JP2003163361A - Light receiving element and optical communication device - Google Patents

Abstract

(57) Abstract: Provided is a light receiving element formed monolithically and inexpensively with a receiving circuit on a Si substrate without using a compound semiconductor, and a miniaturized optical communication device including the light receiving element. . SOLUTION: n-type SiG located on a Si layer 1
an e-graded buffer layer 2, an i-SiGe layer 3 located on the SiGe graded buffer layer and being an intrinsic semiconductor that absorbs light, and a p-conductive SiGe layer 4 located on the i-SiGe layer. Is provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light receiving element and an optical communication device using the light receiving element. More specifically, the present invention has a sufficiently high light absorption efficiency and is formed monolithically with a light receiving circuit on a Si substrate. The present invention relates to a light receiving element that can be used and an optical communication device including the light receiving element.

[0002]

2. Description of the Related Art With the increase in speed and capacity of optical communication, there is a tendency that severe requirements are imposed on the performance improvement of optical receiving devices. In order to meet this strict requirement, a compound semiconductor such as InGaAs is used for the light receiving element for long wavelength.
Therefore, in the light receiving device formed on the Si substrate, the light receiving circuit section is manufactured as a separate chip and incorporated on the Si substrate. Therefore, it has been an obstacle to cost reduction and miniaturization of the optical receiving device.

On the other hand, when SiGe is used as a light receiving element, SiGe has a light receiving sensitivity in the long wavelength band for optical communication in the range of Ge composition x = 0.3 to 0.5. By using Si _1-x Ge _x in this Ge composition range, Si-CMO
It is possible to form a receiving circuit, which is incorporated in an S (Complementary Metal Oxide Semiconductor) process and can operate at high speed, monolithically on a Si substrate. As a result, a one-chip optical receiver can be realized, and cost reduction and miniaturization are promoted. However, since the lattice constant difference between Si and Ge is as high as 4%, SiGe in the Ge composition range having sensitivity in the above long wavelength band has a critical film thickness (crystal defects occur due to lattice mismatch and lattice relaxation occurs). A good quality epitaxially grown film can only be obtained with a thin film having a thickness less than or equal to the film thickness at which it begins to occur. This critical film thickness decreases rapidly as the Ge composition increases. S
Although depending on the method of forming iGe, the critical film thickness is several tens of nm or less when x of Ge composition is around 0.4.

FIG. 7 is a sectional view of a conventional SiGe light receiving element (Huang et al. Appl. Phys. Lett. 67 (4), 566).
(1995)). Referring to FIG. 7, an n conductivity type Si substrate (hereinafter,
A multi-quantum well structure in which a non-doped (intrinsic semiconductor) i-Si layer 120 and an i-Si _0.5 Ge _0.5 layer 121 of an intrinsic semiconductor having a thickness of, for example, about 10 nm are stacked for N periods on an (n-Si substrate) 101. Is formed. A p-Si layer 122 containing p-conductivity type impurities is formed on the multiple structure. A p-i-n structure is formed by sandwiching the multiple quantum well structure from above and below with the p-Si layer 122 and the n-Si layer 101.
At this time, in order to suppress the crystal defect density due to the lattice mismatch of the i-SiGe layer 121, it is necessary to suppress the film thickness to the critical film thickness or less. Note that an electrode that makes ohmic contact with the n-Si layer 101 and an electrode that makes ohmic contact with the p-Si layer 122 are provided for extracting the photocurrent to an external circuit, but they are omitted in FIG. 7.

FIG. 8 is an energy band diagram when a reverse bias voltage is applied to the pin structure.
The band gap of the intrinsic semiconductor i-SiGe layer 121 is smaller than that of the Si layer 120, and the band offset is mainly formed on the valence band side. Since SiGe containing a high concentration of impurities has high conductivity when a reverse bias voltage is applied,
The electric field is applied to the multiple quantum well structure region including the i-Si layer 120 and the i-SiGe layer 121. P-i-n from the outside
Incident light incident on the structure is absorbed by the i-SiGe layer 121, and a photocurrent generated is detected by an external circuit to convert an optical signal into an electrical signal.

[0006]

Since the conventional SiGe light receiving element is formed as described above, it is difficult to sufficiently thicken the i-SiGe layer 121 which becomes the light absorption region. Therefore, there is a problem related to the root of low light absorption efficiency.

The present invention provides a light receiving element which can be formed monolithically with a receiving circuit on a Si substrate at low cost without using a compound semiconductor, and a miniaturized optical communication device including the light receiving element. The purpose is to

[0008]

The light receiving element of the present invention is an S element.
A first conductivity type SiGe graded buffer layer located on the i layer, an i-SiGe layer located on the SiGe graded buffer layer and absorbing light, and a second conductivity type located on the i-SiGe layer. And a SiGe layer (claim 1).

With this structure, i-Si which is the light absorption layer is formed.
It is possible to form the Ge epitaxial film with a sufficiently thick film thickness while suppressing the crystal defect density to a predetermined level or less. In the above structure, since the graded buffer layer is heavily doped with impurities, the conductivity is high and no electric field is applied to this region. Therefore, almost no light absorption current is generated in this region. As a result, the i of the intrinsic semiconductor is
-SiGe can efficiently absorb light. Here, the i-SiGe layer of an intrinsic semiconductor is a semiconductor layer containing only impurities whose concentration is such that its influence can be ignored.

By using the Si layer as a Si substrate, for example, the receiving circuit and the like can be monolithically formed on the Si substrate. As a result, downsizing and cost reduction can be realized.

In the above light receiving element of the present invention, the i-SiGe
The Ge composition of at least one of the impurity-containing layers sandwiching the layer can be made smaller than the Ge composition of the i-SiGe layer (claim 2).

With this structure, when the Ge composition of the impurity-containing layer is reduced, the band gap is increased, so that light absorption can be caused only in the i-SiGe layer.
As a result, the light absorption efficiency can be increased.

Another receiving element of the present invention is a SiGe graded buffer layer of a first conductivity type located on a Si layer,
A laminated structure for absorbing light located on the SiGe graded buffer layer, the semiconductor i-Si _1-y Ge _y layer having the same Ge composition as the Ge composition in the uppermost layer of the SiGe graded buffer layer, and i -Si _1-y Ge _y layers and i-Si _1-x Ge _x layers having different Ge compositions are alternately laminated, and the same Ge composition as the i-Si _1-x Ge _x layers in the laminated structure is provided. And a second conductivity type SiG that is located above the laminated structure
and an e layer (claim 3).

With this structure, i-Si _1-y G
By using the layer quantum well structure including the laminated structure of the e _y layer and the i-Si _1-x Ge _x layer, the density of states at the conduction band edge and the valence band edge in the light absorption layer can be increased.
As a result, the light absorption efficiency can be increased. Further, the lattice constants at each interface of the p-i-n structure can be matched, and the crystal defect density can be greatly reduced to epitaxially grow each film constituting the p-i-n structure.
This good crystallinity is also effective in enabling sharp light absorption according to the wavelength. It should be noted that SiGe having the same Ge composition does not have to be exactly the same composition as the Ge composition of the layers in the multilayer structure, and if it is within a predetermined range so that the crystal irregularity at the interface can be reduced to a predetermined range or less. Good.

In any of the above light receiving elements, Si
The layer can be a Si substrate (claim 4).

With this configuration, any of the above light receiving elements can be monolithically incorporated on a Si substrate to form an optical communication device. As a result, it is possible to obtain an optical communication device that is inexpensive, downsized, and highly reliable.

In any of the above light receiving elements, Si
The layer may be a Si layer forming a surface layer of the SOI substrate (claim 5).

With this structure, a resonance space can be formed between the insulating layer of the SOI substrate and the insulating layer by providing an appropriate reflection layer that allows light to enter from the outside and does not allow light to exit from the inside. As a result, the absorption efficiency of the light absorption layer can be improved. Also, on the Si substrate,
Leakage current can be prevented as compared with the case where the light receiving element is directly formed.

In any of the above-mentioned light receiving elements, on the second conductive type semiconductor layer located on the layer for absorbing light,
A dielectric reflection layer may be further provided (Claim 6).

With the above structure, the dielectric reflection layer and the SOI
By forming a resonator structure between the insulating layer and the insulating layer, the incident light repeats multiple reflection inside the resonator, so that the light absorption efficiency in the non-doped i-SiGe layer can be increased.

In any of the above light receiving elements, Si
The film thickness of the layer can be equal to or smaller than the critical film thickness of the SiGe graded buffer layer located thereabove (claim 7).

With this configuration, for example, S of the SOI substrate is
By making the film thickness of the i layer smaller than the above-mentioned critical film thickness,
The rigidity of the Si layer can be reduced. For this reason,
The lattice strain of the SiGe graded buffer layer formed thereon can be relaxed by generating dislocations in the Si layer. As a result, the film thickness of the SiGe graded buffer layer can be reduced. The critical film thickness means a film thickness at which crystal defects start due to lattice mismatch and lattice relaxation starts.

In the optical communication device of the present invention, any one of the above light receiving elements can be provided in the light receiving section (claim 8).

With this structure, it is possible to obtain an optical communication device equipped with a light receiving element which is inexpensive and has a high absorption efficiency without using a compound semiconductor.

An optical communication device of the present invention can include a Si substrate on which any of the above light receiving elements is formed, and an electronic circuit monolithically formed with the light receiving element on the Si substrate. 9).

With this configuration, it is possible to inexpensively obtain a miniaturized optical communication device including a light receiving element having a high absorption efficiency.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the drawings.

(Embodiment 1) FIG. 1 is a sectional view of a light receiving element according to Embodiment 1 of the present invention. With reference to FIG. 1, a SiGe graded buffer layer 2 doped with an n-conductivity type dopant is formed on an n-conductivity type Si substrate 1. The SiGe graded buffer layer 2 is
For example, it is configured by laminating SiGe thin film layers in which the Ge composition is gradually increased to reach the Ge composition of the i-SiGe layer 3. A non-doped i-SiGe layer 3 is laminated on the SiGe graded buffer layer 2, and a p-SiGe layer 4 having the same Ge composition as the i-SiGe layer 3 is further laminated thereon. Further, an electrode in ohmic contact with the n-conductivity type Si layer 1 or the SiGe graded buffer layer 2 and an electrode in ohmic contact with the p-conductivity type SiGe layer 4 are provided for extracting the photocurrent to the outside, but not shown in FIG. Has been done. Even if the polarities of the p-conductivity type and the n-conductivity type are replaced with each other, functionally the same ones can be manufactured.

FIG. 2 shows an energy band diagram of the pin structure shown in FIG. This energy band diagram is an energy band diagram when a reverse bias voltage V is externally applied to the pin structure of FIG. In the SiGe layer, its band gap decreases as the Ge concentration increases. Therefore,
The band gap of the SiGe graded buffer layer 2 gradually decreases from the Si band gap value to the i-SiGe layer 3 band gap value. n
-Si substrate 1 and n-SiGe graded buffer layer 2
Since the p-SiGe layer 4 is a heavily doped layer,
Most of the electric field is applied to the i-SiGe layer 3.

In the light receiving element according to the present embodiment, incident light is absorbed by the i-SiGe layer 3, and the generated electron-hole pairs are respectively caused by an electric field to the n-SiGe layer 2 and p-SiGe.
It is attracted in the direction of layer 4 and detected as photocurrent.
Since the SiGe graded buffer layer 2 is sufficiently lattice-relaxed, the i-SiGe layer 3 has no defects due to lattice distortion, and its thickness can be made sufficiently thick, so that the light absorption efficiency is increased. be able to.

(Second Embodiment) FIG. 3 is an energy band diagram of a pin structure of a light receiving element according to the second embodiment of the present invention. Referring to FIG. 3, n-SiG
The composition of the e graded buffer layer 2 and the p-SiGe layer 4 is smaller than that of the i-SiGe layer 3. With such a structure, the film thickness of the n-SiGe graded buffer layer 2 can be reduced, so that the height of the entire SiGe light receiving element can be suppressed to be low. This is Si-C
This has the effect of facilitating the process when monolithically integrated with a MOS transistor to form an optical communication device. However, lattice mismatch occurs because the Ge composition of the i-SiGe layer 3 is different from the others. However, since the amount is small, the critical film thickness can be made sufficiently thick,
It is possible to secure the same effect as that of the first embodiment.

(Third Embodiment) FIG. 4 is a sectional view of a light receiving element according to a third embodiment of the present invention. Also, FIG.
4 is an energy band diagram of a pin structure in the light receiving element of FIG. 4. As shown in FIG. 4, in the light receiving element of the present embodiment, the light absorption layer formed on the n-conductivity type SiGe graded buffer layer 2 has a multilayer structure. This multilayer structure has a non-doped i-Si _1-y Ge _y layer 20 and an i-Si _1-y Ge _y layer 20 having the same Ge composition as the uppermost layer of the n-conduction type SiGe graded buffer layer 2.
Undoped i-with a Ge composition greater than that of
It has a multi-quantum well structure in which Si _1-x Ge _x layers 21 are alternately laminated for N periods. Above this multilayer structure 30,
A p-conductivity type Si _1-x Ge _x layer 25 having the same Ge composition as the i-Si _1-y Ge _y layer 21 is arranged.

As shown in FIG. 5, with such a structure, the lattice constant of each interface of the pin structure is matched, and the lattice irregularity of each layer is reduced as much as possible to obtain a film excellent in crystallinity. Can be epitaxially grown. Further, the i-Si _1-x Ge _x layer 21 and the i-Si _1-y G that will become the light absorption layer are formed.
The density of states at the conduction band edge and the valence band edge of the e _y layer 20 can be increased. As a result, the light absorption efficiency can be increased.

(Embodiment 4) FIG. 6 is a sectional view of a light receiving element according to Embodiment 4 of the present invention. Referring to FIG. 6, an n-S layer is formed on an SOI substrate including a Si substrate 1, a SiO ₂ layer 9 and a silicon layer (SOI layer) 10 on the Si substrate 1.
iGe graded buffer layer 2 and i-SiGe layer 3
Then, a p-i-n structure including the p-SiGe layer 4 is formed. Further thereon, a mirror 11 composed of a dielectric film, for example, a laminated structure of Si thin film and SiO ₂ thin film is formed. As a result, the mirror 11 and the S of the SOI substrate are
A resonator structure is constructed using the iO ₂ layer 9 as the other mirror. However, if the SiGe graded buffer layer 2 and i
The film thicknesses of the -SiGe layer and the p-SiGe layer 4 need to be designed so that the resonance wavelength of the resonator matches the wavelength of the absorbed light.

With such a structure, the incident light repeats multiple reflection inside the resonator, so that the i-SiG
The light absorption efficiency of the e-layer 3 can be increased. Needless to say, the same effect can be obtained even if the i-SiGe layer 3 has the structure of the second and third embodiments.

(Embodiment 5) A light receiving element according to Embodiment 5 of the present invention will be described with reference to FIG. In the fifth embodiment, the film thickness of the SOI layer 10 is n-
It is equal to or smaller than the critical film thickness of the SiGe graded buffer layer 2. With such a configuration, the lattice strain of the SiGe layer 2 can be relaxed by generating dislocations in the SOI layer 10, so that the thickness of the SiGe graded buffer layer 2 can be reduced. This also makes it possible to reduce the film thickness of the entire SiGe light receiving element, and has the effect of facilitating the process when monolithically integrating with the Si-CMOS transistor.

Although the embodiments of the present invention have been described above, the embodiments of the present invention disclosed above are merely examples, and the scope of the present invention is not limited to these embodiments. Not limited. For example, the following modifications are possible. (A) The Si layer is not limited to the Si layer of the Si substrate or the SOI substrate, and may be any Si film formed on the Si substrate. (B) Further, in the SiGe graded buffer layer, one surface is not necessarily a Si layer and the other surface is SiGe.
It does not have to be a layer. In order to adjust the difference in lattice constant between Si and SiGe, the Ge composition may be increased from the Si layer toward the i-SiGe layer.

The scope of the present invention is shown by the description of the claims, and includes the meaning equivalent to the description of the claims and all modifications within the scope.

[0039]

By using the light receiving element of the present invention,
An inexpensive light receiving element having high light absorption efficiency can be obtained without using a compound semiconductor. Further, with this light receiving element, it is possible to provide a miniaturized optical communication device which is monolithically formed with the receiving circuit on the Si substrate.

[Brief description of drawings]

FIG. 1 is a sectional view of a light receiving element according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an energy band diagram of the light receiving element shown in FIG.

FIG. 3 is a diagram showing an energy band diagram of a light receiving element according to a second embodiment of the present invention.

FIG. 4 is a sectional view of a light receiving element according to a third embodiment of the present invention.

5 is a diagram showing an energy band diagram of the light receiving element of FIG.

FIG. 6 is a sectional view of a light receiving element according to the fourth and fifth embodiments of the present invention.

FIG. 7 is a cross-sectional view of a conventional light receiving element formed on a Si substrate.

FIG. 8 is a diagram showing an energy band diagram of a conventional light receiving element.

[Explanation of symbols]

1 n-Si substrate, 2 n-SiGe graded buffer layer, 3 i-SiGe layer (light absorption layer), 4 p-Si
Ge layer, 9 SiO ₂ (insulating layer), 10 SOI layer, 1
1 Multi-layered dielectric layer (mirror layer), 20 i-Si _1-y
Ge _y layer (partial layer of a multilayer structure that absorbs light), 21 i-
Si _1-x Ge _x layer (partial layer of a multilayer structure that absorbs light), 2
5 p-Si _1-x Ge _x layer, 30 Multi-layered light absorption layer.

Continued front page    (72) Inventor Yuji Abe             2-3 2-3 Marunouchi, Chiyoda-ku, Tokyo             Inside Ryo Electric Co., Ltd. F-term (reference) 5F049 MA04 MB03 NA18 NA19 NB01                       QA07 RA10 SS03

Claims (9)

[Claims] 1. A first conductivity type SiG located on a Si layer.
e graded buffer layer, i-SiGe layer of intrinsic semiconductor located on the SiGe graded buffer layer and absorbing light, and second conductivity type SiGe located on the i-SiGe layer
A light receiving element comprising a layer. 2. The Ge composition of at least one of the impurity-containing layers sandwiching the i-SiGe is the Ge of the i-SiGe layer.
The light-receiving element according to claim 1, which is smaller than the composition. 3. A first conductivity type SiG located on a Si layer.
An e-graded buffer layer and a laminated structure that is located on the SiGe graded buffer layer and absorbs light, the semiconductor i-Si having the same Ge composition as the Ge composition in the uppermost layer of the SiGe graded buffer layer. A laminated structure in which a _1-y Ge _y layer, the i-Si _1-y Ge _y layer, and an i-Si _1-x Ge _x layer having a different Ge composition are alternately laminated, and i-Si in the laminated structure Second-conductivity-type SiG having the same Ge composition as the _1-x Ge _x layer and located on the laminated structure
A light-receiving element comprising an e-layer. 4. The Si layer is a Si substrate.
4. The light receiving element according to any one of 3 to 3. 5. The Si layer is SOI (Silicon On Insula)
tor) a Si layer forming a surface layer of the substrate,
3. The light receiving element according to any one of 3 above. 6. A second layer overlying the light absorbing layer
The light-receiving element according to claim 1, further comprising a dielectric reflection layer on the conductive SiGe layer. 7. The light-receiving element according to claim 1, wherein the film thickness of the Si layer is equal to or smaller than the critical film thickness of the SiGe graded buffer layer located thereon. 8. An optical communication device, comprising a light receiving element according to claim 1 in a light receiving section. 9. An optical communication device comprising: a Si substrate on which the light receiving element according to claim 1 is formed; and an electronic circuit monolithically formed on the Si substrate with the light receiving element. .