JPH02246380A - Photodiode - Google Patents

Abstract

PURPOSE:To perform a high speed operation even at the time of a large input operation by providing a light absorption layer formed of a semiconductor made of narrower second band gap than a first band gap and having weak p-type conductivity, and a higher concentration p-type region than the layer. CONSTITUTION:A p<-> type light absorption layer 2 having relatively narrow band gap is formed on the surface 7 of an n-type semiconductor substrate 1, a p-type region 3 is formed thereon with a pin type structure as a whole. A light incident surface is provided on the rear face 6 of the substrate 1, its periphery is surrounded by an N-side electrode 8, and a p-side electrode 9 is formed on the region 3. An i-type layer of the layer 2 is formed to be p<-> type, and a p-n junction is formed in the vicinity of a boundary to the substrate 1. Accordingly, a strongest light absorption occurs in a part having a strongest electric field distribution. Thus, the distribution tends to be uniform even at the time of large input operation, and a high speed operation can be expected.

Description

[Detailed Description of the Invention] [Summary] Regarding a photodiode capable of high-input and high-speed operation, the purpose of the present invention is to provide a photodiode that can operate at high-speed even during high-input operation, and is made of a first bandgap semiconductor. , formed of an n-type semiconductor substrate having a light incident surface on the back side and a semiconductor formed on the front side of the semiconductor substrate and having a second bandgap narrower than the first bandgap, and exhibits weak P-type conductivity. and a P-type region having a higher concentration than the light absorption layer formed on the light absorption layer.

[Industrial Field of Application] The present invention relates to a semiconductor device, and particularly to a photodiode with large input and high speed operation.

In a coherent optical communication system, there is a need for a photodiode that can detect small signals at high speed (4GH2 or higher) under irradiation with a large input local oscillation laser beam. A front-illuminated structure and a back-illuminated structure are known as pin-type photodiodes.

[Prior art] With reference to FIGS. 2(A) to (E), pl according to the prior art
An n-type photodiode will be explained.

FIG. 2(A) shows a front-illuminated pin type photodiode, in which an n-type light absorption layer 12 with a relatively narrow bandgap, such as InGaAs, is formed on the surface 17 of an n-type semiconductor substrate 11, such as InP. There is. This n-type light absorption layer corresponds to l of a pin-type photodiode. Light absorption layer 12
An n- layer 14 made of a relatively wide bandgap semiconductor material such as InP is formed thereon, and a p-type region 1 is formed therein.
3 is formed by ln diffusion or the like. n-type semiconductor substrate 1
On the back of 1 is nl! A ft pole 18a is formed, and a pr! An iJ electrode 19a is formed.

In such a front-illuminated pin type photodiode, the electrode 19a is located on the light incident surface, and the incident light is affected by the incident light.If the area occupied by the pole 19a on the p side is made as small as possible, the large input During operation, the voltage drop due to contact resistance becomes large.If the area of the plPI electrode 19a is increased in an attempt to lower the contact resistance, the area of the p-type region 13 must be increased, which is based on the pn junction. This increases the capacity.

As described above, the front-illuminated pin type photodiode is not suitable for large input and high speed operation.

Figure 2(B) shows a back-illuminated pin type photodiode.
An n-type light absorbing layer 12 with a relatively narrow bandgap is formed, on which an n-type layer 14 such as InP is formed, and a p-type region 13 is formed in a part of it by 2n diffusion or the like. There is. On the p-type region 13 there is a p-side electric potential [! 19 was formed,
An n-side electrode 18 is provided in a region surrounding the incident surface of the back surface 16 of the substrate 11.
is formed.

In such a back-illuminated ρ1n-type photodiode, the pn electrode 19 can be formed on almost the entire surface of the p-type region 13, and the problem of increased contact resistance or pn junction area described for the front-illuminated pin type bottom diode can be avoided. will be reduced.

Figure 2 (C) shows the back-illuminated pin shown in Figure 2 (B).
From the right side of Figure 0, which shows an example of impurity distribution of a type photodiode, an n-type 1nP substrate 11 has a silicon concentration of about 1018CI', and an n-type light absorption layer 12 thereon has a silicon concentration of about Ix1.
It has a silicon concentration of about 0.015. The n-type layer 14 above the light absorbing layer 12 has a silicon concentration of approximately IXio 16 cat-3.

P-type region 13 formed within n-type layer 14 has an In concentration of approximately i x t o 18 cm' in surface concentration.

Zn diffuses quickly in the n-type layer 14, which has a relatively high impurity concentration, and slows down when it enters the lower-concentration n(i)-type layer 12 below. Therefore, a pn junction is generated slightly into the light absorption layer 12.

Figure 2 (D) shows the back-illuminated pin shown in Figure 2 (B).
An example of the electric field distribution inside a type photodiode is shown below.
With the peak at the prl junction, the electric field rapidly decreases within the P-type region 13 with a high impurity concentration. Since the n-type light absorption layer 12 has a low impurity concentration, the electric field strength gradually decreases, and after reaching the n-type substrate 11, which has a high impurity concentration, the electric field decreases rapidly.

Figure 2 (E) shows the back-illuminated type pi shown in Figure 2 (B).
An example of the band structure inside an n-type photodiode is shown. For example, JnP with a band gap of about 1.35 eV is used as the n-type substrate 11 and the p-type region 13, and the band gap is about 0.75 eV as the light absorption layer 12. InGa
Assume that As is used to detect light with a wavelength of 1°55 μm.

InGaAs has a band gap of about 0.75 eV, while InP has a band gap of 1.35 eV, so the band offset is about 0.6 eV. It will be distributed in a ratio of approximately 4:6. As the substrate 11 is doped to be n-type, the energy level moves downward in the figure. On the other hand, when the impurity concentration of the ρ type region 13 increases, the level of the ρ type region 13 moves upward.

In this way, a band structure as shown in FIG. 2(E) is realized. In FIG. 2, a small potential barrier is shown when moving from the light absorption layer 12 to the n-type substrate 11.

If such a potential barrier exists, carriers will be accumulated in the potential barrier during irradiation with a large input light, limiting the operating speed.By doping the n-type substrate 11 with a higher concentration, the level of the n-type substrate 11 will be further reduced. can be done.

Similarly, a small potential barrier to holes is also shown in the conduction band when moving from the light absorption layer 12 to the p-type region 13.

This potential barrier also becomes a cause of limiting the operating speed.

By doping p-type region 13 more heavily, the level of p-type region 13 can be moved further upward in the figure.

[Problems to be Solved by the Invention NM] As described above, the front-illuminated pin type photodiode is unsuitable for large input and high speed operation.

On the other hand, back-illuminated pin type photodiodes are also required to further improve their performance. That is, at the time of large input, a large amount of carriers are generated within the light absorption layer. More of these carriers are generated on the incident side, which is closer to the substrate. Since these photoexcited carriers move freely, they function to shield the electric field, and as shown in Figure 2 (D> medium broken line),
Changes the electric field distribution, that is, the light absorption layer 12
The electric field is further weakened at a portion near the substrate where the electric field is relatively weak, which limits the operating speed of the pin photodiode.

An object of the present invention is to provide a pin type photodiode that can operate at high speed even during large input operation.

[Means for Solving the Problems] In FIG. 6 showing the principle explanatory diagram of the present invention in FIG. An absorption layer 2 is formed, and a p-type region 3 is formed thereon, forming a pin-type region as a whole.

A light incident surface is provided on the back surface 6 of the n-type semiconductor substrate 1,
It is surrounded by n1m electric @8. A p-side electrode 9 is formed on the P-type region 3 .

When an n-type buffer layer is provided on the surface of an n-type semiconductor substrate, the surface of the buffer layer may be regarded as the substrate surface 7.

It is also possible to make the back surface 6 of the n-type semiconductor substrate 1 a convex surface to have the function of a convex lens.

[Function] In the structure shown in FIG. 1, the pn junction connects the substrate 1 and the light absorption layer 2.
Formed near the interface with When light enters from the back surface 6 of the substrate 1, the light absorption layer 2 absorbs the light. At this time, since the absorption is stronger toward the incident side, many photoexcited carriers exist in the portion of the light absorption layer near the substrate 1.

The electric field distribution inside the light absorption layer 2 increases closer to the pn junction, so the electric field is stronger in areas closer to the substrate where many photoexcited carriers exist. Therefore, even when the electric field is shielded by photoexcited carriers, Only the parts where the electric field is relatively strong are weakened, and the light absorption layer 2
The electric field tends to be uniform within the . Therefore, high-speed operation can be expected even during large inputs.

[Embodiment] FIGS. 3(A) to 3(D) show back-illuminated pin-type photodiodes according to embodiments of the present invention.

An n-type InP substrate 21 whose cross-sectional structure is shown in FIG.
An n+ type 1nP buffer layer 25 is formed thereon. A light absorption region 22 of p-type InGaAs with a relatively narrow bandgap is formed on the surface 27 of this buffer layer. On this light absorption region 22, an n-type 1nP
A layer 24 is formed, and In, which is a p-type impurity, is diffused into the negative portion of the layer 24 to form a p-type Zn diffusion region 23. InP has a wide bandgap, 1.55 μm
It is transparent to light of 1.3 μm to 1.3 μm.

The composition of the light absorption layer In(3aAs) can be adjusted depending on the target wavelength. For example, when detecting light of 1.55 μm, the band gap of InGaAs is approximately 0.
．． The voltage is set to 75 eV.

FIG. 3(B) shows an example of the impurity distribution in the structure shown in FIG. 3(A). has. The p-type light absorption region 22 has a p-type impurity concentration of, for example, 1×10 16 crp-3. For example, if the absorption coefficient of the light absorption region 22 is 0.
66 μm°1, and if the thickness is, for example, 3 or more, the light absorption region 22 is applied with an appropriate voltage, for example, 5
The impurity concentration is selected so that the impurity is sufficiently depleted at ˜IOV, for example, an impurity concentration of 2×10 16 cm −3 or less is selected.

The n-type 1nP layer 24 above the light absorption region 22 has an n-type impurity concentration of, for example, I x 1016C1l'.

The p-type region 23 formed therein is, for example, 1×10
P-type impurity concentration (for example, Zn concentration) of about “C1l”
has.

In order to reduce the capacitance associated with the pn junction as much as possible, it is preferable to make the p-type region 23 as small as possible and surround it with a region 24 having a low impurity concentration.

FIG. 3(C) shows an example of the electric field distribution in the structure shown in FIG. 3(A). Since the substrate 21 or the buffer layer 25 has a high n-type impurity concentration, the electric field therein is transmitted from the p-n junction. As you move away from it, it decreases rapidly.

Since the p-type light absorption region 22 has a low impurity concentration,
Therein, the electric field distribution gradually decreases as it moves away from the pn junction.

Since p-type region 23 has a high impurity concentration, the electric field therein decreases rapidly. That is, the electric field distribution is highest at the pn junction formed at the boundary between the substrate and the light absorption region, and gradually decreases toward the surface of the light absorption region.

In such an electric field distribution, when light is incident from the back side of the substrate, the strongest light absorption occurs on the substrate side of the light absorption region 22, and when photoexcited carriers generated by light absorption weaken the electric field distribution, the light absorption region 22 The electric field distribution within 22 has a uniform direction.

Therefore, high operating speed can be expected even during large inputs.

FIG. 3(D) shows an example of the band structure in the structure shown in FIG. 3(A), from the n-type substrate 21 to the buffer layer 25.
The p-type region 23 is formed of InP, and the light-absorbing region 22 of InGaAs or the like sandwiched between 1 and 1 having a wide bandgap has a narrow bandgap and exhibits p-type conductivity. A band structure as shown in Figure (D) is obtained.

When compared with the band structure according to the prior art shown in FIG. 2(E), the light absorption region 22 is
Compared to 2, the shape has been moved upward in the figure.

FIG. 4 shows another embodiment of the present invention, in which an n-type buffer layer 25 is formed on an n-type substrate 21, a p-type light absorption region 22 with a relatively narrow band gap is formed thereon, and An n-type layer 24 is formed thereon, and a P-type region 23 is formed within the n-type layer 24. On the p-type region 23 is a p(II electron 1
i29, surrounding the incident surface and forming an n
A st pole 28 is formed. N-type substrate 21 forming the incident surface
The surface of the substrate 21 is processed to have a convex shape, and the substrate 21 forms a meniscus convex lens. Therefore, the incident light that has entered over a wide range is focused within the substrate 21 and directed toward the light absorption region 22, so that P
Even if the mold region 23 has a small area, photoexcited carriers can be collected effectively.

The n-type substrate 21, the n-type buffer layer 25, and the n-type layer 24 are
For example, it can be formed of InP, and the p-type light absorption region 22 can be formed of, for example, InGaAs. Furthermore, the p-type region 23 can be formed, for example, by diffusion of Zn.

Although the present invention has been described above with reference to the embodiments, it will be obvious to those skilled in the art that the present invention is not limited to these and that, for example, various modifications, changes, improvements, combinations, etc. are possible.

[Effects of the Invention] According to the present invention, in a back-illuminated pin type photodiode, the i-type layer, which is a light absorption layer, is p-type, and a pn junction is formed near the interface with the substrate, so that the electric field distribution is reduced. can cause the strongest light absorption at the strongest part,
Even during high input operation, the electric field distribution tends to be uniform.
You can expect high-speed operation.

[Brief explanation of drawings]

FIG. 1 is a diagram for explaining the principle of the present invention, FIGS. 2(A) to (E) are diagrams for explaining the prior art, and FIG. 2 (A> is a cross-sectional view of a front-illuminated pin type photodiode. FIG. 2(B) is a cross-sectional view of a back-illuminated pin type photodiode, and FIG. 2(C) is a graph showing an example of impurity distribution in the back-illuminated pin type photodiode shown in FIG. 2(B). Figure (D) is a graph showing an example of electric field distribution in the back-thinned pin type photodiode shown in Figure 2(B), and Figure 2(E) is a graph showing an example of the electric field distribution in the back-thinned pin type photodiode shown in Figure 2(B). Graphs showing examples of band structures in FIGS. 3A to 3D are diagrams for explaining back-illuminated pin photodiodes according to embodiments of the present invention,
Figure 3 (A) is a cross-sectional view, Figure 3 (B) is a graph showing an example of impurity distribution, Figure 3 (C) is a graph showing an example of electric field distribution, and Figure 3 (D) is a graph of the band structure. Graph showing an example. FIG. 4 is a cross-sectional view showing another embodiment of the present invention. In the figure, 18.18a 19.19a n-type semiconductor substrate P type light absorption layer p-type region The surface of the back substrate of the substrate n-side electrode p (I! It pole n-type semiconductor substrate n-type light absorption layer p-type region n-type layer substrate surface n-side electrode p (l!It pole n-type 1nP substrate InGaAs p-type light absorption region p-type region n + pear-shaped 1nP buff layer back surface n Side electrode P side electrode Figure 1 Figure 2 (Part 1) (B) Back-illuminated type (C) Impurity distribution (E) Band structure Conventional technology Figure 2 (Part 2) (A) Cross-sectional structure (B) Impurity distribution Embodiment of the present invention FIG. 3

Claims (2)

[Claims] (1) an n-type semiconductor substrate (1) made of a first bandgap semiconductor and having a light incident surface (6) on the back surface; A light absorption layer (2) formed of a semiconductor having a second bandgap narrower than the bandgap and having weak p-type conductivity; and a light absorption layer (2) formed on the light absorption layer (2). A photodiode characterized by having a highly concentrated p-type region (3). (2) The photodiode according to claim 1, wherein the light incident surface on the back surface of the semiconductor substrate has a convex shape bulging outward, and functions as a convex lens.