JPS6136988A - Semiconductor laser of single axis mode - Google Patents

Abstract

PURPOSE:To obtain the distribution feedback semiconductor laser of high productivity by removing an active layer and a light guiding layer in nearly center of a resonator, where the phase shift region composed of semiconductor of different refractive index from those of the active and light guiding layers is formed. CONSTITUTION:On an N type InP substrate 1, a P type InGaAsP light guiding layer 4 on which InGaAsP active layer 3 and a diffraction grating 200 are formed is formed and the layers 3 and 4 are removed in nearly center of the substrate 1, where a phase shift region 120 is formed. Over the whole surface of the region 120, a P type InP clad layer 5 is formed and a multilayer film wafer is formed. Further on that, a buried hetero structure is formed with two parallel grooves 50 and 51 present on both sides of a mesa stripe 52, after which a current blocking layer, a current enclosing layer, a buried layer are laminated in order. Then a current injection region 61 is formed on the above layers and a P-side electrode 70 is formed on the region 61 and furthermore, an N-side electrode 71 is formed.

Description

[Detailed description of the invention] (Field of invention) The present invention relates to a distributed feedback semiconductor laser.

(Description of Prior Art) Long-distance, large-capacity transmission experiments using optical fiber communications are underway. High speeds such as 2 Gb/s or 10
In order to realize a long-distance system exceeding 0 km, a semiconductor laser as a light source is required to operate in a single-axis mode in order to avoid the influence of transmission waveform distortion due to wavelength dispersion within the optical fiber.

Until now, distributed feedback semiconductor lasers (hereinafter abbreviated as DFB LD), which have a diffraction grating for wavelength selection inside,
Alternatively, distributed Bragg reflective semiconductor lasers are being developed at a rapid pace.
DPB LD using AsP has high power operation, 2 o
Good characteristics such as nest-axis mode operation during b/s modulation have been obtained.In order to put DFB LD into practical use in the future, it will be important to improve reliability and production yield. In particular, with DFBLDI/fi, there is no axial mode that oscillates at the Bragg wavelength, and the axial modes of the wavelengths on both sides have similar oscillation thresholds, so it is easy to oscillate in multiple axial modes and it is a stable single axial mode. A major problem exists in that it reduces the yield of working devices. Conventionally, as a countermeasure against this problem, as reported by Itaya et al. in Proceedings of the 1981 National Conference of the Institute of Electronics and Communication Engineers, Volume 4, 944, the axial modes on both sides of the Sibrag wavelength were Methods such as making a difference in the oscillation threshold of
As reported in 18, approximately in the center of DFBLD,
A method to enable oscillation at the Sibrag wavelength by providing a region where the phase of the diffraction grating changes has been proposed and experiments have been reported. This method has the advantage of having a small oscillation threshold for the axial mode of the black wavelength and is promising, but it requires electron beam exposure to change the phase of the diffraction grating. The method of fabricating a diffraction grating using the two-beam interference exposure method using an O gas laser as a light source has the disadvantage of being time-consuming and not suitable for mass production. Therefore, if this method can be applied to the production of a diffraction grating in which the phase is changed during the process described above, the productivity of DFB Ll) can be improved.

(Object of the Invention) An object of the present invention is to provide a distributed feedback semiconductor laser having an internal phase shift region with excellent device characteristics and productivity.

(Structure of the Invention) According to the present invention, a semiconductor layer including an active layer and an optical guide layer adjacent to the active layer and having a periodically uneven shape on one surface is formed on a semiconductor substrate, and A phase shift region in which a semiconductor layer having a refractive index different from that of the active layer and the optical guide layer is formed in a portion where the optical guide layer and the optical guide layer are removed is located approximately in the center of the resonator. A single-axis mode semiconductor laser is obtained.

(First Embodiment) (Support) The first part is a perspective view of a buried distributed feedback semiconductor laser showing a first embodiment of the present invention. The manufacturing method will be explained according to the process order shown in Figure 2.
As shown in (001) plane n-type InP substrate 1 (8n
A 5n-type InP buffer layer 2 (3 μm thick
, Sn doped, carrier concentration 5×l Q”cm−3)
, InGaAsT' active layer 3 (non-doped, thickness 0.1
1.55 μm composition in μm2 emission wavelength), p-type InG
a, AsP light guide layer 4 (thickness 0.15 μm, Zn doped, carrier concentration lX10 c1++) is laminated continuously ~
do. Next, as shown in Figure 2 (bl), 4 times diluted A
Z1350 photoresist was applied at a rotation speed of 600 Orpm to form a resist film 100 for a diffraction grating, and then a photoresist diffraction grating 101 with a period of 2350X was formed by three-beam interference exposure using a He-Cd gas laser with an oscillation wavelength of 3250X. The light guide layer 4 is formed so that the repetition is in the (11,0) direction (FIG. 2(C)), and then the optical guide layer 4 is etched using an HBr-based etching solution to form a diffraction grating 200 (FIG. 2(C)). dl) O Next, Figure 2 (stripe with a width of about 4 μm like cl) 11
After forming a photoresist pattern 110't-, which is perpendicular to the plane of the paper in FIG. Shift region 120 is formed (FIG. 2, 1fl) o Next, the entire structure is formed by the second liquid phase bitaxial growth (five rp-type InP cladding layers are formed (FIG. 2 (g)) - this completes the actual implementation. The basic structure of the multilayer film (weno) was completed, but in order to improve the performance of the device characteristics, that is, to reduce the oscillation threshold and increase the differential quantum efficiency, the authors and others next formed a buried heterostructure. yo), Electronici X0 Letters (Electroni
cs Letters) Magazine Vol. 18 No. 22 (1982
In order to form the structure of a double channel buried brainer (DC-PBH) semiconductor laser reported in pages 953 to 954 of 2010 (published in 2013), a width of 10 μm is shown in the perspective view of FIG. 2fh).

After two parallel grooves with a depth of 3 μm were formed parallel to the (110) direction with a mesast 2 groove 52 with a width of 1.5 μm sandwiched between them, the grooves were formed by liquid phase epitaxial growth as shown in Fig. 2 (il). P-type InP current blocking layer 6 (thickness at flat part 0.5 μm, Zn doped, carrier concentration 1×10
), n-type InP current confinement layer 7 (thickness at flat part 0.
5 μrl, Te doped 1, carrier concentration 2×10
), p-type InP buried layer 8 (thickness 1.5μ at flat part)
m, Zn dog, carrier concentration 1×10 cM),
and p-type InQaAsP cap layer 9 (thickness at flat part 05 μm, Zn doping, carrier concentration lXl0
A composition of 1.2 μm is formed with Cm as the emission wavelength. This completes the epitaxial growth.

Next, in the electrode formation process, as shown in FIG. 1, a current injection region 61 with a width of 10 μm for the total 60 SiO□ films 60 is provided on the p side, and a Cr/Aup side electrode 70 is formed thereon. An Au/Ge/Nin side electrode 71 is formed thereon. The chip is folded to a length of approximately 250 μm with the phase shift region 120 located approximately at the center.

The chip was fused to a diamond heat sink and the device characteristics were investigated. The oscillation threshold was 20 mA, and the differential quantum efficiency was 18 cm per side.The maximum CW operating temperature was 110°C.The maximum optical output was 30 mW at room temperature. The oscillation wavelength is 1.550μ in these operating output and operating temperature ranges.
It operated in a single-axis mode without axial mode hopping. The temperature dependence of the oscillation wavelength is 0.
It was 9X/°C.

As described above, good device characteristics were obtained, and the yield of devices having such characteristics was also close to 80%, which was a good result.

The reason why the above-mentioned good device characteristics and yield were obtained was because the phase shift region 120 was provided.

That is, the same effect as that of the distributed feedback semiconductor laser described above in which Sekartejo et al. shifted the phase of the grating using the electron beam exposure method can be achieved by the phase shift region 120. It is explained as follows. Effective refractive index 71 of the entire optical waveguide including the active layer 3 and the optical guide layer 4
a? f is about 33 for light with a wavelength of 1.55 μm.
On the other hand, the phase shift region 120 is composed of an InP semiconductor layer, and the effective refractive index 72 of this region is 3.
．． It is 20. Period Δ of diffraction grating 200, 235o
The Bragg wavelength λB corresponding to X is λ8='71eFF×
It is given by 2A and becomes 1.5510 μm. When light of this wavelength enters the phase shift region 120 of length L, the effective refractive index of this region is as small as 3.20. When the OL is 4 μm, the phase φ of the light wave is delayed by 0.52 compared to when the light wave travels through an optical waveguide in which the guide layer 4 is formed.
It becomes π. That is, when light at the Bragg wavelength passes through the 4 μm phase shift region 120, the phase of the light is shifted by approximately - compared to the case without the phase shift region 120. Distributed feedback semiconductor lasers are capable of oscillation at the black wavelength (as reported by Utaka in Proceedings of the 1986 Institute of Electronics and Communication Engineers of Japan Comprehensive National Conference, Vol. 4, 1017), and the oscillation wavelength and other Since the difference in threshold gain between the single-axis mode and the single-axis mode can be large, the yield of devices operating in a stable single-axis mode is increased. Therefore, the reason why the device according to the first embodiment of the present invention shown in FIG. 1 has good characteristics and the yield of devices operating in a single axis mode is high can be said to be because.

(Second Embodiment) FIG. 3 is a perspective view showing a second embodiment of the present invention. The difference from the first embodiment shown in FIG. 1 is that the diffraction grating 200 is formed directly on the substrate 1. In addition, the n-type InGaAsP optical guide layer 4, the active layer 3. Laminated up to p-type InP cladding layer 5,
Active layer 3. After forming the phase shift region 20 in which the optical guide layer 4 is removed with a length of 4 μm, it is sufficient to form a buried structure in the second growth, which has the advantage of reducing the number of growths by one. . Results similar to those of the first example were obtained in terms of oscillation characteristics, yield, etc.

(Third Embodiment) FIG. 4 is a perspective view of a buried type distributed feedback semiconductor laser showing a third embodiment of the present invention. To explain the structure in the order of fabrication, a IIe-Cd gas laser with an oscillation wavelength of 3250X is used on the surface of a (001)-plane n-type InP substrate 1 to a depth of approximately 1 in the <1.10> direction with a period of 2350X.
A diffraction grating 200 in which 000X unevenness is repeated is formed. Next, perpendicular to the <110> direction, the width is 4 μm and the depth is 1.5 μm.
A separation groove 100 of m is formed using a mask and a methanol etching solution.

A diffraction grating substrate with a separation groove 100 formed in a part thereof,
N-type InGaAsP light guide layer 4 (thickness 0.15 μm, 3n doping, carrier concentration 5XIOeM, composition of 1.3 μm in terms of emission wavelength) by liquid phase epitaxial growth,
Non-doped InGaAsP active layer 3 (thickness 0.11 tm
, composition of 1.55 μm in terms of emission wavelength), one-sided InP cladding layer 5 (thickness 1 μm, Zn doping, carrier concentration 1
×10 cfn) are successively laminated. Separation groove 10
In the portion where 0 is formed, the light guide layer 4 and the active layer 3 are formed to be separated by the flat portion and the bottom of the separation groove 100.

This completed a multilayer film wafer with the basic structure of this example, but in order to improve the performance of the device characteristics, that is, reduce the oscillation threshold, increase the differential quantum efficiency, etc., we next created a buried heterostructure. Form. From 953 negative to 954 in Electronics Letters, Vol. 18, No. 22 (published in 1982) by the authors, etc.
Dual channel brainer embedded type (D
C-PI3H) Width 1 to form the structure of the cylindrical laser
0/Jm, mesa stripe 52k<11.0) with a width of about 1.5 μm between two parallel grooves 50 and 51 with a depth of 3 μm.
After forming parallel to the direction, p is formed by liquid phase epitaxial growth.
InP type current blocking layer 6 (thickness at flat part 0.5 μm
, Zn doped.

Carrier concentration IX1. Oon ), n-type 1nP current confinement; -7 (0.5 μm thick at flat part, Te doped.

carrier temperature 2×10 an ), p-type InP buried layer 8 (thickness 1.5 μm at flat part, Zn doping, carrier visibility l×10 crn ), and p-type InGa layer sr
'F Yap layer 9 (thickness 0.5 μm at flat part, Z
n doped, carrier a degree lXl0 cm + composition of 1.2 μm in terms of emission wavelength) may be sequentially layered in the form shown in FIG.

Next, in the electrode formation process, S'O! 960
A current injection region 61 with a width of 10 μnl is provided using a Cr/Au layer 1, and a pole 70 is formed on the Cr/Au layer 1 on the n-side.
/Ge/N in 9111 electrode 71 is formed. The separation groove 100 is positioned at #MihoNakao, and the bone is made into a chip with a length of about 250 μm. This is the perspective view shown in FIG. This chip was placed on a diamond heat sink with the p-side electrode 70 facing downward, and device characteristics were investigated. The oscillation threshold is 18mA, and the differential quantum efficiency is 17% per side.
It happened. The maximum CW operating temperature was 110°C. The maximum optical output was 30 mW at room temperature, and in these operating optical output and operating temperature ranges, the oscillation wavelength was around 1.550 μm, and the device operated in a single-axis mode, with no axial mode hopping. The temperature dependence of the oscillation wavelength was 0.9X/'C. The yield rate for obtaining the above-mentioned good device characteristics is 80t.
Good results were obtained.

Good device characteristics and high yield were obtained with isolation trench 1.
The separation groove 10 has the same effect as a distributed feedback semiconductor laser in which the phase of the grating is increased by 7 feet using the electron beam exposure method.
This is because it is realized by 0. It is explained in the same way as in the first embodiment! This can be said to be because a λ/4 shift is realized.

(Fourth Embodiment) FIG. 5 is a perspective view showing a fourth embodiment of the present invention. The difference from the third embodiment shown in FIG. 4 is that the separation groove 100 has a triangular shape.

If a hydrochloric acid based etching solution is used, a side surface of the separation groove 100 is inclined at about 30 degrees with respect to the surface of the InP substrate 10, and if a Br-methanol etching solution is used, a surface inclined at about 54 degrees is obtained. Since the side surfaces of these separation grooves 200 are determined by the etching solution used,
The reproducibility when forming the separation grooves 100 is greatly improved.

Regardless of the shape of the separation groove 100, in order to realize the phase shift (1), the active layer 3 and the light guide layer 4 must be formed separately inside and outside the separation groove 100. Good. Further, the width of the separation groove 100 was set to 4 μm in the embodiment of the present invention, but since a phase shift effect can be expected even if there is a variation of ±50%, it is sufficient that the width is in the range of about 2 μm to 6 μm.

(Fifth Embodiment) FIG. 6 is a perspective view of a buried distributed feedback semiconductor laser showing a fifth embodiment of the present invention. To explain the structure in the order of fabrication, a 1(C
/, H, and PO were used to form 300 separated mesa stripes parallel to the <1-0> direction.

(This part corresponds to the phase shift region). Next, using a He-Cd gas laser with an oscillation wavelength of 3250X (11
0) direction with a repeating period of 3250X and a depth of about 10001
A diffraction grating 200 is formed. Separated mesa stripe 30
The diffraction grating 200 may or may not be formed on the upper surface of the 0, but it does not matter which case the structure of this embodiment is concerned.

FIG. 6 shows an example in which a diffraction grating 200 is also formed above the separation mesa stripe 300. An n-type In0aAsP optical guide layer 4 (thickness 0.15 μm, Sn doped, carrier concentration 5X
10 cm, composition of 1.3 μm in terms of emission wavelength).

Non-doped InGaAsP active layer 3 (thickness 0.1 μm1
(1.3 μrn composition in terms of emission wavelength), p-type InP cladding layer 5 (thickness 1 μm, Zn doping, carrier concentration 1×1
0''-Kushi) All are laminated continuously. In the part where the isolation mesa stripe 300 is formed, the light guide layer 4 and the active layer 3 are formed separated by the flat part and the upper part of the isolation mesa stripe 300. A multilayer film wafer with the basic structure of this example was completed, but in order to improve the performance of the device characteristics, that is, reduce the oscillation threshold, increase the differential quantum efficiency, etc., we next formed a buried heterostructure. Electronics Letts by the authors.
ers) magazine, Vol. 18, No. 22 (published in 1982), 953.
To form the structure of the dual channel buried brainer (DC-PBH) semiconductor laser reported on pages 954 to 954, two parallel grooves 50.
Mesa stripe s2v<N with a width of about 1.5 μm between 51
o> direction, and then a p-type InP current blocking layer 6 (thickness 0.5 in the flat part) is formed by liquid phase epitaxial growth.
11m, Zn doped, carrier concentration IX10cm)
, n-type InP current confinement layer 7 (thickness at flat part 0.5
μm, Te doped, carrier concentration 2X10 cm
), p-type InP buried layer 8 (thickness 1.5μ at flat part)
m.

Zn-doped, carrier concentration l
m, Zn doped, carrier concentration lXl0 cm +
(composition of 1.2 μm in terms of emission wavelength) may be sequentially laminated in the form shown in FIG. Next, an electrode process is performed and S
A current injection region 61 with a width of 10 μm is provided using a film 60, a Cr/Au p-side electrode 70 is formed thereon, and an Au/Ge/N! film 60 is formed on the n-side. An n-side electrode 71 is formed.

The separation mesa stripe 300 is placed in the center to form a chip with a length of about 250 μm.

This is the perspective view shown in FIG. This chip was placed on a diamond heat sink with the pm electrode 70 facing downward, and device characteristics were investigated. The oscillation threshold is 18 mA.

The differential doji efficiency was 17 per side. The maximum CW operating temperature was 110°C. Maximum light output is 30 m at room temperature
W, the oscillation wavelength was around 1.550 μm in these operating optical output ranges and operating temperature ranges, and the device operated in a single axial mode, and no axial mode hopping occurred.

The temperature dependence of the oscillation wavelength was o, aX/c. Good results were obtained, with a yield of 80% at which the above-mentioned good device characteristics could be obtained.

Good device characteristics and high yield were achieved by providing the isolation mesa stripes 300. That is, the separation mesa stripe 3oO can achieve the same effect as the distributed feedback semiconductor laser in which the phase of the grating is shifted by the electron beam exposure method by Sekartejo et al. It will be explained in the same way as in the case of the first embodiment (sixth embodiment).
The difference from the fifth embodiment shown in FIG. 6 is that the shape of the separation mesa stripe 300 is triangular. When hydrochloric acid-based etching is used, the separation mesa stripe 300
The plane 1j is inclined at an angle of about 30 degrees with respect to the plane of the InP substrate 1. Since the number +-m+ of the separated mesa stripe 300 is 1-1, which varies depending on the etching solution used, the reproducibility when forming the separated mesa stripe 300e is very good.

Regardless of the shape of the separation mesa stripe 300, in order to realize a phase shift, the active layer 3 and the optical guide layer 4 must be formed separately inside and outside the separation mesa stripe 300. That's fine. Further, the width of the separation mesa stripe 300 was set to 4 μm in the embodiment of the present invention, but since a phase shift effect can be expected even if there is a variation of ±50 −, it is acceptable if the width is in the range of about 2 μm to 6 μm.

(Effects of the Invention) The distributed feedback semiconductor laser of the present invention has an internal region 120 (separation groove 10 in the third embodiment) in which a phase shifter of λ/4 is performed.
0. In the fifth embodiment, the separation mesa stripe 300 corresponds to this), so good characteristics can be obtained, and the device is characterized in that it has a high yield of obtaining a device that operates in a stable single-axis mode.

Furthermore, since ordinary photolithography and two-beam interference exposure are sufficient to form the phase shift mechanism, there is no need to use electron beam exposure, which takes time for drawing, resulting in excellent productivity. Further, the length of the phase shift region 120 was set to 4 μm in the embodiment of the present invention, but ±5 μm.
Since a phase shift effect can be obtained even with a variation of about 0%, that is, a variation from about 2 kn to 6 μm, good reproducibility was obtained with small variations in device characteristics within a wafer and from one wafer to another.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing a first embodiment of the present invention. Fig. 2 is a diagram showing the manufacturing process of the first embodiment, Fig. 3 is a diagram showing the second embodiment of the present invention, Fig. 4 is a diagram showing the third embodiment, and Fig. 5 is a diagram showing the fourth embodiment. For example, FIG. 6 shows a fifth embodiment, and FIG. 7 shows a sixth embodiment. In the figure, %1 is an n-type InP substrate, 2 is an n-type InP buffer layer, 3 is an nGaAsP active layer, %4 is an InGaAsP waveguide layer, 5 is a p-type InP cladding layer, and 6 is a p-type InP layer.
PM? 1L block layer, 7 is n-type InP current confinement layer, 8 is p-type InT' buried layer, 9 is p-type InGaA
sP cap layer, 200 is a diffraction grating, 120 is a phase shift region, 60 is an 8 IO film, 70 is a p-side electrode, 71 is an n-side electrode, 50 and 51 are two parallel uras, 52 is a mesa stripe, 100 is a separation groove, and 300 is a separation mesa stripe.暉

Claims (1)

[Claims] A laminated structure including at least an active layer and a light guide layer adjacent to the active layer and having a periodically uneven shape on one surface is formed on a semiconductor substrate, and the active layer and the light guide layer are removed. and a phase shift region, in which a semiconductor layer having a refractive index different from that of the active layer and the light guide layer is formed in the removed portion, is formed at approximately the center of the resonator. Single-axis mode semiconductor laser.