impurities. In particular, if the strained quantum well active layer has a modulation doping structure, it is expected that a large improvement in characteristics will be obtained.

FIG. 19 shows a semiconductor laser provided with a quantum well active layer having a conventional modulation doping structure (K.
Uomi, T.Mishima, N.Chinione, Jpn. J. Appl. Phys.,
51 (1990) 88). In the semiconductor laser of FIG. 19, an n-type GaAs buffer layer 62 and an n-type InA are formed on a GaAs substrate 61.
lAs clad layer 63, non-doped GRIN-GaAl
A modulation-doped quantum well layer 68, a p-type GaAlAs cladding layer 69, an n-type GaAs current block portion 70, an oxide film 71, and a p-side electrode 73 sandwiched between the As layers 64 are laminated in this order. The oxide film 71 is provided with a stripe-shaped opening, and the p-side electrode 73 is provided through the opening.
Is in contact with part of the n-type GaAs current block portion 70. A p-type GaAlAs cladding layer 69 is formed on the contact portion.
A Zn diffusion region 72 extending therethrough is provided. Board 61
An n-side electrode 74 is provided on the back surface of the.

[0005]

In the above-mentioned conventional semiconductor laser (FIG. 18), it is a long-term high temperature acceleration test (see FIG. 18) that the drive current required for obtaining a constant light output is stable without fluctuation. 20). However, in the early stage of the test, an increase in drive current due to an increase in the laser oscillation threshold value is observed. In order to put the semiconductor laser into practical use, it is necessary to avoid an increase in drive current even in the initial stage of operation.

In the case of forming the current block portion by using the LPE (liquid phase epitaxial growth) method, a method for suppressing the threshold rise has been established. However, as described above, the MOVPE method still has a problem that the threshold current increases.

Further, the conventional semiconductor laser having a quantum well type active layer having a modulation-doped structure (FIG. 19) practically shows almost no improvement in performance as compared with the case where it has no modulation-doping. There's a problem. It is considered that this is because, during crystal growth performed after the step of forming the modulation dope structure, the dopant is diffused from the semiconductor layer growing upward to the active layer, so that the modulation dope structure disappears.

The present invention has been made to solve the above problems, and an object thereof is to provide a semiconductor laser having high reliability and reduced leakage current, and a method for manufacturing the same.

Another object of the present invention is to provide a semiconductor laser and a method of manufacturing the same in which the leakage current is reduced and the modulation dope structure does not disappear during the manufacturing process.

[0010]

A semiconductor laser of the present invention comprises a semiconductor substrate of a first conductivity type, a stripe-shaped multilayer structure formed on the semiconductor substrate and including an active layer, and both sides of the stripe-shaped multilayer structure. And a current block portion formed on the semiconductor substrate, wherein the current block portion is a second current type first current block layer,
A second of a first conductivity type formed on the first current blocking layer;
A current blocking layer, wherein the first current blocking layer has a low-concentration region having a relatively low second-conductivity-type impurity concentration, and a high-concentration region having a higher impurity concentration than the low-concentration region. And the low-concentration region is provided closer to the striped multilayer structure than the high-concentration region, thereby achieving the above object.

In one embodiment, the first and second current blocking layers are layers deposited by metal organic chemical vapor deposition.

In one embodiment, the thickness of the low-concentration region of the first current blocking layer is smaller as it is closer to the striped multilayer structure.

The impurity concentration of the high concentration region of the first current blocking layer is 2 × 10 ¹⁸ cm ⁻³ or less, and the concentration of the first conductivity type impurity of the second current blocking layer is 1 It is preferably × 10 ¹⁸ cm ⁻³ or more.

In one embodiment, the side surface of the stripe-shaped multilayer structure is formed of a crystal plane including a {111} P plane.

It is preferable to further include an undoped semiconductor layer provided between the surface of the semiconductor substrate and the side surface of the stripe-shaped multilayer structure and the current block portion. In one embodiment, the active layer has a multiple quantum well structure.

A second conductive type semiconductor layer which covers both the stripe-shaped multilayer structure and the current block portion and has a band gap different from the band gap of the current block portion may be further provided.

Another semiconductor laser of the present invention is a semiconductor substrate of the first conductivity type, a stripe-shaped multilayer structure formed on the semiconductor substrate and including an active layer, and the semiconductor substrates on both sides of the stripe-shaped multilayer structure. A semiconductor laser comprising a current blocking portion formed on the first current blocking layer, the current blocking portion including a first current blocking layer of a second conductivity type and a first current blocking layer formed on the first current blocking layer. A second current blocking layer of one conductivity type, and one of the two ends of the second current blocking layer which is closer to the striped multilayer structure is 6
It has an apex angle of 0 degrees or more, which achieves the above object.

The concentration of the second conductivity type impurity in the first current blocking layer is 1 × 10 ¹⁸ cm ⁻³ or less, and the concentration of the first conductivity type impurity in the second current blocking layer is It is preferably 1 × 10 ¹⁸ cm ⁻³ or more.

In one embodiment, the side surface of the stripe-shaped multilayer structure is formed of a crystal plane including a {111} InP plane.

It is preferable that the semiconductor device further comprises an undoped semiconductor layer provided between the surface of the semiconductor substrate and the side surface of the stripe-shaped multilayer structure and the current block portion. In one embodiment, one of the two ends of the second current blocking layer, which is closer to the striped multilayer structure, is
It has an apex angle of 80 degrees or more.

In one embodiment, the active layer has a multiple quantum well structure.

A second conductivity type semiconductor layer which covers both the stripe-shaped multilayer structure and the current block portion and has a bandgap different from the bandgap of the current block portion may be further provided.

The method of manufacturing a semiconductor laser according to the present invention is the first
A semiconductor including a conductive type semiconductor substrate, a stripe-shaped multilayer structure formed on the semiconductor substrate and including an active layer, and a current block portion formed on the semiconductor substrate on both sides of the stripe-shaped multilayer structure. A method for manufacturing a laser, comprising a step of depositing a plurality of semiconductor layers including the active layer on the semiconductor substrate, and a cap layer having etching characteristics different from the etching characteristics of the semiconductor layers on the semiconductor layers. Forming step, forming a striped mask layer on the cap layer, and using an etchant that does not substantially etch the mask layer and preferentially etches the cap layer over the semiconductor layer And selectively etching the cap layer and the semiconductor layer to form the striped multilayer structure having a width narrower than that of the cap layer. And grayed step, which includes a step of forming a current blocking section, the object can be achieved.

The width of the striped mask layer is preferably at least twice the width of the striped multilayer structure.

In the etching step, it is preferable that a part of the etchant is inserted between the stripe-shaped mask layer and the cap layer.

The step of forming the cap layer is performed using InG.
forming a cap layer from an aAsP crystal,
The etching process may include a first etching process using an acetic acid etchant and a second etching process using a hydrochloric acid etchant.

It is preferable that the step of forming the current block portion includes a step of epitaxially growing the current block portion at a growth temperature of 600 ° C. or higher using a metal organic chemical vapor deposition method.

The step of forming the current block portion preferably includes the step of raising the temperature of the substrate to the growth temperature in an atmosphere containing a Group 5 element of the active layer.

Another method of manufacturing a semiconductor laser according to the present invention is
A semiconductor substrate of a first conductivity type; a stripe-shaped multilayer structure formed on the semiconductor substrate and including an active layer; and a current block unit formed on the semiconductor substrate on both sides of the stripe-shaped multilayer structure. And a step of depositing a plurality of semiconductor layers including the active layer on the semiconductor substrate, a step of forming a stripe-shaped mask layer on the semiconductor layer, and the step of forming the mask layer substantially. Selectively etching the semiconductor layer using an etchant that does not etch selectively, and forming a stripe-shaped multilayer structure, and growing a second current type first current blocking layer on the semiconductor substrate. And a step of growing, on the first current blocking layer, a second current blocking layer of the first conductivity type having an apex angle of 60 degrees or more at an end closer to the striped multilayer structure, And includes, above objects can be achieved.

It is preferable that the first and second current blocking layers are epitaxially grown at a growth temperature of 600 ° C. or higher by using a metal organic chemical vapor deposition method.

Before forming the first current blocking layer,
It is preferable that the method further includes a step of epitaxially growing the undoped semiconductor layer.

Still another semiconductor laser of the present invention is a semiconductor substrate of the first conductivity type, a stripe-shaped multilayer structure including a quantum well active layer having a modulation doping structure, which is formed on the semiconductor substrate, and the stripe. Block portions formed on both sides of the striped multilayer structure for narrowing current to the striped multilayer structure, and the second conductive type semiconductor layer formed above the striped multilayer structure and the current block portion. And a diffusion suppressing layer that covers both the stripe-shaped multilayer structure and the current block portion and suppresses diffusion of impurities contained in the second conductivity type semiconductor layer into the active layer. The present invention further includes a diffusion suppressing layer having a bandgap smaller than the bandgap of the current block portion, thereby achieving the above object.

It is preferable that the diffusion suppressing layer has a higher solid solubility with respect to impurities doped in the second conductivity type semiconductor layer than that of the second conductivity type semiconductor layer. In one embodiment, the current block portion is formed of InP, the second conductivity type semiconductor layer is formed of Zn-doped InP, and the diffusion suppressing layer is formed of InGaAsP.

The stripe-shaped multi-layer structure has a solid solubility with respect to impurities doped in the second-conductivity-type semiconductor layer that is higher than that of the second-conductivity-type semiconductor layer. It is preferable to include a layer between the active layer and the diffusion suppressing layer.

The method of manufacturing a semiconductor laser according to the present invention is the first
A step of growing a multilayer structure including a quantum well active layer having a modulation-doped structure on a conductive semiconductor substrate, an etching step of processing the multilayer structure into a stripe-shaped multilayer structure, and both sides of the stripe-shaped multilayer structure. A step of growing a current block portion for narrowing an electric current into the stripe-shaped multilayer structure, and forming a band gap smaller than the band gap of the current block portion on the stripe-shaped multilayer structure and the current block portion. The method includes the step of forming the diffusion suppressing layer, and the step of growing the second conductivity type semiconductor layer on the diffusion suppressing layer, thereby achieving the above object.

[0036]

The first current blocking layer of the present invention has a relatively low impurity concentration in a region near the stripe-shaped multilayer film including the active layer. Such a structure is obtained by epitaxially growing the first current blocking layer using the overhanging mask. This utilizes the fact that it is difficult for the growing film to take in the impurity element below the overhanging mask. As a result of the low impurity concentration near the active layer, the impurities do not adversely affect the active layer. Therefore, the reliability of the semiconductor laser is improved. Further, since a high concentration of impurities can be introduced into the first current block layer in a region away from the active layer, a current block portion in which thyristor operation is unlikely to occur can be obtained. As a result, the leak current is reduced.

Further, according to another aspect of the present invention, the end portion of the second current block layer has a shape in which punch through does not easily occur therein. Such a structure can be easily obtained by epitaxially growing the first and second current block portions using a mask that does not overhang. As a result of the film constituent elements coming to the mask surface gathering at both ends of the mask, a raised structure is obtained in the region of the second current blocking layer close to the active layer.

In yet another aspect of the present invention, a second conductive type semiconductor layer having a band gap different from the band gap of the current block portion is provided so as to cover the entire surface of the stripe-shaped multilayer structure and the current block portion. Since this semiconductor layer has a bandgap different from the bandgap of the current block part, an energy barrier is generated between the semiconductor layer and the current block part. As a result, the reactive current flowing into the current block portion is reduced, and the current is efficiently confined in the stripe-shaped multilayer structure.

According to still another aspect of the present invention, diffusion is performed between the quantum well type active layer having a modulation doping structure and the second conductivity type semiconductor layer formed above the stripe-shaped multilayer structure and the current block portion. A suppression layer is provided. The diffusion suppressing layer covers both the stripe-shaped multilayer structure and the current block portion, and has a function of suppressing diffusion of impurities contained in the second conductivity type semiconductor layer into the active layer. Since this diffusion suppressing layer has a band gap smaller than the band gap of the current blocking portion, an energy barrier is generated between the diffusion suppressing layer and the current blocking portion. As a result, the second
The reactive current flowing from the conductive type semiconductor layer to the current block portion is reduced, and the current is efficiently confined in the stripe-shaped multilayer structure.

In order to suppress the diffusion, the diffusion suppressing layer preferably has a higher solid solubility with respect to the impurities doped in the second conductivity type semiconductor layer than the second conductivity type semiconductor layer. . By doing so, the impurities diffused from the second conductivity type semiconductor layer are sufficiently dissolved, and it functions as a diffusion stopper to the active layer. For example, if the current block unit is In
When it is formed of P, I is used as the material of the diffusion suppressing layer.
If nGaAsP is selected, the above condition is satisfied.

The waveguide layer, in which the stripe-shaped multilayer structure has a solid solubility higher than that of the second conductivity type semiconductor layer with respect to impurities doped in the second conductivity type semiconductor layer, When it is included between the active layer and the diffusion suppressing layer, the diffusion suppressing effect is further improved.

[0042]

【Example】

(Embodiment 1) An embodiment of a semiconductor laser according to the present invention will be described below with reference to FIGS. 1 (a) and 1 (b).

First, reference will be made to FIG. The semiconductor laser of this embodiment uses an Sn-doped n-InP substrate 1 having a striped ridge formed on its upper surface. n-I
A striped multilayer structure is formed on the ridge of the nP substrate 1. N-In on both sides of the striped multilayer structure
On the P substrate 1, a current block portion provided with a stripe-shaped groove is formed. This groove reduces the parasitic capacitance and improves the high frequency characteristics of the semiconductor laser.

The stripe-shaped multilayer structure has a thickness of 150 nm.
N-InGaAsP waveguide layer 2, active layer 20, and n
-InGaAsP clad layer (waveguide layer) 13 is included from substrate 1 in this order. The active layer 20 is shown in FIG.
As shown in (b), 4 nm thick InGaAsP
Well layer 3 and InGaAsP having a thickness of 10 nm (λg =
1.15 μm) It has a multiple quantum well structure composed of the barrier layer 4. The number of quantum well layers is 7, and the cavity length is 3
00 μm.

The current block portion is made of p-doped Zn.
-InP first current blocking layer (thickness: about 1 μm) 6,
A Si-doped n-InP second current blocking layer (thickness: about 1 μm) 7 is included in this order from the substrate 1. An undoped InP layer 5 having a thickness of 10 nm is sandwiched between the p-InP first current blocking layer 6 and the substrate 1. On the n-InP second current blocking layer 7, Zn-doped p-InP third current blocking portion (thickness: about 4
μm) 8 is formed, and the p-InP third current block portion 8 also covers the upper surface of the stripe-shaped multilayer structure. The relatively thick p-InP third current block unit 8 has
It may have a two-layer structure. In particular, a lower layer having a relatively low Zn concentration (eg, Zn concentration: 1 × 10 ¹⁸ cm ⁻³ or less) and an upper layer having a relatively high Zn concentration (eg, Zn concentration: 1 × 10 ¹⁸ cm ⁻³ or more). It is preferably composed of

On the p-InP third current block unit 8,
P-InGaAsP for suppressing the Schottky barrier
A barrier relaxation layer 9, a p-GaInAs contact layer 10 for forming an ohmic contact, and a p-side electrode 12 made of Au / Zn are formed. An n-side electrode 11 made of Au / Sn is formed on the back surface of the substrate 1.

FIG. 2A shows in more detail the shape of the n-InP second current blocking layer 7 and the shape of the stripe-shaped multilayer structure of the semiconductor laser of this embodiment. The side surface of the stripe-shaped multilayer structure is a {111} P side surface which is more stable than the {111} In surface and is easy for crystal growth. In order to form a stripe-shaped multi-layer structure having a {111} P plane on its side surface, for example, InGaAsP crystal is used as the material of the cap layer 14 and etching is performed using an acetic acid etchant.
Then, the stripe height may be adjusted by etching using a hydrochloric acid-based etchant. The side surface of the striped multilayer structure may be a {111} In surface. In particular, if the Zn concentration of the p-InP first current blocking layer 6 is reduced to suppress the damage generated on the side surface of the active layer, good characteristics can be obtained even in the case of the {111} In surface. .

The p-InP first current blocking layer 6 is thinned in the vicinity of the stripe-shaped multilayer structure. p-
The upper surface of the thinned portion of the InP first current blocking layer 6 is the (114) plane. The p-InP first current blocking layer 6 includes a low-concentration region in which the concentration of Zn, which is an impurity of p-conductivity type, is relatively low, and a high-concentration region in which the concentration of Zn is higher than the low-concentration region. The low-concentration region is located closer to the stripe-shaped multilayer structure (the thinned portion) than the high-concentration region. In other words, the Zn concentration of the p-InP first current blocking layer 6 is relatively low in the vicinity of the active layer 20 (a region separated from the active layer 20 by only a few μm). When the p-InP first current blocking layer 6 is grown using a normal crystal growth method, the Zn concentration in the vicinity of the active layer 20 (a region separated from the active layer 20 by only a few μm) has a Zn concentration in the flat portion. It tends to be higher than the concentration. In other words, the p-InP first current blocking layer 6 having the Zn concentration distribution of this example cannot be obtained by the conventional manufacturing method.

The low Zn concentration reduces the influence of Zn on the active layer 20. Here, “the effect of Zn in the active layer” means
It means that Zn forms a defect serving as a non-emission center in the active layer or at the interface between the active layer and the first current blocking layer. Reduction of the influence of Zn on the active layer not only increases the optical output and the threshold value, but also produces high reliability. When the active layer has a modulation-doped structure, the diffusion of Zn destroys the modulation-doped structure is also included in the "effect of Zn in the active layer". This point will be described later with reference to the third embodiment.

In this embodiment, the concentration of Zn in the vicinity of the active layer 20 is relatively low, but the concentration of Zn in the flat portion (high concentration region) away from the active layer 20 is relatively low. large. Thus, even if the Zn concentration in the flat portion (high-concentration region) away from the active layer 20 is higher than usual, the influence of Zn on the active layer is reduced. By increasing the Zn concentration in the flat portion (high-concentration region) of the p-InP first current blocking layer 6, the thyristor current can be increased and a high-power semiconductor laser can be realized. p-In
The n-InP second current blocking layer 7 located on the P first current blocking layer 6 has a substantially flat upper surface.
In order to suppress the leak current, it is preferable that the n-InP second current block layer 7 be thick. However, the n-InP second current blocking layer 7 of the present embodiment is thinned in the vicinity of the stripe-shaped multilayer structure, and the angle of the end is an acute angle (for example, 40 degrees).

The operation of the semiconductor laser will be described below with reference to FIG.

To achieve laser oscillation, the p-side electrode 12
It is necessary to sufficiently invert the carrier distribution in the active layer 20 by passing a drive current of a predetermined value (threshold value) or more between the n-side electrode 11 and the n-side electrode 11. In order to obtain a high light output with a low threshold value, in this embodiment, the current flowing between the p-side electrode 12 and the n-side electrode 11 is changed by the current block unit.
It is confined within the striped multilayer structure.

As a result of evaluating the semiconductor laser of this example,
According to the semiconductor laser of FIG. 1A, it was found that laser oscillation was stably performed with a drive current of 20 mA (oscillation threshold:
20 mA). This oscillation threshold is almost the same as the oscillation threshold of the semiconductor laser in which the buried layer is grown by the LPE method.
Further, as a result of reliability evaluation by an acceleration test (70 ° C., 150 mA, 100 hr), it was found that the threshold variation was almost 0%.

These evaluation results show that in the semiconductor laser of this example, diffusion of Zn into the active layer was suppressed. The diffusion of Zn into the active layer is suppressed because the active layer 20 of the p-InP first current block layer 6 is suppressed.
This is because the Zn concentration is relatively low in the vicinity of.
In order to suppress the diffusion of Zn into the active layer, if p-I
When the concentration of Zn is reduced over the entire nP first current blocking layer 6, p-I is generated during operation of the semiconductor laser.
The nP first current blocking layer 6, the n-InP second current blocking layer 7, and the p-InP third current blocking unit 8 cause a thyristor operation, and the output of laser light is reduced.

FIG. 5 shows p for the semiconductor laser of FIG.
The relationship between the dopant concentration of each of the -InP first current blocking layer 6 and the n-InP second current blocking layer 7 and the presence or absence of the thyristor operation is shown. In FIG.
Area a indicates a highly reliable area. Region c is
A region d, which is a region where the thyristor operation does not occur, is a region where the thyristor operation can occur. p-InP 1st
Current blocking layer 6 and n-InP second current blocking layer 7
It is preferable that the dopant concentration of each of the above is set within the region c. In the case of the present embodiment, the reason why it is wide within the range shown and high in reliability is that the Zn concentration is low near the active layer 20. This point is different from the embodiment described later (see FIG. 10).

Next, a method of manufacturing the semiconductor laser of FIG. 1A will be described with reference to FIGS.
First, on a Sn-doped InP substrate 1, a 150 nm-thick n-InGaAsP (λg =
1.15 μm) Growing the waveguide layer 2. Next, film thickness 4n
Ga _0.1 In _0.9 As _0.5 P _0.5 well layer 3, thickness 10 n
GaInAsP (λg = 1.37 μm) barrier layer 4 of m
7 layers of the well layer 3 and the barrier layer 4 are repeatedly grown to form a multiple quantum well active layer having 7 pairs. Then, a p-InP clad layer 13 having a thickness of 400 nm and p-InGaAsP having a thickness of 100 nm (λg = 1.3 μm) are formed.
m) grow the cap layer 14. The p-InP clad layer 13 and the p-InGaAsP cap layer 14 have different etching characteristics with respect to at least one type of etchant.

Next, a silicon nitride film (thickness: 50 to 300)
nm) on the p-InGaAsP cap layer 14, and then the silicon nitride film is dry-etched in a stripe shape to form a stripe silicon nitride film (width: about 3 to 6 μm) 1
Get 5. After that, the stripe-shaped silicon nitride film 15 is not substantially etched, and the p-InP cladding layer 1 is
Using an etchant that preferentially etches the p-InGaAsP cap layer 14 over p.
The sP cap layer 14 to the substrate 1 are etched.
An acetic acid-based etchant is used as such an etchant.

If the adhesion between the p-InGaAsP cap layer 14 and the stripe-shaped silicon nitride film 15 is set to a relatively poor state, lateral etching proceeds rapidly, and the stripe-shaped silicon nitride film 15 is formed. The semiconductor layer located directly below the substrate is also etched. As a result, as shown in FIG. 3B, the stripe-shaped silicon nitride film 15 overhangs. After that, a predetermined area on the upper surface of the substrate 1 is etched with a hydrochloric acid-based etchant to adjust the height of the stripe-shaped ridge formed on the upper surface of the substrate 1, and the total stripe height is set to about 2 μm. The etching conditions are adjusted so that the width of the striped ridge falls within the range of 1 to 1.5 μm.

Next, by the MOVPE method, undoped I
nP layer 5, p-InP first current blocking layer 6, and n-
The InP second current blocking layer 7 is continuously epitaxially grown. Then, the striped silicon nitride film 15 and the cap layer 14 are removed, and the p-InP third current block portion 8 is removed.
And p-GaInAsP barrier relaxation layer 9 and p-GaInAsP
The contact layer 10 is grown by the MOVPE method. After that, the n-side electrode 11 and the p-side electrode 12 are vapor-deposited, and then, as shown in FIG.
The structure shown in FIG.

FIG. 13 is a sectional view schematically showing the growth process of the p-InP first current block layer 6, the n-InP second current block layer 7 and the p-InP third current block portion 8. In this embodiment, the side surface of the ridge provided on the substrate is composed of a plane including the {111} P plane. The angle formed by the side surface of the ridge and the upper surface of the substrate, that is, the (001) plane, exceeds 90 degrees. P-InP near the ridge
The growth surface of the first current blocking layer 6 is composed of the (112) surface to the (114) surface. When the growth surface is the (114) plane, dangling bonds are less likely to be formed in the growth layer as compared with the case where the growth surface is the {111} plane,
Incorporation of impurity elements is also small.

Hereinafter, the step of forming the p-InP first current block layer 6 and the like will be described in detail with reference to FIG. FIG.
7 is a graph showing growth conditions for the p-InP first current blocking layer 6. The vertical axis of the graph represents the substrate 1 in the MOVPE device.
Represents the temperature (Celsius) of the heater for heating, and the horizontal axis represents the time (minutes) measured from the start of heating the heater. In the example of FIG. 4, the heater temperature is 60 when about 5 minutes have passed after the start of heating.
A temperature of 0 ° C is reached and then maintained at 600 ° C. The actual temperature of the substrate 1 does not reach 600 ° C. 5 minutes after the start of heating. The actual temperature of the substrate 1 becomes steady after a few minutes after the heater temperature becomes steady.

In the present embodiment, the substrate 1 was heated to PH3 and AsH in the chamber of the MOVPE device while the temperature was raised for 5 minutes.
Expose to 3 mixed atmosphere. When 5 minutes have passed after the start of heating, the growth of the undoped InP layer 5 is immediately started, and then the p-InP layer 6 is grown. Usually, as in the present embodiment, after the growth of the undoped InP layer 5, the p-I is continuously formed.
The growth of the nP layer 6 may cause the following problems. That is, when the undoped InP layer 5 is being grown, the temperature of the substrate 1 has not yet risen sufficiently, so that the growth of the undoped InP layer 5 remains incomplete and the p-InP layer 6 is not yet grown. The problem is that the growth of If the growth of the undoped InP layer 5 is incomplete, the side surface of the active layer is not sufficiently covered by the undoped InP layer 5. In such a case, p-In
If the dopant (Zn) is contained in the P layer 6 at a high concentration, the excess dopant will pass through the undoped InP layer 5 and reach the active layer. This causes damage to the sides of the active layer. Since the concentration of Zn is relatively low in the vicinity of the active layer 20 in the p-InP first current blocking layer 6 of the present embodiment, excess dopant passes through the undoped InP layer 5 and reaches the active layer. There is a very small risk that it will happen. In this embodiment, since the p-InP layer 6 can be continuously grown after the growth of the undoped InP layer 5, the time required for the growth process is shortened.

As described above, the atmospheric gas flow rate during the temperature rise before InP growth is set to be the same as that of the Group 5 element during the growth of the active layer, and the desorption of the Group 5 element from the active layer 20 is suppressed and the first If the undoped layer is grown before growing the current blocking layer, the dopant in the first current blocking layer can be prevented from aggregating at the interface with the active layer 20.

Generally, the Group 3 element and the dopant that have reached the silicon nitride film 15 during crystal growth are the silicon nitride film 15
It diffuses upward in the lateral direction and contributes to the crystal growth around the silicon nitride film 15. For this reason, normally, the crystal growth rate of the p-InP layer 6 increases and the dopant concentration also increases near the side surface of the striped ridge. However, in the case of this embodiment, the overhanging silicon nitride film 1 is
5 suppresses the supply of the Group 3 element and the dopant to the side surface of the striped ridge due to the mask effect. As a result, the p-InP layer 6 becomes relatively thin under the overhanging silicon nitride film 15 and has a low dopant concentration. The thinning of the p-InP layer 6 and the decrease in the dopant concentration become more remarkable as the side surface of the striped ridge is approached below the overhanging silicon nitride film 15. Therefore, even if the dopant concentration of the p-InP layer 6 located in the flat portion is increased in order to suppress the thyristor current, the dopant concentration becomes low in the vicinity of the active layer, and the solid solubility limit of impurities is not exceeded.
Therefore, the dopant concentration in the flat portion can be sufficiently increased.

Crystal growth of the p-InP layer 6 was performed at 600 degrees as shown in FIG. At temperatures below this, crystals around the mask of the n-InP layer 7 may grow abnormally. If the growth temperature is below 560 degrees,
There is a possibility that the growth selectivity (face orientation dependency) may deteriorate. Further, when the temperature is 560 ° C. or lower, the temperature dependence of the Zn concentration becomes large, so that the dopant concentration in the grown semiconductor layer is not stable. Therefore, the growth temperature is preferably 600 degrees or higher.

The flow rates of PH3 and AsH3 are determined by the waveguide layer 2
Was the same as the flow rate supplied when growing the. When only PH3 was supplied and the undoped InP layer was not inserted, the threshold current was doubled.

Example 2 Hereinafter, FIGS. 6A and 6B are shown.
Another embodiment of the semiconductor laser according to the present invention will be described with reference to FIG.

First, FIG. 6A will be referred to. The semiconductor laser of this embodiment uses an Sn-doped InP substrate 1 having a striped ridge formed on the upper surface. A stripe-shaped multilayer structure is formed on the ridge of the Sn-doped InP substrate 1. S on both sides of the striped multilayer structure
On the n-doped InP substrate 1, as in the embodiment shown in FIG. 1A, a current block portion having stripe-shaped grooves is formed.

The striped multilayer structure has a thickness of 150 nm.
The n-InGaAsP waveguide layer 2, the active layer 20, and the cladding layer 13 are included in this order from the substrate 1. The active layer 20 has a thickness of 4 nm as shown in FIG.
InGaAsP well layer 3 and InG having a thickness of 10 nm
It has a multiple quantum well structure composed of an aAsP (λg = 1.15 μm) barrier layer 4. The number of quantum well layers is 7, and the cavity length is 300 μm.

The current block portion is a Zn-doped p-type.
-InP first current blocking layer (thickness: about 1 μm) 6,
A Si-doped n-InP second current blocking layer (thickness: about 1 μm) 7 is included in this order from the substrate 1. An undoped InP layer 5 having a thickness of 10 nm is sandwiched between the p-InP first current blocking layer 6 and the substrate 1. On the n-InP second current blocking layer 7, Zn-doped p-InP third current blocking portion (thickness: about 4
μm) 8 is formed, and the p-InP third current block portion 8 also covers the upper surface of the stripe-shaped multilayer structure. Relatively thick p-InP third current blocking unit 8
May have a two-layer structure. In particular, a lower layer having a relatively low Zn concentration (for example, Zn concentration: 1 × 10 ¹⁸ cm ⁻³
Lower) and a relatively high Zn concentration in the upper layer (eg,
Zn concentration: higher than 1 × 10 ¹⁸ cm ⁻³ ) is preferable.

On the p-InP third current block unit 8,
P-InGaAsP for suppressing the Schottky barrier
Barrier relaxation layer 9, p- for making ohmic contact
A GaInAs contact layer 10 and a p-side electrode 12 made of Au / Zn are formed. On the back side of substrate 1,
An n-side electrode 11 made of Au / Sn is formed. FIG. 2B shows the n-InP second semiconductor laser of the present embodiment.
The shape of the current blocking layer 7 and the shape of the stripe-shaped multilayer structure are shown in more detail. The side surface of the striped multilayer structure is a {111} In surface.

The n-InP second current blocking layer 7 has a substantially uniform film thickness, but is bent upward in the vicinity of the stripe-shaped multilayer film. This is because, when the n-InP second current block layer 7 is crystal-grown, the crystal growth rate becomes relatively large in the vicinity of the stripe-shaped multilayer film. As a result, as shown in FIG. 2B, the thickness of the n-InP second current block layer 7 in the vicinity of the stripe-shaped multilayer film is about twice the film thickness at other positions.

The angle of the end portion of the n-InP second current block layer 7 of this embodiment is 80 degrees or more. n-InP second
The relationship between the shape of the end of the current blocking layer 7 and the leakage current will be described with reference to FIGS. 7A to 7D.

FIGS. 7A and 7B show the second current blocking layer in which the angle of the end portion close to the active layer is 45 degrees or less. During operation of the semiconductor laser, a depletion layer is formed in the second current block layer. The depletion layer extends from a PN junction formed between the second current blocking layer and a semiconductor layer of another conductivity type that is in contact with the second current blocking layer. The depletion layer is more likely to spread horizontally at the end as the angle of the end of the second current blocking layer is smaller. The depletion layer spreading in the horizontal direction at the end portion leaks a part of the drive current.

FIGS. 7C and 7D show the second current blocking layer in which the angle of the end portion near the active layer is about 90 degrees. In such a second current blocking layer,
Even at the edges, the depletion layer does not easily spread in the horizontal direction. Therefore, the drive current is relatively unlikely to leak at the end portion of the second current block layer.

According to the embodiment of FIG. 6A, the leak current generated by the contact between the depletion layers as described above is reduced. When the leak current is reduced, a large light output can be obtained with a relatively small drive current. Further, since the linear relationship between the optical output and the drive current is improved, the modulation distortion of the laser is reduced.

As a result of evaluating the semiconductor laser of this example,
It was found that laser oscillation was stably performed with a drive current of 20 mA (oscillation threshold: 20 mA). This oscillation threshold is LPE
It is almost the same as the oscillation threshold of the semiconductor laser in which the buried layer is grown by the method. The optical output increased 1.5 times as compared with the conventional example. In addition, accelerated test (70 ℃, 150mA, 1
As a result of reliability evaluation based on 00 hr), it was found that the threshold variation was almost 0%.

FIG. 8 shows the relationship between the variation rate of the threshold current and the Zn concentration of the p-InP first current blocking layer 6.
It can be seen from FIG. 8 that the threshold fluctuation rate increases when the Zn concentration of the p-InP first current blocking layer 6 is 1.0 × 10 ¹⁸ cm ⁻³ or more. It is considered that when the Zn concentration of the p-InP layer 6 exceeds the above value, the Zn concentration exceeds the solid solubility limit in the vicinity of the side surface of the active layer, which causes threshold variation. In order to set the threshold fluctuation rate to 0%, the Zn concentration of the p-InP first current blocking layer 6 is 0.7 × 10 ¹⁸ cm ^−3.
It is preferable to set the following.

FIG. 9 shows the relationship between the thyristor current (which constitutes a reactive current) and the Zn concentration of the p-InP first current block layer 6 and the Si concentration of the n-InP layer 7.
From FIG. 9, in order to reduce the thyristor current, n-InP
It is understood that it is preferable that the Si concentration of the layer 7 is 1 × 10 ¹⁸ cm ⁻³ or more. By setting the impurity concentration in this range, the thyristor current was not generated when the driving current was 300 mA or less, and the characteristics suitable for practical use were obtained. FIG.
0 shows the relationship between the dopant concentration of each of the p-InP first current blocking layer 6 and the n-InP second current blocking layer 7 and the presence or absence of the thyristor operation in the semiconductor laser of FIG. In FIG. 10, a region a indicates a highly reliable region, and a region b indicates a poorly reliable region. Areas c and e are areas where thyristor operation does not occur, and area d is an area where thyristor operation can occur. The dopant concentration of each of the p-InP first current blocking layer 6 and the n-InP second current blocking layer 7 is preferably set within the region c.

FIG. 11 shows the relationship between Zn concentration and hole concentration. In the case of InP, the hole concentration is 1 × 10 ^18.
While saturated at cm ⁻³ , InGaAsP saturates at about 5 × 10 ¹⁸ cm ⁻³ . When the hole concentration is saturated, even if Zn increases, Zn does not enter the lattice position but exists in the lattice. It is necessary to reduce the excess Zn, that is, the Zn located between the lattices, in order to improve the reliability of the semiconductor laser. For that purpose, it is preferable to set the Zn concentration in a range where the hole concentration is not saturated. In the case of InP, the Zn concentration is preferably 7 × 10 ¹⁷ cm ⁻³ or less, which is half the saturation concentration or less.
In the case of GaAsP, the Zn concentration is preferably 3 × 10 ¹⁸ cm ⁻³ or less, which is half the saturation concentration or less.

Referring to FIGS. 12 (a) to 12 (c),
A method of manufacturing the semiconductor laser of FIG. 6A will be described. First, on a Sn-doped InP substrate 1, a 150 nm-thick n-InGaAsP (λg =
1.15 μm) Growing the waveguide layer 2. Next, film thickness 4n
Ga _0.1 In _0.9 As _0.5 P _0.5 (λg = 1.37μ
m) Well layer 3, GaInAsP having a thickness of 10 nm (λg =
1.15 μm) With the barrier layer 4 as one pair, seven layers of the well layer 3 and the barrier layer 4 are repeatedly grown to obtain a multiple quantum well active layer having 7 pairs. Then, p-In with a thickness of 400 nm
The P clad layer 13 is grown.

Next, after depositing the silicon nitride film, the silicon nitride film is dry-etched into a stripe shape to obtain a stripe-shaped silicon nitride film (width: 1.5 μm to 3 μm) 15.
Then, the p-InP clad layer 13 is etched using a hydrochloric acid-based etchant that does not substantially etch the silicon nitride film 15 and etches the p-InP clad layer 13. Next, using an acetic acid-based etchant,
Etching is performed from the active layer to the upper surface of the substrate 1. As a result, as shown in FIG. 12B, an inverted mesa-shaped striped multilayer structure is obtained. Silicon nitride film 1 of this embodiment
5 does not overhang as much as the silicon nitride film 15 of FIG.

Next, by the MOVPE method, undoped I
The nP layer 5, the p-InP layer 6, and the n-InP layer 7 are epitaxially grown. After that, the silicon nitride film 15 is removed, and the p-InP current block portion 8 and the p-GaInAsP are formed.
The barrier relaxation layer 9 and the p-GaInAs contact layer 10 are M
Embedding is grown by the OVPE method. Then, the n-side electrode 11 and the p-side electrode 12 are vapor-deposited to obtain the structure shown in FIG.

FIG. 14 is a sectional view schematically showing the growth process of the p-InP first current blocking layer 6, the n-InP second current blocking layer 7, and the p-InP third current blocking portion 8. In this embodiment, the side surface of the ridge provided on the substrate is composed of a surface including the {111} In surface. The angle formed by the side surface of the ridge and the upper surface of the substrate, that is, the (001) surface is
It is less than 90 degrees. In the vicinity of the ridge, the growth surface of the p-InP first current block layer 6 is the (112) plane. When the growth surface is the (112) plane, the number of dangling bonds is larger and the amount of impurities taken in is larger than when the growth surface is the (114) plane. Above the active layer 20, (00
1) The surface is obtained.

Hereinafter, the method of forming the p-InP first current block layer 6 and the like will be described in detail with reference to FIG. 4 again. Also in this embodiment, the substrate 1 is exposed to the mixed atmosphere of PH3 and AsH3 in the chamber of the MOVPE apparatus during the temperature rise for 5 minutes. When 5 minutes have passed after the start of heating, the growth of the undoped InP layer 5 is immediately started. However, after forming the undoped InP layer 5, a growth interruption period of 10 minutes is set, and then the p-InP layer 6 is grown. This is p-
This is to sufficiently raise the temperature of the substrate 1 before the growth of the InP layer 6. As a result, p-I near the active layer
The increase of the dopant (Zn) in the nP layer 6 is prevented. In this example also, the crystal growth temperature was set to 600 degrees. The above method is more preferable as the method of forming the current blocking layer 6, but any of the methods shown in FIG. 4 may be adopted in both Examples 1 and 2.

In order to suppress the leakage current, the film thickness of the second current block layer should be large. Therefore, in this embodiment, an insulating film stripe having good adhesion is formed on the clad layer so that the cap layer width and the mask width are substantially the same. As a result, there is no shadow effect due to the mask and the elements on the mask are supplied by diffusion, so the growth rate increases. As a result, as shown in FIGS. 7C and 7D, the film thickness in the vicinity of the stripe becomes large,
Leakage current due to contact of the depletion layer, that is, punch-through, generated inside the current block layer is suppressed, and the current is concentrated in the active layer, whereby the light emission current can be increased. Here, the growth temperature is set to 600 degrees, which is the lower limit growth temperature at which a good crystal can be obtained, and an excessive element supplied from the mask is suppressed from being diffused on the crystal. Further, since the cap layer width and the mask width are the same, the side surface of the stripe has a step constituted by a {111} In surface. Since the angle of this plane is 90 degrees or less, as shown in FIG. 2B, the crystal surface near the stripe is close to the (112) plane, and the growth rate is further promoted. In addition, as shown in FIG. 2B, {111} P
In the case of a plane, the growth rate is about 70% of that of the (112) plane because the (114) plane is grown. This stripe shape can be obtained by etching the clad layer with a hydrochloric acid-based etchant and then with an acetic acid-based etchant.

In Examples 1 and 2, InP was used as the material for the p-type current block portions 6 and 8. But I
InGaAsP to InGaAs materials may be used instead of nP. InGaAsP to InGa
Since the materials up to As have higher Zn solid solubility than InP, the influence of Zn diffusion into the active layer 20 can be further reduced.
As a material for the first and second current blocking layers 6 and 7, I
A material having a bandgap larger than nP, for example, In
If a material from GaAsP to InGaP is used, it is easy to suppress the occurrence of thyristor operation. On the contrary, as the material of the third current block portion 8, a material having a bandgap smaller than InP, for example, InGaAsP to InGaP is used.
If materials up to are used, the occurrence of thyristor operation can be easily suppressed, and the leak current can be further reduced.

In Examples 1 and 2, crystals of InP compound semiconductor were used, but other crystals such as Ga are used.
As series, ZnSeS series, InA1As series, A1GaAs
You may use the crystal | crystallization of semiconductor materials, such as a GaInA1AsP type | system | group.

Further, the present invention is a DFB other than the DH laser.
It can also be applied to high value-added lasers such as lasers and DBR lasers. Although the structure of the buried layer is the PBH type, other structures may be used.

Furthermore, in Examples 1 and 2, the active layer 20 was used.
Although the quantum well structure is adopted for the above, a strained quantum well structure may be adopted.

Although the waveguide layer is formed of a simple InGaAsP layer, InGaAsP having a graded composition is used.
You may form from a layer. The crystal growth method is not limited to the MOVPE method, and other growth methods such as a gas source MBE method, a MONBE method, and a hydride VPE method may be used.

The dopant may be a dopant other than Zn and Si. The insulating film is a silicon nitride film, but any material having good selectivity such as an oxide film may be used. Furthermore, although an n-type substrate is used as the substrate, a p-type substrate may be used.

(Embodiment 3) FIG. 15 shows a cross section of still another semiconductor laser according to the present invention. This semiconductor laser includes an n-type InP substrate 31 doped with Sn, an In
And a stripe-shaped multilayer structure including a strained quantum well active layer 35 having a modulation doping structure and formed on a P substrate 31. The active layer 35 is composed of a strain well layer (6 nm) 3 having a compressive strain of 1% and an InGaAsP barrier layer (thickness 15 nm) 34 having a bandgap wavelength of 1.31 μm. The number of wells is 5. The barrier layer 34 is
Undoped InGaAsP layer 34a and Zn-doped InGaAsP layer (Zn concentration: 5 × 10 ¹⁸ cm ⁻³ ) 34
b and an undoped InGaAsP layer 34c.

In the stripe-shaped multilayer structure, the strain modulation doped quantum well active layer 35 is an n-type InGaAsP.
Waveguide layer (thickness: 30 nm) 32 and p-type InGaA
It is sandwiched between sP waveguide layers (thickness: 100 nm) 36. The p-type InGaAsP waveguide layer 36 has 5 × 10 ¹⁷
cm ⁻³ of Zn is doped.

On both sides of the striped multilayer structure, current block portions for narrowing the current into the striped multilayer structure are provided. The current block portion is the p-type InP layer 3
7a and the n-type InP layer 37b, and the PN junction is formed inside. When driving the semiconductor laser, this P
Reverse bias is applied to the N-junction.

A diffusion suppressing layer (thickness: 200 nm, so as to cover both the stripe-shaped multilayer structure and the current block portion).
Zn concentration: 5 × 10 ¹⁷ cm ⁻³ ) 38 is provided.
A p-type InP clad layer (thickness:
4 μm, Zn concentration: 5 × 10 ¹⁷ cm ⁻³ ) 39, p-type In
GaAs cap layer (thickness: 200 nm, Zn concentration: 5)
× 10 ¹⁸ cm ⁻³ ) 40 and the p-side electrode 41 are laminated in this order. An n-side electrode 42 is provided on the back surface of the substrate 31.

When the p-type InP clad layer 39 contains a dopant between the lattices, the p-type InP clad layer 39
Is a source of dopant. Since Zn is used as the p-type dopant in this example, the present invention will be described below with respect to the diffusion of Zn. However, other dopants such as Be, Mg, Cd, Se, S, Te or C are used. Similar arguments hold when using.

The Zn solid solubility of the InP layer changes depending on the InP layer growth conditions (temperature, total gas flow rate, etc.),
It is about 1 × 10 ¹⁸ cm ⁻³ . When the Zn concentration of the p-type InP cladding layer 39 is set to 5 × 10 ¹⁷ cm ⁻³ or less,
The concentration of Zn existing between the lattices of the clad layer 39 is negligibly small, so that Zn diffusion from the p-type InP clad layer 39 to the active layer 35 is considerably suppressed.

In this embodiment, the p-type InP clad layer 39 is used.
Not only lowers the Zn concentration, but also p-type In
Between the P clad layer 39 and the active layer 35, two kinds of In
A GaAsP layer, that is, a p-type InGaAsP waveguide layer 36 and a p-type InGaAsP diffusion suppressing layer 38 are provided. Since InGaAsP has a higher solid solubility than InP, even if Zn located between the lattices of the p-type InP clad layer 39 above diffuses, it sufficiently dissolves Zn.
It functions so as not to substantially diffuse downward. Therefore, the modulation-doped structure of the modulation-doped quantum well active layer 35 is preserved without being finally destroyed.

In order to reduce the resistance, the p-type InGaAsP waveguide layer 36 and the p-type InGaAsP diffusion suppressing layer 38 have 5
× 10 ¹⁷ cm ^-3 Zn is doped, but I
Since the solid solubility of nGaAsP is sufficiently higher than 1 × 10 ¹⁸ cm ⁻³ , the doped Zn exists in the lattice and does not substantially contribute to diffusion.

If, for example, a p-type InGaAsP diffusion suppressing layer 38 is provided between the p-type InP clad layer 39 and the active layer 35.
When only the p-type InGaAsP waveguide layer 36 is provided without providing the above, it is necessary to sufficiently increase the thickness of the p-type InGaAsP waveguide layer 36. However, p-type InGaAs
As the thickness of the P waveguide layer 36 is increased, the p-type I
A leak current easily flows from the nP cladding layer 39 to the p-type InP layer 37a via the side portion of the p-type InGaAsP waveguide layer 36. However, according to the configuration of the present invention, p
While maintaining the thickness of the InGaAsP waveguide layer 36 at a thickness at which the leak current does not increase remarkably, the InGaA having the required thickness between the p-type InP cladding layer 39 and the active layer 35 is formed.
Since the sP layer is provided, the above problem does not occur.

In order to make the light distribution of the semiconductor laser uniform, it is preferable to make the p-type InP cladding layer 39 sufficiently thick (for example, 4 μm or more). In order to thicken the p-type InP clad layer 39, it is necessary to perform crystal growth for a long time at the growth temperature. If there is no diffusion suppressing layer 38, if the p-type InP clad layer 39 is grown thick, a problem of dopant diffusion from the p-type InP clad layer 39 to the active layer 35 will occur during the growth process. However, according to the present invention, since the problem does not occur due to the presence of the diffusion suppressing layer 38, the p-type InP clad layer 39 can be grown to a required thickness, and a semiconductor laser excellent in light distribution can be obtained. To be

Further, the diffusion suppressing layer (In
Since the band gap of GaAsP) 38 is smaller than the band gap of InP, it becomes difficult for an injection current (hole current) to flow into the p-type InP layer 37a forming the current block portion, and a leak current that does not contribute to light emission is reduced. The effect is obtained. Therefore, the linearity of the optical output-injection current characteristic is improved, and the modulation distortion is reduced as compared with the analog modulation.

The reduction of the leak current by the diffusion suppressing layer 38 will be described below with reference to FIGS. 16 (a) to 16 (d).

FIG. 16D is a schematic cross-sectional view corresponding to the cross-sectional view of FIG. 15, showing three types of current paths A, B and C.
Is shown. The current bus C shows a path of an active current contributing to laser oscillation, and the current buses A and B show a path of a reactive current not contributing to laser oscillation. Note that a1
To a5, b1 to b5, and c1 to c5 indicate the regions of the semiconductor through which the current flows for each current path.

As shown in FIG. 16A, since the band gap of the diffusion suppressing layer 38 is smaller than that of the n-type InP layer 37b, there is an energy gap between the regions a4 and a3. Therefore, no current can flow from the region a4 to the region a3, and the current A does not occur until the thyristor state is established. The holes injected from the region a5 to the region a4 are
It will be injected into the region c3.

As shown in FIG. 16B, since the band gap of the diffusion suppressing layer 38 is smaller than that of the p-type InP layer 37a, there is an energy gap between the region b4 and the region b2. Therefore, it is difficult for the current to flow from the region b4 to the region b2, and the current B is extremely small.

As shown in FIG. 16C, the area a5
The holes injected into the regions a4 and b4 from the regions a and b5 respectively flow into the region c3.

As described above, according to this embodiment,
The current introduced from the p-side electrode 41 is applied to the current block unit 3
7 and the diffusion suppressing layer 38 serve to efficiently squeeze the striated multilayer structure. The current squeezed in the stripe-shaped multilayer structure is injected into the active layer 35 and contributes to laser oscillation. In order to obtain light having a wavelength of 1.31 μm, the thickness of the strain well layer 33 of the active layer 35 is set to the strain well layer 3
It is set to a required value according to the composition of the semiconductor material used in No. 3. In this example, Ga0.1In0.9As0.
The well layer 33 is formed from 5P0.5 and its thickness is 6 nm.
Is set to Ga0.1In0.9As0.5P
From the bulk semiconductor material of 0.5, the calculated value is 1.41.
Although the wavelength light of μm can be obtained, this embodiment (resonator length 300
In the case of (μm), it was confirmed that light having a wavelength of 1.30 μm was obtained. This energy shift (70 meV) is due to the quantum size effect. The oscillation threshold is 15
mA, relaxation oscillation frequency was 2.2 GHz / mA ^1/2 , and transmission distortion IM2 was less than -65 dBc (modulation degree 20
%). This is because the leak current is reduced and the modulation-doped structure of the strained quantum well active layer 35 is preserved.

Although the diffusion suppressing layer 38 of this embodiment has a one-layer structure, it may have a structure of two or more layers. In that case, the properties that the energy band gap is lower than that of the current block portion and the property that the impurity solid solubility is higher than that of the p-type cladding layer may be realized by separate layers.

Next, with reference to FIGS. 17A to 17D, a method of manufacturing the semiconductor laser shown in FIG. 15 will be described.

First, on the n-type InP substrate 31 doped with Sn, the n-type InGaAsP is formed by the MOVPE method.
A waveguide layer (λg = 1.31 μm) 32 is grown. Next, Ga _0.3 In _0.7 As _0.5 P _0.5 having a compression strain of 1%
The strain well layer 33 and the InGaAsP barrier layer (λg = 1.
31 μm) 34 is alternately and repeatedly grown five times to form a modulation-doped quantum well type active layer 35 including five quantum well layers. Thereafter, a p-type InGaAsP waveguide layer (λg = 1.31 μm) 36 is further grown. Thus, the structure shown in FIG. 17A is obtained.

Next, using the silicon nitride film patterned in stripes as an etching mask, p-type In
The GaAsP waveguide layer 36, the modulation-doped quantum well active layer 35, and the n-type InGaAsP waveguide layer 32 are selectively mesa-etched. Thus, as shown in FIG. 17B, a stripe-shaped multilayer structure including the active layer 35 is obtained.

Next, p-type In forming the current block section
The P layer 37a and the n-type InP layer 37b are selectively grown on both sides of the stripe-shaped multilayer structure by using the MOVPE method. After removing the silicon nitride film, the diffusion suppressing layer 38, p-type I
The nP clad layer 39 and the p-type InGaAs cap layer 40 are grown using the MOVPE method. Thus, the structure shown in FIG. 17C is obtained.

Finally, by evaporation, the p-side electrode 41 and n
The side electrode 42 is formed.

During the crystal growth step by the MOVPE method, the total flow rate of the gas flowing in the growth chamber was 5 L / min, and the growth temperature was 640 ° C.

The growth of the diffusion suppressing layer 38 on the current block portion is achieved by supplying AsH3 and PH3 into the growth chamber at 640 ° C. AsH3 and P which are flown during the growth of the diffusion suppressing layer 38 until the temperature is raised to 640 ° C.
By supplying H3 into the growth chamber, defects at the interface between the waveguide layer 36 and the diffusion suppression layer 38 are reduced.

As the well layer, a strained well layer having a compressive strain of 1% was used. However, since the modulation doping effect does not depend on the strain amount, the strain amount has another value (for example, a value that causes "tensile"). It may be set. In addition, as the configuration of the semiconductor laser,
A DFB or DBR structure other than the DH structure may be adopted. Furthermore, a configuration other than PBH may be adopted for the current block unit.

The present invention is applicable not only to semiconductor lasers but also to other electronic devices (HEMTs, HFETs, and HBTs), waveguide elements, light-receiving elements, and the like, as long as the elements have a modulation quantum well structure. It is possible. In particular, FIG.
When applied to the semiconductor lasers shown in (a) and 6 (a),
The reliability is further improved and the leak current is reduced.

[0120]

According to the present invention, the impurity concentration of the first current block layer is relatively low in the region near the stripe-shaped multilayer film including the active layer. Therefore, the impurities do not adversely affect the active layer, and the reliability of the semiconductor laser is improved. Further, since a high concentration of impurities can be introduced into the first current block layer in a region away from the active layer, a current block portion in which thyristor operation is unlikely to occur can be obtained. As a result, the leak current is reduced. Further, by increasing the angle of the end portion of the second current block layer, punch-through is less likely to occur inside the end portion, and the leak current is reduced.

According to another aspect of the present invention, due to the function of the diffusion suppressing layer, the modulation doping structure of the quantum well type active layer does not disappear during the manufacturing process and exhibits its original characteristics.
Since the diffusion suppressing layer covers both the striped multilayer structure and the current block portion and has a band gap smaller than the band gap of the current block portion, the reactive current flowing into the current block portion is reduced. Even in the striped multilayer structure, the modulation-doped structure is more stably preserved by providing the waveguide layer formed of the material having the same property as that of the diffusion suppressing layer. Thus, the efficiency of constriction can be improved and the reactive current can be reduced while the advantages of the modulation dope structure are actually exhibited. As a result, the linearity of the optical output with respect to the injection current is improved, and a semiconductor laser having low transmission distortion characteristics is provided.

[Brief description of the drawings]

1A is a sectional view of a semiconductor laser according to the present invention, and FIG. 1B is an energy diagram showing an active layer of the semiconductor laser.

2A is a cross-sectional view of the main part of the semiconductor laser shown in FIG.
FIG. 3B is a sectional view of the main part of another semiconductor laser according to the present invention.

3A to 3C are process cross-sectional views showing a manufacturing process of the semiconductor laser of FIG.

FIG. 4 is a graph for explaining a process before a growth process of a current block part.

5 is a graph showing the relationship between the carrier concentration in the current block section and the thyristor operation in the semiconductor laser of FIG. 1 (a).

6A is a sectional view of a semiconductor laser according to the present invention, and FIG. 6B is an energy diagram showing an active layer of the semiconductor laser.

FIG. 7A to FIG. 7D are diagrams schematically showing the relationship between the shape of the current block portion near the active layer and the leak current.

FIG. 8 is a diagram showing the relationship between the Zn concentration of the first current blocking layer and the fluctuation of the threshold current.

FIG. 9 is a diagram showing the dependence of the thyristor current on the Zn concentration of the first current blocking layer and the Si concentration of the second current blocking layer.

FIG. 10 is a graph showing the relationship between the carrier concentration of the current block section and the thyristor operation in the semiconductor laser of FIG. 6 (a).

FIG. 11 is a graph showing the relationship between Zn concentration and hole concentration.

12A to 12C are process cross-sectional views showing the manufacturing process of the semiconductor laser of FIG. 6A.

FIG. 13 is a cross-sectional view showing the growth process of the current block portion of the semiconductor laser of FIG.

FIG. 14 is a cross-sectional view showing the growth process of the current block portion of the semiconductor laser of FIG. 6 (a).

FIG. 15 is a sectional view showing still another semiconductor laser according to the present invention.

16A is a band diagram along a current path A, FIG. 16B is a band diagram along a current path B, and FIG. 16C is a band diagram along a current path C.
(D) is a schematic cross-sectional view corresponding to the cross-sectional view of FIG. 1.

17A to 17D are process cross-sectional views showing the manufacturing process of the semiconductor laser of FIG.

FIG. 18 is a cross-sectional view showing a conventional semiconductor laser.

FIG. 19 is a sectional view showing another conventional semiconductor laser.

FIG. 20 is a graph showing the results of a long-term high temperature acceleration test on a conventional semiconductor laser.

[Explanation of symbols]

 1 Sn-doped InP substrate 2 InGaAsP waveguide layer 3 InGaAsP well layer 4 InGaAsP barrier layer 5 Undoped InP layer 6 p-InP first current block layer 7 n-InP second current block layer 8 p-InP third current block portion 9 p-InGaAsP barrier relaxation layer 10 p-InGaAsP contact layer 11 n-side electrode made of Au / Sn 12 p-side electrode made of Au / Zn 13 p-InP clad layer 14 p-InGaAsP cap layer 15 silicon nitride film 31 Sn doped N-type InP substrate 32 n-type InGaAsP waveguide layer 33 strained well layer 3 34a undoped InGaAsP barrier layer 34b Zn-doped InGaAsP barrier layer 34c undoped InGaAsP barrier layer 35 strained quantum well active layer having a modulation-doped structure Layer 36 p-type InGaAsP waveguide layer 37a p-type InP layer 37b n-type InP layer 38 diffusion suppression layer 39 p-type InP clad layer) 40 p-type InGaAs cap layer 41 p-side electrode 42 n-side electrode

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasushi Matsui 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) Reference JP-A-5-218585 (JP, A) JP-A-6- 97592 (JP, A) JP-A-5-299764 (JP, A) JP-A-5-198894 (JP, A) JP-A-7-22691 (JP, A)

Claims (13)

(57) [Claims] 1. A semiconductor laser comprising: a substrate; a stripe-shaped multilayer structure formed on the substrate and including an active layer; and a current block section formed on the substrate on both sides of the stripe-shaped multilayer structure. The current blocking portion has a first conductivity type first current block layer and a second conductivity type second current block layer formed on the first current block layer. The second current block layer is bent upward on the stripe-shaped multilayer structure side, and the angle of the end portion of the second current block layer closest to the stripe-shaped multilayer structure is 80 degrees or more. Is a semiconductor laser. 2. The strike of the second current blocking layer.
The edge closest to the strip-shaped multilayer structure is
The semiconductor laser according to claim 1, wherein the semiconductor laser is located above the multilayer structure . 3. The concentration of the impurities of the first conductivity type in the first current blocking layer is 1 × 10 ¹⁸ cm ⁻³ or less, and the concentration of the impurities of the second conductivity type in the second current blocking layer is
The semiconductor laser according to claim 1, having a size of 1 × 10 ¹⁸ cm ⁻³ or more. 4. The side surface of the stripe-shaped multilayer structure is
The semiconductor laser according to claim 1, which is formed of a crystal plane including a {111} P plane . 5. The surface of the substrate and the stripe-shaped poly
Provided between the side surface of the layer structure and the current block portion
The semiconductor laser according to claim 1 , further comprising an undoped semiconductor layer . 6. The angle of the end portion of the second current blocking layer closest to the stripe-shaped multilayer structure is :
The semiconductor laser according to claim 1, wherein the semiconductor laser is 90 degrees. Wherein said active layer has a multiple quantum well structure, a semiconductor laser according to claim 1. 8. The side surface of the active layer has a relative impurity concentration.
2. The semiconductor laser according to claim 1, which is covered by a lower layer . 9. A substrate, a stripe-shaped multilayer structure formed on the substrate and including an active layer, and a first conductivity type first current blocking layer formed on the substrate on both sides of the stripe-shaped multilayer structure. And a current blocking portion including a second current blocking layer of a second conductivity type, the method comprising: forming a semiconductor laminated structure including the active layer on the substrate. And a step of forming a stripe-shaped mask layer on the semiconductor laminated structure, and the semiconductor laminated structure is selectively etched using an etchant that does not substantially etch the mask layer to form the stripe-shaped multilayer structure. Forming a structure, forming the first current blocking layer on the substrate, and bending the second current blocking layer upward on the side of the striped multilayer structure and forming the striped multilayer structure into the latest structure. To have as the angle of the end portion is 8 0 degrees or more, encompasses forming on the first current blocking layer, the manufacturing method of the semiconductor laser. 10. The strut of the second current blocking layer
The edge closest to the ip-shaped multilayer structure is
The method according to claim 9, which is located above the ip-shaped multilayer structure.
Method for manufacturing semiconductor lasers described above. 11. The method of manufacturing a semiconductor laser according to claim 9 , wherein the angle of the edge portion of the second current blocking layer which is closest to the stripe-shaped multilayer structure is 90 degrees. 12. The method of manufacturing a semiconductor laser according to claim 9, wherein the first and second current blocking layers are epitaxially grown at a growth temperature of 600 ° C. or higher using a metal organic chemical vapor deposition method. 13. Prior to forming the first current blocking layer.
To grow an undoped semiconductor layer epitaxially.
10. The semiconductor laser of claim 9, further comprising:
Production method.