A semiconductor laser device

Abstract: A semiconductor laser device comprising an active layer, a layer furnished with a diffraction grating and formed in the vicinity of said active layer, and another layer formed on said layer with the diffrac­ tion grating and having a refractive index smaller than that of said active layer but greater than that of said layer with the diffraction grating.

Description: BACKGROUND OF THE INVENTION: The present invention relates to a distributed feedback type or distributed Bragg reflection type semiconductor laser device which is equipped with a diffraction grating and performs laser oscillation at a wavelength of 660 to 890 nm in a single longitudinal mode. When a semiconductor laser device is used as a light source in an optical information transmission system or an optical measurement system utilizing optical fibers, the laser device is required to have such operation characteristics as to perform its oscillation in a single longitudinal mode. Relative to the laser device structure suited for achieving desired laser oscillation in a single longitudinal mode, there is known a distributed feedback type or distributed Bragg reflection type having a periodically corrugated diffraction grating in the active region or in the vicinity thereof. Fig. 3 shows the structure of a conventional distributed feedback type semiconductor laser device, which comprises an n-InP substrate 10, an n-InP cladding layer (buffer layer) 20, a non-doped InGaPAs active layer 30, a p-InGaAs optical guiding layer 40, a p-InP cladding layer 50 and a p-InGaPAs cap layer 60 formed in sequence. A p-side ohmic electrode 70 and an n-side ohmic electrode 80 are disposed on the cap layer 60 and the substrate 10 respectively, and a diffraction grating 401 for controlling the laser oscillation wavelength is formed on the upper surface of the optical guiding layer 40. This laser device employs an InGaPAs-InP semiconductor configuration with such oscillation characteristics as to effect laser oscillation at a relatively long wavelength of 1,300 nm. Meanwhile, a similar structure can be applied also to a semiconductor laser device for general use at a shorter oscillation wavelength of 890 nm or less. Such laser device comprises, as shown in Fig. 4, an n-GaAs substrate 100, an n-GaAlAs cladding layer 200, a non-doped GaAs or GaAlAs active layer 300, a p-GaAlAs optical guiding layer 400, a p-GaAlAs cladding layer 500 and a p-GaAs cap layer 600 formed in sequence. In this structure, the GaAlAs cladding layer 500 is grown on a diffraction grating 401 which is formed on the GaAlAs optical guiding layer 400. However, since a crystal layer of GaAlAs or the like containing Al as a component is readily oxidised in the atmosphere to form an oxide film momentarily, regrowth of the crystal on GaAlAs is rendered difficult. Consequently, with regard to laser devices operating at any oscillation wavelength shorter than 890 nm, complete requisite technology for the semiconductor lasers with a diffraction grating has not yet been accomplished practically in the present stage. SUMMARY OF THE INVENTION: In view of the problems mentioned above, it is an aim of the present invention to provide a laser device that alleviates some of the aforementioned problems. According to the present invention, a semiconductor laser device which can attain laser oscillation at a wavelength in the range of 660 to 890 nm, comprises a first cladding layer, an active layer formed on said first cladding layer and composed of GA 1-x Al x As (where 0 ≦ x ≦ 0.4) or In 1-y Ga y P 1-z As z (where 0.51 ≦ y ≦ 1, 0 ≦ z ≦ 1, z = 2.04y - 1.04), an optical guiding layer with a diffraction grating, formed on said active layer and composed of In 1-u Ga u P 1-v As v (where v = 2.04u - 1.04), and a second cladding layer formed above said optical guiding layer, said device characterised in that an intermediate layer is disposed between the optical guiding layer and the second cladding layer, said intermediate layer being composed of Ga 1-w Al w As (where 0 ≦ w ≦ 1.0) or In 1-k Ga k P₁₋ As (where 0.51 ≦ k 1 , 0 ≦ ≦ 1, = 2.04k - 1.04) and having a refractive index n selected so as to satisfy a condition n a1 > n > n G > n cl where n al is the refractive index of said active layer, n G is that of said optical guiding layer with the diffraction grating, and n cl is that of said first and second cladding layers. BRIEF DESCRIPTION OF THE DRAWINGS: The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings wherein: Fig. 1 is a typical sectional view of a distributed feedback semiconductor laser device embodying the present invention; Fig. 2 illustrates the distribution of refractive indexes in the direction of thickness of the semiconductor laser device shown in Fig. 1; Figs. 3 and 4 are typical sectional views showing the structure of conventional semiconductor laser devices; and Fig. 5 is a typical sectional view showing the structure of a conventional semiconductor laser device. DETAILED DESCRIPTION OF THE INVENTION: In advance of describing the present invention, an explanation will be first given on the structure of a known exemplary device in view of some problems existing in the conventional semiconductor laser devices, wherein an optical guiding layer forming a diffraction grating thereon consists of In 1-k Ga k P 1-ℓ As ℓ (0.51 ≦ k ≦ 1, 0 ≦ ℓ ≦ 1, ℓ = 2.04k - 1.04) which is free from a disadvantage of oxidation and enables the grating to match with GaAs. This exemplary device was proposed previously by the research group to which the present inventor belongs and was transferred later to the present applicant. The structure which corresponds to the preamble of claim 1 of the present invention is disclosed in United States Patent Application, Serial No. 789,787 filed on October 21, 1985, entitled "A SEMICONDUCTOR LASER DEVICE" by Shinji KANEIWA et al. The European counterpart is EP-A-0 182 508 (Application No. 85307630.5) filed on October 22, 1985. Now the structure of the above laser device will be described below with reference to Fig. 5. An n-GaAlAs cladding layer 2, a non-doped GaAs active layer 3 and a p-InGaPAs optical guiding layer 4 are grown successively on an n-GaAs substrate 1 by liquid phase epitaxy. Subsequently the optical guiding layer 4 is coated with a photo-resist film, which is then subjected to an interference exposure treatment using an ultraviolet laser beam, so that a photo-resist diffraction grating 44 is formed with periodic intervals of 2500Å. Using such diffraction grating 44 as a mask, grooves are formed on the optical guiding layer 4 by a chemical etching technique, and then the photo-resist film is removed. As a result, there is formed on the optical guiding layer 4 a desired diffraction grating 44 with corrugations having a periodicity of 2500Å. Further on the optical guiding layer 4 having such diffraction grating 44 on its surface, a p-GaAlAs cladding layer 5 and a p-GaAs cap layer 6 are successively grown by liquid phase epitaxy in the same manner as mentioned previously, and then ohmic metal electrodes 7 and 8 are formed on the cap layer 6 and the substrate 1, respectively. In the above laser device furnished with double-heterojunction layers constituting a multilayered crystal structure for laser operation, the active layer 3 consists of GaAs, and the cladding layers 2 and 5 consist of mixed crystals of Ga 1-x Al x As in which the compound ratio x is set to be 0.2 or more. The oscillation wavelength in this example is about 890 nm. The optical guiding layer 4 consists of a quaternary mixed crystal of In 1-y Ga y P 1-z As z in which the crystal compound ratios are selected as 0.68 ≦ y ≦ 1, 0.34 ≦ z ≦ 1, z = 2.04y -1.04 Meanwhile the forbidden band width and the refractive index of the optical guiding layer 4 are so set as to become intermediate between those of the active layer 3 and the cladding layer 5. For achieving an oscillation wavelength of 890 nm or less, the compositions of the active layer 3 and the cladding layers 2 and 5 may be so modified that, besides the above example, the active layer 3 is composed of Ga 1-x Al x As with its crystal compound ratio x selected in a range of 0 ≦ x ≦ 0.4, while the cladding layers 2 and 5 are composed of Ga 1-x Al x As with its compound ratio x selected in a range of 0.2 ≦ x ≦ 1 which is higher than that of the active layer 3 to render the band gap energy thereof greater than that of the active layer 3 by 0.3 eV or more. Due to selective setting of the crystal compound ratios in the above-mentioned proper ranges, the laser device obtained has an oscillation wavelength of 660 to 890 nm. For setting the oscillation wavelength within such range, the active layer 3 may be composed of In 1-y Ga y P 1-z As z , instead of the aforesaid material GaAs-GaAlAs, with the individual crystal compound ratios being selected as 0.51 ≦ y ≦ 1, 0 ≦ z ≦ 1 and z = 2.04y - 1.04. Furthermore, the cladding layers 2 and 5 may be composed of In 1-y Ga y P 1-z As z with the individual crystal compound ratios being selected as 0.51 ≦ y ≦ 0.81, 0 ≦ z ≦ 0.6 and z = 2.04y - 1.04. Regarding the optical guiding layer 4, a quaternary mixed crystal of In 1-y Ga y P 1-z As z (where z = 2.04y - 1.04) is employed instead of GaAlAs so as to avert oxidation of Al and its crystal compound ratios are determined by properly selecting the materials of both the active layer 3 and the cladding layers 2 and 5. Since InGaPAs is used for the optical guiding layer 4 in the above structure, its surface is not oxidized in the subsequent steps of photo etching and so forth to form a diffraction grating, so that the succeeding regrowth is executed with facility to consequently accomplish a high-quality distributed feedback laser device with satisfactory reproducibility. However, the semiconductor laser device mentioned hereinabove still has the following disadvantages. It is known that an unstable mixed region is existent in a mixed crystal of InGaAs grating-matched with GaAs. With regard to such unstable mixed region, a detailed description is given in, for example, Kentaro Onabe, "Applied Physics", Vol. 53, p. 802 (1984). At a general growth temperature of 800°C for the semiconductor laser device shown in Fig. 5, it is difficult, due to the existence of such unstable mixed region, to attain proper growth of the InGaAs layer composed of In 1-k Ga k P 1-ℓ As ℓ (where 0.64 ≦ k ≦ 0.72, 0.27 ≦ ℓ ≦ 0.43) with the forbidden band width ranging from 1.77 to 1.68 eV. Although the InGaPAs layer in the vicinity of such composition is allowed to grow to some extent, its crystalline properties are not satisfactory including a low intensity of the photo luminescence and a large half width of the spectrum. It is therefore requisite for the guiding layer to consist of In 0.49 Ga 0.51 P which has the highest crystalline quality out of mixed crystals of InGaPAs to be grating-matched with GaAs. In the visible-ray semiconductor laser of Ga 1-x Al x As system, the most reliable active layer consists of Ga 0.87 Al 0.13 As which achieves an oscillation wavelength of 780 nm. In constituting a distributed feedback semiconductor laser of such oscillation wavelength, the respective thicknesses of the active layer, optical guiding layer and cladding layers need to be determined on the basis of the individual refractive indexes thereof. In such determination, it is necessary to contrive that a great change is induced in the equivalent refractive index by the diffraction grating and a sufficient amount of light is introduced toward the guiding layer to effect feedback. However, in the active layer of AlGaAs where the Al compound ratio x is 0.13, the refractive index at an oscillation wavelength of 780 nm is 3.56. Meanwhile, in the case of using In 0.49 Ga 0.51 P for the optical guiding layer, the refractive index thereof is 3.37. Accordingly, the difference between the refractive index of the active layer and that of the guiding layer becomes excessively great to consequently impede adequate leak of the light from the active layer toward the optical guiding layer, whereby there is raised a problem that sufficient feedback fails to be effected by the diffraction grating. In an attempt to solve the problem relative to such feedback, the present inventor has developed an improved laser device structure on the basis of the semiconductor laser device mentioned hereinabove. There is also known from EP-A-0 149 462 a distributed feedback semiconductor laser having an active layer covered by a first semiconductor layer with a diffraction grating which is in turn covered by a second semiconductor layer having a refractive index larger than that of the first semiconductor layer but smaller than that of the active layer and wherein first and second cladding layers adjoin the active layer and the second semiconductor layer respectively. Fig. 1 shows the structure of a preferred embodiment of the present invention. On an n-GaAs substrate 11, there are successively formed by epitaxial growth an n-Ga 0.5 Al 0.5 As cladding layer 12 for shutting light in, a non-doped Ga 0.89 Al 0.13 As active layer 13 for causing laser oscillation, and a p-In 0.49 Ga 0.51 P optical guiding layer 14 for forming a diffraction grating thereon. In this structure, the optical guiding layer 14 is composed of a ternary mixed crystal with a greater forbidden band width and a smaller refractive index than the active layer 13. Subsequently a diffraction grating is shaped on the optical guiding layer 14 by the same process as in the prior art. First the optical guiding layer 14 is coated with a photo-resist film, which is then subjected to an interference exposure treatment using an ultraviolet laser beam, so that a photo-resist diffraction grating is formed with periodic intervals of 2500Å. Using such diffraction grating as a mask, grooves are formed on the optical guiding layer 14 by a chemical etching technique, and then the photo-resist film is removed. As a result, there is formed on the optical guiding layer 14 a desired diffraction grating with corrugations having a periodicity of 2500Å. Further on the optical guiding layer 14 having such a diffraction grating on its surface, a p-Ga 0.75 Al 0.25 As intermediate layer 15, a p-Ga 0.5 Al 0.5 As cladding layer 16, and a cap layer 17 for ohmic contact with electrodes are successively formed by epitaxial growth. And subsequently ohmic metal electrodes 18 and 19 are formed on the cap layer 17 and the reverse surface of the substrate 11, respectively. Thus, there is produced a semiconductor laser device operating at an oscillation wavelength of 780 nm. The oscillation wavelength can be selectively changed as desired by controlling the crystal compound ratio of Al in the active layer 13. In this exemplary embodiment, the intermediate layer 15 is composed of InGaPAs which is grating-matched with the substrate of GaAlAs or GaAs where the crystal compound ratio is so selected as to obtain a relationship n G < n < n al , in which n, n al and n G are the refractive indexes of the intermediate layer 15, the active layer 13 and the optical guiding layer 14 with the diffraction grating formed thereon, respectively. In the above embodiment, the intermediate layer 15 consists of GaAlAs where the compound ratio of Al is 0.25. Fig. 2 shows changes of the individual refractive indexes in the direction of thickness of each layer in the semiconductor laser device of the above embodiment. Due to insertion of the intermediate layer 15, the light from the active layer 13 is led out toward the intermediate layer 15 having a high refractive index difference and is caused to leak out thereto, so that the optical density of the optical guiding layer 14 having a small refractive index difference is rendered high. As a result, the light is induced to be readily leakable toward the optical guiding layer 14. Consequently, the light furnished with wavelength selectivity by the diffraction grating of the optical guiding layer 14 is fed back by a large amount to the active layer 13, whereby laser oscillation is attained at a single wavelength. It is to be understood here that the present invention is not limited to the distributed feedback type laser device alone and is applicable also to a distributed Bragg reflection type as well. The semiconductor laser device according to the present invention is so fabricated that an intermediate layer having a high refractive index is provided on the reverse side, with respect to the active layer, of the optical guiding layer with a diffraction grating formed thereon. The refractive index n of the intermediate layer is selected to be within a range of nG < n < n al , where n al and n G are respectively the refractive indexes of the active layer and the optical guiding layer. And there are sequentially superposed in the order of the active layer, the optical guiding layer and the layer having a refractive index n, wherein the light from the active layer is led out by the layer of a refractive index n so that the optical density of the guiding layer disposed therebetween is rendered high. As described in detail hereinabove, according to the semiconductor laser device of the present invention equipped with a diffraction grating and an optical guiding layer having a minimum refractive index in comparison with that of the active layer, light can be introduced sufficiently to the optical guiding layer to consequently ensure adequate feedback by the diffraction grating. The laser device of the present invention is so producible that a desired oscillation wavelength can be obtained within a range of 660 to 890 nm. The active layer consists of Ga 1-x Al x As (where 0 ≦ x ≦ 0.4) or In 1-y Ga y P 1-z As z (where 0.51 ≦ y ≦ 1, 0 ≦ z ≦ 1, z = 2.04y - 1.04); the optical guiding layer with a diffraction grating consists of In 1-u Ga u P 1-v As v (0.51 ≦ u ≦ 1, 0 ≦ v ≦ 1, v = 2.04u - 1.04); and the layer formed on the optical guiding layer with a diffraction grating consists of Ga 1-w Al w As (where 0 ≦ w ≦ 1.0) or In 1-k Ga k P 1-ℓ As ℓ (where 0.51 ≦ k ≦ 1, 0 ≦ ℓ ≦ 1, ℓ = 2.04 k - 1.04). The refractive index n of the last-mentioned layer is so set as to satisfy a condition naℓ > n > nG, in which naℓ and nG are the refractive indexes of the active layer and the optical guiding layer with the diffraction grating, respectively. There are described above novel features which the skilled man will appreciate give rise to advantages.

Claims: A semiconductor laser device which can attain laser oscillation at a wavelength in the range of 660 to 890 nm, comprising a first cladding layer (12), an active layer (13) formed on said first cladding layer and composed of GA 1-x Al x As (where 0 ≦ x ≦ 0.4) or In 1-y Ga y P 1-z As z (where 0.51 ≦ y ≦ 1, 0 ≦ z ≦ 1, z = 2.04y - 1.04), an optical guiding layer (14) with a diffraction grating, formed on said active layer and composed of In 1-u Ga u P 1-v As v and where v = 2.04u - 1.04), and a second cladding layer (16) formed above said optical guiding layer, said device being characterised in that an intermediate layer (15) is disposed between the optical guiding layer and the second cladding layer, said intermediate layer (15) being composed of Ga 1-w Al w As (where 0 ≦ w ≦ 1.0) or In 1-k Ga k P 1-ℓ As ℓ (where 0.51 ≦ k ≦ 1, 0 ≦ ℓ ≦ 1, ℓ = 2.04k - 1.04) and having a refractive index n selected so as to satisfy a condition n a1 > n > n G > n cl where n al is the refractive index of said active layer, n G is that of said optical guiding layer with the diffraction grating, and n cl is that of said first and second cladding layers.

Inventors: KANEIWA SHINJI, KUDO HIROAKI, TAKIGUCHI HARUHISA, YOSHIDA TOSHIHIKO, KANEIWA, SHINJI, KUDO, HIROAKI, TAKIGUCHI, HARUHISA, YOSHIDA, TOSHIHIKO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1987
Main classification: H01S3
IPC codes: H01S5