JPH0511437B2 - - Google Patents

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]

Semiconductor lasers are beginning to be put into practical use as light sources for optical fiber communications. The semiconductor laser used for this purpose needs to be able to oscillate with high efficiency and be capable of high-speed modulation. 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. less than,
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 the above requirements, and explain why the above three requirements could not be met at the same time. .

The typical semiconductor laser that has been manufactured in the past is 2
Double groove planar embedded semiconductor laser (Double
Channel Planner Buried Heterostrucure
Laser Diode (abbreviated as DC-PBH LD),
It is detailed in Journal of Lightwave Technology, Volume LT-1, March 1983, pages 195-202. In this semiconductor laser, in order to selectively allow current to flow through the striped active region, pnpn junctions are formed in areas other than the active region, and current is blocked by reverse np junctions. The current blocking structure using np reverse junction, which is represented by this structure, exhibits a very good current blocking effect, so
Oscillates at a high rate of over 50%. Furthermore, since the manufacturing process is based on a combination of highly reproducible steps, it can be manufactured with a high yield. However, in this DC-PBH semiconductor laser, the leakage current, which causes a decrease in efficiency, is
There was a problem that increased at high output due to transistor operation. Furthermore, since the np reverse junction has a capacitance of 10 pF or more, it is difficult to perform modulation at a high speed exceeding 1 Gb/s.

As semiconductor lasers devised to overcome the drawbacks of the DC-PBH semiconductor laser, there are first and second semiconductor lasers described below.

The first semiconductor laser was the Proceedings of the Japan Society of Applied Physics (Vol.
Proceedings of the 46th Japan Society of Applied Physics Academic Support Meeting, 2p-
N-11, p. 206, Autumn 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. 5GHz
It can be modulated at a relatively high frequency. However, this semiconductor laser had insufficient removal of residual capacitance, making it difficult to modulate it above 10 GHz.
This is because in this semiconductor laser, the p-type electrode and the highly doped p-type semiconductor face each other with the SiO ₂ layer sandwiched between them in the flat portions on both sides of the active layer, which generates residual capacitance.

Also, the second semiconductor laser is used for electronics/
Letters, (Electronics Letters), Volume 21, Issue 13,
This is a semiconductor laser called a vapor phase regrowth buried type described in 1985, pp. 577-578. This semiconductor laser is designed to reduce the capacitance of the diode to enable high-speed modulation.
It can be modulated at frequencies as high as 15GHz. or,
In order to oscillate with high efficiency, the structure is designed to reduce leakage current, which causes a decrease in efficiency, and to allow current to flow only through the active layer. However, this semiconductor laser has a major problem in productivity. This is due to the structure of this semiconductor laser. In order to oscillate a semiconductor laser in a single transverse mode, the active layer width must be approximately 1 to 2 microns. However, in this semiconductor laser, the direction of the mesa is <
011>, and has a trapezoidal shape that widens downward. Therefore, it is necessary to form stripes that are below the limit of the photolithography method, or to form wide stripes and then narrow the active layer width by selective etching in which only the active layer is selectively side-etched. However,
This selective etching has very low controllability, and it is difficult to make the active layer width constant. this is,
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 regrown embedded semiconductor laser has a high yield and is not suitable for mass production.

[Problem that the invention seeks to solve]

As described above, conventional semiconductor lasers simultaneously satisfy three conditions: they can oscillate with high efficiency, can be modulated at high speed, and can be manufactured using a highly reproducible manufacturing method that can produce large quantities of semiconductor lasers with high yield. I couldn't do it.

An object of the present invention is to provide a semiconductor laser that can oscillate with high efficiency, perform high-speed modulation, and whose structure can be manufactured by a highly productive method, and a method for manufacturing the same.

[Means for solving problems]

A first aspect of the invention is a buried semiconductor laser in which an active region is surrounded by a semiconductor having a refractive index lower than that of the active region and a forbidden band width larger than the forbidden band width of the previous active region. is located in a bank sandwiched between two grooves, and has a current-carrying layer in the groove having a conductivity type opposite to that of the semiconductor substrate, and a current-blocking layer having the same conductivity type as the substrate on the upper surface of the current-carrying layer; Further, the tip of the embankment and the upper surface of the current blocking layer are covered with a cap layer having a conductivity type opposite to that of the semiconductor substrate, and current insulating layers connected to the current blocking layer are provided on both sides of the cap layer, and the cap layer is connected to the current blocking layer. It is characterized by having an electrically insulating organic film on the upper surface of the current insulating layer except for the upper surface of the layer.

A second invention provides a method for manufacturing a buried semiconductor laser in which an active region is surrounded by a semiconductor 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, a first step of epitaxially growing a double heterostructure containing an active region on a semiconductor substrate; and a second step of etching the two trenches so that the active region is located in a bank between the two trenches. a current-carrying layer having a conductivity type opposite to that of the semiconductor substrate and a current-blocking layer having the same conductivity type as that of the semiconductor substrate are sequentially laminated on the upper surface of the current-carrying layer in the groove using a vapor phase growth method; Further, a third step of laminating the cap layer so that the tip of the embankment and the upper surface of the current blocking layer are covered with a cap layer having a conductivity type opposite to that of the semiconductor substrate, and depositing the current blocking layer on both sides of the cap layer. a fourth step of forming a current insulating layer connected to the cap layer; and a fifth step of forming an electrically insulating organic film on the upper surface of the current insulating layer excluding the upper surface of the cap layer. It is characterized by

[Effect]

The semiconductor laser according to the first aspect of the present invention has a structure in which the current blocking layers in the flat portions on both sides of the active layer of the conventional DC-PBH type semiconductor laser are replaced with insulating films such as SiO ₂ . Therefore, the npn transistor, which contributes to an increase in leakage current, is not formed in the flat part, so the leakage current in this part can be significantly reduced compared to the conventional method, and oscillation with high efficiency is possible. It is. Also,
Compared to conventional DC-PBH semiconductor lasers, the np reverse bias region is limited to only this groove, so the parasitic capacitance generated in this portion is sufficiently small. In addition, by covering the outside of the two grooves with an insulating organic film several micrometers thick, the charge is sufficiently separated compared to conventional SiO ₂ alone, so the parasitic capacitance in this area is reduced to less than 1/10. . In total, it is possible to suppress the parasitic capacitance to 2 pF or less.

The semiconductor laser described above is realized by the manufacturing method according to the second invention. In particular, the first double-hetero growth step, the second etching step, the third buried growth step, and the fourth insulating layer coating step are all highly reproducible methods suitable for mass production. If so, it is possible to provide a semiconductor laser that can oscillate with high efficiency and can be modulated at high speed with a high yield.

〔Example〕

Embodiments of the present invention will be described using the drawings.

FIG. 1 is a sectional view showing an embodiment of a buried semiconductor laser according to the first aspect of the present invention. The active region 11 is located in a bank between two grooves, and is made of InGaAsP crystal with a forbidden band width of 0.95 eV.
Emit light at 1.3μm. This active layer is an n-type InP substrate13
Above, from the top and bottom, there is an n-type InP buffer layer 12 with a forbidden band width of 1.3 eV and a p-type InP cladding layer 14, and from the left and right, a p-type InP layer 15 that constitutes a conductive layer and an n-type InP layer on the upper surface of this layer. Surrounded by a current blocking layer 16, a refractive index waveguide structure is formed. Further, the tip of the embankment and the upper surface of the current blocking layer 16 are covered with a p-type cap layer 17. The upper side of this p-type cap layer 17 is p
SiO ₂ layers are provided on both sides of the p-type cap layer 17 as a current insulating layer 19 covered with an InGaAsP type contact layer 18 and connected to the current blocking layer 16 . A polyimide layer 22 which is an electrically insulating organic film is provided on the upper surface of the SiO ₂ layer 19, and a contact layer 18 and an n-type InP
A p-type electrode 20 and an n-type electrode 21 are provided in contact with the substrate, respectively.

In the buried semiconductor laser having the above structure, the p-type InP layer 15 and the n-type
Due to the np reverse junction formed by the InP current blocking layer 16, the polyimide layer 22 and the SiO ₂ layer 19, the current flowing due to the voltage applied to the p-type electrode 20 and the n-type electrode 21 is transferred to the p-type InGaAsP contact layer. 18, selectively flows to the p-type InP cap layer 17, the p-type InP ground layer 14, the active region 11, and the n-type InP buffer layer 12. At this time, leakage current flows from the p-type InP layer 15 to the p-type outside the groove.
It flows to the InP cladding layer 14, but in this part
Since no npn transistor is formed, the leakage current is not amplified. Therefore, leakage current is very small and oscillation is performed with high efficiency.

Parasitic capacitance mainly occurs in two places. One is np
The other is the reverse junction part, where the p-type electrode 20 and the p-type cladding layer 14 outside the groove are made of polyimide layer 2.
This is a parasitic capacitance formed by 2 and SiO ₂ layer 19 facing each other. Assuming that the laser cavity length is 300 μm and the electrode area is 300 μm x 300 μm,
The parasitic capacitance due to the former is approximately 0.3 pF when the groove width is 5 μm, and the parasitic capacitance due to the latter is approximately 0.3 pF when the groove width is 5 μm.
When the thickness of the SiO2 layer is 2.5 μm and the thickness of the _two SiO layers is 0.2 μm, it is approximately 1 pF.
Therefore, the total parasitic capacitance is approximately 1.3 pF, indicating that sufficiently high-speed modulation is possible.

Next, an embodiment of the manufacturing method, which is the second aspect of the present invention, will be described. In this embodiment, a case will be described in which a buried semiconductor laser having the configuration shown in FIG. 1 is manufactured. FIG. 2 shows cross-sectional views at each stage of the manufacturing process.

First, as a first step, a sulfur-doped n-InP buffer layer 12 (thickness 2.5 μm, n=1×10 ¹⁸ cm ^-3 ) is deposited on a sulfur-doped n-InP substrate 13 by vapor phase growth.
Non-doped InGaAsP active region 11 (band gap 0.95 eV, thickness 0.1 μm), zinc doped p-InP
Cladding layer 14 (thickness 0.3 μm, p=1×10 ¹⁸ cm ^-3 )
A double hetero (DH) structure was formed.

Next, in the second step, such a DH crystal is coated with two layers of SiO and _two layers of eaves approximately 1 μm in length by ordinary photolithography and chemical etching.
form two grooves. To do this, first of all, please refer to Figure 2 a.
Form a SiO ₂ layer 19 on the DH crystal as shown in
Two windows 24 and 25 were opened in a striped pattern. Next, using the SiO ₂ layer 19 as a mask, the DH crystal was deeply etched with a bromine methanol solution until side etching occurred. At this time, striped window 2
4 and 25 are formed parallel to the crystal orientation (011), V-shaped grooves 26 and 27 shown in FIG.
One was formed. A bank containing the active region 11 is formed between these two grooves. Next, a photoresist 23 was applied, a window was opened in the part of the bank containing the active region by conventional photolithography and etching, and the SiO ₂ layer 19 on the top of the bank was removed. This state is shown in FIG. 2c. Furthermore, photoresist 23
When removed, the shape shown in FIG. 2d was formed.

Next, in the third step, a p-type InP layer 1 is formed on the two grooves 26 and 27 by vapor phase growth as shown in FIG.
5 and an n-type InP current blocking layer 16 were sequentially formed. At this time, the InP current blocking layer 16 is
It is formed so as to be connected to the eaves of the SiO ₂ layer 19. Next, p-type InP cap layer 1
7 and p-type InGaAsP contact layer 18, 2
It was formed so as to cover the two grooves and the p-type InP cladding layer 14 on the active region 11 of the embankment. As a characteristic peculiar to the vapor phase growth method, when crystal growth is performed on the W-shaped grooves 26 and 27, the growth on the bank in the center is slowest. By utilizing the unique properties of this vapor phase growth method,
The structure of the diagram is realized. The crystal cap layer 17 and contact layer 18 thus formed are
A mesa having a height of about 3 μm compared to the SiO ₂ layer 19 is formed.

Therefore, in the fourth step, the step was filled with a polyimide layer 22 which was an insulating organic film. In this case, a photosensitive polyimide was used, and a window was opened in the upper part of the mesa using a conventional photolithography method, and the polyimide was cured. Thereafter, a p-type electrode 20 and an n-type electrode 21 were formed by a conventional method.

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, the characteristics of semiconductor lasers manufactured by such a method have little variation and have a high yield.

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

Although vapor phase growth was used in the first step in the above embodiment, the DH 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 present invention oscillated with very high efficiency and was able to respond at high frequencies exceeding 10 GHz. Further, in the manufacturing method according to the present invention, the formation of the groove is stable, and the filling growth is also highly reproducible, so that a very high yield can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a semiconductor laser illustrating an embodiment of a buried semiconductor laser according to the present invention, and FIG. 2 is a cross-sectional view of a semiconductor laser illustrating an embodiment of a method for manufacturing a buried semiconductor laser according to the present invention. It is. DESCRIPTION OF SYMBOLS 11... Active region, 12... N-type InP buffer layer, 13... InP substrate, 14... P-type InP cladding layer, 15... P-type InP layer, 16... N-type InP current blocking layer, 17... ...p-type InP cap layer, 1
8... p-type InGaAsP contact layer, 19...
SiO ₂ layer, 20... p-type electrode, 21... n-type electrode,
22...Polyimide layer, 23...Photoresist, 26, 27...Groove.

Claims (1)

[Scope of Claims] 1. A buried semiconductor laser in which an active region is surrounded by a semiconductor 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, wherein the active region The region is located in a bank sandwiched between two grooves, and the groove includes a current-carrying layer having a conductivity type opposite to that of the semiconductor substrate, and a current-blocking layer having the same conductivity type as the substrate on the upper surface of the current-carrying layer. Further, the tip of the embankment and the upper surface of the current blocking layer are covered with a cap layer having a conductivity type opposite to that of the semiconductor substrate, and current insulating layers connected to the current blocking layer are provided on both sides of the cap layer. 1. A buried semiconductor laser comprising an electrically insulating organic film on the upper surface of the current insulating layer except for the upper surface of the cap layer. 2. In a method for manufacturing a buried semiconductor laser in which an active region is surrounded by a semiconductor 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 first step of epitaxially growing a double heterostructure including an active region; a second step of etching the two trenches so that the active region is located in a bank between the two trenches; Using a phase growth method, a current-carrying layer having a conductivity type opposite to that of the semiconductor substrate and a current-blocking layer having the same conductivity type as that of the semiconductor substrate are sequentially laminated on the upper surface of the current-carrying layer in the groove, and further, the tip of the embankment is laminated. and a third step of laminating the cap layer so that the upper surface of the current blocking layer is covered with a cap layer having a conductivity type opposite to that of the semiconductor substrate, and a current insulating layer connected to the current blocking layer on both sides of the cap layer. a fourth step of forming a layer, and a fifth 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 for manufacturing a type semiconductor laser.