Nitride semiconductor light-emitting element and method for fabricating the same

Provided is a nitride semiconductor light-emitting element having a low contact resistance between an n-type nitride semiconductor layer and an n-side electrode. A portion of the n-type nitride semiconductor layer is removed by a plasma etching process using a gas containing halogen to expose a surface region of the n-type nitride semiconductor layer. Next, such an exposed surface region is further subjected to a plasma treatment using a gas containing oxygen. After that, the n-side electrode formed of aluminum is formed so as to be in contact with the surface region. In the surface region, a carrier concentration is decreased from the inside of the n-type nitride semiconductor layer toward the n-side electrode.

BACKGROUND

1. Technical Field

The present invention relates to a nitride semiconductor light-emitting element and a method for fabricating the same.

2. Description of the Related Art

Recently, a nitride semiconductor light-emitting element having a principal plane of an m-plane has been researched and developed actively to improve an illumination efficiency. This is because a nitride semiconductor light-emitting element having a principal plane of an m-plane is free from piezoelectric field, which decrease luminous efficiency. Hereinafter, a nitride semiconductor light-emitting element having a principal plane of an m-plane is referred to as “m-plane nitride semiconductor light-emitting element”.

US Pre-Grant Patent Application Publication No. 2013/0126902 discloses an m-plane nitride semiconductor light-emitting element. As shown inFIG. 11, this m-plane nitride semiconductor light-emitting element comprises an n-side electrode30, an n-type nitride semiconductor layer21, an active layer22, a p-type nitride semiconductor layer23and a p-side electrode40. A voltage is applied between the n-side electrode30and the p-side electrode40to emit light from the active layer22.

According to the paragraphs 0161-0166 of US Pre-Grant Patent Application Publication No. 2013/0126902, this m-plane nitride semiconductor light-emitting element is fabricated as below. First, the n-type nitride semiconductor layer21, the active layer22and the p-type nitride semiconductor layer23are epitaxially grown in this order on a substrate10.

Then, a portion of the n-type nitride semiconductor layer21, the active layer22and the p-type nitride semiconductor layer23is removed by dry etching using a chlorine-based gas to expose the n-type nitride semiconductor layer21. The p-side electrode40and the n-side electrode30are formed on the p-type nitride semiconductor layer23and the portion of the n-type nitride semiconductor layer21which has been exposed by dry etching, respectively.

SUMMARY

The present invention provides a method for fabricating a nitride semiconductor light-emitting element, the method comprising steps of:

(a) growing an n-type nitride semiconductor layer epitaxially on a substrate, wherein the n-type nitride semiconductor layer has a principal plane of an m-plane;

(b) growing an active layer epitaxially on the n-type nitride semiconductor layer grown in the step (a), wherein the active layer has a principal plane of an m-plane;

(c) growing a p-type nitride semiconductor layer epitaxially on the active layer grown in the step (b) to obtain a nitride semiconductor stacking structure, wherein the p-type nitride semiconductor layer has a principal plane of an m-plane;

(d) removing a portion of the p-type nitride semiconductor layer, a portion of the active layer and a portion of the n-type nitride semiconductor layer by a plasma etching process using a gas containing halogen to the extent to form a plasma-etched surface region exposed on the n-type nitride semiconductor layer;

(e) subjecting the plasma-etched surface region exposed in the step (d) to a plasma treatment using a gas containing oxygen; and

(f) forming an n-side electrode formed of aluminum on the plasma-etched surface region which has been subjected to the plasma treatment using the gas containing oxygen in the step (e), and forming a p-side electrode electrically connected to the p-type nitride semiconductor layer, wherein the n-side electrode is in contact with the plasma-etched surface region.

The present invention provides a nitride semiconductor light-emitting element, comprising:

an n-type nitride semiconductor layer electrically connected to the n-side electrode;

a p-type nitride semiconductor layer electrically connected to the p-side electrode; and

an active layer interposed between the n-type nitride semiconductor layer and the p-type nitride semiconductor layer, wherein

the n-type nitride semiconductor layer, the active layer, and the p-type nitride semiconductor layer each constitutes a nitride semiconductor layer having a principal plane of an m-plane,

the n-side electrode is formed of aluminum,

the n-side electrode contains gallium,

the n-side electrode is in contact with an surface region of the n-type nitride semiconductor layer, and

the surface region has a carrier concentration which decreases from the inside of the n-type nitride semiconductor layer toward the n-side electrode.

The present invention provides a method for forming a nitride semiconductor structure comprising an n-type nitride semiconductor layer and a metal layer which is formed on the n-type nitride semiconductor layer, the method comprising the following steps (d) to (f):

(d) subjecting a surface of the n-type nitride semiconductor layer to a plasma treatment using a gas containing halogen; wherein the n-type nitride semiconductor layer has a principal plane of an m-plane,

(e) subjecting the surface of the n-type nitride semiconductor layer to a plasma treatment using a gas containing oxygen after the step (d); and

(f) forming the metal layer formed of aluminum on the surface of the n-type nitride semiconductor layer subjected to the plasma treatment using the gas containing oxygen in the step (e) to obtain the nitride semiconductor structure, wherein the metal layer is in contact with the surface of the n-type nitride semiconductor layer.

The present invention provides a nitride semiconductor structure comprising:

an n-type nitride semiconductor layer; and

a metal layer which is electrically connected to and is formed on the n-type nitride semiconductor layer, wherein

the n-type nitride semiconductor layer has a principal plane of an m-plane;

the metal layer is formed of aluminum;

the metal layer contains gallium;

the metal layer is in contact with the surface of the n-type nitride semiconductor layer; and

the surface of the n-type nitride semiconductor layer has a carrier concentration which decreases from the inside of the n-type nitride semiconductor layer toward the metal layer.

The present invention provides an m-plane nitride semiconductor light-emitting element having a low contact resistance between the n-type nitride semiconductor layer and the n-side electrode.

DETAILED DESCRIPTION OF THE EMBODIMENTS

First, a method for fabricating the nitride semiconductor light-emitting element shown inFIG. 1according to the present embodiment is described with reference toFIG. 2A-FIG. 2G.

As shown inFIG. 2A, a substrate101is prepared. An example of the substrate101is a sapphire substrate having a principal plane of an a-plane and a gallium nitride substrate having a principal plane of an m-plane. A gallium nitride single-crystalline substrate having a principal plane of an m-plane is desirable. Other substrates101such as an SiC substrate may be used, as long as the nitride semiconductor layer having a principal plane of an m-plane is epitaxially grown on the substrate101.

As shown inFIG. 8, the substrate101may have an off-angle θ. The off-angle θ is formed between the normal direction810of the m-plane and the normal direction820of the principal plane. The off-angle θ may be more than 0 degrees and not more than 5 degrees. When the substrate101has an off-angle θ, the surface of the substrate101is stepwise microscopically. Desirably, the off-angle θ is 0 degrees. In other words, it is desirable that the substrate101does not have an off-angle θ.

Then, as shown inFIG. 2B, an n-type nitride semiconductor layer102having a principal plane of an m-plane is epitaxially grown on the substrate101. An example of the n-type dopant contained in the n-type nitride semiconductor layer102is silicon.

The term “nitride semiconductor” used in the present specification is a compound represented by the chemical formula: AlxInyGazN (0≦x<1, 0≦y<1, 0<z≦1, x+y+z=1).

As shown inFIG. 2C, an active layer103is epitaxially grown on the n-type nitride semiconductor layer102. The active layer103also has a principal plane of an m-plane.

As shown inFIG. 2D, a p-type nitride semiconductor layer104is epitaxially grown on the active layer103. The p-type nitride semiconductor layer104also has a principal plane of an m-plane. An example of the p-type dopant contained in the p-type nitride semiconductor layer104is magnesium. In this way, obtained is a nitride semiconductor stacking structure composed of the n-type nitride semiconductor layer102, the active layer103and the p-type nitride semiconductor layer104.

Then, as shown inFIG. 2E, a portion of the n-type nitride semiconductor layer102, the active layer103and the p-type nitride semiconductor layer104is removed by a plasma etching process using a gas containing halogen. In other words, the portion is dry-etched by the plasma etching process using the gas containing halogen. Thus, a surface region102aof the n-type nitride semiconductor layer102is exposed. Since this surface region102ahas been subjected to the plasma etching process using the gas containing halogen, the surface region102acontains halogen.

An example of halogen is fluorine, chlorine, bromine or iodine. Chlorine is desirable. An example of the gas containing chlorine is a chlorine gas, a hydrogen chloride gas, a carbon tetrachloride gas, a fluorine chloride gas, a germanium gas tetrachloride, a boron trichloride gas or mixture thereof. A chlorine gas is desirable. Therefore, it is desirable that the surface region102acontains chlorine.

The term “halogen plasma treatment” used in the present specification means a plasma etching process using a gas containing halogen.

As shown inFIG. 2F, the thus-exposed surface region102ais further subjected to a plasma treatment using a gas containing oxygen. The plasma treatment using the gas containing oxygen is one of the characteristics of the present invention.

The term “oxygen plasma treatment” used in the present specification means a plasma treatment using a gas containing oxygen.

An example of oxygen is oxygen molecules or ozone molecules. Oxygen molecules are desirable. An example of the gas containing oxygen is an oxygen gas, an ozone gas or mixture thereof. An oxygen gas is desirable.

As understood from the example 1, the comparative example 1 - the comparative example 3, the reference example A1 - the reference example B2, and the reference comparative example A1- the reference comparative example B6, described below with Tables 5 and6, the method of the present invention has the following three requirements (A)-(C):

(Requirement A): the n-side electrode109(see,FIG. 2G) which is in contact with the surface region102ais formed of aluminum. Desirably, the n-side electrode109which is in contact with the surface region102aconsists only of aluminum.

(Requirement B): the n-type nitride semiconductor layer102has a principal plane of an m-plane.

(Requirement C): the surface region102awhich has been subjected to the plasma etching process by the halogen plasma treatment is further subjected to the oxygen plasma treatment.

In case where the requirement (A) is not satisfied, the contact resistance between the n-side electrode109and the n-type nitride semiconductor is high, as understood from the reference comparative examples B2 - B6 (see, Table 6 andFIGS. 6-7E). For example, when a titanium layer is in contact with the n-type nitride semiconductor layer, the contact resistance is high.

In case where the requirement (B) is not satisfied, the surface region102awhich has been subjected to the oxygen plasma treatment has a higher contact resistance than the surface region102awhich has not yet subjected to the oxygen plasma treatment. For example, as understood from the comparative examples 2- 3, when the surface region102ahas a principal plane of a c-plane, the oxygen plasma treatment increases the contact resistance.

In case where the requirement (C) is not satisfied, as understood from the comparative example 1, the contact resistance of the surface region102ais not lowered.

In this way, the requirements (A)-(C) are indivisible.

As understood from the example 1, the comparative example 1-the comparative example 3, the reference example A1-the reference example B2, and the reference comparative example A1-the reference comparative example B6, described below with Tables 5 and 6, the method of the present invention has the following three requirements (A)-(C):

(Requirement A): the n-side electrode109which is in contact with the surface region102ais formed of aluminum. Desirably, the n-side electrode109which is in contact with the surface region102aconsists only of aluminum.

(Requirement B): the n-type nitride semiconductor layer102has a principal plane of an m-plane.

(Requirement C): the surface region102awhich has been subjected to the plasma etching process by the halogen plasma treatment is further subjected to the oxygen plasma treatment.

In case where the requirement (A) is not satisfied, the contact resistance between the n-side electrode109and the n-type nitride semiconductor is high, as understood from the reference comparative examples B2 -B6 (see, Table 6 andFIGS. 6-7E). For example, when a titanium layer is in contact with the n-type nitride semiconductor layer, the contact resistance is high.

After the oxygen plasma treatment, it is desirable that the surface region102is subjected to an acid treatment. In this way, it is desirable to remove the outermost surface of the surface region102a. The acid treatment further lowers the contact resistance.

In particular, it is desirable that the surface region102ais subjected to an acid treatment using buffered hydrofluoric acid.

As shown inFIG. 2G, the n-side electrode109is formed on the thus-formed surface region102a. The p-side electrode108is formed on the p-type nitride semiconductor layer104.

The n-side electrode109is formed of aluminum. In other words, the main component of the n-side electrode109is aluminum. In one embodiment, the n-side electrode109may be one metal layer formed of aluminum. The n-side electrode109is in contact with the surface region102a. The n-side electrode109may consist only of aluminum.

The n-side electrode109may be a laminate in which plural metal layers are stacked. In this case, the metal layer in contact with the surface region102a, namely, the bottom metal layer, is formed of aluminum. In this way, it is desirable that the n-side electrode109has a shape of a layer parallel to the n-type nitride semiconductor layer102.

After the step (f), it is desirable that the obtained nitride semiconductor stacking structure is subjected to a sinter treatment under a temperature of not less than 400 degrees Celsius and not more than 600 degrees Celsius to activate the p-type dopant, such as magnesium, and the n-type dopant, such as silicon.

During the sinter treatment of the nitride semiconductor stacking structure, gallium atoms are diffused from the n-type nitride semiconductor layer102into the n-side electrode109, as understood from the secondary ion mass spectrometry of the examples 1-2 and the comparative example, which are described later. SeeFIG. 9andFIG. 10. This diffusion phenomenon occurs between the surface region102aand the n-side electrode109. Therefore, after the sinter treatment, the n-side electrode109contains not only aluminum but also gallium. Similarly, n-type impurities, namely, silicon, may also be diffused from the n-type nitride semiconductor layer102. Therefore, the n-side electrode109may further contain silicon.

Desirably, the sinter treatment is performed after an aluminum layer consisting only of aluminum is formed on the surface region102a. After the sinter treatment, the n-side electrode109is formed of aluminum and contains gallium. In other words, the n-side electrode109has a main component of aluminum and contains a small amount of gallium. As described above, it is desirable that the n-side electrode109also contains a small amount of silicon.

When the n-side electrode109is the laminate in which the plural metal layers are stacked, a metal component contained in another metal layer formed on the bottom metal layer of aluminum may be diffused into the bottom metal layer of aluminum due to the sinter treatment. Accordingly, in this case, the bottom metal layer of the n-side electrode109may be formed of aluminum and may contain gallium and another metal after the sinter treatment. In other words, the bottom metal layer of the n-side electrode109may have a main component of aluminum and may contain a small amount of gallium and another metal.

In this way, the nitride semiconductor light-emitting element shown inFIG. 1is obtained.

As described above, similarly to a conventional m-plane nitride semiconductor light-emitting element, the nitride semiconductor light-emitting element according to the present embodiment comprises the n-side electrode109, the p-side electrode108, the n-type nitride semiconductor layer102, the p-side nitride semiconductor106and the active layer103. The n-type nitride semiconductor layer102and the p-type nitride semiconductor layer104are electrically connected to the n-side electrode109and the p-side electrode108, respectively. The active layer103is interposed between the n-type nitride semiconductor layer102and the p-type nitride semiconductor layer104. The n-type nitride semiconductor layer102, the active layer103and the p-type nitride semiconductor layer104each has a principal plane of an m-plane.

The n-side electrode109is formed of aluminum and contains gallium. In other words, the n-side electrode109has a main component of aluminum and contains small amount of gallium. The n-side electrode109may contain silicon. The n-side electrode109may contain a small amount of metal other than aluminum and gallium.

As demonstrated in the reference examples A1 -A2, and the reference comparative example A1 (in particular, see Table 5), the surface region102ahas a carrier concentration which decreases from the inside of the n-type nitride semiconductor layer102toward the n-side electrode109after the oxygen plasma treatment. Desirably, the carrier concentration is monotonically decreased from the inside of the n-type nitride semiconductor layer102toward the n-side electrode109. In case where the oxygen plasma treatment is not performed, the carrier concentration is constant.

The theory of the present invention is described below. However, the present inventors do not like to be bound by the following theories.

FIG. 3Ashows a bandgap between the n-side electrode formed of aluminum and the n-type nitride semiconductor layer. As shown inFIG. 3A, a Schottky barrier having a width W is formed between the n-side electrode formed of aluminum and the n-type nitride semiconductor layer.

Generally, N vacancies are formed in the n-type n-side nitride semiconductor layer to lower the contact resistance between the n-side electrode and the n-type nitride semiconductor layer. See the paragraph 0013 of US-PreGrant Patent Application Publication No 2011/0108853.

FIG. 3Bshows a bandgap in a case where N vacancies are formed in the n-type nitride semiconductor layer. As readily understood from the comparison toFIG. 3A, N vacancies narrow the width W of the Schottky barrier. For this reason, compared to the case ofFIG. 3A, electrons supplied from the n-side electrode overcome the Schottky barrier more easily.

However, as is clear from the secondary ion mass spectrometry of the examples 1-2 and the comparative example 1, which are described later, the Ga vacancies are formed in the surface region102ain the present embodiment. SeeFIG. 9andFIG. 10. The Ga vacancies are formed in the surface region102adue to the oxygen plasma treatment. Accordingly, the formation of the N vacancies does not contribute evidence to support the technical effect of the decrease of the contact resistance obtained in the present embodiment.

FIG. 3Cshows a bandgap according to the present embodiment. Defect levels are generated in the Schottky barrier due to the oxygen plasma treatment in the present embodiment. The electrons supplied from the n-side electrode flow through these defect levels to the n-type semiconductor layer. In other words, tunnel current flows from the n-side electrode through these defect levels to the n-type nitride semiconductor layer. The present inventors believe that the reason why the oxygen plasma treatment decreases the carrier concentration in the surface region102awould be that some of carriers are bound by the defect levels generated by the oxygen plasma treatment. In other words, the oxygen plasma treatment would inactivate some of donors.

EXAMPLES

The present invention is described in greater detail with reference to the following examples.

A metalorganic chemical vapor deposition method (hereinafter, referred to as “MOCVD method”) was used as an epitaxial growth method in the following examples and comparative examples.

The raw materials shown in the following Table 1 were used in the examples and comparative examples.

The example 1 is described with reference toFIG. 2A-FIG.2G. First, as shown inFIG. 2A, the n-type GaN substrate101having a principal plane of an m-plane was prepared.

Then, as shown inFIG. 2B, the n-type GaN layer having a thickness of 1 micrometer was epitaxially grown at a growth temperature of 945 degrees Celsius on the n-type GaN substrate101. The n-type GaN layer had a silicon concentration of 2.0×1018cm−3. In this way, the n-type nitride semiconductor layer102was formed.

As shown inFIG. 2C, the active layer103was epitaxially grown at a growth temperature of 720 degrees Celsius on the n-type GaN layer. In greater detail, the active layer103was formed by stacking three InxGa1-xN layers (x=0.16) each having a thickness of 6 nanometers and three GaN layers each having a thickness of 12 nanometers alternately.

Subsequently, an undoped GaN layer (not shown) having a thickness of 75 nanometers was epitaxially grown on the active layer103.

Then, the p-type Alx1Iny1Gaz1N layer (x=0.10, y=0, z=0.90) was epitaxially grown at a growth temperature of 890 degrees Celsius on the undoped GaN layer. The p-type Alx1Iny1Gaz1N layer had a thickness of 20 nanometers. The p-type Alx1Iny1Gaz1N layer had a magnesium concentration of approximately 5.0×1018cm−3-1.0×1019cm−3. This p-type Alx1Iny1Gaz1N layer (x=0.10, y=0, z=0.90) functioned as an overflow-suppression layer. In greater detail, excess electrons supplied from the n-side electrode109to the active layer103were blocked by the p-type Alx1Iny1Gaz1N layer to return to the active layer103.

Furthermore, the p-type GaN layer having a thickness of 85 nanometers was epitaxially grown at a growth temperature of 890 degrees Celsius on the p-type Alx1Iny1Gaz1N layer. In this way, as shown inFIG. 2D, the p-type nitride semiconductor layer104was formed.

Then, as shown inFIG. 2E, a portion of the nitride semiconductor stacking structure shown inFIG. 2Dwas dry-etched by a halogen plasma treatment. In this way, a portion of the p-type nitride semiconductor layer104, the active layer103and the n-type nitride semiconductor layer was removed to expose the surface region102aon the surface of the n-type nitride semiconductor layer102. Needless to say, this surface region102ahad been was subjected to the halogen plasma treatment.

Table 2 shows the condition of the dry-etching by the halogen plasma treatment.

After the dry-etching by the halogen plasma treatment, an SiO2protective film and a photoresist film (both of which are not shown) were formed on the nitride semiconductor stacking structure. The surface region102awas not covered with the SiO2protective film and the photoresist film.

Then, as shown inFIG. 2F, the surface region102awas subjected to an oxygen plasma treatment.

Table 3 shows the condition of the oxygen plasma treatment.

Furthermore, the surface region102awas subjected to an acid treatment using buffered hydrofluoric acid (NH4F:HF=10:1) for 30 minutes. Subsequently the surface region102awas washed using pure water for two minutes.

As shown inFIG. 2G, an aluminum layer having a thickness of 500 nanometers was formed by a vacuum evaporation method on the surface region102a. On the other hand, a silver layer having a thickness of 400 nanometers was formed by the vacuum evaporation method on the p-type nitride semiconductor layer104.

Then, the nitride semiconductor stacking structure was subjected to a sinter treatment under a temperature of 500 degrees Celsius for 20 minutes.

Finally, a titanium layer having a thickness of 40 nanometers and a gold layer having a thickness of 750 nanometers were formed by a vacuum evaporation method on the aluminum layer and on the silver layer. In this way, the nitride semiconductor light-emitting element according to the example 1 was obtained.

The experiment similar to the example 1 was performed except that the acid treatment using buffered hydrofluoric acid was not performed.

Comparative Example 1

The experiment similar to the example 1 was performed except that the oxygen plasma treatment was not performed.

Comparative Example 2

The experiment similar to the example 1 was performed except that a c-plane GaN substrate was used instead of the m-plane GaN substrate. Needless to say, the p-type nitride semiconductor layer102, the active layer103and the n-type nitride semiconductor layer104each have a principal plane of a c-plane in the comparative example 2.

Comparative Example 3

The experiment similar to the example 1 was performed except that a c-plane GaN substrate was used instead of the m-plane GaN substrate and except that the oxygen plasma treatment was not performed. Needless to say, the p-type nitride semiconductor layer102, the active layer103and the n-type nitride semiconductor layer104each have a principal plane of a c-plane also in the comparative example 3.

The current-voltage properties of the nitride semiconductor light-emitting elements according to the example 1 and the comparative examples 1-3 were measured.FIG. 4Ashows the current-voltage properties of the nitride semiconductor light-emitting elements according to the example 1 and the comparative example 1.FIG. 4Bshows the current-voltage properties of the nitride semiconductor light-emitting elements according to the comparative examples 2-3.

The following Table 4 shows series resistance values between 250 milliamperes and 350 milliamperes which were calculated from the current-voltage properties.

As is clear fromFIG. 4Aand Table 4, if the surface region102ahas a principal plane of an m-plane and if the surface region102awhich has been subjected to the dry-etching by the halogen plasma treatment is further subjected to the oxygen plasma treatment, the resistance value is lowered. On the other hand, even if the surface region102ahas a principal plane of an m-plane, however, if the surface region102ais not subjected to the oxygen plasma treatment, the resistance value remains high.

As is clear fromFIG. 4Band Table4, if the surface region102ahas a principal plane of a c-plane, and if the surface region102awhich has been subjected to the dry-etching by the halogen plasma treatment is further subjected to the oxygen plasma treatment, the resistance value is increased. This phenomenon on a c-plane is the exact reverse of the phenomenon of an m-plane. Accordingly, if the surface region102ahas a principal plane of a c-plane, it would be obvious fromFIG. 4Bthat a skilled person would not subject the surface region102awhich has been subjected to the dry-etching by the halogen plasma treatment to an oxygen plasma treatment.

The nitride semiconductor light-emitting elements according to the example 1, the example 2 and the comparative example 1 were subjected to a secondary ion mass spectrometry (hereinafter, referred to as “SIMS”).FIG. 9shows a SIMS profile of the n-side electrode109formed of aluminum and the n-type nitride semiconductor layer102formed of n-GaN before the sinter treatment is performed.FIG. 10shows a SIMS profile of the n-side electrode formed of aluminum and the nitride semiconductor layer102formed of n-GaN after the sinter treatment is performed.

As is clear fromFIG. 9andFIG. 10, compared to the nitride semiconductor light-emitting element according to the comparative example 1, a greater amount of gallium atoms were diffused from the n-type nitride semiconductor layer102to the n-side electrode109in the nitride semiconductor light-emitting elements according to the examples 1-2. This suggests that a greater amount of gallium vacancies were formed in the n-type nitride semiconductor layers102included in the nitride semiconductor light-emitting elements according to the examples 1-2.

(Measurement of Carrier Concentration)

Reference Example A1

In the reference example A1, an n-type GaN layer having a thickness of 1 micrometer was epitaxially grown under a growth temperature of 945 degrees Celsius on an n-type GaN substrate having a principal plane of an m-plane. The n-type GaN layer had a silicon concentration of 2.0×1018cm−3.

Then, the n-type GaN layer was subjected to an oxygen plasma treatment under the condition shown in Table 3. Subsequently, the surface of the n-type GaN layer was subjected to an acid treatment using buffered hydrofluoric acid (NH4F:HF=10:1) for 30 minutes. After the acid treatment, the surface of the n-type GaN layer was washed using pure water for two minutes.

Carrier concentrations at the depth of 100 nanometers, 125 nanometers, 150 nanometers, 175 nanometers and 200 nanometers were measured by a capacity-voltage measurement method (hereinafter, referred to as “C-V measurement”). The following non-patent literatures disclose the details of the C-V measurement.

Reference Example A2

The experiment similar to the reference example A1 was performed except that the n-type GaN layer was not subjected to the acid treatment using buffered hydrofluoric acid.

Reference Comparative Example A1

The experiment similar to the reference example A1 was performed except that the n-type GaN layer was not subjected to the oxygen plasma treatment and except that the n-type GaN layer was not subjected to the acid treatment using buffered hydrofluoric acid.

The following Table 5 shows the carrier concentrations of the n-type nitride semiconductor layers according to the reference example A1, the reference example A2 and the reference comparative example A1. As is clear from Table 5, the oxygen plasma treatment decreases the carrier concentration of the n-type nitride semiconductor layer at a depth of 100-150 nanometers. For this reason, the n-type nitride semiconductor layer which has been subjected to the oxygen plasma treatment, namely, the n-type nitride semiconductor layers according to the reference examples A1-A2, have lower carrier concentration than the n-type nitride semiconductor which has not been subjected to the oxygen plasma treatment, namely, the n-type nitride semiconductor according to the reference comparative example A1.

As understood from the reference examples A1-A2 and the reference comparative example A1, if the oxygen plasma treatment is performed, the carrier concentration decreases monotonically from the inside of the n-type nitride semiconductor layer toward the surface of the n-type nitride semiconductor layer. On the other hand, if the oxygen plasma treatment is not performed, the carrier concentration is constant along the depth direction.

Reference Example B1

FIG. 5AandFIG. 5Bshow a plan view and a cross-sectional view of the reference example B1, respectively.

In the reference example B1, an n-type GaN layer800having a thickness of 1 micrometer was epitaxially grown under a growth temperature of 945 degrees Celsius on an n-type GaN substrate (not shown inFIG. 5AandFIG. 5B) having a principal plane of an m-plane. The n-type GaN layer800had a silicon concentration of 2.0×1018cm−3.

Then, the n-type GaN layer800was subjected to an oxygen plasma treatment under the condition shown in Table 3. Subsequently, the surface of the n-type GaN layer800was subjected to an acid treatment using buffered hydrofluoric acid (NH4F:HF=10:1) for 30 minutes. After the acid treatment, the surface of the n-type GaN layer800was washed using pure water for two minutes. Finally, a first n-side electrode layer802aand a second n-side electrode layer802beach formed of aluminum were formed on the n-type GaN layer800by a vacuum evaporation method.

As shown inFIG. 5A, the first n-side electrode layer802ahad a width W1of 100 micrometers. The first n-side electrode layer802ahad a length L1of 200 micrometers. The interval I1between the first n-side electrode layer802aand the second n-side electrode layer802bwas 20 micrometers. As shown inFIG. 5B, the first n-side electrode layer802ahad a thickness T1of 0.4 micrometers. The second n-side electrode layer802bhad a shape identical to that of the first n-side electrode layer802a.

As shown inFIG. 5A, the current-voltage properties between the first n-side electrode layer802aand the second n-side electrode layer802bwere measured.

Reference Example B2

The experiment similar to the reference example B1 was performed except that the n-type GaN layer800was not subjected to the acid treatment using the buffered hydrofluoric acid.

Reference Comparative Example B1

The experiment similar to the reference example B1 was performed except that the n-type GaN layer800was not subjected to the oxygen plasma treatment and except that the n-type GaN layer800was not subjected to the acid treatment using the buffered hydrofluoric acid.

FIG. 6shows the current-voltage properties according to the reference example B1, the reference example B2 and the reference comparative example B1.

As is clear fromFIG. 6, even when the n-type GaN layer800was not subjected to the acid treatment using the buffered hydrofluoric acid (see the reference example B2), the resistance was low, similarly to the case where the n-type GaN layer800was subjected to the acid treatment using the buffered hydrofluoric acid (see the reference example B1)

On the other hand, when the n-type GaN layer800was not subjected to the oxygen plasma treatment (see the reference comparative example B1), the resistance was high.

As understood fromFIG. 6, the oxygen plasma treatment lowers the contact resistance between the m-plane nitride semiconductor layer and the electrode layer formed of aluminum.

Reference Comparative Examples B2-B6

The experiments similar to the reference example B1 were performed except that the first n-side electrode layer802aand the second n-side electrode layer802beach formed of the materials shown in Table 6 were used instead of those each formed of aluminum. Needless to say, the n-type GaN layer800was subjected to the oxygen plasma treatment in the reference comparative examples B2-B6. The current-voltage properties between the first n-side electrode layer802aand the second n-side electrode layer802bwere measured as shown inFIG. 5A.

TABLE 6Reference comparative example B2MagnesiumReference comparative example B3SilverReference comparative example B4TitaniumReference comparative example B5NickelReference comparative example B6Platinum

FIG. 7A-FIG.7E shows the current-voltage properties according to the reference comparative examples B2-B6, respectively.

As understood fromFIG. 6andFIGS. 7A-7E, when the material of the electrode layer was aluminum, the resistance was low. On the other hand, when the material of the electrode layer was material other than aluminum, the resistance was high.

INDUSTRIAL APPLICABILITY

The nitride semiconductor light-emitting element according to the present invention is a nitride semiconductor light-emitting diode or a nitride semiconductor laser. A nitride semiconductor light-emitting diode is desirable. The nitride semiconductor light-emitting diode according to the present invention is installed in a lighting installation. Desirably, the nitride semiconductor light-emitting element according to the present invention is installed in an automotive headlight.

REFERENTIAL SIGNS LIST

810normal direction of m-plane

820normal direction of principal plane