JP2009260059A - Method of manufacturing ultraviolet sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing an ultraviolet sensor which is excellent in durability, require no thin film growth and reduces a manufacturing cost. <P>SOLUTION: The method of manufacturing the ultraviolet sensor comprises the steps of: producing an gallium oxide single crystal substrate (S1); forming an ohmic electrode on a rear face of the gallium oxide single crystal substrate (S2); performing heat treatment (S3); furthermore forming a Schottky electrode on a front surface of the gallium oxide single crystal substrate (S4); and performing heat treatment (S6). A heat treatment at a temperature of 450 to 550°C for 5 to 15 minutes is performed in a nitrogen atmosphere by adopting an Al/Ti structure or an Au/Ti structure as the ohmic electrode, heat treatment at a temperature of 350 to 450°C for 5 to 15 minutes is performed in the nitrogen atmosphere by using Au as the Schottky electrode, or heat treatment at a temperature of 450 to 550°C for 5 to 15 minutes is performed in the nitrogen atmosphere by using Pt as the Schottky electrode. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for manufacturing an ultraviolet sensor, and particularly detects only ultraviolet rays having a wavelength of 280 nm or less emitted by flames and harmful substances without being affected by sunlight, even in the daytime or outdoors where sunlight exists. The present invention relates to a method for manufacturing a small, simple, and low cost solar blind ultraviolet sensor.

Solar blind ultraviolet sensor is expected to be applied as a solid element type small and simple flame sensor. It is used as a sensor for fire detectors and cigarette detectors, and for automatic control of combustion flames in household combustion equipment and industrial furnaces. Expected to be used.
In addition, it can be applied as an ultraviolet monitor sensor in an ultraviolet exposure apparatus used for the production of next generation VLSI (large scale integrated circuit).

  Conventionally, a phototube exists as a sensor that detects only deep ultraviolet rays with a wavelength of 280 nm or less, and has already been put into practical use as a sensor that detects flickering of flames. ing. However, a sensor for detecting deep ultraviolet rays using a phototube has a problem that its lifetime is short and its cost is high.

In contrast, GaN-based III-nitride semiconductors, which are wide bandgap semiconductors, have attracted attention as solid-state sensors that realize small and simple flame sensors, and applications of AlGaN films have been studied (for example, Non-patent document 1). Further, an ultraviolet sensor using a diamond semiconductor has been studied (for example, see Non-Patent Document 2 below).
Hirano: “Applying GaN-based light-receiving elements to flame sensors” Applied Physics Vol. 68, No. 7 (1999) pp.0805-0809 Yasuo Koide: “Diamond UV Sensor” Materia Vol. 46, No. 4 (2007) pp. 272-277

However, the sensor in Non-Patent Document 1 described above is a thin film epitaxially grown on a substrate, and AlGaN, which is a mixed crystal of GaN and AlN, is in a situation where it is difficult to grow a high-quality film. Many are left behind.
On the other hand, with respect to the diamond film in Non-Patent Document 2, when homoepitaxial growth is performed using a diamond substrate, a high-quality diamond film grows. In this case, however, the problem is that the cost is high because the substrate is expensive. is there.
If a device is formed using a thin film like the sensor in Non-Patent Documents 1 and 2, the quality of the thin film to be grown is affected by the substrate. Therefore, if the device can be formed without using the thin film, the manufacturing process will be In addition to being simple, there is an advantage that the manufacturing cost can be reduced.

  An object of the present invention is to provide a method for manufacturing an ultraviolet sensor that is excellent in durability, does not require thin film growth, and can reduce the manufacturing cost.

  In order to solve the above-described problems, a method of manufacturing an ultraviolet sensor according to the present invention includes forming a first electrode in Schottky contact with a gallium oxide single crystal substrate on the surface of the gallium oxide single crystal substrate, A second electrode is formed on the back surface of the substrate in ohmic contact with the gallium oxide single crystal substrate and through which current flows between the first electrode and the first electrode. The first electrode and the second electrode This is a method of heat-treating an electrode.

  According to the present invention, since the gallium oxide single crystal substrate is used and the first electrode and the second electrode formed on the front surface and the back surface of the gallium oxide single crystal substrate, respectively, are heat-treated, thin film growth is not required and durability is improved. It is possible to manufacture an ultraviolet sensor excellent in the manufacturing cost at a low cost.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 1 shows a cross-sectional structure of an ultraviolet sensor manufactured by the manufacturing method according to the embodiment. A vertical Schottky diode is formed by forming a Schottky electrode 11 and an ohmic electrode 12 on the front and back surfaces of the Ga ₂ O ₃ single crystal substrate 10 respectively. At this time, in the Ga ₂ O ₃ single crystal substrate 10, a depletion layer 10a is formed immediately below the surface Schottky electrode 11, and a conductive layer 10b is formed thereunder.

A manufacturing method for obtaining such an ultraviolet sensor is shown in the flowchart of FIG.
First, in step S1, a Ga ₂ O ₃ single crystal substrate 10 is manufactured. The present inventors have already succeeded in growing a high-quality bulk gallium oxide (Ga ₂ O ₃ ) single crystal. A Ga ₂ O ₃ single crystal is an oxide semiconductor having a wide band gap of about 4.8 eV, and transmits light up to a wavelength of 260 nm. Therefore, it has a characteristic that it can detect ultraviolet rays having a wavelength of 280 nm or less. Furthermore, since the Ga ₂ O ₃ single crystal has conductivity, an electrode can be formed as it is on the Ga ₂ O ₃ single crystal substrate. Furthermore, since the Ga ₂ O ₃ single crystal is an oxide, it is considered that there is no fear of deterioration due to oxidation unlike the above AlGaN, and it is excellent in durability and stability. As a result of diligent research on a method for manufacturing an ultraviolet sensor using such characteristics, an ultraviolet ray is formed by applying heat treatment to electrodes formed on the front and back surfaces of a bulk Ga ₂ O ₃ single crystal substrate without forming a thin film. It came to the present invention which improves detection sensitivity.

A Ga ₂ O ₃ single crystal as a material can be produced by a method already invented by the present inventors, and a single crystal excellent in crystal quality can be produced. This method uses 4N purity Ga ₂ O ₃ powder as a raw material, sealed in a rubber tube, molded with a rubber press, sintered in an electric furnace at 1500 ° C for 10 hours, and used as a raw material rod, FZ (Floating Zone) ) Method to grow a single crystal. As the single crystal growth conditions, for example, the growth rate is 5 to 10 mm / h, the atmosphere is dry air, and the pressure is 1 atm.

  The single crystal produced in this way was sliced with a wire saw on the plane parallel to the (100) plane with the strongest cleavage, and this (100) plane was mirror-polished by chemical mechanical polishing (CMP). And processed into a wafer with a thickness of 0.4 to 0.5 mm.

At this time, the Ga ₂ O ₃ single crystal is adjusted so that the carrier density is 2 × 10 ¹⁶ to 2 × 10 ¹⁷ cm ⁻³ . When the carrier density exceeds 2 × 10 ¹⁷ cm ⁻³ , the width of the depletion layer becomes narrow, so that light is not sufficiently absorbed in the depletion layer region where an electric field exists, and sensitivity is lowered. Conversely, if the carrier density is less than 2 × 10 ¹⁶ cm ⁻³ , it becomes difficult to form a good ohmic electrode, and a Schottky electrode and an ohmic electrode are formed on both sides of the Ga ₂ O ₃ single crystal substrate. It becomes impossible to function as a sensor having a so-called vertical structure. For this reason, the carrier density of the Ga ₂ O ₃ single crystal is preferably 2 × 10 ¹⁶ to 2 × 10 ¹⁷ cm ⁻³ . Such control of the carrier density can be realized by controlling the oxygen concentration in the quartz tube, the growth rate, and the like during single crystal growth by the FZ method.

Thus the Ga ₂ O ₃ single crystal was produced by processing the wafer-shaped to produce a Ga ₂ O ₃ single crystal substrate 10.
In order to detect a current due to electrons and holes generated by photoexcitation, it is necessary to form a high resistance layer sandwiched between electrodes. This is because, in the low resistance layer, a very large number of carriers already exist even in the dark, so that it is difficult to detect an increase in current due to the carriers generated by photoexcitation.
This high resistance layer can be made by using a high resistance thin film, a Schottky contact, or a depletion layer by a pn junction, but the method using a depletion layer has an amplifying effect and is highly sensitive. More preferable. Furthermore, since only n-type is obtained in the case of Ga ₂ O ₃ for producing a depletion layer, it is preferable to use a depletion layer by Schottky contact instead of a pn junction. As a result, the structure of the sensor chip is a Metal-Semiconductor-Metal (MSM) type.

Among the MSM types, there are a horizontal structure and a vertical structure. In the case of a horizontal structure, it is necessary to form a comb-shaped electrode by using photolithography. The comb electrode is difficult to increase in area, and the depletion layer is formed only directly under the electrode, so that the utilization efficiency of Ga ₂ O ₃ is lowered.

On the other hand, in the vertical structure as shown in FIG. 1, the sensor portion has a light receiving surface 11r formed on the surface and a thin Schottky electrode (first electrode) 11 having translucency with respect to ultraviolet rays to be detected. In this case, a simple structure is formed simply by forming the ohmic electrode (second electrode) 12 on the back surface. Unlike the lateral structure, since light can be received by the entire surface of the depletion layer 10a extending under the Schottky electrode 11, the utilization efficiency of the Ga ₂ O ₃ single crystal is high, and it is not necessary to fabricate a comb electrode like the lateral structure. The feature is that the structure is simple and the process is simple.

Therefore, the ohmic electrode 12 is formed on the back surface of the Ga ₂ O ₃ single crystal substrate 10 in step S2. The ohmic electrode 12 employs an Al / Ti structure in which Ti is vapor-deposited on the back surface of the Ga ₂ O ₃ single crystal substrate 10 and then Al is vapor-deposited. The electrode size is the entire back surface. By forming the electrode on the entire back surface, the masking step is omitted, and the process becomes simple. The thickness of Ti is 15 to 25 nm, preferably 20 nm, and the thickness of Al is 90 to 110 nm, preferably 100 nm.

Thereafter, in step S3, heat treatment is performed in a nitrogen atmosphere at a temperature of 450 to 550 ° C. for 5 to 15 minutes, preferably at a temperature of 500 ° C. for 10 minutes. If the temperature is lower than 450 ° C., good ohmic characteristics cannot be obtained, so that the sensitivity characteristics are not sufficiently improved. Conversely, if the temperature exceeds 550 ° C., the sensitivity characteristics deteriorate. If the processing time is shorter than 5 minutes, the sensitivity characteristics are not sufficiently improved, and even if the processing time exceeds 15 minutes, the sensitivity characteristics do not increase any further.
By performing such heat treatment, ohmic contact can be obtained without performing plasma irradiation. For this reason, the manufacturing process is simplified and the manufacturing time is shortened.

Next, the Schottky electrode 11 is formed on the surface of the Ga ₂ O ₃ single crystal substrate 10 in step S4. Au or Pt is formed on the surface of the Ga ₂ O ₃ single crystal substrate 10 as the Schottky electrode 11. The film thickness of Au is 8 to 12 nm, preferably 10 nm. On the other hand, the film thickness of Pt is 3 to 8 nm, preferably 5 nm.

After depositing Au or Pt, heat treatment is performed in step S5. As a heat treatment condition, in the case of Au, heat treatment is performed in a nitrogen atmosphere at a temperature of 350 to 450 ° C. for 5 to 15 minutes, preferably at a temperature of 400 ° C. for 10 minutes. If the temperature is lower than 350 ° C., the sensitivity characteristics are not sufficiently improved. Conversely, if the temperature exceeds 450 ° C., the sensitivity characteristics are degraded. If the processing time is shorter than 5 minutes, the sensitivity characteristics are not sufficiently improved, and even if the processing time exceeds 15 minutes, the sensitivity characteristics do not increase any further.
On the other hand, when Pt is used as the Schottky electrode 11, the conditions for the atmosphere and the heat treatment time are the same as those for Au, but the heat treatment temperature is preferably 450 to 550 ° C. Since this heat treatment temperature is about the same as the heat treatment temperature of the ohmic electrode 12 described above, when the Schottky electrode 11 made of Pt is used, both the ohmic electrode 12 and the Schottky electrode 11 are made of Ga ₂ O ₃ single crystal. After forming on both surfaces of the substrate 10, the ohmic electrode 12 and the Schottky electrode 11 can be subjected to heat treatment at the same time. Thereby, simplification of a manufacturing process and shortening of manufacturing time can be achieved.

According to the present invention, a vertical structure ultraviolet sensor using a bulk Ga ₂ O ₃ single crystal, after forming a Schottky electrode on the surface of the Ga ₂ O ₃ single crystal substrate and an ohmic electrode on the back surface, respectively, heat treatment conditions By optimizing the spectral sensitivity, the spectral sensitivity can be increased. In addition, sensitivity can be adjusted by controlling the carrier density of the bulk Ga ₂ O ₃ single crystal. These effects according to the present invention are considered to be effective as a simple and low-cost solar blind sensor that monitors ultraviolet light having a wavelength of 254 nm, which is effective for sterilization and sterilization.

(Example 1)
Gallium oxide powder (purity 4N) was sealed in a rubber tube and subjected to isostatic pressing, and sintered in the atmosphere at 1500 ° C. for 10 hours. Single crystals were grown using this sintered body as a raw material rod using an optical FZ apparatus. The growth rate was 7.5 mm / hr, and oxygen 80% -nitrogen 20% (flow rate ratio) was used as the atmosphere gas. Single crystals were further produced using the grown single crystals as raw materials.
The (100) plane of the single crystal thus obtained was cut out and polished with CMP to a thickness of 0.4 mm, and the surface was a mirror surface with an average roughness of ~ 0.2 nm to obtain a wafer-like substrate. The substrate size is approximately 7 mm × 7 mm.

As a result of Hall measurement of the Ga ₂ O ₃ single crystal, the resistivity was 6.45 × 10 ⁻¹ Ωcm, the carrier density was 1.19 × 10 ¹⁷ cm ⁻³ , and the mobility was 81.3 cm ² / Vs. As a pretreatment, this substrate was washed with acetone, ethanol, hydrofluoric acid, and pure water.

Next, electrodes were prepared on both sides of the Ga ₂ O ₃ single crystal substrate. First, as an ohmic electrode, Ti was deposited to a thickness of 20 nm on the entire back surface of the Ga ₂ O ₃ single crystal substrate, and then Al was deposited to a thickness of 100 nm. This was heat-treated in a nitrogen atmosphere at a temperature of 500 ° C. for 10 minutes. Furthermore, 10 nm of Au was formed on the surface of the Ga ₂ O ₃ single crystal substrate as a Schottky electrode. The size of the Schottky electrode is 1 mmφ. This was heat-treated at a temperature of 400 ° C. for 10 minutes in a nitrogen atmosphere. In this way, an ultraviolet sensor was manufactured.

  When the spectral sensitivity characteristic was measured for this ultraviolet sensor, the result shown in FIG. 3 was obtained. FIG. 3 also shows, as a comparative example, measurement results for an untreated sensor that does not perform heat treatment on the Schottky electrode. It can be seen that the sensitivity is improved by heat-treating the Au Schottky electrode at a temperature of 400 ° C. In particular, the difference in sensitivity in the vicinity of a wavelength of 250 nm reached three digits or more, and it was confirmed that the heat treatment after electrode formation is effective in improving the sensitivity of the ultraviolet sensor.

(Example 2)
A Ga ₂ O ₃ single crystal was produced in the same manner as in Example 1. In this case, a sintered body was used as a raw material, and oxygen 80% -nitrogen 20% (flow rate ratio) was used as an atmosphere gas. The (100) plane of the obtained single crystal was mirror-finished into a wafer and Hall measurement was performed. As a result, the resistivity was 7.6 × 10 ⁻² Ωcm, the carrier density was 8.3 × 10 ¹⁷ cm ⁻³ , and the mobility was 98.7 cm ² The value of / Vs was obtained. The value of the carrier density is about 7 times larger than that in the first embodiment.

Subsequently, electrodes were prepared on both surfaces of the single crystal substrate. As in Example 1, first, Ti was deposited to a thickness of 20 nm on the entire back surface of the Ga ₂ O ₃ single crystal substrate as an ohmic electrode, and then Al was deposited to a thickness of 100 nm. This was heat-treated in a nitrogen atmosphere at a temperature of 500 ° C. for 10 minutes. Next, 10 nm of Au was formed on the surface of the Ga ₂ O ₃ single crystal substrate as a Schottky electrode. The electrode size is 1 mmφ. This was heated at 100 ° C, 200 ° C, 300 ° C, 400 ° C, 500 ° C and 100 ° C in a nitrogen atmosphere, heat-treated for 10 minutes each, and current-voltage characteristics and spectral sensitivity characteristics were measured. However, the results shown in FIGS. 4 and 5 were obtained. 4 and 5 also show a measurement result for an untreated sensor that does not perform heat treatment on the Schottky electrode as a comparative example.

As shown in FIG. 4, the rectification ratio was 10 ⁶ to 10 ⁷ for any heat treatment temperature, and it was confirmed that good rectification property was exhibited even after the heat treatment.
Furthermore, as shown in FIG. 5, it can be seen that the spectral sensitivity characteristic has a high sensitivity in the vicinity of a wavelength of 200 to 250 nm and functions as a sunlight blind ultraviolet sensor. In addition, when heat treatment is performed at a temperature of 400 ° C., the result that sensitivity is greatly increased is obtained. However, it can be seen that the sensitivity near the wavelength of 250 nm is inferior to the result of the heat treatment temperature of 400 ° C. in Example 1, which is considered to be due to the difference in carrier density of the Ga ₂ O ₃ single crystal substrate. This suggests that the lower the carrier density, the better the sensitivity. That is, it is considered that the sensitivity was improved because the depletion layer width increased due to the decrease in carrier density, and light was sufficiently absorbed in the depletion layer region where an electric field was present.
FIG. 6 shows the bias voltage dependence of the photosensitivity in the ultraviolet sensor of Example 2 in which the heat treatment at a temperature of 400 ° C. was performed on the Schottky electrode. The result that sensitivity is increased by increasing the bias voltage is obtained.

(Example 3)
In Examples 1 and 2, a Schottky electrode made of Au was used, but in Example 3, Pt was used as the material of the Schottky electrode.
As a substrate, the Ga ₂ O ₃ single crystal substrate having a carrier density of 8.3 × 10 ¹⁷ cm ⁻³ used in Example 2 is adopted, and Ti is formed on the entire back surface of the Ga ₂ O ₃ single crystal substrate as an ohmic electrode with a thickness of 20 nm. After deposition, Al was subsequently deposited to 100 nm. This was heat-treated in a nitrogen atmosphere at a temperature of 500 ° C. for 10 minutes. Next, 5 nm of Pt was formed on the surface of the Ga ₂ O ₃ single crystal substrate as a Schottky electrode. The electrode size is 1 mmφ. In the same manner as in Example 2, the temperature was raised every 100 ° C., 100 ° C., 200 ° C., 300 ° C., 400 ° C., 500 ° C. in a nitrogen atmosphere, and heat treatment was performed for 10 minutes. As a result of the measurement, a result as shown in FIG. 7 was obtained. FIG. 7 also shows, as a comparative example, measurement results for an untreated sensor that does not perform heat treatment on the Schottky electrode.

  As can be seen from FIG. 7, when the heat treatment temperature is 500 ° C., the sensitivity increases most. As shown in FIG. 5, the sensitivity of the Schottky electrode made of Au was the highest when heat treatment was performed at a temperature of 400 ° C., suggesting that there is an optimum condition for the heat treatment temperature depending on the electrode material. is doing.

It is a figure which shows the cross-section of the ultraviolet sensor manufactured by the manufacturing method which concerns on embodiment. It is a flowchart which shows the manufacturing method of the ultraviolet sensor which concerns on embodiment. 6 is a graph showing measurement results of spectral sensitivity characteristics for the ultraviolet sensor of Example 1. It is a graph which shows the measurement result of the current-voltage characteristic with respect to the ultraviolet sensor of Example 2. It is a graph which shows the measurement result of the spectral sensitivity characteristic with respect to the ultraviolet sensor of Example 2. It is a graph which shows the bias voltage dependence of the photosensitivity in the ultraviolet sensor of Example 2. It is a graph which shows the measurement result of the spectral sensitivity characteristic with respect to the ultraviolet sensor of Example 3.

Explanation of symbols

10 Ga ₂ O ₃ single crystal substrate, 10a depletion layer, 10b conductive layer, 11 Schottky electrode, 11r light-receiving surface, 12 ohmic electrode.

Claims (8)

  Forming a first electrode in Schottky contact with the gallium oxide single crystal substrate on a surface of the gallium oxide single crystal substrate; forming an ohmic contact with the gallium oxide single crystal substrate on a back surface of the gallium oxide single crystal substrate; An ultraviolet sensor comprising: forming a second electrode through which a current flows between the first electrode and the first electrode through the gallium oxide single crystal substrate; and heat-treating the first electrode and the second electrode. Production method.   The second electrode has an Al / Ti or Au / Ti structure in which a Ti layer is formed as an underlayer between Al or Au and the gallium oxide single crystal substrate. The method for producing an ultraviolet sensor according to claim 1, wherein heat treatment is performed at a temperature of 450 to 550 ° C for 5 to 15 minutes in a nitrogen atmosphere.   3. The method of manufacturing an ultraviolet sensor according to claim 2, wherein the first electrode is made of Au, and the first electrode is heat-treated at a temperature of 350 to 450 ° C. for 5 to 15 minutes in a nitrogen atmosphere.   3. The method of manufacturing an ultraviolet sensor according to claim 2, wherein the first electrode is made of Pt, and the first electrode is heat-treated at a temperature of 450 to 550 ° C. for 5 to 15 minutes in a nitrogen atmosphere.   5. The ultraviolet sensor according to claim 3, wherein after forming the second electrode and heat-treating the second electrode, forming the first electrode and heat-treating the first electrode. 6. Method.   The method for manufacturing an ultraviolet sensor according to claim 4, wherein after forming the first electrode and the second electrode, the first electrode and the second electrode are heat-treated simultaneously. 7. The gallium oxide single crystal substrate has electrical conductivity with a carrier density of 2 × 10 ¹⁶ to 2 × 10 ¹⁷ cm ⁻³ , and uses the (100) plane. The manufacturing method of the described ultraviolet sensor.   The method of manufacturing an ultraviolet sensor according to claim 1, wherein the second electrode is formed on the entire back surface of the gallium oxide single crystal substrate.

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