Semiconductor laser device and method for fabricating the same

A semiconductor laser device according to the present invention has a semiconductor substrate having a first region and a second region adjacent to the first region, a first active layer formed on the first region and made of a compound semiconductor, a first clad layer formed on the first active layer and made of a compound semiconductor containing a first dopant, and a second active region formed on the second region and made of a compound semiconductor containing a second dopant having a diffusion coefficient with respect to the first active region which is higher than that of the first dopant.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor laser deice and to a method for fabricating the same. More particularly, it relates to an increase in the output of the semiconductor laser device.

Recent years have seen rapid widespread use of DVD (Digital Versatile Disk) devices in the fields of AV (Audio-Video) equipment, PCs (Personal Computers), and the like. In particular, great expectations have been placed on the use of recordable DVD devices (such as DVD-RAM and DVD-R) as large-capacity memory devices embedded in PCs and the like and as post-VTR (Video Tape Recorder) devices.

As pickup light sources for the foregoing DVD devices, red semiconductor lasers at wavelengths in the 650 nm band have been used. With the recent increases in the density and capacity of an optical disk, a pick-up light source capable of performing a particularly high output operation over 80 mw has been in growing demand to allow a higher-speed write operation with respect to the optical disk.

If a semiconductor laser device is increased in output, however, each of the laser facets of semiconductor laser device suffers catastrophic optical damage (hereinafter referred to as COD). The catastrophic optical damage is a degradation phenomenon caused by heat resulting from the absorption of a laser beam in the vicinity of the laser facet of the semiconductor laser device. The resulting heat degrades the portion of a semiconductor layer located in the vicinity of the laser facet. Specifically, the heat reduces the band gap of the portion of the semiconductor layer located in the vicinity of the laser facet and increases the absorption coefficient of the portion of the semiconductor layer located in the vicinity of the laser facet. Consequently, the laser beam is further absorbed in the vicinity of the laser facet.

It has been known that, in preventing COD, preliminary provision of a semiconductor layer having a large band gap and transparent to a laser beam emitted from the semiconductor laser device in a region located in the vicinity of each of the laser facets of the semiconductor laser device, i.e., the formation of a so-called window structure is effective. In particular, the formation of the window structure in a semiconductor laser device outputting a red laser beam exceeding 50 mW is inevitable to ensure the reliability of the semiconductor laser device in use.

Thus far, various methods have been proposed each for fabricating a semiconductor laser device having a window structure. One of the methods uses a phenomenon in which diffused Zn alloys a superlattice in an active layer. For example, Japanese Unexamined Patent Publication No. HEI 11-284280 discloses a method in which a window structure is formed by further forming a group III-V compound semiconductor layer containing Zn at a high concentration (hereinafter referred to as a Zn supply layer) over a region located in the vicinity of each of the laser facets of a semiconductor laser device, causing solid-phase diffusion of Zn from the Zn supply layer, and thereby disordering the active layer in the laser facet region. A method of using ZnO as a Zn diffusion source instead of the Zn supply layer is also disclosed in, e.g., Japanese Unexamined Patent Publication No. HEI 10-290043.

FIG. 10is a perspective view showing a structure of a conventional semiconductor laser device.

As shown inFIG. 10, a conventional semiconductor laser device70has a structure (so-called window structure) comprising laser facet regions713and an internal region712.

The internal region712has a multilayer structure composed of: an n-type clad layer701made of n-type AlGaInP; a guide layer702a(with a thickness of 30 nm) made of AlGaInP; an active layer702made of a quantum well consisting of a plurality of GaInP layers and a plurality of AlGaInP layers; a guide layer702b(with a thickness of 30 nm) made of AlGaInP; a first p-type clad layer703made of p-type AlGaInP containing Zn as a dopant; a current block layer704made of n-type AlGaInP; a second p-type clad layer705made of p-type AlGaInP containing Zn as a dopant; and a contact layer706made of p-type GaAs containing Zn as a dopant, which are stacked successively on a substrate700made of n-type GaAs.

The active layer702is composed of a repetition of the structure in which the GaInP layers are sandwiched between the AlGaInP layers.

Each of the laser facet regions713has a multilayer structure composed of: the n-type clad layer701made of n-type AlGaInP; the guide layer702a(with a thickness of 30 nm) made of AlGaInP; an alloyed active layer711made of alloyed GaInP and AlGaInP; the guide layer702b(with a thickness of 30 nm) made of AlGaInP; the first p-type clad layer703made of p-type AlGaInP containing Zn as a dopant; the current block layer704made of n-type AlGaInP; the second p-type clad layer705made of p-type AlGaInP containing Zn as a dopant; and the contact layer706made of p-type GaAs containing Zn as a dopant, which are stacked successively on the substrate700made of n-type GaAs.

An n-side electrode708made of a metal (such as an alloy of Au, Ge, or Ni) making an ohmic contact with the n-type GaAs substrate700is formed on the lower surface of the n-type GaAs substrate700. A p-side electrode709made of a metal (such as an alloy of Cr, Pt, or Au) making an ohmic contact with the contact layer706is formed on the upper surface of the contact layer706.

The alloyed active layer711has been disordered through the solid-phase diffusion of Zn. This increases the band gap of the alloyed active layer711and forms a window structure which is transparent to a laser beam emitted from the semiconductor laser device70.

The formation of the window structure through the diffusion of Zn mentioned above increases the reliability of a semiconductor laser device and provides a semiconductor laser device capable of producing a 50-mW class output.

In either of the cases where the methods disclosed in the foregoing publications are used, thermal treatment should be performed in the steps of causing solid-phase diffusion of Zn from the diffusion source to the active layer and alloying the active layer.

If the thermal treatment is performed in the step of causing solid-phase diffusion of Zn, Zn that has been introduced as a dopant in the first p-type clad layer703, the second p-type clad layer705, and the contact layer706is diffused not only into the portions of the active layer702located in the laser facet regions713of the semiconductor laser device70but also into the portion of the active layer702located in the internal region712thereof. If a dopant such as Zn is diffused into the active layer702, a nonradiative recombination center may be formed within the active layer702to degrade the characteristics of the semiconductor laser device70. Otherwise, a crystal defect may be formed within the active layer702to reduce the lifespan of the semiconductor laser70.

The amount of the dopant diffused into the individual semiconductor lasers composing the semiconductor laser device70is larger as the dopant concentrations of the first p-type clad layer703, the second p-type clad layer705, and the contact layer706are higher. As the concentration of Zn as the dopant is higher, the problems of the degraded characteristics, reduced lifespan, and the like of the semiconductor laser device accordingly become more conspicuous. To suppress the diffusion of Zn into the portion of the active layer702located in the internal region712, therefore, the doping concentrations of Zn in the first p-type clad layer703, the second p-type clad layer705, and the contact layer706are preferably lowered.

However, the doping concentrations of Zn in the first p-type clad layer703, the second p-type clad layer705, and the contact layer706greatly affect the temperature characteristic of the semiconductor laser device70. If the doping concentrations of Zn in the first p-type clad layer703, the second p-type clad layer705, and the contact layer706are lowered as described above, the band offset of the conduction band is reduced between the first p-type clad layer703and the active layer702. This indicates that a sufficiently large band barrier against electrons in the conduction band cannot be formed between the first p-type clad layer703and the active layer702. As a result, electrons overflowing from the active layer702to the first p-type clad layer703are increased. Even if an injected current is increased, an increase in current component contributing to light emission is reduced and a light output is saturated. The problem is encountered particularly at a high temperature that a high output cannot be produced.

FIG. 11shows a measurement profile obtained as a result of secondary ion mass spectroscopy (hereinafter referred to as SIMS) performed with respect to the laser facet regions713and internal region712of the conventional semiconductor laser device70having the first p-type clad layer703doped with Zn at a high concentration. It is to be noted that the dopant had not been introduced into the active layer702.

As shown inFIG. 11, Zn was mixed in the portion of the active layer702located in the internal region712irrespective of the fact the dopant had not been introduced therein intentionally. This is because Zn introduced at a high concentration into the first p-type clad layer703was diffused in the thermal treatment step for forming the window structure. Similar dopant diffusion also occurs during the operation of the semiconductor laser device.

SUMMARY OF THE INVENTION

The present invention has been achieved to solve the foregoing problems and it is therefore an object of the present invention to provide a semiconductor laser device with high reliability.

A first semiconductor laser device according to the present invention comprises: a semiconductor substrate having a first region and a second region adjacent to the first region; a first active layer formed on the first region and made of a compound semiconductor; a first clad layer formed on the first active layer and made of a compound semiconductor containing a first dopant; and a second active region formed on the second region and made of a compound semiconductor containing a second dopant having a diffusion coefficient with respect to the first active region which is higher than that of the first dopant.

The present invention suppresses the diffusion of the first dopant from the first clad layer into the first active layer. This reduces crystal defects in the first active layer and provides the semiconductor laser device according to the present invention with high reliability.

Preferably, a concentration of the first dopant in the first clad layer is in a range of 5×1017atoms cm−3to 1×1019atoms cm−3.

Since the present invention suppresses the diffusion of the first dopant from the first clad layer to the first active layer, a doping concentration in the first clad layer can be set to a high value. Accordingly, the concentration of the first dopant can be set to a high value in the range of 5×1017atoms cm−3to 1×1019atoms cm−3. This suppresses the overflow of electrons and provides a semiconductor laser device capable of performing a high-output operation at a high temperature.

The first active layer may be made of (AlxGa1−x)1−yInyP (0≦x≦1, 0≦y≦1), the first clad layer may be made of (AlcGa1−c)1−dIndP (0≦c≦1, 0≦d≦1), and the first dopant may be at least one element selected from the group consisting of Mg, Be, Cd, and Hg.

The first active layer preferably includes two types of layers made of compound semiconductors with different band gaps and alternately stacked and the second active layer is preferably made of alloyed compound semiconductors with different band gaps.

In the arrangement, the first active layer is larger in band gap than the first active layer and becomes transparent to a laser beam emitted from the semiconductor laser device. Consequently, the laser beam is emitted from the first active layer without being absorbed by the second active layer. This suppresses or prevents COD in the semiconductor laser device.

An uppermost portion of the semiconductor substrate is preferably composed of a second clad layer made of a compound semiconductor of a conductivity type opposite to that of the first clad layer and the second clad layer preferably contains a third dopant having a diffusion coefficient with respect to the first active layer which is lower than that of the second dopant.

The arrangement suppresses or prevents the diffusion of the third dopant from the second clad layer into the first active layer.

A second semiconductor laser device according to the present invention comprises: a semiconductor substrate; an active layer formed on the semiconductor substrate and made of a compound semiconductor; and a first clad layer formed on the active layer and made of a compound semiconductor containing a first dopant having a diffusion coefficient with respect to the active region which is lower than that of Zn.

The present invention suppresses the diffusion of the first dopant from the first clad layer into the active layer. This reduces crystal defects in the active layer and provides the semiconductor laser device according to the present invention with high reliability.

Preferably, a concentration of the first dopant in the first clad layer is in a range of 5×1017atoms cm−3to 1×1019atoms cm−3.

Since the present invention suppresses the diffusion of the dopant from the first clad layer into the active layer, a doping concentration in the first clad layer can be set to a high value. Accordingly, the concentration of the dopant can be set to a high value in the range of 5×1017atoms cm−3to 1×1019atoms cm−3. This suppresses the overflow of electrons and provides a semiconductor laser device capable of performing a high-output operation at a high temperature.

The active layer may be made of (AlxGa1−x)1−yInyP (0≦x≦1, 0≦y≦1), the first clad layer may be made of (AlcGa1−c)1−dIndP (0≦c≦1, 0≦d≦1), and the first dopant may be at least one element selected from the group consisting of Mg, Be, Cd, and Hg.

The active layer preferably has a first region including two types of layers made of compound semiconductors with different band gaps and alternately stacked and a second region adjacent to the first region and made of alloyed compound semiconductors with different band gaps and the second region of the active layer preferably contains a second dopant having a diffusion coefficient with respect to the active layer which is higher than that of the first dopant.

In the arrangement, the second region of the active layer is larger in band gap than the first region of the active layer and becomes transparent to a laser beam emitted from the first region of the active layer. Consequently, the laser beam is emitted from the first region of the active layer without being absorbed by the second region of the active layer. This suppresses or prevents COD in the semiconductor laser device.

An uppermost portion of the semiconductor substrate is preferably composed of a second clad layer made of a compound semiconductor of a conductivity type opposite to that of the first clad layer and the second clad layer preferably contains a third dopant having a diffusion coefficient with respect to the active layer which is lower than that of the first dopant.

The arrangement suppresses or prevents the diffusion of the third dopant from the second clad layer into the first active layer.

A first method for fabricating a semiconductor laser device according to the present invention comprises the steps of: (a) preparing a semiconductor substrate having a first region and a second region adjacent to the first region; (b) depositing an active layer made of a compound semiconductor over the first and second regions; (c) depositing, on the substrate, a first clad layer made of a compound semiconductor containing a first dopant; and (d) diffusing, into a portion of the active layer located in the second region, a second dopant having a diffusion coefficient with respective to the active layer which is higher than that of the first dopant to alloy the portion of the active layer located in the second region.

The present invention suppresses the diffusion of the first dopant from the first clad layer into the portion of the active layer located in the first region. This reduces crystal defects in the portion of the active layer located in the first region. The portion of the active layer located in the second region is larger in band gap than the portion of the active layer located in the first region and becomes transparent to a laser beam emitted from the portion of the active layer located in the first region. Consequently, the laser beam is emitted from the portion of the active layer located in the first region without being absorbed by the portion of the active layer located in the second region. This suppresses or prevents COD in the semiconductor laser device and provides the semiconductor laser device with high reliability.

The first method may further comprise the steps of: (e) after the step (c), depositing a current block layer made of a compound semiconductor on the substrate; (f) forming an opening configured as a stripe in the current block layer; and (g) depositing a second clad layer made of a compound semiconductor on the substrate.

The first method may further comprise the steps of: (h) after the step (c), successively depositing, on the substrate, an etching stop layer and a second clad layer made of a compound semiconductor; (i) after the step (d), forming a mask having an opening on the second clad layer; (j) removing a portion of the second clad layer located in the opening by using the mask to expose the etching stop layer in the opening; and (k) forming a current block layer made of a compound semiconductor on the substrate.

A second method for fabricating a semiconductor laser device according to the present invention comprises the steps of: (a) depositing an active layer made of a compound semiconductor on a semiconductor substrate; and (b) depositing, on the substrate, a clad layer made of a compound semiconductor containing a dopant having a diffusion coefficient with respect to the active layer which is lower than that of Zn.

The present invention suppresses the diffusion of the dopant from the clad layer into the active layer. This reduces crystal defects in the active layer and provides the semiconductor laser device according to the present invention with high reliability.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, the embodiments of the present invention will be described herein below. For the sake of simplicity, components common to the individual embodiments are designated by the same reference numerals.

FIG. 1is a perspective view showing a structure of a semiconductor laser device according to EMBODIMENT 1.FIG. 2Ais a cross-sectional view taken along the line A—A shown inFIG. 1.FIG. 2Bis a cross-sectional view taken along the line B—B shown inFIG. 1.

As shown inFIG. 1, a semiconductor laser device10according to the present embodiment has a structure (so-called window structure) comprising an internal region112and laser facet regions113.

As shown inFIGS. 1 and 2A, the internal region112has a multilayer structure composed of an n-type clad layer101made of n-type AlGaInP; a guide layer102a(with a thickness of 30 nm) made of AlGaInP; an active layer102composed of a quantum well consisting of a plurality of GaInP layers and a plurality of AlGaInP layers; a guide layer102b(with a thickness of 30 nm) made of AlGaInP; a first p-type clad layer103made of p-type AlGaInP containing Mg as a dopant; a current block layer104made of n-type AlGaInP; a second p-type clad layer105made of p-type AlGaInP containing Mg as a dopant; and a contact layer106made of p-type GaAs containing Mg as a dopant, which are stacked successively on a substrate100made of n-type GaAs.

In the present embodiment, the active layer102is composed of a repetition of the structure in which the GaInP layers are sandwiched between the AlGaInP layers.

As shown inFIGS. 1 and 2B, each of the laser facet regions113has a multilayer structure composed of: the n-type clad layer101made of n-type AlGaInP; the guide layer102a(with a thickness of 30 nm) made of AlGaInP; alloyed active layers111each containing Zn diffused therein and made of alloyed GaInP and AlGaInP; the guide layer102b(with a thickness of 30 nm) made of AlGaInP; the first p-type clad layer103made of p-type AlGaInP containing Mg as a dopant; the current block layer104made of n-type AlGaInP; the second p-type clad layer105made of p-type AlGaInP containing Mg as a dopant; and the contact layer106made of p-type GaAs containing Mg as a dopant, which are stacked successively on the substrate100made of n-type GaAs.

In particular, a superlattice in each of the alloyed active layers111according to the present embodiment has been alloyed through the diffusion of Zn. The band gap of each of the alloyed active layers111is larger than that of the active layer102in the internal region112.

An n-side electrode107made of a metal (such as an alloy of Au, Ge, or Ni) making an ohmic contact with the n-type GaAs substrate100is formed on the lower surface of the n-type GaAs substrate100. A p-side electrode108made of a metal (such as an alloy of Cr, Pt, or Au) making an ohmic contact with the contact layer106is formed on the upper surface of the contact layer106.

As shown inFIGS. 1,2A, and2B, the current block layer104is formed with an opening121configured as a rectangle (stripe) reaching the first p-type clad layer103.

Through the opening121, the first and second p-type clad layers103and105are in contact with each other. A current is injected through the opening121into the active layer102to cause laser oscillation.

In the present embodiment, AlGaInP represent (AlxGa1−x)1−yInyP (0≦x≦1, 0≦y≦1). In particular, GaInP represents a material obtained when x=0 is satisfied in (AlxGa1−x)1−yInyP.

Table 1 shows the specific compositions, thicknesses, dopant concentrations (impurity concentrations) of the individual layers of the semiconductor laser device10according to the present embodiment.

In the present embodiment, y representing the composition ratio of In has a value of about 0.5 in each of the n-type clad layer101, the guide layer102a, the active layer102, the guide layer102b, the first p-type clad layer103, the current block layer104, and the second p-type clad layer105each composed of a material represented by (AlxGa1−x)1−yInyP. To achieve lattice matching with the n-type GaAs substrate100, the value of y representing the In composition ratio is preferably in the range of 0.45≦y≦0.55. It will easily be appreciated that, if another substrate is used, the value of y may be varied appropriately depending on the substrate and the value of y is not limited to the foregoing range.

A description will be given to a method for fabricating the semiconductor laser device10according to the present embodiment with reference toFIGS. 3 and 4.FIGS. 3 and 4are perspective views diagrammatically showing the individual process steps of the method for fabricating the semiconductor laser device according to the present embodiment.

First, in the step shown inFIG. 3A, the substrate100made of n-type GaAs is prepared. Then, the n-type clad layer101made of n-type AlGaInP, the guide layer102amade of AIGaInP, the active layer102, the guide layer102bmade of AlGalnP, the first p-type clad layer103made of p-type AlGaInP, the current block layer104made of n-type AlGaInP, and a cap layer110made of GaAs are deposited successively by, e.g., metal organic chemical vapor deposition (hereinafter referred to as MOCVD) or molecular beam epitaxy (hereinafter referred to as MBE). The active layer102is formed by alternately depositing the GaInP layers and the AlGaInP layers.

In forming the first p-type clad layer103made of p-type AlGaInP in the foregoing step, Mg is introduced as a p-type dopant. The introduction of Mg is effected by using a method well known to those skilled in the art, such as one which mixes Mg(C5H5)2in a raw material gas and forms an AlGaInP layer by crystal growth, one which forms an undoped AlGaInP layer and introduces Mg by ion implantation, or one which forms an undoped AlGaInP layer by crystal growth and uses MgO as a diffusion source to introduce Mg by solid-phase diffusion.

Next, in the step shown inFIG. 3B, the cap layer110is etched away, a resist film is patterned into a rectangle (stripe) by photolithography, and the current block layer104is etched by using the resist film as a mask so that the openings121configured as the rectangle (stripe) is formed in the current block layer104.

Subsequently, in the step shown inFIG. 4A, the second p-type clad layer105made of p-type AlGaInP and the contact layer106made of p-type GaAs are deposited successively by, e.g., MOCVD or MBE on the substrate obtained in the previous step. In forming the second p-type clad layer105and the contact layer106also, Mg is introduced as a p-type dopant. The introduction of Mg is effected by the same method as used in forming the first p-type clad layer103in the step shown inFIG. 3A, which is well known to those skilled in the art.

Next, in the step shown inFIG. 4B, a ZnO film is deposited by sputtering or the like on the substrate obtained in the previous step. Subsequently, photolithography and wet etching is performed to remove the portion of the ZnO film located in the internal region112, thereby forming a ZnO diffusion source145. Then, a SiO2film147is deposited by, e.g., CVD on the substrate. Subsequently, the substrate is thermally treated such that Zn is diffused from the ZnO diffusion source145to the active layer102and that intersubstitution occurs between the elements of GaInP and AlGaInP composing the active layer102(alloying process). As a result, the alloyed active layers111each having a band gap larger than that of the active layer102are formed.

In particular, the diffusion coefficient of Zn in a group III–V compound semiconductor is relatively large. This allows effective doping of the active layer102with Zn. In addition, Zn exerts a large effect of alloying a superlattice structure composed of the AlGaInP material when it is diffused. Consequently, the portion of the active layer102located in the laser facet region113is alloyed efficiently.

The SiO2film147has the function of protecting the contact layer106during the diffusion of Zn.

Subsequently, the ZnO diffusion source145and the SiO2film147are etched away. Finally, the p-side electrode108is formed by electron beam vapor deposition or the like on the contact layer106. Likewise, the n-side electrode107is also formed by electron beam vapor deposition or the like on the substrate100, whereby the semiconductor laser device10according to the present embodiment shown inFIG. 1is obtained.

The semiconductor laser device10according to the present embodiment has used Mg as a p-type dopant for the first p-type clad layer103, the second p-type clad layer105, and the contact layer106. In general, the diffusion coefficient of Mg in a semiconductor material is lower than that of Zn used conventionally as a p-type dopant. In diffusing Zn by thermal treatment in the step shown inFIG. 4B, therefore, the diffusion of Mg as a dopant from the first p-type clad layer103to the portion of the active layer102located in the internal region112is suppressed satisfactorily.

It will easily be appreciated that, during the deposition of each of the first p-type clad layer103, the second p-type clad layer105, and the contact layer106, the diffusion of the dopant from the first p-type clad layer103to the active layer102is similarly suppressed or prevented. Similar diffusion of the dopant is also suppressed or prevented during the operation of the semiconductor laser device. This reduces crystal defects in the portion of the active layer102located in the internal region112and provides the semiconductor laser device10according to the present embodiment with high reliability.

In the fabrication of the semiconductor laser device10according to the present embodiment, the diffusion of Mg as a dopant is suppressed when Zn is diffused by thermal treatment in the step shown inFIG. 4B. Accordingly, the doping concentration of the first p-type clad layer103can be set to a high value. Preferably, the doping concentration of the first p-type clad layer103is in the range of 5×1017atoms cm−3to 1×1019atoms cm−3. The high doping concentration of the first p-type clad layer103suppresses the overflow of electrons and provides a red semiconductor laser device capable of performing a high-output operation at a high temperature.

A detailed description will be given herein below to the effect of suppressing the diffusion of the dopant into the active layer102achieved in the semiconductor laser device10according to the present embodiment.

FIG. 5shows a SIMS profile obtained from the n-type clad layer101, guide layer102a, active layer102, guide layer102b, and first p-type clad layer103of the semiconductor laser device10according to the present embodiment.

As shown inFIG. 5, Zn at a concentration of about 2×1018atoms cm−3has been implanted into the alloyed active layer111in each of the laser facet regions113of the semiconductor laser device10according to the present embodiment. The foregoing concentration is sufficient to alloy the active layer and form the window structure. It has been proved that the alloyed active layer111in the laser facet region113is formed when Zn at a concentration of 7×1017atoms cm−3or more is diffused.

On the other hand, the diffusion of Mg to the portion of the active layer102located in the internal region112has not occurred, as shown inFIG. 5, irrespective of the first p-type clad layer103doped with Mg at a high concentration of 2×1018atoms cm−3. This is because Mg having a diffusion coefficient lower than that of Zn is used as the dopant.

In the present embodiment, silicon has been used as an n-type dopant in the n-type clad layer101. As another n-type dopant, selenium can also be used. In general, dopants such as silicon and selenium have diffusion coefficients in a semiconductor material which are lower than the diffusion coefficient of Zn used as a p-type dopant. As a result, the diffusion of an n-type dopant from the n-type clad layer101to the active layer102can also be suppressed or prevented.

A description will be given next to the operation of the semiconductor laser device10according to the present embodiment.

If a voltage is applied between the n-side electrode107and the p-side electrode108such that a current is injected to flow therebetween, the injected current is confined by the current block layer104in the second p-type clad layer105so that a laser beam at an oscillating wavelength of 650 nm is emitted from the active layer102.

The semiconductor laser device10according to the present embodiment has a structure (i.e., window structure) in which the alloyed active layer111is formed in each of the laser facet regions113. The alloyed active layer111has a band gap larger than that of the portion of the active layer102located in the internal region112so that the absorption of the emitted beam is suppressed in each of the laser facet regions113. This increases the level at which damage to the laser facet occurs and allows a high-output operation.

In addition, the semiconductor laser device10according to the present embodiment has used Mg having a diffusion coefficient lower than that of Zn as a dopant for the first p-type clad layer103. As a result, the diffusion of Mg to the portion of the active layer102located in the internal region112is suppressed or prevented. This suppresses the formation of crystal defects in the active layer102and significantly increases the reliability of the semiconductor laser device10.

Moreover, the first p-type clad layer103heavily doped with Mg as a p-type dopant effectively suppresses the overflow of electrons from the active layer to the p-type clad layer.

When the semiconductor laser device10according to the present embodiment was operated actually, an output of 100 mW was obtained without the occurrence of thermal saturation even at an ambient temperature of 70° C.

Although the present embodiment has used a substrate made of n-type GaAs as the substrate100, it is not limited thereto. A p-type substrate (such as a p-type GaAs substrate) may also be used instead.

Although the present embodiment has used an active layer having a multiple quantum well structure as the active layer102, it is not limited thereto. For example, a single quantum well structure or a bulk quantum well structure may also be used instead.

Although the present embodiment has used the current block layer104made of AlGaInP and having an effective index-guided structure, it is not limited thereto. For example, a current block layer made of GaAs and having a complex index-guided structure may also be used instead.

Although the present embodiment has been described by using Zn as the dopant implanted into the laser facet regions113and using Mg as the p-type dopant, it is not limited thereto. It is also possible to use another material having a high diffusion coefficient as the dopant implanted into the laser facet regions113and use another material having a low diffusion coefficient as the p-type dopant. For example, Mg may be used appropriately as the dopant implanted into the laser facet regions113and any of Be, Cd, and Hg may be used appropriately as the p-type dopant.

Although the present embodiment has described the method in which the active layer is alloyed by implanting Zn by solid-phase diffusion, it is not limited thereto. The alloyed active layer can similarly be formed even if a method is adopted in which the active layer is alloyed by ion-implanting Zn into the vicinity of the active layer and a thermal treatment is performed subsequently.

Although the present embodiment has described the semiconductor laser device having the window structure, the semiconductor laser device may also be constructed without the laser facet regions113(i.e., without the window structure).

Even if the semiconductor laser device is constructed without the laser facet regions113, the use of a material having a diffusion coefficient lower than that of Zn, such as Mg, as the p-type dopant also suppresses or prevents the diffusion of the dopant from the first p-type clad layer103to the active layer102during the deposition of each of the first p-type clad layer103, the second p-type clad layer105, and the contact layer106in the same manner as in the semiconductor laser device10according to the present embodiment.

Since the diffusion of the dopant from the first p-type clad layer103to the active layer102is suppressed or prevented in the semiconductor laser device constructed without the laser facet regions113, the doping concentration of the first p-type clad layer103can be set to a high value in the same manner as in the semiconductor laser device10according to the present embodiment. This suppresses the overflow of electrons and provides a red semiconductor laser device capable of performing a high-output operation at a high temperature.

Although EMBODIMENT 1 has described the semiconductor laser device having an inner-stripe waveguide structure, the present invention is not limited thereto. The semiconductor laser device according to the present invention may also has, e.g., a ridge waveguide structure. Referring toFIG. 6, the present embodiment will describe a semiconductor laser device60having a ridge waveguide structure.

As shown inFIG. 6, the semiconductor laser device60according to the present embodiment has a structure (so-called window structure) comprising an internal region112and laser facet region113.

As shown inFIG. 6, the internal region112has a multilayer structure composed of an n-type clad layer101made of n-type AlGaInP; a guide layer102a(with a thickness of 30 nm) made of AlGaInP; an active layer102composed of a quantum well consisting of a plurality of GaInP layers and a plurality of AlGaInP layers; a guide layer102b(with a thickness of 30 nm) made of AlGaInP; a first p-type clad layer103made of p-type AlGaInP containing Mg as a dopant; an etching stop layer130made of GaInP; a current block layer104made of n-type AlGaInP; a second p-type clad layer105made of p-type AlGaInP containing Mg as a dopant; and a contact layer106made of p-type GaAs containing Mg as a dopant, which are stacked successively on a substrate100made of n-type GaAs.

In the present embodiment, the active layer102is composed of a repetition of the structure in which the GaInP layers are sandwiched between the AlGaInP layers.

As shown inFIG. 6, each of the laser facet regions113has a multilayer structure composed of: the n-type clad layer101made of n-type AlGaInP; the guide layer102a(with a thickness of 30 nm) made of AlGaInP; alloyed active layers111each containing Zn diffused therein and made of alloyed GaInP and AlGaInP; the guide layer102b(with a thickness of 30 nm) made of AlGaInP; the first p-type clad layer103composed of p-type AlGaInP containing Mg as a dopant; the etching stop layer130made of GaInP; the current block layer104made of n-type AlGaInP; the second p-type clad layer105made of p-type AlGaInP containing Mg as a dopant; and the contact layer106made of p-type GaAs containing Mg as a dopant, which are stacked successively on the substrate100composed of n-type GaAs.

In particular, a superlattice in each of the alloyed active layers111according to the present embodiment has been alloyed through the diffusion of Zn. The band gap of each of the alloyed active layers111is larger than that of the active layer102in the internal region112.

An n-side electrode107made of a metal (such as an alloy of Au, Ge, or Ni) making an ohmic contact with the n-type GaAs substrate100is formed on the lower surface of the n-type GaAs substrate100. A p-side electrode108made of a metal (such as an alloy of Cr, Pt, or Au) making an ohmic contact with the contact layer106is formed on the upper surface of the contact layer106.

As shown inFIG. 6, the current block layer104is formed with an opening121configured as a rectangle (stripe) reaching the etching stop layer130. Through the opening121, the etching stop layer130and the second p-type clad layer105are in contact with each other. A current is injected through the opening121into the active layer102to cause laser oscillation.

As will be understood from the foregoing description, the individual layers of the semiconductor laser device60according to the present embodiment other than the etching stop layer130are composed of exactly the same materials as composing those of the semiconductor laser device10according to EMBODIMENT 1, which are shown above in Table 1.

A description will be given to a method for fabricating the semiconductor laser device according to the present embodiment with reference toFIGS. 7 and 8.FIGS. 7 and 8are perspective views diagrammatically showing the individual process steps of the method for fabricating the semiconductor laser device according to the present embodiment.

First, in the step shown inFIG. 7A, the substrate100made of n-type GaAs is prepared. Then, the n-type clad layer101made of n-type AlGaInP, the guide layer102amade of AlGaInP, the active layer102, the guide layer102bmade of AlGaInP, the first p-type clad layer103made of p-type AlGaInP, the etching stop layer130made of GaInP, the second p-type clad layer105made of p-type AlGaInP, and the cap layer110made of GaAs are deposited successively by, e.g., MOCVD or MBE. The active layer102is formed by alternately depositing the GaInP layers and the AlGaInP layers.

In forming the first p-type clad layer103and the second p-type clad layer105in the foregoing step, Mg is introduced as a p-type dopant. The introduction of Mg is effected by using a method well known to those skilled in the art, such as one which mixes Mg(C5H5)2in a raw material gas and forms an AlGaInP layer by crystal growth, one which forms an undoped AlGaInP layer and introduces Mg by ion implantation, or one which forms an undoped AlGaInP layer by crystal growth and uses MgO as a diffusion source to introduce Mg by solid-phase diffusion.

Next, in the step shown inFIG. 7B, a ZnO film is deposited by sputtering or the like on the substrate obtained in the previous step. Subsequently, photolithography and wet etching is performed to remove the portion of the ZnO film located in the internal region112, thereby forming a ZnO diffusion source145. Then, a SiO2film147is deposited by, e.g., CVD on the substrate.

Next, in the step shown inFIG. 8A, the substrate obtained in the previous step is thermally treated such that Zn is diffused from the ZnO diffusion source145to the active layer102and that intersubstitution occurs between the elements of GaInP and AlGaInP composing the active layer102(alloying process). As a result, the alloyed active layers111are formed. Subsequently, the ZnO diffusion source145and the SiO2film147are etched away.

Then, an SiO2film is formed on the substrate obtained in the previous step. The SiO2film is further patterned by photolithography and wet etching to form an SiO2mask150configured as a rectangle (stripe) extending crosswise the internal region112and the laser facet regions113. Subsequently, etching is performed by using the SiO2mask150, thereby removing the second p-type clad layer105and the cap layer110.

Next, in the step shown inFIG. 5B, the current block layer104made of n-type AlGaInP is deposited by, e.g., MOCVD or MBE on the substrate obtained in the previous step. During the deposition, the current block layer105is barely deposited on the SiO2mask150composed of an oxide film. Subsequently, the SiO2mask150and the cap layer110are etched away.

Next, the contact layer106made of p-type GaAs is deposited by, e.g., MOCVD or MBE on the substrate obtained in the previous step. In forming the contact layer106made of p-type GaAs, Mg is introduced as a p-type dopant. The introduction of Mg is effected by the same method as used in forming the first p-type clad layer103and the second p-type clad layer105in the step shown inFIG. 7A, which is well known to those skilled in the art.

Subsequently, the p-side electrode108is formed by electron beam vapor deposition or the like on the contact layer106. Likewise, the n-side electrode107is formed by electron beam vapor deposition or the like on the substrate100, whereby the semiconductor laser device60according to the present embodiment shown inFIG. 6is obtained.

Since the operation of the semiconductor laser device60according to the present embodiment is exactly the same as that of the semiconductor laser device10according to EMBODIMENT 1, the description thereof will be omitted.

The semiconductor laser device60according to the present embodiment has also used Mg as a p-type dopant for the first p-type clad layer103, the second p-type clad layer105, and the contact layer106, similarly to the semiconductor laser device10described above. In general, the diffusion coefficient of Mg in a semiconductor material is lower than that of Zn used conventionally as a p-type dopant. In forming the alloyed active layers111by diffusing Zn in the laser facet regions113, therefore, the diffusion of Mg as a dopant from the first p-type clad layer103to the portion of the active layer102located in the internal region112is suppressed satisfactorily.

It will easily be appreciated that, during the deposition of each of the first p-type clad layer103, the second p-type clad layer105, and the contact layer106, the diffusion of the dopant from the first p-type clad layer103to the active layer102is similarly suppressed or prevented. Similar diffusion of the dopant is also suppressed or prevented during the operation of the semiconductor laser device. This reduces crystal defects in the portion of the active layer102located in the internal region112and provides the semiconductor laser device60according to the present embodiment with high reliability.

In the fabrication of the semiconductor laser device60according to the present embodiment, the diffusion of Mg as a dopant is suppressed when Zn is diffused by thermal treatment in the step shown inFIG. 7B. Accordingly, the doping concentration of the first p-type clad layer103can be set to a high value. This suppresses the overflow of electrons and provides a red semiconductor laser device capable of performing a high-output operation at a high temperature.

In the semiconductor laser device60according to the present embodiment, the portion of the current block layer104located in each of the laser facet regions113is formed with an opening121, as shown inFIG. 6. Moreover, Zn diffused in each of the laser facet regions113has lowered the resistance of the entire laser facet region113. As a result, a leakage current which does not contribute to laser oscillation may flow in the laser facet region113through the opening121.

To prevent this, there may be adopted a structure in which the opening121is not formed in either of the portions of the current block layer104located in the laser facet regions113, i.e., only the portions of the current block layer104located in the laser facet regions113are formed to cover up the second p-type clad layer105. A description will be given to the structure.

FIG. 9Ais a perspective view showing a structure of a semiconductor laser device90.FIG. 9Bis a cross-sectional view taken along the line X—X shown inFIG. 9A.

As shown inFIG. 9A, the semiconductor laser device90has nearly the same structure as the semiconductor laser device60. As shown inFIG. 9B, the internal structure of the semiconductor laser device90is exactly the same as that of the semiconductor laser device60so that the current block layer104is formed with the opening121. As shown inFIG. 9A, the semiconductor laser device90is different from the semiconductor laser device60in that only the portions of the current block layer104located in the laser facet regions113are formed to cover up the second p-type clad layer105.

Since the portions of the current block layer104located in the laser facet regions113are formed to cover up the second p-type clad layer105, a leakage current is prevented from flowing in the laser facet regions113.

A method for fabricating the semiconductor laser device90is nearly the same as the method for fabricating the semiconductor laser device60except in that the portions of the cap layer110and the SiO2mask150located in the laser facet regions113are removed immediately before the step shown inFIG. 8B.

If the portions of the cap layer110and the SiO2mask150located in the laser facet regions113are removed immediately before the step shown inFIG. 8B, the portions of the current block layer104located in the laser facet regions113are formed to cover up the second p-type clad layer105.

Although the present embodiment has thus described the semiconductor laser device having the window structure, the semiconductor laser device may also be constructed without the window structure (i.e., without the laser facet regions113).

Even if the semiconductor laser device is constructed without the laser facet regions113, the use of a material having a diffusion coefficient lower than that of Zn, such as Mg, as the p-type dopant also suppresses or prevents the diffusion of the dopant from the first p-type clad layer103to the active layer102during the deposition of each of the first p-type clad layer103, the second p-type clad layer105, and the contact layer106in the same manner as in the semiconductor laser device60according to the present embodiment.

Since the diffusion of the dopant from the first p-type clad layer103to the active layer102is suppressed or prevented in the semiconductor laser device constructed without the laser facet regions113, the doping concentration of the first p-type clad layer103can be set to a high value in the same manner as in the semiconductor laser device60according to the present embodiment. This suppresses the overflow of electrons and provides a red semiconductor laser device capable of performing a high-output operation at a high temperature.