JP5119789B2 - Quantum dot semiconductor laser - Google Patents

Description

  The present invention relates to a quantum dot semiconductor laser suitable for use in, for example, coherent optical communication.

For example, when optical signals are multiplexed in coherent optical communication, a semiconductor laser having a stable and high wavelength purity, that is, excellent in single mode and a narrow spectral line width is required.
In particular, a quantum dot DFB laser (Distributed Feed Back Laser) that uses quantum dots in the light emitting layer (active layer) has excellent single mode characteristics that are characteristic of DFB lasers. As a result, the change in the refractive index with respect to the change in the carrier density becomes small, so that it is expected that an oscillation light with a narrow spectral line width can be obtained. Regarding the quantum dot DFB laser, there is, for example, Non-Patent Document 1.
Jin Soo Kim, et al., "InAs-InAlGaAs Quantum Dot DFB Lasers Based on InP (001)", IEEE PHOTONICS TECHNOLOGY LETTERS, VOL.18, NO.4, pp.595-597, FEBRUARY 15, 2006

By the way, such a quantum dot laser uses a semiconductor material (strained material) having a lattice constant different from that of the semiconductor substrate, and can be grown while being distorted so as to be lattice-matched to the semiconductor substrate (two-dimensional critical film). (Thickness) Self-assembled quantum dots using the Stranski-Krastanov (SK) growth mode, which is three-dimensionalized by growing above, are used for the active layer.
The self-formed quantum dots (SK quantum dots) formed in this way are two-dimensional with a critical film thickness or less (for example, a film thickness of 1 to 2 atomic layers) on the base as shown in FIG. With a wetting layer (Wet layer; WL) 101 spreading.

Moreover, as shown in FIG. 8, the shape of the self-formed quantum dot (SK quantum dot) 100 formed in this way is about 5 to 10 times the horizontal size, and is flat. It has a nice shape.
Furthermore, in general, a semiconductor material having a composition wavelength longer than that of the semiconductor substrate 102 is used for the quantum dots 100, so that the lattice constant of the semiconductor material forming the quantum dots 100 is larger than the lattice constant of the semiconductor substrate 102. Thus, compression distortion is applied inside the quantum dot.

Due to the shape and distortion of the quantum dot 100, the conventional quantum dot laser is a laser that oscillates in a TE mode in which the electric field of light oscillates in the horizontal direction.
However, with such a quantum dot laser, an ideal laser with a narrow spectral line width could not be realized.
The present invention was devised in view of such problems, and an object of the present invention is to provide a quantum dot semiconductor laser that can realize a semiconductor laser having excellent single mode characteristics and a narrow spectral line width.

Therefore, the quantum dot semiconductor laser of the present invention is formed so as to be connected to the semiconductor substrate, the quantum dot having a TM mode gain larger than the TE mode gain at the ground level, and the same material as the quantum dot. An active layer having a semiconductor layer made of a semiconductor material having a composition; and a diffraction grating . The active layer has quantum dots made of a semiconductor material having a lattice constant larger than that of the semiconductor substrate, and a wetting as a semiconductor layer. A quantum dot layer having a gate layer and a barrier layer made of a semiconductor material having a lattice constant smaller than the lattice constant of the semiconductor substrate, and the TM mode gain at the ground level is higher than the TE mode gain. The composite quantum dots are formed by stacking a plurality of layers so that a large composite quantum dot is formed. And the above 3.8 or less, the average strain amount of the quantum dot layer is characterized der Rukoto 1.0% or more.

  Therefore, according to the quantum dot semiconductor laser of the present invention, there is an advantage that a semiconductor laser having excellent single mode characteristics and a narrow spectral line width can be realized.

Hereinafter, a quantum dot semiconductor laser according to an embodiment of the present invention will be described with reference to the drawings.
[First Embodiment]
First, a quantum dot semiconductor laser according to a first embodiment of the present invention will be described with reference to FIGS.

The quantum dot semiconductor laser according to the present embodiment uses, as an active layer (light emitting layer), a quantum dot having a TM mode gain at the ground level larger than the TE mode gain, and includes a diffraction dot inside (in the resonator). Semiconductor lasers [for example, distributed feedback (DFB) lasers].
That is, the quantum dot semiconductor laser includes an n-type InP buffer layer 2 on an n-type InP substrate [here, an n-InP (100) substrate; a semiconductor substrate] 1 as shown in FIGS. N-type InGaAsP diffraction grating layer 3 having diffraction grating 3A, n-type InP lower cladding layer 4, SCH (Separate Confinement Heterostructure) layer (lower barrier layer; optical confinement layer) 5, quantum, Active layer (quantum dot active layer) 6 using dots, SCH layer (upper barrier layer: optical confinement layer) 7, p-type InP upper cladding layer 8, and p-type InP buried layer (block layer; current confinement) Layer) 9, n-type InP buried layer (block layer; current confinement layer) 10, p-type InGaAsP contact layer 11, p-side electrode 12, n-side electrode 13, and SiO ₂ film (insulating film) 14. And be prepared , Mesa structure 15 including the active layer 6 and the diffraction grating 3A is configured as a buried quantum dot semiconductor laser embedded by buried layers 9 and 10.

  Here, as shown in FIG. 3, the active layer 6 includes quantum dots (compressed strain quantum dots) 20 made of a semiconductor material having a lattice constant larger than that of the semiconductor substrate 1, and a wetting layer (quantum dots 20). And a barrier layer (tensile strain barrier) made of a semiconductor material having a lattice constant smaller than that of the semiconductor substrate 1 and a semiconductor layer made of a semiconductor material having the same material and composition as the quantum dots 20. A plurality of quantum dot layers 23 having (layer) 22. In FIG. 3, the active layer 6 is simply shown as a stack of six quantum dots 20, but in reality, 13 or more and 50 or less quantum dots 20 are formed as described later. It is assumed that they are stacked.

Here, as shown in FIG. 3, the active layer includes an InAs composite quantum dot (columnar quantum dot) 24 in which a plurality of InAs quantum dots 20 are stacked in close proximity, an InAs wetting layer 21, and a composite quantum dot 24. And an InGaAsP barrier layer (side barrier layer) 22 formed so as to be in contact with the side surface.
In the present embodiment, a plurality of quantum dot layers 23 are stacked to form an active layer so that the quantum dots 20 are joined vertically and a composite quantum dot 24 having a TM mode gain at the ground level larger than the TE mode gain is formed. 6 is formed.

Specifically, the aspect ratio of the composite quantum dot 24 is set to 1 or more and 3.8 or less. Here, the aspect ratio is obtained by dividing the height from the bottom surface of the lowermost quantum dot 20 to the apex of the uppermost quantum dot 20 by the lateral size of the quantum dot 20.
Further, the average strain amount of the quantum dot layer 23 is set to 0.5% or more. Here, the average strain amount of the quantum dot layer 23 is defined by the following formula (1).
((Ε _ba × t _ba ) + (ε _QD × t _QD )) / (t _ba + t _QD ) (1)
Here, ε _ba is the strain amount (%) of the barrier layer, t _ba is the thickness of the barrier layer (raw material supply amount) (ML), ε _QD is the strain amount (%) of the quantum dot, and t _QD is the quantum dot Raw material supply amount (ML). The sign of the strain amount is + for tensile strain and-for compressive strain.

Hereinafter, the principle of obtaining a semiconductor laser having a narrow spectral line width by setting the aspect ratio of the composite quantum dot 24 and the average distortion amount of the quantum dot layer 23 in this manner will be described.
First, the spectral line width Δν of the semiconductor laser satisfies the relationship of the following formula (2).

  Here, α is called a line width increase coefficient and is given by the following equation (3).

Here, λ is the oscillation wavelength, n _eff is the effective refractive index, g is the gain, and N is the carrier density.
From the above equation (2), it can be seen that the smaller the line width increase coefficient α, the narrower the spectral line width. From the above formula (3), it can be seen that the line width increase coefficient α decreases as the change in effective refractive index (∂n _eff / ∂N) due to the carrier density, called the plasma effect, decreases. Therefore, it can be seen that the smaller the change in effective refractive index (∂n _eff / ∂N), the narrower the spectral line width.

Until now, since the carriers accumulated in the quantum dots are confined three-dimensionally, the carriers do not vibrate due to the electric field of the oscillation light, and ∂n _eff / ∂N is assumed to be 0. Was theoretically considered to have a line width increase coefficient α of 0 and a narrow spectral line width.
However, as a result of detailed studies by the present inventors, the change due to the carrier density of the effective refractive index (∂n _eff / ∂N) includes the component due to the carriers accumulated in the wetting layer (wetting layer; WL). It was found that the component due to the carriers accumulated in the quantum dots was included.

That is, the change (∂n _eff / ∂N) due to the carrier density of the effective refractive index is given by the following equation (4).

  For this reason, in the quantum dot laser, only the second term on the right side of the equation (4) is 0, and the first term on the right side is not necessarily 0. In particular, it was found that in the conventional quantum dot lasers that oscillate in the TE mode, the first term on the right side of the above formula (4) cannot be zero. This is because in the conventional quantum dot laser that oscillates in the TE mode, carriers accumulated in the wetting layer (WL) can vibrate in the horizontal direction so as to follow the electric field vibration.

In contrast, in the case of a quantum dot laser that oscillates in the TM mode, the electric field oscillation of the oscillation light is in the vertical direction, and the carriers accumulated in the wetting layer (WL) are like quantum wells in the wetting layer (WL). Since it cannot move in the vertical direction due to the above properties, it is possible to suppress the plasma effect of the first term on the right side of the above equation (4) [that is, change in effective refractive index due to carrier density (∂n _eff / ∂N)]. As a result, the present inventors have found that the line width increase coefficient α can be reduced and an ideal quantum dot laser with a narrow spectral line width can be realized.

Next, the present inventors examined a quantum dot portion that controls characteristics in order to realize a quantum dot laser that oscillates in the TM mode.
Then, the present inventors use a composite quantum dot (columnar coupled quantum dot; columnar quantum dot) in which a plurality of quantum dots are stacked in close proximity, and is required to actually oscillate in the TM mode. As a result, a quantum dot having a larger TM mode gain than the TE mode gain was successfully formed, and the conditions required for the quantum dot required for TM mode oscillation were obtained. The conditions will be specifically described below.

In order to realize a quantum dot having a TM mode gain larger than the TE mode gain, it has been found that there is a lower limit on the aspect ratio of the columnar quantum dot 24 and the average distortion amount of the quantum dot layer 23.
Here, FIG. 2 shows the case where the average strain amount of the quantum dot layer 23 and the aspect ratio of the columnar quantum dot 24 are changed [that is, the thickness of the barrier layer 22 constituting the quantum dot layer 23 (supply of the raw material of the barrier layer). Amount) and the number of stacked quantum dots 20 (number of stacked quantum dot layers 23)], the change in the emission intensity ratio (TE / TM) of the TE polarization and the TM polarization in decibels (dB). It is shown by.

  Here, an InAs quantum dot 20 (compression strain 3.2%; quantum dot 20 is formed by supplying a raw material corresponding to 2 ML in terms of a two-dimensional film thickness on an n-type InP (100) substrate 1 is configured. And the InAs wetting layer 21 and the tensile strain of 3.7% (the lattice constant of the semiconductor material constituting the barrier layer 22 is the same as that of the semiconductor substrate 1). InAs columnar quantum dots 24 are formed by laminating a plurality of quantum dot layers 23 composed of InGaAsP barrier layers (composition wavelength: 1 μm) 22 which are 3.7% smaller than those. In this case, one quantum dot 20 has a height of about 1.2 nm and a lateral size of about 16 nm.

As shown in FIG. 2, in order to make the TM mode gain larger than the TE mode gain, that is, in order to make the emission intensity ratio (TE / TM) less than 0 dB, the aspect ratio of the columnar quantum dot 24 is set to 0. It has been found that it is necessary to make it .6 or more (preferably 1.0 or more).
This is because, by increasing the shape of the columnar quantum dots 24 and making them vertically long, it becomes more susceptible to compressive strain in the height direction than in-plane compressive strain, and therefore is more susceptible to tensile strain in the in-plane direction. As a result, it is considered that the transition component with light holes increases.

  On the other hand, the upper limit of the aspect ratio of the columnar quantum dots 24 is determined by the condition that the quantum dots 20 have effective quantum confinement in the height direction, and the height must be 50 nm or less. Since the range of the average value of the lateral size of the self-formed quantum dots is about 13 nm to 40 nm, and the minimum is about 13 nm, the upper limit of the aspect ratio of the composite quantum dots 24 is about 3.8. In addition, the range of the average value of the height of the self-formed quantum dot is about 1 to 3 nm.

  Specifically, when the lateral size of the quantum dots 20 is 13 nm and the height is 1 nm (the film thickness of the quantum dot layer 23 is 1 nm), the quantum dot layer 23 is 8 layers or more (preferably 13 layers or more), By laminating within the range of 50 layers or less, the aspect ratio of the composite quantum dots 24 can satisfy the condition of 0.6 or more (preferably 1.0 or more) and 3.8 or less.

Further, as shown in FIG. 2, in order to make the TM mode gain larger than the TE mode gain, that is, to make the emission intensity ratio (TE / TM) less than 0 dB, the average distortion of the quantum dot layer 23 It was found that the amount [see the above formula (1)] needs to be 0.5% or more (preferably 0.6% or more).
This is because by setting the average strain amount of the quantum dot layer 23 to 0.5% or more (preferably 0.6% or more), the inside of the quantum dots can be in a state of receiving tensile strain in the in-plane direction. it is conceivable that.

  Specifically, as described above, when the tensile strain amount of the InGaAsP barrier layer 22 is 3.7% and the raw material supply amount of the InAs quantum dots 20 is 2 ML (in this case, InAs having a compressive strain of 3.2%). (Quantum dots are formed), and by growing the raw material supply amount of the barrier layer 22 to 2.4 ML or more, the above equation (1) indicates that the average strain amount of the quantum dot layer 23 is 0.5% or more. Can be met.

In particular, from the viewpoint of manufacturing a high-efficiency laser, the emission intensity ratio (TE / TM) is preferably set to −3 dB or less. Therefore, the average strain amount of the quantum dot layer 23 is set to 1.0% or more. Is preferred.
Specifically, as described above, when the tensile strain amount of the InGaAsP barrier layer 22 is 3.7% and the raw material supply amount of the InAs quantum dots 20 is 2 ML (in this case, InAs having a compressive strain of 3.2%). (Quantum dots are formed), and by growing the raw material supply amount of the barrier layer 22 to 3.2 ML or more, the above equation (1) indicates that the average strain amount of the quantum dot layer 23 is 1.0% or more. Can be met.

Here, the InGaAsP barrier layer 22 has a tensile strain of 3.7% and a composition wavelength of 1 μm, but is not limited to this. For example, when the columnar quantum dot 24 that emits light in the optical communication wavelength band is formed, the composition wavelength of the barrier layer 22 may be set to about 0.9 to 1.2 μm. Therefore, In _x Ga _1-x As ₁ the composition of _-y P _y barrier layer 22, for example, as shown in FIG. 5, composition wavelength (here 0.9μm is 1.0 .mu.m, 1.1 .mu.m, 1.2 [mu] m) and tensile strain amount (here 3.2 %, 3.7%, 4.2%).

Hereinafter, a method for manufacturing the quantum dot semiconductor laser (embedded quantum dot semiconductor laser having columnar quantum dots) according to the present embodiment will be described.
In addition, for the crystal growth of each layer, for example, a metalorganic vapor-phase epitaxy (MOVPE) method is used.
First, as shown in FIG. 4A, the n-InP (100) substrate (semiconductor substrate) 1 is heated to 600 to 650 ° C. in a phosphine (PH ₃ ) atmosphere, for example, at a pressure in the reaction chamber of 50 Torr ( Temperature).

After the temperature is stabilized, trimethylindium (TMIn) and disilane (Si ₂ H ₆ ) are further supplied in a state where PH ₃ is supplied, whereby an n-InP substrate is obtained as shown in FIG. An n-InP buffer layer 2 is grown to 100 nm, for example, on 1 and then supplied with TMIn, PH ₃ , triethylgallium (TEGa), arsine (AsH ₃ ), and Si ₂ H ₆ to form an n-InGaAsP layer (diffraction grating) Underlayer 3X is grown, for example, by 30 nm, and then n-InP cap layer 4A is grown, for example, by 10 nm.

  Thereafter, part or all of the n-InP cap layer 4A and the n-InGaAsP layer 3X are periodically removed by a method such as lithography and etching, as shown in FIG. 4B, as shown in FIG. Further, the n-InP clad layer 4 is epitaxially grown on, for example, 300 to 500 nm thereon. Thereby, as shown in FIGS. 1 and 4B, a structure in which the n-InP buffer layer 2, the n-InGaAsP diffraction grating layer 3 having the diffraction grating 3A, and the n-InP cladding layer 4 are sequentially laminated is formed. The The n-InP cap layer 4A becomes a part of the n-InP clad layer 4.

Here, the n-type impurity concentration of the n-InP buffer layer 2, the n-InGaAsP diffraction grating layer 3, and the n-InP cladding layer 4 (including the n-InP cap layer 4A) is, for example, 5 × 10 ¹⁷ cm ⁻³ . is there.
Next, as shown in FIG. 1, on the n-InP cladding layer 4, SCH layer (barrier layer) made of _{In x Ga 1-x As 1} -y P y 5 is, for example, 100nm growth. Here, by setting the composition of the In _x Ga _1-x As _1-y P _y barrier layer (SCH layer) 5 to, for example, x = 0.78 and y = 0.52, there is no distortion at a composition wavelength of 1.2 μm. The barrier layer is formed.

Thereafter, the temperature is lowered to, for example, 430 to 450 ° C. which is a quantum dot growth temperature in a PH ₃ atmosphere.
After the temperature is stabilized, as shown in FIG. 3, the active layer including columnar quantum dots 24 on the In _x Ga _1-x As _{1-y Py} barrier layer (SCH layer) 5 ( Quantum dot active layer) 6 is formed.

First, by supplying TMIn and AsH ₃ , InAs quantum dots (self-formed quantum dots) 20 are formed on the In _x Ga _{1 -x} As _{1 -y Py} barrier layer (SCH layer) 5.
In this case, a wetting layer (wetting layer) 21 is formed by two-dimensional growth at the initial stage of growth, but when the critical film thickness is exceeded, three-dimensional island-shaped InAs quantum dots 20 are formed on the wetting layer 21. Will be formed.

Here, the Group III material (TMIn) is two-dimensionally thickened so that the supply ratio (V / III ratio) of the Group V material (AsH ₃ ) and the Group III material (TMIn) is in the range of 5-20. Supply is made at a flow rate (supply amount; supply condition) corresponding to 1 to 3 ML in terms of (layer thickness). Thus, quantum dots 20 having a height of 1 to 3 nm and a lateral size of 13 to 40 nm are formed.
After the InAs quantum dots 20 are formed in this way, TMIn, TEGa, AsH ₃ , and PH ₃ are supplied, whereby a tensile strain In _x Ga _1-x As _1-y P _y barrier is obtained as shown in FIG. Layer 22 is grown. As a result, the quantum dot layer 23 including the InAs quantum dots 20 and the tensile strain In _x Ga _1-x As _{1-y Py} barrier layer 22 is formed.

Here, In the composition of _{x Ga 1-x As 1-} y P y barrier layer 22 is not determined only tensile strain amount, determined in combination with the composition wavelength. For example, when forming the columnar quantum dots 24 that emit light in the optical communication wavelength band, the composition of the In _x Ga _1-x As _1-y P _y barrier layer 22 is, for example, as shown in FIG. 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm) and the amount of tensile strain (3.2%, 3.7%, 4.2% here) may be set.

On the other hand, the raw material supply amount (film thickness) of the tensile strain In _x Ga _{1 -x} As _{1 -y Py} barrier layer 22 can be determined from the raw material supply amount of the quantum dots 20 and the strain amount of the barrier layer 22.
For example, when the tensile strain amount of the tensile strain In _x Ga _{1 -x} As _{1 -y Py} barrier layer 22 is 3.7% and the raw material supply amount of the InAs quantum dots 20 is 2 ML (in this case, the compressive strain of 3 2% InAs quantum dots are formed), and by growing the tensile strain In _x Ga _1-x As _1-y P _y barrier layer 22 to 2.4 ML or more, the above formula (1) indicates that the quantum It is possible to satisfy the condition that the average distortion amount of the dot layer 23 is 0.5% or more.

In particular, from the viewpoint of manufacturing a highly efficient laser, the emission intensity ratio (TE / TM) is preferably −3 dB or less. In this case, the tensile strain In _x Ga _1-x As _1-y P _y barrier is used. When the tensile strain amount of the layer 22 is 3.7% and the raw material supply amount of the InAs quantum dots 20 is 2 ML (in this case, InAs quantum dots having a compressive strain of 3.2% are formed), the barrier layer 22 By growing with the raw material supply amount set to 3.2 ML or more, it is possible to satisfy the condition that the average strain amount of the quantum dot layer 23 is 1.0% or more from the above formula (1).

Thereafter, as shown in FIG. 3, a plurality of InAs quantum dots are grown by repeatedly growing a quantum dot layer 23 composed of InAs quantum dots 20 and tensile strained In _x Ga _1-x As _{1-y Py} barrier layers 22. Columnar quantum dots 24 formed by laminating dots 20 are formed.
Here, the height of the columnar quantum dots 24 can be controlled by the number of stacked quantum dot layers 23 (the number of repetitions). For example, when the lateral size of one quantum dot 20 is 13 nm and the height is 1 nm (the film thickness of the quantum dot layer is 1 nm), the quantum dot layer 23 is 8 layers or more (preferably 13 layers or more), 50 layers By laminating within the following range, the aspect ratio of the composite quantum dot 24 can satisfy the condition of 0.6 or more (preferably 1.0 or more) and 3.8 or less.

After forming the active layer (quantum dot active layer) 6 including the columnar quantum dots 24 in this way, as shown in FIG. 1, an In _x Ga _1-x As _{1-y Py} barrier layer (SCH layer) is formed. 7 is grown to 100 nm, for example, and a part of the p-InP cladding layer 8 is grown to 100 nm, for example. Here, the composition of the In _x Ga _1-x As _{1-y Py} barrier layer (SCH layer) 7 is set to, for example, x = 0.78 and y = 0.52, so that no distortion occurs at a composition wavelength of 1.2 μm. The barrier layer is formed.

Thereafter, a SiO ₂ mask is applied, and the mesa structure 15 is formed by a method such as lithography and etching.
Then, after the p-InP buried layer (block layer) 9 is grown so as to embed the mesa structure 15, an n-InP buried layer (block layer) 10 is further grown.
Next, the SiO ₂ mask is removed, and a part of the p-InP cladding layer 8 is grown on the entire surface by, for example, 2 to 3 μm. The p-type impurity concentration of the p-InP cladding layer 8 is, for example, 1 × 10 ¹⁸ cm ⁻³ .

Next, a p-InGaAsP contact layer 11 is grown on the p-InP cladding layer 8. The p-type impurity concentration of the p-InGaAsP contact layer 11 is 1 × 10 ¹⁹ cm ⁻³ , for example.
Thereafter, an axial end face from which light is emitted is formed by cleavage, and a resonator structure (cavity) is formed. An antireflection film may be formed on both end faces of the element. In this way, a quantum dot semiconductor laser is manufactured.

Therefore, according to the quantum dot semiconductor laser according to the present embodiment, there is an advantage that a semiconductor laser having excellent single mode characteristics and a narrow spectral line width can be realized.
[Second Embodiment]
Next, a quantum dot semiconductor laser according to a second embodiment of the invention will be described with reference to FIG.

The quantum dot semiconductor laser according to this embodiment is different from the above-described first embodiment in that it is a ridge type quantum dot semiconductor laser, whereas the quantum dot semiconductor laser is a buried type quantum dot semiconductor laser.
In other words, this quantum dot semiconductor laser is a quantum dot semiconductor laser (for example, a DFB laser) having a diffraction grating using a quantum dot having a TM mode gain at the ground level larger than the TE mode gain as an active layer (light emitting layer). For example, as shown in FIG. 6, an n-type InP buffer layer (not shown), an n-type InP lower cladding layer 4 and an SCH (Separate) are formed on an n-type InP substrate (semiconductor substrate) 1 as necessary. Confinement Heterostructure (separate confinement heterostructure) layer (lower barrier layer; optical confinement layer) 5, active layer (quantum dot active layer) 6 using columnar quantum dots, and SCH layer (upper barrier layer: optical confinement layer) 7, p-type InP upper cladding layer 8, p-type InGaAs contact layer 32, p-side electrode 12, and n-side electrode 13, and p-type InP upper cladding layer 8, -Type InGaAs contact layer 32 and has a ridge structure 30 comprising a p-side electrode 12, configured as a ridge quantum dot semiconductor laser diffraction grating 31 is formed on the side surfaces of the ridge structure 30. In FIG. 6, the same components as those in the first embodiment (see FIG. 1) are denoted by the same reference numerals.

Next, a manufacturing method of the quantum dot semiconductor laser (embedded quantum dot semiconductor laser having columnar quantum dots) according to the present embodiment will be described with reference to FIG.
First, as shown in FIG. 6, an n-type InP buffer layer (not shown), an n-type InP clad layer 4 and an SCH layer (lower barrier) are formed on an n-type InP substrate 1 as necessary by, for example, MOVPE. Layer) 5, an active layer 6 using columnar quantum dots, an SCH layer (upper barrier layer) 7, a p-type InP cladding layer 8, and a p-type InGaAs contact layer 32 are grown.

Next, as shown in FIG. 6, a ridge structure 30 having a diffraction grating 31 on the side surface is formed by a method such as lithography and etching.
Next, the p-side electrode 12 is formed on the upper surface of the ridge structure 30, that is, on the p-type InGaAs contact layer 32, and the n-side electrode 13 is formed on the back surface of the n-type InP substrate 1.
Thereafter, an axial end face from which light is emitted is formed by cleavage, and a resonator structure (cavity) is formed. An antireflection film may be formed on both end faces of the element. In this way, a quantum dot semiconductor laser is manufactured.

Other configurations and operations are the same as those in the first embodiment described above, and thus description thereof is omitted here.
Therefore, according to the quantum dot semiconductor laser according to the present embodiment, there is an advantage that a semiconductor laser having excellent single mode characteristics and a narrow spectral line width can be realized.
[Others]
In each of the above-described embodiments, a quantum dot having a TM mode gain at the ground level larger than the TE mode gain is realized by the columnar quantum dot. However, the present invention is not limited to this, and the TM at the ground level is not limited thereto. A quantum dot having a mode gain larger than the TE mode gain can also be produced using lithography, for example, as follows.

First, as shown in FIG. 7A, on a semiconductor substrate 40, a buffer layer 41, a quantum dot material layer 42 [film thickness corresponding to a desired quantum dot height (for example, 20 nm)], an unstrained barrier cap layer. 43 (for example, 10 nm) and SiO ₂ film 44 (for example, 100 nm) are grown in this order.
Next, as shown in FIG. 7B, for example, the SiO ₂ film 44, the unstrained barrier cap layer 43, and the quantum dot material layer 42 are removed by lithography and dry etching to form box-shaped quantum dots 42A. To do. Here, a part of the quantum dot material layer 42 remains so as to be continuous with the quantum dots 42A. That is, the semiconductor layer 42B made of a semiconductor material having the same material and composition as the quantum dots 42A is formed so as to be continuous with the quantum dots 42A.

Thereafter, as shown in FIG. 7C, the tensile strain barrier layer 45 is grown on the semiconductor layer 42B to such an extent that the gaps between the quantum dots 42A are filled.
Then, as shown in FIG. 7D, after the SiO ₂ film 44 is removed, an unstrained barrier layer 46 is grown on the entire surface (the unstrained barrier cap layer 43 is formed as a part of the unstrained barrier layer 46). Finally, in order to recover the crystallinity of the quantum dots 42A, annealing is performed at a predetermined temperature of about 750 ° C. for a predetermined time of about 1 minute, for example.

  In this way, for example, using lithography, the quantum dot 42A having a TM mode gain at the ground level larger than the TE mode gain is formed on the semiconductor substrate so as to be connected to the quantum dot 42A. A quantum dot semiconductor laser including an active layer 47 having a semiconductor layer 42B made of a semiconductor material having the same material and composition can also be produced.

In addition, what is necessary is just to set the specific structure of a semiconductor substrate, a quantum dot, and a barrier similarly to the above-mentioned each embodiment. Although not shown here, the quantum dot semiconductor laser is configured to include a diffraction grating as in the above-described embodiments.
Further, in each of the above-described embodiments, a typical quantum dot semiconductor laser (DFB laser) of a buried type and a ridge type is described as an example. However, the present invention is not limited to this, The active layer has a quantum dot having a TM mode gain at the ground level larger than the TE mode gain, and a semiconductor layer formed to be connected to the quantum dot and made of a semiconductor material having the same material and composition as the quantum dot. The present invention can be widely applied to dot semiconductor lasers. For example, the configuration and manufacturing method other than the active layer may be other configurations and manufacturing methods.

In each of the above-described embodiments, the case where the present invention is applied to a DFB laser is described as an example. However, the present invention is not limited to this. For example, the present invention is applied to a distributed BraggReflector laser. You can also
In each of the above-described embodiments, the semiconductor laser formed on the n-type InP substrate (first conductive type semiconductor substrate) is described as an example, but the present invention is not limited to this. For example, it may be formed on a p-type InP substrate (second conductivity type semiconductor substrate) or a high-resistance InP substrate (SI-InP substrate).

  In each of the above-described embodiments, an example in which the present invention is applied when the barrier layer is formed of a semiconductor crystal made of InGaAsP is described. However, the present invention is not limited to this example. , GaInNAs, and other III-V group compound semiconductor materials (III-V group compound semiconductor mixed crystals) can be applied to the present invention.

  Further, in each of the above-described embodiments, an example in which the present invention is applied when the quantum dot is configured by an InAs semiconductor crystal is described. However, the present invention is not limited to this example. The present invention can also be applied to a case where it is constituted by other III-V group compound semiconductor materials (III-V group compound semiconductor mixed crystals).

  In the first embodiment described above, the buried layer is a p-type InP layer and an n-type InP layer, but is not limited to this. For example, a semi-insulating InP buried layer such as an Fe—InP layer is used. (High-resistance semiconductor layer) [SI-PBH (semi-insulating blocked planar buried heterostructure) structure or SI-BH (Semi-Insulating Buried Heterostructure) structure].

The present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention.
Hereinafter, additional notes will be disclosed regarding each of the above-described embodiments.
(Appendix 1)
A semiconductor substrate;
An active layer having a quantum dot having a TM mode gain at the ground level larger than a TE mode gain, and a semiconductor layer formed to be continuous with the quantum dot and made of a semiconductor material having the same material and composition as the quantum dot; ,
A quantum dot semiconductor laser comprising a diffraction grating.

(Appendix 2)
The active layer includes quantum dots made of a semiconductor material having a lattice constant larger than the lattice constant of the semiconductor substrate, a wetting layer as the semiconductor layer, and a semiconductor having a lattice constant smaller than the lattice constant of the semiconductor substrate. A plurality of quantum dot layers having a barrier layer made of a material are laminated so that the quantum dots are joined vertically to form a composite quantum dot having a TM mode gain at the ground level larger than the TE mode gain. 2. The quantum dot semiconductor laser as set forth in appendix 1, wherein

(Appendix 3)
The quantum dot semiconductor laser according to appendix 2, wherein an aspect ratio of the composite quantum dot is 1 or more and 3.8 or less.
(Appendix 4)
4. The quantum dot semiconductor laser according to appendix 2 or 3, wherein an average strain amount of the quantum dot layer is 0.5% or more.

(Appendix 5)
A mesa structure including the active layer and the diffraction grating;
The quantum dot semiconductor laser according to any one of appendices 1 to 4, further comprising an embedded layer in which the mesa structure is embedded.
(Appendix 6)
With ridge structure,
The quantum dot semiconductor laser according to any one of appendices 1 to 4, wherein the diffraction grating is formed on a side surface of the ridge structure.

1 is a schematic diagram showing a configuration of a quantum dot semiconductor laser (embedded quantum dot semiconductor laser) according to a first embodiment of the present invention. In the quantum dot semiconductor laser according to the first embodiment of the present invention, the emission intensity ratio of TE polarization and TM polarization (TE / TM) when the average distortion amount of the quantum dot layer and the aspect ratio of the columnar quantum dot are changed. It is a figure which shows the change of (). It is a typical sectional view showing the composition of the active layer which consists of the columnar quantum dot and barrier layer which constitute the quantum dot semiconductor laser concerning a 1st embodiment of the present invention. (A), (B) is typical sectional drawing which shows the structure of the diffraction grating part which comprises the quantum dot semiconductor laser concerning 1st Embodiment of this invention, and its manufacturing method. Is a diagram showing a composition example in relation to the first pull constituting the quantum dot semiconductor laser according to the embodiment distortion _{In x Ga 1-x As 1} -y P y distortion amount and composition wavelength of the barrier layers of the present invention. It is a schematic diagram which shows the structure of the quantum dot semiconductor laser (ridge type quantum dot semiconductor laser) concerning 2nd Embodiment of this invention. (A)-(D) are typical sectional drawings which show the structure and manufacturing method of the quantum dot semiconductor laser concerning the modification of each embodiment of this invention. It is a schematic diagram which shows the relationship between the SK quantum dot structure used for the conventional quantum dot semiconductor laser, and the polarization direction of light.

Explanation of symbols

1 n-type InP substrate (semiconductor substrate)
2 n-type InP buffer layer 3 n-type InGaAsP diffraction grating layer 3A diffraction grating 3X n-type InGaAsP layer 4 n-type InP lower cladding layer 4A n-type InP cap layer 5 SCH layer (lower barrier layer)
6 Active layer (Quantum dot active layer)
7 SCH layer (upper barrier layer)
8 p-type InP upper cladding layer 9 p-type InP buried layer (block layer)
10 n-type InP buried layer (block layer)
11 p-type InGaAsP contact layer 12 p-side electrode 13 n-side electrode 14 SiO ₂ film (insulating film)
15 Mesa Structure 20 InAs Quantum Dot 21 InAs Wetting Layer 22 InGaAsP Barrier Layer 23 Quantum Dot Layer 24 InAs Composite Quantum Dot (Columna Quantum Dot)
30 ridge structure 31 diffraction grating 32 p-type InGaAs contact layer 40 semiconductor substrate 41 buffer layer 42 quantum dot material layer 42A quantum dot 42B semiconductor layer 43 unstrained barrier cap layer 44 SiO ₂ film 45 tensile strain barrier layer 46 unstrained barrier layer 47 Active layer

Claims (2)

A semiconductor substrate;
An active layer having a quantum dot having a TM mode gain at the ground level larger than a TE mode gain, and a semiconductor layer formed to be continuous with the quantum dot and made of a semiconductor material having the same material and composition as the quantum dot; ,
A diffraction grating ,
The active layer includes quantum dots made of a semiconductor material having a lattice constant larger than the lattice constant of the semiconductor substrate, a wetting layer as the semiconductor layer, and a semiconductor having a lattice constant smaller than the lattice constant of the semiconductor substrate. A plurality of quantum dot layers having a barrier layer made of a material are laminated so that the quantum dots are joined vertically to form a composite quantum dot having a TM mode gain at the ground level larger than the TE mode gain. Configured,
The composite quantum dot has an aspect ratio of 0.8 or more and 3.8 or less,
The average amount of strain of the quantum dot layer, a quantum dot semiconductor laser, characterized in der Rukoto 1.0% or more. A mesa structure including the active layer and the diffraction grating;
The quantum dot semiconductor laser according to claim 1, further comprising an embedded layer in which the mesa structure is embedded.

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