JP2005129628A - Light receiving element and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ultra high-speed light receiving element by narrowing the width of the light receiving element in an evanescent coupling optical waveguide-type and reducing a junction capacity. <P>SOLUTION: In the evanescent coupling joint optical waveguide-type light receiving element, the width of a slab-type optical waveguide formed on the surface of a substrate is made to become narrow gradually toward an upper part. The width of a light absorption layer which is formed on the optical waveguide and to which an evanescent light is led is formed so as to be matched with a part of the narrowest optical waveguide width. An electrode is formed on the light absorption layer. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an improvement in the cut-off frequency of an ultrahigh-speed light receiving element that performs optical-electrical (O / E) signal conversion used in an optical communication system and an optical communication measuring instrument.

  In recent years, the transmission capacity of optical fiber communication has been dramatically increased by a WDM (Wavelength / Division / Multiplexing) optical communication technology having a transmission rate per channel of several Gbps (bit per second) to several tens of Gbps. An ultrahigh-speed light receiving element has been developed as a photoelectric conversion element used in such an optical communication system.

Prior art documents related to such a conventional light receiving element include the following.
Japanese Patent Laid-Open No. 04-241471 L. Giraude et al. : Electron · Lett · 37, pp. 973-975 (2001) T. Takeuchi et al. : Electron 36, pp. 1-2 (2000) Toshitaka Torikai: OPTORONICS, No. 1, pp. 108-112 (2003)

  FIGS. 6A, 6B and 6C are diagrams showing a conventional example of a photodiode having a pin structure having InGaAs as a light absorption layer. As shown in the figure, semiconductor light-receiving elements used in optical fiber-optical communication and the like are roughly classified into a front-incident type (FIG. 6a) and an end-face incident type (FIGS. 6b and 6c) depending on the light incident direction to the light-receiving element. Is done.

In the front-illuminated type of FIG. 6A, a buffer 2 made of n-InP is formed on an n ⁻ InP substrate, and an n-InGaAs layer 3 and a P ⁺ InGaAs layer 4 as light absorption layers are formed on the buffer 2. The light 8 is incident from a direction perpendicular to one plane. In this case, the electrodes 5 and 6 are formed on the front surface and the back surface of the substrate.

  6b and 6c are those in which the incident light 8 is incident on the optical waveguide 9 from a direction parallel to the substrate 1, and the incident light is absorbed into the light absorption layer 10 by the evanescent effect to be photoelectrically converted. It is a thing.

The cutoff frequency of the response frequency characteristic in such a light receiving element is
1) Product of element resistance R and element capacitance C (so-called CR time constant),
2) Carriers in the light absorption layer of photogenerated carriers of hole-electron pairs generated by light absorption in the light absorption layer (n or i-InGaAs layer in FIGS. 6a and 6b, N-Type InGaAs layer in FIG. 6c). running time,
Determined by

For this reason, in the past, attempts have been made to obtain a smaller CR product by reducing the electrode contact resistance, reducing the element area, and reducing the junction capacitance.
On the other hand, in order to shorten the carrier traveling time, it is necessary to make the light absorption layer 10 thin. However, in the surface incident type shown in FIG. 6A, when the light absorption layers 3 and 4 become thin, incident light is not absorbed by the light absorption layer. The amount of light passing therethrough increases and sensitivity decreases.

In order to obtain the internal quantum efficiency of about 80% with the front-illuminated type, the thickness of the light absorption layer needs to be about 2.5 μm. Therefore, in recent years, an end face incident type as shown in FIG. 6b has been developed.
In FIG. 6b, since the light incident from the side surface is confined in the optical waveguide 9 and absorbed by the light absorption layer 10, the thickness of the light absorption layer sufficient (several μm) in the light traveling direction can be obtained. it can.

Therefore, sufficient sensitivity can be obtained even if the thickness of the light absorption layer 10 is 0.5 to 1 μm by the light confinement structure, and at the same time, the traveling time of the carrier traveling in the thickness direction can be reduced.
However, it is known that in the vicinity of the light receiving surface, the light generation carrier concentration is the highest due to light absorption, and the light incident surface of the light receiving element is damaged when the light intensity is high.

  In order to improve the occurrence of damage on the incident surface, a structure has been proposed in which an optical waveguide and a light receiving element are integrated, and an evanescent wave from the optical waveguide is guided to the light absorption layer of the light receiving element portion to reduce the maximum light intensity.

  In the conventional example shown in FIG. 6c, in order to shorten the carrier traveling time, the light absorption layer 10 (for example, i-InGaAs layer) of the light receiving element portion 10a on the optical waveguide 9 is thin (for example, about 0.3 to 0.6 μm). Thickness) is formed.

  When the i layer of the light receiving element portion 10a having the pin structure is thinned, the junction capacitance increases. For this reason, the main limiting factor of the cutoff frequency of the response frequency characteristic of the light receiving element is the CR product again. Therefore, it is necessary to reduce the junction area of the light absorption layer 10 so as to reduce the junction capacitance of the light receiving element portion 10a.

As shown in FIG. 6c ′, the guided (incident) light propagating through the optical waveguide 9 is guided to and absorbed by the light absorption layer 10 by evanescent coupling. At this time, for example, optical communication in a wavelength band of 1.5 μm In order to achieve a quantum efficiency of about 40% or more of a light receiving element having practical sensitivity in the wavelength region, the length of the light receiving element portion 10a in the waveguide direction needs to be about 20 μm or more.
Therefore, in order to reduce the junction capacitance of the light receiving element portion 10a, the width of the light receiving element portion 10a is reduced.

  However, in the conventional example (FIGS. 6b and 6c), the width h of the optical waveguide 9 and the width h ′ of the light receiving element portion 10a are made substantially the same. This is because dry etching processing is used to form the shape of the light receiving element portion material (InP, InGaAs, etc.) and the waveguide portion material (InGaAsP, etc.), and the difference in processing shape due to the difference in material system is reduced. This is because a method is employed in which the side surfaces of the portion and the waveguide portion are processed substantially vertically.

Therefore, an active structure for reducing the light receiving element area has not been proposed. In addition, a structure for reducing the junction capacitance of the light receiving element portion by specifically narrowing the width of the light receiving element portion on the optical waveguide and the manufacturing method itself have not been designed or manufactured.
Therefore, the problem to be solved by the present invention is to realize an ultrahigh speed light receiving element by reducing the junction capacitance by narrowing the width of the light receiving element portion of the evanescent coupling optical waveguide type light receiving element.

In order to achieve such a problem, the invention according to claim 1 of the present invention is:
In an evanescent-coupled optical waveguide type light receiving element, the slab type optical waveguide formed on the surface of the substrate is formed so that the width of the slab optical waveguide becomes gradually narrower upward, and the light absorption is formed on the optical waveguide to guide the evanescent light. The layer was formed so as to coincide with the narrowest portion of the optical waveguide width, and an electrode was formed on the light absorption layer. The propagation loss of the optical waveguide is reduced.

The invention according to claim 2 of the present invention is the light receiving element according to claim 1,
The optical waveguide is formed of a plurality of layers of InGaAsP on an InP substrate, and the refractive index of each layer is formed so as to increase sequentially from the substrate side toward the upper layer. A light absorption layer and a cap layer are formed on the uppermost InGaAsP layer. Formed. Evanescent effect can be obtained efficiently.

The invention according to claim 3 of the present invention is the light receiving element according to claim 1 or 2,
The light absorption layer was an InGaAs layer, and the cap layer was InP. The propagation loss of the optical waveguide is reduced.

The invention according to claim 4 of the present invention is
1) A step of laminating a plurality of InGaAsP layers having different refractive indexes on an InP substrate so that the refractive index becomes higher.
2) forming an InGaAs layer on the uppermost InGaAsP layer;
3) forming an InP layer on the InGaAs layer;
4) forming an insulating film on the InP layer;
5) A process in which InGaAs is etched using an etchant having a higher etching rate than InGaAsP using the insulating film as a mask. It is characterized by including.

As is apparent from the above description, the present invention has the following effects.
According to the first to fourth aspects of the invention, the width of the slab type optical waveguide formed on the surface of the substrate is formed so as to be gradually narrowed upward, and the evanescent light is formed on the optical waveguide. The light absorption layer to be guided is formed so as to coincide with the narrowest portion of the optical waveguide width, and an electrode is formed on the light absorption layer to reduce the junction capacitance of the light receiving element portion. Was formed of InGaAsP with a plurality of layers on an InP substrate, and the refractive index of each layer was formed so as to increase sequentially from the substrate side toward the upper layer, and a light absorption layer and a cap layer were formed thereon. As a result, an evanescent effect was efficiently obtained, and a light receiving element with reduced propagation loss of the optical waveguide could be realized.

  Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a sectional view showing an embodiment of a light receiving element according to the present invention. The plan view is the same as the conventional example shown in FIG.

In FIG. 1, reference numeral 1 denotes a semiconductor substrate made of InP, on which an optical waveguide layer 9, a light absorption layer 10, and PN electrodes 5 and 6 are formed.
2, 3, and 4 are photomicrographs showing examples of the present invention. As shown in FIG. 1, the basic structure is an evanescent coupled optical waveguide type light receiving element structure as in the conventional example.

  2A shows a cross section AA ′ of the optical waveguide portion of FIG. 1, and three InGaAsP layers (InGaAsP (3), InGaAsP (2), InGaAsP (1)) are sequentially formed on the InP substrate 1. FIG. The slab type optical waveguide is formed by etching the top layer InGaAsP (1) so that the etching side surface is inclined.

  The InGaAsP layers (3), (2), and (1) are formed so that the refractive index increases as they go upward, so that incident light oozes out sequentially due to the evanescent effect. FIG. 2B shows a calculation result of the light intensity distribution propagating through the optical waveguide.

  FIG. 3A shows a BB ′ cross section of the optical waveguide portion 10a of the light receiving element of FIG. An InGaAs layer (thickness to 0.5 μm) and an InP cap layer (thickness to 0.1 μm) are formed as the light absorption layer 10 on the uppermost InGaAsP (1) of the optical waveguide 9 shown in FIG.

According to such a configuration, the evanescent light of the waveguide light propagating through the optical waveguide portion 9 reaches the InGaAs light absorption layer 10 and is sufficiently long (˜30 μm) of the light receiving element portion 10a having the InGaAs light absorption layer 10. When passing through the distance, almost all of the waveguide light 8 is absorbed, and a photoelectric signal converted from light to electricity is obtained.
When the light intensity distribution (although very weak) of the light receiving element portion 10a after a sufficiently long distance is calculated, it is as shown in FIG. 3B, and it can be seen that the light is propagated in the InGaAs light absorption layer 10.

  FIG. 4 is a cross-sectional secondary view after the optical waveguide 9 and the light receiving element portion 10a are formed by etching the light receiving element portion 10a when the light receiving element is actually manufactured with the structure of the embodiment (FIG. 1: BB ′ cross section). It is an electron microscope image. The structure shown in FIG. 3 is realized.

Next, a method for manufacturing such a light receiving element will be described.
A three-layer optical waveguide made of InGaAsP material is formed on the InP substrate by MOVPE (metal organic vapor phase epitaxy). A refractive index is formed such that InGaAsP (1)> InGaAsP (2)> InGaAsP (3), and an InGaAs light absorption layer 10 and an InP cap layer 11 are formed thereon.

Next, by using a chemical etching solution (for example, a mixed solution of phosphoric acid, sulfuric acid, and hydrogen peroxide water) having a higher etching rate than InGaAsP and etching using the SiO ₂ film as a mask, FIG. The light receiving element having the structure shown can be realized. A p + InGaAs region is formed by Zn diffusion to a depth of about 0.1 μm in the n-InGaAs layer of the light absorption layer through the InP cap layer of the light receiving element portion, and a pn junction is formed as shown in FIG. An electrode 5 is provided.

  As shown in the structure of the actual embodiment of FIG. 4, in the evanescent coupling optical waveguide type light receiving element formed so that the width of the optical waveguide becomes narrower toward the InGaAs light absorption layer of the light receiving element formed thereon, The evanescent light from the optical waveguide can be guided to the InGaAs light absorption layer 10 without leaking to the outside and being attenuated.

  At the same time, since the width of the PN junction formed in the InGaAs layer can be made narrower than the width of the etched optical waveguide layer 9 of the slab type optical waveguide, the junction capacitance can be reduced. The bottom width of the uppermost InGaAsP (1) layer of the optical waveguide layer to be etched is about 6 to 10 μm. At this time, the width of the InGaAs light absorption layer can be controlled to about 4 to 8 μm. 50% to 80%.

  Accordingly, the junction capacity can be reduced to 50 to 80% compared to the case of the conventional vertical etching method.

FIG. 5 shows the calculation result of the relationship between the cutoff frequency of the light receiving element and the thickness of the n (i) -InGaAs light absorption layer. For example, when the thickness of the light absorption layer is 0.3 μm and the light receiving element area (junction area) is 80 μm ² (width 4 μm × length 20 μm) and 150 μm ² (width 7.5 μm × length 20 μm), that is, about 50 It can be seen that the cut-off frequency changes by about 10 GHz when changed by ~ 80%.

  The above description merely shows a specific preferred embodiment for the purpose of explanation and illustration of the present invention. For example, in the embodiment, the InGaAsP layer of the optical waveguide is three layers, but may be two layers or three or more layers. Therefore, the present invention is not limited to the above-described embodiments, and includes many changes and modifications without departing from the essence thereof.

It is sectional drawing which shows one Example of the light receiving element which concerns on this invention. It is a microscope picture which shows the cross section of the optical waveguide of the light receiving element which concerns on this invention. It is a microscope picture which shows the cross section of the light receiving element part of the light receiving element which concerns on this invention. It is a microscope picture which shows the cross section of the state which formed the electrode on the optical waveguide of the light receiving element which concerns on this invention. It is a calculation result which shows the relationship between the cutoff frequency of a light receiving element, and n (i) -InGaAs light absorption layer thickness. It is sectional drawing which shows an example of the conventional light receiving element.

Explanation of symbols

1 substrate 2 n-InP (buffer)
3 n-InGaAs layer 4 P ⁺ InGaAs layer 5 P electrode (Au—Zn)
6 N electrode (Au-Sn)
7 SiN
8 Incident light 9 Optical waveguide 9a Optical waveguide portion 10 Light absorption layer 10a Light receiving element portion 11 InP layer

Claims (4)

  In an evanescent-coupled optical waveguide type light receiving element, the slab type optical waveguide formed on the surface of the substrate is formed so that the width of the slab optical waveguide becomes gradually narrower upward, and the light absorption is formed on the optical waveguide to guide the evanescent light. A light receiving element, wherein the width of the layer is formed to coincide with the narrowest portion of the optical waveguide width, and an electrode is formed on the light absorption layer.   The optical waveguide is formed of a plurality of layers of InGaAsP on an InP substrate, and the refractive index of each layer is formed so as to increase sequentially from the substrate side toward the upper layer. A light absorption layer and a cap layer are formed on the uppermost InGaAsP layer. The light receiving element according to claim 1, wherein:   3. The light receiving element according to claim 1, wherein the light absorption layer is an InGaAs layer and the cap layer is InP. 1) A step of laminating a plurality of InGaAsP layers having different refractive indexes on an InP substrate so that the refractive index becomes higher.
2) forming an InGaAs layer on the uppermost InGaAsP layer;
3) forming an InP layer on the InGaAs layer;
4) forming an insulating film on the InP layer;
5) A process in which InGaAs is etched using an etchant having a higher etching rate than InGaAsP using the insulating film as a mask. The manufacturing method of the light receiving element characterized by including.

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