Semiconductor laser device

Abstract: A semiconductor laser device has a current confinement structure of a thyristor structure (1,5,6,7) at both sides of an active region (3), and at least a p-n junction in the current confinement structure, to which a forward bias voltage is applied during laser operation, is short-circuited by a metal layer (11) or a low resistance material layer. Therefore, injection of holes from the p layer (7) into the n layer (6), which are short-circuited during the laser operation, is suppressed, whereby the current confinement structure is not likely to be turned on. In addition, the current practically flowing through the blocking layer is decreased. At a result, a current blocking effect is largely improved, preventing reduction in the laser output power and reduction in the linearity of light output vs. current characteristic.

Description: The present invention relates to a semiconductor laser device having a current confinement structure of a thyristor structure (p-n-p-n structure) and, more particularly, to a semiconductor laser with an improved current blocking effect. Figure 8 is a cross-sectional view showing a prior art semiconductor laser having a current confinement structure of a thyristor structure. In figure 8, reference numeral 1 designates an n type InP substrate. An n type InP cladding layer 2, an undoped InGaAsP active layer 3, and a first p type InP cladding layer 4 are successively disposed on the n type InP substrate 1. A ridge structure 15 includes a part of the substrate 1, the n type cladding layer 2, the active layer 3, and the first p type cladding layer 4. A p type InP current blocking layer 5 and an n type InP current blocking layer 6 are successively disposed on the substrate 1 at opposite sides of the ridge structure 15 so as to bury the ridge structure 15. A second p type InP cladding layer 7 is disposed on the n type InP current blocking layer 6 and the first p type cladding layer 4. A p type InGaAsP contact layer 8 is disposed on the second p type InP cladding layer 7. An n side electrode 9 is disposed on the rear surface of the substrate 1 and a p side electrode 10 is disposed on the p type InGaAsP contact layer 8. The second p type InP cladding layer 7, the n type InP current blocking layer 6, the p type InP current blocking layer 5, and the n type InP substrate 1 constitute a current confinement structure having a p-n-p-n structure, i.e., a thyristor structure. This prior art semiconductor laser device operates as follows. When a plus voltage and a minus voltage are applied to the p side electrode 10 and the n side electrode 9, respectively, i.e., a forward bias voltage is applied across the electrodes 10 and 9, a current flows from the p side electrode 10 through the contact layer 8 and the second p type cladding layer 7 toward the substrate 1, and it is injected into the active layer 3, whereby laser oscillation occurs in the active layer 3. Since a p-n junction is formed on opposite sides of the active layer 3 by the current blocking layers 5 and 6, a reverse bias voltage is applied to this p-n junction during operating the laser, so that the current is concentrated in the active layer 3. Since this laser device has the current confinement structure of thyristor structure comprising the current blocking layers 5 and 6, the n type InP substrate 1 and the second p type InP cladding layer 7, leakage current flowing outside the active layer 3 is further reduced and more current is concentrated in the active layer 3. However, even in the semiconductor laser device which has the current confinement structure of thyristor structure at both sides of the active layer as shown in figure 8, the current confinement structure of thyristor structure is sometimes turned on due to rising in ambient temperature, crystal defect, rising in temperature in the vicinity of the active layer during high output operation, or the like. Further, an increase in the driving current sometimes causes a reduction in the current blocking effect, and it may increase the leakage current that may causes a reduction in the laser output power, a reduction in linearity of light output vs. current characteristic or the like. Japanese Published Patent Application No. 61-214591 discloses a semiconductor laser device which further improves the current blocking effect. In this laser device, a base electrode is formed on a base region of a p-n-p transistor constituted by a p type cladding layer, an n type current blocking layer and a p type current blocking layer in a current confinement structure of thyristor structure, and a reverse bias voltage is applied between emitter and base to reduce the injection current into the p-n-p transistor. In this structure, however, an external circuit is required for applying the reverse bias voltage between the emitter and base, resulting in a complicated laser driving circuit and a large-sized device. It is an object of the present invention to provide a semiconductor laser device that prevents a turn-on of a current confinement structure caused by rising in ambient temperature, crystal defect, rising in temperature in the vicinity of an active layer during high output operation or the like and that practically reduces a leakage current flowing through a blocking layer, thereby preventing reduction in the output power of the semiconductor laser itself and reduction in the linearity of light output vs. current characteristic, without complicating the laser driving circuit. Other objects 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. In accordance with a semiconductor laser device of the present invention as claimed in Claim 1, at least a p-n junction in a p-n-p transistor formed in a current confinement structure of thyristor structure, to which a forward bias voltage is applied during laser operation, is short-circuited by a metal electrode or a low resistance material layer. Thus, electric potentials of a p layer and an n layer forming the p-n junction come close to each other, thereby suppressing injection of holes from the p layer into the n layer, so that the current confinement structure of thyristor structure is not likely to be turned on. Therefore, less ratio of a current flowing into the thyristor structure is amplified and an absolute value of the leakage current flowing through blocking layers is suppressed. Figure 1 is a cross-sectional view showing a semiconductor laser device in accordance with a first embodiment of the present invention; Figure 2 is a cross-sectional view showing a semiconductor laser device in accordance with a second embodiment of the present invention; Figure 3 is a cross-sectional view showing a semiconductor laser device in accordance with a third embodiment of the present invention; Figures 4(a) to 4(h) are cross-sectional views in process steps for manufacturing the semiconductor laser device shown in figure 1; Figure 5(a) is a schematic view showing a current confinement structure of the semiconductor laser device shown in figure 1 and figure 5(b) is an equivalent circuit diagram thereof; Figure 6 is a graph showing leakage current dependences of common base current amplification factors of a p-n-p transistor and an n-p-n transistor in the current confinement structure of the semiconductor laser device shown in figure 1; Figure 7 is a graph showing leakage current dependences of common base current amplification factors of a p-n-p transistor and an n-p-n transistor in the current confinement structure of the prior art semiconductor laser device; and Figure 8 is a cross-sectional view showing the prior art semiconductor laser device. Embodiments of the present invention will be described in detail with reference to the drawings. Figure 1 is a cross-sectional view showing a 1.3 to 1.55 microns wavelength band semiconductor laser device in accordance with a first embodiment of the present invention. In figure 1, the same references as those of figure 8 designate the same or corresponding parts. Reference numeral 1 designates an n type InP substrate. An n type InP cladding layer 2 having a thickness of 1 to 1.5 microns and a carrier concentration of 1 X 10¹⁸ cm⁻³ or less, an undoped In x Ga 1-x As y P 1-y active layer 3 having a thickness of 0.05 to 0.15 micron (composition ratio of x and y should be adjusted in ranges of x = 0.255 to 0.410 and y = 0.590 to 0.910 so as to enable lattice-matching with the n type InP substrate 1), and a first p type InP cladding layer 4 having a thickness of 0.5 to 1 micron and a carrier concentration of 1 X 10¹⁸ cm⁻³ or less are successively disposed on the n type InP substrate 1. A ridge structure 15 comprises a part of the n type InP substrate 1, the n type InP cladding layer 2, the undoped InGaAsP active layer 3 and the first p type InP cladding layer 4. A p type InP current blocking layer 5 having a thickness of 1 to 1.5 microns and a carrier concentration of 1 X 10¹⁸ cm⁻³ or less and an n type InP current blocking layer 6 having a thickness of 0.5 micron or less and a carrier concentration of ∼1 X 10¹⁸ cm⁻³ are successively disposed on the substrate 1 at opposite sides of the ridge structure 15 so as to bury the ridge structure 15. A second p type InP cladding layer 7 having a thickness of 0.5 to 1 micron and a carrier concentration of 1 X 10¹⁸ cm⁻³ or less is disposed on the n type InP current blocking layer 6 and the first p type InP cladding layer 4. A p type InGaAsP contact layer 8 having a thickness of 0.5 micron or less and a carrier concentration of 1 X 10¹⁸ cm⁻³ or less is disposed on the second p type InP cladding layer 7. An n side electrode 9 comprising Cr/Au is disposed on the rear surface of the n type InP substrate 1 and a p side electrode 10 comprising AuZn/Ti/Au is disposed on the p type InGaAsP contact layer 8. Stripe grooves 13 are formed penetrating the p type InGaAsP contact layer 8 and the second p type InP cladding layer 7 to reach into the n type InP current blocking layer 6. Electrodes 11 for short-circuiting p-n junction, comprising AuZn/Ti/Au, are disposed covering the surfaces of the stripe grooves 13 and extending on the p type InGaAsP contact layer 8. The n type InP substrate 1, the p type InP current blocking layer 5, the n type InP current blocking layer 6 and the second p type InP cladding layer 7 constitute a thyristor structure (p-n-p-n structure). A method for manufacturing the semiconductor laser structure of figure 1 is illustrated in figures 4(a) to 4(h). First, the n type InP substrate 1 shown in figure 4(a) is prepared. Then, as shown in figure 4(b), the n type InP cladding layer 2, the undoped InGaAsP active layer 3 and the first p type InP cladding layer 4 are successively grown on the substrate 1 by MOCVD (Metal Organic Chemical Vapor Deposition), LPE (Liquid Phase Epitaxy) or the like. Then, as shown in figure 4(c), an insulating film 14 comprising such as SiN₄ or SiO₂ is formed on the first p type InP cladding layer 4 by CVD (Chemical Vapor Deposition) or the like. Thereafter, the insulating film 14 is patterned in a stripe shape using an ordinary photolithography and etching technique. Then, using the stripe-shaped insulating film 14a as a mask, the n type InP substrate 1, the n type InP cladding layer 2, the undoped InGaAsP active layer 3 and the first p type InP cladding layer 4 are partially etched away using a sulfate etchant, for example a mixed solution comprising H₂SO₄ and H₂O₂ mixed in a ratio of 6:1, resulting in the ridge structure 15 shown in figure 4(d). Then, using the mask 14a as a mask for selective growth, the p type InP current blocking layer 5 and the n type InP current blocking layer 6 are successively grown on the substrate 1 by MOCVD, LPE or the like, as shown in figure 4(e). Then, as shown in figure 4(f), the mask 14a is removed and the second p type InP cladding layer 7 and the p type InGaAsP contact layer 8 are grown by MOCVD or the like. Then, as shown in figure 4(g), the n type InP current blocking layer 6, the second p type InP cladding layer 7 and the p type InGaAsP contact layer 8 are partially removed using an ordinary photolithography and etching technique to form the stripe grooves 13. Thereafter, a metal layer comprising AuZn/Ti/Au is formed on the p type InGaAsP contact layer 8 and the stripe grooves 13 by evaporation, sputtering or the like and patterned by etching with a mask, resulting in the p side electrode 10 and the p-n junction short-circuiting electrodes 11 shown in figure 4(h). Thus, the n type InP current blocking layer 6 is short-circuited with the second p type InP cladding layer 7 on the chip. Finally, the n side electrode 9 comprising Cr/Au is formed on the rear surface of the n type InP substrate 1 by evaporation, sputtering or the like, completing the laser structure. The p side electrode 10 and the p-n junction short-circuiting electrodes 11 may be formed by lift-off method. In addition, the short-circuiting between the n type InP current blocking layer 6 and the second p type InP cladding layer 7 is equivalent to short-circuiting between p type emitter and n type base in the pnp transistor in the thyristor structure comprising the n type InP substrate 1, the p type InP current blocking layer 5, the n type InP current blocking layer 6 and the second p type InP cladding layer 7. A description is given of the operation hereinafter. When a plus voltage is applied to the electrode 10 while a minus voltage is applied to the electrode 9, i.e., a forward bias voltage is applied across the electrodes 9 and 10, a current flows from the contact layer 8 and the second cladding layer 7 toward the substrate 1 and is injected into the active layer 3, and laser oscillation occurs in the active layer 3. Figure 5(a) is a schematic view showing the current confinement structure and figure 5(b) is an equivalent circuit diagram thereof. As shown in figures 5(a) and 5(b), the current confinement structure comprising the n type InP substrate 1, the p type InP current blocking layer 5, the n type InP current blocking layer 6 and the second p type InP cladding layer 7 is regarded as a thyristor in which a pnp transistor Tr₁ and an n-p-n transistor Tr₂ are combined, and a current I, represented by the following equation (1), flows through the current confinement structure. I = I₀ / {1 - (α₁ + α₂)} α₁: common base current amplification factor of Tr₁ α₂: common base current amplification factor of Tr₂ I₀: reverse bias leakage current of central pn junction J₂ Figure 6 is a graph showing current (I) dependences of the common base current amplification factors α₁ and α₂ of the p-n-p transistor Tr₁ and the n-p-n transistor Tr₂, respectively, in the current confinement structure having p-n-p-n structure of the semiconductor laser device according to the first embodiment of the present invention and figure 7 is a graph showing those of the prior art semiconductor laser device shown in figure 8. In these graphs, when α₁ + α₂ = 1, the right side, i.e., the current I flowing into the current confinement structure, becomes infinity and then the thyristor is in its turn-on state. It is found from these graphs that the current dependence of the common base current amplification factor α₁ of the p-n-p transistor Tr₁ according to the present invention shown in figure 6 is very small in a region where the current is small and it rises up at considerably large value of current, as compared with that of the p-n-p transistor Tr₁ according to the prior art shown in figure 7. That is, the turn-on condition of the p-n-p-n thyristor of the semiconductor laser device according to the first embodiment of the present invention, i.e., the current value I₂ at which α₁ + α₂ = 1, is very large as compared with the current value I₁ of the semiconductor laser device according to the prior art. Therefore, even when a relatively large current flows through the current confinement structure during the laser operation, the thyristor structure is not turned on, so that the light output of the laser is not lowered and the laser oscillation is not stopped. In addition, in case where the reverse direction leakage current I₀ generated at the junction J₂ in the center of the thyristor structure (p-n-p-n structure) of the semiconductor laser device according to the present invention, to which junction a reverse bias voltage is applied during the laser operation, takes the same value as that of the semiconductor laser device according to the prior art, less current flowing into the thyristor structure is amplified as is evident from the equation (1), so that the current flowing through the current confinement structure is much smaller than that of the prior art device. According to the semiconductor laser device of the first embodiment of the present invention, the ridge structure 15 is formed of a part of the n type InP substrate 1, the n type InP cladding layer 2, the undoped InGaAsP active layer 3 and the first p type InP cladding layer 4, and the current confinement structure having p-n-p-n thyristor structure comprising the n type InP substrate 1, the p type InP current blocking layer 5, the n type InP current blocking layer 6 and the second p type InP cladding layer 7 is formed at both sides of the ridge structure 15 including the active layer 3. Further, the n type InP current blocking layer 6, the second p type InP cladding layer 7 and the p type InGaAsP contact layer 8 are partially removed to form stripe grooves 13 and the p-n junction short-circuiting electrodes 11 are formed in the stripe grooves 13 thereby to short-circuit the n type InP current blocking layer 6 with the second p type InP cladding layer 7. Therefore, injection of holes from the p type InP cladding layer 7 into the n type InP current blocking layer 6 can be suppressed without providing an extra electrode for applying reverse bias voltage or an external circuit, so that the current confinement structure having the thyristor structure is not likely to be turned on. Further, less value of current flowing into this thyristor structure is amplified and an absolute value of the leakage current can be reduced. As the result, the thyristor is not turned on by rising in ambient temperature, crystal defect, rising in temperature in the vicinity of the active layer during high output operation or the like, and the leakage current flowing into the current confinement structure (blocking layers 5 and 6) is reduced, so that reduction in output power of the semiconductor laser itself and reduction in linearity of the light output vs. current characteristic can be suppressed. Figure 2 is a cross-sectional view showing a semiconductor laser device in accordance with a second embodiment of the present invention. In figure 2, the same references as those in figure 1 designate the same or corresponding parts. In this second embodiment, the p side electrode 10 comprising AuZn/Ti/Au serves as the p-n junction short-circuiting electrodes. Manufacturing steps of this laser device is essentially identical to those shown in figures 4(a) to 4(h). Since it is not necessary to form the p-n junction short-circuiting electrodes 11 in the final step of forming electrodes, this step can be simplified. In addition, when crystal defects are formed in the vicinity of interface between the ridge structure 15 and the p type InP cladding layer 7 or the n type InP current blocking layer 6, the current flowing from the p side electrode 10 into the active layer 3 (ridge structure 15) may leak from these layers 7 and 6. Therefore, it is necessary to form the stripe grooves 13 apart from the ridge part 15 by approximately 5 microns or more. Figure 3 is a cross-sectional view showing a semiconductor laser device in accordance with a third embodiment of the present invention. In figure 3, the same references as those in figure 1 designate the same or corresponding parts. In this third embodiment, the n type current blocking layer has a double-layer structure of an n type InP blocking layer 6 and an n type InGaAsP blocking layer 12. Therefore, when the stripe grooves 13 are formed by a selective etching using, for example HCl as etchant, the n type InGaAsP blocking layer 12 serves as an etch stopping layer and the grooves 13 reliably reach the blocking layer 12. Thus, the p type InP cladding layer 7 and the n type InGaAsP blocking layer 12 are short-circuited with each other reliably by the p-n junction short-circuiting electrodes 11 formed in the stripe grooves 13, resulting in a semiconductor laser device with high stability and high reliability. In addition, lesser faulty devices are produced, resulting in an improved production yield. While in the above-described first to third embodiments the p-n junction short-circuiting electrode 11 is formed of metal, it may be formed of a low resistance material such as polysilicon. Also in this case, the same effects as described above can be obtained. In the semiconductor laser devices according to the first to third embodiments, the current confinement structure is formed by successively growing crystal layers on the n type substrate in the order of n-p-n-p at both sides of the ridge structure comprising n type substrate, n type cladding layer and undoped active layer. However, the current confinement structure may be formed by successively growing crystal layers on a p type substrate in the order of p-n-p-n at both sides of the ridge structure comprising p type substrate, p type cladding layer and undoped active layer. Also in this case, the same effects as described above can be obtained. According to the present invention, a semiconductor laser device includes a current confinement structure having a thyristor structure, and at least a p-n junction in the current confinement structure, to which a forward bias voltage is applied during laser operation, is short circuited by a metal electrode or a low resistance material layer. Therefore, injection of holes from the p layer into the n layer, which are short-circuited during the laser operation, is suppressed, so that the current confinement structure is not likely to be turned on. In addition, the current practically flowing through the blocking layer is decreased. As a result, a current blocking effect is largely improved, preventing reduction in the laser output power and reduction in the linearity of light output vs. current characteristic.

Claims: A semiconductor laser device having a current confinement structure of a thyristor structure (7-6-5-1) at both sides of an active region (15), characterised in that at least one p-n junction (7-6) formed in said current confinement structure, to which a forward bias voltage is applied during laser operation, is short-circuited (13) by a low resistance material layer (11). A semiconductor laser device in accordance with claim 1 wherein said active region comprises a ridge structure (15) including an n type cladding layer (2) and an undoped active layer (3) successively formed on an n type substrate (1) and said current confinement structure (5, 6, 7) is formed so as to bury said ridge structure. A semiconductor laser device in accordance with claim 2 wherein said current confinement structure comprises an n type InP substrate (1), a p type InP current blocking layer (5), an n type InP current blocking layer (6), and a p type InP cladding layer (7). A semiconductor laser device in accordance with claim 3 wherein a p-n junction produced by said n type InP current blocking layer (6) and said p type InP cladding layer (7) is short-circuited. A semiconductor laser device in accordance with claim 4 wherein an n type InGaAsP current blocking layer (12) is provided between said n type InP current blocking layer (6) and said p type InP cladding layer (7) and a p-n junction produced by said n type InGaAsP current blocking layer (12) and said p type InP cladding layer (7) is short-circuited. A semiconductor laser device according to any preceding claim wherein said low resistance material layer (11) is a metal layer. A semiconductor laser device in accordance with claim 6 wherein said metal layer (11) for short-circuiting the p-n junction comprises AuZn/Ti/Au. A semiconductor laser device in accordance with any of claims 1 to 5 wherein said low resistance material layer (11) for short-circuiting the p-n junction comprises polysilicon. A semiconductor laser device in accordance with any preceding claim wherein said low resistance material layer (11) for short-circuiting the p-n junction is formed as one layer united with a p side electrode (10) or an n side electrode. A semiconductor laser device in accordance with any preceding claim wherein said low resistance material layer (11) for short-circuiting the p-n junction is formed as one layer united with a p side electrode (10) or an n side electrode and thereafter it is separated from said p side electrode or said n side electrode.

Inventors: MIHASHI YUTAKA C O MITSUBISHI, MIHASHI, YUTAKA, C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1992
Main classification: H01S3
IPC codes: H01S5, H01L29