WO2009075506A2

WO2009075506A2 - Light emitting device using compound semiconductor

Info

Publication number: WO2009075506A2
Application number: PCT/KR2008/007270
Authority: WO
Inventors: Doyeol Ahn; Bun-Hei Koo
Original assignee: Wooree Lst Co., Ltd.
Priority date: 2007-12-11
Filing date: 2008-12-09
Publication date: 2009-06-18
Also published as: WO2009075506A4; KR20090061194A; KR100925704B1; WO2009075506A3

Abstract

Disclosed is a light emitting device using a compound semiconductor. The light emitting device optimizes strain applied to an active layer to minimize a piezoelectric field and a spontaneous polarization field in the active layer and to maximize light emitting efficiency. The light emitting device using a compound semiconductor includes an upper clad layer, an active layer and a lower clad layer, wherein the upper clad layer includes a first clad layer and a second clad layer; the lower clad layer includes a third clad layer and a fourth clad layer; the second clad layer and the third clad layer are disposed on the top and bottom of the active layer, respectively; the first clad layer and the second clad layer are different from each other in their chemical compositions; and the third clad layer and the fourth clad layer are different from each other in their chemical compositions.

Description

LIGHT EMITTING DEVICE USING COMPOUND SEMICONDUCTOR

Technical Field

[1] This disclosure relates to a light emitting device using a compound semiconductor.

More particularly, this disclosure relates to a light emitting device using a compound semiconductor, which optimizes strain applied to an active layer to minimize a piezoelectric field and a spontaneous polarization field in the active layer and to maximize light emitting efficiency.

[2]

Background Art

[3] Light emitting devices using Group III-V nitride semiconductors or Group II- VI oxide semiconductors are capable of realizing a bluish purple color and a bluish green color, and thus have been applied to various industrial fields including flat panel display devices, optical communications, etc.

[4] Such light emitting devices using Group III-V nitride semiconductors or Group II- VI oxide semiconductors include multi-layer thin films having an active layer and a clad layer. Particularly, in the case of a light emitting device using a Group III-V nitride semiconductor, since the active layer and the clad layer have different lattice parameters, the active layer is subjected to stress, thereby causing a piezoelectric field and a spontaneous polarization field, resulting in degradation of the light emitting characteristics. Many attempts have been made to minimize the piezoelectric field and the spontaneous polarization field. Such attempts include the use of a non-polar or semi- polar substrate, and the use of a clad layer having a quaternary film structure and containing an increased proportion of aluminum (Al) to increase a confinement effect for transmitters and to improve light emitting efficiency. The former causes many defects in fabricating devices due to the unripe technology with respect to growth of heterogeneous crystals. The use of a non-polar or semi-polar substrate may provide the devices with light emitting characteristics lower than theoretically expected characteristics. See Park et al., Phys Rev B 59, 4725 (1999), Waltereit et al., Nature 406, 865 (2000), and Park & Ahn, Appl. Phys. Lett. 90, 013505 (2007).

[5] Meanwhile, the latter cannot fundamentally eliminate the problems of the piezoelectric field and the spontaneous polarization field. Moreover, increasing the proportion of aluminum in the clad layer is difficult to realize in practice. See Zhang et al., Appl. Phys. Lett. 77, 2668 (2000), and Lai et al., IEEE Photonics Technol. Lett. 13, 559 (2001). [6] In addition, light emitting devices using Group II- VI oxide semiconductors also require inhibition of a piezoelectric field and a spontaneous polarization field, although such problems are less severe as compared to the light emitting devices using Group III- V nitride semiconductors.

[7]

Disclosure of Invention Technical Problem

[8] Disclosed is a light emitting device capable of solving the above-mentioned problems. More particularly, disclosed herein is a light emitting device using a compound semiconductor, which optimizes strain applied to an active layer to minimize a piezoelectric field and a spontaneous polarization field in the active layer and to maximize light emitting efficiency.

[9]

Technical Solution

[10] In one aspect, there is provided a light emitting device using a compound semiconductor, which includes an upper clad layer, an active layer and a lower clad layer, wherein the upper clad layer includes a first clad layer and a second clad layer; the lower clad layer includes a third clad layer and a fourth clad layer; the second clad layer and the third clad layer are disposed on the top and bottom of the active layer, respectively; the first clad layer and the second clad layer are different from each other in their chemical compositions; and the third clad layer and the fourth clad layer are different from each other in their chemical compositions.

[11] In one embodiment, the light emitting device may be one using a Group III- V nitride semiconductor, or using a Group II- VI oxide semiconductor.

[12] In another embodiment, the first through the fourth clad layers may include a material represented by the formula of In x (Al y Ga 1-y )N (wherein 0 < x < 1, and 0 < y <

1), the second and the third clad layers have a higher indium content as compared to the first and the fourth clad layers, and the indium content in the second and the third clad layers may be 0-10%. Additionally, the indium content in the second and the third clad layers may be varied linearly or non-linearly along the thickness direction of the clad layers. [13] In still another embodiment, the first through the fourth clad layers may include a material represented by the formula of Mg (Cd Zn )O (wherein 0 < x < 0.4, and 0 < y x y 1-y

< 0.4), the second and the third clad layers have a lower magnesium content as compared to the first and the fourth clad layers, and the magnesium content in the second and the third clad layers may be 0-10%. Additionally, the magnesium content in the second and the third clad layers may be varied linearly or non-linearly along the thickness direction of the clad layers.

[14] The second and the third clad layers may have a thickness of 1-5 nm, while the first and the fourth clad layers may have a thickness of 5-30 nm.

[15] The light emitting device may further include a buffer layer and a strain distributing layer interposed between the buffer layer and the lower clad layer, wherein the strain distributing layer includes a strain controlling layer and a strain guiding layer stacked alternately at least once. Additionally, the strain controlling layer may have a single unit strain controlling layer or multiple unit stain controlling layers. For example, the strain controlling layer may have 1-10 unit strain controlling layers.

[16] The unit strain controlling layer may have a thickness of 10-30 nm, and the strain controlling layer may have a thickness of 10-100 nm. Additionally, the unit strain controlling layers may include homogeneous films or a stacked structure having multiple superlattice layers. The superlattice layer may have a thickness of 1-2 nm.

[17] Each of the strain controlling layer and the strain guiding layer may include a material represented by the formula of In x (Al y Ga 1-y )N (wherein 0 < x < 1, and 0 < y <

1). Otherwise, the strain guiding layer may include a material represented by the formula of Mg x Zn 1-x O (wherein 0 < x < 0.4), and the strain controlling layer may include a stacked structure having multiple superlattice layers represented by the formula of Mg y Zn 1-y O/Mg z Zn 1-z O (wherein 0 < y < 0.4, and 0 < z < 0.4).

[18] In addition, each of the buffer layer and the active layer may include a material represented by the formula of In (Al Ga )N (wherein 0 < x < 1, and 0 < y < 1), or a x y 1-y material represented by the formula of Mg (Cd Zn )O (wherein 0 < x < 0.4, and 0 < y x y 1-y

< 0.4).

Advantageous Effects

[19] The light emitting device using a compound semiconductor disclosed herein provides the following effects.

[20] As described above, the upper clad layer and the lower clad layer each have a bilayer structure, the first and the second clad layers forming the upper clad layer are designed to have different chemical compositions, and the third and the fourth clad layers forming the lower clad layer are designed to have different chemical compositions. Due to such a unique structure, step-like energy band gaps are realized to minimize compressive strain applied to the active layer, thereby improving the strain and spontaneous polarization characteristics of the active layer and the clad layer,

[21] In addition, the presence of the strain controlling layer interposed between the buffer layer and the lower clad layer reduces compressive strain applied to the active layer while increasing tensile strain applied to the upper and lower clad layers, thereby minimizing a piezoelectric field and a spontaneous polarization field in the active layer. By virtue of this, the light emitting device provides improved spontaneous emission characteristics.

[22] Further, the strain controlling layer may serve as a distributed Bragg reflector (DBR) due to the difference in dielectric constants of the thin layers forming the light emitting device, so that it performs total reflection of the light generated from the active layer. As a result, the light emitting device realizes improved optical efficiency.

[23]

Brief Description of Drawings

[24] The above and other aspects, features and advantages of the disclosed exemplary embodiments will be more apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

[25] Fig. 1 is a schematic view showing the light emitting device using a compound semiconductor according to one embodiment;

[26] Fig. 2 is a schematic view showing the light emitting device using a compound semiconductor according to another embodiment;

[27] Fig. 3 is an energy band diagram showing the step-like energy band gaps among the lower clad layer, the active layer and the upper clad layer in the light emitting device using a compound semiconductor according to one embodiment;

[28] Fig. 4 is an energy band diagram showing the energy band gaps when the indium contents of the second clad layer and the third clad layer are varied linearly;

[29] Fig. 5 is an energy band diagram showing the energy band gaps when the indium contents of the second clad layer and the third clad layer are varied non-linearly;

[30] Fig. 6 is an energy band diagram showing the energy band gaps when the first and the fourth clad layers have a superlattice structure in the light emitting device using a compound semiconductor according to one embodiment;

[31] Fig. 7 is a graph showing the spontaneous emission characteristics as a function of the indium contents of the second and the third clad layers;

[32] Fig. 8 is an energy band diagram showing the step-like energy band gaps among the lower clad layer, the active layer and the upper clad layer in the light emitting device using a compound semiconductor according to another embodiment;

[33] Fig. 9 is an energy band diagram showing the energy band gaps when the magnesium contents of the second clad layer and the third clad layer are varied linearly;

[34] Fig. 10 is an energy band diagram showing the energy band gaps when the magnesium contents of the second clad layer and the third clad layer are varied non- linearly; and

[35] Fig. 11 is an energy band diagram showing the energy band gaps when the first and the fourth clad layers have a superlattice structure in the light emitting device using a compound semiconductor according to another embodiment. [36]

Best Mode for Carrying out the Invention

[37] Exemplary embodiments now will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. This disclosure may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth therein. Rather, these exemplary embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of this disclosure to those skilled in the art. In the description, details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the presented embodiments.

[38] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of this disclosure. As used herein, the singular forms "a" "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, the use of the terms a, an, etc. does not denote a limitation of quantity, but rather denotes the presence of at least one of the referenced item. It will be further understood that the terms "comprises" and/or "comprising" or "includes" and/or "including" when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof.

[39] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[40] In the drawings, like reference numerals in the drawings denote like elements. The shape, size and regions, and the like, of the drawing may be exaggerated for clarity.

[41]

Mode for the Invention

[42] The examples will now be described. The following examples are for illustrative purposes only and not intended to limit the scope of this disclosure.

[43] According to various embodiments of the light emitting device disclosed herein, strain applied to the active layer of the light emitting device is optimized by controlling energy band gaps while varying the composition of the n-type upper clad layer, thereby minimizing a piezoelectric field and a spontaneous polarization field in the active layer.

[44] Additionally, according to some embodiments, a strain distributing layer having a strain controlling layer and a strain guiding layer is provided in the light emitting device to optimize strain applied to the active layer of the light emitting device, thereby minimizing a piezoelectric field and a spontaneous polarization field in the active layer.

[45] Before describing one of the embodiments of the light emitting devices, description will be made with reference to mathematical analysis of the strain applied to each layer in a multi-layer stacked structure, as well as the piezoelectric field and the spontaneous polarization field generated in the corresponding layer due to the corresponding strain.

[46] First, the strain and stress applied to each layer in a structure having i thin layers will be mathematically considered. For reference, mathematical analysis about the strain and stress applied to each layer follows Nakajima s approach. See Nakajima, J. Appl. Phys. 72, 5213 (1992).

[47] When the stress applied to the ith layer is taken as F , moment of the ith layer as Mi, thickness of the ith layer as d , lattice parameter of the ith layer as a , Young s modulus of the ith layer as E , and the curvature of the structure having i layers as R, the stress applied to the ith layer may be defined by the following Formula 1 :

[48] [Formula 1]

[49] p . _: force per unit length

Af ₁- : moment _M = p x -- _r

[51] Meanwhile, the condition for maintaining an equilibrium state between the ith layer and the (i+l)th layer may be defined by the following Formula 2: [52] [Formula 2] [53]

I₁ = a,(\ + Ot₁T)

/,₊.P +

= /_I[l + e_Irø + e₍(M₍)] E₁ : Young's modulus

' 2R

[54] wherein 1 is the lattice parameter of the ith layer considering heat expansion, T is the temperature of the lattices, and e is the strain applied to the ith layer. [55] The stress and strain applied to the ith layer may be calculated from the combination of Formula 1 with Formula 2, as shown in the following Formula 3:

[56] [Formula 3]

Q

[58] wherein is the effective strain applied to the ith layer. [59] Meanwhile, the curvature R of the structure having i thin layers may be defined by the following Formula 4:

[60] [Formula 4] [61]

R,

R R_t + R₂

[62] In the above formulae, the sum of stresses applied to the structure having multiple thin layers is zero (0) as shown in Formula 1. It can be seen from Formulae 2-4 that strain is adequately distributed over the individual thin layers. When applying the principle to the light emitting device disclosed herein, guiding compressive strain in such a manner that the strain is applied to a specific thin layer (strain controlling layer) reduces compressive strain applied to the other thin layer (active layer) relatively. [63] Meanwhile, the piezoelectric field and the spontaneous polarization field applied to each layer may be calculated by using the strain obtained from Formulae 1-4. Analysis of the piezoelectric field and the spontaneous polarization field is based on Bernardini s approach [see Phys. Stat. Sol. (b) 216, 392 (1999)]. The piezoelectric field and the spontaneous polarization field may be defined by the following Formula 5:

[64] [Formula 5]

= ε^ + P

∑_ld_kE_k — 0; periodic boundary condition k

∑d_kP_ιk /ε_k - P,,∑d_k /ε_k

E = _ k s∑ά_k iε_k

[66] wherein E is the effective electric field caused by the piezoelectric field and the spontaneous polarization field applied to the ith layer.

[67] Hereinafter, the light emitting device using a compound semiconductor according to one embodiment will be described in detail with reference to the accompanying drawings. Fig. 1 is a schematic view showing the light emitting device using a compound semiconductor according to one embodiment. Fig. 2 is a schematic view showing the light emitting device using a compound semiconductor according to another embodiment. The former includes a light emitting device using a Group III- V nitride semiconductor, while the latter includes a light emitting device using a Group II- VI oxide semiconductor.

[68] As shown in Fig. 1, the light emitting device according to one embodiment includes a buffer layer, and a strain distributing layer, a lower clad layer, an active layer and an upper clad layer stacked successively on the buffer layer. For reference, the active layer may have multiple layers, and each of the upper clad layer and the lower clad layer may have multiple layers accordingly. In this case, the lower clad layer may be present below the upper clad layer. However, also in this case, a structure of lower clad layer-active layer-upper clad layer may be repeated. Otherwise, in some cases, a structure of lower clad layer-active layer-lower (or upper) clad layer-active layer-upper clad layer may be present. It will be noted that when multiple active layers are provided, the clad layer disposed on the top of the uppermost active layer is the upper clad layer, and the clad layer disposed on the bottom of the lowermost active layer is the lower clad layer. In the following description, Figs. 3-6 and Figs. 8-11 illustrate example embodiments of the light emitting device using a single active layer, i.e., having a structure of lower clad layer- active layer-upper clad layer.

[69] The buffer layer, the strain distributing layer, the upper clad layer and the lower clad layer each include a Group M-V nitride semiconductor, specifically a material represented by the formula of In (Al Ga )N (wherein 0 < x < 1, and 0 < y < 1). x y 1-y

[70] The strain distributing layer includes a strain controlling layer and a strain guiding layer stacked alternately. The strain distributing layer will be described later.

[71] Meanwhile, each of the upper clad layer and the lower clad layer has a bilayer structure so as to reduce compressive strain applied to the active layer. More particularly, the upper clad layer includes the first clad layer and the second clad layer, while the lower clad layer includes the third clad layer and the fourth clad layer. For convenience, the clad layers adjacent to the active layer are designated as the second and the third clad layers and those not adjacent to the active layer are designated as the first and the fourth clad layers, as shown in Fig. 1.

[72] In the lower clad layer and the upper clad layer having a bilayer structure, the first clad layer and the second clad layer have different chemical compositions, and the third clad layer and the fourth clad layer have different chemical compositions. Particularly, in the first through the fourth clad layers having a chemical composition represented by the formula of In (Al Ga )N (wherein 0 < x < 1, and 0 < y < 1), the x y 1-y first clad layer and the second clad layer have different indium (In) contents, and the third clad layer and the fourth clad layer have different indium contents.

[73] Due to such different indium contents between the first clad layer and the second clad layer, and between the third clad layer and the fourth clad layer, step-like energy band gaps are formed in the upper clad layer and the lower clad layer, as shown in Fig. 3, thereby reducing compressive strain and spontaneous polarization field applied to the active layer.

[74] To form the step-like energy band gaps, the second and the third clad layers have higher indium content as compared to the first and the fourth clad layers. For example, the first through the fourth clad layers may have an indium content defined by the formula of In x (Al y Ga 1-y )N (wherein 0 < x < 1, and 0 < y < 1).

[75] Thus, the step-like energy band gaps are formed by controlling the indium contents in the upper and the lower clad layers, thereby improving compressive strain and spontaneous polarization characteristics in the active layer. Examples of the control of the indium contents will be explained later. In addition to the control of the indium contents, the thickness of the second clad layer and that of the third clad layer may be adjusted to control compressive strain and spontaneous polarization characteristics. Examples of the control of the thickness will also be explained later.

[76] Besides the step-like energy band gaps as shown in Fig.3, according to some embodiments wherein the indium contents are controlled to realize the upper clad layer or the lower clad layer with a bilayer structure, the indium contents in the second and the third clad layers may be varied linearly or non-linearly along the direction of the thickness of the clad layers to realize energy band gaps as shown in Figs. 4 and 5. Fig. 4 is an energy band diagram showing the energy band gaps when the indium contents of the second clad layer and the third clad layer are varied linearly. Fig. 5 is an energy band diagram showing the energy band gaps when the indium contents of the second clad layer and the third clad layer are varied non-linearly.

[77] In addition to the method for forming the upper clad layer and the lower clad layer with a bilayer structure by controlling the indium content in each clad layer, the first clad layer and the fourth clad layer may be formed to have a superlattice structure, as shown in Fig. 6, thereby realizing the upper and lower clad layers with a bilayer structure, and thus minimizing compressive strain applied to the active layer.

[78] More particularly, the second clad layer and the third clad layer may include materials having the same chemical composition, for example, materials represented by the formula of In Ga N/GaN (wherein 0 < x < 1), while the first clad layer and the x 1-x fourth clad layer may have one or more superlattice layers. The superlattice layers include materials represented by the formula of In Ga N/GaN (wherein 0 < y < 1). y i-y Particularly, each superlattice layer may be designed to have a thickness of 1-2 nm.

[79] In one example embodiment, the active layer may include In Ga N (x = 0.2) with a x 1-x thickness of 30 A the second clad layer and the third clad layer include GaN with a thickness of 30 A and the first clad layer and the fourth clad layer have a stacked structure including multiple superlattice layers of In Ga N/GaN (y = 0.3). According y i-y to the above example embodiment, the strain applied to the active layer decreases from

-2.08% to -1.084% and the spontaneous polarization field in the active layer decreases from 2.98 MV/cm to 1.73 MV/cm, as compared to a similar structure wherein the first clad layer and the fourth clad layer include a homogeneous material, GaN.

[80]

[81] *Hereinabove, description was made with reference to the construction and function of the upper clad layer and the lower clad layer with a bilayer structure. Meanwhile, as mentioned above, a strain distributing layer having a strain controlling layer and a strain guiding layer is provided between the buffer layer and the lower clad layer. Hereinafter, description will be made with reference to the construction and function of the strain distributing layer.

[82] The strain controlling layer and the strain guiding layer are alternately stacked at least once. When multiple strain controlling layers are used, the strain guiding layer may be interposed between one strain controlling layer and another strain controlling layer.

[83] In the strain distributing layer, the strain guiding layer serves to distribute and apply compressive strain, which otherwise might be applied to the active layer, to the strain controlling layer. Since the compressive strain is applied to a certain portion of the strain controlling layer, the amount of compressive strain applied to the active layer is reduced, while the amount of tensile strain applied to the upper and the lower clad layers is increased accordingly.

[84] As a result, piezoelectric field and spontaneous polarization field in the interface between the lower clad layer and the active layer, and those in the interface between the upper clad layer and the active layer have opposite signs, thereby minimizing the piezoelectric filed and spontaneous polarization field applied to the active layer.

[85] As mentioned above, the strain controlling layer and the strain guiding layer include a material represented by the formula of In x (Al y Ga 1-y )N (wherein 0 < x < 1, and 0 < y

< 1). Additionally, the strain controlling layer may have a single layer or multiple layers, wherein unit strain controlling layer forming the single layer or multiple layers may have a thickness of 10-30 nm, and the total thickness of the strain controlling layer may be 10-100 nm. In the case of a multi-layer type strain controlling layer, 2-10 unit strain controlling layers may be present. Further, when the strain controlling layer has a stacked structure of multiple superlattice layers, each superlattice layer may have a thickness of 1-2 nm.

[86] Hereinabove, description was made with reference to the construction and function of the upper clad layer and the lower clad layer, as well as those of the strain distributing layer. Hereinafter, detailed discussion will be made with reference to the strain and spontaneous polarization characteristics depending on the indium contents in the second and the third clad layers, and on the thicknesses of the second and the third clad layers.

[87] Strain and Spontaneous Polarization Characteristics Depending on Indium Contents

[88] The following test is carried out to investigate strain and spontaneous polarization characteristics depending on the indium contents in the second and the third clad layers. The following Table 1 shows the results of strain and spontaneous polarization characteristics applied to the active layer and the clad layers, when the indium content (y) in each of the second clad layer and the third clad layer is set at 0, 2.5% and 5%, in the case of a light emitting device provided with the active layer including In Ga N (x = 0.2) with a thickness of 3 nm, and the second and the third clad layers including In Ga N with a thickness of 4 nm. l-y

[89] [90] Table 1 [Table 1] [Table ]

[91] As shown in Table 1, when the indium content in the second and the third clad layers increases, the strain, i.e., compressive strain, applied to the active layer decreases, while the strain, i.e., tensile strain, applied to the upper and the lower clad layers increases. Additionally, when the indium content increases, the spontaneous polarization field in the active layer decreases, and the spontaneous polarization fields in the upper and the lower clad layers increase. As a result, the spontaneous polarization fields at the interface between the lower clad layer and the active layer and at the interface between the upper clad layer and the active clad layer decrease.

[92] Meanwhile, as shown in Fig. 7, spontaneous emission characteristics are improved as the indium contents in the second and the third clad layers increase. This demonstrates that the second and the third clad layers cause a change in strain applied to the active layer and the n-type and upper clad layer, thereby reducing the effective piezoelectric fields and spontaneous polarization fields, resulting in significant improvement of optical properties. For reference, analysis of optical characteristics as shown in Table 1 and Fig.7 is performed on the basis of Ahn's model [see Ahn, IEEE J. Quantum Electron. 34, 344 (1998) and Ahn et al., IEEE J. Quantum Electron. 41, 1253 (2005)].

[93] Strain and Spontaneous Polarization Characteristics Depending on Thickness [94] The following test is carried out to investigate strain and spontaneous polarization characteristics depending on the thicknesses of the second and the third clad layers. The following Table 2 shows the results of strain and spontaneous polarization characteristics applied to the active layer and the clad layers, when the thickness (D) of each of the second clad layer and the third clad layer is set at 2 nm, 4 nm and 6 nm, in the case of a light emitting device provided with the active layer including In Ga N (x = x 1-x

0.2) with a thickness of 3 nm, and the first and the fourth clad layers including In Ga y i-y N (y = 0.025) (wherein the thickness of each of the lower clad layer and the upper clad layer is 15 nm).

[95] [96] Table 2 [Table 2] [Table ]

[97] As shown in Table 2, when the thickness of the second and the third clad layers increases, the compressive strain applied to the active layer decreases, while the tensile strain applied to the n-type and upper clad layer increases. Additionally, the spontaneous polarization field in the active layer decreases as the thickness increases. Meanwhile, the spontaneous polarization fields in the upper and the lower clad layers decrease as the thickness of the second and the third clad layers increase. This suggests that electrons and holes are less constrained in the upper and the lower clad layers. Based on the above results, the second and the third clad layers may be designed to have a thickness of 1-5 nm. Meanwhile, the first and the fourth clad layers may be designed to have a thickness of 5- 30 nm.

[98] Hereinabove, description was made with reference to one embodiment of the light emitting device using a compound semiconductor. Hereinafter, detailed description will be made with reference to another embodiment of the light emitting device using a compound semiconductor.

[99] As mentioned above, according to another embodiment, provided is a light emitting device using a Group II- VI oxide semiconductor. The light emitting device includes a buffer layer, and a strain distributing layer, a lower clad layer, an active layer and an upper clad layer stacked successively on the buffer layer.

[100] The upper clad layer has a bilayer structure including a first clad layer and a second clad layer, while the lower clad layer has a bilayer structure including a third clad layer and a fourth clad layer. Additionally, the strain distributing layer includes a strain controlling layer and a strain guiding layer, which are alternately stacked. [101] The active layer, the buffer layer, the upper clad layer and the lower clad layer (the first through the fourth clad layers) each include a material represented by the formula of Mg (Cd Zn )O (wherein 0 < x < 0.4, and 0 < y < 0.4). The strain guiding layer x y 1-y includes a material represented by the formula of Mg Zn O (0 < x < 1), and the strain x 1-x controlling layer has a stacked structure of multiple superlattice layers including a material represented by the formula of Mg Zn O/Mg Zn O (wherein 0 < y < 0.4, and y 1-y z 1-z

0 < z < 0.4). Herein, x and y are greater than z.

[102] Like the light emitting device as described earlier, the first clad layer and the second clad layer, and the third clad layer and the fourth clad layer are different from each other in chemical compositions. Particularly, in the first through the fourth clad layers having a chemical composition of Mg (Cd Zn )O (wherein 0 < x < 0.4, and 0 < y < x y 1-y

0.4), the first clad layer and the second clad layer, and the third clad layer and the fourth clad layer are different from each other in the magnesium (Mg) contents.

[103] Due to such different magnesium contents between the first clad layer and the second clad layer, and between the third clad layer and the fourth clad layer, step-like energy band gaps are formed in the upper clad layer and the lower clad layer, as shown in Fig. 8, thereby reducing compressive strain and spontaneous polarization field applied to the active layer.

[104] To form the step-like energy band gaps, the second and the third clad layers have a lower magnesium content as compared to the first and the fourth clad layers. For example, the first through the fourth clad layers may have a magnesium content defined by the formula of Mg (Cd Zn )O (wherein 0 < x < 0.1, and 0 < y < 0.4). Ad- x y 1-y ditionally, the second and the third clad layers have a thickness of 1-5 nm.

[105] Besides the step-like energy band gaps as shown in Fig.8, according to some embodiments wherein the magnesium contents are controlled to realize the n-type or upper clad layer with a bilayer structure, the magnesium contents in the second and the third clad layers may be varied linearly or non-linearly along the direction of the thickness of the clad layers to realize energy band gaps as shown in Figs. 9 and 10. Fig. 9 is an energy band diagram showing the energy band gaps when the magnesium contents of the second clad layer and the third clad layer are varied linearly. Fig. 10 is an energy band diagram showing the energy band gaps when the magnesium contents of the second clad layer and the third clad layer are varied non-linearly.

[106] In addition to the method for forming the upper clad layer and the lower clad layer with a bilayer structure by controlling the magnesium content in each clad layer, the first clad layer and the fourth clad layer may be formed to have a superlattice structure, as shown in Fig. 11, thereby realizing the upper and lower clad layers with a bilayer structure, and thus minimizing compressive strain applied to the active layer. [107] More particularly, the second clad layer and the third clad layer may include materials represented by the formula of Mg Zn O (wherein 0 < x < 0.4), while the x 1-x first clad layer and the fourth clad layer may have one or more superlattice layers. The superlattice layers include materials represented by the formula of Mg Zn O/Mg Zn y 1-y z 1-z

O (wherein 0 < y < 0.4, and 0 < z < 0.4). Particularly, each superlattice layer may be designed to have a thickness of 1-2 nm. [108]

Industrial Applicability

[109] The light emitting device using a compound semiconductor disclosed herein may be applied to various industrial fields including flat panel display devices, traffic lights, interior lightings, high-resolution output systems, optical communications, or the like.

[110] While the exemplary embodiments have been shown and described, it will be understood by those skilled in the art that various changes in form and details may be made thereto without departing from the spirit and scope of this disclosure as defined by the appended claims.

[I l l] In addition, many modifications can be made to adapt a particular situation or material to the teachings of this disclosure without departing from the essential scope thereof. Therefore, it is intended that this disclosure not be limited to the particular exemplary embodiments disclosed as the best mode contemplated for carrying out this disclosure, but that this disclosure will include all embodiments falling within the scope of the appended claims.

[112]

Claims

[I] A light emitting device using a compound semiconductor, which comprises an upper clad layer, an active layer and a lower clad layer, wherein the upper clad layer comprises a first clad layer and a second clad layer; the lower clad layer comprises a third clad layer and a fourth clad layer; the second clad layer and the third clad layer are disposed on the top and bottom of the active layer, respectively; the first clad layer and the second clad layer are different from each other in their chemical compositions; and the third clad layer and the fourth clad layer are different from each other in their chemical compositions.

[2] The light emitting device using a compound semiconductor according to claim 1, which uses a Group III- V nitride semiconductor.

[3] The light emitting device using a compound semiconductor according to claim 1, which uses a Group II- VI oxide semiconductor.

[4] The light emitting device using a compound semiconductor according to claim 1, wherein the first through the fourth clad layers comprise materials represented by the formula of In (Al Ga )N (wherein 0 < x < 1, and 0 < y < 1). x y 1-y

[5] The light emitting device using a compound semiconductor according to claim 4, wherein the second and the third clad layers have a higher indium content as compared to the first and the fourth clad layers

[6] The light emitting device using a compound semiconductor according to claim 4, wherein the indium content in the second and the third clad layers is 0-10%.

[7] The light emitting device using a compound semiconductor according to claim 4, wherein the indium content in the second and the third clad layers is varied linearly along the thickness direction of the clad layers.

[8] The light emitting device using a compound semiconductor according to claim 4, wherein the indium content in the second and the third clad layers is varied non- linearly along the thickness direction of the clad layers.

[9] The light emitting device using a compound semiconductor according to claim 1, wherein the first through the fourth clad layers comprise materials represented by the formula of Mg (Cd Zn )O (wherein 0 < x < 0.4, and 0 < y < 0.4). x y 1-y

[10] The light emitting device using a compound semiconductor according to claim 9, wherein the second and the third clad layers have a lower magnesium content as compared to the first and the fourth clad layers

[I I] The light emitting device using a compound semiconductor according to claim 9, wherein the magnesium content in the second and the third clad layers is 0-10%.

[12] The light emitting device using a compound semiconductor according to claim 9, wherein the magnesium content in the second and the third clad layers is varied linearly along the thickness direction of the clad layers.

[13] The light emitting device using a compound semiconductor according to claim 9, wherein the magnesium content in the second and the third clad layers is varied non-linearly along the thickness direction of the clad layers.

[14] The light emitting device using a compound semiconductor according to claim 1, wherein the second clad layer or the third clad layer has a thickness of 1-5 nm.

[15] The light emitting device using a compound semiconductor according to claim 1, which further comprises a buffer layer and a strain distributing layer interposed between the buffer layer and the lower clad layer.

[16] The light emitting device using a compound semiconductor according to claim

15, wherein the strain distributing layer comprises a strain controlling layer and a strain guiding layer, which are alternately stacked at least once.

[17] The light emitting device using a compound semiconductor according to claim

16, wherein the strain controlling layer comprises a single unit strain controlling layer or multiple unit strain controlling layers.

[18] The light emitting device using a compound semiconductor according to claim

17, wherein the strain controlling layer comprises 1-10 unit strain controlling layers.

[19] The light emitting device using a compound semiconductor according to claim

17, wherein the unit strain controlling layer has a thickness of 10-30 nm.

[20] The light emitting device using a compound semiconductor according to claim

16, wherein the strain controlling layer has a thickness of 10-100 nm.

[21] The light emitting device using a compound semiconductor according to claim

17, wherein the unit strain controlling layer comprises homogeneous films, or has a stacked structure comprising multiple superlattice layers.

[22] The light emitting device using a compound semiconductor according to claim

21, wherein the superlattice layer has a thickness of 1-2 nm.

[23] The light emitting device using a compound semiconductor according to claim

16, wherein the strain controlling layer and the strain guiding layer comprise a material represented by the formula of In (Al Ga )N (wherein 0 < x < 1, and 0 x y 1-y

≤ y ≤ i).

[24] The light emitting device using a compound semiconductor according to claim

16, wherein the strain guiding layer comprises a material represented by the formula of Mg x Zn 1-x O (wherein 0 < x < 0.4), and the strain controlling layer comprises a stacked structure having multiple superlattice layers represented by the formula of Mg Zn O/Mg Zn O (wherein 0 < y < 0.4, and 0 < z < 0.4). y l-y z l-z

[25] The light emitting device using a compound semiconductor according to claim

15, wherein the buffer layer and the active layer comprise a material represented by the formula of In (Al Ga )N (wherein 0 < x < 1, and 0 < y < 1). x y 1-y

[26] The light emitting device using a compound semiconductor according to claim

15, wherein the buffer layer and the active layer comprise a material represented by the formula of Mg (Cd Zn )O (wherein 0 < x < 0.4, and 0 < y < 0.4). x y 1-y

[27] The light emitting device using a compound semiconductor according to claim 1, wherein the first clad layer or the fourth clad layer has a thickness of 5-30 nm.

[28] The light emitting device using a compound semiconductor according to claim 1, which comprises at least one active layer.

[29] The light emitting device using a compound semiconductor according to claim 1, wherein the first clad layer and the fourth clad layer comprise one or more su- perlattice layers.

[30] The light emitting device using a compound semiconductor according to claim

29, wherein the first clad layer and the fourth clad layer comprise a material represented by the formula of In x (Al y Ga 1-y )N (wherein 0 < x < 1, and 0 < y < 1).

[31] The light emitting device using a compound semiconductor according to claim

29, wherein the second clad layer and the third clad layer comprise materials having the same chemical composition.

[32] The light emitting device using a compound semiconductor according to claim

29, wherein the superlattice layer comprises a material represented by the formula of In y Ga 1-y N/GaN (0 < y < 0.1).

[33] The light emitting device using a compound semiconductor according to claim

29, wherein the superlattice layer comprises a material represented by the formula of Mg Zn 0/ Mg Zn O (0 < y < 0.4, 0 < z < 0.4). y 1-y z 1-z

[34] The light emitting device using a compound semiconductor according to claim

29, wherein the superlattice layer has a thickness of 1-2 nm.