JP2008282991A - Semiconductor laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor laser whose characteristic can be improved by suppressing reduction in efficiency of optical absorption in an etching stop layer and optical confinement to an active layer. <P>SOLUTION: An etching stop layer 30 of an AlGaInP-based semiconductor laser is composed of an AlInAs mixed crystal. By adjusting a composition ratio, a band gap of the etching stop layer 30 can be made larger than that of an active layer 23, and optical absorption in the etching stop layer 30 is suppressed. An optical field present in a laser structure can be set to be symmetric to the active layer 23 by setting the refractive index of the etching stop layer 30 to be smaller than or equal to those of a p-type first clad layer 25A and a p-type second clad layer 25B. Therefore, a reduction in the efficiency of optical confinement to the active layer 23 is suppressed. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a semiconductor laser suitable for a red semiconductor laser having an oscillation wavelength in the 600 nm band.

  At present, a semiconductor laser having a so-called ridged stripe structure in which a part of a clad layer has a protrusion is dominant. This is because the ridged stripe structure has various advantages such as improved current injection, laser transverse mode control, and emission angle characteristic control.

In the production of the ridged stripe structure, it is necessary to provide an etching stop layer in the cladding layer in order to stop the etching of the cladding layer at a desired thickness during the etching process. Conventionally, in a red semiconductor laser, GaInP is used as a material for an etching stop layer (see, for example, Patent Document 1).
JP 2006-49420 A S. S. Tiwari, 1 other, Empirical fit to band discontinuities and barrier heights in III-V alloy systems, “Applied Physics Letters "(USA), American Institute of Physics, February 3, 1992, Vol. 60, No. 5, p. 630-632, FIG. 1

  However, GaInP may not be transparent to the light emitted from the active layer, and there is a problem that light absorption occurs in the etching stop layer and the laser performance may be deteriorated. That is, as shown in FIG. 6, the light hν generated in the active layer 123 spreads in the vertical direction of the laser structure, and a part thereof is applied to the etching stop layer 130. If this light is absorbed by the etching stop layer 130, the laser light is consumed by the etching stop layer 130, and becomes invalid energy, leading to a decrease in laser efficiency.

  In addition, since the refractive index of the GaInP etching stop layer is higher than that of AlGaInP used for the cladding layer, the light field near the active layer becomes asymmetric with respect to the active layer, and the light confinement efficiency (Γ factor) in the active layer There is also a problem that the performance of the laser is lowered. That is, since the light field existing in the laser structure depends on the vertical structure of the laser, if the laser structure is symmetrical on both the n side and the p side with respect to the active layer, the light field has a Gaussian distribution and In contrast, if there is an asymmetric layer in the laser structure, the light field is also asymmetric. When there is no etching stop layer 130, the light field is symmetric as shown by the solid line in FIG. 7, but when there is the etching stop layer 130, the light field is asymmetric as shown by the dotted line in FIG. . FIG. 7 shows that the light intensity in the active layer 123 decreases when the etching stop layer 130 is present compared to when the etching stop layer 130 is absent. This means that the optical confinement efficiency that determines the laser characteristics is reduced, which may cause deterioration of the laser characteristics.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a semiconductor laser capable of improving the characteristics by suppressing the light absorption in the etching stop layer and the light confinement efficiency in the active layer. There is.

  The semiconductor laser according to the present invention includes an AlGaInP-based substrate including at least one of aluminum (Al), gallium (Ga), and indium (In) among group 3B elements and phosphorus (P) among group 5B elements on a substrate made of GaAs. A first conductivity type cladding layer made of a compound semiconductor, an active layer, a second conductivity type first cladding layer, an etching stop layer, and a second conductivity type second cladding layer, which are in this order, and comprising a second conductivity type second cladding The layer is a protrusion for defining the current injection region of the active layer, and the etching stop layer contains aluminum (Al), indium (In), and arsenic (As).

  In this semiconductor laser, the etching stop layer is configured to contain aluminum (Al), indium (In), and arsenic (As), so that the band gap of the etching stop layer is made higher than that of the active layer by adjusting the composition ratio. The light absorption in the etching stop layer can be suppressed. Further, by making the refractive index of the etching stop layer smaller or equal to that of the cladding layer, the light field existing in the laser structure can be made symmetric with respect to the active layer. Therefore, a decrease in light confinement efficiency in the active layer can be suppressed.

  According to the semiconductor laser of the present invention, since the etching stop layer includes aluminum (Al), indium (In), and arsenic (As), light absorption in the etching stop layer and light to the active layer are performed. The decrease in confinement efficiency can be suppressed and the characteristics can be improved.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows a configuration of a semiconductor laser according to an embodiment of the present invention. This semiconductor laser is used for medical purposes such as a solid-state laser excitation light source, a projector light source, or photodynamic therapy, and is a red laser having an oscillation wavelength in a wavelength range of 630 nm to 690 nm, for example.

  The semiconductor laser has a semiconductor layer 20 made of an AlGaInP compound semiconductor on a substrate 11 made of n-type gallium arsenide (GaAs) to which an n-type impurity such as silicon (Si) or selenium (Se) is added. Yes. Note that the AlGaInP-based compound semiconductor here refers to at least one of aluminum (Al), gallium (Ga), and indium (In) among the group 3B elements in the short-period periodic table and phosphorus (P) among the group 5B elements. For example, an AlGaInP mixed crystal, a GaInP mixed crystal, an AlInP mixed crystal, or the like can be given. These contain an n-type impurity such as silicon (Si) or selenium (Se), or a p-type impurity such as magnesium (Mg), zinc (Zn), or carbon (C) as necessary.

  The semiconductor layer 20 includes, for example, an n-type cladding layer 21, a first guide layer 22, an active layer 23, a second guide layer 24, a p-type first cladding layer 25A, a p-type second cladding layer 25B, and a p-side contact layer 26. Are stacked in this order from the substrate 11 side. An etching stop layer 30 is provided between the p-type first cladding layer 25A and the p-type second cladding layer 25B. A layer above the etching stop layer 30, that is, the p-type second cladding layer 25B and The p-side contact layer 26 is formed as a thin strip-shaped protrusion (ridge) 27. The protrusion 27 is for defining the current injection region 23A of the active layer 23, and the portion corresponding to the protrusion 27 of the active layer 23 is the current injection region 23A.

  The n-type cladding layer 21 has a thickness of 2.0 μm, for example, and is made of an n-type AlInP mixed crystal or AlInGaP mixed crystal to which an n-type impurity such as silicon or selenium is added. For example, the first guide layer 22 has a thickness of 120 nm and is made of AlGaInP mixed crystal. The first guide layer 22 may not contain impurities, or may contain an n-type impurity such as silicon or selenium.

  The active layer 23 has a thickness of 12 nm, for example, and is composed of a GaInP mixed crystal.

  The second guide layer 24 has, for example, a thickness of 120 nm and is composed of AlGaInP mixed crystal. The second guide layer 24 may not contain impurities, or may be added with a p-type impurity such as zinc (Zn) or magnesium (Mg). The p-type first cladding layer 25A has, for example, a thickness of 0.4 μm and is composed of a p-type AlInGaP mixed crystal to which a p-type impurity such as zinc or magnesium is added. The p-type second cladding layer 25B has, for example, a thickness of 1.6 μm and is composed of a p-type AlInGaP mixed crystal to which a p-type impurity such as zinc or magnesium is added. The p-side contact layer 26 has, for example, a thickness of 0.3 μm and is made of p-type GaAs to which a p-type impurity such as zinc or magnesium is added.

An insulating layer 28 made of, for example, silicon dioxide (SiO ₂ ) or silicon nitride (SiN) is formed on the surface of the p-side contact layer 26. In the insulating layer 28, an opening is provided on the upper surface of the protrusion 27, and a p-side electrode 41 is formed in the opening. The p-side electrode 41 has a structure in which, for example, titanium (Ti), platinum (Pt), and gold (Au) are sequentially stacked, and is electrically connected to the p-side contact layer 26 through the opening of the insulating layer 28. It is connected. An n-side electrode 42 is formed on the back side of the substrate 11. The n-side electrode 42 has a structure in which, for example, AuGe: Ni and gold (Au) are sequentially stacked and alloyed by heat treatment, and is electrically connected to the substrate 11.

  Further, in this semiconductor laser, a pair of side surfaces facing each other in the direction of the resonator is a resonator end surface, and a pair of reflector films (not shown) are formed on the pair of resonator end surfaces. The reflectance is adjusted so that one of the pair of reflecting mirror films has a low reflectance and the other has a high reflectance. Thereby, the light generated in the active layer 23 is amplified by reciprocating between the pair of reflecting mirror films, and is emitted as a laser beam from the reflecting mirror film on the low reflectance side.

  The etching stop layer 30 is for suppressing variation in the thickness of the p-type first cladding layer 25A in the manufacturing process described later. The etching stop layer 30 includes aluminum (Al), indium (In), and arsenic (As), that is, is composed of an AlInAs mixed crystal. Thereby, in this semiconductor laser, the light absorption in the etching stop layer 30 and the light confinement efficiency in the active layer 23 are suppressed, and the characteristics can be improved.

  First, light absorption in the etching stop layer 30 will be described with reference to FIG. 2 is shown in FIG. 1 was created. FIG. FIG. 1 is a graph summarizing experimental data including discontinuities, barrier heights, and band gaps in a strain-free state for binary and ternary III-V group compound semiconductors.

  As can be seen from FIG. 2, the band gap of the AlInAs mixed crystal is about 2.1 eV at the maximum. On the other hand, in the conventional GaInP etching stop layer, the band gap of the GaInP mixed crystal is about 2.0 eV. That is, the AlInAs mixed crystal has a wide band gap comparable to that of the conventional GaInP etching stop layer, and light absorption in the etching stop layer 30 can be suppressed.

  Moreover, since the AlInAs mixed crystal has a larger lattice constant than GaAs constituting the substrate 11, when it is applied to a laser structure, compressive strain is introduced into the layer. At that time, the band gap of the semiconductor widens. By this effect, the etching stop layer 30 made of AlInAs mixed crystal can be made more difficult to absorb light.

  Furthermore, AlInAs mixed crystal is a material that can be either indirect transition or direct transition depending on the composition. Therefore, by adjusting the composition of the AlInAs mixed crystal, an indirect transition semiconductor is formed, and by forming the etching stop layer 30 with this indirect transition AlInAs mixed crystal, it is possible to reduce light absorption and improve characteristics. .

  As can be seen from FIG. 2, the GaPSb mixed crystal has a wide band gap equivalent to that of the conventional GaInP etching stop layer, and is considered to be applicable to the etching stop layer 30.

  Next, the light confinement efficiency in the active layer 23 will be described. The aluminum composition ratio included in the etching stop layer 30 is adjusted to be smaller or equal to the refractive index of the n-type cladding layer 21, the p-type first cladding layer 25A, and the p-type second cladding layer 25B. Is preferred. Furthermore, if the aluminum composition ratio contained in the etching stop layer 30 is adjusted so as to be equal to the refractive indexes of the n-type cladding layer 21, the p-type first cladding layer 25A, and the p-type second cladding layer 25B, preferable. The refractive index distributions of the n-type cladding layer 21, the first guide layer 22, the active layer 23, the second guide layer 24, the p-type first cladding layer 25A, the etching stop layer 30 and the p-type second cladding layer 25B are represented by the active layer. This is because the optical field existing in the laser structure can be symmetric with respect to the active layer 23, and a decrease in light confinement efficiency can be suppressed. Here, “being equal” includes not only the case where the refractive indexes are exactly equal, but also the case where they are equal. For example, the refractive index of the etching stop layer 30 is extremely larger than the refractive indexes of the n-type cladding layer 21, the p-type first cladding layer 25A, and the p-type second cladding layer 25B, but rather close to the active layer 23. In this case, the light field becomes asymmetrical and is excluded. However, it is acceptable if the refractive index of the etching stop layer 30 is slightly larger than that of the n-type cladding layer 21, the p-type first cladding layer 25A, and the p-type second cladding layer 25B and the symmetry of the optical field is maintained. Is done. Similarly, “symmetric” includes not only the case of being exactly symmetric but also the case of being almost symmetric.

  Also in this regard, the above-described AlInAs mixed crystal is preferable as a constituent material of the etching stop layer 30. This is because the AlInAs mixed crystal mixed crystal can obtain a refractive index smaller or equal to the constituent material of the n-type cladding layer 21 and the p-side cladding layer 25 described above depending on the composition.

Specifically, for example, when the wavelength is around 640 nm and the n-type cladding layer 21, the p-type first cladding layer 25A, and the p-type second cladding layer 25B are composed of Al _0.5 In _0.5 P mixed crystal, etching is performed. The refractive index of the stop layer 30 is 3.35 or less, and the aluminum composition ratio contained in the etching stop layer 30 is preferably 0.7 or more. This is because an effect equivalent to that of a conventional GaInP etching stop layer can be obtained.

  Furthermore, the refractive index of the etching stop layer 30 is 3.18 or less, and the aluminum composition ratio contained in the etching stop layer 30 is more preferably 0.9 or more. This is because an effect superior to that of the conventional GaInP etching stop layer can be obtained. FIG. 3 shows the band structure and refractive index distribution in this case. In this semiconductor laser, the refractive index of the etching stop layer 30 is the same as that of the p-type first cladding layer 25A and the p-type second cladding layer 25B, and the optical field existing in the laser structure is centered on the active layer 23. The light confinement efficiency can be increased.

Further, for example, the wavelength is around 660 nm used for DVD (Digital Versatile Disk), and the n-type cladding layer 21, the p-type first cladding layer 25A and the p-type second cladding layer 25B are (Al _0.7 Ga _0.3 ) _0.5 In _0.5. When composed of P mixed crystals, the refractive index of the etching stop layer 30 is preferably 3.27 or less, and the aluminum composition ratio contained in the etching stop layer 30 is preferably 0.85 or more. This is because an effect superior to that of the conventional GaInP etching stop layer can be obtained. FIG. 4 shows the band structure and refractive index distribution in this case. Also in this semiconductor laser, the refractive index of the etching stop layer 30 is equal to that of the p-type first cladding layer 25A and the p-type second cladding layer 25B, and the light field existing in the laser structure is centered on the active layer 23. Symmetry can be achieved, and light confinement efficiency can be increased.

  In this case, in order to obtain the same effect as the conventional GaInP etching stop layer, the aluminum composition ratio included in the etching stop layer 30 may be 0.65 or more.

  FIG. 5 is a diagram showing the critical film thickness of the AlInAs mixed crystal constituting the etching stop layer 30. As can be seen from FIG. 5, the critical film thickness of the AlInAs mixed crystal is 40 nm (400 最大) at the maximum when the aluminum composition ratio is 0.9, and can be an optimum thickness as the etching stop layer 30. Further, when the aluminum composition ratio is 0.7, the maximum thickness is 13 nm (130 Å), and in this case as well, the thickness can be made sufficient as the etching stop layer 30.

  This semiconductor laser can be manufactured as follows.

  First, for example, an n-type cladding layer 21 made of the above-described thickness and material is formed on the substrate 11 made of the above-described thickness and material, for example, by MOCVD (Metal Organic Chemical Vapor Deposition). A first guide layer 22, an active layer 23, a second guide layer 24, and a p-type first cladding layer 25A are stacked in this order. Next, the group V raw material is switched to form the etching stop layer 30 made of the above-described thickness and material. Subsequently, the Group V raw material is switched again, and the p-type second cladding layer 25B and the p-side contact layer 26 made of the above-described thickness and material are sequentially stacked.

  After that, etching using the etching stop layer 30 is performed to selectively remove a part of the p-side contact layer 26 and the p-type second cladding layer 25B, thereby forming a thin strip-shaped protrusion 27. At that time, the AlInAs mixed crystal forming the etching stop layer 30 and the AlGaInP mixed crystal forming the p-type second cladding layer are sulfuric acid-based etching used for etching the protrusions in the conventional red semiconductor laser production for DVD. It has an advantage that it has etching selectivity with a liquid and can apply a conventional manufacturing process.

  After the protrusion 27 is formed, the insulating layer 28 made of the above-described material is formed on both sides thereof by, eg, CVD (Chemical Vapor Deposition).

  After forming the insulating layer 28, for example, the back side of the substrate 11 is ground to a thickness of the substrate 11 of about 110 μm, and the n-side electrode 42 is formed on the back side of the substrate 11. Further, an opening is provided in the insulating layer 28 corresponding to the p-side contact layer 26 by, for example, etching, and a p-side electrode 41 is formed on the p-side contact layer 26 and the insulating layer 28. After the n-side electrode 42 and the p-side electrode 41 are formed, the substrate 11 is adjusted to a predetermined size, and a reflecting mirror film (not shown) is formed on a pair of resonator end faces facing in the length direction of the p-side contact layer 26. . Thereby, the semiconductor laser shown in FIG. 1 is formed.

  In this semiconductor laser, when a predetermined voltage is applied between the n-side electrode 42 and the p-side electrode 41, the current is confined by the projecting portion 27, and a current is injected into the active layer 23, so that electron-hole Luminescence occurs due to recombination. This light is reflected by a pair of reflecting mirror films (not shown), reciprocates between them to generate laser oscillation, and is emitted to the outside as a laser beam. Here, since the etching stop layer 30 is composed of an AlInAs mixed crystal, the band gap of the etching stop layer 30 can be made wider than that of the active layer by adjusting the composition ratio, and light absorption in the etching stop layer 30 is possible. Is suppressed. Further, the refractive index of the etching stop layer 30 is set to be smaller or equivalent to that of the p-type first cladding layer 25A and the p-type second cladding layer 25B, and the light field existing in the laser structure is symmetric with respect to the active layer 23. It becomes possible. Accordingly, a decrease in light confinement efficiency in the active layer 23 can be suppressed.

  Furthermore, since the light absorption in the etching stop layer 30 is suppressed, the heat generation site in the laser structure can be reduced, and the reliability can be improved.

  As described above, in this embodiment, the etching stop layer 30 is made of AlInAs mixed crystal, so that the light absorption in the etching stop layer 30 and the light confinement efficiency in the active layer 23 are suppressed, and the characteristics are improved. Can be made.

  While the present invention has been described with reference to the embodiment, the present invention is not limited to the above embodiment, and various modifications can be made. For example, although the case where the etching stop layer 30 is composed of an AlInAs mixed crystal has been described in the above embodiment, the etching stop layer 30 may be composed of an AlInAsP mixed crystal in addition to the AlInAs mixed crystal. Since inclusion of phosphorus (P) means expansion of the band gap, the etching stop layer 30 made of AlInAsP mixed crystal can widen the controllable range of the refractive index and the band gap. Even when the etching stop layer 30 is made of an AlInAs mixed crystal, phosphorus (P) may be included along with the switching of the group V raw material in the manufacturing process. In particular, an AlInAsP mixed crystal may be formed. That is, the etching stop layer 30 only needs to contain aluminum (Al), indium (In), and at least arsenic (As) among arsenic (As) and phosphorus (P).

  Further, for example, the material and thickness of each layer described in the above embodiment, the film formation method and the film formation conditions are not limited, and other materials and thicknesses may be used, or other film formation methods and Film forming conditions may be used. For example, in the embodiment described above, the case where the semiconductor layer 20 made of an AlGaInP-based compound semiconductor is formed on the substrate 11 made of GaAs by the MOCVD method has been described. ) Method or the like.

  Furthermore, for example, in the above-described embodiment, the case where the composition of the p-type first cladding layer 25A and the p-type second cladding layer 25B is the same has been described, but the p-type first cladding layer 25A and the p-type second cladding are The layer 25B does not necessarily have the same composition, and may have a different composition.

  In addition, although the semiconductor laser has been described as an example in the above embodiment, the present invention can be applied to other semiconductor light emitting elements such as a superluminescent diode in addition to the semiconductor laser.

It is a perspective view showing the structure of the semiconductor laser which concerns on one embodiment of this invention. It is a figure for demonstrating the light absorption in an etching stop layer, and is an experiment including the discontinuity in an undistorted state, a barrier height, and a band gap about a binary and ternary III-V group compound semiconductor. It is the figure which aggregated data. It is a figure showing an example of the band structure and refractive index distribution of the semiconductor laser shown in FIG. It is a figure showing the other example of the band structure and refractive index distribution of the semiconductor laser shown in FIG. It is a figure showing the critical film thickness of an AlInAs mixed crystal. It is a figure for demonstrating the problem of the conventional etching stop layer. It is a figure for demonstrating the other problem of the conventional etching stop layer.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Semiconductor laser, 11 ... Substrate, 20 ... Semiconductor layer, 21 ... N-type cladding layer, 22 ... First guide layer, 23 ... Active layer, 24 ... Second guide layer, 25A ... First p-type cladding layer, 25B ... p-type second cladding layer, 26 ... p-side contact layer, 27 ... protrusion, 28 ... insulating film, 30 ... etching stop layer, 41 ... p-side electrode, 42 ... n-side electrode

Claims (7)

First conductivity made of an AlGaInP-based compound semiconductor containing at least one of aluminum (Al), gallium (Ga) and indium (In) among group 3B elements and phosphorus (P) among group 5B elements on a substrate made of GaAs. A semiconductor laser having a mold cladding layer, an active layer, a second conductivity type first cladding layer, an etching stop layer, and a second conductivity type second cladding layer in order,
The second conductivity type second cladding layer is a protrusion for defining a current injection region of the active layer;
The etching stop layer includes aluminum (Al), indium (In), and arsenic (As). The etching stop layer is configured such that a refractive index of the etching stop layer is smaller than or equal to a refractive index of the first conductive type cladding layer, the second conductive type first cladding layer, and the second conductive type second cladding layer. The semiconductor laser according to claim 1, wherein an aluminum composition ratio contained in the layer is adjusted. Included in the etching stop layer such that the refractive index of the etching stop layer is equal to the refractive index of the first conductivity type cladding layer, the second conductivity type first cladding layer, and the second conductivity type second cladding layer. The semiconductor composition according to claim 1 or 2, wherein an aluminum composition ratio to be adjusted is adjusted. The refractive index of the etching stop layer is 3.35 or less, and the aluminum composition ratio contained in the etching stop layer is 0.7 or more. The semiconductor laser described. The refractive index of the etching stop layer is 3.27 or less, and the aluminum composition ratio contained in the etching stop layer is 0.85 or more. The semiconductor laser described. The refractive index of the etching stop layer is 3.18 or less, and the aluminum composition ratio contained in the etching stop layer is 0.9 or more. The semiconductor laser described. The semiconductor laser according to any one of claims 1 to 6, wherein the semiconductor laser has an oscillation wavelength in a wavelength range of 630 nm to 690 nm.

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