JP4617684B2 - Semiconductor laser element - Google Patents

Description

  The present invention relates to a buried semiconductor laser device, and more particularly to a GaAs buried semiconductor laser device using an active layer containing nitrogen.

In current optical communications, semiconductor lasers with wavelengths of 1.3 μm and 1.55 μm are used, and InP semiconductor lasers have been put into practical use in such wavelength bands (see Non-Patent Document 1). ). However, InP semiconductor lasers are expensive due to their high material costs and poor temperature characteristics, which necessitates a cooling device.
Recently, it is clear that an inexpensive and large-diameter substrate can be produced because a GaAs substrate is used, and that a light emission with a wavelength of 1.3 μm or more can be obtained by using an active layer of GaInNAs on the GaAs substrate. (See Non-Patent Document 2).
Thus, it has been found that a semiconductor laser having an active layer of GaInNAs exhibits excellent temperature characteristics. That is, by using a semiconductor laser having a GaInNAs active layer on a GaAs substrate, there is a possibility that the problems of conventional InP semiconductor lasers can be solved.

In an optical device for communication, a buried (BH) semiconductor laser element is generally used in many cases (see Non-Patent Document 1). A semiconductor laser device using GaInNAs as an active layer has also been proposed (see Patent Document 1).
JP-A-13-223437 Optical communication element optics (Hiroo Yonezu, optical books, pages 78, 240-241) 1.3um continuous-wave lasing operation in GaInNAsquantum-well lasers Nakahara, K .; Kondow, M .; Kitatani, T .; Larson, MC; Uomi, K .; Photonics Technology Letters, IEEE, Volume: 10 Issue: 4, April 1998 Page (s): 487 -488

The embedded semiconductor laser device can reduce the threshold current and can bring the aspect ratio, which is important for improving the coupling efficiency to the optical fiber, to close to 1. In addition, since lateral light confinement can be controlled, single transverse mode oscillation can be achieved by suppressing spatial hole burning.
However, in an optical device using a GaAs substrate such as Patent Document 1 and Non-Patent Document 2, it is difficult to realize a buried semiconductor laser with high reliability. In particular, GaAs-based devices often use an AlGaAs layer to confine light. However, when an Al-containing layer is used for the guide layer, oxygen is taken into the semiconductor laser device because the Al-containing layer is exposed to the surface and is violently oxidized. The deterioration of the semiconductor laser element is promoted, and the reliability is greatly reduced.
On the other hand, when an InGaP layer that does not contain Al is used for the guide layer, the thermal resistance is increased, which causes a deterioration in temperature characteristics, which is a merit of the GaAs device.

  In view of the above-described points, the present invention provides a particularly excellent and highly reliable GaAs-based embedded semiconductor laser device using a compound semiconductor layer containing nitrogen (N) as an active layer.

The buried type semiconductor laser device of the present invention includes a first conductivity type AlGaAs first cladding layer, a non-doped GaAs first guide layer, a GaInNAs, GaInNAsSb, GaAsSbN, or GaNAs on a first conductivity type GaAs substrate. An active layer, a second guide layer of non-doped GaAs, and a second cladding layer of second conductivity type AlGaAs are sequentially stacked. The second conductive type AlGaAs or GsAs grown sequentially from the Al-free recrystallized growth surface of the mesa portion formed by mesa etching from the surface of the second guide layer to the middle of the first guide layer. A first conductive layer formed between the second current confinement layer and the second clad layer; a buried layer configured to block a current composed of one current confinement layer and a second current confinement layer of a first conductivity type AlGaAs or GaAs; And a cap layer of type GaAs.

The buried semiconductor laser device of the present invention can be configured as a distributed feedback type (DFB) having a diffraction grating layer. The diffraction grating layer can be formed of an AlGaAs layer and a GaAs layer, a GaInNAsSb layer or a GaInNAs layer and a GaAs layer, or a GaAsSbN layer, a GaAsSb layer or a GaAsN layer and a GaAs layer.

  In the buried type semiconductor laser device of the present invention, the refractive index increases because GaInNAs, GaInNAsSb, GaAsSbN, or GaNAs is used for the active layer. Therefore, it is possible to increase the light confinement in the active layer even if the guide layer is thickened. As a result, mesa etching is performed up to the middle of the guide layer, and the regrowth surface when the buried layer is formed can be a surface that does not contain Al.

  According to the buried semiconductor laser device of the present invention, since it is a GaAs semiconductor laser, the thermal resistance can be lowered, and since an active layer of GaInNAs, GaInNAsSb, GaAsSbN, or GaNAs is used, a guide layer ( Since ΔEc, which is the difference in band gap between the valence electrons of the barrier layer and the active layer, can be increased, the temperature characteristics are superior to those of conventional InP semiconductor laser elements. Thus, the embedded semiconductor laser of the present invention is suitable for an uncooled semiconductor laser element that does not require a cooling device. Further, the GaAs substrate is inexpensive and can be increased in diameter, and from these, a semiconductor laser having excellent characteristics can be realized at low cost.

Further, since the active layer of GaInNAs, GaInNAsSb, GaAsSbN, or GaNAs has a high refractive index, the GaAs guide layer can be made thick.
Thus, an embedded semiconductor laser element can be manufactured without exposing the layer containing Al to the atmosphere. That is, it is possible to fabricate an embedded semiconductor laser element without leaving oxygen that causes element degradation at the interface of the guide layer.

  That is, by using GaInNAs, GaInNAsSb, GaAsSbN, or GaNAs as the active layer, a buried semiconductor laser element with a low threshold current and high reliability can be manufactured.

  The embedded semiconductor laser device of the present invention can be manufactured as a DFB semiconductor laser device. When configured as a DFB semiconductor laser element, a 1.3 μm-band semiconductor laser having excellent temperature characteristics can be obtained. For example, the structure is suitable for an analog laser in which linearity of light output is important.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 shows an embodiment of an embedded semiconductor laser device according to the present invention.
The buried type semiconductor laser device 1 according to the present embodiment sequentially has a first cladding layer 3 made of AlGaAs of the first conductivity type and a first conductivity type first made of non-doped GaAs on the first conductivity type GaAs substrate 2. A light guide layer 4, an active layer made of GaAs containing nitrogen (N), an active region 5 having an active layer made of GaInNAs in this example, and a second light guide layer 6 of the second conductivity type made of non-doped GaAs are laminated. Then, a buried layer for confining current, that is, a so-called current confinement layer (also referred to as a current blocking layer for blocking current) 13 is formed in the removed portion by mesa etching, and further, in contact with the second light guide layer 6, the second conductive A second cladding layer 11 made of AlGaAs of the type and a cap layer 12 made of GaAs of the second conductivity type are stacked.
In this embodiment, in particular, mesa etching is performed from the second light guide layer 6 that does not contain Al to the middle of the first light guide layer 4 that does not contain Al, and the first and second light guide layers 4 and 6. The current confinement layer 13 is formed by growing from the regrowth surface 14 that does not contain Al, including the active layer 5.

The current confinement layer 13 includes, for example, a pn junction j formed by a second conductivity type first semiconductor layer 8 in contact with the first conductivity type first light guide layer 4 and a first conductivity type second semiconductor layer 9 thereon. Can be formed. The first semiconductor layer 8 and the second semiconductor layer 9 of the current confinement layer 13 can be formed of AlGaAs or GaAs. In addition, the current confinement layer 13 can be formed of an insulating layer.
Further, a cap layer 10 made of GaAs of the first conductivity type is formed between the current confinement layer 13 and the second clad layer 11.

The active region 5 can be formed, for example, with a quantum well structure such as a GaInNAs active layer and a GaAs barrier layer, for example, a double quantum well (DQW) structure. The active layer constituting the active region 5 can be formed of GaInNAsSb, GaAsSbN, GaNAs, or the like in addition to the GaInNAs described above.
The mesa portion by mesa etching is formed in a stripe shape in the resonator length direction. Although not shown, one electrode is formed on the surface of the second conductivity type cap layer 12, and the other electrode is formed on the back surface of the first conductivity type GaAs substrate 2.

Next, an embodiment of a manufacturing method of the embedded semiconductor laser device 1 of the present embodiment will be described with reference to FIGS. Although the figure shows a preferred embodiment, the composition of each layer is not limited to this as described above, and the composition ratio, film thickness, etc. of each layer are not limited to this.
In the buried semiconductor laser device 1 of the present embodiment, the first cladding layer 3 of n-type Al _0.2 Ga _0.8 As is formed on the n-type GaAs substrate 2 having the main surface (100) crystal plane. In this example, the first cladding layer 3 of n-type Al _0.2 Ga _0.8 As is grown to a thickness of 2 μm (see FIG. 2A). Next, a first guide layer 4 of non-doped GaAs is formed. In this example, the first guide layer 4 is grown to a thickness of 500 nm (see FIG. 2B). Next, the active region 5 having a GaInNAs layer is grown (see FIG. 2C). The active region 5 is formed in a double quantum well (DQW) structure including an active layer of GaInNAs and a barrier layer of GaAs. Next, a non-doped GaAs second guide layer 6 is formed. In this example, the second guide layer 6 is grown to a thickness of 500 nm. In this way, the substrate 7 in which the n-type first cladding layer 3, the first light guide layer 4, the active region 5, and the second light guide layer 6 are laminated on the substrate 2 by the MOCVD (metal organic chemical vapor deposition) method. Form (see FIG. 2D).

Next, the substrate 7 is taken out, and an etch-resistant mask 15 having a long stripe pattern in the resonator length direction (direction perpendicular to the paper surface) is formed on the upper surface of the second guide layer 6 by photolithography. In this example, an SiO ₂ mask 15 having a stripe pattern with a width of, for example, about 3 μm is formed on the second guide layer 6 of the substrate 7 (see FIG. 3E). The mesa width for oscillating in the single transverse mode, that is, the stripe pattern width can be changed by lateral light confinement.

Next, the mesa portion 16 is formed by mesa etching by wet etching. At this time, it is important that the mesa etching is stopped at the first guide layer 4 of GaAs (see FIG. 3F). When etching is performed up to the first cladding layer 3 of n-type Al _0.2 Ga _0.8 As, the first cladding layer 3 of n-type Al _0.2 Ga _0.8 As that contains Al is exposed to the atmosphere. , Combined with oxygen. It is difficult to remove oxygen bonded to Al. On the surface of the first guide layer 3 of GaAs, if annealing is performed at 700 ° C. or higher in AsH ₃ , the oxygen concentration can be reduced to a level below the detection limit by SIMS (Secondary Ion Mass Spectrometry). Further, in order to stop etching accurately, an etching stop layer made of InGaP may be used. By this mesa etching, the surface of the etching removal portion, that is, the recrystallized growth surface 14 on both sides of the mesa portion 16, the entire surface including the first and second guide layers 4 and 6 and the active region 5 does not contain Al. .

Next, after the mesa portion 16 is formed, it is introduced into the MOCVD furnace without removing the SiO ₂ mask 15, and an n-type AlGaAs first semiconductor layer (so-called first current confinement layer) is formed on both sides of the mesa portion 16. ) 8 and a second semiconductor layer (so-called second current confinement layer) 9 of p-type AlGaAs are sequentially selectively grown to form a buried layer 13 which becomes a current confinement layer. The AlGaAs first current confinement layer 8 and the second current confinement layer 9 desirably have an Al composition of 0.3 or less. Above that, the selective growth of the current confinement layer 13 becomes difficult, and the thermal resistance of the laser element also increases. In this example, the composition ratio of the first and second current confinement layers 8 and 9 is Al _0.1 Ga _0.9 As. Next, a first cap layer 10 of n-type GaAs is formed (see FIG. 3G).

Next, after the selective growth, the substrate 7 is taken out of the MOCVD furnace, and the SiO ₂ mask 15 is removed. After removing the SiO ₂ mask 15, the substrate 7 is once again introduced into the MOCVD furnace to form the second cladding layer 11 of p-type AlGaAs. In this example, the second cladding layer 11 having a composition ratio of p-type Al _0.2 Ga _0.8 As is grown to a thickness of about 2 μm.
Next, a second cap layer 12 of p-type GaAs is grown on the second cladding layer 11. Next, a p-type substrate 17 is formed on the upper surface of the second cap layer 12, and an n-type electrode 18 is formed on the back surface of the n-type GaAs substrate 2, thereby completing the target embedded semiconductor laser device 1 (FIG. 4).

  In a typical buried semiconductor laser device, if the GaAs guide layer is thickened, the light confinement in the active layer becomes weak, so the guide layer cannot be made so thick. However, since the active layer of GaInNAs has a lower refractive index than that of a normal GaAs-based material, in this embodiment, even if the first and second guide layers 4 and 6 of GaAs become thick, sufficient light can be obtained. It can be confined in the active layer 5. Therefore, an increase in threshold current due to a decrease in optical confinement is small. The thickness of the first and second guide layers 4 and 6 of GaAs needs to be such that the p-type first current confinement layer 8 and the n-type second current confinement layer 9 can function. For example, the thickness of the first and second guide layers 4 and 6 of GaAs is preferably 100 nm or more. When the first and second guide layers 4 and 6 are thick, there is no limitation, but it is considered that a practical thickness is preferably 1 μm or less.

FIG. 5 is a graph showing a comparison of optical confinement with an embedded semiconductor laser using an active layer of conventional In _0.2 Ga _0.8 As, using an embedded semiconductor laser using an active layer of GaInNAs.
FIG. 5 is a characteristic diagram showing the dependence of the optical confinement ratio on the guide layer thickness, where the vertical axis indicates the optical confinement ratio (%) and the horizontal axis indicates the guide layer thickness (μm). However, the graph of FIG. 5 is represented by data obtained by estimating the optical confinement when the buried semiconductor laser is made from the refractive index calculated from the FFP (Far Field Pattern) of a normal ridge semiconductor laser.
(Solid line) 30 is a characteristic curve (dashed line) 31 of a semiconductor laser of a GaInNAs active layer having a wavelength of 1.3 μm band, and a characteristic curve 31 is a characteristic of a semiconductor laser of a conventional In _0.2 Ga _0.8 As active layer. According to this graph, the GaInNAs active layer laser (curve 30) is at any stage of the barrier thickness compared to the conventional In _0.2 Ga _0.8 As active layer laser (curve 31). It can be seen that the light confinement rate is improved.
The buried type semiconductor laser device provided with the GaInNAs active layer is highly reliable and can realize a low threshold current.

  According to the embedded semiconductor laser device 1 according to the above-described embodiment, since it is a GaAs laser, the thermal resistance is low, and an active layer having nitrogen (N) such as a GaInNAs active layer can have a large ΔEc. In addition, it has superior temperature characteristics compared to conventional InP-based semiconductor laser elements. Since the temperature characteristics are excellent, the embedded semiconductor laser element 1 of the present embodiment has been an uncooled semiconductor laser element without a cooling device. Further, the GaAs substrate 2 is inexpensive and can have a large diameter. Therefore, an embedded semiconductor laser element having excellent characteristics can be realized at low cost.

  Further, in the buried semiconductor laser device 1 according to the present embodiment, the GaInNAs layer serving as the active layer has a small refractive index, and thus the GaAs guide layers 4 and 6 can be formed thickly. As a result, the current confinement layer 13 by the buried layer can be formed without exposing the Al-containing layer, that is, the AlGaAs first cladding layer 3 to the atmosphere, and a highly reliable buried semiconductor laser device 1 can be realized. . That is, an embedded semiconductor laser device can be manufactured without leaving oxygen that causes device deterioration at the regrowth interface.

  According to the manufacturing method of the semiconductor laser device 1 according to the present embodiment, by using GaInNAs in the active region 5, the first guide layer 4 and the second guide layer 6 can be formed thick, and a highly reliable embedding is achieved. Type semiconductor laser device 1 can be manufactured.

FIG. 6 shows a second embodiment using the embedded semiconductor laser device according to the present invention.
Usually, in optical communication, a DFB (Distributed Feedback) laser that oscillates at a single wavelength is used. The present embodiment is a case where the present invention is applied to a buried semiconductor laser device that is made into a DFB (distributed feedback type).
The buried semiconductor laser device 21 according to the present embodiment sequentially has a first conductivity type AlGaAs first clad layer 3 and a non-doped GaAs first conductivity type first on a first conductivity type GaAs substrate 2. One light guide layer 4a is grown partway. Thereafter, a diffraction grating is formed by etching, and then the AlGaAs layer is grown at a growth rate reduced so that the diffraction grating is filled flat, thereby forming the diffraction grating layer 22. Thereafter, a GaAs light guide layer 4b is added on the diffraction grating layer 22, followed by an active layer 5 made of GaAs containing nitrogen (N), in this example, an active region 5 made of GaInNAs, and a non-doped GaAs first layer. A second conductivity type second light guide layer 6 is laminated. Thereafter, a buried layer for confining current, that is, a so-called current confinement layer (also referred to as a current blocking layer for blocking current) 13 is formed in the removed portion by mesa etching. At this time, the etching is not performed up to the diffraction grating 22 containing Al. Thereby, the surface containing Al is not exposed to the atmosphere. Further, a laser structure is configured by laminating a second cladding layer 11 made of AlGaAs of the second conductivity type and a cap layer 12 made of GaAs of the second conductivity type.

As shown in FIG. 6B, the diffraction grating layer 22 is formed by repeatedly arranging, for example, a GaAs layer 23 and an AlGaAs layer (for example, an Al _0.1 Ga _0.9 As layer) 24 in the resonator length direction. The width d in the arrangement direction of the layers 23 and 24 can be set to about 10 nm to 50 nm. The one layer 24 constituting the diffraction grating layer 22 is not limited to AlGaAs, and may be any material layer that has a refractive index difference with the other GaAs layer 23 and can form a diffraction grating. For example, the layer 23 may be a GaAs (Sb) (N) layer (that is, a GaAsSbN layer, a GaAsSb layer or a GaAsN layer) and a GaAs layer, or a GaInNAs (Sb) layer (that is, a GaInNAsSb layer or a GaInNAs layer) and a GaAs layer. Good.
In addition, as shown in FIG. 6C, since the surface area of the diffraction grating portion is very small, if the Al composition is 0.2 or less, the reliability is greatly reduced even if the diffraction grating is formed on the mesa of the active layer. Never do. Furthermore, when the diffraction grating portion is made of a material not containing Al, there is no problem in reliability even if the diffraction grating is formed on the active layer.

  In the present embodiment, mesa etching is performed from the second light guide layer 6 containing no Al to the middle of the first light guide layer 4b containing no Al, and the first and second light guide layers 4a, 4b, and 6 are activated. The current confinement layer 13 is formed by growing from the regrowth surface 14 containing no Al including the layer 5 and the diffraction grating layer 22. Here, even when the diffraction grating layer 22 includes the AlGaAs layer 24, the area is extremely small and the distance from the active layer does not greatly affect the reliability.

The current confinement layer 13 is preferably formed of the second conductivity type first semiconductor layer 8 and the first conductivity type second semiconductor layer 9 thereon, as described above. A semiconductor layer having a pn junction j is formed. In addition, an insulating layer can be used. A cap layer 10 made of GaAs of the first conductivity type is formed between the current confinement layer 13 and the second cladding layer 11.
The active region 5 can be formed in a quantum well structure such as a GaInNAs active layer and a GaAs barrier layer, for example, a double quantum well (DQW) structure, as described above. The active layer constituting the active region 5 can be formed of GaInNAsSb, GaAsSbN, GaNAs, or the like in addition to the GaInNAs described above.

Next, a manufacturing method of the embedded semiconductor laser element 21 of the present embodiment that is DFB will be described.
First, in the same manner as the above-described buried type semiconductor laser device 1, an n-type AlGaAs first cladding layer 3 and a non-doped GaAs first guide layer 4a are formed on an n-type GaAs substrate 2 whose main surface is a (100) crystal plane. To grow halfway through. Thereafter, a photoresist layer is formed on the surface, exposed to a diffraction grating pattern by an electron beam (EB) exposure device, and developed. That is, a resist mask having openings at a required interval in the resonator length direction is formed. Next, the surface portion of the non-doped GaAs first guide layer 4a is dry-etched through a resist mask to form a concave portion that repeats at a predetermined interval. In this example, dry etching is performed with a diffraction grating pattern of about 10 to 50 nm.

Next, after removing the resist and pre-treating, an AlGaAs layer is grown at a slow growth rate so as to fill the recesses in the surface of the uneven first guide layer 4a. This growth layer is not limited to the AlGaAs layer. It is only necessary to have a refractive index difference from the GaAs layer and to form the diffraction grating 22. For example, a GaInNAs layer or a GaNASSb may be used. When these materials are used for the diffraction grating, an absorption diffraction grating type DFB can be produced. Changing the depth of the diffraction grating 22 and the composition of the buried layer changes the characteristics of the DFB. It is desirable to determine parameters according to the application. Thereafter, the remaining GaAs optical guide layer 4b, the active region 5 having a GaInNAs layer, and the second guide layer 6 of non-doped GaAs are formed, thereby completing the second epitaxial growth.
Thereafter, the target DFB-embedded embedded semiconductor laser element 21 shown in FIG. 6 can be obtained through the same steps as those shown in FIGS. 3E to 4 described above. At this time, if the diffraction grating layer is not etched, a layer containing Al does not appear on the regrown surface.

  According to the embedded semiconductor laser element 21 according to the present embodiment, it is possible to provide a DFB semiconductor laser element having excellent temperature characteristics and high reliability. Further, by using the active layer having nitrogen (N) equivalent to GaInNAs, the guide layer can be formed thick, so that a highly reliable BH-DFB semiconductor laser element can be manufactured.

  Although the embodiment of the semiconductor laser device according to the present invention has been described above, the present invention is not limited to this embodiment, and various other configurations can be taken without departing from the elements of the present invention.

1 is a cross-sectional view showing an embodiment of an embedded semiconductor laser device according to the present invention. 1A to 1D are process diagrams showing an embodiment of an embedded semiconductor laser device according to the present invention. EG is a process drawing showing an embodiment of an embedded semiconductor laser device according to the present invention. It is process drawing which shows one Embodiment of the embedded type semiconductor laser element concerning this invention. It is a graph which shows the optical confinement rate of a buried type semiconductor laser element, and the thickness of a guide layer. A is a cross-sectional view showing an embodiment of a BH-DFB semiconductor laser device according to the present invention. B is an enlarged view showing a cross-sectional view of the diffraction grating layer of FIG. 6A. C is a cross-sectional view showing another embodiment of a BH-DFB semiconductor laser device according to the present invention. FIG.

Explanation of symbols

1 .. Embedded semiconductor laser element 2... First conductivity type GaAs substrate 3.. First cladding layer 4, 4a, 4b... First conductivity type first light guide layer 5. Region, 6... Second conductivity type second light guide layer, 7... Substrate, 8... First current confinement layer, 9... Second current confinement layer, 10. 2nd cladding layer, 12 ... cap layer, 13 ... current confinement layer, 14 ... re-growth surface, 15 ... SiO ₂ mask, 16 ... mesa part, 17 ... p-type substrate, 21 ... embedded type Semiconductor laser element, 22 ·· Diffraction grating layer, 23 ·· GaAs layer, 24 ·· AlGaAs layer,

Claims (2)

On the first conductivity type GaAs substrate,
A first cladding layer of AlGaAs of the first conductivity type;
A first guide layer of non-doped GaAs;
An active layer of GaInNAs, GaInNAsSb, GaAsSbN or GaNAs;
A second guide layer of non-doped GaAs;
A stacked body in which second cladding layers of second conductivity type AlGaAs are sequentially stacked;
The first current of the second conductivity type AlGaAs or GsAs sequentially grown from the recrystallized growth surface not containing Al in the mesa portion formed by mesa etching from the surface of the second guide layer to the middle of the first guide layer. A buried layer for blocking current comprising a constriction layer and a second current confinement layer of a first conductivity type AlGaAs or GaAs;
A cap layer of a first conductivity type GaAs formed between the second current confinement layer and the second cladding layer;
Buried type semiconductor laser device having a. 2. The buried semiconductor laser device according to claim 1, further comprising a diffraction grating layer formed of an AlGaAs layer and a GaAs layer, a GaInNAsSb layer or a GaInNAs layer and a GaAs layer, or a GaAsSbN layer, a GaAsSb layer or a GaAsN layer and a GaAs layer .

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