Semiconductor laser device and method of producing the same

In an AlGaInP semiconductor laser device, at least a first conductivity type first cladding layer, an active layer and a second conductivity type second cladding layer are formed on a semiconductor substrate. The second cladding layer forms a stripe-shaped ridge on a side opposite from the substrate, and a first conductivity type current block layer is disposed on both sides of the ridge. The first conductivity type current block layer has a lattice mismatch rate of −0.20% or more but not more than 0% relative to the semiconductor substrate. The lattice mismatch rate may be uniform within the current block layer. Alternatively, the lattice mismatch rate may increase continuously or stepwise with an increasing distance from a portion of the second conductivity type second cladding layer other than the ridge.

This nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application No(s). 2003-277410 filed in Japan on Jul. 22, 2003, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor laser device to be used for optical disks, and the like, and a method of producing the same. Particularly, the present invention relates to a semiconductor laser device having a window structure, which is superior in high power operation characteristics, and a method of producing the same.

In recent years, various types of semiconductor lasers have widely been used as light sources for optical disc devices. In particular, high-power semiconductor lasers have been used as light sources for writing information to disks in a DVD (digital versatile disc) player and a CD-RAM (random access memory) drive, and there is a need for reduction of driving current and further improvement in power.

One of factors that increase the driving current of a semiconductor laser is diffusion of impurity atoms from a cladding layer into an active layer. Further, one of factors that restrict increase in power of a semiconductor laser is catastrophic optical damage (COD), which tends to occur with increase of the optical power density in regions of an active layer in proximity of end faces of a laser cavity.

One method for reducing the driving current in a semiconductor laser device by suppressing the diffusion of impurity atoms into an active layer is adopted in a semiconductor light emitting device disclosed in Patent Document 1 (JP 11-87831 A). Further, as one method for increasing output power by reducing the COD level, there is a method utilizing a window structure in which an active layer of a multiquantum well structure is disordered, as adopted in a semiconductor laser disclosed in Patent Document 2 (JP 3-208388 A).

First, prior art for suppressing diffusion of impurity atoms into an active layer disclosed in Patent Document 1 will be described.FIG. 14is a cross-sectional view showing an AlGaInP semiconductor laser device as a semiconductor light-emitting device disclosed in Patent Document 1.

An upper part of the p-type cladding layer6, the p-type intermediate layer7and the p-type cap layer has a stripe shape extending in one direction, and both sides of the stripe portion are filled with an n-type GaAs current constriction layer (current block layer)9. Se atoms are used as n-type conductivity impurities to be introduced into the n-type cladding layer2and the n-type current constriction layer9, and Zn atoms are used as p-type conductivity impurities to be introduced into the p-type cladding layer6, the p-type intermediate layer7and the p-type cap layer8.

In the AlGaInP semiconductor laser device with the above construction, the n-type cladding layer2has a lattice mismatch rate of −0.15% or more but not more than −0.02% relative to the n-type substrate1. The p-type cladding layer6has a lattice mismatch rate of 0.02% or more but not more than 0.3% relative to the n-type substrate1.

Next, prior art for reducing the COD level disclosed in Patent Document 2 will be described.FIG. 15AandFIG. 15Bare cross-sectional views showing the structure of a semiconductor laser device with a window structure disclosed in Patent Document 2.

FIG. 15Ais a cross-sectional view of the semiconductor laser device in an excitation region (active region), andFIG. 15Bis a cross-sectional view of the semiconductor laser device in an impurity diffusion region (window region). Reference numeral21denotes an n-type GaAs substrate, reference22denotes an n-type GaAs buffer layer, reference numeral23denotes an n-type AlGaInP cladding layer, reference numeral24denotes an undoped GaInP active layer, reference numeral25denotes a p-type AlGaInP inner cladding layer, reference numeral27denotes a p-type AlGaInP outer cladding layer, reference numeral29denotes a p-type GaAs cap layer, reference numeral30denotes a GaAs block layer, reference numeral31denotes a p-type GaAs contact layer, reference numeral32denotes a p-side electrode, and reference numeral33denotes an n-side electrode.

FIG. 16AthroughFIG. 16Dare process drawings showing a conventional method of producing a semiconductor laser device disclosed in Patent Document 2. In accordance withFIG. 16AthroughFIG. 16D, the conventional method of producing a semiconductor laser device will be described.

As shown inFIG. 16A, an n-type GaAs buffer layer22, an n-type AlGaInP cladding layer23, an undoped GaInP active layer24, a p-type AlGaInP inner cladding layer25, a p-type GaInP etching stopper layer26, a p-type AlGaInP outer cladding layer27, a p-type GaInP hetero-barrier layer28, and a p-type GaAs cap layer29are formed in sequence on an n-type GaAs substrate21at a growth temperature of 660° C. by an MOVPE (metal organic vapor phase epitaxy) method. Zn atoms are doped, as p-type impurities, into each of the layers having p-type conductivity from the p-type inner cladding layer25to the p-type cap layer29.

Next, a dielectric film34is deposited on the p-type cap layer29, and, after patterning the dielectric film in a stripe shape by photolithography, Zn atoms are diffused by a sealed tube diffusion method using ZnAs2as an impurity diffusion source. Thereby, highly concentrated Zn atoms are diffused into a region of the undoped active layer24, which becomes an impurity diffusion region, so that the bandgap energy of the undoped active layer24increases.

Next, as shown inFIG. 16B, using the photolithography again, a resist stripe mask35is formed on the dielectric film34and the p-type cap layer29. Thereafter, the dielectric film34, the p-type cap layer29, the p-type hetero-barrier layer28and the p-type outer cladding layer27are sequentially removed by a chemical etching treatment as shown inFIG. 16C, so as to form a ridge.

Next, as shown inFIG. 16D, after removing the resist stripe mask35, using the dielectric film34as a mask, an n-type GaAs block layer30(seeFIG. 15AandFIG. 15B) is selectively grown at a growth temperature of 660° C. by the MOVPE method. Thereby, the n-type block layer30is formed in regions on both sides of the ridge, and also on the impurity diffusion regions. Current injection into the regions where the n-type block layer30is formed is prevented.

Next, after removing the dielectric film34, a p-type GaAs contact layer31is formed at a growth temperature of 660° C. by the MOVPE method (seeFIGS. 15A and 15B). Thereafter, as shown inFIG. 15AandFIG. 15B, a p-side electrode32is formed on the p-type contact layer31, and an n-side electrode33is formed on the underside of the n-type substrate21. Then, the wafer is cleaved, and a semiconductor laser device shown inFIG. 15AandFIG. 15Bis obtained.

However, the conventional semiconductor laser devices have the following problems. Specifically, in the semiconductor laser device disclosed in Patent Document 1 in which diffusion of impurity atoms into the active layer is suppressed, in order to suppress diffusion of Zn atoms contained in the p-type cladding layer6into the active layer4, a strain is provided to the p-type cladding layer6so that the p-type cladding layer6has a lattice mismatch rate of 0.02% or more but not more than 0.3% relative to the n-type substrate1.

However, in the conventional semiconductor laser device in which the diffusion of impurity atoms into the active layer is suppressed, mere provision of a strain in the p-type cladding layer6does not make it possible to sufficiently suppress diffusion of p-type conductivity impurity atoms (Zn atoms) contained in the p-type cladding layer6into the active layer4. In the case where the p-type conductivity impurities contained in the cladding layer6are Be atoms, if a positive strain is applied to the p-type cladding layer6, a large number of Be atoms are diffused into the active layer4, which will invite an increase of driving current at high-power operation.

In the semiconductor laser device in which the diffusion of impurity atoms into the active layer is suppressed, laser light is prone to be absorbed in regions in proximity of end faces of a cavity, and therefore COD is liable to occur in regions of an active layer in proximity of the emission end faces. For that reason, a reduction of maximum optical output during high power operation is caused. Consequently, sufficient long-term reliability cannot be obtained.

In the semiconductor laser device having a conventional window structure, which is disclosed in Patent Document 2, diffusion of Zn atoms into the undoped active layer24is performed by the sealed tube diffusion method using, as an impurity diffusion source, ZnAs2containing Zn atoms having a relatively large diffusion constant with respect to AlGaInP materials, so that the bandgap energy of the impurity diffusion regions (window regions) is larger than the bandgap energy corresponding to the laser oscillation wavelength. Thereby, absorption of laser light in the regions in proximity of the cavity is suppressed, and the occurrence of COD in the regions of the active layer in proximity of the emission end faces is prevented.

However, in the semiconductor laser device with the conventional window structure, for the bandgap energy of the active layer in proximity of the emission end faces to be larger than the bandgap energy corresponding to the laser oscillation wavelength, Zn atoms are diffused into the impurity diffusion regions (window regions) of the undoped active layer24in proximity of end faces of the laser cavity, as described above. At this time, disadvantageously, a large number of Zn atoms present in the p-type inner cladding layer25are diffused even into the excitation region (active region) of the undoped active layer24, which invites the increase of driving current at high-power operation and deterioration of long-term reliability.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a semiconductor laser device having a reduced driving current at high-power operation and superior long-term reliability, and a method of producing the same.

In order to accomplish the above object, there is provided, according to an aspect of the present invention, an AlGaInP semiconductor laser device comprising at least a first conductivity type first cladding layer, an active layer and a second conductivity type second cladding layer formed on a semiconductor substrate, said second conductivity type second cladding layer having a stripe-shaped ridge on a side opposite from the substrate, said device having a first conductivity type current block layer formed on both lateral sides of the ridge, wherein the first conductivity type current block layer has a lattice mismatch rate of −0.20% or more but not more than 0% relative to the semiconductor substrate.

With the above construction, provision of a negative lattice mismatch to the first conductivity type current block layer indirectly imparts a negative strain to the second conductivity type second cladding layer. Consequently, a diffusion rate of impurity atoms having second conductivity present in the second cladding layer is reduced. As a result, the diffusion of impurity atoms having second conductivity into the active layer is reduced, so that driving current at a high-power operation is reduced. In that case, even if the second conductivity type impurity atoms are Be atoms, their diffusion rate is reduced, so that the diffusion of impurity atoms into the active layer is reduced.

In one embodiment, a photoluminescence peak wavelength of the active layer in a laser cavity end face proximity region is smaller than a photoluminescence peak wavelength of the active layer in a laser cavity internal region.

According to the embodiment, in the active layer in the laser cavity end face proximity region, a window region where there is no absorption of laser light is formed, so that COD in the active layer in the laser cavity end face proximity regions is suppressed. Therefore, a COD-free semiconductor laser device superior in long-term reliability in high power operation is obtainable.

In one embodiment, the lattice mismatch rate of the first conductivity type current block layer relative to the semiconductor substrate is uniform within the current block layer.

According to the above construction, a negative lattice mismatch is uniformly provided within the first conductivity type current block layer whereby a diffusion rate of impurity atoms having second conductivity present in the second conductivity type second cladding layer is reduced. As a result, the diffusion of impurity atoms having second conductivity into the active layer is reduced, so that driving current at a high-power operation is reduced.

In one embodiment, the lattice mismatch rate of the first conductivity type current block layer relative to the semiconductor substrate changes within the current block layer.

According to the embodiment, a negative strain is partially introduced by changing the negative lattice mismatch rate within the first conductivity type current block layer. This makes it possible to adjust the diffusion rate of impurity atoms having second conductivity present in the second conductivity type second cladding layer and to adjust the quantity of strain indirectly introduced into the active layer. As a result, driving current in a high-power operation is optimally reduced, and a semiconductor laser device superior in long-term reliability is obtainable.

In one embodiment, the lattice mismatch rate of the first conductivity type current block layer relative to the semiconductor substrate increases with an increasing distance thereof from a portion of the second conductivity type second cladding layer other than the ridge.

According to the embodiment, because the lattice mismatch rate of the first conductivity type current block layer decreases as the distance of the layer to the second conductivity type second cladding layer decreases. Thus, the quantity of strain indirectly introduced into the active layer is reduced while reducing the diffusion rate of impurity atoms having second conductivity present in the second conductivity type second cladding layer. Therefore, the diffusion of impurity atoms having the second conductivity into the active region is reduced, and deterioration of crystallinity in the active layer is suppressed. As a result, driving current in high-power is reduced, and a semiconductor laser device superior in long-term reliability is obtainable.

In one embodiment, the lattice mismatch rate of the first conductivity type current block layer changes stepwise within the current block layer.

According to the embodiment, a first conductivity type current block layer having a smaller lattice mismatch rate is disposed in proximity of the second conductivity type second cladding layer, and a first conductivity type current block layer having a larger lattice mismatch rate is disposed at a position away from the second cladding layer. This makes it easier to control the diffusion rate of impurity atoms having second conductivity present in the second cladding layer as well as to control the quantity of strain introduced into the active layer. Therefore, driving current in a high-power operation is reduced, and a semiconductor laser device superior in long-term reliability is stably obtained.

In one embodiment, the first conductivity type current block layer is composed of AlxInzP (0≦x≦1, 0≦z≦1)

According to the embodiment, by controlling the mole fractions in the AlxInzP, the lattice mismatch rate of the first conductivity type current block layer relative to the semiconductor substrate is controlled to a desired value. Therefore, driving current in high-power is reduced, and a semiconductor laser device superior in long-term reliability is stably obtained.

In one embodiment, the first conductivity type current block layer is composed of AlxInzAsαPβ(0≦x≦1, 0≦z≦1, 0≦α≦1, 0≦β≦1).

According to the embodiment, by controlling the mole fractions in the AlxInzAsαPβ, the lattice mismatch rate of the first conductivity type current block layer relative to the semiconductor substrate is controlled to a desired value. Therefore, driving current in high-power is reduced, and a semiconductor laser device superior in long-term reliability is stably obtained.

In one embodiment, an Al mole fraction x and an In mole fraction z in the current block layer are fixed, and an As mole fraction α in the current block layer in proximity of the second conductivity type second cladding layer is 0.

According to the embodiment, the As mole fraction α in the first conductivity type AlxInzAsαPβcurrent block layer is changed so that the lattice mismatch rate increases with the increasing distance thereof from a portion of the second conductivity type second cladding layer other than the ridge, namely, in a direction substantially perpendicular to the substrate. Thus, the lattice mismatch rate of the AlxInzAsαPβcurrent block layer in proximity of the second cladding layer is smaller. In this manner, a negative strain to be introduced into the active layer is reduced although a negative strain is indirectly provided to the second cladding layer. Therefore, diffusion of impurity atoms having second conductivity into the active layer is reduced, and deterioration of crystallinity in the active layer is suppressed. As a result, driving current in high-power operation is reduced, and a semiconductor laser device superior in long-term reliability in high power operation is obtained.

In one embodiment, impurity atoms contained in the second conductivity type second cladding layer are Be atoms.

According to the embodiment, Be atoms having a smaller diffusion constant in the AlGaInP materials is used and therefore driving current in high-power operation is reduced, and a COD-free semiconductor laser device superior in long-term reliability in high power operation is obtained.

In one embodiment, the second conductivity type second cladding layer has a lattice mismatch rate of −0.15% or more but not more than 0.05% relative to the semiconductor substrate.

According to the embodiment, provision of a negative lattice mismatch in the first conductivity type current block layer indirectly imparts a negative strain into the second conductivity type second cladding layer. Furthermore, a negative lattice mismatch is introduced into the second cladding layer, so that the diffusion rate of impurity atoms having second conductivity present in the second cladding layer is further reduced. As a result, the diffusion of impurity atoms into the active layer is further reduced and driving current in high-power operation is further reduced.

In one embodiment, the lattice mismatch rate of the first conductivity type current block layer is smaller than a lattice mismatch rate of the second conductivity type second cladding layer.

According to the embodiment, a negative strain is effectively introduced into the second conductivity type second cladding layer, so that the diffusion rate of impurity atoms having second conductivity present in the second cladding layer is further reduced. As a result, the diffusion of impurity atoms into the active layer is more reduced, and driving current in high-power is further reduced.

According to another aspect of the present invention, there is provided a method of producing a semiconductor laser device, comprising:

forming a layered structure composed of AlGaInP materials including at least a first conductivity type first cladding layer, an active layer and a second conductivity type second cladding layer on a semiconductor substrate;

processing the second cladding layer on a side thereof opposite from the substrate into a stripe-shaped ridge; and

filling both sides of the ridge with a first conductivity type current block layer having a lattice mismatch rate of −0.20% or more but not more than 0% relative to the semiconductor substrate.

With the above construction, provision of a negative lattice mismatch to the first conductivity type current block layer indirectly imparts a negative strain to the second conductivity type second cladding layer. Consequently, a diffusion rate of impurity atoms having second conductivity present in the second cladding layer is reduced. As a result, the diffusion of impurity atoms having second conductivity into the active layer is reduced, so that driving current at a high-power operation is reduced. In that case, even if the second conductivity type impurity atoms are Be atoms, their diffusion rate is reduced, so that the diffusion of impurity atoms into the active layer is reduced.

In one embodiment, Be atoms are contained in the second conductivity type second cladding layer as impurity atoms having second conductivity, and the method further comprises, after forming the layered structure and before forming the stripe-shaped ridge,

forming an impurity diffusion source film containing Zn atoms having second conductivity in a laser cavity end face proximity region in a wafer having the layered structure; and

annealing the wafer formed with the impurity diffusion source film such that Be atoms having second conductivity contained in the second conductivity type second cladding layer in the laser cavity end face proximity region and Zn atoms having second conductivity contained in the impurity diffusion source film in the laser cavity end face proximity region into the active layer in the laser cavity end face proximity region, to make a photoluminescence peak wavelength of the active layer in the laser cavity end face proximity region smaller than a photoluminescence peak wavelength of the active layer in a laser cavity internal region.

According to the embodiment, in the active layer in the laser cavity end face proximity region, a window region where substantially no laser light is absorbed is formed and therefore COD in the active layer in the laser cavity end face proximity region is suppressed. Therefore, driving current in high-power operation is reduced, and a COD-free semiconductor laser device superior in long-term reliability in high power operation is obtained.

In one embodiment, in the step of filling both sides of the ridge with the first conductivity type current block layer, AlxInzAsαPβis used as the current block layer, and with Al mole fraction x and an In mole fraction z in the AlxInzAsαPβbeing fixed, an As mole fraction α in the AlxInzAsαPβis controlled to be increased with an increasing distance thereof from a portion of the second conductivity type second cladding layer other than the ridge.

According to the embodiment, the lattice mismatch rate of the AlxInzAsαPβcurrent block layer is small in proximity of portions of the second conductivity type second cladding layer positioned below the ridge. In this manner, a negative strain introduced into the active layer is alleviated in spite that a negative strain is indirectly imparted to the second cladding layer. Therefore, the diffusion of impurity atoms having second conductivity into the active layer is reduced, and deterioration of crystallinity in the active layer is suppressed. As a result, driving current in high-power operation is reduced, and a semiconductor laser device superior in long-term reliability in high power operation is obtained.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will hereinafter be described in detail by embodiments illustrated.

First Embodiment

FIG. 1is a perspective view of a semiconductor laser device according to a first embodiment of the present invention, showing a light emission end face of the device.FIG. 2Ais a cross-sectional view taken along line2A-2A ofFIG. 1, showing parts including a waveguide. And,FIG. 2Bis a cross-sectional view taken along line2B-2B ofFIG. 1.

The p-type third cladding layer47, the p-type intermediate layer48and the p-type protective layer49constitute a stripe-shaped ridge54. Si atoms are contained, as n-type impurities, in each of the layers from the n-type substrate41to the n-type first cladding layer43, while Be atoms are contained, as p-type impurities, in each of the layers from the p-type second cladding layer45to the p-type protective layer49.

Reference numeral50denotes an n-type AlxInzP current block (constriction) layer (carrier concentration: 1×1018cm−3, x=0.538, z=0.462, film thickness: about 1.2 μm) The n-type current block layer50is formed so that both sides of the ridge54are filled with the n-type current block layer50. Reference numeral51denotes a p-type GaAs contact layer (carrier concentration: 1×1019cm−3, film thickness: about 4 μm), reference numeral52denotes a p-side electrode, and reference numeral53denotes an n-side electrode.

In the semiconductor laser device with the above construction, the n-type current block layer50uniformly has a lattice mismatch rate of −0.10% relative to the n-type substrate41. In this case, the n-type current block layer50has a smaller lattice constant a2than the lattice constant a1of the n-type substrate41(a2<a1).

In the semiconductor laser device, the n-type buffer layer42, the p-type etching stopper layer46, the p-type intermediate layer48and the p-type protective layer49have a lattice mismatch rate of 0% relative to the n-type substrate41. In this case, these layers have a lattice constant a1equal to that of the n-type substrate41.

In the semiconductor laser device, the n-type first cladding layer43, the p-type second cladding layer45and the p-type third cladding layer47have a lattice mismatch rate of +0.05% relative to the n-type substrate41. The barrier layers and the light guide layers constituting parts of the MQW active layer44have a lattice mismatch rate of +0.03% relative to the n-type substrate41. The well layers constituting the other parts of the MQW active layer44have a lattice mismatch rate of +0.22% relative to the n-type substrate41. In this case, lattice constants of the n-type first cladding layer43, the barrier layers, the well layers, the light guide layers, the p-type second cladding layer45, and the p-type third cladding layer47are larger than the lattice constant a1of the n-type substrate41.

In the n-type current block layer50, the lattice mismatch rate [Δa/a (%)] is given by adjusting an In mole fraction z of AlxInzP constituting the n-type current block layer50.

Relationship between the lattice mismatch rate relative to the n-type GaAs substrate41and the Al, Ga, and In mole fractions will be described. The relational expression for calculating the lattice mismatch rate (Δa/a) relative to the n-type substrate41from the Al, Ga, and In mole fractions is given by equations (1) and (2) below:
Δa/a=−3.5912+7.4z+(0.13135−0.13z)x/(x+y)  (1)
x+y+z=1  (2)

The In mole fraction z of the n-type current block layer50for its lattice mismatch rate of 0% is z=0.476 because Δa/a=0 and y=0 in equations (1) and (2). Therefore, the composition of the n-type current block layer50is Al0.524In0.476P. On the other hand, the In mole fraction z of the n-type current block layer50in the case where the lattice mismatch rate is −0.10% is z=0.462 because Δa/a=−0.10 and y=0 in equations (1) and (2). Therefore, the composition of the n-type current block layer50is Al0.538In0.462P. That is, the In mole fraction z2in the case where the n-type AlxInzP current block layer50has a negative lattice mismatch rate relative to the n-type GaAs substrate41is smaller than the In mole fraction z, in the case where the n-type AlxInzP current block layer50is lattice matched with the n-type GaAs substrate41(i.e., the case where the lattice mismatch rate is 0%).

Next, a method of producing a semiconductor laser device with the above constitution will be described in accordance withFIGS. 3A-3E. First, as shown inFIG. 3A, an n-type GayInzP buffer layer42, an n-type AlxGayInzP first cladding layer43, a MQW active layer44, a p-type AlxGayInzP second cladding layer45, a p-type etching stopper layer46, a p-type AlxGayInzP third cladding layer47, a p-type GayInzP intermediate layer48and a p-type GaAs protective layer49are sequentially grown on an n-type GaAs substrate41epitaxially by molecular beam epitaxy (MBE).

Next, using a known photolithography technique, a stripe-shaped resist mask55extending in a direction perpendicular to end faces of a laser cavity is formed on the p-type protective layer49. Then, as shown inFIG. 3B, using a known etching technique, the p-type protective layer49, the p-type intermediate layer48and the p-type third cladding layer47are etched until the p-type etching stopper layer46is reached, so that a stripe-shaped ridge54having a width of about 3 μm is formed.

Next, the stripe-shaped resist mask55formed on the p-type protective layer49is removed, and both sides of the ridge54consisting of the p-type third cladding layer47, the p-type intermediate layer48and the p-type protective layer49are filled with an n-type AlxInzP current block layer50grown by the second MBE process, as shown inFIG. 3C. At this time, the In mole fraction is adjusted so that the lattice mismatch rate of the n-type current block layer50relative to the n-type substrate41is uniform within the n-type current block layer50.

Thereafter, using a known photolithography technique, as shown inFIG. 3D, resist masks56,56are formed on the n-type current block layer50formed on the sides of the ridge54. Then, using a known etching technique, only a part of the n-type current block layer50that is present at an opening between the resist masks56is selectively etched. Next, the resist masks56formed on the n-type current block layer50are removed, and the third MBE process is performed to form a p-type GaAs contact layer51as shown inFIG. 3E. Furthermore, although not shown, a p-side electrode52is formed on the top of the p-type contact layer51, and an n-side electrode53is formed on the underside of the n-type substrate41.

After that, the wafer thus obtained is divided into bars such that the cavity length is 800 μm. Light emission end faces of each bar at both sides thereof are coated with a reflection film (not shown). Lastly, each bar is divided into chips to form individual semiconductor laser devices.

Evaluation of characteristics was conducted for the semiconductor laser devices formed by the production method in the present embodiment so that the lattice mismatch rate of the n-type current block layer50relative to the n-type substrate41was uniformly 0.10%. For comparison, 12 types of semiconductor laser devices in which the lattice mismatch rate of the n-type current block layer50relative to the n-type substrate41was uniformly −0.30%, −0.25%, −0.20%, −0.15%, −0.05%, 0%, +0.05%, +0.10%, +0.15%, +0.20%, +0.25% and +0.30%, respectively, were fabricated, and the measurement of characteristics of these devices was also conducted at the same time.

First, the oscillation wavelength (λ) at CW 50 mW was measured. As a result, both the semiconductor laser devices of the present embodiment and the 12 types of the semiconductor laser devices for comparison had an oscillation wavelength of 655 nm. Next, the driving current (Iop) at CW 50 mW of the semiconductor laser devices of the present embodiment and the 12 types of the semiconductor laser devices for comparison were measured.

FIG. 4shows the relationship between the lattice mismatch rate of the N-type current block layer50relative to the n-type substrate41and the driving current (Iop) at CW 50 mW regarding the semiconductor laser devices fabricated by the production method in the present embodiment and the 12 types of semiconductor laser devices for comparison. InFIG. 4, the axis of ordinates shows Iop (mA) at CW 50 mW, while the axis of abscissas shows the lattice mismatch rate (%) of the n-type current block layer50relative to the n-type substrate41.

As seen fromFIG. 4, in the range in which the lattice mismatch rate of the n-type current block layer51relative to the n-type substrate41is −0.20% or more but not more than 0%, the driving current (Iop) is as low as not more than 85 mA. On the other hand, in the range in which the lattice mismatch rate is less than −0.20%, or more than 0%, the driving current (Iop) suddenly increases.

By uniformly providing the negative lattice mismatch within the n-type current block layer50, a negative strain is indirectly imparted to the p-type second cladding layer45. As a result, the diffusion rate of impurity atoms (Be atoms) having p-type conductivity, which are present in the p-type second cladding layer45, is lowered. Consequently, the diffusion of Be atoms into the MQW active layer44is reduced. Accordingly, the driving current (Iop) is lowered in the range in which the lattice mismatch rate is not more than 0%. However, in the case where the lattice mismatch rate of the n-type current block layer50relative to the n-type substrate41is less than −0.20%, the lattice of the n-type current block layer50is relaxed relative to the n-type substrate41and the p-type second cladding layer45, so that the effect of the strain introduced into the p-type second cladding layer45disappears. Because of that, the diffusion of Be atoms into the active layer increases and the driving current (Iop) increases.

That is, if the lattice mismatch rate is in the range of from −0.20% to 0%, inclusive, the driving current (Iop) is as low as 85 mA or less. On the other hand, if the lattice mismatch rate is in the range of less than −0.20%, or more than 0%, the driving current (Iop) suddenly increases.

It can be seen from the above that reduction of the driving current is realized by setting the lattice mismatch rate of the n-type current block layer50relative to the n-type substrate41to the range of −0.20% or more but not more than 0%, preferably −0.15% or more but not more than −0.05%. This is why, in the semiconductor laser device of the present embodiment, in which the lattice mismatch rate of the entire n-type current block layer50is uniformly −0.10%, the driving current is reduced.

Second Embodiment

FIG. 5is a perspective view of a semiconductor laser device according to a second embodiment of the present invention, showing a light emission end face of the device.FIG. 6Ais a cross-sectional view taken along line6A-6A ofFIG. 5, showing parts including a waveguide. And,FIG. 6Bis a cross-sectional view taken along line6B-6B ofFIG. 5.

InFIG. 5, reference numeral61denotes an n-type GaAs substrate (carrier concentration: 2×1018cm−3), reference numeral62denotes an n-type GayInzP (0≦y, z≦1) buffer layer (carrier concentration: 1×1018cm−3, y=0.515, z=0.485, film thickness: about 0.2 μm), and reference numeral63denotes an n-type AlxGayInzP (0≦x≦1) first cladding layer (carrier concentration: 1×1018cm−3, x=0.360, y=0.155, z=0.485, film thickness: about 2 μm). Reference numeral64denotes an active layer (MQW active layer) constructed of a multiquantum well structure of alternating AlxGayInzP barrier layers (x=0.258, y=0.257, z=0.485; each having a film thickness of about 50 Å) and GayInzP well layers (y=0.485, z=0.515; each having a film thickness of about 50 Å), and two AlxGayInzP light guide layers (x=0.258, y=0.257, z=0.485, film thickness: about 500 Å) between which the multiquantum well structure is interposed. Of the MQW active layer64, reference numeral64a denotes an MQW active layer region inside of a laser cavity (hereinafter referred to as “active region”), and reference numeral64bdenotes an MQW active layer region which is in proximity of an end face of the laser cavity and which has a photoluminescence peak wavelength smaller than that of the active region64a(hereinafter referred to as “window region”).

The p-type third cladding layer67, the p-type intermediate layer68and the p-type protective layer69constitute a stripe-shaped ridge74. Reference numeral75denotes a current non-injection region formed by removing the p-type intermediate layer68and the p-type protective layer69. Si atoms are contained, as n-type impurities, in each of the layers from the n-type substrate61to the n-type first cladding layer63, while Be atoms are contained, as p-type impurities, in each of the layers from the p-type second cladding layer65to the p-type protective layer69.

Reference numeral70denotes an n-type AlxInzP current block (constriction) layer (carrier concentration: 1×1018cm−3, x=0.538, z=0.462, film thickness: about 1.2 μm). The n-type current block layer70is formed so that both sides of the ridge74are filled with the n-type current block layer70. Reference numeral71denotes a p-type GaAs contact layer (carrier concentration: 1×1019cm−3, film thickness: about 4 μm), reference numeral72denotes a p-side electrode, and reference numeral73denotes an n-side electrode.

In the semiconductor laser device with the above construction, the p-type second cladding layer65and the p-type third cladding layer67have a lattice mismatch rate of −0.05% relative to the n-type substrate61. The n-type current block layer70has a lattice mismatch rate of −0.10% relative to the n-type substrate61. In this case, lattice constants of the p-type second cladding layer65, the p-type third cladding layer67and the n-type current block layer70are smaller than the lattice constant of the n-type substrate61.

In the semiconductor laser device, the n-type buffer layer62, the p-type etching stopper layer66, the p-type intermediate layer68and the p-type protective layer69have a lattice mismatch rate of 0% relative to the n-type substrate61. In this case, these layers have a lattice constant a, equal to the lattice constant of the n-type substrate61.

In the semiconductor laser device, the n-type first cladding layer63has a lattice mismatch rate of +0.05% relative to the n-type substrate61. The barrier layers and the light guide layers constituting parts of the MQW active layer64have a lattice mismatch rate of +0.03% relative to the n-type substrate61. The well layers constituting the other parts of the MQW active layer64have a lattice mismatch rate of +0.22% relative to the n-type substrate61. In this case, lattice constants of the n-type first cladding layer63, the barrier layers, the well layers, the light guide layers, the p-type second cladding layer65and the p-type third cladding layer67are larger than the lattice constant a1of the n-type substrate61.

In the p-type second cladding layer65and the p-type third cladding layer67, the lattice mismatch rate (Δa/a) is given by adjusting, in AlxGayInzP constituting these layers, the ratio of the Al mole fraction to the sum of the Al mole fraction and the Ga mole fraction (x/(x+y)) so that the ratio is always constant (i.e., x/(x+y)=0.7), and also adjusting the In mole fraction z.

In the case where the lattice mismatch rates of the p-type second cladding layer65and the p-type third cladding layer67are −0.05%, the Al mole fraction x, the Ga mole fraction y and the In mole fraction z in the p-type second and third cladding layers65and67are x=0.370, y=0.158, and z=0.472 because Δa/a=−0.050 and x/(x+y)=0.7 from equations (1) and (2) above. Therefore, the composition of the p-type second cladding layer65and the p-type third cladding layer67is Al0.370Ga0.158In0.472P.

Next, a method of producing a semiconductor laser device with the above constitution will be described in accordance withFIGS. 7A-7F. First, as shown inFIG. 7A, an n-type GayInzP buffer layer62, an n-type AlxGayInzP first cladding layer63, a MQW active layer64, a p-type AlxGayInzP second cladding layer65, a p-type etching stopper layer66, a p-type AlxGayInzP third cladding layer67, a p-type GayInzP intermediate layer68and a p-type GaAs protective layer69are epitaxially grown on an n-type GaAs substrate61by the molecular beam epitaxy (MBE) method.

In that case, the In mole fraction is controlled so that the lattice mismatch rates of the p-type second cladding layer65and the p-type third cladding layer67relative to the n-type GaAs substrate61are uniform in the p-type second cladding layer65and the p-type third cladding layer67, respectively.

Next, using a known photolithography technique, a 60 μm wide stripe-shaped ZnvOw(v, w≧1) film76serving as an impurity diffusion source is formed on a surface of the p-type protective layer69in regions in proximity of end faces of the laser cavity, as shown inFIG. 7B, in such a manner that the stripes of the film76extend in a direction perpendicular to the ridge stripe. The ZnvOwfilm76has a film thickness of 35 nm, and the pitch of the stripes of the film76is 800 μm, which is the same as the cavity length. Then, a SitOu(t, u≧1) film77that is a dielectric film is formed on the entire surface of the wafer having the ZnvOwfilm76, to have a film thickness of 200 nm.

Next, annealing is performed on the wafer of which the surface is covered with the SitOufilm77that is a dielectric film, in a nitrogen atmosphere under the condition that the temperature is 510° C. and the holding time is two hours. Thereby, in the laser cavity end face proximity regions, where the ZnvOwimpurity diffusion source film76is formed, Zn atoms from the ZnvOwfilm76are let to be diffused into the MQW active layer64. At the same time, Be atoms contained in each of the layers from the p-type second cladding layer65through the p-type protective layer69are also diffused into the MQW active layer64so that the photoluminescence peak wavelength of the MQW active layer (window regions)64bin the laser cavity end face proximity regions is made smaller than that of the MQW active layer (active region)64ain the laser cavity internal region.

Using a part of the wafer after forming the active region64aand the window regions64bin the MQW active layer64by the annealing, the wavelengths of the MQW active layer (window regions)64bin the laser cavity end face proximity regions and the MQW active layer (active region)64ain the laser cavity internal region were measured by the PL method. As a result, it was confirmed that the emission spectrum of the window region64bwas shifted to the short wavelength side by38nm from the emission spectrum of the active region64a.

Thereafter, the ZnvOwfilm76and the SitOufilm77formed on the p-type protective layer69are removed, and using a known photolithography technique, a stripe-shaped resist mask78extending in a direction perpendicular to the end faces of the laser cavity is formed on the p-type protective layer69as shown inFIG. 7C. Then, using a known etching technique, the p-type protective layer69, the p-type intermediate layer68and the p-type third cladding layer67are etched until the p-type etching stopper layer66is reached, to thereby form a stripe-shaped ridge74having a width of about 2 μm.

Next, the stripe-shaped resist mask78formed on the p-type protective layer69is removed, and both sides of the ridge74consisting of the p-type third cladding layer67, the p-type intermediate layer68and the p-type protective layer69are filled with an n-type AlxInzP current block layer70by the second MBE process, as shown inFIG. 7D. At this time, the In mole fraction in AlxInzP is controlled so that the entire n-type current block layer70has a uniform lattice mismatch rate relative to the n-type substrate61.

Thereafter, using a known photolithography technique, a resist mask (not shown) is formed on the n-type current block layer70at the lateral sides of the ridge74. Then, using a known etching technique, only a part of the n-type current block layer70that is present at an opening of the resist mask and formed on the ridge74is selectively etched. After that, the resist mask formed on the n-type current block layer70is removed. Using the known photolithography technique again, a resist mask79is formed such that a 740 μm wide resist mask stripe is formed at the laser cavity internal region, as shown inFIG. 7E, and the p-type protective layer69and the p-type intermediate layer68in openings of the resist mask79, which are located in the laser cavity end face proximity regions, are selectively removed. The openings of the resist mask79are formed so as to be positioned immediately above the MQW active layer (window regions)64bin the laser cavity end face proximity regions.

Thereby, there will be a bandgap energy difference between the p-type third cladding layer67and a p-type contact layer71(to be formed later) in the laser cavity end face proximity regions, so that current non-injection regions75will be formed. The thus formed current non-injection regions75will be positioned immediately above the window regions64b.Therefore, current injection into the widow regions64bis prevented, and reactive current not contributing to light emission is reduced.

The resist mask79formed in the laser cavity internal region is then removed. Then, a p-type GaAs contact layer71is formed by the third MBE process, as shown inFIG. 7F. Further, although not shown, a p-side electrode72is formed on the top of the p-type contact layer71, while an n-side electrode73is formed on the underside of the n-type substrate61.

Next, the thus obtained wafer is scribed substantially at the center of the 60 μm wide laser cavity end face proximity regions, and divided into bars having the cavity length. Further, both sides of each bar serving as light emission end faces are coated with a reflection film. Then, the bars are divided into chips to form semiconductor laser devices each having about 30 μm wide window regions and current non-injection regions at the end faces of the laser cavity having a length of 800 μm.

Evaluation of characteristics was conducted for the semiconductor laser devices formed by the production method in the present embodiment so that the lattice mismatch rates of the p-type second cladding layer65and the p-type third cladding layer67relative to the n-type substrate61were uniformly −0.05%. For comparison, 8 types of semiconductor laser devices in which the lattice mismatch rates of the p-type second and third cladding layers65and67relative to the n-type substrate61were uniformly −0.20%, −0.15%, −0.10%, 0%, +0.05%, +0.10%, +0.15% and +0.20%, respectively, were fabricated, and the measurement of characteristics of these devices was also conducted at the same time.

First, the oscillation wavelength (λ) at CW 50 mW was measured. As a result, both the semiconductor laser devices of the present embodiment and the 8 types of the semiconductor laser devices for comparison had an oscillation wavelength of 660 nm. Further, as a result of maximum optical output experiments, neither the semiconductor laser devices of the present embodiment nor the semiconductor laser devices for comparison suffered from COD even at an optical output power of 300 mW or more.

From the above results, the following was confirmed. That is, in the semiconductor laser devices of the present embodiment in which the photoluminescence peak wavelength of the window regions64bis smaller than that of the active region64a,regions where laser light absorption is prevented are formed in the window regions64band as a result, the occurrence of COD in the window regions64bis suppressed.

Next, driving currents (Iop) at CW 50 mW of the semiconductor laser devices of the present embodiment and the 8 types of semiconductor laser devices for comparison were measured.

FIG. 8shows the relationship between the lattice mismatch rate of the p-type second and third cladding layer65and67relative to the n-type substrate61and the driving current (Iop) at CW 50 mW regarding the semiconductor laser devices fabricated by the production method in the present embodiment and the 8 types of semiconductor laser devices for comparison. InFIG. 8, the axis of ordinates shows Iop (mA) at CW 50 mW, while the axis of abscissas shows the lattice mismatch rate (%) of the p-type second and third cladding layers65and67relative to the n-type substrate61.

As seen fromFIG. 8, in the range in which the lattice mismatch rates of the p-type second cladding layer65and the p-type third cladding layer67relative to the n-type substrate61are −0.15% or more but not more than +0.05%, the driving current (Iop) is as low as not more than 100 mA. On the other hand, in the range in which the lattice mismatch rates thereof are less than −0.15%, or more than +0.05%, the driving current (Iop) suddenly increases.

By providing the negative lattice mismatch within the n-type current block layer70, a negative strain is indirectly imparted to the p-type second cladding layer65. For that reason, the diffusion rate of Be atoms present in the p-type second cladding layer65is reduced. Furthermore, by providing the negative lattice mismatch in the p-type second cladding layer65and the p-type third cladding layer67, the diffusion rate of Be atoms present in the p-type second cladding layer65is further reduced. As a result, diffusion of Be atoms into the MQW active layer64is reduced. Therefore, in the range in which the lattice mismatch rate is not more than 0%, the driving current (Iop) is reduced. However, in the case where the lattice mismatch rates of the p-type second cladding layer65and the p-type third cladding layer67relative to the n-type substrate61are less than −0.15%, crystal defects are developed in the MQW active layer64due to the negative strain in the n-type current block layer70and the p-type second cladding layer65. Therefore, the driving current (Iop) suddenly increases. Furthermore, in the case where the lattice mismatch rates of the p-type second cladding layer65and the p-type third cladding layer67relative to the n-type substrate61are more than +0.05%, the effect of strain introduced into the p-type second cladding layer65is reduced. Because of that, diffusion of Be atoms into the active layer increases.

That is, if the lattice mismatch rate is in the range of from −0.15% to +0.05%, inclusive, the driving current (Iop) is as low as 100 mA or less. On the other hand, if the lattice mismatch rate is in the range of less than −0.15%, or more than +0.05%, the driving current (Iop) suddenly increases.

It can be seen from the above that reduction of the driving current is realized by setting the lattice mismatch rates of the p-type second cladding layer65and the p-type third cladding layer67relative to the n-type substrate61to the range of −0.15% or more but not more than 0.05%, preferably −0.10% or more but not more than 0%. This is why, in the semiconductor laser device of the present embodiment in which the lattice mismatch rates of the p-type second cladding layer65and the p-type third cladding layer67relative to the n-type current block layer61are −0.05%, the driving current is reduced.

In the semiconductor laser device having a window structure of the present embodiment, wherein the photoluminescence peak wavelength of the MQW active layer (window regions)64bin the laser cavity end face proximity regions is made smaller than that of the MQW active layer (active region)64ain the laser cavity internal region, the lattice mismatch rates of the p-type second cladding layer65and the p-type third cladding layer67relative to the n-type substrate61are set to −0.05%. However, the invention is not limited to this. The reduction of the driving current can be realized also by a semiconductor laser device of the first embodiment if the lattice mismatch rates of the p-type second cladding layer45and the p-type third cladding layer47relative to the n-type substrate41are set to the range of −0.15% or more but not more than +0.05%, preferably −0.10% or more but not more than 0%.

In the present embodiment, the lattice mismatch rate of the n-type current block layer70is uniformly −0.10% within the n-type current block layer70. However, if the lattice mismatch rate of the n-type AlxInzP current block layer70relative to the n-type substrate61is in the range of −0.20% or more but not more than 0%, preferably −0.15% or more but not more than −0.05%, the same effect as that of the present embodiment can be obtained.

Third Embodiment

FIG. 9is a perspective view of a semiconductor laser device according to a second embodiment of the present invention, showing a light emission end face of the device.FIG. 10Ais a cross-sectional view taken along line10A-10A ofFIG. 9, showing parts including a waveguide. And,FIG. 10Bis a cross-sectional view taken along line10B-10B ofFIG. 9.

InFIG. 9, reference numeral81denotes an n-type GaAs substrate (carrier concentration: 2×1018cm−3), reference numeral82denotes an n-type GayInzP (0≦y, z≦1) buffer layer (carrier concentration: 1×1018cm−3, y=0.515, z=0.485, film thickness: about 0.2 μm), and reference numeral83denotes an n-type AlxGayInzP (0≦x≦1) first cladding layer (carrier concentration: 1×1018cm−3, x=0.360, y=0.155, z=0.485, film thickness: about 2 μm). Reference numeral84denotes an active layer (MQW active layer) constructed of a multiquantum well structure of alternating AlxGayInzP barrier layers (x=0.258, y=0.257, z=0.485; each having a film thickness of about 50 Å) and GayInzP well layers (y=0.485, z=0.515; each having a film thickness of about 50 Å), and two AlxGayInzP light guide layers (x=0.258, y=0.257, z=0.485, film thickness: about 500 Å) between which the multiquantum well structure is interposed. Of the MQW active layer84, reference numeral84adenotes an MQW active layer region inside of a laser cavity (hereinafter referred to as “active region”), and reference numeral84bdenotes an MQW active layer region which is in proximity of an end face of the laser cavity and which has a photoluminescence peak wavelength smaller than that of the active region84a(hereinafter referred to as “window region”).

The p-type third cladding layer87, the p-type intermediate layer88and the p-type protective layer89constitute a stripe-shaped ridge94. Reference numeral95denotes current non-injection regions formed by removing the p-type intermediate layer88and the p-type protective layer89. Si atoms are contained, as n-type impurities, in each of the layers from the n-type substrate81to the n-type first cladding layer83, while Be atoms are contained, as p-type impurities, in each of the layers from the p-type second cladding layer85to the p-type protective layer89.

Reference numeral90denotes a current block (constriction) layer that is formed so that both sides of the ridge stripe94are filled with the current block layer90. The current block layer90consists of an n-type AlxInzP first current block layer90a(carrier concentration: 1×1018cm−3, x=0.545, z=0.455, film thickness: about 0.3 μm) and an n-type AlxInzP second current block layer90b(carrier concentration: 1×1018cm−3, x=0.531, z=0.469, film thickness: about 0.9 μm). Reference numeral91denotes a p-type GaAs contact layer (carrier concentration: 1×1019cm−3, film thickness: about 4 μm), reference numeral92denotes a p-side electrode, and reference numeral93denotes an n-side electrode.

In the semiconductor laser device, the n-type first current block layer90ahas a lattice mismatch rate of −0.15% relative to the n-type substrate, and the n-type second current block layer90bhas a lattice mismatch rate of −0.05% relative to the n-type substrate81. That is, the lattice mismatch rate of the current block layer increases stepwise with the increasing distance from the p-type second cladding layer85in a direction substantially perpendicular to the n-type substrate81. In this case, lattice constants of the n-type first current block layer90aand the n-type second current block layer90bare smaller than a lattice constant of the n-type substrate81, and the lattice constant of the n-type first current block layer90ais less far from the lattice constant of the n-type substrate81than the lattice constant of the n-type second current block layer90bis.

In the semiconductor laser device, the p-type second cladding layer85and the p-type third cladding layer87have a lattice mismatch rate of −0.05% relative to the n-type substrate81. In this case, lattice constants of the p-type second cladding layer85and the p-type third cladding layer87are smaller than the lattice constant of the n-type substrate81.

In the semiconductor laser device, the n-type buffer layer82, the p-type etching stopper layer86, the p-type intermediate layer88and the p-type protective layer89have a lattice mismatch rate of 0% relative to the n-type substrate81. In this case, these layers have a lattice constant a1equal to that of the n-type substrate81.

In the semiconductor laser device, the n-type first cladding layer83has a lattice mismatch rate of +0.05% relative to the n-type substrate81. The barrier layers and the light guide layers constituting parts of the MQW active layer84have a lattice mismatch rate of +0.03% relative to the n-type substrate81. The well layers constituting the other parts of the MQW active layer84have a lattice mismatch rate of +0.22% relative to the n-type substrate81. In this case, lattice constants of the n-type first cladding layer83, the barrier layers, the well layers, the light guide layers, the p-type second cladding layer85and the p-type third cladding layer87are larger than the lattice constant of the n-type substrate81.

In the p-type second cladding layer65and the p-type third cladding layer67, the lattice mismatch rate (Δa/a) is given by adjusting, in AlxGayInzP constituting these layers, the ratio of the Al mole fraction to the sum of the Al mole fraction and the Ga mole fraction (x/(x+y)) so that the ratio is always constant (i.e., x/(x+y)=0.7), and also adjusting the In mole fraction z. Furthermore, in the n-type first current block layer90aand the n-type second current block layer90b,the lattice mismatch rate (Δa/a) is given by adjusting the In mole fraction z in AlxInzP constituting these layers.

In the case where the lattice mismatch rates of the p-type second cladding layer85and the p-type third cladding layer87are −0.05%, the Al mole fraction x, the Ga mole fraction y and the In mole fraction z in the p-type second and third cladding layers85and87are x=0.370, y=0.158, and z=0.472 because Δa/a=−0.050 and x/(x+y)=0.7 from equations (1) and (2) above. Therefore, the composition of the p-type second cladding layer85and the p-type third cladding layer87is Al0.370Ga0.158In0.472P.

On the other hand, the In mole fraction z in the n-type first current block layer90ahaving the lattice mismatch rate of −0.15% is z=0.455 because Δa/a=−0.15 and y=0 from equations (1) and (2). Therefore, the composition of the n-type first current block layer90ais Al0.545In0.455P. Further, the In mole fraction z in the n-type second current block layer90bhaving the lattice mismatch rate of −0.05% is z=0.469 because Δa/a=−0.05 and y=0 from equations (1) and (2). Therefore, the composition of the n-type second current block layer90bis Al0.531In0.469P.

The measurement of characteristics of semiconductor laser devices of the present embodiment was conducted. For comparison, the measurement of characteristics of semiconductor laser devices of the second embodiment was also conducted. First, the oscillation wavelength (λ) at CW 50 mW was measured. As a result, both the semiconductor laser devices of the present embodiment and the semiconductor laser devices of the second embodiment had an oscillation wavelength of 660 nm. Next, the driving current (Iop) at CW 50 mW was measured. As a result, both the semiconductor laser devices of the present embodiment and the semiconductor laser devices of the second embodiment had a driving current (Iop) of 85 mA. Maximum optical output tests were conducted. As a result, neither the semiconductor laser devices of the present embodiment nor the semiconductor laser devices of the second embodiment suffered from COD even at an optical output power of 300 mW or more.

Furthermore, reliability tests at 70° C. and 50 mW were conducted on the semiconductor laser devices. While the semiconductor laser devices of the second embodiment had an average lifetime of about 2000 hours, the semiconductor laser devices of the present embodiment had an average lifetime of about 3000 hours, demonstrating that the average lifetime has improved.

It can be understood from the above that in a semiconductor laser device wherein the lattice mismatch rates of the n-type first current block layer90aand the n-type second current block layer90brelative to the. n-type substrate81are −0.15% and −0.05%, respectively, so that the lattice mismatch rate of the current block layer90increases stepwise as a whole as the distance from the p-type second cladding layer85increases, as in the present embodiment, reduction of driving current and further improvement in long-term reliability have been realized.

The reduction of the driving current and the further improvement in the long-term reliability are realized by the following reason. That is, the n-type first current block layer90awhose lattice mismatch rate is smaller is disposed in the vicinity of the p-type second cladding layer85, while at a position farther away from the p-type second cladding layer85, the n-type first current block layer90bwhose lattice mismatch rate is larger is disposed, whereby a negative strain introduced into the MQW active layer84can be reduced while a negative strain is indirectly imparted to the p-type second cladding layer85. As a result, the diffusion of Be atoms into the active layer is reduced, and deterioration of crystallinity in the MQW active layer84is suppressed.

In the present embodiment, the lattice mismatch rates of the n-type first current block layer90aand the n-type second current block layer90brelative to the n-type substrate81are set to −0.15% and −0.05%, respectively, so that the lattice mismatch rate of the current block layer90increases stepwise as a whole as the distance thereof from the p-type second cladding layer85increases. However, the effect of the present embodiment can also be obtained by setting the lattice mismatch rates of the n-type first current block layer90aand the n-type second current block layer90brelative to the n-type substrate81to fall within the range of −0.20% or more but not more than 0%, preferably within the range of −0.15% or more but not more than −0.05%, and also adjusting the lattice mismatch rate of the current block layer90such that it increases stepwise as the current block layer90goes farther from the p-type second cladding layer85.

In the present embodiment, the n-type first and second current block layers90a,90bare composed of AlxInzP, although the effect of the present embodiment can also be obtained if the above layers are composed of AlxInzAsαPβ(0≦α, β≦1). Further, the effect of the present embodiment can also be obtained if the n-type first current block layer90ais composed of AlxInzP, and the n-type second current block layer90bis composed of AlxInzAsαPβ.

In the present embodiment, the n-type current block layer consists of two layers having different lattice mismatch rates relative to the n-type substrate81. However, the effect of the present embodiment can also be obtained if the n-type current block layer consists of three or more layers having different lattice mismatch rates relative to the n-type substrate81, if the lattice mismatch rate of each layer is set within the range of −0.20% or more but not more than 0%, preferably −0.15% or more but not more than −0.05%, and if the lattice mismatch rates of the three or more layers are adjusted such that the layer at a larger distance from the p-type second cladding layer85has a larger lattice mismatch rate so that the lattice mismatch rate of the current block layer increases stepwise as a whole.

According to the present embodiment, in a window structure semiconductor laser device in which the photoluminescence peak wavelength of the MQW active layer (window regions)84bin the laser cavity end face proximity regions is made smaller than that of the MQW active layer (active region)84ain the laser cavity internal region, the lattice mismatch rate of the n-type first current block layer90arelative to the n-type substrate81is set to −0.15%, the lattice mismatch rate of the n-type second current block layer90brelative to the n-type substrate81is set to −0.05%, so that the lattice mismatch rate increases stepwise as the distance of the current block layer from the p-type AlxGayInzP second cladding layer85increases. However, the invention is not limited to this, and in the semiconductor laser device of the first embodiment as well, the n-type current block layer50can be formed in two layers, and the lattice mismatch rates of the n-type first and second current block layers relative to the n-type substrate41are set within the range of −0.20% or more but not more than 0%, preferably −0.15% or more but not more than 0.05% such that the lattice mismatch rate of the n-type current block layer50increases stepwise with the increasing distance thereof from the p-type second cladding layer45. As a result, reduction of the driving current and improvement in the long-term reliability can be realized.

Fourth Embodiment

FIG. 11is a perspective view of a semiconductor laser device according to a first embodiment of the present invention, showing a light emission end face of the device.FIG. 12Ais a cross-sectional view taken along line12A-12A ofFIG. 11, showing parts including a waveguide. And,FIG. 12Bis a cross-sectional view taken along line12B-12B ofFIG. 11.

InFIG. 11, reference numeral101denotes an n-type GaAs substrate (carrier concentration: 2×1018cm−3), reference numeral102denotes an n-type GayInzP (0≦y, z≦1) buffer layer (carrier concentration: 1×1018cm−3, y=0.515, z=0.485, film thickness: about 0.2 μm, and reference numeral103denotes an n-type AlxGayInzP (0≦x≦1) first cladding layer (carrier concentration: 1×1018cm−3, x=0.360, y=0.155, z=0.485, film thickness: about 2 μm). Reference numeral104denotes an active layer (MQW active layer) constructed of a multiquantum well structure of alternating AlxGayInzP barrier layers (x=0.258, y=0.257, z=0.485; each having a film thickness of about 50 Å) and GayInzP well layers (y=0.485, z=0.515; each having a film thickness of about 50 Å), and two AlxGayInzP light guide layers (x=0.258, y=0.257, z=0.485, film thickness: about 500 Å) between which the multiquantum well structure is interposed. Of the MQW active layer104, reference numeral104adenotes an MQW active layer region inside of a laser cavity (hereinafter referred to as “active region”), and reference numeral104bdenotes an MQW active layer region which is in proximity of an end face of the laser cavity and which has a photoluminescence peak wavelength smaller than that of the active region64a(hereinafter referred to as “window region”).

The p-type third cladding layer107, the p-type intermediate layer108and the p-type protective layer109constitute a stripe-shaped ridge114. Reference numeral115denotes a current non-injection region formed by removing the p-type intermediate layer108and the p-type protective layer109. Si atoms are contained, as n-type impurities, in each of the layers from the n-type substrate101to the n-type first cladding layer103, while Be atoms are contained, as p-type impurities, in each of the layers from the p-type second cladding layer105to the p-type protective layer109.

In the semiconductor laser device, the n-type current block layer110has a lattice mismatch rate of −0.15% or more but not more than −0.05% relative to the n-type substrate101. The lattice mismatch rate in the n-type current block layer110increases with the increasing distance from the p-type second cladding layer105. In proximity of the p-type second cladding layer105, the As mole fraction α of the n-type current block layer110is 0, and the Al mole fraction x and the In mole fraction thereof are fixed. In this case, the n-type current block layer110has a smaller lattice constant than that of the n-type substrate101.

In the semiconductor laser device, the p-type second cladding layer105and the p-type third cladding layer107have a lattice mismatch rate of −0.05% relative to the n-type substrate101. In this case, lattice constants of the p-type second cladding layer105and the p-type third cladding layer107are smaller than the lattice constant of the n-type substrate101.

In the semiconductor laser device, the n-type buffer layer102, the p-type etching stopper layer106, the p-type intermediate layer108and the p-type protective layer109have a lattice mismatch rate of 0% relative to the n-type substrate101. In this case, these layers have lattice constants a, equal to that of the n-type substrate101.

In the semiconductor laser device, the n-type first cladding layer103has a lattice mismatch rate of +0.05% relative to the n-type substrate101. The barrier layers and the light guide layers constituting parts of the MQW active layer104have a lattice mismatch rate of +0.03% relative to the n-type substrate101. The well layers constituting the other parts of the MQW active layer104have a lattice mismatch rate of +0.22% relative to the n-type substrate101. In this case, lattice constants of the n-type first cladding layer103, the barrier layers, the well layers, the light guide layers, the p-type second cladding layer105, and the p-type third cladding layer107are larger than the lattice constant a1of the n-type substrate101.

In the n-type current block layer110, the lattice mismatch rate (Δa/a) is given by adjusting the As mole fraction α of AlxInzAsαPβconstituting the current block layer110.

The relationship between the lattice mismatch rate relative to the n-type substrate101and the Al, In, As, and P mole fractions will be described below. The relational expression for calculating the lattice mismatch rate (Δa/a) relative to the n-type substrate101from the Al, In, As, and P mole fractions is given by equation (3) below:
Δa/a=−3.45985+7.27Xz+3.491Xα(3)

If the In mole fraction z is set to z=0.455, the As mole fraction α of the n-type AlxInzAsαPβcurrent block layer110in proximity of the p-type second cladding layer105for the lattice mismatch rate of −0.15% is α=0 from equation (3). Therefore, the composition of the n-type current block layer110is Al0.545In0.455P. Further, with the In mole fraction set to z=0.455, the As mole fraction α of the n-type current block layer110for the lattice mismatch rate of −0.05% is α=0.029 from equation (3). Therefore, the composition of the n-type current block layer110is Al0.545In0.455As0.029P0.971.

Next, a method of producing a semiconductor laser device with the above constitution will be described in accordance withFIGS. 13A-13F. First, as shown inFIG. 13A, an n-type GayInzP buffer layer102, an n-type AlxGayInzP first cladding layer103, a MQW active layer104, a p-type AlxGayInzP second cladding layer105, a p-type etching stopper layer106, a p-type AlxGayInzP third cladding layer107, a p-type GayInzP intermediate layer108and a p-type GaAs protective layer49are epitaxially grown on an n-type GaAs substrate101by the molecular beam epitaxy method.

During this process, the In mole fraction is controlled so that the p-type second cladding layer105and the p-type third cladding layer107have uniform lattice mismatch rates relative to the n-type GaAs substrate101, respectively.

Next, using the known photolithography technique, a 60 μm wide stripe-shaped ZnvOw(v, w≧1) film116serving as an impurity diffusion source is formed on a surface of the p-type protective layer109in regions in proximity of end faces of a laser cavity (“laser cavity end face proximity regions”) in such a manner that the stripes of the film116extend in a direction perpendicular to the ridge stripe. The ZnvOwfilm116has a film thickness of 35 nm, and the pitch of the stripes is 800 μm, which is the same as the cavity length. Then, a SitOu(t, u≧1) film177that is a dielectric film is formed on the entire surface of the wafer having the ZnvOwfilm116,to have a film thickness of 200 nm.

Next, annealing is performed on the wafer of which the surface is covered with the SitOudielectric film177in a nitrogen atmosphere under the condition that the temperature is 510° C. and the holding time is two hours. Thereby, in the laser cavity end face proximity regions, where the ZnvOwimpurity diffusion source film116is formed, Zn atoms from the ZnvOwfilm116are diffused into the MQW active layer104. At the same time, Be atoms contained in each of the layers from the p-type second cladding layer105to the p-type protective layer109are also diffused into the MQW active layer104so that the photoluminescence peak wavelength of the MQW active layer (window regions)104bin the laser cavity end face proximity regions is made smaller than that of the MQW active layer (active region)104ain the laser cavity internal region.

Using a part of the wafer after forming the active region104aand the window regions104bin the MQW active layer104by the annealing, the wavelengths of the MQW active layer (window regions)104bin the laser cavity end face proximity regions and the MQW active layer (active region)104ain the laser cavity internal region were measured by the PL method. As a result, it was confirmed that the emission spectrum from the window region104bwas shifted to the short wavelength side by38nm from the emission spectrum from the active region104a.

After that, the ZnvOwfilm116and the SitOufilm117formed on the p-type protective layer109are removed, and a stripe-shaped resist mask118extending in a direction perpendicular to the end faces of the laser cavity is formed on the p-type protective layer109using a known photolithography technique, as shown inFIG. 13C. Then, using the known etching technique, the p-type protective layer109, the p-type intermediate layer108and the p-type third cladding layer107are etched until the p-type etching stopper layer106is reached, to form a stripe-shaped ridge114having a width of about 2 μm.

Next, the stripe-shaped resist mask118formed on the p-type protective layer109is removed, and both sides of the ridge114consisting of the p-type third cladding layer107, the p-type intermediate layer108and the p-type protective layer109are filled with an n-type AlxInzAsαPβcurrent block layer110by performing the second MBE process, as shown inFIG. 13D. At this time, the lattice mismatch rate of the n-type current block layer110relative to the n-type substrate101is changed within the n-type current block layer110. The lattice mismatch rate of the n-type current block layer110is controlled by setting first the As mole fraction α in the n-type AlxInzAsαPβcurrent block layer110in proximity of the p-type second cladding layer105to 0, and then changing the As mole fraction α, with the Al mole fraction x and the In mole fraction z fixed, so that the lattice mismatch rate increases as the distance from the p-type second cladding layer105increases.

Thereafter, using a known photolithography technique, a resist mask (not shown) is formed on the n-type current block layer110at the lateral sides of the ridge114. Then, using a known etching technique, only a part of the n-type current block layer110that is present at an opening of the resist mask and formed on the ridge114is selectively etched. After that, a resist mask119is formed such that a 740 μm wide resist mask stripe is formed at the laser cavity internal region, as shown inFIG. 13E, and the p-type protective layer109and the p-type intermediate layer108in openings of the resist mask119, which are located in the laser cavity end face proximity regions, are selectively removed. The openings of the resist mask119are formed so as to be positioned immediately above the MQW active layer (window regions)104bin the laser cavity end face proximity regions.

Thereby, there will occur a bandgap energy difference between the p-type third cladding layer107and a p-type contact layer111(to be formed later) in the laser cavity end face proximity regions occurs, so that current non-injection regions115will be formed. The thus formed current non-injection regions115will be positioned immediately above the window regions104b.Therefore, current injection into the widow regions104bis prevented, and reactive current not contributing to light emission is reduced.

The resist mask119formed in the laser cavity internal region is then removed. Then, the p-type GaAs contact layer111is formed by the third MBE process, as shown inFIG. 13F. Further, although not shown, a p-side electrode112is formed on the top of the p-type contact layer111, while an n-side electrode113is formed on the underside of the n-type substrate101.

Next, the thus obtained wafer is scribed substantially at the center of the 60 μm wide laser cavity end face proximity regions, and divided into bars having the cavity length. Further, both sides of each bar serving as light emission end faces are coated with a reflection film. Then, the bars are divided into chips to form semiconductor laser devices each having about 30 μm wide window regions and current non-injection regions at the end faces of the laser cavity having a length of 800 μm.

The measurement of characteristics of semiconductor laser devices formed by the method of the present embodiment was conducted. For comparison, the measurement of characteristics of semiconductor laser devices of the second embodiment was also conducted.

First, the oscillation wavelength (λ) at CW 50 mW was measured. As a result, both the semiconductor laser devices of the present embodiment and the semiconductor laser devices of the second embodiment had an oscillation wavelength of 660 nm. Further, the driving current (Iop) at CW 50 mW was measured. As a result, both the semiconductor laser devices of the present embodiment and the semiconductor laser devices of the second embodiment had a driving current (Iop) of 85 mA. Maximum optical output tests were conducted. As a result, neither the semiconductor laser devices of the present embodiment nor the semiconductor laser devices of the second embodiment suffered from COD even at an optical output power of 300 mW or more.

COD did not occur in both of the semiconductor laser devices of the present embodiment and the semiconductor laser devices of the second embodiment even at an optical output power of 300 mW or more.

Furthermore, reliability tests at 70° C. and 50 mW were conducted on the semiconductor laser devices. While the semiconductor laser devices of the second embodiment had an average lifetime of about 2000 hours, the semiconductor laser devices of the present embodiment had an average lifetime of about 3000 hours, demonstrating that the average lifetime has improved.

It can be understood from the above that in a semiconductor laser device wherein the lattice mismatch rate of the n-type AlxInzAsαPβcurrent block layer110relative to the n-type substrate101changes within a range of from −0.15% to −0.05% so as to increase with the increasing distance from p-type second cladding layer105, and wherein the As mole fraction α of the n-type AlxInzAsαPβcurrent block layer110in proximity of the p-type second cladding layer105is 0 with the Al mole fraction x and the In mole fraction z fixed, reduction of the driving current and further improvement in the long-term reliability have been realized.

The reduction of the driving current and the further improvement in the long-term reliability are realized by the following reason. That is, changing or varying the negative lattice mismatch rate within the n-type current block layer110acontributes to partial introduction of a negative strain. Also, changing the As mole fraction α of the n-type AlxInzAsαPβcurrent block layer110so that the lattice mismatch rate therein increases with the increasing distance from the p-type second cladding layer105contributes to reduction of the lattice mismatch rate of the n-type current block layer110in proximity of the p-type second cladding layer105. That is, though a negative strain is indirectly provided in the p-type second cladding layer105, a negative strain introduced into the MQW active layer104is reduced. As a result, diffusion of Be atoms into the active layer is reduced, and deterioration of crystallinity in the MQW active layer104is suppressed.

In the present embodiment, the lattice mismatch rate of the n-type current block layer110relative to the n-type substrate101is set to −0.15% to −0.05%, the lattice mismatch rate increases as the distance from the p-type second cladding layer105increases, the As mole fraction α of the n-type AlxInzAsαPβcurrent block layer110in proximity of the p-type second cladding layer105is 0, and the Al mole fraction and the In mole fraction are fixed. However, the effect of the present embodiment can also be obtained if the lattice mismatch rate of the n-type current block layer110relative to the n-type substrate101is set within the range of −0.20% or more but not more than 0%, preferably −0.15% or more but not more than −0.05%, and if the lattice mismatch rate is adjusted so as to increase with the increasing distance from the p-type second cladding layer105, with the As mole fraction α of the n-type current block layer110in proximity of the p-type second cladding layer105being 0 and with the Al mole fraction x and the In mole fraction z fixed.

According to the present embodiment, in a window structure semiconductor laser device in which the photoluminescence peak wavelength of the MQW active layer (window regions)104bin the laser cavity end face proximity regions is made smaller than that of the MQW active layer (active region)104ain the laser cavity internal region, the lattice mismatch rate of the n-type first current block layer110relative to the n-type substrate101is set to −0.15% to −0.05% so that the lattice mismatch rate increases as the distance from p-type second cladding layer105increases, the As mole fraction α of the n-type AlxInzAsαPβin proximity of the p-type second cladding layer105is 0, and the Al mole fraction x and the In mole fraction z are fixed. However, the invention is not limited to this, and in the semiconductor laser device of the first embodiment as well, the effect of the present embodiment can also be obtained if the lattice mismatch rate of the n-type current block layer50relative to the n-type substrate41is set within the range of −0.20% or more but not more than 0%, preferably −0.15% or more but not more than −0.05%, and if the lattice mismatch rate is adjusted so as to increase with the increasing distance from the p-type second cladding layer45, with the Al mole fraction x and the In mole fraction z of the n-type AlxInzP current block layer50in proximity of the p-type second cladding layer45being fixed.