Gain-Coupling Distributed Feedback Semiconductor and Method of Producing the Same

1. Field of the Invention

The present invention relates to a single-mode oscillation distributed feedback semiconductor laser, and in particular, to a gain-coupling distributed feedback semiconductor laser, which operates in a single mode due to a gain coupling, and a method of producing the gain-coupling distributed feedback semiconductor laser.

2. Related Background Art

A distributed feedback laser is conventionally known as a single-mode oscillation laser. In a Bragg waveguide whose equivalent refractive index is modulated at a periodicity or pitch .LAMBDA., advancing and retreating waves of light having a wavelength in the vicinity of a Bragg wavelength are coupled to each other and reflected. When a gain exists in the waveguide, a laser oscillation occurs in the laser. However, its oscillation wavelength is not the Bragg wavelength, and the single-mode oscillation is effected at an outside end of a stop band in a longitudinal mode.

In order to achieve a stabler single-mode operation, a method is known, according to which a structure for shifting a phase of a grating is formed at a central portion of the laser to oscillate a mode of the Bragg condition wavelength. The amount of the phase shift is in most cases set at a quarter of the Bragg wavelength, but a complicated step is needed to fabricate the phase-shift diffraction grating.

Further, the above-discussed refractive index-coupling distributed feedback laser has an inherent problem: The single-mode operation of the distributed feedback laser is likely to be affected by a spatial hole burning and an end-surface reflection as well as returning light. Moreover, such a laser is subject to a chirping at a high-speed modulation time, and thus its line-width increases.

A gain-coupling distributed feedback laser is proposed to solve those problems. Features of the gain-coupling distributed feedback laser are as follows: A single-mode operation under a Bragg condition is possible without using a phase-shift structure. Adverse influence of returning light is small in the gain-coupling type, as compared to the index-coupling type. The amount of chirping is small in the gain-coupling type. Decrease in yield due to a phase position of its facet plane does not exist in the gain-coupling type.

For example, a structure, in which an absorptive body is periodically formed on an active layer to impart a periodical absorption loss and thus achieving a gain-coupling operation, is disclosed in "Applied Physics Letters", vol. 55, No. 16, pp. 1606-1608.

The above-mentioned gain-coupling distributed feedback laser, however, has the drawback that an oscillation threshold increases due to an increase in its internal loss. Furthermore, "IEEE Journal of Quantum Electronics", vol. 27, No. 6, pp. 1724-1731 discloses a gain-coupling distributed feedback laser which is fabricated by regrowing the buffer layer and the active layer on a second-order diffraction grating of the AlGaAs series. This structure, however, has disadvantages. Reduction of its threshold current is not easy because of a regrown interface in the vicinity of the active layer and adoption of a quantum well structure in the active layer is difficult.

It is an object of the present invention to provide a gain-coupling distributed feedback laser which operates in a single mode due to gain-coupling. The gain-coupling distributed feedback laser can be readily fabricated and its recombination current at a regrown interface can be reduced since the regrown interface can be positioned away from its active layer. Further, its freedom in design can be increased since a quantum well structure can be employed as the active layer.

Further, it is an object of the present invention to provide a method of fabricating a gain-coupling distributed feedback laser.

The object of the present invention is achieved by a gain-coupling distributed feedback semiconductor laser which performs a gain-coupling distributed feedback operation by a structure which comprises a waveguide including an active layer, and a current restraint layer fomed along the waveguide. The current restraint layer includes p- and n-type semiconductor portions which are periodically arranged alternately along a light propagation direction.

Further, the object of the present invention is achieved by a method of fabricating a gain-coupling distributed feedback semiconductor laser which performs a gain-coupling distributed feedback operation, which comprises a step of forming an uneven surface having a plurality of kinds of surfaces with different surface indices, and a step of forming a current restraint layer on the uneven surface by simultaneously forming p- and n-type semiconductor portions on the uneven surface using a gas-phase crystal growth method.

More specifically, the p- and n-type semiconductor portions may be arranged at a pitch which is equal to 1/2.times.(natural number).times.(a wavelength of light propagating along said waveguide). The p- and n-type semiconductor portions may be respectively Si-doped semiconductor layers grown on layer portions having different surface orientations. The active layer may comprise a multi-quantum well structure. Light confinement layers may be respectively formed under and above the active layer. A carrier block layer may be formed on the light confinement layer. A light guide layer may be formed on the carrier block layer. The current restraint layer may be formed on the carrier block layer. The current restraint layer may be formed on the light guide layer. The current restraint layer may be grown on an uneven surface. The uneven surface may comprise a periodical (or, periodic) uneven surface whose pitch is equal to a pitch of modulation of carrier density of current injected into the current restraint layer. The uneven surface may comprise a periodical uneven surface whose pitch is equal to half of a pitch of modulation of carrier density of current injected into the current restraint layer. The uneven surface may have a triangular sectional shape. The uneven surface may have a trapezoidal sectional shape. The uneven surface may have an asymmetric sectional shape.

The vapor-phase crystal growth method may be one of molecular beam epitaxy (MBE), chemical beam epitaxy (CBE), metal organic molecular beam epitaxy (MOMBE), and gas source molecular beam epitaxy (GSMBE) methods, and an amphi-conductivity impurity of group IV may be used as an impurity. The vapor-phase crystal growth method may be metal organic vapor phase epitaxy (MOVPE) method, and an amphi-conductivity impurity of group IV may be used as an impurity. The uneven surface may be formed using wet etching with sulphuric acid series etchant. The uneven surface may be formed using light wet etching with sulphuric acid series etchant that is performed after dry etching.

FIGS. 1A-1C are respectively cross-sectional views illustrating steps for fabricating a first embodiment of the present invention which is directed to a single-mode distributed feedback laser having a buried heterostructure.

FIG. 1D is a graph illustrating a carrier density distribution in an active layer of the first embodiment.

FIG. 2 is a partially cut-away perspective view illustrating the first embodiment.

FIG. 3 is a cross-sectional view illustrating a modification of a diffraction grating formed on a light guide layer.

FIGS. 4A-4C are respectively cross-sectional views illustrating steps for fabricating a second embodiment of the present invention which is directed to a single-mode distributed feedback laser having a buried heterostructure.

FIGS. 5A-5C are respectively cross-sectional views illustrating steps for fabricating a third embodiment of the present invention which is directed to a single-mode distributed feedback laser having a buried heterostructure.

FIG. 5D is a graph illustrating a carrier density distribution in an active layer of the third embodiment.

FIGS. 6A-6C are respectively cross-sectional views illustrating steps for fabricating a fourth embodiment of the present invention which is directed to a single-mode distributed feedback laser having a buried heterostructure.

FIGS. 7A-7C are respectively cross-sectional views illustrating steps for fabricating a fifth embodiment of the present invention which is directed to a single-mode distributed feedback laser having a buried heterostructure.

Embodiments of the present invention will be hereinafter described with reference to the figures.

A first embodiment of the present invention will be described with reference to FIGS. 1A-1D.

As shown in FIG. 1A, an n-GaAs buffer layer 102 having a thickness of 0.5 .mu.m is initially formed on a Si-doped n-GaAs (001) substrate 101, and a Si-doped Al.sub.0.5 Ga.sub.0.5 As cladding layer 103 having a thickness of 1.5 .mu.m is formed on the buffer layer 102. Then, a Si-doped Al.sub.0.3 Ga.sub.0.7 As layer 104 of a light confinement region having a thickness of 100 nm is layered, and an active layer 105 of a multi-quantum well is formed on the Si-doped layer 104. The multi-quantum well layer 105 is composed of five undoped Al.sub.0.3 Ga.sub.0.7 As barrier layers each having a thickness of 10 nm and five undoped GaAs well layers each having a thickness of 6 nm, which are alternately layered. Then, a Be-doped Al.sub.0.3 Ga.sub.0.7 As layer 106 of an upper light confinement region having a thickness of 100 nm is layered, and a Be-doped Al.sub.0.4 Ga.sub.0.6 As carrier block layer 107 having a thickness of 40nm on the Be-doped layer 106. Further, Be-doped Al.sub.0.15 Ga.sub.0.85 As light guide layer 108 is formed on the carrier block layer 107. Thus, a first growth is completed.

Next, as shown in FIG. 1B, a resist pattern having a pitch of 250 nm is formed on the carrier block layer 107 using a laser interference exposure method, and a grating 109 is formed by etching. The grating 109 is a second-order diffraction grating for an AlGaAs series laser having an oscillation wavelength of approximately 830 nm. The direction of grooves of the grating 109 is aligned to a direction of ›110!. By using wet etching with an etchant of sulphuric acid series or light wet etching with an etchant of sulphuric acid series, which is performed after dry etching, slanting portions of (111)A surfaces 110 and 111 and flat portions of (001) surfaces 112 are formed as shown in FIG. 1B.

After remaining resist and oxide on the surface are removed by thermal cleaning and electron cyclotron resonance (ECR) hydrogen plasma process performed in an MBE chamber, an Si-doped Al.sub.0.5 Ga.sub.0.85 As layer having a thickness of 0.2 .mu.m is grown on the surface by using organic metal and arsine (AsH.sub.3) as material and a solid Si as a dopant, using the chemical beam epitaxy (CBE) method. Thus, a current restraint layer 113 is formed as shown FIG. 1C. When the growth conditions are appropriately selected, conduction types of the Si-doped AlGaAs layers differ from each other, depending on the surface orientation of the underlying layer on which those Si-doped AlGaAs layers are grown. That is, an n-type layer 114 appears on a (100) surface and a p-type layer 115 appears on a (111)A surface, so n- and p-type AlGaAs layers 114 and 115 can be alternately arranged at the same pitch as that of the diffraction grating 109, as shown in FIG. 1C. Then a Be-doped Al.sub.0.5 Ga.sub.0.5 As cladding layer 116 having a thickness of 1.5 .mu.m is formed on the n- and p-type AlGaAs layers 114 and 115, and a Be-doped GaAs cap layer 117 having a thickness of 0.5 .mu.m is deposited on the cladding layer 116.

Further, as shown in FIG. 2, a stripe is formed by etching, using an ordinary photolithography method, and a p-Al.sub.0.4 Ga.sub.0.5 As 118 and an n-Al.sub.0.4 Ga.sub.0.6 As 119 are buried at side portions. Then, an upper p-side electrode 120 and a lower n-side electrode 121 are formed, and a transverse single mode is realized by confinement in a transverse direction.

The operation of this embodiment will be described below. When current is injected, the p-type portions 115 of the current restraint layer 113 serve as a path of current, while the n-type portions 114 thereof act as a current block portion. Further, a curve of potential at p-n junction portions slightly curve potentials of the upper light guide layer 108 and the active layer 105, so that holes injected into the active layers 105 are weakly confined by a potential difference though spread by diffusion. Therefore, the holes are unevenly injected into the active layer 105, and a density distribution of holes is periodically modulated.

The carrier density distribution in the active layer 105 is shown in FIG. 1D. The gain and equivalent refractive index of the active layer 105 are modulated due to the carrier density distribution. Furthermore, physical unevenness at an interface between current restraint layer 113 and cladding layer 116 causes modulation of the equivalent refractive index in the waveguide. Although a difference in conduction type in the current restraint layer 113 results in a difference in the refractive index, this difference is small, compared to the modulation amount of the refractive index due to the structural modulation. Thus, a Bragg waveguide is established by these structural features and uneven current injection, and a stable oscillation in a single longitudinal mode is attained.

In the first embodiment, when a device length is set to 500 .mu.m and a mesa width is set to 2 .mu.m, the oscillation threshold value is approximately 10 mA and a single-mode oscillation is performed at a wavelength 835 nm. In this embodiment, recombination at the regrown interface is prevented by the carrier block layer 107, and this fact contributes to reduction of the threshold.

Although the buried heterostructure is used as a transverse confinement structure in the first embodiment, other conventional carrier confinement type waveguide structures may be employed if they are applicable to a laser having an active layer of a quantum well structure. Further, the shape of the second-order diffraction grating 109 is not limited to a triangular shape shown in FIG. 1B, but a trapesoidal shape as shown in FIG. 3 may be adopted, for example, only if different crystalline surfaces, on which n-AlGaAs and p-AlGaAs can be simultaneously grown, are exposed. In FIG. 3, portions corresponding to the portions in FIG. 1B are designated by reference numerals which are respectively created by adding a hundred to the reference numerals in FIG. 1B.

FIGS. 4A-4C show a second embodiment of the present invention, which relates to a gain-coupling distributed feedback semiconductor laser having a structure wherein a current restraint layer is formed under an active layer. Detailed structure and fabrication methods of the second embodiment are described below.

As shown in FIG. 4A, an n-GaAs buffer layer 302 having a thickness of 0.5 .mu.m is initially formed on a Si-doped n-GaAs (001) substrate 301, and a Si-doped Al.sub.0.5 Ga.sub.0.5 As cladding layer 303 having a thickness of 1.5 .mu.m is formed on the buffer layer 302. Then, a Si-doped Al.sub.0.2 Ga.sub.0.8 As light guide layer 304 having a thickness of 100 nm is layered.

Next, as shown in FIG. 4B, a resist pattern having a pitch of 250 nm is formed on the light guide layer 304 using a laser interference exposure method, and a second-order grating 305 is formed by etching. The direction of grooves of the grating 305 is aligned to a direction of ›110!. By using wet etching with an etchant of sulphuric acid series or light wet etching with an etchant of sulphuric acid series, which is performed after dry etching, slanting portions of (111)A surfaces 306 and 307 and flat portions of (001) surfaces 308 are formed as shown in FIG. 4B, similar to the first embodiment.

After remaining resist and oxide on the surface are removed by thermal cleaning and electron cyclotron resonance (ECR) hydrogen plasma process in an MBE chamber, a Si-doped Al.sub.0.2 Ga.sub.0.8 As layer having a thickness of 0.2 .mu.m is grown on the surface, using the MBE method. Thus, a current restraint layer 309 is formed as shown in FIG. 4C. When the growth conditions are appropriately selected, conduction types of the Si-doped AlGaAs layers differ from each other, depending on surface orientations of the underlying layer on which those Si-doped AlGaAs layers are grown. That is, an n-type conductive layer 310 appears on a (001) surface and a p-type block layer 311 appears on a (111)A surface.

Then, a Sn-doped Al.sub.0.2 Ga.sub.0.8 As layer 312 having a thickness of 0.2 .mu.m, which is a lower light confinement region, is formed on the p- and n-type AlGaAs layers 311 and 310, and an undoped GaAs active layer 313 having a thickness of 0.1 .mu.m is formed on the Sn-doped layer 312. Further, a Be-doped Al.sub.0.2 Ga.sub.0.8 As layer 314 having a thickness of 100 nm, which is an upper light confinement region, is formed on the active layer 313, and a Be-doped Al.sub.0.5 Ga.sub.0.5 As cladding layer 315 having a thickness of 1.5 .mu.m is formed. On the cladding layer 315, a Be-doped GaAs cap layer 316 having a thickness of 0.5 .mu.m is formed as shown in FIG. 4C.

Since the Sn-doped AlGaAs layer 312 becomes an n-type layer, independent of the surface orientation, the conduction type of the lower light confinement layer 312 is an n-type in its entirety.

Furthermore, on this wafer, a waveguide structure in a transverse direction, a carrier confinement structure, an upper p-side electrode and a lower n-side electrode are formed, and a single-mode laser is realized, similar to the first embodiment.

In the second embodiment, the p-type layer becomes the carrier block layer 311 and performs a current restraint for injected current. Further, since the active layer 313 is unevenly formed, the modulation of the refractive index-coupling coefficient is increased, compared to the first embodiment. The single-mode operation is thus accomplished, similar to the first embodiment.

A third embodiment of the present invention will be described with reference to FIGS. 5A-5D. The third embodiment relates to a distributed feedback semiconductor laser in which a first-order coupling is conducted in an AlGaAs series semiconductor laser. Detailed structure and fabrication methods of the third embodiment will be described in the following.

As shown in FIG. 5A, an n-GaAs buffer layer 402 having a thickness of 0.5 .mu.m is initially formed on a Si-doped n-GaAs (001) substrate 401, and a Si-doped Al.sub.0.5 Ga.sub.0.5 As cladding layer 403 having a thickness of 1.5 .mu.m is formed on the buffer layer 402. Then, a Si-doped Al.sub.0.3 Ga.sub.0.7 As layer 404 of a light confinement region having a thickness of 100 nm is layered, and an active layer 405 of a multi-quantum well structure is formed on the Si-doped layer 404. The multi-quantum well active layer 405 is composed of five undoped Al.sub.0.3 Ga.sub.0.7 As barrier layers each having a thickness of 10 nm and five undoped GaAs well layers each having a thickness of 6 nm, which are alternately layered. Then, a Be-doped Al.sub.0.3 Ga.sub.0.7 As layer 406 of an upper light confinement region having a thickness of 100 nm is layered, and a Be-doped Al.sub.0.4 Ga.sub.0.6 As carrier block layer 407 having a thickness of 40 nm is formed on the Be-doped layer 406. Further, a Be-doped Al.sub.0.15 Ga.sub.0.85 As light guide layer 408 is formed on the carrier block layer 407. Thus, a first growth is completed.

Next, as shown in FIG. 5B, a resist pattern having a pitch of 250 nm is formed on the light guide layer 408 using a laser interference exposure method, and a second-order grating 409 is formed by etching. The direction of grooves of the grating 409 is aligned to a direction of ›110!. By using light wet etching with an etchant of sulphuric acid series, which is performed after dry etching, slanting portions of (111)A surfaces 410 and 411 and flat portions of (001) surfaces 412 and 413 are formed as shown in FIG. 5B, and thus the grating 409 having a trapezoidal sectional shape is formed. Considering the effect of carrier diffusion, the etching condition is controlled so that the upper flat portion 413 is wider than the lower flat portion 412.

After remaining resist and oxide on the surface are removed by thermal cleaning and plasma process in an MBE chamber, a Si-doped Al.sub.0.2 Ga.sub.0.8 As layer having a thickness of 0.2 .mu.m is grown on the surface by using organic metal and arsine (AsH.sub.3) as material, employing the CBE method. Thus, a current restraint layer 414 is formed.

Different conduction types appear depending on surface orientations of the underlying layer, and p- and n-type AlGaAs layers 415 and 416 can be alternately arranged as shown in FIG. 5C, similar to the first embodiment. However, as shown in FIG. 5D, a modulation pitch .lambda. of carrier density at the time of current injection is a half of a pitch .LAMBDA. of the diffraction grating 409. This is a main feature of the third embodiment.

Further, a Be-doped Al.sub.0.5 Ga.sub.0.5 As cladding layer 417 having a thickness of .sub.1.5 .mu.m is formed. On the cladding layer 417, a Be-doped GaAs cap layer 418 having a thickness of 0.5 .mu.m is formed. Furthermore, on this wafer, a waveguide structure in a transverse direction, a carrier confinement structure, an upper p-side electrode and a lower n-side electrode are formed, and a single mode laser is attained, similar to the first embodiment.

FIGS. 6A-6C shows a fourth embodiment of the present invention, which relates to a distributed feedback semiconductor laser which is obtained by applying the present invention to an AlGaAs series semiconductor laser using a (111)A substrate. Detailed structure and fabrication methods of the fourth embodiment are described in the following.

As shown in FIG. 6A, an n-GaAs buffer layer 502 having a thickness of 0.5 .mu.m is initially formed on a Si-doped n-GaAs (001) substrate 501, and a Si-doped Al.sub.0.5 Ga.sub.0.5 As cladding layer 503 having a thickness of 1.5 .mu.m is formed on the buffer layer 502. Then, a Si-doped Al.sub.0.3 Ga.sub.0.7 As layer 504 of a light confinement region having a thickness of 100 nm is layered, and an active layer 505 of a multi-quantum well structure is formed on the Si-doped layer 504. The multi-quantum well active layer 505 is composed of five undoped Al.sub.0.3 Ga.sub.0.7 As barrier layers each having a thickness of 10 nm and five undoped GaAs well layers each having a thickness of 6 nm, which are alternately layered. Then, a Be-doped Al.sub.0.3 Ga.sub.0.7 As layer 506 of an upper light confinement region having a thickness of 100 nm is layered, and a Be-doped Al.sub.0.4 Ga.sub.0.6 As carrier block layer 507 having a thickness of 40 nm is formed on the Be-doped layer 506. Further, a Be-doped Al.sub.0.15 Ga.sub.0.85 As light guide layer 508 is formed on the carrier block layer 507. Thus, a first growth is completed.

Next, as shown in FIG. 6B, a resist pattern having a pitch of 250 nm is formed on the light guide layer 508 using a laser interference exposure method, and a second-order grating 509 is formed by etching. The direction of grooves of the grating 509 is aligned to a direction of ›110!. By using light wet etching with an etchant of sulphuric acid series, which is performed after dry etching, slanting portions of (001) surface 510, slanting portions of (110) surfaces 511 and flat portions of (111) surface 512 are formed as shown in FIG. 6B, and thus the grating 509 having an asymmetrical sectional shape is formed.

After remaining resist and oxide on the surface are removed by thermal cleaning and plasma process in an MBE chamber, a Si-doped Al.sub.0.2 Ga.sub.0.8 As layer having a thickness of 0.2 .mu.m is grown on the surface by using organic metal and arsine (AsH.sub.3) as material, employing the CBE method. Thus, a current restraint layer 513 is formed.

Different conduction types appear depending on surface orientations of the underlying layer, and p- and n-type AlGaAs layers 514 and 515 can be alternately arranged as shown in FIG. 6C, similar to the first embodiment. Dissimilar to the first embodiment, the p-type layer 514 is formed on the flat portion and becomes a path of current. This is a main feature of the fourth embodiment.

Further, a Be-doped Al.sub.0.5 Ga.sub.0.5 As cladding layer 516 having a thickness of 1.5 .mu.m is formed. On the cladding layer 516, a Be-doped GaAs cap layer 517 having a thickness of 0.5 .mu.m is formed. Furthermore, on this wafer, a waveguide structure in a transverse direction, a carrier confinement structure, an upper p-side electrode and a lower n-side electrode are formed, and a single mode laser is realized, similar to the first embodiment.

FIGS. 7A-7C shows a fifth embodiment of the present invention, which relates to another model of an AlGaAs series single-mode distributed feedback semiconductor laser. Detailed structure and fabrication methods of the fifth embodiment are described in the following.

As shown in FIG. 7A, an n-GaAs buffer layer 602 having a thickness of 0.5 .mu.m is initially formed on a Si-doped n-GaAs (001) substrate 601, and a Si-doped Al.sub.0.5 Ga.sub.0.5 As cladding layer 603 having a thickness of 1.5 .mu.m is formed on the buffer layer 602. Then, a Si-doped Al.sub.0.3 Ga.sub.0.7 As layer 604 of a light confinement region having a thickness of 100 nm is layered, and an active layer 605 of a multi-quantum well is formed on the Si-doped layer 604. The multi-quantum well active layer 605 is composed of five undoped Al.sub.0.3 Ga.sub.0.7 As barrier layers each having a thickness of 10 nm and five undoped GaAs well layers each having a thickness of 6 nm, which are alternately layered. Then, a Be-doped Al.sub.0.3 Ga.sub.0.7 As layer 606 of an upper light confinement region having a thickness of 100 nm is layered, and a Be-doped Al.sub.0.4 Ga.sub.0.6 As carrier block layer 607 having a thickness of 120 nm is formed on the Be-doped layer 606. Thus, a first growth is completed.

Next, as shown in FIG. 7B, a resist pattern having a pitch of 250 nm is formed on the carrier block layer 607 using a laser interference exposure method, and a second-order grating 608 is formed by etching. The direction of grooves of the grating 608 is aligned to a direction of ›110!. By using wet etching with an etchant of sulphuric acid series, slanting portions of (111)A surfaces 609 and 610, and flat portions of (001) surface 611, which is an exposed surface of the upper light confinement layer 606, are formed as shown in FIG. 7B, and thus the grating 608 is formed. At this time, the etching rate varies depending on a difference between Al mole fractions of the light confinement layer 606 and the carrier block layer 607, so that the shaping of the grating 608 is very easy.

After remaining resist and oxide on the surface are removed by thermal cleaning and plasma process in an MBE chamber, a Si-doped Al.sub.0.4 Ga.sub.0.6 As layer having a thickness of 0.2 .mu.m is grown on the surface by using organic metal and arsine (AsH.sub.3) as material, using the CBE method. Thus, a current restraint layer 612 is formed.

Different conduction types appear depending on surface orientations of the underlying layer, and p-type AlGaAs current injection portion 613 and n-type AlGaAs current block layer 614 can be alternately arranged as shown in FIG. 7C, similar to the first embodiment. However, recombination at the regrown interface is prevented by the following mechanism. In the current injection portion 613 formed by growing the p-type AlGaAs on the (111)A surfaces 609 and 610, current injection into the regrown interface is prevented by the action of the carrier block layer 607. In the current block portion 614 formed by growing the n-type AlGaAs on the upper light confinement region portion 606, recombination is diminished since a p-n junction is fabricated at the regrown interface and carriers are swept out by an internal electric field.

Further, a Be-doped Al.sub.0.5 Ga.sub.0.5 As cladding layer 615 having a thickness of 1.5 .mu.m is formed. On the cladding layer 615, a Be-doped GaAs cap layer 616 having a thickness of 0.5 .mu.m is formed. Furthermore, on this wafer, a waveguide structure in a transverse direction, a carrier confinement structure, an upper p-side electrode and a lower n-side electrode are formed, similar to the first embodiment, and a single-mode laser is attained.

In the above-discussed embodiments, the crystalline growth on an n-GaAs substrate is used, and an AlGaAs series semiconductor laser is attained. However, a p-GaAs substrate may be used to fabricate a laser. In this case, block and current transmission portions of a current restraint layer are replaced with each other, but its structure and operation are substantially the same.

Furthermore, while only an AlGaAs series semiconductor laser is discussed in the foregoing, the present invention can be also applied to, for example, InGaAsP series laser, InAlGaP series laser, InGaAs series laser and InAlGaAs series laser since they are group III-V compound semiconductor lasers and those materials can be grown in a vapor phase using the MBE or MOMBE method.

Moreover, dopant impurity is not limited to Si, but any amphi-conductivity impurity of group IV, which creates p- or n-type layer depending on orientations of a growth surface, may be employed. Further, the MOVPE method and the like, which use group IV material as an impurity, may be used.

There is a phenomenon that, when epitaxial growth is performed on an uneven surface, such as a periodical surface or grating surface, having surfaces with different surface indices, p- and n-type layer portions can be formed depending on growth surface orientations if an impurity such as Si or group IV elements is used. In the present invention, that phenomenon is utilized, and a current restraint layer, which includes alternately-arranged p- and n-type layer portions, is formed in the vicinity of an active layer. Thus, current is unevenly injected into the active layer, and a periodical structure of gain can be established. As a result, a distributed feedback structure is achieved, which performs a gain-coupling operation. That is, the carrier density during current injection time is periodically modulated along a light propagation direction, and a periodical modulation is imparted to refractive index and gain in the waveguide due to the modulated carrier density. Thus, the waveguide becomes a Bragg waveguide, and a single-mode operation is attained by current above an oscillation threshold.

As described in the foregoing, according to the present invention, a gain-coupling distributed feedback laser, which operates in a single mode due to gain coupling, can be fabricated. Further, a laser, in which recombination current at a regrowth interface is small, can be realized since the regrowth interface can be formed away from an active layer. In addition, freedom in design is great because a quantum well structure can be employed in the active layer. In other words, a threshold can be decreased, an oscillation wavelength can be tuned by controlling parameters of the quantum well, an oscillation in a transverse magnetic (TM) mode can be oppressed, and an oscillation spectrum can be narrowed by introducing a strained structure.

While the present invention has been described with respect to what is presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. The present invention is intended to cover various modifications and equivalent arrangements included within the sprit and scope of the following claims.