JPH0575160A - Avalanche photodiode and its operation - Google Patents

Abstract

PURPOSE:To realize high-speed operation and suppress the increase in excess noise while keeping a carrier multiplication factor necessitated by changing the electric field intensity positively in the avalanche photodiode capable of receiving optical signals with high sensitivity. CONSTITUTION:On a light absorbing layer of the first conductive type with low impurity concentration are formed an electric field drop layer 4 of the same type with high impurity concentration and small thickness, avalanche multiplication layer 5 of the same type with intermediate impurity concentration and the specified thickness, and reverse conductive type semiconductor layer 7 of reverse conductive type with high impurity concentration. When the holes generated in the layer 2 are implanted into the layer 5, the area adjacent to the layer 2 is mainly ionized by means of the holes, and the area adjacent to the layer 7 is intensively ionized, so that high carrier multiplication factor is obtained. On the other hand, the part near the layer 7 shows high ionization percentage, and the part near the layer 2 is only ionized by means of the holes, thereby excess noises are suppressed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor light receiving device, and more particularly to an avalanche photodiode capable of receiving a high speed optical signal with high sensitivity in an optical communication using an optical fiber and an operating method thereof.

In recent years, with the advancement of communication, there has been a demand for speeding up of optical communication. For this purpose, it is required to speed up the light receiving element that converts an optical signal into an electric signal. The avalanche photodiode (APD) is widely used as a highly sensitive light-receiving element because it has a current multiplication mechanism inside.

[0003]

2. Description of the Related Art FIG. 5 shows the structure of an APD according to the prior art and the electric field distribution inside the APD. In FIG. 5A, n
A light absorption layer 52 made of n ⁻ type InGaAs is disposed on a ⁺ type InP substrate 51. InP is transparent to light in the 1 μm band, but InGaAs absorbs light. By injecting light of 1 μm band into the light absorption layer 52 from a fiber or the like, an electron-hole pair can be formed inside the light absorption layer 52. The composition of InGaAs is InP.
To be lattice matched with.

A composition of InGaA is formed on the light absorption layer 52.
An InGaAsP heterobarrier relaxation layer 53 that relaxes the barrier due to the heterojunction by gradually changing from s to InP is provided, and n for rapidly changing the electric field strength is provided thereon.
^{A +} type InP electric field drop layer 54 is arranged.

An i-type or n-type is formed on the electric field drop layer 54.
^{A −} type InP window layer 56 is arranged, and p ⁺ type impurities are doped to a predetermined depth from the surface thereof to form ap ⁺ type InP reverse conductivity type region 57.

Below the reverse conductivity type region 57, i
The avalanche multiplication layer 55 of type or n ⁻ type InP is left. Further, an electric field concentration relaxation region 58 of p ⁻ type InP is formed so as to surround an end of the opposite conductivity type region 57 of p ⁺ type InP. A p-side electrode 61 is formed on the peripheral surface of the opposite conductivity type region 57, and SiN is formed on the other surface, and a passivation film 59 also serving as an antireflection film is formed.

In such a semiconductor structure, the pn junction 6
3 is a p-type opposite conductivity type region 57, an electric field concentration relaxation region 58 surrounding the region 57, and n-type regions 54 and 5 around the region 57.
The light absorption layer 52 is formed at the interface with the layers 5 and 56, and constitutes a region that absorbs light having the narrowest band gap.

In the structure shown in the figure, when a reverse bias voltage is applied between the p-side electrode 61 and the n-side electrode 62, the depletion layer becomes p
It extends from the n-junction 63 toward the peripheral n-type region. When light enters from the upper side of the semiconductor structure through the passivation film 59, the light is absorbed by the light absorption layer 52, and electrons and
Generates hole pairs.

Of these electron-hole pairs, the electron is n ⁺
The holes are attracted toward the substrate 51 made of type InP, and the holes are attracted toward the p-type regions 57 and 58. If there is a region where a high electric field is formed among the regions through which holes pass, ionization occurs there and multiplication of carriers is performed.

FIG. 5B shows the electric field distribution generated when a reverse bias is applied in the main region of FIG. 5A. FIG. 5 (B)
5A, the horizontal axis represents the position in the depth direction of the semiconductor structure in FIG. 5A, and the vertical axis represents the electric field strength.

The depletion due to the reverse bias voltage is caused by the pn junction 6
Although extending from 3 to both sides, the opposite conductivity type region 57 has a high impurity concentration, and therefore the depletion width is extremely narrow. On the other hand, since the avalanche multiplication layer 55 arranged on the opposite side has an extremely low impurity concentration, the entire region thereof is easily depleted,
Moreover, the change of the electric field strength inside thereof is extremely small.

The depletion further proceeds to the electric field drop layer 54 which is the n ⁺ type region. The electric field abruptly changes in the electric field drop layer 54 having a high impurity concentration, and the electric field strength abruptly decreases toward the light absorption layer 52.

Within the hetero barrier drop layer 53 and the light absorption layer 52, which are low impurity concentration regions, the change in electric field strength is small. In this way, the electric field distribution as shown in FIG. 5B is formed.

The structure shown in FIG. 5A has an electric field relaxation layer 54.
Has a high impurity concentration, and the avalanche multiplication layer 55 and the light absorption layer 52 or the hetero barrier relaxation layer 53 arranged on both sides thereof have a low impurity concentration region, so that a low-high-low structure is formed with respect to the impurity concentration.

As a factor that limits the response speed of the APD,
There is time required for avalanche multiplication to rise.
In order to speed up APD, it is desired to shorten the avalanche rise time. In order to shorten the avalanche rise time, it is effective to reduce the thickness of the avalanche multiplication layer 55 as shown by the broken line in FIG.

In order to obtain a desired carrier multiplication factor in the thinned avalanche multiplication layer 55, it is necessary to increase the electric field strength of the avalanche multiplication layer 55 in order to increase the ionization rate. ..

FIG. 3 shows I constituting the avalanche multiplication layer.
6 is a graph showing the ionization rate of nP as a function of electric field strength. In FIG. 3, the horizontal axis represents the reciprocal of the electric field strength, and the vertical axis represents the ionization rate. ○ indicates the ionization rate of holes,
● indicates the ionization rate of electrons.

As shown in the figure, the ionization rate increases as the electric field strength increases. Further, the ionization rate of holes is higher than the ionization rate of electrons. However, as the electric field strength increases, the ratio of the ionization rate of electrons to the ionization rate of holes tends to gradually approach 1.

By the way, as shown by the broken line in FIG. 5B, when the electric field intensity in the avalanche multiplication layer 55 increases, the ratio of the ionization rate of electrons to the ionization rate of holes gradually approaches 1.

To reduce noise in the avalanche photodiode, it is desirable that only carriers of holes or electrons cause ionization, but in the avalanche multiplication layer 55, both ions and holes cause ionization. become.

When electron-hole pairs are generated in the light absorption layer 52 and the holes are injected into the avalanche multiplication layer 55, avalanche multiplication occurs and the avalanche multiplication layer 55.
Electrons and holes are generated inside. Of these, electrons travel from the avalanche multiplication layer 55 toward the light absorption layer 52, but when the electric field strength is high, ionization occurs again in the avalanche multiplication layer 55.

That is, when the electric field strength in the avalanche multiplication layer 55 increases, the positive feedback due to the ionization of electrons increases. This increases the noise (excessive noise) generated in the avalanche process.

That is, if the avalanche multiplication layer 55 is thinned in order to increase the speed of the APD, the response speed is increased and the noise characteristic, and thus the sensitivity, is deteriorated.

FIG. 6 illustrates another prior art avalanche photodiode. FIG. 6A shows the structure in cross section, and FIG. 6B shows the electric field intensity distribution in the main structure. The structure shown in FIG. 6A corresponds to the structure shown in FIG. 5A with the avalanche multiplication layer of n ⁻ type InP removed.

That is, on a substrate 51 formed of n ⁺ type InP, a light absorption layer 52 formed of n ⁻ type InGaAs, a hetero barrier relaxation layer 53 formed of n ⁻ type InGaAsP, and an n ⁺ type InP. The avalanche multiplication layer 64 formed by the above method and the window layer 56 formed by n ⁻ type InP are epitaxially laminated, and the avalanche multiplication layer 6 is formed in the window layer 56.
4, a p-type impurity is doped at a high concentration to form a reverse conductivity type region 57, and a p ⁻ -type electric field concentration relaxation region 58 is formed around it.

The main part of the pn junction 63 is a p ⁺ -n ⁺ junction formed between the p ⁺ type reverse conductivity type region 57 and the n ⁺ type avalanche multiplication layer 64. Since this structure has the n ⁻ type light absorption layer 52 and the hetero barrier relaxation layer 53 disposed under the n ⁺ type avalanche multiplication layer 64, it is called a high-low structure from the distribution of the impurity concentration.

The p-side electrode 61 and n having the structure shown in FIG.
When a reverse bias voltage is applied between the side electrodes 62, a depletion layer develops around the pn junction 63. The electric field distribution at this time is shown in FIG. In FIG. 6B, the horizontal axis represents the position in the depth direction of the configuration of FIG. 6A, and the vertical axis represents the electric field strength.

Since the n-side region in contact with the pn junction 63 is the n ⁺ -type avalanche multiplication layer 64, the electric field strength is pn.
The shape decreases from the junction 63 to both sides. As in the case of FIG. 5, the change in electric field strength is small because the impurity concentration is low in the hetero barrier relaxation layer 53 and the light absorption layer 52, which are n ⁻ -type regions.

By the way, in order to shorten the avalanche rise time of the APD shown in FIG. 6, the thickness of the avalanche multiplication layer 64 is set as shown by the broken line in FIG. 6B as in the configuration shown in FIG. It is effective to make it thin. At this time, in order to obtain a desired carrier multiplication factor, it is necessary to increase the electric field strength in the avalanche multiplication layer 64, as in the configuration of FIG. Therefore, excess noise increases.

Further, in the structure shown in FIG. 6, the electric field strength greatly changes in the avalanche multiplication layer 64. Therefore, when the average electric field strength is increased, the maximum electric field strength becomes extremely large. Will end up. In such a region, the ionization rate of holes and the ionization rate of electrons are close to each other, which increases excess noise.

Further, as can be seen from the electric field distribution of FIG. 6B, the light absorption layer 52 of the avalanche multiplication layer 64.
The electric field strength is low in the area close to the area, and the ionization rate itself becomes small in such area.

That is, in the structure shown in FIG. 6, there is a region that does not contribute to carrier multiplication or light absorption.
For this reason, the carrier transit time increases, which is disadvantageous in high-speed characteristics as compared with the low-high-low structure shown in FIG.

[0033]

As described above,
In the conventional technology, there is a problem that the noise characteristic deteriorates when trying to improve the high speed characteristic.

An object of the present invention is to provide an avalanche photodiode capable of improving high speed characteristics and reducing deterioration of noise characteristics. Another object of the present invention is to provide a method of operating an avalanche photodiode, which can improve high-speed characteristics and reduce deterioration of noise characteristics.

[0035]

The avalanche photodiode of the present invention is formed of a first compound semiconductor having a first band gap capable of absorbing light of a wavelength to be detected, and has a first low conductivity type impurity. A light-absorbing layer having a concentration and a second compound semiconductor formed on the light-absorbing layer and having a second bandgap wider than the first bandgap, and having a high first conductivity type impurity. An electric field drop layer having a concentration and an extremely thin thickness, and a third compound semiconductor formed adjacent to the electric field drop layer and having a third bandgap wider than the first bandgap, About 3 × 10 ¹⁶ cm of the first conductivity type
Avalanche multiplication layer having an intermediate impurity concentration of ⁻³ or more, and a fourth compound semiconductor formed adjacently on the avalanche multiplication layer and having a fourth bandgap wider than the first bandgap And a reverse conductivity type semiconductor layer having a high impurity concentration of a second conductivity type opposite to the first conductivity type.

FIG. 1 is a diagram for explaining the principle of the present invention. Figure 1
(A) is a schematic sectional view showing a configuration. First conductivity type low
With the same conductivity type on the light absorption layer 2 having a high impurity concentration.
Electric field drop layer 4 having high impurity concentration and extremely thin thickness
And about 3 × 10 of the same conductivity type ¹⁶cm^-3Intermediate impure above
An avalanche multiplication layer 5 having a material concentration and a predetermined thickness,
Reverse-conductivity type semiconductor layer 7 having a high impurity concentration
And are formed.

The band gap is narrow in the light absorption layer 2 and wide in other regions. These regions are made of a compound semiconductor. In this structure, it is particularly important that the avalanche multiplication layer 5 has an intermediate impurity concentration of about 3 × 10 ¹⁶ cm ⁻³ or more.

[0038]

When a reverse bias voltage is applied to the structure shown in FIG. 1 (A), an electric field distribution as shown in FIG. 1 (B) is generated. Since the opposite conductivity type semiconductor layer 7 has a high impurity concentration, the electric field strength sharply rises.

Since the avalanche multiplication layer 5 has an intermediate impurity concentration, the electric field strength gradually decreases toward the light absorption layer 2. Since the electric field drop layer 4 has a high impurity concentration, the electric field strength sharply decreases. Since the light absorption layer 2 has a low impurity concentration, the electric field strength changes extremely gradually.

In the avalanche multiplication layer 5, since the electric field strength changes considerably, it can be considered that the area is divided into a relatively weak electric field strength area 5a and a relatively strong electric field strength area 5b. How the carriers move in such an electric field strength distribution is schematically shown in FIG.

When the incident light is absorbed in the light absorption layer 2, electron-hole pairs are generated by the light absorption. Among the electron-hole pairs, holes travel toward the opposite conductivity type semiconductor layer 7. In the region 5a of the avalanche multiplication layer 5 having a low electric field intensity, the ionization rate of holes has a certain value.
Generation of hole pairs is performed.

In the region 5a having a comparatively weak electric field intensity, the electrons have a low ionization rate, so that the electrons generated here do not cause ionization and are extracted from the light absorption layer 2 toward the substrate.

The holes are injected into the region 5b having a higher electric field strength, and further generate electron-hole pairs. Among these, the electrons are extracted toward the light absorption layer 2, but in the region 5a where the electric field strength is relatively weak, the ionization rate of the electrons is low, so that they are not ionized so much and are extracted to the light absorption layer 2. In this way, an operation is performed in which the ionization rate of holes is kept at a relatively high value and the ionization rate of electrons is kept at a relatively low value.

Although the above analysis is an average movement of carriers, it is clear that, on average, the ionization of holes is performed in a predetermined manner, while the ionization of electrons is suppressed.

Compared with the conventional technique shown in FIG. 5, the electric field intensity of 5b having a relatively high electric field intensity can be set high while keeping the noise low, so that the carrier multiplication factor can be increased.

Compared with the conventional technique shown in FIG. 6, the electric field strength of the region 5a having a relatively low electric field strength is increased to reduce the area which does not contribute to the ionization, and the area 5b having a relatively high electric field strength. The electric field strength can be reduced, the ionization rate of electrons can be reduced, and noise can be reduced.

In this way, the carrier multiplication factor as a whole is kept high and the ionization rate of electrons is kept low, whereby the increase of excess noise can be suppressed.

[0048]

FIG. 2 shows an avalanche photodiode according to an embodiment of the present invention. Impurity concentration about 1 × 10 ¹⁸ cm ^-3
And n ⁻ type In having an impurity concentration of about 2 × 10 ¹⁵ cm ⁻³ on the n ⁺ type InP substrate 1 having a thickness of about 300 μm.
The GaAs light absorption layer 2 is epitaxially grown to a thickness of about 1.5 μm, and the hetero barrier relaxation layer 3 for relaxing the potential barrier due to the heterojunction is further formed thereon with an impurity concentration of about 2 × 10.
N ^- type compositional gradient I with ¹⁵ cm ^-3 and thickness about 0.1 μm
It is formed by an epitaxial layer of nGaAsP. InGa
If the composition is changed continuously from As to InP, the band structure also changes continuously. The composition may be changed stepwise.

Furthermore, an impurity concentration of about 5.3 × 10 ¹⁷ c
an n ⁺ -type InP field drop layer 4 having m ^-3 and a thickness of about 64 nm, an n-type InP avalanche multiplication layer 5 having an impurity concentration of about 8.9 × 10 ¹⁶ cm ^-3 and a thickness of about 0.2 μm,
An n ⁻ type InP window layer 6 having an impurity concentration of about 2 × 10 ¹⁵ cm ⁻³ and a thickness of about 1.3 μm is sequentially epitaxially grown.

From the surface of the n ⁻ type InP window layer 6, n type In
Impurity concentration reaching the P avalanche multiplication layer 5 about 1 × 1
A 0 ⁺¹⁸ cm ^-3 p ⁺ type diffusion layer 7 is formed by Cd vapor phase diffusion, and the electric field concentration reaching the n ⁺ type InP field drop layer 4 from the surface of the n ⁻ type InP window layer 6 is mitigated around the diffusion layer 7. The p ⁻ -type guard ring 8 is formed by implanting Be ions so as to have an impurity concentration of about 1 × 10 ¹⁶ cm ⁻³ .

A p-side electrode 9 of, for example, Au / Zn / Au is formed on the periphery of the p ⁺ type diffusion layer 7 thus formed, and the other regions are formed by a passivation film of SiN film having a thickness of about 200 nm. Cover with membrane 11. The passivation film 11 also serves as an antireflection film. Further, the n-side electrode 10 of Au / AuGe is formed on the back surface of the semiconductor substrate 1.

Each epitaxial growth layer can be formed by MOCVD continuous growth or the like. With such a configuration, the n-type InP avalanche multiplication layer 5 adjacent to the p-type diffusion layer 7 which is the opposite conductivity type region is formed.
However, since it has an intermediate impurity concentration of, for example, about 8.9 × 10 ¹⁶ cm ⁻³ , an appropriate electric field strength gradient is generated in the avalanche multiplication layer 5.

Therefore, when the holes generated in the light absorption layer 2 are injected into the avalanche multiplication layer 5, ionization is mainly performed by the holes in the region near the light absorption layer 2, and the reverse conductivity type region is formed. In the region close to the p ⁺ -type diffusion layer 7 which is, strong ionization is performed by the high electric field strength, and a high carrier multiplication factor can be obtained.

Since the avalanche multiplication layer 5 has an appropriate impurity concentration, it has a high ionization rate in the portion close to the opposite conductivity type region 7 and almost holes in the portion close to the light absorption layer 2. Excess noise can be suppressed by performing only the ionization by.

The n-type impurity concentration of the electric field drop layer 4 is about 5.3 × 10 ¹⁷ cm ⁻³ , and the n-type avalanche multiplication layer 5 has n-type impurity concentration.
Although the case where the type impurity concentration is about 8.9 × 10 ¹⁶ cm ⁻³ has been described, the electric field strength rapidly increases in the electric field drop layer 4, and the effective avalanche multiplication layer 5 is effective in almost the entire region. If the impurity concentration is such that a specific ionization rate is obtained and the ionization by one carrier is effectively performed in the portion of the avalanche multiplication layer 5 close to the light absorption layer 2, another impurity The concentration may be selected.

The thickness of these layers can also be selected in relation to the impurity concentration. It is preferable to select such structural parameters by referring to the electric field strength dependence of the ionization rate as shown in FIG.

FIG. 4 is a graph plotting an example of how the obtained excess noise coefficient changes when the impurity concentration of the avalanche multiplication layer 4 is changed. The multiplication factor M is 10, and the width of the avalanche multiplication layer 5 is 0.2 μm.
The dependence of the excess noise coefficient on the impurity concentration when changing from m to 0.35 μm is shown.

In general, as the impurity concentration of the avalanche multiplication layer 5 is increased, the electric field strength changes in the avalanche multiplication layer, and as described above, only the ionization by carriers of one polarity is emphasized. The excess noise factor gradually decreases.

However, if the impurity concentration is further increased, the excess noise coefficient will increase. It is considered that this is because ionization by carriers of both polarities becomes significant in the region where the electric field strength is high. Therefore, the higher the impurity concentration of the avalanche multiplication layer 5, the better, and the optimum range exists.

The thin avalanche multiplication layer is preferable for high-speed operation. The thickness of the avalanche multiplication layer is preferably about 0.28 μm or less. The optimum value of the impurity concentration of the avalanche multiplication layer varies as a function of the width of the avalanche multiplication layer as shown in the figure, but it is preferably at least about 3 × 10 ¹⁶ cm ⁻³ or more.

The impurity concentration of the avalanche multiplication layer is about 7
× 10¹⁶cm^-3Above the avalanche multiplication layer
When the width is about 0.25 μm or more, the excess noise coefficient is reversed.
Impurity of the avalanche multiplication layer
Material concentration is about 7 × 10¹⁶cm ^-3Preferably below
Yes.

The characteristic of FIG. 4 is that the multiplication factor of the carrier is about 10
Avalanche multiplication layer 5
If the width is too thin, it is necessary to increase the electric field strength in order to obtain a desired carrier multiplication factor, and the excess noise coefficient tends to increase.

Further, in order to effectively multiply the carriers in the avalanche multiplication layer 5, it is desirable to increase the electric field strength in the electric field drop layer 4 to some extent. For this purpose, the impurity concentration of the electric field drop layer 4 is about 6 × 10 ¹⁷ cm ⁻³ or more, and the thickness thereof is about 0.05 μm.
The following is desired.

The present invention has been described above with reference to the embodiments.
The present invention is not limited to these. For example,
The compound semiconductor to be used may be InP or InGaAs or less. It will be apparent to those skilled in the art that other various modifications, improvements, combinations, and the like can be made.

[0065]

As described above, according to the present invention,
By positively changing the electric field strength in the avalanche multiplication layer, while ensuring the necessary carrier multiplication factor,
It is possible to increase the speed and suppress the increase of excess noise.

[Brief description of drawings]

FIG. 1 is a diagram illustrating the principle of the present invention. 1A is a schematic cross-sectional view showing the structure, FIG. 1B is a graph showing the electric field distribution, and FIG. 1C is a conceptual diagram showing the movement of carriers.

FIG. 2 is a sectional view showing an embodiment of the present invention.

FIG. 3 is a graph showing the ionization rate of InP as a function of electric field strength.

FIG. 4 is a graph showing dependency of excess noise on impurity concentration.

FIG. 5 shows a conventional technique. 5A is a cross-sectional view showing the structure, and FIG. 5B is a graph showing electric field distribution.

FIG. 6 shows a conventional technique. FIG. 6A is a cross-sectional view showing the structure, and FIG. 6B is a graph showing electric field distribution.

[Explanation of symbols]

 1 substrate 2 light absorption layer 3 hetero barrier relaxation layer 4 electric field drop layer 5 avalanche multiplication layer 6 window layer 7 reverse conductivity type semiconductor layer 8 guard ring 9 p-side electrode 10 n-side electrode 11 passivation film

Claims (3)

[Claims] 1. A light absorption layer (2) formed of a first compound semiconductor having a first band gap capable of absorbing light of a wavelength to be detected, and having a low impurity concentration of a first conductivity type. The second compound semiconductor is formed on the light absorption layer (2) and has a second band gap wider than the first band gap, and has a high impurity concentration of the first conductivity type and an extremely thin thickness. And a first electric field drop layer (4) adjacent to the first electric field drop layer (4),
An avalanche multiplication layer formed of a third compound semiconductor having a third band gap wider than that of the first conductivity type and having an intermediate impurity concentration of about 3 × 10 ¹⁶ cm ⁻³ or more of the first conductivity type ( 5) and adjacently formed on the avalanche multiplication layer (5),
A reverse conductivity type semiconductor layer formed of a fourth compound semiconductor having a fourth band gap wider than the first band gap and having a high impurity concentration of a second conductivity type opposite to the first conductivity type ( 7) An avalanche photodiode having and. 2. A light absorption layer (2) mainly formed of InGaAs capable of absorbing light of a wavelength to be detected and having a low n-type impurity concentration; and a light absorption layer (2) formed on the light absorption layer (2), An n-type electric field drop layer having an n-type impurity concentration of about 6 × 10 ¹⁷ cm ⁻³ or more and a thickness of about 0.05 μm or less (4)
And an n-type impurity concentration of about 3 × 10 ¹⁶ cm ⁻³ or more and about 7 × 10 ¹⁶ cm ⁻³ or less, which is formed adjacent to the electric field drop layer (4) and is mainly made of InP. An avalanche multiplication layer (5) having a thickness of 0.28 μm or less, and formed adjacent to the avalanche multiplication layer (5),
An avalanche photodiode, which is mainly made of InP and has a reverse conductivity type semiconductor layer (7) having a high p-type impurity concentration. 3. An avalanche photo diode according to claim 1, wherein the avalanche multiplication layer (5) is almost completely depleted and the electric field drop layer (4) is at least partially depleted. How to operate a photodiode.