JP2018056225A - Light receiving element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light receiving element having an extremely low current value in a dark state and also having good light receiving sensitivity when light is incident.SOLUTION: A light receiving element 1 of the present invention includes a substrate 2, a first nitride semiconductor layer 3 formed on the substrate 2, and a second nitride semiconductor layer 4 formed on the first nitride semiconductor layer 3. The first nitride semiconductor layer 3 comprises a nitride semiconductor containing at least one of Al, In and Ga. The second nitride semiconductor layer 4 comprises a nitride semiconductor containing at least one of Al, In and Ga, has a lattice constant smaller than that of the first nitride semiconductor layer 3, and includes a recessed portion 41 on a surface 40 on a side opposite to the first nitride semiconductor layer 3.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a light receiving element.

  As an example of the light receiving element, an ultraviolet light receiving element including a p-type gate optical FET using a GaN / AlGaN heterostructure is known (see, for example, Patent Document 1).

International Publication No. 2007/135739 Pamphlet

In the light receiving element described above, a p-type GaN-based semiconductor layer that depletes the two-dimensional electron gas layer is present on the outermost surface of the light receiving surface (light incident surface). Due to the presence of this layer, the current value in the dark state (the state in which light to be received is not incident) can be made extremely low, but this layer is present when light is incident (when the light to be received is incident). As a result, a part of the incident light is absorbed, so that the light receiving sensitivity is lowered.
An object of the present invention is to provide a light receiving element having a very low current value in a dark state and good light receiving sensitivity at the time of light incidence.

  In order to solve the above problems, a light-receiving element that is one embodiment of the present invention includes a substrate, a first nitride semiconductor layer formed over the substrate, and a second nitride formed over the first nitride semiconductor layer. A semiconductor layer. The first nitride semiconductor layer is made of a nitride semiconductor containing at least one of Al, In, and Ga. The second nitride semiconductor layer is made of a nitride semiconductor containing at least one of Al, In, and Ga, has a lattice constant smaller than that of the first nitride semiconductor layer, and is opposite to the first nitride semiconductor layer. The surface has a recess. Only one recess may be formed on the upper surface of the second nitride semiconductor layer, or a plurality of recesses may be formed.

  According to one embodiment of the present invention, a light-receiving element having a very low current value in a dark state and excellent light-receiving sensitivity at the time of light incidence can be provided.

It is sectional drawing which shows the light receiving element of embodiment.

[Light receiving element of one aspect]
In the light receiving element which is one embodiment of the present invention (hereinafter referred to as “one light receiving element”), the first nitride semiconductor layer functions as a channel layer, and the second nitride semiconductor layer functions as a barrier layer. There is a two-dimensional electron gas (2DEG) layer at the interface between the barrier layer and the barrier layer. The second nitride semiconductor layer has a recess on the surface opposite to the first nitride semiconductor layer (the upper surface of the second nitride semiconductor layer), so that the portion immediately below the recess at the interface between these layers is depleted. Layers on both sides of the recess become source / drain regions.

  Therefore, in the dark state, since the 2DEG layer is electrically disconnected by the depletion layer, only a very small current flows between the source and the drain. That is, when this light receiving element is used with the upper surface of the second nitride semiconductor layer as the light receiving surface, the current value in the dark state can be made extremely low as compared with a light receiving element that is different only in that there is no recess. Further, the light receiving sensitivity at the time of light incidence can be equivalent to that of a light receiving element that differs only in that it does not have a recess. Further, by not providing a layer that absorbs light on the bottom surface of the recess, the light receiving sensitivity at the time of light incidence can be increased.

<Preferred embodiment 1>
In the light receiving element of one embodiment, the ratio (T2 / T1) of the thickness (T2) at the bottom surface position of the recess to the thickness (T1) of the second nitride semiconductor layer at a position other than the recess is 0.05. It is preferable that it is 0.4 or more.
A light receiving element having a film thickness ratio (T2 / T1) in the range of 0.05 or more and 0.4 or less is light receiving in which the film thickness ratio (T2 / T1) is out of this range, as described in Examples below. The device is superior to the device in that both the current value in the dark state is extremely low and the light receiving sensitivity at the time of light incidence is high.

<Preferred embodiment 2>
In one embodiment, a light receiving element has a two-dimensional electron gas having an electron concentration of 1 × 10 ⁸ cm ⁻² or more at a portion of the interface between the first nitride semiconductor layer and the second nitride semiconductor layer except for the portion immediately below the recess. A layer (electron transit layer) is preferably present. As a result, a large amount of current flows between the source and the drain when light is incident, so that the light receiving sensitivity when light is incident is increased.
<Preferred embodiment 3>
In the light receiving element of one embodiment, it is preferable that a depletion layer having an electron concentration of 1 × 10 ⁷ cm ⁻² or less exists in a portion immediately below the recess in the interface. Thereby, the current value in the dark state can be extremely reduced.

<Preferred Aspect 4: Regarding First Nitride Semiconductor Layer and Second Nitride Semiconductor Layer>
The material of the first nitride semiconductor layer constituting the light receiving element of one embodiment is not particularly limited as long as it contains at least one element of Al and Ga, but examples include AlN, GaN, AlGaN, and the like. Can be mentioned.
The material of the second nitride semiconductor layer constituting the light receiving element of one embodiment is not particularly limited as long as it is a nitride semiconductor layer containing at least one element of Al, In, and Ga. As an example, AlN, Examples thereof include GaN and AlGaN.

In the light receiving element of one embodiment, the first nitride semiconductor layer is a layer made of Al _x Ga _(1-X) N (0 ≦ X <1), and the second nitride semiconductor layer is Al _Y Ga _{(1-Y ) A} layer composed of N (0 ≦ X <Y ≦ 1) is preferable. Thereby, the gas concentration of the two-dimensional electron layer existing at the interface between the first nitride semiconductor layer and the second nitride semiconductor layer can be increased. Due to the presence of a high-concentration two-dimensional electron gas at this interface, a large amount of current flows between the source and the drain when light is incident, so that the light receiving sensitivity when light is incident is increased.
From the viewpoint of ensuring crystallinity, the first nitride semiconductor layer is undoped (for example, the impurity concentration is less than 1 × 10 ¹⁶ cm ⁻³ ) Al _X Ga _(1-X) N (0 ≦ X <1 The second nitride semiconductor layer is preferably a layer made of undoped Al _Y Ga _(1-Y) N (0 ≦ X <Y ≦ 1).

<Preferred embodiment 5>
In the light receiving element of one embodiment, the first nitride semiconductor layer is a layer made of Al _x Ga _(1-X) N (0.3 ≦ X ≦ 0.6), and the second nitride semiconductor layer is Al _Y Ga. _{A layer made of (1-Y)} N (0.6 <Y ≦ 0.9) is preferred. Thereby, a high-concentration two-dimensional electron gas layer is formed at the interface between the first nitride semiconductor layer and the second nitride semiconductor layer.
The first nitride semiconductor layer is a layer made of Al _X Ga _(1-X) N (0 ≦ X <1), and the second nitride semiconductor layer is Al _Y Ga _(1-Y) N (0 ≦ X < _1). In the case of a layer composed of Y ≦ 1), the greater the difference between X and Y, the higher the concentration of the two-dimensional electron gas layer. However, if this difference becomes too large, a defect may occur at the interface between the first nitride semiconductor layer and the second nitride semiconductor layer due to the difference in lattice constant, which may increase the current value in the dark state. On the other hand, by satisfying 0.3 ≦ X ≦ 0.6 and 0.6 <Y ≦ 0.9, that is, satisfying 0.3 <Y−X ≦ 0.3, The current value can be reduced.

<Preferred Aspect 6>
In one aspect of the light receiving element, a portion of the interface immediately below the recess serves as a gate region, and both portions sandwiching the recess serve as a source / drain region, but the size of the recess serving as a gate length is 0. It is preferably 5 μm or more and 5 μm or less. As a result, a high electric field can be applied between the source and the drain, and a large amount of current can flow between the source and the drain when light is incident.
<Preferred embodiment 7>
In the light receiving element of one embodiment, the thickness of the second nitride semiconductor layer is preferably 10 nm or more and 50 nm or less. Thereby, it becomes easy to adjust the carrier concentration of the two-dimensional electron gas layer.
<Preferred embodiment 8>
In the light receiving element of one embodiment, it is preferable that the bottom surface of the concave portion is exposed. Thereby, the light receiving sensitivity when the light receiving element of one embodiment is used with the bottom surface of the recess as the light receiving surface is increased.

<Preferred Aspect 9: Regarding Substrate>
The substrate constituting the light receiving element of one embodiment is not particularly limited as long as it can form a first nitride semiconductor layer containing at least one of Al, In, and Ga. Specific examples of the substrate material include Si, SiC, MgO, Ga ₂ O ₃ , Al ₂ O ₃ , ZnO, GaN, InN, AlN, or a mixed crystal thereof. Further, impurities may be mixed in the substrate.
When the material forming the substrate is sapphire, AlN, or GaN, the difference in lattice constant from the first nitride semiconductor layer is small, and the substrate can be grown in a lattice matching system, so that threading dislocations can be reduced.
That is, in the light receiving element of one embodiment, the substrate is preferably a sapphire substrate, an AlN substrate, or a GaN substrate.

<Preferred embodiment 10>
The light receiving element of one embodiment preferably further includes a buffer layer made of AlN between the substrate and the first nitride semiconductor layer. Thereby, the crystallinity of the first nitride semiconductor layer can be improved, and the light receiving sensitivity at the time of light incidence can be further increased.
<Preferred embodiment 11>
The light receiving element of one embodiment preferably further includes a source electrode and a drain electrode on both sides of the recess on the second nitride semiconductor layer.
The source electrode and the drain electrode are formed on the second nitride semiconductor layer, can apply a bias voltage to the light receiving element, and can extract current generated by incident light. It is not limited.

<Preferred embodiment 12>
In the light receiving element of one embodiment, the source electrode and the drain electrode are preferably made of a material containing at least one of Ti, Al, Au, Ni, V, Mo, and Zr. Thereby, contact resistance can be reduced.
<Preferred embodiment 13>
In the light receiving element of one embodiment, it is preferable that no gate electrode exists on the second nitride semiconductor layer. The absence of the gate electrode eliminates the need to apply a bias voltage. In addition, since the gate electrode formation step is not required, the manufacturing cost for element formation is reduced.

<Preferred embodiment 14>
In the light receiving element of one embodiment, it is preferable that a current of 10 ⁻⁸ A / mm or more flows between the source electrode and the drain electrode only when light having a wavelength of 200 nm to 365 nm is incident. Thereby, the selectivity of the light receiving wavelength is high.
<Preferred embodiment 15>
The light receiving element of one embodiment preferably has a response speed of 1 ms or less when light having a wavelength of 250 nm is incident.

<Preferred Aspect 16>
The light receiving element of one embodiment preferably has a light receiving wavelength of 200 nm to 365 nm and a current value in a dark state of 10 ⁻⁹ A / mm or less.
<Preferred embodiment 17>
The light receiving element of one embodiment preferably has a light receiving sensitivity of 1 × 10 ² A / W or more when light having a wavelength of 200 nm to 365 nm is incident.

<Surface protective layer>
The light receiving element of one embodiment may include a surface protective layer. The surface protective layer _{SiO 2, SiN, Al 2 O} 3, but including AlN and the like, not limited.
<Method for confirming that first nitride semiconductor layer contains Al or Ga>
The fact that the first nitride semiconductor layer contains Al or Ga means that an X-ray fluorescence elemental analyzer (XRF), a Rutherford backscattering spectrometer (RBS), a secondary ion mass spectrometer (SIMS), and an X-ray photoelectron spectrometer ( XPS).

<Measurement Method of Al Composition Ratio X of Al _X Ga _(1-X) N in First Nitride Semiconductor Layer>
The Al composition ratio (X) of Al _X Ga _(1-X) N in the first nitride semiconductor layer is determined by 2θ-ω scan and reciprocal lattice mapping measurement (RSM) by X-ray diffraction (XRD) method. Can be measured.
Specifically, first, a 2θ-ω scan of the plane index of the surface corresponding to the plane orientation of the substrate surface is performed by X-ray diffraction, and the Al _X Ga _{(1-X of} the first nitride semiconductor layer is determined from the peak position. ₎ Obtain the lattice constant of N.

Here, in the case where the substrate is a substrate (just substrate) accurately cut in a predetermined plane orientation, as described above, from the peak position in the 2θ-ω scan of the plane index of the plane corresponding to the plane orientation of the just substrate. The lattice constant can be obtained. When the substrate is a substrate cut off by applying an off angle from a predetermined plane orientation (off substrate), X-rays are incident from an angle shifted from the surface of the off substrate by the off angle, and 2θ-ω scan is performed. Need to do.
Then, the lattice constants of the resulting _{Al X Ga (1-X)} N , to determine the _{Al X Ga (1-X)} N of X (Al composition ratio) using Vegard law. Specifically, the Vegard rule is expressed by the following formula (1).
a _AB = Xa _A + (1-X) a _B (1)

In the formula (1), a _A is the lattice constant of AlN, a _B is the lattice constant of GaN, and a _AB is the lattice constant of Al _X Ga _(1-X) N. Here, a _A and a _B are “S. Strite and H. Morko, GaN, AIN, and InN: A review Journal of Vacuum Science & Technology B 10, 1237 (1992); doi: 10.1161 / 1.585897”. The stated values (a _A = 3.112 Å, a _B = 3.189 Å) can be used.
Therefore, using a _A = 3.112 Å, a _B = 3.189 Å, and the value of the lattice constant (a _AB ) of the obtained Al _X Ga _(1-X) N, Equation (1) to X (Al The composition ratio can be determined.

On the other hand, since the relaxation rate cannot be obtained only by the 2θ-ω scan, an accurate X (Al composition ratio) cannot be calculated. Therefore, it is useful to perform reciprocal lattice mapping on asymmetric surfaces such as the (105) plane and the (204) plane. Specifically, the point at which the 2θ-ω peak of the substrate is maximized is measured on the (204) plane where the substrate and the Al _X Ga _(1-X) N layer are separated in the reciprocal space. Then, 2θ−ω is scanned while changing ω at intervals of 0.01 °. By repeating this and mapping the obtained Qx and Qy, it is possible to calculate how much the Al _X Ga _(1-X) N layer is relaxed with respect to the substrate. Based on this relaxation rate and the lattice constant calculated above, accurate X (Al composition ratio) can be obtained.

<Method for manufacturing light receiving element>
A method for manufacturing the light receiving element of one embodiment will be described.
A method for manufacturing a light receiving element includes a step of depositing a first nitride semiconductor layer on a substrate by a metal organic deposition method (MOCVD method), and a step of depositing a second nitride semiconductor layer on the first nitride semiconductor layer. Forming a recess on the surface (upper surface) opposite to the first nitride semiconductor layer of the second nitride semiconductor layer. As a method for forming the concave portion on the upper surface of the second nitride semiconductor layer, various methods such as etching may be mentioned.

When forming the first nitride semiconductor layer and the second nitride semiconductor layer, at least one of an Al material, an In material, and a Ga material, and an N material can be used. These materials can also be used when a buffer layer is formed between the substrate and the first nitride semiconductor layer.
For example, trimethylaluminum (TMAl) can be used as the Al material. For example, trimethylindium (TMIn) can be used as the In raw material. As the Ga material, for example, trimethylgallium (TMGa), triethylgallium (TEGa), or the like can be used. As the N raw material, for example, ammonia (NH 3) can be used.

Embodiment
Hereinafter, although the form for implementing this invention (henceforth "embodiment") is demonstrated, this invention is not limited to embodiment shown below. In the embodiment described below, a technically preferable limitation is made for carrying out the present invention, but this limitation is not an essential requirement of the present invention. Further, the figure is schematic, the thickness of each layer is different from the actual one, and the ratio of the thickness of each layer is also different from the actual one. Specific thicknesses and dimensions should be determined in consideration of the description of this embodiment and examples.

As shown in FIG. 1, the light receiving element 1 of the embodiment includes a substrate 2, a first nitride semiconductor layer 3 formed on the substrate 2, and a second nitride formed on the first nitride semiconductor layer 3. And a source electrode 5 and a drain electrode 6 formed on the second nitride semiconductor layer 4.
The substrate 2 is made of a sapphire substrate. The first nitride semiconductor layer 3 is a layer made of Al _X Ga _(1-X) N (0.3 ≦ X ≦ 0.6), and AlN is interposed between the substrate 2 and the first nitride semiconductor layer 3. A buffer layer. The second nitride semiconductor layer 4 is made of Al _Y Ga _(1-Y) N (0.6 <Y ≦ 0.9). A recess 41 is formed on the upper surface 40 of the second nitride semiconductor layer 4 (the surface opposite to the first nitride semiconductor layer 3).

Ratio (T2 / T1) of the thickness (T2) of the second nitride semiconductor layer 4 at the position of the bottom surface 41a of the recess 41 to the thickness (T1) at a position other than the recess 41 of the second nitride semiconductor layer 4 Is 0.05 or more and 0.4 or less.
Of the interface between the first nitride semiconductor layer 3 and the second nitride semiconductor layer 4, a depletion layer having an electron concentration of 1 × 10 ⁷ cm ⁻² or less exists in a portion 34 a immediately below the recess 41. In the portion 34b except immediately below, there is a two-dimensional electron gas layer having an electron concentration of 1 × 10 ⁸ cm ⁻² or more. That is, of the interface between the two layers, a portion 34a immediately below the recess 41 serves as a gate region, and a portion excluding the portion directly below the recess 41 (a portion on both sides of the recess 41) serves as a source / drain region. The source electrode 5 and the drain electrode 6 are formed on both sides of the upper surface 40 across the recess 41.
In the light receiving element 1 of the embodiment, the upper surface 40 of the second nitride semiconductor layer 4 is a light receiving surface, and the bottom surface 41a of the recess 41 is exposed.

According to the light receiving element 1 of the present embodiment, in the dark state, the 2DEG layer is electrically disconnected by the depletion layer, so that only a very small current flows between the source electrode 5 and the drain electrode 6. Further, since the bottom surface 41a is exposed, the light receiving sensitivity at the time of light incidence can be made higher than that of a light receiving element having a layer that absorbs light on the bottom surface 41a.
Further, according to the light receiving element 1 of the present embodiment, since the film thickness ratio (T2 / T1) of the recess 41 of the second nitride semiconductor layer 4 is in the range of 0.05 to 0.4, the film thickness ratio Compared with the case where (T2 / T1) is outside this range, the current value in the dark state is extremely low and the light receiving sensitivity is high, which is particularly excellent.
Even when the film thickness ratio (T2 / T1) is out of the range of 0.05 or more and 0.4 or less, the current value in the dark state is extremely low as compared with that having no recess 41. And excellent light receiving sensitivity.

Examples of the present invention and comparative examples will be described below.
The light receiving elements of Examples 1 to 6 are the substrate 2 made of sapphire, the buffer layer, the first nitride semiconductor layer 3, the second nitride semiconductor layer 4, and the source, similarly to the light receiving element 1 of the embodiment shown in FIG. The electrode 5 and the drain electrode 6 are provided, and a recess 41 is formed on the upper surface of the second nitride semiconductor layer 4. The light receiving element of Comparative Example 1 has the same layer configuration as this, but the recess 41 is not formed on the upper surface of the second nitride semiconductor layer 4.

[Example 1]
An AlN layer (buffer layer) of 3 μm was grown on the substrate 2 made of a 2-inch sapphire wafer by an organic metal deposition method while maintaining the surface temperature of the substrate at 1250 ° C. Next, with the surface temperature of the AlN layer kept at 1100 ° C., the first nitride semiconductor layer 3 made of Al _x Ga _(1-x) N (X = 0.52) is grown to 500 nm on the AlN layer. I let you. Next, with the surface temperature of the first nitride semiconductor layer 3 kept at 1100 ° C., the first nitride semiconductor layer 3 is made of Al _Y Ga _(1-Y) N (Y = 0.7). The second nitride semiconductor layer 4 was grown to 25 nm.

When CV measurement was performed on the substrate 2 in this state, the carrier concentration of the two-dimensional electron gas (2DEG) layer induced at the interface between the first nitride semiconductor layer 3 and the second nitride semiconductor layer 4 is It was 8 × 10 ¹² cm ⁻² .
Next, the substrate 2 in this state was washed, and a resist mask having a plurality of openings of 30 μm × 100 μm was formed on the first nitride semiconductor layer 3. Next, mesa separation was performed up to the AlN layer using this resist mask to electrically insulate a plurality of elements formed on the wafer.

Next, for each element, the source electrode 5 and the drain electrode 6 were formed on the second nitride semiconductor layer 4 at an interval of 8 μm by an evaporation method. The source electrode 5 and the drain electrode 6 have a laminated structure made of Ti / Al / Ni / Au, and the thickness of each layer is Ti: 20 nm, Al: 80 nm, Ni: 35 nm, Au: 100 nm.
Next, a resist mask having a plurality of openings of 2 μm (gate length) × 100 μm (gate width) was formed on the second nitride semiconductor layer 4 of the substrate 2 in this state. The plurality of openings are provided at positions between the source electrode 5 and the drain electrode 6 of each element. Next, by performing RIE (reactive ion etching) on the second nitride semiconductor layer 4 existing in the opening of the resist mask, the recess 41 is formed on the upper surface 40 of the second nitride semiconductor layer 4 of each element. Formed.

Next, the resist mask was removed, and the sapphire wafer was diced to obtain a plurality of light receiving elements 1.
When a cross section perpendicular to the substrate surface of the obtained light receiving element was analyzed using a transmission electron microscope (TEM), the thickness (T2) of the second nitride semiconductor layer 4 at the position of the bottom surface 41a of the recess 41 was It was 2.0 nm. Since the thickness (T1) of the second nitride semiconductor layer 4 other than the recess 41 is 25 nm, the film thickness ratio (T2 / T1) is 0.08.
The characteristics of the obtained light receiving element were measured by the following method.

For the measurement of current (photocurrent) at the time of light incidence, a pseudo solar light source was used as a light source, and a spectroscope was used in combination. Then, the upper surface 40 of the second nitride semiconductor layer 4 was irradiated with ultraviolet light having a wavelength of 250 nm at an intensity of 45 μW / cm ² , and the current flowing between the source and drain electrodes when the source / drain voltage was 3 V was measured. Measurement of current (dark current) in a dark state without ultraviolet irradiation was also performed with a source / drain voltage of 3V. In addition, the parameter analyzer and the probe measuring device were used for the current voltage measurement.
As a result of the measurement, the dark current was 1.0 × 10 ⁻¹⁰ A / mm, and the photocurrent was 3.0 × 10 ⁻⁵ A / mm. The light receiving sensitivity obtained by converting the photocurrent was 3.3 × 10 ⁵ A / W. The ratio (photocurrent / dark current) of the photocurrent value (current value when irradiated with 250 nm ultraviolet light) to the obtained dark current value was 3.0 × 10 ⁵ . Moreover, the response speed at the time of ultraviolet light irradiation (at the time of light incidence) was 1 microsecond or less.

[Example 2]
In the step of forming the recess 41 on the upper surface 40 of the second nitride semiconductor layer 4, the etching depth by RIE is made shallower than that of the first embodiment, and the second nitride semiconductor layer 4 at the position of the bottom surface 41 a of the recess 41 is formed. The thickness (T2) was set to 3.0 nm. Since the thickness (T1) of the second nitride semiconductor layer 4 other than the recess 41 is 25 nm, the film thickness ratio (T2 / T1) is 0.12. Except for this point, the light receiving element 1 was obtained in the same manner as in Example 1.
When the characteristics of the obtained light receiving element were measured by the method described in Example 1, the dark current was 3.0 × 10 ⁻¹⁰ A / mm, and the photocurrent was 3.0 × 10 ⁻⁵ A / mm. there were. The light receiving sensitivity obtained by converting the photocurrent was 3.3 × 10 ⁵ A / W. The ratio (photocurrent / dark current) of the photocurrent value (current value when irradiated with 250 nm ultraviolet light) to the obtained dark current value was 1.0 × 10 ⁵ . Moreover, the response speed at the time of ultraviolet light irradiation (at the time of light incidence) was 1 microsecond or less.

[Example 3]
In the step of forming the recess 41 on the upper surface 40 of the second nitride semiconductor layer 4, the etching depth by RIE is made shallower than that of the first embodiment, and the second nitride semiconductor layer 4 at the position of the bottom surface 41 a of the recess 41 is formed. The thickness (T2) was set to 5.0 nm. Since the thickness (T1) of the second nitride semiconductor layer 4 other than the recess 41 is 25 nm, the film thickness ratio (T2 / T1) is 0.20. Except for this point, the light receiving element 1 was obtained in the same manner as in Example 1.
When the characteristics of the obtained light receiving element were measured by the method described in Example 1, the dark current was 5.0 × 10 ⁻¹⁰ A / mm and the photocurrent was 3.0 × 10 ⁻⁵ A / mm. there were. The light receiving sensitivity obtained by converting the photocurrent was 3.3 × 10 ⁴ A / W. The ratio (photocurrent / dark current) of the photocurrent value (current value when irradiated with 250 nm ultraviolet light) to the obtained dark current value was 6.0 × 10 ⁴ . Moreover, the response speed at the time of ultraviolet light irradiation (at the time of light incidence) was 1 microsecond or less.

[Example 4]
In the step of forming the recess 41 on the upper surface 40 of the second nitride semiconductor layer 4, the etching depth by RIE is made shallower than that of the first embodiment, and the second nitride semiconductor layer 4 at the position of the bottom surface 41 a of the recess 41 is formed. The thickness (T2) was set to 9.0 nm. Since the thickness (T1) of the second nitride semiconductor layer 4 other than the recess 41 is 25 nm, the film thickness ratio (T2 / T1) is 0.36. Except for this point, the light receiving element 1 was obtained in the same manner as in Example 1.
When the characteristics of the obtained light receiving element were measured by the method described in Example 1, the dark current was 8.0 × 10 ⁻¹⁰ A / mm, and the photocurrent was 3.0 × 10 ⁻⁵ A / mm. there were. The light receiving sensitivity obtained by converting the photocurrent was 3.3 × 10 ⁴ A / W. The ratio (photocurrent / dark current) of the photocurrent value (current value when irradiated with 250 nm ultraviolet light) to the obtained dark current value was 3.8 × 10 ⁴ . Moreover, the response speed at the time of ultraviolet light irradiation (at the time of light incidence) was 1 microsecond or less.

[Example 5]
In the step of forming the recess 41 on the upper surface 40 of the second nitride semiconductor layer 4, the etching depth by RIE is made shallower than that of the first embodiment, and the second nitride semiconductor layer 4 at the position of the bottom surface 41 a of the recess 41 is formed. The thickness (T2) was 12.5 nm. Since the thickness (T1) of the second nitride semiconductor layer 4 other than the recess 41 is 25 nm, the film thickness ratio (T2 / T1) is 0.50. Except for this point, the light receiving element 1 was obtained in the same manner as in Example 1.
When the characteristics of the obtained light receiving element were measured by the method described in Example 1, the dark current was 3.0 × 10 ⁻⁹ A / mm, and the photocurrent was 3.0 × 10 −5 A / mm. It was. The light receiving sensitivity obtained by converting the photocurrent was 3.3 × 10 ⁵ A / W. The ratio (photocurrent / dark current) of the photocurrent value (current value when irradiated with 250 nm ultraviolet light) to the obtained dark current value was 1.0 × 10 ⁴ . Moreover, the response speed at the time of ultraviolet light irradiation (at the time of light incidence) was 1 microsecond or less.

[Example 6]
In the step of forming the recess 41 on the upper surface 40 of the second nitride semiconductor layer 4, the etching depth by RIE is made deeper than that of the first embodiment, and the second nitride semiconductor layer 4 at the position of the bottom surface 41 a of the recess 41 is formed. The thickness (T2) was set to 1.0 nm. Since the thickness (T1) of the second nitride semiconductor layer 4 other than the recess 41 is 25 nm, the film thickness ratio (T2 / T1) is 0.04. Except for this point, the light receiving element 1 was obtained in the same manner as in Example 1.
When the characteristics of the obtained light receiving element were measured by the method described in Example 1, the dark current was 9.0 × 10 ⁻¹¹ A / mm, and the photocurrent was 8.0 × 10 ⁻⁶ A / mm. there were. The light receiving sensitivity obtained by converting the photocurrent was 8.9 × 10 ⁴ A / W. The ratio (photocurrent / dark current) of the photocurrent value (current value when irradiated with 250 nm ultraviolet light) to the obtained dark current value was 8.9 × 10 ⁴ . Moreover, the response speed at the time of ultraviolet light irradiation (at the time of light incidence) was 1 microsecond or less.

[Comparative Example 1]
The step of forming the recess 41 on the upper surface 40 of the second nitride semiconductor layer 4 was not performed. Except for this point, the light receiving element 1 was obtained in the same manner as in Example 1.
When the characteristics of the obtained light receiving element were measured by the method described in Example 1, the dark current was 3.0 × 10 ⁻⁵ A / mm, and the photocurrent was 3.0 × 10 ⁻⁵ A / mm. there were. The light receiving sensitivity obtained by converting the photocurrent was 3.3 × 10 ⁵ A / W. The ratio (photocurrent / dark current) of the photocurrent value (current value when irradiated with 250 nm ultraviolet light) to the obtained dark current value was 1.0. Moreover, the response speed at the time of ultraviolet light irradiation (at the time of light incidence) was 1 microsecond or less.

  The structure of the second nitride semiconductor layer constituting the light receiving elements of Examples 1 to 6 and Comparative Example 1 and the performance of each light receiving element obtained by measurement are summarized in Table 1 below.

From this result, the following can be understood.
The light receiving elements of Examples 1 to 6 having the recesses 41 on the upper surface 40 of the second nitride semiconductor layer 4 have an extremely low dark current value as compared with the light receiving element of Comparative Example 1 that does not have the recesses 41. The photocurrent value and the light receiving sensitivity of the light receiving elements of Examples 1 to 6 are the same as the photocurrent value and the light receiving sensitivity of the light receiving element of Comparative Example 1. That is, the light receiving elements of Examples 1 to 6 are light receiving elements that have extremely low current values in the dark state and high light receiving sensitivity when light is incident.

Among the light receiving elements of Examples 1 to 6, the dark current value of the light receiving element of Example 5 is one digit larger than that of the light receiving elements of Examples 1 to 4. In the light receiving element of Example 6, the dark current value is an order of magnitude smaller than that of the light receiving elements of Examples 1 to 4, but the photocurrent value and the light receiving sensitivity are an order of magnitude smaller than those of the light receiving elements of Examples 1 to 5. And in the light receiving element of Examples 1-4, the film thickness ratio (T2 / T1) of the recessed part 41 of the 2nd nitride semiconductor layer 4 exists in the range of 0.05-0.4, but Example 5 and In the light receiving element of Example 6, the film thickness ratio (T2 / T1) is outside this range.
Therefore, a light receiving element having a film thickness ratio (T2 / T1) in the range of 0.05 to 0.4 is in a darker state than a light receiving element having a film thickness ratio (T2 / T1) outside this range. This is excellent in that both the current value is extremely low and the light receiving sensitivity at the time of light incidence is high.

DESCRIPTION OF SYMBOLS 1 Light receiving element 2 Board | substrate 3 1st nitride semiconductor layer 34a The part directly under a recessed part 34b among the interface of a 1st nitride semiconductor layer and a 2nd nitride semiconductor layer 34b 1st nitride semiconductor layer and a 2nd nitride semiconductor layer 4 of the interface except for the portion immediately below the recess 4 Second nitride semiconductor layer 40 Upper surface of the second nitride semiconductor layer (surface opposite to the first nitride semiconductor layer)
41 recess 41a bottom of recess 5 source electrode 6 drain electrode

Claims (18)

A substrate,
A first nitride semiconductor layer formed on the substrate and made of a nitride semiconductor including at least one of Al, In, and Ga;
A second nitride semiconductor layer formed on the first nitride semiconductor layer, comprising a nitride semiconductor containing at least one of Al, In, and Ga, and having a lattice greater than that of the first nitride semiconductor layer A second nitride semiconductor layer having a small constant and having a recess on a surface opposite to the first nitride semiconductor layer;
A light receiving element.   The ratio (T2 / T1) of the thickness (T2) at the bottom surface position of the recess to the thickness (T1) at a position other than the recess of the second nitride semiconductor layer is 0.05 or more and 0.4 or less. The light receiving element according to claim 1. A two-dimensional electron gas layer having an electron concentration of 1 × 10 ⁸ cm ⁻² or more exists in a portion of the interface between the first nitride semiconductor layer and the second nitride semiconductor layer except for a portion directly below the recess. Item 3. The light receiving element according to item 1 or 2. The light receiving element according to claim 3, wherein a depletion layer having an electron concentration of 1 × 10 ⁷ cm ⁻² or less exists in a portion of the interface immediately below the recess. The first nitride semiconductor layer is a layer made of Al _x Ga _(1-x) N (0 ≦ X <1),
5. The light receiving element according to claim 1, wherein the second nitride semiconductor layer is a layer made of Al _Y Ga _(1-Y) N (0 ≦ X <Y ≦ 1). The first nitride semiconductor layer is a layer made of Al _x Ga _(1-x) N (0.3 ≦ X ≦ 0.6),
5. The light receiving element according to claim 1, wherein the second nitride semiconductor layer is a layer made of Al _Y Ga _(1-Y) N (0.6 <Y ≦ 0.9).   Of the interface between the first nitride semiconductor layer and the second nitride semiconductor layer, the portion immediately below the recess is a gate region, the portions on both sides sandwiching the recess are source / drain regions, and the gate length The light receiving element according to any one of claims 1 to 6, wherein the size of the recess is 0.5 μm or more and 5 μm or less.   The light receiving element according to claim 1, wherein the second nitride semiconductor layer has a thickness of 10 nm to 50 nm.   The light receiving element according to claim 1, wherein a bottom surface of the concave portion is exposed.   The light receiving element according to claim 1, wherein the substrate is a sapphire substrate, an AlN substrate, or a GaN substrate.   The light receiving element according to claim 1, further comprising a buffer layer made of AlN between the substrate and the first nitride semiconductor layer.   The light receiving element according to any one of claims 1 to 11, further comprising a source electrode and a drain electrode on both sides of the concave portion on the second nitride semiconductor layer.   The light receiving element according to claim 12, wherein the source electrode and the drain electrode are made of a material containing at least one of Ti, Al, Au, Ni, V, Mo, and Zr.   The light receiving element according to claim 1, wherein no gate electrode is present on the second nitride semiconductor layer. 14. The light receiving element according to claim 12, wherein a current of 10 ⁻⁸ A / mm or more flows between the source electrode and the drain electrode only when light having a wavelength of 200 nm to 365 nm is incident.   The light receiving element according to any one of claims 1 to 15, wherein a response speed when light having a wavelength of 250 nm is incident is 1 ms or less. The light receiving element according to claim 1, wherein the light receiving wavelength is 200 nm or more and 365 nm or less, and the current value in the dark state is 10 ⁻⁹ A / mm or less. The light receiving element according to claim 1, wherein the light receiving sensitivity when light having a wavelength of 200 nm to 365 nm is incident is 1 × 10 ² A / W or more.

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