JP2013239682A - Semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor laser for an environmental gas monitor which suppresses optical output reduction and has increased temperature change during current sweep drive.SOLUTION: A semiconductor laser comprises: a semiconductor substrate (8); and a laser epitaxial layer having a buffer layer (5); a first SCH layer (2); an active layer (1) (which includes quantum well layer or quantum well layer and barrier layer); a second SCH layer (3); a clad layer (6) and a cap layer (7) sequentially formed on the semiconductor substrate. The clad layer (6) is a p-type semiconductor with an acceptor concentration of 1×10cmor more and 4×10cmor less.

Description

  The present invention relates to a semiconductor laser, and more particularly to a semiconductor laser light source for gas concentration measurement using an absorption line.

  In recent years, a single longitudinal mode oscillation semiconductor laser (LD: Laser Diode) has attracted attention as a light source for environmental gas monitoring. This is because the use of a practical semiconductor laser as the light source of the gas measurement system is expected to further reduce the size, power consumption, and cost of the system.

  Semiconductor lasers have already been developed and commercialized for communication and CD / DVD recording / reproduction. Among these, a communication InGaAsP (active layer) / InP (substrate) -based DFB (Distributed feedback) laser oscillates in a single longitudinal mode at near infrared wavelengths of 1.5 μm and 1.3 μm. Therefore, if the active layer composition can be changed and a high-quality active layer can be realized even at wavelengths longer than 1.6 μm, a 2 μm wavelength band semiconductor laser can be developed using the communication semiconductor laser manufacturing technology using the InP substrate as it is. Is possible. Here, the 2 μm wavelength band semiconductor laser has an oscillation wavelength longer than 1.7 μm, which is not used in optical fiber communication, and can be realized by a communication semiconductor laser active layer manufacturing technique using an InP substrate. A semiconductor laser having an oscillation wavelength shorter than 4 μm.

  So far, in order to realize a high-quality high-strain active layer, techniques for controlling the lattice strain of the barrier layer, adding a small amount of Sb, and reducing the growth temperature have been developed, and 2.1 μm band (high-strain InGaAs) and 2. A 3 μm band (high strain InAs) buried structure type DFB laser has been realized.

FIG. 1 shows a structure of a p-InP / n-InP buried structure type DFB laser shown in Non-Patent Document 1 as an example of a conventional 2 μm wavelength band semiconductor laser. The DFB laser shown in FIG. 1 includes an n-InP buffer layer 5, a lower SCH layer (1.3 μm composition n-InGaAsP layer) 2, and an MQW (Multiple Quantum Well) that are sequentially stacked on an n-InP substrate 8. Well) layer 1, upper SCH layer (p-InGaAsP layer 1.3 μm composition) 3, p-InP cladding layer 6 and p ⁺ -InGaAsP cap layer 7. The n-InP buffer layer 5, the lower SCH layer 2, the MQW layer 1, the upper SCH layer 3 and the p-InP cladding layer 6 have a mesa structure 12. A grating 4 is formed on the upper SCH layer 3, and a p-InP cladding layer 6 is laminated on the grating 4. The MQW layer 1 constitutes an active layer. For example, the MQW layer 1 constitutes an active layer (InGaAs active layer) together with an InGaAs barrier layer (not shown).

  Further, the DFB laser shown in FIG. 1 includes a p-InP buried layer 15 in contact with both sides of the mesa structure 12 between the n-InP buffer layer 5 and the p-InP cladding layer 6 and the p-InP buried layer 15. An n-InP buried layer 16 stacked thereon is provided.

Further, the DFB laser shown in FIG. 1 includes a p-electrode 10 formed on the p ⁺ -InGaAsP cap layer 7 along the longitudinal direction of the mesa structure 12 (laser emission direction). Is connected. An n electrode 9 is provided on a surface different from the surface on which the n-InP buffer layer 5 of the InP substrate 8 is laminated.

  Furthermore, the DFB laser shown in FIG. 1 includes a low reflection film (AR (Antireflection) film) 17 on the laser emission end face, and a high reflection film (HR (High reflection) film) on the end face facing the low reflection film 17. 18 is provided.

  The current is injected from the p-electrode 10 and is blocked by the p-InP buried layer 15 and the n-InP buried layer 16 so that the current is efficiently concentrated on the active layer 1. Further, the laser beam generated in the active layer 1 is guided around the active layer and is mainly emitted from the low reflection film 17. The laser chip is mounted on an AlN heat sink, and the entire mount is mounted on a 5.6 mm TO Can (Transistor Outline Can).

  FIG. 2 shows the drive current dependence of the optical output and voltage depending on the ambient temperature. The threshold voltage is about 0.7 V, which reflects the low band gap of the InGaAs active layer. The optical output increases linearly with an increase in driving current, and the p-InP buried layer 15 and the n-InP buried layer 16 have good current blocking characteristics. As a result, the maximum light output is 8 mW or more at an ambient temperature of 25 ° C., and laser oscillation up to 60 ° C. is obtained.

FIG. 3 shows the oscillation wavelength dependence depending on the driving current of the conventional 2 μm wavelength band DFB laser. The oscillation wavelength 2.1μm have absorption lines wavelength of _{N 2} O, a useful light source for detection of _{N 2} O gas. As shown in FIG. 3, when the drive current is increased from 20 mA to 140 mA, the oscillation wavelength is shifted to a long wavelength of about 0.49 nm. Here, assuming that the thickness is 0.1 nm / K, in this case, it is estimated that the temperature is increased by 4.9 ° C. inside the laser.

T.A. Takeshita, T .; Sato, M.M. Mitshara, Y. et al. Kondo, M .; Sugo and K.K. Kato, “Degradation Analysis of 2-μm DFB Laser Using Optical Beam-Induced Current Technique,” IEEE TRANSACTIONS ON ELECTRON DEVICES. 54, no. 10, pp. 2644-2649, October 2007 H. Casey and P.M. Carter, “Variation of interference band absorption with hole contention in p-type InP,” Applied Physics Letters, Vol. 44, no. 1, pp. 82-83, January 1984

  It is desirable that a communication semiconductor laser having an oscillation wavelength of 1.26 μm to 1.675 μm is operated with a constant optical output or a constant current and has almost no wavelength change. Specifically, in a laser, a layer structure that suppresses the influence of heat generation due to an injection current is required.

  On the other hand, a 2 μm wavelength band semiconductor laser, which is a light source for environmental gas monitoring, is desirably different in oscillation wavelength. This is because the environmental gas monitor sweeps the wavelength and uses the fact that light is absorbed by the gas at a wavelength that matches the gas absorption line wavelength and the transmittance decreases. The oscillation wavelength of the laser can be swept by changing the temperature of the heat sink on which the laser element is mounted or changing the injection current to the laser. The former method of changing the temperature of the heat sink takes a long time for the wavelength sweep, but the latter method of changing the injection current enables the wavelength sweep in a short time. For this reason, in a practical light source for environmental gas monitoring, it is necessary to perform wavelength sweep by the latter injection current. However, the conventional 2 μm wavelength band semiconductor laser has a problem that the change in the oscillation wavelength due to the injection current is small and the wavelength range that can be swept is narrow.

In order to achieve the above object, the present invention provides a semiconductor substrate and a buffer layer, a first SCH layer, an active layer, and a buffer layer, which are sequentially formed on the semiconductor substrate. A semiconductor laser comprising a laser epitaxial layer comprising a second SCH layer, a cladding layer, and a cap layer, wherein the cladding layer has an acceptor concentration of 1 × 10 ¹⁷ cm ⁻³ or more and 4 × 10 ¹⁷ cm ^−3. It is the following p-type semiconductor.

  The invention according to claim 2 is the semiconductor laser according to claim 1, characterized in that the oscillation wavelength is longer than 1.7 μm and shorter than 2.4 μm.

As described above, by using a p-type semiconductor having an acceptor concentration of 1 × 10 ¹⁷ cm ⁻³ or more and 4 × 10 ¹⁷ cm ⁻³ or less as a cladding layer, the resistance of the cladding layer becomes several Ω, By increasing / decreasing the injection current, the temperature of the active layer can be increased / decreased more effectively than the conventional structure. As a result, the temperature change of the semiconductor laser during current sweep driving increases, and the oscillation wavelength changes greatly. be able to. Furthermore, by using a p-type semiconductor having an acceptor concentration of 1 × 10 ¹⁷ cm ⁻³ or more and 4 × 10 ¹⁷ cm ⁻³ or less as a cladding layer, optical loss in a p-type semiconductor that is noticeable in a 2 μm wavelength band semiconductor laser is also suppressed. You can also In the conventional 2 μm wavelength band semiconductor laser, there is no concept of efficiently generating heat in the cladding layer by the injection current and greatly changing the oscillation wavelength, and there is no concept of suppressing optical loss by reducing the acceptor concentration. It was.

The suppression of optical loss in the p-type semiconductor, which is noticeable in the above-described 2 μm wavelength band semiconductor laser, will be described below. It is known that light absorption in a p-type semiconductor increases with an increase in the acceptor concentration, and further increases as the wavelength of light increases (for example, see Non-Patent Document 2). The laser light is absorbed by this increase in light absorption, so that light loss increases. FIG. 4 is a diagram showing the relationship between the light absorption coefficient and the acceptor concentration of light having wavelengths of 1.55 μm and 2 μm in InP used for the cladding layer of a semiconductor laser using an InP substrate based on Non-Patent Document 2. is there. FIG. 4 shows that when the acceptor concentrations are the same, the light absorption coefficient at a wavelength of 2 μm is larger than the light absorption coefficient at a wavelength of 1.55 μm. On the other hand, as described above, in the semiconductor laser for communication, a layer configuration that suppresses the influence of heat generation due to the injection current is required, and it is necessary to increase the acceptor concentration of the InP cladding layer and reduce the resistance. Specifically, an acceptor concentration of 5 × 10 ⁷ cm ⁻³ or more is required for a p-type InP clad layer of a general communication semiconductor laser. However, as shown in FIG. 4, when the acceptor has a concentration of 5 × 10 ⁷ cm ⁻³ , the light absorption coefficient at the wavelength of 2 μm is the light absorption coefficient (about 11 cm ⁻¹ ) at the wavelength of 1.55 μm, which is the communication wavelength band. 1.5 times as much. For this reason, in order to make the 2 μm wavelength band semiconductor laser have a light absorption coefficient comparable to that of the communication wavelength band laser, it is necessary to set the acceptor concentration to 4 × 10 ¹⁷ cm ⁻³ or less. Note that when the acceptor concentration is 1 × 10 ¹⁷ cm ⁻³ or less, the resistance becomes 10 Ω or more, and further, in this region, the resistance changes abruptly depending on the acceptor concentration, so that a problem remains in reproducibility. For this reason, the lower limit of the acceptor concentration is 1 × 10 ¹⁷ cm ⁻³ .

It is a structural diagram of a conventional p-InP / n-InP buried structure type 2 μm wavelength band DFB laser. It is a figure which shows the optical output and voltage, and a drive current characteristic by the atmospheric temperature of the conventional p-InP / n-InP buried structure type 2 micrometer wavelength band DFB laser. It is a figure which shows the oscillation wavelength dependence by the drive current of the 2 micrometer wavelength band DFB laser concerning the past and this invention. It is the figure which showed the relationship between the optical absorption coefficient of light with a wavelength of 1.55 micrometers and 2 micrometers in InP, and an acceptor density | concentration. 1 is a structural diagram of a ridge waveguide structure type 2 μm wavelength band DFB laser according to the present invention. FIG. It is a figure which shows the drive current dependence of the optical output of the 2 micrometer wavelength band DFB laser concerning the past and this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The same or similar elements are denoted by the same or similar reference numerals, and detailed description thereof is omitted.

[First Embodiment]
FIG. 5 shows a structural diagram of a ridge waveguide structure type 2 μm wavelength band DFB laser of one embodiment of a semiconductor laser according to the present invention. The DFB laser shown in FIG. 5 includes an n-InP buffer layer 5, a lower SCH layer (1.3 μm composition n-InGaAsP layer) 2, an MQW (Multiple Quantum Well) that are sequentially stacked on an n-InP substrate 8. Well) layer 1, upper SCH layer (p-InGaAsP layer 1.3 μm composition) 3, p-InP cladding layer 6 and p ⁺ -InGaAsP cap layer 7. A grating 4 is formed on the upper SCH layer 3, and a p-InP cladding layer 6 is laminated on the grating 4. The MQW layer 1 constitutes an active layer (InGaAs active layer) together with an InGaAs barrier layer (not shown). The 2 μm wavelength band DFB laser of this embodiment is different from the semiconductor laser shown in FIG. 1 in that the p-InP cladding layer 6 and the p ⁺ -InGaAsP cap layer 7 have a ridge structure 13.

  Further, the 2 μm wavelength band DFB laser of the present embodiment is different from that shown in FIG. 1 in that the n-InP buffer layer 5, the lower SCH layer 2, the MQW layer 1, the upper SCH layer 3, and the p-InP cladding layer 6 are not mesa structures. Different from the semiconductor laser shown in FIG. That is, in the 2 μm wavelength band DFB laser of the present embodiment, the p− which is in contact with both sides of the mesa structure 12 between the n-InP buffer layer 5 and the p-InP clad layer 6 as shown in FIG. The InP buried layer 15 and the n-InP buried layer 16 stacked on the p-InP buried layer 15 are not formed.

In the 2 μm wavelength band DFB laser of the present embodiment, the SiO ₂ film (insulating film) 14 is formed on the portion of the upper SCH layer 3 where the ridge 13 is not formed. The SiO ₂ film (insulating film) 14 is formed so as to cover the side surface of the ridge 13.

Furthermore, the 2 μm wavelength band DFB laser of this embodiment includes a p-electrode 10 formed in contact with the top of the ridge 13 (upper part of the p ⁺ -InGaAsP cap layer 7), and an electrode pad 11 is provided on the p-electrode 10. It is connected. An n electrode 9 is provided on a surface different from the surface on which the n-InP buffer layer 5 of the InP substrate 8 is laminated.

  Furthermore, the 2 μm wavelength band DFB laser of this embodiment includes a low reflection film (AR (Antireflection) film) 17 on the laser emission end face, and a high reflection film (HR (High reflection) film on the end face facing the low reflection film 17. ) Membrane) 18.

In the 2 μm wavelength band DFB laser of the present embodiment, the doping concentration of the p-InP cladding layer 6 is 4 × 10 ¹⁷ cm ⁻³ . In the waveguide, the width of the ridge 13, which has a smaller heat dissipation than the buried structure, is 2 μm. The ridge 13 includes a p-InP cladding layer 6 and a p ⁺ -InGaAsP cap layer 7. Further, a SiO ₂ insulating film 14 is provided on the semiconductor surface except for the top of the ridge, and the semiconductor and the p-electrode 10 are insulated. The element length is 600 μm. Current is injected from the top of the ridge 13. Laser light generated in the active layer 1 is confined and guided around the active layer 1 in the vertical direction and centered on the ridge region 13 in the horizontal direction. The laser chip was mounted on an AlN heat sink, and the entire mount was mounted on a 5.6 mm TO Can (Transistor Outline Can). For comparison, a 2 μm wavelength band DFB laser (hereinafter referred to as a conventional 2 μm wavelength band DFB laser) shown in FIG. 1 was also fabricated. A conventional 2 μm wavelength band DFB laser was manufactured by changing only the doping concentration of the p-InP cladding layer 6 to 5 × 10 ¹⁷ cm ⁻³ . The mounting method of the conventional 2 μm wavelength band DFB laser is the same as that of the 2 μm wavelength band DFB laser of this embodiment.

  FIG. 6 shows the drive current dependence of the optical output of the conventional 2 μm wavelength band DFB laser and the 2 μm wavelength band DFB laser of this embodiment. FIG. 6 shows that the 2 μm wavelength band DFB laser of this embodiment has an increase in slope efficiency of 1.07 times (around 50 mA where the drive current is low) compared to the conventional 2 μm wavelength band DFB laser, and the optical loss is reduced. Show. Furthermore, the threshold current is reduced reflecting the reduction in optical loss. The optical output of the 2 μm wavelength band DFB laser of this embodiment is 7.8 mW when the driving current is 140 mA, which is consistent with the conventional 2 μm wavelength band DFB laser. This saturation tendency of the optical output is due to the fact that heat is generated as the drive current increases and the temperature rise of the 2 μm wavelength band DFB laser of this embodiment is large. Therefore, it was confirmed that the slope efficiency and the threshold current are improved in the 2 μm wavelength band DFB laser of the present embodiment, and the light output decrease due to the device temperature increase is compensated.

  FIG. 3 shows the oscillation wavelength dependence of the driving current of the conventional 2 μm wavelength band DFB laser and the 2 μm wavelength band DFB laser of this embodiment. In the DFB laser of this embodiment, when the drive current is increased from 20 mA to 140 mA, the oscillation wavelength is shifted to a long wavelength of about 0.61 nm. Assuming 0.1 nm / K, in this case, it is estimated that the temperature has increased by 6.1 ° C. inside the laser. Therefore, it is confirmed that the wavelength fluctuation is about 1.24 times larger and the wavelength fluctuation is larger in the ridge waveguide type 2 μm wavelength band DFB laser of the present embodiment than the conventional embedded structure type 2 μm wavelength band DFB laser. It was done.

  Furthermore, when the aging of the 2 μm wavelength band DFB laser of this embodiment was performed with the temperature set at 45 ° C. and the light output constant at 3 mW, it was confirmed that the operation was stable even after 5000 hours.

  As described above, in the 2 μm wavelength band DFB laser of the present embodiment, the decrease in the optical output is compensated by the reduction in the doping concentration of the p-InP cladding layer 6 and the wavelength fluctuation in the current sweep is improved. Furthermore, it was confirmed that there was no problem with reliability.

In the present embodiment, an example in which the doping (acceptor) concentration of the p-InP cladding layer 6 is 3 × 10 ¹⁷ cm ⁻³ has been described, but the acceptor concentration is 1 × 10 ¹⁷ or more and 4 × 10 4 as described above. The same effect can be obtained at ¹⁷ cm ⁻³ or less.

[Second Embodiment]
The structure of the ridge waveguide structure type 2 μm wavelength band DFB laser of this embodiment is the same as the structure shown in the first embodiment shown in FIG. However, in the DFB laser of the present embodiment, the doping concentration of the p-InP cladding layer 6 is 3 × 10 ¹⁷ cm ⁻³ . The waveguide has a ridge structure with less heat dissipation than the buried structure, the width is 2 μm, and the element length is 600 μm. The laser chip was mounted on an AlN heat sink, and the entire mount was mounted on a 5.6 mm TO Can (Transistor Outline Can). In addition, by adopting the ridge waveguide structure, production defects related to embedding are eliminated, and the yield is improved.

  FIG. 6 shows the drive current dependence of the optical output of the conventional 2 μm wavelength band DFB laser described in the first embodiment and the 2 μm wavelength band DFB laser of this embodiment. FIG. 6 shows that the 2 μm wavelength band DFB laser of this embodiment has an increase in slope efficiency of 1.07 times (around 50 mA where the drive current is low) compared to the conventional 2 μm wavelength band DFB laser, and the optical loss is reduced. Show. Furthermore, the threshold current is reduced reflecting the reduction in optical loss. The output of the 2 μm wavelength band DFB laser in this embodiment is 7 mW at 130 mA, which matches the conventional 2 μm wavelength band DFB laser, indicating that the temperature rise is larger than that of the 2 μm wavelength band DFB laser in the first embodiment. . As described above, in the 2 μm wavelength band DFB laser of this embodiment, it was confirmed that the slope efficiency and the threshold current were improved, and the light output decrease due to the device temperature increase was compensated.

  FIG. 3 shows the oscillation wavelength dependence of the driving current of the conventional 2 μm wavelength band DFB laser and the 2 μm wavelength band DFB laser of this embodiment. In the 2 μm DFB laser of this embodiment, when the drive current is increased from 20 mA to 140 mA, the oscillation wavelength is shifted to a long wavelength of about 0.79 nm. Assuming 0.1 nm / K, in this case, it is estimated that the temperature is increased by 7.9 ° C. inside the laser. As a result, compared with the conventional embedded structure type 2 μm wavelength band DFB laser, the wavelength fluctuation of the ridge waveguide type 2 μm wavelength band DFB laser of this embodiment is about 1.6 times larger, and a large wavelength fluctuation was confirmed. .

  In addition, the temperature was kept at 45 ° C., the light output was kept constant at 3 mW, and the 2 μm wavelength band DFB laser of this embodiment was aged, and it was confirmed that the operation was stable even after 3000 hours.

  As described above, in the 2 μm wavelength band DFB laser of this embodiment, the decrease in the optical output is compensated by the reduction in the doping concentration of the p-InP cladding layer 6, and the wavelength fluctuation in the current sweep becomes large. Furthermore, it was confirmed that there was no problem with reliability.

  As described in the first and second embodiments, the decrease in the doping concentration of the p-InP cladding layer compensates for the decrease in light output even when the device temperature rises, and the semiconductor laser during current sweep driving is compensated. The temperature change could be increased. It was also confirmed that there was no problem with reliability.

It is obvious that the same effect can be obtained even if the resistance of the p-electrode is increased. Here, although a ridge waveguide structure type DFB laser with 2.1 μm wavelength oscillation is shown as a light source having an absorption line wavelength of N ₂ O gas, a ridge waveguide structure type DFB laser with 2.05 μm oscillation is used for CO ₂ gas detection. It is clear that the same effect can be obtained with a ridge waveguide structure type DFB laser with 2.33 μm oscillation for CO gas detection, and the same effect can be obtained even with a wavelength band other than the 2 μm wavelength band (1 μm). It is.

1 MQW layer 2 Lower SCH layer (1.3 μm composition n-InGaAsP layer)
3 Upper SCH layer (1.3 μm composition p-InGaAsP layer)
4 grating 5 n-InP buffer layer 6 p-InP cladding layer 7 p ⁺ -InGaAsP cap layer 8 n-InP substrate 9 n electrode 10 p electrode 11 electrode pad 12 mesa 13 ridge 14 SiO ₂ film (insulating film)
15 p-InP buried layer 16 n-InP buried layer 17 Low reflection film (AR film)
18 High reflective film (HR film)

Claims (2)

A semiconductor substrate;
A semiconductor laser comprising: a laser epitaxial layer comprising a buffer layer, a first SCH layer, an active layer, a second SCH layer, a clad layer, and a cap layer, which are sequentially formed on the semiconductor substrate;
2. The semiconductor laser according to claim 1, wherein the clad layer is a p-type semiconductor having an acceptor concentration of 1 × 10 ¹⁷ cm ⁻³ or more and 4 × 10 ¹⁷ cm ⁻³ or less.   2. The semiconductor laser according to claim 1, wherein the oscillation wavelength is longer than 1.7 [mu] m and shorter than 2.4 [mu] m.

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