JP6879273B2 - Light receiving element - Google Patents

Description

The techniques disclosed herein relate to light receiving elements.

A Schottky type light receiving element made of a semiconductor and a metal as disclosed in Patent Document 1 is known.

Japanese Unexamined Patent Publication No. 2011-171519

The Schottky type light receiving element of Patent Document 1 does not have an amplification function. Therefore, it is difficult to detect minute light.

The light receiving element disclosed in the present specification includes a first layer which is a first conductive type semiconductor. The light receiving element is provided on the surface layer portion of the first layer, and is a second layer which is a first conductive type semiconductor having a first thickness and has a lower impurity concentration than the first layer. Be prepared. The light receiving element includes a third layer, which is a second conductive type semiconductor, formed in a part of the surface layer portion of the second layer with a second thickness thinner than the first thickness. The light receiving element includes a metal layer arranged across the surface of the second layer and the surface of the third layer. The light receiving element is located in the region where the third layer is not formed and is arranged in the region of the second layer when viewed from above perpendicular to the surfaces of the second layer and the third layer. It has a specific region that is a type semiconductor. The impurity concentration in the specific region is higher than the impurity concentration in the second layer. The specific region is separated from the third layer by the second layer. The distance between the specific region and the third layer is closer than the distance between the first layer and the third layer.

A Schottky junction is formed between the second layer and the metal layer. Since the depletion layer extends from the second conductive type third layer to the first conductive type second layer, the electric field applied to the joint between the second layer and the metal layer can be relaxed. Further, the distance between the specific region and the third layer is shorter than the distance between the first layer and the third layer. Therefore, the high electric field region is generated in the vicinity of the region close to the third layer in the specific region. When light is absorbed by free electrons in the metal layer and electrons are emitted to the second layer side, it is accelerated in a high electric field region near a specific region and avalanche amplification occurs. As a result, the amplification function can be realized, so that minute light can be detected.

The second layer may be surrounded by the third layer when viewed from above perpendicular to the surfaces of the second and third layers. Details of the effect will be described in Examples.

The width of the opening of the third layer surrounding the second layer may be larger inside the third layer than on the surface of the third layer. Details of the effect will be described in Examples.

The specific region and the first layer may be separated by the second layer. Details of the effect will be described in Examples.

The specific area and the first layer may be connected. Details of the effect will be described in Examples.

A reverse bias voltage for the Schottky junction formed by the metal layer and the second layer may be applied between the metal layer and the first layer. Details of the effect will be described in Examples.

It is a cross-sectional view of the light receiving element which concerns on Example 1, and the figure which shows the potential gradient in the depth direction. It is a top view of the light receiving element. FIG. 3 is a cross-sectional view taken along the line III-III of FIG. 1 (A). It is a top view of the photodiode. It is sectional drawing of the light receiving element which concerns on Example 2. FIG. It is sectional drawing of the light receiving element which concerns on Example 3. FIG.

FIG. 1A shows a schematic cross-sectional view of the light receiving element 1. The light receiving element 1 is a device made by using a silicon substrate. The light receiving element 1 includes a lower electrode 10, a high-concentration n-type layer 11, a low-concentration n-type layer 12, a high-concentration p-type layer 13, a Schottky electrode 14, an upper electrode 15, and a specific region 16.

The lower electrode 10 is made of aluminum (Al). A high-concentration n-type layer 11 is arranged on the upper surface of the lower electrode 10. The high-concentration n-type layer 11 is in ohmic contact with the lower electrode 10. The impurities in the high-concentration n-type layer 11 are phosphorus or arsenic, and the concentration thereof is 1 × 10 ¹⁸ cm- ² or more. A low-concentration n-type layer 12 is arranged on the surface layer portion of the high-concentration n-type layer 11. The low-concentration n-type layer 12 has a first thickness T1. The first thickness T1 is in the range of 1 to 1000 μm. The impurity concentration of the low-concentration n-type layer 12 is lower than the impurity concentration of the high-concentration n-type layer 11.

The high-concentration p-type layer 13 is arranged on a part of the surface layer portion of the low-concentration n-type layer 12. The high-concentration p-type layer 13 includes an opening 13a. A low-concentration n-type layer 12 is arranged in the opening 13a. The width W1 of the opening 13a in the X direction is 2 μm or less. The second thickness T2 of the high-concentration p-type layer 13 is thinner than the first thickness T1. The second thickness T2 is in the range of 1 to 3 μm. The impurity of the high-concentration p-type layer 13 is boron, and its concentration is 1 × 10 ¹⁸ cm- ² or more.

The Schottky electrode 14 is arranged so as to straddle the surfaces of the low-concentration n-type layer 12 and the high-concentration p-type layer 13. The Schottky electrode 14 is gold (Au). The thickness of the Schottky electrode 14 is 200 nm or less. More preferably, it is 50 nm to 100 nm. An Au and Si Schottky junction surface SJ is formed at the interface between the Schottky electrode 14, the low-concentration n-type layer 12 and the high-concentration p-type layer 13.

The upper electrode 15 is arranged on a part of the surface layer portion of the Schottky electrode 14. The upper electrode 15 is not arranged in the region where the opening 13a is formed. The upper electrode 15 is aluminum (Al). FIG. 2 shows a top view of the light receiving element 1 as viewed from the direction of arrow Y1. The upper electrode 15 includes a substantially circular window portion 15a. The Schottky electrode 14 is exposed inside the window portion 15a.

The explanation is returned to FIG. The specific region 16 is a high-concentration n-type region. The impurity concentration of the specific region 16 is higher than the impurity concentration of the low-concentration n-type layer 12. The lower end portion 16b of the specific region 16 is connected to the high-concentration n-type layer 11. The tip portion 16a of the specific region 16 projects in the positive direction of the Z axis (upward in FIG. 1A) toward the Schottky electrode 14. The tip portion 16a is a region close to the high-concentration p-type layer 13. The tip portion 16a of the specific region 16 is separated from the high-concentration p-type layer 13 by the low-concentration n-type layer 12.

FIG. 3 shows a cross-sectional view taken along the line III-III of FIG. 1 (A). FIG. 3 is a cross-sectional view at the interface between the low-concentration n-type layer 12 and the high-concentration p-type layer 13. That is, FIG. 3 is a view seen from above (the Z-axis positive direction) perpendicular to the surfaces of the low-concentration n-type layer 12 and the high-concentration p-type layer 13. The low-concentration n-type layer 12 is arranged in the region of the opening in which the high-concentration p-type layer 13 is not formed. That is, the low-concentration n-type layer 12 is surrounded by the high-concentration p-type layer 13. The low-concentration n-type layer 12 functions as a vertical current path. A specific region 16 is arranged in the low-concentration n-type layer 12.

FIG. 4 shows a schematic top view of the photodiode 2 provided with a plurality of light receiving elements 1. The photodiode 2 has a chip shape. The chip is surrounded by a low-concentration n-type layer 12, and a high-concentration p-type layer 13 is arranged inside the low-concentration n-type layer 12. A Schottky electrode 14 and an upper electrode 15 are laminated on the upper surface of the high-concentration p-type layer 13. A plurality of window portions 15a are formed on the upper electrode 15. The portion where the window portion 15a is formed functions as the light receiving element 1. That is, in the photodiode 2 of FIG. 4, 15 light receiving elements 1 are arranged. The cross-sectional view of the main part of FIG. 1 described above corresponds to the cross section of the I-I portion in FIG. Further, a region for wire bonding is arranged on the upper electrode 15.

(Operation of light receiving element 1)
A positive high voltage VH is applied to the lower electrode 10 with respect to the upper electrode 15. As a result, a high voltage VH is applied between the Schottky electrode 14 and the high concentration n-type layer 11. The high voltage VH is a reverse bias voltage with respect to the Schottky junction surface SJ formed by the Schottky electrode 14 and the low concentration n-type layer 12. 1 (B) and 1 (C) show the potential gradient in the depth direction (Z-axis negative direction) of the light receiving element 1 when a high voltage VH is applied. FIG. 1 (B) is a potential gradient in line BB of FIG. 1 (A). That is, it shows the potential gradient of the region where the high-concentration p-type layer 13 is formed. FIG. 1 (C) is a potential gradient in line CC of FIG. 1 (A). That is, it shows the potential gradient of the region where the opening 13a and the specific region 16 are formed. The depth positions of FIGS. 1B and 1C correspond to the adjacent cross-sectional views of FIG. 1A. In FIGS. 1B and 1C, the horizontal axis represents the electric potential. The vertical axis indicates the depth position up to the lower electrode 10 when the Schottky electrode 14 is used as a reference.

As shown in FIG. 1 (B), in the region where the high-concentration p-type layer 13 is formed, the position P1 at the lower end of the high-concentration p-type layer 13 has the same potential as the Schottky electrode 14. Therefore, since the high-concentration p-type layer 13 functions as a buffer region, a high electric field is not applied to the Schottky junction surface SJ.

On the other hand, as shown in FIG. 1C, in the region where the opening 13a and the specific region 16 are formed, the electric field of the low-concentration n-type layer 12 in the opening 13a has a high concentration arranged around it. It is relaxed by the depletion layer extending from the p-type layer 13 (region R2). Therefore, a high electric field is not applied to the Schottky joint surface SJ.

The distance D1 between the tip portion 16a of the specific region 16 and the high-concentration p-type layer 13 is closer than the distance D2 between the high-concentration n-type layer 11 and the high-concentration p-type layer 13. Further, since the specific region 16 is a high-concentration n-type region, the depletion layer is less likely to grow than the low-concentration n-type layer 12. Therefore, as shown in FIG. 1C, a high electric field region HF can be formed in the vicinity of the tip portion 16a of the specific region 16.

(effect)
When eye-safe band light (example: 1550 nm, energy: 0.8 eV) is incident from the window portion 15a of the light receiving element 1 and the light is absorbed by the Schottky electrode 14 as free electrons, electrons are emitted to the low-concentration n-type layer 12 side. It is released. The emitted electrons can be accelerated in the high electric field region HF near the tip portion 16a of the specific region 16. Since avalanche amplification can occur in the high electric field region HF, the light receiving sensitivity of the light receiving element 1 can be greatly increased.

Autonomous vehicles and ADAS (Advanced driver-assistance systems) systems use infrared cameras and LiDAR (Light Detection and Ranging) systems to recognize the surrounding environment. In these systems, it is preferable to use eye-safe band light (1300 nm to 1600 nm light) for safety. However, since the eye-safe bandlight is light having an energy lower than the bandgap energy of Si, it is necessary to prepare a light receiving element using a substrate or the like other than Si. In this case, the light receiving element is created using a Ge substrate or the like, and the signal processing circuit is created using a Si substrate. Since it is necessary to mount multiple chips in the light receiving system, this leads to an increase in cost. Since the light receiving element 1 described in the present specification has an avalanche amplification function, it is possible to detect eye-safe band light using a Si substrate. The light receiving element and the signal processing circuit can be monolithically integrated on the Si substrate. It is possible to reduce the manufacturing cost of the light receiving system.

FIG. 5 shows a cross-sectional view of the light receiving element 1a according to the second embodiment. The high-concentration p-type layer 13M of the light-receiving element 1a of Example 2 has a different cross-sectional shape from the high-concentration p-type layer 13 of the light-receiving element 1 of Example 1. Since the other structure of the light receiving element 1a of the second embodiment is the same as that of the light receiving element 1 of the first embodiment, the description thereof will be omitted.

The high-concentration p-type layer 13M includes an upper layer L1 and a lower layer L2. The width W12 of the opening of the lower layer L2 is larger than the width W11 of the opening of the upper layer L1. Further, the tip portion 16a of the specific region 16 exceeds the lower surface B2 of the lower layer L2 and reaches the upper side (the positive direction side of the Z axis), and is located below the lower surface B1 of the upper layer L1. .. The width W11 is 2 μm or less.

(effect)
The leakage current of the light receiving element 1a becomes smaller as the width of the low-concentration n-type layer 12 in contact with the Schottky electrode 14 becomes narrower. That is, the narrower the opening width of the high-concentration p-type layer 13M, the smaller the leakage current can be. On the other hand, as the opening width becomes narrower, the distance between the specific region 16 and the high-concentration p-type layer 13 in the X direction becomes closer, which makes manufacturing difficult. In the light receiving element 1a according to the second embodiment, the high-concentration p-type layer 13M is divided into an upper layer and a lower layer. Then, the leakage current can be reduced by narrowing the width W11 of the opening of the upper layer L1. Further, by widening the width W12 of the opening of the lower layer L2, the distance between the specific region 16 and the high-concentration p-type layer 13M in the X direction can be increased. It is possible to achieve both reduction of leakage current and facilitation of manufacturing.

FIG. 6 shows a cross-sectional view of the light receiving element 1b according to the third embodiment. The specific region 16M of the light receiving element 1b of the third embodiment has a different cross-sectional shape from the specific region 16 of the light receiving element 1 of the first embodiment. Since the other structure of the light receiving element 1b of the third embodiment is the same as that of the light receiving element 1 of the first embodiment, the description thereof will be omitted.

(effect)
The specific region 16M and the high-concentration n-type layer 11 are separated by the low-concentration n-type layer 12, but are electrically connected by the low-concentration n-type layer 12. Therefore, in the same manner as in the first embodiment, a high electric field region can be formed in the vicinity of the tip portion 16Ma of the specific region 16M. The specific region 16M of Example 3 has an advantage that it is easy to manufacture because it is shallower than the specific region 16 (FIG. 1) of Example 1.

Although specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of claims. The techniques described in the claims include various modifications and modifications of the specific examples illustrated above. In addition, the technical elements described in the present specification or the drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the techniques illustrated in the present specification or drawings can achieve a plurality of purposes at the same time, and achieving one of the purposes itself has technical usefulness.

(Modification example)
In the present specification, an example in which a Schottky electrode is formed by using Au for an n-type layer has been described, but the present invention is not limited to this form, and other metals may be used. For example, metals such as Ni, Pb, Rh, Co, Re, Te, Ir, Pt, and Se can be used.

In the structure of the light receiving element described in the present specification, the p-type and the n-type may be interchanged. In this case, a negative high voltage may be applied to the lower electrode 10. Examples of the metal forming the Schottky electrode with respect to the p-type layer include Zr, Mn, Ti and the like.

In Example 2, the high-concentration p-type layer 13M having a two-layer structure of the upper layer L1 and the lower layer L2 has been described, but the present invention is not limited to this form. Any structure may be used as long as the inside of the high-concentration p-type layer 13 has a larger opening width than the surface of the high-concentration p-type layer 13. For example, a tapered cross section may be provided so that the width of the opening continuously increases toward the lower side (negative side of the Z axis).

The high-concentration p-type layer 13M shown in Example 2 may be combined with the specific region 16M shown in Example 3.

The n type is an example of the first conductive type. The p-type is an example of the second conductive type. The high-concentration n-type layer 11 is an example of the first layer. The low-concentration n-type layer 12 is an example of the second layer. The high-concentration p-type layer 13 is an example of the third layer. The Schottky electrode 14 is an example of a metal layer.

1: Light receiving element 2: Photodiode 10: Lower electrode 11: High-concentration n-type layer 12: Low-concentration n-type layer 13: High-concentration p-type layer 14: Schottky electrode 15: Upper electrode 16: Specific region T1: First Thickness T2: Second thickness

Claims (8)

It is a light receiving element
The first layer, which is the first conductive type semiconductor,
The second layer, which is a first conductive type semiconductor having a first thickness and is provided on the surface layer portion of the first layer, and has an impurity concentration lower than that of the first layer.
A third layer, which is a second conductive semiconductor, formed on a part of the surface layer portion of the second layer with a second thickness thinner than the first thickness.
A metal layer arranged across the surface of the second layer and the surface of the third layer, and
When viewed from above perpendicular to the surfaces of the second layer and the third layer, the first conductive layer is located in the region where the third layer is not formed and is arranged in the region of the second layer. A specific area that is a type semiconductor and
Is equipped with
The impurity concentration in the specific region is higher than the impurity concentration in the second layer.
The specific region is separated from the third layer by the second layer.
A light receiving element in which the distance between the specific region and the third layer is shorter than the distance between the first layer and the third layer. The light receiving element according to claim 1, wherein the second layer is surrounded by the third layer when viewed from above perpendicular to the surfaces of the second layer and the third layer. The light receiving element according to claim 2, wherein the width of the opening of the third layer surrounding the second layer is larger inside the third layer than on the surface of the third layer. The light receiving element according to any one of claims 1 to 3, wherein the specific region and the first layer are separated by the second layer. The light receiving element according to any one of claims 1 to 3, wherein the specific region and the first layer are connected. According to any one of claims 1 to 5, a reverse bias voltage for a Schottky junction formed by the metal layer and the second layer is applied between the metal layer and the first layer. The light receiving element described. The light receiving element according to any one of claims 1 to 6, wherein the semiconductor is silicon. The light receiving element according to any one of claims 1 to 7, wherein the metal layer is Au.

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