JP7060009B2 - Laser radar device - Google Patents

Description

The present invention relates to a laser radar device that detects scattered light due to laser irradiation and measures the distance to an object. Specifically, the present invention is a laser radar device using a light receiving element having a germanium (Ge) absorbing layer, having high productivity and low manufacturing cost, and capable of efficiently receiving light from free space. Regarding.

Conventionally, in a measuring device such as a laser radar (rider), for example, light in a wavelength range of 1400 to 2600 nm, which is difficult to reach the retina of the eye, is projected from a light source, and the light is received by a light receiving element to receive the object. Is being measured. Currently, there are various options for the light source, but the options for the light receiving element are limited, and there are many problems.

As a conventional light receiving element having light receiving sensitivity to these near infrared lights, for example, a compound semiconductor such as indium gallium arsenic (InGaAs) may be used from the viewpoint of low noise and high response speed. many.
However, the method using indium gallium arsenic (InGaAs) has a problem that the productivity is very poor and the manufacturing cost is high. Therefore, there is a demand for a new light receiving element having high productivity and suppressing manufacturing cost.

By the way, as a light receiving element having light receiving sensitivity in the near infrared region near a wavelength of 1550 nm without using indium gallium arsenic (InGaAs), a light receiving element using germanium (Ge) as an absorption layer is known.

As such a light receiving element, germanium (Ge) or silicon (Si) -germanium (Ge) is used as an intrinsic semiconductor to absorb light having a wavelength in the near infrared region, which is suitable for applications such as optical communication. (Patent Document 1) discloses an optical element that can be used in the above. Patent Document 1 discloses an avalanche photodiode (APD) having a p-doped region, an intrinsic region, and an n-doped region in which at least one of the p-doped region and the n-doped region is arranged in an array. ..

As an example of another light receiving element, an avalanche photodiode (APD) having germanium (Ge) as an absorption layer and silicon (Si) as an amplification layer by growing germanium (Ge) on a silicon (Si) layer. ) Is disclosed (Non-Patent Document 1). According to the light receiving element of Non-Patent Document 1, it is known that germanium (Ge) has a lot of noise, but by using silicon (Si) as an amplification layer, it has low noise and the above-mentioned near infrared region. It is stated that it is possible to manufacture a sensor that is sensitive to the wavelength of.

Since these optical elements are intended to be used for optical communication applications, they are configured to have low power consumption and a high response speed. Therefore, the structure is usually such that light is propagated and absorbed by using an absorption layer formed in a waveguide shape (see FIG. 15). Since the waveguide-like absorption layer can have a long interaction length (L2 in FIG. 15) for absorbing light even if the layer thickness is thin, noise caused by dark current or the like can be suppressed and a response can be achieved. The speed can be increased. Further, since the applied voltage can be suppressed, the power consumption can also be suppressed.

However, these optical elements are intended to be used for applications for optical communication, and it is difficult to use them for applications that receive light from free space.
In particular, germanium (Ge) has a very large refractive index of about 4, and since many of the light from free space has a large incident angle, the rate of reflection on the surface of the absorption layer is large, and the efficiency of the absorption layer is high. Cannot be absorbed.
Further, as described above, the light receiving element used for optical communication has a thin absorption layer, and therefore, when used as a light receiving element for receiving light from a free space, it absorbs light. Therefore, the interaction length (L1 in FIG. 8) becomes short, and there arises a problem that the absorption layer cannot sufficiently absorb light. In addition, since the absorption layer made of germanium (Ge) has a very large noise, simply increasing the thickness of the absorption layer slows down the response speed and causes the noise to become very large. It cannot be used for receiving light.

Japanese Unexamined Patent Publication No. 2014-107562

Nature Photonics, 2010, 4,527-534.

The present invention has been made in view of the above problems and situations, and the solution thereof is to provide a laser radar device having high productivity, low manufacturing cost, and efficiently receiving light from free space. That is.

As a result of diligent studies to solve the above problems, the present inventors have an amplification layer, an absorption layer and an antireflection layer in this order on the substrate, and the amplification layer is at least a p-Si layer and an n-Si layer. By using a light receiving element having at least a p-Ge layer as the absorbing layer, it has been found that near-infrared light having a high light receiving sensitivity of the absorbing layer can be efficiently received from a free space, leading to the present invention. rice field.
That is, the above-mentioned problem according to the present invention is solved by the following means.

1. 1. A laser radar device including a light projecting unit that irradiates an object with a laser beam and a light receiving unit that receives the scattered light of the laser light scattered by the object.
The wavelength of the laser beam is in the range of 1400 to 2600 nm.
The light receiving portion has an amplification layer containing silicon (Si), an absorption layer containing germanium (Ge), and an antireflection layer laminated in this order on a substrate, and has a light receiving element that receives the scattered light. death,
The amplification layer has an n-type doped n—Si layer and a p-type doped p—Si layer on the substrate in at least this order.
The absorption layer has an i-Ge layer, which is an intrinsic region, and a p-type-doped p-Ge layer on the amplification layer in at least this order.
A laser radar device having a thickness of the absorption layer that absorbs 80% or more of the laser beam .

2 . The laser radar apparatus according to item 1 , wherein the absorption layer has a second p-Ge layer between the i-Ge layer and the amplification layer.

3 . The first item or that the absorption layer has a p + -Ge layer doped in a p-type at a higher concentration than the p-Ge layer, and the p + -Ge layer is laminated on the p-Ge layer. The laser radar device according to paragraph 2 .

4 . The laser radar apparatus according to any one of items 1 to 3 , wherein the amplification layer has an i-Si layer which is an intrinsic region between the n-Si layer and the p-Si layer.

5 . The laser radar apparatus according to any one of items 1 to 4 , wherein the refractive index of the material forming the antireflection layer is in the range of 1.2 to 3.5.

6 . The laser radar device according to any one of items 1 to 5 , wherein a fine uneven structure is formed on the surface of the antireflection layer.

7 . The laser radar device according to item 6 , wherein the fine uneven structure is a moth-eye structure.

8 . The laser radar device according to any one of items 1 to 7 , wherein the antireflection layer has a multilayer structure in which a plurality of antireflection layers are laminated.

9 . Items 1 to 8 are formed on the side of the substrate opposite to the side on which the absorption layer is provided, in which a light reflection layer that reflects at least a part of the light to be received by the absorption layer is formed. The laser radar device according to any one item.

10 . The laser radar apparatus according to any one of items 1 to 9 , wherein the light receiving unit includes a near-infrared photodetector in which a plurality of the light receiving elements are arranged in a one-dimensional or two-dimensional array. ..

11 . The laser radar device according to any one of items 1 to 10 , wherein the laser light source of the laser light is a semiconductor laser or a fiber laser.

12 . The laser radar device according to any one of items 1 to 11 , further comprising a scanning unit for scanning the laser light emitted from the light projecting unit in the main scanning direction.

13 . Item 12. The laser radar apparatus according to Item 12, wherein a polygon mirror or a MEMS mirror is used as the scanning unit.

According to the above means of the present invention, it is possible to provide a laser radar device having high productivity, low manufacturing cost, and capable of efficiently receiving light from a free space.
The mechanism of action of the above effects is as follows.

In the light receiving element of the present invention, an amplification layer containing silicon (Si), an absorption layer containing germanium (Ge), and an antireflection layer are laminated in this order on a substrate.
Since the absorption layer containing germanium (Ge) has a very large refractive index, light from free space is easily reflected on the surface of the absorption layer, but the antireflection layer prevents reflection on the surface of the light receiving element. Therefore, the amount of light entering the inside of the light receiving element can be increased.
Further, in the light receiving element of the present invention, the absorption layer has at least a p-Ge layer doped in a p-type. Since the p-Ge layer moves slowly but has less noise, for example, by increasing the proportion of the p-Ge layer and providing a thick absorption layer, the light receiving sensitivity (quantum efficiency) is improved and noise is also generated. It can be suppressed.
Further, since the present invention has an amplification layer containing silicon (Si), it is possible to amplify the movement of carriers that have moved from the absorption layer and allow a larger current to flow. Further, by using Si as an amplification layer, it is possible to obtain a sensor having low noise while having sensitivity to light having an absorption wavelength of germanium (Ge).

Further, since the light receiving element of the present invention is a light receiving element in which germanium (Ge) is laminated on a silicon (Si) layer, it can be produced by using a silicon wafer having a large wafer size. Therefore, the productivity is higher and the manufacturing cost can be suppressed lower than the method using silicon indium gallium arsenic (InGaAs) having a small wafer size.

Block diagram showing the configuration of a laser radar device Schematic diagram showing an example of a schematic configuration of a laser radar device Schematic diagram for explaining the scanning of the laser in the scanning unit. Schematic diagram showing another example of the schematic configuration of a laser radar device Schematic diagram showing another example of the schematic configuration of a laser radar device A plan view showing a schematic configuration of a near-infrared photodetector in which optical elements are arranged in an array. Cross-sectional view of the VII-VII portion of the near-infrared photodetector of FIG. A cross-sectional view schematically showing how the absorption layer of an optical element absorbs free light. Cross-sectional view showing the layer structure of the light receiving element Bandgap diagram in the layer configuration of the light receiving element of FIG. Cross-sectional view showing another example of the layer structure of the light receiving element Cross-sectional view showing another example of the layer structure of the light receiving element Graph showing the relationship between the thickness of the absorption layer and the light absorption rate Graph showing the relationship between the presence or absence of an antireflection layer and the light reflectance A cross-sectional view schematically showing how the absorption layer absorbs light in an optical element according to a conventional example having a waveguide-like absorption layer.

The laser radar device of the present invention is a laser radar device including a light projecting unit that irradiates an object with laser light and a light receiving unit that receives the scattered light of the laser light scattered by the object. The wavelength of the laser light is in the range of 1400 to 2600 nm, and the light receiving portion has an amplification layer containing silicon (Si), an absorption layer containing germanium (Ge), and an antireflection layer on the substrate. Are laminated in this order, and have a light receiving element that receives the scattered light, and the amplification layer includes an n-Si layer doped in an n-type and a p-Si layer doped in a p-type. The absorption layer has at least an i-Ge layer which is an intrinsic region and a p-type doped p-Ge layer on the amplification layer in at least this order on the substrate. The absorption layer is characterized in that it absorbs 80% or more of the laser beam . This feature is a technical feature common to or corresponding to the following embodiments.

Further, in an embodiment of the present invention, the absorption layer has a p ⁺ -Ge layer doped in a p-type at a higher concentration than the p-Ge layer, and the p ⁺ is placed on the p-Ge layer. -It is preferable that the Ge layer is laminated. As a result, the mobility of the carrier can be improved and the response speed can be increased. Further, in the band structure, since the Fermi level is different between the p-Ge layer and the p ⁺ -Ge layer, a inclination occurs between the bands, and it becomes easy to take out electrons from the electrode. Further, when the p ⁺ -Ge layer is laminated on the p-Ge layer, it can be expected that electrons can be easily introduced to the amplification layer side. Further, the contact resistance with the electrode can be reduced.

Further, as an embodiment of the present invention, from the viewpoint of obtaining a larger amplification effect by forming the amplification layer into a pin structure, the amplification layer is formed between the n—Si layer and the p—Si layer in an intrinsic region. It is preferable to have a certain i—Si layer.

Further, as an embodiment of the present invention, the refractive index of the material forming the antireflection layer is preferably in the range of 1.2 to 3.5 from the viewpoint of suppressing reflection and improving the light receiving sensitivity. ..

Further, as an embodiment of the present invention, it is preferable that a fine uneven structure is formed on the surface of the antireflection layer from the viewpoint of suppressing reflection and improving the light receiving sensitivity. Further, the fine uneven structure is preferably a moth-eye structure.

Further, as an embodiment of the present invention, from the viewpoint of improving the light receiving sensitivity by improving the antireflection performance, it is preferable that the antireflection layer has a multilayer structure in which a plurality of antireflection layers are laminated.

Further, in an embodiment of the present invention, a light reflection layer that reflects at least a part of light to be received by the absorption layer is formed on the side of the substrate opposite to the side where the absorption layer is provided. It is preferable to have. As a result, the light that has passed through the absorption layer can be reflected and passed through the absorption layer again, so that the amount of light absorbed can be increased and the light receiving sensitivity can be improved.

Further, as an embodiment of the present invention, from the viewpoint of more effectively obtaining the effect of the present invention, the light receiving unit is a near-infrared photodetection in which a plurality of the light receiving elements are arranged in a one-dimensional or two-dimensional array. It is preferable to have a vessel.

Further, as an embodiment of the present invention, it is preferable that the laser light source of the laser beam is a semiconductor laser or a fiber laser from the viewpoint of being compact and capable of high output.

Further, as an embodiment of the present invention, from the viewpoint of widening the angle at which the light can be projected, it is preferable to further include a scanning unit for scanning the laser light emitted from the light projecting unit in the main scanning direction.

Further, as an embodiment of the present invention, it is preferable to use a polygon mirror or a MEMS mirror as the scanning unit from the viewpoint of widening the angle at which light can be projected.

Hereinafter, the present invention, its constituent elements, and modes and embodiments for carrying out the present invention will be described in detail. In the present application, "-" indicating a numerical range is used to mean that the numerical values described before and after the numerical range are included as the lower limit value and the upper limit value.

[Laser radar device]
As shown in FIG. 1, the laser radar device 100 according to the present embodiment has, for example, a light projecting unit 200 that irradiates the object 500 with the laser light L, and scattered light of the laser light L scattered by the object 500. It includes a light receiving unit 300 that receives S, and a scanning unit 400 that scans the laser beam L emitted from the light projecting unit 200 in the main scanning direction D1.
Further, the laser radar device 100 of the present embodiment may be configured to directly irradiate the object 500 with the laser light L emitted from the light projecting unit 200 without providing the scanning unit 400. However, from the viewpoint of widening the angle at which light can be projected, it is preferable that the laser radar device 100 is provided with a scanning unit 400.

Further, in FIG. 1, the direction of the arrow indicates the direction in which the laser light L or the scattered light S travels. Specifically, the laser light L emitted from the light projecting unit 200 passes through the scanning unit 400 and is an object. This means that after the 500 is irradiated, the scattered light S scattered by the object 500 returns to the light receiving unit 300 via the scanning unit 400.

Further, in the laser radar device 100, the time from the start of irradiation of the laser beam L from the light projecting unit 200 to the reception of the scattered light S by the light receiving unit 300 is measured, and the measured time and the speed of light are used. The distance to the object 500 can be calculated.

[Light projector]
The light projecting unit 200 includes a laser light source 210 and a light projecting optical system (for example, a collimator lens 220) (see FIG. 2 and the like).
The laser light L emitted from the laser light source 210 is preferably an eye-safe laser having a wavelength in the range of 1400 to 2600 nm (eye-safe wavelength). The eye-safe laser is a general term for lasers having a large damage threshold for the eyes. Safety standards have been set by the International Electrotechnical Commission (IEC) and the American Standards Association for the intensity of laser light that does not damage the eyes, and the maximum allowable exposure for laser light is laser light. It depends on the wavelength of the laser and the operating conditions of the laser. For example, according to the IEC 60825: 1: 2007 standard, near-infrared laser light having a wavelength of 1400 to 2600 nm shows a higher tolerance than other wavelengths even if the pulse width, repetition frequency, etc. are changed. Refers to this wavelength.

The laser light source 210 is preferably a semiconductor laser or a fiber laser from the viewpoint of being compact and capable of high output. It is preferable that high output is possible because it is possible to detect an object farther away.
As the semiconductor laser, it is preferable to use a VCSEL (Vertical Cavity Surface Emitting LASER) from the viewpoint that a plurality of light sources can be arranged two-dimensionally to irradiate a high-power laser beam L.
Examples of the fiber laser include an erbium fiber laser.

Further, in the light projecting unit 200, the laser light L emitted from the laser light source 210 is incident on the collimator lens 220 as the light projecting optical system. The laser beam L incident on the collimator lens 220 is converted into parallel rays by adjusting the position of the focal point of the laser beam L, and is emitted to the scanning unit 400.
Further, the configuration of the floodlight optical system can be changed as appropriate. For example, parallel light is transmitted by passing only a part of the parallel light emitted from the collimator lens 220 between the collimator lens 220 and the scanning unit 400. It may be shaped and provided with an aperture that adjusts the amount of light.

[Scanning unit]
The scanning unit 400 scans (scans) the laser beam L emitted from the light projecting unit 200 in the main scanning direction D1. By providing the scanning unit 400, the angle at which light can be projected and received can be widened, and light can be projected and received over a wide range. As the scanning unit 400, it is preferable to use a two-dimensional mirror such as a polygon mirror or a MEMS mirror.
FIG. 2 shows an example in which a polygon mirror is used as the scanning unit 400. Further, FIG. 3 shows a schematic diagram illustrating scanning of the laser beam L in the scanning unit 400. As shown in FIG. 3, in the polygon mirror, the laser light L is reflected in a predetermined direction by rotating the polygon mirror around the central axis 410, so that the laser light L is transmitted in the main scanning direction D1. Scan to. This makes it possible to project the laser beam L with respect to the object 500 at a higher and wider angle.
In FIG. 3, the laser beam L before being reflected by the polygon mirror is shown by a solid line, and the laser beam L after being reflected by the polygon mirror is shown by a single point chain line.

Further, FIG. 4 shows an example in which a MEMS mirror is used as the scanning unit 400. In the MEMS mirror, the laser beam L is scanned in the main scanning direction D1 by rotating the angle of the flat plate-shaped mirror around the central axis portion 410, as in the case of the polygon mirror shown in FIG. be able to.

Further, although the scanning in the main scanning direction D1 has been described by using the polygon mirror and the MEMS mirror, the present invention is not limited to this, and for example, another reflection mirror is provided to provide a sub-scanning in a direction intersecting the main scanning direction. The direction may also be scanned.

Further, the scanning unit 400 is not limited to the polygon mirror or the MEMS mirror, and may be any as long as it can scan the laser beam. For example, if the scanning unit 400 is affected by vibration, such as when mounted on a car, or if the scanning unit 400 itself can interfere with light projection or light reception, instead of these mirrors, for example, a voltage is used. The laser light may be propagated through an element whose refractive index changes accordingly, and a voltage may be applied to the element to change the refractive index, thereby changing the traveling direction of the laser light and scanning the laser light.

Further, although the angle at which the light can be projected becomes narrower, as shown in FIG. 5, the laser radar device 100 is not provided with the scanning unit 400, and the laser light emitted from the light projection optical system is directly directed to the object 500. It is also possible to irradiate.

[Light receiving section]
The light receiving unit 300 includes a near infrared photodetector 310 and a light receiving optical system (for example, a condenser lens 320).
The condenser lens 320 as a light-receiving optical system collects the scattered light S scattered by the object 500 on the light-receiving surface 310s of the near-infrared light detector 310.
Further, the configuration of the light receiving optical system can be appropriately changed, and for example, the configuration may further include an imaging lens, an optical filter, and the like.

The near-infrared photodetector 310 is provided with a light-receiving element 10 that receives near-infrared light and converts it into electricity. Further, in the near-infrared photodetector 310, it is preferable that the light receiving elements 10 are arranged in a one-dimensional or two-dimensional array. As an example, FIG. 6 shows a configuration in which a total of 10 light receiving elements 10 in 2 rows × 5 columns are arranged in an array. Further, FIG. 7 shows a cross-sectional view of the VII-VII portion of FIG.
Since each light receiving element 10 of the near infrared photodetector 310 has a germanium (Ge) absorbing layer 40, it can be suitably used for applications in which near infrared light from a free space is received and detected. ..

The near-infrared photodetector 310 can be manufactured, for example, by patterning an SOI (Silicon on Insulator) wafer using a known method.
Specifically, for example, as described in US Pat. No. 6,816,495 and US Pat. No. 6,946,318, germanium is used on a silicon (Si) substrate 20 using a known UHV-CVD method. It can be produced by growing (Ge).

[Light receiving element]
In the light receiving element 10 of the present invention, an amplification layer 30 containing silicon (Si), an absorption layer 40 containing germanium (Ge), and an antireflection layer 50 are laminated in this order on a substrate 20, and the amplification layer 30 is formed. However, the n-Si layer 31 doped in the n-type and the p-Si layer 33 doped in the p-type are provided on the substrate 20 in at least this order, and the absorption layer 40 is doped in the p-type. It is characterized by having at least a p-Ge layer 42.

Specific examples of the layer structure of the light receiving element 10 include, but are not limited to, the following examples.
(i) Substrate / n-Si layer / p-Si layer / p-Ge layer / antireflection layer
(ii) Substrate / n-Si layer / p-Si layer / i-Ge layer / p-Ge layer / antireflection layer
(iii) Substrate / n-Si layer / p-Si layer / i-Ge layer / p-Ge layer / p ⁺ -Ge layer / antireflection layer
(iv) Substrate / n-Si layer / p-Si layer / p-Ge layer / i-Ge layer / p-Ge layer / antireflection layer
(v) Substrate / n-Si layer / p-Si layer / p-Ge layer / i-Ge layer / p-Ge layer / p ⁺ -Ge layer / antireflection layer
(vi) Substrate / n-Si layer / i-Si layer / p-Si layer / p-Ge layer / antireflection layer
(vii) Substrate / n-Si layer / i-Si layer / p-Si layer / i-Ge layer / p-Ge layer / antireflection layer
(viii) Substrate / n-Si layer / i-Si layer / p-Si layer / i-Ge layer / p-Ge layer / p ⁺ -Ge layer / antireflection layer
(ix) Substrate / n-Si layer / i-Si layer / p-Si layer / p-Ge layer / i-Ge layer / p-Ge layer / antireflection layer
(x) Substrate / n-Si layer / i-Si layer / p-Si layer / p-Ge layer / i-Ge layer / p-Ge layer / p ⁺ -Ge layer / antireflection layer

Further, as shown below as an example, it is also preferable that the light reflection layer 60 is further laminated on the bottom surface side of the substrate 20 (the side opposite to the side where the absorption layer 40 is provided).
(xi) Light reflection layer / substrate / n-Si layer / i-Si layer / p-Si layer / i-Ge layer / p-Ge layer / p ⁺ -Ge layer / antireflection layer

In FIG. 9, as an example, an amplification layer 30 formed of an n—Si layer 31, an i—Si layer 32, and a p—Si layer 33 on a substrate 20, which is the layer structure of the above (viii), and i. The light receiving element 10 in which the absorption layer 40 formed from the −Ge layer 41, the p—Ge layer 42, and the p ⁺ −Ge layer 43 and the antireflection layer 50 are laminated in this order is shown.
Further, as shown in FIG. 9, for example, electrodes 70 and 71 are provided at a portion in contact with the n—Si layer 31 and on the upper surface of the absorption layer 40, respectively. These electrodes 70 and 71 form a circuit by wiring or the like (not shown) so that a potential difference can be generated between the electrodes and the electrons generated by the absorption layer 40 absorbing light can be taken out. It has become.
As described above, the positions where the electrodes 70 and 71 are provided can be appropriately changed as long as a potential difference can be generated and the electrons generated by absorbing the light can be taken out.

Further, FIG. 10 shows the band structure of the light receiving element 10 having the layer structure of the above (viii) shown in FIG. 9 when a reverse bias voltage is applied.

The substrate 20 is not particularly limited as long as the effects of the present invention can be obtained, but for example, a silicon substrate is used.

The amplification layer 30 has an n-type doped n—Si layer 31 and a p-type doped p—Si layer 33 on the substrate 20 in at least this order, and carriers transferred from the absorption layer 40. It has the function of amplifying the movement of silicon and allowing a larger current to flow.
Further, from the viewpoint of increasing the amplification amount, the amplification layer 30 has a configuration in which the i-Si layer 32, which is an intrinsic region, is provided between the n—Si layer 31 and the p-Si layer 33 doped with the p-type. , It is preferably formed by a pin structure.

Further, it is preferable to operate as an avalanche photodiode (APD) by applying a high reverse bias between the electrodes provided in the light receiving element 10. Due to the avalanche effect, an amplification effect such as a magnification of about 10 to 100 times can be obtained.
The dope region of the n—Si layer 31 and the p—Si layer 33 can be formed by, for example, a known ion implantation method or a heat diffusion method.

The thickness of the amplification layer 30 can be appropriately changed according to the applied voltage, and is not particularly limited as long as a sufficient amplification effect can be obtained according to the application.

The absorption layer 40 has at least a p-Ge layer 42 doped with a p-type, and functions to absorb light having an absorption wavelength of germanium (Ge). The absorption layer 40 of the present invention is particularly suitable for absorbing light in the wavelength range of 1400 to 1550 nm, which is a near-infrared region.

Further, it is preferable to use the absorption layer 40 by appropriately changing the layer structure as follows depending on the noise level and the response speed required according to the intended use.
For example, when it is required to reduce noise, it is preferable to increase the proportion of the p-Ge layer 42 in the absorption layer 40, and all of the absorption layer 40 may be formed by the p-Ge layer 42.
Further, when it is required to increase the response speed, the absorption layer 40 is configured to have the i-Ge layer 41 which is an intrinsic region, and specifically, the i-Ge layer 41 is placed on the amplification layer 30. , P-Ge layers 42 are preferably laminated in this order. Since the i-Ge layer 41 is located between the p-Ge layer 42 and the p-Si layer 33, the reverse bias voltage is caused by the difference in the Fermi level between the p-Ge layer 42 and the p-Si layer 33. When applied, the band structure is tilted as shown in FIG. Therefore, in the i-Ge layer 41, the moving speed of the carrier can be increased and the response speed can be increased.

Further, the absorption layer 40 may be configured to have a second p-Ge layer 44 between the i-Ge layer 41 and the amplification layer 30 (FIG. 11).

Further, it is preferable that the p-Ge layer 42 has a p ⁺ -Ge layer 43 doped in a p-type at a higher concentration than the p-Ge layer 42. As a result, the mobility of the carrier can be improved and the response speed can be increased. Further, in the band structure, since the Fermi level is different between the p-Ge layer 42 and the p ⁺ -Ge layer 43, an inclination occurs between the bands, so that electrons can be easily taken out from the electrode 71. Further, when the p ⁺ -Ge layer 43 is laminated on the p-Ge layer 42, it can be expected that electrons can be easily introduced to the amplification layer 30 side. Further, the contact resistance with the electrode 71 can be reduced.
Further, as described above, the p ⁺ -Ge layer 43 referred to in the present specification is defined as a Ge layer doped with a p-type at a higher concentration than the p-Ge layer 42.

The dope region of the p-Ge layer 42 and the p ⁺ -Ge layer 43 can be formed by, for example, a known ion implantation method or a heat diffusion method.

For the absorption layer 40, for example, the substrate 20 and the amplification layer 30 are heated to about 600 ° C., and Ge is deposited on the amplification layer 30 by epitaxial growth using GeH ₄ , which is a raw material gas for germanium (Ge). Can be formed by.

The thickness L of the absorption layer 40 preferably satisfies the following formula, where α is the absorption coefficient of germanium (Ge) at the wavelength of the light to be received.
exp (-L x α)> 0.8
[Α represents the absorption coefficient of germanium (Ge) at the wavelength of the light to be received. ]
Further, when the above equation is calculated for L, it becomes the following equation (1).
Equation (1): L <(ln0.8) / α

Satisfying the above formula (1) means that when the thickness of the absorption layer 40 is L, 80% of the light to be received is absorbed by the absorption layer 40.

Further, when the thickness of the absorption layer 40 is 200 nm, 500 nm, 3 μm (3000 nm), and 5 μm (5000 nm), k = 0.123 is used for the imaginary part of the complex refractive index to determine the absorption wavelength (nm). The result of calculating the relationship of absorbance is shown in FIG. As can be seen from FIG. 13, for example, when the absorption of light at 1550 nm is calculated, it is possible to absorb more than 90% and nearly 100% of light with a thickness of 3 μm.
From the above, from the viewpoint of sufficiently absorbing light and improving the light receiving sensitivity, the thickness L of the absorption layer 40 is preferably 3 μm or more.

By the way, if all of the absorption layer 40 is a p-Ge layer 42, if an electric field is not applied to the absorption layer 40, electrons will move in the absorption layer 40 at a diffusion rate. In this case, if the electrons move at the diffusion rate during the average time (so-called minority carrier lifetime) until the electrons recombine with the holes and disappear, the moving distance is about 7 μm. Therefore, from the viewpoint of facilitating the transfer of carriers from the absorption layer 40 to the amplification layer 30, the thickness of the absorption layer 40 is preferably 7 μm or less. Further, by setting the thickness of the absorption layer 40 to 7 μm or less, a sufficient response speed can be obtained when used in a device for measurement.

As the antireflection layer 50, the refractive index of the material forming the antireflection layer 50 is preferably in the range of 1.2 to 3.5 from the viewpoint of efficiently suppressing reflection on the surface of the absorption layer 40. , 1.4 to 3.0 is particularly preferable.
Here, FIG. 14 shows a graph showing the relationship between the presence / absence of the antireflection layer 50 and the light reflectance. The light reflectance in the absorption layer 40 when the antireflection layer 50 is not provided is about 36% as shown in FIG. 14 (a). Further, the material has a refractive index of (b) 1.2, (c) 1.4, (d) 2.0, (e) 3.0, and (f) 3.5, respectively, and the thickness is optimum. 14 shows the light reflectance (%) when the antireflection layer 50 is provided. As can be seen from (d) of FIG. 14, in the antireflection layer 50 made of a material having a refractive index of 2.0, the reflectance of light having a wavelength of about 1550 nm can be suppressed to about 0, and the reflection on the surface of the absorption layer 40 can be suppressed. Can be efficiently suppressed. Further, when the antireflection layer 50 formed of a material having a refractive index of 1.2 to 3.5 is provided, the reflection of light within the wavelength range of 1400 to 1550 nm suitable for the absorption layer 40 according to the present invention. Can be suitably suppressed.
Examples of the material having a refractive index in the range of 1.2 to 3.5 include silicon nitride (SiN) having a refractive index of about 2.0, silicon dioxide (SiO ₂ ) having a refractive index of about 1.5, and a refractive index. It is preferable to use about 3.5 silicon (Si).

Further, as the antireflection layer 50, it is also preferable that a fine concavo-convex structure 51 is formed from the viewpoint of efficiently suppressing reflection on the surface of the absorption layer 40. The fine concavo-convex structure 51 preferably has a shape in which the refractive index substantially increases as it approaches the absorption layer 40, and it is preferable to use a moth-eye structure as such a concavo-convex structure 51.
As shown in the schematic diagram in FIG. 12, the moth-eye structure can be formed, for example, by providing a plurality of cone-shaped convex portions.
The pyramid shape in the moth-eye structure is not particularly limited, and is a cone shape having an antireflection function such as a cone shape, a pyramid shape, a truncated cone shape, a pyramid cone shape, a bell shape, and an elliptical pyramid shape. If there is, it can be selected as appropriate.

Since the substantial refractive index in the moth-eye structure is determined by the material of the material forming the moth-eye structure, the rate of change in the ratio of the structure to the space in the thickness direction of the cone shape, the pitch and depth of the unevenness, and the like. By appropriately adjusting these, the refractive index may be adjusted to be within the range of 1.2 to 3.5 described above. The pitch of the unevenness is preferably, for example, 1000 to 1600 nm, and the depth of the unevenness is preferably 0.5 to 5 times the pitch, and more preferably 1 to 3 times the pitch.

Further, the antireflection layer 50 is preferably configured to have a multilayer structure in which a plurality of antireflection layers 50 are laminated, from the viewpoint of improving the light receiving sensitivity by improving the antireflection performance.
Further, from the viewpoint of efficiently suppressing reflection on the surface of the absorption layer 40, when the wavelength of the light to be received light is λ, the antireflection layer 50 having an optical layer thickness of (λ / 4) an odd multiple is formed. It is preferable that one layer or a plurality of layers are laminated. As a result, the light reflected on the upper surface and the lower surface of each layer provided on the antireflection layer 50 cancel each other out, so that the reflection of the light can be effectively prevented.

(Relationship between reflectance and signal-to-noise ratio)
When the optical element is reverse-biased to operate as an avalanche photodiode (APD), the SN ratio can be calculated by the following equation (A1).

In the above equation (A1), S: signal, N: noise, q: charge, η: quantum efficiency, _Popt : power of incident light, h: Planck's constant, ν: optical frequency, _Ip : shot noise current, IB. : Background light noise current, _ID : Dark current, F (M): Noise factor, B: Band, k: Boltzmann constant, T: Absolute temperature, R _eq : Load resistance, M: Multiplier.
Since the noise in the amplification layer 30 made of silicon (Si) is 1/100 or less of the noise in the absorption layer 40 made of germanium (Ge), it is ignored in the above calculation.

In the light receiving element 10 of the present invention, as shown in FIG. 14, when the antireflection layer 50 is not provided, the reflectance of light having a wavelength of 1550 nm in the absorption layer 40 is about 36%, and the refractive index is about 2. When the antireflection layer 50 formed of the silicon nitride (SiN) of 0.0 is provided, the reflectance can be set to almost 0%.
At this time, when the power (W) of the incident light having an SN ratio of 1 is calculated by the above equation (A1), it is about 100 nW when the reflectance is 40%, and when the reflectance is 0%, it is about 100 nW. It will be 20 nW. Further, when the reflectance is 40% and the reflectance is 0%, the intensity of the light entering the absorption layer 40 is (1.0-0.4) :( 1.0-0) = 3: 5. Here, since the power of the incident light is (Popt) squared, the SN ratio is effective, so when the reflectance is changed from 40% to 0% by the antireflection layer 50, the light receiving sensitivity is 52/32 times. That is, it can be improved by about 2.8 times.

The light reflection layer 60 is provided on the lower surface of the substrate 20 (the side opposite to the side where the absorption layer 40 is provided), and when there is light that has passed through the absorption layer 40, the light that has passed through the substrate 20 At least a part of it is reflected so that it can pass through the absorption layer 40 again. Thereby, the absorption rate in the absorption layer 40 can be improved.
The light reflecting layer 60 is not particularly limited as long as it can reflect at least a part of the near-infrared light to be received, and may be formed by using either an inorganic or organic material, and the forming method is not particularly limited. ..
Specifically, for example, ITO (indium tin oxide), ATO (antimony-doped tin oxide) and the like can be used as the inorganic material, and polycarbonate resin and the like can be used as the organic material.

It should be considered that the embodiments of the present invention described above are exemplary in all respects and not restrictive. That is, the scope of the present invention is shown not by the above description but by the scope of claims, and is intended to include all modifications within the meaning and scope equivalent to the scope of claims.

The present invention can be used in a laser radar device that detects scattered light due to laser irradiation and measures the distance to an object.

10 Light receiving element 20 Substrate 30 Amplification layer 31 n-Si layer 32 i-Si layer 33 p-Si layer 40 Absorption layer 41 i-Ge layer 42 p-Ge layer 43 p ⁺ -Ge layer 44 Second p-Ge layer 50 Anti-reflection layer 51 Concavo-convex structure 60 Light-reflecting layer 100 Laser radar device 200 Light-emitting unit 210 Laser light source 300 Light-receiving unit 310 Near-infrared light detector 400 Scanning unit 500 Object L Laser light S Scattered light D1 Main scanning direction

Claims (13)

A laser radar device including a light projecting unit that irradiates an object with a laser beam and a light receiving unit that receives the scattered light of the laser light scattered by the object.
The wavelength of the laser beam is in the range of 1400 to 2600 nm.
The light receiving portion has an amplification layer containing silicon (Si), an absorption layer containing germanium (Ge), and an antireflection layer laminated in this order on a substrate, and has a light receiving element that receives the scattered light. death,
The amplification layer has an n-type doped n—Si layer and a p-type doped p—Si layer on the substrate in at least this order.
The absorption layer has an i-Ge layer, which is an intrinsic region, and a p-type-doped p-Ge layer on the amplification layer in at least this order.
A laser radar device having a thickness of the absorption layer that absorbs 80% or more of the laser beam . The laser radar apparatus according to claim 1 , wherein the absorption layer has a second p-Ge layer between the i-Ge layer and the amplification layer. The claim that the absorption layer has a p ⁺ -Ge layer doped in a p-type at a higher concentration than the p-Ge layer, and the p ⁺ -Ge layer is laminated on the p-Ge layer. 1 or the laser radar apparatus according to claim 2 . The laser radar apparatus according to any one of claims 1 to 3 , wherein the amplification layer has an i-Si layer which is an intrinsic region between the n—Si layer and the p—Si layer. The laser radar apparatus according to any one of claims 1 to 4 , wherein the refractive index of the material forming the antireflection layer is in the range of 1.2 to 3.5. The laser radar device according to any one of claims 1 to 5 , wherein a fine uneven structure is formed on the surface of the antireflection layer. The laser radar device according to claim 6 , wherein the fine uneven structure is a moth-eye structure. The laser radar apparatus according to any one of claims 1 to 7 , wherein the antireflection layer has a multilayer structure in which a plurality of antireflection layers are laminated. The first to eighth aspects of the substrate, wherein a light reflecting layer that reflects at least a part of the light to be received by the absorbing layer is formed on the side opposite to the side where the absorbing layer is provided. The laser radar device according to any one. The laser radar apparatus according to any one of claims 1 to 9 , wherein the light receiving unit includes a near-infrared photodetector in which a plurality of the light receiving elements are arranged in a one-dimensional or two-dimensional array. .. The laser radar device according to any one of claims 1 to 10 , wherein the laser light source of the laser light is a semiconductor laser or a fiber laser. The laser radar apparatus according to any one of claims 1 to 11 , further comprising a scanning unit for scanning the laser light emitted from the light projecting unit in the main scanning direction. The laser radar device according to claim 12 , wherein a polygon mirror or a MEMS mirror is used as the scanning unit.

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