JP2005286000A - Photodetector and avalanche photodiode - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a photodetector and an avalanche photodiode which are capable of suppressing dark current in the vicinity of mesa side surface, and improving the reliability of the detector without effecting the re-growth of crystal. <P>SOLUTION: The avalanche diode 30 is constituted of a plurality of semiconductor layers comprising a buffer layer 2, a light absorbing layer 3, an electric field mitigating layer 4, a multiplication layer 5, and a contact layer 6 which are laminated in a mesa structural unit formed on a semiconductor substrate 1. In such an avalanche photodiode 30, the contact layer 6 is laminated as the remotest layer mostly separated from the semiconductor substrate among a plurality of semiconductor layers and the surface of a nextly remote layer (the multiplication layer 5) is constituted in the vicinity of an interface between the nextly remote layer (the multiplication layer 5) contacted with the remotest layer so as to be larger than the surface of the contact layer 6 and so as to have a part without contacting with the contact layer 6. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a light receiving element and an avalanche photodiode, and more particularly to a light receiving element and an avalanche photodiode for converting an optical signal into an electric signal in an optical communication network or the like.

  In recent years, in optical fiber transmission systems, there has been an increasing demand for higher speed and higher capacity of optical signal transmission. In order to meet the demand for higher speed and larger capacity, it is necessary to increase the operation speed of the light receiving element used in the optical fiber transmission system. Here, an avalanche photodiode with high detection sensitivity is widely used in an optical receiver for optical communication for medium and long distances so that a weak optical signal can be detected.

  Avalanche photodiodes can be broadly classified into planar avalanche photodiodes in which elements are formed almost flat on a substrate surface and mesa type avalanche photodiodes in which elements having a mesa structure are formed on a substrate. . A planar avalanche photodiode has a medium (semiconductor part) with a high dielectric constant around the element and is affected by the capacitance of this medium. Has a large capacitance and slow operation speed. For this reason, mesa-type avalanche photodiodes are used as light receiving elements that operate at high speed.

However, in the mesa type light receiving element, a number of local energy levels different from the energy level of the bulk crystal are present in the vicinity of the mesa side surface due to a number of crystal defects existing on the mesa side surface. Due to the presence of such local energy levels, dark current, noise, and the like that do not contribute to the signal current are generated at the mesa interface, and the device performance is greatly adversely affected. As a technique for reducing dark current, noise, etc. on the mesa side, the shape of the electric field relaxation layer is made such that the electric field strength at the center of the multiplication layer is higher than the electric field strength near the mesa side, Some attempt to efficiently multiply carriers in the center of the multiplication layer (see, for example, Patent Document 1).
JP-A-8-181349

  However, in such a conventional mesa-type light receiving element, in order to form the electric field relaxation layer having the shape as described above, the crystal growth is temporarily stopped and processing for obtaining this shape is performed. Therefore, there has been a problem that crystal re-growth must be performed, crystallinity at the re-growth interface is impaired, dark current, noise, etc. are increased, and device reliability is lowered.

  The present invention has been made to solve such a problem, and it is possible to suppress the dark current in the vicinity of the side surface of the mesa and to improve the reliability of the device without performing crystal regrowth. An element and an avalanche photodiode are provided.

  In view of the above points, the invention according to claim 1 is a light receiving element in which a plurality of semiconductor layers including a contact layer for obtaining ohmic contact are stacked in a mesa structure formed on a semiconductor substrate. The contact layer is stacked as the furthest layer farthest from the semiconductor substrate among the plurality of semiconductor layers, and the surface of the next layer is in the vicinity of the interface with the next layer contacting the outermost layer. And having a portion larger than the surface of the contact layer and not in contact with the contact layer.

  With this configuration, in the vicinity of the interface with the next delamination layer that is in contact with the outermost delamination layer, the surface of the next delamination layer is larger than the surface of the contact layer and has a portion that does not contact the contact layer. The current is concentrated in the center of the mesa, reducing the photocurrent and dark current flowing near the side of the mesa without reducing the contact layer smaller than the next delamination, and reducing the reliability of the device. A light receiving element that can be improved can be realized.

  According to a second aspect of the present invention, a mesa structure formed on a semiconductor substrate absorbs incident light to generate carriers, and among the carriers generated by the light absorption layer. In the avalanche photodiode in which a plurality of semiconductor layers including a multiplication layer for multiplying at least one carrier as a main carrier and a contact layer for obtaining an ohmic contact are stacked, the contact layer includes a plurality of the semiconductors In the vicinity of the interface with the next release layer in contact with the most separated layer, the surface of the next release layer is larger than the surface of the contact layer. The structure has a portion that does not contact the contact layer.

  With this configuration, in the vicinity of the interface with the next separation layer in contact with the outermost separation layer, the surface of the next separation layer is larger than the contact layer surface and has a portion that does not contact the contact layer. The current is concentrated in the center of the mesa, reducing the photoreception current and dark current flowing near the mesa side without reducing the contact layer smaller than the next delamination, and reducing the reliability of the device. An avalanche photodiode that can be improved can be realized.

  The invention according to claim 3 is the structure according to claim 2, wherein holes in the electron-hole pair generated in the light absorption layer serve as main carriers and are multiplied in the multiplication layer. have.

  According to this configuration, in addition to the effect of claim 2, the electron hole pair generated in the light absorption layer becomes a main carrier, and a multiplication layer having excellent crystallinity that is not a superlattice layer is provided. As a result, it is possible to realize an avalanche photodiode capable of further improving the reliability of the element as compared with an avalanche photodiode using electrons as a main carrier.

  According to a fourth aspect of the present invention, in the second aspect of the present invention, the optical signal incident on the light absorption layer is incident from the rear surface side of the semiconductor substrate, which is the surface opposite to the surface on which the semiconductor layer is formed. It has the structure made to do.

  With this configuration, in addition to the effect of the second aspect, since the optical signal is incident on the light absorption layer from the back side, the incident light is reflected and reflected by the p electrode after passing through the light absorption layer and absorbed again. Since it is absorbed by the layer, an avalanche photodiode having high sensitivity can be realized.

  In the present invention, in the vicinity of the interface with the next separation layer in contact with the outermost separation layer, the surface of the next separation layer is larger than the surface of the contact layer and has a portion not in contact with the contact layer. The current concentrates in the center of the mesa, and the light receiving element has an effect that the light receiving current and dark current flowing near the side of the mesa can be suppressed and the reliability of the element can be improved without performing crystal regrowth. And an avalanche photodiode can be provided.

  Hereinafter, embodiments of the present invention will be described by taking an avalanche photodiode as an example of a light receiving element. FIG. 1 is a schematic cross-sectional view showing an example of an avalanche photodiode according to an embodiment of the present invention.

In Figure 1, the avalanche photodiode ^30, n + on the semiconductor substrate 1 (100) plane consisting ^-InP, the light absorbing layer 3 made of the buffer layer 2, i-InGaAs consisting n + -InP, ^{n +} field relaxation layer ⁴ composed of -InP, p - multiplication layer 5 made of -InP, ^and a contact layer 6 made of p + -InGaAs has a stacked layer structure in this order.

  Here, of the semiconductor layers 2 to 6, the contact layer 6 stacked most distant from the semiconductor substrate 1 is referred to as a most separated layer, and the multiplication layer 5 in contact with the most separated layer is referred to as a next separated layer. In the layer configuration shown in FIG. 1, the multiplication layer 5 is laminated so as to be in contact with the contact layer 6, but a layer configuration in which a known block layer or the like is interposed between the multiplication layer 5 and the contact layer 6. In this case, among the layers interposed between the multiplication layer 5 and the contact layer 6, the layer in contact with the contact layer 6 that is the most separated layer is referred to as the next separation layer. The contact layer 6 is not limited to a single layer, and may be composed of a plurality of layers as long as an ohmic contact can be obtained. In this case, a plurality of layers constituting the contact layer 6 are used as the outermost layer. For convenience of explanation, in the following, semiconductor layers are stacked as shown in FIG. 1, and the multiplication layer 5 is referred to as a next delamination.

  Here, the avalanche photodiode 30 is configured such that holes in the electron-hole pairs generated in the light absorption layer 3 become main carriers and are multiplied in the multiplication layer 5. Further, it is known that the i-InGaAs film is usually a low concentration n-type, and a pn junction is formed at the interface between the electric field relaxation layer 4 and the multiplication layer 5.

  A semiconductor film (hereinafter referred to as an epitaxial film) for each of the layers 2 to 6 is formed using a technique such as MOVPE (Metal Organic Vapor Phase Epitaxy) or MBE (Molecular Beam Epitaxy). Further, for example, Sn or S may be used as the n-type impurity, and Zn or the like may be used as the p-type impurity.

The thicknesses of the epitaxial films for the layers 2 to 6 are, for example, 0.5 to 1 μm, 0.5 to 2 μm, 0.01 to 0.1 μm, and 0 from the semiconductor substrate 1 side toward the contact layer 6, respectively. .1 to 1 μm and 0.1 to 0.5 μm. The impurity concentrations of the buffer layer 2, the electric field relaxation layer 4, the multiplication layer 5, and the contact layer 6 are 1 × 10 ¹⁸ (cm ⁻³ ), 5 × 10 ¹⁷ (cm ⁻³ ), and 5 × 10 respectively. ¹⁶ (cm ⁻³ ) and 5 × 10 ¹⁸ (cm ⁻³ ). Note that these values of film thickness and impurity concentration are examples, and are not limited to these. Further, the impurity concentration of the contact layer 6 may be further increased so that ohmic contact can be easily obtained.

  The mesa structure of the avalanche photodiode 30 is formed in a truncated cone shape as shown in FIG. 1, but the mesa structure is not limited to a predetermined shape. In the following description, the formation of the contact layer 6 will be described with reference to the drawings, taking a truncated conical forward mesa structure as an example, with the contact layer 6 having a circular shape. FIG. 2 is a process diagram for explaining a method of forming the contact layer 6. FIG. 2 shows an example in which the epitaxial film includes the block film L7, but the block film L7 is not an essential epitaxial film.

First, as shown in FIG. 2A, a SiNx film is formed on the epitaxial film L6 for the contact layer 6 using a technique such as CVD (Chemical Vapor Deposition), and a photolithography technique, an etching technique, or the like is used. Then, a circular mask 21 having a diameter w _m is formed from the SiNx film. Hereinafter, the mask forming method is the same. Here, the other epitaxial films L2 to L5 are epitaxial films for the buffer layer 2, the light absorption layer 3, the electric field relaxation layer 4, and the multiplication layer 5, respectively.

  Next, as shown in FIG. 2B, the epitaxial film L6 for the contact layer 6 other than the region masked by the mask 21 is removed by etching using a sulfuric acid-based etchant. The etching is stopped when the block film L7 appears. The etching technique performed using the block film L7 is known and will not be described. After the etching, as shown in FIG. 2C, the mask 21 is removed to obtain the contact layer 6 having a diameter w.

Next, formation of the forward mesa structure will be described. FIG. 3 is a process diagram for explaining a method of forming a forward mesa structure. First, as shown in FIG. 3 (a), the surface of the contact layer 6 is formed by the contact layer 6 and substantially concentrically to form a mask 22 of larger diameter d _m than the diameter w of the contact layer 6. Next, as shown in FIG. 3B, using bromine-based etchant, the epitaxial films L <b> 2 to L <b> 5 other than the region masked by the mask 22 are removed by etching until the semiconductor substrate 1 appears. After the etching, as shown in FIG. 3C, the mask 22 is removed to obtain a forward mesa structure.

Hereinafter, the portion having the forward mesa structure is referred to as a forward mesa structure portion. The forward mesa structure portion has a side surface in which a region near the apex of the cone is cut, and a substantially flat portion of the surface on which the contact layer 6 is formed. Approximately the diameter of the flat portion is generally smaller than the diameter d _m of the mask 22, to the diameter as d. The diameter d _m of the mask 22, the diameter d of the substantially flat portion of the mesa structure obtained by the above etching is set to be larger than the diameter w of the contact layer 6.

  As described above, the formation of the contact layer 6 that is the outermost layer as described above is that the surface of the next delamination layer is in the vicinity of the interface between the outermost layer and the multiplication layer 5 that is the next delamination layer. It is to have a portion larger than the surface of the outermost layer and not in contact with the outermost layer. Therefore, the present invention forms the outermost layer in the vicinity of the interface between the outermost layer and the next outer layer so that the surface of the next outer layer is larger than the surface of the outermost layer and does not contact the outermost layer. To do.

  Next, processing after forming the forward mesa structure by etching will be described. First, the protective film 8 made of SiNx is formed on the surface having the forward mesa structure portion of the semiconductor substrate 1. Next, a contact hole 10 having a diameter v is opened in the protective film 8 on the contact layer 6 (see FIG. 1). The contact hole 10 is formed using a photolithography technique and a dry etching technique. Hereinafter, the contact hole formation method is the same. Next, the p-electrode 7 is formed on the contact layer 6 by, for example, a vacuum evaporation method through the contact hole 10 provided in the protective film 8 on the contact layer 6.

  Next, a contact hole is opened in the protective film 8 on the semiconductor substrate 1. This contact hole may be provided when the contact hole 10 is provided in the protective film 8 on the contact layer 6 described above. The n electrodes 9a and 9b are formed on the semiconductor substrate 1 by, for example, a vacuum evaporation method through a contact hole provided in the protective film 8 on the semiconductor substrate 1. For the p-electrode 7 and the n-electrodes 9a and 9b, materials that can make ohmic contact with the contact layer 6 and the semiconductor substrate 1 were used, respectively. Here, the method of providing the wirings connected to the p-electrode 7 and the n-electrodes 9a and 9b is outside the spirit of the present invention, and thus the description thereof is omitted.

  The n electrodes 9a and 9b are formed on the semiconductor substrate 1 by, for example, a vacuum evaporation method through a contact hole provided in the protective film 8 on the semiconductor substrate 1. For the p-electrode 7 and the n-electrodes 9a and 9b, materials that can make ohmic contact with the contact layer 6 and the semiconductor substrate 1 were used, respectively.

Here, it is assumed that the optical signal is incident from the surface opposite to the surface on which the forward mesa structure portion of the surface of the semiconductor substrate 1 is formed (hereinafter referred to as the back surface of the semiconductor substrate). On the back surface of the semiconductor substrate 1, in order to reduce the reflection of the optical signal, mirror processing is performed by wet etching, and a SiO ₂ antireflection film is formed. The description of the method of providing the wiring connected to the p-electrode 7 and the n-electrodes 9a and 9b is omitted.

  Next, the operation of the avalanche photodiode 30 will be described using the avalanche photodiode 30 as an example of a light receiving element. Here, it is assumed that the p electrode 7 is at a negative potential and a predetermined voltage equal to or higher than the avalanche voltage is applied to the multiplication layer 5 between the p electrode 7 and the n electrodes 9a and 9b.

The optical signal incident from the back side of the semiconductor substrate 1 passes through the semiconductor substrate 1 made of n ⁺ -InP and the buffer layer 2 made of n ⁺ -InP because the energy gap of InP is larger than the energy of the optical signal. It reaches the light absorption layer 3 made of i-InGaAs. Since the energy gap of InGaAs is smaller than the energy determined according to the wavelength of the optical signal (hereinafter referred to as optical signal energy), the optical signal forms electron-hole pairs in the light absorption layer 3 made of i-InGaAs. Absorbed.

  The holes of the electron-hole pairs formed in the light absorption layer 3 made of i-InGaAs pass through the electric field relaxation layer 4 and move to the multiplication layer 5. Since a predetermined electric field strength required for causing avalanche multiplication is applied to the multiplication layer 5, holes that have entered the multiplication layer 5 are multiplied and flow into the contact layer 6. .

  Here, the effect brought about by making the diameter w of the contact layer 6 smaller than the diameter d of the substantially flat portion of the forward mesa structure portion will be described. The diameter of the substantially flat portion of the forward mesa structure is, for example, about 30 to 40 μm, whereas the thickness of the forward mesa structure is, for example, about 2 to 3 μm. Since the electric field is applied in the thickness direction, the main carrier multiplied by the multiplication layer 5 is accelerated and moved in the thickness direction.

  Then, since the contact layer 6 is formed so as to be located in the central portion that is a region away from the side surface of the forward mesa structure portion, the electrical resistance value of the central portion of the forward mesa structure is lower than the electrical resistance value in the vicinity of the side surface, As shown by arrows in the forward mesa structure portion in FIG. 1, current flows into the contact layer 6 mainly from the central portion of the forward mesa structure portion. As a result, the current flowing in the vicinity of the side surface of the forward mesa structure portion can be reduced, and the dark current generated in the vicinity of the side surface of the forward mesa structure portion can be reduced.

FIG. 4 shows an avalanche photo when the diameter d of the substantially flat portion of the forward mesa structure portion is 33 μm and the diameter w of the contact layer 6 is 25 μm (Example 1), 29 μm (Example 2) and 33 μm (Example 3). 3 is a graph showing an example of current-voltage characteristics obtained by a diode 30. When the voltage is applied to the pn junction in the forward mesa structure with a reverse bias polarity and the applied voltage is 20 V, the dark currents in Examples 1, 2 and 3 are 2.0 × 10 ^{− 10} A, 2.3 × 10 ⁻¹⁰ A, and 5.0 × 10 ⁻¹⁰ A.

As described above, the avalanche photodiodes (Examples 1 and 2) of the present invention have less dark current than the conventional avalanche photodiodes (Example 3) having the contact layer (example 3). , Less than half.) Further, when the voltage applied when the dark current of 1.0 × 10 ⁻⁴ A flows is used as the breakdown voltage, the breakdown voltages in the above-described examples 1, 2 and 3 are 27 respectively. .9V, 26.4V, and 25.5V.

  Regarding the breakdown voltage, the avalanche photodiode (Example 1 and Example 2) of the present invention is higher than the avalanche photodiode (Example 3) having the contact layer of the conventional configuration. As a result, only by reducing the size of the contact layer, not only the dark current value was reduced but also the breakdown voltage value was improved.

Next, the influence on the frequency characteristics when the diameter of the contact layer 6 is further reduced will be described with reference to FIGS. First, a 3 dB cut-off frequency _fCR is used as a measure for influencing the frequency characteristics. Here, f _CR = 1 / (2πC (R _L + R _C )), C is the capacitance of the pn junction, R _L is a load resistance, 50Ω, R _C is the contact layer 6 and the p electrode 7 Is the contact resistance between.

FIG. 5 is a graph showing the dependency of the contact resistance _RC on the diameter d of the contact layer 6 when the specific contact resistance is 5 × 10 ⁻⁷ Ωcm ⁻² . Here, in order to exclude the influence of the contact area between the contact layer 6 and the p-electrode 7, it is assumed that the contact hole 10 has the same diameter as the contact layer 6 and is provided immediately above the contact layer 6. . The specific contact resistance varies depending on the impurity distribution and is not necessarily limited to the above value. However, the above value is used as an example of the value obtained for the group III-V compound semiconductor. FIG. 5 shows how the contact resistance changes with the square of the diameter w of the contact layer 6.

Figure 6 is a graph showing the calculation results of the above 3dB cut-off frequency _{f CR.} Here, the calculation of the 3dB cut-off frequency f _CR, using a contact resistance values shown in FIG. 5, using the value of 0.3pF as the capacitance of the pn junction, was assumed to be constant. Note that the capacitance C of the pn junction has a value of about 0.1 to 0.3 pF by the CV measurement method.

Here, when the diameter of the contact layer 6 at which the 3 dB cutoff frequency _fCR is 10 GHz is the lower limit of the diameter of the contact layer 6 of the avalanche photodiode 30 to be used, the lower limit of the diameter of the contact layer 6 is about 4.5 μm. It becomes. However, the present invention provides an avalanche photodiode 30 capable of suppressing light reception current and dark current flowing in the vicinity of the mesa side surface and improving light reception sensitivity and element reliability. It is not limited. In addition, the present invention is applied when the effects of the present invention are useful depending on the utilization method and purpose of the avalanche photodiode 30.

  As described above, in the light receiving element and the avalanche photodiode according to the embodiment of the present invention, the surface of the next delamination layer is larger than the surface of the most delamination layer in the vicinity of the interface with the next delamination layer in contact with the most separation layer. Because it has a part that is not in contact with the outermost layer, current flows into the outermost layer from the center of the mesa, and it is near the mesa side without performing crystal regrowth just by making the outermost layer smaller than the next outer layer. It is possible to suppress the received light current and the dark current flowing through the element, and to improve the reliability of the element.

  In addition, since the holes of the electron-hole pairs generated in the light absorption layer are the main carriers and a multiplication layer having excellent crystallinity that is not a superlattice layer is used, the electrons are separated from the main carriers. Compared with an avalanche photodiode, the reliability of the element can be further improved.

  In addition, since an optical signal is incident on the light absorption layer from the back side, incident light is transmitted and absorbed through the light absorption layer, then reflected by the p electrode and absorbed again by the light absorption layer. Can be obtained.

  In addition, simply by making the contact layer surface smaller than the multiplication layer surface as described above, an avalanche photodiode having a low dark current value and a higher breakdown voltage can be produced. Since the voltage applied to the layer is increased, the range of usable multiplication factors is widened, and the reliability can be further improved.

  The current flowing into the contact layer can be concentrated on the current from the good crystallinity part in the forward mesa structure, which can improve the BER (Bit Error Rate) and improve the device life and improve the device reliability. It can be improved.

  In the embodiment of the present invention, an avalanche photodiode having a forward mesa structure portion has been described as an example. However, the present invention can be similarly applied to a light receiving element and an avalanche photodiode having an inverted mesa structure. In this case, the portion corresponding to the forward mesa structure portion in the light receiving element and the avalanche photodiode of the reverse mesa structure is referred to as the reverse mesa structure portion, and the forward mesa structure portion and the reverse mesa structure portion are collectively referred to as the mesa structure portion.

  The light-receiving element and the avalanche photodiode according to the present invention have an effect that the light-receiving current and dark current flowing in the vicinity of the mesa side surface can be suppressed and the reliability of the element can be improved without performing crystal regrowth. It can also be applied to uses such as useful light receiving elements and avalanche photodiodes.

Schematic sectional view showing an example of an avalanche photodiode according to an embodiment of the present invention Process diagram for explaining a method of forming a contact layer Process diagram for explaining a method for forming a forward mesa structure The graph which shows an example of the current-voltage characteristic obtained by the avalanche photodiode when the diameter of the substantially flat part of the forward mesa structure part is 33 μm and the diameter of the contact layer 6 is 25 μm, 29 μm and 33 μm The graph which shows the diameter d dependence of the contact layer of contact resistance _RC Graph showing the calculation results of the 3dB cut-off frequency _{f CR}

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semiconductor substrate 2 Buffer layer 3 Light absorption layer 4 Electric field relaxation layer 5 Multiplication layer 6 Contact layer 7 P electrode 8 Protective film 9a, 9b N electrode 10 Contact hole 21, 22 Mask 30 Avalanche photodiode L2 Semiconductor film for buffer layer L3 of the substantially flat portion of the semiconductor film L7 block film d order mesa structure for the semiconductor film L6 contact layer for the semiconductor layer L5 multiplication layer for the semiconductor film L4 electric field relaxation layer for light absorption layer diameter d _m mask 22 diameters of v contact hole having a diameter of w contact layer diameter w _m mask 21

Claims (4)

In a light receiving element (30) in which a plurality of semiconductor layers (2 to 6) including a contact layer (6) for obtaining ohmic contact are stacked in a mesa structure formed on a semiconductor substrate (1).
The contact layer is stacked as the most distant layer farthest from the semiconductor substrate among the plurality of semiconductor layers, and in the vicinity of the interface with the next dissociation layer in contact with the distant layer, the surface of the next separation layer is A light receiving element having a portion larger than a surface of the contact layer and not in contact with the contact layer. In the mesa structure formed on the semiconductor substrate (1), a light absorption layer (3) that absorbs incident light to generate carriers, and at least one of the carriers generated by the light absorption layer In an avalanche photodiode in which a plurality of semiconductor layers (2 to 6) including a multiplication layer (5) that multiplies as a main carrier and a contact layer (6) for obtaining ohmic contact are laminated,
The contact layer is stacked as the most distant layer farthest from the semiconductor substrate among the plurality of semiconductor layers, and in the vicinity of the interface with the next dissociation layer in contact with the distant layer, the surface of the next separation layer is An avalanche photodiode having a portion larger than a surface of the contact layer and not in contact with the contact layer.   The avalanche photodiode according to claim 2, wherein holes in the electron-hole pair generated in the light absorption layer serve as main carriers and are multiplied in the multiplication layer.   The optical signal incident on the light absorption layer is incident on a back surface side of the semiconductor substrate, which is a surface opposite to the surface on which the semiconductor layer is formed. Avalanche photodiode.

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