WO2013132783A1

WO2013132783A1 - Nitride semiconductor laminate structure, nitride semiconductor light emitting element provided with nitride semiconductor laminate structure, and method for producing nitride semiconductor laminate structure

Info

Publication number: WO2013132783A1
Application number: PCT/JP2013/001120
Authority: WO
Inventors: 満明大屋; 横川　俊哉
Original assignee: パナソニック株式会社
Priority date: 2012-03-07
Filing date: 2013-02-26
Publication date: 2013-09-12

Abstract

This nitride semiconductor light emitting element is provided with: an n-side electrode (40); a p-side electrode (30); an n-type nitride semiconductor layer (22) that has a nonpolar or semi-polar surface; a p-type nitride semiconductor layer (26) that is electrically connected to the p-side electrode; and an active layer (24) that is sandwiched between the n-type nitride semiconductor layer and the p-type nitride semiconductor layer. The n-side electrode (40) has an aluminum part, and the aluminum part is in contact with the surface of the n-type nitride semiconductor layer. The nitrogen atom concentration in the surface of the n-type nitride semiconductor layer, said surface being in contact with the aluminum part, is higher than the gallium atom concentration in the surface of the n-type nitride semiconductor layer, said surface being in contact with the aluminum part.

Description

Nitride semiconductor multilayer structure, nitride semiconductor light emitting device including the nitride semiconductor multilayer structure, and method for manufacturing the nitride semiconductor multilayer structure

The present invention relates to a nitride semiconductor multilayer structure including an n-type nitride semiconductor layer having a nonpolar or semipolar surface, a nitride semiconductor light emitting device including the nitride semiconductor multilayer structure, and the nitride semiconductor The present invention relates to a method for manufacturing a laminated structure.

A nitride semiconductor having nitrogen (N) as a group V element is considered promising as a material for a short-wavelength light-emitting element because of its band gap. In particular, research on gallium nitride-based compound semiconductors (GaN-based semiconductors: Al _x Ga _y In _z N (where x + y + z = 1, 0 ≦ x, z <1, 0 <y ≦ 1)) is active. Blue light emitting diode (LED) elements, green LED elements, and blue semiconductor laser elements have also been put into practical use.

A GaN-based semiconductor has a wurtzite crystal structure. FIG. 1 schematically shows a unit cell of GaN. In the GaN-based semiconductor, a part of gallium (Ga) shown in FIG. 1 can be replaced with at least one of aluminum (Al) and indium (In).

2 (a), 2 (b), and 2 (c) show the plane orientation of the wurtzite crystal structure in four-index notation (hexagonal crystal index). In the 4-index notation, the crystal plane and its plane orientation are represented using basic vectors represented by a ₁ , a ₂ , a ₃ and c. The basic vector c extends in the [0001] direction, and this direction is called “c-axis”. A plane perpendicular to the c-axis is called a “c-plane” or “(0001) plane”. FIG. 2A shows an a-plane “= (11-20) plane” and an m-plane “= (1-100) plane” in addition to the c-plane. FIG. 2B shows the r plane “= (1-102) plane”, and FIG. 2C shows the (11-22) plane. In the present specification, the symbol “-” attached to the left side of the number in parentheses representing the Miller index represents the inversion of the index for convenience, and corresponds to “bar” in the figure.

Conventionally, when manufacturing a semiconductor element using a GaN-based semiconductor, a c-plane substrate, that is, a substrate having a (0001) plane as a main surface is used as a substrate on which a GaN-based semiconductor crystal is grown. However, in the c-plane, since the arrangement of the atomic layer of gallium (Ga) and the atomic layer of nitrogen (N) is slightly shifted in the c-axis direction, spontaneous polarization (Electrical Polarization) is generated. For this reason, the “c plane” is also called a “polar plane”. As a result of polarization, a piezo electric field is generated along the c-axis direction in the quantum well layer made of InGaN constituting the active layer of the nitride semiconductor light emitting device. When a piezo electric field is generated in the active layer, a position shift occurs in the distribution of electrons and holes in the active layer, and the internal quantum efficiency of the active layer is reduced due to the quantum confinement Stark effect of carriers. For this reason, in the case of a semiconductor laser element, an increase in threshold current is caused. If it is an LED element, the increase in power consumption and the fall of luminous efficiency will be caused. In addition, since the piezo electric field screening occurs as the injected carrier density increases, the emission wavelength also changes.

Therefore, in recent years, it has been studied to use a substrate (m-plane GaN-based substrate) that has a (10-10) plane called a m-plane, which is a nonpolar plane, for example, perpendicular to the [10-10] direction. ing. As shown in FIG. 2A, the m plane is a plane parallel to the c axis (basic vector c) and is orthogonal to the c plane. In the m plane, Ga atoms and N atoms are located on the same atomic plane, and therefore no polarization occurs in a direction perpendicular to the m plane. For this reason, if a semiconductor multilayer structure is formed in a direction perpendicular to the m-plane, a piezoelectric field is not generated in the active layer, so that the problem of a decrease in internal quantum efficiency due to the quantum confinement Stark effect of carriers can be solved. This is the same for non-polar surfaces other than the m-plane, and a similar effect can be obtained for a semi-polar surface.

The m-plane is a general term for the (10-10) plane, the (-1010) plane, the (1-100) plane, the (-1100) plane, the (01-10) plane, and the (0-110) plane. In this specification, “X-plane growth” means that epitaxial growth occurs in a direction perpendicular to the X-plane (X = c, m, etc.) of the hexagonal wurtzite structure. In X-plane growth, the X plane may be referred to as a “growth plane”, and a semiconductor layer formed by the X plane growth may be referred to as an “X plane semiconductor layer”.

Japanese Patent No. 4605193 Japanese Patent Laid-Open No. 55-9442 Japanese Patent No. 2783349 Japanese Patent No. 2967743

However, in the above conventional technique, reduction of contact resistance has been demanded.

The present invention has been made in view of this problem, and an object thereof is to reduce contact resistance.

In order to solve the above problems, the nitride semiconductor multilayer structure of one embodiment of the present invention includes an n-side electrode and an n-type nitride semiconductor layer having a nonpolar or semipolar surface. The n-side electrode includes an aluminum portion, the aluminum portion is in contact with the surface of the n-type nitride semiconductor layer, and the concentration of nitrogen atoms on the surface of the n-type nitride semiconductor layer in contact with the aluminum portion is It is higher than the concentration of gallium atoms on the surface of the n-type nitride semiconductor layer that is in contact.

Another embodiment of the present invention is a nitride semiconductor light emitting device comprising the following: an n-side electrode, a p-side electrode, an n-type nitride semiconductor layer having a nonpolar or semipolar surface, A p-type nitride semiconductor layer electrically connected to the p-side electrode, and an active layer sandwiched between the n-type nitride semiconductor layer and the p-type nitride semiconductor layer. The n-side electrode includes an aluminum portion. The aluminum portion is in contact with the surface of the n-type nitride semiconductor layer. The concentration of nitrogen atoms on the surface of the n-type nitride semiconductor layer in contact with the aluminum portion is higher than the concentration of gallium atoms on the surface of the n-type nitride semiconductor layer in contact with the aluminum portion.

Further, another aspect of the present invention is a light source device including a nitride semiconductor light emitting device and a wavelength conversion unit including a fluorescent material that converts the wavelength of light emitted from the nitride semiconductor light emitting device.

Another aspect of the present invention is a method for manufacturing a nitride semiconductor multilayer structure, which includes the following steps: (a) an n-type nitride semiconductor layer having a nonpolar or semipolar surface. Irradiating the surface of the substrate with oxygen plasma, (b) contacting the surface irradiated with the oxygen plasma in step (a) with acid, and (c) contacting the surface with the acid in step (b) with aluminum. Forming a portion;

According to the present invention, the contact resistance can be reduced.

FIG. 1 is a diagram showing a crystal structure of a GaN-based semiconductor in a stick ball model. FIG. 2A is a perspective view showing the basic vectors a ₁ , a ₂ , a ₃ and c of the wurtzite crystal structure and the a, c and m planes. FIG. 2B is a perspective view showing the r-plane of the wurtzite crystal structure. FIG. 2C is a perspective view showing the (11-22) plane of the wurtzite crystal structure. FIG. 3A is a schematic cross-sectional view showing a nitride semiconductor light emitting device according to an embodiment. FIG. 3B is a diagram showing the crystal structure of the m-plane in the nitride semiconductor by a stick ball model. FIG. 3C is a diagram showing the crystal structure of the c-plane in the nitride semiconductor by a stick ball model. FIG. 4 is a schematic cross-sectional view showing a light source device according to an embodiment. 5 (a) and 5 (b) are reference examples showing the electrical characteristics of an n-side electrode in which aluminum (Al) is applied to m-plane GaN, and FIG. 5 (a) shows the voltage between the n-side electrodes. FIG. 5B is a graph showing the current-voltage characteristics between the n-side electrodes for each sinter temperature. FIG. 5C is a schematic diagram showing an evaluation method for evaluating electrical characteristics. 6 (a) and 6 (b) are reference examples, showing electrical characteristics of an n-side electrode in which a laminated film of titanium (Ti) / aluminum (Al) is applied to m-plane GaN, and FIG. ) Is a graph showing the relationship between the voltage between the n-side electrodes and the sintering temperature, and FIG. 6B is a graph showing the current-voltage characteristics between the n-side electrodes for each sintering temperature. 7A and 7B are schematic cross-sectional views in order of steps showing the method for manufacturing a nitride semiconductor multilayer structure according to one embodiment. 8A and 8B show the electrical characteristics of the n-side electrode in the nitride semiconductor multilayer structure according to one embodiment, and FIG. 8A shows the relationship between the voltage between the n-side electrodes and the sintering temperature. FIG. 8B is a graph showing the current-voltage characteristics between the n-side electrodes for each sintering temperature. FIG. 9 shows a comparison between the electrical characteristics of the n-side electrode in the nitride semiconductor multilayer structure according to one embodiment and the electrical characteristics of the n-side electrode to which Al is applied and the n-side electrode to which Ti / Al is applied. It is a graph. FIG. 10 is a graph comparing current-voltage characteristics of a nitride semiconductor light emitting device according to one embodiment with a nitride semiconductor light emitting device having an n-side electrode to which Ti / Al is applied. FIGS. 11A and 11B are reference examples, and the electrical characteristics of an n-side electrode in which a Ti / Al laminated film is applied to m-plane GaN subjected to “O ₂ plasma irradiation + hydrofluoric acid treatment”. FIG. 11A is a graph showing the relationship between the voltage between the n-side electrodes and the sintering temperature, and FIG. 11B is a graph showing the current-voltage characteristics between the n-side electrodes for each sintering temperature. is there. FIG. 12A is a reference example, in which no sintering treatment is performed on an n-side electrode in which a Ti / Al laminated film is applied to m-plane GaN subjected to “O ₂ plasma irradiation + hydrofluoric acid treatment”. Are photographs observed with an optical microscope from the back side according to the sintering temperature. FIG. 12B is a schematic cross-sectional view showing an observation method. FIGS. 13A and 13B are reference examples, and show the electrical characteristics of an n-side electrode in which Ti is applied to m-plane GaN subjected to “O ₂ plasma irradiation + hydrofluoric acid treatment”. (A) is a graph showing the relationship between the voltage between the n-side electrodes and the sintering temperature, and FIG. 13 (b) is a graph showing the current-voltage characteristics between the n-side electrodes for each sintering temperature. 14 (a) and 14 (b) show the electrical characteristics of an n-side electrode in which various metal materials are applied to m-plane GaN subjected to “O ₂ plasma irradiation + hydrofluoric acid treatment”. ) Is a graph showing respective voltages between the n-side electrodes, and FIG. 14B is a graph showing current-voltage characteristics between the n-side electrodes for each metal material. 15 (a) and 15 (b) show an example of m-plane GaN subjected to “no O ₂ plasma”, “O ₂ plasma irradiation” and “O ₂ plasma irradiation + hydrofluoric acid treatment” for m-plane GaN. FIG. 15A is a graph showing each voltage between the n-side electrodes, and FIG. 15B is a current-voltage characteristic between the n-side electrodes. It is a graph which shows. 16 (a) to 16 (c) show the depth dependence of oxygen (O) concentration and carbon (C) concentration in m-plane GaN having an n-side electrode in which Al is applied to m-plane GaN. (A) is a graph when not irradiating m-plane GaN with O ₂ plasma, FIG. 16 (b) is a graph when m-plane GaN is irradiated with O ₂ plasma, and FIG. 16 (c) is m-plane. it is a graph when subjected to irradiation and hydrofluoric acid treatment of the O ₂ plasma GaN. FIG. 17 (a) not irradiated with O ₂ plasma samples, the sample was irradiated with O ₂ plasma, and with respect to O ₂ plasma irradiation and each sample subjected to hydrofluoric acid treatment, indicating the depth dependence of the carrier density It is a graph. FIG. 17B is a graph in which the vertical axis of FIG. 17A is normalized with no O ₂ plasma. Figure 18 is a sample not irradiated with O ₂ plasma, the sample was irradiated with O ₂ plasma, and with respect to O ₂ plasma irradiation and each sample subjected to hydrofluoric acid treatment, is a graph showing the evaluation results of evaluating the photoluminescence . FIG. 19 is a graph showing a profile in the depth direction of Ga atoms before heat treatment. FIG. 20 is a graph showing a profile of Ga atoms in the depth direction after sintering. FIG. 21 is a graph showing the relationship between the voltage generated between the test patterns constituting the n-side electrode and the plasma processing conditions. FIG. 22 is a graph showing the relationship between plasma input power and contact resistance value. FIG. 23 is an enlarged partial cross-sectional view of region A shown in FIG. FIG. 24 is a partial cross-sectional view according to a first modification of the configuration shown in FIG. 25 is a partial cross-sectional view according to a second modification of the configuration shown in FIG. FIG. 26 is a partial cross-sectional view according to a third modification of the configuration shown in FIG.

One form of the present disclosure is a nitride semiconductor multilayer structure including the following: an n-side electrode, and an n-type nitride semiconductor layer having a nonpolar or semipolar surface, where the n-side electrode Comprises an aluminum portion, the aluminum portion is in contact with the surface of the n-type nitride semiconductor layer, and the concentration of nitrogen atoms on the surface of the n-type nitride semiconductor layer in contact with the aluminum portion is in contact with the aluminum portion. It is higher than the concentration of gallium atoms on the surface of the n-type nitride semiconductor layer.

In the nitride semiconductor multilayer structure according to one embodiment, the aluminum portion may be made of only aluminum.

In the nitride semiconductor multilayer structure according to one embodiment, the aluminum portion may have a layer shape parallel to the n-type nitride semiconductor layer.

A plurality of aluminum portions may be provided in the nitride semiconductor multilayer structure according to one embodiment.

In the nitride semiconductor multilayer structure according to one embodiment, the surface of the n-type nitride semiconductor layer in contact with the aluminum portion may have gallium vacancies.

In the nitride semiconductor multilayer structure according to one embodiment, the surface region of the n-type nitride semiconductor layer in contact with the aluminum portion may have a thickness of 10 nm or more and 150 nm or less.

In the nitride semiconductor multilayer structure according to one embodiment, the n-type nitride semiconductor layer may have an m-plane surface.

In the nitride semiconductor multilayer structure according to one embodiment, the surface of the n-type nitride semiconductor layer in contact with the aluminum portion has a lower carrier density than the portion of the n-type nitride semiconductor layer not in contact with the aluminum portion. You may do it.

Further, another embodiment of the present disclosure is a nitride semiconductor light emitting device including the following: an n-type electrode, a p-side electrode, an n-type nitride semiconductor layer having a nonpolar surface or a semipolar surface, a p-type nitride semiconductor layer electrically connected to the p-side electrode, and an active layer sandwiched between the n-type nitride semiconductor layer and the p-type nitride semiconductor layer, wherein the n-side electrode is an aluminum portion And the aluminum portion is in contact with the surface of the n-type nitride semiconductor layer, and the concentration of nitrogen atoms in the surface of the n-type nitride semiconductor layer in contact with the aluminum portion is n-type in contact with the aluminum portion. It is higher than the concentration of gallium atoms on the surface of the nitride semiconductor layer.

In the nitride semiconductor light emitting device according to another embodiment, the aluminum portion may be made of only aluminum.

In the nitride semiconductor light emitting device according to another embodiment, the aluminum portion may have a layer shape parallel to the n-type nitride semiconductor layer.

The nitride semiconductor light emitting device according to another embodiment may be provided with a plurality of aluminum portions.

In the nitride semiconductor light emitting device according to another embodiment, the surface of the n-type nitride semiconductor layer in contact with the aluminum portion may have gallium vacancies.

In the nitride semiconductor light emitting device according to another embodiment, the surface region of the n-type nitride semiconductor layer in contact with the aluminum portion may have a thickness of 10 nm or more and 150 nm or less.

In the nitride semiconductor light emitting device according to another embodiment, the n-type nitride semiconductor layer may have an m-plane surface.

In another aspect of the nitride semiconductor light emitting device, the surface of the n-type nitride semiconductor layer in contact with the aluminum portion has a lower carrier density than the portion of the n-type nitride semiconductor layer not in contact with the aluminum portion. You may have.

Another embodiment of the present disclosure is a method for manufacturing a nitride semiconductor multilayer structure, which includes the following steps: (a) an n-type nitride semiconductor layer having a nonpolar or semipolar surface. Irradiating the surface of the substrate with oxygen plasma, (b) contacting the surface irradiated with the oxygen plasma in step (a) with acid, and (c) contacting the surface with the acid in step (b) with aluminum. Forming a portion;

The method according to another embodiment, wherein the aluminum portion may be made of only aluminum.

In another method, the aluminum portion may have a layer shape parallel to the n-type nitride semiconductor layer.

In the method according to another embodiment, a plurality of aluminum portions may be provided.

In another method, the surface of the n-type nitride semiconductor layer in contact with the aluminum portion may have gallium vacancies.

In another method, the surface region of the n-type nitride semiconductor layer in contact with the aluminum portion may have a thickness of 10 nm or more and 150 nm or less.

In another method, the n-type nitride semiconductor layer may have an m-plane surface.

In this specification, a nitride semiconductor multilayer structure having an n-type GaN layer having an m-plane as a main surface will be described as an example. However, the same can be said for a nitride semiconductor multilayer structure having another n-type nitride semiconductor layer having an m-plane as a main surface. Also, semipolar planes such as -r plane, r plane, (20-21) plane, (20-2-1) plane, (10-1-3) plane and (11-22) plane, or a plane, etc. The same can be said for a nitride semiconductor multilayer structure having an n-type GaN layer or other n-type nitride semiconductor layer whose main surface is a non-polar surface.

By the way, although the m-plane GaN-based semiconductor element can exhibit a remarkable effect as compared with the c-plane GaN-based semiconductor element, the technology for forming the n-side ohmic electrode for the m-plane GaN has not yet been completed.

For example, Patent Document 1 describes that formation of an n-side ohmic electrode for m-plane GaN is more difficult than formation of an n-side ohmic electrode for c-plane GaN. As a solution to this, the main surface of the m-plane GaN is etched, for example, in a stripe shape to intentionally expose the c-plane GaN on the stripe-shaped sidewall, and the exposed portion is effective for the c-plane GaN. A technique is described in which an electrode made of Ti / Al is formed, and thereafter, sintering is performed at a temperature of about 500 ° C.

That is, the technique described in Patent Document 1 is not a direct n-side ohmic electrode formation technique for m-plane GaN. In this method, since the etching is performed on the m-plane GaN, the process flow at the time of manufacturing the device is limited. In addition, since a sintering process at a temperature of about 500 ° C. is performed in order to obtain ohmic characteristics, the manufacturing process is further restricted.

Therefore, the present inventors have intensively studied to newly develop a technique for forming an n-side ohmic electrode for a nitride semiconductor layer having a nonpolar or semipolar surface such as m-plane GaN. The present inventors have found a method capable of forming an n-side ohmic electrode with extremely low resistance.

(One embodiment)
Hereinafter, a nitride semiconductor multilayer structure according to an embodiment, a nitride semiconductor light-emitting element using the same, and a light source device using the same will be described with reference to the drawings. In the following drawings, components having substantially the same function are denoted by the same reference numerals for the sake of brevity. Note that the present disclosure is not limited to the following embodiments.

FIG. 3A schematically shows a cross-sectional configuration of the nitride semiconductor light emitting device according to this embodiment. A nitride semiconductor light emitting device 100 shown in FIG. 3A is a semiconductor device formed of a GaN-based semiconductor, and includes an n-type nitride semiconductor layer 22, an active layer 24 formed of a nitride semiconductor, and p-type nitride. The semiconductor multilayer structure 20 includes a physical semiconductor layer 26.

Specifically, the nitride semiconductor light emitting device 100 according to this embodiment includes a GaN-based substrate 10 having an m-plane as a main surface 10a, and a semiconductor multilayer structure 20 formed on the main surface 10a of the GaN-based substrate 10. A p-side electrode 30 formed on the p-type nitride semiconductor layer 26 in the semiconductor multilayer structure 20, and an n-side electrode 40 formed on the exposed n-type nitride semiconductor layer 22. It has.

The p-type nitride semiconductor layer 26 is, for example, an Al _d Ga _e N (where d + e = 1, 0 ≦ d, 0 <e) layer. The surface modification layer 23 may be a contact layer. The GaN-based substrate 10 is, for example, a GaN substrate. The n-side electrode 40 may be a metal part. The n-side electrode 40 is formed on the surface modification layer 23 provided on the exposed surface of the n-type nitride semiconductor layer 22 exposed from the recess 42 obtained by etching the semiconductor multilayer structure 20. The n-type nitride semiconductor layer 22 is, for example, an Al _u Ga _v In _w N (where U + V + W = 1, 0 ≦ U, 0 ≦ W, 0 <V) layer.

In this embodiment, the semiconductor multilayer structure 20 is an m-plane semiconductor multilayer structure formed by m-plane growth, and its main surface is an m-plane. There are cases where a-plane GaN grows on the r-plane sapphire substrate, and depending on the growth conditions, it is not always necessary that the main surface of the GaN-based substrate 10 is the m-plane. Recess 42 of n-type nitride semiconductor layer 22 in contact with n-side electrode 40 has a surface that is an m-plane.

The nitride semiconductor light emitting device 100 according to this embodiment includes the GaN-based substrate 10 that holds the semiconductor multilayer structure 20, but may include another holding substrate instead of the GaN-based substrate 10. Further, the holding substrate may be removed.

FIG. 3B schematically shows a crystal structure in a nitride semiconductor cross section (a cross section in a direction perpendicular to the surface of the substrate) of the nitride semiconductor whose main surface is the m-plane. Since Ga atoms and N atoms are located on the same atomic plane parallel to the m-plane, no polarization occurs in the direction perpendicular to the m-plane. That is, the m-plane is a nonpolar plane, and no piezo electric field is generated in the active layer grown in the direction perpendicular to the m-plane. The added In and Al are located at the Ga site and replace Ga. Even if at least part of Ga is substituted with In or Al, no polarization occurs in the direction perpendicular to the m-plane.

A GaN-based substrate having an m-plane as a main surface is also referred to as an “m-plane GaN-based substrate” in this specification. To obtain an m-plane nitride semiconductor stacked structure grown in a direction perpendicular to the m-plane, typically, an m-plane GaN substrate is used, and a nitride semiconductor can be grown on the m-plane of the m-plane GaN substrate. That's fine. This is because the plane orientation of the main surface of the m-plane GaN-based substrate is reflected in the plane orientation of the semiconductor product structure. However, as described above, the main surface of the substrate is not necessarily the m-plane, and the substrate may be removed from the final device.

For reference, FIG. 3C schematically shows a crystal structure of a nitride semiconductor cross section (cross section in a direction perpendicular to the surface of the substrate) of the nitride semiconductor whose principal surface is the c plane. Ga atoms and N atoms do not exist on the same atomic plane parallel to the c-plane. As a result, polarization occurs in a direction perpendicular to the c-plane. A GaN-based substrate having a c-plane as a main surface is also referred to as a “c-plane GaN-based substrate” in this specification.

The c-plane GaN-based substrate is a general substrate for growing GaN-based semiconductor crystals. As described above, since the positions of the Ga atomic layer and the N atomic layer parallel to the c-plane are slightly shifted in the c-axis direction, polarization is formed along the c-axis direction.

Again, the detailed structure of the semiconductor stacked structure 20 will be described with reference to FIG.

On the main surface 10a of the GaN-based substrate 10, a semiconductor multilayer structure 20 is formed by epitaxial growth. The semiconductor stacked structure 20 includes an n-type nitride semiconductor layer 22 formed on the GaN-based substrate 10 and an Al _a In _b Ga _c N layer (a + b + c = 1) formed on the n-type nitride semiconductor layer 22. , A ≧ 0, b ≧ 0, c ≧ 0), and a p-type nitride semiconductor layer 26 formed on the active layer 24. Here, the active layer 24 is an electron injection region in the nitride semiconductor light emitting device 100.

An undoped GaN layer may be provided between the active layer 24 and the p-type nitride semiconductor layer 26. The upper portion of the p-type nitride semiconductor layer 26, that is, the upper surface portion of the semiconductor multilayer structure 20, may be composed of a GaN layer having an Al composition d of zero. Thereby, contact resistance can be reduced.

In the p-type nitride semiconductor layer 26, the Al composition d need not be uniform in the thickness direction. In the p-type nitride semiconductor layer 26, the Al composition d may change continuously or stepwise in the thickness direction. That is, the p-type nitride semiconductor layer 26 may have a multilayer structure in which a plurality of layers having different Al compositions d are stacked, and the dopant concentration may also change in the thickness direction.

A p-side electrode 30 is formed on the semiconductor multilayer structure 20. The p-side electrode 30 according to the present embodiment is an electrode formed from silver (Ag). In the present embodiment, the p-side electrode 30 is in contact with the p-type nitride semiconductor layer 26 doped with a dopant whose conductivity type is p-type. The p-type nitride semiconductor layer 26 is doped with, for example, magnesium (Mg) as a dopant. As a p-type dopant other than Mg, for example, zinc (Zn) or beryllium (Be) may be doped.

As shown in FIG. 3A, in the configuration according to this embodiment, a recess 42 exposing a part of the n-type nitride semiconductor layer 22 formed on the GaN-based substrate 10 is formed. Here, the thickness of the n-type nitride semiconductor layer 22 is not less than 0.2 μm and not more than 2 μm, for example. A surface modification layer 23 is provided on a part of the exposed surface of the n-type nitride semiconductor layer 22 exposed from the recess 42. This exposed surface is an m-plane. The n-side electrode 40 is formed on the surface modified layer 23 in the exposed n-type nitride semiconductor layer 22 in contact with the surface modified layer 23.

The surface modification layer 23 provided on the n-type nitride semiconductor layer 22 is formed by irradiating the n-type nitride semiconductor layer 22 with oxygen (O ₂ ) plasma. For example, in the case of a reactive ion etching (RIE) type O ₂ plasma apparatus (for example, RIE-10NR manufactured by samco), the pressure of O ₂ is 10 Pa and the gas flow rate is 40 ml / min (standard state). ) And an RF power of 40 W, and irradiation with O ₂ plasma for 30 seconds makes it possible to form a suitable surface modified layer 23.

In order to reduce the contact resistance between the n-type nitride semiconductor layer 22 and the n-side electrode 40, the surface modification layer 23 is formed, and then the surface of the formed surface modification layer 23 is subjected to acid treatment. Also good. For the acid treatment, for example, hydrofluoric acid or buffered hydrofluoric acid can be used. The acid treatment time when hydrofluoric acid or buffered hydrofluoric acid is used can be set to 30 seconds, for example.

The n-side electrode 40 is made of, for example, a single layer of aluminum (Al), and the thickness thereof can be, for example, 5 nm or more and 1000 nm or less. A single layer of Al can be deposited by, for example, a vacuum deposition method (such as a resistance heating method or an electron beam method).

In order to further reduce the contact resistance, sintering may be performed after the n-side electrode 40 is formed. For example, the n-side electrode 40 can be made a more suitable n-side ohmic electrode by performing a sintering process at 300 ° C. for 10 minutes in a nitrogen (N ₂ ) atmosphere. The sintering temperature can be arbitrarily selected within a range of 200 ° C. or more and 700 ° C. or less, for example. The sintering temperature may be 300 ° C. or more and 600 ° C. or less. In this sintering temperature region, the contact resistance takes the lowest value (specific contact resistance 3 × 10 ⁻⁵ Ωcm ² ). The sintering temperature may be 300 ° C. or lower or 500 ° C. or lower. As a result, the manufacturing process of the light-emitting element is facilitated, and the reliability of the m-plane light-emitting element that is easily affected by heat can be improved. Further, the sintering process may not be performed.

The effect of reducing the n-side contact resistance according to the present embodiment is obtained when aluminum (Al) atoms come into contact with the surface modification layer 23. Therefore, the n-side electrode 40 does not necessarily need to be a single layer or a single Al, and may be a metal layer containing Al as a main component, that is, an Al alloy. The concentration of Al is, for example, 50% by mass or more. Alternatively, a metal multilayer film in which Al is in contact with the surface modification layer 23 may be used. Since Al has a higher reflectance and thermal conductivity than Ti, when an Al electrode is used, light absorption can be reduced and heat dissipation can be improved compared to the case where a Ti / Al electrode is used. Thereby, the light output can be improved.

Even if the n-side electrode 40 does not form an Al alloy at the time of formation, the n-side electrode 40 finally becomes an Al alloy in the sintering process after the n-side electrode 40 is formed, and Al atoms If they are in contact with the surface modification layer 23, they belong to the technical scope of this embodiment.

For example, as will be described later, when the n-side electrode 40 is composed of a Ti / Al film in which titanium (Ti) having a thickness of 10 nm and aluminum (Al) having a thickness of 500 nm are stacked thereon, a sinter is used. Unless the treatment is performed, the surface modified layer 23 and Ti are in direct contact and the surface modified layer 23 and Al are not in contact with each other. However, by performing a sintering process at a high temperature of 500 ° C. or higher, Al is mixed with Ti and alloyed, and as a result, Al atoms come into contact with the surface modification layer 23. In this case, an n-side ohmic electrode having a low contact resistance can be realized. Note that another electrode layer or another wiring layer made of a metal or an alloy may be formed on the p-side electrode 30 and the n-side electrode 40.

The thickness of the GaN-based substrate 10 having the m-plane main surface 10a is, for example, 100 μm to 400 μm. If the thickness of the substrate is approximately 100 μm or more, handling of the wafer becomes easy. Note that the GaN-based substrate 10 according to the present embodiment may have a laminated structure as long as it has an m-plane main surface 10a made of a GaN-based material. That is, the GaN-based substrate 10 according to this embodiment includes a substrate in which at least the main surface 10a that is the uppermost surface is an m-plane. Therefore, the entire substrate may be GaN-based or a combination with other materials.

The active layer 24 according to the present embodiment is, for example, a multiple quantum well (MQW) made of GaInN / GaN in which well layers made of Ga _0.9 In _0.1 N and barrier layers made of GaN are alternately stacked. It has a structure. Here, the thicknesses of the well layer and the barrier layer are each 9 nm, for example, and the thickness of the active layer 24 is 81 nm, for example.

A p-type nitride semiconductor layer 26 is formed on the active layer 24. The thickness of the p-type nitride semiconductor layer 26 is, for example, not less than 0.2 μm and not more than 2 μm. As described above, an undoped GaN layer may be provided between the active layer 24 and the p-type nitride semiconductor layer 26.

Further, for example, a p-type GaN layer can be formed on the p-type nitride semiconductor layer 26. A contact layer made of p ⁺ -GaN may be formed on the p-type GaN layer, and the p-side electrode 30 may be formed on the contact layer. Note that the contact layer made of GaN may be considered as a part of the p-type nitride semiconductor layer 26 instead of being a semiconductor layer different from the p-type nitride semiconductor layer 26.

The nitride semiconductor light emitting device 100 according to this embodiment may be used as a light source device in this state. However, when combined with a resin material or the like in which a fluorescent material for wavelength conversion is dispersed, it can be used as a light source device with an expanded wavelength band, such as a white light source device.

FIG. 4 schematically shows a cross-sectional configuration of an example of the light source device. The light source device shown in FIG. 4 converts the nitride semiconductor light emitting device 100 according to this embodiment shown in FIG. 3A and the wavelength of light emitted from the nitride semiconductor light emitting device 100 into a longer wavelength. And a resin layer 200 in which a phosphor (for example, YAG: Yttrium Aluminum Garnet) is dispersed. The nitride semiconductor light emitting device 100 is fixed on a holding member 220 having a wiring pattern 210 formed on the surface thereof. On the holding member 220, a reflecting member 240 is disposed and fixed so as to surround the nitride semiconductor light emitting element 100. The resin layer 200 is formed inside the reflecting member 240 so as to cover the nitride semiconductor light emitting element 100.

As described above, the LED element has been described as the nitride semiconductor light emitting element 100 as an example of the embodiment, but the effect of reducing the contact resistance in the n-side electrode 40 is also obtained in a light emitting element other than the LED element, for example, a semiconductor laser element. be able to. Further, this effect is not limited to the light emitting element, and can naturally be obtained in a device such as a transistor or a light receiving element.

Incidentally, the main surface or surface of the m-plane semiconductor does not have to be a plane that is completely parallel to the m-plane, and may be inclined at a predetermined angle from the m-plane. The tilt angle is determined by the angle formed by the normal of the main surface in the nitride semiconductor layer and the normal of the non-tilted m-plane. The main surface or surface of the m-plane nitride semiconductor layer can be inclined from the non-inclined m-plane toward the vector direction represented by the c-axis direction and the a-axis direction. The absolute value of the inclination angle θ may be 5 ° or less or 1 ° or less in the c-axis direction. Further, it may be 5 ° or less or 1 ° or less in the a-axis direction. That is, in the present invention, the “m-plane” includes a plane inclined in a predetermined direction from the non-inclined m-plane within a range of ± 5 °. Within such a tilt angle range, the main surface or surface of the nitride semiconductor layer is entirely tilted from the m-plane, but a large number of m-plane regions are exposed microscopically. Conceivable. Thereby, it is considered that the surface inclined at an angle of 5 ° or less from the m-plane has the same properties as the m-plane.

In addition, about the -r plane, the r plane, the (20-21) plane, the (20-2-1) plane, the (10-1-3) plane and the (11-22) plane, and the a-plane The same can be said for. Accordingly, in the present invention, the “−r plane”, “r plane”, “(20-21) plane”, “(20-2-1) plane”, “(10-1-3) plane” and “( The “11-22) plane” and the “a plane” include a plane inclined in a predetermined direction within a range of ± 5 ° from these non-inclined planes.

Next, the nitride semiconductor multilayer structure according to this embodiment will be described in more detail, including the background to the present disclosure.

Non-Patent Document 1 describes that aluminum (Al) can be applied as a material of an n-side electrode for GaN (considering c-plane GaN because it is conventional GaN). Patent Document 2 also describes that Al can be applied as a material for the n-side electrode for GaN.

Therefore, the present inventors first applied aluminum (Al) as an n-side electrode material for m-plane GaN as a basic study experiment, and evaluated its electrical characteristics. The film thickness of Al was 500 nm. When the sintering process was performed, it was performed in a nitrogen atmosphere, and the sintering process time was 10 minutes.

5 (a) and 5 (b) show the results of electrical characteristics when a current of 20 mA is applied using Al as the electrode material of the n-side electrode provided on the m-plane GaN. FIG. 5C shows a schematic configuration of an electrical property evaluation method. As shown in FIG. 5C, on the main surface of the m-plane GaN layer 50, two n-side electrodes 51 each having a planar rectangular shape with a thickness of 500 nm, a length of 200 μm, and a width of 100 μm are only 20 μm from each other. Formed apart. The probe needle 55 connected to the current source 53 was brought into contact with the n-side electrode 51, and the current-voltage characteristics of the n-side electrode 51 were evaluated using a voltmeter 54 connected in parallel with the current source 53. FIG. 5A shows the relationship between the voltage between the n-side electrodes 51 and the sintering temperature when a current of 20 mA is applied between the n-side electrodes 51. FIG. 5B shows current-voltage characteristics between the n-side electrodes 51 for each sintering temperature. Since the measurement limit voltage of the evaluation apparatus used in this evaluation is 10V, data exceeding 10V is a reference value. In this specification, unless otherwise specified, the electrical characteristics of the n-side electrode are evaluated by the method described above.

As is clear from FIGS. 5A and 5B, ohmic characteristics can be obtained when aluminum (Al) is used as the n-side electrode material on the surface of m-plane GaN without any special treatment. It turned out not to be. Sintering was also attempted, but the properties deteriorated and there was no sign of ohmic properties. Thus, it has been clarified that an ohmic electrode cannot be formed for m-plane GaN even if Al used for c-plane GaN is directly applied as an n-side electrode material.

Therefore, in order to obtain ohmic characteristics, it is conceivable to change the n-side electrode material from aluminum (Al) to another metal. For example, it is conceivable to use titanium (Ti) / aluminum (Al) (see Patent Document 2), which is an n-side electrode material provided on c-plane GaN, for m-plane GaN. The present inventors applied Ti / Al as it is to m-plane GaN as an n-side electrode material provided on c-plane GaN, and evaluated the electrical characteristics. A Ti film and an Al film were sequentially deposited on the main surface of the m-plane GaN. Here, the thickness of the Ti film was 10 nm, and the thickness of the Al film was 500 nm. When the sintering process was performed, it was performed in a nitrogen atmosphere, and the sintering process time was 10 minutes.

6 (a) and 6 (b) show the results of electrical characteristics when a current of 20 mA is applied to an n-side electrode in which Ti / Al is applied to m-plane GaN. Here, the thickness of the Ti film is 10 nm, and the thickness of the Al film is 500 nm. A certain amount of n-side ohmic electrode could be formed by performing the sintering process in a high temperature region of 500 ° C. to 600 ° C. However, further reduction of contact resistance is required. That is, one of the greatest merits of using m-plane GaN as a light-emitting device is that a decrease in output due to a piezoelectric field is suppressed in a large current density region. For this reason, in consideration of operating in a large current density region not used in c-plane GaN, it is important to further reduce contact resistance.

Here, referring to FIGS. 7A and 7B, a configuration and a manufacturing method thereof for forming an n-side ohmic electrode with reduced contact resistance with respect to m-plane GaN according to the present embodiment. explain.

First, as shown in FIG. 7A, the m-plane GaN layer 50 is selectively irradiated with O ₂ plasma to form a surface modification layer 52 on the m-plane GaN layer 50. For example, in the case of an RIE type O ₂ plasma apparatus (for example, RIE-10NR manufactured by samco), the pressure of O ₂ is 10 Pa, the gas flow rate is 40 ml / min (standard state), and the RF power is 40 W. A suitable surface modification layer 52 can be formed by irradiating O ₂ plasma for 30 seconds. In the present embodiment, the entire surface of the m-plane GaN layer 50 is not irradiated with O ₂ plasma, but only the n-side electrode formation region is irradiated. Specifically, the resist film covers a portion excluding the n-side electrode formation region. Instead of this, the entire surface of the m-plane GaN layer 50 may be irradiated with O ₂ plasma.

Subsequently, the surface of the surface modification layer 52 is subjected to an acid treatment. For example, hydrofluoric acid or buffered hydrofluoric acid can be used for the acid treatment. The acid treatment time when using hydrofluoric acid or buffered hydrofluoric acid can be set to 30 seconds, for example.

Next, as shown in FIG. 7B, Al is deposited on the surface modification layer 52 as an electrode material. For example, the Al film can be deposited by a vacuum evaporation method (such as a resistance heating method or an electron beam method). Thereby, it will be in the state which the surface modification layer 52 and Al atom contact. The electrode material is not necessarily made of Al alone, but may be a metal layer containing Al as a main component, that is, an Al alloy.

FIGS. 8A and 8B show the n electrical characteristics of the n-side electrode formed by “O ₂ plasma irradiation + hydrofluoric acid treatment + Al” according to this embodiment when a current of 20 mA is applied. Yes. The thickness of the Al film is 500 nm. As can be seen from FIGS. 8A and 8B, a good n-side ohmic electrode is formed on the m-plane GaN without performing the sintering process. Further, the contact resistance is further reduced by performing a sintering process for 10 minutes at a temperature of 200 ° C. or more and 700 ° C. or less in a nitrogen atmosphere. The sintering temperature may be 300 ° C. or more and 600 ° C. or less. In this sintering temperature region, the contact resistance showed the lowest value (specific contact resistance 3 × 10 ⁻⁵ Ωcm ² ). The value of the specific contact resistance was separately evaluated using a transmission line matrix (TLM) method.

For comparison, FIG. 9 shows the electrical characteristics of the n-side electrode (corresponding to FIG. 8) according to this embodiment “O ₂ plasma irradiation + hydrofluoric acid treatment + Al”, and the electrical characteristics of the n-side electrode when Al is applied (FIG. 9). 5), and the electrical characteristics of the n-side electrode when Ti / Al is applied (corresponding to FIG. 6). From FIG. 9, it is clear that the n-side electrode according to the present embodiment has a great advantage.

In this embodiment, the sintering process for the Al film for the n-side electrode may not be performed. Even if the sintering process is performed at a temperature lower than usual, for example, 300 ° C. or lower, or 500 ° C. or lower, an n-side ohmic electrode having a low contact resistance can be formed. This greatly increases the degree of freedom of design related to the device manufacturing process flow. Also, according to the study by the present inventors, m-plane GaN is more susceptible to heat than c-plane GaN. It is very significant that an n-side ohmic electrode with low contact resistance can be formed without performing sintering on such m-plane GaN.

FIG. 10 shows the current-voltage characteristics of the nitride semiconductor light emitting device according to this embodiment, here the LED device. As can be seen from FIG. 10, by applying the above-described n-side electrode to the nitride semiconductor light emitting device, a significant reduction in operating voltage and a significant variation in electrical characteristics are realized.

Thus, it was demonstrated that the device characteristics can be significantly improved by actually applying the n-side electrode according to the present embodiment to a device.

Next, the fact that the n-side electrode according to the present embodiment cannot be easily conceived by those skilled in the art will be described below in two broad categories.

<First difficult difficulty>
As the first conceived difficulty, as shown in FIG. 5, as the n-side electrode material provided on the m-plane GaN, although the experimental results and evidence that aluminum (Al) cannot be applied at all have been obtained, For m-plane GaN whose surface has been modified by irradiation with O ₂ plasma, conversely, it has been found that a remarkable effect is manifested only in Al.

If a normal engineer examines Al as an n-side electrode material for m-plane GaN and obtains an experimental result that this cannot be applied, in the subsequent examination, as an n-side electrode material for m-plane GaN, It seems natural to remove Al from the candidate.

However, the present inventors dare to obtain extremely low contact resistance by using Al as the n-side electrode material and contacting Al atoms with m-plane GaN whose surface has been modified by O ₂ plasma irradiation. did it.

(Comparative Experiment 1)
As a comparative experiment 1, in FIGS. 11 (a) and 11 (b), n-plane GaN is irradiated with O ₂ plasma and hydrofluoric acid treatment to modify its surface, and then provided on the c-plane GaN. The electrical characteristics of the n-side electrode when a side electrode material of Ti / Al is deposited and a current of 20 mA is applied are shown.

From FIGS. 11A and 11B, no ohmic characteristics are obtained when the sintering process is not performed and when the sintering process is performed up to about 400 ° C. It can be seen that even if the modified layer is formed, an effect cannot be obtained if the metal in contact with the surface modified layer is titanium (Ti).

In the high-temperature sintering temperature region where the temperature is 500 ° C. or more and 600 ° C. or less, even in the case of Ti / Al, it looks as if it shows the same effect as aluminum (Al). However, this is due to solid phase diffusion or alloying between Ti / Al metals caused by high temperature sintering. As a result, Al atoms break through the Ti film, and the surface modification layer of m-plane GaN and Al atoms are in contact with each other. This is clear from the following verification result.

FIG. 12A is a photograph of an optical microscope observed from the back surface of the m-plane GaN layer 50 with respect to a sample in which the presence or absence of sintering treatment and the temperature of sintering treatment were changed in three ways. Specifically, as shown in FIG. 12B, the main surface of the m-plane GaN layer 50 is irradiated with O ₂ plasma and hydrofluoric acid to form a surface modification layer 52 on the surface. Thereafter, an n-side electrode 51 made of Ti / Al was formed. Thereafter, the sample not subjected to the sintering process and the sample subjected to the sintering process at temperatures of 400 ° C., 500 ° C., and 600 ° C. were observed with an optical microscope from the back surface of the m-plane GaN layer 50. Here, the thickness of the Ti film is 10 nm, and the thickness of the Al film is 500 nm.

As shown in FIG. 12A, in the sample not subjected to the sintering process, Ti is in contact with the surface modification layer 52 entirely. However, by performing a high-temperature sintering process at 500 ° C. or higher, Al atoms constituting the n-side electrode 51 are mixed with the Ti film by solid phase diffusion or alloying to break through the Ti film, It can be clearly seen that they are in contact. In the case where the sintering process is performed at 600 ° C., Al atoms break through almost the entire surface of the Ti film, both atoms are mixed and alloyed, and almost the entire surface of the interface with the m-plane GaN layer 50 is in contact with the Al atoms. It turns out that it is in a state.

(Comparative experiment 2)
As Comparative Experiment 2, in FIG. 13 (a) and FIG. 13 (b), after performing O ₂ plasma irradiation and hydrofluoric acid treatment, an n-side electrode made of only titanium (Ti) was formed, and a current of 20 mA was applied. The result of having investigated the electrical property at the time of applying is shown. When the sintering process is not performed, the metal in contact with GaN is Ti, as in the case of the sample not performing the sintering process on the n-side electrode made of Ti / Al, and thus its electrical characteristics are composed of Ti / Al. It was exactly the same as the case of the n-side electrode. However, when the high temperature sintering treatment at 500 ° C. or higher was performed, unlike the case of the n-side electrode made of Ti / Al, the ohmic characteristics were not shown at all, and the resistance remained high.

(Comparative Experiment 3)
As comparative experiment 3, in FIGS. 14 (a) and 14 (b), electrical characteristics when various electrode materials are deposited and a current of 20 mA is applied after O ₂ plasma irradiation and hydrofluoric acid treatment are performed. The result of having investigated is shown.

In the usual “metal-semiconductor interface junction theory”, a metal having a smaller work function value is more suitable as an n-side electrode material. Thus, in order to obtain ohmic characteristics, it is necessary to select an electrode material having a work function smaller than the work function value in the semiconductor, in this case, the work function value in the m-plane GaN. According to Non-Patent Document 2 described above, the work function of n-type m-plane GaN is 3.7 eV. Therefore, as an n-side electrode material, an electrode material having a work function smaller than 3.7 eV, that is, in the examination experiment shown in FIG. 14, magnesium (Mg) having a work function value of 3.66 eV is n-side electrode material. Seems to be optimal.

However, from FIG. 14 (a) and FIG. 14 (b), the case where Al having a work function value of 4.28 eV is used as the n-side electrode material is better than the case where Mg is used as the n-side electrode material. It can be seen that the electrical characteristics are remarkably excellent.

The above results cannot be logically derived from a simple “metal-semiconductor interface junction theory”. That is, it shows that the combination of m-plane GaN with modified surface and Al atoms is very specific.

Thus, it is clear from Comparative Experiment 1, Comparative Experiment 2, and Comparative Experiment 3 that the combination of m-plane GaN and Al atoms whose surfaces have been modified by O ₂ plasma irradiation and hydrofluoric acid treatment is specific. became. That is, when the Al atoms are not in contact with the surface-modified m-plane GaN, the contact resistance becomes extremely high, and when the Al atoms are in contact with the surface-modified m-plane GaN, It was discovered that an n-side ohmic electrode with extremely low contact resistance can be realized.

<Second difficult difficulty>
The second conceivable difficulty is that contact resistance is further reduced by adding acid treatment after the O ₂ plasma irradiation.

In Patent Document 4, as a technique for forming an n-side electrode provided on c-plane GaN, a method of depositing Ti / Al after performing O ₂ plasma irradiation is disclosed. Here, the purpose of irradiating with O ₂ plasma is to form an oxygen (O) doped layer on the outermost surface of GaN by irradiating with O ₂ plasma. Here, the O-doped layer is a compound layer of GaN and O in which O is introduced into GaN as a donor. That is, it is expected that a high concentration n-type carrier layer is formed on the outermost surface of GaN by O atoms contributing as donors. Therefore, a technique for reducing the contact resistance of the n-side electrode by increasing the carrier density on the outermost surface is widely known.

Based on this conventional technical common sense, the O-doped layer on the outermost surface of GaN is important. Therefore, after surface treatment for removing the O-doped layer, that is, after irradiating O ₂ plasma, the surface of GaN is treated with an acid. It is totally unreasonable to apply processing. In patent document 4, the examination of performing acid treatment after irradiating O ₂ plasma is also performed. In this case, as expected, an experimental result was obtained that the electrical characteristics of the n-side electrode deteriorated, and as a result, acid treatment after irradiation with O ₂ plasma was completely denied. Thus, acid treatment after irradiation with O ₂ plasma is not possible as normal technical common sense.

However, the present inventors perform acid treatment on the surface of GaN after being irradiated with O ₂ plasma. Moreover, the acid treatment is performed using hydrofluoric acid (HF), which has an extremely high ability to remove oxygen compounds among acids. According to conventional common general knowledge, if such an acid treatment is performed, the O-doped layer in the vicinity of the outermost surface of GaN is removed, and the electrical characteristics are considered to be greatly deteriorated. On the other hand, the present inventors can realize an n-side electrode having a contact resistance lower than that before hydrofluoric acid treatment by performing hydrofluoric acid treatment on the surface of GaN after being irradiated with O ₂ plasma. I found out.

15 (a) and 15 (b) show an n-side electrode using Al as the electrode material in each sample without O ₂ plasma, O ₂ plasma irradiation, and O ₂ plasma irradiation + hydrofluoric acid treatment. The result of an electrical property is shown. Here, the sintering process was performed at a temperature of 400 ° C. for 10 minutes. As described above, a remarkable effect is exhibited by the combination of depositing an Al film on m-plane GaN whose surface has been modified by O ₂ plasma irradiation.

What should be noted here is that the voltage is further reduced by almost an order of magnitude by performing acid treatment with hydrofluoric acid after irradiation with O ₂ plasma, that is, further reduction of contact resistance is realized. That's what it means. This is a result opposite to that of Patent Document 4 and cannot be theoretically derived from ordinary technical common sense.

[About the mechanism]
In order to elucidate the mechanism by which an n-side ohmic electrode with extremely low contact resistance with respect to m-plane GaN is realized, various analyzes and evaluations were performed.

Using a secondary ion mass spectrometer (SIMS), changes in the concentration of elements near the surface of GaN before and after the O ₂ plasma irradiation and before and after hydrofluoric acid treatment were analyzed.

FIG. 16A plots the oxygen (O) concentration and the carbon (C) concentration in the depth direction of the GaN layer in the first sample in which the Al film was deposited without irradiating the O ₂ plasma. It is a graph. The interface between the Al film and the GaN layer was defined as depth 0, and the GaN layer side was defined as the positive direction. That is, the negative side on the horizontal axis indicates the position in the Al film (electrode), and the positive side indicates the position in the GaN layer.

FIG. 16B is a graph in which the O concentration and the C concentration in the second sample in which the Al film is deposited after the O ₂ plasma irradiation are plotted in the depth direction of the GaN layer. It can be seen that O atoms increase while C atoms decrease due to the O ₂ plasma irradiation.

FIG. 16C is a graph in which the concentration of O and the concentration of C in the third sample in which an Al film is deposited after O ₂ plasma irradiation and hydrofluoric acid treatment are plotted in the depth direction of the GaN layer. It is. C concentration has not changed after the irradiation of the O ₂ plasma, it can be seen that O atoms which is increased by irradiation of O ₂ plasma is removed by hydrofluoric acid treatment.

As can be seen from FIG. 16 (b), the since the C atoms have been removed by irradiation of O ₂ plasma, the removal of carbon (C) contamination in the surface of the GaN layer due to irradiation with O ₂ plasma, i.e. the potential barrier due to impurity It is conceivable that the removal of this is one factor in the ohmic characteristics of the n-side electrode and the reduction in contact resistance.

Further, as can be seen from FIG. 16C, O atoms are removed after the hydrofluoric acid treatment. This is because, from the experimental results that the electrical characteristics of the n-side electrode were better after the hydrofluoric acid treatment than before the treatment, the removed O atoms were not caused by the “O-doped layer” but the potential. This is considered to be caused by a surface oxide film or a surface amorphous film that contributes as a barrier. Here, both the surface oxide film and the surface amorphous are expressed as Ga _x O _y .

Next, in order to clarify whether or not an O-doped layer is formed by O ₂ plasma irradiation, the depth direction distribution of the carrier density in the vicinity of the surface of the GaN layer is determined using a capacitance-voltage (CV) evaluation method. Examined.

FIG. 17 (a), the first sample is a m-plane GaN is not irradiated with _{O 2} plasma, the second sample of _{O 2} plasma is irradiated m-plane GaN, as well as _{O 2} plasma exposure, and hydrofluoric acid treatment of the in each went third sample is a m-plane GaN, a plot of carrier density _(N D -N _a) in the depth direction of the GaN layer. From the second sample, it can be seen that the carrier density in the vicinity of the surface of the GaN layer, that is, the region from the surface to about 150 nm is reduced by the O ₂ plasma irradiation. Further, it can be seen from the third sample that the carrier density is almost unchanged even when hydrofluoric acid treatment is performed after the O ₂ plasma is irradiated.

As described in Patent Document 4, if, as long as the O-doped layer is formed by irradiation of O ₂ plasma, should the carrier density increases near the surface of the GaN layer after irradiation with O ₂ plasma It is. Further, as described in Patent Document 4, if the O-doped layer is removed by the acid treatment after the plasma irradiation, the carrier density should return to the original after the hydrofluoric acid treatment.

However, as the experimental result according to the present embodiment, let alone be irradiated with O ₂ plasma with respect to m-plane GaN layer carrier density is increased, it is reduced to the contrary. Also, the carrier density is almost unchanged by the subsequent hydrofluoric acid treatment. These results mean that the O-doped layer is not formed even if the m-plane GaN layer is irradiated with O ₂ plasma. Therefore, it strongly suggests that the physical phenomenon occurring is substantially different from the case of irradiation of O ₂ plasma to c-plane GaN described in Patent Document 4.

FIG. 17 (b), a second sample was irradiated with O ₂ plasma, and O ₂ plasma irradiation and the respective carrier densities of the third sample subjected to hydrofluoric acid treatment, first without irradiating the O ₂ plasma It is the graph which normalized the carrier density of the sample of, ie, showed the value of ratio of carrier density. For example, it can be seen that the carrier density is reduced by about 0.6 times compared to the first sample at a position where the depth is 90 nm from the surface of the m-plane GaN layer.

Although the cause of the decrease in carrier density is not clear, for example, as one physical model, crystal defects are formed near the surface of the m-plane GaN layer by O ₂ plasma irradiation, and the formed crystal defects trap donor carriers. Therefore, it can be considered that the carrier density decreases.

Next, in order to clarify the presence or absence of the formation of crystal defects by irradiation with O ₂ plasma, the surface of the m-plane GaN layer was evaluated by photoluminescence.

Figure 18 is a first sample not irradiated with O ₂ plasma, a second sample was irradiated with O ₂ plasma, and for each of the third sample subjected to irradiation and hydrofluoric acid treatment of the O ₂ plasma, photo The result of performing luminescence evaluation (PL evaluation) is shown. In the case of the second sample irradiated with O ₂ plasma, the band edge emission in GaN was reduced by about one digit. This result suggests that crystal defects that can act as non-luminescent centers were formed by O ₂ plasma irradiation. Here, no yellow luminescence is observed in the wavelength region of 500 nm to 600 nm due to nitrogen vacancies which are typical crystal defects in GaN. From this, it is highly possible that some complex defects other than nitrogen vacancies are formed. In addition, even if hydrofluoric acid treatment is performed after irradiation of O ₃ plasma, which is the third sample, the PL spectrum is unchanged, so that it can be seen that the crystal defect region is not removed by hydrofluoric acid treatment.

From the above experimental results, it is considered that crystal defects were formed near the surface of the GaN layer by irradiating the surface of the GaN layer with O ₂ plasma. The present inventors have sought a mechanism that a good n-side ohmic electrode is realized with respect to m-plane GaN by a tunnel current through which a crystal defect level is interposed. However, even if the surface of the m-plane GaN layer is modified by irradiation with O ₂ plasma, no effect will be produced unless Al atoms are in contact therewith. I can't. It is important to elucidate why Al atoms are essential for the manifestation of effects.

Here, the “surface modified layer” formed in the vicinity of the GaN surface by the O ₂ plasma irradiation is defined again.

From the evaluation of photoluminescence shown in FIG. 18, it is considered that the surface modified layer contains crystal defects. When it is assumed that one crystal defect traps one carrier, referring to FIG. 17A, for example, the carrier density is 0.7 nm at a depth of 90 nm from the surface of the GaN layer. Since it is decreased by × 10 ¹⁷ cm ⁻³ , the crystal defect density in this region is considered to be 0.7 × 10 ¹⁷ cm ⁻³ . However, it is difficult to determine the crystal defect density as long as the influence of O ₂ plasma irradiation on the surface of the GaN layer has not been elucidated.

In the present embodiment, the surface modified layer is defined as a region having a carrier density lower than the carrier density when not irradiated with O ₂ plasma. More specifically, for example, it can be defined as a region having a carrier density of 0.9 times or less of the carrier density when not irradiated with O ₂ plasma. It is believed that the decrease in carrier density is caused by crystal defects. For example, assuming that one crystal defect traps one carrier, the value of the carrier density ratio becomes 0.9 by forming a crystal defect density of 10% of the donor density.

Further, the thickness of the surface modification layer is defined as the depth to a region having a carrier density lower than the carrier density when not irradiated with O ₂ plasma. More specifically, for example, it can be defined as the depth to the portion where the value of the carrier density ratio becomes 0.9 as described above. If it defines in this way, in this embodiment, the thickness of a surface modification layer is about 120 nm from FIG.17 (b).

Generally, when a semiconductor layer is irradiated with plasma, it is said that about 10 nm from the surface of the irradiated semiconductor layer has some direct influence. Therefore, the thickness of the surface modification layer may be about 10 nm, for example. Of course, it may be 10 nm or more like 120 nm in the present embodiment. Even when the thickness is 10 nm or less, if the surface modification layer is formed, the contact resistance reduction effect due to the contact between the surface modification layer and the Al atoms can be appropriately obtained.

Finally, the features of this embodiment will be summarized. In order to form an ohmic electrode having a small contact resistance, a technique for increasing the carrier density on the outermost surface on which the electrode is formed is widely used. For example, there is a technique of forming a high carrier density layer by doping a dopant at a high concentration on the outermost surface of a semiconductor layer and forming an electrode thereon. However, in the present embodiment, contrary to normal technical common sense, an electrode is formed on m-plane GaN having a reduced carrier density on the outermost surface, that is, m-plane GaN having a modified surface.

Another feature of the present embodiment is that the electrode material in contact with the surface modified layer exhibits a particularly remarkable effect in a metal mainly composed of aluminum (Al) or aluminum (Al). According to the experiments by the present inventors, when an Al electrode was provided for m-plane GaN that was not irradiated with O ₂ plasma, the contact resistance was very high. Therefore, it seemed quite inappropriate to apply Al to m-plane GaN. However, when an Al electrode was provided for m-plane GaN whose surface was modified, a remarkable effect that could not be obtained with an electrode formed from another metal was obtained.

A further feature of this embodiment is that the m-plane GaN layer whose surface has been modified by irradiation with O ₂ plasma is subjected to acid treatment. As shown in Patent Document 4, in general technical common sense, if an acid treatment is performed after irradiating O ₂ plasma, an O-doped layer is removed, and thus a region having a high carrier density is removed. The characteristics of the n-side electrode deteriorate. However, in the present embodiment, on the contrary, the contact resistance can be further reduced by performing the acid treatment.

The true mechanism of the combination of “m-plane GaN surface-modified by O ₂ plasma irradiation” and “Al atoms” according to the present embodiment has not yet been elucidated. It is possible that the present disclosure may be realized by other essential mechanisms other than those described above. However, a remarkable effect is exhibited with high reproducibility in a special combination of “m-plane GaN surface-modified by O ₂ plasma irradiation” and “Al atoms”.

As described above, when the material of the n-side electrode is Al alone (that is, a metal having an Al concentration of 100%), good contact resistance can be obtained. However, since this effect is obtained by contact between the surface-modified m-plane GaN and Al atoms, the material of the n-side electrode does not necessarily need to be Al alone. A metal mainly composed of Al may be used. That is, the material of the n-side electrode may be a metal containing Al as a main component and atoms other than Al at the impurity level. Moreover, the metal which occupies 90 mass% or more of the whole with the Al atom may be sufficient. Moreover, the metal which occupies more than half (namely, 50 mass% or more) of the whole with Al atom may be sufficient. Furthermore, even if the concentration of Al in the electrode material is less than 50% by mass, if the situation where Al atoms are in contact with the surface-modified m-plane GaN is realized, in principle The effect of the embodiment is considered to be manifested. Therefore, it is included in the scope of the present disclosure if a situation in which Al atoms are in contact with the surface-modified m-plane GaN is realized regardless of the Al concentration of the electrode material.

When the material of the n-side electrode is an Al alloy, for example, AlSi or AlSiCu may be used. Use of these Al alloys for the n-side electrode can be expected to improve the reliability of the electrode in addition to reducing the contact resistance. In this case, the Al concentration may be 98% by mass or more, and the Si and Cu concentrations may be 2% by mass or less.

FIG. 19 shows a profile in the depth direction of Ga atoms before the heat treatment. Here, the result of elemental analysis of Ga by a secondary ion mass spectrometer (Secondary ion mass spectrometer: SIMS) is shown. The position indicated by a broken line near the depth of 200 nm from the surface corresponds to the interface between the Al electrode and n-GaN. Compared to the case where oxygen plasma treatment is not performed (○ mark), when oxygen plasma treatment is performed (■ mark), and after oxygen plasma treatment using BHF (buffered hydrofluoric acid) (▲ mark) Then, it is observed that the concentration of Ga atoms at the interface between the Al electrode and n-GaN is increased. On the other hand, when the depth is in the vicinity of 250 nm to 300 nm, the concentration of Ga atoms in n-GaN decreases when oxygen plasma treatment is performed and when treatment with BHF is performed after oxygen plasma treatment. Observed. From this result, it is considered that Ga atoms diffuse from the n-GaN to the interface between the Al electrode and n-GaN when the oxygen plasma treatment is performed and when the treatment is performed with BHF after the oxygen plasma treatment. This suggests that Ga vacancies are formed in n-GaN due to the diffusion of Ga atoms.

FIG. 20 shows the result when a sintering process is performed at a temperature of 500 ° C. for 10 minutes. As shown in FIG. 20, Ga atoms diffused at the interface between the Al electrode and n-GaN before the sintering process are further diffused to the Al electrode side, and no pile-up is observed. However, at a depth of about 250 nm, the concentration of Ga atoms in n-GaN decreases when oxygen plasma treatment is performed (marked with ■) and when treated with GHF after oxygen plasma treatment (marked with ▲). You can see that This result also suggests that Ga vacancies are formed in n-GaN due to the diffusion of Ga atoms.

FIG. 21 shows a change in voltage during energization of 100 mA between the test patterns constituting the n-side electrode when the plasma power, the oxygen flow rate, and the plasma processing time are changed. The arrangement of the n-side electrode test pattern is shown in the figure. The distance between the electrodes here is 20 μm. In this experiment, the voltage between these two n-side electrodes is measured, and the voltage reflects the contact resistance at the n-side electrode. A low voltage suggests a low contact resistance. First, a case where plasma processing times are different is compared. As the first treatment (I), the plasma power is 10 W, the oxygen flow rate is 30 ml / min (0 ° C., 1 atm), and the treatment time is 30 s. As the second treatment (II), the plasma power is 10 W, the oxygen flow rate is 30 ml / min (0 ° C., 1 atm), and the treatment time is 300 s. Comparing the first process (I) and the second process (II), it can be seen that the voltage of the second process (II) is lower. This indicates that the contact resistance of the n-side electrode is decreased by increasing the processing time of the oxygen plasma. Furthermore, the case where plasma power differs is compared. Here, when the first process is compared with the third process (III) when the plasma power is 40 W, the oxygen flow rate is 30 ml / min (0 ° C., 1 atm), and the process time is 30 s, It can be seen that the voltage is lower in the process (III). This also indicates that the contact resistance of the n-side electrode decreases with increasing plasma power in oxygen plasma. Furthermore, the case where oxygen flow rates differ is compared. Here, comparing the third process (III) and the fourth process (IV) with a plasma power of 40 W, an oxygen flow rate of 10 ml / min (0 ° C., 1 atm) and a processing time of 30 s, It can be seen that the voltage is lower in the third process (III). This also indicates that the contact resistance of the n-side electrode is decreased by increasing the oxygen flow rate in the oxygen plasma treatment.

In summary, the contact resistance of the n-side electrode decreases by increasing the plasma power, increasing the oxygen flow rate, and increasing the processing time. Increasing the plasma power, increasing the oxygen flow rate, and increasing the processing time is nothing but increasing the impact energy by physical sputtering imparted to the n-GaN layer by the plasma processing. 19 and 20 together with the results of elemental analysis, it is shown that Ga vacancies are formed in the n-GaN layer by plasma treatment, the Ga vacancies increase, and the contact resistance decreases. ing.

The results showing the same tendency are shown in FIG. FIG. 22 shows the relationship between plasma input power and contact resistance value. When the plasma power is increased from 10 W to around 40 W, a decrease in contact resistance is observed. Very good low contact resistance is exhibited up to a plasma power of about 120 W. However, when the plasma power increases beyond 120 W, the contact resistance starts to increase. That is, the plasma power has an optimum value in the vicinity of 40W to 120W. This is considered that when the plasma power is extremely low, the Ga vacancy concentration is low and the contact resistance cannot be reduced. On the other hand, if the plasma power is too high and the impact energy is too high, the concentration of defects other than Ga vacancies, such as nitrogen vacancies and their composite defects, will increase, and the effect of Ga vacancies Screening for defects is thought to increase contact resistance.

FIG. 23 shows the area A shown in FIG. 3 in an enlarged manner.

As shown in FIG. 23, the nitride semiconductor multilayer structure according to this embodiment includes an n-side electrode 40 and an n-type nitride semiconductor layer 22 having a surface modification layer 23 having a nonpolar surface or a semipolar surface. To do. The n-side electrode 40 includes an aluminum portion 40a. Aluminum portion 40 a is in contact with the surface of n-type nitride semiconductor layer 22. As apparent from FIGS. 19 to 22 and the description thereof, the surface modified layer of the n-type nitride semiconductor layer 22 in contact with the aluminum portion 40a due to Ga vacancies formed in the surface modified layer 23. The concentration of nitrogen (N) atoms in 23 is higher than the concentration of gallium (Ga) atoms in the surface modification layer 23 of the n-type nitride semiconductor layer 22 in contact with the aluminum portion 40a.

Here, the following surface layer 23 having Ga vacancies may have a thickness of 10 nm or more and 150 nm or less.

In FIG. 23, the n-side electrode 40 is composed of an aluminum portion 40a and a metal portion 40b formed thereon. The aluminum part 40a may be formed only from aluminum (Al). The metal part 40b can also be formed only from aluminum (Al). In other words, the entire n-side electrode 40 can be formed only from aluminum. However, the metal part 40b can be formed from metals other than aluminum. Examples of such metals are platinum (Pt), nickel (Ni), gold (Au), titanium (Ti), or palladium (Pd).

23, the aluminum portion 40a and the metal portion 40b may be layered, and the interface thereof may be parallel or substantially parallel to the n-type nitride semiconductor layer 22. In other words, the aluminum portion 40 a having a layer shape may be sandwiched between the metal portion 40 b having a layer shape and the n-type nitride semiconductor layer 22. However, the metal portion 40 b is not in contact with the n-type nitride semiconductor layer 22.

FIG. 24 shows a first modification of the configuration shown in FIG. As shown in FIG. 24, the aluminum alloy part 40c may be formed between the aluminum part 40a and the metal part 40b. For example, a part of the aluminum part 40a and a part of the metal part 40 may be alloyed at the interface between the aluminum part 40a and the metal part 40b to form the aluminum alloy part 40c. The aluminum alloy portion 40c is formed of an aluminum-platinum (AlPt) alloy, an aluminum-nickel (AlNi) alloy, an aluminum-gold (AlAu) alloy, an aluminum-titanium (AlTi) alloy, or an aluminum-palladium (AlPd) alloy. Can do. The aluminum alloy portion 40c may also have a layer shape.

25 and 26 show a second modification and a third modification. As shown in FIGS. 25 and 26, a plurality of divided aluminum portions 40a may be formed on the surface modification layer 23 of the n-type nitride semiconductor layer 22, respectively. In other words, in the configuration according to the second modification and the third modification, the surface modification layer of the n-type nitride semiconductor layer 22 so that the metal portion 40b or the aluminum alloy portion 40c covers the plurality of aluminum portions 40a. 23 may be in contact.

The present invention can be used for light emitting elements such as LED elements, for example.

DESCRIPTION OF SYMBOLS 10 GaN-type board | substrate 10a Main surface 20 Semiconductor laminated structure 22 N type nitride semiconductor layer 23 Surface modification layer 24 Active layer 26 P type nitride semiconductor layer 30 P side electrode 40 N side electrode 40a Aluminum part 40b Metal part 40c Aluminum alloy Portion 42 Recess 50 m-plane GaN layer 51 n-side electrode 52 surface modification layer 53 current source 54 voltmeter 55 probe needle 100 nitride semiconductor light emitting element 200 resin layer 210 wiring pattern 220 holding member 240 reflecting member

Claims

A nitride semiconductor light emitting device comprising:
n-side electrode,
p-side electrode,
An n-type nitride semiconductor layer having a nonpolar or semipolar surface,
A p-type nitride semiconductor layer electrically connected to the p-side electrode, and an active layer sandwiched between the n-type nitride semiconductor layer and the p-type nitride semiconductor layer, wherein
The n-side electrode comprises an aluminum portion,
The aluminum portion is in contact with the surface of the n-type nitride semiconductor layer,
The concentration of nitrogen atoms on the surface of the n-type nitride semiconductor layer in contact with the aluminum portion is higher than the concentration of gallium atoms on the surface of the n-type nitride semiconductor layer in contact with the aluminum portion.
The nitride semiconductor light emitting device according to claim 1,
The aluminum portion is made of only aluminum.
The nitride semiconductor light emitting device according to claim 1,
The aluminum portion has a layer shape parallel to the n-type nitride semiconductor layer.
The nitride semiconductor light emitting device according to claim 2,
The aluminum portion has a layer shape parallel to the n-type nitride semiconductor layer.
The nitride semiconductor light emitting device according to claim 1,
A plurality of the aluminum portions are provided.
The nitride semiconductor light emitting device according to claim 1,
The surface of the n-type nitride semiconductor layer in contact with the aluminum portion has gallium vacancies.
The nitride semiconductor light emitting device according to claim 1,
The surface region of the n-type nitride semiconductor layer in contact with the aluminum portion has a thickness of 10 nm or more and 150 nm or less.
The nitride semiconductor light emitting device according to claim 1,
The n-type nitride semiconductor layer has an m-plane surface.
The nitride semiconductor light emitting device according to claim 1,
The surface of the n-type nitride semiconductor layer that is in contact with the aluminum portion has a lower carrier density than the portion of the n-type nitride semiconductor layer that is not in contact with the aluminum portion.
A nitride semiconductor multilayer structure comprising:
an n-type nitride semiconductor layer having an n-side electrode and a nonpolar or semipolar surface, wherein
The n-side electrode comprises an aluminum portion,
The aluminum portion is in contact with the surface of the n-type nitride semiconductor layer,
The concentration of nitrogen atoms on the surface of the n-type nitride semiconductor layer in contact with the aluminum portion is higher than the concentration of gallium atoms on the surface of the n-type nitride semiconductor layer in contact with the aluminum portion.
The nitride semiconductor multilayer structure according to claim 10,
The aluminum portion is made of only aluminum.
The nitride semiconductor multilayer structure according to claim 10,
The aluminum portion has a layer shape parallel to the n-type nitride semiconductor layer.
The nitride semiconductor multilayer structure according to claim 11,
The aluminum portion has a layer shape parallel to the n-type nitride semiconductor layer.
The nitride semiconductor multilayer structure according to claim 10,
A plurality of the aluminum portions are provided.
The nitride semiconductor multilayer structure according to claim 10,
The surface of the n-type nitride semiconductor layer in contact with the aluminum portion has gallium vacancies.
The nitride semiconductor multilayer structure according to claim 10,
The surface region of the n-type nitride semiconductor layer in contact with the aluminum portion has a thickness of 10 nm or more and 150 nm or less.
The nitride semiconductor multilayer structure according to claim 10,
The n-type nitride semiconductor layer has an m-plane surface.
The nitride semiconductor multilayer structure according to claim 10,
The surface of the n-type nitride semiconductor layer that is in contact with the aluminum portion has a lower carrier density than the portion of the n-type nitride semiconductor layer that is not in contact with the aluminum portion.
A method for manufacturing a nitride semiconductor multilayer structure comprising the following steps:
(A) irradiating the surface of the n-type nitride semiconductor layer having a nonpolar or semipolar surface with oxygen plasma;
(B) a step of bringing the surface irradiated with oxygen plasma in step (a) into contact with an acid; and (c) a step of forming an aluminum portion on the surface in contact with the acid in step (b).
20. The method according to claim 19, comprising
The aluminum portion is made of only aluminum.
20. The method according to claim 19, comprising
The aluminum portion has a layer shape parallel to the n-type nitride semiconductor layer.
The method of claim 20, comprising:
The aluminum portion has a layer shape parallel to the n-type nitride semiconductor layer.
20. The method according to claim 19, comprising
A plurality of the aluminum portions are provided.
20. The method according to claim 19, comprising
The surface of the n-type nitride semiconductor layer in contact with the aluminum portion has gallium vacancies.
20. The method according to claim 19, comprising
The surface region of the n-type nitride semiconductor layer in contact with the aluminum portion has a thickness of 10 nm or more and 150 nm or less.
20. The method according to claim 19, comprising
The n-type nitride semiconductor layer has an m-plane surface.