JP5170869B2 - Optical semiconductor device and method for manufacturing optical semiconductor device - Google Patents

Description

  The present invention relates to an optical semiconductor device and a method for manufacturing the optical semiconductor device, and more particularly to a structure of an optical semiconductor device having an active layer formed in a high mesa shape.

  Along with the spread and progress of optical communication networks, higher performance of optical components used in optical communication systems is progressing. In particular, optical semiconductor elements based on semiconductor materials have features such as compactness, high-speed operation, high efficiency, and low power consumption. Therefore, development and commercialization of various applications including semiconductor lasers and semiconductor light-receiving elements have been promoted. It has been. With the recent demand for higher functionality of optical components, the element structure and configuration of optical semiconductor elements have become more complex, and not only optical semiconductor devices having a single function such as light emission only or light reception only. Development of monolithic integrated technology for manufacturing optical semiconductor elements with two or more different functions such as light emission, light reception, light modulation, optical amplification, optical waveguide, multiplexing and demultiplexing on the same semiconductor substrate (wafer) Is progressing.

  As a waveguide structure in a functional element (active element) of an optical semiconductor element, typically, three types of high mesa type, low mesa type, and embedded mesa type are often used. The high mesa type waveguide structure has a structure in which at least the active layer is formed in a mesa shape, and the mesa protrudes upward from the outer semiconductor stack. The low mesa type waveguide structure has a structure in which the layers up to the upper layer of the active layer are formed in a mesa shape, and the mesa protrudes upward from the semiconductor stack on the outer peripheral side. The embedded mesa waveguide structure has a structure in which a semiconductor layer is stacked on the outer periphery of a high mesa or low mesa type mesa and the mesa is embedded by this semiconductor layer.

  In an optical waveguide that constitutes a passive element (passive element) of an optical semiconductor element, when a high mesa structure is adopted, since the difference in refractive index in the lateral direction is particularly large, there is an advantage that the radius of curvature in the bending waveguide can be reduced. . On the other hand, when this waveguide structure is applied to an optical functional element, the active layer surface is exposed on the mesa side, and thus there is a problem that it is difficult to improve the reliability of the functional element.

  An optical semiconductor element employing a low-mesa waveguide structure has the advantage that no embedded crystal growth is required and the active layer surface of the functional element is not exposed, so that non-radiative recombination on the surface is small. However, there are also drawbacks such as an increase in leakage current due to the spread of the injection current and difficulty in controlling the remaining thickness during etching when forming a low mesa.

  In an optical semiconductor device using a buried mesa waveguide structure, both sides of the active layer are etched to form a mesa shape, and then the mesa shape is embedded by crystal growth of a current blocking semiconductor layer It has a structure. For this reason, there is an advantage that the leakage current that flows outside the active layer can be reduced and that the surface of the active layer is covered with the semiconductor layer, so that non-radiative recombination on the surface can be suppressed. For example, in an optical functional element such as a laser, high reliability can be realized. Therefore, conventionally, a buried mesa type is often used as the waveguide structure of a single semiconductor optical functional element.

  In the embedded mesa type optical semiconductor element, the crystal growth of the blocking semiconductor layer is necessary, and the crystal growth is dependent on the crystal plane orientation. There were drawbacks such as limited direction. Since buried crystal growth greatly affects current blocking characteristics, it is necessary to perform crystal growth by accurately controlling the thickness of the buried layer, the doping concentration, and the like.

  In addition, since the difference in refractive index between the active layer region and the buried semiconductor layers on both sides of the active layer region is small, if the curvature radius is reduced when manufacturing a bending waveguide or the like, bending loss may be reduced to other structures. There is also a disadvantage that it becomes larger than the case. In the realization of a single optical semiconductor device, the above-mentioned drawbacks were not so big, but as monolithic integration progressed, embedded crystal growth was necessary, the mesa direction was limited, and the radius of curvature was limited. As a result, the problem that compact integration is more difficult than other structures becomes prominent.

A semiconductor laser or optical waveguide having a high-mesa waveguide structure is described in, for example, Patent Document 1 or Patent Document 2, and a semiconductor laser having an embedded waveguide structure is described in, for example, Patent Document 3. ing. In Patent Document 1, a buried mesa or a low mesa (ridge) structure is used for the optical functional element (active) region instead of a high mesa structure, and a high mesa structure is used for the bent waveguide portion serving as the optical waveguide region. Therefore, the manufacturing process becomes complicated and high accuracy is required.
JP 2002-118324 A JP 2003-207665 A JP 2001-320125 A

  In view of the above background, a high-mesa structure is required as a structure that does not require buried crystal growth, does not depend on crystal plane orientation, can adopt a bent waveguide shape with a small radius of curvature, and can be compactly integrated. The benefits of are attracting attention. For this reason, it is an urgent task to overcome the above-described reduction in quantum efficiency due to non-radiative recombination due to surface exposure of the active layer and deterioration of characteristics.

  The present invention has been made to solve the problems of conventional high-mesa optical semiconductor elements. Unlike embedded mesa-type optical semiconductor elements, the present invention provides strictly controlled embedded crystal growth and surface growth. A waveguide structure that does not require orientation-dependent buried crystal growth, has a high mesa type advantage that enables compact integration, and also has the high reliability that was the advantage of a buried mesa type It is an object of the present invention to provide a high-performance optical semiconductor element that can be compactly monolithically integrated with a small number of crystal growth times and a high degree of freedom in fabrication.

  It is another object of the present invention to provide a method for manufacturing an optical semiconductor device having the above-described waveguide structure.

In order to achieve the above object, an optical semiconductor element of the present invention is an optical semiconductor element in which a laminate including an active layer of an optical functional element is formed in a high mesa structure.
A semiconductor protective layer having a band gap larger than the band gap of the active layer and a thickness of 1.5 μm or less is formed at least on the side surface of the active layer.

The method for manufacturing an optical semiconductor element of the present invention includes an optical semiconductor element comprising a functional element region having an optical functional element including an active layer and an optical waveguide region having an optical waveguide including a core layer on a semiconductor substrate. A method of manufacturing comprising:
Depositing a stack including an active layer on a semiconductor substrate;
Removing a laminated portion including at least an active layer among the laminated layers formed in the optical waveguide region;
Depositing a laminate including a core layer at the location of the removed laminate portion;
Etching the stack of the functional element region and the optical waveguide region to form a high mesa structure including the active layer and the core layer;
Depositing a semiconductor protective layer having a band gap larger than the band gap of the active layer and having a thickness of 1.5 μm or less on the exposed side surfaces of the active layer and the core layer in the high mesa structure;
Forming an optical functional element and an optical waveguide from the high mesa structure of the functional element region and the optical waveguide region on which the semiconductor protective layer is deposited, respectively.

  In the optical semiconductor device of the present invention and the optical semiconductor device manufactured by the manufacturing method of the present invention, by forming the semiconductor protective layer on the side surface of the active layer, non-light-emitting re-generation due to pairs of electrons and holes generated in the active layer is achieved. Prevent binding.

  Further, in the optical semiconductor device of the present invention in which the optical semiconductor device manufactured by the manufacturing method of the present invention and the optical functional device and the optical waveguide are formed on the same substrate, the influence of the semiconductor protective layer on the bending loss is high. Since the structure is comparable or the loss is sufficiently low compared with the embedded mesa structure, the optical waveguide can be bent with a small radius of curvature, so that an optical semiconductor device having a compact structure can be obtained.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a perspective view of an optical semiconductor device (optical semiconductor integrated device) according to an embodiment of the present invention. The optical semiconductor device 100 of the present embodiment is formed on a semiconductor substrate 11, and includes a laser region 10A having a semiconductor laser 31 as an optical functional device, and an optical waveguide region 10B having an optical waveguide 32 as a passive device. The semiconductor laser 31 and the optical waveguide 32 are monolithically integrated on the same substrate. The optical waveguide 32 has a bent portion 33 bent into a U-shape, bends light incident from the semiconductor laser 31 at the bent portion 33, and guides light in a direction opposite to the incident direction.

  2A and 2B show the A-A ′ and B-B ′ cross sections of FIG. 1, respectively. As shown in FIG. 2A, the semiconductor laser 31 formed in the laser region 10A includes an n-InP buffer layer 12, a laser active layer (MQW active layer) 13, which are sequentially formed on the semiconductor substrate 11. The clad layers 14 and 18 and the contact layer 19 are laminated, and the entire laminated portion from a part of the buffer layer 12 forms a mesa structure. The laser region 10 </ b> A including the side surface of the mesa is covered with a semiconductor thin film protective layer (hereinafter referred to as a thin film protective layer) 20.

  As shown in FIG. 2B, the optical waveguide 32 in the optical waveguide region 10B has a stack of the buffer layer 12, the core layer 16, and the cladding layers 17 and 18 that are sequentially formed on the semiconductor substrate 11. The entire laminated portion from a part of the buffer layer 12 forms a mesa structure. The optical waveguide region 10 </ b> B including the side surface of the mesa is covered with the semiconductor thin film protective layer 20.

  The thin film protective layer 20 has a band gap larger than that of the active layer and a thickness of 1.5 μm or less, and is formed of, for example, a p-type semiconductor layer. The thin-film protective layer 20 has a band gap larger than the band gap of the active layer 13 of the semiconductor laser 31, thereby suppressing non-radiative recombination due to electron and hole pairs generated on the side surface of the laser active layer 13, This prevents an increase in threshold current and deterioration of characteristics of the active layer that are particularly likely to occur in an active layer having a high mesa structure.

  A method for manufacturing the optical semiconductor element 100 according to this embodiment will be described with reference to FIG. 3A to 3E are cross-sectional views sequentially showing a cross section taken along line C-C ′ of FIG. 1 for each process step. First, the following crystal growth is performed at a growth temperature of 600 ° C. using an MOCVD crystal growth apparatus. First, an n-InP buffer layer 12, a multiple quantum well (MQW-SCH) active layer 13 made of InGaAsP, and a p-InP upper cladding layer 14 are crystal-grown on the n-InP substrate 11 (FIG. 3 ( a)). Next, the laser region 10A for forming the semiconductor laser is protected by a hard mask 15 made of a SiNx layer, and the upper cladding layer 14 and the MQW-SCH active layer 13 are formed in the laminated optical waveguide region 10B. It is removed by etching (FIG. 3B).

  Next, the core layer 16 of the optical waveguide made of InGaAsP and the p-InP upper clad layer 17 are formed by the butt joint growth method in the portion removed by the etching (FIG. 3C). After the SiNx hard mask 15 is removed, the p-InP cladding layer 18 and the InGaAs contact layer 19 are crystal-grown in both the laser region 10A and the optical waveguide region 10B (FIG. 3D). Next, after the contact layer 19 in the optical waveguide region 10B is removed by etching (FIG. 3E), etching is performed by dry etching in both the laser region 10A and the optical waveguide region 10B using a SiNx film (not shown) as a mask. A high mesa structure having a width of 2 μm is formed.

  In dry etching for forming a high mesa structure, etching is performed to a depth that penetrates the cladding layer 18, the cladding layer 14 and the cladding layer 17, the active layer 13, and the core layer 16 and reaches a part of the upper side of the n-InP buffer layer 12. . Therefore, the side surface of the active layer 13 is exposed immediately after etching.

Next, the SiNx film (not shown) used in the etching for forming the mesa shape is used as a crystal growth mask as it is, and the InP thin film protective layer 20 is grown on the side of the high mesa shape mesa. Sections DD ′ and EE ′ of FIG. 3 (e) are shown in FIGS. 4 (a) and 4 (b). The thickness of the thin film protective layer 20 is, for example, 0.1 μm at the side surface portion of the active layer 13. The conductivity of the thin film protective layer 20 is p-type, and the doping amount is about 5 × 10 ¹⁷ cm ⁻³ . The thickness, conductivity, and doping concentration of the thin film protective layer 20 are determined in consideration of the following points.

  FIG. 5 shows the calculation result of the change in bending loss of the core layer 16 when the thickness of the InP thin film protective layer 20 on the side surface of the active layer 13 is changed. When the thin-film protective layer 20 is not formed (graph (i)), the bending loss of the core layer starts to increase when the radius of curvature is about 210 μm. When the thin film protective layer 20 has a thickness of 0.1 μm (graph (ii)), the bending loss of the core layer 16 starts to increase in the vicinity of 240 μm, and the loss is not much inferior to the case without the thin film protective layer 20. Is shown. As the thickness of the thin film protective layer 20 is increased, the radius of curvature at which bending loss begins to increase increases. When the thickness is 0.5 μm (vi), the thickness of the curvature is about 360 μm and the thickness is 1.5 μm. In the case of (graph (viii)), the bending loss starts to increase at a curvature radius of around 470 μm. On the other hand, in the embedded mesa type (graph (ix)), the bending loss starts to increase when the radius of curvature is around 530 μm.

  Therefore, when the thickness of the thin film protective layer 20 is 1.5 μm or less, the high mesa type optical waveguide formed with the thin film protective layer 20 is superior to the embedded mesa type optical waveguide in terms of bending loss. In particular, in the high mesa structure in which the thickness of the thin film protective layer 20 is 0.5 μm or less, the radius of curvature can be significantly reduced as compared with the embedded mesa structure.

  When the side surface of the active layer 13 is exposed, the electron and hole carrier pair injected into the active layer 13 disappears by performing non-radiative recombination through the surface level. , Reduce the luminous efficiency. In the present embodiment, a thin film protective layer 20 having a band gap larger than that of the active layer 13 is crystal-grown on the side surface of the active layer 13 processed into a mesa shape, and the carriers appearing at the interface of the active layer 13 are controlled, thereby preventing non-light emission. Suppresses recombination. As described above, in order to realize a low bending loss like a normal high mesa type optical waveguide, the thickness of the thin film protective layer 20 on the side surfaces of the active layer 13 and the core layer 16 is preferably as small as possible. On the other hand, the smaller the thickness of the thin film protective layer 20, the more the carrier in the active layer 13 diffuses to the surface and the possibility of contributing to non-radiative recombination increases. Therefore, it is preferable to reduce the thickness of the thin film protective layer 20 as much as possible and to effectively prevent diffusion of carriers in the active layer 13 to the side surface.

By performing p-type or n-type doping on the thin film protective layer 20, diffusion of electrons or holes to the side of the active layer is significantly suppressed, and non-radiative recombination generated on the side is further reduced. Is possible. Since the diffusion length is shorter as the doping concentration is higher, an appropriate doping concentration is preferably about 1 × 10 ¹⁷ cm ⁻³ or more in terms of suppression of diffusion, although it depends on the thickness of the thin film protective layer 20. If the doping concentration exceeds 1 × 10 ¹⁸ cm ⁻³ , the increase in waveguide loss is particularly strongly affected, so it is preferable to avoid excessive doping. When doping is performed, it is preferable that the thickness of the thin protective layer 20 is about 0.05 μm or more even if it is thin. In general, the higher the doping concentration, the greater the waveguide loss. Therefore, when the highest priority is given to reducing the waveguide loss, the thin film protective layer 20 is preferably non-doped.

  When doping the thin-film protective layer 20, there is a concern that it may become a current leakage path. Usually, since the electron mobility is larger than that of holes, it is particularly preferable to suppress electron leakage. Therefore, it is preferable from this viewpoint to dope the thin film protective layer 20 with a p-type impurity.

In consideration of the above, the thickness and the doping concentration range of the thin film protective layer 20 are about 0.05 to 0.5 μm, the doping concentration is about 1 × 10 ¹⁷ to 1 × 10 ¹⁸ cm ⁻³ , and the conductivity type. Is preferably p-type. In this embodiment, the thickness of the thin film protective layer is 0.1 μm, the doping is a p-type impurity, and its concentration is 5 × 10 ¹⁷ cm ⁻³ .

  After the thin film protective layer 20 is grown, passivation is performed with a SiNx film, only the contact layer portion on the mesa in the laser region 10A is opened, Ti / Pt / Au is deposited, and the p-type electrode 21 (FIG. 2 (a )). Further, after polishing the back surface of the wafer, an n-type electrode (not shown) of AuGeNi is formed on the back surface.

  The optical integrated device of the above embodiment was manufactured as a sample of the present invention. For comparison, a high-mesa optical semiconductor device that has been subjected to SiNx passivation without growing the thin film protective layer 20 and a buried hetero type that has the same active layer structure and the same active layer width of 2 μm. Each of the laser elements was manufactured as a sample for a comparative example. The three types of samples produced were evaluated for the characteristics of the laser region as a Fabry-Perot laser with a cavity length of 300 μm and cleaved at both ends. As a result, the sample of the high mesa type semiconductor laser of the above embodiment oscillated with a threshold current as low as about 8 mA, which is almost the same as the sample of the buried hetero type laser element. On the other hand, in the sample of the comparative example in which the thin film protective layer was not grown, the threshold current was about 12 mA, which was higher than the sample in which the thin film protective layer was grown. This is presumably because a part of the injected carriers is non-radiatively recombined through the surface level, thereby reducing the luminous efficiency.

  In addition, as a result of conducting an energization test at a high temperature of 85 ° C. for the above three types of samples, a high mesa type laser element sample and a buried hetero type laser element sample in which a thin film protective layer was grown are prominent. While no degradation was observed, the high-mesa laser element sample without thin film protective layer growth showed a significant decrease in long-term reliability, such as an increase in threshold current and a decrease in light emission efficiency. .

Furthermore, as a result of evaluating the bending loss of the optical waveguide portion of the high-mesa optical semiconductor element on which the thin film protective layer has been grown, the curvature radius is about 3 dB / mm when the radius of curvature is 250 μm, and the loss is as low as 1 dB / mm when the radius of curvature is 300 μm. It was confirmed that the calculation results were almost the same. In addition, the surface recombination rate when the thin film protective layer is not grown is 1.3 × 10 ⁵ cm / s in the result of measuring the carrier lifetime depending on whether the thin film protective layer is grown or not. On the other hand, when the thin film protective layer was grown, the surface recombination rate was greatly reduced to 4.0 × 10 ² cm / s, and the thin film protective layer caused non-radiative recombination. It was confirmed that it could be greatly suppressed.

  As described above, the optical semiconductor element of this embodiment example has a light emitting characteristic and long-term reliability equivalent to those of a normal embedded mesa type as a functional element portion, and the waveguide portion is a normal high mesa waveguide. It was found that they have almost the same low bending loss characteristics at the same time.

  The thin film protective layer growth in this embodiment can be performed by using the mask used in the mesa etching as it is, so that the process can be manufactured by almost the same process as a normal buried hetero laser. It is. In addition, the buried hetero laser usually requires a buried growth of 2 to 3 μm. On the other hand, in the above embodiment, the growth film thickness is as small as submicron, so that the crystal growth is very easy and in a short time. It can be carried out. Further, compared with the buried growth of a normal buried hetero laser, it is not necessary to strictly control the thickness and doping concentration of the thin film protective layer.

  Although not particularly mentioned in the above description of the manufacturing process, if it is necessary to remove the etching damage layer due to the mesa formation by dry etching, the mesa shape is not changed before the thin film protective layer growth. Mild wet etching, ammonium sulfide treatment, heat treatment, or the like may be performed.

  In the above embodiment, an InGaAsP-based MQW structure is used for the active layer. However, the optical semiconductor element of the present invention is not limited to this material or the MQW structure. For example, it is considered that the same non-radiative recombination reduction effect can be obtained also in materials such as InGaAs and InAlGaAs, and active layers made of bulk, quantum wires, and quantum dots. In particular, when an Al-based material that is easily surface oxidized, such as InAlGaAs, is used for the active layer, the semiconductor thin film growth in the present invention is considered to be particularly effective.

  In the embodiment described above, the thin film protective layer 20 is a single layer of p-doped InP, but may have a multilayer structure in which a doped layer and a non-doped layer are combined. However, as already described, since the total film thickness of the thin film protective layer is preferably 0.5 μm or less, it is preferable to suppress the total film thickness below this. In the above embodiment, the thin film protective layer was grown after mesa etching to a depth penetrating the active layer. However, even if the mesa etching is performed to a depth in the middle of the active layer, the thin film protective layer is grown. Thus, the effect of suppressing surface non-radiative recombination can be obtained. Note that the bending loss is expected to be somewhat larger than the high mesa shape.

  As mentioned above, although this invention was demonstrated based on the suitable embodiment, the optical semiconductor element of this invention and the manufacturing method of an optical semiconductor element are not limited only to the structure of the said embodiment, The said implementation is carried out. What carried out various correction | amendment and change from the structure of an aspect is also contained in the scope of the present invention.

1 is a perspective view of an optical semiconductor element according to an embodiment of the present invention. (A) And (b) is sectional drawing of the optical semiconductor element of FIG. 1, respectively. (A)-(e) is sectional drawing which shows the cross section for each process step in order in the process which manufactures the optical semiconductor element of FIG. 1, respectively. (A) And (b) is sectional drawing which follows the D-D 'and E-E' line | wire of FIG.3 (e), respectively. The graph which shows the relationship between the thickness of the thin film protective layer in the optical semiconductor element of FIG. 1, and the curvature dependence of bending loss.

Explanation of symbols

100: optical semiconductor element 10A: laser region 10B: optical waveguide region 11: semiconductor substrate 12: buffer layer 13: MQW active layer 14: cladding layer 15: SiNx hard mask 16: core layer 17: cladding layer 18: cladding layer 19: Contact layer 20: thin film protective layer 21: p-side electrode 31: laser 32: optical waveguide 33: bent portion

Claims (7)

In an optical semiconductor element in which a stack including an active layer of an optical functional element is formed in a high mesa structure on a semiconductor substrate,
A semiconductor protective layer having a band gap larger than the band gap of the active layer and having a thickness in the range of 0.05 μm to 0.5 μm is formed on all sides of the high mesa structure .
The semiconductor protective layer has the same conductivity type in all layers of the semiconductor protective layer, and the semiconductor protective layer is a layer that suppresses diffusion of electrons or holes to the side of the active layer,
A cross section perpendicular to the semiconductor substrate of the high mesa structure is rectangular,
On the semiconductor substrate, in addition to the optical functional element, an optical waveguide including a core layer is formed in the high mesa structure, and the semiconductor protective layer is formed on both side surfaces of the active layer and the core layer,
An optical semiconductor element in which the optical functional element and the optical waveguide are optically coupled . 2. The optical semiconductor device according to claim 1 , wherein the optical waveguide has a bent portion that is bent with a curvature radius of 360 μm or less parallel to the substrate surface. The optical semiconductor element according to claim 1 , wherein the optical waveguide has a bent portion that is bent with a curvature radius of 240 μm or less parallel to the substrate surface. The semiconductor protective layer is a p-type semiconductor, an optical semiconductor device according to any one of claims 1-3. Wherein a semiconductor protective layer is p-type or n-type semiconductor, the doping concentration is in the range of ^{1 × 10 17 ~1 × 10 18} cm -3, an optical semiconductor device according to any one of claims 1-3. The optical semiconductor element according to claim 5 , wherein the semiconductor protective layer is a layer that suppresses diffusion of electrons or holes to the side surface of the active layer depending on the doping concentration. A method of manufacturing an optical semiconductor element comprising a functional element region having an optical functional element including an active layer and an optical waveguide region having an optical waveguide including a core layer on a semiconductor substrate,
Depositing a stack including an active layer on the semiconductor substrate;
Removing a laminated portion including at least an active layer among the laminated layers formed in the optical waveguide region;
Depositing a laminate including a core layer at the location of the removed laminate portion;
Etching the stack of the functional element region and the optical waveguide region to form a high mesa structure including the active layer and the core layer;
The exposed side surfaces of the active layer and the core layer in the high mesa structure, the band gap being larger than the band gap of the active layer and the thickness being in the range of 0.05 μm to 0.5 μm on all side surfaces of the high mesa structure. Depositing a semiconductor protective layer;
Forming the optical functional element and the optical waveguide from the high mesa structure of the functional element region and the optical waveguide region where the semiconductor protective layer is deposited, respectively,
The semiconductor protective layer has the same conductivity type in all layers of the semiconductor protective layer, and the semiconductor protective layer is a layer that suppresses diffusion of electrons or holes to the side of the active layer,
The cross section perpendicular to the high mesa structure substrate is rectangular,
On the semiconductor substrate, in addition to the optical functional element, an optical waveguide including a core layer is formed in the high mesa structure, and the semiconductor protective layer is formed on both side surfaces of the active layer and the core layer,
Method for manufacturing an optical semiconductor element and the optical waveguide and the optical functional element that will be coupled optically.

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