JP5524517B2 - Light receiving element - Google Patents

Description

  The present invention relates to an increase in sensitivity of a light receiving element, for example, an effective technique that can be used for a photodiode for signal reception in optical communication.

  Semiconductor light-receiving elements using compound semiconductors are widely used for optical communication elements and the like. An example of the light receiving element for optical communication is an InGaAs PIN photodiode. Here, the PIN photodiode is composed of a p-type semiconductor, an undoped semiconductor, and an n-type semiconductor. When incident light is incident, the light is absorbed by an undoped semiconductor layer to which a reverse bias electric field is applied, converted into electrons and holes, and detected as an electrical signal. With the recent increase in optical communication transmission capacity, it is required to increase the operation speed.

  There are the following two methods for operating the PIN photodiode at high speed. One is to reduce the thickness of the undoped absorption layer. Electrons and holes generated by absorbing light travel (cross) the undoped absorption layer and are detected as current. Therefore, the shorter the traveling distance, the shorter the current is converted into the current, and the higher speed operation becomes possible. The other is to reduce the capacitance of the junction region by reducing the light receiving area (junction area: here the junction area is the junction area between the undoped absorption layer and the n and p doped layers above and below it). Then, the response delay time determined by the product of the capacitance and the resistance is reduced to increase the speed.

  Among the methods for realizing the high speed, the method of thinning the absorption layer deteriorates the light receiving sensitivity. In particular, in a photodiode in which an optical signal is incident perpendicularly to the absorption layer, such as a surface photodiode, the light receiving sensitivity is deteriorated when the thickness of the absorption layer is reduced. As a method for solving the trade-off between the operation speed and the light receiving sensitivity, there is a waveguide type photodiode. In the waveguide type photodiode, an optical signal is incident from the end face of the absorption layer, and light propagates along (in parallel with) the absorption layer. Therefore, in this case, even if the absorption layer is thinned, the deterioration of the light receiving sensitivity can be suppressed small by increasing the propagation distance. However, in the case of a waveguide type photodiode, the optical coupling efficiency between a normal single mode fiber and the waveguide is small, and the light receiving sensitivity (external quantum efficiency) including the coupling efficiency is worse than that of a planar photodiode. As a method for solving this problem, a tip-end fiber can be used. However, when considering practical application, the tip-end fiber is expensive, and the optical alignment tolerance is as small as 1 μm or less. Is difficult to install. Basically, a package called a BOX type is used for a waveguide type photodiode package, which is more expensive than a CAN type applied to a planar photodiode.

  In a planar photodiode, as a method of avoiding the deterioration of the light receiving sensitivity due to the thin absorption layer, the light that has not been absorbed by the absorption layer is reflected by the reflection mirror and passed through the absorption layer again to absorb the absorption efficiency. There is a way to raise. In this case, if the reflectance of the reflecting mirror is 100%, this corresponds to effectively doubling the thickness of the absorbing layer, and high sensitivity can be achieved while maintaining the operation speed.

  When this reflection mirror is used, a back-illuminated type in which light is incident from a substrate is generally used. In Patent Document 1, a highly reflective mirror is used in which a dielectric and a metal (this metal also serves as an electrode) are laminated in this order on a part of the light receiving portion region. If only the metal is used for the reflection mirror, it becomes difficult to achieve good flatness at the interface between the semiconductor and the metal, and the reflection mirror is not good. Therefore, high-quality flatness is realized by sandwiching a dielectric between the semiconductor and the metal. In Patent Documents 2 and 3, a reflection mirror made of a dielectric multilayer film is formed immediately below an electrode. This dielectric multilayer film has a structure in which two pairs of layers made of materials with different refractive indexes are periodically stacked, and is called a distributed Bragg reflector (DBR). . In this structure, the thickness of each layer is λ / 4 / n. Here, λ is the wavelength of incident light, and n is the refractive index. Non-Patent Document 1 has a structure in which a DBR mirror made of a semiconductor multilayer film is formed on a substrate side under a light receiving portion, and light incident from the surface is reflected.

Japanese Patent Laid-Open No. 5-218488 Japanese Patent Laid-Open No. 2001-308369 JP 2002-252366 JP

Journal of Applied Physics 78, 1995, 607-639

  In Patent Document 1, the flatness with a semiconductor is improved by sandwiching a dielectric layer directly under an electrode, and scattering at a semiconductor interface is reduced, so that the reflectance of a reflecting mirror made of a dielectric film and an electrode metal is reduced. Has improved. In this case, basically the reflectance is determined by the reflectance of the electrode metal. However, in practice, titanium (Ti) is inserted between the dielectric and the metal to prevent the electrode from peeling off, so that the reflectivity decreases due to absorption by Ti or the like, and it is not easy to realize a reflectivity of 70% or more. Absent.

  In Patent Documents 2 and 3, the reflection is based only on the DBR structure formed immediately below the electrode. In this case, it is easy to set the reflectance to 70% or more, but 20 layers or more are necessary. In order to make electrical contact with the semiconductor in the light receiving part, it is necessary to form a through hole (hole) in the DBR structure to form a contact region where the electrode and the semiconductor are in direct contact, but the number of layers is large. Then, this through hole formation process becomes difficult. In addition, when the direction of the reflection mirror using the DBR structure (hereinafter referred to as DBR mirror) is inclined with respect to the layer, there is a drawback that the reflectance varies greatly depending on the polarization direction of light.

  In Non-Patent Document 1, since a reflection mirror using a DBR structure is formed on the semiconductor substrate side, the DBR is formed of a semiconductor material. In a semiconductor, films having different refractive indexes can be formed by changing the composition ratio of materials. However, since it is difficult to increase the difference in refractive index, it is necessary to increase the number of layers in order to achieve a high reflectance. When an InP / InAlGaAs material, which is a material for a light receiving element for optical communication, is used, the refractive index difference is about 0.2, and the number of layers of about 40 layers is required to realize a reflectance of 70% or more. Necessary.

  As described in “Problems to be Solved by the Invention”, Patent Document 1 uses reflection of only metal for electrodes. On the other hand, in Patent Documents 2 and 3, only a DBR mirror is used. In order to solve these problems, a structure using reflection from both the electrode metal and the dielectric multilayer film is used. It is possible to increase the reflectivity by aligning the phases of the reflected light from the metal and the dielectric multilayer film. FIG. 1 shows a structural diagram of a reflecting mirror composed of a metal and a two-layer dielectric multilayer film, and FIG. 2 shows an example of a calculation result of the thickness dependence of the dielectric film on the reflectance of the reflecting mirror. Here, the refractive index of the first dielectric 2a is 1.92, the refractive index of the second dielectric 2b is 1.4, the metal film 1 is Ti / Pt / Ti / Pt / Au, and the wavelength of light is 1.31 μm. It can be seen that by optimizing the film thickness of the dielectric (in a state where the phases of the reflected light from the metal and the dielectric multi-layers are aligned), a high reflectivity of 70% or more can be easily obtained. Here, the thicknesses of the first dielectric film and the second dielectric film necessary for realizing high reflectivity are 300 nm and 200 nm, respectively, and are different from the film thickness of each layer of the DBR structure, λ / 4 / n (Λ is the wavelength of light and n is the refractive index). By using this structure, it is possible to realize a mirror having a higher reflectance than the metal-only reflection described in Patent Document 1. In addition, by using this structure, as in Patent Documents 2 and 3, the number of layers of the dielectric multilayer film can be reduced, and a through hole for electrical contact can be easily formed. Since it does not depend on the reflection from only the dielectric multilayer film, the polarization dependency of the reflectance at the time of oblique incidence like a DBR mirror is reduced.

  According to the present invention, a high-speed and high-sensitivity photodiode can be realized in a planar photodiode.

FIG. 2 is a structural diagram of a metal / dielectric multilayer mirror according to the present invention (two dielectric films). The figure which shows the calculation result of the thickness dependence of the dielectric film of the reflectance of the reflective mirror comprised from a metal and a dielectric multilayer film of two layers. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory diagram of an embodiment of a back-incident surface type photodiode having a metal / dielectric multilayer mirror according to the present invention. 1 is a basic structural diagram of a metal / dielectric multilayer mirror according to the present invention. The figure which shows the calculation result of the dependence of two types of dielectric film thicknesses of the reflectance of a metal and dielectric multilayer film mirror when a dielectric multilayer film is comprised from two different dielectric films. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory diagram of an embodiment of a back-incident surface type photodiode having a metal / dielectric multilayer mirror and an integrated lens on a substrate side according to the present invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory diagram of an embodiment of a surface incident surface type photodiode having a metal / dielectric multilayer mirror according to the present invention and an optical path changing mirror formed on a substrate side. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory diagram of an embodiment of a surface incident surface type photodiode having a metal / dielectric multilayer mirror according to the present invention, an optical path changing mirror formed on a substrate side, and an integrated lens on the opposite side of the substrate. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory view of an embodiment of an end face incident type photodiode having a metal / dielectric multilayer mirror, a substrate side, and an optical path conversion mirror formed on the opposite side of the substrate according to the present invention.

Hereinafter, examples will be described in detail with reference to the drawings.
Example 1
With reference to FIGS. 3 and 4, an embodiment of a back-illuminated surface type photodiode having a reflection mirror composed of a metal and a dielectric multilayer film in the present invention will be described.

  The photodiode according to the present invention includes an n-type InP n-contact layer 7, an n-type InGaAlAs n-clad layer 8, an undoped InGaAs light absorption layer 9, and a p-type InGaAlAs formed on a substrate 6 (for example, an InP substrate). The light receiving element is composed of a p-clad layer 10, a p-type InGaAs p-contact layer 11, a metal / dielectric multilayer mirror 12, a p-electrode 13, and an n-electrode 14. Incident light 5 incident from the substrate (back surface) side is once absorbed by the light absorption layer 9, and the light that has not been absorbed is reflected by the metal / dielectric multilayer mirror 12 and absorbed by the light absorption layer 9 again. Is done. Although a PIN photodiode is described here, the light receiving portion may have an avalanche photodiode structure.

The metal / dielectric multilayer mirror 12 used here includes a dielectric multilayer film 3 and an electrode metal film 1 as shown in FIG. The metal film for electrodes is, for example, Ti / Pt / Ti / Pt / Au. FIG. 5 shows two kinds of dielectrics of reflectivity of the metal / dielectric multilayer mirror when the metal / dielectric multilayer mirror 12 is composed of two different dielectric films. The calculation result of the dependence of body thickness is shown. Here, a metal / dielectric multilayer mirror was assumed in which a SiNx film, a SiO2 film, and a Ti / Pt / Ti / Pt / Au layer were stacked on the p contact layer 11. The refractive indexes of the SiNx film and the SiO2 film were set to 1.92 and 1.4, respectively. The wavelength of light was assumed to be 1.31 μm. From this result, in the figure, points A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, and A11 in the white line formed by connecting the points in this order, and B0, B1 , B2, B3, B4, B5, a region 2 in the white line formed by connecting the points in this order can achieve a high reflectance of 70% or more. This condition can be described in a more general form by applying the light scaling law: Now, on the p-contact layer 11, a first dielectric film having a refractive index n1, a second dielectric film having a refractive index n2, and a metal / dielectric multilayer laminated in the order of Ti / Pt / Ti / Pt / Au Assume a membrane mirror. Let λ be the wavelength of light. In this case, the points A0 to A11 in FIG. 5 are A0 = (0, 500a), A1: (0, 440a), A2: (115b, 295a), A3: (185b, 235a), A4: ( 275b, 195a), A5: (425b, 150a), A6: (500b, 60a), A7: (500b, 175a), A8: (445b, 200a), A9: (300b, 400a), A10: (220b, 440a), A11: (100b, 500a), and B0-B5 points are B0: (0, 190a), B1: (0, 100a), B2: (75b, 0), B3: (345b , 0), B4: (250b, 100a), B5: (160b, 140a). Here, a = (1.92 / n1) × (λ / 1.31 μm), b = (1.4 / n2) × (λ / 1.31 μm), and λ is the wavelength of light. By using the combination of the dielectric films in the regions 1 and 2 in FIG. 5, a high reflectance of 70% or more can be realized. A point indicated by a white triangle in FIG. 5 is a DBR condition, that is, each film thickness satisfies λ / 4 / n. In this case, it is understood that when the DBR condition is satisfied as in Patent Document 2 or 3, high reflectance cannot be obtained.

  The photodiode according to the present invention includes an n-type InP n-contact layer 7, an n-type InGaAlAs n-clad layer 8, and an undoped InGaAs light-absorbing layer 9 on a substrate 6 (for example, an InP substrate) on which an integrated lens 15 is formed. This is a light receiving element comprising a p-type InGaAlAs p-cladding layer 10, a p-type InGaAs p-contact layer 11, a metal / dielectric multilayer mirror 12, a p-electrode 13 and an n-electrode 14. Incident light 5 incident from the substrate side is collected by the integrated lens 15 and incident on the light absorption layer 9 to be absorbed. Thereafter, the light that has not been absorbed is reflected by the metal / dielectric multilayer mirror 12 and again absorbed by the light absorption layer 9. Although a PIN photodiode is described here, the light receiving portion may have an avalanche photodiode structure. The metal / dielectric multilayer mirror 12 used here is a high reflection mirror composed of the metal film and the dielectric multilayer film described in the first embodiment.

The integrated lens 15 can reduce the beam spot size when incident on the light receiving portion, and can greatly improve the optical coupling tolerance. Further, the high-reflection film region composed of the metal / dielectric multilayer film described in Example 1 except for the p-contact region 16 (the portion where the p-contact layer 11 and the p-electrode 13 are in contact), which is a low-reflectance region. By concentrating the beam on the surface, it is possible to prevent a decrease in reflectance.
(Example 3)
7 and 8, in the present invention, a front-illuminated surface type photodiode composed of a reflection mirror composed of a metal and a dielectric multilayer film and two optical path conversion mirrors formed on the back surface of the substrate The embodiment will be described.

  The photodiode according to the present invention includes an n-type InP n-contact layer 7, an n-type InGaAlAs n-clad layer 8, an undoped InGaAs light on a substrate 6 (for example, an InP substrate) on which two optical path conversion mirrors 17 are formed. This is a light receiving element including an absorption layer 9, a p-type InGaAlAs p-clad layer 10, a p-type InGaAs p-contact layer 11, a metal / dielectric multilayer mirror 12, a p-electrode 13, and an n-electrode 14. Incident light 5 incident from the surface on the opposite side of the substrate has its optical path converted by an optical path converting mirror 17 formed on the substrate, and is incident on the light absorption layer 9 from the substrate side and absorbed. Thereafter, the light that has not been absorbed is reflected by the metal / dielectric multilayer mirror 12 and again absorbed by the light absorption layer 9. Although a PIN photodiode is described here, the light receiving portion may have an avalanche photodiode structure. The metal / dielectric multilayer mirror 12 used here is a high reflection mirror composed of the metal film and the dielectric multilayer film described in the first embodiment.

Further, as shown in FIG. 8, in addition to the structure shown in FIG. 7, a structure in which the integrated lens 16 is formed on the opposite side of the substrate is also conceivable. In this case, the integrated lens 15 can reduce the beam spot size at the time of incidence on the light receiving portion, and can greatly improve the optical coupling tolerance. Further, the high-reflection film region composed of the metal / dielectric multilayer film described in Example 1 except for the p-contact region 16 (the portion where the p-contact layer 11 and the p-electrode 13 are in contact), which is a low-reflectance region. By concentrating the beam on the surface, it is possible to prevent a decrease in reflectance.
Example 4
With reference to FIG. 9, an embodiment of an end face incident type photodiode constituted by a reflection mirror constituted by a metal and a dielectric multilayer film and five optical path conversion mirrors formed on the back surface of the substrate in the present invention will be described. explain.

  The photodiode according to the present invention includes an n-type InP n-contact layer 7, an n-type InGaAlAs n-cladding layer 8, an undoped InGaAs light on a substrate 6 (for example, an InP substrate) on which five optical path conversion mirrors 17 are formed. This is a light receiving element comprising an absorption layer 9, a p-type InGaAlAs p-clad layer 10, a p-type InGaAs p-contact layer 11, a metal / dielectric multilayer mirror 12, a p-electrode 13 and an n-electrode 14. The incident light 5 incident from the substrate end surface is converted in its optical path by the optical path converting mirror 17 formed on the substrate side and the opposite side of the substrate, and is incident on the light absorption layer 9 from the substrate side and absorbed. Thereafter, the light that has not been absorbed is reflected by the metal / dielectric multilayer mirror 12 and again absorbed by the light absorption layer 9. Although a PIN photodiode is described here, the light receiving portion may have an avalanche photodiode structure. The metal / dielectric multilayer mirror 12 used here is a high reflection mirror composed of the metal film and the dielectric multilayer film described in the first embodiment.

The surface-type high-speed and high-sensitivity photodiode is suitable for an inexpensive package as compared with the waveguide-type photodiode that easily satisfies high-speed and high-sensitivity.
Furthermore, since the optical alignment tolerance is large when the module is assembled, the assembly is easy and the assembling cost can be reduced.
As the optical communication capacity increases, the integration of optical devices in an array becomes an important technology as the optical communication system with multiple channels (multiple wavelengths) advances. In the case of a photodiode, compared to a waveguide photodiode, a planar photodiode is suitable for planar arraying and is advantageous in terms of integration.

DESCRIPTION OF SYMBOLS 1 ... Metal film, 2a ... 1st dielectric film, 2b ... 2nd dielectric film, 3 ... Dielectric multilayer film, 4 ... Semiconductor, 5 ... Incident light, 6 ... Substrate, 7 ... n contact layer, 8 ... n Cladding layer, 9 ... light absorption layer, 10 ... p cladding layer, 11 ... p contact layer, 12 ... metal / dielectric multilayer mirror, 13 ... p electrode, 14 ... n electrode, 15 ... integrated lens, 16 ... p contact Area, 17 ... Mirror for optical path conversion.

Claims (5)

In the light receiving element in which a light receiving portion region is provided on the first main surface of the substrate and light enters from the second main surface side facing the first main surface,
A plurality of dielectric films having different dielectric constants between adjacent layers at a part of a light receiving portion region provided on the first main surface, and a side farther than the first main surface of the plurality of dielectric films metal film have a reflecting mirror which reflects the light provided on the main surface,
The light receiving region has an absorption layer that absorbs light incident from the second main surface side,
The reflection mirror is provided at a position that has passed through the absorption layer,
The plurality of dielectric films are in order of a first dielectric film having a first dielectric constant and a second dielectric film having a dielectric constant different from the first dielectric constant from a side closer to the absorption layer. Laminated,
A refractive index n1 of the first dielectric film is greater than a refractive index n2 of the second dielectric film;
The combination of the film thicknesses of the first dielectric film and the second dielectric film is on a two-dimensional plane (X, Y) with these film thicknesses as variables.
(X, Y) is (0, 500a), (0, 440a), (115b, 295a), (185b, 235a), (275b, 195a), (425b, 150a), (500b, 60a), ( 500b, 175a), (445b, 200a), (300b, 400a), (220b, 440a), (100b, 500a)
The points (X, Y) are (0, 190a), (0, 100a), (75b, 0), (345b, 0), (250b, 100a), (160b, 140a) A light receiving element, wherein the light receiving element is located in any one of the second regions surrounded by the formed line. (Here, a = (1.92 / n1) × (λ / 1.31 μm), b = (1.4 / n2) × (λ / 1.31 μm), and λ [μm] is the wavelength of light.) The light receiving element according to claim 1,
The light receiving element, wherein the first dielectric film is a SiNx film, and the second dielectric film is a SiO2 film. In the light receiving element according to claim 1 or 2 ,
A condensing lens provided on the first main surface;
The light receiving element, wherein the light receiving region receives incident light that has passed through the condenser lens. In the light receiving element according to claim 1 or 2 ,
Comprising two reflecting mirrors provided on the first main surface;
The light incident from the second main surface side is reflected by one of the two reflection mirrors, and the optical path of the light is converted in a direction substantially orthogonal to the normal line of the second main surface, and the two reflection mirrors The light receiving element, which is incident on the light receiving portion region while being further converted in the direction of the normal line of the second main surface by another reflection mirror. In the light receiving element according to claim 1 or 2 ,
A plurality of first reflecting mirrors provided on the first main surface;
A plurality of second reflecting mirrors provided on the second main surface,
Light incident from the side end surface of the substrate is reflected by one of the plurality of first reflection mirrors, and the optical path of the light is converted in the normal direction of the second main surface, and the plurality of second reflection mirrors Further, the optical path is further changed in a direction substantially orthogonal to the normal line of the second main surface, and alternately alternates between the other one of the plurality of first reflection mirrors and the other one of the plurality of second reflection mirrors. A light receiving element that is incident on the light receiving portion region while repeating reflection.

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