Photo-detector

Abstract

To provide a photo-detector having no (or sufficiently low) sensitivity to visible light, UV-A light, and UV-B light so as not to require an optical filter that cuts visible light, UV-A light, and UV-B light, and having characteristics enabling high-sensitivity detection of light with wavelengths shorter than UV-B light.SOLUTION: The photo-detector according to the present disclosure includes, as a photo-detecting element, an α-gallium oxide film which is plane-oriented to either a-plane, m-plane, n-plane, or r-plane.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to photodetectors. [Background technology]

[0002] Among ultraviolet rays, UV-C light (wavelength 200-280 nm) is known as solar blind light and does not reach the earth. The only things that emit UV-C light on earth are mercury lamps and high-temperature objects that exceed 800°C. Therefore, using UV-C light makes it possible to perform optical communications, flame detection, and biosensing without being affected by ambient light such as sunlight.

[0003] Conventional ultraviolet photodetectors have used silicon (Si), silicon carbide (SiC), gallium nitride (GaN), aluminum gallium nitride (AlGaN), or the like. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2023-156732 [Non-patent literature]

[0005] [Non-Patent Document 1] M. Ohta, H. Tamura, K. Uno, “Growth of α-(AlGa)2O3 alloy thin films on c-sapphire substrates by mist chemical vapor deposition using acetylacetonated Al and Ga solutions”, Applied Physics Express 2022, 15, 055502. Summary of the Invention

[0006] Conventional ultraviolet light detectors are sensitive to visible light, UV-A light (wavelengths 320-400 nm), and UV-B light (wavelengths 280-320 nm). Therefore, when trying to detect light with shorter wavelengths than UV-B light, such as UV-C light, it was necessary to use an optical filter to block light with wavelengths of 280 nm or more. However, such filters are difficult to fabricate, expensive, and prone to deterioration over time. Furthermore, increasing the sensitivity of a photodetector can result in sensitivity to even the smallest light components that the filter was unable to remove.

[0007] If aluminum gallium nitride is used, it is possible to create a photodetector that is insensitive to visible light, UV-A light, and UV-B light. However, current technology has the problem of low crystal quality. Even if the crystal quality improves in the future, a thin film containing a mixture of aluminum and gallium atoms cannot be expected to significantly improve sensitivity. For this reason, aluminum gallium nitride is not suitable for use in highly sensitive photodetectors.

[0008] Therefore, what is desired is a photodetector that has no sensitivity (or has sufficiently low sensitivity) to visible light, UV-A light, and UV-B light, thereby eliminating the need for optical filters that cut out visible light, UV-A light, and UV-B light, and that also has the ability to sensitively detect light with wavelengths shorter than UV-B light.

[0009] One aspect of the present disclosure is a photodetector, which includes an α-gallium oxide film oriented in any one of the a-plane, m-plane, n-plane, and r-plane as a photodetector element.

[0010] Further details will be described in the following embodiments. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram of a photodetector. [Figure 2] FIG. 2 shows the surface atomic arrangement of sapphire and α-gallium oxide. [Figure 3]FIG. 3 shows the X-ray 2θ-θ diffraction profile of α-gallium oxide on a sapphire substrate. [Figure 4] FIG. 4 is an explanatory diagram of the structural distortion of α-gallium oxide on a substrate. [Figure 5] FIG. 5 is an explanatory diagram of the measurement of the photoresponse characteristics. [Figure 6] FIG. 6 shows the photoresponse characteristics (current-voltage characteristics) of the c-plane orientation. [Figure 7] FIG. 7 shows the photoresponse characteristics (current-voltage characteristics) of the a-plane orientation. [Figure 8] FIG. 8 shows the photoresponse characteristics (current-voltage characteristics) of the m-plane orientation. [Figure 9] FIG. 9 shows the photoresponse characteristics (current-voltage characteristics) of the n-plane orientation. [Figure 10] FIG. 10 shows the photoresponse characteristics (current-voltage characteristics) of the r-plane orientation. [Figure 11] FIG. 11 shows the optical response characteristics for each plane orientation when irradiated with a D2 lamp. DETAILED DESCRIPTION OF THE INVENTION

[0012] <1. Overview of the photodetector>

[0013] (1) The photodetector according to the embodiment includes, as a photodetecting element, an α-gallium oxide film oriented in any one of the a-plane, m-plane, n-plane, and r-plane.

[0014] (2) The α-gallium oxide film is preferably a substantially single-phase film of α-gallium oxide.

[0015] (3) When the full width at half maximum (FWHM) of the X-ray diffraction rocking curve of the α-gallium oxide film is Δω [deg] and the full width at half maximum (FWHM) of the X-ray diffraction φ scan of the α-gallium oxide film is Δφ [deg], it is preferable that both Δω and Δφ are less than 1 [deg].

[0016] (4) It is preferable that Δω and Δφ are both less than 0.5 degrees.

[0017] (5) The absolute value of the difference between Δω and Δφ (|Δω−Δφ|) is preferably less than 0.6 degrees.

[0018] (6) The absolute value of the difference between Δω and Δφ (|Δω−Δφ|) is preferably less than 0.3 [deg].

[0019] (7) The α-gallium oxide film is preferably oriented to any one of the m-plane, the n-plane, and the r-plane.

[0020] (8) The α-gallium oxide film is substantially a single-phase film of α-gallium oxide, and when the full width at half maximum (FWHM) of the X-ray diffraction rocking curve of the α-gallium oxide film is Δω [deg] and the full width at half maximum (FWHM) of the X-ray diffraction φ scan of the α-gallium oxide film is Δφ [deg], it is preferable that both Δω and Δφ are less than 1 [deg].

[0021] (9) The α-gallium oxide film is preferably oriented to any one of the m-plane, the n-plane, and the r-plane.

[0022] <2. Example of a photodetector>

[0023] Examples of photodetectors will now be described in more detail with reference to the drawings.

[0024] 1 has a planar structure and includes a substrate 11 and a semiconductor film 12 formed on the substrate 11.

[0025] An electrode 13 is formed on the surface of the semiconductor film 12. The photodetector 10 shown in FIG. 1 is configured as an MSM (metal / semiconductor / metal) type photodetector, for example. The MSM type photodetector is a photodetector with a metal-semiconductor contact. The metal-semiconductor contact may be a Schottky contact or an ohmic contact. The Schottky contact type has a fast response speed, while the ohmic contact type has high sensitivity.

[0026] 1A and 1B, the electrode 13 has a line width of approximately 20 μm, an electrode spacing of 100 μm, and an entire electrode 13 has a comb-shaped pattern of 1.1 mm square.

[0027] The semiconductor film 12 functions as a photodetector (photoelectric conversion element). The semiconductor film 12 here is a gallium oxide (Ga2O3) film. Because gallium oxide is composed of two types of atoms, it is easier to improve the crystal quality compared to aluminum gallium nitride, which is composed of three or more types of atoms, making it preferable.

[0028] Gallium oxide (GaO) is known to have five crystalline phases: α, β, γ, δ, and κ. Of these, α-phase gallium oxide (α-gallium oxide; α-Ga2O3) has a forbidden band width of 5.3 to 5.6 eV. α-gallium oxide has the widest forbidden band width of the five crystalline phases.

[0029] α-Gallium oxide (α-Ga2O3) is insensitive to visible light, UV-A light, and UV-B light, but is sensitive to UV-C light, which is solar blinding light. Therefore, using α-gallium oxide to detect UV-C light makes optical filters that cut visible light, UV-A light, and UV-B light unnecessary.

[0030] In contrast, gallium oxide in a crystal phase other than the α-phase, such as β-gallium oxide, is sensitive to UV-B light as well as UV-C light. Therefore, if β-gallium oxide is used to detect UV-C light, an optical filter that cuts UV-B light is required. Therefore, a single-phase film of α-gallium oxide is suitable for the semiconductor film 12. Note that α-gallium oxide can also be sensitive to light with wavelengths shorter than UV-C light (e.g., VUV or gamma rays), and therefore can be used to detect not only UV-C light but also light with wavelengths shorter than UV-C light.

[0031] The substrate 11 has a corundum structure. The substrate 11 having the corundum structure may be, for example, a sapphire (α-Al2O3) substrate or a chromium oxide (α-Cr2O3) substrate. In the following description, the substrate 11 will be described as a sapphire substrate as an example, but the substrate 11 is not limited to sapphire.

[0032] The α-gallium oxide constituting the semiconductor film 12 has a corundum structure. The α-gallium oxide thin film is formed by epitaxial growth on the substrate 11 having a corundum crystal structure. The film thickness t of the α-gallium oxide (see FIG. 1(A)) is, for example, in the range of 100 nm to 500 nm. The upper limit of the film thickness t may be 400 nm or less, or even 300 nm or less.

[0033] The α-gallium oxide film 12 according to the embodiment is formed on any one of an a-plane sapphire substrate 11, an m-plane sapphire substrate 11, an n-plane sapphire substrate 11, and an r-plane sapphire substrate 11.

[0034] Here, the a-plane sapphire substrate 11 is a sapphire substrate with an a-plane (11-20) orientation. The a-plane orientation is an orientation (plane orientation) in which the a-plane is parallel to the surface of the substrate 11. In this specification, "parallel" means that it is sufficient if the a-plane is substantially parallel.

[0035] Similarly, the m-plane sapphire substrate 11 is a sapphire substrate with an m-plane (10-10) orientation. The m-plane orientation is an orientation (plane orientation) in which the m-plane is parallel to the surface of the substrate 11. The n-plane sapphire substrate 11 is a sapphire substrate with an n-plane (11-23) orientation. The n-plane orientation is an orientation (plane orientation) in which the n-plane is parallel to the surface of the substrate 11. The r-plane sapphire substrate 11 is a sapphire substrate with an r-plane (01-12) orientation. The r-plane orientation is an orientation (plane orientation) in which the r-plane is parallel to the surface of the substrate 11.

[0036] The α-gallium oxide film 12 is formed by epitaxial growth on the substrate 11 oriented to any one of the a-plane, m-plane, n-plane, and r-plane, and therefore its orientation generally coincides with that of the substrate 11.

[0037] That is, the α-gallium oxide film 12 formed on the a-plane sapphire substrate 11 is plane-oriented to the a-plane. With respect to the α-gallium oxide film 12, "plane-oriented to the a-plane" means that the a-plane (11-20) of α-gallium oxide is oriented parallel to the film surface of the α-gallium oxide film 12 (plane-oriented).

[0038] Similarly, the α-gallium oxide film 12 formed on the m-plane sapphire substrate 11 is plane-oriented in the m-plane. With respect to the α-gallium oxide film 12, "plane-oriented in the m-plane" means that the m-plane (10-10) of α-gallium oxide is oriented (plane-oriented) parallel to the film surface of the α-gallium oxide film 12. The α-gallium oxide film 12 formed on the n-plane sapphire substrate 11 is plane-oriented in the n-plane. With respect to the α-gallium oxide film 12, "plane-oriented in the n-plane" means that the n-plane (11-23) of α-gallium oxide is oriented (plane-oriented) parallel to the film surface of the α-gallium oxide film 12. The α-gallium oxide film 12 formed on the r-plane sapphire substrate 11 is plane-oriented in the r-plane. With respect to the α-gallium oxide film 12, "plane-oriented in the r-plane" means that the r-plane (01-12) of the α-gallium oxide is oriented parallel to the film surface of the α-gallium oxide film 12 (plane-oriented).

[0039] Figure 2 shows the surface atomic arrangements of a sapphire (α-Al2O3) substrate 11 and an α-gallium oxide (α-Ga2O3) thin film 12. In addition to the surface atomic arrangements of the a-plane, m-plane, n-plane, and r-plane, Figure 2 also shows the atomic arrangement of the c-plane.

[0040] As shown in Figure 2, when the crystal orientation is c-plane, sapphire and α-gallium oxide crystals have a hexagonal structure. In contrast, when the crystal orientation is a-plane, m-plane, n-plane, or m-plane, they have a tetragonal structure. α-gallium oxide oriented to the a-plane, m-plane, n-plane, or r-plane has a tetragonal structure, which reduces defects and improves the sensitivity of light detection compared to α-gallium oxide oriented to the c-plane, which has a hexagonal structure. This point will be discussed later.

[0041] Defects in the α-gallium oxide film 12 can be caused by a lattice mismatch between sapphire and α-gallium oxide. Figure 2 also shows the lattice constants of sapphire and α-gallium oxide.

[0042] As shown in FIG. 2, the lattice constants of sapphire and gallium oxide on the c-plane are 4.759 Å and 4.9825 Å, respectively, in the a-axis direction, and the lattice mismatch between them in the a-axis direction is 4.696%.

[0043] As shown in Figure 2, the lattice constant of sapphire in the a-plane is 12.991 Å along the long side of the rectangle and 8.243 Å along the short side. The lattice constant of α-gallium oxide in the a-plane is 13.433 Å along the long side of the rectangle and 8.6299 Å along the short side. The lattice mismatch between the two is 3.290% along the long side and 4.694% along the short side. In the a-plane orientation, the ratio of the long side to the short side of the α-gallium oxide rectangle (long side / short side) is approximately 1.56.

[0044] As shown in Figure 2, the lattice constant of sapphire in the m-plane is 12.991 Å along the long side of the rectangle and 4.759 Å along the short side. The lattice constant of α-gallium oxide in the m-plane is 13.433 Å along the long side of the rectangle and 4.9825 Å along the short side. The lattice mismatch between the two is 3.290% along the long side and 4.694% along the short side. In the m-plane orientation, the ratio of the long side to the short side of the α-gallium oxide rectangle (long side / short side) is approximately 2.70.

[0045] As shown in Figure 2, the lattice constant of sapphire on the n-plane is 11.383 Å along the long side of the rectangle and 8.243 Å along the short side. The lattice constant of α-gallium oxide on the n-plane is 11.797 Å along the long side of the rectangle and 8.6299 Å along the short side. The lattice mismatch between the two is 3.637% along the long side and 4.694% along the short side. In the n-plane orientation, the ratio of the long side to the short side of the α-gallium oxide rectangle (long side / short side) is approximately 1.37.

[0046] As shown in Figure 2, the lattice constant of sapphire in the r-plane is 15.385 Å along the long side of the rectangle and 4.759 Å along the short side. The lattice constant of α-gallium oxide in the r-plane is 15.966 Å along the long side of the rectangle and 4.98259 Å along the short side. The lattice mismatch between the two is 3.776% along the long side and 4.696% along the short side. In the r-plane orientation, the ratio of the long side to the short side of the α-gallium oxide rectangle (long side / short side) is approximately 3.20.

[0047] An example of a method for manufacturing the photodetector 10 shown in Fig. 1 will now be described. An α-gallium oxide thin film according to the embodiment is formed by mist CVD on a substrate 11 oriented to any one of the a-plane, m-plane, n-plane, and r-plane.

[0048] Before the growth of α-gallium oxide by mist CVD, the substrate 11 is pretreated to have an atomic layer step-terrace structure on the surface of the substrate 11 .

[0049] After the pretreatment, a mist CVD apparatus is used to grow a thin film 12 of α-gallium oxide on the substrate 11. The mist CVD apparatus is a film formation apparatus using a mist CVD method. The mist CVD method is atmospheric pressure CVD using dry mist. In mist CVD, for example, a raw material solution is turned into a mist, and fine particles of the mist raw material solution are transported by a carrier gas to a film formation chamber, where a thin film is formed on the substrate 11 in the film formation chamber. The mist CVD method can be used to grow a continuous film of metastable α-gallium oxide. The mist CVD method can grow an α-gallium oxide thin film with high crystallinity. The mist CVD apparatus may be the mist CVD apparatus described in Patent Document 1.

[0050] Here, the term "mist" refers to a general term for fine particles of a liquid dispersed in a gas, and includes what is called fog, droplets, etc. The mechanism for forming the mist is not particularly limited as long as it can form the raw material solution into a mist.

[0051] To grow α-gallium oxide using the mist CVD method, a solution containing gallium ions is used as a raw material solution (Ga solution; Ga source). As the raw material solution, an acetylacetonated solution (gallium acetylacetonate) is preferably used. The use of an acetylacetonated raw material solution is preferable because it strongly picks up the atomic arrangement of the substrate crystal through ligand exchange, resulting in strong epitaxial growth.

[0052] The gallium ion concentration in the raw material solution is not particularly limited, and [Ga 3+ ] can be 0.005 to 1 mol / L, for example.

[0053] The growth temperature of α-gallium oxide by mist CVD can be, for example, 500° C. or higher. The growth temperature is preferably set within the range of 500° C. to 700° C., for example.

[0054] The growth temperature of α-gallium oxide by mist CVD is preferably set within a range of, for example, 60 to 120 minutes.

[0055] The photodetector 10 is obtained by forming an electrode 13 on the thin film 12 of α-gallium oxide by an appropriate method.

[0056] Below, we will explain a more specific example of a method for manufacturing the photodetector 10. The manufacturing method explained below can be applied regardless of the surface orientation of the substrate 11. The manufacturing method explained below will be used in the experiments described later.

[0057] First, to obtain a step-terrace structure, the sapphire substrate 11 is pretreated by heating it at 1050°C for 10 hours, and then etching it for 1 hour with a piranha solution (98% H2SO4:30% H2O2=3:1 in volume, Fujifilm-Wako) to remove hydrocarbon impurities remaining on the substrate surface.

[0058] An α-gallium oxide thin film 12 is grown on a pretreated substrate 11 using a mist CVD apparatus. The mist CVD apparatus is equipped with a mist generator having four 2.4 MHz ultrasonic vibrators to turn the raw material solution into mist. The film formation chamber is composed of a hot-wall furnace equipped with a channel flow-type quartz susceptor. In the mist CVD apparatus, the flow rate control of the carrier gas and the heater control for heating the film formation chamber are managed by a microcomputer.

[0059] To grow a single-phase film of α-gallium oxide, a gallium acetylacetonate solution is used as the raw material solution for mist CVD. To acetylacetonate the raw material solution, a concentrated aqueous solution of metallic gallium in hydrochloric acid is diluted and acetylacetone is added to form the acetylacetonate complex. This method ensures that the aqueous solution of the acetylacetonate complex can be produced. For example, the concentration of the raw material solution is [Ga 3+]=0.02 mol / L. In the mist CVD method using gallium acetylacetonate solution, the metal acetylacetonate complex fixes the surface hydroxyl groups, and gallium oxide is formed by a ligand exchange mechanism. This mechanism results in a strong epitaxial growth.

[0060] Nitrogen gas was used as the carrier gas for transporting the mist of the raw material solution at a flow rate of 0.5 L / min, and oxygen gas was used as the dilution gas for diluting the carrier gas at a flow rate of 0.1 L / min.

[0061] The growth temperature of the α-gallium oxide thin film is 600° C., and the growth time is 60 minutes.

[0062] An electrode 13 having a comb-shaped pattern is formed on the surface of the α-gallium oxide thin film. The electrode 13 is formed by photolithography and lift-off processes. A laser exposure device and a positive photoresist are used to form the electrode 13.

[0063] <3. Experiment>

[0064] In the experiment, the above manufacturing method was used to form a thin film 12 of α-gallium oxide on each of an a-plane sapphire substrate 11, an m-plane sapphire substrate 11, an n-plane sapphire substrate 11, and an r-plane sapphire substrate 11. The film thickness t of the α-gallium oxide thin film 12 was 250 to 400 nm. In addition, a substrate in which α-gallium oxide was formed on a c-plane sapphire substrate 11 was manufactured as a comparative example.

[0065] Figure 3 shows X-ray 2θ-θ diffraction profiles. Figure 3(a) is an X-ray diffraction profile of α-gallium oxide grown on a-plane sapphire. Figure 3(b) is an X-ray diffraction profile of α-gallium oxide grown on m-plane sapphire. Figure 3(c) is an X-ray diffraction profile of α-gallium oxide grown on n-plane sapphire. Figure 3(d) is an X-ray diffraction profile of α-gallium oxide grown on r-plane sapphire.

[0066] As is clear from FIG. 3, the plane orientation of the α-gallium oxide film 12 roughly coincides with the plane orientation of the substrate 11. Therefore, it can be seen that the above-described manufacturing method results in an α-gallium oxide film with plane orientations in the a-plane, m-plane, n-plane, and r-plane. The α-gallium oxide film 12 (comparative example) formed on a c-plane sapphire substrate has a c-plane orientation. In the X-ray diffraction measurement shown in FIG. 3, the measurement axis was aligned with the plane orientation of the gallium oxide, not with the plane orientation of the sapphire substrate. This is because the crystal axis of the gallium oxide crystal grown on the sapphire substrate is slightly tilted in a specific direction relative to the crystal axis of the sapphire substrate.

[0067] 3 also shows that the thin film formed on substrate 11 is substantially a single-phase film of α-gallium oxide. Here, "substantially a single-phase film of α-gallium oxide" means that in the X-ray diffraction profile, no peaks other than those attributable to α-gallium oxide and the material of substrate 11 are present, or even if such other peaks are present, the magnitude of such other peaks is less than 1 / 10 of the magnitude of the peak attributable to α-gallium oxide. More preferably, the magnitude of such other peaks is less than 1 / 100 of the magnitude of the peak attributable to α-gallium oxide. Note that the magnitude of such other peaks refers to the magnitude of peaks attributable to the material causing such other peaks.

[0068] For example, in Figure 3(a), a peak due to κ-gallium oxide appears. However, in Figure 3(a), the peak due to κ-gallium oxide is sufficiently smaller than the peak due to α-gallium oxide. Therefore, the α-gallium oxide is essentially a single-phase film.

[0069] As mentioned above, there is a large lattice mismatch between α-gallium oxide and sapphire. This causes structural fluctuations in the α-gallium oxide grown on the sapphire substrate 11. The structural fluctuations arise due to the lattice mismatch and occur during the growth of the α-gallium oxide thin film as the lattice mismatch is resolved. The structural fluctuations in the α-gallium oxide film are evaluated by two factors.

[0070] Here, (a1) and (a2) in Figure 4 show a c-plane sapphire substrate, (a3) ​​shows the structural distortion of α-gallium oxide formed on the c-plane sapphire substrate, (b1) and (b2) in Figure 4 show an a-plane sapphire substrate, and (b3) shows the structural distortion of α-gallium oxide formed on the a-plane sapphire substrate.

[0071] The first element for evaluating the structural distortion is the fluctuation Δω in the normal direction N of the film surface (see Figures 4(a3) and (b3)). Δω indicates the extent of the mosaic. Δω [deg] is calculated as the full width at half maximum (FWHM) of the X-ray diffraction rocking curve of the α-gallium oxide film.

[0072] The second element for evaluating the structural distortion is the rotational fluctuation Δφ around the normal direction N (see Figures 4(a3) and (b3)). Δφ is evaluated by a φ-scan in the skew configuration in X-ray diffraction measurement. That is, Δφ [deg] is calculated as the full width at half maximum (FWHM) of the X-ray diffraction φ-scan of the α-gallium oxide film.

[0073] As shown in FIG. 4(c), α-gallium oxide formed on a c-plane sapphire substrate 11 (α-gallium oxide oriented to the c-plane; comparative example) has a small Δω and a large Δφ, with a Δω of 0.0096 degrees and a Δφ of 1.32 degrees. That is, in the c-plane oriented α-gallium oxide (comparative example), the structural strain is biased toward Δω (fluctuation in the normal direction N) and is large enough to exceed 1 degree, while the Δφ (rotational fluctuation around the normal direction N) is small. Furthermore, in the c-plane oriented α-gallium oxide, the absolute value of the difference between Δω and Δφ (|Δω-Δφ|) is 1.3104 degrees, which is very large. Furthermore, the ratio of Δω to Δφ, Δφ / Δω, is 137.5, which is very large.

[0074] On the other hand, in α-gallium oxide oriented to the a-plane, m-plane, n-plane, and r-plane, Δω and Δφ do not exceed 1 [deg], and Δω is prevented from becoming so large that it exceeds 1 [deg]. In other words, in α-gallium oxide thin films oriented to the a-plane, m-plane, n-plane, and r-plane, the difference between Δω and Δφ is small.

[0075] Here, Δω and Δφ are both preferably less than 1 degree, more preferably less than 0.5 degrees, and may be even smaller. The absolute value of the difference between Δω and Δφ (|Δω-Δφ|) is preferably less than 0.6 degrees, more preferably less than 0.3 degrees, and may be even smaller.

[0076] As shown in Figure 4(c), in α-gallium oxide with a-plane orientation, Δω is 0.2435 [deg] and Δφ is 0.229 [deg], both of which are less than 0.25 [deg]. Furthermore, in α-gallium oxide with a-plane orientation, the absolute value of the difference between Δω and Δφ (|Δω-Δφ|) is 0.0145 [deg], less than 0.02 [deg], and the difference between the two is small. Furthermore, Δφ / Δω is approximately 0.94, which is relatively small and falls within the range of 0.8 to 1.2 (0.9 to 1.1).

[0077] In α-gallium oxide with an m-plane orientation, Δω is 0.3617° and Δφ is 0.347°, both of which are less than 0.4°. Furthermore, in α-gallium oxide with an m-plane orientation, the absolute value of the difference between Δω and Δφ (|Δω-Δφ|) is 0.0147°, less than 0.02°, making the difference between the two small. Furthermore, Δφ / Δω is approximately 0.96, which is relatively small and falls within the range of 0.8 to 1.2 (0.9 to 1.1).

[0078] In α-gallium oxide with n-plane orientation, Δω is 0.4593 [deg] and Δφ is 0.953 [deg], both of which are less than 1 [deg]. Furthermore, in α-gallium oxide with n-plane orientation, the absolute value of the difference between Δω and Δφ (|Δω-Δφ|) is 0.4937 [deg], which is less than 0.6 [deg] (less than 0.5 [deg]), and the difference between the two is small. Furthermore, Δφ / Δω is approximately 2.01, which is less than 3 and is relatively small.

[0079] In α-gallium oxide with a plane orientation to the r-plane, Δω is 0.3487 [deg] and Δφ is 0.478 [deg], both of which are less than 0.5 [deg]. In α-gallium oxide with a plane orientation to the n-plane, the absolute value of the difference between Δω and Δφ (|Δω-Δφ|) is 0.1293 [deg], which is less than 0.3 [deg] (less than 0.2 [deg]), and the difference between the two is small. Furthermore, Δφ / Δω is approximately 1.37, which is less than 2 (less than 1.5), which is relatively small.

[0080] As described above, the difference between Δω and Δφ is smaller in the a-plane, m-plane, n-plane, and r-plane orientations than in the c-plane orientation. The reason for the small difference between Δω and Δφ is that the degree of rotational freedom around the normal direction N is lower in the a-plane, m-plane, n-plane, and r-plane orientations than in the c-plane orientation.

[0081] As shown in Figure 2, in the c-plane orientation, α-gallium oxide has a hexagonal crystal structure in the top view of the film. A hexagon is closer to a circle than a square, and therefore has a higher degree of rotational freedom than a square. Therefore, in the c-plane orientation, structural fluctuations tend to occur as large, biased rotational fluctuations Δφ around the normal direction N. As a result of Δφ becoming large, Δω becomes very small.

[0082] As shown in Figure 2, in the a-plane, m-plane, n-plane, and r-plane orientations, α-gallium oxide has a square (rectangular) crystal structure with short and long sides in the top view of the film. A square has a lower degree of rotational freedom than a hexagon. Therefore, in the a-plane, m-plane, n-plane, and r-plane orientations, the occurrence of structural fluctuations as rotational fluctuations Δφ during film growth is suppressed compared to the c-plane orientation. As a result, in the a-plane, m-plane, n-plane, and r-plane orientations, structural fluctuations occur in both Δω and Δφ, with less bias toward one side.

[0083] When the structural fluctuations are biased toward Δφ, as in the case of a c-plane orientation, the number of crystal defects increases, and current can flow through the α-gallium oxide even in the dark or when irradiated with visible light. This tends to reduce the visible light rejection ratio of the photodetector 10. In contrast, by dispersing the structural fluctuations toward both Δω and Δφ, as in the case of a-plane orientation, m-plane orientation, n-plane orientation, and r-plane orientation, the number of defects decreases, and the visible light rejection ratio of the photodetector 10 can be increased.

[0084] Furthermore, while both α-gallium oxide and sapphire crystals have a corundum structure, the lattice mismatch rate differs between the a-axis and c-axis directions. Therefore, when grown on a c-plane sapphire substrate, where the lattice mismatch rate in the a-axis direction is dominant, the lattice mismatch rate is approximately 4.8%, but on other planes (a-plane, m-plane, n-plane, r-plane), the lattice mismatch rate in the c-axis direction is smaller, resulting in a lattice mismatch rate of 3.3 to 3.8%. Therefore, there are fewer lattice defects on the a-plane, m-plane, n-plane, and r-plane.

[0085] In the experiment, the photoresponse characteristics (current-voltage characteristics) of the fabricated photodetector 10 were measured. Figure 5 shows how the photoresponse characteristics were measured. In the experiment, as shown in Figure 5(a), light from a deuterium (D2) lamp (wavelength: 220-440 nm) or a halogen lamp (400-900 nm) was irradiated onto the photodetector 10 using an optical fiber. The D2 lamp irradiates ultraviolet light (mainly UV-C), and the halogen lamp irradiates mainly visible light. The characteristics of the D2 lamp and halogen lamp used in the experiment are as shown in Figure 5(b), and the intensity of the D2 lamp was 337 μmW / cm 2 and the halogen lamp intensity is 320 μW / cm 2 The current-voltage characteristics of the photodetector 10 were measured using a source meter (Keysight B2902A). A pair of styli (HiSol HP40 standard probe) were used to apply a voltage to the electrodes of the photodetector 10.

[0086] The current-voltage characteristics were determined when irradiated with light from a D2 lamp, when irradiated with light from a halogen lamp, and in the dark (when not irradiated with light).

[0087] 6 to 10 show the measurement results of the photoresponse characteristics. In Fig. 6 to 10, the vertical axis represents the photocurrent [A] and the horizontal axis represents the voltage applied between the electrodes. The applied voltage was changed between -40V and +40V.

[0088] Figure 6 shows the photoresponse characteristics of a c-plane oriented photodetector. Figure 6(a) shows the current-voltage characteristics when irradiated with a D2 lamp and a halogen lamp, respectively, and Figure 6(b) shows the current-voltage characteristics in the dark.

[0089] As shown in Figure 6(a), when ultraviolet light (UV-C) is irradiated onto a photodetector 10 with a c-plane orientation by a D2 lamp, a current flows. The sensitivity to ultraviolet light (UV-C) is about 1 [A / W]. In the c-plane orientation, the visible light rejection ratio, which is expressed as the magnitude of ultraviolet light sensitivity relative to visible light sensitivity, is 10 when 10 V is applied. 3.8 When 24V is applied, the 4.3However, the visible light rejection ratio is 10 4 Preferably greater than 10 5 It is more preferable that it is greater than .

[0090] Fig. 7 shows the photoresponse characteristics of the a-plane oriented photodetector 10 (film thickness t: 258.66 nm). Fig. 7(a) shows the current-voltage characteristics when irradiated with a D2 lamp and a halogen lamp, and Fig. 7(b) shows the current-voltage characteristics in the dark.

[0091] As shown in Figure 7(a), in the a-plane orientation, the current value was 2.32 × 10 when 10 V was applied. -6 [A], and the visible light rejection ratio is 10 4.4 The photosensitivity was 0.71 [A / W]. When 24 V was applied, the current was 5.58 × 10 -6 [A], and the visible light rejection ratio is 10 4.8 The photosensitivity was 1.71 [A / W].

[0092] In the case of a-plane orientation, the visible light rejection ratio is improved compared to the case of c-plane orientation.

[0093] Fig. 8 shows the photoresponse characteristics of the m-plane oriented photodetector 10 (film thickness t: 299.42 nm). Fig. 8(a) shows the current-voltage characteristics when irradiated with a D2 lamp and a halogen lamp, and Fig. 8(b) shows the current-voltage characteristics in the dark.

[0094] As shown in Figure 8(a), in the m-plane orientation, the current value was 8.55 × 10 when 10 V was applied. -4 [A], and the visible light rejection ratio is 10 6.9 The photosensitivity was 262 [A / W]. When 24 V was applied, the current value was 2.51 × 10 -3 [A], and the visible light rejection ratio is 10 7.4 The photosensitivity was 768 [A / W].

[0095] The m-plane orientation has a significantly improved visible light rejection ratio compared to the c-plane orientation, and the m-plane orientation also has a significantly improved light sensitivity compared to the c-plane and a-plane orientations.

[0096] Fig. 9 shows the photoresponse characteristics of the n-plane oriented photodetector 10 (film thickness t: 421.87 nm). Fig. 9(a) shows the current-voltage characteristics when irradiated with a D2 lamp and a halogen lamp, and Fig. 9(b) shows the current-voltage characteristics in the dark.

[0097] As shown in Figure 9(a), in the n-plane orientation, the current value was 3.21 × 10 when 10 V was applied. -4 [A], and the visible light rejection ratio is 10 6.5 The photosensitivity was 98.3 [A / W]. When 24 V was applied, the current value was 8.24 × 10 -4 [A], and the visible light rejection ratio is 10 6.9 The photosensitivity was 252 [A / W].

[0098] The n-plane orientation has a higher visible light rejection ratio than the c-plane orientation, and the n-plane orientation also has a significantly higher light sensitivity than the c-plane and a-plane orientations.

[0099] Fig. 10 shows the photoresponse characteristics of the photodetector 10 (film thickness t: 253.77 nm) with an r-plane orientation. Fig. 10(a) shows the current-voltage characteristics when irradiated with a D2 lamp and a halogen lamp, and Fig. 10(b) shows the current-voltage characteristics in the dark.

[0100] As shown in Figure 10(a), in the r-plane orientation, the current value was 3.67 × 10 when 10 V was applied. -4 [A], and the visible light rejection ratio is 10 6.5 The photosensitivity was 112 [A / W]. When 24 V was applied, the current value was 1.05 × 10 -3 [A], and the visible light rejection ratio is 10 6.9 The photosensitivity was 321 [A / W].

[0101] The r-plane orientation has a much higher visible light rejection ratio than the c-plane orientation, and the r-plane orientation also has a much higher light sensitivity than the c-plane and a-plane orientations.

[0102] As described above, in the cases of the m-plane orientation, n-plane orientation, and r-plane orientation, the current value improved by two orders of magnitude compared to the a-plane orientation, and the light-receiving sensitivity increased.

[0103] In particular, for the m-plane orientation, the maximum visible light rejection ratio of 6.9 digits was obtained when 24 V was applied. Therefore, as shown in Figure 11, among the a-plane, m-plane, n-plane, and r-plane orientations, the m-plane orientation is the most suitable. Furthermore, the n-plane and r-plane orientations are next most suitable. Furthermore, the photocurrent exceeded 1 mA for the m-plane, and photocurrents close to 1 mA were obtained for the n-plane and r-plane. Furthermore, for all plane orientations, the dark current was below the sensitivity limit of the measurement system.

[0104] The present invention is not limited to the above-described embodiments and experimental examples, and various modifications are possible. [Explanation of symbols]

[0105] 10: Photodetector 11: Substrate 12: Gallium oxide film 13: Electrode N: Normal direction t: Film thickness

Claims

1. A photodetector includes, as a photodetecting element, an α-gallium oxide film oriented in any one of the a-plane, m-plane, n-plane, and r-plane.

2. The α-gallium oxide film is substantially a single-phase film of α-gallium oxide. The photodetector of claim 1 .

3. When the full width at half maximum (FWHM) of the X-ray diffraction rocking curve of the α-gallium oxide film is Δω [deg] and the full width at half maximum (FWHM) of the X-ray diffraction φ scan of the α-gallium oxide film is Δφ [deg], Both Δω and Δφ are less than 1 [deg]. The photodetector of claim 1 .

4. Both Δω and Δφ are less than 0.5 degrees.

4. The photodetector of claim 3.

5. The absolute value of the difference between Δω and Δφ (|Δω−Δφ|) is less than 0.6 [deg].

5. The photodetector according to claim 3 or claim 4.

6. The absolute value of the difference between Δω and Δφ (|Δω−Δφ|) is less than 0.3 [deg].

5. The photodetector according to claim 3 or claim 4.

7. The α-gallium oxide film is oriented to any one of the m-plane, the n-plane, and the r-plane. The photodetector of claim 1 .

8. the α-gallium oxide film is substantially a single-phase film of α-gallium oxide, When the full width at half maximum (FWHM) of the X-ray diffraction rocking curve of the α-gallium oxide film is Δω [deg] and the full width at half maximum (FWHM) of the X-ray diffraction φ scan of the α-gallium oxide film is Δφ [deg], Δω and Δφ are both less than 1 [deg]. The photodetector of claim 1 .

9. The α-gallium oxide film is oriented to any one of the m-plane, the n-plane, and the r-plane.

9. The photodetector of claim 8.

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