JP2009238913A - Distributed feedback semiconductor laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a distributed feedback semiconductor laser, having a structure that improves the planarity of crystal layer growth in the upper layer of a diffraction grating, and further, improves the reliability of an element by reducing the density of crystal defects. <P>SOLUTION: A first active layer 1A including a first diffraction grating 3a and a second active layer 1B including a second diffraction grating 3b are separated and arranged above and below so that the arrangement period for the first and second diffraction gratings 3a and 3b can be expanded. As a result, the area of grooves among the diffraction gratings are expanded so as to improve the removal effect of a mask material to be used during the formation of the diffraction gratings, reduce the density of internal dislocation (crystal defect), and attain the improved reliability for an element. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a structure of a semiconductor laser, particularly a distributed feedback semiconductor laser that oscillates in a single axis mode and is used in an optical communication system.

  Distributed feedback semiconductor lasers that oscillate in a single axis mode have been put into practical use, particularly in the trunk line system, due to the demand for an ultra-high-speed and wide-band optical communication system. Single-axis mode oscillation in this distributed feedback semiconductor laser is achieved by providing a diffraction grating layer in the vicinity of the active layer (optical waveguide) that is the core of the distributed feedback semiconductor laser, thereby causing periodic perturbations in the refractive index in the active layer. In addition, the oscillation spectrum is narrowed.

  However, in the configuration in which the diffraction grating layer is provided inside the element of the distributed feedback semiconductor laser, the crystal itself becomes a starting point of dislocation (crystal defect) to the crystal layer during crystal growth on the upper part of the diffraction grating layer. It is conceivable that the process problem for making it difficult to grow the crystal layer on the upper part of the diffraction grating layer. This crystal defect penetrates through the active layer at the time of crystal growth or device operation, and there is a problem that the device characteristics deteriorate and the device reliability deteriorates, for example, a non-light emitting region due to the crystal defect occurs in the active layer.

Regarding the difficulty of growing a crystal layer on the upper part of the diffraction grating layer, in the distributed feedback semiconductor laser manufacturing method disclosed in Patent Document 1 below, arsine supplied during the growth of the diffraction grating layer and the active layer is used. We pay attention to the partial pressure of (AsH ₃ ) and phosphine (PH ₃ ). Specifically, by making the partial pressure of phosphine (PH ₃ ) at the time of growing the diffraction grating larger than that at the time of growing the active layer, the size of the diffraction grating alone can be increased, and it grows on the upper part of the diffraction grating layer. The flatness of the active layer is improved.

However, in this distributed feedback semiconductor laser manufacturing method, the flatness can be improved when the diffraction grating layer is formed. However, in the process of removing the mask material used when forming the diffraction grating layer, the mask material cannot be completely removed due to the narrow gap between the diffraction grating grooves and the high viscosity of the solution used for the removal process. As a result, the mask material residue becomes a cause of crystal defects, which may reduce the reliability of the distributed feedback semiconductor laser.
JP 2004-71678 A

The problem to be solved by the present invention is that when the partial pressure of phosphine (PH ₃ ) during growth of the diffraction grating is made larger than that during the growth of the active layer, the flatness of the active layer can be improved, but it is used when forming the diffraction grating. In the mask material removal step, the mask material residue becomes a cause of crystal defects, and the element reliability is lowered.

  Accordingly, an object of the present invention is to provide a distributed feedback having a structure capable of improving the element reliability by improving the flatness of crystal layer growth in the upper layer of the diffraction grating layer and reducing the crystal defect density. It is to provide a type semiconductor laser.

  The distributed feedback semiconductor laser according to the present invention includes a semiconductor substrate, a first active layer provided on the semiconductor substrate, and a second active layer provided on the first active layer. The first active layer includes a first diffraction grating provided at a predetermined arrangement period, and the second active layer is shifted from the arrangement period of the first diffraction grating at a predetermined arrangement period. A second diffraction grating is provided.

  According to the distributed feedback semiconductor laser based on the present invention, by arranging the diffraction gratings so as to be divided into upper and lower parts, the interval between the diffraction gratings provided in each active layer can be increased as compared with the prior art. As a result, the mask material removal effect in the diffraction grating formation process formed using the mask material or the like can be enhanced. Furthermore, it is possible to improve the device reliability of the distributed feedback semiconductor laser by improving the flatness of crystal layer growth and reducing the crystal defect density.

  Embodiments of the present invention will be described below. The same or corresponding parts are denoted by the same reference numerals, and the description thereof may not be repeated.

(Reference technology)
First, referring to FIGS. 1 to 5, as a reference technique, a structure of a distributed feedback semiconductor laser 100B including a diffraction grating in an active layer and a manufacturing method thereof will be described. 1 is a cross-sectional view showing the structure of the distributed feedback semiconductor laser 100B. FIGS. 2 to 5 show first to fourth methods of manufacturing the distributed feedback semiconductor laser 100B having the structure shown in FIG. It is process sectional drawing.

(Structure of distributed feedback semiconductor laser 100B)
With reference to FIG. 1, the structure of the distributed feedback semiconductor laser 100 in the reference technique will be described. An n-type InP buffer layer 2, an n-type InGaAsP (indium: gallium: arsenic: phosphorus) diffraction grating layer 31, and an n-type InP cap layer 41 are provided on one side of an n-type InP (Indium Phosphide) substrate 1. Prepare.

  The n-type InP buffer layer 2, the n-type InGaAsP (indium: gallium: arsenic: phosphorus) diffraction grating layer 31, and the n-type InP cap layer 41 constitute an active layer diffraction grating 3, and the diffraction grating period is an arbitrary Bragg wavelength ( In the figure, λ) is set.

  Next, an n-type InP diffraction grating buried layer 51 is provided so as to cover the active layer diffraction grating 3, and the multiple quantum well active layer 6 and the p-type AlInAs cladding layer are formed on the n-type InP diffraction grating buried layer 51. 7 and a p-type InP contact layer 8 are provided. In the multiple quantum well active layer 6, a plurality of AlGaInAs well layers 11 and AlGaInAs barrier layers 12 are alternately stacked.

  A p-type electrode 9 is provided on the p-type InP contact layer 8, and an n-type electrode 10 is provided on the other surface side of the n-type InP substrate 1.

(Manufacturing method of distributed feedback semiconductor laser 100B)
Next, an outline of a manufacturing method of the distributed feedback semiconductor laser 100B having the above configuration will be described. Referring to FIG. 2, an n-type InP buffer layer 2 and an n-type InGaAsP diffraction grating layer are formed on one side of an n-type InP substrate 1 using MOCVD (Metal Organic Chemical Vapor Deposition) method or the like. 31, n-type InP cap layer 41 is formed.

  Next, referring to FIG. 3, after applying a resist on n-type InP cap layer 41, the resist is patterned by EB (electron beam) exposure or the like to form a mask. Thereafter, the n-type InP buffer layer 2, the n-type InGaAsP diffraction grating layer 31, and the n-type InP cap layer 41 are patterned by wet etching using sulfuric acid or the like, and the active layer diffraction grating 3 is patterned. The diffraction grating period formed at this time is set to an arbitrary Bragg wavelength (λ / 2 in the figure).

  Next, referring to FIG. 4, an n-type InP diffraction grating buried layer 51, a multiple quantum well active layer 6, a p-type AlInAs cladding layer 7, and a p-type InP contact layer 8 are formed on the active layer diffraction grating 3. Are sequentially formed. In the multiple quantum well active layer 6, a plurality of AlGaInAs well layers 11 and AlGaInAs barrier layers 12 are alternately stacked.

  Next, referring to FIG. 5, p-type electrode 9 is formed on p-type InP contact layer 8, and n-type electrode 10 is formed on the other surface side of n-type InP substrate 1. Thereby, the distributed feedback semiconductor laser 100 is completed.

  In the distributed feedback semiconductor laser 100B having the above-described configuration, the current confinement structure (not shown) in the multiple quantum well active layer 6 is not particularly mentioned. A current confinement structure may be applied, and the present invention is not limited to a specific current confinement structure.

  Strictly speaking, there is no longitudinal oscillation mode of the Bragg frequency corresponding to the Bragg wavelength by simply arranging the diffraction grating with a certain periodic interval λ / 2 in the distributed feedback semiconductor laser, and the Bragg frequency is equal to the frequency from the Bragg frequency. There are two longitudinal oscillation modes with minimum threshold gains at different low and high frequencies.

  In order to oscillate at a Bragg wavelength corresponding to the Bragg period of the diffraction grating, a configuration such as providing a λ / 4 phase shift unit in the laser element waveguide is required. However, distributed feedback in the embodiment of the present invention is required. In the type semiconductor laser, there is no problem even if these configurations are adopted.

(Embodiment)
Next, the structure of the distributed feedback semiconductor laser 100A in the embodiment of the present invention and the manufacturing method thereof will be described in comparison with the distributed feedback semiconductor laser 100B in the above-described reference technique. 6 is a sectional view showing the structure of the distributed feedback semiconductor laser 100A. FIGS. 7 to 11 show first to fifth methods of manufacturing the distributed feedback semiconductor laser 100A having the structure shown in FIG. It is process sectional drawing.

(Structure of distributed feedback semiconductor laser 100A)
Referring to FIG. 6, a first active layer 1A and a second active layer 1B provided on the first active layer 1A are provided on one surface side of an n-type InP substrate 1 which is a semiconductor substrate. Yes. The first active layer 1A includes an n-type InP buffer layer 2, an n-type InGaAsP diffraction grating layer 31, and an n-type InP cap layer 41 constituting an active layer lower diffraction grating 3a that is a first diffraction grating, and is below the active layer. The side diffraction grating 3 a is covered with an n-type InP diffraction grating buried layer 51. A multiple quantum well active layer 6 is provided on the n-type InP diffraction grating buried layer 51. The multiple quantum well active layer 6 has a plurality of AlGaInAs well layers 11 and AlGaInAs barrier layers 12 alternately stacked.

  The second active layer 1B is provided on the multiple quantum well active layer 6, and includes a p-clat layer 13, a p-type InGaAsP diffraction grating layer 32, and a p-type InP constituting the active layer upper diffraction grating 3b as the second diffraction grating. A cap layer 42 is provided, and the active layer upper diffraction grating 3 b is covered with a p-type InP diffraction grating buried layer 52. A p-type InP clad layer 7 and a p-type InP contact layer 8 are provided on the p-type InP diffraction grating buried layer 52. A p-type electrode 9 is provided on the p-type InP contact layer 8, and an n-type electrode 10 is provided on the multi-sided side of the n-type InP substrate 1.

  The diffraction grating periods of the active layer lower diffraction grating 3a and the active layer upper diffraction grating 3b are each set to a period of a Bragg wavelength (λ) corresponding to the oscillation frequency of a single longitudinal mode to be oscillated. The active layer lower diffraction grating 3a and the active layer upper diffraction grating 3b are arranged so that the phase is shifted by a half of the Bragg wavelength (λ).

(Manufacturing method of distributed feedback semiconductor laser 100A)
Next, an outline of a manufacturing method of the distributed feedback semiconductor laser 100A having the above configuration will be described. Referring to FIG. 7, an n-type InP buffer layer 2, an n-type InGaAsP diffraction grating layer 31, and an n-type InP cap layer 41 are formed on one side of an n-InP substrate 1 using MOCVD or the like. Note that the MOCVD method has been described as an example of the film formation method, but other methods such as the MBE (Molecular Beam Epitaxy) method may be used as long as the film formation method has few crystal defects.

  Next, referring to FIG. 8, after a resist is applied on n-type InP cap layer 41, the resist is patterned by EB exposure or the like to form a mask. Thereafter, the n-type InP buffer layer 2, the n-type InGaAsP diffraction grating layer 31, and the n-type InP cap layer 41 are patterned by wet etching using sulfuric acid or the like, and the active layer lower diffraction grating 3a is patterned. The diffraction grating period formed at this time is λ (= 2 · λ / 2). At this time, apart from the diffraction grating mask, a positioning marker is formed so that the mask position can be relatively specified.

  Next, referring to FIG. 9, an n-type InP diffraction grating buried layer 51 and a multiple quantum well active layer 6 are formed on the active layer lower diffraction grating 3 a. In the multiple quantum well active layer 6, a plurality of AlGaInAs well layers 11 and AlGaInAs barrier layers 12 are alternately stacked. Further, a p-type InP cladding layer 13, a p-type InGaAsP diffraction grating layer 32, and a p-type InP cap layer 42 are sequentially formed on the multiple quantum well active layer 6.

  Next, referring to FIG. 10, after applying a resist on p-type InP cap layer 42, the resist is patterned by EB exposure or the like to form a mask. Thereafter, the p-type InP cladding layer 13, the p-type InGaAsP diffraction grating layer 32, and the p-type InP cap layer 42 are patterned by wet etching using sulfuric acid or the like, and the active layer upper diffraction grating 3b is patterned.

  At this time, the position of the EB exposure is controlled by a positioning marker provided in the lower n-type InP buffer layer 2 and the like, and the diffraction grating periods of the active layer lower diffraction grating 3a and the active layer upper diffraction grating 3b are arbitrary single axes. It is formed so as to be shifted by λ / 2 of the Bragg wavelength of the Bragg frequency for mode oscillation.

  Next, referring to FIG. 11, a p-type InP diffraction grating buried layer 52, a p-type AlInAs cladding layer 7, and a p-type InP contact layer 8 are sequentially formed on the active layer upper diffraction grating 3b. Thereafter, a p-type electrode 9 is formed on the p-type InP contact layer 8 and an n-type electrode 10 is formed on the other surface side of the n-type InP substrate 1. Thereby, the distributed feedback semiconductor laser 100A shown in FIG. 6 is completed.

  As described above, according to the distributed feedback semiconductor laser 100A in the present embodiment, the active layer 1A including the first diffraction grating 3a and the second active layer 1B including the second diffraction grating 3b are arranged separately in the upper and lower sides. Thus, the arrangement period of each of the first diffraction grating 3a and the second diffraction grating 3b can be expanded. As a result, the groove area between the diffraction gratings can be expanded, improving the removal effect of the mask material used during diffraction grating formation, reducing the internal dislocation (crystal defect) density, and improving device reliability. The improvement effect can be obtained.

  Further, the structure of the distributed feedback semiconductor laser 100B described above includes a diffraction grating having a Bragg wavelength period λ on both the upper side and the lower side of the active layer, and the upper and lower phases are shifted by λ / 2. The upper and lower diffraction grating groove intervals (3X) are approximately three times the diffraction grating groove interval (X) in the case of the distributed feedback semiconductor laser 100B.

  Compared to the case of the distributed feedback semiconductor laser 100B, the gap width of the diffraction grating groove is increased by about three times. This means that even if the solution used for removing the mask is a highly viscous solution, it is sufficient in the diffraction grating groove portion. A mask material removal effect can be obtained.

  FIG. 12 is a diagram showing the relationship between the surface carbon (C) detection intensity ratio and the diffraction grating spacing obtained from the results of Auger electron spectroscopy measurement of the diffraction grating grooves. The vertical axis is an arbitrary scale, and the horizontal axis is the length of the diffraction grating interval. By widening the diffraction grating interval, the mask material residue can be remarkably reduced and the embedded crystal growth with reduced crystal defects becomes possible, so that a distributed feedback semiconductor laser having excellent reliability can be obtained. Note that the current confinement structure is not limited to a specific structure as described above.

  The active layer lower diffraction grating 3a and the active layer upper diffraction grating 3b included in the distributed feedback semiconductor laser 100A in the present embodiment have a Bragg wavelength corresponding to the oscillation frequency (Bragg frequency) of a single longitudinal mode to be oscillated. It has a period of λ, and the phase is different by λ / 2 between the upper side and the lower side, and the influence on the active layer (optical waveguide) is the same.

  In other words, the periodic change of the refractive index applied to the active layer is characterized by changing by λ / 4 in the extending direction of the active layer using both the upper and lower diffraction grating layers. This means that the influence on the extending direction of the active layer is equivalent on the upper side and the lower side. However, it only needs to have the same effect on the extending direction of the active layer, and does not specify anything about the distance of the diffraction grating from the active layer in the vertical direction, the size of the diffraction grating, or the like.

  In addition, it should be thought that the said embodiment disclosed this time is an illustration and restrictive at no points. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is sectional drawing which shows the structure of the distributed feedback semiconductor laser in a reference technique. It is 1st process sectional drawing which shows the manufacturing method of the distributed feedback semiconductor laser in a reference technique. It is 2nd process sectional drawing which shows the manufacturing method of the distributed feedback semiconductor laser in a reference technique. It is 3rd process sectional drawing which shows the manufacturing method of the distributed feedback semiconductor laser in a reference technique. It is a 4th process sectional view showing a manufacturing method of a distributed feedback type semiconductor laser in reference technology. It is sectional drawing which shows the structure of the distributed feedback type semiconductor laser in embodiment of this invention. It is 1st process sectional drawing which shows the manufacturing method of the distributed feedback type semiconductor laser in embodiment of this invention. It is 2nd process sectional drawing which shows the manufacturing method of the distributed feedback semiconductor laser in embodiment of this invention. It is 3rd process sectional drawing which shows the manufacturing method of the distributed feedback semiconductor laser in embodiment of this invention. It is a 4th process sectional view showing a manufacturing method of a distributed feedback type semiconductor laser in an embodiment of the invention. It is 5th process sectional drawing which shows the manufacturing method of the distributed feedback type semiconductor laser in embodiment of this invention. It is a figure which shows the relationship between the carbon (C) detection intensity ratio of the surface obtained from the result of the Auger electron spectroscopy measurement of the diffraction grating groove part, and a diffraction grating space | interval.

Explanation of symbols

  1 n-type InP substrate, 2 n-type InP buffer layer, 3a active layer lower diffraction grating, 3b active layer upper diffraction grating, 6 multiple quantum well active layer, 7 p-type InP cladding layer, 8 p-type InP contact layer, 9 p-type electrode, 10 n-type electrode, 11 AlGaInAs well layer, 12 AlGaInAs barrier layer, 13 p clat layer, 31 n-type InGaAsP diffraction grating layer, 32 p-type InGaAsP diffraction grating layer, 41 n-type InP cap layer, 42 p-type InP cap layer, 51 n-type InP diffraction grating buried layer, 52 p-type InP diffraction grating buried layer, 100A distributed feedback semiconductor laser.

Claims (2)

A semiconductor substrate;
A first active layer provided on the semiconductor substrate;
A second active layer provided on the first active layer,
The first active layer includes a first diffraction grating provided at a predetermined arrangement period,
The distributed active semiconductor laser, wherein the second active layer includes a second diffraction grating provided at a predetermined arrangement period in a state shifted from the arrangement period of the first diffraction grating. The arrangement period of the first diffraction grating and the second diffraction grating is arranged to have a Bragg wavelength period corresponding to the oscillation frequency of a single longitudinal mode to be oscillated,
2. The distributed feedback semiconductor laser according to claim 1, wherein the first diffraction grating and the second diffraction grating are arranged with a phase shifted by a half of a Bragg wavelength.

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