WO2000019543A1

WO2000019543A1 - Photodiode and optical communication system

Info

Publication number: WO2000019543A1
Application number: PCT/JP1999/005170
Authority: WO
Inventors: Shojiro Kitamura; Takeo Kawase; Takeo Kaneko
Original assignee: Seiko Epson Corporation
Priority date: 1998-09-29
Filing date: 1999-09-21
Publication date: 2000-04-06
Also published as: JP2000114580A

Abstract

A photodiode comprising a light absorbing layer (102) and a contact layer (103) both formed one on the other on a semiconductor substrate (101), the bandwidth of the contact layer (103) being wider than that of the light absorbing layer (102), and the contact layer (103) being thick enough to substantially absorb light having a wavelength less than that corresponding to the energy of the bandwidth of the contact layer.

Description

Description Photodiodes and optical communication systems

The present invention relates to a photodiode having wavelength selectivity and suitable for use in wavelength division multiplexing optical communication, and an optical communication system using the same. Background art

In recent years, wavelength division multiplexing optical communication capable of high-speed and large-capacity communication has attracted attention. In this wavelength division multiplexing optical communication, since a plurality of lights having different wavelengths are used as signals, in order to realize low crosstalk communication, it is necessary to use a photodiode having wavelength selectivity on the receiver side. desirable.

A photodiode having such wavelength selectivity is described in the literature (J. App 1. Phys. 78 (2), 15 Jul 1995 pp. 607-639). A raised photodiode is disclosed. According to this photodiode, since the distributed reflection type multilayer mirror has wavelength selectivity, it is possible to achieve high wavelength selectivity with a small Q value for a specific wavelength. However, since this photodiode uses a distributed reflection multilayer mirror, it is necessary to accurately control the Mi of each layer. In addition, the thickness of the light absorbing layer as well as the layer of the multilayer mirror needs to be set to a length that allows light to resonate between the upper and lower reflective multilayer mirrors. Needs to be controlled. Therefore, this photodiode has a drawback in that strict TO control of each layer is required, and fabrication is not easy. Disclosure of the invention

The present invention has been made in view of the above problems, and an object of the present invention is to provide a photodiode having wavelength selectivity and easy to manufacture. Another object of the present invention is to enable wavelength division multiplexing optical communication using an extremely simple optical circuit without using optical components such as a multiplexer and a demultiplexer using the photodiode according to the present invention. Another object of the present invention is to provide a simple optical communication system.

The photodiode of the present invention includes a semiconductor substrate, a light absorbing layer and a contact layer laminated on the semiconductor substrate, wherein the contact layer has a larger bandwidth than the light absorbing layer, and The contact layer has a thickness JJ capable of absorbing light having a wavelength equal to or less than the wavelength corresponding to the energy of the band width of the contact layer.

According to the photodiode of the present invention, a film thickness capable of absorbing light having a wavelength equal to or less than the wavelength (human _gc ) corresponding to the bandwidth of the contact layer, preferably absorbing 95% or more of such light. Since the contact layer has the largest possible thickness, light having a wavelength equal to or less than the specific wavelength _gc is absorbed in the contact layer. Then, the light transmitted through the contact layer enters the light absorbing layer. Therefore, the contact layer functions as a filter that absorbs light in a specific wavelength range.

More specifically, assuming that the wavelength corresponding to the energy of the bandwidth of the contact layer is え and the wavelength corresponding to the energy of the bandwidth of the light absorbing layer is _gi , the light absorbing layer has the following formula:

λ g _C- pan ≤ λ gi

The light having a wavelength that satisfies is absorbed.

This will be further described with reference to FIG. In FIG. 7, the horizontal axis represents the wavelength, and the vertical axis represents the photoelectric conversion efficiency of the pin-type photodiode. First, looking at a single photodiode _100-1 , the light of wavelength _s (person _s ≤ input _gcl ) less than the wavelength A _gcl corresponding to the energy of the bandwidth of the contact layer is partially The light is absorbed by the contact layer, and the light that has passed through without being absorbed by the contact layer (the light corresponding to the portion indicated by the symbol b1 in FIG. 7) is absorbed by the light absorbing layer. On the other hand, the wavelength corresponding to the energy of the bandwidth of the light absorption layer is longer than human _gil.

Part of the incident light L (incident _g n <J) is partially absorbed (the light corresponding to the portion indicated by the symbol a1 in FIG. 7) by the light absorbing layer, and the remaining light is absorbed by the contact layer and the contact layer. Light absorption The light passes through the layer and enters the semiconductor chip.

Then, the light in the predetermined wavelength region (including the input _{gcl and the} input _gil ) that is larger than the wavelength input _{gcl and} is equal to or less than _gil , including the wavelength, is mainly absorbed by the light absorption layer and converted into a photocurrent. In the layers other than the light absorption layer, that is, the contact layer and the semiconductor substrate, electrons and holes excited by light absorption are annihilated because there is no electric field, and do not contribute to generation of photocurrent.

As described above, in the photodiode of the present invention, each of the contact layer and the light absorption layer functions as a filter for light in a specific wavelength region, and defines a wavelength region that can contribute to photocurrent to a specific range. And thus has excellent wavelength selectivity.

Since the photodiode has wavelength selectivity, for example, even if light of wavelength, and light of wavelength λ ₂ are on the same optical path, a photodiode having wavelength selectivity corresponding to each wavelength is used. For example, it is possible to independently detect the light of wavelength λ and the light of wavelength λ ₂ . In other words, since the photodiode has wavelength selectivity, for example, as shown in FIG. 7, the wavelength region of light that can be detected by the first photodiode is c1, and the wavelength region of light that can be detected by the second photodiode is c1, as shown in FIG. By setting the wavelength region to be c 2, light of each wavelength can be detected independently.

In the photodiode of the present invention, a detectable wavelength region can be specified by controlling the composition and expansion of the contact layer and the light absorbing layer. The control of the yarn and the film thickness is easier than the film formation of a distributed reflection type multilayer mirror or the like, and the photodiode can be manufactured by a simple process.

By using a plurality of photodiodes having different wavelength ranges to be selected, a plurality of lights having different wavelengths can be detected individually (or independently). For example, as shown in FIG. 7, in the use of the photodiode 1 0 0 1 and 1 0 0 2 wavelength region c 1 and c 2 containing the detection light Hachoe i and input ₂ do not overlap Thus, two lights can be detected. The same applies to the case where the number of detected light is three or more.

Letting the absorption coefficient of the contact layer be d and the fl J * of the contact layer be d The following formula

e- ^axd <0. 0 5

Is preferably satisfied.

By satisfying the above expression, good communication can be achieved with a crosstalk of -26 dB or less in wavelength division multiplexing optical communication using two or more signals using the photodiode according to the present invention as a light receiving element. .

The photodiode according to the present invention is desirably formed of a direct transition semiconductor in that a good wavelength cutoff characteristic can be obtained. Examples of direct transition type semiconductors include AlGaAs, GalInP, ZnSSe and

A semiconductor such as an InGaN system can be used.

An optical communication system according to the present invention includes a light emitting element, an optical waveguide, and a light receiving element having the above-described photodiode, and the light emitting element and the light receiving element are directly optically connected to each other by the optical waveguide.

According to this optical communication system, a simple configuration consisting of a light-emitting element, an optical waveguide, for example, an optical fiber, and a light-receiving element having a photodiode according to the present invention is provided. A wavelength division multiplexing optical communication system or the like can be configured with a configuration that does not require optical components such as a device.

And an element mounted with a plurality of surface emitting lasers having different oscillation wavelengths as the light emitting element, and a light receiving device having a plurality of photodiodes according to the present invention, wherein the light of the plurality of wavelengths can be detected by individual elements. By using the element, and further, by optically directly coupling each emission port of the plurality of surface emitting lasers constituting the light emitting element, the core of the optical waveguide, and each light receiving surface of the photodiode according to the present invention, Unlike conventional optical communication systems, there is no need for optical components such as lenses, multiplexers, and demultiplexers. As a result, an optical communication system with a simple configuration, easy optical adjustment, and low-cost wavelength division multiplexing transmission can be configured.

By using an element configured by arranging a plurality of surface emitting lasers having different oscillation wavelengths, preferably a vertical cavity surface emitting laser, as the light emitting element, it is possible to control the threshold value and the current-light emission characteristics. Excellent and easy to assemble. Another optical communication system according to the present invention includes a first light receiving / emitting element including a light emitting element and a light receiving element having the above-described photodiode, an optical waveguide, and a light emitting element; A second light receiving / emitting element including a light receiving element having the light emitting element, wherein the first light receiving / emitting element and the second light receiving / emitting element are directly optically connected by an optical waveguide.

According to this optical communication system, a simple structure including a first light receiving / emitting element, an optical waveguide, for example, an optical fiber and a second light receiving / emitting element, that is, a lens, a multiplexer, a duplexer A wavelength division multiplexing optical communication system can be configured with a configuration that does not require optical components such as the above.

In the first and second light emitting and receiving elements, it is preferable that the light emitting element is a surface emitting laser, preferably a vertical cavity surface emitting laser. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a sectional view schematically showing a photodiode according to Embodiment 1 of the present invention.

FIG. 2 is a cross-sectional view schematically showing a manufacturing process of the photodiode shown in FIG. FIG. 3 is a cross-sectional view schematically showing a manufacturing process performed subsequently to FIG.

FIG. 4 is a diagram showing an optical communication system using the photodiode of the present invention according to Embodiment 2 of the present invention.

FIG. 5 is a diagram showing an optical communication system using the photodiode of the present invention according to Embodiment 3 of the present invention.

FIG. 6 is a diagram showing an optical communication system using the photodiode of the present invention according to Embodiment 4 of the present invention.

FIG. 7 is a diagram showing the relationship between wavelength and photoelectric conversion efficiency for indicating the photoselectivity of the photodiode according to the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Difficult form 1)

(Device structure)

FIG. 1 is a cross-sectional view schematically showing a structure of a photodiode 100 according to one embodiment of the present invention.

In the photodiode 100 shown in FIG. _{1, a} light absorption layer 102 made of i-type Al _a Ga _1-a As and an n-type semiconductor substrate 101 made of n-type

A p _- type contact layer 103 made of p _- type Al _b Ga _1-b As is sequentially laminated to form a pin-type photodiode.

Further, a dielectric film 106 made of a silicon oxide film, a silicon nitride film, or the like is formed around a deposition layer formed of a plurality of semiconductor layers formed on the n-type semiconductor substrate 101. The lower end of the dielectric film 106 is formed so as to reach the n-type semiconductor substrate 101. In the light receiving section 110 on the upper surface of the p-type contact layer 103, the dielectric film 106 constitutes an incident surface, and is therefore at least optically transparent.

Further, on the upper surface of the p-type contact layer 103, a p-type ohmic electrode 107 is formed so as to surround the light receiving section 110. On the lower surface of the n-type semiconductor substrate 101, an n-type ohmic mil 09 is formed.

Here, the composition ratio of A1 in each layer of the light absorption layer 102 and the p-type contact layer 103 preferably has a relationship of 0 ≦ a <b. That is, assuming that the wavelengths corresponding to the energy of the bandwidth of each layer of the light absorption layer 102 and the p-type contact layer 103 are _{gi and} _gc , respectively, a relationship of human _gc <human _gi is established. Contact layer 103, e absorb light of _s ≤ e wavelengths having a relationship of _{gc (λ} _8), in order for this light does not reach the light-absorbing layer 102 without a sufficient thickness, preferably, e_ ^axd < 0.05 (h: absorption coefficient of contact layer, d: thickness of contact layer). This is, as mentioned above, two or more different wavelengths, for example the two wavelengths shown in Figure 7! (A _gcl <A! ≤A _gil ) and λ ₂ (A _gc2 <A ₂ ≤A _gi2 ) It means there is.

Of the light incident from the light receiving unit 110, light Hachoe _gc following wavelengths e _s (e _s ≤A _gc) is substantially fully in contact layer 103 is (for example, preferably 95% or more) absorption. Hachoe wavelength's less long wavelength a and the wavelength e _gi than _gc, the light (human _gc rather human ^ ON _gi) through the contact layer 103, is absorbed by the light absorbing layer 102. Furthermore, the light of wavelength incident _gi than the long wavelength light e _{_L} (A _gi <A _L) is transmitted through the contact layer 103 and Hikari吸Osamuso 102. Then, only the light absorbed by the light absorption layer 102 contributes to the photocurrent and is detected.

That is, light of a wavelength (person _s ) having a relationship of _s ≤ person _ge is almost absorbed by the contact layer 103. The electron and hole pairs excited by this light absorption disappear because of no electric field in the contact layer, and do not contribute to the generation of photocurrent. Light of a wavelength (incident _L ) having a relation of _{incident gi} <enter _t passes through the contact layer 103 and the light absorbing layer 102 and is absorbed or transmitted by the n-type semiconductor substrate 101. The light absorbed by the semiconductor substrate 101 does not contribute to the generation of a photocurrent similarly to the light absorbed by the contact layer 103.

(Device manufacturing process)

Next, an example of a manufacturing process of the photodiode 100 shown in FIG. 1 will be described with reference to FIG. 2 and FIG.

(a) First, as shown in FIG. 2, on the n-type semiconductor substrate 101 made of n-type GaAs, eight 1 _{_0.07} 0 & ₃ 3 light absorbing layer 102 made of, and

P-type A10.085 G a _0. ₉₁₅ to p-type contact layer 103 of As it then 4 are sequentially Epitakisharu grown thigh zm by MOVPE (Me ta 1 Organic Vapor Phase E pit axy) method. In this embodiment, the MO VP E method is used for epitaxial growth, but the MBE (Molecular Beam Epitaxial) method or LPE (Liguid Phase Epitaxial) method may be used.

Next, a dielectric film consisting of SiO _{2 of} about 25 nm is deposited on the epitaxial growth layer by atmospheric pressure thermal CVD (Chemi ca 1 Vapor Deposition). Form 105. The dielectric film 105 prevents surface contamination during the process of forming the epitaxial growth layer.

(b) Next, as shown in FIG. 3, using a photoresist (not shown) as a mask, the n-type semiconductor substrate 101 is etched into a circular shape when viewed from the upper surface of the epitaxy forming layer until it is in a columnar shape. Form part 1 1 2 The plane の of the columnar portion 112 is circular in the present embodiment, but is not limited to this.

Next, the photoresist is removed, and the etching cross section is treated with ammonium sulfide or the like. Then, a dielectric film 106 made of SiO ₂ is formed on the epitaxial growth layer and the etching cross section by the atmospheric pressure thermal CVD method. Further formed. Here, since the upper surface of the columnar portion 112 becomes the light receiving portion 110 (see FIG. 1), the dielectric film 106 also functions as a protective film and an anti-reflection film in the light receiving portion 110. As described above, the optical thickness of the dielectric film 106 in the light-receiving section 110 is set to be approximately one-fourth the wavelength of the light (detected light) used as the light source. Set to be.

(c) Next, as shown in FIG. 1, a ring-shaped contact hole surrounding the light receiving section 110 is opened in the dielectric film 106 on the p-type contact layer 103 to form a p-type ohmic electrode. After forming 107, the n-type semiconductor substrate 101 is polished to a thickness of 50 to 150 1m, which is a thickness that is easy to cleave, and then an n-type semiconductor S109 is formed. I do. Finally, each element is cleaved to complete the element shown in FIG. When the device is formed by dicing or the like instead of cleavage, polishing of the n-type semiconductor substrate is not necessary.

Here, the n-type semiconductor substrate 101 is replaced with a high-resistance GaAs semiconductor substrate and an n-type contact layer, and the n-type contact layer is exposed by etching to form a dielectric film. A contact hole may be formed in the film to form an n-type semiconductor 109.

As described above, the photodiode of the present embodiment has wavelength selectivity. In this embodiment, when a = 0.07 and b = 0.085 in the composition of the semiconductor layer,

A _gi = 8 2 O nm

λ _gc = 8 10 nm Was confirmed. Therefore, input the wavelength j and input _gc <input! By setting, for example,! = 8 15 nm to satisfy ≤ λ _gi , the wavelength can be increased by the photodiode 100! Of light can be detected.

Form 2)

FIG. 4 shows an embodiment of an optical communication system using the photodiode according to the present invention. The optical communication system includes a light emitting element 100, an optical fiber 20 for transmitting light emitted from the light emitting element 100 0, and a light receiving element 20 for receiving light from the optical fiber 20. 0 and 0.

The light emitting device 1000 includes a plurality (two in this example) of surface emitting lasers 10-1 and 10-2 each having a different wavelength of emitted light.

That is, in this embodiment, two surface emitting lasers 10-1 (oscillation wavelength: human,) and 10-2 (oscillation wavelength: ₂ ) having different wavelengths are arranged close to each other, and One exit port is arranged so as to face the core section 22 of the optical fiber 20. It is very difficult for an edge-emitting semiconductor laser to arrange a plurality of light-emitting components having different wavelengths close to each other. In contrast, a vertical cavity surface emitting laser having a resonance path in a direction perpendicular to the substrate has a high degree of freedom in in-plane arrangement, and the light emitting portions of a plurality of surface emitting lasers having different wavelengths. Are easily arranged close to each other, and are one of the most suitable as the light emitting element of the present invention.

The light receiving element 2 0 0 0, at least previous remarks third wavelengths's t and person ₂ photodetection used possible two photodiodes 1 0 0 1 and 1 0 0 2. The photodiodes 100-1 and 100-2 are arranged with their light receiving surfaces facing the core portion 22 of the optical fiber 20.

Further, in the present scythe configuration, the wavelength corresponding to the energy of the bandwidth of the light absorption layer and the contact layer of the first photodiode 100-1 constituting the light receiving element 2000 is deviated. and human _gil Oyobie _gcl, when is its wavelength corresponding to the energy of the second photodiode 1 0 0 2 bandwidth of the light absorption layer and the contact layer and it's _si2 Oyobie _gc2, shown in FIG. 7 like,

_{_{λ, Εΐ <λ gil <λ}} gc2 <λ gi2 of relationship is established. Further, assuming that the wavelengths of the light emitted from the first surface emitting laser 10-1 and the second surface emitting laser 10-2 constituting the light emitting element 1000 are and 2 ₂ , respectively, input _gcl <input i ≤ The relationship of input _gil and A _gc2 <A ₂ ≤A _gi2 is established.

The light emitting element 1000 and the light receiving element 2000 are directly optically connected by the optical waveguide 20, and two-wavelength wavelength division multiplexing optical communication can be performed. The same applies when three or more different wavelengths are used. Of course, it can be applied to the case of a single wavelength. As described above, as the light emitting device used in the present S mode, a device mounted with a plurality of surface emitting lasers having different oscillation wavelengths is used. (I-L characteristics) are desirable, but the present invention is not limited to this, and a monolithic surface-emitting laser that can emit light of multiple wavelengths can be used.

Also, as the optical fiber 20, it is preferable to use a GI (Graded Index) type fluoroplastic fiber or a GI type HPCF (Hard Polymer Clad Fiber) having a large core diameter, small light loss and small dispersion. .

According to the optical communication system according to the present embodiment, an element mounted with a plurality of surface emitting lasers having different oscillation wavelengths as a light emitting element, and a plurality of photodiodes according to the present invention used as a light receiving element can emit light. It is possible to configure a wavelength division multiplexing optical communication system having a simple configuration including three elements: an element, an optical fiber, and a light receiving element. Then, by optically aligning the emission ports of the plurality of surface emitting lasers constituting the light emitting element, the core of the optical fiber, and the light receiving surface of the photodiode according to the present invention, the optical communication system of It does not require optical components such as lenses, multiplexers, and demultiplexers. As a result, an optical communication system with a simple configuration, easy optical adjustment, and low-cost wavelength division multiplex transmission can be configured.

(Experimental example)

The first surface emitting laser 10-1 and the second surface emitting laser 102 are used as light emitting elements, and the first photodiode 100 according to the present invention is used as a light receiving element. When wavelength division multiplexing optical communication was performed using the first and second photodiodes 100-2, it was confirmed that good communication with a crosstalk of no more than 26 dB was possible. The following were used as each light emitting element and light receiving element.

The first photodiode 100-1;

the n-type G a As _{_{_{substrate, A 1 0. 07 Ga 0}}} . 93 light absorbing layer of IS4〃m of As and P-type Al _{_0.} ₀₈₅ Ga _0. p-type fl Jf 4〃M consisting ₉₁₅ As It is a _pin- type photodiode with a stacked contact layer, with a _{gcl of 810} nm and a human _gil of 820 nm. Second photodiode 100-2;

On a n-type GaAs substrate, a 4 m TO absorption layer made of GaAs

In p-type Al _0. ₀₁₄ Ga ₈₆ consisting As fl i ¥ 4 m pin type photodiodes stacked p-type contact layer, a person _gc2 860 nm, human _gi2 is 870 nm. First surface emitting laser 10-1;

Wavelength, 815 Ι1Π1 \ input gci, input 1 / ^ gi

Second surface emitting laser 10-2;

Wavelength λ ₂ = 865 nm

(Difficult form 3)

FIG. 5 shows another embodiment of the optical communication system using the photodiode according to the present invention. This optical communication system includes a first light receiving / emitting element 3000, an optical fiber 20, and a second light receiving / emitting element 4000.

In the first light emitting / receiving element 3000, light of wavelength, can be detected by the first photodiode 100-1, and light of wavelength _z can be detected by the second surface emitting laser 10-2. Can be emitted. Further, in the second light emitting / receiving element 4000, light of wavelength ₂ can be detected by the second photodiode 100-2, and the wavelength λ can be detected by the first surface emitting laser 10-1. ! Of light can be emitted.

Also, in the present embodiment, the wavelength corresponding to the energy of the bandwidth of the light absorption layer and the contact layer of the first photodiode 100-1 constituting the first light emitting / receiving element 3000 as in the second embodiment. And _gil and _λgcl , respectively. Assuming that the wavelengths corresponding to the energy of the bandwidth of the light absorption layer and the contact layer of the second photodiode 100-2 constituting the second light-receiving / emitting element ₄₀₀₀₀ are _gi2 and human _gc2 , respectively. As shown in FIG. 7, the _relationship of human _{gcl and} human _gil <human _gc2 <A _gi2 is established.

Further, the wavelengths of the light emitted from the second surface emitting laser 10-2 and the first surface emitting laser 10-1 which constitute the light emitting and receiving elements 30000 and 40000, respectively, If they are ₂ and i, then person _gcl <^ person _gi ! _Satisfies person _gc2 <person ₂ ≤ _{λ gi2} . Full-duplex optical communication can be performed by directly optically connecting the first light emitting / receiving element 3000 and the second light emitting / receiving element 4000 with the optical waveguide 20. The same applies when three or more different wavelengths are used.

Since the surface emitting laser and the optical fiber are the same as those in the second embodiment, detailed description thereof will be omitted.

According to the optical communication system of the present embodiment, the surface emitting laser and the photodiode according to the present invention capable of detecting light having a wavelength different from the oscillation wavelength of the surface emitting laser are used as the light receiving and emitting elements. Accordingly, a wavelength division multiplexing optical communication system having a simple configuration including three members, a first light receiving / emitting element, an optical fino, and a second light receiving / emitting element can be configured. Then, each light-emitting or emitting surface of one or more surface-emitting lasers constituting each light-receiving / emitting element, the light-receiving surface of one or more photodiodes according to the present invention, and the core portion of the optical waveguide are optically positioned. By combining them, there is no need for optical components such as lenses, multiplexers, and demultiplexers as in conventional optical communication systems. As a result, it is possible to configure a multiplex optical communication system having a simple configuration, easy optical adjustment, and low-cost full-duplex optical communication.

(Embodiment 4)

FIG. 6 shows a light-receiving / emitting element 500 that can be used instead of the light-receiving / emitting element in the optical communication system shown in FIG.

The light emitting / receiving element 500 0 has a first monolithic light emitting / receiving element 200-1 and a second monolithic light emitting / receiving element 200-2. The first monolithic light-receiving / emitting element 200-1 comprises a first photodiode 100-1 and a first surface-emitting laser 100-1. 1 are monolithically laminated and formed. The second monolithic light emitting / receiving element 200-2 is formed by monolithically laminating a second photodiode 100-2 and a second surface emitting laser 10-2. By directly optically connecting the light receiving / emitting element 500 to both ends of the optical waveguide, bidirectional wavelength division multiplex optical communication can be performed.

As described above, the embodiments of the present invention have been described, but the present invention is not limited thereto, and can take various forms within the scope of the present invention.

Claims

The scope of the claims

(1) a semiconductor substrate, including a light absorbing layer and a contact layer laminated on the semiconductor substrate, wherein the contact layer has a larger bandwidth than the light absorbing layer, and the contact layer has A photodiode having a thickness capable of absorbing light having a wavelength equal to or less than the wavelength corresponding to the energy of the bandwidth of the contact layer.

(2) _Assuming that a wavelength corresponding to the energy of the bandwidth of the contact layer is _gc and a wavelength corresponding to the energy of the bandwidth of the light absorbing layer is I _gi , the light absorbing layer has the following formula:

2. The photodiode according to claim 1, wherein light having a wavelength satisfying the following condition is absorbed.

(3) When the absorption coefficient of the contact layer is taken as d and the thickness of the contact layer is d,

e- «xd _<0 . o 5

The photodiode according to claim 1, wherein the following holds.

(4) the semiconductor substrate is an n-type semiconductor substrate, and the light absorbing layer is

Claim range wherein the contact layer is a pin-type photodiode composed of i-type Al _a Ga _1-a As and the contact layer is composed of p-type A 1 _b G an As, and a relationship of 0≤a <b is satisfied. The photodiode according to item 1.

(5) The dielectric film is provided in the light receiving section, and the thickness of the dielectric film is set so that the optical thickness thereof is substantially 1/4 times the wavelength of the light to be detected. Photodiode as described.

(6) A light-emitting element, an optical waveguide, and a light-receiving element having the photodiode according to any one of claims 1 to 5, wherein the light-emitting element and the light-receiving element are directly connected by an optical waveguide. Optical communication system optically connected to

(7) The light emitting element is configured by arranging a plurality of surface emitting lasers having different oscillation wavelengths, and the light receiving element is configured by arranging a plurality of photodiodes capable of detecting at least a transmission wavelength of the surface emitting laser. The optical communication system according to claim 6, wherein the optical communication system is configured as follows. Tem.

(8) A first light receiving / emitting element, an optical waveguide, and a light emitting element each including a light emitting element and a light receiving element having a photodiode according to any one of claims 1 to 5. A light receiving element having the photodiode according to any one of items 1 to 5, and a second light receiving and emitting element including

An optical communication system, wherein the first light receiving and emitting element and the second light receiving and emitting element are directly optically connected by an optical waveguide.

(9) The optical communication system according to claim 8, wherein the first and second light emitting and receiving elements are surface emitting lasers.

(10) The optical communication system according to claim 7 or 9, wherein the surface emitting laser is a vertical cavity surface emitting laser.

(11) The optical communication system according to any one of claims 6 to 10, wherein the optical waveguide is an optical fiber.

(12) The optical communication system according to claim 11, wherein the optical fiber is a fluoroplastic fiber or HPCF.