WO2014003402A1

WO2014003402A1 - Near uv light emitting device

Info

Publication number: WO2014003402A1
Application number: PCT/KR2013/005576
Authority: WO
Inventors: Chang Suk Han; Hwa Mok Kim; Hyo Shik Choi; Mi So Ko; A Ram Cha Lee; Jung Hwan Hwang
Original assignee: Seoul Viosys Co., Ltd.
Priority date: 2012-06-28
Filing date: 2013-06-25
Publication date: 2014-01-03
Also published as: KR20140002910A

Abstract

A near ultraviolet light emitting device is disclosed. The light emitting device includes an n-type contact layer including a gallium nitride layer, a p-type contact layer including a gallium nitride layer, and an active region of a multiple quantum well structure disposed between the n-type contact layer and the p-type contact layer, wherein the active region emits near ultraviolet light in a wavelength range from 360 nm to 390 nm.

Description

NEAR UV LIGHT EMITTING DEVICE

The present invention relates to an inorganic semiconductor light emitting device, and more particularly, to a near ultraviolet (UV) light emitting device.

Generally, gallium nitride-based semiconductors are widely applied to full-color displays, traffic signals, light sources for general illumination and optical communication apparatuses, ultraviolet and blue/green light emitting diodes, laser diodes, and the like. Particularly, indium gallium nitride (InGaN) compound semiconductors have attracted attention due to narrow band gap thereof.

Light emitting devices using such gallium nitride-based compound semiconductors are utilized in various application fields, such as large natural color flat panel display apparatuses, light sources for backlight units, traffic signals, indoor illumination, high-density light sources, high-resolution output systems, optical communication systems, and the like. Particularly, near UV light emitting devices are used in the fields of counterfeit bill identification, resin curing, ultraviolet treatment, and the like, and can realize various colors of visible light in combination with phosphors.

Near ultraviolet light generally refers to ultraviolet light in a wavelength range from about 320 nm to about 390 nm. Devices employing InGaN as a well layer can be used to emit light having wavelengths of about 360 nm or more, that is, near ultraviolet light having wavelengths from 360 nm to 390 nm depending on the amount of In.

Here, since light generated in the well layer is emitted outside through a barrier layer and a contact layer, a plurality of semiconductor layers is placed on a light travelling path and absorbs light. Particularly, when the semiconductor layers have narrower or similar band gaps to that of the well layer, significant light loss occurs. Particularly, it is necessary to control light absorption caused by an n-type contact layer and a p-type contact layer, which occupy most thickness of the light emitting device.

For this reason, in a typical near ultraviolet light emitting device, an electron blocking layer, a barrier layer, an n-type contact layer and a p-type contact layer are formed of AlGaN having a wider band gap than InGaN. However, since the n-type contact layer is formed of AlGaN, formation of an active layer having good crystallinity is difficult. Thus, near ultraviolet light emitting devices are inferior to blue light emitting devices in terms of electrical and optical properties, and are more expensive than blue/green light emitting diodes.

It is an aspect of the present invention to provide a gallium nitride-based near ultraviolet light emitting device which has improved light output.

It is another aspect of the present invention to provide a near ultraviolet light emitting device which allows improvement in crystallinity of an active layer.

In accordance with one aspect of the invention, a light emitting device includes: an n-type contact layer including an AlGaN layer; a p-type contact layer including an AlGaN layer; an active region of a multiple quantum well structure disposed between the n-type contact layer and the p-type contact layer; and a superlattice layer disposed between the n-type contact layer and the active region. Here, the superlattice layer has a structure in which a first AlInGaN layer and a second AlInGaN layer are alternately stacked one above another, and the active region of the multiple quantum well structure emits near ultraviolet light within a wavelength range from 360 nm to 390 nm.

The active region of the multiple quantum well structure may include barrier layers and well layers. The barrier layers are formed of AlInGaN. The barrier layer includes In, thereby mitigating lattice mismatch between the well layer and the barrier layer.

A first barrier layer closest to the n-type contact layer among the barrier layers may include 2% to 4% more Al than other barrier layers. The first barrier layer is formed of AlInGaN having a smaller lattice constant than the other barrier layers, thereby improving light output of the light emitting device. In the specification, the content of a metallic element represented in percent is a composition ratio of each metallic component to the sum of composition ratios of metallic components of the gallium nitride-based layer in percent. That is, the content of Al of the gallium nitride-based layer represented by Al_xIn_yGa_zN is calculated by 100×x/(x+y+z) and is represented in %.

The well layers are formed of InGaN, and the other barrier layers except for the first barrier layer may include 10% to 12% of Al and 1% or less of In. In addition, the first barrier layer may be formed of AlInGaN including 12% to 16% of Al and 1% or less of In.

In some embodiments, the p-type contact layer may include a lower doped layer with higher concentration, an upper doped layer with higher concentration, and a doped layer with lower concentration disposed between the lower doped layer with higher concentration and the upper doped layer with higher concentration. In addition, the doped layer with lower concentration is thicker than the lower and upper doped layers with higher concentration. As the doped layer with lower concentration is formed to a relatively thick thickness, light absorption due to the p-type contact layer is prevented.

Further, the n-type contact layer may include a lower gallium nitride layer, an upper aluminum gallium nitride layer, and an intermediate layer of a multilayer structure disposed between the lower gallium nitride layer and the upper aluminum gallium nitride layer. The intermediate layer of the multilayer structure is inserted into the middle of the n-type contact layer, thereby improving crystallinity of epitaxial layers formed on the n-type contact layer. Particularly, the intermediate layer of the multilayer structure may have a structure in which AlInN and GaN are alternately stacked one above another.

Further, the n-type contact layer may include a modulation doped AlGaN layer. The upper aluminum gallium nitride layer may be a modulation doped layer.

The light emitting device may further include an electron implantation layer disposed between the superlattice layer and the active region. Here, the electron implantation layer has a higher n-type impurity doping concentration than the superlattice layer. Since electrons are well implanted into the active region by the electron implantation layer, the light emitting device may exhibit improved luminous efficacy.

In one embodiment, the electron implantation layer may be formed of AlGaN.

An undoped AlGaN layer may be disposed between the n-type contact layer and the superlattice layer. The undoped AlGaN layer may adjoin the n-type contact layer, and restores deteriorated crystallinity of the n-type contact layer depending on impurity doping.

The light emitting device may further include an AlGaN layer with lower concentration disposed between the undoped AlGaN layer and the superlattice layer and doped in a lower n-type impurity concentration than the n-type contact layer; and an AlGaN layer with higher concentration disposed between the AlGaN layer with lower concentration and the superlattice layer and doped in a higher n-type impurity concentration than the AlGaN layer with lower concentration.

An AlInGaN/AlInGaN-stacked superlattice layer is disposed between an n-type contact layer including an AlGaN layer and an active region to improve crystallinity of the active region, thereby improving light output. In addition, a first barrier layer includes more Al than other barrier layers, thereby further improving light output.

The above and other aspects, features, and advantages of the present invention will become apparent from the detailed description of the following embodiments in conjunction with the accompanying drawings, in which:

Figure 1 is a sectional view of a light emitting device according to one embodiment of the present invention;

Figure 2 is a sectional view of a multiple quantum well structure of a light emitting device according to one embodiment of the present invention;

Figure 3 is a graph depicting light output depending on use of a superlattice layer;

Figure 4 is a graph depicting light output depending on the amount of In in a superlattice layer; and

Figure 5 is a graph depicting light output depending on thickness of a well layer.

Now, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be understood that the present invention is not limited to the following embodiments and may be embodied in different ways, and that the embodiments are provided for complete disclosure and thorough understanding of the invention by those skilled in the art. In the drawings, widths, lengths, thicknesses and the like of components may be exaggerated for convenience. Like components will be denoted by like reference numerals throughout the specification.

Figure 1 is a sectional view of a light emitting device according to one embodiment of the present invention, and Figure 2 is an enlarged sectional view of a multiple quantum well structure of the light emitting device.

Referring to Figure 1, the light emitting device includes an n-type contact layer 27, a superlattice layer 35, an active region 39, and a p-type contact layer 43. In addition, the light emitting device may include a substrate 21, a nuclear layer 23, a buffer layer 25, an undoped GaN layer 29, an AlGaN layer 31 with lower concentration, a AlGaN layer with higher concentration 33, an electron implantation layer 37, an electron blocking layer 41, or a delta doped layer 45.

The substrate 21 is a substrate for growth of a gallium nitride-based semiconductor layer, and may be formed of sapphire, SiC, spinel, and the like, without being limited thereto. For example, the substrate 21 may be a patterned sapphire substrate (PSS).

The nuclear layer 23 may be formed of (Al, Ga)N at a low temperature from 400℃ to 600℃ in order to grow the buffer layer 25 on the substrate 21. Preferably, the nuclear layer is formed of GaN or AlN. The nuclear layer 23 may be formed to a thickness of about 25 nm. The buffer layer 25 serves to mitigate defects such as dislocation between the substrate 21 and the n-type contact layer 27, and is grown at relatively high temperatures. For example, the buffer layer 25 may be formed of undoped GaN to a thickness of about 1.5 ㎛.

The n-type contact layer 27 is formed as a gallium nitride-based semiconductor layer doped with an n-type impurity such as Si, and may have, for example, a thickness of about 3 ㎛. The n-type contact layer 27 includes an AlGaN layer and may be formed as a single layer or multiple layers. For example, as shown in Figure 1, the n-type contact layer 27 may include a lower GaN layer 27a, an intermediate layer 27b, and an upper AlGaN layer 27c. Here, the intermediate layer 27b may be formed of AlInN, or may have a multilayer structure (including a superlattice structure) in which AlInN and GaN are alternately stacked one above another, for example, about 10 times. The lower GaN layer 27a may be formed to a thickness of about 1.5 ㎛, and the upper AlGaN layer 27c may be formed to a thickness of about 1 ㎛. For example, the upper AlGaN layer 27c may have a composition ratio of about 3% Al.

The intermediate layer 27b is formed to a smaller thickness than the upper AlGaN layers 27c, and may be formed to a thickness of about 80 nm. The intermediate layer 27b is formed on the lower GaN layer 27a, and the upper AlGaN layer 27c is formed thereon, thereby improving crystallinity of the upper AlGaN layer 27c.

Particularly, the lower GaN layer 27a and the upper AlGaN layer 27c are doped in a high Si impurity concentration, and the intermediate layer 27b may be doped in the same or lower impurity concentration than the upper AlGaN layer 27c, or may be intentionally left undoped. In addition, the upper AlGaN layer 27c may be formed as a modulation doped layer by repeating doping and un-doping. The lower GaN layer 27a and the upper AlGaN layer 27c are doped in a high impurity concentration, thereby reducing resistance of the n-type contact layer 27. An electrode, which contacts the n-type contact layer 27, may contact the upper AlGaN layer 27c. Particularly, when the light emitting device of a vertical structure is formed by removing the substrate 21, the lower GaN layer 27a and the intermediate layer 27b may be removed.

The undoped AlGaN layer 29 is formed of impurity-free AlGaN, and may be formed to a smaller thickness than the upper AlGaN layer 27c, for example, a thickness from 80 nm to 300 nm. As the n-type contact layer 27 is doped with an n-type impurity, residual stress is generated in the n-type contact layer 27, thereby deteriorating crystallinity thereof. Thus, when another epitaxial layer is grown on the n-type contact layer 27, it is difficult to grow an epitaxial layer having good crystallinity thereon. However, since the undoped AlGaN layer 29 is free from impurities, the undoped AlGaN layer 29 functions as a restoring layer which restores deteriorated crystallinity of the n-type contact layer 27. Thus, it is advantageous that the undoped AlGaN layer 29 is directly formed on the n-type contact layer 27 and adjoins the n-type contact layer 27. In addition, since the undoped AlGaN layer 29 has higher resistivity than the n-type contact layer 27, electrons flowing from the n-type contact layer 27 into the active layer 39 may be uniformly dispersed in the n-type contact layer 27 before passing through the undoped AlGaN layer 29.

The AlGaN layer with lower concentration 31 is disposed on the undoped GaN layer 29 and has a lower n-type impurity doping concentration than the n-type contact layer 27. For example, the AlGaN layer with lower concentration 31 may have a Si doping concentration in the range from 5×10¹⁷/cm³ to 5×10¹⁸/cm³, and may be formed to a smaller thickness than the undoped AlGaN layer 29, for example, to a thickness from 50 nm to 150 nm. On the other hand, the AlGaN layer with higher concentration 33 is disposed on the AlGaN layer with lower concentration 31 and has a higher n-type impurity doping concentration than the AlGaN layer with lower concentration 31. The AlGaN layer with higher concentration 33 may have a substantially similar Si doping concentration to the n-type contact layer 27. The AlGaN layer with higher concentration 33 may have a smaller thickness than the AlGaN layer with lower concentration 31, and for example, may be formed to a thickness of about 30 nm.

The n-type contact layer 27, the undoped AlGaN layer 29, the AlGaN layer with lower concentration 31 and the AlGaN layer with higher concentration 33 may be continuously grown by supplying metallic source gases into a chamber. As raw materials of the metallic source gases, organic materials of Al, Ga and In, such as TMA, TMG and/or TMI, and the like, are used. SiH₄ may be used as the Si source gas. These layers may be grown at a first temperature, for example, at a temperature from 1050℃ to 1150℃.

The superlattice layer 35 is disposed on the AlGaN layer with higher concentration 33. The superlattice layer 35 may be formed by alternately stacking a first AlInGaN layer and a second AlInGaN layer having different composition ratios. For example, 20 Å thick first AlInGaN layers and 20 Åthick second AlInGaN layers may be alternately stacked about 30 times one above another to form the superlattice layer 35. The first AlInGaN layer and the second AlInGaN layer have a wider band gap than well layers 39w (Figure 2) in the active region 39. The first AlInGaN layer and the second AlInGaN layer may have a lower composition ratio of In than the well layers 39w, without being limited thereto, and at least one layer of the first AlInGaN layer and the second AlInGaN layer may have a higher composition ratio of In than the well layers 39w. For example, between the first AlInGaN layer and the second AlInGaN layer, one layer including more In than the other layer may have a composition ratio of about 1% In and a composition ratio of about 4% Al. The superlattice layer 35 may be formed as an undoped layer which is free from impurities. The superlattice layer 35 is formed as the undoped layer, thereby reducing leakage current of the light emitting device.

The superlattice layer 35 may function as a buffer layer for an epitaxial layer formed thereon, thereby improving crystallinity of the epitaxial layer.

The electron implantation layer 37 has a higher n-type impurity doping concentration than the superlattice layer 35. In addition, the electron implantation layer 37 may have substantially the same n-type impurity doping concentration as that of the n-type contact layer 27. Preferably, the electron implantation layer 37 has an n-type impurity doping concentration ranging from 2×10¹⁸/cm³ to 2×10¹⁹/cm³, more preferably from 1×10¹⁹/cm³ to 2×10¹⁹/cm³. The electron implantation layer 37 has a higher impurity doping concentration and thus facilitates electron injection into the active region 39. The electron implantation layer 37 may be formed to a thickness similar to or thinner than that of the doped layer with higher concentration 33, for example, to a thickness of about 20 nm. For example, the electron implantation layer 37 may be formed of AlGaN.

The active region 39 is disposed on the electron implantation layer 37. Figure 2 is an enlarged sectional view of the active region 39.

Referring to Figure 2, the active region 39 has a multiple quantum well structure including alternately stacked barrier layers 39b and well layers 39w. The well layers 39w have a composition to emit near ultraviolet light in the wavelength range from 360 nm to 390 nm. For example, the well layers 39w may be formed of GaN, InGaN or AlInGaN, particularly InGaN. Here, the amount of In included in the well layer 39w is determined depending on a desired wavelength of near ultraviolet light. For example, the well layer 39w may include 1% or less of In. The well layers 39w may be formed to a thickness from about 20 Å to about 30 Å.

The barrier layers 39b may be formed of gallium nitride-based semiconductor having a wider band gap than the well layer 39b, such as GaN, InGaN, AlGaN, and AlInGaN. Particularly, the barrier layers 39b may be formed of AlInGaN, thus including In, and thereby mitigating lattice mismatch between the well layer 39w and the barrier layer 39b.

Among barrier layers 39b1, 39b, 39bn, a first barrier layer 39b1 closest to the electron implantation layer 37 or the superlattice layer 35 may have more Al than other barrier layers. For example, the first barrier layer 39b1 may include 2% to 4% more Al than the other barrier layers 39b. For example, when the other barrier layers 39b, 39bn include about 15% of Al, the first barrier layer 39b1 may include about 18% of Al. The barrier layers 39b1, 39b, 39bn include about 1% or less of In.

Generally, in the light emitting device, the barrier layers have the same composition. However, in embodiments of the present invention, the first barrier layer 39b1 has 2% to 4% higher Al than the other barrier layers 39b. The first barrier layer 39b1 is formed to have a wider band gap than the other barrier layers 39b1, and thus can confine carriers in the active region 39. If the Al content in the first barrier layer 39b1 exceeds about 4%, lattice mismatch between the first barrier layer 39b1 and the electron implantation layer 37 and between the first barrier layer 39b1 and the well layer 39w can increase, thereby deteriorating crystallinity of the active region 39.

It is desirable that the first barrier layer have substantially the same thickness as the other barrier layers except for the last barrier layer closest to the electron blocking layer 41 or the p-type contact layer 43. For example, the first barrier layer may have a thickness from 40 Å to 60 Å particularly, about 50 Å.

The active region 39 may adjoin the electron implantation layer 37. Although the barrier layers and the well layers of the active region 39 may be formed as undoped layers in order to improve crystallinity of the active region, the active region 39 may be partially or entirely doped with impurities to decrease forward voltage.

Referring to Figure 1 again, the p-type contact layer 43 may be disposed on the active region 39, and the electron blocking layer 41 may be disposed between the active region 39 and the p-type contact layer 43. The electron blocking layer 41 may be formed of AlGaN or AlInGaN, preferably AlInGaN, in order to mitigate lattice mismatch between the active region 39 and the electron blocking layer 41. Here, the electron blocking layer 41 may include about 35% of Al. The electron blocking layer 41 may be doped with a p-type impurity such as Mg, or may be free of impurities. The electron blocking layer 41 may be formed to a thickness of about 15 nm.

The p-type contact layer 43 may be an Mg-doped AlGaN layer, and may include, for example, about 8% of Al and have a thickness of 100 nm. The p-type contact layer 43 may be formed as a single layer, without being limited thereto, and may include a lower doped layer with higher concentration 43a, a doped layer with lower concentration 43b, and an upper doped layer with higher concentration 43c, as shown. The doped layer with lower concentration 43b has a lower doping concentration than the lower and upper doped layer with higher concentrations 43a, 43c, and is disposed between the lower doped layer with higher concentration 43a and the upper doped layer with higher concentration 43c. The doped layer with lower concentration 43b may be grown without supply of Mg source gas such as Cp₂Mg during growth. In addition, during growth of the doped layer with lower concentration 43b, the impurity content may be reduced using N₂ gas excluding H₂ gas as a carrier gas. Further, the doped layer with lower concentration 43b may be formed to a greater thickness than the lower and upper doped layer with higher concentrations 43a, 43c. For example, the doped layer with lower concentration 43b may be formed to a thickness of about 60 nm, and each of the lower and upper doped layer with higher concentrations 43a, 43c may be formed to a thickness of 10 nm. Thus, crystallinity of the p-type contact layer 43 is improved and an impurity concentration thereof is decreased, thereby preventing or mitigating loss of near ultraviolet light due to the p-type contact layer 43.

The delta doped layer 45 may be disposed on the p-type contact layer 43 in order to reduce ohmic contact resistance. The delta doped layer 45 is doped in a high n-type or p-type impurity concentration, thereby reducing ohmic resistance between an electrode formed thereon and the p-type contact layer 43. The delta doped layer 45 may be formed to a thickness from about 2 Å to about 5 Å.

On the other hand, a light emitting device of a horizontal structure or a flip chip structure may be prepared by patterning the epitaxial layers on the substrate 21, or a light emitting device of a vertical structure may be prepared by removing the substrate 21.

(Experimental Example 1)

To investigate light output depending on the presence of the superlattice layer 35 and the thickness thereof, epitaxial layers were grown on a patterned sapphire substrate by MOCVD under the same conditions as shown in Figure 1. The superlattice layer 35 was omitted in Comparative Example, the superlattice layer 35 had a thickness of 60 nm in Example 1, and the superlattice layer 35 had a thickness of 120 nm in Example 2. Each of the first AlInGaN layer and the second AlInGaN layer had a thickness of 20 Å. The superlattice layer 35 was formed by changing a flow rate of TMI as an In source. Composition ratios of Al and In in the superlattice layer 35 were measured using an atomic probe. The first layer and the second layer had a relatively thin thickness, and thus did not show great difference in composition ratio from the measurement results by the atomic probe. On the whole, the composition ratio of In was about 1% and the composition ratio of Al was about 4%.

In each of Comparative Example and Examples 1 and 2, three wafers were prepared, and light output of each light emitting device was measured at wafer level. An average value of each three wafers having the same layer structure is shown in Figure 3.

Referring to Figure 3, the light emitting devices employing the superlattice layer 35 in Examples 1 (60 nm) and 2 (120 nm) exhibited 20% higher light output than the light emitting device prepared in Comparative Example (0 nm). In addition, the light emitting device of Example 1 exhibited higher light output than the light emitting device of Example 2.

(Experimental Example 2)

To investigate change in light output depending on the content of In included in the superlattice layer 35, epitaxial layers were grown on a patterned sapphire substrate by MOCVD under the same conditions except for conditions for growth of the superlattice layer 35. Upon growth of the superlattice layer 35, the composition ratio of In in the superlattice layer 35 was changed by changing a flow rate of TMI, while maintaining constant flow rates of TMA and TMG.

Light output of each sample having a different composition ratio of In was measured at wafer level, and results are shown in Figure 4. Here, the composition ratio of In of each sample was measured using an atomic probe. As shown in Figure 4, light output was improved with increasing composition ratio of In from 0.2% to 1%.

(Experimental Example 3)

To investigate light output depending on the thickness of the well layer, epitaxial layers were grown under the same conditions except for the thickness of the well layer. Light output of each sample having a different thickness of the well layer was measured, and the results are shown in Figure 5. The thickness of the barrier layer was fixed to 5 nm, and the thickness of the well layer was changed from 3 nm to 12 nm. The superlattice layer 35 was formed under the same conditions for all of the samples.

Referring to Figure 5, the light emitting device having a well layer thickness of 3 nm exhibited the highest light output among the prepared samples and light output was decreased with increasing thickness of the well layer.

Claims

A light emitting device comprising:

an n-type contact layer comprising an AlGaN layer;

a p-type contact layer comprising an AlGaN layer;

an active region of a multiple quantum well structure disposed between the n-type contact layer and the p-type contact layer; and

a superlattice layer disposed between the n-type contact layer and the active region,

wherein the superlattice has a structure in which a first AlInGaN layer and a second AlInGaN are alternately stacked one above another, and the active region of the multiple quantum well structure emits near ultraviolet light in a wavelength range from 360 nm to 390 nm.
The light emitting device according to claim 1, wherein the active region of the multiple quantum well structure comprises barrier layers and well layers, the barrier layers being formed of AlInGaN, a first barrier layer closest to the n-type contact layer among the barrier layers comprising 2% to 4% more Al than other barrier layers.
The light emitting device according to claim 2, wherein the well layers are formed of InGaN, and the other barrier layers except for the first barrier layer are formed of AlInGaN comprising 10% to 12% of Al and 1% or less of In.
The light emitting device according to claim 3, wherein the first barrier layer is formed of AlInGaN comprising 12% to 16% of Al and 1% or less of In.
The light emitting device according to claim 1, wherein the p-type contact layer comprises a lower doped layer with higher concentration, an upper doped layer with higher concentration, and a doped layer with lower concentration disposed between the lower doped layer with higher concentration and the upper doped layer with higher concentration.
The light emitting device according to claim 5, wherein the doped layer with lower concentration has a greater thickness than the lower and upper doped layers with higher concentration.
The light emitting device according to claim 1, wherein the n-type contact layer comprises a lower gallium nitride layer, an upper aluminum gallium nitride layer, and an intermediate layer of a multilayer structure disposed between the lower gallium nitride layer and the upper aluminum gallium nitride layer.
The light emitting device according to claim 7, wherein the intermediate layer of the multilayer structure has a structure in which AlInN and GaN are alternately stacked one above another.
The light emitting device according to claim 1, further comprising: an electron implantation layer disposed between the superlattice layer and the active region, the electron implantation layer having a higher n-type impurity doping concentration than the superlattice layer.
The light emitting device according to claim 9, wherein the electron implantation layer is formed of AlGaN.
The light emitting device according to claim 9, further comprising: an undoped AlGaN layer disposed between the n-type contact layer and the superlattice layer.
The light emitting device according to claim 11, further comprising:

an AlGaN layer with lower concentration disposed between the undoped AlGaN layer and the superlattice layer and doped in a lower n-type impurity concentration than the n-type contact layer; and

an AlGaN layer with higher concentrationdisposed between the AlGaN layer with lower concentration and the superlattice layer and doped in a higher n-type impurity concentration than the AlGaN layer with lower concentration.
The light emitting device according to claim 1, wherein the n-type contact layer comprises a modulation doped AlGaN layer.