JP7296845B2 - MODULATION DOPED SEMICONDUCTOR LASER AND MANUFACTURING METHOD THEREOF - Google Patents

Description

The present invention relates to modulation doped semiconductor lasers and methods of manufacturing the same.

A modulation-doped semiconductor laser in which an acceptor is added to a barrier layer of a multiple quantum well (MQW) is known (Patent Document 1).

JP-A-10-284799 JP 2012-220530 A

Zn (zinc) is mainly used as a p-type dopant in the MOCVD (Metal Organic Chemical Vapor Deposition) method, which is often used in the crystal growth of optical semiconductor devices. Zn tends to diffuse during crystal growth. Therefore, the carrier concentration of the p-side SCH decreases and the carrier concentration of the MQW increases. Patent Document 1 discloses the use of beryllium or the like, which has low diffusion properties, instead of Zn, but beryllium cannot be used for MOCVD. It is also known that the p-type dopant diffuses into the MQW, increasing the carrier concentration in the MQW (Patent Document 2).

An object of the present invention is to secure an appropriate carrier concentration.

(1) A modulation doped semiconductor laser according to the present invention comprises a plurality of layers including a plurality of first layers and a plurality of second layers, which are alternately stacked, a multiple quantum well containing acceptors and donors, and the plurality of layers and an n-type semiconductor layer contacting the bottom layer of the plurality of layers, wherein the plurality of first layers have a p-type carrier concentration in the p-type semiconductor layer The acceptors are contained so as to be 10% or more and 150% or less of the p-type carrier concentration, and the plurality of second layers have a p-type carrier concentration of 10% or more and 150% or less of the p-type semiconductor layer. The plurality of second layers contain the acceptor, and the plurality of second layers further contain the donor, and in the plurality of second layers, the effective carrier concentration corresponding to the difference between the p-type carrier concentration and the n-type carrier concentration is equal to the above It is characterized by being 10% or less of the p-type carrier concentration of the plurality of second layers.

According to the present invention, since the multiple quantum well has a high p-type carrier concentration, diffusion of acceptors from the p-type semiconductor layer can be suppressed and an appropriate carrier concentration is ensured. On the other hand, the second layer of multiple quantum wells has a lower effective carrier concentration to make the electric field uniform.

(2) The modulation doped semiconductor laser described in (1) may be characterized in that the p-type semiconductor layer and the n-type semiconductor layer are for forming a separate confinement heterostructure.

(3) The modulation doped semiconductor laser according to (1) or (2), wherein the p-type carrier concentration is 1× in each of the plurality of first layers and each of the plurality of second layers. It may be characterized by being 10 ¹⁷ cm ⁻³ or more.

(4) The modulation doped semiconductor laser according to any one of (1) to (3), wherein each of the top layer and the bottom layer of the plurality of layers corresponds to the first layers of the plurality of layers. It may be characterized as being one to do.

(5) The modulation doped semiconductor laser according to any one of (1) to (3), wherein each of the top layer and the bottom layer of the plurality of layers corresponds to the plurality of second layers. It may be characterized as being one to do.

(6) The modulation doped semiconductor laser according to any one of (1) to (5), wherein each of the plurality of first layers is a barrier layer, and each of the plurality of second layers may be characterized as a quantum well layer.

(7) In the modulation doped semiconductor laser according to any one of (1) to (5), each of the plurality of first layers is a quantum well layer, and each of the plurality of second layers is a quantum well layer. Each may be characterized as a barrier layer.

(8) The modulation doped semiconductor laser according to any one of (1) to (6), wherein the acceptor is at least one of Zn and Mg, and the donor is Si. may be

(9) The modulation doped semiconductor laser according to any one of (1) to (8), wherein the plurality of second layers has a p-type carrier concentration lower than that of the p-type semiconductor layer. It may be characterized by

(10) The modulation doped semiconductor laser according to any one of (1) to (9), wherein the plurality of second layers has a lower n-type carrier concentration than the n-type semiconductor layer. It may be characterized by

(11) A method for manufacturing a modulation doped semiconductor laser according to the present invention comprises a step of forming an n-type semiconductor layer, a plurality of layers including a plurality of first layers and a plurality of second layers alternately laminated, and an acceptor layer. and a donor, forming multiple quantum wells such that the bottom layer of the plurality of layers overlies in contact with the n-type semiconductor; forming a p-type semiconductor layer by vapor deposition, wherein the plurality of first layers have a p-type carrier concentration of 10% or more and 150% or less of the p-type semiconductor layer, The plurality of second layers containing the acceptor contain the acceptor such that the p-type carrier concentration is 10% or more and 150% or less of the p-type semiconductor layer, and the plurality of second layers further contains the donor, and in the plurality of second layers, an effective carrier concentration corresponding to the difference between the p-type carrier concentration and the n-type carrier concentration is greater than the p-type carrier concentration of the plurality of second layers It is characterized by being 10% or less.

According to the present invention, since the multiple quantum well has a high p-type carrier concentration, diffusion of acceptors from the p-type semiconductor layer can be suppressed and an appropriate carrier concentration is ensured. On the other hand, the second layer of multiple quantum wells has a lower effective carrier concentration to make the electric field uniform.

(12) The method of manufacturing a modulated doped semiconductor laser described in (11) may be characterized in that the multiple quantum wells are formed by the metal-organic chemical vapor deposition method.

1 is a plan view of a modulation doped semiconductor laser according to a first embodiment; FIG. 2 is a cross-sectional view of the modulation doped semiconductor laser shown in FIG. 1 taken along the line II-II. FIG. It is a figure which shows the carrier concentration of a donor (n-type dopant). It is a figure which shows the carrier concentration of an acceptor (p-type dopant). It is a figure which shows the difference (effective carrier concentration) of a p-type carrier density|concentration and an n-type carrier density|concentration. FIG. 10 is a diagram showing carrier concentrations of donors (n-type dopants) in the second embodiment. FIG. 10 is a diagram showing carrier concentrations of acceptors (p-type dopants) in the second embodiment; FIG. 10 is a diagram showing the difference (effective carrier concentration) between the p-type carrier concentration and the n-type carrier concentration in the second embodiment;

Hereinafter, embodiments of the present invention will be described specifically and in detail with reference to the drawings. Members denoted by the same reference numerals in all drawings have the same or equivalent functions, and repeated description thereof will be omitted. Note that the size of the figure does not necessarily match the magnification.

[First Embodiment]
FIG. 1 is a plan view of a modulated doped semiconductor laser according to a first embodiment. FIG. 2 is a cross-sectional view of the modulation doped semiconductor laser shown in FIG. 1 taken along line II-II.

The modulation doped semiconductor laser may be a distributed feedback semiconductor laser (DFB laser). The modulation doped semiconductor laser may be a modulator-integrated semiconductor optical device (for example, a modulator-integrated laser) in which a modulation section (not shown) is monolithically integrated. A modulator integrated laser modulates continuous light emitted by injecting a driving current into a laser section in a modulation section to output a signal light. The modulator may be an electro-absorption modulator (EA (Electro-Absorption) modulator). The electro-absorption modulator has advantageous characteristics such as small chirp (wave modulation), large extinction ratio, which is the difference between the ON level and the OFF level of the optical signal, and a wide band. It is widely used due to its cost.

A modulation doped semiconductor laser has a ridge structure. The ridge structure is a structure in which multiple quantum wells, which will be described later, spread laterally, and a semiconductor mesa stripe (waveguide) is formed above them. A buried heterostructure (BH structure) may also be used. The BH structure refers to a structure in which multiple quantum wells are located within a mesa stripe and buried layers are provided on both sides of the mesa stripe structure. The BH structure has a strong effect of confining light in the horizontal direction, and the FFP (Far Field Pattern) is more circular, so it has the advantage of high coupling efficiency with optical fibers, and has the advantage of excellent heat dissipation. There is

[n-type semiconductor layer]
The modulation doped semiconductor laser has an n-type semiconductor layer 12 (InGaAlAs layer) on a lower clad layer 10 (n-type InP layer). A donor (n-type dopant) of the n-type semiconductor layer 12 is Si. Si is known to have little diffusion during crystal growth. The n-type semiconductor layer 12 is for constructing a separate confinement heterostructure (SCH).

[Multiple quantum well]
The modulation doped semiconductor laser has a Multiple-Quantum Well (MQW) 14 . Multiple quantum well 14 contains acceptors (p-type dopants) and donors (n-type dopants). The multiple quantum well 14 is composed of multiple layers, and the bottom layer is in contact with the n-type semiconductor layer 12 .

The multiple layers that make up multiple quantum well 14 include multiple first layers 16 . The top layer of the multiple layers is the first layer 16 . The bottom layer of the multiple layers is also the first layer 16 . Each of the plurality of first layers 16 is a barrier layer (InGaAlAs layer). The multiple layers that make up multiple quantum well 14 include multiple second layers 18 . Each of the plurality of second layers 18 is a quantum well layer (InGaAlAs layer). The plurality of first layers 16 and the plurality of second layers 18 are alternately laminated.

The first layer 16 (barrier layer) contains an acceptor (p-type dopant). The acceptor is at least one of Zn and Mg. The second layer 18 (quantum well layer) also contains an acceptor (p-type dopant). The second layer 18 (quantum well layer) also contains donors (n-type dopants). The donor is Si (the same material as the donor of the n-type semiconductor layer 12).

[p-type semiconductor layer]
The modulation doped semiconductor laser has a p-type semiconductor layer 20 (InGaAlAs layer). The acceptor of the p-type semiconductor layer 20 is, for example, at least one of Zn and Mg (the same material as the acceptor of the first layer 16), and it is extremely difficult to suppress diffusion. The p-type semiconductor layer 20 is in contact with the top layer (first layer 16) of the multiple quantum wells 14. As shown in FIG. The p-type semiconductor layer 20 is for forming a separate confinement heterostructure (SCH). An upper clad layer 22 (p-type InP layer) is laminated on the p-type semiconductor layer 20 . A diffraction grating 28 is formed above the p-type semiconductor layer 20 .

There is an electrode 24 (eg, cathode) on the back surface of the lower cladding layer 10 . Above the upper cladding layer 22 is an electrode 26 (eg, anode) for applying a voltage facing an electrode 24 (eg, cathode).

[Carrier concentration]
3A to 3C are diagrams showing carrier concentrations near multiple quantum wells (MQW) in the first embodiment. Here, the carrier concentration indicates the density of the added impurity. Strictly speaking, not all doped impurities function as carriers, but for the sake of simplification of explanation, it is assumed here that all impurities function as carriers. Even when impurities are not intentionally added, semiconductors contain very small amounts of various impurities. 1×10 ¹⁵ cm ⁻³ or less.

In FIGS. 3A to 3C, the width of the multiple quantum well 14 (MQW) is enlarged on the horizontal axis for ease of explanation, and the actual ratio differs from that of other layers. Furthermore, although the actual profile gradually changes at the interface of each layer due to the diffusion of dopants due to the growth of multiple layers, the change in carrier concentration is shown here for the sake of explanation.

FIG. 3A is a diagram showing carrier concentrations of donors (n-type dopants). The upper clad layer 22 (p-clad) and the p-type semiconductor layer 20 (p-SCH) are not doped with Si as a donor. A quantum well layer (W2), which is the second layer 18 of the multiple quantum well 14 (MQW), is doped with Si (2×10 ¹⁷ cm ⁻³ ) as a donor. The n-type semiconductor layer 12 (n-SCH) is doped with 1×10 ¹⁸ cm ⁻³ of Si as an impurity. The quantum well layer (W2) has a lower n-type carrier concentration than the n-type semiconductor layer 12 . An n-type dopant is added to the lower clad layer 10 (n-clad) from the beginning. An example of Si is shown here, but other donors may be used.

FIG. 3B is a diagram showing carrier concentrations of acceptors (p-type dopants). Zn, which is a p-type dopant, is not added to the n-type semiconductor layer 12 (n-SCH). Zn, which is a p-type dopant, is added to the upper clad layer 22 (p-clad). Note that the acceptor may be Mg.

The p-type semiconductor layer 20 (p-SCH) and the upper clad layer 22 (p-clad) are each doped with 1×10 ¹⁸ cm ⁻³ of Zn as an impurity. In the multiple quantum well 14 (MQW), both the barrier layer (B1) and the quantum well layer (W2), which are the first layers 16, are doped with 2×10 ¹⁷ cm ⁻³ of Zn.

In the multiple quantum well 14 (MQW), both the barrier layer (B1) and the quantum well layer (W2) have a p-type carrier concentration of 1×10 ¹⁷ cm ⁻³ or more. In the multiple quantum well 14 (MQW), the p-type carrier concentration in both the barrier layer (B1) and the quantum well layer (W2) is 10% or more and 150% or less of the p-type semiconductor layer 20 (p-SCH). (For example, lower than the p-type semiconductor layer 20).

FIG. 3C is a diagram showing the difference (effective carrier concentration) between the p-type carrier concentration and the n-type carrier concentration. As mentioned above, the p-type carrier is Zn doped and the n-type carrier is Si doped. In the quantum well layer (W2), both Zn and Si are doped at approximately the same concentration, so the two carriers cancel each other out, and the effective carrier concentration (the difference between the p-type carrier concentration and the n-type carrier concentration) is very high. lower.

In the quantum well layer (W2), the effective carrier concentration is 10% or less of the p-type carrier concentration of the quantum well layer (W2). On the other hand, the barrier layer (B1) is doped with only Zn, leaving p-type carriers. Thus, a modulated doped semiconductor laser is constructed.

In conventional modulation doped semiconductor lasers, when the barrier layer is doped with only p-type carriers, the quantum well layer is generally not doped with anything. As described above, Zn and Mg, which are p-type impurities, have the property of diffusing. Therefore, if the p-type dopant is added only to the barrier layer, the p-type dopant may diffuse into the quantum well layer. As a result, the desired (as designed) carrier density decreases in the barrier layer and increases in the quantum well layer, making it impossible to obtain desired characteristics.

In this embodiment, the quantum well layer (W2) is also doped with the p-type dopant at the same density as the barrier layer (B1), so that diffusion is difficult to occur. In this state, the quantum well layer (W2) is also p-type, but since the n-type dopant Si is doped at the same time, the effective carrier concentration becomes very small, and substantially only the barrier layer (B1) is p-type. It is possible to realize a modulated doped semiconductor laser with It is known that diffusion of Si is very small, and there is no need to consider the diffusion of Si. And, since it is not affected by diffusion, it is possible to obtain characteristics as designed.

Furthermore, Zn also diffuses from the p-type semiconductor layer 20 (p-SCH) and the upper clad layer 22 (p-clad) on the multiple quantum well 14 (MQW). However, because the multiple quantum well 14 (MQW) and its upper layers are relatively close in carrier concentration, the amount of diffusion can be sufficiently reduced. Therefore, it is possible to obtain desired characteristics that are less likely to be affected by diffusion.

Although it is preferable that the densities of Zn and Si doping the quantum well layer (W2) are the same, if the density is within ±10%, the modulation doped semiconductor laser can sufficiently function. The modulation doped semiconductor laser according to this embodiment operates as a directly modulated semiconductor laser in the 1.3 μm band and is a device excellent in high-speed response. This structure may be applied to a continuous oscillation semiconductor laser or a 1.55 μm band semiconductor laser.

[Production method]
Next, a method for manufacturing a modulation doped semiconductor laser according to this embodiment will be described. An n-type semiconductor layer 12 is formed of InGaAlAs on the lower clad layer 10 shown in FIG. A multi-quantum well layer 14 of, for example, five layers is formed thereon, in which the first layer 16 (barrier layer) and the second layer 18 (quantum well layer) are respectively made of InGaAlAs. A p-type semiconductor layer 20 made of InGaAlAs and a diffraction grating 28 are formed thereon.

These formations are each performed using metal organic chemical vapor deposition (MOCVD). In the formation of the n-type semiconductor layer 12, multiple layers are grown while doping with Si of 1×10 ¹⁸ cm ⁻³ as an impurity. Similarly, 2×10 ¹⁷ cm −3 of Zn is used to form the first layer 16 (barrier layer) ^of the multiple quantum well 14, and 2×10 ¹⁷ cm ⁻³ of Zn is used to form the second layer 18 (quantum well layer). of Zn and 2×10 ¹⁷ cm ⁻³ of Si are both grown while doping. Also, the multiple quantum well 14 is configured with a composition corresponding to a wavelength band of 1.3 μm. In forming the p-type semiconductor layer 20 and the diffraction grating 28, Zn is doped at 1×10 ¹⁸ cm ⁻³ .

Next, the diffraction grating 28 is processed into a diffraction grating shape, and the upper cladding layer 22 doped with Zn of 1×10 ¹⁸ cm ⁻³ and a contact layer (not shown) are grown in multiple layers. Further, the upper cladding layer 22 is processed into a ridge shape, a passivation film (not shown) is formed, an electrode 26 is formed on the upper surface, and an electrode 24 is formed on the rear surface. Finally, by dividing into a plurality of chips and coating the facets, a modulation doped semiconductor laser is completed. Other details are self-explanatory from the details of the modulation doped semiconductor laser provided above.

[Second embodiment]
4A to 4C are diagrams showing carrier concentrations near multiple quantum wells (MQW) in the second embodiment. In this embodiment, contrary to the first embodiment, the quantum well layer (W1) is the first layer and the barrier layer (B2) is the second layer. The uppermost barrier layer (B2) of the multiple quantum well (MQW) is the second layer, and the lowermost barrier layer (B2) is also the second layer.

FIG. 4A is a diagram showing carrier concentrations of donors (n-type dopants). The n-type semiconductor layer (n-SCH) is doped with 1×10 ¹⁸ cm ⁻³ of Si as an impurity. The barrier layer (B2) of the multiple quantum well (MQW) is doped with Si (2×10 ¹⁷ cm ⁻³ ) as a donor. The barrier layer (B2) has a lower n-type carrier concentration than the n-type semiconductor layer. In contrast, the upper clad layer 22 (p-clad) and the p-type semiconductor layer (p-SCH) are not doped with Si as a donor. Si is not added to the quantum well layer (W1) either.

FIG. 4B is a diagram showing carrier concentrations of acceptors (p-type dopants). In the multiple quantum well (MQW), both the barrier layer (B2) and the quantum well layer (W1) are doped with 2×10 ¹⁷ cm ⁻³ of Zn. Since other details are the same as the contents shown in FIG. 3B, description thereof is omitted.

FIG. 4C is a diagram showing the difference (effective carrier concentration) between the p-type carrier concentration and the n-type carrier concentration. In the barrier layer (B2) of the multiple quantum well (MQW), since both Zn and Si are doped at approximately the same concentration, the two carriers cancel each other out, and the effective carrier concentration (p-type carrier concentration and n-type carrier concentration difference) becomes very low. In the barrier layer (B2), the effective carrier concentration is 10% or less of the p-type carrier concentration of the barrier layer (B2). On the other hand, in the quantum well layer (W1), only Zn is doped, leaving p-type carriers. Thus, a modulated doped semiconductor laser is constructed.

The present invention is not limited to the above-described embodiments, and various modifications are possible. For example, the configurations described in the embodiments can be replaced with configurations that are substantially the same, configurations that produce the same effects, or configurations that can achieve the same purpose.

10 lower clad layer, 12 n-type semiconductor layer, 14 multiple quantum well, 16 first layer, 18 second layer, 20 p-type semiconductor layer, 22 upper clad layer, 24 electrode, 26 electrode, 28 diffraction grating.

Claims (12)

a multiple quantum well comprising multiple layers including multiple first layers and multiple second layers stacked alternately and containing acceptors and donors;
a p-type semiconductor layer in contact with the uppermost layer of the plurality of layers;
an n-type semiconductor layer in contact with the bottom layer of the plurality of layers;
has
The plurality of first layers contain the acceptor such that the p-type carrier concentration is 10% or more and 150% or less of the p-type semiconductor layer,
The plurality of second layers contain the acceptor such that the p-type carrier concentration is 10% or more and 150% or less of the p-type semiconductor layer,
The plurality of second layers further contain the donor,
In the plurality of second layers, an effective carrier concentration corresponding to the difference between the p-type carrier concentration and the n-type carrier concentration is 10% or less of the p-type carrier concentration in the plurality of second layers. A modulated doped semiconductor laser characterized by: A modulation doped semiconductor laser according to claim 1, comprising:
A modulation doped semiconductor laser, wherein said p-type semiconductor layer and said n-type semiconductor layer are for forming a separate confinement heterostructure. 3. A modulation doped semiconductor laser according to claim 1 or 2,
A modulation doped semiconductor laser, wherein the p-type carrier concentration is 1×10 ¹⁷ cm ⁻³ or more in each of the plurality of first layers and each of the plurality of second layers. A modulated doped semiconductor laser according to any one of claims 1 to 3,
A modulation doped semiconductor laser, wherein each of said top layer and said bottom layer of said plurality of layers is a corresponding one of said plurality of first layers. A modulated doped semiconductor laser according to any one of claims 1 to 3,
A modulation doped semiconductor laser, wherein each of said top layer and said bottom layer of said plurality of layers is a corresponding one of said plurality of second layers. A modulated doped semiconductor laser according to any one of claims 1 to 5,
each of the plurality of first layers is a barrier layer;
A modulation doped semiconductor laser, wherein each of said plurality of second layers is a quantum well layer. A modulated doped semiconductor laser according to any one of claims 1 to 5,
each of the plurality of first layers is a quantum well layer;
A modulation doped semiconductor laser, wherein each of said plurality of second layers is a barrier layer. A modulated doped semiconductor laser according to any one of claims 1 to 7,
the acceptor is at least one of Zn and Mg;
A modulated doped semiconductor laser, wherein said donor is Si. A modulated doped semiconductor laser according to any one of claims 1 to 8,
A modulation doped semiconductor laser, wherein the plurality of second layers has a p-type carrier concentration lower than that of the p-type semiconductor layer. A modulated doped semiconductor laser according to any one of claims 1 to 9,
A modulation doped semiconductor laser, wherein the plurality of second layers have a lower n-type carrier concentration than the n-type semiconductor layer. forming an n-type semiconductor layer;
a plurality of layers comprising a plurality of first layers and a plurality of second layers stacked alternately, containing acceptors and donors, the bottom layer of the plurality of layers resting in contact with the n-type semiconductor; forming a quantum well;
forming a p-type semiconductor layer by metalorganic vapor phase epitaxy overlying and in contact with the topmost layer of the plurality of layers;
including
The plurality of first layers contain the acceptor such that the p-type carrier concentration is 10% or more and 150% or less of the p-type semiconductor layer,
The plurality of second layers contain the acceptor such that the p-type carrier concentration is 10% or more and 150% or less of the p-type semiconductor layer,
The plurality of second layers further contain the donor,
In the plurality of second layers, an effective carrier concentration corresponding to the difference between the p-type carrier concentration and the n-type carrier concentration is 10% or less of the p-type carrier concentration in the plurality of second layers. A method of manufacturing a modulated doped semiconductor laser, characterized in that: 12. A method of manufacturing a modulated doped semiconductor laser according to claim 11, comprising:
A method for manufacturing a modulation doped semiconductor laser, wherein the multiple quantum well is formed by the metal-organic chemical vapor deposition method.

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