A METHOD OF PRODUCING A SEMICONDUCTOR LASER

Abstract: A method of producing a semiconductor laser is provided which comprises a first time crystal growth process for producing a crystal growth layer (1, 2 3) including at least a semiconductor layer (2) including aluminum on a substrate (1), a process for producing a stripe groove (10) by conducting a selective etching to the crystal growth layer to expose a portion of the semiconductor layer (2) including aluminum, and a second time crystal growth process for producing a semiconductor layer (4) including no aluminum on the entire wafer surface including the exposed surface of the semiconductor layer (2) and further producing a semiconductor layer (5) including aluminum on the semiconductor layer (4) including no aluminum. The production of a high resistance layer (9) having bad crystallinity at the second time crystal growth is thus prevented.

Description: The present invention relates to a method of producing a semiconductor laser, and more particularly to that enables obtaining a high reproducibility and high reliability semiconductor laser. A prior art SBA laser is reported by Mihashi et al in a preprint of Associated Spring Meeting of Applied Physics Society related societies of 1985 30a-ZB-4. Figure 2 shows a cross-sectional view of an SBA (Self aligned laser with Bent Active layer) laser that is produced by utilizing MO-CVD method. In this figure, the reference numeral 1 designates a p type GaAs substrate, the reference numeral 2 designates a buffer layer comprising p type Al 0.43 Ga 0.57 As, the reference numeral 3 designates a current blocking layer comprising n type GaAs, the reference numeral 5 designates a first cladding layer comprising p type Al 0.43 Ga 0.57 As, the reference numeral 6 designates an active layer comprising undoped Al 0.07 Ga 0.93 As, the reference numeral 7 designates a second cladding layer comprising n type Al 0.43 Ga 0.57 As, the reference numeral 8 designates a contact layer comprising n type GaAs, the reference numeral 9 designates an absorbed oxygen film, the reference numeral 10 designates a stripe groove which is produced in a reverse trapezoid cross section at the current blocking layer 3. The reference numeral 11 designates an active region, the reference numerals 12 and 13 designate an n electrode and a p electrode fixed to the contact layer 8 and the substrate 1, respectively. The method of producing this SBA laser will be described briefly. At first, a buffer layer 2 and a current blocking layer 3 are grown on the substrate 1 by MO-CVD method in a first crystal growth. After the growth, a stripe groove 10 having a reverse trapezoid cross section is produced at the current blocking layer 3 using an etchant for etching only GaAs selectively. As a result of this selective etching, the buffer layer 2 comprising p type Al 0.43 Ga 0.57 As is exposed at the bottom of the strip groove. Next, in the second crystal growth using MO-CVD method a first cladding layer 5, an active layer 6, a second cladding layer 7, and a contact layer 8 are grown successively on the wafer having the strip groove 10. After the second crystal growth, an n type electrode 12 and a p type electrode 13 are produced on the contact layer 8 and at the surface of the substrate 1, respectively, by a method of such as vapor plating or spattering, thereby completing a SBA laser. In this SBA laser, a current flows through the stripe groove 10 produced at the current blocking layer 3, and the portion parallel with the substrate 1 positioned above the stripe groove 10 of the active layer 6 becomes an active region 11. Furthermore, by using a MO-CVD method, the active layer 6 can be bent in a configuration close to that of the stripe groove 10. In this bent portion, there arises a refractive index difference in the transverse direction, and thus a light confinement in the transverse direction is conducted effectively, thereby resulting in a low threshold current and a high efficiency. In this prior art method of producing a SBA laser, a buffer layer 2 comprising p type Al 0.43 Ga 0.57 As is exposed at the bottom of the stripe groove 10 at the etching for producing the stripe groove 10. Meanwhile, as Al has a nature to be easily combined with oxygen, a thin oxide layer is produced at the surface of the buffer layer 2 by an etching or a water washing process. Further, it is exposed to the air until a second crystal growth is conducted, and oxidation proceeds with the passage of time. Generally such an oxide film is not a dense one, and almost all of it is dispersed during the subsequent temperature increases that follow with MOCVD growth processing. Occasionally quite a thin oxide layer can be retained or, even if such an oxide layer is not retained, an absorbed oxygen film 9 can result from a high concentration of oxygen that will combine with the aluminium. Oxygen is taken into the crystal when a first cladding layer 5 comprising p type Al 0.43 Ga 0.57 As is directly grown on the buffer layer 2, thereby producing a high resistance layer including oxygen which deteriorates the crystalinity. Such a reduction in crystalinity and production of a high resistance layer worsen the reproducibility of the laser characteristics as well as lead to deterioration in other characteristics such as by increasing threshold current and operational current. Furthermore, the temperature rising at operation is increased, thereby leading to a further deterioration of the element, and affecting an unfavourable influence on reliability. The present invention is intended as a remedy. According to the present invention as defined in claim 1, a stripe groove is produced by selectively etching the semiconductor layer obtained by the first crystal growth, a semiconductor layer including Al is exposed, and a semiconductor layer including Al is produced on the semiconductor layer including Al, after growth of a semiconductor layer including no Al, during a second crystal growth process. Subsequent oxidation of material grown at the bottom of the stripe groove is substantially avoided. The production of a high resistance layer having poor crystalinity in the second crystal growth is thus prevented, and a semiconductor laser superior in reproducibility and reliability is obtained. Figure 1 is a diagram showing a cross-section of a semiconductor laser produced by a method embodying the present invention; and Figure 2 is a diagram showing a cross-section of a prior art semiconductor laser. An embodiment of the present invention will be described with reference to figure 1. Figure 1 shows a semiconductor laser produced by a method embodying the present invention. In Figure 1, the same reference numerals designate the same elements as those shown in Figure 2. The reference numeral 4 designates a buffer layer comprising p type GaAs as a semiconductor layer including no Al. Next, the production method will be described. At first, in the first crystal growth a buffer layer 2 and a current blocking layer 3 are successively grown on a substrate 1. After the growth, a stripe groove 10 is produced by photolithography technique and an etching in a similar manner to that in the prior art method. Since the buffer layer 2 is exposed by this etching, an absorbed oxygen film 9 combined with Al is produced at the surface thereof. A buffer layer 4, a first cladding layer 5, an active layer 6, a second cladding layer 7, and a contact layer 8 are successively grown thereon in the second crystal growth. After the growth of these layers, an n type electrode 12 and a p type electrode 13 are produced on the contact layer 8 and at the surface of the substrate 1, respectively, by vapour plating or sputtering. The thickness of the buffer layer 4 is desirable to be larger than 0.01 µm and less than the depth of the stripe groove 10, that is, the thickness of the current blocking layer 3. Also, in a similar manner to that in the prior art method, either a high concentration of oxygen is absorbed at the surface of the buffer layer 2 or an oxide film is produced thereon by etching or water washing during the production of the stripe groove 10. Furthermore, as the buffer layer 2 at the bottom of the groove is exposed to the outside air after the stripe groove 10 is produced, further oxygen is absorbed. A resulting absorbed oxygen film 9 is thus present at the start of the second crystal growth. In the present embodiment, however, since a buffer layer 4 comprising p type GaAs is grown on the buffer layer 2 before growing a first cladding layer 5 comprising p type Al 0.43 Ga 0.57 As, the production of a high resistance layer at the surface of the crystal growth is suppressed. This is because the oxygen absorbed in the buffer layer 2 is less taken up by the crystal due to the fact that the buffer layer 4 does not include Al that is easily oxidised. This makes it possible to obtain a semiconductor laser having improved characteristics. Al is described in the above-illustrated embodiment in which an SBA laser is employed. The present invention is applicable, for example, when an AlGaAs layer is regrown by the second crystal growth on the AlGaAs layer produced by the first crystal growth. This regrowing of layers over a GaAs buffer layer makes it possible to enhance the element characteristics easily.

Claims: A method of producing a semiconductor laser comprising: a first crystal growth process producing on a substrate (1) a crystal growth layer (2,3) including semiconductor layers at least one of which includes aluminium; a process for producing a stripe groove (10) for the active region (11) by conducting a selective etching of said crystal growth layer to expose a portion of said semiconductor layer (2) including aluminium; and a second crystal growth process producing a semiconductor layer (4) including no aluminium on the entire wafer surface including the exposed surface of said semiconductor layer (2), further producing a semiconductor layer (5) including aluminium on said semiconductor layer (4) including no aluminium, and producing further layers (6,7,8) completing the semiconductor laser. A method of producing a semiconductor laser as defined in claim 1, wherein: said substrate (1) comprises a first conductivity type GaAs; a buffer layer (2) comprising a first conductivity type Al x Ga 1-x As and a current blocking layer (3) comprising a second conductivity type GaAs or Al y Ga 1-y As are grown during said first crystal growth; and a buffer layer (4) comprising a first conductivity type GaAs, a first cladding layer (5) comprising a first conductivity type Al z Ga 1-z As, an active layer (6) comprising Al α Ga 1-α As, a second cladding layer (7) comprising a second conductivity type Al β Ga 1-β As, and a contact layer (8) comprising a second conductivity type GaAs are successively grown during said second crystal growth.

Inventors: MIHASI, YUTAKA MITSUBISHI DENKI KABUSHIKI KAISHA, NAGAI, YUTAKA MITSUBISHI DENKI KABUSHIKI KAISHA, OTA, YOICHIRO MITSUBISHI DENKI KABUSHIKI KAISHA, YAGI, TETSUYA MITSUBISHI DENKI KABUSHIKI KAISHA
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1987
Main classification: H01S3
IPC codes: H01S5