WO2013191406A1

WO2013191406A1 - Light emitting device having electron blocking layer

Info

Publication number: WO2013191406A1
Application number: PCT/KR2013/005124
Authority: WO
Inventors: Jung Whan JUNG; Seung Kyu Choi; Chae Hon Kim
Original assignee: Seoul Viosys Co., Ltd.
Priority date: 2012-06-18
Filing date: 2013-06-11
Publication date: 2013-12-27
Also published as: KR20130141945A; KR101923670B1

Abstract

A light emitting device having an electron blocking layer is disclosed. The light emitting device includes an n-type semiconductor layer, a p-type semiconductor layer, an active layer disposed between the n-type semiconductor layer and the p-type semiconductor layer, and an electron blocking layer disposed between the active layer and the p-type semiconductor layer. The electron blocking layer includes first, second, third and fourth layers, wherein the first layer is disposed closer to the active layer than the second to fourth layers and has a wider band gap than the second to fourth layers, the second layer adjoins the first layer, and the third layer is disposed between the second layer and the fourth layer and has a narrower band gap than the second and fourth layers. Thus, the light emitting device has improved hole injection efficiency and can efficiently prevent electron overflow.

Description

LIGHT EMITTING DEVICE HAVING ELECTRON BLOCKING LAYER

The present invention relates to a light emitting device using a nitride semiconductor, and more particularly, to a light emitting device having an electron blocking layer with an improved structure.

GaN-based nitride semiconductors have a direct transition type energy band structure, and can provide a light emitting device having a wide range of emission wavelengths from visible light to ultraviolet light by adjusting a composition ratio of Al, In, and Ga. Particularly, nitride-based semiconductors are widely applied to full-color displays, traffic signals, light sources for general illumination and optical communication apparatuses, blue/green light emitting diodes, laser diodes, and the like.

Such nitride-based light emitting devices include an active layer of a multi-quantum well structure disposed between n-type and p-type nitride semiconductor layers, and generate light through recombination of an electron and a hole in a quantum well layer of the active layer.

Figure 1 is a sectional view of a typical semiconductor light emitting device, and Figure 2 is a schematic band diagram of the light emitting device shown in Figure 1.

Referring to Figures 1 and 2, the light emitting device includes a substrate 11, an n-type semiconductor layer 13, a superlattice layer 15, an active layer 17, an electron blocking layer 19, and a p-type semiconductor layer 21.

Such a typical light emitting device includes the active layer 17 of a multi-quantum well structure disposed between the n-type semiconductor layer 13 and the p-type semiconductor layer 21 to improve luminous efficacy, and may emit light having a desired wavelength by adjusting the amount of In in an InGaN quantum well layer within the multi-quantum well structure. In addition, the electron blocking layer 19 is disposed between the p-type semiconductor layer 21 and the active layer 17 to block electron overflow, thereby improving luminous recombination rate.

When electric current is applied to the light emitting device, the n-type semiconductor layer 13 and the p-type semiconductor layer 21 provide electrons and holes, respectively, whereby light can be emitted through recombination of the electrons and the holes in the active layer 17. The electron blocking layer 19 is formed of a material having a greater energy band gap than the p-type semiconductor layer 21 to prevent electron overflow, and is generally formed of AlGaN.

Although the electron blocking layer 19 can prevent electron overflow, the electron blocking layer has a problem of decreasing hole injection efficiency by blocking the holes injected into the active layer 17. In addition, since ionization energy of a dopant increases with increasing fraction of Al in the AlGaN layer, it is difficult to obtain high hole concentration in the electron blocking layer 19. Further, a large difference between the AlGaN electron blocking layer 19 and the active layer 17 in terms of lattice constant makes it difficult to grow the electron blocking layer 19 or the p-type semiconductor layer 21 having with good crystal quality. Thus, the electron blocking layer 19 provides a limited improvement in luminous efficacy through increase in the recombination rate between electrons and holes in the active layer 17.

It is an aspect of the invention to provide a semiconductor light emitting device which can prevent electron overflow and improve luminous efficacy by increasing a concentration of holes injected into an active layer.

It is another aspect of the invention to provide a semiconductor light emitting device which includes an electron blocking layer and a p-type semiconductor layer both exhibiting good crystal quality.

In accordance with one aspect of the invention, a light emitting device includes: an n-type semiconductor layer; a p-type semiconductor layer; an active layer disposed between the n-type semiconductor layer and the p-type semiconductor layer; and an electron blocking layer disposed between the active layer and the p-type semiconductor layer. The electron blocking layer includes first, second, third and fourth layers, wherein the first layer is disposed closer to the active layer than the second to fourth layers and has a wider band gap than the second to fourth layers, the second layer adjoins the first layer, and the third layer is disposed between the second layer and the fourth layer and has a narrower band gap than the second and fourth layers.

The electron blocking layer may prevent electron overflow while improving hole injection efficiency. With the first layer of a relatively large band gap disposed close to the active layer, the second layer having a wider band gap than the third layer is disposed to adjoin the first layer, thereby efficiently preventing electron overflow. In addition, the third layer having a relatively narrow band gap is disposed inside the electron blocking layer, whereby the total hole concentration in the electron blocking layer can be increased. Further, holes can be dispersed due to the two-dimensional hole gas effect in the third layer, thereby improving current spreading capabilities of the light emitting device.

Particularly, the first layer may have a widest band gap in the electron blocking layer, and the third layer may have a narrowest band gap therein.

In addition, the third layer may adjoin the second and fourth layers. Further, the p-type semiconductor layer may adjoin the fourth layer.

The first to fourth layers may be formed of an AlInGaN-based nitride semiconductor. In addition, the second and fourth layers may be formed of an In-containing AlInGaN-based nitride semiconductor, thereby increasing a doping concentration of Mg. For example, the first layer may be formed of AlGaN, the third layer may be formed of GaN, and the second and fourth layers may be formed of AlInGaN.

The first, second and fourth layers may have a wider band gap than the p-type semiconductor layer. The third layer may have the same band gap as the p-type semiconductor layer, without being limited thereto. For example, the third layer may have a narrower band gap than the p-type semiconductor layer.

The first layer may have a lower p-type impurity concentration than the second to fourth layers, and may be an undoped layer that is intentionally impurity-free. Thus, it is possible to prevent p-type impurities from flowing into the active layer.

In addition, the third layer may have a higher p-type impurity concentration than the second and fourth layers.

The first layer may have a thinner thickness than each of the second to fourth layers. The first layer has a relatively thin thickness, thereby further improving hole injection efficiency. In addition, the third layer may have a thinner thickness than each of the second and fourth layers.

The light emitting device may further include an undoped semiconductor layer between the active layer and the electron blocking layer, wherein the undoped semiconductor layer may be formed of GaN. By growing the undoped semiconductor layer before forming the electron blocking layer, the electron blocking layer may have improved crystal quality.

In addition, the light emitting device may further include a c-plane GaN substrate. The n-type semiconductor layer is disposed closer to the substrate than the p-type semiconductor layer. The c-plane GaN substrate used as a growth substrate may further improve crystal quality of a GaN-based semiconductor layer grown thereon.

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 schematic sectional view of a typical semiconductor light emitting device;

Figure 2 is a schematic band diagram of the semiconductor light emitting device of Figure 1;

Figure 3 is a schematic sectional view of a semiconductor light emitting device according to one embodiment of the present invention;

Figure 4 is an enlarged sectional view of an electron blocking layer 59 of the semiconductor light emitting device of Figure 3; and

Figure 5 is a schematic band diagram of the semiconductor light emitting device of Figure 3.

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, the 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 3 is a sectional view of a semiconductor light emitting device according to one embodiment of the present invention, Figure 4 is an enlarged sectional view of an electron blocking layer 59 of the semiconductor light emitting device of Figure 3, and Figure 5 is a schematic band diagram of the semiconductor light emitting device of Figure 3.

Referring to Figs. 3 to 5, the light emitting device includes an n-type semiconductor layer 53, an active layer 57, an electron blocking layer 59, and a p-type semiconductor layer 61. In addition, the light emitting device may include a substrate 51, a superlattice layer 55, a spacer layer 58, and electrodes (not shown) for connection to an external power supply.

The substrate 51 is a substrate for growth of a gallium nitride-based semiconductor layer and may be a c-plane gallium nitride (GaN) substrate. Use of a gallium nitride substrate 51 as a growth substrate may improve crystal quality of the gallium nitride-based semiconductor layer formed thereon. Comparing a gallium nitride layer grown on the gallium nitride substrate with a gallium nitride layer grown on a sapphire substrate in terms of XRD main peak width, it can be ascertained that the width of the gallium nitride layer grown on the gallium nitride substrate is reduced to about 1/3 the width of the gallium nitride layer grown on a sapphire substrate. Further, use of the c-plane gallium nitride substrate as the growth layer allows elimination of a nuclear layer or buffer layer generally used in the art.

The n-type semiconductor layer 53 may be formed as a gallium nitride-based semiconductor layer, for example, a GaN layer, which is doped with n-type impurities such as Si, Ge, and the like. The n-type semiconductor layer 53 may be formed as a single layer, without being limited thereto, or as multiple layers.

The superlattice layer 55 is disposed between the n-type semiconductor layer 53 and the active layer 57. The superlattice layer 55 may be formed by alternately stacking gallium nitride-based semiconductor layers having different compositions. For example, a 20 Å thick GaN layer and a 20 Å thick InGaN layer may be alternately stacked several times one above another to form the superlattice layer 55. The uppermost layer in the superlattice layer 55 is doped with a high concentration by Si. The doping concentration of Si in the uppermost layer may be, for example, about 4 to about 5 times higher than that of a doping impurity, such as Si, in the n-type contact layer 53. In addition, the remaining layers in the superlattice layer 55 may be formed as undoped layers. The superlattice layer 55 may prevent crystal defects, such as dislocations, in the n-type semiconductor layer 53 from being transferred to the active layer 57, and may function as a buffer layer for growth of the active layer 57.

The active layer 57 may have a multi-quantum well structure in which a barrier layer and a quantum well layer are alternately stacked several times one above another. A composition of the quantum well layer is selected depending on a required wavelength of light, and is generally formed of InGaN. A composition ratio of In in the InGaN quantum well layer is determined based upon desired light emission wavelength. The barrier layer may be formed of a gallium nitride-based semiconductor, such as GaN, InGaN, AlGaN, or AlInGaN, which has a wider band gap than the quantum well layer.

The p-type semiconductor layer 61 may be formed as a single layer or multiple layers. For example, the p-type semiconductor layer 61 may include a p-type GaN layer doped with Mg.

The electron blocking layer 59 is disposed between the active layer 57 and the p-type semiconductor layer 61, and the spacer layer 58 is disposed between the active layer 57 and the electron blocking layer 59. The spacer layer 58 may be formed as an undoped semiconductor layer which is intentionally impurity-free, and may be formed of, for example, GaN. The spacer layer 58 may restore reduction in crystal quality due to growth of the active layer 57, and may function as a buffer layer for growth of the electron blocking layer 59.

The electron blocking layer 59 includes a first layer 59a, a second layer 59b, a third layer 59c, and a fourth layer 59d. As shown in Figs. 4 and 5, the first layer 59a is disposed closer to the active layer 57 than the second to fourth layers 59b to 59d, and has a wider band gap than the second to fourth layers 59b to 59d. In addition, the first layer 59a may adjoin the spacer layer 58. Further, the first layer 59a may have a widest band gap in the electron blocking layer 59. By disposing a layer having a widest band gap 59a closer to the active layer 57, electron overflow can be efficiently prevented. Furthermore, the first layer 59a may have a lower impurity doping concentration than the second to fourth layers 59b to 59d, and may be formed as an undoped layer. Thus, it is possible to prevent an impurity, such as Mg, from being diffused into the active layer 57.

The second layer 59b adjoins the first layer 59a, and the third layer 59c is disposed between the second layer 59b and the fourth layer 59d and has a narrower band gap than the second and

fourth layers

59b, 59d. The third layer 59c may adjoin the second and

fourth layers

59b, 59d. In addition, the third layer 59c may have a narrowest band gap in the electron blocking layer 59.

The second layer 59b is disposed between the first layer 59a and the third layer 59c, and has a band gap between the band gap of the first layer 59a and the band gap of the third layer 59c. The fourth layer 59d is disposed between the third layer 59c and the p-type semiconductor layer 61, and has a band gap between the band gap of the first layer 59a and the band gap of the third layer 59c. In addition, the second and

fourth layers

59b, 59d have a wider band gap than the p-type semiconductor layer 61.

By disposing the second layer 59b having a relatively wide band gap to adjoin the first layer 59a, electrons can be prevented from passing through the first layer 59a by a tunneling effect, and thus can be effectively confined in the active layer 57.

The first to fourth layers 59a to 59d may be formed of an AlInGaN-based nitride semiconductor. Particularly, the second and

fourth layers

59b, 59d may be formed of an In-containing nitride semiconductor. Since In traps Mg, an impurity concentration can be increased in the second and

fourth layers

59b, 59d. For example, the first layer 59a may be formed of AlGaN, the third layer 59c may be formed of GaN, and the second and

fourth layers

59b, 59d may be formed of AlInGaN. The third layer 59c may have a higher p-type impurity concentration than the second and

fourth layers

59b, 59d, without being limited thereto.

The first layer 59a may have a thinner thickness than each of the second to fourth layers 59b to 59d. The first layer 59a is formed in a relatively thin thickness, thereby improving hole injection efficiency. In addition, the third layer 59c may have a thinner thickness than the second and

fourth layers

59b, 59d.

According to the embodiments of the invention, the light emitting device employs the electron blocking layer 59 including the first to fourth layers 59a to 59d, thereby improving hole injection efficiency while preventing electron overflow. In addition, the light emitting device employs the c-plane GaN substrate 51 and/or the spacer layer 58 to improve crystal quality of semiconductor layers, thereby improving luminous efficacy.

By patterning the semiconductor layers and forming electrodes (not shown), light emitting devices having various structures, such as a horizontal type and a vertical type structure, may be prepared.

(Experimental Example)

To investigate effects of an electron blocking layer on optical and electrical properties of a light emitting device, a light emitting device (inventive example) including an electron blocking layer was prepared, in which a first layer 59a of AlGaN was formed to a thickness of about 15 Å, each of second and

fourth layers

59b, 59d of AlInGaN was formed to a thickness of about 30 Å, and a third layer 59c of GaN was formed to a thickness of about 25 Å to form an electron blocking layer 59 having a total thickness of about 100 Å. In a comparative example, a light emitting device was prepared in the same manner as in Example except that the electron blocking layer 59 was formed as a single AlGaN layer having a thickness of 100 Å.

Light output and forward voltage of these light emitting devices were measured. Results showed that the light emitting device prepared in the inventive example had about 6% higher light output and about 9% lower forward voltage than the light emitting device prepared in the comparative example.

From these results, it can be seen that the use of the electron blocking layer 59 according to the present invention increases hole injection efficiency of the light emitting device while efficiently preventing electron overflow.

Claims

A light emitting device comprising:

an n-type semiconductor layer;

a p-type semiconductor layer;

an active layer disposed between the n-type semiconductor layer and the p-type semiconductor layer; and

an electron blocking layer disposed between the active layer and the p-type semiconductor layer,

wherein the electron blocking layer comprises first, second, third and fourth layers,

the first layer being disposed closer to the active layer than the second to fourth layers and having a wider band gap than the second to fourth layers,

the second layer adjoining the first layer,

the third layer being disposed between the second layer and the fourth layer and having a narrower band gap than the second and fourth layers.
The light emitting device according to claim 1, wherein the first layer has a widest band gap in the electron blocking layer, and the third layer has a narrowest band gap therein.
The light emitting device according to claim 1, wherein the third layer adjoins the second and fourth layers.
The light emitting device according to claim 1, wherein the first to fourth layers are formed of an AlInGaN-based nitride semiconductor.
The light emitting device according to claim 4, wherein the first layer is formed of AlGaN, the third layer is formed of GaN, and the second and fourth layers are formed of AlInGaN.
The light emitting device according to claim 1, wherein the first, second and fourth layers have a wider band gap than the p-type semiconductor layer.
The light emitting device according to claim 1, wherein the first layer has a lower p-type impurity concentration than the second to fourth layers.
The light emitting device according to claim 7, wherein the first layer is an undoped layer.
The light emitting device according to claim 7, wherein the third layer has a higher p-type impurity concentration than the second and fourth layers.
The light emitting device according to claim 1, wherein the first layer has a thinner thickness than each of the second to fourth layers.
The light emitting device according to any one of claims 1 to 10, further comprising: an undoped semiconductor layer disposed between the active layer and the electron blocking layer.
The light emitting device according to claim 11, wherein the undoped semiconductor layer is formed of GaN.
The light emitting device according to any one of claims 1 to 10, further comprising:

a c-plane GaN substrate,

wherein the n-type semiconductor layer is disposed closer to the substrate than the p-type semiconductor layer.