KR101019134B1 - Light emitting device and manufacturing method thereof - Google Patents

Abstract

A light emitting device and a method of manufacturing the same are disclosed. Such a light emitting device includes a substrate, an active layer, an N-type contact layer, and a P-type contact layer. The active layer is formed on the substrate and includes a nitride semiconductor layer in which crystals are formed to be inclined with respect to the substrate. The N-type contact layer injects electrons into the active layer. The P-type contact layer is disposed to face the N-type contact layer with respect to the active layer, and injects holes into the active layer. For example, an angle formed between the c-axis of the crystal of the nitride semiconductor layer and the normal of the substrate may range from 40 degrees to 90 degrees. Thus, polarization characteristics and light emission efficiency are increased.

Nitride semiconductor, polarization, spontaneous emission, crystal, piezoelectric field

Description

Light Emitting Device and Method of Manufacturing the Same

The present invention relates to a light emitting device and a method for manufacturing the same, and more particularly, to a light emitting device for improving internal quantum efficiency and emitting polarized light and a method of manufacturing the same.

Semiconductor Group III-V nitride compound semiconductor structure constituting the blue-violet and cyan light emitting devices is significantly lower than that of the other III-V semiconductors due to the piezoelectric field and spontaneous polarization caused by stress applied to the active layer, which is one of the essential characteristics. Fall [Park et al., Appl. Phys. Lett. 75, 1354 (1999).

In particular, there have been many attempts to minimize piezoelectric fields and spontaneous polarization, which are inherent weaknesses of III-V nitride semiconductors. Among them, the representative method

1) Method for minimizing spontaneous polarization and piezo effect using non-polar or semi-polar substrate [Park & Chuang, Phys. Rev. B59, 4725 (1999), Waltereit et al., Nature 406, 865 (2000);

2) The cladding layer is formed into a four-layer film and the composition ratio of double aluminum (Al) is increased to increase the confining effect of the transmitter to increase luminous efficiency [Zhang et al., Appl. Phys. Lett. 77, 2668 (2000), Lai et al., IEEE Photonics Technol Lett. 13, 559 (2001).

In case of 1), the growth technology for the direction of dissimilar crystal growth is not yet mature, so there are many defects in device fabrication. Therefore, the device characteristics are not known as theoretically expected and the manufacturing process is very difficult [K. Nishizuka et al., Appl. Phys. Lett. 87, 231901 (2005)]. Also, in case of 2), spontaneous polarization and piezo effect cannot be eliminated fundamentally, so it cannot be a fundamental solution.

In addition, in the case of II-VI oxide semiconductors, which have recently been in the limelight, piezo and spontaneous polarizations are also present, and a method of improving optical characteristics by controlling them has become important [S.-H. Park and D. Ahn, Appl. Phys. Lett. 87, 253509 (2005); D. Ahn et al., Photonics Technol. Lett. 18, 349 (2006).

Existing researches have been mainly aimed at improving light emission characteristics, and are not well known about polarization characteristics important in a backlight unit (BLU).

Accordingly, the first problem to be solved by the present invention is to minimize the piezoelectric field and spontaneous polarization inside the crystal in the nitride semiconductor to improve the internal quantum efficiency, light emission that can be utilized in the backlight of the liquid crystal display device by emitting polarized light It is to provide an element.

The second problem to be solved by the present invention is to provide a method of manufacturing such a light emitting device.

The light emitting device according to the exemplary embodiment of the present invention includes a substrate, an active layer, an N-type contact layer, and a P-type contact layer. The active layer is formed on the substrate and includes a nitride semiconductor layer in which crystals are formed to be inclined with respect to the substrate. The N-type contact layer injects electrons into the active layer. The P-type contact layer is disposed to face the N-type contact layer with respect to the active layer, and injects holes into the active layer.

For example, an angle formed between the c-axis of the crystal of the nitride semiconductor layer and the normal of the substrate may range from 40 degrees to 90 degrees.

Alternatively, an angle formed between the c-axis of the crystal of the nitride semiconductor layer and the normal of the substrate may range from 50 degrees to 70 degrees.

Alternatively, an angle formed between the c-axis of the crystal of the nitride semiconductor layer and the normal of the substrate may be 56 degrees.

For example, the active layer includes Al _x Ga _y In _z N (where x + y + z = 1, 0 ≦ x, y, z ≦ 1).

According to an exemplary embodiment of the present invention, a method of manufacturing a light emitting device includes forming an N-type contact layer on a substrate, and forming a nitride semiconductor layer on the N-type contact layer inclined with respect to the substrate. Forming an active layer comprising; and forming a P-type contact layer on the active layer.

According to the light emitting device of the present invention, when the polarized light is emitted and applied to the backlight of the liquid crystal display device, the light utilization efficiency can be improved.

In addition, by increasing the spontaneous emission, it is possible to improve the internal quantum efficiency, it is possible to reduce the crystal internal electric field to improve the optical properties.

In the drawings below, in order to distinguish each layer, the thickness of each layer is exaggerated and the thickness of each layer is not limited to the drawings. In addition, the expression "A layer is formed on the B layer" in this specification means that not only the A layer is formed directly on the B layer, but also the C layer may be formed between the A layer and the B layer.

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

1 is a schematic diagram of a light emitting device according to an embodiment of the present invention.

The light emitting device 100 according to an exemplary embodiment of the present invention includes a substrate 101, an N-type contact layer 102, an active layer 103, and a P-type contact layer 104.

As the substrate 101, for example, a sapphire (Al ₂ O ₃ ) substrate or a silicon carbide substrate (SiC) may be used.

Optionally, a buffer layer (not shown) may be further formed between the substrate 101 and the N-type contact layer 102. The buffer layer is formed for high quality nitride crystal growth.

The N-type contact layer 102 may be formed of, for example, one or more nitride semiconductor layers. The N-type contact layer 102 injects a donor into the active layer 103.

The P-type contact layer 104 injects the acceptor into the active layer 103.

The active layer 103 is formed between the N-type contact layer 102 and the P-type contact layer 104. For example, the active layer may include Al _x Ga _y In _z N (where x + y + z = 1, 0 ≦ x, y, z ≦ 1). When voltage is applied through the N-type electrode 105 and the P-type electrode 106, the donor of the N-type contact layer 102 meets at the acceptor of the P-type contact layer 104 and the active layer 103 to emit light. It emits light.

As a feature of the present invention, crystals of the active layer 103 are formed to be inclined with respect to the substrate 101.

As shown in FIG. 2, the crystals of the active layer have a hexagonal column shape. The crystals of the conventional active layer have a c-plane parallel to the substrate 101, but the crystals of the active layer 103 according to the present invention are formed to be inclined relative to the substrate 101, as shown in FIG.

For example, an angle formed between the c-axis of the crystal of the nitride semiconductor layer and the normal of the substrate 101 may range from 40 degrees to 90 degrees. Alternatively, an angle formed between the c-axis of the crystal of the nitride semiconductor layer and the normal of the substrate 101 may be in a range of 50 degrees to 70 degrees. Alternatively, an angle formed between the c-axis of the crystal of the nitride semiconductor layer and the normal of the substrate 101 may be 56 degrees.

In this way, when the crystal of the nitride semiconductor layer of the active layer is formed to be inclined around the substrate 101, the polarization component of the light generated from the active layer is improved. Therefore, when used for the backlight of the liquid crystal display device, the utilization efficiency of light can be improved. That is, the backlight generally includes polarized light in all directions, of which, as the polarizer, uses light polarized in only one direction, and the remaining lights are filtered by the polarizer, but are thus polarized from the beginning. In the case of emitting light, the utilization efficiency of light may be improved.

In addition, the spontaneous emission is improved, the internal quantum efficiency of the light emitting device is increased, the internal electric field is reduced, and the light emission characteristic is improved.

Hereinafter, the theoretical background concerning this invention is demonstrated.

the valence band (valence-band) Hamiltonian (Hamiltonian) of the structure induced by k and p way is represented by Equation 1 below (see SLChung and CS Chang, "The k and p method for strained wurtzite semiconductors". Phys. Rev. B, vol. 54, pp. 2491-2504, 1996).

c-plane analysis

In transplantation,

to be. In addition, Ai is a valence versus effective mass parameter similar to the Lutinger parameter in the ZB crystal, Di is the strain potential of the Wurtzite crystal, ki is the wave vector, εij is the strain tensor Δ1 is the split energy of the crystal field, and Δ2 and Δ3 are the corrections for the spin-orbit interaction.

의 식으로 표현되는데, 이것은 양자우물(Quantum well)의 격자상수(lattice constant)의 미스매치(mismatch)에 기인한다. 상기 수학식 1의 베이스벡터들(고유벡터)은 아래의 수학식 2로 나타난다.Also, in the above formula

This is due to the mismatch of the lattice constant of the quantum well. The base vectors (unique vectors) of Equation 1 are represented by Equation 2 below.

Meanwhile, the optical momentum matrix of the c-plane is given by Equation 3 below.

In the above formula, Ψ ^c and Ψ ^v represent the wave function of the conduction band and the valence band, respectively, and the superscript η represents the spin up and spin down states.

The interband momentum matrix depending on polarization is expressed by the following equation (4) in the case of spin-up of TE polarization.

In addition, in the case of spin down of TE polarization, it is represented by following formula (5).

이다.In Equations 4 and 5,

to be.

In addition, the interband momentum matrix depending on polarization is expressed by the following equation (6) in the case of spin-up of TM polarization.

In addition, in the case of spin down of TM polarization, it is represented by following formula (7).

이다.In Equations 6 and 7,

to be.

은 x, y, z좌표계에서 m-번째 서브밴드의 파동함수이며, P_x, P₁ ², P₂ ²는 아래의 수학식 8과 같이 표현된다.Further, in the above Equation 4 to Equation 7,

Is a wave function of the m-th subband in the x, y, z coordinate system, and P _x , P ₁ ² , P ₂ ² are expressed by Equation 8 below.

interpretation in m-plane

Then, each physical quantity in the m-plane of the nitride semiconductor element shown in FIG. 2 will be analyzed.

First, the Euler matrix described by Equation 9 below is used to rotate the Hamiltonian described in Equation 1 in any crystal direction.

The polar angle θ and the azimuth angle φ in the Euler matrix rotate the respective physical quantities from the x, y, z coordinate system to the x ', y', z 'coordinate system (see Fig. 3). The z-axis corresponds to the c-axis shown in FIG. 2, and the z'-axis is the axis corresponding to the growth direction of the crystal.

When the Hamiltonian shown in Equation 1 is rotated using Equation 9, Equation 10 is expressed as follows.

In the transplantation, in the case of the m-plane, when azimuth angle φ = π / 6 and polar angle θ = π / 2 are substituted, each matrix component of Equation 10 is expressed as Equation 11 below.

to be.

Note that in Equation 10, the base vector used in Equation 2 is used.

Meanwhile, the m-plane optical momentum matrix is given by Equation 12 below.

In the above equation, Ψ ' ^c and Ψ' ^v represent the wave function of the conduction band and the valence band, respectively, and the superscript η represents the spin up and spin down states.

The interband momentum matrix depending on polarization is expressed by the following equation (13) in the case of spin-up of TE polarization.

In addition, in the case of spin down of TE polarization, it is represented by following formula (14).

이다.In equation (13) and (14),

to be.

In addition, the interband momentum matrix depending on polarization is expressed by the following equation (15) in the case of spin-up of TM polarization.

In addition, in the case of spin down of TM polarization, it is represented by following formula (16).

이다.In Equations 15 and 16,

to be.

In addition, in the above formulas (13) to (16), Is the wavefunction of the m-th subband in the x ', y', z 'coordinate system.

a-plane interpretation

Then, each physical quantity in the a-plane of the nitride semiconductor element shown in FIG. 2 will be analyzed. In the case of the a-plane, when azimuth angle φ = 0 and polar angle θ = π / 2 are substituted, each matrix component of Equation 10 is expressed by Equation 17 below.

The interband momentum matrix depending on polarization is expressed by the following equation (13) in the case of spin-up of TE polarization.

In addition, in the case of spin down of TE polarization, it is represented by following formula (19).

이다.In Equations 18 and 19,

to be.

In addition, the interband momentum matrix depending on polarization is expressed by the following equation (20) in the case of spin-up of TM polarization.

In addition, in the case of spin down of TM polarization, it is represented by the following formula (21).

이다.In Equations 20 and 21,

to be.

은 x', y', z'좌표계에서 m-번째 서브밴드의 파동함수이다.In addition, in the above formulas (13) to (16),

Is the wavefunction of the m-th subband in the x ', y', z 'coordinate system.

Where P is polarization, superscripts w and b are wells and barriers, L is layer thickness, and ε is the dielectric constant.

Non-Markovian Optical Gain with Multibody Effect

The optical gain spectrum is calculated using a non-Markovian gain model with multibody effects (see SH Park, SL Chung, and D Ahn, "Interband relaxation time effects on non-Markovian gain with many-body effects and comparison with experiment ", Semicond. Sci. Technol., vol. 15 pp. 2003-2008). The optical gain with the multibody effect including the effect of the anisotropy of valence versus dispersion is expressed by the following equation (23).

Where ω is the angular velocity, μ ₀ is the permeability in vacuum, ε is the dielectric constant, σ = U (or L) is the upper (or lower) block of the effective mass Hamiltonian, and e is the electron's Charge, m ₀ is the mass of free electrons, k _|| Is the magnitude of the surface wave vector in the quantum well plane, Lw is the width of the well, and | M _lm | ² is the matrix component of the strained quantum well. In addition, f _l ^c and f _m ^v are Fermi functions for the probability of electron occupancy in the conduction band and valence band, respectively. The subscripts l and m represent the electron and hole states in the conduction band, respectively.

Parameters of GaN and InN materials required for the calculation are shown in Table 1 below.

Parameters GaN InN Lattice constant a () 3.1892 3.53 Energy
Parameter Eg (ev) 3.44 1.89 Δcr = Δ ₁ (meV) 22.0 41.0 Δso = 3Δ ₂ (meV) 15.0 1.0 Δ ₃ = Δ ₂ Conduction band
effective masses m _ez ^w / m ₀ (= m _et ^w / m ₀ ) 0.20 0.11 Valence band effective mass parameters A1 -6.4 -9.09 A2 -0.5 -0.63 A5 -2.56 -4.36 A3 = A2-A1, A4 = A3 / 2, A6 = (A3 + 4A5) / √2 Deformation
potentials (eV) a _c = -6.4 + a _v -4.60 -1.40 D ₁ -1.70 -1.76 D ₂ 6.30 3.43 D ₅ -4.00 -2.33 D ₃ = D ₂ -D ₁ D ₄ = D _3/2 Dielectric constant ε 10.0 15.3 Elastic stiffness
constant
(10 ¹¹ dyn / cm ² ) C ₁₁ 39.0 27.1 C ₁₂ 14.5 12.4 C ₁₃ 10.6 9.4 C ₃₃ 39.8 20.0 C ₄₄ 10.5 4.6 C ₆₆ 12.3 7.4 Piezoelectric constant d ₃₁ (x 10 ^-12 m / V) -1.7 -1.1 Spontaneous polarization constant P (C / m ² ) -0.029 -0.032

4 is a graph showing x-polarized light and y-polarized light with respect to the change in the polar angle θ formed between the crystal axis of the nitride semiconductor and the normal of the substrate, and FIG. 5 is the polar angle formed by the normal of the substrate and the crystal axis of the nitride semiconductor. It is a graph showing the polarization anisotropy with respect to the change of). 4 is a result of numerical calculation using equations (13) and (14).

The anisotropy p in FIG. 5 is defined as ρ = (| M'x |-| M'y |) / (| M'x | + | M'y |) as follows.

4 and 5, when the polar angle θ formed between the crystal axis of the nitride semiconductor and the normal of the substrate increases, the X-polarized light gradually decreases, and the Y-polarized light gradually starts to increase so that the polar angle θ is about 56 degrees. When it reaches degrees, saturation is reached. That is, when the polar angle θ is 0, the light of the X-polarized light and the light of the Y-polarized light are emitted in the same amount, but as the polar angle θ gradually increases, the light of the Y-polarized light increases and the light of the X-polarized light This reduction can yield polarized light.

FIG. 6 is a graph showing the internal electric field of the change of the polar angle θ formed between the crystal axis of the nitride semiconductor and the normal of the substrate, and FIG. 7 is a change of the polar angle θ formed between the crystal axis of the nitride semiconductor and the normal of the substrate. A graph showing the synthesis of wave functions. The internal electric field of FIG. 6 is a result of numerical calculation using Equation 22, and FIG. 7 is a result of numerical calculation using Equation 10. FIG.

6 and 7, when the polar angle θ formed between the crystal axis of the nitride semiconductor and the normal of the substrate increases, the absolute value of the internal electric field begins to decrease, and reaches zero when it reaches about 56 degrees, and then increases again. It starts, and when it reaches about 70 degrees, it decreases again. When the size of the internal electric field decreases, the optical characteristic is improved. Therefore, it is possible to determine the internal electric field range of the allowable value and the range of the polar angle θ corresponding thereto.

8 and 9 are graphs showing the spontaneous emission according to the wavelength change. 8 and 9 were numerically calculated using Equation 23.

8 and 9, when the polar angle θ formed between the crystal axis of the nitride semiconductor and the normal of the substrate increases, the spontaneous emission gradually increases. In particular, in the case of Y-polarized light, it is seen to increase continuously, but in the case of X-polarized light, it begins to decrease after about 24 degrees.

While the present invention has been described in connection with what is presently considered to be practical and exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the invention. Therefore, the above description and the drawings below should be construed as illustrating the present invention, not limiting the technical spirit of the present invention.

1 is a schematic diagram of a light emitting device according to an embodiment of the present invention.

FIG. 2 is a perspective view illustrating a nitride semiconductor crystal of the active layer illustrated in FIG. 1.

3 is a perspective view illustrating the rotation of the crystal axis illustrated in FIG. 2.

4 is a graph showing x-polarized light and y-polarized light with respect to the change in the polar angle θ formed between the crystal axis of the nitride semiconductor and the normal of the substrate.

FIG. 5 is a graph showing polarization anisotropy with respect to the change in the polar angle θ formed between the crystal axis of the nitride semiconductor and the normal of the substrate.

FIG. 6 is a graph showing the internal electric field with respect to the change in the polar angle θ formed between the crystal axis of the nitride semiconductor and the normal of the substrate.

FIG. 7 is a graph showing the synthesis of a wave function with respect to the change in the polar angle θ formed between the crystal axis of the nitride semiconductor and the normal of the substrate.

8 and 9 are graphs showing the spontaneous emission according to the wavelength change.

Claims (10)

Board; An active layer including a nitride semiconductor layer formed on the substrate, wherein the c-axis of the crystal of the nitride semiconductor layer is inclined with respect to the normal of the substrate; An N-type contact layer injecting electrons into the active layer; And And a P-type contact layer disposed to face the N-type contact layer on the active layer and injecting holes into the active layer. The angle formed by the c-axis of the crystal of the nitride semiconductor layer and the normal of the substrate is in the range of 40 degrees to 90 degrees. delete The light emitting device of claim 1, wherein an angle formed between the c-axis of the crystal of the nitride semiconductor layer and the normal of the substrate is in a range of 50 degrees to 70 degrees. 4. The light emitting device according to claim 3, wherein an angle formed between the c-axis of the crystal of the nitride semiconductor layer and the normal of the substrate is 56 degrees. The light emitting device of claim 1, wherein the active layer comprises Al _x Ga _y In _z N, wherein x + y + z = 1, 0 ≦ x, y, z ≦ 1. delete delete delete delete delete

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