JPS59127889A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To form a non-injection region easily at the step of crystal growth, and to improve the reproducibility of characteristics, yield on manufacture and reliability by forming sections, height thereof differs in the laser resonant-axis direction, in a mesa stripe containing an active layer and forming a current block layer with the exception of only a section, mesa height thereof is high. CONSTITUTION:A mesa stripe 9 containing an active layer, which emits light and recombines, and two parallel etching grooves 7, 8 holding the mesa stripe are formed in parallel in the <011> direction to a DH wafer. In this case, the width of the mesa stripe 9 is brought to 1.5mum in the vicinity of the active layer 4, and depth is brought to approximately 2.5mum and width approximately 10mum in both the etching grooves 7, 8. A photo-resist film is formed in parallel in the <011> direction, and the mesa strip 9 is etched so as to form a stepped difference. N-InP Current block layers 10, 11 are formed with the exception of only the upper surface of a section, height thereof is high, of the mesa stripe 9, and a P-InP buried layer 12 and an electrode layer 13 are laminated on the whole surface.

Description

Detailed Description of the Invention The present invention relates to a buried heterostructure semiconductor laser (BH-LD) in which an active layer is surrounded by a semiconductor layer with a large energy gap and a small refractive index, especially in laser resonance. The present invention relates to a distributed feedback type buried semiconductor laser having a partial I carrier non-injection region in the axial direction.

Buried heterostructure semiconductor lasers (BH-LDs) have excellent properties such as low oscillation end value current, stabilized oscillation transverse mode, high optical output operation, and high temperature operation.
It is attracting attention as a light source for optical fiber communications. By the way, in a normal BH-LD, when pulse modulated at high speed, the wavelength is no longer single, and even when used with direct current, temperature fluctuations,
The wavelength jumps discontinuously due to changes in the injected current. BH-L
When D is modulated at high speed and the laser light is input to one end of an optical fiber, the waveform of the light emitted from the output end of the optical fiber is distorted due to material dispersion of the optical fiber. On the other hand, a distributed feedback semiconductor laser (DFB-T
,D) has been proposed.

A normal BH-L I) has a Fabry-Bello resonator structure, and the light confined in the active layer is resonated using the resonator mirror surfaces at both ends of the LD chip, causing laser oscillation. - In the LD, a diffraction grating is provided near the active layer, and light waves reciprocate within the diffraction grating and resonate. It's been like that recently with FB-LD and BH-
Various semiconductor lasers having a structure combining an LD have been developed, and results have been obtained that oscillate at a single wavelength even when subjected to high-speed pulse modulation at 500 Mb/sec. However, in a DFB-LD, if a laser resonator surface is formed by cleavage when cutting out individual laser beams from a laser wafer, single-axis mode oscillation by the diffraction grating cannot be obtained. In other words, it is important to suppress the Fapry-Perot mode. Conventionally, for this purpose, one laser end face was obliquely etched to prevent the formation of a resonator, or the final electrode layer was made n-type and p-type impurities were diffused in only a portion, or 8i0
2. A method of forming an insulating film such as Si3N4 and removing a portion of it to form an electrode has been considered.

Broadly speaking, two methods have been used: one in which one laser end face is formed obliquely, and the other in which the laser end face is cleaved to expose the crystal plane and a portion of the laser stripe is made into a non-injected region during electrode formation. However, in the former case, for example, even if a mixed etching solution such as Br 2 methanol is used, a completely oblique etched surface cannot always be obtained in the vicinity of the active layer. A disadvantage is that a very small portion forms a laser resonator together with the crystal cleavage plane at the other end, and as a result, a small axial mode oscillates near one central wavelength mode. In the latter case, slight pinholes in the insulating film may cause an increase in reactive current that does not contribute to laser oscillation, leading to a rise in the laser oscillation threshold, or heat treatment of the electrode with the insulating film attached may cause an increase in reactive current that does not contribute to laser oscillation. Alloy spikes may occur at the edge of the insulating film, which may adversely affect the reliability of the semiconductor laser.

Similarly, when impurities are partially diffused, there are problems such as pinholes and diffusion edges existing in the diffusion mask, and there are drawbacks such as poor reproducibility and manufacturing yield.

Therefore, if a non-implanted region is formed in advance at the stage of crystal growth, the above-mentioned problem can be eliminated.

An object of the present invention is to eliminate the above-mentioned problems and provide a distributed feedback buried structure semiconductor laser in which a non-implanted region can be formed at the stage of crystal growth, and the reproducibility of characteristics and manufacturing yield are greatly improved. .

The structure of the semiconductor laser according to the present invention is a buried heterostructure semiconductor laser in which a multilayer structure semiconductor wafer including at least an active layer is etched deeper than the active layer to form a mesa stripe, and then buried and grown. The mesa stripe has portions with different heights in the laser resonance axis direction, and a current blocking layer is formed in the lower height portion of the mesa stripe.

The present invention will be explained below using drawings showing examples.

FIG. 1 shows an embodiment of the present invention.

FIG. 2 is a plan view of a semiconductor double hetero (DH) wafer before in-depth growth of DFB-BHLD. Fig. 2 is a cross-sectional view of the section taken at the center line X in Fig. 1, that is, a cross-sectional view in the direction of the laser resonance axis including the mesa stripe portion, of the DFB-BHLD fabricated by performing buried growth on the DH wafer shown in Fig. 1. be. Similarly, FIG. 3(a) shows the prepared DF
It is the 1st sectional view of 'B-BHLD. Mainly in Figure 3 (a
The manufacturing process will be explained below using cross-sectional views of c and fb+.

First, a (100) n-InP substrate such as He-Cd
Laser interferometry.

A diffraction grating 2 with a pitch of 0.23 μm is formed using the above.

This is repeated in the direction of the laser resonance axis in the (011> direction, and it is easy to understand in Figure 2, which shows a cross section in the direction of the laser resonance axis.On the n-InP substrate 1 on which such a diffraction grating is formed, n-1nOaA corresponding to .1am
sP light guide layer 0.85 0.15
0.33 0.673 is a non-doped InO, 59G with a thickness of 0.3 am and an emission wavelength of 1.55 μm.
aO, 41AsO, 90PO, 10 active layer 4 with a thickness of 0.
p"0.72 corresponding to 1 μm 1 emission wavelength 1.3 μm
”0.28 AaO, 61PO, 39me, A= ) /
(The anti-slip layer 5 has a thickness of 0.155 m, and further p-1nP
The cladding layer 6 is sequentially laminated to a thickness of 1 nm. Note that in order to prevent the diffraction grating at a depth of about 1ooo X from melting back,
PO, 67 light guide layer 3 has supersaturation degree △T-10'C
Using a supercooling solution with a moderate temperature, continue to
The crystal growth of the O,10 active layer 4 is preferably performed using the overseeding method, which allows easy control of the film thickness and provides a good heterointerface. The DH wafer thus obtained is etched with a mesa containing an active layer that recombines one light in parallel to the <011> direction, as shown in FIGS. A stripe 9 and two parallel etching grooves 7.8 sandwiching the stripe L are formed. At this time, the width of the mesa stripe 90 is set to 1.5I1m near the active layer, and by using side etching during etching to form vertical etched sides, the reproducibility will be very good during the subsequent buried growth. The etching grooves 7.8 may each have a depth of 2.5 am and a width of about 10 μm.

After mesa etching is performed in this way to form the mesa stripe 9 and the two etching grooves 7 and 8, an etching resist film 7 is formed parallel to the (011> direction to form a step in the mesa stripe 9. Perform etching.

This can be done with good reproducibility by using a mixed etching solution of water and hydrochloric acid. In this case the exposed I
The entire np surface is etched, but the width is only 1
．． The etching rate of the protruding mesa stripe 9 of about 5 Iam is particularly high compared to other parts.

Actually, by etching at 3°C for 10 seconds using an etching solution containing a mixture of hydrochloric acid and pure water at a volume ratio of 4:1, the p-InP layer on the mesa stripe 9 and the cladding layer 6 are etched at 0.0000. 6Im, the p-InP layer in the flat part and the InP layer inside the etching groove 7.8 are both 0.6Im.
Embedded growth is performed on a DH wafer which has been etched by 1 to 015 .mu.m to form a nine-metast two-wavelength groove 9.2 with a step in the direction of the laser beam axis and an etched groove 7.8 sandwiching it. First, the p-InP current blocking layer 11 and the rllnP current blocking layer 11 are removed by removing only the upper surface of the high part of the mesa stripe, and then the p-InP buried layer 12 and the -In07□”0.28 AsO,6I Po,3
9* Pole layers 13 are laminated over the entire surface. p
−4nP current blocking layer 10 and n −I nP
Preventing the current process layer 11 from being partially laminated as described above can be done easily and with good reproducibility by a liquid phase growth method using an ordinary two-phase solution. This takes advantage of the growth conditions in liquid phase growth, and in the narrow and tall mesa stripe part, the crystal growth rate on the mesa sides is high, so the minority lattice in the growth solution grows on the mesa top surface. This is because atoms, in this case P atoms in the In solution, decrease, making it extremely difficult for crystal growth to occur only on the upper surface of the mesa. In actual device fabrication, by controlling the growth start temperature, cooling rate, and growth time of the two-phase growth solution, it is easy to avoid stacking only on the high height portions of the mesa stripes 9. In the low height part of the mesa, the degree of decrease of the minority lattice atoms in the solution on the top surface of the mesa is smaller than in the high mesa height part. Therefore, in the low part of the mesa stripe 9, there are two current blocking layers. This means that it is easy to layer the layers and at the same time avoid layering on the top surface of the areas with high redness. In this example, it is 30°
Cooling gray with C) 0.7”C/'m1n
Growth start temperature 62 of Xp-InP E flow blocking layer 10
By growing at 0 C for 90 seconds, and then growing two layers of n-(nP current blocks for 1160 seconds), the crystal growth described above was easily performed and with good P>Facility.I&
Afterwards, electrode formation was carried out to obtain the desired DFB-BHI, D.

The impurity diffusion for forming the low resistance ohmic electrode can be carried out over the entire surface of the device, and there are no problems such as the diffusion edges of the impurity diffusion or the occurrence of alloy spikes during alloying heat treatment with gold as in the conventional example.

As mentioned above, InGaAsP/InPD F which oscillates in a single axis mode at a wavelength of 1.55 am, which is an embodiment of the present invention,
In B-B HT and D, the flow σ for suppressing the Fabry-Perot mode could be easily formed at the stage of crystal growth and with extremely high efficiency. The mesa stripe 9 including the active layer that undergoes radiative recombination has portions with different heights in the laser colossal axis direction, and a current blocking layer is formed except for only the top surface of the portion with a high mesa height. The above-mentioned non-current injection region was easily formed, and the reproducibility of characteristics and manufacturing yield were significantly improved compared to the conventional example. In the DFB-BHLD manufactured in this way, the length of the current injection region is 300+m, and the length of the non-injection region is 200μ.
m, and the cw optical oscillation threshold current at room temperature is 30 mAXI
The temperature change in the wavelength during c w oscillation is o, 5X70c, 5
Even during high-speed pulse modulation of 00 Mbits/see, laser fluorine in a single DFB axis mode was obtained with good reproducibility.

In the embodiment of the present invention, InP is used as the substrate, and
n1. ．． -xGaxASy Although an optical device with an oscillation wavelength of 1 nm using Pl-y as an active layer and a light guide layer has been shown, the present invention is of course not limited to this material system, and can be applied to wavelengths from the visible light region to the far infrared region. There is no problem in using other semiconductor materials to cover the range concavity.

The diffraction grating used in the DFB-BHLD also showed a diffraction grating with a pitch of 0.23 am for the laser oscillation light of 1.55j1m, but it is not limited to this, and may be 0.46+5y11.
Any diffraction grating may be used as long as it has a pitch that is an integral multiple of 1/2 of the oscillation wavelength in the active layer.

The structure for controlling the transverse mode of the BH-LD is shown in the example as a structure in which a mesa stripe is sandwiched between two etching grooves, but the present invention is of course not limited to this. This includes all BH-LDs whose active layer is covered with another semiconductor material. Further, regarding the current blocking layer structure, although a blocking structure using a PNPN junction has been shown, the present invention is not limited to this, and there is no problem in using a semi-insulating semiconductor layer as a current blocking layer, for example.

The feature of the present invention is that in the Dr'B-BHLD, the mesa stripe including the active layer that emits light and recombines has portions with different heights in the direction of the laser resonance axis, and current is blocked except for the portion with the high mesa height. This is due to the formation of layers. This allows a non-implanted region to be easily formed during the crystal growth stage.
Its reproducibility is also extremely good. Therefore, compared to the conventional method of etching the laser resonator end face obliquely or forming a non-implanted region by using an insulating film or impurity diffusion, the reproducibility of characteristics, manufacturing yield, and reliability of DFBLD have been significantly improved.

[Brief explanation of drawings]

FIG. 1 shows the DH of a DFB-BHLD which is an embodiment of the present invention.
FIG. 2 is a plan view of the wafer before buried growth; FIG. 2 is a cross-sectional view taken along the line A-A' in FIG.
3(b) is a sectional view taken along the line C-C. In the figure, 1 is an n-InP substrate, 2 is a diffraction grating, and 3
” ”0.85 ”0.15”0.33 PO,6
7 Optical guide layer-4 is made of "0.59 GaO, 41As0.
90PO11o active layer, 5 is p"0.72 GaO
,28"0.61 PO439 Meltback Prevention Layer, 6
is a p-InP cladding layer, 7.8 is an etching layer, 9 is a mesa stripe, 10 is a p-InP current blocking layer, 1
1 is an n-4nP current blocking layer, 12 is a p-InP buried layer, 13 is p Ino, 72 GaO, 28As
O, 61 PO, 39a Pier 46 1,000 1 Zuo 3 Zu Otsu! ! 0

Claims (1)

[Claims] In a buried heterostructure semiconductor laser formed by forming a mesa stripe on a multilayer semiconductor wafer including at least an active layer by etching it deeper than the active layer, and then growing the mesa stripe, the mesa stripe is aligned with the laser resonance axis. 1. A semiconductor laser comprising portions having different heights in different directions, and a current blocking layer being formed in a lower height portion of the mesa slab.