JPS59198786A - Distributed feedback type semiconductor laser - Google Patents

Abstract

PURPOSE:To restrain Fabry-Perot mode and at the same time reduce the leakage current other than in an active layer by a method wherein one end of a mesa stripe including the active layer emitting light and recombines is covered with buried semiconductor, and the mesa stripe is formed by being placed between two grooves. CONSTITUTION:An optical guide layer 3, an active layer 4, and a P-InP clad layer 5 are successively laminated on an N-InP substrate wherein diffraction grating 2 is formed, thus manufacturing a semiconductor wafer of DH structure. The mesa stripe 6 and the two parallel etching grooves 7 and 8 pinching it are formed by etching this DH wafer. The mesa stripe 6 has a structure that one end 9 is cut in the halfway. Thereby, the Fabry-Perot mode in DFB-LD can be restrained. Subsequently, a P-InP current blocking layer 10 and an N-InP blocking layer 11 are laminated by excluding only on the mesa stripe 6 by performing burial growth, and further a P-InP buried layer 12 and an electrode layer 13 are laminated over the entire surface.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a buried heterostructure semiconductor laser in which an active layer is surrounded by a semiconductor layer having a larger energy gap and a lower refractive index than the active layer, in particular, a buried heterostructure semiconductor laser on one side of the active layer. The present invention relates to a distributed feedback semiconductor laser in which a waveguide layer having a diffraction grating is formed.

Buried heterostructure semiconductor lasers (BH-LDs) have excellent characteristics such as low oscillation threshold current, stabilized oscillation transverse mode, and high-temperature operation, and are attracting attention as light sources for optical fiber communications. There is.

By the way, when a normal BH-LD is modulated at high speed, the wavelength is not single, and even when used with direct current, the wavelength becomes discontinuous (jumps) due to temperature rise or changes in the injection current.BH-LD
When the LD 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 (D
FB-LD). In normal BH-LD, Fabry
It has a Perot cavity structure, and the light confined in the active layer is oscillated using resonant mirrors at both ends of the chip, whereas the DFB-LD has a diffraction grating near the active layer, and the inside of the diffraction grating is The waves reciprocate and resonate. Recently, various semiconductor lasers with a structure combining FB-LD and BH-LD have been developed, and 500Mbi
Results have been obtained that oscillation occurs at a single wavelength even when high-speed modulation is performed at t/see. As an example of such a semiconductor laser, In1-xGaxAs, Pl-, DFB-
BH-LD will be explained. In the case of an oscillation wavelength of 1.55 μm, a diffraction grating with a pitch of 0.23 μm is formed in advance on an n-InP substrate. This can be produced with relatively good controllability, for example, using He--Cd laser interferometry.

For example, on the substrate on which this diffraction grating is formed, the emission wavelength composition 1
．． 3 μm n InO, 72Ga, 28AsO
, 61P0.39 The waveguide layer is made of non-doped In O, 59 Ga O, 4 with a thickness of 0.2 μm and an emission wavelength of 1.55 μm.
1AS O, 90P O, 10 active layer with thickness Q, 2 μm
.

p-In (,,72G) with an emission wavelength composition of 1.3 pm
a (1,211AS O, 61P O, 39 meltback prevention layer and p-■nP cladding layer are sequentially laminated\). The semiconductor wafer thus obtained is mesa-etched to form an active layer with an active layer width of about 2 μm. After forming the mesa stripe, a current blocking layer etc. is buried and grown to form an InGaAsP/InPDFB-BH-
LD has been obtained.

However, in the DFB-LD, if a laser resonator surface is formed due to cleavage when cutting the manufactured laser wafer into individual laser pellets, single-axis mode oscillation that resonates in the diffraction grating cannot be obtained. That is, D.F.
In B-LD, suppression of Fabry-Perot mode is one of the important issues. Conventionally, various structures have been adopted for the above-mentioned purpose, but one of the most effective methods is a buried structure provided with a window area.

As reported in No. 278 of the 1986 Autumn Light and Radio Division National Conference of the Institute of Electronics and Communication Engineers, one end of the mesa stripe containing the InGaAsP active layer that recombines the light emission of a buried structure semiconductor laser is A DFB-BH-LD with an embedded structure was fabricated. In this method, a DH structure semiconductor wafer on which a diffraction grating for distributed feedback is formed is fabricated, and then a mesa stripe is formed by etching deeper than the active layer, and a current blocking layer and the like are buried and grown using InP. In such a window region DFB-BH-LD, Utakahi et al.
An element was fabricated in which single or FB mode oscillation was obtained in the temperature range up to 65° C., and the Fabry-Perot mode was sufficiently suppressed. For the purpose of suppressing the Fabry-Perot mode, which is one of the important issues in DFB-LD, it is effective to adopt a so-called window structure in which one end of the mesa stripe is buried with a buried semiconductor layer as described above. It can be said that this is a good method.

By the way, DFB-LD aiming at single-axis mode oscillation
Also, stabilization of the transverse mode is an essential condition.
In particular, the buried hetero (BH) structure is superior in terms of low oscillation threshold current and high temperature operation. DFB-L
By combining D and B H-L D, it is possible to obtain an element that stably oscillates in a single axis mode even during high-speed modulation, including only the transverse mode, but when adopting the BH structure, radiative recombination occurs. flows outside the active layer,
The key point is how to reduce leakage current. DF'B-BH- prepared by Utakahi et al.
In an LD, a mesa stripe with an active layer that recombines light is buried with an InP layer, which is a semiconductor with a larger energy gap than the InGaAsP active layer.
-InP, n -InP current blocking layer, p-I
By sequentially stacking nP buried layers, p -n
A thyristor with a -pn structure is formed to prevent current from leaking to areas other than the mesa stripe. However, in this case, the current flowing from the p-1nP cladding layer formed on the InGaAsP active layer to the p-InP current blocking layer stacked beside the mesa stripe flows into the n-InP layer below, and as described above. p −n −p
- It acts as a gate current for the n-structure silicon risk, significantly weakening the effect of the current blocking layer. In other words, even if the injection current of the semiconductor laser is increased, the leakage current flowing through areas other than the InGaAsP active layer that undergoes radiative recombination will rapidly increase, and therefore, when looking at the device as a whole, the oscillation threshold current may not become sufficiently small. However, there was a drawback that the desired optical output could not be obtained.

The object of the present invention is to sufficiently suppress the Fabry-Perot mode, and at the same time, the leakage current flowing through areas other than the active layer that undergoes radiative recombination is sufficiently small, so that the oscillation threshold is low.
DFB-BH with high quantum efficiency and high optical output
- To provide LD.

The structure of the distributed feedback semiconductor laser according to the present invention includes at least an active layer on a semiconductor substrate, an energy gap larger than that of the active layer, and a period of n·λ/2 on one surface.
(where n is an integer and λ is the oscillation wavelength in the pre-active layer). It is formed by rubbing between two grooves formed deeper than the layer, and the groove part has p-n-p.
- N-type and p-type current blocking layers are provided to form an "n" structure, and at least one end of the mesa stripe is embedded in a semiconductor layer with a larger energy gap than the optical guide layer. It is characterized by

The present invention will be described in detail below using drawings showing examples (IJI
I will explain this.

FIG. 1 is a perspective view showing the manufacturing process of a distributed feedback semiconductor laser according to an embodiment of the present invention. In order to manufacture F B -B H-L DI, as shown in FIG. n Ing, 72Gag, 2BAS0,61 corresponding to
PO, 3g photoactive layer 3, non-doped Ino, 5gGa (, 41As) corresponding to the emission wavelength of 155 μm 6.
goPO, 1o active layer 4, p-InP cladding layer 5
A semiconductor wafer with DI-I grooves is manufactured by sequentially stacking the DI-I grooves. Diffraction grating 2 is n-InP with (100) plane orientation.
On the substrate, a pattern is formed that repeats in the (011>crystal direction),
It has a periodicity of 240 OA and a depth of about 1200 Å, which was performed using laser interferometry using a He-Cd gas laser and conventional photolithography and chemical etching techniques.The thickness of each layer is nInO, 7.
2oaO, 28AS0.6ip0.39 light guide layer 3 and InO,5g GaO,41ASO,90P
o, 10 active layer 4, both 0.1 μnl, p −
I (1p cladding layer 5 is 1 μm, melt back after growth of active layer and diffraction grating 2 during melt soak time)
The growth was performed at a low temperature of around 600° C. to prevent loss due to heat. The DH wafer prepared in this way is etched using photolithography and chemical etching techniques as shown in FIG. , 8. The mesa stripe 6 has a width of 1.5μIn in the active layer portion, and the etching grooves 7°8 each have a width of 10μIn.
The depth was 3 μm. Actually, etching was carried out at 3° C. for about 3 minutes using a Br methanol etching solution. As mentioned above, it is important that the mesa stripe 6 has a structure in which one end 9 is cut in the middle.
This means that the mode can be sufficiently suppressed.

Subsequently, as shown in FIG. 1(C), buried growth is performed to form a p-In1' current process layer 10, n-in
All of the P current blocking layers 11 except for the top surface of the mesa stripe 6 are further covered with a p-1nP buried layer 12.
, emission wavelength 1.1 pm E corresponds to p-1n0.8
5'''0.15As o, 33F 6.67 Electrode Layer 1
3 over the entire surface.

p-InP and n-InP current block IX old 0
Both samples No. 11 were grown by a two-phase solution growth method in which InP source crystals were suspended in the growth melt. The reason why it does not grow only on the top surface of the mesa is that the growth rate at mesa 11 ((1) is high, and P(IJn), which is a minority atom in the growing melt, decreases near the top surface of the mesa.Growth temperature and cooling rate By appropriately setting and determining the growth conditions, the special crystal growth described above can be performed with good reproducibility.Finally, Zn is diffused over the entire surface of the device to form a low resistance ohmic electrode, and the electrode metal is formed. , the desired DFB is obtained by performing cleavage into individual laser pellets.
-B ) (-L l) is obtained.

InGaA, sP/inP prepared in this way
1) F) 3-B H-L D t oscillation threshold current 4Q mA at room temperature CW, -20°C to 50
We were able to obtain a device that oscillates in a single-axis mode over a temperature range of ℃ with excellent reproducibility. This warmth again,
In the c range, the oscillation wavelength temperature IW gradation rate is 0.9 A,/d.
eg s Also single-sided optical output up to 8mW or more in Murori! It showed sleeve mode oscillation.

InGaAs single layer InP D which is an embodiment of the present invention
In FB-B H-L I), the Fabry-Perot mode is suppressed by burying one end 9 of the mesa stripe 6 containing the active layer that recombines light with InP, which is a buried semiconductor.
By adopting a structure in which Q is sandwiched between two grooves 7 and 8, leakage current flowing through areas other than the active layer is reduced. The laser beam emitted from one end 9 of the mesa stripe 6 travels through the InP layer, but since the beam is emitted at a certain angle, even if it is reflected by the InP crystal plane, The reflected light did not enter the active layer for radiative recombination, and the Fabry-Perot mode was sufficiently suppressed, resulting in stable single-axis mode oscillation up to high optical output levels. In addition, a p-n-p-n current blocking layer structure is formed in the areas other than the mesa stripe 6 and etching grooves 7 and 8, and this current blocking layer structure includes a single InGaAs layer with a small energy gap.
Mr. Mito's report (Electronics-L/Tars (Ele)
ctronics Letters), Vol. 18, No. 22, pp. 953-954).
Since the p-n thyristor structure causes breakdown, the injected current flows into the active layer where it effectively recombines light, and the optical output does not saturate, especially up to a high optical output level, resulting in sufficiently good characteristics. was gotten.

In the embodiment of the present invention, when forming the etching grooves 7.8 and the mesa stripes 6, an etching pattern as shown in FIG. Mesa stripes 6 and etched grooves 7 are created using the etched pattern.
, 8, etc., a mesa stripe and an etching groove sandwiching the mesa stripe are formed, and one end of the mesa stripe is buried in the buried semiconductor. It suffices if it has a suitable structure. For example, if etching is performed in the pattern shown in Figure 2, the laser beam emitted at one end 9 of the mesa stripe will be absorbed by the semiconductor layer made of the same material as the active layer before reaching the end face 14 of the device. , which is more effective in suppressing the Fabry-Perot mode.Furthermore, if etching is performed as shown in Section 3, the etching depth will be shallower in the area where the groove width of the etching grooves 7 and 8 is narrower. Current blocking layers are stacked to form non-injected regions, i.e. mesa stripes 6.
The vicinity of one end 9 becomes a non-injected region, where the laser beam is absorbed. In this case as well, the element end face 1
4, the laser light reaches this point, and therefore the effect of suppressing the Fabry-Perot mode becomes even stronger.

The embodiment shown in FIG. 1 (in this case, inP is the substrate,
We used a semiconductor material with a wavelength of 1 μm with 1nGaAsP as the active layer, but of course the semiconductor materials used include InGaP and InGaP to cover the visible light to far infrared light range.
Any other semiconductor material may be used, such as AIP, GaAlAsSb, etc. The semiconductor substrate used may also be a semi-insulating semiconductor substrate or the like.

The features of the present invention are that in the DF'B-BH-LD, one end of the mesa stripe containing the active layer that recombines light is covered with a buried semiconductor, and the mesa stripe is
It was formed by sandwiching it between the grooves of a book. By embedding one end of the mesa stripe in the semiconductor, the DFB
-Fabry-Perot mode in the LDi was sufficiently suppressed.

In addition, by adopting a structure in which the mesa stripe is sandwiched between two grooves, p −nl) −n
'? [The break-town withstand voltage of the flow block layer structure is improved, and therefore the leakage current in the BH-LD can be reduced, and the DFB-BH-LD with single-axis mode oscillation can be obtained, which is stable up to high output levels. Ta.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing the manufacturing process of a DFB-HH-LD according to an embodiment of the present invention, and FIGS. 2 and 3 are views showing etching patterns showing modified examples. In the figure, 1 is an n-InP substrate, 2 is a diffraction grating, and 3 is an n-In0
, 72GaO, 2BAsO, 61PO, 3g The light guide layer 14 is I n 05g Ga 041 As O, 90
PO,1o active layer, 5 is rz-InP cladding layer, 6
7 and 8 are mesa stripes, 7 and 8 are etching grooves, 9 is the edge of the mesa stripe, 10 is a p-InP current block layer, and 11 is an n-InP'@flow blotter block layer p-In.
P buried layer, 13 is p-I n O, 85 Ga o,
15As o33P o, 67 electrode layers, and 14 are respectively coated on the element end faces. Figure 1 Ca) O1l No. ? figure

Claims (1)

[Claims] At least an active layer on a semiconductor substrate, which has a larger energy gap than the active layer, and has a periodic force on one surface.
A light guide layer in which a diffraction grating of λ/2 (where n is an integer and λ is an oscillation wavelength in the active layer) is formed, and the semiconductor laser includes the active layer that emits light and recombines. A mesa stripe is formed between two grooves formed deeper than the active layer, and n-type and p-type current blocking layers are formed in the grooves to form a p-n-p-n structure. A distributed feedback semiconductor laser comprising stacked layers, and further characterized in that at least one end of the mesa stripe is embedded in a semiconductor layer having a larger energy gap than the optical guide layer.