Divided electrode type semiconductor laser device

Abstract: To provide a divided electrode type semiconductor layer device designed to improve the separation of elect­ rodes without reducing the amount of doping for carrier injection layers, a double-hetero-structure is formed on a semi-insulating substrate, and at least two pairs of carrier injection clad layers (109a, 109b; 111a, 111b) are thereafter buried like islands while a high-resistance portion (105) is left between the carrier injection clad layers to electrically separate these layers. Electrodes (113a,113b; 115a,115b) are respectively formed on the separated carrier injection layers (109a,109b; 111a,111b)

Description: This invention relates generally to semiconductor laser devices for producing laser light and, more particularly, to a divided electrode type semiconductor laser device designed to facilitate and improve the separation of the electrodes. Fig. 2 shows the structure of a lateral junction type embedded semiconductor laser device of the kind shown in EP 0 342 018, which is a document falling within the term defined by Art 54(3) EPC. This laser device includes a double-hetero-structure which is formed on a semi-insulating substrate 131 and in which a GaAs active layer 137 is interposed between undoped AlGaAs clad layers 133 and 135. For carrier injection, a p-AlGaAs clad layer 139, a p-GaAs cap layer 140, an n-AlGaAs clad layer 141 and an n-GaAs cap layer 142 are formed and embedded on the left-hand and right-hand sides of the GaAs active layer 137 respectively as viewed in Fig. 2, to form carrier injection layers 113 and 115 and electrodes 143 and 145 are formed on these layers. In the thus-constructed laser device, a current flowing through the electrodes 143 and 145 is confined between the undoped AlGaAs clad layers 133 and 135 having a high resistance, and carriers are thereby injected into the GaAs active layer 137 having a smaller energy band gap, so that light is generated by recombination of electrons and holes. The generated light is confined between and guided by the upper and lower, left and right clad layers, thereby emitting laser light in the direction perpendicular to the plane of projection of Fig. 2. This laser device is specifically suitable for integration because of the construction in which the device is constructed on the semi-insulating substrate 131, the carrier injection clad layers are positioned on the opposite sides, and the electrodes are independent of the substrate. Fig. 3 is a top view of the semiconductor laser device shown in Fig. 2. As shown in Fig. 3, leads 147 and 149 are connected to the electrodes 143 and 145, a current is supplied through these leads to inject carriers from the buried carrier injection layers 115 and 113 into the active layer, thereby emitting laser light from opposite ends of the device. This lateral-junction embedded semiconductor laser device may be divided into a plurality of elements which are independently used for oscillation, control and so on to effect pulse oscillation or the like by controlling laser oscillation. For the manufacture of such a tandem electrode type laser, it is necessary to divide each electrode of the semiconductor laser device into a plurality of elements. Conventionally, the electrodes are divided in such a manner that, as shown in Fig. 4, the carrier injection clad layer and the electrodes are partially removed by gas-phase or liquid-phase etching using HCl gas or H₂O₂ to form grooves 151 which separate the electrode 143 into electrodes 143a and 143b and the electrode 145 into electrodes 145a and 145b. However, in a case where a lateral junction type buried semiconductor laser has electrodes divided by etching in this way, the active layer into which electrons and holes are injected from the left and right clad layers has a small thickness of about 0.2 µm and a large width of about 2 µm, and the forward-direction resistance tends to increase. It is therefore necessary to increase the amount of doping for the n-type and p-type clad layers and to form n-type and p-type GaAs cap layers for reducing the resistance of contact with the electrodes. To satisfy these requirements, the facility with which the electrodes are separated as shown in Fig. 4 is sacrificed. To improve the separation based on etching, it is necessary to etch the outer layers to a depth very close to the active layer. A portion of the active layer may be removed if the extent of etching is excessive, or the electrode cannot be suitably separated if the extent of etching is insufficient. The process of separating the electrodes is thus difficult. US-A-4 700 353 discloses lateral junction type quantum well lasers arranged in tandem on a conducting substrate, wherein modulation is achieved by applying voltage between a gate electrode and the substrate. In view of these problems, an object of the present invention is to provide a semiconductor laser device designed for suitable electrical separation of the electrodes without reducing the amount of doping for the n-type and p-type clad and cap layers and without increasing the forward-direction resistance of the p-n junction. To achieve this object, according to the present invention, there is provided a lateral junction type embedded semiconductor laser device as defined in claim 1. An embodiment of the present invention will be described by way of example below with reference to the accompanying drawings in which: Fig. 1 is a diagrammatic plan view of the structure of a lateral junction type embedded semiconductor laser device in accordance with the present invention; Fig. 2 is a cross-sectional view taken along the line A - B of Fig. 1, showing the part of the structure which is common to the known device the direction of the section being indicated by the presence of the letters A and B in Fig. 2. Fig. 3 is a plan view of a known laser device having the cross-section shown in Fig. 2; and Fig. 4 is a diagram of a state of the art laser device of Fig. 3 in which the electrodes are divided by etching. Referring to the drawings, the semiconductor laser device shown in Figs.land 2 has a semi-insulating substrate 131, undoped AlGaAs clad layers 133 and 135, a GaAs active layer 137, a p-AlGaAs clad layer 139, p-GaAs cap layer 140, an n-AlGaAs clad layer 141, an n-GaAs cap layer 142, an n-electrode 143 and a p-electrode 145. As shown in Fig. 2, a double-hetero-structure in which the GaAs active layer 137 is interposed between the undoped AlGaAs clad layers 133 and 135 is formed on the semi-insulating substrate 131, left and right portions of this structure is removed, and the p-AlGaAs clad layer 139, the p-GaAs cap layer 140, the n-AlGaAs clad layer 141 and the n-GaAs cap layer 142 are selectively grown as carrier injection clad layers 113 and 115 in place of the removed portions. This selective growth of the carrier clad injection layers is effected in such a manner that the carrier injection clad layers are embedded like islands as shown in Fig. 1 so that the high-resistance undoped clad layer 135,105 remains between the clad layers 139 and 141. The electrodes are formed on each of the carrier injection clad layers thus electrically separated. That is, the embedded p-type carrier injection layer 113 is separated into layers 113a and 113b while the buried n-type carrier injection layer is separated into layers 115a and 115b, and p-electrodes 145a and 145b and n-electrodes 143a and 143b are respectively formed on these separated layers. The electrodes are suitably separated electrically since the high-resistance undoped clad layer 135,105 remains between the electrodes 145a, 145b, 143a and 143b. It is thus possible to improve the electrical separation of the electrodes in a simple manner without reducing the amount of doping for the embedded p-type layers 113a and 113b and the embedded n-type layers 115a and 115b and, hence, without increasing the forward-direction resistance of the p-n junction. In the above-described embodiment, the n- and p-clad layers are separated into two elements but, needless to say, it is possible to freely select the number of elements into which these layers are separated. In accordance with the present invention, as described above, each of the electrodes respectively connected to the p-type and n-type layers can be easily divided without reducing the amount of doping for the embedded n-type and p-type carrier injection layers and, hence, without increasing the forward-resistance of the p-n junction.

Claims: A semiconductor laser device comprising a double-hetero-structure formed on a semi-insulating substrate (131) and in which a single active layer (137) is interposed between a lower undoped layer (133) and an upper undoped AlGaAs clad layer (135,105) and carrier injection clad layers (113,115) are embedded like islands to form a plurality of lateral junction type embedded semiconductor laser structures arranged in tandem, the said carrier injection clad layers (113,115) of each said laser structure including a p-type and an n-type AlGaAs clad layer (139,141) on each of which a GaAs cap layer (140,142) is formed, and the arrangement being such that the said carrier injection clad layers of each laser structure are separated by a first area (135) of said undoped AlGaAs clad layer and that said carrier injection clad layers of adjacent tandem laser structures are separated by further areas (105) of said undoped AlGaAs clad layer, and that electrodes associated with said cap layers form the only electrodes of the device.

Inventors: GOTOH HIDEKI, SHIMADA JUNICHI, SHIMOYAMA KENJI, SUZUKI YOSHIHIRO, YAJIMA HIROYOSHI, GOTOH, HIDEKI, SHIMADA, JUNICHI, SHIMOYAMA, KENJI, SUZUKI, YOSHIHIRO, YAJIMA, HIROYOSHI, GOTOH, HIDEKI MITSUBISHI KASEI CORPORATION, SHIMADA, JUNICHI AGENCY OF IND.SCIENCE AND TECHN., SHIMOYAMA, KENJI MITSUBISHI KASEI CORPORATION, SUZUKI, YOSHIHIRO AGENCY OF IND.SCIENCE AND TECHN., YAJIMA, HIROYOSHI AGENCY OF IND.SCIENCE AND TECHN.
Applicants: AGENCY IND SCIENCE TECHN, MITSUBISHI CHEM CORP, DIRECTOR-GENERAL OF THE AGENCY OF INDUSTRIAL SCIENCE AND TECHNOLOGY, MITSUBISHI CHEMICAL CORPORATION
Application year: 1989
Main classification: H01S3
IPC codes: H01S5