JPH11340571A - Nitride semiconductor element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve adhesion between an n-electrode and a supporting body by providing a first layer of metal obtaining ohmic contact with an n-type nitride semiconductor, a second layer containing a metal whose melting point is higher than that of Al and a third layer containing Sn in the n-electrode from a side close to a second main face. SOLUTION: An element structure having an n-side clad layer 14, formed of the superlattice structure of an undoped AlGaN layer and a Si doped n-type GaN layer and a p-side clad layer 18, formed of the super lattice structure of an Mg doped AlGaN layer and an undoped GaN layer, is formed in the first main face side of a nitride semiconductor substrate 10 formed of an n-type nitride semiconductor. An n-electrode 30 is formed on the second main face side of the nitride semiconductor substrate 10. A first layer 31 of a metal for obtaining ohmic contact with the n-type nitride semiconductor, a second layer 32 containing metal whose melting point is higher than Al, and a third layer 33 containing Sn or In, are stacked and formed in the n-electrode 30 from a side close to the second main face.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a nitride semiconductor (In _XA).
a light emitting diode (LED), a laser diode (LD) composed of l _Y Ga _1-XY N, 0 ≦ X, 0 ≦ Y, X + Y ≦ 1);
The present invention relates to a light emitting element such as a super luminescent diode (SLD), an optical sensor, a light receiving element such as a solar cell, or a nitride semiconductor element used for an electronic device such as a transistor or a power device.

[0002]

2. Description of the Related Art We have manufactured a nitride semiconductor laser device including an active layer on a GaN substrate and have achieved the world's first continuous oscillation of over 10,000 hours at room temperature (ICNS'97). Proceedings, October 27-31, 1997, P444-446, and
Jpn.J.Appl.Phys.Vol.36 (1997) pp.L1568-1571). Furthermore, it was announced that continuous oscillation of 10,000 hours or more was achieved even at a power of 5 mW by removing sapphire from the laser element and using GaN alone. (Jpn.J.Appl.Phys.
Vol.37 (1998) pp.L309-L312 and Appl.Phys.Lett.Vol.7
2 (1998) No. 16, 2014-2016)

The above laser device has a structure in which the n-electrode and p-electrode are taken out from the nitride semiconductor surface side without taking out the n-electrode from the GaN back surface side because the carrier concentration of the undoped GaN substrate is insufficient. Has become. In such a structure in which n, p and two types of electrodes are taken out from the same surface side, the chip size becomes large. Therefore, in order to reduce the chip size, it is necessary to take out the electrodes from the back surface side of the substrate. However, since undoped GaN has a low carrier concentration, an electrode cannot be formed. In order to obtain a certain carrier concentration, n-type impurities must be doped during GaN substrate growth.

[0004]

When a GaN substrate is doped with an n-type impurity to obtain a carrier concentration of, for example, 10 ¹⁷ / cm ³ or more, an n-electrode can be taken out from the back surface of the substrate. When an electrode is provided on the back surface of the substrate, the back surface of the substrate is often polished to a mirror surface before forming the electrode. In polishing, for example, a diamond abrasive is used, so that the n-type nitride semiconductor growth surface (as-g
The damage to the surface is greater than that of the rown) surface. Therefore, a special device is required to obtain a good ohmic between the substrate surface and the n-electrode.

When the n-electrode side is die-bonded to the support, the ohmic property of the electrode may be lost due to the die-bonding material. In particular, in an element such as a laser element, which has a locally high temperature, if the ohmic property is lost due to heat with the passage of time, the driving voltage increases and directly affects the element life. In addition, in a device having a novel structure using a nitride semiconductor substrate, a bonding technique for firmly bonding an n-electrode and a support is not well known.

Accordingly, when the present invention is applied to the practical use of an element using a GaN substrate, the reliability of maintaining the adhesiveness between the n-electrode provided on the back side of the GaN substrate and the support and the ohmic property of the n-electrode can be maintained. Another object of the present invention is to realize a nitride semiconductor device having excellent characteristics.

[0007]

A nitride semiconductor device according to the present invention comprises an n-type nitride semiconductor layer and a p-type nitride semiconductor on a first main surface side of a nitride semiconductor substrate made of an n-type nitride semiconductor. An element structure having a layer is formed, an n-electrode is formed on the second main surface side of the nitride semiconductor substrate, and the n-electrode and the support face each other, and the element is die-bonded to the support. A semiconductor device, wherein the n-electrode comprises a second
A first layer containing a metal that provides good ohmic contact with the n-type nitride semiconductor, a second layer containing a metal having a higher melting point than Al, and Sn or In And at least a three-layer structure including a third layer.

In the device according to the present invention, a nitride semiconductor layer doped with an n-type impurity is grown between the n-electrode and the second main surface. It is desirable that the n-type impurity concentration of the nitride semiconductor layer be higher than the n-type impurity concentration of the nitride semiconductor substrate near the second main surface.

The first layer is made of W, Al, Ti, Zr, V, N
b. At least one metal selected from the group consisting of b. As a preferred specific example, W / A
1, Ti / Al, Ti / Au, V / Al, and V / Au.

The second layer is made of W, Ti, Zr, Pt, Mo,
It contains at least one metal selected from the group consisting of Au and Ni.

The third layer is characterized by containing at least one metal selected from the group consisting of Au, Ge, Si, and Ag, and Sn or In. Preferred combinations are Au / Sn (In), Au / Ge / Sn
(In) and Au / Ag / Sn (In).

A first main surface of the nitride semiconductor substrate;
A nitride semiconductor layer containing at least In is formed between the second semiconductor layer and the second main surface. The nitride semiconductor containing In is preferably a layer having an InGaN layer, and may be a single layer or a multilayer film in which InGaN and a layer not containing In are stacked. By forming this layer, the nitride semiconductor substrate tends to be easily cleaved. When cleaving the substrate, the M-plane (11-00) of the nitride semiconductor, that is,
When the crystal form of the nitride semiconductor is approximated by a hexagonal prism, it is desirable to cleave at six types of planes corresponding to the side surfaces.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, at least three layers are provided on a second main surface (hereinafter, the second main surface is sometimes referred to as a back surface) of an n-type nitride semiconductor substrate doped with an n-type impurity. An n-electrode having a structure is provided. The first layer is a layer containing an n-type nitride semiconductor on the back surface of the substrate and an electrode material for obtaining a good ohmic. The first layer does not necessarily need to be formed in contact with the back surface of the nitride semiconductor substrate. For example, even if the first layer is grown on the back surface of the nitride semiconductor substrate via the nitride semiconductor layer further grown. good. It is desirable that the n-type impurity concentration of the nitride semiconductor layer further grown on the back surface side be higher than the n-type impurity concentration (for example, 5 μm) near the back surface of the nitride semiconductor substrate. This effect is achieved by newly growing a nitride semiconductor on the back surface side, so that the damage on the back side due to polishing, peeling, etc. is reduced.
-Can be recovered with grown nitride semiconductor. Further, when the n-type impurity concentration of the newly grown nitride semiconductor layer is made higher than that near the rear surface and the layer is used as a contact layer, the ohmic property is further improved and the forward voltage can be reduced. .

The n-type impurity concentration of the nitride semiconductor substrate is 5 ×
It is adjusted to 10 ¹⁶ / cm ³ or more and 5 × 10 ¹⁸ / cm ³ or less, preferably 1 × 10 ¹⁸ / cm ³ or less. If it is more than 5 × 10 ¹⁸ / cm ^3, the crystallinity of the nitride semiconductor substrate tends to deteriorate, and the number of crystal defects tends to increase. If it is less than 5 × 10 ¹⁶ / cm ³ , a sufficient carrier concentration cannot be imparted to the nitride semiconductor substrate, and if an electrode is formed on the back surface side, the driving voltage becomes high. The preferred impurity concentration of the newly formed nitride semiconductor layer on the back side is 5 × 10 ¹⁷ / cm ³ or more, preferably 1 × 10 ¹⁸ / cm ³ or more, and more preferably 3 × 1 / cm ³ or more.
0 ¹⁸ / cm ³ or more. Further, between the nitride semiconductor having a high n-type impurity concentration and the second main surface, a nitride semiconductor having an undoped or n-type impurity concentration lower than that near the back surface is formed to a thickness of 0.1 μm or less. good. Undoped,
The nitride semiconductor layer having a small amount of n-type impurity concentration recovers damage received on the back surface side, and facilitates the growth of a high carrier concentration n-type nitride semiconductor thereon. Examples of the n-type impurities include Si, Ge, Sn, S, Ti, and Zr.
Most preferably, Si is used. Although the thickness of the nitride semiconductor having a high impurity concentration is not particularly limited, it is generally preferable to grow the nitride semiconductor with a thickness of 10 Å to 10 μm.

When an element structure is formed on the first main surface of the nitride semiconductor substrate, an electrode is formed on the second main surface side, and the second main surface side is die-bonded, the adhesiveness is high. In addition, the stability of the electrode material is very important. In the device of the present invention,
Good ohmic contact with the n-type nitride semiconductor is obtained by the first layer. Further, the second layer is a barrier layer, which prevents the electrode material from diffusing due to heat treatment during element driving, electrode formation, metallizing, or the like, and does not damage ohmic. Further, the third layer is a low-temperature adhesion-enhancing layer with the support, and is made of a layer containing Sn or In to improve the adhesion of the support such as a heat sink, a submount, or a lead frame. Can be done. However, as described above, Sn, I
If n diffuses, the ohmic property of the n electrode may be deteriorated. In the present invention, a second metal having a metal having a higher melting point than Al is used.
Since the layer functions as a barrier layer, Sn and In contained in the third layer are not diffused, and a stable ohmic can be obtained. Furthermore, when a chip-shaped element is manufactured from a normal wafer, an n-electrode is formed on the back surface of the substrate and then separated by cleavage, dicing, or the like. In the device of the present invention, since the n-electrode has a three-layer structure, even when stress is applied to the interface between the electrode and the substrate due to a physical action such as dicing during cleavage, the electrode is hardly peeled off from the substrate. .

The first layer is made of W, Al, Ti, Zr, V, N
At least one metal selected from the group consisting of b, preferably at least two of these, and at least two of at least one and Au added. The first layer may be in an alloy state or a multilayer structure. Preferred specific examples include W / Al, Ti
/ Al, Ti / Au, V / Al, V / Au,
In these combinations, the metal ratio is not particularly limited.

The second layer is composed of W, Ti, Zr, Pt, Mo,
It contains at least one metal selected from the group consisting of Au and Ni, and particularly preferably uses W, Ti, Pt and Ni. These metals function as barrier layers, and can prevent Sn and In of the third layer from diffusing into the first layer. The second layer is preferably formed thicker than the first layer as a barrier layer.

The third layer is preferably a layer containing at least one metal selected from the group consisting of Au, Ge, Si, and Ag, and Sn or In. Preferred combinations are Au / Sn (In), Au / Ge /
Sn (In), Au / Ag / Sn (In),
The third layer may be in the form of an alloy or a multilayer. Layers composed of these combinations have a particularly strong adhesion to the support.

For example, when the substrate is separated into chips so that at least one end face is exposed by cleavage of the substrate,
Even if the n-electrode is formed on almost the entire back surface of the substrate, the electrode having the three-layer structure of the present invention is hardly peeled off from the back surface.

[0020]

FIG. 1 is a schematic sectional view showing the structure of a laser device according to one embodiment of the present invention, and shows a view when the device is cut in a direction parallel to a resonance surface. Hereinafter, Embodiment 1 will be described with reference to FIG. The device of the present invention is not limited to a laser device.

[Example 1] 2 inch square Si-doped Ga
A nitride semiconductor substrate 10 made of N is prepared. This nitride semiconductor substrate 10 is grown as follows.

(Nitride semiconductor substrate 10) 2 inch φ, C
MOV of a heterogeneous substrate 1 made of sapphire whose main surface is
It was set in a PE reaction vessel, and at 500 ° C., trimethylgallium (TMG) and ammonia (NH ₃ ) were used.
A low temperature buffer layer of N is grown to a thickness of 200 Å. 1050 ° C after low temperature buffer layer growth
Then, an underlayer made of GaN is also grown to a thickness of 4 μm. After the growth of the underlayer, the wafer is taken out of the reaction vessel, and a protective film made of SiO ₂ having a stripe width of 10 μm and a stripe interval (window portion) of 2 μm is formed on the surface of the underlayer. After the formation of the protective film, the wafer was set again in the MOVPE reaction vessel, the temperature was set to 1050 ° C., and the TM
An undoped GaN layer is grown to a thickness of 5 μm using G and ammonia to cover the surface of SiO ₂ .

First GaN layer 11: After growth, the wafer is transferred from the MOVPE apparatus to the HVPE apparatus, and Ga metal, ammonia, HCl, and silane gas are used as impurity gases, and n is doped with Si at 3 × 10 ¹⁷ / cm ^3. Type GaN
A first GaN layer 11 composed of a layer is grown to a thickness of 200 μm.

Intermediate layer 12: Next, at 800 ° C., TM
Using I (trimethylindium), TMG, and ammonia, an intermediate layer 12 made of undoped In _0.3 Ga _0.7 N is grown to 500 Å.

Second GaN layer 13: Next, at 900 ° C., a second GaN layer 13 made of GaN doped with 1 × 10 ¹⁸ / cm ^{3 of} Si is grown to a thickness of 1 μm. Second
GaN layer 13 is substantially the same as the growth temperature of the intermediate layer 12,
Alternatively, by growing at a temperature higher than the growth temperature of the intermediate layer and lower than the growth temperature of the n-side cladding layer 21, decomposition of the intermediate layer containing In can be prevented, and a layer having good crystallinity can be grown. It is desirable that the second GaN layer has the same composition as the first nitrided GaN layer. Second Ga
After growing the N layer, the sapphire substrate is polished from the sapphire substrate side to remove the sapphire substrate, the low-temperature growth buffer layer, the underlayer, the protective film, and the undoped GaN layer.
m of the first GaN layer 11, the intermediate layer 12, and the second GaN
The nitride semiconductor substrate 10 including the layer 13 is manufactured. Note that the intermediate layer 12 and the second GaN layer 13 are formed of the first Ga
After growing the N layer 11, undoped GaN is deposited from the sapphire substrate.
After removing up to the layer, it may be formed on the surface of the first GaN layer on the as-grown side.

(Element Structure Growth) Next, the second GaN layer 1
3 on top of 30 Angstroms of undoped Al _0.16
N having a total thickness of 1.2 μm and having a superlattice structure of a Ga _0.84 N layer / 30 Å Si-doped n-type GaN layer
Side cladding layer 14 0.1 μm undoped GaN n-side light guide layer 15, 100 Å Si-doped In _0.02 G
a _0.98 N barrier layer / 40 Å undoped In _0.15 Ga _0.85 N well layer and a multiple quantum well structure with a total film thickness of 380 Å 16 active layer 16 0.1 μm undoped GaN p-side optical guide layer 17 30 Angstrom Mg-doped Al _0.16 Ga _0.84 N
A total thickness of 0.6 μmp-side cladding layer 18 having a superlattice structure with a layer / 30 Å undoped GaN layer A p-side contact layer 19 made of 150 Å Mg-doped p-type GaN is sequentially laminated.

(N-side contact layer 40) After the growth of the device structure having the double hetero structure including the active layer, the wafer is turned upside down, and the nitride semiconductor substrate 10 on the side from which the sapphire substrate is removed faces upward. Then, on the polished nitride semiconductor substrate 10, GaN doped with 3 × 10 ¹⁸ / cm ^{3 of} Si was used.
The n-side contact layer 40 is grown to a thickness of 5 μm.

(P-electrode forming step) The wafer on which the nitride semiconductor has been grown as described above is taken out of the reaction vessel, and the width of the wafer is formed on the surface of the uppermost p-side contact layer 19 through a mask having a predetermined shape. A protective film made of SiO ₂ consisting of 1.5 μm stripes is formed. After forming the protective film,
As shown in FIG. 1, the etching was performed to the vicinity of the interface between the p-side cladding layer 18 and the p-side light guide layer 17 and the width was 1.5 μm.
An m-shaped striped waveguide is formed.

After forming the stripe waveguide, the nitride semiconductor layer
ZrO on the surface of_TwoAn insulating film 100 is formed. Absolute
After the edge film 100 is formed, the S film formed on the p-side contact layer
iO _TwoIs dissolved and removed by lift-off method._TwoWith
The ZrO on the p-side contact layer_TwoIs removed.

After the formation of the insulating film 100, a p-electrode 20 made of Ni / Au is formed so as to obtain a good ohmic contact with the p-side contact layer 19 via the insulating film 100 as shown in FIG.

(Step of Forming N-Electrode 30) First Layer 31: Next, Ti is formed on substantially the entire surface of the n-side contact layer 40 at 0.01 μm, and Al is formed thereon at 0.05 μm. Second layer 32: the same area as the first layer, Ti0.05μ
m is formed, and Ni and 0.05 μm are formed thereon to form the second layer 32 having a thickness of 0.1 μm. Third layer 33: Au (80%) / Sn on the second layer
(20%) A third layer of an alloy is formed to a thickness of 1 μm.

Thereafter, the wafer is heat-treated at 300.degree. After the heat treatment, the electrodes were partially etched, and the current and voltage between the electrodes were measured for the ohmic contact. As a result, it was confirmed that a substantially ohmic contact was obtained, indicating that a good ohmic contact was obtained.

After the formation of the p and n electrodes as described above, the substrate 10 is cleaved at the M plane of the nitride semiconductor substrate 10 to form a bar into a wafer, and resonance occurs at the cleaved surface of the bar. Make a surface. Further, the bar is diced in a direction perpendicular to the resonance surface to obtain a laser chip of 400 μm (resonator length) × 400 μm square. After the fabrication of the laser chip, none of the n-electrodes was peeled off from the n-side contact layer 40.

After the fabrication of the laser chip, the n-electrode 30 side is Au
Is heat-pressed onto the metalized heat sink, and an Au wire is wire-bonded as shown in FIG. 1 to form a laser element. When laser oscillation was attempted on this laser device at room temperature, the oscillation wavelength was 408.5 nm, and the threshold current density was 2 kA.
/ Cm ² showed continuous oscillation at room temperature, and the voltage at the threshold decreased by about 10% as compared with the conventional one. Even after the current value was further increased and the output was increased, and continuous oscillation was performed at 40 mW for 20 hours, both the voltage and the current at 40 mW hardly changed from the first. Further, when an attempt was made to peel the element by applying a load of 1 kg to the side from the long side of the element and the element was peeled off, the element was firmly attached and there was no peeling of the element.

Further, in Example 1, the second layer was made of Ti
When W, Zr, Pt, Mo, and Au were each formed to a thickness of 0.1 μm instead of / Ni, a laser element having almost the same characteristics as in Example 1 was obtained.

On the other hand, when the second layer was made of Al in Example 1, the threshold current and the voltage increased over time,
The life of the device was extended at 0 hours. When the n-electrode is analyzed, the first layer does not have a laminated structure, but has a state in which Ti and Al are partially alloyed.
Was spreading. In this example, the first layer was in a state where Ti and Al were partially alloyed, and the second layer was in a state where Ni and Ti were partially alloyed.

[Embodiment 2] In the step of forming the n-electrode 30 of the embodiment 1, the first layer is formed of W (0.01 μm) / Al
(0.05 μm), and the second layer has W
(0.1 μm), and Au (80%) in the third layer,
A laser device was manufactured in the same manner as in Example 1 except that a layer containing Sn (20%) was used. As a result, an element having substantially the same characteristics as in Example 1 was obtained. Furthermore, when Ti, Zr, Pt, Mo, Au, and Ni were formed in the intermediate layer to have the same thickness, respectively, an element having substantially the same characteristics was obtained.

[Embodiment 3] In the step of forming the n-electrode 30 of the embodiment 1, the first layer is formed of W (0.01 μm) / Al
(0.05 μm), and the second layer is made of Pt.
(0.1 μm), and Au (80%) in the third layer,
A laser device was manufactured in the same manner as in Example 1 except that a layer containing Si (10%) and In (10%) was obtained. As a result, an element having substantially the same characteristics as in Example 1 was obtained. Furthermore, when W, Ti, Zr, Mo, and Au were formed in the intermediate layer in the same film thickness, respectively, an element having substantially the same characteristics was obtained. Note that the first layer is in a state in which W and Al are partially alloyed, and the second layer is a Pt layer, and the third layer is Au, Si,
It had a three-layer structure made of an In alloy.

[Embodiment 4] In the step of forming the n-electrode 30 of the embodiment 1, the first layer is formed of Ti (0.01 μm) / V
(0.01 μm) and Au (0.08 μm), Zr (0.1 μm) is formed in the second layer, and Au (80%), Ge (10%), and Sn (10%)
When a laser device was manufactured in the same manner as in Example 1 except that the layer was formed, a device having substantially the same characteristics as in Example 1 was obtained. Further, instead of Zr for the intermediate layer, W, T
When i, Pt, and Mo were formed with the same film thickness,
An element having substantially the same characteristics was obtained. The reason why Au was not tried in the second layer is that the Au
This is because u is laminated.

[Embodiment 5] In the step of forming the n-electrode 30 of the embodiment 1, the first layer is formed of Zr (0.01 μm) / Nb.
(0.01) / Au (0.04 μm), Mo (0.1 μm) is formed in the second layer, and Au (80%), Ag (5%), When a laser device was manufactured in the same manner as in Example 1 except that the layer containing Sn (15%) was used, an element having substantially the same characteristics as in Example 1 was obtained. Further, instead of Mo, W, Ti, P
When t and Zr were formed to have the same film thickness, an element having substantially the same characteristics was obtained although the ohmic property was slightly inferior to that of Example 1.

Example 6 A laser device was obtained in the same manner as in Example 1, except that the n-electrode 30 was formed directly on the back surface of the nitride semiconductor substrate without forming the n-side contact layer 40. Although the ohmic property was slightly inferior to that of Example 1, an element having substantially the same characteristics was obtained.

[0042]

As described above, according to the device of the present invention, Ga
It is possible to provide a stable n-electrode which has good ohmic properties with an N-substrate, has good adhesive strength, and is hardly deteriorated. In this specification, the laser element used under the most severe conditions has been described, but the present invention is not limited to the laser element,
The present invention is applicable to all nitride semiconductor devices in which a GaN substrate is used and an n-electrode is formed on the back surface of the GaN substrate.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing the structure of a laser device according to one embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 10: nitride semiconductor substrate (11: first GaN layer) (12: intermediate layer) (13: second GaN layer) 14: n-side cladding layer 15: n-side light guide layer 16 ... active layer 17 ... p-side light guide layer 18 ... p-side cladding layer 19 ... p-side contact layer 40 ... n-side contact layer 20 ... p-electrode 30 ... n electrode (31 ... first layer) (32 ... second layer) (33 ... third layer) 100 ... insulating film

Claims (7)

[Claims] An element structure having an n-type nitride semiconductor layer and a p-type nitride semiconductor layer is formed on a first main surface side of a nitride semiconductor substrate made of an n-type nitride semiconductor, and the nitride semiconductor An n-electrode is formed on the second principal surface side of the substrate, and the n-electrode is formed on the n-electrode.
An electrode and a support are opposed to each other, and the device is die-bonded to the support. The n-electrode includes an n-type nitride semiconductor and an n-type nitride semiconductor. A first layer containing a metal that provides a good ohmic contact, a second layer containing a metal having a higher melting point than Al,
A nitride semiconductor device having at least a three-layer structure including a third layer containing Sn or In. 2. The nitride semiconductor device according to claim 1, wherein a nitride semiconductor layer doped with an n-type impurity is grown between said n-electrode and a second main surface. . 3. The nitride semiconductor according to claim 2, wherein an n-type impurity concentration of the nitride semiconductor layer is higher than an n-type impurity concentration of the nitride semiconductor substrate near the second main surface. element. 4. The first layer is made of W, Al, Ti, Zr,
The nitride semiconductor device according to claim 1, wherein the nitride semiconductor device includes at least one metal selected from the group consisting of V and Nb. 5. The second layer is made of W, Ti, Zr, Pt,
The nitride semiconductor device according to claim 1, wherein the nitride semiconductor device includes at least one metal selected from the group consisting of Mo, Au, and Ni. 6. The third layer is made of Au, Ge, Si, Ag.
At least one metal selected from the group consisting of
2. The nitride semiconductor device according to claim 1, comprising n or In. 7. A first main surface of the nitride semiconductor substrate,
The nitride semiconductor device according to any one of claims 1 to 6, wherein a nitride semiconductor layer containing at least In is formed between the nitride semiconductor layer and the second main surface.