Method for Manufacturing a Buried Hetero-Junction Type Semiconductor Laser

Abstract: A semiconductor laser has a semiconductor substrate (1) with a feature (1a) on its surface. The feature (1a), either a projection or a groove, forms a mesa or a mesa groove having relatively sharp edges. Active layers (4) and cladding layers (2, 5) are deposited on this semiconductor substrate (1). The cladding layer (2) deposited directly on the semicon­ ductor substrate (1) has at least one slope (2a) opposite the mesa or mesa groove, on which slope (2a) no active layer is formed during epitaxial growth. The active layer (4) im­ mediately above the cladding layer (2) is prevented from depositing on the slope (2a) of the cladding layer (2) so that no etching process for removing sections of the active layer (4) is necessary. This allows deposition of the active layers (4) and cladding layers (2, 5) on the semiconductor substrate (1) in an uninterrupted sequence of single epitaxial growth steps.

Description: The present invention relates to a method for manufacturing a buried hetero-junction type (BH) semiconductor laser, preferably made of AlGaAs. A method for manufacturing such lasers by liquid phase epitaxy is described in an article published by M. Sugimoto et al. in Journal of Lightwave Technology, Vol. LT-2, No. 4, August 1984, pages 496 - 503 under the title "InGaAsP/InP Current Confinement Mesa Structure Buried Heterostructure Laser Diode Fabricated by One-Step Liquid-Phase Epitaxy". Another such laser as described in an article by Kishino et al., published in IEEE Journal of Quantum Electronics, Vol. OE-16, No. 2, February 1980, pages 160 - 164, under the title "Fabrication and Lasing Properties of Mesa Substrate Buried Heterostructure GaInAsP/InP Lasers at 1.3 µm Wavelength" comprises an insulating layer located between a cap layer and a first electrode with a channel passing through the insulating layer at a position opposite the mesa-structure of the first surface of the semiconductor substrate. It is the object of the present invention to provide a method for manufacturing a BH semiconductor laser having current confinement means and having an active layer of well defined narrow width and being isolated as well as possible within the cladding material. The method of the present invention is defined in claim 1. Embodiments of the method of the present invention are defined in the dependent claims. The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to limit the invention to the specific embodiment, but are for explanation and understanding only. Fig. 1 is a cross-section through a semiconductor laser as grown by an embodiment of the method of the present invention; and Fig. 2(A) to 2(G) show the sequence of production steps of the embodiment of the method of the present invention used to fabricate the semiconductor laser of Fig. 1. Referring now to the drawings, particularly to Fig. 1, a semiconductor laser manufactured with the method of the present invention has a conductive n-type semiconductor substrate 1. The semiconductor substrate 1 is an n-type compound, for example and its surface is a (100) crystal face. An etch-mask mesa is deposited on the surface of the semiconductor substrate 1. In practice, a plurality of mesas la run parallelly across the surface of the semiconductor substrate in a direction perpendicular to the section plane. A first cladding layer 2 is deposited on the (100) face of the semiconductor substrate. The first cladding layer 2 is preferably a conductive n-type layer. An optical waveguide layer 3 and an active layer 4 are deposited on the first cladding layer 2. The optical waveguide layer 3 is not always necessary and may be used when desired. The active layer 4 is of lightly doped or undoped material. A second cladding layer 5 lies above the active layer 4. The second cladding layer 5 is of the conductive p-type. A conductive p-type cap layer 6 is formed above the second cladding layer. A conductive n-type auxiliary current-channelling layer 7 covers the cap layer 6 incompletely, as is explained below. The first and second cladding layers 2 and 5, the optical waveguide layer 3, the active layer 4, the cap layer 6 and the auxiliary current-channelling layer 7 are all deposited on the semiconductor layer 1 by epitaxial growth performed by MOCVD, metal-organic chemical vapour deposition. The first and second cladding layers 2 and 5 have a smaller refraction index and a wider inhibiting band than the active layer. The form of the mesa 1a on the (100) crystal face of the semiconductor substrate 1 is selected with respect to the crystal structure so that the first cladding layer 2 has sloping faces 2a on both sides of the mesa 1a. The sloping faces 2a extend along the longitudinal edges of the mesa and are of the (111) B crystal face. The (111) B crystal face grows relatively slowly in epitaxial growth, i.e. as much as 100 times as slow as the (100) crystal face. Therefore, during epitaxial growth of the aforementioned layers, there will be substantially no growth on this (111) B crystal face. Therefore, the optical waveguide layer 3 and the active layer 4 which are much thinner than the other layers do not appear at all on the (111) B crystal face. Therefore, the optical waveguide layer 3 and the active layer 4 have discontinuous breaks on the (111) B crystal faces. The second cladding layer 5 can be grown on the first cladding layer 2 at points opposite the (111) B crystal faces simply by overgrowth from the portions which do not oppose the (111) B crystal faces. The thickness of the second cladding layer 5 overall is chosen so as to deposit a sufficient thickness of the second cladding layer at the portions opposite the (111) B crystal faces. The auxiliary current-channelling layer has cut-out 7a. An electrode 8 is deposited over the auxiliary current-channelling layer. Part of the electrode 8 passes through the cut-out 7a and makes an ohmic connection with cap layer 6. In order to form oscillation region or resonating space on the mesa 1a, the cut-out of the auxiliary current-channelling layer directly opposes the active layer 4 above the mesa 1a. Therefore, the cut-out 7a in the auxiliary current-channelling layer extends longitudinally along the axis of the mesa la of the semiconductor substrate 1. As shown in Fig. 1, another electrode 9 is deposited on the opposite surface of the semiconductor substrate 1. Figs. 2(A) to 2(G) illustrate the process of producing the aforementioned preferred embodiment of the semiconductor laser according to the invention, for facilitating better understanding of the invention. The process shown in Figs. 2(A) to 2(G) is specifically adapted to form a AlGaAs (III-V family compound) semiconductor laser. First, the semiconductor substrate 1 is fabricated, which is an n-type, III-V family compound semiconductor substrate, for example n-type GaAs compound semiconductor substrate. The major surface 1b of the semiconductor substrate 1 is a (100) crystal face. An etching mask ribbon 11 of a desired width W is formed on the major surface 1b of the semiconductor substrate 1. This etching mask 11 is fabricated by well-known processes, including the steps of depositing a photo-resist layer on the semiconductor substrate, subjecting the photo-resist layer the pattern exposure, and developing. In this case, the face lying parallel to the plane through which the section of Fig. 1 is given is selected to be the (011) face. The longitudinal axis of the etching mask 11 is perpendicular to the (011) face, as shown in Fig. 2(a). Then, etching is performed by means of an etching agent, such as phosphoric etching solution. The etching agent is applied to the major surface 1b of the semiconductor substrate 1 on which the photo-resist layer was deposited and pattern-exposure and developing were performed. The portion of the semiconductor substrate 1 which was not masked by the etching mask 11 is eroded to form numerous parallel mesas 1a, each as shown in Fig. 2(B). The mesa 1a thus formed has (111) A faces on both receding faces. After this, the etching mask 11 is removed and the first cladding layer 2 is formed by epitaxial growth. The first cladding layer 2 is made of n-type Al x Ga 1-x As. (111) B crystal faces naturally grow on from the receding faces of the mesa 1a. The slopes 2a with the (111) B crystal faces lie at an angle ϑ relative to the horizontal upper surface of the mesa 1a. The angle ϑ is approximately 55°. Once the epitaxial growth reaches the state shown in Fig. 2(C) growth of the first cladding layer 2 is stopped. At this time, the deposition material gas is switched and subsequently MOCVD is performed to deposit the optical waveguide layer 3 and the active layer 4, as shown in Fig. 2(D). The active layer 4 is made of Al y Ga 1-y As. As described above, the epitaxial growth rate on (111) B crystal faces is relatively slow as set forth above. Therefore, the optical waveguide layer 3 and the active layer 4 deposited on the (111) B crystal face slopes 2a of the first cladding layer 2 will be of negligible thickness. Therefore, the optical waveguide layer and the active layer 4 will have breaks along the slopes 2a of the first cladding layer. Then, the deposition material is again changed and the 2nd cladding layer 5 is added by epitaxial growth. In this embodiment, the second cladding layer 5 is made of p-type Al x Ga 1-x As. In this case, as shown in Fig. 2(E), the second cladding layer 5 is not deposited on the (111) B crystal face slopes 2a of the first cladding layer 2 in the initial stages of growth. During this initial growth period, the second cladding layer 5 is built up over the active layer 4. Atop the mesa, the second cladding layer 5 grows to have slopes substantially in alignment with the slopes of the first cladding layer 2. At the same time, the second cladding layer 5 on either side of the mesa 1a starts to build up slowly along the slopes 2a at the portions c in Fig. 2(E). By further growth, the second cladding layer 5 comes to cover the slopes 2a of the first cladding layer 2, as shown in Fig. (F). Thus the second cladding layer 5 covers the entire surface including the active layer 4. Then, again the deposition material gas is changed for deposition of the cap layer 6. In the preferred embodiment, the cap layer 6 is made of highly-doped p-type GaAs. Thereafter, the auxiliary current-channelling layer of Al z Ga 1-z As is deposited over the cap layer 6. The cut-out 7a in the auxiliary current-channelling layer 7 is formed by etching, as shown in Fig. 2(G). The cut-out 7a exposes a small area of the second cladding layer 5. Then, as shown in Fig. 1, the first and second electrodes 8 and 9 are formed on both surfaces of the finished assembly. Part of the electrode 8 passes through the cut-out 7a and is in contact with the second cladding layer 5. As will be appreciated herefrom, according to the present invention, MOCVD can be performed continuously to form different layers only by changing the gaseous deposition material. Thus, fabrication involves only a sequence of continuous MOCVD steps, which simplifies the production process of the semiconductor laser. According to embodiments of the method of the present invention, the first and second cladding layers 2 and 5 are made of Al x Ga 1-x As and the active layer 4 is made of Al y Ga 1-y As, where x > y. It should be noted that the optical waveguide layer 3 can be made of a material richer in A1 than the active layer 4. In this case, the AlGaAs layer forming the optical waveguide layer 3 will have a wider inhibiting band. Furthermore, as set forth above, the present invention is applicable to semiconductor lasers with no optical waveguide layers. As will be appreciated herefrom, the thickness of the second cladding layer is determined by the width of the top surface of the mesa and the thickness of the first cladding layer. By selecting these dimensions appropriately, an adequately narrow resonating region above the mesa can be ensured without the need for an etching process.

Claims: A method for manufacturing a buried hetero-junction type semiconductor laser, said method comprising the following steps: etching a (100) surface of a substrate (1) along the [011] direction to form an inverted mesa-structure; growing a first lower cladding layer (2) onto said substrate, which cladding layer grows with (111)B surfaces along said mesa-structure; growing an active layer (4) onto the (100) surfaces of said lower cladding layer whereby discontinuous breaks are induced in said active layer on said (111)B surfaces of said first lower cladding layer; growing a second upper cladding layer (5) onto and so as to completely cover the so far grown layers; growing a cap layer (6) onto said second upper cladding layer; forming a first electrode (8); and forming a second electrode (9) at the surface of said semiconductor substrate which is opposite to the surface onto which said layers are grown; wherein all crystal growing steps are performed by MOCVD; and an insulating layer (7) is formed atop said cap layer before said first electrode is formed with a channel (7a) passing through said insulating layer (7) at a position opposite and along said inverted mesa-structure, through which channel said first electrode can oppose a section of said active layer atop said inverted mesa-structure, thereby forming a resonating region therebetween. A method according to claim 1, characterized in that a semiconductor substrate (1) of an n-type III-V-compound is used. A method according to claim 2, characterized in that said first lower cladding layer (2) is made of n-type Al x Ga 1-x As and said second upper cladding layer (5) is made of p-type Al x Ga 1-x As. A method according to claim 3, characterized in that said active layer (4) is made of Al y Ga 1-y As, whereby x > y. A method according to one of the claims 1 to 4, charac terized in that an optical waveguide layer (3) is grown onto the (100) surfaces of said first lower cladding layer (2), and said active layer (4) is grown onto said optical waveguide layer (3).

Inventors: MORI YOSHIFUMI, OKADA TSUNEKAZU, MORI, YOSHIFUMI, OKADA, TSUNEKAZU
Applicants: SONY CORP, SONY CORPORATION
Application year: 1986
Main classification: H01S3
IPC codes: H01S5