JP2012084661A - Semiconductor laser, optical module having the semiconductor laser, optical communication device, and optical communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the occurrence of non-light-emitting recombination in an interface of a GaAs layer in which a diffraction grating is formed.SOLUTION: A semiconductor laser of the present invention comprises: an active layer 18 that is provided above a substrate 10 and includes at least a portion having a smaller band gap energy than GaAs; GaAs layers (a GaAs waveguide layer 20 and a diffraction grating layer 22) that are provided above the active layer 18 and serve as a waveguide for transmitting light oscillated from the active layer 18; a diffraction grating 26 provided on the top surface of the diffraction grating layer 22; a buried layer 24 provided so as to bury the diffraction grating 26; and a carrier stop layer 34 that is provided between the active layer 18 and the diffraction grating 26 and prevents carriers from overflowing from the active layer 18 to the interface between the diffraction grating layer 22 and the buried layer 24.

Description

  The present invention relates to a semiconductor laser, an optical module including the semiconductor laser, an optical communication apparatus, and an optical communication system, and more particularly, to a semiconductor laser including a diffraction grating, an optical module including the semiconductor laser, an optical communication apparatus, and an optical communication system. .

  In order to reduce transmission signal degradation due to dispersion of an optical fiber, a semiconductor laser that emits laser light having a single wavelength is required. As such a semiconductor laser, a distributed feedback semiconductor laser (hereinafter referred to as a DFB laser) that realizes a dynamic single mode using light oscillated in an active layer by using Bragg reflection at a diffraction grating is known. Yes. For example, Patent Document 1 discloses a DFB laser in which a diffraction grating is formed on the upper surface of a GaAs layer and an AlGaInP layer is provided so as to be embedded in the diffraction grating.

JP 2007-299791 A

  A DFB laser having a structure shown in Patent Document 1 is generally manufactured by performing GaAs layer growth, GaAs layer etching to form a diffraction grating, and AlGaInP layer growth to embed the diffraction grating. The For this reason, the surface of the GaAs layer is oxidized, and the AlGaInP layer is regrown on the surface of the oxidized GaAs layer. That is, the interface between the GaAs layer and the AlGaInP layer is a regrowth interface. For example, GaAs is more easily oxidized than InGaP or the like, and a level that causes non-radiative recombination is formed at the regrowth interface that is an oxidized surface of the GaAs layer. For this reason, when the carriers in the active layer overflow and reach the regrowth interface of the GaAs layer, non-radiative recombination occurs at the regrowth interface of the GaAs layer, resulting in a decrease in quantum efficiency, a decrease in reliability, etc. End up.

  The present invention has been made in view of the above problems, and a semiconductor laser capable of suppressing non-radiative recombination from occurring at an interface where a diffraction grating of a GaAs layer is formed, an optical module including the semiconductor laser, and an optical communication device An object of the present invention is to provide an optical communication system.

  The present invention provides an active layer provided on a substrate and having at least a part of a band gap energy smaller than that of GaAs, and provided on the active layer to propagate light oscillated from the active layer. A GaAs layer that is a waveguide; a diffraction grating provided on the top surface of the GaAs layer; a buried layer provided to embed the diffraction grating; and provided between the active layer and the diffraction grating, A semiconductor laser comprising: a carrier stop layer for suppressing carrier overflow from an active layer to an interface between the GaAs layer and the buried layer. According to the present invention, since carriers from the active layer can be prevented from reaching the interface where the diffraction grating of the GaAs layer is formed, minority carriers and majority carriers are formed at the interface where the diffraction grating of the GaAs layer is formed. Non-radiative recombination can be suppressed.

  In the above configuration, the diffraction grating is provided between the active layer and the p-type cladding layer, and the carrier stop layer has a conduction band edge energy larger than that of GaAs. be able to.

  In the above configuration, the diffraction grating is provided between the active layer and the n-type cladding layer, and the carrier stop layer has a valence band edge energy smaller than that of GaAs. It can be.

  The said structure WHEREIN: The said carrier stop layer can be set as the structure provided between the said active layer and the said GaAs layer.

  The said structure WHEREIN: The thickness of the said carrier stop layer can be set as the structure which is 50 nm or more and 500 nm or less.

  The said structure WHEREIN: The said carrier stop layer can be set as the structure which consists of AlGaAs or InGaP.

  The said structure WHEREIN: The said active layer can be set as the structure which is a quantum well active layer or a quantum dot active layer.

  The present invention is an optical module comprising the semiconductor laser described above. According to the present invention, since a semiconductor laser in which non-radiative recombination is suppressed at the interface where the diffraction grating of the GaAs layer is formed is used, an optical module with improved quantum efficiency and improved reliability can be obtained. Obtainable.

  The present invention is an optical communication device comprising the semiconductor laser described above. According to the present invention, since a semiconductor laser in which non-radiative recombination is suppressed at the interface where the diffraction grating of the GaAs layer is formed is used, an optical communication device with improved quantum efficiency and improved reliability Can be obtained.

  The present invention is an optical communication system comprising the semiconductor laser described above. According to the present invention, an optical communication system in which quantum efficiency is improved and reliability is improved because a semiconductor laser in which non-radiative recombination is suppressed at the interface where the diffraction grating of the GaAs layer is formed is used. Can be obtained.

  According to the present invention, since carriers from the active layer can be prevented from reaching the interface where the diffraction grating of the GaAs layer is formed, minority carriers and majority carriers are formed at the interface where the diffraction grating of the GaAs layer is formed. Non-radiative recombination can be suppressed.

FIG. 1 is an example of an energy band diagram of an epi structure included in the semiconductor laser according to Comparative Example 1. FIG. 2 is an example of a perspective view of the semiconductor laser according to the first embodiment. FIG. 3A and FIG. 3B are examples (part 1) of perspective views illustrating the method of manufacturing the semiconductor laser according to the first embodiment. 4A and 4B are examples (part 2) of a perspective view illustrating the method of manufacturing the semiconductor laser according to the first embodiment. FIG. 5A shows the result of the reliability test of the semiconductor laser according to Comparative Example 2, and FIG. 5B shows the result of the reliability test of the semiconductor laser according to Example 1. FIG. 6 is an example of an energy band diagram from the n-type GaAs substrate to the contact layer in FIG. FIG. 7 is an example of an energy band diagram of an epi structure included in the semiconductor laser according to Example 3. FIG. 8 is an example of a block diagram of an optical module according to the fourth embodiment. FIG. 9 is an example of a block diagram of an optical communication apparatus according to the fifth embodiment. FIG. 10 is an example of a block diagram of an optical communication system according to the sixth embodiment.

  First, the semiconductor laser according to Comparative Example 1 will be described. A semiconductor laser according to Comparative Example 1 includes an n-type AlGaAs cladding layer, an undoped GaAs waveguide layer, a multiple quantum well active layer of an InGaAs well layer and a GaAs barrier layer, and a p-type GaAs waveguide on an n-type GaAs substrate. An epitaxial structure in which a layer, a p-type GaAs diffraction grating layer, a p-type InGaP buried layer, a p-type AlGaAs cladding layer, and a p-type GaAs contact layer are sequentially stacked. A diffraction grating is formed on the upper surface of the diffraction grating layer, and an embedded layer is formed so as to embed the diffraction grating.

  The diffraction grating of the semiconductor laser according to Comparative Example 1 has a growth of a p-type GaAs diffraction grating layer, etching the upper surface of the diffraction grating layer to form a diffraction grating, and a growth of a p-type InGaP buried layer so as to embed the diffraction grating. It is formed by carrying out in this order. Therefore, the interface between the diffraction grating layer and the buried layer becomes a regrowth interface.

  FIG. 1 is an example of an energy band diagram of an epi structure included in the semiconductor laser according to Comparative Example 1. As shown in FIG. 1, since the diffraction grating layer and the p-type GaAs waveguide layer are both made of GaAs, the band gap is equal from the p-type GaAs waveguide layer to the diffraction grating, and the magnitude of the conduction band band edge energy is equal. . For this reason, for example, during high-temperature operation, the electrons overflowing from the active layer to the p-type GaAs waveguide layer side reach the regrowth interface of the diffraction grating layer.

  Since the diffraction grating layer is a GaAs layer, a level that causes non-radiative recombination is formed at the regrowth interface as described above. In addition, since the diffraction grating layer and the buried layer are p-type semiconductors, electrons that have reached the regrowth interface of the diffraction grating layer are minority carriers. Such electrons, which are minority carriers, recombine non-radiatively with holes, which are majority carriers, at the regrowth interface. Non-radiative recombination causes thermal energy, which causes defects in the crystal. Crystal defects grow by repeated non-radiative recombination and reach the active layer. Since there are a large number of electrons and holes in the active layer, non-radiative recombination of these electrons and holes may cause crystal defects to grow at a stretch in the active layer and cause the semiconductor laser to fail. Therefore, in order to solve the failure of the semiconductor laser due to such a mechanism, an embodiment capable of suppressing the occurrence of non-radiative recombination at the interface where the diffraction grating of the GaAs layer is formed will be described.

  FIG. 2 is an example of a perspective view of the semiconductor laser according to the first embodiment. As shown in FIG. 2, an n-type GaAs buffer layer 12, an n-type AlGaAs n-type cladding layer 14, and an undoped GaAs waveguide layer 16 are sequentially stacked on a substrate 10 that is an n-type GaAs substrate. On the GaAs waveguide layer 16, there is provided an active layer 18, which is a quantum dot active layer in which seven quantum dot layers comprising InAs quantum dots and a GaAs barrier layer provided so as to cover the quantum dots are stacked. It has been.

  On the active layer 18, a carrier stop layer 34 made of p-type AlGaAs and suppressing an electron overflow of the active layer 18 and a p-type GaAs waveguide layer 20 are sequentially laminated. The undoped GaAs waveguide layer 16 and the p-type GaAs waveguide layer 20 are waveguides for propagating light oscillated in the active layer 18, and the carrier stop layer 34 also has a function as a waveguide. A diffraction grating layer 22 made of p-type GaAs is provided on the GaAs waveguide layer 20, and the upper surface of the diffraction grating layer 22 has periodic unevenness in the waveguide direction in which light oscillated in the active layer 18 propagates. A diffraction grating 26 is formed. On the diffraction grating layer 22, an embedded layer 24 is provided so as to embed the diffraction grating 26 made of p-type InGaP. The diffraction grating layer 22 has a function as a waveguide, and the buried layer 24 has a function as a cladding.

A p-type AlGaAs p-type cladding layer 30 and a p-type GaAs contact layer 32 are sequentially stacked on the buried layer 24. Table 1 shows the material, film thickness, dopant, and doping concentration of each layer.

  The p-type cladding layer 30 and the contact layer 32 form a ridge portion 36. On both sides of the ridge portion 36, a concave portion where the buried layer 24 is exposed is formed, and a BCB (bisbenzocyclobutene) film 38 is provided so as to be buried in the concave portion. A p-electrode 40 in contact with the contact layer 32 is provided on the ridge portion 36 and the BCB film 38. An n-electrode 42 is provided on the lower surface of the n-type GaAs substrate 10.

  The front end surface from which the laser light stimulated and emitted by the active layer 18 emits is provided with a low reflection film (not shown) with respect to the wavelength of the laser light, and the rear end surface located on the opposite side of the front end surface is provided with a laser. A highly reflective film (not shown) is provided for the wavelength of light. The resonator length that is the distance between the front end face and the rear end face is, for example, 300 μm.

Next, a method for manufacturing a semiconductor laser according to Example 1 will be described with reference to FIGS. First, a substrate 10 which is an n-type GaAs substrate having a thickness of, for example, 450 μm and doped with Si at a concentration of 2 × 10 ¹⁸ cm ⁻³ is prepared. Next, for example, by MBE (molecular beam epitaxy) method using As, Ga, Al, and In as raw materials and Si and Be as dopants, a buffer layer 12, an n-type cladding layer 14, undoped GaAs are formed on the substrate 10. The waveguide layer 16, the active layer 18, the carrier stop layer 34, the p-type GaAs waveguide layer 20, and the diffraction grating layer 22 are sequentially deposited. The buffer layer 12 to the diffraction grating layer 22 are continuously deposited without removing the substrate 10 from the MBE apparatus. Below, manufacture of each layer is demonstrated in detail.

As shown in FIG. 3A, an n-type GaAs buffer on a substrate 10 at a growth temperature of 600 ° C., a film thickness of 500 nm, and Si (silicon) doped at a concentration of 1 × 10 ¹⁸ cm ⁻³ Layer 12 is formed. Subsequently, the n-type cladding layer 14 of n-type AlGaAs having a thickness of 1500 nm and doped with Si at a concentration of 1 × 10 ¹⁸ cm ⁻³ is formed on the buffer layer 12 with the growth temperature of 600 ° C. Subsequently, an undoped GaAs waveguide layer 16 having a film thickness of 50 nm is formed on the n-type cladding layer 14 while maintaining the growth temperature at 600 ° C.

As shown in FIG. 3B, the growth temperature is lowered to 500 ° C., and an InAs quantum dot is supplied on the GaAs waveguide layer 16 by self-forming an InAs quantum dot. A barrier layer made of GaAs having a film thickness of 30 nm is deposited thereon to embed and flatten the quantum dots. Seven such quantum dots and barrier layers are repeatedly deposited to form an active layer 18 having a quantum dot structure. Next, the growth temperature is raised to 600 ° C., and a p-type AlGaAs carrier stop layer 34 having a thickness of 170 nm and Be doped at a concentration of 4 × 10 ¹⁷ cm ⁻³ is formed on the active layer 18. Subsequently, the p-type GaAs waveguide layer 20 having a film thickness of 30 nm and Be (beryllium) doped at a concentration of 4 × 10 ¹⁷ cm ⁻³ is formed on the carrier stop layer 34 at a growth temperature of 600 ° C. Form. Subsequently, a p-type GaAs diffraction grating layer 22 having a film thickness of 20 nm and Be doped at a concentration of 1 × 10 ¹⁸ cm ⁻³ is formed on the GaAs waveguide layer 20 with the growth temperature of 600 ° C. .

  As shown in FIG. 4A, after the substrate 10 having been deposited up to the diffraction grating layer 22 is taken out from the MBE apparatus, a photoresist is applied onto the diffraction grating layer 22, and then a pattern for the diffraction grating is formed using interference exposure technology. Form on photoresist. Thereafter, the diffraction grating layer 22 is wet-etched using a photoresist as a mask to form a diffraction grating 26 having a pitch of 200 nm, a depth of 20 nm, and a duty ratio of 0.5.

After forming the diffraction grating 26, the substrate 10 is introduced into, for example, an MOCVD (metal organic vapor phase epitaxy) apparatus, and TMGa (trimethyl gallium), TMIn (trimethyl indium), PH ₃ (phosphine), DMZn (dimethyl zinc) are used as raw materials. The buried layer 24, the p-type cladding layer 30, and the contact layer 32 are sequentially deposited by MOCVD using CBr ₄ (carbon tetrabromide). From the buried layer 24 to the contact layer 32, the n-type GaAs substrate 10 is continuously deposited without being taken out of the MOCVD apparatus. Specifically, at a growth temperature of 640 ° C., a film thickness of 50 nm and a Zn (zinc) concentration of 1 × 10 ¹⁸ cm ⁻³ are formed on the diffraction grating layer 22 so that the diffraction grating 26 is embedded and flattened. A doped p-type InGaP buried layer 24 is formed. By such a manufacturing process, the upper surface of the diffraction grating layer 22 on which the diffraction grating 26 is formed is oxidized, and a buried layer 24 is formed on the upper surface of the oxidized diffraction grating layer 22. The interface with the buried layer 24 becomes a regrowth interface. Next, the growth temperature is raised to 680 ° C., and a p-type cladding layer 30 of p-type AlGaAs doped with a thickness of 1500 nm and C (carbon) at a concentration of 1 × 10 ¹⁸ cm ⁻³ is formed on the buried layer 24. Form. Subsequently, a p-type GaAs contact layer 32 having a film thickness of 300 nm and C doped at a concentration of 8 × 10 ¹⁹ cm ⁻³ is formed on the p-type cladding layer 30 with the growth temperature kept at 680 ° C. .

  As shown in FIG. 4B, after the substrate 10 deposited up to the contact layer 32 is taken out from the MOCVD apparatus, a photoresist is applied on the contact layer 32, and then a stripe pattern is formed on the photoresist using an exposure technique. Thereafter, the contact layer 32 and the p-type cladding layer 30 are wet-etched using a photoresist as a mask to form a ridge portion 36 composed of the p-type cladding layer 30 and the contact layer 32. Next, after the concave portions on both sides of the ridge portion 36 are filled with the BCB film 38, the substrate 10 is polished to a thickness of, for example, 150 μm. Thereafter, a p-electrode 40 is formed on the ridge portion 36 and the BCB film 38, and an n-electrode 42 is formed on the lower surface of the substrate 10. Finally, the substrate 10 is cleaved to form a resonator having a resonator length of 300 μm, and a low reflection film is formed on the front end surface of the cleaved surface that emits laser light, and a light reflection film is formed on the rear end surface. Such a semiconductor laser is completed.

Here, for comparison, a semiconductor laser according to Comparative Example 2 having the layer structure shown in Table 2 that does not have the carrier stop layer 34 was manufactured by the same manufacturing method as the semiconductor laser according to Example 1.

  The oscillation wavelength of the semiconductor laser according to Example 1 at room temperature was 1305 nm. In addition, the semiconductor laser according to Example 1 had a result that the threshold current was reduced by 15% and the differential quantum efficiency was improved by 12% compared with the semiconductor laser according to Comparative Example 2.

  Next, a reliability test was performed on the semiconductor laser according to Example 1 and the semiconductor laser according to Comparative Example 2. The reliability test was performed by performing a constant light output drive of 15 mW by APC (Automatic Power Control) control in a temperature environment of 85 ° C. In addition, for both the semiconductor laser according to Example 1 and the semiconductor laser according to Comparative Example 2, a reliability test was performed on eight elements.

  FIG. 5A shows the result of the reliability test of the semiconductor laser according to Comparative Example 2, and FIG. 5B shows the result of the reliability test of the semiconductor laser according to Example 1. 5A and 5B, the horizontal axis represents drive time (arbitrary coordinates), and the vertical axis represents drive current (arbitrary coordinates). As shown in FIG. 5A, in the semiconductor laser according to Comparative Example 2, an increase in drive current that appears to be a sudden failure occurs in two of the eight elements, and the drive current increases in other elements as the drive time elapses. As a result, satisfactory reliability was not obtained. When the cross section of the failed element was observed with a TEM (transmission electron microscope), the growth of crystal defects was observed around the diffraction grating 26 which is the regrown interface, and part of the defect reached the active layer 18. It was confirmed that crystal defects were growing at a stretch. Such growth of crystal defects is due to the mechanism described in the semiconductor laser according to Comparative Example 1.

  On the other hand, as shown in FIG. 5B, in the semiconductor laser according to Example 1, no increase in driving current was observed in all the eight elements, and a substantially constant driving current was maintained, and sufficient reliability was obtained. As a result. Further, when the cross section of the element was observed with a TEM, no crystal defects as found in the semiconductor laser according to Comparative Example 1 were found.

  FIG. 6 is an example of an energy band diagram from the substrate 10 to the contact layer 32 in FIG. As shown in FIG. 6, since the carrier stop layer 34 provided between the active layer 18 and the diffraction grating 26 is made of AlGaAs, the band gap energy is larger than that of the GaAs waveguide layer 20 and the conduction band edge. Energy is big. For this reason, it is difficult for the electrons of the active layer 18 to exceed the carrier stop layer 34, and the arrival of the regrown interface of the diffraction grating layer 22 is suppressed. That is, the carrier stop layer 34 functions to suppress the overflow of electrons from the active layer 18 to the interface between the diffraction grating layer 22 and the buried layer 24.

  Thus, according to the first embodiment, the p-type GaAs waveguide layer 20 and the p-type GaAs diffraction grating layer 22 are provided on the active layer 18 provided on the substrate 10. A diffraction grating 26 is formed on the upper surface. A p-type InGaP buried layer 24 is provided so as to embed the diffraction grating 26, and a p-type cladding layer 30 is provided on the buried layer 24. That is, the diffraction grating 26 is provided between the active layer 18 and the p-type cladding layer 30. A carrier stop layer 34 having a conduction band edge energy larger than that of GaAs is provided between the active layer 18 and the diffraction grating 26. With such a structure, as described with reference to FIG. 6, it is possible to prevent electrons as minority carriers from reaching the regrowth interface of the diffraction grating layer 22. For this reason, it is possible to suppress non-radiative recombination of electrons, which are minority carriers, and holes, which are majority carriers, at the regrowth interface of the diffraction grating layer 22 made of GaAs even during high-temperature operation. Thereby, reduction of threshold current, improvement of differential quantum efficiency, etc. can be realized. Further, as shown in FIG. 5B, the reliability can be improved.

  As shown in Table 1, the carrier stop layer 34 is an example of an AlGaAs layer having an Al composition ratio of 0.2, but is not limited thereto. The carrier stop layer 34 only needs to have conduction band edge energy that is large enough to prevent electrons in the active layer 18 from exceeding the carrier stop layer 34 during high-temperature operation or the like. Therefore, when the carrier stop layer 34 is an AlGaAs layer, for example, the Al composition ratio is preferably 0.1 or more and less than 1, and more preferably 0.1 or more and 0.4 or less. The conduction band edge offset amount, which is the difference between the conduction band edge energy of the carrier stop layer 34 and the conduction band edge energy of GaAs, is preferably, for example, 70 meV or more, and more preferably 140 meV or more. .

  The thickness of the carrier stop layer 34 is preferably not less than a thickness that does not allow electrons of the active layer 18 to tunnel, for example, preferably not less than 50 nm, and more preferably not less than 150 nm. Further, since the distance between the active layer 18 and the diffraction grating 26 is designed based on the optical confinement coefficient Γ, the coupling coefficient κ, and the like, considering the optical confinement coefficient Γ, the coupling coefficient κ, and the like, The distance from the diffraction grating 26 is 1 μm or less. Accordingly, the thickness of the carrier stop layer 34 provided between the active layer 18 and the diffraction grating 26 is 1 μm or less. However, when the thickness of the carrier stop layer 34 is increased, the carrier stop layer 34 becomes a waveguide. Therefore, the thickness of the carrier stop layer 34 is preferably thin. For example, the thickness is preferably 500 nm or less, and more preferably 300 nm or less. From the above, the thickness of the carrier stop layer 34 is preferably 50 nm or more and 500 nm or less, and more preferably 150 nm or more and 300 nm or less.

  Although the case where the carrier stop layer 34 is provided between the active layer 18 and the GaAs waveguide layer 20 is shown as an example, the carrier stop layer 34 may be provided so as to be sandwiched between the GaAs waveguide layers 20. That is, the GaAs waveguide layer 20, the carrier stop layer 34, and the GaAs waveguide layer 20 may be stacked on the active layer 18 in this order. However, if the carrier stop layer 34 is provided in contact with the active layer 18, it is possible to suppress the leakage of electrons from the active layer 18 to the outside of the active layer 18, thereby improving quantum efficiency. Therefore, the carrier stop layer 34 is preferably provided between the active layer 18 and the GaAs waveguide layer 20. That is, it is preferable that the carrier stop layer 34 and the GaAs waveguide layer 20 are stacked in this order on the active layer 18. Further, by adopting a structure in which the carrier stop layer 34 and the GaAs waveguide layer 20 are sequentially laminated on the active layer 18, the number of times of changing the growth conditions during the thin film growth can be reduced, and the manufacture becomes easy.

  The buried layer 24 is not limited to being made of InGaP, and may be made of a material having a band gap larger than that of GaAs and causing a difference in refractive index from that of GaAs, for example, made of AlGaAs. Thereby, a refractive index modulation type diffraction grating is obtained.

  The case where the active layer 18 is a quantum dot active layer composed of an InAs quantum dot and a GaAs barrier layer provided so as to cover the quantum dot is shown as an example, but the present invention is not limited thereto. Since the GaAs waveguide layers 16 and 20 are provided so as to sandwich the active layer 18, the active layer 18 only needs to have a band gap energy smaller than the band gap energy of GaAs at least in part. The wavelength of the light oscillated from the layer 18 may be longer than that of GaAs. For example, instead of InAs quantum dots, InGaAs quantum dots, GaInNAs quantum dots, and GaInNAsSb quantum dots can be used. The barrier layer is preferably made of GaAs made of the same material as the GaAs waveguide layers 16 and 20.

  Although the carrier stop layer 34 is shown as an example of AlGaAs, the carrier stop layer 34 is more than the conduction band edge energy of GaAs so that the electrons of the active layer 18 can be prevented from reaching the regrowth interface of the diffraction grating layer 22. Any material having a large conduction band edge energy may be used. For example, InGaP can be used for the carrier stop layer 34.

  In the first embodiment, the case where the GaAs waveguide layer 16 and the GaAs waveguide layer 20 are provided so as to sandwich the active layer 18 from above and below is shown as an example. The waveguide layer 16 may not be provided.

The semiconductor laser according to Example 2 is a case where the active layer 18 is a multiple quantum well active layer. In the semiconductor laser according to the second embodiment, the active layer 18 has a multiple quantum well structure in which two layers of InGaAs well layers and GaAs barrier layers are alternately stacked. The dopant of the carrier stop layer 34, the GaAs waveguide layer 20, and the diffraction grating layer 22 is C. Furthermore, the resonator length is 800 μm. Except for these points, the structure is the same as that of the semiconductor laser according to Example 1 shown in FIG. Table 3 shows the material, film thickness, dopant, and doping concentration of each layer of the semiconductor laser according to Example 2.

Next, a method for manufacturing a semiconductor laser according to Example 2 will be described. First, a substrate 10 that is an n-type GaAs substrate having a thickness of, for example, 450 μm and doped with Si (silicon) at a concentration of 2 × 10 ¹⁸ cm ⁻³ is prepared. Next, for example, a buffer layer 12 is formed on the substrate 10 by MOCVD using TMGa, TMAl (trimethylaluminum), TMIn, AsH ₃ (arsine) as raw materials and SiH ₄ (silane), CBr ₄ , DMZn as dopants. The n-type cladding layer 14, the undoped GaAs waveguide layer 16, the active layer 18, the carrier stop layer 34, the p-type GaAs waveguide layer 20, and the diffraction grating layer 22 are sequentially deposited. The buffer layer 12 to the diffraction grating layer 22 are continuously deposited without removing the substrate 10 from the MOCVD apparatus. Below, each layer is demonstrated in detail.

An n-type GaAs buffer layer 12 is formed on the substrate 10 at a growth temperature of 690 ° C., a film thickness of 500 nm, and Si doped at a concentration of 1 × 10 ¹⁸ cm ⁻³ . Subsequently, the n-type cladding layer 14 of n-type AlGaAs having a thickness of 1500 nm and doped with Si at a concentration of 1 × 10 ¹⁸ cm ⁻³ is formed on the buffer layer 12 with the growth temperature of 690 ° C. Subsequently, an undoped GaAs waveguide layer 16 having a film thickness of 50 nm is formed on the n-type cladding layer 14 with the growth temperature kept at 690 ° C.

The growth temperature is lowered to 560 ° C., and an InGaAs well layer having a thickness of 6 nm and a GaAs barrier layer having a thickness of 30 nm are repeatedly deposited on the GaAs waveguide layer 16 to have a multiple quantum well structure. An active layer 18 is formed. Thereafter, the growth temperature is raised to 630 ° C., and a p-type AlGaAs carrier stop layer 34 having a thickness of 170 nm and C doped at a concentration of 4 × 10 ¹⁷ cm ⁻³ is formed on the active layer 18. Subsequently, the p-type GaAs waveguide layer 20 having a film thickness of 30 nm and C doped at a concentration of 4 × 10 ¹⁷ cm ⁻³ is formed on the carrier stop layer 34 with the growth temperature of 630 ° C. Subsequently, a p-type GaAs diffraction grating layer 22 having a film thickness of 20 nm and C doped at a concentration of 1 × 10 ¹⁸ cm ⁻³ is formed on the GaAs waveguide layer 20 with the growth temperature of 630 ° C. .

  The substrate 10 deposited up to the diffraction grating layer 22 is taken out from the MOCVD apparatus, and a photoresist is applied on the diffraction grating layer 22, and then a pattern for the diffraction grating is formed on the photoresist by using an interference exposure technique. Thereafter, the diffraction grating layer 22 is wet-etched using a photoresist as a mask to form a diffraction grating 26 having a pitch of 310 nm, a depth of 20 nm, and a duty ratio of 0.5.

After forming the diffraction grating 26, the substrate 10 is again introduced into the MOCVD apparatus, and the buried layer 24, the p-type cladding layer 30, and the contact layer 32 are sequentially deposited. The buried layer 24 to the contact layer 32 are continuously deposited without removing the substrate 10 from the MOCVD apparatus. Specifically, at a growth temperature of 640 ° C., the diffraction grating 26 is buried and planarized so that the film thickness is 50 nm and the Zn is doped at a concentration of 1 × 10 ¹⁸ cm ^−3. A p-type InGaP buried layer 24 is formed. By such a manufacturing process, the upper surface of the diffraction grating layer 22 on which the diffraction grating 26 is formed is oxidized, and a buried layer 24 is formed on the upper surface of the oxidized diffraction grating layer 22. The interface with the buried layer 24 becomes a regrowth interface. Next, the growth temperature is raised to 680 ° C., and a p-type cladding layer 30 of p-type AlGaAs doped with a film thickness of 1500 nm and C of 1 × 10 ¹⁸ cm ⁻³ is formed on the buried layer 24. Subsequently, a p-type GaAs contact layer 32 having a film thickness of 800 nm and C doped at a concentration of 8 × 10 ¹⁹ cm ⁻³ is formed on the p-type cladding layer 30 while the growth temperature is 680 ° C.

  The substrate 10 deposited up to the contact layer 32 is taken out from the MOCVD apparatus, and after applying a photoresist on the contact layer 32, a stripe pattern is formed on the photoresist using an exposure technique. Thereafter, the contact layer 32 and the p-type cladding layer 30 are wet-etched using a photoresist as a mask to form a ridge portion 36 composed of the p-type cladding layer 30 and the contact layer 32. Next, after the BCB film 38 is embedded in the concave portions on both sides of the ridge portion 36, the substrate 10 is polished to a thickness of 150 μm, for example. Thereafter, a p-electrode 40 is formed on the ridge portion 36 and the BCB film 38, and an n-electrode 42 is formed on the lower surface of the substrate 10. Finally, the substrate 10 is cleaved to form a resonator having a resonator length of 800 μm, and a low reflection film (not shown) is provided on the front end face of the cleaved face that emits laser light, and a high reflection film (not shown) is provided on the rear end face. Thus, the semiconductor laser according to Example 2 is completed.

Here, for comparison, a semiconductor laser according to Comparative Example 3 having a layer structure shown in Table 4 that does not have the carrier stop layer 34 was manufactured by the same manufacturing method as the semiconductor laser according to Example 2.

  The oscillation wavelength of the semiconductor laser according to Example 2 at room temperature was 1063 nm. Further, the semiconductor laser according to Example 2 had a result that the threshold current was reduced by 21% and the differential quantum efficiency was improved by 14% compared with the semiconductor laser according to Comparative Example 3. Also, the semiconductor laser according to Example 2 was sufficiently reliable as with the semiconductor laser according to Example 1.

  Thus, even when the active layer 18 is a quantum well active layer having a quantum well structure, a conduction band larger than the conduction band edge energy of GaAs between the active layer 18 and the diffraction grating 26. By providing the carrier stop layer 34 having edge energy, it is possible to suppress non-radiative recombination at the regrowth interface of the diffraction grating layer 22 made of GaAs.

  In the second embodiment, the active layer 18 is a quantum well active layer composed of an InGaAs well layer and a GaAs barrier layer. However, the present invention is not limited to this. For example, a quantum well structure of a GaAsSb well layer and a GaAs barrier layer, a quantum well structure of a GaInNAs well layer and a GaAs barrier layer, or a quantum well structure of a GaInNAsSb well layer and a GaAs barrier layer. It may be a well structure. In the second embodiment, the active layer 18 is a multi-quantum well active layer having two layers. However, the active layer 18 may be a multi-quantum well active layer having two or more layers such as three or four layers. A single quantum well active layer may be used.

  In Example 1, the diffraction grating layer 22 is formed by the MBE method, and in Example 2, the diffraction grating layer 22 is formed by the MOCVD method. As described above, regardless of whether the diffraction grating layer 22 is formed by using the MBE method or the MOCVD method, or when another formation method is used, the diffraction grating layer 22 made of GaAs is used. A level that causes non-radiative recombination is formed at the regrowth interface. Therefore, regardless of the method of forming the diffraction grating layer 22, by providing the carrier stop layer 34 having a conduction band edge energy larger than that of GaAs between the active layer 18 and the diffraction grating 26, the diffraction grating layer made of GaAs. It is possible to suppress non-radiative recombination at 22 regrowth interfaces.

  The semiconductor laser according to Example 3 is a case where the diffraction grating 26 is provided between the active layer 18 and the n-type cladding layer 14. The semiconductor laser according to Example 3 includes a p-type GaAs substrate 10, a p-type GaAs buffer layer 12, a p-type AlGaAs p-type cladding layer 30, an undoped GaAs waveguide layer 16, and an InAs quantum dot. Active layer 18 having a quantum dot layer of GaAs and a GaAs barrier layer, n-type AlGaAs carrier stop layer 34, n-type GaAs waveguide layer 20, n-type GaAs diffraction grating layer 22, and n-type InGaP buried layer 24 , An n-type AlGaAs n-type cladding layer 14 and an n-type GaAs contact layer 32 are sequentially stacked. On the upper surface of the diffraction grating layer 22, a diffraction grating 26 having periodic unevenness is formed in the waveguide direction in which light oscillated in the active layer 18 propagates. The buried layer 24 is embedded so as to embed the diffraction grating 26. Is provided. The perspective view of the semiconductor laser according to the third embodiment is the same as the perspective view of the semiconductor laser according to the first embodiment shown in FIG.

  FIG. 7 is an example of an energy band diagram of the epi structure of the semiconductor laser according to Example 3. As shown in FIG. 7, a carrier stop layer 34 having a band gap energy larger than GaAs and a valence band edge energy smaller than that of GaAs is provided between the active layer 18 and the diffraction grating 26. This suppresses the holes in the active layer 18 from reaching the regrowth interface of the diffraction grating layer 22.

  As described above, according to Example 3, the diffraction grating 26 is provided between the active layer 18 and the n-type cladding layer 14, and the valence band is higher than that of GaAs between the active layer 18 and the diffraction grating 26. A carrier stop layer 34 having a small edge energy is provided. With such a structure, as described with reference to FIG. 7, it is possible to suppress holes that are minority carriers from reaching the regrowth interface of the diffraction grating layer 22. Thereby, even during high-temperature operation, non-radiative recombination of holes, which are minority carriers, and electrons, which are majority carriers, can be suppressed at the regrowth interface of the diffraction grating layer 22 made of GaAs, and the threshold current can be reduced. Improvement of differential quantum efficiency can be realized. In addition, reliability can be improved.

  In the third embodiment, the case where the active layer 18 has a quantum dot structure as in the first embodiment has been described as an example. However, the active layer 18 may have a quantum well structure as in the second embodiment.

  Example 4 is an example of an optical module including the semiconductor laser according to any one of Examples 1 to 3. FIG. 8 is an example of a block diagram of an optical module according to the fourth embodiment. As illustrated in FIG. 8, the optical module 200 includes the semiconductor laser 100 according to any one of the first to third embodiments, an optical waveguide unit 210, and an optical modulation unit 220. Laser light emitted from the semiconductor laser 100 is guided to the light modulation unit 220 through the optical waveguide unit 210. The light modulator 220 modulates the incident light. The modulated light is incident on the single mode fiber optically coupled by the optical fiber coupling unit 230 through the optical waveguide unit 210 and propagates in the single mode fiber.

  According to the fourth embodiment, since the semiconductor laser 100 in which non-radiative recombination is suppressed at the regrowth interface of the diffraction grating layer 22 made of GaAs is used, quantum efficiency is improved and reliability is improved. An optical module can be obtained.

  Example 5 is an example of an optical communication apparatus including the optical module according to Example 4. FIG. 9 is an example of a block diagram of an optical communication apparatus according to the fifth embodiment. As illustrated in FIG. 9, the optical communication device 300 includes the optical module 200 according to the fourth embodiment that functions as a transmission unit, a reception unit 310, and a control unit 320. The optical module 200 converts the transmission data signal from the control unit 320 into light and emits it. The emitted light enters the single mode fiber and propagates through the single mode fiber. The receiving unit 310 receives the light propagating through the single mode fiber and outputs the received data to the control unit 320 as received data.

  According to Example 5, since the semiconductor laser 100 in which non-radiative recombination is suppressed at the regrowth interface of the diffraction grating layer 22 made of GaAs is used, quantum efficiency is improved and reliability is improved. An optical communication device can be obtained.

  Example 6 is an example of an optical communication system including the optical communication apparatus according to Example 5. FIG. 10 is an example of a block diagram of an optical communication system according to the sixth embodiment. As shown in FIG. 10, the optical communication system 400 includes a single optical communication device 300a, a second optical communication device 300b, and a single optical device that connects the first optical communication device 300a and the second optical communication device 300b. Mode fiber 410. The first optical communication device 300a includes an optical module 200a, a receiving unit 310a, and a control unit 320a. The second optical communication device 300b includes an optical module 200b, a receiving unit 310b, and a control unit 320b. For example, when the optical module 200a of the first optical communication apparatus 300a converts the transmission data signal from the control unit 320a into light and emits the light, the emitted light propagates through the single mode fiber 410, and the second light Light is received by the receiving unit 310b of the communication device 300b. When light is received by the receiving unit 310b, received data is output to the control unit 320b. Similarly, when the optical module 200b of the second optical communication device 300b converts the transmission data signal from the control unit 320b into light and emits it, the emitted light propagates through the single mode fiber 410, and the first Light is received by the receiving unit 310a of the optical communication device 300a. When light is received by the reception unit 310a, reception data is output to the control unit 320a. Thereby, data communication can be performed between the first optical communication device 300a and the second optical communication device 300b.

  According to Example 6, since the semiconductor laser 100 in which non-radiative recombination is suppressed at the regrowth interface of the diffraction grating layer 22 made of GaAs is used, quantum efficiency is improved and reliability is improved. An optical communication system can be obtained.

  As in the sixth embodiment, the optical communication system 400 that performs data communication using the single mode fiber 410 between the first optical communication device 300a and the second optical communication device 300b is an FTTH (Fiber To The Home). And is preferably used for an optical communication backbone network. Alternatively, the first optical communication device 300a and the second optical communication device 300b may perform data communication by receiving light emitted into the space without using the single mode fiber 410. In this case, the first optical communication device 300a and the second optical communication device 300b can be personal computers, for example, and one of the first optical communication device 300a and the second optical communication device 300b is A personal computer may be used, and the other may be an electronic device such as a mobile phone, a digital camera, or a video camera, or a projector.

  The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific embodiments, and various modifications and changes can be made within the scope of the gist of the present invention described in the claims. It can be changed.

10 substrate 12 buffer layer 14 n-type cladding layer 16 GaAs waveguide layer 18 active layer 20 GaAs waveguide layer 22 diffraction grating layer 24 buried layer 26 diffraction grating 30 p-type cladding layer 32 contact layer 34 carrier stop layer 36 ridge portion 38 BCB Film 40 P electrode 42 N electrode 100 Semiconductor laser 200 Optical module 210 Optical waveguide unit 220 Optical modulation unit 230 Optical fiber coupling unit 300 Optical communication device 310 Reception unit 320 Control unit 400 Optical communication system 410 Single mode fiber

Claims (10)

An active layer provided on a substrate and having at least a part of a band gap energy smaller than that of GaAs;
A GaAs layer which is provided on the active layer and is a waveguide for propagating light oscillated from the active layer;
A diffraction grating provided on the upper surface of the GaAs layer;
An embedded layer provided to embed the diffraction grating;
And a carrier stop layer provided between the active layer and the diffraction grating and suppressing carrier overflow from the active layer to the interface between the GaAs layer and the buried layer. . The diffraction grating is provided between the active layer and the p-type cladding layer,
2. The semiconductor laser according to claim 1, wherein the carrier stop layer has a conduction band edge energy larger than a conduction band edge energy of GaAs. The diffraction grating is provided between the active layer and the n-type cladding layer,
2. The semiconductor laser according to claim 1, wherein the carrier stop layer has a valence band edge energy smaller than that of GaAs.   4. The semiconductor laser according to claim 1, wherein the carrier stop layer is provided between the active layer and the GaAs layer. 5.   5. The semiconductor laser according to claim 1, wherein a thickness of the carrier stop layer is not less than 50 nm and not more than 500 nm.   6. The semiconductor laser according to claim 1, wherein the carrier stop layer is made of AlGaAs or InGaP.   The semiconductor laser according to claim 1, wherein the active layer is a quantum well active layer or a quantum dot active layer.   An optical module comprising the semiconductor laser according to claim 1.   An optical communication apparatus comprising the semiconductor laser according to claim 1.   An optical communication system comprising the semiconductor laser according to claim 1.

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