GB2225671A

GB2225671A - Semiconductor laser

Info

Publication number: GB2225671A
Application number: GB8929323A
Authority: GB
Inventors: Takahiro Suyama; Toshiro Hayakawa; Kohsei Takahashi; Masafumi Kondo; Saburo Yamamoto
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 1986-04-30
Filing date: 1989-12-29
Publication date: 1990-06-06
Anticipated expiration: 2007-04-27
Also published as: GB2225671B; GB8929323D0

Abstract

After sequentially forming a first clad layer 3, active layer 4, second clad layer 10, cap layer 11, and further layers 12, 13, on a substrate to form a laser, the further layers 12, 13, are selectively removed to form a window and in this window grooves 14 are formed from the surface of the cap layer 11 down into the second clad layer 10, an insulation layer 16 being formed in the groove, to form a current stripe structure, in which the grooves are used to confine current and the regions between adjacent grooves serve as light emitting regions. A structure may have only two grooves, or if an array is required, more than two grooves. <IMAGE>

Description

SEMICONDUCTOR LASER BACKGROUND OF TISE INVENTION Field of the Invention This invention relates to a semiconductor laser device possessing a refractive index waveguide mechanism and capable of operating at high output at low threshold current.

Description of the Prior Art: For the semiconductor laser used in optical information processing apparatus such as light signal and optical disc player, it is indispensable to possess a refractive index waveguide mechanism in its property.

Such semiconductor laser was, conventionally, fabricated by liquid phase growth process, and various structures were proposed for incorporating said refractive index waveguide mechanism. For example, as the construction using GaAlAs compound as the material, the VSIS (Vchanneled substrate inner stripe) laser with the substrate processed in a groove form, CSP (channeled substrate planar) laser, and BH (buried heterostructure) laser having the light-emitting region buried with clad layer are known.

In these laser devices, however, since the liquid phase growth process is used in the manufacture, it is difficult to control the film thickness or composition of each layer to make up the laser device, and it is also difficult to form very thin active layers in a clean shape free from lattice defects, and it is hence extremely difficult to realize a high performance semiconductor laser low in threshold current density.

Besides, the manufacturing yield is poor. To solve these demerits of the liquid phase growth process, as the crystal growth method capable of controlling the film thickness very strictly, molecular beam epitaxial (MBE) process controlling the vacuum deposition technology at extremely high precision, and metalorganic chemical vapor deposition are being introduced recently.

Particularly, in the MBE process, it is possible to control the film thickness in the order of atom layer using together with vacuum analyzers and electronic computers. In these method, however, since the crystal growth mechanism of the semiconductor laser is different from that of the liquid phase growth process, the device structure possessing various refractive index waveguide mechanisms proposed in the liquid phase growth process cannot be applied in production, and other new device structures must be employed. As such new device structures, for example, the ridge waveguide type semiconductorlaser shown in Fig. 1 and semiconductor laser device shown in Fig. 2 are proposed.

In the ridge waveguide type laser device in Fig. 1, n-type GaAlAs clad layer 32, GaAs active layer 33, ptype GaAlAs clad layer 34, and p-type GaAs cap layer 37 are sequentially formed on a flat n-type GaAs substrate 31, and all of right and left sides of cap layer 37 and part of right and left sides of clad layer 34 are removed by etching, and SiO2 insulation layer 36 is formed on the surface of both right and left sides of this clad layer 34, and finally a p-side electrode 38 and an n-type electrode 39 are respectively formed on the upper surface and lower surface of the convex form, respectively.In the semiconductor laser in Fig. 2, which is manufactured by metalorganic chemical vapor deposition, n-type GaAlAs clad layer 22, GaAs active layer 23, p-type GaAlAs clad layer 24, and n-type GaAs current narrowing layer 25 are sequentially formed on a flat n-type GaAs substrate 21, and the middle part of this current narrowing layer 25 is removed in stripes by etching, and p-type GaAlAs clad layer 26 and p-type GaAs cap layer 27 are formed thereon, and finally a p-type electrode 28 and an n-type electrode 29 are formed on the upper surface and lower surface of the concave form, respectively.

In the laser device in Fig. 2, since it is taken out of a quartz reaction tube or the like for etching, the surface of the n-type GaAs current narrowing layer 25 is exposed to the atmosphere to be oxidized. In particular, since the surface 20 is close to the lightemitting region 23a of active layer 23, and devIce deterioration derived from oxygen is likely to occur, and moreover since the current narrowing layer 25 is made of GaAs, light absorption occur in this area, and IL is r.ot desired for reduction of threshold current density. Besides, whenGaAlAs is used, for example, as this current narrowing layer 25, crystal growth on the t surface is extremely difficult due to oxidation of the GaAlAs surface at the time of etching.

Therefore, in the structure in Fig. 2, it is difficult to sufficiently utilize the low threshold current density characteristics of the MQW (multi quantum well) laser and GRIN-SCH laser making use of the excellent film thickness and controllability of composition of the MBE process. On the other hand, the ridge waveguide type semiconductor laser is hard to handle as it is because ridges are exposed on the surface, and it is hard to mount on the growth layer side where oscillation region is present. If mounted by settinq the growth layer upper, cooling performance is poor, and it causes problems in the aspects of device reliability and high output operation. Therefore, as the laser deice of ridge waveguide type, actually, it seems effective to form a supporting part of the same height as the ridge at both sides of the ridge as shown in Fig.

3, but the following problems are involves in the structure in Fig. 3. That is, in the ridge waveguide type laser device, the device characteristic greatly depends on the ridge shape (ridge width, or thickness from active layer at both sides of ridge to the surface), and to control this ridge shape as accurately as possible, it is desired that the total thickness of the second clad layer 34 and cap layer 37 may be small to an extent that the device characteristics may not be -worsened. In this case, however, when mounting the device, the solder such as In goes up along the element end surface to be adhered, and the possibility of shortcircuiting of the pn junction of device becomes very high.

OBJECTS AND SUMMARY OF THE INVENTION Objects of the Invention: It is hence a primary object of this invention to present a semiconductor laser device possessing a refractive index waveguide mechanism, free from light absorption, capable of realizing low threshold current operation or high output operation, and easy in fabrreatisn and mounting cf device without any problem.

Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

Summary of the Invention: In order to achieve the above obects, the semiconductor laser device of this invention is manufactured by sequentially forming a first clad layer, active layer, second clad layer, and cap layer, for example, AlGaAs layer with AlAs mixing ratio of > 0.4, on a substrate of GaAs or the like developed, for example, by MBE, MOCVD or other high precision growth process.

selectively removing the AlGaAs layer only in the vicinity of the ridge forming area, forming in that area grooves above two stripes from the cap layer surface, leaving, for example. the second clad layer by a thick o ness of 3000 A. and forming an insulation layer made of SiN or the like in the groove part and AlGaAs layer region, so that only the region provided between the two grooves in the current stripe structure are used as current passage, that is, light-emittion region. In this structure, the thickness of the mesa part (ridge part) between the two grooves may be minimum, and the processing precision may be raised, while by properly selecting the thickness of the AlGaAs layer, the distance between the mount surface and active layer may be sufficiently set long, and problem of climbing-up of the solder along the device end surface may not occur.Incidentally, in this AlGaAs layer, when the AlAs mixing ratio is set at 0.4, it is A possible to remove selectively by etching, using, for example, hydrogen fluoride.

The two stripe grooves may possess a refractive index waveguide mechanism by processing the second clad, leaving about 3000 as mentioned above, and also, as shown in Fig. 5, by processing so as to cross the active layer In the part slightly remote from the mesa part, it is possible to lower the current, and this is particularly effective for lowering of threshold.

Or by designing the AlGaAs layer as the conductive type opposite to the cap layer, current of this area is possible by the AlGaAs layer, and it is not necessary to form any insulation layer thereon. Instead, by forming GaAs layer on the AlGaAs layer, the electrode may be formed in the same process as on the mesa (on the cap layer), and problems such as removal of insulation layer and aggregation of electrodes do not occur, and moreover since this process may be done after mesa forming, it is easy to reduce the mesa width, and a lower threshold is expected.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein: Fig. 1 is a structural drawing of a conventional ridge waveguide type semiconductor laser device; Fig. 2 is a structural drawing of a conventional self-aligned semiconductor laser manufactured by MOCVD process or the~like: Fig. 3 is a structural drawing of a conventional ridge waveguide type semiconductor laser possessing a supporting part; Fig. 4 is a structural drawing of a semiconductor laser device showing one of the embodiments of this invention; and Fig. 5 is a structural drawing of a semiconductor laser device showing other embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 4 is a sectional view of a GaAs-GaAlAs semiconductor laser showing one of the embodiments of this invention. To form this semiconductor laser, to begin with, n-GaAs buffer layer 2, n-AlGaAs first clad layer 3, and active layer 4 are sequentially deposited on a flat n-GaAs substrate 1 by molecular beam epitaxy (MBE). In this embodiment, particularly, as the active layer 4 te t-the laser oscillation operating part, a multilayer structure laminating the following layers is sed. That is, it is a so-called quantum well structure of GRIN-SCH structure consisting of a laminate of AlxGal-xAs layer 5 where the mixing ratio x varies gradually from 0.7 to 0.28, a superlattice layer 8 alternately laminating three layers of Al028Ga072As layer 6 of an extremely thin layer thickness and four layers of GaAs layer 7 with similar thin layer thick ne, and AlyGal-yAs layer 9 where mixing ratio y varies gradually from 0.28 to 0.7. On this active layer 4, P-A107Ga03As second clad layer 10, P-GaAs cap layer 11, n-A105Ga05As layer 12, and GaAs layer 13 are sequentially deposited.

This is taken out of the MBE apparatus, and coated with photoresist, and a stripe window of 30 um in width is formed by photolithography, and by the mixed solution of NH40H and N202,and llF, the GaAs layer 13 and n AlQ5Ga0.5. As layer 12 are selectively removed in. stripe corresponding to the above window.

Next, the inside of the window removed in stripe is coated again with photoresist, and plural stripe-shaped parallel grooves 14 of a narrow width-are shown in the drawing are carved by photolithography, and a resist pattern for forming mesa pattern 15 between second 'clad layer 10 and cap layer 11 is formed between groove.

14 and groove 14, and the grooves 14 are formed in such a thickness that'the second clad layer 10 is left over o --by about 2000 A by the reactive ion beam etching process After forming grooves 14, the resist is removed, and the entire surface is coated withSiN film 16 by plasma CVD process. Again by photolithography, the groove part 14 is removed, and SiN film 16 is taken away to pass the current passage. Furthermore, after polishing the wafer to a proper thickness, P-side electrodes 17 are formed on the exposed surfaces of GaAs layer 13, cap layer 11, and SiN film 16, while n-side electrode 18 is formed on the back side of the GaAs substrate, so that individual laser chips are formed by dividing by the cleavage process.

This embodiment is an array structure possessing three mesa'patterns 15 (ridges), but the number of mesa pattn-rns is not limited to this. Besides, the active layer 4 is not limited to the GRIN-SCH structure, and various double heterojunction structures can be applied.

When a driving current is injected through the p-type electrode 17 and n-type electrode 18, the current flows into the active layer 4 from right above the retFed mesa pattern 15 of the SiN film 16, and reaches up to the n-type electrode 18. When the current flows into the active layer 4, laser oscillation is started in this part. This SiN film 16 is an insulator, and the n-AlGaAs layer 12 is biased in reverse polarity, and current does not flow in this part.

Fig. 5 is a structural.drawing of a semiconductor laser device showing other embodiment of this invention.

In this embodiment, the stripe parallel grooves 14 formed within the strip-shaped window are carved by two pieces at both sides-, and the depth of parallel grooves 14 is enough to penetrate through the active layer 4.

The other constitution is same as in the embodiment shown in Fig. 4, and the current passage is formed in the direction of active layer 4 from right above the mesa pattern 15 formed between two parallel grooves 14.

Since the active layer 4 is -separated by the parallel grooves 14, the current flowing in this part does not spread outward, so that generation of reactive current not contributing to the oscillation of laser may be prevented. A flat part is present between the foot area oi the mesa pattern 15 and the parallel grooves 14, -and by this flat part, the laser device of this embodiment becomes a refractive index waveguide type semiconductor laser.

As clear from the explanation above, this invention can bring about a semiconductor laser possessing an excellent heat releasing property, high processing precision, ease of mounting, and secure high performance, and in particular, a high performance, refractive index g waveguide type semiconductor laser can be obtained by using the semiconductor crystal layers with the film thickness and composition controlled at high degree being developed by molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MDCVD) process.

This invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications are intended to be included within the scope of the following claims.

Claims

CLAIMS:

1. A semiconductor laser device composed by forming a dent in the middle part of a multilayer crystal layer containing an active layer for laser oscillation laminated on a substrate, and forming a current passage in a mesa pattern region enclosed by two parallel stripe grooves in the bottom of said dent, wherein the laser oscillation region is limited with respect to said current passage.

2. The semiconductor laser of claim 1, wherein the multilayer crystal layer possesses a pair of clad layers to enclose the active layer between them and a cap layer for deposition of electrodes.

3. A semiconductor laser comprising: a GaAs substrate; an n-GaAs buffer layer formed on said GaAs substrate; an n-AlGaAs first clad layer formed on said n-GaGa buffer layer; an actIve layer formed on said n-AlGaAs first clad layer; a P-AlGaAs second clad layer formed on said active layer; a P-GaAs cap layer formed on said second clad layer; an n-AlGaAs layer formed on said n-GaAs cap layer; a GaAs layer formed on said n-AlGaAs layer; a stripe window formed in said GaAs layer and n-AlGaAs layer; and a stripe groove formed in said stripe window, reaching up to said second clad layer through said P-GaAs cap layer.

4. The semiconductor laser of claim 3, wherein said second clad layer has a thickness of about 2000 A at a position where said stripe groove is formed.

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5. A semiconductor laser device which has a multi layer structure which includes an active layer, and in which said structure is adapted to provide a strip shaped zone for current passage through the active layer to define a region for laser oscillation, the adaptation comprising a strip recess and at least two grooves formed in and extending along the floor of the recess to define one or more elongate ridges at the level of the recess floor.

6. A semiconductor laser device according to claim 5 wherein more than two said grooves are provided so as to define a plurality of said elongate ridges.

7. A semiconductor laser device according to claim 5 or claim 6 wherein said active layer is a quantum well structure of the multi-layer GRIN-SCH type.

7. A semiconductor laser device according to claim 5 or claim 6 wherein said grooves extend to the active layer to confine the current passage from the or each ridge to the part of the active layer lying between the grooves defining that ridge.

8. A semiconductor laser device substantially as hereinbefore described with reference to figure 4 of the accompanying drawings.

9. A semiconductor laser device substantially as hereinbefore described with reference to figure 5 of the accompanying drawings.

Amendments to the claims have been filed as follows 1. A semiconductor laser device comprising a multi layer crystal structure formed on a substrate and comprising in order upwardly from the substrate, a first clad layer, an active layer, a second clad layer, and a cap layer, the device including a recess formed by removal of portions of two layers overlying said cap layer down to the level of said cap layer, and a plurality of parallel grooves formed in the floor of said recess through the cap layer and at least partly through said second clad layer with one or more ridges between said grooves each constituting a mesa region defining a zone of current flow from an electrode portion overlying said cap layer on said ridge to said active layer, the region of laser oscillation in said active layer being limited in accordance with said zone or zones of current flow.

2. A semiconductor laser device according to claim 1 wherein said substrate is a GaAs substrate, said first clad layer is a n-AlGaAs clad layer, said second clad layer is a P-AlGaAs clad layer, said cap layer is a P GaAs cap layer and said two layers overlying said cap layer comprise an n-AlGaAs layer, and a GaAs layer.

3. A semiconductor laser device according to claim 1 or claim 2 wherein said grooves extend down to the active layer.

4. A semiconductor laser device according to any preceding claim wherein more than two said grooves are provided so as to provide a plurality of said elongate ridges defining respective said zones of current flow.

5. A semiconductor laser device comprising: a GaAs substrate; an n-GaAs buffer layer formed on said GaAs substrate; an n-AlGaAs first clad layer formed on said n GaAs buffer layer; an active layer formed on said n-AlGaAs first clad layer; a P-AlGaAs second clad layer formed on said active layer; a P-GaAs cap layer formed on said second clad layer; an n-AlGaAs layer formed on said P-GaAs cap layer; a GaAs layer formed on said n-AlGaAs layer; a strip-shaped window formed in said GaAs layer and n-AlGaAs layer; and a plurality of strip-snaped grooves formed in and extending along said window, reaching said second clad layer through said P-AlAs cap layer.

6. A semiconductor laser device according to claim 5, wherein said second clad layer has a thickness of about 2000 A at a position where said strip-shaped grooves are formed.