Laser diode and method for fabricating of the same

Abstract: A laser diode comprises a substrate (1), a first blocking layer (2) having a groove (18), an alternate combination of a lower cladding layer (7) and a second blocking layer (2a), an active layer (9) formed in the groove (18), and an upper cladding layer (11). A light-amplifying region and a light-absorbing region are formed alternately in the waveguide. Both regions are disposed periodically at a period corresponding to bragg wavelength.

Description: This invention relates to a laser diode and a fabrication method thereof, more particularly to a laser diode which radiates with a selected light wavelength. Laser diodes are employed for light source in optical transmission systems for example. In order to decrease influence of wavelength dispersion in optical fibers, laser diodes are required to radiate with a single wavelength. For the requirement, diffraction gratings are fabricated in waveguides of the diodes. A distributed feedback laser diode (DFB-LD) employing the grating is described in "Optical Integrated Circuit - foundation and application", April 1988, Asakura shoten, Japan, for example. In the prior DFB-LD, thickness of a medium is periodically changed to perform the grating. Thereby refractive index of a waveguide region is periodically changed. Then a light wavelength is selected in correspondence with the period of the grating, so that the DFB-LD radiates with the selected wavelength. However, the prior DFB-LD has a stop band with bragg wavelength corresponding to the grating. Accordingly radiation is induced with two wavelengths around the bragg wavelength. SUMMARY OF THE INVENTION It is an object of the present invention to provide a laser diode which radiates with a single wavelength, and a method for fabricating the same. Another object of the invention is to provide a DFB-LD which selects single wavelength, and a method for fabricating the same. According to the present invention, a light amplification region and a light absorption region are formed alternately in the waveguide. Both regions are disposed at a period Λ corresponding to the bragg wavelength. Thus light feedback utilizing gain coupling is performed, so that the DFB-LD of this invention radiates with single wavelength. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 to 3 are a plan view and sectional views of an embodiment of a DFB-LD according to the present invention. Fig. 4A to 4F and Fig. 5A to 5F illustrate a fabrication process of the embodiment according to the invention. DETAILED DESCRIPTION OF THE DRAWINGS An embodiment of DFB-LD according to the present invention is illustrated in Fig. 1 to 3. Fig. 1 shows a plan view, Fig. 2 shows a sectional view along A-A line in Fig. 1, and Fig. 3 shows a sectional view along B-B line in Fig. 2. A first blocking layer 2 consisting of Fe-doped InP is formed on an n-Inp substrate 1. A stripe-like groove 18 is fabricated into the first blocking layer 2. As shown in Fig. 3, in the groove 18 and on the blocking layer 2, a guide layer 8, an active layer 9 and a buffer layer 10 are formed successively. The three layers consist of undoped InGaAsP. The guide layer 8 and the buffer layer 10 are employed in consideration of a characteristic of a DFB-LD. The active layer 9 on the first blocking layer 2 is not utilized for laser radiation. In the groove 18, as shown in Fig. 2, a lower cladding layer 7 consisting of n-InP and a second blocking layer 2a are disposed along the active layer 9. The second blocking layer 2a is a part of the first blocking layer 2. The lower cladding layer 7 and the second blocking layer 2a are disposed alternately at a specified same period Λ=mλ/2n e . Where m is positive integer; λ is light wavelength; n e is effective index. The period Λ corresponds to bragg wavelength of the DFB-LD. An upper cladding layer 11 consiting of p-InP is formed on the buffer layer 10, and a contact layer 12 consisting of p-InGaAsP is formed thereon. An SiN flim 13 having a contact window 14 is formed on the contact layer 12. An AuZn electrode 15 is connected to the contact layer 12 through the window 14. An AnGeNi electrode 16 is formed on a bottom surface of the substrate 1. In the structure described above, when prescribed voltage is applied to the electrodes 15 and 16, a current flows to the active layer 9 through only the lower cladding layer 7. In the active layer 9, a current injection region occurs above the lower cladding layer 7, and a non-injection region occurs above the second blocking layer 2a. Accordingly, amplification and absorption of light are repeated in light proceeding direction. As described above, light feedback by such alternate formation, i.e. gain coupling is utilized in the embodiment. The use of the gain coupling does not occur a stop band, so that single wavelength of laser radiation can be selected. Next a fabrication method of the invention will be described. Fig. 4A to 4F and Fig. 5A to 5F are diagrams illustraring the fabrication process of the embodiment. Fig. 4A to 4F are sectional views corresponding to Fig. 2. Fig. 5A to 5F are sectional views corresponding to Fig. 3. Firstly, as shown in Fig. 4A and 5A, a first blocking layer 2(2.5 µm) consisting of Fe-doped InP is formed on a n-InP substrate 1 by chemical vapor deposition (CVD). An SiN film 31 (2000Å) is formed on the first blocking layer 2 by plasma CVD. A positive resist 41 as a mask(1 µm) is coated on the SiN film 31 and is patterned into prescribed pattern by photolithography. Then the SiN film 31 is removed selectively by dry etching with the pattern of the resist 41, so that a stripe-like opening 17 (1.5 µm width) is formed. As shown in Fig. 4B and 5B, the resist 41 is removed. Then the first blocking layer 2 is dry-etched with the SiN film 31 as a mask to fabricate a stripe-like groove 18. The groove 18 is 0.5 µm depth for example and does not pass through the blocking layer 2. Combination gas of Ar and Cl₂ for example is utilized for the dry etching. As shown in Fig. 4C and 5C, a second SiN film 32 (1000Å) is formed in the groove 18 and on the first SiN film 31 by plasma CVD. Then a positive resist 42 (500 Å) for electron beam is coated on the above structure. Then a resist pattern 5 (2400Å pitch) for a grating is formed by interference exposure method and development. As shown in Fig. 4D and 5D, the second SiN film 32 is removed selectively by etching with the pattern of the resist 42 as a mask, utilizing HF solution. The resist 42 is then removed. By dry etching with the second SiN film 32 as a mask, a recess 6 reaching the substrate 1 is formed. Therefore a second blocking layer 2a is formed periodically as shown in Fig. 4D. The etching gas is combination of Ar and Cl₂. In Fig. 4D, the recess 6 is about 1200Å width and about 3 µm depth. As shown in Fig. 4E and 5E, a lower cladding layer 7 consisting of n-InP is formed in the recess 6 by MOCVD. In Fig. 5E, a top of the cladding layer 7 is below a top of the first blocking layer 2. The gap is about 0.3 µm. The SiN films 31 and 32 are then removed. As shown in Fig. 4F and 5F, in the groove 18 and on the first blocking layer 2, a guide layer 8 (0.1 µm thick, λg = 1.3 µm), an active layer 9(0.1 µm thick, λg = 1.55 µm) and a buffer layer 10 (0.1 µm thick, λg = 1.3 µm) are formed successively by liquid phase epitaxy. These layers 8,9 and 10 consist of undoped InGaAsP. Where λg is wavelength corresponding to a bandgap of each layer. Then an upper cladding layer 11 (about 1 µm) consisting of p-InP and a contact layer 12 (0.5 µm) consisting of p-InGaAsP are formed. Then a SiN film 13 (2000 Å) is formed on the contact layer 12 by plasma CVD. The SiN film 13 is removed selectively by photolithography, so that a stripe-like contact window 14 is formed. Finally an AuZn electrode 15 is formed on the contact layer 12, and an AuGeNi electrode 16 is formed on a bottom of the substrate 1. The scope of this invention is not limited to the embodiment described above, but includes many modifications and variations which will be obvious to one skilled in the art. For example, the lower cladding layer 7 may be fabricated in the groove 18 without recess 6, by impurity diffusion or ion implantation.

Claims: A laser diode comprising a substrate (1), a first blocking layer (2) disposed over said substrate (1), said first blocking layer (2) having a stripe-like groove (18), a wave guide layer disposed in said groove (18), an active layer (9) disposed over said wave guide layer in said groove (18), and a first cladding layer (11) disposed over the active layer (9), characterized in that said wave guide layer comprises a second blocking layer (2a) and a second cladding layer (7), said second blocking layer (2a) and said second cladding layer (7) being disposed alternately in the longitudinal direction in the groove (18) at a period of the Bragg wavelength. A laser diode according to claim 1, further comprising a contact layer (12) disposed over said first cladding layer (11), a first electrode (15) electrically connected to said contact layer (12), and a second electrode (16) electrically connected to said substrate (1). A process for fabricating a laser diode comprising the steps of: providing a substrate (1) having a first surface and a second surface being substantially opposite said first surface; forming a first blocking layer (2) on said first surface of said substrate (1); forming a stripe-like groove (18) in said first blocking layer (2); forming a second cladding layer (7) and a second blocking layer (2a) alternately in the longitudinal direction in said groove (18) at a period of the Bragg wavelength; forming an active layer (9) over said first cladding layer (7) and said second blocking layer (2a); and forming a first cladding layer (11) over said active layer (9). A process for fabricating a laser diode according to claim 3, further comprising the steps of: forming a contact layer (12) on said first cladding layer (11); forming a first electrode (15) over said contact layer (12) to be electrically connected therewith; forming a second electrode (16) on said second surface of said substrate (1).

Inventors: HOSOI YOJI C O OKI ELECTRIC IN, TSUBOTA TAKASHI C O OKI ELECTR, HOSOI, YOJI, C/O OKI ELECTRIC IND. CO.-LTD., TSUBOTA, TAKASHI, C/O OKI ELECTRIC IND. CO.-LTD.
Applicants: OKI ELECTRIC IND CO LTD, OKI ELECTRIC INDUSTRY CO., LTD.
Application year: 1990
Main classification: H01S3
IPC codes: H01S5