JP7506872B2 - Avalanche Photodiode - Google Patents

Description

The present invention relates to an avalanche photodiode.

Conventionally, an avalanche photodiode has been known that has a _{Ga2O3 -} based single crystal layer, a dielectric layer laminated on the _{Ga2O3 -} based single crystal layer, an anode electrode connected to the dielectric layer, and a _cathode electrode connected to the _Ga2O3 -based single crystal layer (see Patent Document 1).

According to the avalanche photodiode described in Patent Document 1, avalanche breakdown can be generated at the junction between the _{Ga2O3 -} based single crystal layer and the dielectric layer by applying a reverse voltage between the anode electrode and the cathode electrode. This makes it possible to obtain a large current relative to the intensity of the received light, resulting in excellent light receiving sensitivity.

Furthermore, the avalanche photodiode described in Patent Document 1 uses a _{Ga2O3 -} based single crystal layer having a wide band gap, and therefore can realize an ultra-sensitive solar blind sensor that responds only to light with short wavelengths and detects only flames in an environment where sunlight is pouring down, for example.

JP 2017-220550 A

However, in the avalanche photodiode described in Patent Document 1, a current flows through the dielectric layer after light is received, and this dielectric layer may be degraded by the repeated flow of current, raising concerns about its long-term reliability.

An object of the present invention is to provide an avalanche photodiode that uses a Ga ₂ O ₃ based semiconductor, responds to light of a short wavelength, and has excellent long-term reliability.

In order to achieve the above object, one aspect of the present invention provides an avalanche photodiode according to any one of [1] to [ 7 ].

[1] An avalanche photodiode comprising: an n-type first semiconductor layer made of a _{Ga2O3 -} based semiconductor; a p-type second semiconductor layer made of a semiconductor having a band gap larger than that of the _{Ga2O3 -} based semiconductor and stacked on the first semiconductor layer; an anode electrode connected to the second semiconductor layer; and a cathode electrode connected to the first semiconductor layer, wherein an effective acceptor concentration of the second semiconductor layer is higher than an effective donor concentration of the first semiconductor layer, and an avalanche breakdown can be caused between the first semiconductor layer and the second semiconductor layer by applying a reverse voltage between the anode electrode and the cathode electrode.
[2] An avalanche photodiode comprising: a first n-type semiconductor layer made of a _{Ga2O3 -} based semiconductor; a second p-type semiconductor layer made of a semiconductor having a band gap larger than that of the _{Ga2O3 -} based semiconductor; a third n-type semiconductor layer made of a _{Ga2O3 -} based semiconductor sandwiched between the first semiconductor layer and the second semiconductor layer, the third semiconductor layer being i-type or having an effective donor concentration lower than that of the first semiconductor layer; an anode electrode connected to the second semiconductor layer; and a cathode electrode connected to the first semiconductor layer, wherein the effective acceptor concentration of the second semiconductor layer is higher than the effective donor concentration of the third semiconductor layer, and an avalanche breakdown can be caused between the first semiconductor layer and the second semiconductor layer by applying a reverse voltage between the anode electrode and the cathode electrode.
[3] The avalanche photodiode according to the above [2], wherein the third semiconductor layer is in contact with the first semiconductor layer and the second semiconductor layer, and an effective donor concentration of the third semiconductor layer is 1×10 ¹⁷ cm ⁻³ or less.
[ 4 ] The avalanche photodiode according to any one of the above [1] to [3] , wherein the second semiconductor layer is made of diamond.
[ 5 ] The avalanche photodiode according to any one of the above [1] to [ 4 ], wherein the Ga ₂ O ₃ based semiconductor layer includes a guard ring.
[ 6 ] The avalanche photodiode according to the above [1], wherein the stack of the first semiconductor layer and the second semiconductor layer has a mesa shape.
[ 7 ] The avalanche photodiode according to the above [2] or [3] , wherein a stack of the first semiconductor layer, the second semiconductor layer, and the third semiconductor layer has a mesa shape.

According to the present invention, it is possible to provide an avalanche photodiode that uses a Ga ₂ O ₃ semiconductor and responds to light of a short wavelength, and that has excellent long-term reliability.

FIG. 1 is a vertical cross-sectional view of an avalanche photodiode according to a first embodiment. FIG. 2 is a diagram comparing the emission spectra of various light sources. FIG. 3 is a vertical cross-sectional view of an avalanche photodiode according to the second embodiment. 4A and 4B are vertical sectional views of an avalanche photodiode according to the third embodiment. 5A to 5C are vertical cross-sectional views of an avalanche photodiode according to the fourth embodiment.

First Embodiment
(Avalanche photodiode structure)
FIG. 1 is a vertical cross-sectional view of an avalanche photodiode 1 according to a first embodiment.

The avalanche photodiode 1 is a vertical light receiving element and has a _{Ga2O3 -} based semiconductor layer 10, a diamond layer ₁₁ laminated on the _Ga2O3 -based semiconductor layer 10, an anode electrode 12 connected to the diamond layer 11, and a cathode electrode 13 connected to the _{Ga2O3 -} based semiconductor layer 10.

The Ga ₂ O ₃ based semiconductor layer 10 is an n-type semiconductor layer made of a Ga ₂ O ₃ based semiconductor. The Ga ₂ O ₃ based semiconductor layer 10 contains a group IV element such as Si or Sn as a donor. The effective donor concentration (donor concentration Nd minus acceptor concentration Na) of the Ga ₂ O ₃ based semiconductor layer 10 is appropriately set according to the magnitude of the operating voltage of the avalanche photodiode 1, and is set to, for example, approximately 1×10 ¹⁸ cm ⁻³ or less. The thickness of the Ga ₂ O ₃ based semiconductor layer 10 is set, for example, within a range of 10 μm or more and 1000 μm or less.

In addition, since an electric field close to 5 to 8 MV/cm, which is predicted as the dielectric breakdown field strength _{of Ga2O3} during operation of the avalanche photodiode 1, can be applied to the _{Ga2O3 -} based semiconductor layer 10, the _Ga2O3 -based semiconductor constituting the _Ga2O3 -based semiconductor layer 10 is preferably a _single crystal having excellent resistance to the electric field. Also, any high-quality _Ga2O3 -based semiconductor polycrystal that can withstand an electric field of 5 to ₈ MV/cm can be used as the material for _{the Ga2O3 -} based semiconductor layer 10.

Here, the _{Ga2O3 -} based semiconductor is _Ga2O3 or _Ga2O3 doped with elements such as Al and In, and has _a composition expressed as ( _{GaxAlyIn (1-xy)} ) _2O3 (0<x≦1, 0≦y<1, 0<x+y≦1). When _Al is added, the band gap becomes wider, and when In is added, the band gap becomes narrower. The above _{Ga2O3 - based} semiconductor has _a β-type crystal structure.

Since the characteristics of the avalanche photodiode 1 are hardly dependent on the plane orientation of the primary surface of the _{Ga2O3 -} based semiconductor layer 10, the plane orientation of the primary surface of the _{Ga2O3 -} based semiconductor layer 10 is not particularly limited. However, from the viewpoint of, for example, crystal growth rate, it is preferable that the plane orientation be (001), (010), (110), (210), (310), (610), (910), (101), (102), (201), (401), (-101), (-201), (-102) or (-401).

The diamond layer 11 is a p-type semiconductor layer made of diamond having a band gap larger than that of the Ga ₂ O ₃ semiconductor constituting the Ga ₂ O ₃ semiconductor layer 10. The diamond layer 11 contains III group elements such as B, Al, Ga, In, and Tl as acceptors. The effective acceptor concentration (acceptor concentration Na minus donor concentration Nd) of the diamond layer 11 is set higher than the effective donor concentration of the Ga ₂ O ₃ semiconductor layer 10 in order to form a depletion layer mainly on the Ga ₂ O ₃ semiconductor layer 10 side, and is set within a range of, for example, about 1×10 ¹⁸ cm ⁻³ or more and 1×10 ²³ cm ⁻³ or less. If the depletion layer is widely spread not only on the Ga ₂ O ₃ semiconductor layer 10 side but also on the diamond layer 11 side, the voltage required for the operation of the avalanche photodiode 1 becomes high, making it difficult to handle. Furthermore, in order to effectively suppress the spread of the depletion layer towards the diamond layer 11, it is preferable that the effective acceptor concentration of the diamond layer 11 is 1×10 ¹⁸ cm ⁻³ or more. The thickness of the diamond layer 11 is set within the range of, for example, 100 nm or more and 1 mm or less.

In addition, instead of the diamond layer 11, a p-type semiconductor layer made of a semiconductor other than diamond, such as AlGaN, AlN, MgO, etc., which has a larger band gap than _{the Ga 2 O 3} -based semiconductor that constitutes the Ga ₂ O ₃ -based semiconductor layer 10, may be used. The effective acceptor concentration of the p-type semiconductor layer is also set higher than the effective donor concentration of the Ga 2 O ₃ -based semiconductor layer 10, similar to the effective acceptor concentration of the diamond layer 11, and is set within a range of, for example, about 1×10 ¹⁸ or more and 1×10 ²¹ cm ⁻³ or less. However, since diamond is easier to be made p-type than the above-mentioned AlGaN, AlN, MgO, etc., it is preferable to use the diamond layer 11.

The diamond constituting the diamond layer 11 and the semiconductor constituting the p-type semiconductor layer used in place of the diamond layer 11 can operate as an avalanche photodiode 1 whether they are single crystal or polycrystalline. From the viewpoint of device reliability, however, single crystal is preferable, and from the viewpoint of manufacturing costs, polycrystalline is preferable.

The wavelength of light absorbed by the _Ga2O3 - _based semiconductor layer 10 depends on its composition (concentration of Al and In), but for example, _{Ga2O3 ,} which is a typical composition, has a band gap of about 4.5 eV and therefore absorbs light with a wavelength shorter than about 280 nm. Also, the diamond layer 11 has a band gap of about 5.5 eV and therefore absorbs light with a wavelength shorter than about 225 nm.

Therefore, when the avalanche photodiode 1 receives light from the diamond layer 11 side, only light having a wavelength longer than 225 nm passes through the diamond layer 11, and among the light, light having a wavelength shorter than the absorption edge wavelength of the _Ga2O3 -based semiconductor layer 10 (280 nm when _{the Ga2O3 -} based semiconductor layer ₁₀ is made of _Ga2O3 ) is absorbed by the _{Ga2O3 -} based semiconductor layer 10. That is, the wavelength range of light that can be received by the avalanche photodiode 1 is approximately 225 nm or more and the absorption edge wavelength of the _{Ga2O3 -} based semiconductor layer 10 or less.

In addition, it is difficult to make an avalanche photodiode without using a pn junction (pin junction). It is difficult to make a p-type _{Ga2O3 -} based semiconductor, and it is difficult to make an n-type diamond. For this reason, it is difficult to make the semiconductor layer of an avalanche photodiode only from a _{Ga2O3 -} based semiconductor or only from diamond.

Figure 2 is a comparison of the emission spectra of various light sources. A, B, C, D, and E in the figure are the emission spectra of a gas flame, sunlight, a white LED, an incandescent bulb, and a fluorescent lamp, respectively.

As shown in Figure 2, the spectral intensity of sunlight is nearly zero below 300 nm. On the other hand, although not shown in the figure, it has been confirmed that the spectral intensity of a gas flame also approaches zero below 200 nm.

As described above, the wavelength range of light that the avalanche photodiode 1 can receive is approximately 225 to 280 nm, so it is able to respond to light from a gas flame but not to light from the sun, white LEDs, incandescent bulbs, or fluorescent lights. For this reason, the avalanche photodiode 1 can be used, for example, as an ultra-sensitive solar blind sensor that detects only flames in an environment where sunlight is pouring down.

When an impurity such as In that shifts the absorption edge to the long wavelength side is added to the Ga ₂ O ₃ -based semiconductor layer constituting the Ga ₂ O ₃ -based semiconductor layer 10, the amount of the impurity added should be suppressed to a level that does not absorb solar light.

The anode electrode 12 is formed on the surface of the diamond layer 11 opposite to the Ga ₂ O ₃ -based semiconductor layer 10, and is connected to the diamond layer 11. Here, the connection to the diamond layer 11 includes not only a direct connection but also an indirect connection. That is, the anode electrode 12 and the diamond layer 11 may be connected via a layer that does not interfere with the operation of the avalanche photodiode 1 as a light receiving element.

The material of the anode electrode 12 is not particularly limited, and may be, for example, Ti, Mo, Ta, Pt, Au, Ni, or Al. However, when the anode electrode 12 is directly connected to the diamond layer 11, it is preferable to use Ti, Mo, Ta, or the like, which is in ohmic contact with the diamond layer 11, in order to suppress device losses (suppress an increase in the operating voltage).

Since the avalanche photodiode 1 receives light by passing through the anode electrode 12, the thickness of the anode electrode 12 is preferably thin, for example, about 1 to 10 nm. The shape of the anode electrode 12 is typically circular or rectangular.

The surface of the anode electrode 12 may be covered with an anti-reflection film. By using this anti-reflection film, it is possible to suppress the reflection of light on the surface of the anode electrode 12 and increase the amount of light received by the avalanche photodiode 1.

The cathode electrode 13 is formed on the surface of the _{Ga2O3 -} based semiconductor layer 10 opposite to the diamond layer 11, and is connected to the _Ga2O3 -based semiconductor layer 10. Here, the connection to _{the Ga2O3 -} based semiconductor layer 10 includes not only a direct connection but also an indirect connection. That is, the cathode electrode 13 and the _{Ga2O3 -} based semiconductor layer 10 may be connected via a layer that does not interfere with the operation of the avalanche photodiode 1 as a light receiving element.

The cathode electrode 13 has a structure in which, for example, a 300 nm thick Au film is laminated on a 50 nm thick Ti film. The shape of the cathode electrode 13 is typically rectangular, the same as the shape of the bottom surface of the Ga ₂ O ₃ -based semiconductor layer 10.

(Avalanche Photodiode Operation)
By applying a reverse voltage (a voltage that makes the potential of the cathode electrode 13 higher than the potential of the anode electrode 12) slightly smaller than the voltage at which avalanche breakdown occurs between the anode electrode 12 and the cathode electrode 13 of the avalanche photodiode 1, it is possible to cause an avalanche breakdown between the _{Ga2O3 -} based semiconductor layer 10 and the diamond layer 11 when light is received.

More specifically, when the avalanche photodiode 1 receives light, electrons are excited to generate electron-hole pairs in a depletion layer formed in the vicinity of the interface between the _{Ga2O3 -} based semiconductor layer 10 and the diamond layer 11. When a reverse voltage is applied, the electrons excited in the depletion layer are accelerated by a strong electric field and collide with and ionize atoms constituting the _{Ga2O3 -} based semiconductor layer 10, generating new electron-hole pairs (impact ionization).

The electrons generated by this impact ionization cause further impact ionization, and this process is repeated to generate many electron-hole pairs. The multiple holes generated in this way move across the diamond layer 11 to the anode electrode 12 due to the strong electric field in the depletion layer. The multiple electrons also move to the cathode electrode 13 due to the strong electric field in the depletion layer. This generates a large current (avalanche breakdown).

In this way, the avalanche photodiode 1 can generate a large current relative to the intensity of the irradiated light by utilizing the avalanche breakdown. This allows the avalanche photodiode 1 to have excellent light receiving sensitivity.

(Manufacturing method of avalanche photodiode)
The following describes an example of a method for manufacturing the avalanche photodiode 1. First, a surface of a _Ga2O3 - _based semiconductor single crystal substrate serving as the _{Ga2O3 -} based semiconductor layer 10 and a surface of a diamond single crystal substrate serving as the diamond layer 11 are bonded together.

For example, first, a diamond single crystal substrate is immersed in _a 75°C _H2SO4 / _H2O2 (4:1) mixed solution (so-called sulfuric acid/hydrogen peroxide) for 10 _minutes , and its surface is terminated with OH groups. Next, the surface of a _{Ga2O3 -} based semiconductor single crystal substrate is irradiated with oxygen plasma for 30 seconds under the conditions of an output of 200W and a pressure of 60Pa, and is terminated with OH groups. Next, the diamond single crystal substrate and the _{Ga2O3 -} based semiconductor single crystal substrate, which are terminated with OH groups, are brought into contact with each other in the atmosphere, and are stored for about 3 days to dry. After that, a heat treatment is performed at 250°C for 24 hours, and the bonding is completed.

In addition to bonding, a method may be used in which _{Ga2O3 -} based semiconductor crystals are grown on the surface of a diamond single crystal substrate as the diamond layer 11 using a CVD (Chemical Vapor Deposition) method or the like to form the _{Ga2O3 -} based semiconductor layer 10, or diamond crystals are grown on the surface of a _{Ga2O3 -} based semiconductor single crystal substrate as the _{Ga2O3 -} based semiconductor layer 10 to form the diamond layer 11. However, since these crystal growth methods are not easy, it is preferable to use a bonding method.

Thereafter, the anode electrode 12 and the cathode electrode 13 are formed on the surfaces of the diamond layer 11 and the _{Ga2O3 -} based semiconductor layer 10, respectively, by vacuum deposition, RF (radio frequency) sputtering, etc. The anode electrode 12 and the cathode electrode 13 are ohmic-contacted to the diamond layer 11 and the _{Ga2O3 -} based semiconductor layer 10, respectively.

Second Embodiment
The second embodiment differs from the first embodiment in that a semiconductor layer is used to control the width of the depletion layer. In the description of this embodiment, the same or substantially the same members as those used in the above-mentioned embodiments are denoted by the same reference numerals, and the description thereof may be omitted or simplified.

(Avalanche photodiode structure)
FIG. 3 is a vertical cross-sectional view of an avalanche photodiode 2 according to the second embodiment.

The avalanche photodiode 2 is a vertical light receiving element and has a _{Ga2O3 -} based semiconductor layer 20, a diamond layer 11, a _Ga2O3 -based semiconductor layer 21 sandwiched between the _{Ga2O3 -} based _{semiconductor} layer 20 and the diamond layer 11, an anode electrode 12 connected to the diamond layer 11, and a cathode electrode 13 connected to the _{Ga2O3 -} based semiconductor layer 20.

The _{Ga2O3 -} based semiconductor layer 20 is an n-type semiconductor layer made of a _{Ga2O3 -} based semiconductor. The _Ga2O3- based semiconductor layer 20 contains a group IV element such as Si or Sn as a donor. The effective donor concentration of _{the Ga2O3 -} based semiconductor layer 20 is set to, for example, approximately 1× ¹⁰¹⁸ cm ^-3 or more. The thickness of the _{Ga2O3 -} based semiconductor layer 10 is set to, for example, within a range of 10 μm or more and 1000 μm or less.

Since the characteristics of the avalanche photodiode 2 are hardly dependent on the plane orientation of the primary surface of the _{Ga2O3 -} based semiconductor layer 20, the plane orientation of the primary surface of the _{Ga2O3 -} based semiconductor layer 20 is not particularly limited. However, from the viewpoint of, for example, crystal growth rate, it is preferable that the plane orientation be (001), (010), (110), (210), (310), (610), (910), (101), (102), (201), (401), (-101), (-201), (-102) or (-401).

The Ga ₂ O ₃ based semiconductor layer 21 is an i-type or n-type semiconductor layer made of a Ga ₂ O ₃ based semiconductor. The effective donor concentration of the Ga ₂ O ₃ based semiconductor layer 21 is set lower than the effective donor concentration of the Ga ₂ O ₃ based semiconductor layer 20, for example, set to about 1×10 ¹⁷ cm ⁻³ or less, so that the Ga ₂ O ₃ based semiconductor layer 21 is almost completely depleted without almost depleting the Ga ₂ O ₃ based semiconductor layer 20. In order to almost completely deplete the Ga ₂ O ₃ based semiconductor layer 21 without almost depleting the Ga ₂ O ₃ based semiconductor layer 20, for example, it is preferable that the effective donor concentrations of the Ga ₂ O ₃ based semiconductor layer 20 and the Ga ₂ O ₃ based semiconductor layer 21 are different by one digit or more.

The Ga ₂ O ₃ -based semiconductor layer 21 behaves as an i-type or n-type depending on the magnitude of the effective donor concentration, but may be either i-type or n-type. When a donor is added to the Ga ₂ O ₃ -based semiconductor layer 21, a group IV element such as Si or Sn is used as the donor. Basically, if no donor is added, the Ga ₂ O ₃ -based semiconductor layer 21 becomes an i-type, but in order to prevent the Ga 2 O 3 -based semiconductor layer 21 from becoming an n-type due to the inclusion of unintended impurities, a deep acceptor (an acceptor that forms a deep level) such as Mg, N, Zn, or Fe may be added to make the Ga ₂ O 3 -based semiconductor layer 21 an i-type. In addition, the Ga 2 O ₃ -based semiconductor layer 21 can also be formed by diffusing oxygen on the surface of the n-type Ga ₂ O ₃ -based semiconductor layer 20 by oxygen annealing treatment or the like. The thickness of the Ga ₂ O ₃ -based semiconductor layer 21 is set, for example, within a range of 0.2 μm or more and 2 μm or less.

The _{Ga2O3 -} based semiconductor constituting the Ga2O3-based semiconductor layers 20 and ₂₁ is preferably a single crystal having excellent resistance to an electric field, similar to _{the Ga2O3} - _based semiconductor constituting the _Ga2O3 - _based semiconductor layer 10. Any high-quality _{Ga2O3 -} based semiconductor polycrystal capable of withstanding an electric field of 5 to 8 MV/cm can be used as the material for the _{Ga2O3 -} based semiconductor layers 20 and 21.

The diamond layer 11 is a p-type semiconductor layer made of diamond, similar to the first embodiment. The effective acceptor concentration of the diamond layer 11 is set higher than the effective donor concentration of the Ga ₂ O ₃ -based semiconductor layer 21 in order to form a depletion layer mainly on the Ga ₂ O ₃ -based semiconductor layer 21 side, and is set within a range of, for example, approximately 1×10 ¹⁸ cm ⁻³ or more and 1×10 ²³ cm ⁻³ or less. In order to effectively suppress the spread of the depletion layer toward the diamond layer 11 side, it is preferable that the effective acceptor concentration of the diamond layer 11 is 1×10 ¹⁸ cm ⁻³ or more. The thickness of the diamond layer 11 is set within a range of, for example, 100 nm or more and 1 mm or less.

Also, similarly to the first embodiment, instead of the diamond layer 11, a p- _type semiconductor layer made of a semiconductor other than diamond, such as AlGaN, AlN, MgO, etc., having a band gap larger than that of the _{Ga2O3 -} based semiconductor constituting the _Ga2O3 -based semiconductor layer 10, may be used.

As in the first embodiment, the diamond constituting the diamond layer 11 and the semiconductor constituting the p-type semiconductor layer used in place of the diamond layer 11 can operate the avalanche photodiode 2 whether they are single crystal or polycrystalline, but from the viewpoint of device reliability, single crystal is preferable, and from the viewpoint of manufacturing costs, polycrystalline is preferable.

The wavelength range of light that can be received by the avalanche photodiode 2 is approximately 225 nm or more and the absorption edge wavelength or less of the _Ga2O3 -based semiconductor layer 21. _{The Ga2O3 -based} semiconductor layer 21 is made of _{a Ga2O3} -based semiconductor like the _Ga2O3 -based semiconductor layer 10 of the avalanche photodiode ₁ , so the avalanche photodiode 2 can respond to light in the same wavelength range as the avalanche photodiode 1. Therefore, the avalanche photodiode 2 can be used for the same applications as the avalanche photodiode 1.

The avalanche photodiode 2 has a _Ga2O3- based semiconductor layer 21 between the _{Ga2O3 -} based semiconductor layer 20 and the diamond layer 11, the _Ga2O3 -based semiconductor layer 21 having a lower effective donor concentration than the _Ga2O3 -based _{semiconductor} layer 20, so that the _Ga2O3 -based semiconductor layer 21 almost becomes a depletion layer. That is, in the avalanche photodiode 2, the thickness of _{the Ga2O3 -} based semiconductor layer 21 almost becomes the width of the depletion layer, so that the width of the depletion layer can be easily controlled.

In the avalanche photodiode 1 according to the first embodiment, the width of the depletion layer is controlled by the effective donor concentration of the _{Ga2O3 -} based semiconductor layer 10. However, it is not easy to control the effective donor concentration of the _{Ga2O3 -} based semiconductor layer 10 with high accuracy.

Therefore, compared to the avalanche photodiode 1, the avalanche photodiode 2 has a higher manufacturing cost due to the provision of the _{Ga2O3 -} based semiconductor layer 21, but has the advantage that the width of the depletion layer can be easily controlled and therefore the variation in the depletion layer width can be further suppressed or the depletion layer width can be made larger. By suppressing the variation in the depletion layer width, the variation in the light receiving sensitivity of each product of the avalanche photodiode 2 can be suppressed and the yield can be improved. Furthermore, by increasing the depletion layer width, light can be efficiently absorbed and the sensitivity of the avalanche photodiode 2 can be improved.

(Avalanche Photodiode Operation)
By applying a reverse voltage slightly smaller than the voltage at which avalanche breakdown occurs between the anode electrode 12 and the cathode electrode 13 of the avalanche photodiode 2, it is possible to cause avalanche breakdown between the _{Ga2O3 -} based semiconductor layer 20 and the diamond layer 11 when light is received.

More specifically, when the avalanche photodiode 2 receives light, electrons are excited to generate electron-hole pairs in a depletion layer formed in the _{Ga2O3 -} based semiconductor layer _21. When a reverse voltage is applied, the electrons excited in the depletion layer are accelerated by a strong electric field and collide with and ionize atoms constituting the _Ga2O3 -based semiconductor layer 21, generating new electron-hole pairs (impact ionization).

The electrons generated by this impact ionization further cause impact ionization, and this process is repeated to generate many electron-hole pairs. The multiple holes generated in this manner move across the diamond layer 11 to the anode electrode 12 due to the strong electric field in the depletion layer. The multiple electrons also move across the _{Ga2O3 -} based semiconductor layer 20 to the cathode electrode 13 due to the strong electric field in the depletion layer. This generates a large current (avalanche breakdown).

In this way, the avalanche photodiode 2 can generate a large current relative to the intensity of the irradiated light by utilizing the avalanche breakdown. This allows the avalanche photodiode 2 to have excellent light receiving sensitivity.

(Manufacturing method of avalanche photodiode)
Hereinafter, a description will be given of an example of a method for manufacturing the avalanche photodiode 2. First, a single crystal film of a _{Ga2O3 - based} semiconductor as the _Ga2O3 -based semiconductor layer 21 is epitaxially grown by a CVD method or the like on a single crystal substrate of a _{Ga2O3 -} based semiconductor as the _{Ga2O3 -} based semiconductor layer 20.

Here, when forming the i-type _Ga2O3 -based semiconductor layer 21, oxygen may be diffused into the surface of the n-type _{Ga2O3 - based} semiconductor layer 20 by oxygen annealing treatment or the like, and a portion near the surface of the _{Ga2O3 -} based semiconductor layer 20 that has changed to i-type may be used as _{the Ga2O3 -based semiconductor layer 21. When the method of diffusing oxygen into the surface of the Ga2O3-based semiconductor layer 20 by oxygen annealing treatment or the like is used to form the i-type Ga2O3 - based semiconductor} layer 21, the manufacturing cost of the avalanche photodiode 2 can be reduced compared to the case of epitaxially growing a crystal by a CVD method or the like.

Next, the surface of the _{Ga2O3 -} based semiconductor layer 21 formed on the _{Ga2O3 -} based semiconductor layer ₂₀ is bonded to the surface of the diamond single crystal substrate as the diamond layer 11. This bonding can be performed in the same process as the bonding of the _Ga2O3 - _based semiconductor single crystal substrate as the _Ga2O3 -based semiconductor layer 10 in the first embodiment and the diamond single crystal substrate as the diamond layer 11. In addition, the diamond layer 11 may be formed by growing diamond crystal on the _Ga2O3 - _based semiconductor layer 21 by CVD or the like, or conversely, the _Ga2O3 -based semiconductor layers 21, ₂₀ may be formed by growing _Ga2O3 -based semiconductor crystal on the diamond single crystal substrate as _the diamond layer 11.

Thereafter, the anode electrode 12 and the cathode electrode 13 are formed on the surfaces of the diamond layer 11 and the _{Ga2O3 -} based semiconductor layer 20, respectively, by vacuum deposition, RF (radio frequency) sputtering, etc. The anode electrode 12 and the cathode electrode 13 are ohmic-contacted to the diamond layer 11 and the _{Ga2O3 -} based semiconductor layer 20, respectively.

Third embodiment
The third embodiment differs from the first embodiment in that the avalanche photodiode has a field plate structure and a guard ring structure. In the description of this embodiment, the same or substantially the same members as those used in the above-mentioned embodiments are denoted by the same reference numerals, and the description thereof may be omitted or simplified.

(Avalanche photodiode structure)
4A and 4B are vertical cross-sectional views of avalanche photodiodes 3 and 4 according to the third embodiment, respectively.

The avalanche photodiode 3 shown in FIG. 4( a) has a _Ga2O3 -based semiconductor layer 10, a diamond layer 11 laminated on the _Ga2O3 -based semiconductor layer 10, an _anode electrode ₁₂ connected to the diamond layer 11, and a cathode electrode ₁₃ connected to the _Ga2O3 -based semiconductor layer 10, similar to the avalanche photodiode 1 according to the first embodiment.

In the avalanche photodiode 3 , a dielectric film 30 made of SiO ₂ , Al ₂ O ₃ or the like is provided along the edge of the upper surface of the diamond layer 11 , and the edge of the anode electrode 12 rides up onto the dielectric film 30 .

By providing such a field plate structure, it is possible to suppress electric field concentration near the end of the anode electrode 12. In addition, the dielectric film 30 also functions as a passivation film that suppresses surface leakage current flowing on the upper surface of the second diamond layer 11.

4A, a pad electrode 32 is connected to the anode electrode 12. The pad electrode 32 is formed on an insulator 31 made of _SiO2 or the like that is formed on a dielectric film 30. The pad electrode 32 has a structure in which an Au film with a thickness of 5000 nm is laminated on a Ti film with a thickness of 5 nm, for example.

The avalanche photodiode 3 has a structure in which a field plate structure is added to the avalanche photodiode 1 according to the first embodiment, but may also have a structure in which a field plate structure is added to the avalanche photodiode 2 according to the second embodiment.

The avalanche photodiode 4 shown in FIG. 4(b) has a _Ga2O3 -based semiconductor layer 10, a diamond layer 11 laminated on the _Ga2O3 -based semiconductor layer 10, an _anode electrode ₁₂ connected to the diamond layer 11, and a cathode electrode ₁₃ connected to the _Ga2O3 -based semiconductor layer 10, similar to the avalanche photodiode 1 according to the first embodiment.

In the avalanche photodiode 4, a guard ring 40 is formed in the vicinity of the interface between the _{Ga2O3 -} based semiconductor layer 10 and the diamond layer 11. By using this guard ring 40, electric field concentration in the vicinity of the end of the anode electrode 12 can be alleviated.

The guard ring 40 is an oxidized region or an acceptor ion-containing region formed by selectively performing oxygen annealing or ion implantation of acceptor ions such as Mg, N, Zn, or Fe in a ring-shaped region on the upper surface of _{the Ga2O3} -based semiconductor layer 10. The shape and number of the guard rings 40 are not particularly limited as long as they are within a range in which electric field concentration near the end of the anode electrode 12 can be alleviated.

Also, as shown in FIG. 4(b), the avalanche photodiode 4 may have a field plate structure similar to the avalanche photodiode 3.

The avalanche photodiode 4 has a structure in which a guard ring structure is added to the avalanche photodiode 1 according to the first embodiment, but may have a structure in which a guard ring structure is added to the avalanche photodiode 2 according to the second embodiment.

Fourth embodiment
The fourth embodiment is different from the first embodiment in that the laminate of the _{Ga2O3 -} based semiconductor layer and the diamond layer has a mesa shape.In the description of this embodiment, the same or substantially the same members as those used in the above-mentioned embodiment are given the same reference numerals, and the description thereof may be omitted or simplified.

(Avalanche photodiode structure)
5(a) to 5(c) are vertical cross-sectional views of an avalanche photodiode 5 according to the fourth embodiment.

The avalanche photodiode 5 has a _{Ga2O3 -} based semiconductor layer 10, a diamond layer ₁₁ laminated on the _Ga2O3 -based semiconductor layer 10, an anode electrode 12 connected to the diamond layer 11, and a cathode electrode 13 connected to the _{Ga2O3 -} based semiconductor layer 10.

The laminate of the _{Ga2O3 -} based semiconductor layer 10 and diamond layer 11 of the avalanche photodiode 5 has a mesa shape with a convex portion facing the anode electrode 12. The anode electrode 12 is formed on the upper surface of the diamond layer 11 processed into a mesa shape, and the position of the end of the anode electrode 12 is approximately aligned with the position of the end of the upper surface of the mesa shape. With this structure, it is possible to reduce electric field concentration near the end of the anode electrode 12.

In the example shown in Fig. 5(a), the laminate of the _{Ga2O3 -} based semiconductor layer 10 and the diamond layer 11 has a mesa shape with vertical sides. In the example shown in Fig. 5(b), the laminate of the _{Ga2O3 -} based semiconductor layer 10 and the diamond layer 11 has a mesa shape with tapered sides. In the example shown in Fig. 5(c), the laminate of the _{Ga2O3 -} based semiconductor layer 10 and the diamond layer 11 has a mesa shape with reverse tapered sides. Of the three types of mesa shapes that have an effect of reducing electric field concentration, the mesa shape shown in Fig. 5(c) has the greatest effect.

The side surface of the mesa-shaped portion of the laminate of the Ga ₂ O ₃ based semiconductor layer 10 and the diamond layer 11 of the avalanche photodiode 5 is covered with an insulator 50 made of SiO ₂ or the like.

In the example shown in FIGS. 5(a) to (c), a pad electrode 51 is connected to the anode electrode 12. The pad electrode 51 is formed on an insulator 50 and has a configuration similar to that of the pad electrode 32 of the avalanche photodiode 3 according to the third embodiment, for example.

The avalanche photodiode 5 has a structure in which the laminate of the _{Ga2O3 -} based semiconductor layer 10 and the diamond layer 11 of the avalanche photodiode 1 according to the first embodiment is processed into a mesa shape, but may have a structure in which the laminate of the _{Ga2O3 -} based semiconductor layers 20, 21 and the diamond layer 11 of the avalanche photodiode 2 according to the second embodiment is processed into a mesa shape.

(Effects of the embodiment)
According to the first to fourth embodiments, it is possible to provide an avalanche photodiode that uses Ga ₂ O ₃ single crystal and responds to light of a short wavelength. This makes it possible to realize, for example, an ultra-sensitive solar blind sensor that detects only flames in an environment where sunlight is pouring down. In addition, since the avalanche photodiodes according to the first to fourth embodiments do not use an insulator (dielectric) in the layer that becomes the current path, they are less likely to deteriorate even if current flows repeatedly, and have excellent long-term reliability.

Although the embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and various modifications are possible without departing from the spirit of the invention. Furthermore, the components of the above embodiment can be combined in any manner without departing from the spirit of the invention.

Furthermore, the embodiments and examples described above do not limit the invention according to the claims. It should also be noted that not all of the combinations of features described in the embodiments and examples are necessarily essential means for solving the problems of the invention.

REFERENCE SIGNS LIST 1, 2, 3, 4, 5...avalanche photodiode, 10, 20, 21... _{Ga2O3 -} based semiconductor layer, 11...diamond layer, 12...anode electrode, 13...cathode electrode, 40...guard ring

Claims (7)

An n-type first semiconductor layer made of a _{Ga2O3 -} based semiconductor;
a p-type second semiconductor layer made of a semiconductor having a band gap larger than that of the Ga ₂ O ₃ -based semiconductor and laminated on the first semiconductor layer;
an anode electrode connected to the second semiconductor layer;
a cathode electrode connected to the first semiconductor layer;
Equipped with
an effective acceptor concentration of the second semiconductor layer is higher than an effective donor concentration of the first semiconductor layer;
By applying a reverse voltage between the anode electrode and the cathode electrode, an avalanche breakdown can be caused between the first semiconductor layer and the second semiconductor layer.
Avalanche photodiode. An n-type first semiconductor layer made of a _{Ga2O3 -} based semiconductor;
a p-type second semiconductor layer made of a semiconductor having a band gap larger than that of the _{Ga2O3 -} based semiconductor;
a third semiconductor layer sandwiched between the first semiconductor layer and the second semiconductor layer, the third semiconductor layer being made of a _{Ga2O3 -} based semiconductor and being of i-type or of n-type having an effective donor concentration lower than that of the first semiconductor layer;
an anode electrode connected to the second semiconductor layer;
a cathode electrode connected to the first semiconductor layer;
Equipped with
an effective acceptor concentration of the second semiconductor layer is higher than an effective donor concentration of the third semiconductor layer;
By applying a reverse voltage between the anode electrode and the cathode electrode, an avalanche breakdown can be caused between the first semiconductor layer and the second semiconductor layer.
Avalanche photodiode. the third semiconductor layer is in contact with the first semiconductor layer and the second semiconductor layer;
The effective donor concentration of the third semiconductor layer is 1×10 ¹⁷ cm ^-3 Below is the
3. The avalanche photodiode according to claim 2. The second semiconductor layer is made of diamond.
4. The avalanche photodiode according to claim 1 . The Ga ₂ O ₃ -based semiconductor layer includes a guard ring;
The avalanche photodiode according to any one of claims 1 to 4 . a stack of the first semiconductor layer and the second semiconductor layer having a mesa shape;
2. The avalanche photodiode according to claim 1. a stack of the first semiconductor layer, the second semiconductor layer, and the third semiconductor layer has a mesa shape;
4. The avalanche photodiode according to claim 2 or 3 .

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