Optical sensor using gallium nitride

Abstract

An optical sensor (100) comprises: a gallium nitride-based semiconductor layer (104) that has crystallinity and is disposed on a substrate (102) having an insulating surface; and a first electrode (106) and a second electrode (108) that are disposed separately from each other on the same surface of the gallium nitride-based semiconductor layer and that make an ohmic contact with the gallium nitride-based semiconductor layer. A region in which the first electrode and the second electrode are separated from each other is a light-receiving surface.

Description

Gallium nitride optical sensors

[0001] One embodiment of the present invention relates to an optical sensor in which a photoelectric conversion layer is formed of a gallium nitride based semiconductor layer such as gallium nitride.

[0002] Gallium nitride is a compound of gallium and nitrogen and is known as one of the III-V group compound semiconductors. Gallium nitride is a direct transition semiconductor with a wide band gap, and is therefore used as a material for forming semiconductor optical devices such as light-emitting diodes, laser diodes, and photoelectric conversion elements (solar cells). Single-crystal gallium nitride layers are fabricated by metal organic chemical vapor deposition (MOCVD). In recent years, attempts have been made to fabricate crystalline gallium nitride films by sputtering (see, for example, Patent Document 1). Photodiodes and photoelectric conversion elements using gallium nitride have also been disclosed (see, for example, Patent Document 2).

[0003] International Publication No. 2020 / 075599 Japanese Patent Application Laid-Open No. 2009-278003 Japanese Patent Application Laid-Open No. 2017-034189

[0004] When gallium nitride crystals are irradiated with light with a wavelength of 365 nm or less, which is greater than the band gap, electron-hole pairs are generated. Taking advantage of this property, it is expected that they will be applied to optical sensors that detect ultraviolet light. However, conventional optical sensors using gallium nitride employ a stacked structure with a pn junction consisting of a p-type gallium nitride layer and an n-type gallium nitride layer, and a pin junction with an i-type (intrinsic) layer added, which poses a problem of complex device structure.

[0005] An optical sensor according to one embodiment of the present invention comprises a crystalline gallium nitride semiconductor layer on a substrate having an insulating surface, and a first electrode and a second electrode that are spaced apart and disposed on the same surface of the gallium nitride semiconductor layer and that are in ohmic contact with the gallium nitride semiconductor layer, with the region where the first electrode and the second electrode are spaced apart serving as a light-receiving surface.

[0006] An optical sensor according to one embodiment of the present invention comprises: a crystalline gallium nitride based semiconductor layer on a substrate having an insulating surface; and a first element and a second element, each of which is disposed on the same surface of the gallium nitride based semiconductor layer but spaced apart and includes a first electrode and a second electrode in ohmic contact with the gallium nitride based semiconductor layer; the region where the first electrode and the second electrode of the first element are spaced apart serves as a light-receiving surface, and the second element is light-shielded.

[0007] FIG. 1 is a cross-sectional view showing the structure of an optical sensor according to one embodiment of the present invention. FIG. 2 is a diagram showing the configuration of a sputtering apparatus used in the fabrication of an optical sensor according to one embodiment of the present invention. FIG. 3 is a graph showing a rocking curve obtained by ω-scanning by X-ray diffraction of a gallium nitride film used as a photoelectric conversion layer of an optical sensor according to one embodiment of the present invention. FIG. 4 is a graph showing the photoluminescence characteristics of a gallium nitride film used as a photoelectric conversion layer of an optical sensor according to one embodiment of the present invention. FIG. 5 is a graph showing the current-voltage characteristics of an optical sensor according to one embodiment of the present invention. FIG. 6 is a graph showing the temperature characteristics of the resistance of an optical sensor according to one embodiment of the present invention. FIG. 7 is a graph showing the change in resistance of an optical sensor according to one embodiment of the present invention after blocking light irradiation. FIG. 8 is a graph showing the current-voltage characteristics of an optical sensor fabricated by a sputtering method. FIG. 9 is a graph showing the temperature characteristics of the resistance of an optical sensor fabricated by a sputtering method. FIG. 10 is a graph showing the change in resistance of an optical sensor fabricated by a sputtering method after blocking light irradiation. A conceptual diagram showing the relationship between the full width at half maximum of the rocking curve obtained by ω-scanning by X-ray diffraction and photosensitivity is shown. FIG. 11 is a cross-sectional view showing the structure of an optical sensor according to one embodiment of the present invention. FIG. 12 is a diagram showing the configuration of a measurement system using an optical sensor according to one embodiment of the present invention. FIG. 13 is a flowchart for explaining the operation of an optical sensor according to one embodiment of the present invention. 4 shows a flowchart for explaining the operation of an optical sensor according to an embodiment of the present invention.

[0008] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention can be implemented in many different forms, and should not be construed as being limited to the description of the embodiments exemplified below. For clarity of explanation, the drawings may show the width, thickness, shape, etc. of each part schematically compared to the actual form, but these are merely examples and do not limit the interpretation of the present invention. Furthermore, in this specification and each drawing, elements similar to those previously described with reference to the previous drawings are designated by the same reference numerals (or reference numerals with a, b, etc. suffixed thereto), and detailed descriptions may be omitted as appropriate. Furthermore, the letters "first" and "second" attached to each element are convenient labels used to distinguish each element and have no further meaning unless otherwise specified.

[0009] In this specification, when a component or region is referred to as being "on (or under)" another component or region, unless otherwise specified, this includes not only the case where it is directly above (or directly under) the other component or region, but also the case where it is above (or under) the other component or region, i.e., the case where another component is included between the component or region and above (or under) the other component or region.

[0010] 1 shows a cross-sectional view of an optical sensor 100 according to a first embodiment of the present invention. The optical sensor 100 has a gallium nitride semiconductor layer 104, a first electrode 106, and a second electrode 108 formed on a substrate 102. The gallium nitride semiconductor layer 104 forms a light receiving portion PA and functions as a photoelectric conversion layer. The first electrode 106 and the second electrode 108 are provided on the same surface of the gallium nitride semiconductor layer 104.

[0011] The substrate 102 has a flat insulating surface. A sapphire substrate is used as the substrate 102. Use of a sapphire substrate allows crystallization of the gallium nitride based semiconductor layer 104. For example, gallium nitride can be grown as a crystal on the sapphire substrate, and the gallium nitride layer can be used as the gallium nitride based semiconductor layer 104.

[0012] Furthermore, a glass substrate can be used as the substrate 102. For example, a glass substrate made of aluminoborosilicate glass or aluminosilicate glass can be used. Furthermore, a synthetic quartz glass substrate can be used as the substrate 102. When a glass substrate or a synthetic quartz glass substrate is used as the substrate 102, it is preferable to provide an orientation control layer (not shown) having crystallinity between the substrate 102 and the gallium nitride based semiconductor layer 104. For example, aluminum nitride (AlN), aluminum oxide (Al ２ O ３ ), lithium niobate (LiNbO), BiLaTiO, SrFeO, SrFeO, BiFeO, BaFeO, ZnFeO, PMnN-PZT, or biological apatite (BAp) can be used. An orientation control layer formed from these materials is preferably c-axis oriented. By providing such an orientation control layer, gallium nitride crystals can be grown in the same manner as on a sapphire substrate, and the gallium nitride layer can be used as the gallium nitride-based semiconductor layer 104.

[0013] As will be described later, when the gallium nitride based semiconductor layer 104 is fabricated by a sputtering method, it is also possible to use, as the substrate 102, a flexible resin substrate such as a polyimide substrate, an acrylic substrate, a siloxane substrate, or a fluororesin substrate, in addition to a glass substrate.

[0014] The gallium nitride based semiconductor layer 104 is formed of a compound semiconductor material such as gallium nitride (GaN), indium gallium nitride (InGaN), or aluminum gallium nitride (AlGaN). The gallium nitride, indium gallium nitride, or aluminum gallium nitride preferably has a stoichiometric composition, but may deviate from the stoichiometric composition. The gallium nitride based semiconductor layer 104 is formed of one compound semiconductor material selected from these. For example, the gallium nitride based semiconductor layer 104 is formed of a gallium nitride layer.

[0015] The gallium nitride semiconductor layer 104 preferably has crystallinity. The gallium nitride semiconductor layer 104 is preferably single crystalline, but may be polycrystalline, microcrystalline, or nanocrystalline. The gallium nitride semiconductor layer 104 may include an amorphous structure near the interface with the substrate 102, but preferably has crystallinity in the region (bulk) away from the interface. The crystalline structure of the gallium nitride semiconductor layer 104 preferably has a wurtzite structure. The gallium nitride semiconductor layer 104 preferably has a c-axis orientation.

[0016] The conductivity type of the gallium nitride semiconductor layer 104 is preferably n-type. The conductivity type of the gallium nitride semiconductor layer 104 may be determined by an impurity element that is inevitably mixed in, or may be controlled by an intentionally added dopant (impurity element). When a dopant is intentionally added, silicon (Si) or germanium (Ge) is used as a dopant for controlling the conductivity to n-type. Even when a dopant is intentionally added, the concentration of the dopant is preferably within a range in which the gallium nitride semiconductor layer 104 does not degenerate.

[0017] When a sapphire substrate is used as the substrate 102, the gallium nitride based semiconductor layer 104 is provided directly on the sapphire substrate. When a glass substrate, a synthetic quartz glass substrate, or a resin substrate is used as the substrate 102, the gallium nitride based semiconductor layer 104 is preferably provided in contact with the above-mentioned orientation control layer.

[0018] The gallium nitride semiconductor layer 104 is fabricated by a sputtering method. The substrate temperature (set temperature) when fabricating the gallium nitride semiconductor layer 104 by a sputtering method is controlled within a range from room temperature to 600°C. By using a sapphire substrate as the substrate 102, a crystalline gallium nitride semiconductor layer 104 can be fabricated even at a substrate temperature of 600°C or lower. Even when a glass substrate or a synthetic quartz glass substrate is used as the substrate 102, a crystalline gallium nitride semiconductor layer 104 can be fabricated by providing an orientation control layer. Note that room temperature refers to a state in which the substrate is set in a sputtering apparatus without being intentionally heated or cooled. Room temperature is, for example, 25°C, and may further include a range of ±10°C.

[0019] When the gallium nitride semiconductor layer 104 is formed by sputtering, a sintered body of a gallium nitride semiconductor is used as a sputtering target. For example, when a gallium nitride layer is formed, a sintered body of gallium nitride is used as a sputtering target. The gas introduced during film formation by sputtering (sputtering gas) is argon (Ar), or a mixture of argon (Ar) and nitrogen (N ２ As the sputtering apparatus, a two-pole sputtering apparatus, a magnetron sputtering apparatus, a dual magnetron sputtering apparatus, a facing target sputtering apparatus, an ion beam sputtering apparatus, an inductively coupled plasma (ICP) sputtering apparatus, or the like can be used.

[0020] When a sapphire substrate is used as the substrate 102, the gallium nitride based semiconductor layer 104 may be formed by metal organic chemical vapor deposition (MOCVD).

[0021] Gallium nitride (GaN) is known to have two crystal faces, a Ga-polar face and an N-polar face. In this embodiment, the gallium nitride layer fabricated by sputtering may have either a Ga-polar face or an N-polar face, or may have both polar faces mixed therein.

[0022] The gallium nitride semiconductor layer 104 contains oxygen as an impurity element. The oxygen concentration in the gallium nitride semiconductor layer 104 is 3×10 １９ / cm ３ 3x10 or more ２０ / cm ３ It is preferable that the concentration of oxygen contained in the gallium nitride based semiconductor layer 104 is within this range, thereby making the layer photoconductive.

[0023] The first electrode 106 and the second electrode 108 are provided on the same surface of the gallium nitride semiconductor layer 104. FIG. 1 shows a structure in which the first electrode 106 and the second electrode 108 are provided separately on the upper surface of the gallium nitride semiconductor layer 104 (the surface opposite the surface facing the substrate 102). The first electrode 106 and the second electrode 108 may be single layers, or may have a structure in which multiple layers are stacked. It is preferable that at least the layer of the first electrode 106 and the second electrode 108 that contacts the gallium nitride semiconductor layer 104 be formed from a material that forms ohmic contact. For example, when the gallium nitride semiconductor layer 104 is an n-type gallium nitride layer, ohmic contact can be formed by using titanium (Ti) or chromium (Cr) as the metal material for forming the first electrode 106 and the second electrode 108.

[0024] The inset in Fig. 1 shows an example of a stacked structure of the first electrode 106 and the second electrode 108. The structure (A) shown in the inset in Fig. 1 has a structure in which a first metal layer 105A, a second metal layer 105B, a third metal layer 105C, and a fourth metal layer 105D are stacked from the gallium nitride semiconductor layer 104 side. Here, the materials forming each metal layer are titanium (Ti) for the first metal layer 105A, aluminum (Al) for the second metal layer 105B, titanium (Ti) for the third metal layer 105C, and gold (Au) for the fourth metal layer 105D. The structure (B) shown in the inset in Fig. 1 shows a structure in which the first metal layer 105A, the second metal layer 105B, and the third metal layer 105C are stacked from the gallium nitride semiconductor layer 104 side. Here, the materials forming each metal layer are chromium (Cr) for first metal layer 105A, nickel (Ni) for second metal layer 105B, and gold (Au) for third metal layer 105C. By forming first electrode 106 and second electrode 108 in such a stacked structure, ohmic contact with gallium nitride based semiconductor layer 104 can be formed, thereby achieving low resistance.

[0025] The distance between the first electrode 106 and the second electrode 108 can be appropriately selected depending on the resistance value of the gallium nitride semiconductor layer 104. The distance between the first electrode 106 and the second electrode 108 can be set within a range of 0.1 mm to 10 mm, for example, 5 mm. The region sandwiched between the first electrode 106 and the second electrode 108 forms the light receiving portion PA where light is incident. Because the gallium nitride semiconductor layer 104 is photoconductive, its resistance value changes when light strikes the light receiving portion PA. The resistance value of the optical sensor 100 changes depending on the light intensity. Therefore, the intensity (illuminance) of irradiated light can be measured by measuring the resistance value of the optical sensor 100. The gallium nitride semiconductor layer 104 has a wider band gap than single-crystal silicon. For example, the band gap of gallium nitride is approximately 3.4 eV. Therefore, the optical sensor 100 according to this embodiment is capable of detecting light in the ultraviolet wavelength band.

[0026] Sapphire has a very large band gap compared to the band gap of the gallium nitride based semiconductor layer 104. For example, the band gap of gallium nitride is about 3.4 eV, while the band gap of sapphire is 8.5 eV. Furthermore, the band gap of quartz is about 8 eV to 9 eV, the band gap of aluminum nitride (AlN) used as the orientation control layer is about 6 eV, and the band gap of aluminum oxide (AlN) is about 9 eV. ２ O ３ ) has a band gap of approximately 6 eV. Therefore, by using a sapphire substrate or a synthetic quartz glass substrate as the substrate 102, the light incident surface of the light receiving unit PA can be on the side of the gallium nitride based semiconductor layer 104, or on the side of the substrate 102. On the other hand, when a glass substrate is used as the substrate 102, the transmittance in the deep ultraviolet band decreases, so that a structure in which light is incident from the gallium nitride based semiconductor layer 104 side is preferable depending on the wavelength band of light to be detected.

[0027] As described above, the optical sensor 100 has a planar structure in which electrodes are arranged on the same surface. This allows for a wider electrode spacing, thereby increasing dielectric breakdown resistance. In other words, compared to a structure in which electrodes are arranged above and below a gallium nitride layer, the optical sensor 100 according to this embodiment can have higher dielectric breakdown resistance.

[0028] 1 , a passivation film may be provided to cover the gallium nitride semiconductor layer 104, the first electrode 106, and the second electrode 108. When providing the passivation film, it is preferable to use a material that is transparent (has high transmittance) to the wavelength of light to be detected. The passivation film may also have a structure in which multiple layers with different refractive indices are stacked so as to function as an anti-reflection film.

[0029] The photosensor 100 according to this embodiment has a simple structure in which a first electrode 106 and a second electrode 108 are provided on a gallium nitride semiconductor layer 104. By using the gallium nitride semiconductor layer 104 as a photoelectric conversion layer and forming the first electrode 106 and the second electrode 108 from a high-melting-point metal, it is possible to improve heat resistance and enable operation at high temperatures. Furthermore, by forming the electrodes into a planar structure, it is possible to increase the breakdown voltage and enable operation at high voltages.

[0030] Next, an example of a method for fabricating the gallium nitride based semiconductor layer 104 that constitutes the optical sensor 100 will be described. The gallium nitride based semiconductor layer 104 can be fabricated by a sputtering method. As described above, there are no limitations on the sputtering method, and methods that can be used include bipolar sputtering, magnetron sputtering, dual magnetron sputtering, facing target sputtering, ion beam sputtering, and inductively coupled plasma (ICP) sputtering.

[0031] 2 is a schematic diagram showing the arrangement of a sputtering target TG and a substrate stage SS in a sputtering apparatus used to form the gallium nitride based semiconductor layer 104. The sputtering target TG and the substrate stage SS are arranged facing each other. When the sputtering target TG and the substrate stage SS are viewed from above, the substrate stage SS has a larger area than the sputtering target TG. In other words, the sputtering target TG is arranged facing the substrate stage SS at a predetermined distance so as to overlap a portion of the substrate stage SS. The distance Ts between the sputtering target TG and the substrate stage SS is set appropriately.

[0032] The substrate stage SS can accommodate a plurality of substrates 102. FIG. 2 shows an example in which a first substrate 102A, a second substrate 102B, a third substrate 102C, and a fourth substrate 102D are arranged at four positions, positions A to D. Here, the second substrate 102B is arranged in a region overlapping with the sputtering target TG in a plan view, and the third substrate 102C is arranged at a distance W from the second substrate 102B. The sputtering apparatus shown in FIG. 2 can control the crystallinity and electrical characteristics of the deposited gallium nitride semiconductor layer 104 by varying the position at which the substrate 102 is placed. In other words, the sputtering apparatus used in this embodiment is configured to be able to control the crystallinity and electrical characteristics of the deposited gallium nitride semiconductor layer 104 by varying the distance from the sputtering target TG to the substrate 102 depending on the position at which the substrate 102 is placed.

[0033] A gallium nitride target is used when a gallium nitride film is formed as the gallium nitride-based semiconductor layer 104. The gallium nitride target is made of a sintered body of gallium nitride. The substrate stage SS may be provided with a heater for heating the substrate 102.

[0034] When forming the gallium nitride based semiconductor layer 104, it is preferable to perform pre-sputtering, pre-annealing, and reverse sputtering (plasma treatment of the substrate 102) in order to remove residual oxygen in the film formation chamber and moisture adsorbed on the inner wall of the film formation chamber, the surface of the sputtering target TG, the substrate 102, etc. The sputtering gas is argon (Ar), or a mixture of argon (Ar) and nitrogen (N ２ ) mixed gas is used.

[0035] Table 1 below shows an example of the deposition conditions for a gallium nitride film as an example of the gallium nitride based semiconductor layer 104. Note that the deposition conditions shown in Table 1 are just an example, and the deposition conditions for the gallium nitride based semiconductor layer 104 according to this embodiment are not limited to the deposition conditions shown in Table 1. Note that pre-sputtering was also performed under the same conditions.

[0036] The gallium nitride film formed on a sapphire substrate under the deposition conditions shown in Table 1 is crystalline. Figure 3 shows the results of evaluating the crystallinity of the gallium nitride film on the sapphire substrate by X-ray diffraction rocking curve measurement (ω scan). Figure 3 shows the results of rocking curve measurement for the (0002) plane of gallium nitride (GaN), with samples A, B, C, and D corresponding to positions A, B, C, and D on the substrate 102 shown in Figure 2, respectively. Because the thickness of the gallium nitride film differs for each sample, the characteristics shown in Figure 3 are normalized by the peak value of the rocking curve. As shown in Figure 2, position B is closest to the sputtering target TG, followed by positions A, C, and D, in that order.

[0037] As is clear from the characteristics shown in Figure 3, the full width at half maximum of the rocking curve of each sample is different. The full width at half maximum of the rocking curve correlates with crystallinity, with a narrower full width at half maximum indicating better crystallinity. The full width at half maximum of the rocking curve obtained from the characteristics of each sample is shown in Table 2.

[0038] As is clear from the data shown in Table 2, samples C and D, which are positioned farther from the sputtering target TG, have smaller full widths at half maximum than samples A and B, which are positioned relatively closer to the sputtering target TG. Therefore, it can be determined that samples C and D have better crystallinity than samples A and B. From the full width at half maximum data shown in Table 2, it can be said that in the ω scan in the X-ray diffraction of the gallium nitride based semiconductor layer 104, it is preferable that the full width at half maximum of the GaN (0002) plane be 1300 arcsec or less. The full width at half maximum conditions for the optical sensor using gallium nitride according to the present invention will be explained in detail below.

[0039] The oxygen concentration in the gallium nitride film was evaluated by secondary ion mass spectrometry. As a result, the oxygen concentration in the samples A, B, C, and D was 3×10 １９ / cm ３ From 3 x 10 ２０ / cm ３The conductivity of samples A, B, C, and D was found to be n-type by Hall effect measurements.

[0040] Figure 4 shows the photoluminescence characteristics of Samples B and C prepared as described above. A helium-cadmium laser with a wavelength of 325 nm, which has energy greater than the band gap of gallium nitride, was used as the excitation light source for the photoluminescence measurements. Gallium nitride is known to exhibit a photoluminescence called the yellow peak around 550 nm. This peak is thought to be due to defect levels caused by nitrogen vacancies.

[0041] In contrast, in the characteristics shown in Figure 4, both Sample B and Sample C exhibit photoluminescence with a peak near 500 nm (2.47 eV), a wavelength shorter than the yellow peak. Since the band gap of gallium nitride is approximately 3.4 eV, it can be assumed that this photoluminescence peak is generated by carrier recombination via intra-gap levels. Secondary ion mass spectrometry (SIMS) analysis revealed that the gallium nitride films of Sample B and Sample C contain oxygen. Therefore, it can be assumed that the photoluminescence with a peak near 500 nm is due to defect levels caused by oxygen. Comparing Sample C and Sample B, Sample C exhibits stronger photoluminescence, suggesting that Sample C has higher crystallinity.

[0042] From the results of X-ray diffraction and photoluminescence characteristics, it is inferred that sample C, placed outside the sputtering target TG, will produce a gallium nitride film with better crystallinity than sample B, placed between the sputtering target TG and the substrate stage SS.

[0043] 1 was fabricated, and the current-voltage characteristics were evaluated. In the following description, an optical sensor in which the gallium nitride based semiconductor layer 104 was fabricated under the same conditions as those of sample B will be referred to as optical sensor 100B, and an optical sensor fabricated under the same conditions as those of sample C will be referred to as optical sensor 100C.

[0044] 5A shows the current-voltage characteristics of the optical sensor 100C. The current-voltage characteristics in the dark state and in the irradiated state are shown in FIG. 5A. For the measurement in the irradiated state, a halogen lamp was used as the light source. The irradiated light intensity of the halogen lamp was 0.5 mW / cm. ２ The temperature during measurement was 25°C.

[0045] 5A, the current-voltage characteristics of the optical sensor 100C, in which the gallium nitride based semiconductor layer 104 was fabricated under the same conditions as those of sample C, are linear, indicating that the first electrode 106 and the second electrode 108 form ohmic contact. Furthermore, it is clear that the optical sensor 100C has a very high resistance in the dark, but that the resistance decreases due to the photoelectric effect when illuminated with light.

[0046] 5B shows the temperature dependence of the resistance value in the dark state and in the irradiated state obtained from the current-voltage characteristics of the optical sensor 100C. The measurement temperatures were 25°C, 40°C, 60°C, and 80°C. As is clear from the characteristics shown in FIG. 5B, in the dark state, ８ Ω to 1.8 x 10 ８ Ω range, while under light irradiation it shows a resistance of 2.1 × 10 ７ From 1.1 x 10 ７ The resistance values ​​are within the range of 1 / 100 of the original value, and the resistance value is about one order of magnitude lower in the light-irradiated state than in the dark state. Furthermore, a tendency for the resistance value to decrease with increasing temperature is observed, suggesting that electrical conduction is thermally activated. Thus, it has been found that the optical sensor 100C fabricated using a gallium nitride layer fabricated under the conditions of sample C has the property of changing its resistance value when irradiated with light, and is optically sensitive.

[0047] FIG. 5C is a graph showing the change in resistance over time after light irradiation of the optical sensor 100C is cut off. The measurement observed the change in resistance over 600 seconds after light was cut off. The results are from measurements at temperatures of 25°C and 80°C, and in both cases, a tendency for the resistance to increase by approximately 1.5 times over time was observed. This characteristic of the resistance gradually increasing over a relatively long period of time is thought to be due to the fact that photoexcited carriers generated in the gallium nitride semiconductor by light irradiation are released to the electrode when light irradiation is stopped, resulting in a decrease in carrier concentration and an increase in resistance over time.

[0048] 6A shows the current-voltage characteristics of optical sensor 100B in the dark and illuminated states. It can be seen that the current value in the dark state is larger than that of optical sensor 100C, and that there is almost no change in the current value even in the illuminated state.

[0049] 6B shows the temperature dependence of the resistance value in the dark state and in the irradiated state obtained from the current-voltage characteristics of the optical sensor 100B. The resistance value is 8×10 ４ 6B, the resistance value hardly changes in the dark state and in the illuminated state, and the temperature dependency is also small.

[0050] 6C is a graph showing the change in resistance over time after light irradiation of the optical sensor 100B is blocked. The measurement was conducted to observe the change in resistance over 600 seconds after light was blocked. The results are shown for measurement temperatures of 25°C and 80°C, and it can be seen that in both cases, the resistance hardly changes over time.

[0051] Comparing the characteristics of optical sensor 100C shown in Figures 5A to 5C with the characteristics of optical sensor 100B shown in Figures 6A to 6C, it can be seen that optical sensor 100C has a higher light-to-dark ratio of the current value (resistance value) and is therefore more sensitive as an optical sensor. It can also be seen that optical sensor 100C has a unique characteristic in which its resistance value gradually decreases over a relatively long period of time.

[0052] 7 is a graph illustrating the characteristics of a gallium nitride film fabricated on a sapphire substrate, and is a graph conceptually showing the relationship between the full width at half maximum of the rocking curve obtained by ω scanning of X-ray diffraction and the photosensitivity, based on the findings obtained from the above measurement results. In the graph shown in FIG. 7, the horizontal axis represents the full width at half maximum of the GaN (0002) plane, the vertical axis represents the photosensitivity, and the oxygen concentration is shown as a parameter. The oxygen concentration is 3×10 ２０ / cn ３ , 1×10 ２０ / cm ３ , 3 x 10 １９ / cm ３ It is assumed that the gallium nitride film has a mixture of Ga polarity and N polarity. It is known that the larger the full width at half maximum, the lower the photosensitivity, and the smaller the full width at half maximum, the higher the photosensitivity. In actual measurement results, sample C having a full width at half maximum of 1300 arcsec achieved a photosensitivity of at least one digit, so it is considered preferable that the full width at half maximum be less than this value for use as an optical sensor. It can also be inferred that the lower the oxygen concentration, the higher the photosensitivity. In other words, since sample C having a full width at half maximum of 1300 arcsec has been confirmed to function as an optical sensor, if the photosensitivity threshold T is set at that value, it can be considered that the crystallinity of the gallium nitride film requires a full width at half maximum of 1300 arcsec or less.

[0053] As shown in this embodiment, when forming a gallium nitride film by sputtering, the substrate 102 is positioned so as not to face the sputtering target TG and not to be exposed to plasma, thereby obtaining a gallium nitride film with excellent crystallinity and photosensitivity. By forming planar electrodes on the gallium nitride layer thus produced, a photosensor can be formed with a very simple structure. The distance between the two electrodes provided on the gallium nitride layer can be adjusted as needed and can be wider than the thickness of the gallium nitride film, allowing for the application of high voltages and increasing the breakdown voltage. Furthermore, the gallium nitride layer has high heat resistance, and forming ohmic contacts with refractory metals such as titanium (Ti) and chromium (Cr) as electrodes enables high-temperature operation.

[0054] Furthermore, the optical sensor 100 according to this embodiment can be used as an ultraviolet sensor because the photoelectric conversion layer is formed from a wide bandgap material such as gallium nitride. Also, as described with reference to FIG. 4, since the optical sensor 100 has a photoluminescence peak at a wavelength of around 500 nm, it can also be used as a sensor for detecting light in the visible light band.

[0055] Second Embodiment Fig. 8 is a cross-sectional view showing the structure of an optical sensor 100 according to a second embodiment. The optical sensor 100 has a light-receiving element (first element) PS and a reference element (second element) RE. The light-receiving element PS and the reference element RE have a structure in which first electrodes 106A and 106B and second electrodes 108A and 108B are provided on a gallium nitride based semiconductor layer 104, similar to the optical sensor 100 shown in Fig. 1 .

[0056] The light-receiving element PS has a light-receiving portion PA that is irradiated with light. The reference element RE is covered with a light-shielding layer 112. The light-shielding layer 112 is provided on the first electrode 106B and the second electrode 108B, with an insulating layer 110 sandwiched between them. The light-shielding layer 112 is provided to block external light. The light-shielding layer 112 may be formed of a metal film or an organic resin film containing a black pigment. Furthermore, the light-shielding layer 112 may be replaced with a plate-like member such as a metal plate or a plastic plate instead of the thin film described above.

[0057] In the optical sensor 100 shown in Figure 8, the light receiving element PS is used to output an electrical signal corresponding to the light intensity through photoelectric conversion, and the reference element RE is used to compensate for temperature characteristics. Therefore, it is preferable that the light receiving element PS and the reference element RE are arranged in close proximity. It is preferable that the light receiving element PS and the reference element RE are fabricated on the same substrate 102 and made of the same gallium nitride-based semiconductor layer 104. With this configuration, the characteristics of the light receiving element PS and the reference element RE can be made similar to each other, thereby improving the accuracy of temperature compensation.

[0058] Although not shown in detail, a shutter SR may be provided on the light receiving surface of the light receiving element PS, and a mechanism for controlling the timing at which light is incident on the light receiving element PS may be provided.

[0059] 9 shows an example of a measurement system using an optical sensor 100 equipped with a light-receiving element PS and a reference element RE. This system includes a parameter analyzer 202 connected to the light-receiving element PS and the reference element RE, a data processing device 204 for performing arithmetic processing on data output from the parameter analyzer 202, a storage device 206 for measuring in advance the relationship between the resistance value of the light-receiving element PS and the illuminance and storing the measured result as a look-up table, and an output device 208 for outputting the measurement results. Note that the parameter analyzer 202, data processing device 204, storage device 206, and output device 208 are merely exemplary and may be replaced with other devices or configurations that can perform functions similar to those described below.

[0060] FIG. 10 is a flowchart illustrating a measurement method using the measurement system shown in FIG. 9 . First, the optical sensor 100 is installed at a location where illuminance is to be measured, and a bias voltage is applied to the light-receiving element PS and the reference element RE under the same conditions using the parameter analyzer 202 (S200). Then, the parameter analyzer 202 is used to measure the electrical resistance of the light-receiving element PS and the reference element RE (S202). As described with reference to FIG. 5C , the resistance value of the light-receiving element PS is temperature-dependent, so a correction is required. Specifically, the data processing device 204 subtracts the resistance value in the dark state, obtained as the characteristic of the reference element RE, from the resistance value in the illuminated state, obtained as the characteristic of the light-receiving element PS (S204). Because the light-receiving element PS and the reference element RE have the same temperature characteristics, this data processing results in a temperature-compensated resistance value of the light-receiving element PS.

[0061] As described with reference to FIG. 9 , a lookup table in which resistance values ​​measured at known illuminances are recorded is stored in the storage device 206 (see (A) in FIG. 10 ). The data processing device 204 refers to the lookup table stored in the storage device 206 and searches for a matching resistance value within a predetermined range, taking into account an error (S206). If matching data within the predetermined range is not found, the process returns to step S202 and performs measurement again. If matching data within the predetermined range is found, the data is output to the output device 208 as the measured illuminance (S208). The output device 208 may be a monitor screen or a printer. The output device 208 may also have a function for outputting data to another terminal device.

[0062] By using the optical sensor 100 shown in Fig. 8 and performing measurements according to the procedure shown in Fig. 10, it is possible to measure temperature-compensated illuminance without being affected by the external environment at the time of measurement. Note that the reference table is not limited to data of resistance value versus illuminance (lux: lx), but can also be used for data of resistance value versus light intensity (W / cm ２ ) data may be stored.

[0063] The optical sensor 100 according to this embodiment is similar to that of the first embodiment and, despite its simple structure, is capable of high-temperature and high-voltage operation. Furthermore, the photoelectric conversion layer of the optical sensor 100 is formed of a wide-gap material such as gallium nitride, enabling it to detect ultraviolet light. Furthermore, as described with reference to FIG. 4 , the optical sensor 100 has a photoluminescence peak at a wavelength of around 500 nm, and therefore can also be used as a sensor for detecting light in the visible light band.

[0064] 5C, the optical sensor 100 according to an embodiment of the present invention has a characteristic in which the resistance value changes transiently over a relatively long period of time after light irradiation is interrupted. This embodiment illustrates an example of a method for measuring illuminance by utilizing the characteristic of the transient change in resistance value.

[0065] Fig. 11 shows a flowchart for explaining the method for measuring illuminance in this embodiment. The measurement system shown in Fig. 9 is used to measure illuminance. First, the optical sensor 100 is installed at a location where illuminance is to be measured, and the measurement timing is set (S201). Measurement begins after the shutter SR blocks the light while irradiating the optical sensor 100. Therefore, here, the timing for blocking the shutter SR is set, and at the same time, the timing for starting measurement by the parameter analyzer 202 is set. The timing for driving the shutter SR and the parameter analyzer 202 can be controlled by a personal computer.

[0066] The optical sensor 100 is installed at a location where illuminance is to be measured. At this time, the shutter SR is open, and the light-receiving element PS is illuminated with light. Next, the shutter SR is closed at the predetermined timing set above (S203). Then, a bias voltage is applied to the light-receiving element PS and the reference element RE under the same conditions using the parameter analyzer 202, and the electrical resistances of the light-receiving element PS and the reference element RE are measured (S205). As in the second embodiment, the electrical resistance is measured by the data processing device 204 by subtracting the resistance value of the reference element RE from the resistance value of the light-receiving element PS.

[0067] Measurements are performed multiple times at predetermined time intervals. There are no limitations on the time intervals or number of measurements, but for example, 10 measurements are performed at 60-second intervals. The number of measurements is counted, and it is determined whether the set number of measurements has been reached (S207). If not, the process returns to step S205. If the set number of measurements has been reached,

[0068] As described with reference to FIG. 9 , the storage device 206 stores a lookup table recording resistance values ​​measured at known illuminances. The lookup table records data on the change in resistance over time after blocking light, with temperature as a parameter, and data on the change in resistance over time after blocking light, with illuminance as a parameter (see (B) in FIG. 10 ). The data processing device 204 references the lookup table stored in the storage device 206 and searches for a time-varying resistance characteristic that matches within a predetermined range, taking into account error (S209). The data processing device 204 then determines the illuminance of the actual irradiated light from the time-varying characteristic that matches closely, and outputs the determined illuminance to the output device 208 (S211). The time-varying resistance characteristic of the optical sensor 100 shown in FIG. 5C changes gradually over a relatively long period of time. Therefore, by measuring this change over time and fitting it to the data in the lookup table, highly accurate illuminance can be obtained.

[0069] The optical sensor 100 according to this embodiment is similar to that of the second embodiment, and in addition to achieving the same effects, highly accurate measurements can be performed by utilizing the time-dependent change in resistance value as described above.

[0070] The various configurations of the optical sensor exemplified as one embodiment of the present invention can be combined as appropriate as long as they are not mutually inconsistent. Furthermore, optical sensors in which a person skilled in the art appropriately adds or deletes components or modifies the design, or adds or omits steps or modifies conditions based on the optical sensor disclosed in this specification and drawings, are also included in the scope of the present invention as long as they include the gist of the present invention.

[0071] Even if there are other effects and advantages different from those brought about by the aspects of the embodiments disclosed in this specification, if they are clear from the description in this specification or can be easily predicted by a person skilled in the art, they are naturally understood to be brought about by the present invention.

[0072] 100: optical sensor, 102: substrate, 102A: first substrate, 102B: second substrate, 102C: third substrate, 102D: fourth substrate, 104: gallium nitride based semiconductor layer, 106: first electrode, 108: second electrode, 105A: first metal layer, 105B: second metal layer, 105C: third metal layer, 105D: fourth metal layer, 110: insulating layer, 112: light-shielding layer, 202: parameter analyzer, 204: data processing device, 206: storage device, 208: output device, 210: data table, PD: light-receiving unit, TG: sputtering target, SS: substrate stage, SR: shutter, PS: light-receiving element, RE: reference element

Claims

1. An optical sensor comprising: a crystalline gallium nitride semiconductor layer on a substrate having an insulating surface; and a first electrode and a second electrode disposed on the same surface of the gallium nitride semiconductor layer but spaced apart and making ohmic contact with the gallium nitride semiconductor layer, wherein the area where the first electrode and the second electrode are spaced apart serves as a light-receiving surface.

2. The gallium nitride semiconductor layer contains oxygen, and the oxygen content is 3×10 １９ / cm ３ 3x10 or more ２０ / cm ３ 2. The optical sensor of claim 1, wherein:

3. The optical sensor according to claim 2, wherein in an ω scan in X-ray diffraction of said gallium nitride based semiconductor layer, the full width at half maximum of the GaN (0002) plane is 1300 arcsec or less.

4. The optical sensor according to claim 3, wherein the gallium nitride based semiconductor layer is a gallium nitride layer, and the gallium nitride layer has Ga polarity or N polarity.

5. The optical sensor according to claim 3, wherein the gallium nitride based semiconductor layer is a gallium nitride layer, and the gallium nitride layer has a mixture of Ga polarity and N polarity.

6. The optical sensor according to claim 1, wherein the first electrode and the second electrode are made of a plurality of metal layers, and the layer in contact with the gallium nitride based semiconductor layer is a titanium layer or a chromium layer.

7. An optical sensor comprising: a crystalline gallium nitride semiconductor layer on a substrate having an insulating surface; a first element and a second element, each including a first electrode and a second electrode spaced apart from each other and arranged on the same surface of the gallium nitride semiconductor layer and making ohmic contact with the gallium nitride semiconductor layer; wherein the area of ​​the first element where the first electrode and the second electrode are spaced apart is used as a light-receiving surface; and the second element is light-shielded.

8. The gallium nitride semiconductor layers of the first element and the second element contain oxygen, and the oxygen content is 3×10 １９ / cm ３ 3x10 or more ２０ / cm ３ 8. The optical sensor of claim 7, wherein:

9. The optical sensor according to claim 8, wherein in an ω scan in X-ray diffraction of the gallium nitride based semiconductor layer of the first element and the second element, the full width at half maximum of the GaN (0002) plane is 1300 arcsec or less.

10. The optical sensor according to claim 9, wherein the gallium nitride based semiconductor layers of the first element and the second element are gallium nitride layers, and the gallium nitride layers have Ga polarity or N polarity.

11. The optical sensor according to claim 9, wherein the gallium nitride based semiconductor layers of the first element and the second element are gallium nitride layers, and the gallium nitride layers have a mixture of Ga polarity and N polarity.

12. The optical sensor according to claim 7, wherein the first electrode and the second electrode of the first element and the second element are made of a plurality of metal layers, and the layer in contact with the gallium nitride based semiconductor layer is a titanium layer or a chromium layer.

13. The optical sensor according to claim 7, wherein the intensity of the irradiated light is measured by calculating the difference between the resistance value of the first element when the light is irradiated and the resistance value of the second element measured at the same time.

14. The optical sensor according to claim 7, which has a shutter on the light-receiving surface of the first element that blocks incident light, and which measures the intensity of the irradiated light by calculating the difference between the change in resistance of the first element over time after the shutter is opened to irradiate the first element with light, and the resistance of the second element that is measured at the same time after the shutter is closed to block the light.

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