Semiconductor photo detector

A configuration of a semiconductor photo detector in example 1 is shown in Table 1 and FIG. 2 .

The semiconductor photo detector of example 1 is manufactured as follows. An n-type selective etching layer 202 , an n-type buffer layer 203 , a multiplier layer 204 , a p-type field-relaxing layer 205 , a p-type light absorption layer 206 , a p-type selective etching layer 207 , a p-type buffer layer 208 , a p-type contact layer 209 and a p-type electrode 210 are sequentially deposited on the semiconductor substrate 201 in this order from the semiconductor substrate 201 to form an epitaxial crystal.

First of all, a first mesa-etching process is conducted to partially remove the p-type buffer layer 208 and the p-type contact layer 209 above the p-type selective etching layer 207 through a selective etching to form a circular first mesa. In such case, several circular first mesas having a diameter d 1 of: d 1 =10, 20, 30, 50 and 100 μm, respectively; are manufactured.

Then, a second mesa etching process is conducted through a mask that is concentric with the first mesa to form a second mesa including a p-n junction. In such case, several circular second mesas having a diameter d 2 of: d 2 =d 1 +2, 5, 10, 20 μm, respectively; are manufactured. The height of the mesa may be designed to be larger than the thickness of the epitaxial layer.

A vertical difference between the deepest position of the etched depth and the top of the mesa may be assumed to be about 3 to 5 μm, and in the case of the present example, not smaller than 2.6 μm.

A passivation film 212 is formed over the entire mesa. Apertures for electrodes are formed in the passivation film 212 , and then the p-type electrode 210 is formed in the upper section of the mesa, and the n-type electrode 211 is formed to be in contact with an n-type semiconductor layer (or n-substrate) in the outer circumference of the mesa and associated interconnects are formed to the top of the mesa for the future installation.

The semiconductor element formed in this way is configured that the electric field right under the outer circumference of the selective etching layer is increased over the regions of the p-type light absorption layer 206 and the p-type field-relaxing layer 205 , and the multiplier layer 204 , as compared with the side wall section having the interface between the semiconductor element and the passivation film. Such effect is larger for the element having larger d 2 . This allows relatively reducing amperage of an electric current generated in the interface between the semiconductor element and the passivation film to obtain an advantageous effect of an improved reliability.

A dark current generated in the inside of the layer structure is increased in relation to the flow of a dark current in the condition containing no photocarrier, as compared with a dark career caused in the interface between the semiconductor element and the passivation film, and an operation with relatively smaller side wall current can be achieved in a condition without a light incidence. On the other hand, since amperage of a dark current depends on d 1 and d 2 , larger d 1 , or in the cases of the same d 1 , larger d 2 , likely provides larger dark current, which provides a tendency of decreased sensibility in these elements. Therefore, d 1 and d 2 may be suitably determined according to the purposes (applications) of the elements.

Second Exemplary Embodiment

Configurations of second exemplary embodiment according to the present invention will be described in reference to FIG. 3 . The semiconductor photo detector of second exemplary embodiment has a layer-structure, including a selective etching layer 302 of the first-type conductivity, a buffer layer 303 of the first-type conductivity, a multiplier layer 304 (undoped layer), a field-relaxing layer 305 of the second-type conductivity, a light absorption layer 306 of the second-type conductivity, a selective etching layer 307 of the second-type conductivity, a buffer layer 308 of the second-type conductivity, a contact layer 309 of the second-type conductivity, an electrode in the side of the second-type conductivity 310 , which are sequentially deposited on a semiconductor substrate 301 .

A second mesa is formed on the semiconductor substrate 301 , and a first mesa is formed on the second mesa. The first mesa includes the buffer layer 308 of the second-type conductivity, the contact layer 309 of the second-type conductivity, and the electrode 310 in the side of the second-type conductivity. The second mesa includes the selective etching layer 302 of the first-type conductivity, the buffer layer 303 of the first-type conductivity, the multiplier layer 304 (undoped layer), the field-relaxing layer 305 of the second-type conductivity, the light absorption layer 306 of the second-type conductivity, and the selective etching layer 307 of the second-type conductivity.

The outer periphery of the second mesa is located outside of the outer periphery of the first mesa in two-dimensional view, and the side surfaces of the first mesa and the second mesa are covered with a passivation film.

The semiconductor photo detector of second exemplary embodiment is manufactured as described below. First of all, the selective etching layer 302 of the first-type conductivity, the first buffer layer 303 , the multiplier layer 304 , the field-relaxing layer 305 of the second-type conductivity, the light absorption layer 306 of the second-type conductivity, the selective etching layer 307 of the second-type conductivity, the buffer layer 308 of the second-type conductivity, and the contact layer 309 of the second-type conductivity are sequentially deposited on the semiconductor substrate 301 in this order from the semiconductor substrate 301 to form an epitaxial crystal. For example, the semiconductor substrate 301 may be an InP substrate.

Subsequently, a first mesa-etching process is conducted to partially remove the buffer layer 308 of the second-type conductivity and the contact layer 309 of the second-type conductivity above the selective etching layer 307 of the second-type conductivity through a selective etching to form a circular first mesa (buffer layer 308 of the second-type conductivity, contact layer 309 of the second-type conductivity, electrode in the side of the second-type conductivity 310 ).

Here, the multiplier layer 304 , the field-relaxing layer 305 of the second-type conductivity and the light absorption layer 306 of the second-type conductivity may be formed of an As-containing material (for example, InAlAs, InGaAs, InAlGaAs or the like).

In such case, the selective etching layer 307 of the second-type conductivity may be formed of either InP or In x Ga (1-x) As y P (1-y) . More specifically, a P-related material or a P-containing material (for example, InP, InGaAsP) may be employed for the selective etching layer 307 of the second-type conductivity. While the use of these materials allows ensuring larger selectivity for a wet etching process, the selectivity is not necessarily larger in such case.

In such process, acetic acid, phosphoric acid, hydrogen peroxide or the like may be employed as a wet etchant solution to carry out the first mesa etching process.

Then, a mask having a mesa diameter that is larger than the dimension of the first mesa is formed, and then a second mesa etching process is carried out to etching the whole epitaxial layer, thereby forming the second mesa having a p-n junction (the selective etching layer 302 of the first-type conductivity, the buffer layer 303 of the first-type conductivity, the multiplier layer 304 , the field-relaxing layer 305 of the second-type conductivity, and the selective etching layer 307 of the second-type conductivity). In such case, the thickness of the mask formed on the surface of the first mesa may be not smaller than 0.5 μm. The upper limit of the thickness of the mask is not particularly determined, and may preferably be not larger than 1.0 μm.

After second mesa etching process, a selective etching process may be conducted for the selective etching layer 302 of the first-type conductivity to provide a structure having a reduced dimension in the transverse direction (surface direction). In such case, a structure having a reduced dimension in the transverse direction (surface direction) may also be provided for the etching layer 307 of the second-type conductivity, depending on the combination of the layers.

Subsequently, after the second mesa etching process, a hydrogen treatment is carried out from the side of the front surface to form a hydrogen passivation 313 . The hydrogen treatment may be conducted via a hydrogen annealing process, or a plasma irradiation with a source material containing hydrogen. Alternatively, such hydrogen treatment operation may be omitted if a passivation film 312 that provides a hydrogen passivation is formed. In the hydrogen treatment, a hydrogenation of the first mesa can be avoided if a thick mask is formed on the first mesa. This allows providing the similar advantageous effects, even if a hydrogen treatment is carried out between the first mesa etching process and second mesa etching process.

When an effect of hydrogen passivation is exhibited in the upper surface of the first mesa during the formation of the hydrogen passivation 313 by the hydrogen treatment and during the formation of the passivation film 312 for providing the hydrogen passivation, the contact resistance with the electrode 310 in the side of the second-type conductivity or the resistance of the contact layer 309 of the second-type conductivity may be possibly increased. Consequently, in case of the formation of the hydrogen passivation 313 by the hydrogen treatment, or before the process for forming the passivation film 312 for providing a hydrogen passivation, a passivation film 314 that provides no hydrogen passivation effect is formed in the upper section of the first mesa. This allows minimizing the passivation effect for the second-type conductivity, avoiding unwanted increased in the resistances of the electrode 310 in the side of the second-type conductivity and the contact layer 309 of the second-type conductivity.

Alternatively, another technique may involves forming the selective etching layer 307 of the second-type conductivity with In x Al y Ga (1-x-y) As where y≧0.25. As described above, a layer containing aluminum (Al) at a larger concentration may be employed as the selective etching layer 307 of the second-type conductivity to allow using a mask exhibiting no hydrogen passivation effect in the first mesa etching process. In such configuration, a layer containing Al at smaller content (<25%) or containing no Al may be employed for the buffer layer 308 of the second-type conductivity and the contact layer 309 of the second-type conductivity. Then, a dry etching process with a recipe containing hydrogen gas (for example, CH 4 :H 2 :Cl 2 etchant gas) is conducted to proceed the etching process to reach the selective etching layer 307 of the second-type conductivity.

In such process, an increased Al content in the selective etching layer 307 of the second-type conductivity provides an increased selectivity of the etching process. When such process is employed, it is possible to provide a hydrogen passivation over the upper surface of the second mesa created by the etchant gas, depending upon the scheme for the apparatus (reactive ion etching (RIE), induced coupled plasma reactive ion etching (ICP-RIE) or the like), the deposition condition (radio frequency (RF) power, gas recipe, etch rate, etching time) or the substrate temperature (25 degrees C. (room temperature) to 300 degrees C.).

Apertures for the electrodes are formed in the passivation film formed by the above-described variety of processes, and then, the electrode 311 in the side of the first type conductivity is formed in the peripherals of the mesa, and then other process such as a polishing process, a process for forming an anti-reflection film (AR coat) and a process for forming an element isolation to produce the semiconductor photo detector of second exemplary embodiment.

In the semiconductor photo detector of the second exemplary embodiment, in order to obtain the effect of an improved reliability, the electric field in the second mesa may be designed so as to be higher in the bottom part of the first mesa and lower in the peripheral part of the first mesa. Consequently, the relation of the structure and the concentration of impurity may be determined as follows.

Assume that the diameter of the first mesa is d 1 (μm) and the diameter of the second mesa is d 2 (μm), the relation may be 0<d 2 −d 1 <d 1 ×5, when 10≦d 1 ≦100.

A value of the field-relaxing level ΔE 1b of the buffer layer 303 of the first-type conductivity may be suitably selected so as to provide a relaxation by at least 90% of the multiplied electric field. Since the field-relaxing quantity of ΔE 1b is determined by a product of the thickness d 1b of the buffer layer 303 of the first-type conductivity and the concentration of impurity N 1b , a combination of, for example, d 1b =0.1 μm and N 1b =1×10 −8 cm −3 , may provide sufficient field-relaxing level.

The impurity concentration N 1se in the selective etching layer 302 of the first-type conductivity may be suitable selected to provide sufficiently lower resistance. For example, the impurity concentration N 1se of the selective etching layer 302 of the first-type conductivity may be N 1se >2×10 17 cm −3 .

In addition, the light absorption layer 306 of the second-type conductivity and the selective etching layer 307 of the second-type conductivity may be suitably configured so as to satisfy the relation of ΔE ab +ΔE 2se ≦100 kV/cm, where the field-relaxing level of the light absorption layer 306 of the second-type conductivity is represented by ΔE ab and the field-relaxing level of the selective etching layer 307 of the second-type conductivity is represented by ΔE 2se . In order to achieve such relation, a product of the thickness d ab of the light absorption layer 306 of the second-type conductivity and the concentration N ab may be presented as being smaller than a certain value.

The effect of the hydrogen passivation is, depending on the type of the treatment process, exhibited as being exponentially decreased from the surface toward the inside thereof after the hydrogen treatment is conducted, or as having a peak at a certain depth and being exponentially decreased toward the inside from the certain depth. The hydrogen concentration in surface of the second mesa, depending on the type of the treatment process, may be within a range of equal to or higher than 1×10 16 cm −3 and equal to or lower than 5×10 18 cm −3 . In addition, the hydrogen concentration in the surface of the light absorption layer 306 of the second-type conductivity may be more preferably within a range of equal to or higher than 1×10 16 cm −3 and equal to or lower than 5×10 18 cm −3 . Having such ranges, the hydrogen passivation effect can be preferably obtained. In the doped region of the second-type conductivity that applies the hydrogen passivation, the operation is made so as to decrease the impurity concentration of the second-type conductivity by the hydrogen passivation effect. Assume that the hydrogen passivation has the maximum concentration of 5×10 17 cm −3 in the surface thereof, and a gradient influence for decreasing by one digit at a depth of 1 μm from the surface is provided, the impurity concentration can be increased by such effect. Therefore, it is preferable that the maximum value in the distributed concentration of hydrogen in the second mesa is equal to or higher than 5×10 17 cm −3 , and it is more preferable that the maximum value of hydrogen concentration in the light absorption layer 306 of the second-type conductivity is equal to or higher than 5×10 17 cm −3 .

The impurity concentration of the light absorption layer 306 of the second-type conductivity may be provided as about 1×10 16 cm −3 , and if a hydrogen passivation is presented, N ab may be determined in the side of the surface of the second mesa in the light absorption layer so as to satisfy the following relations.

(1) N ab ≦7.0×10 17 cm −3 in the side of the field-relaxing layer 305 of the second-type conductivity, where d ab ≦0.5 μm;

(2) N ab ≦6×10 16 cm −3 in the side of the field-relaxing layer 305 of the second-type conductivity, where 0.5 μm≦d ab ≦1.0 μm; and

(3) N ab ≦5.6×10 17 cm −3 in the regions at the depth of equal to or higher than 1.0 μm from the surface, where 1.0 μm≦d ab ≦2.0 μm.

As described above, in the region at the depth of within 1.0 μm from the surface, it is possible that N ab is increased to a concentration equivalent to the hydrogen passivation level. If a concentration-gradient configuration is added similarly as in the effect of the hydrogen passivation 313 in such case, N ab can be further increased.

As shown in FIG. 3 , in the semiconductor photo detector formed in this way, the electric field right under the outer circumference (or side wall) of the buffer layer 308 of the second-type conductivity is increased over the regions of the light absorption layer 306 of the second-type conductivity and the field-relaxing layer 305 of the second-type conductivity, and the multiplier layer 304 , as compared with the side wall section having the interface of the passivation film 312 for providing the semiconductor element/hydrogen passivation. The modulation effect of the electric field distribution may be increased by conducting the hydrogen treatment. Focusing on the bias, this effect can be clearly found in the bias that is higher than that in the condition, in which the depleted layer extends through the light absorption layer 306 of the second-type conductivity (punchthrough). This effect allows relatively reducing an amperage of an electric current generated in the interface of the passivation film 312 for providing the semiconductor/hydrogen passivation, as compared with the electric current right under the first mesa during multiplication operation irrespective of a light entered therein or not entered, to prevent a deterioration of the semiconductor/passivation interface and to obtain an advantageous effect of an improved reliability.

Since the equipotential surface has a curved profile in the internal electric field in the section right under the outer circumference (or side wall) of the buffer layer 308 of the second-type conductivity, a local increase in the electric field is generated. Consequently, in order to prevent an increase in the dark current in this section, a use of a material (In x Al y Ga (1-x-y) As: y<1, In x Ga (1-x) AS y P (1-y) : y<1) having sufficiently larger band gap (Eg) for the material of the second selective etching layer 307 as compared with the material (InGaAs) employed for the light absorption layer 306 of the second-type conductivity, may be effective.

A configuration of a semiconductor photo detector in example 2 is shown in Table 2 and FIG. 4 . A semiconductor photo detector of example 2 is configured that a control of electric field is conducted by utilizing a hydrogen passivation to enhance internal electric field and decrease electric field with the interface of the passivation.

The semiconductor photo detector of example 2 is manufactured as follows. An n-type selective etching layer 402 , an n-type buffer layer 403 , a multiplier layer 404 , a p-type field-relaxing layer 405 , a p-type light absorption layer 406 , a p-type selective etching layer 407 , a p-type buffer layer 408 and a p-type contact layer 409 are sequentially deposited on a semiconductor substrate 401 in this order from the semiconductor substrate 401 to form an epitaxial crystal.

Subsequently, a first mesa-etching process is conducted to partially remove the p-type buffer layer 408 , the p-type contact layer 409 and a p-type electrode 410 above the p-type selective etching layer 407 through a selective etching to form a circular first mesa. Here, several circular first mesas having a diameter d 1 of: d 1 =10, 20, 30, 50 and 100 μm, respectively; are manufactured.

Then, a second mesa etching process is conducted through a mask of a passivating film 414 that is concentric with the first mesa and has a large mesa diameter d 2 and provides no hydrogen passivation such as SiO 2 and the like to form a second mesa including a p-n junction. The height of the mesa may be designed to be larger than the thickness of the epitaxial layer. In the case of the present example, the height of the mesa may be not smaller than 2.6 μm, and a vertical difference between the deepest position of the etched depth and the top of the mesa may be assumed to be about 3 to 5 μm.

A hydrogen treatment is conducted from the mesa surface to provide the hydrogen passivation 413 , and then the passivating film 412 for providing a hydrogen passivation is formed over the entire mesa. Apertures for electrodes are formed in the two passivation films, and then the p-type electrode 410 is formed in the upper section of the mesa, and the n-type electrode 411 is formed to be in contact with an n-type semiconductor layer (or n-substrate) in the outer circumference of the mesa and associated interconnects are formed to the top of the mesa for the future installation.

The semiconductor photo detector formed in this way is configured that the electric field right under the outer circumference of the selective etching layer is increased over the regions of the p-type light absorption layer 406 and the p-type field-relaxing layer 405 , and the multiplier layer 404 , as compared with the side wall section having the interface between the semiconductor element and the passivation film.

Such effect is larger for the element having larger d 2 . This allows relatively reducing amperage of an electric current generated in the interface between the semiconductor element and the passivation film during multiplication operation, to obtain an advantageous effect of an improved reliability. A dark current generated in the inside of the layer structure is increased in relation to the flow of a dark current in the condition containing no photocarrier, as compared with a dark career caused in the interface between the semiconductor element and the passivation film, and an operation with relatively smaller side wall current can be achieved in a condition without a light incidence. On the other hand, amperage of a dark current depends on d 1 and d 2 . Larger d 1 , or in the cases of the same d 1 , larger d 2 , likely provides larger dark current, and the maximum multiplication factor is likely decreased.

Thus, d 1 and d 2 may be suitably determined according to the purposes (applications) of the elements.

A configuration of a semiconductor photo detector in example 3 is shown in Table 3 and FIG. 5 . A semiconductor photo detector of example 3 is configured to have a PIN-PD structure, in which a control of electric field is conducted by utilizing a hydrogen passivation to enhance internal electric field and decrease electric field with the interface of the passivation.

The semiconductor photo detector of example 3 is manufactured as follows. An n-type selective etching layer 502 , an n-type buffer layer 503 , an n-type electron transport layer 504 , a p-type light absorption layer 506 , a p-type selective etching layer 507 , a p-type buffer layer 508 and a p-type contact layer 509 are sequentially deposited on the semiconductor substrate 501 in this order from the semiconductor substrate 501 to form an epitaxial crystal.

Subsequently, a first mesa-etching process is conducted to partially remove the p-type buffer layer 508 and the p-type contact layer 509 above the p-type selective etching layer 507 through a selective etching to form a circular first mesa.

Here, several circular first mesas having a diameter d 1 of: d 1 =10, 20, 30, 50 and 100 μm, respectively; are manufactured.

Then, a second mesa etching process is conducted through a mask of a passivating film 514 that is concentric with the first mesa and has a large mesa diameter d 2 (d 2 =d 1 +2, 5, 10, 20 μm) and provides no hydrogen passivation such as SiO 2 and the like to form a second mesa including a p-n junction. The height of the mesa may be designed to be larger than the thickness of the epitaxial layer. In the case of the present example, the height of the mesa may be not smaller than 2.6 μm, and a vertical difference between the deepest position of the etched depth and the top of the mesa may be assumed to be about 3 to 5 μm.

A hydrogen treatment is conducted from the mesa surface to provide the hydrogen passivation 513 , and then the passivating film 512 for providing a hydrogen treatment is formed over the entire mesa. Apertures for electrodes are formed in the two passivation films, and then the p-type electrode 510 is formed in the upper section of the mesa, and the n-type electrode 511 is formed to be in contact with an n-type semiconductor layer (or n-substrate) in the outer circumference of the mesa and associated interconnects are formed to the top of the mesa for the future installation. The semiconductor element formed in this way is configured that the electric field right under the outer circumference of the selective etching layer is increased over the regions of the p-type light absorption layer 506 and the n-type electron transport layer 504 , as compared with the side wall section having the interface between the semiconductor element and the passivation film. Such effect is larger for the element having larger d 2 . This allows relatively reducing amperage of an electric current generated in the interface between the semiconductor element and the passivation film to obtain an advantageous effect of an improved reliability. A dark current generated in the inside of the layer structure is increased in relation to the flow of a dark current in the condition containing no photocarrier, as compared with a dark career caused in the interface between the semiconductor element and the passivation protective film, and an operation with relatively smaller side wall current can be achieved in a condition without a light incidence. On the other hand, amperage of a dark current depends on d 1 and d 2 . Since larger d 1 , or in the cases of the same d 1 , larger d 2 , likely provides larger dark current, the sensibility in these elements are likely decreased. Thus, d 1 and d 2 are suitably determined according to the purposes (applications) of the semiconductor elements.

While the preferred exemplary embodiments of the present invention have been described above in reference to the annexed figures, it should be understood that the disclosures above are presented for the purpose of illustrating the present invention, and various modifications other than that described above are also available.

For example, while the example illustrates that the hydrogen treatment is conducted from the surface of the mesa after the second mesa is formed to provide the hydrogen passivation, the hydrogen treatment may be alternatively carried out before the second mesa is formed. Even if the hydrogen treatment is conducted before forming the second mesa, the obtained structure is almost the same as the structure that the present invention is intended to include, and the advantageous effects may be similarly obtained.

The semiconductor photo detector of the present exemplary embodiment may alternatively be an APD. Conventional APDs cannot provide an easier manufacturing while ensuring better long term reliability. On the contrary, according to the present invention, an APD for ensuring the long term reliability, providing the rapid change of the design and the specification and achieving reduced cost can be presented.

The semiconductor photo detector of the present invention may alternatively be a PIN photodiode. Conventional PIN photodiodes cannot provide an easier manufacturing while ensuring better long term reliability. On the contrary, according to the present invention, a PIN photodiode for ensuring the long term reliability, providing the rapid change of the design and the specification and achieving reduced cost can be presented.

The following configurations may be adopted for the present invention.

(1) A semiconductor photo detector of a mesa-structure semiconductor photo detector, having a structure having a layer of a first-type conductivity, an undoped layer, a light absorption layer of a second-type conductivity, a selective etching layer of the second-type conductivity, and a buffer layer of the second-type conductivity, which are deposited on a semiconductor substrate in this order, a whole PN junction being formed to be a mesa-shape,

wherein semiconductor photo detector includes two mesas of different sizes, which includes a first mesa in the buffer layer of the second-type conductivity and sections above thereof, and a second mesa larger than the first mesa in the selective etching layer and sections under thereof.

(2) The semiconductor photo detector as set forth in the above (1), which is formed on an InP substrate.

(3) The semiconductor photo detector as set forth in the above (2), which is an APD.

(4) The semiconductor photo detector as set forth in the above (2), in which a concentration in the buffer layer of the second-type conductivity is equal to or higher than 2×10 17 cm −3 .

(5) The semiconductor photo detector as set forth in the above (1), in which a thickness d 2se of the selective etching layer of the second-type conductivity is presented as: 0.02 μm≦d 2se .

(6) The semiconductor photo detector as set forth in the above (1), in which a thickness d 2se of the selective etching layer of the second-type conductivity is presented as: d 2se ≦0.2 μm.

(7) The semiconductor photo detector as set forth in the above (1), in which a thickness d 2se of the selective etching layer of the second-type conductivity is presented as: 0.02 μm≦d 2se ≦0.2 μm.

(8) The semiconductor photo detector as set forth in the above (1), in which the selective etching layer of the second-type conductivity is formed of InP or In x Ga (1-x) As y P (1-y) .

(9) The semiconductor photo detector as set forth in the above (8), in which the band gap of the selective etching layer of the second-type conductivity is sufficiently larger, as compared with InGaAs (In x Ga (1-x) As y P (1-y) , y<0.5).

(10) The semiconductor photo detector as set forth in the above (8), in which sections above the buffer layer of the second-type conductivity contain no phosphorus (P).

(11) The semiconductor photo detector as set forth in the above (1), in which the selective etching layer of the second-type conductivity is formed of InAlAs or In x Ga (1-x) As y P (1-y) .

(12) The semiconductor photo detector as set forth in the above (10), in which the band gap of the selective etching layer of the second-type conductivity is sufficiently larger, as compared with InGaAs (In x Ga (1-x) As y P (1-y) , y<0.5) or (In x Al y Ga (1-x-y) As, y>0.25).

(13) The semiconductor photo detector as set forth in the above (10), in which sections above the buffer layer of the second-type conductivity is formed of InP or In x Ga (1-x) As y P (1-y) .

(14) The semiconductor photo detector as set forth in the above (1), in which a total of a field-relaxing level ΔE ab of the light absorption layer and a field-relaxing level ΔE 2se of the second selective etching layer is selected to be equal to or lower than 100 kV/cm.

(15) The semiconductor photo detector as set forth in the above (1), in which a total of a field-relaxing level ΔE ab of the light absorption layer and a field-relaxing level ΔE 2se of the second selective etching layer is selected to be equal to or lower than 100 kV/cm, and a relation of N 2se ≦2×10 16 cm −3 is satisfied for d 2se =0.1 μm, where a thickness d ab of the light absorption layer thickness is: d ab =0.5 μm, and a concentration N 2ab is: N 2ab =1.0×10 16 cm −3 .

(16) The semiconductor photo detector as set forth in the above (1), in which a total of a field-relaxing level ΔE ab of the light absorption layer and a field-relaxing level ΔE 2se of the second selective etching layer is selected to be equal to or lower than 100 kV/cm, and a relation of N 2se ≦2×10 16 cm −3 is satisfied for d 2se =0.1 μm, where d ab =1.0 μm, and a concentration N 2ab is: N 2ab =5.0×10 15 cm −3 .

(17) The semiconductor photo detector as set forth in the above (1), in which a total of a field-relaxing level ΔE ab of the light absorption layer and a field-relaxing level ΔE 2se of the second selective etching layer is selected to be equal to or lower than 100 kV/cm, and a relation of N 2se ≦1.2×10 16 cm −3 is satisfied for d 2se =0.1 μm, where d ab =1.5 μm, and a concentration N 2ab is: N 2ab =4×10 15 cm −3 .

(18) The semiconductor photo detector as set forth in the above (1), in which sections of the second mesa section disposed outside of the circumference of the first mesa contains hydrogen at a concentration of equal to or higher than at least about 1×10 16 cm −3 .

(19) The semiconductor photo detector as set forth in the above (1), in which sections of the second mesa section disposed outside of the circumference of the first mesa contains hydrogen with a concentration distribution having a maximum value of equal to or higher than at least about 5×10 17 cm −3 .

(20) The semiconductor photo detector as set forth in the above (1), in which a mask having a thickness of equal to or larger than 0.5 μm is formed on the first mesa in a process of a hydrogen treatment for the second mesa.