JP4652061B2 - Semiconductor laser - Google Patents

Description

  The present invention relates to a semiconductor laser used for optical fiber communication, and more particularly to a high-power semiconductor laser necessary for exciting optical fiber amplification.

  In optical fiber communication, amplification of an optical signal that has been attenuated by propagating through a fiber is performed using, for example, a fiber amplifier using an erbium-doped fiber. In a fiber amplifier, a method of amplifying an attenuated optical signal with pumping light is used. A semiconductor laser that generates a high output is required for the light source of the excitation light.

  For example, in a Fabry-Perot type semiconductor laser, in order to obtain a high output, it is common to provide a low reflection film on the end surface on the light extraction side and a high reflection film on the other end surface.

  In such a structure having an asymmetric end face reflectivity, the electric field concentrates more strongly on the low reflection film side in the laser resonator. For this reason, a phenomenon called spatial hole burning occurs in which the carrier density on the low reflection film (outgoing) side decreases compared to the high reflection film side.

  Due to this phenomenon, there is a problem that the light emission efficiency from the low reflection film side of the laser is lower than in the case where the carrier density is constant in the resonator direction.

  In view of this, a means for optimizing the reflectivity of both end faces has been proposed in consideration of the spatial hole burning occurring in the laser cavity direction (see, for example, Patent Document 1). ).

  However, in this conventional example, since the optimization is performed under the influence of spatial hole burning, there is a problem that the emission efficiency becomes lower than when spatial hole burning is suppressed.

  In addition, for the purpose of suppressing the decrease in light emission efficiency due to spatial hole burning, there is a means to solve the problem by dividing the electrode in the direction of the resonator and causing more current to flow through the low reflection film side where a lot of carriers are consumed. (For example, refer to Patent Document 2).

However, in this conventional example, there is a problem that a process of forming two electrodes is required and two power supply devices are required for one laser, and the control device is complicated.
JP-A-6-85382 Japanese Patent Application Laid-Open No. 6-268312

  In the present invention, it is possible to inject current non-uniformly in the direction of the resonator by adopting a simple means of modifying a semiconductor laser having an active layer made of self-formed quantum dots, as in the prior art. In addition, a complicated configuration such as forming two or more electrodes for one laser or using two or more power supplies is not required.

  In a semiconductor laser using self-forming quantum dots according to the present invention, an active layer composed of self-forming quantum dots is formed, a high reflection film on one end face, and a low reflection film on the other end face The quantum dot density on the high reflection film side is larger than the quantum dot density on the low reflection film side.

  By adopting the above means, in the semiconductor laser with the asymmetric reflectance of the end face of the waveguide, the light emission efficiency of the semiconductor laser is formed by forming quantum dots with relatively low density on the low reflection film side. Can be increased. As a result, in a semiconductor laser in which the waveguide end-face reflectance is asymmetrical, it is possible to suppress a decrease in light emission efficiency due to spatial hole burning that occurs during laser oscillation, and such an effect is achieved by using one electrode and a power source. It can be easily achieved by the apparatus, and a high-power and high-efficiency semiconductor laser can be realized without using a complicated control apparatus.

  In the present invention, in the semiconductor laser having a structure in which the light reflectivity of the waveguide end face is asymmetrical, the low reflection film side is modified by changing the density of the self-forming quantum dots constituting the active layer. The light emission efficiency is improved.

  In order to change the density of the self-forming quantum dots, the lattice constants in the underlying semiconductor on which the self-forming quantum dots are formed can be selectively made different, or the manufacturing process can be modified. Realized.

FIG. 1 is a sectional side view showing a principal part of a semiconductor laser using a self-formed quantum dot according to the present invention. In FIG. 1, 1 is an n-type GaAs substrate, 2 is an n-type GaAs buffer layer, and 3 is an n-type buffer layer. Type Al _0.4 Ga _0.6 As cladding layer, 4 is a GaAs-SCH (separate confinement heterostructure) layer, 5A is an In _0.15 Ga _0.85 As first base region, 5B is a GaAs second base region, and 6 is an InAs quantum dot that is an active layer In _0.15 Ga _0.85 As cap layer covering 6A, 7 is a GaAs-SCH layer, 8 is a p-type Al _0.4 Ga _0.6 As cladding layer, 9 is a p-type Al _0.2 Ga _0.8 As lattice constant difference relaxation and cladding layer, 10 is p GaAs contact layer, 11 is a p-side electrode, 12 is an n-side electrode, HR is a high reflection film, and LR is a low reflection film.

In the semiconductor laser of FIG. 1, in order to form the In _0.15 Ga _0.85 As first ground region 5A and the GaAs second ground region 5B, an In _0.15 Ga _0.85 As layer to be the first ground region 5A is formed on the entire surface. Then, using a hard mask film having an opening in a portion where the second base region 5B is to be formed, etching the In _0.15 Ga _0.85 As layer by dry etching, and forming a GaAs second base region 5B on the etched portion The first ground region 5A and the second ground region 5B are completed by carrying out.

Thereafter, if quantum dots are formed on the first base region 5A and the second base region 5B, the density of the formed quantum dots 6A is 3.0 × 10 ¹⁰ cm ⁻² on the second base region 5B, In addition, it becomes 4.0 × 10 ¹⁰ cm ⁻² on the first base region 5A.

  Thereafter, each upper layer is formed on the active layer, and the density of the quantum dots 6A is small, that is, the low reflection film LR is formed on the side where the base is the GaAs second base region 5B, and the density of the quantum dots A is The high reflection film HR is formed on the side that is high, that is, the side where the base is the first base region 5A.

  At the time of laser oscillation, the photon density is increased on the laser emission side, so that the carrier density is insufficient on the front end face. However, in the structure according to the present invention, the optical confinement coefficient and gain in the forward direction of the laser resonator are low. Since it is small, the decrease in carrier density can be suppressed, and the light emission efficiency of the laser can be increased.

  2 to 4 show the semiconductor laser described in connection with FIG. 1, that is, the semiconductor laser in the process essential point for explaining the process of manufacturing the Fabry-Perot type semiconductor laser using the quantum dots in the active layer. FIG. 2 is a partial cutaway side view, and will be described below with reference to these drawings and FIG. 2 to 4, the same symbols as those used in FIG. 1 represent the same parts or have the same meanings.

In the semiconductor laser described here, the base layer on the front end face (output end face) side is, for example, GaAs, the base layer on the rear end face side is, for example, In _0.15 Ga _0.85 As, and the density of InAs quantum dots on the front end face side is on the rear end face side. The structure is relatively small compared to

See Fig. 2 (1)
An n-type GaAs buffer layer 2, an n-type AlGaAs cladding layer 3, a non-doped GaAs-SCH layer 4, and a non-doped InGaAs underlayer 5 are formed on the n-type GaAs substrate 1.

Here, the main data of each layer is illustrated as follows:
(A) n-type GaAs buffer layer 2 thickness: 500 nm
Doping concentration: 1.0 × 10 ¹⁸ cm ⁻³
(B) Al composition of n-type AlGaAs cladding layer 3: 40%
Doping concentration: 6.0 × 10 ¹⁷ cm ⁻³
Thickness: 1400nm
(C) Thickness of the non-doped GaAs-SCH layer 4: 150 nm
(D) In composition of non-doped InGaAs underlayer 5: 0.15
Thickness: 4nm

See Fig. 3 (2)
A resist pattern having a width of 300 μm and a length of 150 μm is formed by EB exposure or interference exposure, and then by applying a sputtering method, a SiO ₂ film is formed on the entire surface, and then the resist pattern is peeled off, The SiO ₂ film thereon is lifted off to form a mask pattern 13 made of SiO ₂ covering the first base region formation scheduled portion. L1 and L2 are both 150 μm.

See Fig. 4 (3)
After removing the resist, by applying a dry etching method or a wet etching method, the InGaAs underlayer 5 that is not covered with the mask pattern 13 is etched to form a step that is to become a portion where the second underlayer is to be formed. Form a region. Through this process, the first base region 5A is formed. Next, a second base region 5B made of GaAs having a thickness of 4 nm is formed to fill the stepped portion.

See Fig. 1 (4)
InAs quantum dots 6A are formed on the base layer composed of the first base region 5A and the second base region 5B. Next, a non-doped InGaAs cap layer 6 having a thickness of 4 nm and an In composition of 0.15 is formed. Next, a non-doped GaAs-SCH layer 7 having a thickness of 150 nm is formed. Next, a p-type AlGaAs cladding layer 8 having an Al composition of 0.4, a doping concentration of 1.0 × 10 ¹⁸ cm ⁻³ and a thickness of 1400 nm is formed. Next, a p-type AlGaAs cladding layer 9 having an Al composition of 0.2, a doping concentration of 2.0 × 10 ¹⁹ cm ⁻³ and a thickness of 20 nm is formed. Further, a p-type GaAs contact layer 10 having a doping concentration of 2.0 × 10 ¹⁹ cm ⁻² and a thickness of 400 nm is formed thereon.

(5)
Thereafter, a resist pattern having a groove having a width of 3.0 μm and a length of 300 μm is formed by applying an EB exposure method or an interference exposure method, and then a SiO ₂ film is applied by applying a sputtering method. The SiO ₂ film is patterned by lift-off that dissolves the resist pattern, and only the SiO ₂ film that covers the mesa formation planned portion is left and the others are removed, and the remaining SiO ₂ film is used as a mask to dry. An etching method is applied to form a ridge mesa having a height of 1500 nm and a width of 3 μm, and then the mask made of the SiO ₂ film is removed. Finally, a p-side electrode 11 is formed on the p-type GaAs contact layer 10, an n-side electrode 12 is formed on the back surface of the n-type GaAs substrate 1, and a low reflection film LR and a high reflection film HR are further formed. .

  FIG. 5 is a cutaway side view of an essential part for explaining an example in which the present invention is applied to a distributed feedback (DFB) type semiconductor laser using quantum dots in an active layer. The same symbols used indicate the same parts or have the same meaning.

The difference between the semiconductor laser of Example 2 and the semiconductor laser of Example 1 is that
(A) An uneven structure having a period of 200 nm and a height of 20 nm is formed on the surface of the n-type AlGaAs cladding layer 3 and buried with the non-doped GaAs-SCH layer 4 to form a diffraction grating 14. Note that 14A represents a λ / 4 phase shift.
(B) The base layer formed on the non-doped GaAs-SCH layer 4 is the non-doped InGaAs first base region 5A on the front end face (outgoing end face) side and the rear end face side, and the portion between them (λ / 4 The phase shift 14A and the vicinity thereof is the non-doped GaAs second base region 5B.
(C) As a result, the quantum dots 6A are formed at a high density on the front end face side and the rear end face side, and are formed at a low density in the central portion. An antireflective film AR is formed on the front end face and the rear end face.
It is in.

  Since the process for manufacturing the DFB type semiconductor laser of Example 2 can apply most of the process for manufacturing the semiconductor laser of Example 1, it relates to the above (a), (b), and (c). The process will be described.

(A) Forming a concavo-convex structure for producing a diffraction grating on the surface of the n-type AlGaAs cladding layer 3 can be realized by applying a well-known technique, for example, an EB exposure method and a dry etching method. It can be realized by applying and.
(B) To form the non-doped InGaAs first base region 5A and the non-doped GaAs second base region 5B, an undoped InGaAs layer having an In composition of 0.15 and a thickness of 4 nm is formed on the non-doped GaAs-SCH layer 4; Next, an EB exposure method or an interference exposure method is applied to form a resist pattern having a width of 300 μm and a length of 100 μm in the second base region formation scheduled portion, and then a SiO ₂ film is formed on the entire surface by sputtering, Then, the resist is peeled off, and the SiO ₂ film on the resist is lifted off to form a mask pattern made of SiO ₂ covering the first base region formation scheduled portions on the front end face side and rear end face side of the semiconductor laser, and dry By applying the etching method or wet etching method, the part where the mask pattern is not applied, that is, the central part A groove is formed by etching the non-doped InGaAs layer that is exposed in minutes, and then the second base region 5B is formed by selectively growing and embedding GaAs in the groove, and the first base region 5B is formed on both sides thereof. A structure in which the base region 5A is formed is produced.
(C) InAs quantum dots 6A on a base layer composed of a first base region 5A present at two locations on the front end surface side and the rear end surface side and a second base region 5B sandwiched between the two first base regions 5A. Is formed, the density of the quantum dots 6A in the central portion is low, and the density of the quantum dots 6A on both sides thereof is high.

  In the DFB type semiconductor laser that is the second embodiment described above, except for the points described in (a), (b), and (c), for example, the dimensions, impurity concentration, processing technique, etc. of the semiconductor layer are all the first embodiment. Is the same.

  Example 3 is a Fabry-Perot type semiconductor laser using quantum dots in the active layer, and the quantum dots have a low density on the front end face (emission end face) side as in Example 1, and on the rear end face side The density is increasing. Nevertheless, the underlying layer of these quantum dots is characterized only by a uniform GaAs layer. In order to realize such a configuration, it is necessary to devise the manufacturing process.

  FIG. 6 is a cutaway side view showing a principal part of a Fabry-Perot type semiconductor laser of Example 3. The same symbols as those used in FIGS. 1 to 5 represent the same parts or have the same meanings. And

  The feature of the semiconductor laser of Example 3 is that the underlayer of quantum dots having different densities is a GaAs-SCH layer 4 having a uniform composition.

  FIGS. 7 to 10 are sectional side views of the main part of the semiconductor laser in the process key points for explaining the process of manufacturing the Fabry-Perot type semiconductor laser of Example 3, and refer to these figures hereinafter. However, it will be explained. The symbols used in FIG. 1 to FIG. 6 represent the same parts or have the same meaning.

(1) An n-type GaAs buffer layer 2, an n-type AlGaAs cladding layer 3, and a non-doped GaAs-SCH layer 4 are formed on an n-type GaAs substrate 1. The dimensions, impurity concentration, composition, etc. of each layer are exactly the same as those in Example 1.

(2) Using the non-doped GaAs-SCH layer 4 as an underlayer, InAs quantum dots 6A having a surface density of 3.0 × 10 ¹⁰ cm ⁻² are formed. Of course, in this case, the density of the quantum dots 6A is uniform.

(3)
A non-doped InGaAs cap layer 6 for embedding the quantum dots 6A is formed.

(4)
After the non-doped GaAs-SCH layer 7 is formed, a mask pattern 13 made of SiO ₂ having a length L1 of 150 μm and a width of 300 μm is formed thereon.

Refer to FIG. 8 (5)
By applying the dry etching method, the non-doped GaAs-SCH layer 7, the non-doped InGaAs cap layer 6, and the quantum dots 6A are etched using the mask pattern 13 as a mask. According to this process, a step portion is generated in the portion not covered with the mask pattern 13.

Refer to FIG. 9 (6)
An InAs quantum dot 6A having a surface density of 1.5 × 10 ¹⁰ cm ⁻² is formed using the non-doped GaAs-SCH layer 4 exposed at the step portion as a base. Here, in order to change the surface density during the growth of the quantum dots 6A, the growth is performed at a temperature higher than the growth temperature of the quantum dots applied in the step (2) or the supply amount of InAs is reduced. Can be realized.

(7)
A non-doped InGaAs cap layer 6 and a non-doped GaAs-SCH layer 7 are stacked on the stepped portion.

Refer to FIG. 10 (8)
After removing the mask pattern 13, a p-type AlGaAs cladding layer 8, a p-type AlGaAs cladding layer 9, and a p-type GaAs contact layer 10 are formed.

(9)
A mask pattern made of SiO ₂ having a width of 3.0 μm and a length of 300 μm is formed, and a ridge mesa having a width W of 3.0 μm and a height H of 1500 nm is formed by applying a dry etching method.

(10)
A p-side electrode 11 is formed on the ridge mesa, and an n-side electrode 12 is formed on the n-type GaAs substrate side.

(11)
Finally, a high reflection film HR having a reflectance of 90% is provided on the end face on the side where the density of quantum dots is high (3.0 × 10 ¹⁰ cm ⁻² ), and the density of quantum dots is low (1.5 × 10 10 A low reflection film LR having a reflectivity of 5% is formed on the end face on the ¹⁰ cm ⁻² ) side.

  The third embodiment described above is an application example to a Fabry-Perot type semiconductor laser, but can also be applied to a DFB type semiconductor laser as described in the second embodiment. The formation of InAs quantum dots in a GaAs / AlGaAs-based material has been described, but this can be similarly applied to other materials such as InP / InGaAsP-based and InP / AlInGaAs-based materials. Furthermore, the quantum dot layer as the active layer may be multilayered, and in this case, the underlayer made of two or more kinds of semiconductors having different lattice constants may be multilayered.

  In the present invention, the present invention can be implemented in many forms including the above-described embodiment, which will be exemplified below as supplementary notes.

(Appendix 1)
In a semiconductor laser in which an active layer composed of self-forming quantum dots is formed, a high reflection film is formed on one end face, and a low reflection film is formed on the other end face.
A semiconductor laser characterized in that the density of quantum dots on the high reflection film side is larger than the density of quantum dots on the low reflection film side.

(Appendix 2)
A base layer of self-formed quantum dots having a plurality of base regions having different lattice constants is formed on the high-reflection film side and the low-reflection film side in the laser cavity axis direction ( The semiconductor laser according to Appendix 1).

(Appendix 3)
The semiconductor laser according to (Appendix 1) or (Appendix 2), comprising an active layer composed of a plurality of self-forming quantum dots.

(Appendix 4)
Underlayer of self-forming quantum dots composed of a plurality of underlayers having different lattice constants on the high-reflection film side and the low-reflection film side in the laser cavity axis direction, and self-formation generated on the underlayer A semiconductor laser as set forth in (Appendix 2), wherein a plurality of sets of layers are formed by using an active layer made of type quantum dots as a set of layers.

(Appendix 5)
Distributed feedback with a diffraction grating with a phase shift of λ / 4 formed at an arbitrary position in the resonator axis direction, an active layer made of self-forming quantum dots, and antireflection films formed on both end faces Type semiconductor laser,
The density of the self-molding quantum dots in the vicinity of the position where the phase shift exists is smaller than the density of the self-forming quantum dots extending from the position where the phase shift exists toward both end faces. A distributed feedback semiconductor laser.

(Appendix 6)
Self-deposited on an underlying layer composed of two types of semiconductors having different lattice constants between the underlying region near the position where the phase shift exists and the underlying region extending from the position where the phase shift exists toward both end faces. The distributed feedback semiconductor laser according to (Appendix 5), wherein an active layer made of a formed quantum dot is formed.

(Appendix 7)
The distributed feedback semiconductor laser according to (Appendix 5) or (Appendix 6), comprising an active layer made of a plurality of self-forming quantum dots.

(Appendix 8)
A base layer composed of two types of semiconductors having different lattice constants between a base region in the vicinity of the position where the phase shift exists and a base region extending in the direction of both end faces from the position where the phase shift exists, and the underlying layer The distributed feedback semiconductor laser according to (Appendix 6), wherein a plurality of sets of layers are formed with an active layer made of self-formed quantum dots generated on the formation as a set of layers.

It is a principal part cutting side view showing the semiconductor laser using the self-forming type | mold quantum dot by this invention. FIG. 2 is a cutaway side view showing a main part of a semiconductor laser at a process point for explaining a process of manufacturing the semiconductor laser described with reference to FIG. 1. FIG. 2 is a cutaway side view showing a main part of a semiconductor laser at a process point for explaining a process of manufacturing the semiconductor laser described with reference to FIG. 1. FIG. 2 is a cutaway side view showing a main part of a semiconductor laser at a process point for explaining a process of manufacturing the semiconductor laser described with reference to FIG. 1. It is a principal part cutting side view for demonstrating the example which implemented this invention to the distributed feedback semiconductor laser which used the quantum dot for the active layer. FIG. 6 is a cutaway side view showing a main part of a Fabry-Perot type semiconductor laser of Example 3. FIG. 6 is a side view of a main part of a semiconductor laser cut at a process key point for explaining a process of manufacturing a Fabry-Perot type semiconductor laser of Example 3. FIG. 6 is a side view of a main part of a semiconductor laser cut at a process key point for explaining a process of manufacturing a Fabry-Perot type semiconductor laser of Example 3. FIG. 6 is a side view of a main part of a semiconductor laser cut at a process key point for explaining a process of manufacturing a Fabry-Perot type semiconductor laser of Example 3. FIG. 6 is a side view of a main part of a semiconductor laser cut at a process key point for explaining a process of manufacturing a Fabry-Perot type semiconductor laser of Example 3.

Explanation of symbols

1 n-type GaAs substrate 2 n-type GaAs buffer layer 3 n-type Al _0.4 Ga _0.6 As cladding layer 4 GaAs-SCH layer 5A In _0.15 Ga _0.85 As first base region 5B GaAs second base region 6 In _0.15 Ga _0.85 As cap layer 7 GaAs-SCH layer 8 p-type Al _0.4 Ga _0.6 As cladding layer 9 p-type Al _0.2 Ga _0.8 As lattice constant difference relaxation and cladding layer 10 p-type GaAs contact layer 11 p-side electrode 12 n-side electrode HR high reflection film LR low Reflective film

Claims (2)

In a semiconductor laser in which an active layer composed of self-forming quantum dots is formed, a high reflection film is formed on one end face, and a low reflection film is formed on the other end face.
A semiconductor laser, wherein the density of in the quantum dots to the high reflection film side is large compared to the density of in the quantum dots to the low reflective film side. And wherein the base layer of self-assembled quantum dots of a plurality of base regions having different lattice constants at the laser resonator axis direction in the high reflection film side and the low-reflection film side is formed The semiconductor laser according to claim 1.

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