Nitride semiconductor device and method of manufacturing the same

Abstract

To provide a nitride semiconductor device capable of coping with a request for a large current by reducing contact resistance between a semiconductor substrate and a back electrode without performing high-temperature heat treatment.SOLUTION: In the nitride semiconductor device 1, the back surface electrode 10, the semiconductor substrate 20, the semiconductor layer, and the front surface element are stacked in this order. The concentration of the donor element is 1 * 100nm - 3 or more at least in the 1019cm of the depth from the boundary surface 21.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a nitride semiconductor device and a method for manufacturing the same. [Background technology]

[0002] Conventionally, when manufacturing a nitride semiconductor device having a front surface element and a back surface electrode, the back surface electrode is generally formed after the front surface element is fabricated. In this case, in order to reduce the contact resistance between the semiconductor substrate and the back surface electrode, a heat treatment is performed after the back surface electrode is formed to promote alloying at the interface between the semiconductor substrate and the back surface electrode. However, the heat treatment temperature reaches 400 to 500°C, which may degrade the device characteristics of the front surface element. Furthermore, the heat treatment increases manufacturing costs. Therefore, it is desirable to reduce the contact resistance between the semiconductor substrate and the back surface electrode without performing a heat treatment.

[0003] Therefore, for example, Patent Document 1 discloses the following method for reducing the contact resistance between a semiconductor substrate and a back electrode by low-temperature heat treatment: That is, by performing oxygen plasma treatment on the back surface of the semiconductor substrate and then forming the back electrode, the peak concentration of oxygen contained at the interface between the semiconductor substrate and the back electrode is increased and the peak concentration of carbon is reduced, thereby enabling alloying at a low temperature in the subsequent heat treatment, and reducing the contact resistance between the semiconductor substrate and the back electrode to 1×10 -4 Ωcm 2 We have achieved this to a certain extent. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-225767 Summary of the Invention [Problem to be solved by the invention]

[0005] However, to meet the demand for higher current in nitride semiconductor devices, it is necessary to further reduce the contact resistance between the semiconductor substrate and the backside electrode. However, the method disclosed in Patent Document 1 makes it difficult to further reduce the contact resistance. Furthermore, achieving a high oxygen concentration through oxygen plasma treatment is difficult to control and has low reproducibility. Furthermore, the oxygen plasma treatment may cause physical damage to the backside of the semiconductor substrate, which may worsen the contact resistance. Furthermore, cleaning the backside of the semiconductor substrate after oxygen plasma treatment makes it impossible to maintain a high oxygen concentration, which can worsen the contact resistance if unintended contaminants or oxides are present on the backside of the semiconductor substrate. For these reasons, the method disclosed in Patent Document 1 leaves room for further improvement.

[0006] The present invention has been made in view of the above-mentioned problems, and aims to provide a nitride semiconductor device that can meet the demand for larger currents by reducing the contact resistance between the semiconductor substrate and the back electrode without performing high-temperature heat treatment. [Means for solving the problem]

[0007] One aspect of the present invention is A nitride semiconductor device in which a back electrode, a semiconductor substrate, a semiconductor layer, and a front surface element are stacked in this order, In the semiconductor substrate, the concentration of the donor element in a range of at least 100 nm deep from the interface with the back electrode is 1×10 19 cm -3 This is the nitride semiconductor device.

[0008] Another aspect of the present invention is A method for manufacturing a nitride semiconductor device in which a back electrode, a semiconductor substrate, a semiconductor layer, and a front surface element are stacked in this order, comprising the steps of: In the semiconductor substrate, the concentration of the donor element in a range of at least 100 nm deep from the interface with the back electrode is 1×10 19 cm -3 a semiconductor substrate forming step of growing crystals on the semiconductor substrate so as to form the semiconductor substrate; a semiconductor layer forming step of forming the semiconductor layer on one surface of the semiconductor substrate; a surface element forming step of forming the surface element on the semiconductor layer; an acid washing step of acid washing a surface of the semiconductor substrate opposite to a side on which the semiconductor layer is formed; and forming a back electrode on the surface of the semiconductor substrate opposite to the surface on which the semiconductor layer is formed. [Effects of the Invention]

[0009] In the nitride semiconductor device according to the above aspect, the concentration of the donor element in the semiconductor substrate within a range of at least 100 nm in depth from the interface with the back electrode is 1×10 19 cm -3 For these reasons, a region in the semiconductor substrate where the concentration of the donor element is high exists at a sufficiently deep position from the boundary surface. This makes it possible to obtain a sufficient effect of reducing the contact resistance between the semiconductor substrate and the back electrode. As a result, a nitride semiconductor device capable of handling large currents can be obtained without high-temperature heat treatment.

[0010] In the method for manufacturing a nitride semiconductor device according to the other aspect described above, the concentration of the donor element in the semiconductor substrate within a range of at least 100 nm in depth from the interface with the back electrode is 1×10 19 cm -3 The semiconductor substrate is grown as described above. As a result, a region in the semiconductor substrate where the concentration of the donor element is high is present at a position sufficiently deep from the boundary surface, and the effect of reducing the contact resistance between the semiconductor substrate and the back electrode can be sufficiently obtained. As a result, a nitride semiconductor device capable of handling large currents can be manufactured without performing high-temperature heat treatment.

[0011] As described above, according to the above aspect, it is possible to provide a nitride semiconductor device that can meet the demand for larger currents by reducing the contact resistance between the semiconductor substrate and the back electrode without performing high-temperature heat treatment. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a conceptual diagram showing the configuration of a nitride semiconductor device according to a first embodiment. [Figure 2] 3A and 3B are diagrams showing the analysis results of the element contents of the semiconductor substrate and the back surface electrode in the first embodiment. [Figure 3] FIG. 2 is a flow diagram showing a method for manufacturing the nitride semiconductor device of the first embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] In the nitride semiconductor device, the concentration of the donor element in the semiconductor substrate within a range of at least 100 nm in depth from the interface with the back electrode is 1×10 19 cm -3 More than 1×10 22 cm -3 Preferably, the concentration of the donor element in the semiconductor substrate is 1×10 or less. 22 cm -3 If the concentration of the donor element exceeds 1×10, the crystallinity of the semiconductor substrate may be reduced. 22 cm -3 By satisfying the following conditions, it is possible to prevent the crystallinity of the semiconductor substrate from being deteriorated, which can contribute to improving the performance of the nitride semiconductor device.

[0014] In the nitride semiconductor device, the contact resistance between the semiconductor substrate and the back electrode is 1×10 -5 Ωcm 2 In this case, the contact resistance between the semiconductor substrate and the back electrode is sufficiently low, making it possible to meet the demand for even larger currents.

[0015] In the nitride semiconductor device, the contact resistance between the semiconductor substrate and the back electrode is 2×10 -6 Ωcm 2In this case, the contact resistance between the semiconductor substrate and the back electrode is further reduced, making it possible to meet the demand for even larger currents.

[0016] In the nitride semiconductor device, the donor element in the semiconductor substrate may be at least one of O, Si, and Ge, which makes it easy to increase the concentration of the donor element in the semiconductor substrate.

[0017] In the nitride semiconductor device, the concentration of the donor element in the back electrode within a range of at least 100 nm in depth from the interface with the semiconductor substrate is 1×10 19 cm -3 In this case, the donor element is highly concentrated not only in the semiconductor substrate but also in the back electrode, so that the contact resistance between the semiconductor substrate and the back electrode can be further reduced.

[0018] In the method for manufacturing a nitride semiconductor device, in the semiconductor substrate forming step, a concentration of the donor element in the semiconductor substrate within a range of a depth of at least 100 nm from the boundary surface with the back surface electrode is 1×10 19 cm -3 More than 1×10 22 cm -3 It is preferable to grow the crystal of the semiconductor substrate so that the following is satisfied: In this case, deterioration of the crystallinity of the semiconductor substrate can be prevented, which contributes to improving the performance of the nitride semiconductor device.

[0019] In the method for manufacturing a nitride semiconductor device, the semiconductor substrate forming step is preferably performed by OVPE. In this case, when forming the semiconductor substrate, oxygen (O) as a donor element is present at a concentration of 1×10 19 cm -3 As described above, crystal growth of the semiconductor substrate is facilitated and the reproducibility is sufficiently high. Furthermore, since the conventional oxygen plasma treatment is not required, damage to the semiconductor substrate due to the oxygen plasma treatment is not caused and an increase in manufacturing costs can be prevented.

[0020] In the above-described method for manufacturing a nitride semiconductor device, the acid cleaning step and the back surface electrode formation step are preferably performed at an ambient temperature of 150° C. or less. In this case, damage to the semiconductor layer and front surface element formed on the front surface side of the semiconductor substrate due to heat during the acid cleaning step and the back surface electrode formation step can be prevented, and the performance of the nitride semiconductor device can be maintained.

[0021] In the method for manufacturing a nitride semiconductor device, it is preferable not to perform a heat treatment in the back electrode formation step, which can prevent damage to the semiconductor layer and front surface elements formed on the front surface side of the semiconductor substrate due to heat and maintain the performance of the nitride semiconductor device.

[0022] (Embodiment 1) 1. Overview of the configuration of nitride semiconductor device 1 The configuration of the nitride semiconductor device 1 of Embodiment 1 will be described below. As shown in Fig. 1, the nitride semiconductor device 1 of Embodiment 1 has a structure in which a back electrode 10, a semiconductor substrate 20, a semiconductor layer 30, and a front surface element 40 are stacked in this order. Each component and a method of formation will be described in detail below.

[0023] 1-1. Semiconductor substrate 20 The semiconductor substrate 20 is a substrate made of a group III nitride semiconductor. In this embodiment, a GaN (gallium nitride) substrate is used as the semiconductor substrate 20, and contains a donor element as an impurity. The donor element can be, for example, O (oxygen), Si (silicon), or Ge (germanium). In this embodiment, the semiconductor substrate 20 contains O as the donor element.

[0024] A back electrode 10 (described later) is laminated on the back surface side of the semiconductor substrate 20, and the semiconductor substrate 20 has an interface 21 with the back electrode 10. The semiconductor substrate 20 has a donor element concentration of 1×10 in a range of at least 100 nm from the interface 21. 19 cm -3The results of analyzing the depth position from the surface of the back electrode 10 and the contained elements of a structure in which a back electrode 10 (described later) is formed on a semiconductor substrate 20 are shown in FIG. 2. Here, the boundary surface 21 is the depth position where the Ga detection intensity suddenly drops, and specifically, the Ga detection intensity (4×10) at the depth position (depth 550 nm) before the drop begins is 3 ) is 50%. In this embodiment, as shown in FIG. 2, the semiconductor substrate 20 has a donor element concentration of 5×10 in a range D1 of at least 100 nm from the interface 21. 20 cm -3 Furthermore, in this embodiment, the semiconductor substrate 20 has a donor element concentration of 5×10 throughout the entire semiconductor substrate 20, including a region deeper than 100 nm from the interface 21 beyond the range D1. 20 cm -3 2, Ga is derived from GaN that constitutes the semiconductor substrate 20, and Ti and Al are derived from the metals that constitute the back electrode 10.

[0025] The thickness of the semiconductor substrate 20 is not limited, but can be 1 μm or more, and can be 150 μm or more. The thickness of a semiconductor substrate is generally 300 to 400 μm, and the thickness of the semiconductor substrate 20 in this embodiment is sufficiently thin. By making the thickness of the semiconductor substrate 20 sufficiently thin, even if a Schottky barrier is formed between the semiconductor substrate 20 and the back surface electrode 10, the barrier thickness can be made sufficiently thin, and the contact resistance with the back surface electrode 10 can be reduced.

[0026] In the semiconductor substrate 20, the concentration of the donor element is 1×10 22 cm -3 If the concentration of the donor element exceeds 1×10, the crystallinity of the semiconductor substrate 20 may be reduced. 22 cm -3 It is preferable that the following be satisfied: This can prevent the deterioration of the crystallinity of the semiconductor substrate 20, and contribute to improving the performance of the nitride semiconductor device 1.

[0027] As described above, the method for forming the semiconductor substrate 20 may be any method that can incorporate a high concentration of donor elements into the semiconductor substrate 20. For example, OVPE ( O xide V apor P hase E Among them, it is preferable to adopt the OVPE method, which can incorporate O as a donor element into the semiconductor substrate 20 at a high concentration.

[0028] In the OVPE method, a mixed gas of a group III element-containing gas (e.g., GaO gas) and a nitrogen element-containing gas (e.g., NH, NO, NO, NH, NH, etc.) is sprayed toward a seed substrate made of GaN to cause a reaction between the two gases, thereby growing a group III nitride semiconductor crystal on the seed substrate. Note that, in order to grow a group III nitride semiconductor crystal, it is preferable to maintain the temperature at 1000°C or higher and 1400°C or lower. In the OVPE method, the concentration of the donor element (O) can be controlled by adjusting the supply amounts of both gases.

[0029] After forming the semiconductor substrate 20, it is preferable to clean the back surface of the semiconductor substrate 20, which will become the interface 21. The method for cleaning the semiconductor substrate 20 is not limited, but acid cleaning using a chemical solution such as hydrofluoric acid (HF), a dilute hydrofluoric acid (DHF) solution, or a buffered hydrofluoric acid (BHF) solution can be used. The cleaning time is also not limited and can be 0.5 minutes or more. By performing this acid cleaning, organic substances and oxides unintentionally attached to the back surface of the semiconductor substrate 20 can be removed. This reduces the contact resistance between the semiconductor substrate 20 and the back surface electrode 10.

[0030] In this embodiment, the carbon concentration at the rear surface (interface 21) of the semiconductor substrate 20 is 1×10 21 cm -3 More preferably, it is less than 5×10 20 cm -3As a result, the amount of carbon that increases the contact resistance is small at the interface 21, and therefore the contact resistance can be reduced.

[0031] In this embodiment, the hydrogen concentration at the rear surface (interface 21) of the semiconductor substrate 20 is 1×10 21 cm -3 As a result, the interface 21 is hydrogen-terminated, making it less susceptible to oxidation, and inhibiting the formation of oxides on the rear surface of the semiconductor substrate 20 after cleaning, which can contribute to reducing the contact resistance.

[0032] 1-2. Rear electrode 10 The back electrode 10 can be made of a metal electrode material having a work function of 2.0 to 5.7 eV. Examples of the electrode material that can be used include Ti, Al, Ni, Mg, Mo, V, Au, Ag, and Cu, and compounds such as TiN and AlCu may also be used. Among these, it is preferable that the back electrode 10 contains at least one of Au, Ag, and Cu. These electrode materials can improve the heat dissipation properties of the nitride semiconductor device 1. The back electrode 10 may be formed as a single layer, or may be formed by laminating two or more layers.

[0033] The back electrode 10 may contain the above-mentioned donor element. The concentration of the donor element in the back electrode 10 is 1×10 in a range of at least 100 nm from the interface 21 between the back electrode 10 and the semiconductor substrate 20. 19 cm -3 The donor element contained in the back electrode 10 may be a donor element contained in the semiconductor substrate 20 that leaks out from the semiconductor substrate 20 during the formation of the back electrode 10. By including the donor element in the back electrode 10, the work function of the back electrode 10 can be further reduced, and the contact resistance between the semiconductor substrate 20 and the back electrode 10 can be reduced.

[0034] The method for forming the back electrode 10 is not limited, but sputtering, electron beam evaporation, and resistive heating can be used. In this embodiment, the back electrode 10 is formed by sputtering, introducing an inert gas such as Ar or N into a vacuum environment. After the back electrode 10 is formed, no heat treatment for alloying is required. The back electrode 10 can be formed at an ambient temperature of less than 450°C, preferably 150°C or less. This can prevent performance degradation of the semiconductor layer 30, which will be described later.

[0035] 1-3.Contact resistance The contact resistance at the interface 21 between the back electrode 10 and the semiconductor substrate 20 is 1×10 -5 Ωcm 2 It can be less than or equal to 2×10 -6 Ωcm 2 It can be as follows:

[0036] 1-4. Semiconductor layer 30 and surface element 40 A semiconductor layer 30 is formed on the upper surface 22 of the semiconductor substrate 20. The configuration of the semiconductor layer 30 is not limited, and it can be any desired semiconductor layer. In this embodiment, a group III nitride semiconductor layer is formed. Furthermore, a surface element 40 is formed on the semiconductor layer 30. The configuration of the surface element 40 is not limited, but it includes an electrode.

[0037] The method for forming the semiconductor layer 30 and the surface element 40 is not limited, and they can be formed by any desired method. The semiconductor layer 30 and the surface element 40 are formed after the semiconductor substrate 20 is formed and before the back surface electrode 10 is formed. After the semiconductor layer 30 and the surface element 40 are formed, the back surface of the semiconductor substrate 20 can be cleaned.

[0038] 2. Method for manufacturing nitride semiconductor device 1 Next, a method for manufacturing the nitride semiconductor device 1 according to this embodiment will be described with reference to the flowchart shown in Fig. 3. The method for manufacturing the nitride semiconductor device 1 according to this embodiment includes a semiconductor substrate formation step S1, a semiconductor layer formation step S2, a front surface element formation step S3, an acid cleaning step S4, and a back surface electrode formation step S5.

[0039] First, in the semiconductor substrate formation step S1, the above-described semiconductor substrate 20 is formed. In this embodiment, the semiconductor substrate 20 is formed by the OVPE method as described above. As a result, the semiconductor substrate 20 is formed to contain a high concentration of O as a donor element.

[0040] Next, in a semiconductor layer formation step S2, a semiconductor layer 30 is formed on the upper surface 22 of the semiconductor substrate 20, and then, in a front surface element formation step S3, a front surface element 40 is formed on the semiconductor layer 30. Then, in an acid cleaning step S4, the rear surface 21 of the semiconductor substrate 20 is acid cleaned. In this embodiment, the acid cleaning is performed at room temperature.

[0041] Thereafter, in a back surface electrode formation step S5, a back surface electrode 10 is formed on the upper surface 22 of the semiconductor substrate 20. In this embodiment, the back surface electrode 10 is formed by a sputtering method. In this embodiment, the back surface electrode formation step S5 is performed at room temperature. After the back surface electrode formation step S5 is performed, the flow ends.

[0042] 3. Effects According to the nitride semiconductor device 1 of this embodiment, in the semiconductor substrate 20, the concentration of the donor element in the range D1 at a depth of at least 100 nm from the interface 21 with the back surface electrode 10 is 1×10 19 cm -3 For the above reasons, a region in the semiconductor substrate 20 where the concentration of the donor element is high exists from the boundary surface 21 to a sufficiently deep position. This makes it possible to obtain a sufficient effect of reducing the contact resistance between the semiconductor substrate 20 and the back surface electrode 10. As a result, the nitride semiconductor device 1 is capable of handling large currents without performing high-temperature heat treatment.

[0043] In this embodiment, the concentration of the donor element in the semiconductor substrate 20 within a range of at least 100 nm deep from the interface 21 with the back surface electrode 10 is 1×10 19 cm -3 More than 1×10 22 cm -3 This makes it possible to prevent the crystallinity of the semiconductor substrate 20 from being deteriorated, which contributes to improving the performance of the nitride semiconductor device 1.

[0044] In this embodiment, the contact resistance between the semiconductor substrate 20 and the back surface electrode 10 is 1×10 -5 Ωcm 2 or less, and further, 2×10 -6 Ωcm 2 As a result, the contact resistance between the semiconductor substrate 20 and the back surface electrode 10 becomes sufficiently low, making it possible to meet the demand for even larger currents.

[0045] In this embodiment, the donor element in the semiconductor substrate 20 is O. This makes it possible to easily increase the concentration of the donor element in the semiconductor substrate 20.

[0046] In this embodiment, the concentration of the donor element in the range D2 of the back electrode 10 from the interface 21 with the semiconductor substrate 20 to a depth of at least 100 nm is 1×10 19 cm -3 As a result, the donor element is concentrated in the back surface electrode 10 as well as the semiconductor substrate 20, so that the contact resistance between the semiconductor substrate 20 and the back surface electrode 10 can be further reduced.

[0047] In addition, in this embodiment, the method for manufacturing the nitride semiconductor device 1 includes the above-described semiconductor substrate formation step S1, semiconductor layer formation step S2, front surface element formation step S3, acid cleaning step S4, and back surface electrode formation step S5, and the concentration of the donor element in the semiconductor substrate 20 within a range of a depth of at least 100 nm from the interface 21 with the back surface electrode 10 is 1×10 19 cm -3The semiconductor substrate is grown as described above. As a result, a region in the semiconductor substrate 20 where the concentration of the donor element is high exists at a position sufficiently deep from the interface 21, and the effect of reducing the contact resistance between the semiconductor substrate 20 and the back surface electrode 10 can be sufficiently obtained. As a result, a nitride semiconductor device 1 that is compatible with large currents can be manufactured without performing high-temperature heat treatment.

[0048] In this embodiment, in the semiconductor substrate formation step S1, the concentration of the donor element in the semiconductor substrate 20 within a range of at least 100 nm from the interface 21 with the back surface electrode 10 is 1×10 19 cm -3 More than 1×10 22 cm -3 The semiconductor substrate 20 is grown as follows: This prevents the deterioration of the crystallinity of the semiconductor substrate 20, which contributes to improving the performance of the nitride semiconductor device 1.

[0049] In the present embodiment, in the method for manufacturing the nitride semiconductor device 1, the semiconductor substrate formation step S1 is performed by OVPE. As a result, when forming the semiconductor substrate 20, oxygen (O) as a donor element is introduced at a concentration of 1×10 19 cm -3 As described above, it is easy to grow crystals on the semiconductor substrate 20, and the reproducibility is sufficiently high. Furthermore, since the conventional oxygen plasma treatment is not required, damage to the semiconductor substrate 20 due to the oxygen plasma treatment does not occur, and an increase in manufacturing costs can be prevented.

[0050] In this embodiment, the acid cleaning step S4 and the back surface electrode formation step S5 are performed at an ambient temperature of 150° C. or less. This makes it possible to prevent damage to the semiconductor layer 30 and the front surface element 40 formed on the front surface (upper surface 22) of the semiconductor substrate 20 due to heat during the acid cleaning step S4 and the back surface electrode formation step S5, and to maintain the performance of the nitride semiconductor device 1.

[0051] In this embodiment, heat treatment is not performed in the back surface electrode formation step S5, which prevents damage to the semiconductor layer 30 and the front surface element 40 formed on the front surface (upper surface 22) of the semiconductor substrate 20 due to heat, and maintains the performance of the nitride semiconductor device 1.

[0052] As described above, according to this embodiment, it is possible to provide a nitride semiconductor device 1 that can meet the demand for larger currents by reducing the contact resistance between the semiconductor substrate 20 and the back surface electrode 10 without performing high-temperature heat treatment.

[0053] The present invention is not limited to the above-described embodiments, and can be applied to various embodiments within the scope of the present invention. [Explanation of symbols]

[0054] 1. Nitride semiconductor devices 10 Back electrode 20 Semiconductor substrate 21 Boundary (back side) 22 Top surface (front surface) 30 Semiconductor layer 40 Surface Elements

Claims

1. A nitride semiconductor device in which a back electrode, a semiconductor substrate, a semiconductor layer, and a front surface element are stacked in this order, In the semiconductor substrate, the concentration of the donor element in a range of at least 100 nm deep from the interface with the back electrode is 1×10 １９ cm -３ This completes the nitride semiconductor device.

2. In the semiconductor substrate, the concentration of the donor element in a range of at least 100 nm deep from the interface with the back electrode is 1×10 １９ cm -３ 1x10 or more ２２ cm -３ 2. The nitride semiconductor device according to claim 1, wherein:

3. The contact resistance between the semiconductor substrate and the back electrode is 1×10 －５ Ω cm ２ 3. The nitride semiconductor device according to claim 1, wherein:

4. The contact resistance between the semiconductor substrate and the back electrode is 2×10 －６ Ω cm ２ 3. The nitride semiconductor device according to claim 1, wherein:

5. 3. The nitride semiconductor device according to claim 1, wherein the donor element in said semiconductor substrate is at least one of O, Si, and Ge.

6. In the back electrode, the concentration of the donor element in a range of at least 100 nm deep from the interface with the semiconductor substrate is 1×10 １９ cm -３ 3. The nitride semiconductor device according to claim 1 or 2.

7. A method for manufacturing a nitride semiconductor device in which a back electrode, a semiconductor substrate, a semiconductor layer, and a front surface element are stacked in this order, comprising the steps of: In the semiconductor substrate, the concentration of the donor element in a range of at least 100 nm deep from the interface with the back electrode is 1×10 １９ cm -３ a semiconductor substrate forming step of growing crystals on the semiconductor substrate so as to form the semiconductor substrate; a semiconductor layer forming step of forming the semiconductor layer on one surface of the semiconductor substrate; a surface element forming step of forming the surface element on the semiconductor layer; an acid washing step of acid washing a surface of the semiconductor substrate opposite to a side on which the semiconductor layer is formed; and forming a back electrode on the surface of the semiconductor substrate opposite to the surface on which the semiconductor layer is formed.

8. In the semiconductor substrate forming step, the concentration of the donor element in the semiconductor substrate within a range of at least 100 nm in depth from the interface with the back surface electrode is 1×10 １９ cm -３ 1x10 or more ２２ cm -３ The method for manufacturing a nitride semiconductor device according to claim 7 , wherein the semiconductor substrate is grown as follows:

9. 9. The method for manufacturing a nitride semiconductor device according to claim 7, wherein the semiconductor substrate forming step is performed by OVPE.

10. The method for manufacturing a nitride semiconductor device according to claim 7 or 8, wherein the acid cleaning step and the back surface electrode forming step are performed at an ambient temperature of 150° C. or less.

11. The method for manufacturing a nitride semiconductor device according to claim 7 or 8, wherein no heat treatment is performed in said back surface electrode forming step.

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