INTEGRATED LIGHT EMITTING/RECEIVING AMPLIFIER ELEMENT

Abstract: The integrated light emitting/receiving amplifier element comprises a phototransistor (3) over a semiconductor layer (107) at the top of a semiconductor laser (2) of a multilayered structure (101 to 107) formed over a semiconductor substrate (100). This integrated light emitting/receiving amplifier ele­ ment permits ready crystalline growth and requires only a simple manufacturing process.

Description: The present invention relates to an integrated light emitting/receiving amplifier element, which is a principal constituent element of optical communication, optical exchange and optical data processing systems. One of the basic circuits used for optical communications, optical exchange and optical data processing is a light amplifier circuit. Basic study is under way on this kind of circuit, which is expected to permit simpler structure than other conventional electronic circuits. Light amplifier circuits can be classified into combinations of a light receiving element and a semiconductor laser and others whose principal constituent element is a semiconductor laser. A light amplifier circuit of the first category, which is relatively easier to package and assemble and in itself less dependent on temperature can be even more simply structured by integrating the elements. An example of the integration of a light receiving element and a semiconductor laser is proposed by H. Beneking et al. in "GaAs-GaAlAs Phototransistor/Laser Light Amplifier" published in ELECTRONICS LETTERS, Vol. 16, No. 15, 1980, pp. 602 - 603. This report proposes, as a method to integrate the two kinds of element, to grow a semiconductor laser on one end face and a phototransistor on the other of a semiconductor substrate. An optical signal incident upon a light receiving face formed on the emitter side of the phototransistor is converted to a photo-current which is amplified in the phototransistor region and is supplied to the semiconductor laser through the common semiconductor substrate. Since about 2,000 electrons (or positive holes) are generated per photon in a usual phototransistor, even very weak optical signals can be regenerated and amplified. However, the structure wherein the semiconductor substrate is sandwiched between the semiconductor laser and the phototransistor requires the division of crystalline growth into at least two phases. Therefore, in this fabricating process, grinding into the desired wafer thickness is carried out before the second phase of crystalline growth, and the light receiving face, electrodes and so forth are formed after the crystalline growth. As a consequence, there arises the problem of a thinned wafer thickness and fabricating-difficulty. Accordingly there has been a call for a light amplifier element whose structure permits ready crystalline growth and integration by a usual process. In the article "Full Solid State Image Converter Based on Integration of Phototransistors and LEDs" of H. Beneking published in IEEE Electron Device Letters, Vol. EDL-2, No. 4, April 1981, pages 99 and 100, a multilayer structure is disclosed comprising on a substrate a phototransistor, an absorbtion layer and a LED. In this device phototransistors have been integrated with hetero-luminescent diodes to form an array. The high internal gain allows to construct an image converter with optical amplification. In the article "Monolithic Integrated Device for Light Amplification" of A. Sasaki et al.; proceedings of the 14th conference (1982 international) on solid state devices, Tokyo, 1982; Japanese Journal of Applied Physics, Vol. 22 (1983) supplement 22-1, pages 279 - 282, a monolithic light amplifier is described. This known device comprises a multilayered structure in which LED is arranged at the top of a phototransistor formed over a semiconductor substrate. An object of the present invention, therefore, is to provide an integrated light emitting/receiving amplifier element which permits ready crystalline growth and requires only a simple manufacturing process. This object is achieved by the features of the independent claim. First part of claim 1 corresponds to the teaching of US-A-4,349,906. The subclaims are directed to preferred embodiments of the invention. The subject matter of claims differs in particular from US-A-4,349,906, in that the phototransistor is formed over a first portion of the laser to extend an end phase of the semiconductor laser from the mid-point of the mesa and that the third electrode is provided in a portion of the laser other than the first portion. In this structure, by applying a bias current in a forward direction between the electrodes of the laser in the vicinity of the threshold of laser oscillation, the whole photocurrent from the phototransistor can be effectively utilized for laser oscillation. In the absence of such an electrode, a substantial part of photocurrent from the phototransistor, i.e. an optical signal does not serve as a signal because such a part is utilized to put the laser into the threshold of oscillation. Since the electrode on top of the laser feeds the biascurrent of the threshold to the laser, or the photocurrent from the phototransistor can function as a signal current. Fig. 1 shows an oblique view of a prior art embodiment of an integrated light emitting/receiving amplifier element; Fig. 2A is an energy diagram illustrating the state of the embodiment of Fig. 1 under bias when no light is coming in; Fig. 2B is another energy diagram illustrating the state of same when light is coming in, referring to light absorption, photocurrent and light emitting process at the same time; and Figs. 3 and 4 show oblique views of a first and a second (in part) preferred embodiment, respectively, of the present invention. Referring to Fig. 1, a prior art integrated light emitting/receiving amplifier element 1 involves a semiconductor laser 2 and a phototransistor 3. The semiconductor laser 2 further involves semiconductor layers 101 through 107, and the phototransistor 3, semiconductor layer 108 through 110. To manufacture this element 1, over a p-type (hereinafter abbreviated to p-) InP substrate 100, there are successively grown a p-InP clad layer 101, a non-doped InGaAsP (1.3 microns in bandgap wavelength) active layer 102 and an n-InP clad layer 103 to form a double hetero substrate, over which is formed a mesa 104, positioned between two grooves, by applying photoresist and using conventional photolithographic and etching techniques. Next, over this wafer are grown, by the liquid phase growth method, an n-InP first current blocking layer 105, a p-InP second current blocking layer 106, an n-InP burying layer 107, an n-InGaAs (1.6 microns in bandgap wavelength, 10¹⁵ cm⁻³ in impurity concentration) collector layer 108, a p-InGaAs (1.6 microns in bandgap wavelength, 10¹⁸ cm⁻³ in impurity concentration) base layer 109 and an n-InP (10¹⁷ cm⁻³ in impurity concentration) emitter layer 110 to complete crystalline growth. After forming an AuGeNi vaporized film 112 over the n-InP emitter layer 110, the central part of the vaporized film is etched with KI+I₂ solution by the usual photolithographic technique to expose the emitter layer through which light is to be injected into the phototransistor 3 and thereby to form a light receiving face 111. Then, in order to reduce the junction capacity of the phototransistor 3, the vaporized film 112, the n-InP emitter layer 110, the p-InGaAs base layer 109 and the n-InGaAs collector layer 108 are successively so etched by the conventional photolithographic and etching techniques with KI+I₂ solution, HCl-H₂O mixture, H₂SO₄-H₂O₂-H₂O mixture, and H₂SO₄-H₂O₂-H₂O mixture, respectively, that each of these layers form a 30-micron wide stripe over the mesa 104. The part of the film 112 surviving this etching process constitutes a first n-electrode 112. Then the bottom of the p-InP substrate is ground to a desired thickness, usually 100 to 150 microns, to form a p-electrode 113. After that, both ends of the mesa 104 are cleaved to form a resonator. Typical dimensions of this embodiment are 1.5 microns for the width of the mesa 104, 300 microns for the resonator length, 3 microns for the thickness of the collector layer 108, 0.5 micron for that of the base layer 109, 3 microns for that of the emitter layer 110, and 20 microns x 30 microns for the area of the light receiving face 111. By so applying a bias voltage to this integrated element 1 as to make the p-electrode 113 positive and the n-electrode 112 negative, having the light receiving face of the phototransistor 3 receive incident light and using a photocurrent multiplied by the phototransistor as injection current to the semiconductor laser, a laser output is provided from an end face of the mesa 104. Fig. 2A is an energy diagram illustrating the state of the embodiment of Fig. 1 when a bias voltage is applied in the absence of optical input, while Fig. 2B is another energy diagram illustrating the state of the embodiment in the presence of an optical input, referring to light absorption, photocurrent and light emitting process at the same time. In the absence of optical input, as shown in Fig. 2A, electrons in the n-emitter are prevented by the p-base barrier from flowing toward the n-collector. In the presence of incident light, as shown in Fig. 2B, the optical input is absorbed within the p-base, made of a material having a narrow bandgap for the wavelength of the incident light, resulting in the formation of electron-positive hole pairs. The positive holes so formed remain in the base to lower the emitter-base barrier against electrons, so that the electrons in the emitter massively flow toward the collector and are supplied to the semiconductor laser. In the active layer of the semiconductor laser, these electrons rejoin the positive holes supplied from the p-electrode formed on the p-type semiconductor substrate side, and generate light. Since this structure requires only the laying of a semiconductor laser and a phototransistor over a semiconductor substrate, crystalline growth can be achieved by the conventional method, needing no more than one stage of crystalline growth unless the semiconductor laser is buried or two stages even if it is buried. The manufacturing procedure is simple because what is needed is, after electrode formation on one face following crystalline growth, merely to grind the other face to a desired thickness. Fig. 3 shows an oblique view of a first preferred embodiment of the present invention, wherein an electrode 115 for injecting electrons into a semiconductor laser 2 is newly formed in the axial direction of a resonator to reduce the load on its phototransistor. Thus the phototransistor 3 is formed over approximately half the length of a mesa stripe 104 from an end face of the semiconductor laser 2; an n-InP layer 107 is subjected to Zn diffusion in a direction normal to that of the mesa stripe approximately in the middle of the mesa stripe to be converted into a p-type, and an electric separation layer (10 microns wide) 114 is formed as far as it reaches a second current blocking layer 106. A second n-electrode 115 is formed over an n-InP buried layer 107. The bottom of a substrate 100 includes the p-electrode 113. If, in this structure, a bias current in a forward direction is applied between the second n-electrode 115 and the p-electrode 113 in the vicinity of the threshold of laser oscillation, the whole photocurrent from the phototransistor 3 can be effectively utilized for laser oscillation. In the following a second preferred embodiment of the present invention is described. It combines the structures shown in figures 3 and 4. Fig. 4 shows an oblique view of one part of the preferred embodiment of the present invention, wherein another electrode is formed in a base layer 109 to control the current flowing through a phototransistor 3 with not only optical signals but also the current of the base layer 109. Crystalline growth and the formation of an n-electrode 112 and a light receiving face 111 of the phototransistor 3 are achieved in respectively the same manners as in the embodiment of Fig. 1. This embodiment involves the phototransistor 3, whose light receiving face 111 and electrode 112 are formed, behind the mesa axis of a multi-layered structure comprising a collector layer 108, a base layer 109 and an emitter layer 110, and an AuZn base electrode 116 formed over the base layer 109 before the mesa axis, where the emitter layer 110 is removed. In this structure, since the base electrode 116 enables a base current to flow as well, it is possible to control the current flowing through the phototransitor 3 with not only optical signals but also the base current. Accordingly, in the second preferred embodiment a second n-electrode 115 for injecting electrons into the semiconductor laser 2 and a base electrode 116 for controlling the current in the phototransistor 3 are provided. The embodiments of the present invention can have many different versions. The dimensions of the light receiving face 111 of the phototransistor 3 are not particularly limited. Nor are the shape, dimensions and material of the n-electrode 112. The stripe of the semiconductor laser need not be buried, but can be structured in many other ways, for instance in a planar stripe form. In the foregoing embodiments, the n-type and p-type can be reversed. Although InP/InGaAsP-based semiconductor materials are used in these embodiments, other semiconductor materials, including GaAlAs/GaAs-based ones, can also be used.

Claims: An integrated light emitting/receiving element comprising: a semiconductor substrate (100) of a first conductivity type; a semiconductor laser (2), formed on said semiconductor substrate (100), comprising a first semiconductor layer of (101) said first conductivity type, a second semiconductor layer (102) emitting light upon injection of an electric current, and a third semiconductor layer (103) of a second conductivity type opposite to said first conductivity type; a phototransistor (3) comprising a collector layer (108) of said second conductivity type, a base layer (109) of said first conductivity type and an emitter layer (110) of said second conductivity type, successively formed on said third semiconductor layer (103) of said semiconductor laser (2); a first electrode (113) formed on a side of said semiconductor substrate (100) opposite said first semiconductor layer (101); a second electrode (112) formed on said emitter layer (110), leaving an opening (111) through which to let incident light in, said light emitting/receiving element amplifying light by injecting into said semiconductor laser (2) a multiplied photocurrent generated by said phototransistor in response to incident light coming in through said opening; characterized in that said phototransistor (3) is formed on a first portion of said laser (2) and a third electrode (115) is formed on said third semiconductor layer (103) in a portion of said laser (2) other than said first portion, said laser (2) being biased substantially at the threshold level of laser oscillation through said third electrode (115). An integrated light emitting/receiving element as recited in claim 1, further comprising an electric separation layer (114) of a semiconductor material of said second conductivity type extending between said third electrode (115) and said phototransistor (3) and contacting said third semiconductor layer (103). An integrated light emitting/receiving element of claim 1 or 2, wherein said first, second and third semiconductor layers (101,102,103) are formed in a mesa structure, and further comprising at least one current blocking layer (105,106,107) formed to a side of said mesa structure and extending perpendicular to a direction of formation of the layers of said laser (2).-- An integrated light emitting/receiving element of any of claims 1 to 3, wherein the emitter layer (110) is formed on a first portion of the base layer (109) and a fourth electrode (116) is formed on a portion of the base layer (109) other than said first portion of the base layer (109).

Inventors: ODAGIRI, YUICHI, TERAKADO, TOMOJI
Applicants: NEC CORPORATION
Application year: 1985
Main classification: H01L31
IPC codes: H01S5, H01L27, H01L31, H03K17