JP7645653B2 - Semiconductor laser element - Google Patents

Description

The present invention relates to a semiconductor laser element.

A technology has been disclosed for a distributed feedback (DFB) laser element that employs a layered structure in which a diffraction grating layer is provided on the opposite side of the substrate, sandwiching an active layer (Non-Patent Document 1).

In addition, a technology has been disclosed in which a layer with a high refractive index is provided in the n-type cladding layer of a semiconductor laser element, and the distribution of the electric field of the laser light propagating through the active layer is biased toward the n-type cladding layer (Patent Document 1). This makes the laser light less susceptible to the effects of light absorption by p-type impurities (e.g. zinc (Zn)) contained in the p-type cladding layer, making it possible to realize a semiconductor laser element with less internal loss and higher output.

JP 2005-72402 A

C.J.Armistead et.al., “DFB RIDGE WAVEGUIDE LASERS AT λ=1.5μm WITH FIRST ORDER GRATINGS FABRICATED USING ELECTRON BEAM LITHOGRAPHY”, Electronics Letters, Vol.23, No.11(1987), pp.592-593.

However, when a diffraction grating layer is provided in the configuration of the semiconductor laser element of Patent Document 1 to fabricate a semiconductor laser element with a DFB structure, the output characteristics of the semiconductor laser element may not be stable. Here, "not stable" means, for example, that the characteristics vary among multiple elements, or that the characteristics change depending on the operating conditions of the semiconductor laser element, environmental conditions, etc.

The present invention has been made in consideration of the above, and aims to provide a semiconductor laser element with a DFB structure that has stable output characteristics.

One aspect of the present invention is a semiconductor laser element comprising a first cladding layer having one conductivity type, a second cladding layer having a second conductivity type, and an active layer interposed between the first cladding layer and the second cladding layer, the first cladding layer including at least one electric field control layer and a diffraction grating layer disposed between the electric field control layer and the active layer, in which low refractive index portions and high refractive index portions having a refractive index higher than the low refractive index portions are periodically arranged along the extension direction of the active layer, and the refractive index of the electric field control layer and the refractive index of the diffraction grating layer are higher than the refractive index of the region of the first cladding layer other than the electric field control layer and the diffraction grating layer.

The refractive index of the diffraction grating layer may be lower than the refractive index of the electric field control layer.

The first conductivity type may be n-type and the second conductivity type may be p-type.

If the laser oscillation wavelength, which is determined depending on the period of the diffraction grating layer, is λc, the composition wavelength of the electric field control layer may be shorter than λc.

The first cladding layer or the second cladding layer may have a striped ridge portion.

The first cladding layer or the second cladding layer may have layers covering the active layer on both sides of the ridge portion.

The present invention has the effect of realizing a semiconductor laser element with a DFB structure that has stable output characteristics.

FIG. 1 is a schematic cross-sectional view of a semiconductor laser device according to an embodiment. FIG. 2 is a cross-sectional view taken along line AA of FIG. FIG. 3 is a schematic diagram showing an example of the distribution of the refractive index and the electric field intensity of a known semiconductor laser element. FIG. 4 is a schematic diagram showing the distribution of the refractive index and the electric field intensity of an assumed semiconductor laser element. FIG. 5 is a schematic diagram showing an example of the distribution of the refractive index and the electric field intensity in the embodiment. 6A to 6C are schematic views showing a method for manufacturing a semiconductor laser device according to an embodiment. 7A to 7C are schematic views showing a method for manufacturing a semiconductor laser device according to an embodiment. 8A to 8C are schematic views showing a method for manufacturing a semiconductor laser device according to an embodiment. 9A to 9C are schematic views showing a method for manufacturing a semiconductor laser device according to an embodiment. 10A to 10C are schematic views showing a method for manufacturing a semiconductor laser element according to an embodiment.

Below, an embodiment will be described with reference to the drawings. Note that the present invention is not limited to this embodiment. In addition, in the description of the drawings, the same or corresponding elements are appropriately given the same reference numerals. It should be noted that the drawings are schematic, and the dimensional relationships and ratios of each element may differ from reality. There may also be parts in which the dimensional relationships and ratios differ between the drawings. In addition, directions may be explained using the X-axis in the drawings.

(Embodiment)
1 is a schematic diagram of a semiconductor laser device according to an embodiment of the present invention, and FIG 2 is a cross-sectional view taken along line AA in FIG 1.

[Structure of semiconductor laser element]

The semiconductor laser element 100 includes a first cladding layer 120 with a first electrode 110 formed on the back surface thereof, an active layer 130, a second cladding layer 140, and a second electrode 150. The semiconductor laser element 100 outputs laser light from the active layer 130 in the positive X-direction. The wavelength of the laser light is, for example, in the 1.55 μm band. Indium phosphide (InP)-based materials are known as semiconductor materials for making the wavelength of the laser light in the 1.55 μm band.

The first cladding layer 120 is a semiconductor layer having an n-type conductivity. The second cladding layer 140 is a semiconductor layer having a p-type conductivity. The active layer 130 is interposed between the first cladding layer 120 and the second cladding layer 140. The n-type is an example of the first conductivity type, and the p-type is an example of the second conductivity type.

The first cladding layer 120 has a structure in which a buffer layer made of n-InP and multiple semiconductor layers are layered on a substrate made of n-type InP (hereinafter referred to as n-InP) by epitaxial growth or the like.

The first cladding layer 120 includes, as multiple semiconductor layers, a semiconductor layer made of n-InP (hereinafter, appropriately referred to as an n-InP layer), an electric field control layer 121, and a diffraction grating layer 122. There is at least one electric field control layer 121, and in this embodiment, multiple electric field control layers 121, for example 5 to 7 layers, are provided.

The specific configuration of the first cladding layer 120 will be described. The first cladding layer 120 has a structure in which a buffer layer and an n-InP layer are sequentially stacked on a substrate, an electric field control layer 121 and an n-InP layer are alternately stacked on top of that, a diffraction grating layer 122 is stacked on top of that, and an n-InP layer is stacked on top of that. The n-type semiconductor layer in this specification may contain, for example, silicon (Si), sulfur (S), or selenium (Se) as an n-type impurity.

The refractive index of the electric field control layer 121 is higher than the refractive index of the region of the first cladding layer 120 other than the electric field control layer 121 and the diffraction grating layer 122, i.e., the refractive index of the n-InP layer. The electric field control layer 121 is made of, for example, GaInAsP, which is an InP-based quaternary semiconductor material. The composition ratio of the semiconductor material constituting the electric field control layer 121 is set according to the desired laser oscillation wavelength for the semiconductor laser element 100. Specifically, the composition is designed so that the composition wavelength λECL (also called the band gap wavelength), which is the wavelength of light equivalent to the band gap energy of the semiconductor material, is shorter than the laser oscillation wavelength λc. In other words, λECL < λc is satisfied. This makes it possible to suppress the absorption of laser light by the electric field control layer 121. Furthermore, if λECL [μm] < (λc-0.2) [μm] is satisfied, the absorption of laser light by the electric field control layer 121 can be effectively suppressed even when taking into consideration that the absorption edge wavelength of the electric field control layer 121 becomes longer than the band gap wavelength due to the influence of temperature changes in the semiconductor laser element 100, the electric field control layer 121 being doped with impurities, the interface state, and the like.

The semiconductor material that constitutes the electric field control layer 121 is preferably lattice-matched with InP. Lattice matching means that the lattice constants may be perfectly the same, or the lattice constants of the semiconductor layers, such as the active layer 130, stacked on top of it may differ to the extent that they provide the desired crystal quality. In this case, crystal quality refers to the degree of dislocations, distortion, and defects, for example.

The diffraction grating layer 122 has a configuration in which high refractive index portions 122a and low refractive index portions 122b are periodically arranged along the direction in which the active layer 130 extends, i.e., the X direction. The period of the arrangement of the high refractive index portions 122a and low refractive index portions 122b is determined and set depending on the desired laser oscillation wavelength. The high refractive index portions 122a have a higher refractive index than the low refractive index portions 122b. The high refractive index portions 122a are made of, for example, GaInAsP. The low refractive index portions 122b are made of, for example, n-InP.

The refractive index of the diffraction grating layer 122 is higher than the refractive index of the area of the first cladding layer 120 other than the electric field control layer 121 and the diffraction grating layer 122, i.e., the refractive index of the n-InP layer. Here, the refractive index of the diffraction grating layer 122 is the refractive index of the high refractive index portion 122a.

The composition ratio of the semiconductor material that constitutes the high refractive index portion 122a is set according to the desired laser oscillation wavelength. Specifically, the composition is designed so that the composition wavelength λgrating, which is the wavelength of light equivalent to the band gap energy of the semiconductor material, is shorter than the laser oscillation wavelength λc. In other words, λgrating<λc is satisfied.

The active layer 130 has an MQW-SCH structure consisting of a multi-quantum well (MQW) layer of a multi-quantum well structure consisting of multiple barrier layers and multiple well layers, and two separate confinement heterostructure (SCH) layers arranged to sandwich the MQW layer. The active layer 130 is made of, for example, GaInAsP. The composition ratio of the semiconductor material constituting the well layer of the active layer 130 is set so as to emit light at the desired laser oscillation wavelength λc. The composition ratio of the semiconductor material constituting the barrier layer and the SCH layer is set so as to fulfill the respective functions. The active layer 130 may be a single quantum well structure.

The second cladding layer 140 has a structure in which a contact layer (not shown) made of a p-type semiconductor layer is laminated on a semiconductor layer made of p-type InP (hereinafter referred to as p-InP as appropriate). The contact layer is made of, for example, GaInAsP, and is in ohmic contact with the second electrode 150.

The second cladding layer 140 includes a base layer 141 that covers the active layer 130, and a ridge portion 142 that protrudes from the base layer 141. The ridge portion 142 is a striped mesa that extends in the X direction. The width (mesa width) of the ridge portion 142 in the direction perpendicular to the X direction may be equal or unequal.

The second electrode 150 is provided on the ridge portion 142 of the second cladding layer 140 so as to be in ohmic contact with the p-type contact layer. The second electrode 150 includes, for example, titanium, platinum, gold, etc.

The first electrode 110 is provided so as to be in ohmic contact with the substrate of the first cladding layer 120. The first electrode 110 contains, for example, gold or nickel.

In addition, both end faces in the X direction of the semiconductor laser element 100 in FIG. 2 are end faces formed by cleavage, and a HR (High Reflection) film with a relatively high reflectance is formed on the negative side in the X direction, and an AR (Anti-Reflection) film for preventing reflection is formed on the positive side in the X direction. The HR film and the AR film form a laser resonator.

The semiconductor laser element 100 is a DFB type laser element by including the diffraction grating layer 122. When a forward bias current is supplied between the first electrode 110 and the second electrode 150, the supplied current is injected into the active layer 130 across the width of the ridge portion 142. In the semiconductor laser element 100, laser oscillation occurs with the region of the active layer 130 that overlaps with the ridge portion 142 in the stacking direction as the waveguide region, and laser light of the desired wavelength is output from the AR film side.

[Electric field distribution of laser light]
Here, the electric field distribution of the laser light in the semiconductor laser element 100 will be described in comparison with known semiconductor laser elements.

The inventors of the present invention have conducted extensive research into the cause of unstable output characteristics of semiconductor laser elements when a semiconductor laser element with a DFB structure is fabricated by providing a diffraction grating layer to the configuration of the semiconductor laser element of Patent Document 1. As a result, they have found that while the diffraction grating layer should be provided at a position where it overlaps with the electric field of the laser light propagating through the active layer, the stability of the coupling coefficient between the laser light and the diffraction grating layer changes depending on the positional relationship between the diffraction grating layer and the electric field control layer.

Figure 3 is a schematic diagram showing an example of the distribution of the refractive index and electric field strength in a known semiconductor laser element. Figure 3 shows the relationship between the position in the stacking direction of the semiconductor layers and the refractive index and the electric field strength of the laser light. The configuration of this known semiconductor laser element is a configuration in which the electric field control layer 121 is removed from the semiconductor laser element 100, and the diffraction grating layer 122 is made p-type and moved to the second cladding layer 140 side. In addition, as the refractive index, the refractive index distribution n130 of the active layer 130 and the refractive index distribution n122 of the diffraction grating layer 122 are shown. Note that, for the refractive index distribution n130 of the active layer 130, in order to simplify the drawing, the average refractive index of the well layer and barrier layer in the MQW structure is shown.

In the example shown in FIG. 3, the electric field intensity distribution EF1 of the laser light has a shape that is approximately symmetrical with respect to the active layer 130 in the stacking direction.

Figure 4 is a schematic diagram showing an example of the distribution of refractive index and electric field strength in an assumed semiconductor laser element, which is assumed to be a semiconductor laser element having a DFB structure by providing a diffraction grating layer to the configuration of the semiconductor laser element of Patent Document 1. The assumed semiconductor laser element has a configuration in which an electric field control layer 121 is added to the known semiconductor laser element shown in Figure 3. In addition, as the refractive index, the refractive index distribution n130 of the active layer 130, the refractive index distribution n122 of the diffraction grating layer 122, and the refractive index distribution n121 of the electric field control layer 121 are shown.

In the example shown in FIG. 4, the electric field intensity distribution EF2 of the laser light is biased toward the electric field control layer 121 due to the effect of the electric field control layer 121, compared to the electric field intensity distribution EF1 shown by the dashed line in FIG. 4.

However, in the electric field intensity distribution EF2, the overlap with the diffraction grating layer 122 is smaller than in the electric field intensity distribution EF1. As a result, even a slight change in the thickness of the semiconductor layer can easily cause the coupling state between the diffraction grating layer 122 and the laser light, which is represented by the coupling coefficient, to fluctuate, making the coupling unstable.

In contrast, in the semiconductor laser element 100, the diffraction grating layer 122 is disposed between the electric field control layer 121 and the active layer 130, so that the overlap of the electric field intensity distribution EF3 with the diffraction grating layer 122 is large, as shown in the refractive index and electric field intensity distribution EF3 shown in FIG. 5. As a result, even if the thickness or composition of the semiconductor layer varies, for example, the coupling state between the diffraction grating layer 122 and the laser light is unlikely to fluctuate, and the coupling state is stable. As a result, the semiconductor laser element 100 has stable characteristics such as output characteristics. Furthermore, due to this stability, the semiconductor laser element 100 is easy to manufacture.

Furthermore, this allows greater freedom in designing the electric field control layer 121 in the semiconductor laser element 100 compared to the assumed example shown in FIG. 4. For example, even if the bias in the electric field intensity distribution EF3 is adjusted relatively significantly by increasing or decreasing the number of electric field control layers 121 or changing their positions in the stacking direction, the fluctuation in the coupling state between the diffraction grating layer 122 and the laser light is smaller than in the assumed example.

Furthermore, in the semiconductor laser element 100, the electric field control layer 121 is included in the n-type first cladding layer 120, and the electric field of the laser light is biased toward the n-type semiconductor layer side, so that the laser light is less susceptible to the influence of light absorption by the p-type impurities included in the p-type second cladding layer 140. As a result, the semiconductor laser element 100 has less internal loss and achieves higher output.

Furthermore, in the semiconductor laser element 100, the thickness of the second cladding layer 140, particularly the thickness of the base layer 141, can be made relatively thinner than when the diffraction grating layer 122 is included on the second cladding layer 140 side, so the current injection efficiency can be made relatively high. As a result, in the semiconductor laser element 100, the ratio of the power of the laser light to the supplied electric power can be made high.

Furthermore, in the case of a ridge laser such as the semiconductor laser element 100, the current path of the injected carriers, the holes, from the ridge portion 142 to the active layer 130 is the main factor that determines the element resistance. Therefore, if an interface where the band gap changes, such as a diffraction grating layer, is present here, the energy barrier felt by the injected carriers becomes higher, which causes the element resistance to increase. When the element resistance increases, the amount of heat generated by the element also increases, leading to a decrease in optical output. In contrast, according to the structure of this embodiment, the element resistance and amount of heat can be reduced compared to known structures or assumed structures, and high output can be achieved.

The refractive index of the diffraction grating layer 122 is preferably lower than that of the electric field control layer 121 in order to reduce the effect on the control of the electric field of the laser light by the action of the electric field control layer 121. To achieve this, for example, λgrating<λECL can be set. This makes it possible to more accurately design the control of the electric field of the laser light by the electric field control layer 121, and to fabricate an electric field control layer 121 with less error in settings.

[Production method]
Next, an example of a method for manufacturing the semiconductor laser device 100 will be described with reference to FIGS.

First, as shown in FIG. 6, a buffer layer 124, a layered structure in which an electric field control layer 121 and multiple n-InP layers are alternately arranged, and an n-InP layer 125 are layered on a substrate 123 in this order by MOCVD (Metal Organic Chemical Vapor Deposition) or the like, and then a high refractive index layer 126 for forming the high refractive index portion 122a of the diffraction grating layer 122, for example a layer made of GaInAsP, is layered.

Next, as shown in FIG. 7, a portion of the high refractive index layer 126 is removed by photolithography to form the high refractive index portion 122a.

Next, as shown in FIG. 8, the gaps between the high refractive index sections 122a are filled with n-InP to form the low refractive index sections 122b, and the n-InP layer 127 is laminated so as to obtain a growth surface with the desired flatness. This forms the structure of the first cladding layer 120.

Next, as shown in FIG. 9, the active layer 130 and the second cladding layer 140 are laminated in this order.

Next, as shown in FIG. 10, photolithography is applied to the second cladding layer 140 to form a structure of a base layer 141 and a ridge portion 142. At this time, the thickness of the base layer 141 is set to d by etching. d is also called the remaining thickness.

In order to control the remaining thickness of the base layer 141, an etch stop layer made of, for example, GaInAsP may be provided in the second cladding layer 140. In this case, the etch stop layer is provided on the base layer 141, and etching to form the ridge portion 142 is first performed up to the etch stop layer, after which the etch stop layer is removed, but the etch stop layer remains at the bottom of the ridge portion 142. Therefore, care should be taken to ensure that the increase in element resistance due to the remaining etch stop layer is within an acceptable range.

Then, the second electrode 150 is formed. Furthermore, after polishing the substrate 123 to the desired thickness, the first electrode 110 is formed on the entire back surface. Then, known processes such as cleavage, formation of the HR film and the AR film, and cutting into individual element pieces are performed to complete the manufacture of the semiconductor laser element 100.

In the above embodiment, the waveguide region is formed by the ridge structure of the ridge portion 142, but the structure that forms the waveguide region is not limited to the ridge structure, and it is also possible to adopt known structures such as a SAS structure (Self-Aligned Structure) or a BH structure (Buried-Hetero Structure).

In addition, in the above embodiment, the first conductivity type of the first cladding layer 120 including the electric field control layer 121 and the diffraction grating layer 122 is n-type, but it may be p-type if the degree of light absorption by p-type impurities is tolerable. In this case, the second cladding layer 140 is n-type.

In addition, in the above embodiment, the second cladding layer 140 has a ridge portion 142, but the first cladding layer may have a ridge portion.

Furthermore, the present invention is not limited to the above-mentioned embodiment. The present invention also includes configurations in which the above-mentioned components are appropriately combined. Furthermore, further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspects of the present invention are not limited to the above-mentioned embodiment, and various modifications are possible.

100: Semiconductor laser element 110: First electrode 120: First cladding layer 121: Electric field control layer 122: Diffraction grating layer 122a: High refractive index portion 122b: Low refractive index portion 123: Substrate 124: Buffer layers 125, 127: n-InP layer 126: High refractive index layer 130: Active layer 140: Second cladding layer 141: Base layer 142: Ridge portion 150: Second electrodes EF1, EF2, EF3: Electric field intensity distribution n121, n122, n130: Refractive index distribution

Claims (3)

a first cladding layer of a first conductivity type, that is, an n-type;
a second cladding layer of a second conductivity type, that is, a p-type;
an active layer interposed between the first cladding layer and the second cladding layer;
Equipped with
the first cladding layer includes a substrate, a plurality of electric field control layers located between the substrate and the active layer, a diffraction grating layer disposed between the electric field control layer and the active layer, in which low refractive index portions and high refractive index portions having a refractive index higher than that of the low refractive index portions are periodically arranged along an extension direction of the active layer, and a plurality of intermediate layers made of the same material as the substrate and alternately arranged with the plurality of electric field control layers;
the plurality of electric field control layers and the plurality of intermediate layers form a laminated structure,
the laminated structure is provided only on a surface of the diffraction grating layer facing the substrate,
the refractive index of the electric field control layer and the refractive index of the diffraction grating layer are higher than the refractive indexes of the regions of the first cladding layer other than the electric field control layer and the diffraction grating layer,
The second cladding layer has a base portion and a stripe-shaped ridge portion protruding from the base portion, the base portion extending on both sides of the ridge portion, covering the active layer, and being thinner than the ridge portion.
Semiconductor laser element. The semiconductor laser device according to claim 1 , wherein the refractive index of the diffraction grating layer is lower than the refractive index of the electric field control layer. If the laser oscillation wavelength, which depends on the period of the diffraction grating layer, is λc, then
3. The semiconductor laser device according to claim 1, wherein the bandgap wavelength of said electric field control layer is shorter than λc.

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