JPS6346790A - Buried semiconductor laser and manufacture thereof - Google Patents

Abstract

PURPOSE:To efficiently oscillate a buried semiconductor laser thereby to be able to modulate it at a high speed by providing a current block layer, and an organic film having an electric insulation on the upper surface of a current insulating layer except the upper surface of a cap layer. CONSTITUTION:A double hetero structure that an active region 11 is interposed between semiconductor layers having lower refractive index than that of the region 11 and larger forbidden band widths than those of the region 11 is epitaxially grown on a semiconductor substrate 13. A current block layer 15 made of a semi-insulating semiconductor is laminated in a groove by using a vapor growing method for forming two grooves so formed by etching as to dispose the region 11 in a banking interposed between the two grooves, and an organic film 21 having an electric insulation is formed on the upper surface of a current insulating layer 18 except the upper surface of a cap layer 16 covered with a reverse conductivity type cap layer 16 to a substrate on the top end of the banking and the upper surface of the layer 15. Thus, leakage currents at the flat parts at both sides of the active region are stopped by the insulating films, and leakage currents passing the two grooves are stopped by the current block layer to be able to oscillate in a high efficiency.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor laser for optical communications that can oscillate with high efficiency and modulate at high speed, and a method for manufacturing the same.

[Conventional technology and its problems]

Semiconductor lasers have begun to be put into practical use as light sources for optical fiber communications. Semiconductor lasers used as light sources for optical fiber communications must be able to oscillate at a high rate and be modulated at high speed. Furthermore, with the progress of practical application, there is a strong demand for a highly reproducible manufacturing method that can produce a large amount of semiconductor lasers at a high yield. However, conventional semiconductor lasers have not been able to simultaneously satisfy the above three conditions. Below, we will explain typical semiconductor lasers that have been manufactured in the past, as well as semiconductor lasers that have been recently manufactured in an attempt to improve these laser characteristics and that are closest to meeting the above requirements, and explain why the above three requirements could not be met at the same time. Explain.

A typical semiconductor laser that has been manufactured in the past is a double channel planar embedded semiconductor laser (Double Channel).
Planar Buried 1terost
It is detailed in Journal of Lightwave Technology, Vol. LT-1, March 1983 issue, pp. 195-202. In this semiconductor laser, in order to selectively flow current through the stripe-shaped active region, the area other than the active region is p-n-p.
'-n junction is formed, and current is blocked by the n-p reverse junction.

A current blocking structure using an n-p reverse junction, typified by this structure, exhibits a very good current blocking effect, and therefore oscillates with high efficiency exceeding 50%. Furthermore, since the manufacturing process is a combination of highly reproducible steps, it is possible to manufacture with a high yield.

However, there is a problem in that leakage current, which causes a decrease in efficiency, increases at high output due to the operation of the n, -p-n) transistor formed in the block layer. In addition, since the n-p reverse junction has a capacitance of 10 pF or more, I G
It was difficult to modulate at high speeds exceeding b/s.

Next, two semiconductor lasers devised to overcome the drawbacks of the semiconductor lasers described above will be described. The first semiconductor laser is the Proceedings of the Japan Society of Applied Physics (Proceedings of the 46th Japan Society of Applied Physics Academic Conference, 2p-N-II, p. 20).
6. Fall 1985). To enable high-speed modulation, this semiconductor laser reduces the capacitance of the diode by forming grooves in the flat block layers on both sides of the active layer that reach the substrate. Modulation was possible at a relatively high frequency of 5 GHz.

However, even with this semiconductor laser, the reduction in parasitic capacitance was not sufficient and modulation exceeding 10 GHz was difficult. In this semiconductor laser, there is a p-type electrode and a high concentration p
This is because the type semiconductors face each other, which generates residual capacitance.

Further, the second semiconductor laser is described in Electronics Letters, 21
Vol. 13, pp. 577-578, 1985), and is a semiconductor laser formed by vapor phase embedding. This semiconductor laser is designed to reduce the capacitance of the diode to enable high-speed modulation.
It was possible to modulate at frequencies as high as GHz.

In addition, the leakage current that causes efficiency reduction is reduced so that the oscillation occurs at a high rate, and the f
It is made of III. However, this semiconductor laser has a major problem in productivity. This is due to the structure of this semiconductor laser. In order to cause a semiconductor laser to oscillate in a single transverse mode, the width of the active layer must be approximately 1 to 2 microns. However, in this semiconductor laser, the mesa direction is <011. ＞, and has a trapezoidal shape that expands downward. Therefore, it is necessary to form stripes of 1 μm or less, which is below the limit of the photophosphorography method, or
Alternatively, the width of the active layer must be sandwiched by selective etching in which wide stripes of several μm are formed and then only the active layer is selectively side-etched. However, this selective etching has very low controllability, making it difficult to make the active layer width constant. This is because the etching rate largely depends on the temperature and concentration of the etching solution and the composition of the active layer to be etched. For this reason, the vapor phase implantable semiconductor laser has a high yield and is not suitable for mass production.

As described above, conventional semiconductor lasers have not been able to simultaneously satisfy three conditions: oscillate with high efficiency, be capable of high-speed modulation, and be able to be manufactured with high yield and good reproducibility. An object of the present invention is to provide a semiconductor laser having a structure that oscillates with high efficiency, allows high-speed modulation, and has a high productivity, and a method for manufacturing the same.

[Means for solving problems]

According to the present invention, a double heterostructure is provided on a semiconductor substrate in which an active region is sandwiched between semiconductor layers having a refractive index lower than the refractive index of the active region and a forbidden band width larger than the forbidden band width of the active region. In addition, the active region is located in a bank sandwiched between two grooves, a current blocking layer made of a semi-insulating semiconductor is provided in the two grooves, and the tip of the bank and the top surface of the current blocking layer are connected to the substrate. It is covered with a cap layer of the opposite conductivity type, has a current insulating layer connected to a current blocking layer on both sides of the cap layer, and has electrical insulation on the top surface of the current insulating layer except for the top surface of the cap layer. A semiconductor laser having a @ film can be obtained.

Further, according to the present invention, a double heterostructure is provided on a semiconductor substrate in which an active region is sandwiched between semiconductor layers having a refractive index lower than the refractive index of the active region and a forbidden band width larger than the forbidden band width of the active region. The first step is epitaxial growth, the second step is to form two vortices by etching so that the active region is located in the bank between the two grooves, and the second step is to form half a layer in the groove using a vapor phase growth method. A third step of laminating a current blocking layer made of an insulating semiconductor, further covering the tip of the embankment and the top surface of the current blocking layer with a cap layer having a conductivity type opposite to that of the substrate, and forming a current insulating layer excluding the top surface of the cap layer. and a fourth step of forming an electrically insulating organic film on the upper surface of the semiconductor laser.

[Effect]

In the semiconductor laser of the present invention, leakage current at the flat portions on both sides of the active region is blocked by an insulating film such as 5i02, and leakage current passing through the two vortices is blocked by a current blocking layer made of a semi-insulating semiconductor. Ru. Therefore, the leakage current in this part can be significantly reduced compared to the conventional case, and oscillation with high efficiency is possible. Furthermore, by arranging current blocking layers made of semi-insulating semiconductor on both sides of the active region, the n-p reverse bias region required in conventional DC-PBH lasers becomes unnecessary. Therefore, the parasitic capacitance generated in this portion can be sufficiently reduced. In addition, by covering the outside of the two grooves with an insulating organic film several μIn thick, the parasitic capacitance in this area is less than 1/10 compared to the conventional case of using only 5i02 film, which requires several thousand people. become.

In total, it is possible to suppress the parasitic capacitance to 2 pF or less.

- The first double-hetero growth step, second etching step, third buried growth step, and fourth insulating layer coating step in the semiconductor laser manufacturing method are all performed by highly reproducible methods suitable for mass production. According to this method, a semiconductor laser which can oscillate with high efficiency and can be modulated at high speed can be provided with a high yield.

('Example〕 The present invention will be explained in detail using the drawings.

FIG. 1 is a diagram showing an embodiment of a buried semiconductor laser which is the first invention of the present invention. The active region 11 is a 1nGaAsP crystal with a forbidden band width of 0.95 eV and emits light at 1.3 μm. This active layer is formed on an n-type InP substrate 13 by an n-type InP buffer layer 12 with a forbidden band width of 1.3e and a p-type InP cladding layer 14 from the top and bottom, and a Fe layer from the left and right.
It is surrounded by a semi-insulating current blocking layer 15 having a doped InP composition, and a refractive index guided waveguide structure is formed. Further, the tip of the embankment and the upper surface of the current blocking layer 15 are covered with a p-type 1nP cap layer 16. The upper side of this p-type InP cap layer 16 is made of p-type 1nGaAs.
covered with a P contact layer 17 and a p-type InP cap layer 1
On both sides of 6, S i 02 layers 18 are provided as current insulating layers connected to the current blocking layer 15 . 5i
A polyimide layer 21, which is an electrically insulating organic film, is provided on the upper surface of the 02 layer 18, and a p-type electrode 19 and an n-type electrode are provided in contact with the contact layer 17 and the n-type InP substrate 13, respectively. 20 are provided. In the buried semiconductor laser having the above structure, the current blocking layer 15 and the polyimide layer 21 are embedded in the two grooves.
and the S z 02 layer 18, the current flowing due to the voltage applied to the n-type electrode 19 and the n-type electrode 20 is caused by the p-type InGa
AsP contact layer 17, p-type InP cap layer 16,
p-type 1nP cladding layer 14, active region 11 and n-type I
It selectively flows to the nP buffer layer 12. therefore,
Leakage current is very small and oscillates at a high rate.

Parasitic capacitance mainly occurs in two places. One is the parasitic capacitance of the double heterostructure placed outside the groove, and the other is the parasitic capacitance of the n-type electrode 19 and the p-type rad layer 14 outside the vortex, sandwiching the polyimide layer 21 and the 8102 layer 18. This is a parasitic capacitance formed by opposing each other. The cavity length of the semiconductor laser is 30 C) μm, and the electrode area is 300 t, t.
m x 300 μm, the parasitic capacitance due to the former is sufficiently small and can be ignored because the electrical resistance of the semi-insulating current blocking layer 15 is large, and the parasitic capacitance due to the latter is due to the polyimide layer thickness of 2.5 μm and the SiO2 layer thickness Q , 2μ
When rn, it is approximately 1 pF. Therefore, the total parasitic capacitance is approximately 1p, which is ~ compared to a conventional semiconductor laser.
I can do it in 1/10.

Next, an embodiment of the method for manufacturing the semiconductor laser of the present invention shown in FIG. 1 will be described. FIG. 2 shows cross-sectional views at each stage of the manufacturing process.

First, as a first step, the sulfur-doped n-1nP substrate 13
A sulfur-doped n-InP buffer layer 1 is formed on top by vapor phase growth.
2 (thickness 2.5 μm, n=IX region 11 (band gap 0.95 eV, thickness 0°1 μm), dumbbell-doped p-In
A double hetero (DH) structure is formed using the P cladding layer 14.

Next, in the second step, such H crystals are etched to about 1 μm by ordinary photolithography and chemical etching.
S i 02 layer with eaves of length and two grooves 25.
Form 26. For this purpose, first, as shown in FIG.
Using No. 8 as a mask, the DH crystal is deeply etched with a bromine methanol solution until side etching occurs. At this time, the striped windows 23 and 24 are aligned with the crystal orientation (011).
When formed parallel to , the V-shaped ? shown in Figure 2(b) is formed.
Two 425.26 are formed. A bank extending over the active region 11 is formed between these two grooves. Next, a photoresist 22 is applied, and by conventional photolithography and etching, a window is opened in the portion of the bank containing the active region, and the entire SiO□ layer 18 above the bank is formed. This state is shown in FIG. 2(C). Further, when the photoresist 22 is removed, the shape shown in FIG. 2(d) is formed.

Next, in a third step, a current blocking layer 15 is formed on the two i25.26 layers by vapor phase growth as shown in FIG. At this time, the InP current blocking layer 15 is made of SiO2
It is formed to be connected to the eaves of layer 18. Subsequently, p-type InP cap layer 16 and p-type I
An nGaAs contact layer 17 is selectively formed thereon. As a characteristic peculiar to the vapor phase growth method, when crystal growth is performed on the W-shaped region having the following characteristics, the growth on the bank in the center is slowest. The structure shown in FIG. 1 is realized by utilizing the characteristics peculiar to this vapor phase growth method. The crystalline cap layer 16 and contact I layer 17 thus formed form a mesa with a height of about 3 μm compared to the SiO2 layer 18.

Therefore, in the fourth step, the step is filled with a polyimide layer 2 which is an insulating organic film.

In this case, a five-light polyimide is used, applied by a normal photolithography method, and cured after removing the upper part of the mesa and opening a window.

Thereafter, the n-type electrode 19 and the n-type electrode 2 are connected by a normal method.
form 0.

All of the first to fourth steps described above are methods suitable for mass production. In particular, the buried growth by vapor phase growth in the third step has high reproducibility and high uniformity. Therefore, it has been found that the characteristics of semiconductor lasers manufactured by such a method have little variation and can be obtained at a high yield.

In the above example, a laser was obtained that oscillated at a wavelength of 1.3 μm and the forbidden band width of the InGaAsP crystal in the active region 11 was 0.95 eV.
It can be set to oscillate at any wavelength from μm to 1.65 μm.

In the above example, a vapor phase growth method was used in the first step, but DI
The -1 structure may be formed by other growth methods such as liquid phase growth and MBE.

〔Effect of the invention〕

The semiconductor laser according to the invention oscillated with very high efficiency of more than 50% and was capable of responding at high frequencies exceeding OG Hz. Further, in the manufacturing method according to the present invention, the formation of the groove and the growth filling the groove can be performed with good reproducibility, and therefore a very high yield can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of an embodiment of the buried semiconductor laser of the present invention, and FIGS. 2(a) to 2(d) are cross-sectional views showing the manufacturing process of the buried semiconductor laser of the present invention. DESCRIPTION OF SYMBOLS 11... Active region, 12... N-type InP buffer layer, 13... N-type 1nP substrate, 14... P-type InP cladding layer, 15... Fe-doped InP current block 2
Layer 16... p-type InP cap layer, 17... p-type I
nGaAsP contact layer, 18=・5i02 layer, 19
... n-type electrode, 20 ... n-type electrode, 21 ... polyimide layer. 1N1ノ(-a-2 (C) (f) Support 2 Possible

Claims (2)

[Claims] (1) having a double heterostructure on a semiconductor substrate in which an active region is sandwiched between semiconductor layers having a refractive index lower than the refractive index of the active region and a forbidden band width larger than the forbidden band width of the active region; The active region is located in a bank sandwiched between two grooves, and has a current blocking layer made of a semi-insulating semiconductor in the groove, and further, the tip of the bank and the upper surface of the current blocking layer are located in the substrate. is covered with a cap layer having a conductivity type opposite to that of the cap layer, has a current insulating layer connected to the current blocking layer on both sides of the cap layer, and has an electrically conductive layer on the top surface of the current insulating layer except for the top surface of the cap layer. An embedded semiconductor laser characterized by having an organic film with insulating properties. (2) A first step of epitaxially growing a double heterostructure on a semiconductor substrate, in which an active region is sandwiched between semiconductor layers having a refractive index lower than that of the active region and a forbidden band width larger than the forbidden band width of the active region. a second step of forming the two trenches by etching so that the active region is located in a bank between the two trenches; a third current blocking layer made of a conductivity semiconductor, and further covering the tip of the bank and the upper surface of the current blocking layer with a cap layer having a conductivity type opposite to that of the substrate.
and a fourth step of forming an electrically insulating organic film on the upper surface of the current insulating layer except for the upper surface of the cap layer.
A method of manufacturing an embedded semiconductor laser, comprising the steps of: