Nitride semiconductor device and method of manufacturing the same

Abstract

To provide a nitride semiconductor device having a high degree of freedom in material selection of a metal electrode capable of reducing contact resistance to n-type gallium nitride.SOLUTION: The nitride semiconductor device 1 includes a GaN substrate 10 having n-type conductivity, a metal electrode 14, and a Ga layer 12 provided in contact with each of a first main surface 10a of the GaN substrate 10 and the metal electrode 14, a carrier concentration of the GaN substrate 10 is 3 * 10 18 cm -3 or more, and the Ga layer 12 is formed by modifying a part of the GaN substrate.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to nitride semiconductor devices and methods for fabricating the same. [Background technology]

[0002] Group III nitride semiconductors are used in fields such as optical devices such as semiconductor lasers and light-emitting diodes, as well as high-frequency or high-power electronic devices. Group III nitride semiconductors have attracted particular attention in recent years because they are expected to reduce switching losses during power conversion compared to silicon-based devices. In order to fabricate high-frequency or high-power electronic devices, it is necessary to fabricate the devices on high-quality Group III nitride substrates that can suppress the occurrence of crystal defects in the device layer.

[0003] Methods for producing Group III nitride crystals include, for example, hydride vapor phase epitaxy (hereinafter also referred to as HVPE), ammonothermal, sodium flux, and oxide vapor phase epitaxy.

[0004] In the HVPE method, which is the most commonly used method for manufacturing substrates, hydrogen halide gas is introduced onto a single Group III source to generate a halide gas, which is then used as a source gas for crystal growth. For example, when growing gallium nitride crystals, a gallium chloride (e.g., GaCl)-containing gas is used as the Group III source and an ammonia-containing gas is used as the Group V source, allowing for high-speed growth of over 1 mm / h. It is known that the HVPE method can produce Group III nitride crystals with n-type conductivity by primarily adding silicon to the Group III nitride crystals.

[0005] The Group III nitride crystals produced by the above-described methods can be processed to obtain Group III nitride substrates, and devices can be fabricated by forming epitaxial films of Group III nitride semiconductors on the Group III nitride substrates.

[0006] Furthermore, after fabricating the device, it is necessary to form the electrodes necessary for the device to operate. At the interface between the semiconductor and the metal electrode, if the material is not selected appropriately, a Schottky barrier will occur, resulting in high contact resistance. Therefore, it is necessary to achieve ohmic contact by combining appropriate materials. For example, Patent Document 1 and Non-Patent Document 1 disclose that titanium (Ti) and aluminum (Al) are used as metal materials that can achieve ohmic contact with n-type gallium nitride (GaN). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 7-45867 [Non-patent literature]

[0008] [Non-Patent Document 1] BP Luther et al., “Investigation of the mechanism for Ohmic contact formation in Al and Ti / Al contacts to n-type GaN”, Appl. Phys. Lett., Jan. 1997, Vol. 70, No. 1, pp. 57-59 Summary of the Invention [Problem to be solved by the invention]

[0009] Conventionally, there has been a problem in that there is little freedom in selecting metal materials that can reduce contact resistance with n-type gallium nitride.

[0010] An object of the present disclosure is to provide a nitride semiconductor device that can reduce the contact resistance between n-type gallium nitride and a metal electrode and that allows for a high degree of freedom in the selection of materials that can be used for the metal electrode, and a manufacturing method that can easily fabricate the nitride semiconductor device. [Means for solving the problem]

[0011] A nitride semiconductor device according to one aspect of the present disclosure comprises a gallium nitride substrate having n-type conductivity, a metal electrode, and a gallium layer provided between and in contact with the gallium nitride substrate and the metal electrode.

[0012] A method for manufacturing a nitride semiconductor device according to one embodiment of the present disclosure includes the steps of forming a gallium layer by irradiating a portion of a gallium nitride substrate having n-type conductivity with laser light, and forming a metal electrode in contact with the gallium layer. [Effects of the Invention]

[0013] According to the present disclosure, it is possible to obtain a nitride semiconductor device that can reduce the contact resistance between n-type gallium nitride and a metal electrode and that allows a high degree of freedom in selecting materials that can be used for the metal electrode, as well as a manufacturing method that can easily fabricate the nitride semiconductor device. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a cross-sectional view schematically showing an example of a nitride semiconductor device according to an embodiment. [Figure 2] FIG. 2 is a flowchart showing an example of a method for manufacturing a nitride semiconductor device according to the embodiment. [Figure 3] FIG. 3 is a graph showing the relationship between the carrier concentration in gallium nitride and the Fermi energy when the peak of the valence band is used as the reference. DETAILED DESCRIPTION OF THE INVENTION

[0015] (Summary of the Disclosure) A nitride semiconductor device according to a first aspect of the present disclosure comprises a gallium nitride substrate having n-type conductivity, a metal electrode, and a gallium layer provided between and in contact with the gallium nitride substrate and the metal electrode.

[0016] As a result, the work function of gallium (Ga) is close to the electron affinity of gallium nitride (GaN), so a Schottky barrier is unlikely to form near the interface between the Ga layer and the n-type GaN substrate. Therefore, a good ohmic contact between the metal electrode and the GaN substrate is achieved via the Ga layer, thereby reducing contact resistance. Since the Ga layer is provided between the metal electrode and the GaN substrate, the work function constraints on the metal material that can be used as the metal electrode are relaxed, increasing the freedom of material selection. Therefore, according to this embodiment, a nitride semiconductor device can be realized that can reduce the contact resistance between the n-type gallium nitride and the metal electrode and also allows for a high degree of freedom in the selection of materials that can be used as the metal electrode.

[0017] However, forming an electrode made of a metal such as Al or Ti, whose work function is close to the electron affinity of GaN, on the semiconductor surface does not immediately result in low contact resistance. This is because electrons are easily trapped in levels caused by microscopic defects on the surface of the GaN substrate. Electrons trapped in defect levels during operation increase electrical resistance. Conventionally, thermal annealing has been widely used to suppress the occurrence of electron traps. Thus, when forming electrodes, it is necessary to form electrodes that will not deteriorate even after the thermal annealing process, and this also places restrictions on the materials used. Furthermore, the need for a thermal annealing process is one of the factors that complicate the manufacturing process.

[0018] In contrast, the nitride semiconductor device according to the second aspect of the present disclosure is the nitride semiconductor device according to the first aspect, wherein the carrier concentration of the gallium nitride substrate is 3×10 18 cm -3 That's all.

[0019] This allows defect levels that may occur near the interface between the Ga layer and the GaN substrate to be easily filled with electrons contained in the GaN substrate. This prevents electrons from being trapped in defect levels during operation, which would increase electrical resistance. Since a thermal annealing step is no longer necessary, melting of the Ga layer can be avoided, stabilizing the shape of the Ga layer. This allows for a good ohmic contact between the metal electrode and the GaN substrate, further reducing contact resistance.

[0020] A nitride semiconductor device according to a third aspect of the present disclosure is the nitride semiconductor device according to the first or second aspect, wherein the gallium layer is formed by modifying a part of the gallium nitride substrate.

[0021] This makes it possible to fabricate nitride semiconductor devices with low contact resistance between the metal electrode and the GaN substrate using a simple manufacturing method.

[0022] A nitride semiconductor device according to a fourth aspect of the present disclosure is the nitride semiconductor device according to any one of the first to third aspects, wherein the metal electrode contains at least one of Cu and Ag as a main component.

[0023] Since Cu and Ag have higher thermal conductivity than Ti and Al, a nitride semiconductor device with high heat dissipation properties can be realized, and therefore the nitride semiconductor device according to this embodiment is suitable for use as a device that handles high power, such as a power device for power conversion.

[0024] A method for manufacturing a nitride semiconductor device according to a fifth aspect of the present disclosure includes the steps of forming a gallium layer by irradiating a portion of a gallium nitride substrate having n-type conductivity with laser light, and forming a metal electrode in contact with the gallium layer.

[0025] This allows for a simple manufacturing method for nitride semiconductor devices that can reduce the contact resistance between n-type gallium nitride and a metal electrode and allows for a high degree of freedom in the selection of materials that can be used for the metal electrode, thereby reducing the cost of nitride semiconductor devices.

[0026] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. However, the present disclosure is not limited in any way by the use of numerical values ​​or specific elements used in the following description.

[0027] The embodiments described below are all comprehensive or specific examples. The numerical values, shapes, materials, components, component placement and connection configurations, steps, and step order shown in the following embodiments are merely examples and are not intended to limit the present disclosure. Furthermore, among the components in the following embodiments, components not described in the independent claims are described as optional components.

[0028] Furthermore, each figure is a schematic diagram and is not necessarily an exact illustration. Therefore, for example, the scales of the figures do not necessarily match. Furthermore, in each figure, substantially the same components are given the same reference numerals, and redundant explanations are omitted or simplified.

[0029] Furthermore, in this specification, terms indicating the positional relationship between elements, such as "parallel," terms indicating the shape of elements, such as "rectangle," and numerical ranges are not expressions that only express a strict meaning, but are expressions that also include a substantially equivalent range, for example, a difference of about a few percent.

[0030] In this specification, the "thickness direction" of a substrate refers to the direction perpendicular to the main surface of the substrate. The thickness direction is the same as the stacking direction of the semiconductor layers, and is also referred to as the "vertical direction." The direction parallel to the main surface of the substrate may be referred to as the "lateral direction." A "vertical" semiconductor device refers to a device in which the main path of current, such as drain current or forward current, is vertical, i.e., a device in which the main current passes vertically through the substrate. A "lateral" semiconductor device refers to a device in which the main path of current, such as drain current or forward current, is horizontal, i.e., a device in which the main current does not pass through the substrate.

[0031] Furthermore, in this specification, the terms "above" and "below" do not refer to the upward direction (vertically upward) and downward direction (vertically downward) in absolute spatial recognition, but are used as terms defined by a relative positional relationship based on the stacking order in a stacked configuration. Furthermore, the terms "above" and "below" are applied not only to a case where two components are arranged with a gap between them and another component exists between them, but also to a case where two components are arranged closely together and the two components are in contact with each other.

[0032] In addition, in this specification, "A contains B as a main component" means that the content of B (for example, mass ratio or atomic ratio) is the largest when compared with all elements constituting A.

[0033] Furthermore, in this specification, ordinal numbers such as "first" and "second" do not refer to the number or order of components unless otherwise specified, but are used for the purpose of avoiding confusion and distinguishing between components of the same type.

[0034] (Embodiment) [composition] First, the configuration of a nitride semiconductor device according to an embodiment of the present disclosure will be described with reference to FIG.

[0035] Fig. 1 is a cross-sectional view schematically showing an example of a nitride semiconductor device 1 according to the present embodiment. As shown in Fig. 1, the nitride semiconductor device 1 includes a GaN substrate 10, a Ga layer 12, and a metal electrode 14. In the following, an example will be described in which the nitride semiconductor device 1 is a vertical diode including the metal electrode 14 as a cathode electrode. As shown in Fig. 1, the nitride semiconductor device 1, which is a vertical diode, includes a p-type semiconductor layer 16 and an anode electrode 18.

[0036] The GaN substrate 10 is a gallium nitride substrate having n-type conductivity. Specifically, the GaN substrate 10 is a substrate made of GaN containing n-type impurities. The n-type impurities are, for example, Si, but may also be Ge. The thickness of the GaN substrate 10 is, for example, 300 μm, but is not particularly limited thereto. The planar shape of the GaN substrate 10 is, for example, rectangular, but is not limited thereto.

[0037] The carrier concentration of the GaN substrate 10 is 3×10 18 cm -3 This completes the process. This reduces the defect level at the interface of the GaN substrate 10, thereby reducing electron traps and the resistance value. Details will be described later. The upper limit of the carrier concentration of the GaN substrate 10 is not particularly limited, but may be, for example, 4.4×10 22 cm -3 is.

[0038] GaN substrate 10 has first and second major surfaces 10a and 10b. Second major surface 10b is the major surface opposite to first major surface 10a. Note that the term "major surface" refers to the surface with the largest area among multiple surfaces of a flat-shaped component such as a substrate or semiconductor layer, or the surface located opposite to that surface and having a size equivalent to that of the surface.

[0039] The Ga layer 12 is a gallium layer provided between the GaN substrate 10 and the metal electrode 14 in contact with each other. Specifically, the Ga layer 12 is provided in contact with the first major surface 10a. The shape and size of the Ga layer 12 in a planar view are the same as the shape and size of the GaN substrate 10 in a planar view. The Ga layer 12 is formed by modifying a part of the GaN substrate 10. A specific modification method will be described later. The thickness of the Ga layer 12 is, for example, not less than 10 nm and not more than 1 μm, but may be not less than several tens of nm and not more than several hundreds of nm.

[0040] The metal electrode 14 is an electrode containing a metal as a main component. Specifically, the metal electrode 14 contains substantially only a metal, but may contain unavoidable impurities that are unavoidable during manufacturing. For example, the metal electrode 14 contains at least one of Cu and Ag as a main component. The metal electrode 14 may be a single-layer film made of one type of metal, a single-layer film made of an alloy of two or more types of metals, or a stacked structure of metal films having different compositions. The metal electrode 14 is ohmically connected to the GaN substrate 10 via the Ga layer 12. In this embodiment, the metal electrode 14 functions as a cathode electrode of the diode.

[0041] The p-type semiconductor layer 16 is a nitride semiconductor layer having p-type conductivity. Specifically, the p-type semiconductor layer 16 is a layer made of GaN containing p-type impurities. The p-type impurities are, for example, Mg, but may also be Zn. The thickness of the p-type semiconductor layer 16 is not particularly limited, but is, for example, not less than several tens of nanometers and not more than several hundreds of nanometers.

[0042] The p-type semiconductor layer 16 is provided in contact with the second main surface 10b of the GaN substrate 10, and forms a pn junction with the GaN substrate 10. An undoped GaN layer may be provided between the p-type semiconductor layer 16 and the GaN substrate 10.

[0043] The anode electrode 18 is an electrode containing a metal as a main component. Specifically, the anode electrode 18 contains substantially only a metal, but may contain unavoidable impurities that are unavoidable during manufacturing. For example, the anode electrode 18 is formed using a metal material that forms ohmic contact with p-type GaN. The anode electrode 18 contains at least one of Pd, Ni, W, and Au as a main component. The anode electrode 18 may be a single-layer film made of one type of metal, a single-layer film made of an alloy of two or more types of metal, or a laminate structure of metal films having different compositions.

[0044] The nitride semiconductor device 1 configured as described above functions as a vertical diode and performs a rectifying operation. For example, when a forward bias voltage higher than the threshold voltage is applied between the metal electrode 14, which is the cathode electrode, and the anode electrode 18, a current can flow from the anode electrode 18 to the metal electrode 14.

[0045] [Manufacturing method] Next, a method for manufacturing the nitride semiconductor device 1 according to this embodiment will be described with reference to Fig. 2. Fig. 2 is a flowchart showing an example of the method for manufacturing the nitride semiconductor device 1 according to this embodiment.

[0046] 2, first, a GaN substrate 10 having n-type conductivity is prepared (S10). Note that the method for manufacturing the nitride semiconductor device 1 may include a step of thinning the substrate by slicing or polishing, and therefore the prepared GaN substrate 10 may be thicker than the GaN substrate 10 included in the nitride semiconductor device 1, which is the final product.

[0047] Next, the main structure of the device is formed on the prepared GaN substrate 10 (S12). In the case of a pn diode, the main structure of the device includes a structure forming a pn junction. In this embodiment, as the main structure of the device, a p-type semiconductor layer 16 and an anode electrode 18 are formed in this order on the second main surface 10b of the GaN substrate 10. The p-type semiconductor layer 16 is formed, for example, by epitaxial growth of GaN. The anode electrode 18 is formed, for example, by forming a metal film by sputtering or EB (Electron Beam) deposition, and then patterning it by etching, lift-off, or the like as necessary.

[0048] Next, a part of the GaN substrate 10 is irradiated with laser light to form a Ga layer 12 (S14). The laser light is irradiated, for example, from the first main surface 10a side of the GaN substrate 10. The laser light is irradiated onto the surface of the GaN substrate 10 to remove N atoms, thereby forming a Ga layer 12 of a predetermined thickness. Alternatively, the GaN substrate 10 may be sliced ​​(laser sliced) by irradiating the interior of the GaN substrate 10 with laser light.

[0049] Next, a metal electrode 14 is formed in contact with the Ga layer 12 (S16). Specifically, the metal electrode 14 is bonded to the Ga layer 12. At least one of the GaN substrate 10 and the metal electrode 14 may be heated during bonding.

[0050] Through the above steps, the nitride semiconductor device 1 according to this embodiment can be fabricated.

[0051] [Major characteristic configuration and effects] Next, the main characteristic configuration and effects of the nitride semiconductor device 1 will be described.

[0052] To solve the above-mentioned problems, it is necessary to bring an appropriate metal material into contact with a group III nitride semiconductor. Specifically, it is required to form an ohmic contact at the junction interface between the group III nitride semiconductor and the metal without forming a Schottky barrier. For this purpose, it is generally considered preferable that the work function of the metal be equal to or less than the electron affinity of the group III nitride semiconductor.

[0053] The electron affinity of gallium nitride, which has n-type conductivity, is known to be about 4.2 eV. Therefore, by using a metal with a work function lower than 4.2 eV as the material for the metal electrode, a good ohmic contact can be achieved. For example, since the work function of Al is 4.125 eV, Patent Document 1 discloses the use of Al as the electrode material.

[0054] Incidentally, the work function of metallic gallium (Ga) is 4.2 eV. Therefore, it is expected that Ga can be used to obtain the same effect as Al or Ti. However, Ga has a low melting point of approximately 30°C. For this reason, Ga has not been used as an electrode material in the past because it cannot withstand the thermal annealing process and its shape is unstable when Ga becomes liquid. On the other hand, Ga has the advantage that it can be easily formed by decomposing gallium nitride. Therefore, the present inventors came up with the idea that if the thermal annealing process could be eliminated, Ga could be used as a metal to be in direct contact with gallium nitride.

[0055] Furthermore, Ga easily melts and forms alloys with other metals. Therefore, even when a metal other than Ga is used as an electrode, it is possible to easily bond Ga and the other metal while forming an alloy at the interface between them. This increases the freedom of selection of metal materials that can be used for the metal electrode 14. For example, metals such as copper (Cu) or silver (Ag) have a large work function, so it is not preferable to bring them into direct contact with gallium nitride. However, by bonding to gallium nitride via Ga, Cu or Ag can be used for the metal electrode 14. Cu or Ag has high thermal conductivity and excellent heat dissipation properties, making it possible to promote heat dissipation during operation of the nitride semiconductor device 1.

[0056] When the Ga layer 12 is provided between the GaN substrate 10 and the metal electrode 14, a thermal annealing step is not performed after the Ga layer 12 is formed. For this purpose, a GaN substrate having a structure for reducing the influence of defect levels at the interface of the GaN substrate 10 may be used. Specifically, the carrier concentration of the GaN substrate 10 having n-type conductivity is set to 3×10 18 cm -3 This is the end of the explanation.

[0057] Figure 3 is a graph showing the relationship between the carrier concentration in gallium nitride and the Fermi energy when the peak of the valence band is used as a reference. Specifically, Figure 3 shows the relationship between the carrier concentration in the saturated region of the temperature range (specifically, 300 K) in which the nitride semiconductor device 1 is used and the Fermi energy when the peak of the valence band is used as a reference. The band gap energy of GaN is approximately 3.39 eV to 3.42 eV. As shown in Figure 3, when the carrier concentration is 3 x 10 18 cm -3 If this is the case, the Fermi energy becomes larger than 3.42 eV, and gallium nitride has electrons in the conduction band, which makes it possible to fill the defect levels at the interface and sufficiently reduce the effect of the defect levels at the interface trapping electrons during operation of the nitride semiconductor device 1.

[0058] In this embodiment, the Ga layer 12 is formed by modifying a portion of the GaN substrate 10. Specifically, the modification is performed by laser processing. This simplifies the manufacturing process. By applying high energy to GaN, it is possible to separate the GaN into gallium and nitrogen gas. This is because GaN can be easily separated into gallium and nitrogen gas by the energy of the laser light or by heat generation in the surrounding area irradiated with the laser light.

[0059] When Ga layer 12 is formed by partially modifying GaN substrate 10, the surface roughness of first main surface 10a of GaN substrate 10 is large when Ga layer 12 is removed. For example, the surface roughness Ra of first main surface 10a is larger than the surface roughness Ra of second main surface 10b. This is presumably because the strong bonds between Ga atoms and N atoms are broken, causing the N atoms to be desorbed.

[0060] Furthermore, the above-described laser processing of the GaN substrate 10 may be performed on the interior of the GaN substrate 10 rather than on the surface of the GaN substrate 10. In this case, the nitride semiconductor device composed of the anode electrode 18, the p-type semiconductor layer 16, and the GaN substrate 10, and the remainder of the GaN substrate 10 can be separated by laser slicing. The remainder of the GaN substrate 10 can be used as a GaN substrate for fabricating another semiconductor device. The resulting nitride semiconductor device has a Ga layer 12 at the location irradiated with the laser light, and therefore can be bonded with a metal electrode 14 formed using a metal with high thermal conductivity, such as Cu.

[0061] Laser processing is performed by irradiating the GaN substrate with laser light whose energy is lower than the bandgap energy of GaN. For example, a YAG laser with a wavelength of 532 nm can be used. Laser processing is performed by shifting the focus in the thickness direction of the GaN substrate 10. This allows N atoms to be removed within a certain thickness range, forming the Ga layer 12.

[0062] Plasma may be used to modify the GaN substrate 10. For example, by subjecting the surface of the GaN substrate 10 to plasma treatment in an inert gas atmosphere such as nitrogen gas or argon gas, N atoms can be removed to form the Ga layer 12. Alternatively, the Ga layer 12 may be formed by processing the surface of the GaN substrate 10.

[0063] To bond the metal electrode 14 and the Ga layer 12, Ga may be liquefied and then bonded to the metal. To achieve this, the Ga layer 12 must be heated to a melting point. The melting point of Ga is approximately 30°C, but to maintain a sufficient liquid state, the Ga layer 12 is heated to, for example, 40°C or higher. Depending on the heating method, it is convenient to heat the nitride semiconductor device 1 while the Ga layer 12 is exposed to the atmosphere. Furthermore, the metal electrode 14 to be bonded may also be heated to a similar temperature before bonding to prevent Ga from rapidly solidifying. This allows the entire bonding surface between the Ga layer 12 and the metal electrode 14 to be uniformly bonded, enabling stable thermal and electrical conduction.

[0064] External pressure may be applied when bonding the metal electrode 14 and the Ga layer 12. By keeping Ga in a sufficiently liquid state, the surface tension causes Ga to fill the gap between the GaN substrate 10 and the metal electrode 14. This allows good thermal and electrical conduction to be maintained.

[0065] A specific example will be described below.

[0066] [Example 1] In this example, the group III nitride semiconductor substrate is a semiconductor substrate having n-type conductivity and a carrier concentration of 5×10 18 cm -3 The gallium nitride substrate was used to form a group III nitride semiconductor film for a vertical diode using metalorganic vapor phase epitaxy (MOVPE), and then laser slicing was performed inside the gallium nitride substrate. After that, a metal electrode made of copper was bonded to the Ga layer formed on the gallium nitride substrate.

[0067] To bond the metal electrodes to the gallium nitride substrate, the gallium nitride substrate was first heated to 60°C and held there for 30 minutes to ensure a uniform temperature across the entire gallium nitride substrate. Next, a 1mm thick copper plate, preheated to 60°C, was placed in close contact with the Ga layer under a pressure of 49.9kPa, and the temperature was lowered to room temperature over 30 minutes for bonding.

[0068] [Example 2] In this example, the group III nitride semiconductor substrate is a semiconductor substrate having n-type conductivity and a carrier concentration of 5×10 18 cm -3 The gallium nitride substrate was used to form a group III nitride semiconductor film for a vertical diode using metalorganic vapor phase epitaxy (MOVPE), and then the interior of the gallium nitride substrate was laser sliced.A metal electrode made of silver was then bonded to the Ga layer formed on the gallium nitride substrate.

[0069] The metal electrodes to be bonded were silver plates with a thickness of 1 mm, and were bonded in the same manner as in Example 1.

[0070] Table 1 below shows the resistivity of the contact resistance component of the metal electrodes fabricated in Examples 1 and 2, as well as the resistivity described in Non-Patent Document 1. The two values ​​shown in Non-Patent Document 1 represent the results of forming an electrode with a two-layer structure of Al and Ti on a gallium nitride layer having n-type conductivity formed on a sapphire substrate, followed by a thermal annealing process at 600°C for 15 seconds or at 400°C for 5 minutes.

[0071] The resistivity of the contact resistance component in Examples 1 and 2 is the same as that in Non-Patent Document 1, and is 10 -4 Ωcm 2 It was confirmed that the resistivity was below 0.05, which did not increase power consumption during device operation.

[0072] [Table 1]

[0073] [Comparative Example] In the comparative example, a group III nitride semiconductor substrate having n-type conductivity and a carrier concentration of 1×10 18 cm -3 The gallium nitride substrate was used to form a group III nitride semiconductor film for a vertical diode by metalorganic vapor phase epitaxy (MOVPE), and then the interior of the gallium nitride substrate was laser sliced. An electrode made of copper was then bonded to the Ga layer formed on the gallium nitride substrate. The metal electrode to be bonded was a copper plate with a thickness of 1 mm, and was bonded in the same manner as in Example 1.

[0074] The resistivity of the contact resistance component in the comparative example is 2.3 × 10 -2 Ωcm 2 This indicates that a high carrier concentration in the substrate is necessary to achieve low resistivity. As shown above, it has become clear that the nitride semiconductor device according to the present disclosure has a sufficiently low contact resistance even without undergoing a thermal annealing process.

[0075] (Other embodiments) While nitride semiconductor devices and manufacturing methods thereof according to one or more aspects have been described based on the embodiments, the present disclosure is not limited to these embodiments. As long as they do not deviate from the gist of the present disclosure, various modifications conceivable by those skilled in the art to the present embodiments and configurations constructed by combining components of different embodiments are also included within the scope of the present disclosure.

[0076] For example, in the above embodiment, the nitride semiconductor device 1 is a vertical pn junction diode, but this is not limiting. For example, the nitride semiconductor device 1 may be a vertical Schottky diode. In the case of a Schottky diode, the p-type semiconductor layer 16 is not provided, and the anode electrode 18 is provided so as to be directly or indirectly Schottky-connected to the n-type GaN substrate 10. The anode electrode 18 Schottky-connected to the n-type GaN substrate 10 is the main structure formed in step S12 of FIG. 2 .

[0077] The nitride semiconductor device 1 may be a vertical transistor. In the case of a vertical transistor, a gate electrode, a source electrode, and a channel layer are provided on the second main surface 10b side of the GaN substrate 10, instead of the anode electrode 18 and the p-type semiconductor layer 16. The gate electrode, the source electrode, the channel layer, and the like are the main structure formed in step S12 of FIG. 2. The channel layer may be a layer containing two-dimensional electron gas due to a heterostructure of GaN and AlGaN. The nitride semiconductor device 1 may also be a pn diode including a layer containing two-dimensional electron gas as the n-type layer of a pn junction.

[0078] The nitride semiconductor device 1 may also be a lateral device such as a lateral diode or a lateral transistor. In the case of a lateral device, the metal electrode 14 is used, for example, as a ground electrode for fixing the potential of the GaN substrate 10 to ground potential (0 V), but is not limited thereto. In the case of a lateral transistor, a gate electrode, a source electrode, a drain electrode, and a channel layer are provided on the second main surface 10b of the GaN substrate 10. The channel layer may be a layer containing two-dimensional electron gas due to a heterostructure of GaN and AlGaN. In the case of a lateral diode, an anode electrode, a cathode electrode, a p-type semiconductor layer, and an n-type semiconductor layer are provided on the second main surface 10b of the GaN substrate 10. Instead of the n-type semiconductor layer, a layer containing two-dimensional electron gas due to a heterostructure of GaN and AlGaN may be provided.

[0079] The nitride semiconductor device 1 may also be a light-emitting device such as an LED (Light Emitting Diode) or a laser element.

[0080] The carrier concentration of the GaN substrate 10 is 3×10 18 cm -3 As described above, the carrier concentration of the GaN substrate 10 may be less than 3×10 18 cm -3 The above configuration can suppress the influence of defect levels at the interface, but is not essential.

[0081] Furthermore, Ga layer 12 may be a coating film formed by coating or the like on first main surface 10a of GaN substrate 10. In other words, Ga layer 12 may be a layer formed by a method other than partial modification of GaN substrate 10.

[0082] In addition to Cu or Ag, Au, Ti, Al, or the like may be used as the material of the metal electrode 14. For example, the metal electrode 14 may contain at least one of Cu, Ag, Au, Ti, and Al as a main component.

[0083] Furthermore, various modifications, substitutions, additions, omissions, etc. can be made to the above-described embodiments within the scope of the claims or their equivalents. [Industrial Applicability]

[0084] The present disclosure can be used in power devices used in power supply circuits or inverter circuits of electrical equipment. [Explanation of symbols]

[0085] 1. Nitride semiconductor devices 10 GaN substrate 10a First principal surface 10b Second principal surface 12 Ga layers 14 Metal electrode (cathode electrode) 16 p-type semiconductor layer 18 Anode electrode

Claims

1. a gallium nitride substrate having n-type conductivity; a metal electrode; a gallium layer provided between the gallium nitride substrate and the metal electrode in contact with each other, Nitride semiconductor devices.

2. The carrier concentration of the gallium nitride substrate is 3×10 １８ cm －３ That's all. The nitride semiconductor device of claim 1 .

3. the gallium layer is formed by modifying a part of the gallium nitride substrate; The nitride semiconductor device according to claim 1 or 2.

4. The metal electrode contains at least one of Cu and Ag as a main component. The nitride semiconductor device according to claim 1 or 2.

5. forming a gallium layer by irradiating a portion of a gallium nitride substrate having n-type conductivity with laser light; forming a metal electrode in contact with the gallium layer; A method for manufacturing a nitride semiconductor device.

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