JP5307750B2 - Semiconductor photo detector - Google Patents

Description

  The present invention relates to a semiconductor light receiving element, and more particularly to a semiconductor light receiving element that enables high-speed and highly efficient operation.

  A photodiode is generally used as a semiconductor light receiving element used for optical communication. The photodiode performs photoelectric conversion by generating electrons and holes by light incidence with a wavelength equal to or greater than the band gap to the semiconductor absorption layer, and taking out the generated electrons and holes as electrical energy.

  In operation, the photodiode depletes the light absorption layer by applying a voltage across a pin junction composed of a p-type semiconductor layer, an i-type light absorption layer, and an n-type semiconductor layer. In this state, the electrons and holes induced in the light absorption layer by the incident light drift in the light absorption layer because of the voltage applied, and then reach the electrode. This is the basic principle of photoelectric conversion in a pin photodiode.

  The response speed of a semiconductor light-receiving element typified by the photodiode described above is generally governed by the CR time constant of the element and the carrier travel time. Therefore, in order to improve the response speed of the semiconductor light receiving element, it is important to improve both.

  First, in order to shorten the traveling time of the carrier, it is conceivable to make the light absorption layer thin. However, if the light absorption layer is made thin, incident light cannot be sufficiently absorbed, and the photoelectric conversion efficiency is lowered. In order to improve this decrease in photoelectric conversion efficiency, a method of forming a mirror made of a metal or dielectric multilayer film on the back surface of the substrate has been proposed for a top-illuminated semiconductor light-receiving element that makes light incident on the substrate from the top surface. ing. By forming the mirror in this way, the incident light that has passed through the light absorption layer can be reflected by the mirror and returned to the light absorption layer again. Even with a thin light absorption layer, the optical path length in the light absorption layer of the incident light Can be effectively increased, and the traveling time can be shortened while suppressing a decrease in photoelectric conversion efficiency.

  Next, in order to reduce the CR time constant, it is necessary to make the light receiving portion small to reduce the capacitance of the element. However, if the light receiving portion is made small by the structure using the reflection on the back surface of the substrate, the optical path length is long due to the thickness of the substrate, and the light reflected by the mirror on the back surface of the substrate greatly spreads, so it returns. The light receiving unit cannot receive light. On the other hand, there is a method of forming a DBR mirror of a semiconductor multilayer film immediately below the light absorption layer (see, for example, Non-Patent Document 1). However, the semiconductor DBR has a large wavelength dependency and is not easy to apply because it is necessary to optimize the mirror configuration depending on the wavelength band to be used.

As a semiconductor light-receiving element that solves the problem of the substrate-side reflection configuration described above, there is a bottom surface incident type in which light is incident from the substrate side. Hereinafter, a bottom-illuminated pin photodiode used for high-speed optical communication will be described as an example. As shown in FIG. 7, this photodiode includes an n-electrode contact layer 702 made of n ⁺ -InP and a light absorption layer 703 made of undoped InGaAs on a semi-insulating substrate 701 made of InP. ^A p-electrode contact layer 704 made of ⁺ -InGaAs is sequentially formed by epitaxial growth. On the p-electrode contact layer 704, a p-electrode / reflection layer 705 made of a Pt / Ti / Pt / Au / Pt / Ti multilayer film is formed. An n electrode 706 made of a Ti / Pt / Au / Pt / Ti multilayer film is formed on a part of the n electrode contact layer 702 exposed by etching.

  In the above-described bottom-illuminated photodiode, incident light that enters from the substrate 701 side and passes through the light absorption layer 703 is reflected by the p-electrode / reflection layer 705 formed on the uppermost surface, and again the light absorption layer 703. I'm trying to get it back. In this structure, an object much thicker than each semiconductor layer such as the substrate 701 does not enter between the light absorption layer 703 and the p electrode / reflection layer 705. For this reason, even if the reflected light spreads by the p electrode / reflective layer 705, the amount of light that reaches the light absorbing layer 703 is very small, and most of the light returns to the light absorbing layer 703 again. For this reason, by adopting a bottom-illuminated type, it is possible to suppress a decrease in photoelectric conversion efficiency even if the light receiving portion is made small.

E. Yagyu, et al., "Guardring-Free Planar AlInAs Avalanche Photodiodes for 2.5-Gb / s Receivers With High Sensitivity", IEEE PHOTONICS TECHNOLOGY LETTERS, Vol.19, No.10, pp.765-767, 2007. Y. Muramoto and T. Ishibashi, "InP = InGaAs pin photodiode structure maximising bandwidth and efficiency", ELECTRONICS LETTERS 27th, Vol.39, No.24, 2003.

  However, in the above-described bottom-illuminated type, the mirror (reflective layer) also serves as an electrode, so that the metal layer serving as the mirror is formed in contact with the semiconductor layer. When the metal layer and the semiconductor layer are in contact with each other in this manner, the metal element of the metal layer diffuses into the semiconductor layer as a result of thermal history due to various heat treatments in the manufacturing process, and between the semiconductor layer and the metal layer. An alloy is formed on the contact surface. In such a state, there is a problem that the steepness at the interface between the semiconductor and the metal layer is lost, and the effect as a mirror cannot be sufficiently exhibited.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to suppress deterioration in performance of a reflective layer in a bottom-illuminated semiconductor light-receiving element having a reflective layer. .

A semiconductor light receiving element according to the present invention includes a first semiconductor layer made of a first conductivity type semiconductor formed on a substrate, a light absorption layer made of a semiconductor formed on the first semiconductor layer, A second semiconductor layer made of a second conductivity type semiconductor formed on the light absorption layer, a first electrode formed in contact with a peripheral portion on the second semiconductor layer, and formed on the first semiconductor layer At least a reflective layer made of a metal formed on a second semiconductor layer inside the first electrode via a dielectric layer, and the light absorption layer has a wavelength of light of interest. The first semiconductor layer and the second semiconductor layer are made of a semiconductor having a larger band gap energy than the semiconductor constituting the light absorption layer, and the first semiconductor layer and The second semiconductor layer has impurities Is the respective conductivity type by introducing the light absorbing layer, impurity concentration than the first semiconductor layer and the second semiconductor layer is a low state, the dielectric layer is a layer of TiO ₂ of the second semiconductor layer side And a stack of SiO ₂ layers formed on the TiO ₂ layer .

  The semiconductor light receiving element includes a second conductivity type light absorption layer made of a second conductivity type semiconductor formed on the light absorption layer, and the second conductivity type light absorption layer is made of the same semiconductor as the light absorption layer. The first conductivity type may be n-type, and the second conductivity type may be p-type. Moreover, you may make it equip the side part of a board | substrate with the light-incidence surface which consists of a facet formed in the inside of a board | substrate and made an acute angle with respect to the plane of a board | substrate.

  As described above, according to the present invention, the reflective layer made of metal formed on the second semiconductor layer inside the first electrode via the dielectric layer is provided, so the reflective layer is provided. An excellent effect is obtained in that deterioration of the performance of the reflective layer in the bottom-illuminated semiconductor light-receiving element can be suppressed.

FIG. 1A is a cross-sectional view showing the configuration of the semiconductor light receiving element in the first exemplary embodiment of the present invention. FIG. 1B is a cross-sectional view showing a configuration of the semiconductor light receiving element in the first exemplary embodiment of the present invention. FIG. 2 is a characteristic diagram showing a change in reflectance depending on the layer thickness of the SiO ₂ layer constituting the dielectric layer 108. FIG. 3 is a characteristic diagram showing the wavelength dependence of the reflectance in the reflective layer 109 when the dielectric layer 108 is formed with the thickness of the TiO ₂ layer being 6 nm and the thickness of the SiO ₂ layer being 0.23 μm. is there. FIG. 4 is a correlation diagram showing the light absorption layer thickness dependence of the reflectance and the light receiving sensitivity in the reflective layer. FIG. 5A is a cross-sectional view showing the configuration of the semiconductor light receiving element according to the second embodiment of the present invention. FIG. 5B is a cross-sectional view showing the configuration of the semiconductor light receiving element in the second exemplary embodiment of the present invention. FIG. 6 is a cross-sectional view showing the configuration of the semiconductor light receiving element in the present embodiment. FIG. 7 is a cross-sectional view showing a configuration of a bottom-illuminated pin photodiode.

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

[Embodiment 1]
First, Embodiment 1 of the present invention will be described. 1A and 1B are cross-sectional views showing the configuration of the semiconductor light receiving element according to the first embodiment of the present invention. 1B is a cross section taken along line bb in FIG. 1A.

  The semiconductor light receiving element in the present embodiment includes a first semiconductor layer 102 made of a first conductivity type semiconductor formed on a substrate 101, and a light absorption layer made of a semiconductor formed on the first semiconductor layer 102. 103, a second semiconductor layer 104 made of a semiconductor of the second conductivity type formed on the light absorption layer 103, a first electrode 105 formed in contact with the peripheral portion on the second semiconductor layer 104, At least a second electrode 106 formed on the first semiconductor layer 102 and a reflective layer 109 made of metal formed on the second semiconductor layer 104 inside the first electrode 105 via a dielectric layer 108 are provided. .

  Here, the light absorption layer 103 is composed of a semiconductor having a band gap energy corresponding to the wavelength of light of interest, and the first semiconductor layer 102 and the second semiconductor layer 104 constitute the light absorption layer 103. The first semiconductor layer 102 and the second semiconductor layer 104 are made of each conductivity type by introducing impurities, and the light absorption layer 103 is made of the first semiconductor layer 102. In addition, the impurity concentration may be lower than that of the second semiconductor layer 104.

For example, the substrate 101 may be a semiconductor substrate made of semi-insulating InP. The first semiconductor layer 102 only needs to be made of n ⁺ -InP into which an n-type impurity is introduced at a high concentration. Moreover, the light absorption layer 103 should just be comprised from undoped InGaAs. The second semiconductor layer 104 only needs to be made of p ⁺ -InGaAsP into which a p-type impurity is introduced at a high concentration. In these cases, the first conductivity type described above is n-type, and the second conductivity type is p-type.

  The light absorption layer 103 and the second semiconductor layer 104 are patterned into a desired shape, a part of the first semiconductor layer 102 is exposed, and the second electrode 106 is formed in the exposed region. For example, the light absorption layer 103 and the second semiconductor layer 104 are processed into a cylindrical shape having a diameter of about 22 μm. Further, in the present embodiment, the side portion of the semiconductor light receiving element formed of the substrate 101, the first semiconductor layer 102, the light absorption layer 103, and the second semiconductor layer 104 is covered with a protective film 107 made of polyimide, for example. It has been broken. In the present embodiment, the dielectric layer 108 extends from between the reflective layer 109 and the second semiconductor layer 104, between the reflective layer 109 and the first electrode 105, and almost all over the outer side surface of the protective film 107. Forming. Although not shown in the drawing, the first electrode 105 and the second electrode 106 are connected to lead wires, respectively.

  Hereinafter, a method for manufacturing the semiconductor light receiving element in the present embodiment will be briefly described. First, n-type InP (first semiconductor layer 102), undoped InGaAs (light absorption layer 103), and p-type InGaAsP (second semiconductor layer 104) are formed on a substrate 101 made of semi-insulating InP. Deposit sequentially. These may be formed by a well-known metal organic chemical vapor deposition method. For example, Si may be used as an impurity for the n-type layer, and Zn may be used as an impurity for the p-type layer, for example.

  Next, the first electrode 105 is formed on the InGaAsP layer to be the second semiconductor layer 104. For example, a resist pattern in which the formation region of the first electrode 105 is opened is formed, and Pt, Ti, and Au are sequentially deposited thereon by an electron beam evaporation method to form a Pt / Ti / Au multilayer film. Thereafter, if the resist pattern is removed, the first electrode 105 composed of a Pt / Ti / Au multilayer film can be formed (lift-off method). The first electrode 105 may be formed in a circular ring shape having an inner diameter of 18 μm and an outer diameter of about 22 μm, for example, in plan view.

  Next, the undoped InGaAs layer and the p-type InGaAsP layer are patterned by a known lithography technique and wet etching to form the cylindrical light absorption layer 103 and the second semiconductor layer 104. Next, a second electrode composed of a Ti / Pt / Au / Pt / Ti multilayer film is formed at a desired location of the first semiconductor layer 102 exposed by this patterning in the same manner as the first electrode 105 described above. 106 is formed.

  Next, a protective film 107 is formed to cover the substrate 101, the first semiconductor layer 102, the light absorption layer 103, the side portions of the second semiconductor layer 104, and the exposed portion of the first semiconductor layer 102. For example, the protective film 107 can be formed by applying a polyimide resin and heating and curing the coating film at about 200 ° C.

Next, the dielectric layer 108 is formed. For example, a TiO ₂ layer and a SiO ₂ layer may be sequentially deposited by sputtering to form the dielectric layer 108 composed of a laminate of TiO ₂ and SiO ₂ . Here, in order to develop the function of the dielectric layer 108, only SiO ₂ is sufficient, but for the purpose of improving the adhesion and wettability of the interface between the second semiconductor layer 104 and SiO _2. Insert a layer of TiO ₂ .

  Next, the reflective layer 109 is formed. For example, the reflective layer 109 may be formed by selectively depositing Au by an electron beam method. The reflective layer 109 is formed in a region inside the region where the first electrode 105 is formed. Note that the above-described lead-out wiring and the like are also formed.

  As described above, according to the present embodiment, the reflective layer 109 made of metal is formed on the second semiconductor layer 104 via the dielectric layer 108, and thus heat generated by various heat treatments in the manufacturing process. Even if a history is added, diffusion of metal elements from the reflective layer 109 to the dielectric layer 108 and formation of an alloy at the interface are suppressed. As a result, it is suppressed that the steepness at the interface in the reflective layer 109 is impaired, performance deterioration can be suppressed, and the function as the reflective layer 109 is sufficiently exhibited.

Next, the dielectric layer 108 will be described in more detail. First, in the semiconductor light receiving element in the present embodiment, an incident wave incident from the substrate 101 side (back surface side) and a reflected wave in which incident light is reflected by the reflective layer 109 may be attenuated by interference. . FIG. 2 shows the change in reflectivity depending on the thickness of the SiO ₂ layer constituting the dielectric layer 108. FIG. 2 shows three cases in which the thickness of the TiO ₂ layer constituting the dielectric layer 108 is 6 nm (solid line), 30 nm (one-dot chain line), and 60 nm (dotted line). The wavelength of incident light is 1.55 μm.

Regardless of the thickness of the TiO ₂ layer, there is an optimum value for the maximum reflectance in the thickness of the SiO ₂ layer, and the reflectance decreases both when the thickness is thicker and thinner than this maximum value. I understand that. It can also be seen that the smaller the TiO ₂ layer thickness, the higher the maximum reflectance. As described above, the reflectivity varies depending on the layer thickness of the dielectric layer 108. Therefore, the layer thickness of the dielectric layer 108 may be set as appropriate by obtaining a value by which the maximum reflectivity can be obtained through experiments. The structure without the TiO ₂ layer has the best reflectivity, but the TiO ₂ layer is important for improving the adhesion and wettability between the semiconductor layer and the SiO ₂ layer. For this reason, the TiO ₂ layer is required to have a minimum layer thickness that can reliably cover the entire surface by sputtering or the like.

In the present embodiment, the TiO ₂ layer has a thickness of about 6 nm that can be reliably deposited by sputtering. By setting the thickness of the TiO ₂ layer, the maximum reflectance can be obtained when the thickness of the SiO ₂ layer is 0.23 μm. If the uniform layer can be made thinner by the film forming method used, the TiO ₂ layer may be made thinner than 6 nm.

FIG. 3 is a characteristic diagram showing the wavelength dependence of the reflectance in the reflective layer 109 when the dielectric layer 108 is formed with the thickness of the TiO ₂ layer being 6 nm and the thickness of the SiO ₂ layer being 0.23 μm. is there. In the optimized layer thickness of the SiO ₂ layer, the reflectance is about 96.3% at a wavelength of 1.55 μm and about 95.5% at a wavelength of 1.3 μm. As described above, the semiconductor light receiving element in the present embodiment is applicable to both wavelengths of 1.55 μm band and 1.3 μm band that are generally used for optical communication. .

  As an example of the effect of improving the reflectance as described above, the junction area (light receiving area) of the element is set to 24 μm in diameter and the thickness of the light absorption layer is set in order to obtain a semiconductor light receiving element having a 3 dB band of 20 GHz or more as an example. The case of 0.8 μm will be described. Here, the semiconductor light receiving element described with reference to FIG. 7 is compared with the semiconductor light receiving element in the present embodiment described above. In the semiconductor light receiving element in which the electrode / reflecting mirror is directly formed on the semiconductor layer, the reflectance becomes 50% or less and the light receiving sensitivity becomes 0.65 A / W or less through the temperature history during the manufacturing process.

  On the other hand, in the semiconductor light receiving element in the present embodiment in which the reflective layer is formed through the dielectric layer, a high reflectance of 95% or more is obtained even after a temperature history during the manufacturing process, and the light receiving sensitivity is 0. 80 A / W or more.

  Next, the dependence of the reflectance and light receiving sensitivity on the thickness of the light absorption layer will be described. FIG. 4 is a correlation diagram showing the light absorption layer thickness dependence of the reflectance and the light receiving sensitivity in the reflective layer. In FIG. 4, a white circle indicates a case where the reflectance is 100%, a white triangle indicates a reflectance of 50%, and a white square indicates a case where the reflectance is 0%. When the thickness of the light absorption layer is as thick as 3 μm, most of the incident light can be absorbed by the light absorption layer, so that there is no significant difference in light receiving sensitivity even if the reflectance is poor. However, when the light absorption layer is made thin in order to obtain a semiconductor light-receiving element capable of high-speed response, a large difference in quantum efficiency occurs depending on the reflectance. This is because the incident light that has not been absorbed by the light absorption layer is increased, and the contribution ratio of the reflected light from the reflection mirror is relatively increased.

  As is clear from these facts, according to the present embodiment that makes it possible to improve the reflectivity of the reflecting mirror, particularly when the light absorption layer is made thin in order to obtain a semiconductor light-receiving element capable of high-speed response. Compared with the semiconductor light receiving element described with reference to FIG. 7, it is possible to achieve higher speed and higher sensitivity.

[Embodiment 2]
Next, a second embodiment of the present invention will be described. 5A and 5B are cross-sectional views showing the configuration of the semiconductor light receiving element according to the second embodiment of the present invention. FIG. 5B is a cross section taken along line bb of FIG. 5A.

  The semiconductor light receiving element in this embodiment includes a first semiconductor layer 502 made of an n-type semiconductor formed on a substrate 501, a light absorption layer 503 made of a semiconductor formed on the first semiconductor layer 502, and the like. A p-type light absorption layer 504 made of a p-type semiconductor formed on the light absorption layer 503, a second semiconductor layer 505 made of a p-type semiconductor formed on the p-type light absorption layer 504, and The first electrode 506 formed in contact with the peripheral portion on the second semiconductor layer 505, the second electrode 507 formed on the first semiconductor layer 502, and the second semiconductor layer 505 inside the first electrode 506 At least a reflective layer 510 made of metal formed on a dielectric layer 509 is provided.

  Here, the light absorption layer 503 and the p-type light absorption layer 504 are composed of a semiconductor having a band gap energy corresponding to the wavelength of light of interest, and the first semiconductor layer 502 and the second semiconductor layer 505 are: The first semiconductor layer 502, the p-type light absorption layer 504, and the second semiconductor layer 505 are each made conductive by introducing impurities into the semiconductor that has a larger band gap energy than the semiconductor constituting the light absorption layer 503. The light absorption layer 503 may have a lower impurity concentration than the first semiconductor layer 502, the p-type light absorption layer 504, and the second semiconductor layer 505.

For example, the substrate 501 may be a semiconductor substrate made of semi-insulating InP. The first semiconductor layer 502 only needs to be made of n ⁺ -InP into which an n-type impurity is introduced at a high concentration. Further, the light absorption layer 503 may be made of undoped InGaAs. Further, the p-type light absorption layer 504 may be made of InGaAs into which p-type impurities are introduced. The second semiconductor layer 505 only needs to be made of p ⁺ -InGaAsP into which a p-type impurity is introduced at a high concentration.

  The light absorption layer 503, the p-type light absorption layer 504, and the second semiconductor layer 505 are patterned into a desired shape, and a part of the first semiconductor layer 502 is exposed, and the second electrode 507 is exposed in this exposed region. Is formed. For example, the light absorption layer 503, the p-type light absorption layer 504, and the second semiconductor layer 505 are processed into a cylindrical shape having a diameter of about 22 μm. In this embodiment, the side portion of the semiconductor light-receiving element including the substrate 501, the first semiconductor layer 502, the light absorption layer 503, the p-type light absorption layer 504, and the second semiconductor layer 505 is, for example, polyimide. It is covered with a protective film 508 made of In this embodiment mode, the dielectric layer 509 extends from between the reflective layer 510 and the second semiconductor layer 505 to between the reflective layer 510 and the first electrode 506 and almost the entire outer side surface of the protective film 508. Forming. Although not shown, the first electrode 506 and the second electrode 507 are connected to lead wires, respectively.

  Hereinafter, a method for manufacturing the semiconductor light receiving element in the present embodiment will be briefly described. First, on a substrate 501 made of semi-insulating InP, n-type InP (first semiconductor layer 502), undoped InGaAs (light absorption layer 503), p-type InGaAs (p-type light absorption layer 504), Then, p-type InGaAsP (second semiconductor layer 505) is sequentially deposited. These may be formed by a well-known metal organic chemical vapor deposition method. For example, Si may be used as an impurity for the n-type layer, and Zn may be used as an impurity for the p-type layer, for example.

  Next, the first electrode 506 is formed on the InGaAsP layer to be the second semiconductor layer 505. For example, a resist pattern in which the formation region of the first electrode 506 is opened is formed, and Pt, Ti, and Au are sequentially deposited thereon by an electron beam vapor deposition method to form a Pt / Ti / Au multilayer film. Thereafter, if the resist pattern is removed, the first electrode 506 composed of a Pt / Ti / Au multilayer film can be formed (lift-off method). The first electrode 506 may be formed in a ring shape having a plan view, an inner diameter of 18 μm, and an outer diameter of about 22 μm, for example.

  Next, the undoped InGaAs layer, the p-type InGaAs layer, and the p-type InGaAsP layer are patterned by a known lithography technique and wet etching to form a cylindrical light absorption layer 503 and a p-type light absorption layer 504. , And a second semiconductor layer 505 are formed. Next, a second electrode composed of a Ti / Pt / Au / Pt / Ti multilayer film is formed at a desired portion of the first semiconductor layer 502 exposed by this patterning in the same manner as the first electrode 506 described above. 507 is formed.

  Next, a protective film 508 is formed to cover the substrate 501, the first semiconductor layer 502, the light absorption layer 503, the p-type light absorption layer 504, the side portions of the second semiconductor layer 505, and the exposed portion of the first semiconductor layer 502. . For example, the protective film 508 can be formed by applying a polyimide resin and curing the coating film by heating to about 200 ° C.

Next, the dielectric layer 509 is formed. For example, a TiO ₂ layer and a SiO ₂ layer may be sequentially deposited by sputtering to form a dielectric layer 509 composed of a laminate of TiO ₂ and SiO ₂ . Here, as the function of the dielectric layer 509, SiO ₂ alone is sufficient, but for the purpose of improving the adhesion and wettability of the interface between the second semiconductor layer 505 and SiO ₂ , the function of TiO ₂ is improved. Insert a layer.

  Next, the reflective layer 510 is formed. For example, the reflective layer 510 may be formed by selectively depositing Au by an electron beam method. The reflective layer 510 is formed in a region inside the region where the first electrode 506 is formed. Note that the above-described lead-out wiring and the like are also formed.

  As described above, also in this embodiment, the reflective layer 510 made of metal is formed on the second semiconductor layer 505 with the dielectric layer 509 interposed therebetween, so that the thermal history due to various heat treatments in the manufacturing process. However, the diffusion of the metal element from the reflective layer 510 to the dielectric layer 509 and the formation of an alloy at the interface are suppressed. As a result, it is suppressed that the steepness at the interface in the reflective layer 510 is impaired, performance deterioration can be suppressed, and the effect as the reflective layer 510 can be sufficiently obtained.

  In addition, the present embodiment includes a p-type light absorption layer 504 unlike the above-described embodiment. The p-type light absorption layer 504 is configured so that the entire region of the p-type light absorption layer 504 is not depleted even when a voltage is applied to operate the semiconductor light receiving element and the light absorption layer 503 made of undoped InGaAs is depleted. The doping concentration distribution is determined according to the applied voltage and the layer thickness. However, if an attempt is made to prevent the connection surface with the light absorption layer 503 from becoming depleted, it is necessary to dope at a high concentration until it becomes a state close to a semimetal.

  In this embodiment mode, light absorption occurs in both the light absorption layer 503 and the p-type light absorption layer 504, and the transport mechanism of the generated carriers is independent. Therefore, due to the characteristics of the semiconductor light receiving element itself, even if the layer thickness of the light absorption layer (the combined thickness of the undoped light absorption layer 503 and the p-type light absorption layer 504) is increased in order to increase the quantum efficiency, the high speed response is achieved. (For example, refer nonpatent literature 2). In addition, according to the present embodiment, since the reflection layer 510 is configured to improve the reflectivity (a reduction in the reflectivity can be suppressed), the light is reduced in a state where the decrease in the light receiving sensitivity is suppressed. It is possible to further increase the speed by thinning the absorption layer.

  For example, in the case of a semiconductor light receiving element having a 3 dB band of 20 GHz or more, the junction area of the element is 24 μm in diameter, the thickness of the undoped light absorbing layer is 0.7 μm, and the thickness of the light absorbing layer doped with p-type impurities is 0. .5 μm. As a result, the total thickness of the light absorption layer is 1.2 μm. Even with this thickness, since the light absorption layer doped with the p-type impurity is provided, a faster response is possible than in the case of the first embodiment.

  Here, as described above, by adding the light absorption layer doped with the p-type impurity to increase the total thickness of the light absorption layer, the reflection layer can also be used as an electrode as described with reference to FIG. Even when formed directly on a semiconductor, the quantum efficiency is about 0.85 A / W.

  On the other hand, the light receiving sensitivity is improved to about 1.0 A / W by forming the reflective layer on the semiconductor layer through the dielectric layer so that the reflectance of 95% or more can be maintained. Higher sensitivity. On the other hand, when the light receiving sensitivity may be about 0.85 A / W, the entire thickness of the light absorption layer can be reduced to 0.8 μm, and a further high-speed response is possible.

[Embodiment 3]
Next, a third embodiment of the present invention will be described. FIG. 6 is a cross-sectional view showing the configuration of the semiconductor light receiving element in the present embodiment. The semiconductor light receiving element in the present embodiment includes a first semiconductor layer 602 made of a first conductivity type semiconductor formed on a substrate 601 and a light absorption layer made of a semiconductor formed on the first semiconductor layer 602. 603, a second semiconductor layer 604 made of a second conductivity type semiconductor formed on the light absorption layer 603, a first electrode 605 formed in contact with a peripheral portion on the second semiconductor layer 604, At least a second electrode 606 formed on the first semiconductor layer 602 and a reflective layer 609 made of metal formed on the second semiconductor layer 604 inside the first electrode 605 via a dielectric layer 608 are provided. .

  Here, the light absorption layer 603 is composed of a semiconductor having a band gap energy corresponding to the wavelength of light of interest, and the first semiconductor layer 602 and the second semiconductor layer 604 constitute the light absorption layer 603. The first semiconductor layer 602 and the second semiconductor layer 604 are made to have respective conductivity types by introducing impurities, and the light absorption layer 603 includes the first semiconductor layer 602. In addition, the impurity concentration may be lower than that of the second semiconductor layer 604.

For example, the substrate 601 may be a semiconductor substrate made of semi-insulating InP. The first semiconductor layer 602 only needs to be made of n ⁺ -InP into which an n-type impurity is introduced at a high concentration. Moreover, the light absorption layer 603 should just be comprised from undoped InGaAs. The second semiconductor layer 604 may be made of p ⁺ -InGaAsP into which a p-type impurity is introduced at a high concentration. In these cases, the first conductivity type described above is n-type, and the second conductivity type is p-type.

  Note that the light absorption layer 603 and the second semiconductor layer 604 are patterned into a desired shape, a part of the first semiconductor layer 602 is exposed, and a second electrode 606 is formed in the exposed region. For example, the light absorption layer 603 and the second semiconductor layer 604 are processed into a cylindrical shape having a diameter of about 22 μm. In this embodiment, the side portion of the semiconductor light-receiving element formed of the substrate 601, the first semiconductor layer 602, the light absorption layer 603, and the second semiconductor layer 604 is covered with a protective film 607 made of, for example, polyimide. It has been broken. In the present embodiment, the dielectric layer 608 extends from between the reflective layer 609 and the second semiconductor layer 604 to between the reflective layer 609 and the first electrode 605 and almost the entire outer side surface of the protective film 607. Forming. Although not shown, the first electrode 605 and the second electrode 606 are connected to lead-out wirings, respectively.

  The above configuration is almost the same as that of the first embodiment. In this embodiment mode, the dielectric layer 608 is disposed between the second semiconductor layer 604 and the reflective layer 609 so that the reflective layer 609 is in contact with the first electrode 605. The reflective layer 609 may be formed without being in contact with the semiconductor layer, and there is no problem even if it is in contact with the first electrode 605.

  Further, in the present embodiment, a known end-face incidence refracting type semiconductor light-receiving element is provided that has a facet surface 611 formed inwardly on the side of the substrate 601 and has the facet surface 611 as a light incident surface. .

  The facet 611 can be formed, for example, by processing the side wall of the substrate 601 made of InP by wet etching so as to form an acute angle with the surface of the light absorption layer 603 (substrate plane). Note that the substrate 601 side (inner side) surface of the facet surface 611 may be formed to have an acute angle with respect to the plane of the substrate 601 on the first semiconductor layer 602 side. The incident light is refracted at the facet 611 having an acute angle with respect to the substrate plane and is incident on the light absorption layer 603. Incident light is incident obliquely with respect to the plane of the light absorption layer 603. By doing so, the optical path lengths of incident light and reflected light in the light absorption layer 603 are increased, and light receiving sensitivity substantially equivalent to that obtained when the thickness of the light absorption layer 603 is increased can be obtained.

  Here, an example of a method for forming the facet 611 will be briefly described. An InP substrate having a principal plane orientation equivalent to the (001) plane is used, one end of the equivalent plane is left on the InP (−110) plane serving as a side wall, and the other portions are protected with a resist or the like. Next, the exposed surface is anisotropic by wet chemical etching using an etching solution such as a mixed solution of bromine and methanol, a mixed solution of odorous acid, hydrochloric acid and phosphoric acid, and a mixed solution of sulfuric acid and hydrogen peroxide. Etch. As a result, the (111) B surface or an equivalent surface is newly exposed, and the facet 611 can be formed in which the angle formed with the surface of the substrate 601 on the first semiconductor layer 602 side is about 54.7 degrees. .

  When incident light parallel to the plane of the substrate 601 is incident on the facet 611 formed by the above-described wet etching, the incident light is refracted by the facet 611 and taken into the light absorption layer 603. Here, when the reflective layer 609 and the dielectric layer 608 are configured in the same manner as in the first embodiment, a reflectance of 95% or more can be obtained, so that the light receiving sensitivity is further improved. Further, according to the present embodiment, since incident light can be introduced from the side surface, there is an advantage that it is not necessary to make a bottom incident hole when mounting on the circuit board of the optical module, and it is easy to mount.

  The present invention is not limited to the embodiment described above, and many modifications and combinations can be implemented by those having ordinary knowledge in the art within the technical idea of the present invention. It is obvious.

  For example, in the above description, the semiconductor light receiving element of the pin diode type has been described as an example, but the present invention is not limited to this. The present invention can be similarly applied to any semiconductor light-receiving element including avalanche photodiodes and single-carrier traveling photodiodes that emit light in the layer stacking direction.

  The semiconductor light receiving element of the present invention includes a first semiconductor layer made of a first conductivity type semiconductor, a light absorption layer made of a semiconductor formed on the first semiconductor layer, and a light absorption layer, which are sequentially stacked on a substrate. A second semiconductor layer made of a semiconductor of a second conductivity type formed on the layer, a first electrode formed in contact with a peripheral portion on the second semiconductor layer, and a second electrode formed on the first semiconductor layer And a reflective layer made of metal formed on the second semiconductor layer inside the first electrode via a dielectric layer.

Further, the dielectric layer has been described as a laminated structure of TiO ₂ / SiO ₂ , but other oxides such as Al ₂ O ₃ and Ta ₂ O ₅ as long as they do not have a large absorption coefficient in the wavelength band of incident light, Further, Si ₃ N _4, TiN, and may be composed of nitrides such as AlN. In addition, when there is sufficient adhesion and wettability with the semiconductor surface on which the dielectric layer is formed, it is not necessary to have a multilayer structure like TiO ₂ / SiO ₂ . Even when the material is changed, the layer thickness of the dielectric layer is the layer thickness that is more than the minimum necessary to prevent troubles in the process as in the case of TiO ₂ / SiO ₂ and has the maximum reflectance. You can make it thick.

  In addition, although the metal constituting the reflective layer has been described as Au, it is a metal material that can sufficiently ensure the reflectance with respect to the wavelength of incident light, and does not diffuse and alloy with the dielectric layer used in combination. I just need it. For example, examples of the material constituting the reflective layer include Ag, Pt, Al, Ni, Co, W, Cu, and Ti.

  Further, although the light receiving area has been described as having a diameter of 24 μm, the light receiving area may have other dimensions depending on a required band. In the above description, the InGaAs / InP compound semiconductor is used. However, the present invention is not limited to this. For example, a compound semiconductor such as P-based, As-based, N-based, or Sb-based may be used, or a single-element semiconductor material such as Ge-based or Si-based may be used.

  Needless to say, in the first and third embodiments, the conductivity types may be interchanged. In the second embodiment, a p-type semiconductor layer, a p-type light absorption layer, an i-type light absorption layer, and an n-type semiconductor layer are stacked in this order from the substrate side to form an n-type semiconductor. A reflective layer may be formed on the layer via a dielectric layer.

  In the above description, the electrode on the side where the reflective layer is provided is formed in a circular ring shape, but the present invention is not limited to this. For example, it may be formed in a frame shape having an outer shape such as a quadrangle and a hexagon, and a region capable of reflecting light incident from the substrate side is provided in the reflective layer forming the electrode. That's fine. Further, although polyimide is used as the protective film, the present invention is not limited to this, and an organic insulating material such as BCB (Benzocyclobutene) may be used.

  DESCRIPTION OF SYMBOLS 101 ... Substrate, 102 ... First semiconductor layer, 103 ... Light absorption layer, 104 ... Second semiconductor layer, 105 ... First electrode, 106 ... Second electrode, 107 ... Protective film, 108 ... Dielectric layer, 109 ... Reflection layer.

Claims (3)

A first semiconductor layer made of a first conductivity type semiconductor formed on a substrate;
A light absorption layer made of a semiconductor formed on the first semiconductor layer;
A second semiconductor layer made of a second conductivity type semiconductor formed on the light absorption layer;
A first electrode formed in contact with a peripheral portion on the second semiconductor layer;
A second electrode formed on the first semiconductor layer;
A reflection layer made of metal formed on the second semiconductor layer inside the first electrode via a dielectric layer, and
The light absorption layer is composed of a semiconductor having a band gap energy corresponding to the wavelength of light of interest,
The first semiconductor layer and the second semiconductor layer are made of a semiconductor having a larger band gap energy than the semiconductor constituting the light absorption layer,
The first semiconductor layer and the second semiconductor layer are each of the conductivity type by introducing impurities,
The light absorption layer has a lower impurity concentration than the first semiconductor layer and the second semiconductor layer ,
The dielectric layer is composed of a stacked layer of a TiO ₂ layer on the second semiconductor layer side and a SiO ₂ layer formed on the TiO ₂ layer . The semiconductor light-receiving element according to claim 1.
A second conductivity type light absorption layer made of a second conductivity type semiconductor formed on the light absorption layer;
The second conductivity type light absorption layer is composed of the same semiconductor as the light absorption layer,
The semiconductor light receiving element, wherein the first conductivity type is n-type and the second conductivity type is p-type. The semiconductor light receiving element according to claim 1 or 2,
A semiconductor light-receiving element comprising a light incident surface formed of a facet formed on a side portion of the substrate so as to enter the inside of the substrate and having an acute angle with respect to a plane of the substrate.

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