KR20140102422A - Nitride-based semiconductor light emitting device - Google Patents

Abstract

A nitride-based semiconductor light-emitting device is provided. The nitride-based semiconductor light-emitting device includes a first semiconductor layer having a first conductivity, a current diffusion layer having a nitride-based superlattice structure, the current diffusion layer being disposed on the first semiconductor layer and containing aluminum, A second semiconductor layer having conductivity, an active layer disposed on the second semiconductor layer, and a third semiconductor layer disposed on the active layer and having a second conductivity, wherein the current diffusion layer includes a non-doped nitride- And a second nitride based semiconductor layer disposed on the undoped nitride based semiconductor layer, the first nitride based semiconductor layer including aluminum having a constant component ratio and the first nitride based semiconductor layer disposed on the undoped nitride based semiconductor layer, 2 nitride-based semiconductor layer. Therefore, current diffusion in the horizontal and vertical directions of the semiconductor layer having the first conductivity can be improved.

Description

[0001] NITRIDE-BASED SEMICONDUCTOR LIGHT EMITTING DEVICE [0002]

The present invention relates to a nitride-based semiconductor light-emitting device, and more particularly, to a nitride-based semiconductor light-emitting device having improved current diffusion in an n-type nitride-based semiconductor layer.

A light-emitting diode (LED) is a type of p-n junction diode, and is a semiconductor device using electroluminescence, which is a phenomenon in which monochromatic light is emitted when voltage is applied in the forward direction.

The operation of the light emitting diode is a mechanism in which a voltage is applied to two electrodes represented by an anode and a cathode, and a light emitting operation is performed by supplying a current according to application of a voltage. Particularly, in the active layer in which the multiple quantum well structure is formed, the n-type semiconductor layer and the p-type semiconductor layer are in contact with the upper and lower portions. The n-type semiconductor layer supplies electrons to the active layer, and the p-type semiconductor layer supplies holes to the active layer. Electrons and holes injected into the multiple quantum well structure are defined inside the well layer by the quantum confinement effect, and the light emitting operation is performed by recombination.

Nitride compound semiconductors typified by gallium nitride (GaN) have high thermal stability and a wide bandgap (0.8 to 6.2 eV), attracting much attention in the field of high-output electronic component development including LEDs come.

One of the reasons for this is that GaN can be combined with other elements (indium (In), aluminum (Al), etc.) to produce semiconductor layers emitting green, blue and white light.

These nitride-based light emitting diodes are generally grown in a p-i-n diode structure on a different substrate such as sapphire, silicon carbide, or silicon, using metal organic chemical vapor deposition. Since there is a difference between the lattice constant and the thermal expansion coefficient between the nitride thin film and the dissimilar substrate, cracks occur at a critical thickness or more due to an increase in applied tensile or compressive stress as the thickness increases while growing.

The n-type gallium nitride thin film occupies more than 80-90% of the thickness of the blue light emitting nitride light emitting diode structure. In particular, when the silicon substrate is used, it is known that the critical thickness of the n-type gallium nitride thin film is drastically reduced due to a large difference in thermal expansion coefficient have.

As the thickness of the n-type gallium nitride thin film is increased, current diffusion is improved in the light-emitting diode device. Such improved current diffusion contributes to improvement of performance of various devices such as light output, operating voltage and reliability.

However, recently, various horizontal and vertical type light emitting diodes have a structural need to minimize the thickness of the thin film to minimize the absorption loss of the light formed in the active layer.

Therefore, superlattice structure is actively studied as a way to improve current diffusion while minimizing the thickness of the thin film.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a nitride-based semiconductor light-emitting device in which current diffusion is improved while minimizing the thickness of an n-type nitride-based semiconductor layer to improve light emission distribution, operating voltage and optical output.

According to one aspect of the present invention, there is provided a nitride-based semiconductor light emitting device. The nitride-based semiconductor light-emitting device includes a first semiconductor layer having a first conductivity, a current diffusion layer having a nitride-based superlattice structure and located on the first semiconductor layer, the current diffusion layer including aluminum, 1 conductive semiconductor layer, an active layer located on the second semiconductor layer, and a third semiconductor layer located on the active layer and having a second conductivity. In this case, in order to improve the conductivity in the vertical and horizontal directions and to increase the current diffusion effect, the current diffusion layer is formed on the undoped nitride based semiconductor layer, the first non-doped nitride based semiconductor layer containing aluminum Based semiconductor layer and a second nitride-based semiconductor layer located on the first nitride-based semiconductor layer and having a composition ratio of aluminum reduced toward the active layer.

The first nitride-based semiconductor layer at this time is characterized in that the composition ratio of aluminum in the thickness direction is constant and the composition ratio of aluminum in the direction of the active layer of the second nitride-based semiconductor layer is gradually or gradually reduced.

In order to further improve the current diffusion effect, the second nitride semiconductor layer is n-type doped.

The n-type delta-doped layer may be disposed between the first and second nitride-based semiconductor layers to maximize the current diffusion effect.

According to the present invention, by providing the current diffusion layer having a nitride-based superlattice structure including aluminum in the inside of the n-type nitride-based semiconductor layer, the nitride-based semiconductor light-emitting device having improved current diffusion while minimizing the thickness of the n-type nitride- Device can be provided.

Further, in the current diffusion layer of the present invention, by utilizing a layer which gradually changes the aluminum component ratio, a high internal electric field generated at the interface of the nitride semiconductor superlattice structure is transferred to the inside of the thin film to reduce the energy barrier, It is possible to maximize horizontal and vertical conduction characteristics.

In addition, the n-type doping can be performed on the layer in which the aluminum component ratio in the current diffusion layer is changed, thereby further improving the mobility characteristics in the superlattice structure two-dimensional electron gas structure.

In addition, the current diffusion layer of the present invention further includes an n-type delta-doped layer to further cancel the energy barrier to improve the conductivity in the vertical direction and contribute to the improvement of conduction characteristics by the doping region of high concentration .

The technical effects of the present invention are not limited to those mentioned above, and other technical effects not mentioned can be clearly understood by those skilled in the art from the following description.

1 is a cross-sectional view of a nitride-based semiconductor light emitting device according to an embodiment of the present invention.
2 is a cross-sectional view showing an example of the unit structure of the current diffusion layer.
3 is a cross-sectional view showing an example of the unit structure of the current diffusion layer.
4 is a graph showing a result of energy band calculation of a current diffusion layer of a nitride semiconductor light emitting device according to an embodiment of the present invention.
5 is a graph showing a voltage-current relationship in a vertical direction of a nitride semiconductor light emitting device according to an embodiment of the present invention.
6 is a graph illustrating an operating voltage and an optical output of a nitride semiconductor light emitting device according to an embodiment of the present invention.
7 is a graph illustrating current distribution characteristics of a nitride semiconductor light emitting device according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. Rather, the intention is not to limit the invention to the particular forms disclosed, but rather, the invention includes all modifications, equivalents and substitutions that are consistent with the spirit of the invention as defined by the claims.

It will be appreciated that when an element such as a layer, region or substrate is referred to as being present on another element "on," it may be directly on the other element or there may be an intermediate element in between .

Although the terms first, second, etc. may be used to describe various elements, components, regions, layers and / or regions, such elements, components, regions, layers and / And should not be limited by these terms.

1 is a cross-sectional view of a nitride-based semiconductor light emitting device according to an embodiment of the present invention.

1, a nitride-based semiconductor light emitting device includes a substrate 100, a buffer layer 200, a first semiconductor layer 300, a current diffusion layer 400, a second semiconductor layer 500, an active layer 600, A blocking layer 700 and a third semiconductor layer 800.

The substrate 100 may be a growth substrate for growing a nitride semiconductor including such a diode structure, and may be any material capable of easily growing an n-type semiconductor layer with a predetermined light transmittance. For example, the substrate 100 may be made of sapphire (Al ₂ O ₃ ), silicon carbide (SiC), spinel or silicon substrate. Optionally, a patterned substrate can be used. A substrate having such a pattern can improve the light extraction structure of the light emitting device.

The buffer layer 200 is located on the substrate 100. The buffer layer 200 is formed on the first semiconductor layer 300 to minimize the occurrence of crystal defects due to mismatch of lattice constant and thermal expansion coefficient between the substrate 100 and the first semiconductor layer 300. [ , It can be grown at a relatively low temperature. When the first semiconductor layer 300 is an n-GaN layer, the buffer layer 200 may be an undoped GaN layer (u-GaN layer). On the other hand, depending on the case, the buffer layer 200 may be omitted.

The first semiconductor layer 300 is located on the buffer layer 200 and has a first conductivity. The first semiconductor layer 300 having the first conductivity may include a nitride-based material such as gallium nitride. The first semiconductor layer 300 having the first conductivity may be implemented as an n-type semiconductor layer. In this case, a Group 4 element is used as the dopant, and for example, Si or Ge can be used as a dopant.

The current diffusion layer 400 is located between the first semiconductor layer 300 and the second semiconductor layer 500 and has a nitride-based superlattice structure including aluminum.

2, which will be described later, the current diffusion layer 400 includes an undoped nitride-based semiconductor layer 410, a first nitride-based semiconductor layer 420 containing aluminum, and a second nitride- The semiconductor layers 430 may be alternately repeatedly stacked.

The composition ratio of aluminum in the thickness direction of the first nitride semiconductor layer 420 may be constant. In addition, the second nitride based semiconductor layer 430 may have a reduced composition ratio of aluminum in the active layer 600 direction. For example, in the second nitride semiconductor layer 430, the composition ratio of aluminum in the direction of the active layer 600 may be gradually or gradually reduced.

At this time, in order to further improve the horizontal / vertical conduction characteristics, the thickness of the undoped nitride based semiconductor layer 410 and the total thickness of the first and second nitride based semiconductor layers 420 and 430 The thickness to thickness ratio can range from 3: 1 to 1: 3. The ratio of the thickness of the first nitride semiconductor layer 420 to the thickness of the second nitride semiconductor layer 430 may range from 2: 1 to 1: 2.

In addition, the alternately repeatedly stacked structures may be repeatedly stacked one to fifty times.

The gallium nitride thin film including the nitride semiconductor superlattice structure including aluminum in this way exhibited a conductivity improvement of about 2.5 times or more in the horizontal direction as compared with the conventional n-type gallium nitride thin film not including the superlattice structure, It can contribute to diffusion.

Therefore, it is possible to improve device characteristics such as light output, operating voltage, and reliability by inducing uniform current diffusion based on the high conductivity characteristic of the superlattice structure.

However, the general aluminum gallium nitride-gallium nitride semiconductor superlattice structure has a high conduction characteristic by a two-dimensional electron gas (2DEG) in a horizontal direction, but a high energy barrier in the vertical direction has a drawback Have.

This is because the energy barrier in the vertical direction is closely related to the internal electric field generated by the stress at the interface between the gallium nitride thin film and the gallium nitride thin film in the superlattice structure.

Therefore, in the current diffusion layer 400 of the present invention, by utilizing a layer that gradually changes the aluminum component ratio, a high internal electric field generated at the interface of the nitride semiconductor superlattice structure is transferred to the inside of the thin film to reduce the energy barrier, It is possible to maximize horizontal / vertical conduction characteristics of the lattice structure.

The second semiconductor layer 500 having the first conductivity is positioned on the current diffusion layer 400. The second semiconductor layer 500 may include a nitride-based material such as gallium nitride. The second semiconductor layer 500 having the first conductivity may be formed of, for example, an n-type semiconductor layer. In this case, a Group 4 element is used as the dopant, and for example, Si or Ge can be used as a dopant.

For example, the second semiconductor layer 500 may be the same nitride-based material as the first semiconductor layer 300.

The active layer 600 is located on the second semiconductor layer 500. This active layer 600 first grows a quantum barrier layer (not shown) and then a quantum well layer (not shown). Thus, the active layer 600 is formed such that a quantum well layer is positioned between the quantum barrier layers, and the quantum well layer may include a GaN or InGaN layer. The In composition of the quantum barrier layer and the quantum well layer in the multiple quantum well structure and the number of times of lamination of each layer can be arbitrarily set according to the intended emission wavelength of the light emitting device.

An electron blocking layer (EBL) 700 is located on the active layer 600. Such an electron blocking layer 700 may be made of AlGaN, for example. The thickness and the Al composition of the electronic block layer 700 can be arbitrarily set, and can be omitted depending on the case.

The third semiconductor layer 800 having the second conductivity is located on the electron blocking layer 700. If the electron blocking layer 700 is omitted, the third semiconductor layer 800 will be located on the active layer 600.

The third semiconductor layer 800 has a second conductivity. The third semiconductor layer 800 having the second conductivity may be a p-type semiconductor layer. For example, it may be doped with an impurity such as magnesium (Mg) to have a p-type characteristic.

It goes without saying that the first conductivity may be the p-type and the second conductivity may be the n-type, as well as the case where the first conductivity is n-type and the second conductivity is p-type.

Hereinafter, such a current diffusion layer will be described in detail with reference to Figs. 2 and 3.

2 is a cross-sectional view showing an example of the unit structure of the current diffusion layer.

2, the current diffusion layer 400 includes a non-doped nitride based semiconductor layer 410, a first nitride based semiconductor layer 410 including a predetermined proportion of aluminum located on the undoped nitride based semiconductor layer 410, Based semiconductor layer 430 and a second nitride based semiconductor layer 430 disposed on the first nitride based semiconductor layer 420 and having a reduced proportion of aluminum in the thickness direction.

For example, the undoped nitride based semiconductor layer 410 may be an undoped GaN layer, and the first nitride based semiconductor layer 420 may be an AlGaN layer having a constant aluminum proportion in the thickness direction. In addition, the second nitride semiconductor layer 430 may be an AlGaN layer in which the proportion of aluminum in the thickness direction is reduced.

In this case, the Al composition of the AlGaN layer having a constant aluminum component ratio in the thickness direction of the first nitride semiconductor layer 420 may be 5% to 30% of the total of Al and Ga.

In addition, the Al composition ratio of the AlGaN layer in which the aluminum component ratio of the second nitride semiconductor layer 430 is reduced can be reduced in the thickness direction, for example, in the direction of the active layer 600 in FIG. At this time, the composition ratio of Al to be reduced can be reduced to the range of 0% to 5% in the range of 5% to 30% of the total amount of Al and Ga. For example, the second nitride-based semiconductor layer 430 may be an aluminum composition ratio of reduced to Al _{0 .01} Ga _{0 .99} N from Al _{0 .2} Ga _{0 .8} N in the thickness direction.

At this time, the composition ratio of aluminum in the second nitride-based semiconductor layer 430 can be gradually or stepwise reduced.

Therefore, the internal electric field generated at the interface of such a superlattice structure can be gradually relaxed by changing the aluminum component ratio.

That is, the internal electric field generated between the undoped nitride based semiconductor layer 410 and the nitride based semiconductor layer 420 containing aluminum is generated at the interface by using the nitride based semiconductor layer 430 in which the composition ratio of aluminum is changed Can be dispersed throughout the thin film to reduce its size.

On the other hand, the second nitride semiconductor layer 420 is n-type doped. That is, in order to optimize the mobility of the two-dimensional electron gas (2DEG) formed between the aluminum gallium nitride layer and the gallium nitride layer, an n-type dopant such as silicon can be doped at a distance of several nanometers from the interface between the two layers have. Therefore, the doped n-type dopant can increase the mobility of electrons because it causes scattering in the electron migration path.

The n-type doping concentration is preferably in the range of 5 × 10 ¹⁷ cm ^-3 to 5 × 10 ¹⁹ cm ^-3 .

3 is a cross-sectional view showing an example of the unit structure of the current diffusion layer.

Referring to FIG. 3, the current diffusion layer 400 'is formed in the unit structure of the current diffusion layer 400 of FIG. 2, and the n < th > diffusion layer 400' is located between the first nitride semiconductor layer 420 and the second nitride semiconductor layer 430 Type delta-doped layer 440 is further included.

The n-type delta-doped layer 440 may include Si or Ge.

Further, the n-type delta-doping concentration is preferably 5 × 10 ¹⁸ cm ^-3 to 1 × 10 ²⁰ cm ^-3 .

For example, silicon delta doping can additionally offset the energy barrier by applying a V-shaped potential, and a high concentration of doped regions can contribute to improved conduction characteristics. Thus, by inserting this delta doped layer 440, the vertical energy barrier of the superlattice structure can be minimized.

Manufacturing example  One

A 10 nm thick undoped u-GaN layer, a 5 nm thick Al _{0 .15} Ga _{0 .85} N layer and a 5 nm thick Al _x Ga _{1 - x} N layer with decreasing aluminum content Reduction) were alternately repeated eight times, and a current diffusion layer was formed by MOCVD.

Manufacturing example  2

A 10 nm thick undoped u-GaN layer, a 5 nm thick Al _{0 .15} Ga _{0 .85} N layer and a 5 nm thick Al _x Ga _{1 - x} N layer with decreasing aluminum content Reduction) were alternately repeated eight times, and a current diffusion layer was formed by MOCVD.

And, Al _{0 .15} Ga _{0 .85} N layer and the Al _x Ga _{1 - x} N layer delta-doped silicon were locally at the interface.

Comparative Example

An undoped u-GaN layer having a thickness of 10 nm and an Al _{0 .15} Ga _{0 .85} N layer having a thickness of 10 nm were alternately laminated eight times repeatedly to form a current diffusion layer by MOCVD.

Experimental Example  One

The energy bands of the current diffusion layers of Production Example 1, Production Example 2 and Comparative Example were calculated.

4 is a graph showing a result of energy band calculation of a current diffusion layer of a nitride semiconductor light emitting device according to an embodiment of the present invention.

Referring to FIG. 4, the upper graph is a graph of energy bands calculated according to a distance at which the AlGaN / GaN superlattice structure is repeated. Referring to the upper graph, a piezo-electric field generated at the AlGaN-GaN interface bends the band shape.

The lower graph of FIG. 4 is a graph of the distribution of electrons along the distance. Referring to the upper graph and the lower graph, electrons can be gathered by the point where the energy band is downward.

At this time, the position of 230 nm to 240 nm is the GaN layer, the position of 240 nm to 250 nm is the AlGaN layer, and the same structure is repeated thereafter.

It can be seen that a high energy barrier of about 0.3 eV is formed in the current diffusion layer of the comparative example. Therefore, it can be seen that the movement of the electrons in the vertical direction is disturbed.

On the other hand, the energy barrier of the current diffusion layer of Production Example 1 is reduced to about 0.1 eV. Therefore, it can be seen that the movement of electrons in the vertical direction is improved.

Further, it can be seen that the energy barrier is further reduced to about 0.05 eV in the current diffusion layer of Production Example 2. Therefore, it can be seen that the electron mobility in the vertical direction is further improved by the delta doping of silicon. Further, referring to the lower graph, it can be seen that the electron concentration in Production Example 2 is increased as compared with Production Example 1 and Comparative Example. It can be seen that the electron concentration is further improved by the addition of the electrons to the delta-doped layer.

Experimental Example  2

The voltage-current in the vertical direction of the n-type gallium nitride layer in which the current diffusion layers of Production Example 1, Production Example 2 and Comparative Example were inserted was measured.

First, an n-type gallium nitride layer as a unit thin film was formed. Then, a part of the n-type gallium nitride layer was etched, and Cr / Ni / Au electrodes were respectively formed on the upper surface of the n-type gallium nitride layer and the etched surface. At this time, the etched height of the n-type gallium nitride layer is 450 nm.

In addition, the current diffusion layers of Comparative Example, Production Example 1 and Production Example 2 described above formed a structure inserted in the above-described n-type gallium nitride layer to compare the amount of current flowing in the vertical direction.

5 is a graph showing a voltage-current relationship in a vertical direction of a nitride semiconductor light emitting device according to an embodiment of the present invention.

Referring to FIG. 5, it can be seen that the structure in which the current diffusion layers are inserted in Manufacturing Example 1 and Manufacturing Example 2 has better electrical characteristics than the structure in which the current diffusion layer in the comparative example is inserted.

Further, in the case of Production Example 2, it is found that the electric characteristics in the vertical direction are better than those of the general n-type gallium nitride layer without the current diffusion layer inserted therein.

It can be seen that the electrical characteristics in the vertical direction are better as the size of the energy barrier decreases in association with the calculation result of the energy band in FIG.

The sheet concentration, mobility and conductivity of the n-type gallium nitride layer in which the current diffusion layers of Production Example 1, Production Example 2 and Comparative Example were inserted were measured using a Hall effect measurement system.

Table 1 below shows the sheet concentration, mobility and conductivity of the nitride based semiconductor light emitting device according to one embodiment of the present invention.

rescue Sheet concentration
(/ cm ² ) Mobility
(cm ² / Vs) conductivity
(/ Ωcm) The n-type gallium nitride layer - 237 197 Comparative Example 6.61 × 10 ¹³ 649 289 Production Example 1 6.25 × 10 ¹³ 960 340 Production Example 2 1.02 x 10 ¹⁴ 1075 569

Referring to Table 1, the current diffusion of the LED device is closely related to the horizontal conductivity. In other words, the conductivity is proportional to the product of sheet concentration and mobility.

It can be seen that the conductivity of the n-type gallium nitride layer inserted with the current diffusion layer of Comparative Example, Production Example 1 and Production Example 2 is higher than that of the conventional n-type gallium nitride layer.

On the other hand, in the comparative example, Si doped in the AlGaN layer interferes with the movement of electrons, indicating that mobility is lower than that in Structural Examples 1 and 2. In the case of Production Example 1 and Production Example 2, since the undoped AlGaN layer is Si doped after forming about 5 nm, the Si scattering at the AlGaN / GaN interface is relatively low and the mobility is high.

The sheet concentration of Production Example 2 is higher than that of Comparative Example and Production Example 1. In the case of Production Example 2, the amount of electrons supplied from the Si delta-doped layer is relatively larger than that of Production Example 1, resulting in a higher sheet concentration.

Therefore, it can be seen that the conductivity in the horizontal direction is improved as compared with the conventional n-type gallium nitride layer in Comparative Example, Production Example 1 and Production Example 2. In addition, in the case of Production Example 2, the conductivity is improved to be larger than that of Comparative Example and Production Example 1. [

5 and Table 1, it can be seen that, in the case of Production Example 1 and Production Example 2, current diffusion is improved due to high horizontal / vertical conductivity.

Experimental Example  3

A horizontal LED having the current diffusion layers of Production Example 1, Production Example 2 and Comparative Example inserted in the reference LED using only the conventional n-type gallium nitride layer (n-GaN) and the n-type gallium nitride layer was manufactured, Respectively.

FIG. 6 is a graph illustrating an operating voltage and an optical output of a nitride semiconductor light emitting device according to an embodiment of the present invention. FIG. 7 is a graph illustrating current distribution characteristics of a nitride semiconductor light emitting device according to an embodiment of the present invention. admit.

Referring to FIG. 6, in the comparative example, the operating voltage is increased compared to the reference LED, but it is found that the operating voltage is lower than that of the reference LED in Production Example 1 and Production Example 2 (see dotted line).

In addition, it can be seen that the output power of Comparative Example, Production Example 1 and Production Example 2 is increased as compared with the reference LED. Further, it can be seen that the light output is further improved in the case of Production Example 1 and Production Example 2 as compared with the Comparative Example (see solid line).

Referring to FIG. 7, the current diffusion image of each LED can be confirmed. It can be seen that the area between the images of Production Example 1 and Production Example 2 is larger than that of the reference LED. Thus, it can be seen that the current diffusion characteristics of Manufacturing Example 1 and Manufacturing Example 2 are further improved as compared with the reference LED.

Therefore, it can be seen that the improved current diffusion characteristics contributed to the increase of the light output and the reduction of the operating voltage.

It should be noted that the embodiments of the present invention disclosed in the present specification and drawings are only illustrative of specific examples for the purpose of understanding and are not intended to limit the scope of the present invention. It will be apparent to those skilled in the art that other modifications based on the technical idea of the present invention are possible in addition to the embodiments disclosed herein.

100: substrate 200: buffer layer
300: first semiconductor layer 400, 400 ': current diffusion layer
410: a non-doped nitride based semiconductor layer;
420: first nitride semiconductor layer 430: second nitride semiconductor layer
440: n-type delta doping layer 500: second semiconductor layer
600: active layer 700: electronic block layer
800: Third semiconductor layer

Claims (11)

A first semiconductor layer having a first conductivity;
A current diffusion layer located on the first semiconductor layer and having a nitride-based superlattice structure including aluminum;
A second semiconductor layer located on the current diffusion layer and having a first conductivity;
An active layer located on the second semiconductor layer; And
And a third semiconductor layer located on the active layer and having a second conductivity,
The current diffusion layer
An undoped nitride based semiconductor layer;
A first nitride based semiconductor layer including aluminum located on the undoped nitride based semiconductor layer; And
And a second nitride based semiconductor layer located on the first nitride based semiconductor layer and having a composition ratio of aluminum reduced toward the active layer. The method according to claim 1,
Wherein the first nitride based semiconductor layer has a constant component ratio of aluminum in the thickness direction. The method according to claim 1,
Wherein a ratio of aluminum in the direction of the active layer of the second nitride based semiconductor layer is gradually or stepwise reduced. The method according to claim 1,
Wherein the undoped nitride based semiconductor layer, the first nitride based semiconductor layer, and the second nitride based semiconductor layer are alternately and repeatedly stacked. The method according to claim 1,
And the second nitride based semiconductor layer is n-type doped. 6. The method of claim 5,
And the n-type doping concentration is 5 x 10 ¹⁷ cm ^-3 to 5 x 10 ¹⁹ cm ^-3 . The method according to claim 1,
And an n-type delta-doped layer located between the first nitride-based semiconductor layer and the second nitride-based semiconductor layer. 8. The method of claim 7,
Wherein the n-type delta-doped layer comprises Si or Ge. 8. The method of claim 7,
Wherein the n-type delta-doping concentration is in the range of 5 × 10 ¹⁸ cm ^-3 to 1 × 10 ²⁰ cm ^-3 . The method according to claim 1,
Wherein the first nitride based semiconductor layer is an AlGaN layer having a constant aluminum component ratio in the thickness direction and the Al composition ratio of the AlGaN layer having a constant aluminum component ratio is 5% to 30% based on the total of Al and Ga. The method according to claim 1,
Wherein the second nitride based semiconductor layer is an AlGaN layer in which the proportion of aluminum in the direction of the active layer is reduced and the Al composition of the AlGaN layer in which the proportion of aluminum is reduced is in the range of 5% to 30% To 5%. ≪ RTI ID = 0.0 > 8. < / RTI >

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