JP2007035923A - Semiconductor laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor laser that cannot be damaged easily even if it is exposed to excessive electric stress during handling, or the like in a manufacturing process. <P>SOLUTION: The semiconductor laser in an embedded structure comprises a first embedded layer 5 formed on a semiconductor substrate 9 at both the sides of a part machined in a mesa shape such as an active layer 1, and a second embedded layer 6 formed on the first embedded layer; and applies a reverse-bias voltage to a pn junction comprising first and second embedded layers, when a forward-bias voltage is applied between a p electrode 12 and an n electrode 11. In the semiconductor laser, a thin film 14 is provided on the second embedded layer, with the film thickness of one portion of the second embedded layer thinner than that of the other parts of the second embedded layer. Or, a low-doping concentration part is provided on the second embedded layer, with the doping concentration of one part of the second embedded layer lower than that of the other parts of the second embedded layer 6. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to suppression of failure due to forward overvoltage in a semiconductor laser used for optical fiber communication, optical information processing, an optical disk, and the like.

  InGaAsP semiconductor lasers are widely used as optical transmitter light sources with high reliability, high speed, small size, and low power consumption in access, metro, and long distance optical transmission systems.

FIG. 6 is a cross-sectional view showing an example of the structure of a conventional p-InP / n-InP buried type semiconductor laser. In FIG. 6, 1 is a 1.55 μm composition InGaAsP active layer, 2 is a first 1.3 μm composition i-InGaAsP-SCH (Separate-Confinement Heterostructure) layer, and 3 is a second 1.3 μm composition i-InGaAsP-SCH. 4 is a p-InP cover layer, 5 is a first (p-InP) buried layer, 6 is a second (n-InP) buried layer, 7 is a p-InP cladding layer, and 8 is p ⁺ -InGaAsP. A cap layer, 9 is an n-InP substrate, 10 is a SiO ₂ insulating film, 11 is an n-electrode, and 12 is a p-electrode.

  More specifically, the semiconductor substrate 9, the first SCH layer 2 formed on the semiconductor substrate 9, the active layer 1 formed on the first SCH layer 2, and the active layer 1 The second SCH layer 3 formed above and the cover layer 4 formed on the second SCH layer 3 are processed into a mesa shape. A first buried layer 5 formed on the semiconductor substrate 9 on both sides of the portion processed into the mesa shape, and a second buried layer 6 formed on the first buried layer 5. A clad layer 7 formed on the second buried layer 6 and the cover layer 4, and a cap layer 8 formed on the clad layer 7. . The n electrode 11 is provided on the semiconductor substrate 9 side and is in contact with the semiconductor substrate 9. The p-electrode 12 is provided on the cap layer 8 side and is in contact with the cap layer 8. The insulating film 10 is interposed between both sides between the p-electrode 12 and the cap layer 8 so that the p-electrode 12 contacts the cap layer 8 only at a position corresponding to the portion processed into the mesa shape. Thus, a drive current is efficiently passed through the active layer 1.

  In such a p-InP / n-InP buried semiconductor laser, a forward bias voltage (positive on the p-electrode 12 side and negative on the n-electrode 11 side) is applied between the p-electrode 12 and the n-electrode 11 and the active layer 1 Since a reverse bias voltage is applied to the regions (pn junctions) of the first buried layer 5 and the second buried layer 6 when a drive current is supplied to the first buried layer 5 and the second buried layer 5, Current is blocked in the region (pn junction) of the second buried layer 6. As a result, current is efficiently injected into the active layer 1.

  Further, in such a semiconductor laser, a low reflection film is coated on the front side (one end surface in a direction orthogonal to the paper surface of FIG. 6), and a high reflection film is coated on the rear surface side (the other end surface in a direction orthogonal to the paper surface of FIG. 6). The structure is such that laser light is efficiently input to an optical fiber (not shown) from the front side where the light output is large. FIG. 7 shows an example of the light output / voltage characteristics depending on the driving current of a conventional p-InP / n-InP buried semiconductor laser. As shown in FIG. 7, in the p-InP / n-InP buried semiconductor laser, when the drive current of the active layer 1 is increased, the voltage is increased and the light output is increased. Thus, good light output characteristics and electrical characteristics are obtained. Is shown. Note that the rollover of light output characteristics is due to heat generation. Thus, unlike the short-wave semiconductor laser using GaAs as a substrate, the InGaAsP-based semiconductor laser is thermally saturated before reaching the end-face melt breakdown, and therefore, the breakdown due to a surge of about several volts is avoided.

  However, even with an InGaAsP semiconductor laser, damage to the electrostatic discharge (ESD) exceeding several hundred volts cannot be avoided. When a short pulse voltage of about 0.1 .mu.s is applied at a forward bias of 1 to 2 kV as represented by ESD, a long wave system 1.3 .mu.m band or 1.55 .mu.m band InGaAsP strained quantum well laser using InP as a substrate. However, the light density is high on the front end face on the low reflection side, and a catastrophic optical damage (COD) catastrophic optical damage (COD) may occur as in the case of an AlGaAs laser, GaAs laser, or InGaAs strained quantum well laser using GaAs as a substrate. It has been reported. Such ESD is classified as one of electrical overstress (EOS), which damages electronic components and causes a sudden failure of the transmission system. Therefore, an ESD protection circuit is generally arranged. The

  On the other hand, there is an element that has good laser characteristics at the arrival inspection, and has a sudden failure, deterioration of threshold current / slobe efficiency, and oscillation wavelength shift despite taking electrostatic countermeasures in product production. It is known to appear. It has also been reported that the number of sudden failures increases when new packages are introduced. These are considered to be caused by electrical overstress during handling of the manufacturing process, particularly during mounting and testing of each laser, and electrostatic discharge (ESD) has been pointed out.

As prior art documents related to ESD, there are the following.
Y. Twu et. Al., Journal of Applied Physics, Vol. 74, No. 3, pp. 1510, 1993

  For such a failure, it is necessary to ensure reliability related to ESD of electronic components including a package. In recent years, it has been pointed out that ESD damage may cause a sudden failure, and further, an inherent failure cannot be detected by a failure determination standard. Among them, the element having the initial ESD damage is not removed by screening, and an inherent failure cannot be confirmed until a complete failure occurs in the field use. Therefore, it is important not to give ESD damage and to raise the threshold of ESD damage.

  The present invention solves such problems, and an object of the present invention is to provide a semiconductor laser that is not easily damaged even when it is exposed to electrical overstress during handling of the manufacturing process.

A semiconductor laser according to a first invention that achieves the above object includes a semiconductor substrate, a first SCH layer formed on the semiconductor substrate, and an active layer (quantum) formed on the first SCH layer. A well structure / bulk structure), a second SCH layer formed on the active layer, and a cover layer formed on the second SCH layer are processed into a mesa shape, Alternatively, a semiconductor substrate, an active layer (including a quantum well structure / bulk structure) formed on the semiconductor substrate, and a cover layer formed on the active layer are processed into a mesa shape, Alternatively, a semiconductor substrate, a first SCH layer formed on the semiconductor substrate, an active layer (including a quantum well structure and a bulk structure) formed on the first SCH layer, and the activity A second SCH layer formed on the layer, and the second S A cover layer formed on the H layer, and the processed portion to the grating layer and the mesa shape is formed between the cover layer and the second SCH layer,
A first buried layer formed on the semiconductor substrate on both sides of the portion processed into the mesa shape;
A second buried layer formed on the first buried layer;
A cladding layer formed on the second buried layer and the cover layer;
A cap layer formed on the cladding layer;
A first electrode provided on the cap layer side;
A second electrode provided on the semiconductor substrate side,
When a forward bias voltage is applied between the first electrode and the second electrode, a reverse bias voltage is applied to the pn junction formed by the first buried layer and the second buried layer. In a semiconductor laser having a buried structure configured to be applied,
In the second buried layer, the thickness of one or a plurality of portions of the second buried layer is set to be greater than the thickness of a portion other than the one or a plurality of portions of the second buried layer. Further, one or a plurality of thin film portions are provided which are made thinner.

According to a second aspect of the present invention, there is provided a semiconductor laser comprising: a semiconductor substrate; a first SCH layer formed on the semiconductor substrate; and an active layer (quantum well structure, formed on the first SCH layer). Including a bulk structure), a second SCH layer formed on the active layer, and a cover layer formed on the second SCH layer in a mesa shape, or a semiconductor A portion in which a substrate, an active layer (including a quantum well structure / bulk structure) formed on the semiconductor substrate, and a cover layer formed on the active layer are processed into a mesa shape, or a semiconductor A substrate, a first SCH layer formed on the semiconductor substrate, an active layer (including a quantum well structure and a bulk structure) formed on the first SCH layer, and the active layer And a second SCH layer formed on the second SCH layer And made a cover layer, and the processed portion to the grating layer and the mesa shape is formed between the cover layer and the second SCH layer,
A first buried layer formed on the semiconductor substrate on both sides of the portion processed into the mesa shape;
A second buried layer formed on the first buried layer;
A cladding layer formed on the second buried layer and the cover layer;
A cap layer formed on the cladding layer;
A first electrode provided on the cap layer side;
A second electrode provided on the semiconductor substrate side,
When a forward bias voltage is applied between the first electrode and the second electrode, a reverse bias voltage is applied to the pn junction formed by the first buried layer and the second buried layer. In a semiconductor laser having a buried structure configured to be applied,
In the second buried layer, the doping concentration of one or a plurality of portions of the second buried layer is set to be higher than the doping concentration of a portion other than the one or a plurality of portions of the second buried layer. It is characterized in that one or a plurality of low-doping concentration portions are provided.

  According to the semiconductor laser of the first aspect of the present invention, when a forward bias voltage is applied between the first electrode and the second electrode, a pn junction comprising the first buried layer and the second buried layer. The portion is biased in the reverse direction, but at this time until the breakdown voltage at which the depletion layer in the thin film portion of the second buried layer extends to the vicinity of the cladding layer (this breakdown voltage is adjusted by the film thickness and width of the thin film portion) When the bias voltage is increased, the current starts to flow in a region other than the active layer (the pn junction between the first buried layer and the second buried layer), so that the current starts to increase rapidly, resulting in a voltage clamp, The applied voltage can be limited, and an increase in light output can be suppressed. For this reason, it is possible to suppress degradation of the end face of the semiconductor laser even when exposed to electrical excessive stress such as ESD during handling of the manufacturing process.

  Further, according to the semiconductor laser of the second invention, when a forward bias voltage is applied between the first electrode and the second electrode, the semiconductor laser comprises the first buried layer and the second buried layer. The pn junction is biased in the reverse direction. At this time, the breakdown voltage in which the depletion layer in the low doping concentration portion of the second buried layer extends to the vicinity of the cladding layer (this breakdown voltage is the doping concentration and width of the low doping concentration portion). If the bias voltage is increased until the current is adjusted to the pn junction between the first buried layer and the second buried layer, the current starts to increase rapidly. It becomes a clamp, the forward bias applied voltage can be limited, and an increase in light output can be suppressed. For this reason, it is possible to suppress degradation of the end face of the semiconductor laser even when exposed to electrical excessive stress such as ESD during handling of the manufacturing process.

Next, embodiments of the present invention will be described with reference to the drawings.
[First Embodiment]
FIG. 1 is a sectional view showing an example of the structure of a semiconductor laser in which a partial region of a second buried layer according to the first embodiment of the present invention is etched.

In FIG. 1, 1 is a 1.55 μm composition InGaAsP active layer, 2 is a first 1.3 μm composition i-InGaAsP-SCH (Separate-Confinement Heterostructure) layer, and 3 is a second 1.3 μm composition i-InGaAsP-SCH. 4 is a p-InP cover layer, 5 is a first (p-InP) buried layer, 6 is a second (n-InP) buried layer, 7 is a p-InP cladding layer, and 8 is p ⁺ -InGaAsP. A cap layer, 9 is an n-InP substrate, 10 is a SiO ₂ insulating film, 11 is an n-electrode, 12 is a p-electrode, and 13 is a thin film portion left after etching a part of the second buried layer 6.

  More specifically, the semiconductor laser according to the first embodiment includes a semiconductor substrate 9, a first SCH layer 2 formed on the semiconductor substrate 9, and the first SCH layer 2. The formed active layer 1, the second SCH layer 3 formed on the active layer 1, and the cover layer 4 formed on the second SCH layer 3 were processed into a mesa shape. A first buried layer 5 formed on the semiconductor substrate 9 on both sides of the mesa-shaped portion, and formed on the first buried layer 5. A cladding layer 7 formed on the second buried layer 6 and the cover layer 4, and a cap layer 8 formed on the cladding layer 7. have. The n electrode 11 is provided on the semiconductor substrate 9 side and is in contact with the semiconductor substrate 9. The p-electrode 12 is provided on the cap layer 8 side and is in contact with the cap layer 8. The insulating film 10 is interposed between the p electrode 12 and the cap layer 8 so that the p electrode 12 contacts the cap layer 8 only at a position corresponding to the portion processed into the mesa shape. Thus, a drive current is efficiently passed through the active layer 1.

  Such a p-InP / n-InP buried type semiconductor laser is activated by applying a forward bias voltage (positive on the p-electrode 12 side and negative on the n-electrode 11 side) between the p-electrode 12 and the n-electrode 11. Since a reverse bias voltage is applied to the regions (pn junctions) of the first buried layer 5 and the second buried layer 6 when a driving current is passed through the layer 1, the first buried layer 5 and the region of the second buried layer 6 (pn junction) block current. As a result, current is efficiently injected into the active layer 1. In this semiconductor laser, the front side (one end surface perpendicular to the paper surface of FIG. 1) is coated with a low reflection film, and the rear surface (the other end surface perpendicular to the paper surface of FIG. 1) is coated with a high reflection film. A structure is employed in which laser light is efficiently input into an optical fiber (not shown) from the front side having a large output.

  In the second buried layer 6 in the semiconductor laser of the first embodiment, a film thickness of a part of the second buried layer 6 is set to a film thickness of a part other than the part of the second buried layer 6. A thin film portion 13 is provided which is thinner than the thickness. The thin film portion 13 is obtained by re-growing the first buried layer 5 and the second buried layer 6 (second growth of the film following the first growth of the active layer 1 and the like) and then the second buried layer. A resist pattern in which a partial region of the layer 6 (a portion that becomes the thin film portion 13) serves as a window is produced, and a portion of the second buried layer 6 exposed from the window of the resist pattern is etched by dry etching, and thereafter This was prepared by removing the resist pattern.

  By this dry etching, the film thickness d1 of the thin film portion 13 was set to 0.5 μm. The film thickness d2 of the second embedded layer 6 other than the thin film portion 13 is, for example, 0.8 to 1 μm, and the thin film portion 13 has a width w1 of, for example, 2 to 3 μm.

FIG. 2 shows the calculation result of the voltage dependence of the thickness of the depletion layer depending on the doping concentration of the second (n-InP) buried layer 6. In FIG. 2, the doping concentration of the first (p-InP) buried layer 5 is 1 × 10 ¹⁸ cm ⁻³ . In FIG. 2, dn is the thickness of the depletion layer formed on the second buried layer 6 side which is an n-type semiconductor layer, and dp is formed on the first buried layer 5 side which is a p-type semiconductor layer. This is the thickness of the depletion layer.

As shown in FIG. 2, when the doping concentration of the second (n-InP) buried layer 6 (including the thin film portion 13) is 1 × 10 ¹⁸ cm ⁻³ , the second buried layer 6 (the thin film portion 13 is formed). The thickness of the depletion layer is about 0.5 μm when a reverse bias voltage of −2 V is applied to the pn junction between the first buried layer 5 and the second buried layer 6. Accordingly, since the film thickness d1 of the thin film portion 13 is set to 0.5 μm, the thin film portion is applied when a reverse bias voltage of −2 V is applied to the pn junction between the first buried layer 5 and the second buried layer 6. In 13, the depletion layer is substantially in contact with the adjacent p-InP cladding layer 7.

  FIG. 3 shows optical output / voltage characteristics depending on the drive current of the semiconductor laser of the first embodiment. As shown in FIG. 3, when the drive current is increased, the voltage rises and the light output increases. However, when the voltage rises to near 1.8 V, the rise in voltage is suppressed, and at the same time, the light output drops and gradually decreases thereafter. The increase in the voltage is suppressed because the current suddenly started to flow in the region other than the active layer 1 (the pn junction between the first buried layer 5 and the second buried layer 6). The current started to flow when the depletion layer of the thin film portion 13 increased as the reverse bias voltage applied to the pn junction of the first buried layer 5 and the second buried layer 6 increased to 1.8V. This is because it extends to the vicinity of the p-InP clad layer 7 of the adjacent layer and finally reaches the breakdown voltage with a reverse bias voltage of 1.8 V, which is almost consistent with the design.

  As described above, in the semiconductor laser according to the first embodiment, when the voltage rises to the breakdown voltage of the thin film portion 13 of the second buried layer 6, the region other than the active layer 1 (the first buried layer 5). And the pn junction of the second buried layer 6), current starts to flow, voltage clamping occurs, the forward bias voltage can be limited, and the increase in light output can be suppressed (the increase in light output is stopped). To do). According to the first embodiment, it was confirmed that the optical output does not increase to 300 mW or more, and it is possible to manufacture a semiconductor laser with suppressed end face deterioration. By suppressing the increase in light output, end face melting is suppressed, and sudden failures due to COD are suppressed. Therefore, according to the first embodiment, the protection circuit is incorporated in the semiconductor laser, which is effective in suppressing end face deterioration caused by ESD due to handling or the like.

  The breakdown voltage is adjusted by the film thickness d1 of the thin film portion 13 of the second buried layer 6. The thinner the film thickness d1 of the thin film portion 13, the easier it is to yield (lower breakdown voltage). In other words, the film thickness d1 of the thin film portion 13 may be set as appropriate according to a predetermined breakdown voltage. That is, a breakdown voltage suitable for suppressing deterioration of the end face of the semiconductor laser due to electrical overstress such as ESD is set, and the reverse applied to the pn junction between the first buried layer 5 and the second buried layer 6. The film thickness d1 of the thin film portion 13 may be set so that when the bias voltage reaches the predetermined breakdown voltage, the thin film portion 13 of the second buried layer 6 breaks down and current flows.

  Further, in FIG. 1, the thin film portions 13 are provided only in one place in the second buried layers 6 on both the left and right sides of the mesa-shaped portion such as the active layer 1, but the present invention is not limited to this. In the second embedded layer 6, thin film portions 13 may be provided at a plurality of locations. In this case, for example, when the thin film portion 13 is provided only at one place, the width w1 of the thin film portion 13 is set to 2 μm, while when the thin film portion 13 is provided at five places, a current that is five times flows and the effect of voltage clamping is further increased. Rise.

  In the above description, the n substrate 1.55 μm band semiconductor laser is shown. However, the present invention is not limited to this, and the present invention (configuration in which a thin film portion is provided in the second buried layer) is applied to the p substrate 1.55 μm band. The present invention can also be applied to semiconductor lasers, n-substrates, and p-substrate 1.3 μm band semiconductor lasers, and it is clear that the same effects as described above can be obtained with these semiconductor lasers.

  Further, in the above, the semiconductor substrate 9, the first SCH layer 2 formed on the semiconductor substrate 9, the active layer 1 formed on the first SCH layer 2, and the active layer 1 Although a semiconductor laser having a portion in which the second SCH layer 3 formed on the top and the cover layer 4 formed on the second SCH layer 3 are processed into a mesa shape is shown, the present invention is not limited to this. Instead, the present invention (configuration in which a thin film portion is provided in the second buried layer) includes a semiconductor substrate, an active layer formed on the semiconductor substrate, and a cover formed on the active layer. The present invention can also be applied to a semiconductor laser having a layer and a portion processed into a mesa shape.

  Further, the present invention (configuration in which a thin film portion is provided in the second buried layer) is not limited to a Fabry-Perot type semiconductor laser, but can also be applied to a DFB semiconductor laser. In this case, the DFB semiconductor laser has a structure in which a grating layer is formed between the second SCH layer 3 and the cover layer 4 in the structure shown in FIG. 1, that is, the semiconductor substrate 9, the first SCH layer 2, The active layer 1, the second SCH layer 3, the grating layer, and the cover layer 4 have a structure processed into a mesa shape.

[Second Embodiment]
FIG. 4 is a cross-sectional view showing an example of the structure of a semiconductor laser in which a low doping concentration region is formed in a part of the second buried layer according to the second embodiment of the present invention.

In FIG. 4, 1 is a 1.55 μm composition InGaAsP active layer, 2 is a first 1.3 μm composition i-InGaAsP-SCH (Separate-Confinement Heterostructure) layer, and 3 is a second 1.3 μm composition i-InGaAsP-SCH. 4 is a p-InP cover layer, 5 is a first (p-InP) buried layer (doping concentration: 1 × 10 ¹⁸ cm ⁻³ ), and 6 is a second (n-InP) buried layer (doping concentration). : 2 × 10 ¹⁸ cm ⁻³ ), 7 is a p-InP cladding layer, 8 is a p ⁺ -InGaAsP cap layer, 9 is an n-InP substrate, 10 is a SiO ₂ insulating film, 11 is an n-electrode, and 12 is a p-electrode , 14 are low doping concentration portions provided in the second buried layer 6.

  More specifically, the semiconductor laser according to the second embodiment has a semiconductor substrate 9, a first SCH layer 2 formed on the semiconductor substrate 9, and the first SCH layer 2. The formed active layer 1, the second SCH layer 3 formed on the active layer 1, and the cover layer 4 formed on the second SCH layer 3 were processed into a mesa shape. A first buried layer 5 formed on the semiconductor substrate 9 on both sides of the mesa-shaped portion, and formed on the first buried layer 5. A cladding layer 7 formed on the second buried layer 6 and the cover layer 4, and a cap layer 8 formed on the cladding layer 7. have. The n electrode 11 is provided on the semiconductor substrate 9 side and is in contact with the semiconductor substrate 9. The p-electrode 12 is provided on the cap layer 8 side and is in contact with the cap layer 8. The insulating film 10 is interposed between the p electrode 12 and the cap layer 8 so that the p electrode 12 contacts the cap layer 8 only at a position corresponding to the portion processed into the mesa shape. Thus, a drive current is efficiently passed through the active layer 1.

  Such a p-InP / n-InP buried type semiconductor laser is activated by applying a forward bias voltage (positive on the p-electrode 12 side and negative on the n-electrode 11 side) between the p-electrode 12 and the n-electrode 11. Since a reverse bias voltage is applied to the regions (pn junctions) of the first buried layer 5 and the second buried layer 6 when a driving current is passed through the layer 1, the first buried layer 5 and the region of the second buried layer 6 (pn junction) block current. As a result, current is efficiently injected into the active layer 1. In this semiconductor laser, the front side (one end surface perpendicular to the paper surface of FIG. 4) is coated with a low reflection film, and the rear surface (the other end surface perpendicular to the paper surface of FIG. 4) is coated with a high reflection film. A structure is employed in which laser light is efficiently input into an optical fiber (not shown) from the front side having a large output.

In the second buried layer 6 in the semiconductor laser according to the second embodiment, the doping concentration of a part of the second buried layer 6 is set so that the part of the second buried layer 6 other than the part is doped. A low doping concentration portion 14 is provided which is lower than the concentration. This low doping concentration portion 14 is obtained by re-growing the first buried layer 5 and the second buried layer 6 (second growth of the film following the first growth of the active layer 1 and the like), An SiO ₂ pattern in which a partial region of the buried layer 6 (portion that becomes the low doping concentration portion 14) becomes a window is formed, and ions are formed in a part of the second buried layer 6 exposed from the window of the SiO ₂ pattern. It produced by injecting.

By this ion implantation, the doping concentration of the low doping concentration portion 14 of the second buried layer 6 is also set to 1 × 10 ¹⁸ cm ⁻³ . The film thickness d3 of the second buried layer 6 (including the low doping concentration portion 14) was 0.5 μm. The width w2 of the low doping concentration portion 14 is, for example, 2 to 3 μm.

As described above, FIG. 2 shows the calculation result of the voltage dependence of the thickness of the depletion layer depending on the doping concentration of the second (n-InP) buried layer 6. The doping concentration of the first (p-InP) buried layer was 1 × 10 ¹⁸ cm ⁻³ . As shown in FIG. 2, when the doping concentration of the low doping concentration portion 14 of the second buried layer 6 is 1 × 10 ¹⁸ cm ⁻³ , the thickness of the depletion layer of the low doping concentration portion 14 is −2V. When a reverse bias voltage is applied to the pn junction between the first buried layer 5 and the second buried layer 6, it becomes about 0.5 μm. Accordingly, since the film thickness d3 of the second buried layer 6 (including the low doping concentration portion 14) is 0.5 μm, a reverse bias voltage of −2V is applied to the first buried layer 5 and the second buried layer 6. In the low doping concentration portion 14, the depletion layer is substantially in contact with the adjacent p-InP cladding layer 7 when applied to the pn junction. The portions other than the low doping concentration portion 14 of the second buried layer 6 have a doping concentration higher than the doping concentration of the low doping concentration portion 14, so that the depletion layer has a thickness of the depletion layer of the low doping concentration portion 14. Less than the thickness.

  FIG. 5 shows the optical output / voltage characteristics depending on the drive current of the semiconductor laser of the second embodiment. As shown in FIG. 5, when the drive current is increased, the voltage increases and the light output increases. However, when the voltage rises to near 1.8 V, the voltage rise is suppressed, and at the same time, the optical output drops and decreases. The rise in voltage is suppressed because current suddenly started to flow in a region other than the active layer 1 (the pn junction between the first buried layer 5 and the second buried layer 6). The current started to flow when the low bias concentration portion 14 was depleted as the reverse bias voltage applied to the pn junction of the first buried layer 5 and the second buried layer 6 increased to 1.8V. This is because the layer extended to the vicinity of the p-InP cladding layer 7 of the adjacent layer, and finally reached the breakdown voltage at a reverse bias voltage of 1.8 V, which is almost consistent with the design.

  As described above, in the semiconductor laser according to the second embodiment, when the voltage rises to the breakdown voltage of the low doping concentration portion 14 of the second buried layer 6, the region other than the active layer 1 (first buried layer). A current starts to flow in the pn junction between the layer 5 and the second buried layer 6, resulting in a voltage clamp, so that the forward bias voltage can be limited and the increase in light output can be suppressed (increase in light output). Will stop). According to the second embodiment, it was confirmed that the optical output does not increase to 300 mW or more, and it is possible to fabricate a semiconductor laser with suppressed end face deterioration. By suppressing the increase in light output, end face melting is suppressed, and sudden failures due to COD are suppressed. Therefore, according to the second embodiment, the protection circuit is incorporated in the semiconductor laser, which is effective in suppressing end face deterioration caused by ESD due to handling or the like.

  The breakdown voltage is adjusted by the doping concentration of the low doping concentration portion 14 of the second buried layer 6. The lower the doping concentration of the low doping concentration portion 14, the easier the breakdown (the lower the breakdown voltage). In other words, the doping concentration of the low doping concentration portion 14 may be set as appropriate according to a predetermined breakdown voltage. That is, a breakdown voltage suitable for suppressing deterioration of the end face of the semiconductor laser due to electrical overstress such as ESD is set, and the reverse applied to the pn junction between the first buried layer 5 and the second buried layer 6. The doping concentration of the low doping concentration portion 14 may be set so that when the bias voltage reaches the predetermined breakdown voltage, the low doping concentration portion 14 of the second buried layer 6 breaks down and current flows.

  In FIG. 4, the low-doping concentration portions 14 are provided only in one place in the second buried layers 6 on the left and right sides of the mesa-shaped portion such as the active layer 1. However, the present invention is not limited to this. In the second buried layers 6 on both sides, low doping concentration portions 14 may be provided at a plurality of locations. In this case, for example, when the low doping concentration portion 14 is provided only at one location, the width w2 of the low doping concentration portion 14 is set to 2 μm, while when the low doping concentration portion 14 is provided at 5 locations, a current that is five times larger flows. The effect of voltage clamping is further enhanced.

  In the above description, the n substrate 1.55 μm band semiconductor laser is shown. However, the present invention is not limited to this, and the present invention (configuration in which a low doping concentration portion is provided in the second buried layer) is applied to the p substrate 1. The present invention can also be applied to 55 μm band semiconductor lasers, n-substrates and p-substrate 1.3 μm band semiconductor lasers, and it is clear that the same effects as described above can be obtained with these semiconductor lasers.

  In the above description, the semiconductor substrate 9, the first SCH layer 2 formed on the semiconductor substrate 9, the active layer 1 formed on the first SCH layer 2, and the active layer 1 Although a semiconductor laser having a portion in which the second SCH layer 3 formed on the top and the cover layer 4 formed on the second SCH layer 3 are processed into a mesa shape is shown, the present invention is not limited to this. Instead, the present invention (configuration in which the second buried layer is provided with the low doping concentration portion) is formed on the semiconductor substrate, the active layer formed on the semiconductor substrate, and the active layer. The present invention can also be applied to a semiconductor laser having a portion in which the cover layer is processed into a mesa shape.

  Further, the present invention (a configuration in which a low doping concentration portion is provided in the second buried layer) is not limited to a Fabry-Perot type semiconductor laser, but can also be applied to a DFB semiconductor laser. In this case, the DFB semiconductor laser has a structure in which a grating layer is formed between the second SCH layer 3 and the cover layer 4 in the structure shown in FIG. 4, that is, the semiconductor substrate 9, the first SCH layer 2, The active layer 1, the second SCH layer 3, the grating layer, and the cover layer 4 have a structure processed into a mesa shape.

It is sectional drawing which shows one example of the structure of the semiconductor laser which etched the partial area | region of the 2nd embedding layer based on the 1st Example of this invention. It is a figure which shows the calculation result of the voltage dependence of the thickness of a depletion layer by the doping concentration of a 2nd (n-InP) buried layer. It is a figure which shows the optical output and voltage characteristic by the drive current of the semiconductor laser which etched the partial area | region of the 2nd embedding layer based on the 1st Example of this invention. It is sectional drawing which shows an example of the structure of the semiconductor laser which produced the area | region with low doping concentration in a part of 2nd embedding layer based on the 2nd Example of this invention. It is a figure which shows the optical output and voltage characteristic by the drive current of the semiconductor laser which produced the area | region with low doping concentration in a part of 2nd embedding layer based on the 2nd Example of this invention. It is sectional drawing which shows an example of the conventional p-InP / n-InP buried semiconductor laser. It is a figure which shows an example of the optical output and voltage characteristic by the drive current of the conventional p-InP / n-InP buried semiconductor laser.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 1.55 micrometer composition InGaAsP active layer 2 1st 1.3 micrometer composition i-InGaAsP-SCH layer 3 2nd 1.3 micrometer composition i-InGaAsP-SCH layer 4 p-InP cover layer 5 1st (p-InP ) Buried layer 6 second (n-InP) buried layer 7 p-InP clad layer 8 p ⁺ -InGaAsP cap layer 9 n-InP substrate 10 SiO ₂ insulating film 11 n electrode 12 p electrode 13 second buried layer Thin film portion 14 Low doping concentration portion of second buried layer

Claims (2)

Semiconductor substrate, first SCH layer formed on the semiconductor substrate, active layer formed on the first SCH layer, and second SCH layer formed on the active layer And a cover layer formed on the second SCH layer is processed into a mesa shape, or a semiconductor substrate, an active layer formed on the semiconductor substrate, and the active layer. The cover layer formed on the semiconductor substrate is formed into a mesa-shaped part, or a semiconductor substrate, a first SCH layer formed on the semiconductor substrate, and a first SCH layer. An active layer, a second SCH layer formed on the active layer, a cover layer formed on the second SCH layer, and between the second SCH layer and the cover layer The formed grating layer is processed into a mesa shape, and
A first buried layer formed on the semiconductor substrate on both sides of the portion processed into the mesa shape;
A second buried layer formed on the first buried layer;
A cladding layer formed on the second buried layer and the cover layer;
A cap layer formed on the cladding layer;
A first electrode provided on the cap layer side;
A second electrode provided on the semiconductor substrate side,
When a forward bias voltage is applied between the first electrode and the second electrode, a reverse bias voltage is applied to the pn junction formed by the first buried layer and the second buried layer. In a semiconductor laser having a buried structure configured to be applied,
In the second buried layer, the thickness of one or a plurality of portions of the second buried layer is set to be greater than the thickness of a portion other than the one or a plurality of portions of the second buried layer. A semiconductor laser comprising a thin film portion at one place or a plurality of places, which is made thinner. Semiconductor substrate, first SCH layer formed on the semiconductor substrate, active layer formed on the first SCH layer, and second SCH layer formed on the active layer And a cover layer formed on the second SCH layer is processed into a mesa shape, or a semiconductor substrate, an active layer formed on the semiconductor substrate, and the active layer. The cover layer formed on the semiconductor substrate is formed into a mesa-shaped part, or a semiconductor substrate, a first SCH layer formed on the semiconductor substrate, and a first SCH layer. An active layer, a second SCH layer formed on the active layer, a cover layer formed on the second SCH layer, and between the second SCH layer and the cover layer The formed grating layer is processed into a mesa shape, and
A first buried layer formed on the semiconductor substrate on both sides of the portion processed into the mesa shape;
A second buried layer formed on the first buried layer;
A cladding layer formed on the second buried layer and the cover layer;
A cap layer formed on the cladding layer;
A first electrode provided on the cap layer side;
A second electrode provided on the semiconductor substrate side,
When a forward bias voltage is applied between the first electrode and the second electrode, a reverse bias voltage is applied to the pn junction formed by the first buried layer and the second buried layer. In a semiconductor laser having a buried structure configured to be applied,
In the second buried layer, the doping concentration of one or a plurality of portions of the second buried layer is set to be higher than the doping concentration of a portion other than the one or a plurality of portions of the second buried layer. A semiconductor laser characterized in that one or a plurality of low-doping concentration portions are provided.

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