Semiconductor laser and method for manufacturing same

What is provided are: an active-layer ridge which is composed of an n-type cladding layer, an active layer, a first p-type cladding layer and a second n-type blocking layer that are stacked in this order on an n-type InP substrate, and which is formed to project from a position lower than the active layer; burying layers by which both side portions of the active-layer ridge are buried up to a position higher than the active layer; first n-type blocking layers which are each stacked on a front-surface side of each of the burying layers, to be placed on the both sides of the ridge; and a second p-type cladding layer by which an end portion of the active-layer ridge and the first n-type blocking layers are buried thereunder; wherein a current narrowing window for allowing a hole current to pass therethrough is provided in and at a center of the second n-type blocking layer placed at a top of the active-layer ridge.

TECHNICAL FIELD

The present application relates to a semiconductor laser having a semi-insulative burying and blocking layer, and a manufacturing method thereof.

BACKGROUND ART

A conventional semiconductor laser is so configured that: side surfaces of a ridge stripe structure in which an n-type InP cladding layer, an active layer and a first p-type InP cladding layer are stacked in this order on an n-type InP substrate, are buried by semi-insulative InP layers and n-type InP blocking layers placed thereon; and further, the ridge stripe structure and such burying layers consisting of the semi-insulative InP layers and the n-type InP blocking layers placed thereon, are buried by a second p-type InP cladding layer (see, for example, Patent Document 1).

With application of a current to such a semiconductor laser, holes are supplied from the second p-type InP cladding layer and flow toward the n-type InP substrate. On this occasion, a part of the holes is blocked by a potential barrier placed at the interface between the second p-type InP cladding layer and each n-type InP blocking layer, to thereby flow along the n-type InP blocking layer toward the active layer placed in the ridge stripe. In the active layer, electrons supplied from the n-type InP cladding layer are recombined with holes supplied from the second p-type InP cladding layer, making it possible to achieve light power and gain. In order to increase the power, it is effective to inject holes efficiently into the active layer by shortening the distance between the n-type InP blocking layer and the active layer (width of a hole leakage path).

For example, according to Patent Document 2, the width of the hole leakage path is narrowed in such a manner that, after the side surfaces of the ridge stripe are buried by the semi-insulative InP-burying layers and the n-type InP blocking layers, both side portions of the p-type InP cladding layer in the ridge stripe are etched and then n-type InP blocking layers are newly grown thereover, so that the hole leakage that is not contributing to light emission is suppressed.

CITATION LIST

Patent Document

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

According to the semiconductor laser in Patent Document 1, at the time of application of the current to the semiconductor laser, holes blocked by the n-type InP blocking layer will, during flowing through the first p-type InP cladding layer toward the active layer, get spread widthwise. Thus, there is a problem that a part of these holes is not injected into the active layer, resulting in increase of a useless hole-leakage current that is not contributing to light emission. Further, according to the semiconductor laser in Patent Document 2 the object of which is to solve the problem associated with Patent Document 2, there is a problem that crystal growth of n-type InP blocking layer has to be performed twice separately, resulting in increase of the number of production steps and increase of the manufacturing cost.

The present application is presented to disclose a technique for solving the problems as described above, and an object thereof is to provide a semiconductor laser which is capable of increasing the power by reducing the hole-leakage current, with no increase in the number of times of the crystal growth, and a manufacturing method thereof.

Means for Solving the Problems

A semiconductor laser disclosed in this application is characterized by comprising: a ridge which includes semiconductor layers of an n-type cladding layer, an active layer, a first p-type cladding layer and a second n-type blocking layer that are stacked in this order on an n-type substrate, and which is formed to project from a position nearer to the n-type substrate than to the active layer; burying layers by which both side portions of the ridge are buried up to a position higher than the active layer; first n-type blocking layers which are each stacked on a front-surface side of each of the burying layers, to be placed on both sides of the ridge; and a second p-type cladding layer by which a top of the ridge and the first n-type blocking layers are buried thereunder; wherein a current narrowing window for allowing a hole current to pass therethrough is provided in and at a center of the second n-type blocking layer placed at the top of the ridge.

Further, a semiconductor laser disclosed in this application is characterized by comprising: a ridge which is composed of an n-type cladding layer, an active layer and a first p-type cladding layer that are stacked in this order on an n-type substrate, and which is formed to project from a position nearer to the n-type substrate than to the active layer; burying layers by which both side portions of the ridge are buried up to a position higher than the active layer; first n-type blocking layers which are each stacked on a front-surface side of each of the burying layers, to be placed on both sides of the ridge; a second p-type cladding layer by which a top of the ridge and the first n-type blocking layers are buried thereunder; and a current narrowing window for allowing a hole current to pass therethrough, which is provided in and at a center of the first p-type cladding layer placed at the top of the ridge; wherein the current narrowing window is formed in such a manner that, in the first p-type cladding layer, a region corresponding to the region other than the current narrowing window is converted into an insulator.

A method for manufacturing a semiconductor laser disclosed in this application is characterized by comprising: a step of stacking an n-type cladding layer, an active layer, a first p-type cladding layer and a second n-type blocking layer, in this order, on an n-type substrate, and thereafter, etching both side portions of thus stacked layers up to a position nearer to the n-type substrate than to the active layer, to thereby forma ridge; a step of burying both side portions of the ridge by burying layers, up to a position higher than the active layer; a step of growing a first n-type blocking layer on each of the burying layers; and a step of providing a current narrowing window in and at a center of the second n-type blocking layer placed at a top of the ridge.

Further, a method for manufacturing a semiconductor laser disclosed in this application is characterized by comprising: a step of stacking an n-type cladding layer, an active layer, a first p-type cladding layer and a second n-type blocking layer, in this order, on an n-type substrate, and thereafter, providing a current narrowing window in and at a center of the second n-type blocking layer; a step of etching both side portions of thus stacked layers up to a position nearer to the n-type substrate than to the active layer, to thereby form a ridge so that the current narrowing window is placed at a center of that ridge; a step of burying both side portions of the ridge by burying layers, up to a position higher than the active layer; and a step of growing a first n-type blocking layer on each of the burying layers.

Effect of the Invention

According to the present application, since the current narrowing window for allowing a hole current to pass therethrough is provided at the top of ridge, it is possible to provide a semiconductor laser which is capable of increasing the power by reducing the hole-leakage current, and which can be manufactured without increasing the number of times of the crystal growth.

MODES FOR CARRYING OUT THE INVENTION

FIG. 1is a cross-sectional view showing a configuration of a semiconductor laser101according to Embodiment 1. As shown inFIG. 1, the semiconductor laser101has a structure provided with an active-layer ridge6composed of an n-type cladding layer2, an active layer3, a first p-type cladding layer4and a second n-type blocking layer5that are stacked in this order on an n-type InP substrate1. The side surfaces of the active-layer ridge6are buried by burying layers7up to a position higher than the active layer3, and first n-type blocking layers8are provided on the burying layers7. Furthermore, the active-layer ridge6, the burying layers7and the first n-type blocking layers8are buried by a second p-type cladding layer10. Electrodes11are provided on the lower side of the n-type InP substrate1and on the upper side of the second p-type cladding layer10.

The n-type InP substrate1is a substrate whose main plane is (001) plane and in which S is doped in a concentration of 4.0×1018cm−3. The n-type cladding layer2is made of 0.3 μm-thick InP which is placed on the n-type InP substrate1and in which S is doped in a concentration of 4.0×1018cm−3. The active layer3is made of an AlGaInAs-based or InGaAsP-based material including a multiple quantum well. The first p-type cladding layer4is made of 0.1 μm-thick InP in which Zn is doped in a concentration of 1.0×1018cm−3. The second n-type blocking layer5is made of 0.1 μm-thick InP in which S is doped in a concentration of 7.0×1018cm−3. The width of the active-layer ridge6is generally from about 0.8 to 1.4 μm; however, it is not limited to in this range.

The burying layers7are each made of InP in which Fe is doped in a concentration of 5.0×1016cm−3, as a semi-insulative material. The first n-type blocking layers8are each made of 0.4 μm-thick InP which is placed on the burying layer7and in which S is doped in a concentration of 7.0×1018cm−3. Note that the burying layer7may be of another semi-insulative material, such as, InP doped with another material of Ti, Co, Ru or the like. Further, the burying layer7may be formed as a combination with another semiconductor layer having a different impurity concentration or conductivity type.

At the center of the upper portion (top center) of the active-layer ridge6, there is a 0.7 μm-width region, namely, a current narrowing window9, which is sandwiched between portions of the second n-type blocking layer5. The width of the current narrowing window9is generally from about 0.5 to 1.0 μm; however, it is not limited to in this range so far as it is narrower than the width of the active-layer ridge6. Further, side surfaces of the portions of the second n-type blocking layer5, which are placed just on both sides of the current narrowing window9, may be non-perpendicular to the n-type InP substrate1, for example, may be inclined surfaces as shown inFIG. 2.

With respect a hole shape that constitutes the current narrowing window9, its bottom plane is provided in and at the center of an upper portion of the active-layer ridge6so that it is lower than the second n-type blocking layer5but is placed in a range up to a position higher than the active layer3. Furthermore, the hole shape that constitutes the current narrowing window9is, together with the active-layer ridge6, the burying layers7and the first n-type blocking layers8, buried by the second p-type cladding layer10made of 2.0-μm thick InP in which Zn is doped in a concentration of 3.0×1018cm−3. The electrodes11on the lower side of the n-type InP substrate1and on the upper side of the second p-type cladding layer10are each made of a metal, such as Au, Ge, Zn, Pt, Ti or the like.

The front-end facet and the rear-end facet of the semiconductor laser101are each in (110) plane formed by cleavage, and thus form a resonator. Emitted light obtained in the active layer, due to injection of a current, is amplified in the resonator resulting in laser oscillation. Although the length of the resonator is set to from 150 μm to 300 μm in many cases, it is not limited to in this range.

When the current is injected through the upper and lower electrodes11, electrons and holes are supplied, respectively, from the n-type InP substrate1and the n-type cladding layer2and from the first p-type cladding layer4and the second p-type cladding layer10. The holes supplied from the second p-type cladding layer10flow out toward the n-type InP substrate1. A part of these holes is blocked by a potential barrier placed at the interface between the second p-type cladding layer10and the first n-type blocking layer8or at the interface between the second p-type cladding layer10and the second n-type blocking layer5.

The current of holes blocked as described above will pass through between the first n-type blocking layer8and the second n-type blocking layer5, or through the current narrowing window9, to flow toward the n-type InP substrate1. At this time, the portion between the first n-type blocking layer8and the second n-type blocking layer5is 0.4 μm or less, thus narrower than the width of the current narrowing window9, and further, it is in contact with the burying layer7that is higher in resistance than the p-type InP layer, so that its resistance is higher than that inside the current narrowing window9. Accordingly, the hole current is likely to pass through the current narrowing window9having a lower resistance.

After passing through the current narrowing window9, the hole current will, during passing through the first p-type cladding layer4to be injected into the active layer, get spread widthwise (hole-current flow A shown inFIG. 1). However, since the width of the current narrowing window9is narrower than the width of the active-layer ridge6, holes spread widthwise can also be injected into the active layer3. As a result, it is possible to improve the injection efficiency of holes in comparison with a conventional structure without having the second n-type blocking layer5.

FIG. 3is a diagram showing a current characteristic of the semiconductor laser101according to Embodiment 1. InFIG. 3, the abscissa represents a current supplied to the semiconductor laser, and the ordinate represents output power of the semiconductor laser. Note that the operation temperature was set to 95° C., and such a current to light-power characteristic was determined by calculation. As shown inFIG. 3, according to Current Characteristic B of the semiconductor laser101, light power is obtained that is higher than that of Current Characteristic C of a conventional semiconductor laser, even under application of the same current. This is due to improvement in injection efficiency of the hole current into the active layer, of the semiconductor laser101.

FIG. 4is a diagram showing a calculation result of a lateral gain distribution in the active layer3of the semiconductor laser101according to Embodiment 1. InFIG. 4, the abscissa represents a lateral-direction distance from the center of the active-layer ridge, and the ordinate represents a gain. Symbol Xa corresponds to a side surface of the active-layer ridge. Note that the calculation result is that obtained under operation at a current value of 60 mA and at the operation temperature set to 95° C. As shown inFIG. 4, according to Gain E of the conventional semiconductor laser, the gain becomes larger as the distance to the side surface of the active-layer ridge becomes shorter; whereas, according to Gain D of the semiconductor laser101, the gain distribution in the active layer as a whole is nearly uniform. When the gain distribution is so uniform, the injected amount of carriers required for laser oscillation also becomes uniform in the active layer as a whole. Thus, for example, in the case where intensity modulation is performed based on the current injection amount, the temporal fluctuation (jitter) becomes smaller, and this acts favorably in transmission of the modulated signal waveform.

The shape of the gain distribution of the semiconductor laser101can be explained as follows. In a light-intensity distribution of a general semiconductor laser which causes oscillation in a single mode, the intensity is largest at the center of the active layer (x=0) and attenuates with distance from the center. Therefore, at the center of the active layer where the light density is high, stimulated emission is more likely to occur than at the end portion of the active layer, and thus the hole density is likely to become lower. Thus, according to the semiconductor laser as shown in Patent Document 1, holes are injected almost uniformly and thus the hole density at the center of the active layer is reduced, so that the gain at the center of the active layer is reduced.

In contrast, the semiconductor laser as shown in Patent Document 2 has a structure in which the hole density at the center of the active layer is likely to become higher, because the current narrowing window that is narrower than the width of the active layer is placed on the active layer. Accordingly, even if the hole density at the center of the active layer is reduced due to carrier absorption, the hole density in the active layer as a whole is eventually more uniform than that of the semiconductor laser as shown in Patent Document 1, and thus the gain distribution is likely to be uniform as well.

In addition to achieving such effects, the semiconductor laser101according to Embodiment 1 does not require growing twice the n-type blocking layer separately in a manner as is done according to the semiconductor laser as shown in Patent Document 2. Accordingly, it is not required to increase the number of times of the crystal growth and thus, it is possible to improve characteristics of the semiconductor laser without causing reduction of the productivity and increase of the manufacturing cost.

Next, a manufacturing method of the semiconductor laser101according to Embodiment 1 will be described based onFIGS. 5A to 5D.FIGS. 5A to 5Dare cross-sectional views showing manufacturing steps of the semiconductor laser101according to Embodiment 1.

First, as shown inFIG. 5A, after the n-type cladding layer2, the active layer3, the first p-type cladding layer4and the second n-type blocking layer5are stacked in this order on the n-type InP substrate1, an SiO2 mask12is deposited with a width corresponding to the active-layer ridge6and then, using the SiO2 mask12, etching is performed up to an intermediate position in the n-type InP substrate1, to thereby form the active-layer ridge6. Note that, here, etching is performed up to an intermediate position in the n-type InP substrate1; however, the position may instead be any position as long as it is lower than the active layer.

Subsequently, as shown inFIG. 5B, both side portions of the active-layer ridge6are buried by the burying layers7and the first n-type blocking layers8, to thereby form a current narrowing structure.

Then, as shown inFIG. 5C, after removing the SiO2 mask12by using buffered hydrofluoric acid or hydrofluoric acid, an SiO2 mask13is deposited and a hole for the current narrowing window9is created by etching in the SiO2 mask13at a position corresponding to the center of the upper portion of the active-layer ridge6and then, using the SiO2 mask13in which the hole is created, the current narrowing window9is formed in and at the center of the upper portion of the active-layer ridge6. At this time, the bottom plane of the hole shape that constitutes the current narrowing window9is created in and at the center of the upper portion of the active-layer ridge6so that it is placed in a range from the back surface of the second n-type blocking layer5up to a position higher than the active layer3.

Subsequently, as shown inFIG. 5D, after removing the SiO2 mask13by using buffered hydrofluoric acid or hydrofluoric acid, the upper portion of the active-layer ridge6and the hole shape that constitutes the current narrowing window9are buried by the second p-type cladding layer10. Lastly, the electrodes11are formed on the lower side of the n-type InP substrate1and the upper side of the second p-type cladding layer10, so that the semiconductor laser101shown inFIG. 1is completed.

It is noted that, in the foregoing case of Embodiment 1, the active-layer ridge6is formed first by etching and thereafter, the current narrowing window9is formed; however, it is allowed that the current narrowing window9is formed first and thereafter, the active-layer ridge6is formed. Another manufacturing method of the semiconductor laser101according to Embodiment 1 will be described based onFIGS. 6A to 6D.FIGS. 6A to 6Dare cross-sectional views showing other manufacturing steps of the semiconductor laser101according to Embodiment 1.

First, as shown inFIG. 6A, after the n-type cladding layer2, the active layer3, the first p-type cladding layer4and the second n-type blocking layer5are stacked in this order on the n-type InP substrate1, an SiO2 mask12is deposited and a hole for the current narrowing window9is created by etching in the SiO2 mask12at a position corresponding to the center of the upper portion of the active-layer ridge6to be provided and then, using the SiO2 mask12in which the hole is created, the current narrowing window9is formed at the center of the upper portion of the active-layer ridge6. At this time, the bottom plane of the hole shape that constitutes the current narrowing window9is created in and at the center of the upper portion of the active-layer ridge6so that it is placed in a range from the back surface of the second n-type blocking layer5up to a position higher than the active layer3.

Subsequently, as shown inFIG. 6B, after removing the SiO2 mask12by using buffered hydrofluoric acid or hydrofluoric acid, an SiO2 mask13is deposited on the second n-type blocking layer5, with a width corresponding to the active-layer ridge6and then, using the mask, etching is performed up to an intermediate position in the n-type InP substrate1, to thereby form the active-layer ridge6. Note that, here, etching is performed up to an intermediate position in the n-type InP substrate1; however, the position may instead be any position as long as it is lower than the active layer.

Then, as shown inFIG. 6C, both side portions of the active-layer ridge6are buried by the burying layers7and the first n-type blocking layers8, to thereby form a current narrowing structure.

Subsequently, as shown inFIG. 6D, after removing the SiO2 mask13by using buffered hydrofluoric acid or hydrofluoric acid, the upper portion of the active-layer ridge6and the current narrowing window9are buried by the second p-type cladding layer10. Lastly, the electrodes11are formed on the lower side of the n-type InP substrate1and the upper side of the second p-type cladding layer10, so that the semiconductor laser101shown inFIG. 1is completed.

As described above, according to the semiconductor laser101according to Embodiment 1, it comprises: the active-layer ridge6which is composed of the n-type cladding layer2, the active layer3, the first p-type cladding layer4and the second n-type blocking layer5that are stacked in this order on the n-type InP substrate1, and which is formed to project from a position lower than the active layer3(a position nearer to the n-type InP substrate1); burying layers7by which both side portions of the active-layer ridge6are buried up to a position higher than the active layer3; first n-type blocking layers8which are each stacked on a front-surface side of each of the burying layers7, to be placed on both sides of the active-layer ridge6; and the second p-type cladding layer10by which a top of the active-layer ridge and the first n-type blocking layers8are buried thereunder; wherein the current narrowing window9for allowing a hole current to pass therethrough is provided in and at the center of the second n-type blocking layer5placed at the top of the active-layer ridge6. Thus, it is possible to provide a semiconductor laser which is capable of increasing the power by reducing the hole-leakage current, and which can be manufactured without increasing the number of times of the crystal growth.

It is noted that, in Embodiment 1, the first n-type blocking layer8and the second n-type blocking layer5are so configured that the burying layer7of high resistance is sandwiched therebetween; however, this is not limitative. As shown inFIG. 7, the first n-type blocking layer8and the second n-type blocking layer5may be so configured that the burying layer7is not sandwiched therebetween and thus, the first n-type blocking layer8and the second n-type blocking layer5are in contact with each other.

With this configuration, when current injection is performed, holes supplied from the second p-type cladding layer10are blocked by a potential barrier at the interface between the second p-type cladding layer10and the first n-type blocking layer8or at the interface between the second p-type cladding layer10and the second n-type blocking layer5. The current of holes blocked as described above will pass through the current narrowing window9and will be then injected into the active layer3. Accordingly, unlike the foregoing case of Embodiment 1, it is possible to prevent a hole-leakage current from passing through between the first n-type blocking layer8and the second n-type blocking layer5to flow into the burying layer7. This improves the injection efficiency of holes into the active layer more than in the foregoing case of Embodiment 1, and thus the power of laser can be more increased.

In Embodiment 1, the current narrowing window9sandwiched between portions of the second n-type blocking layer5is used; however, in Embodiment 2, a case will be described where the formation of the second n-type blocking layer5is omitted and a current narrowing window is formed in such a manner that an insulative region is provided in the first p-type cladding layer4.

FIG. 8is a cross-sectional view showing a configuration of a semiconductor laser102according to Embodiment 2. As shown inFIG. 8, in Embodiment 2, a current narrowing window9is formed in such a manner that, with respect to an active-layer ridge6in which an n-type cladding layer2, an active layer3and a first p-type cladding layer4are stacked in this order, an insulative region14is provided to be placed in both end portions of the first p-type cladding layer4as an upper portion of the active-layer ridge, and in portions of the burying layers7each placed between the active-layer ridge6and the first n-type blocking layer8. The configuration of the semiconductor laser102other than the above is the same as in the semiconductor laser101of Embodiment 1, so that description thereof will be omitted.

In Embodiment 2, the insulative region14is prepared, for example, as follows.

After the side surfaces of the active-layer ridge6are buried by the burying layers7and the first n-type blocking layers8, a mask using SiO2is formed on and at the center of the first p-type cladding layer4as the upper portion of the active-layer ridge6, to be placed at a position corresponding to a position where the current narrowing window9is to be provided, followed by injection of protons from the upper side thereof. This converts a portion of the first p-type cladding layer4other than that beneath the SiO2mask, into an insulator. On the other hand, the portion beneath the SiO2mask remains the same as the first p-type cladding layer4and thus functions as the current narrowing window9. Except for how the current narrowing window9is formed, the manufacturing method of the semiconductor laser102is the same as that of the semiconductor laser101of Embodiment 1, so that description thereof will be omitted.

As described above, according to the semiconductor laser102according to Embodiment 2, the current narrowing window9is formed in such a manner that, in the first p-type cladding layer4, a region corresponding to the region other than the current narrowing window9is converted into an insulator. Thus, it is possible to achieve effects similar to those in Embodiment 1, and further to provide a semiconductor laser that is superior in flatness of the outermost surface of the second p-type cladding layer because the current narrowing window can be prepared without the step to be buried by that layer.

In Embodiment 2, the current narrowing window9is formed in such a manner that both end portions of the first p-type cladding layer4as the upper portion of the active-layer ridge6are converted into insulative portions; however, in Embodiment 3, a case will be described where a current narrowing window is formed in such a manner that a diffused region of p-type dopants is provided in the second p-type blocking layer.

FIG. 9is a cross-sectional view showing a configuration of a semiconductor laser103according to Embodiment 3. As shown inFIG. 9, in Embodiment 3, a current narrowing window9is formed in such a manner that, a diffused region15of p-type dopants of, for example, Zn, is provided in and at the center of the second n-type blocking layer5as the upper portion of the active-layer ridge6. The configuration of the semiconductor laser103other than the above is the same as in the semiconductor laser101of Embodiment 1, so that description thereof will be omitted.

In Embodiment 3, the diffused region15is prepared, for example, as follows.

Using a photoresist, its patterning corresponding to a hole shape is performed on and at the center of the second n-type blocking layer5, and thereafter, ZnO is deposited by a sputtering method on the upper side and then, portions of ZnO and the resist, other than those at the position corresponding to the diffused region to be provided, are removed by lift-off. Thereafter, annealing is performed, so that the p-type dopant diffused region15can be formed only at the center of the second n-type blocking layer5. With respect to the sequence of preparation, it does not matter whichever the active-layer ridge6is formed first or the active-layer ridge6is formed after formation of the p-type dopant diffused region15. Except for how the current narrowing window9is formed, the manufacturing method of the semiconductor laser103is the same as that of the semiconductor laser101of Embodiment 1, so that description thereof will be omitted.

As described above, according to the semiconductor laser103according to Embodiment 3, the current narrowing window9is formed in such a manner that p-type dopants are diffused in a region of the second n-type blocking layer5corresponding to the current narrowing window. Thus, it is possible to achieve effects similar to those in Embodiment 1, and further to provide a semiconductor laser that is superior in flatness of the outermost surface of the second p-type cladding layer because the current narrowing window can be prepared without the step to be buried by that layer.

In Embodiment 1, the current narrowing window9provided with a hole shape is formed in the second n-type blocking layer5; while, in Embodiment 4, a case will be described where a third p-type cladding layer16is formed that covers the second n-type blocking layer5in which the current narrowing window9is provided.

FIG. 10is a cross-sectional view showing a configuration of a semiconductor laser104according to Embodiment 4. As shown inFIG. 10, in Embodiment 4, the second n-type blocking layer5in which the current narrowing window9is provided, is covered with the third p-type cladding layer16made of 0.1 μm-thick InP in which Zn is doped in a concentration of 3.0×1018cm−3. The configuration of the semiconductor laser104other than the above is the same as in the semiconductor laser101of Embodiment 1, so that description thereof will be omitted.

In Embodiment 4, the third p-type cladding layer16that covers the second n-type blocking layer5is prepared, for example, as follows.FIG. 11is a cross-sectional view of the semiconductor laser104before formation of the second p-type cladding layer10. As shown inFIG. 11, after the third p-type cladding layer16is stacked on the second n-type blocking layer5, a hole for providing the current narrowing window9is created. Subsequently, when the second p-type cladding layer10is to be formed, in the crystal-growing furnace for MOCVD, the in-furnace temperature is elevated up to about 500° C., so that, as shown inFIG. 10, portions of the semiconductor layers that constitute both walls of the current narrowing window9, are deformed due to mass transport. On this occasion, if the uppermost portion of the active-layer ridge6is the second n-type blocking layer5, there is such a risk that the deformed n-type InP portions fall into the current narrowing window9and onto the first p-type cladding layer4, to thereby close a hole-current injection path toward the active layer3. In this respect, when the uppermost portion of the active-layer ridge6is the third p-type cladding layer16as in Embodiment 4, if it falls into the current narrowing window, since it is made of p-type InP with the same polarity as that of the second p-type cladding layer10for burying the current narrowing window, the hole-current injection path is never closed. Except for how the current narrowing window9is formed, the manufacturing method of the semiconductor laser104is the same as that of the semiconductor laser101of Embodiment 1, so that description thereof will be omitted.

As described above, according to the semiconductor laser104according to Embodiment 4, the second n-type blocking layer5in which the current narrowing window9is provided, is covered with the third p-type cladding layer16. Thus, it is possible to provide a semiconductor laser which does not cause the hole-current injection path to be closed, and is thus superior in current characteristics.

In Embodiment 1, the current narrowing window9is buried by the second p-type cladding layer10; however, in Embodiment 5, a case will be described where it is buried by a fourth p-type cladding layer17.

FIG. 12is a cross-sectional view showing a configuration of a semiconductor laser105according to Embodiment 5. As shown inFIG. 12, in Embodiment 5, the current narrowing window9is buried by the fourth p-type cladding layer17that is higher in carrier concentration than the second p-type cladding layer10. For example, the fourth p-type cladding layer17is 0.2 μm-thick p-type InP in which Zn is doped in a concentration of 3.0×1018cm−3, and the second p-type cladding layer10is 1.8 μm-thick p-type InP in which Zn is doped in a concentration of 1.0×1018cm−3. The configuration of the semiconductor laser105other than the above is the same as in the semiconductor laser101of Embodiment 1, so that description thereof will be omitted.

According to Embodiment 5, after the current narrowing window9sandwiched between portions of the second n-type blocking layer5is buried by the fourth p-type cladding layer17, the second p-type cladding layer10that is lower in carrier concentration than the fourth p-type cladding layer17is crystal-grown. Except for the burying step by the fourth p-type cladding layer17, the manufacturing method of the semiconductor laser105is the same as that of the semiconductor laser101of Embodiment 1, so that description thereof will be omitted.

In comparison with a semiconductor laser without having the current narrowing window9, the semiconductor laser having the current narrowing window9is smaller in cross-section of the current flowing region, and thus has a problem that the element resistance becomes higher. In order to solve the problem, it would be appropriate to increase the carrier concentration of the second p-type cladding layer for burying the current narrowing window9, to thereby reduce the resistance of the current flowing region. However, if the carrier concentration is so increased, light absorption loss becomes larger, to thereby reduce the light power. For that reason, the light generated from the active layer must be prevented as much as possible from leaking out to a semiconductor layer whose carrier concentration is high. In Embodiment 5, only the carrier concentration of the fourth p-type cladding layer for burying the current narrowing window9is made high, and the carrier concentration of the second p-type cladding layer thereon is made lower than that of the fourth p-type cladding layer, so that reduction of the element resistance and suppression of the light absorption loss are both ensured.

As described above, according to the semiconductor laser105according to Embodiment 5, since the carrier concentration of the second p-type cladding layer10is set lower than that of the fourth p-type cladding layer17for burying the current narrowing window9, the light absorption loss can be made lower than that of the semiconductor laser of Embodiment 1. Thus, such effects are provided that the threshold current is reduced and that the light power is increased.

In this application, a variety of exemplary embodiments and examples are described; however, every characteristic, configuration or function that is described in one or more embodiments, is not limited to being applied to a specific embodiment, and may be applied singularly or in any of various combinations thereof to another embodiment. Accordingly, an infinite number of modified examples that are not exemplified here are supposed within the technical scope disclosed in the description of this application. For example, such cases shall be included where at least one configuration element is modified; where any configuration element is added or omitted; and furthermore, where at least one configuration element is extracted and combined with a configuration element of another embodiment.

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