Opto-electronic integrated circuit having a transmitter of long wavelength

Abstract: A long wavelength transmitter OEIC includes a transverse direction current injection type semiconductor laser and a high electron mobility transistor which are integrated on a semi-insulating substrate. The semiconductor laser includes at least an AlGaInAs lower cladding layer, a quantum well active layer and a high resistivity AlGaInAs upper cladding layer successively grown on the semi-insulating substrate, disordered regions formed in the quantum well active layer by diffusions of p type and n type dopant, and an active region sandwiched by the disordered regions. The transistor includes an operating layer and a carrier supplying layer both comprising AlGaInAs series material and formed on the high resistivity AlGaInAs upper cladding layer. This transistor uses the upper cladding layer as a leakage current preventing layer. This structure can be formed by only one epitaxial growth, resulting in low costs. In addition, since the above layers are successively grown on a flat substrate, photolithography process for forming a gate of HEMT can be performed on a flat surface, so that a fine gate pattern can be formed at high precision. As a result, a transmitter OEIC performing high speed modulation can be expected.

Description: The present invention relates to a method for producing transmitter opto-electronic integrated circuit (hereinafter referred to as transmitter OEIC) in which a semiconductor laser and an electronic device are integrated, particularly a long wavelength transmitter OEIC in which a semiconductor laser element which oscillates at a wavelength of invisible light band and an electronic switching element are integrated. It also relates to an integrated circuit of this kind. BACKGROUND OF THE INVENTION Figure 3 is a cross-sectional view showing a prior art long wavelength transmitter OEIC described in pages 190 and 191 of Second Optoelectronics Conference (OEC' 88) Technical Digest. In figure 3, reference numeral 1 designates a semi-insulating InP substrate. A semiconductor laser 17 and a field effect transistor 18 (hereinafter referred to as FET) for driving the semiconductor laser 17 are integrated on the substrate 1. An n type InGaAsP layer 14 on which an n side electrode 9 of the semiconductor laser is attached is disposed on the concave part of the InP substrate 1 which is formed by an etching. An n type InP cladding layer 2' is disposed on the n type InGaAsP layer 14. An InGaAsP active layer 3' is disposed on the ridge part of the cladding layer 2'. A p type InP cladding layer 4' is disposed on the active layer 3'. A current blocking layer 15 is disposed on the cladding layer 2' to bury the ridge part. A contact layer 16 is disposed on the current blocking layer 15. A p side electrode 8 of the semiconductor laser is disposed on the contact layer 16 and an n side electrode 9 is disposed on a region of the n type InGaAsP layer 14 where the laminated-layers of the laser are not present. An undoped GaAs buffer layer 13 is disposed on a part remaining as a convex part of the substrate 1. An n type GaAs operating layer 5' is disposed on the buffer layer 13. A gate electrode 12 of the FET is disposed on the bottom of recess groove formed on the operating layer 5'. A source electrode 10 and a drain electrode 11 are disposed on opposite sides of the recess groove. Description is given of the operation hereinafter. This transmitter OEIC operates in a state where the n side electrode 9 of semiconductor laser 17 is connected with the source electrode 10 of FET 18 by a wiring. The operations of the semiconductor laser 17 and the FET 18 are the same as those of the independent elements. More specifically, the driving current of semiconductor laser 17 is subjected to a modulation such as an interruption by a voltage applied to the gate 12 of FET 18 and the light output of semiconductor laser 17 is modulated correspondingly, thereby resulting in an operation as a transmitter OEIC. A method for manufacturing the transmitter OEIC of figure 3 is illustrated in figures 4(a) to 4(i). A portion of InP substrate 1 shown in figure 4(a) is etched away as shown in figure 4(b) to form a lower stage part on which the semiconductor laser 18 is to be produced. That is, in order to arrange the p side electrode 8 of semiconductor laser and the source/drain electrodes 10 and 11 of FET approximately on the same plane, the surface of substrate 1 on which the semiconductor laser comprising thick layers is to be formed is made lower than the surface on which the FET is to be formed. Then, as shown in figure 4(c), an n type InGaAsP layer 14 on which an electrode of the semiconductor laser is to be produced, an n type InP cladding layer 2', an InGaAsP active layer 3' and a p type InP cladding layer 4' are grown by liquid phase epitaxy (LPE), metal-organic chemical vapor deposition (MOCVD) or the like (first crystal growth). Then, unnecessary portions of cladding layer 2', active layer 3' and cladding layer 4' other than portions to be an active region of semiconductor laser are removed by photolithography technique and etching, resulting in a stripe-shaped mesa shown in figure 4(d). Thereafter, as shown in figure 4(e), a current blocking layer 15 and a contact layer 16 are grown by LPE (second crystal growth). Then, as shown in figure 4(f), unnecessary portions of the contact layer 16 and the current blocking layer 15 are removed by an etching. Then, as shown in figure 4(g), a buffer layer 13 and a GaAs operating layer 5' of FET are successively grown by molecular beam epitaxy (MBE) (third crystal growth). Then, as shown in figure 4(h), a recess portion of FET is formed by etching. Thereafter, as shown in figure 4(i), p side and n side electrodes 8 and 9 of semiconductor laser and the source, drain and gate electrodes 10, 11, 12 of FET are formed, thereby completing a transmitter OEIC. As described above, three crystal growths are required for manufacturing the prior art long wavelength transmitter OEIC. More specifically, the first crystal growth is for growing crystals for the semiconductor laser, i.e., the n type InGaAsP layer 14, the n type InP cladding layer 2', the InGaAsP active layer 3' and the p type InP cladding layer 4', the second crystal growth is for growing the current blocking layer 15 and the contact layer 16 after etching those layers 14, 2', 3' and 4' to form a ridge, and the third crystal growth is for growing crystals for the FET, i.e., the undoped GaAs buffer layer 13 and the n type GaAs operating layer 5'. Such many crystal growth processes complicate the manufacturing process, resulting in a poor production yield and a high cost. Furthermore, in the manufacturing process, a step difference approximately equal to the entire thickness of the laser crystals, i.e., 5 microns or more, inevitably arises. Such a step difference induces nonuniformity of photoresist in the photolithography process and, therefore, it is difficult to form a fine pattern of FET. As a result, a high performance OEIC, for example, a high speed OEIC cannot be expected. SUMMARY OF THE INVENTION It is an object of the present invention to provide a structure of a long wavelength transmitter OEIC that can realize the crystal growths for the semiconductor laser and for the electronic device for driving the laser by a single crystal growth process, and also that has no step difference in the manufacturing process. Other object and advantages of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific embodiment are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description. It is mentioned that Patent Abstracts of Japan, vol.12 No. 115 (E-599)(2962), 12 April 1988 and JP-A-62244167 discloses an OEIC combining both a lateral injection QW laser and a field effect transistor defined in a layer atop the laser structural layers. The substrate is of GaAs, the cladding layers of high resistivity AlGaAs, and the quantum well active layer of AlGaAs and GaAs sublayers. Also, DE-A-3709302 discloses an OEIC combining both a vertical injection laser and a high electron mobility transistor. The transistor structural layers are formed atop of the laser structural layers and the electrodes of the laser and of the transistor are co-planar. The method defined in the claims appended provides an alternative. In the method defined in claim 1 of the claims appended, the structural layers of the laser and of the drive transistor (an HEMT) of the OEIC are produced by a single process of epitaxial growth. In addition, the electrical separation between the semiconductor laser and the HEMT can be achieved by slightly etching the thin crystal layers of HEMT (approximately 100 nm (1000 angstroms or less)) at the final stages of the manufacturing process. Therefore, the substrate has a flat surface in a lithography process for forming the gate electrode of the HEMT, and no phenomenon, such as nonuniformity of photoresist, that would reduce the performance of the device would occur. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1(a) and 1(b) are a perspective view and a cross-sectional view showing a long wavelength transmitter OEIC produced in accordance with an embodiment of the present invention; Figures 2(a) to 2(f) are cross-sectional views showing a method for manufacturing the long wavelength transmitter OEIC of figure 1; Figure 3 is a cross-sectional view showing a known long wavelength transmitter OEIC; and Figures 4(a) to 4(i) are cross-sectional views showing a method for manufacturing the long wavelength transmitter OEIC of figure 3 in accordance with the prior art. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be described in detail with reference to the drawings. Figures 1(a) and 1(b) are a perspective view and a cross-sectional view showing a long wavelength transmitter OEIC produced in accordance with an embodiment of the present invention. In these figures, a semiconductor laser 17 and a HEMT 18 are integrated on a semi-insulating InP substrate 1. A lower AlGalnAs cladding layer 2 of the semiconductor laser is disposed on the substrate 1. A quantum well active layer 3 comprising, for example, an AlGaInAs or AlInAs barrier layer and an InGaAs well layer is disposed on the lower cladding layer 2. A high resistivity AlGaInAs cladding layer 4 is disposed on the quantum well active layer 3. An undoped InGaAs operating layer 5 of the HEMT is disposed on the upper cladding layer 4. An n type AlInAs carrier supplying layer 6 is disposed on the operating layer 5. An n type InGaAs contact layer 7 is disposed on the carrier supplying layer 6. A p type dopant diffused region 19 and an n type dopant diffused region 21 of the semiconductor laser are opposed to each other at a predetermined interval and penetrate through the layers from the contact layer 7 to the quantum well active layer 3 to reach into the lower cladding layer 2. A p side electrode 8 of the semiconductor laser is disposed on the contact layer 7 in the p type dopant diffused region region 19 and an n side electrode 9 is disposed on the contact layer 7 in the n type region 21. A gate electrode 12 of the HEMT is disposed on the bottom of the recess groove penetrating the contact layer 7 and reaching into the carrier supplying layer 6. Source and drain electrodes 10 and 11 of the HEMT are disposed on the contact layer 7. Regions 20 and 22 of the quantum well active layer 3 are disordered by diffusions of p type and n type dopants, respectively. An active region 23 of the semiconductor laser is sandwiched by the disordered regions 20 and 22. The p type region 19 and the n type region 21 of the semiconductor laser are separated by the groove 24 and the semiconductor laser 17 and the HEMT 18 are separated by the groove 25. Figures 2(a) to 2(f) illustrate a method for manufacturing the transmitter OEIC of figure 1. On a semi-insulating InP substrate 1 shown in figure 2(a), an AlGaInAs lower cladding layer 2, a quantum well active layer 3 comprising AlGaInAs or AlInAs barrier layer and InGaAs well layer, a high resistivity AlGaInAs upper cladding layer 4, an undoped InGaAs operating layer 5, an n type AlInAs carrier supplying layer 6 and an n type InGaAs contact layer 7 are successively grown by MOCVD as shown in figure 2(b). Then, as shown in figure 2(c), p type dopant such as Zn is diffused or bombarded from the surface of the growth layer to form a p type region 19. The p type dopant disorders the quantum well structure of the quantum well active layer 3, resulting in region 20. Then, as shown in figure 2(d), n type dopant such as Si is diffused or bombarded from the surface at a predetermined interval from the p type region 19 to form an n type region 21. The n type dopant disorders the quantum well structure of quantum well active layer 3, resulting in region 22. Then, as shown in figure 2(e), a recess groove of the HEMT is formed by etching and then a gate electrode 12 is formed on the bottom of the recess groove. Then, as shown in figure 2(f), a groove 24 for separating the p type region 19 from the n type region 21 and a groove 25 for separating the semiconductor laser 17 from the HEMT 18 are formed by etching. Finally, p side and n side electrodes 8 and 9 of the semiconductor laser and source/drain electrodes 10 and 11 of the HEMT are formed, completing a transmitter OEIC. Description is given of the operation hereinafter. Operation of the semiconductor laser 17 will be described first. In the semiconductor laser 17, since n type dopant such as Si and p type dopant such as Zn are diffused from the surface of the epitaxy layers, the quantum well active layer 3 is disordered in the respective dopant diffused regions, resulting in the disordered regions 20 and 22 each having a larger effective energy band gap and smaller refractive index than those of the non-diffused region. When a positive voltage is applied to the p side electrode 8 and a negative voltage is applied to the n side electrode 9, a current only flows through the active layer 23 in the transverse direction because the upper and lower cladding layers 2 and 4 have high resistivity. Then, holes are injected to active region 23 from the disordered region 20, electrons are injected to active region 23 from the disordered region 22, and the holes and electrons are recombined to generate a light. When the current level is increased, induced emission takes place and then laser oscillation occurs. When the construction of the quantum well layer 3, i.e., the thickness of InGaAs well layer and the value of effective energy band gap of AlGaInAs barrier layer are appropriately established, the oscillation wavelength can be set within a long wavelength region of 1.3 to 1.55 µm in which the absorption loss due to optical fiber is small. Since the active region 23 is sandwiched by the disordered regions 20 and 22 having small refractive index, the light is confined in the direction parallel to the active region 23. When the width of active region 23 is set to approximately 2 µm or less, a fundamental transverse mode oscillation can be obtained. Description is given of the operation of the HEMT 18 hereinafter. In the HEMT 18, the InGaAs operating layer 5 is disposed direct on the high resistivity AlGaInAs layer 4, i.e., on the cladding layer of the semiconductor laser 17. It is necessary for HEMT to provide a high resistivity buffer layer beneath the operating layer to reduce the leakage current. In the present invention, the high resistivity cladding layer of the semiconductor laser 17 also serves as the leakage current preventing layer of the HEMT 18. The principle of operation of HEMT 18 is as same as that of the conventional one. More specifically, electrons are supplied from the AlInAs carrier supplying layer 6 to the InGaAs operating layer 5 having low impurity concentration (approximately 10 15 cm -3 or less) and less impurity dispersion, and two-dimensional electron gas is generated at the interface between the operating layer 5 and the carrier supplying layer 6. When the voltage applied to the gate electrode 12 is varied, the level and shape of potential in the vicinity of the interface between the operating layer 5 and the carrier supplying layer 6 vary accordingly, and the concentration of the two-dimensional electron gas also varies. That is, the current is subjected to a modulation by the variation in the gate voltage. The transmitter OEIC of the present invention, in which the semiconductor laser 17 and the HEMT 18 are integrated on a substrate 1, operates in a state that the n side electrode 9 of the semiconductor laser is connected with the source electrode 10 of the HEMT by a wiring. Thus, the HEMT 18 controls the operating current of semiconductor laser 17 and gives signal modulations such as modulations of brightness or strength of light output of the semiconductor laser 17. As described above, in the present invention, a transverse direction current injection type semiconductor laser having a high resistivity upper cladding layer and a HEMT having an operating layer on the high resistivity upper cladding layer are integrated on a substrate. More specifically, the semiconductor laser includes an AlGaInAs lower cladding layer, a quantum well active layer and a high resistivity AlGaInAs upper cladding layer successively grown on the substrate, disordered regions formed in the quantum well active layer by diffusions of p type and n type dopants, and an active region sandwiched by the disordered regions. The HEMT includes an operating layer formed on the high resistivity AlGaInAs upper cladding layer and uses the upper cladding layer as a leakage current preventing layer. Therefore, it is not necessary to provide a special leakage current preventing film for the HEMT. In addition, the above structure can be formed by a single epitaxial growth. As a result, the epitaxial process can be simplified and the costs can be reduced. Furthermore, since the crystals for the semiconductor laser and those for the HEMT are successively grown on a flat substrate having no step difference, photolithography can be performed on a flat surface with no step difference in the later process for forming a gate of HEMT, so that a gate pattern of 1 micron or less can be easily formed at high precision. As a result, a transmitter OEIC performing high speed modulation can be realized. In addition, since AlGaInAs series material is used in the present invention, the oscillation wavelength of the semiconductor laser can be set within a long wavelength region of 1.3 to 1.55 µm in which the absorption loss due to transmitter optical fiber is small, and a high speed HEMT can be constituted. In addition, such a long wavelength semiconductor laser of 1.3 to 1.55 µm band and originally high speed InGaAs/AlInAs series HEMT can be easily integrated on an InP substrate, resulting in a further high speed transmitter OEIC.

Claims: A method for producing a long-wavelength transmitter opto-electronic integrated circuit, which method includes: providing a semi-insulating InP substrate (1); growing successively the following epitaxial layers (2-7) on the surface of said substrate: (i) an AlGalnAs lower cladding layer (2); (ii) a quantum well layer (3); (iii) a high resistivity AlGaInAs upper cladding layer (4); (iv) an undoped InGaAs operating layer (5); (v) an n-type AlInAs carrier supplying layer (6); and (vi) an n-type InGaAs contact layer (7); forming respective n- and p-type longitudinally spaced diffusion regions (21,19) extending from the upper surface of said contact layer (7) into said lower cladding layer (2) thereby sandwiching an active layer portion (23) of said quantum well layer (3); forming a first groove extending from the upper surface of said contact layer (7) into said carrier supplying layer (6); forming a gate electrode (12) on the surface of said carrier supplying layer (6) exposed by said first groove; forming second and third grooves (24,25) extending from the upper surface of said contact layer (7) to reach said upper cladding layer (4), which second and third grooves (24,25) separate the upper portions (5,6,7) of said diffusion regions (21,19) and separate the region (21,19) wherein said diffusion regions (21,19) are formed from the region including said gate electrode (12); and forming electrodes (8,9,10,11) on said contact layer (7) each side of said active layer portion (23) and each side of said gate electrode (12). A method as claimed in claim 1 wherein said step of growing successively the layers (2-7) is performed by metal-organic chemical vapour deposition. A method as claimed in claim 1 or 2 wherein said step of forming respective n- and p-type diffusion regions (21,19) is performed by diffusing or ion implanting Si and Zn, respectively. A method as claimed in any of claims 1 to 3 wherein in said step of growing successively the layers (2-7), the growing of said quantum well layer (3) includes growing an InGaAs well layer and an AlGaInAs or AlInAs barrier layer. A long wavelength transmitter opto-electronic integrated circuit, OEIC, comprising: a semi-insulating InP substrate (1); an AlGaInAs lower cladding layer (2), a quantum well layer (3), a high resistivity AlGaInAs upper cladding layer (4), an undoped InGaAs operating layer (5), an n type AlIAs carrier supplying layer (6) and an n type InGaAs contact layer (7), successively disposed on said semi-insulating InP substrate (1); a p type doped region (19) in a region of said OEIC, penetrating through said layers (7 to 3) from contact layer (7) to quantum well active layer (3) and reaching into said lower cladding layer (2); an n type doped region (21) in another region of said OEIC at a predetermined interval from said p type doped region (19), penetrating through said layers (7 to 3) from contact layer (7) to quantum well layer (3) and reaching into said lower cladding layer (2); a p side electrode (8) on said p type doped region (19); an n side electrode (9) on said n type doped region (21); a groove in a region adjacent to said p and n type doped regions (19,21), penetrating through said contact layer (7) and reaching into said carrier supplying layer (6); a gate electrode (12) on said carrier supplying layer (6) in said groove; source and drain electrodes (10,11) on said contact layer (7) at opposite sides of said groove; a first separation groove (24) separating said p type doped region (19) from said n type doped region (21), reaching said high resistivity AlGaInAs upper cladding layer (4); and a second separation groove (25) separating the region of said doped regions (19,21) from the region of said source/drain electrodes (10,11), reaching said high resistivity AlGaInAs upper cladding layer (4). A long wavelength transmitter OEIC in accordance with claim 5 wherein said p type doped region (19) contain Zn dopant and said n type doped region (21) contains Si dopant. A long wavelength transmitter OEIC in accordance with claim 5 wherein said quantum well layer (3) comprises An AlGaInAs or an AlInAs barrier layer and an InGaAs well layer. A long wavelength transmitter OEIC in accordance with claim 7 wherein the oscillation wavelength of said quantum well layer (3) is set at 1.3 to 1.55 µm.

Inventors: MIHASHI YUTAKA, MIHASHI, YUTAKA, Mihashi, Yutaka, c/o Mitsubishi Denki K.K. Optoel
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1991
Main classification: H01L27
IPC codes: H01L27, H01L29, H01S5, H01L21