KR101012636B1 - Light generating device - Google Patents

Abstract

A light emitting device having improved optical characteristics is disclosed. Such a light emitting device includes an active layer, an N-type contact layer and a P-type contact layer. The active layer includes a quantum well layer and a barrier layer. The quantum well layer includes a first compound semiconductor, and the barrier layer includes a second compound semiconductor. In addition, the quantum well layer includes sub-quantum well layers forming an energy bandgap of a step shape. The N-type contact layer injects electrons into the active layer, and 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. At this time, the thickness of the first sub quantum well layer forming the first band gap is located between the first sub quantum well layer and the barrier layer and the second sub quantum well forming a second band gap larger than the first band gap. Smaller than the thickness of the layer.

Description

Light-Emitting Element {LIGHT GENERATING DEVICE}

The present invention relates to a light emitting device, and more particularly to a compound semiconductor light emitting device.

Group III-V nitride and group II-VI compounds constituting the semiconductor blue violet and cyan light emitting devices The semiconductor structure is group III-V having different light emission characteristics due to the piezoelectric field and spontaneous polarization caused by stress applied to the active layer, which is one of the essential characteristics. It is well known that it is significantly lower than a semiconductor [Park et al., Appl. Phys. Lett. 75, 1354 (1999). In particular, there is no known method for eliminating spontaneous polarization until now by changing the growth direction of the substrate to use a non-polar or semi-polar substrate [Park & Chuang, Phys. Rev. B59, 4725 (1999), Waltereit et al., Nature 406, 865 (2000).

Attempts have been made to minimize piezo and spontaneous polarization, an inherent weakness of III-V nitride semiconductors. Among them, the representative method

1) Method to minimize spontaneous polarization and piezo effect using non-polar or semi-polar substrate [Park et al., Phys Rev B 59, 4725 (1999) and Waltereit et al., Nature 406, 865 (2000) ]

2) The cladding layer is a four-layered film, and the composition ratio of Al is increased to increase the confinement effect of the transmitter to increase the 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 mature yet, 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)].

In case of 2), spontaneous polarization and piezo effect cannot be eliminated fundamentally, so it cannot be a fundamental solution. However, recent studies [Ahn et al., IEEE J. Quantum Electron. 41, 1253 (2005)] showed that the use of a four-barrier barrier has the effect of improving light gain due to the effect of confining the transmitter of a quantum well.

3) Alternatively, based on the theoretical study, the composition ratio of quaternary membranes in which the internal electric field due to piezo and spontaneous polarization disappears can be found in the InGaN / InGaAlN quantum well structure having the quaternary membrane barrier. A method for remarkably improving the light emission characteristics of optical devices such as quantum well LEDs and LDs has been proposed [S. H Park, D. Ahn, J. W. Kim, Applied Physics Letters 92, 171115 (2008). However, this method has the disadvantage that the growth conditions of the four-layer barrier are extremely difficult.

Accordingly, an object of the present invention is to provide a light emitting device having easier growth conditions and improved optical characteristics.

The light emitting device according to the exemplary embodiment of the present invention for solving this problem includes an active layer, an N-type contact layer and a P-type contact layer. The active layer includes a quantum well layer and a barrier layer. The quantum well layer includes a first compound semiconductor, and the barrier layer includes a second compound semiconductor. In addition, the quantum well layer includes sub-quantum well layers forming an energy bandgap of a step shape. The N-type contact layer injects electrons into the active layer, and 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. At this time, the thickness of the first sub quantum well layer forming the first band gap is located between the first sub quantum well layer and the barrier layer and the second sub quantum well forming a second band gap larger than the first band gap. Smaller than the thickness of the layer.

For example, the first compound semiconductor may be composed of indium gallium nitride (InGaN), and the second compound semiconductor may be composed of gallium nitride (GaN).

For example, the first sub quantum well layer is represented by In _x Ga _{1 - x} N, the second sub quantum well layer is represented by In _y Ga _{1 - y} N, x has a range of 0.15 to 0.25, y may have a range greater than 0 and less than 0.075.

The ratio of the thickness of the first sub quantum well layer and the thickness of the second sub quantum well layer is preferably in the range of 8/42 to 22/28. Further, it is more preferable that the ratio of the thickness of the first sub quantum well layer and the thickness of the second sub quantum well layer has 15/35.

According to the present invention, it is possible to facilitate the growth conditions by using the quantum well layer of the three-element film, and by adopting the step-shaped quantum well structure to control the wave function of electrons and holes spatially and at the same time by lattice mismatch By dispersing the strain, the piezoelectric field is reduced to improve the optical properties.

As the inventive concept allows for various changes and numerous embodiments, particular embodiments will be illustrated in the drawings and described in detail in the text. However, this is not intended to limit the present invention to the specific disclosed form, it should be understood to include all modifications, equivalents, and substitutes included in the spirit and scope of the present invention.

The terms first, second, etc. may be used to describe various elements, but the elements should not be limited by the terms. The terms are used only for the purpose of distinguishing one component from another. For example, without departing from the scope of the present invention, the first component may be referred to as the second component, and similarly, the second component may also be referred to as the first component.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this application, the terms "comprise" or "having" are intended to indicate that there is a feature, number, step, action, component, part, or combination thereof described in the specification, and that one or more other features It should be understood that it does not exclude in advance the possibility of the presence or addition of numbers, steps, actions, components, parts or combinations thereof.

Unless defined otherwise, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art. Terms such as those defined in commonly used dictionaries are to be interpreted as having a meaning consistent with the meaning in the context of the relevant art and are to be interpreted as ideal or overly formal in meaning unless explicitly defined in the present application Do not.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

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

Referring to FIG. 1, a light emitting device according to an exemplary embodiment of the present invention includes 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.

The N-type contact layer 102 is formed on the substrate 101. Optionally, a buffer layer (not shown) may be further formed between the substrate 101 and the N-type contact layer 102. This buffer layer (not shown) typically does not include a dopant, such as a donor or acceptor. For example, the buffer layer is formed of a nitride semiconductor containing no dopant. A buffer layer (not shown) 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.

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.

The active layer 103 includes a multi-quantum well layer in which a quantum well layer (not shown) including a first compound semiconductor and a barrier layer (not shown) including a second compound semiconductor are alternately formed. For example, the first compound semiconductor is composed of gallium nitride (InGaN), and the second compound semiconductor is composed of gallium nitride (GaN).

The quantum well layer of the active layer 103 according to the present invention includes sub-quantum well layers having a stepped energy band gap due to different composition ratios of the first compound semiconductors. For example, the quantum well layer includes a first sub quantum well layer having a first band gap and a second sub quantum well layer having a second band gap larger than the first band gap.

The first sub quantum well layer includes In _x Ga _{1 - x} N, where x ranges from 0.15 to 0.25. In addition, the second sub-quantum well layer is Ga _y In _{1 -} comprises N _y, In this case, y has a range between greater than 0 and 0.075. When the value of y is larger than 0.075, the band gap difference with the barrier layer containing gallium nitride (GaN) becomes too large, resulting in less strain reduction effect. In addition, when the range of the x value is out of the above range in conjunction with the range of the y value, light of a wavelength other than blue light is emitted in the range of the emission wavelength.

2 is a graph showing an energy band gap of an active layer having a conventional single quantum well structure and an active layer having a stepped quantum well structure according to an exemplary embodiment of the present invention. More specifically, the graph (a) of Figure 2 is a graph showing the energy bandgap of the active layer having a conventional single quantum well structure, the graph (b) of Figure 3 is a stepped form according to an exemplary embodiment of the present invention A graph showing an energy band gap of an active layer having a quantum well structure.

Referring to Figure 2, as shown in the graph (b) by giving a step in the middle of the quantum well, electrons and holes are constrained in a position closer to the space to improve the light transition characteristics. In addition, the strain dispersion effect is also generated by introducing a step structure having a different lattice constant.

As the strain is dispersed, the piezoelectric field is reduced, thus improving the mineral properties. For example, since the lattice constants of the barrier layer including GaN and the InGaN layer in the active layer 103 of FIG. 1 are greatly different from each other, strain is generated, and the GaN and InN content are controlled by adjusting the indium (In) content of InGaN. By introducing a stepped structure between InGaN, strain can be dispersed.

3 is a graph illustrating a light gain curve of an active layer having a conventional single quantum well structure and an active layer having a stepped quantum well structure according to an exemplary embodiment of the present invention. More specifically, the graph (a) of FIG. 3 is a graph showing a light gain curve of an active layer having a conventional single quantum well structure, and the graph (b) of FIG. 3 is a stepped shape according to an exemplary embodiment of the present invention. It is a graph showing the light gain curve of the active layer having a quantum well structure.

3 is a result of numerical calculation using the equations described below.

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). An optical gain having a multibody effect including an anisotropy of valence versus dispersion is represented by Equation 1 below.

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 and valence bands, respectively. The subscripts l and m represent the electron and hole states in the conduction band, respectively.

In addition, the re-standardized transition energy of electrons and constant space is represented by Equation 2 below.

In the above equation, Eg is the bandgap, ΔE _SX and ΔE _CH are the screened exchange and Coulomb-hole contribution to the bandgap renormalization, respectively (see WW Chow, M. Hagerott, A). Bimdt, and SW Koch, "Threshold coditions for an ultraviolet wavelength GaN quantum-well laser", IEEE J. Select. Topics Quantum Electron., Vol. 4, pp. 514-519, 1998).

A Gaussian line shape function L (ω, k _|| , φ) is expressed by Equation 3 below.

는 엑시토닉(exitonic) 또는 밴드간 전이의 쿨롱상승의 원인이 된다. 상기의 라인형상 함수는 논-마코비안 퀀텀 키네틱스(Non-Marcobian Quantum kinetics)의 가장 간단한 가우시안(Gaussian)이고, 아래의 수학식 4 및 수학식 5로 기술된다.At the top

Causes coulomb rise in exitonic or interband transitions. The line shape function is the simplest Gaussian of Non-Marcobian Quantum kinetics and is described by Equations 4 and 5 below.

The interband relaxation time τ _in and the correlation time τ _c are regarded as constants and are calculated to be 25fs and 10fs, respectively. The parameters of GaN and InN materials required for the calculation are given by 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

Referring to FIG. 3, it can be seen that the effect of the stepped quantum well layer increases as the wavelength goes from blue to green, or as the wavelength becomes longer. In addition, it can be seen that as the thickness of the quantum well layer increases, the effect of the step-shaped quantum well layer increases.

Referring to the graphs (a) and (b) of FIG. 3, in the case of 440 nm of blue wavelength, the light gain (blue dot line) and the first sub quantum well layer corresponding to the conventional single quantum well layer (Lw1 in FIG. 2). Section) and the second sub quantum well layer (Lw2 section in FIG. 2), and the light gain (blue solid line) corresponding to the quantum well layer having a step shape is not large, but the green wavelength is 530 nm (green solid line). ), It can be seen that the light gain is greatly increased compared to the light gain (green dotted line) corresponding to the conventional single quantum well layer.

Referring to the graphs (a) and (b) of FIG. 3, the light gain is reduced in the graph (b) having a thickness of 5 nm of the quantum well layer compared to the graph (a) having a thickness of 3 nm of the quantum well layer. This is because holes and electrons are distributed in a wider area, and thus the bonding probability decreases relatively. However, in the case of the graph (b) in which the thickness of the quantum well layer is 5 nm compared to the graph (a) in which the thickness of the quantum well layer is 3 nm, the optical gain (blue dotted line) and the first corresponding to the conventional single quantum well layer are The difference in light gain (blue solid line) corresponding to the quantum well layer having a step shape is divided into one sub quantum well layer (Lw1 section in FIG. 2) and a second sub quantum well layer (Lw2 section in FIG. 2). You can check it.

4 is a graph (a) of measuring the indium composition ratio of the first sub quantum well layer while varying the thickness of the first sub quantum well layer having a small band gap and a graph (b) illustrating the self-luminescence efficiency.

The graph (a) of FIG. 4 is fixed to have a wavelength of 400 nm, and the total thickness of the first sub quantum well layer (Lw1 section of FIG. 2) and the second sub quantum well layer (Lw2 section of FIG. 2) is 50 angstrom ( I), the thickness Lw1 of the first sub quantum well layer is changed, and the indium content x of the first sub quantum well layer including In _x Ga _1-x N is calculated. In this calculation, In _y Ga _{1 -} a second indium content y of the sub-quantum well layer comprising a N _y was fixed at 0.05.

According to the calculation result, it can be seen that the indium content x of the first sub quantum well layer decreases as the thickness Lw1 of the first sub quantum well layer having a relatively small band gap is increased.

When the thickness w1 of the first sub quantum well layer having the relatively small band gap is increased, the influence of the relatively small band gap is increased, and thus the amplitude of the light emitted tends to increase. However, since the required emission wavelength is fixed at 400 nm, the band gap of the first sub quantum well layer should be increased to compensate for this. On the other hand, as the indium content decreases, the band gap increases. Therefore, in order to generate light having a fixed emission wavelength, the indium (In) content of the first sub quantum well layer should be reduced. In the graph (a) of Figure 4 it can be seen well these results.

Graph (b) of FIG. 4 fixes the overall thickness of the first sub quantum well layer (Lw1 section in FIG. 2) and the second sub quantum well layer (Lw2 section in FIG. 2) to 50 angstroms, and the first Spontaneous emission rate is calculated while changing the thickness of the sub quantum well layer (Lw1 section in FIG. 2).

According to the calculation result, the first sub quantum well layer forming the first band gap (Lw1 section in FIG. 2) is the second sub quantum well layer forming the second band gap larger than the first band gap (Lw2 in FIG. 2). Luminous efficiency increases when smaller than the thickness of the section).

More specifically, in a section where the thickness of the first sub quantum well layer (Lw1 section in FIG. 2) is 8 angstroms and the thickness of the second sub quantum well layer (Lw2 section in FIG. 2) is 42 angstroms Luminous efficiency in a section where the thickness of the first sub quantum well layer (Lw1 section in FIG. 2) is 22 angstroms and the thickness of the second sub quantum well layer (Lw2 section in FIG. 2) is 28 angstroms This is excellent. In other words, the ratio of the thickness of the first sub quantum well layer (Lw1 section in FIG. 2) to the thickness of the second sub quantum well layer (Lw2 section in FIG. 2) has an excellent spontaneous emission rate in the range of 8/42 to 22/28. .

More preferably, the thickness of the first sub quantum well layer (Lw1 section in FIG. 2) is 15 angstroms and the thickness of the second sub quantum well layer (Lw2 section in FIG. 2) is 35 angstroms. It has a spontaneous release rate. In other words, the ratio of the thickness of the first sub quantum well layer (Lw1 section in FIG. 2) to the thickness of the second sub quantum well layer (Lw2 section in FIG. 2) has a maximum spontaneous emission rate at 15/35.

On the other hand, when most of the light emission wavelength is generated in the first sub quantum well layer (Lw1 section in Fig. 2), it is preferable to reduce the width of the second sub quantum well layer (Lw2 section in Fig. 2) as much as possible. That is, when most of the light emission wavelength is generated in the first sub quantum well layer (Lw1 section of FIG. 2), when the width of the second sub quantum well layer (Lw2 section of FIG. 2) is increased, the wave function causes the second sub quantum The emission wavelength tends to increase due to the influence of the well layer (Lw2 section in Fig. 2). Accordingly, when the first sub quantum well layer (Lw1 section of FIG. 2) is designed to generate most of the emission wavelengths, the width of the second sub quantum well layer (Lw2 section of FIG. 2) is reduced as much as possible and the second sub quantum well is formed. The well layer (Lw2 section in FIG. 2) may be responsible only for the function of dispersing strain.

However, manufacturing the thin second sub quantum well layer (Lw2 section in FIG. 2) is not easy, and therefore, the first sub quantum well layer (Lw1 section in FIG. 2) and the second sub quantum well layer ( It is preferable to design so that all Lw2 sections of FIG. 2 contribute to light emission. In this case, it is preferable that the width Lw1 of the first sub quantum well layer is smaller than the width Lw2 of the second sub quantum well layer.

According to the present invention, it is possible to facilitate the growth conditions by using the quantum well layer of the three-element film, and by adopting the step-shaped quantum well structure to control the wave function of electrons and holes spatially and at the same time by lattice mismatch By dispersing the strain, the piezoelectric field is reduced to improve the optical properties.

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 cross-sectional view schematically showing a light emitting device according to an exemplary embodiment of the present invention.

2 is a graph showing an energy band gap of an active layer having a conventional single quantum well structure and an active layer having a stepped quantum well structure according to an exemplary embodiment of the present invention.

3 is a graph illustrating a light gain curve of an active layer having a conventional single quantum well structure and an active layer having a stepped quantum well structure according to an exemplary embodiment of the present invention.

4 is a graph (a) of measuring the indium composition ratio of the first sub quantum well layer while varying the thickness of the first sub quantum well layer having a small band gap and a graph (b) illustrating the self-luminescence efficiency.

<Short Description of Main Drawing Numbers>

100 semiconductor light emitting element 101 substrate

102: N-type contact layer 103: active layer

104: P-type contact layer 105: N-type electrode

106: P-type electrode

Claims (5)

An active layer including a quantum well layer including a first compound semiconductor and a barrier layer including a second compound semiconductor, wherein the quantum well layer includes sub quantum well layers forming a stepped energy band gap; An N-type contact layer injecting electrons into the active layer; And A P-type contact layer disposed opposite the N-type contact layer on the active layer and injecting holes into the active layer, The thickness of the first sub quantum well layer constituting the first band gap is between the first sub quantum well layer and the barrier layer, and the thickness of the second sub quantum well layer constituting a second band gap larger than the first band gap. A compound semiconductor light emitting device, characterized in that less than the thickness. The method of claim 1, And the first compound semiconductor is made of indium gallium nitride (InGaN), and the second compound semiconductor is made of gallium nitride (GaN). The method of claim 2, The first sub quantum well layer is represented by In _x Ga _1-x N, and the second sub quantum well layer is represented by In _y Ga _{1 - y} N, x has a range of 0.15 to 0.25, and y is A compound semiconductor light emitting device, characterized in that it has a range greater than zero and less than 0.075. The method of claim 3, The ratio of the thickness of the first sub quantum well layer and the thickness of the second sub quantum well layer is 8/42 to 22/28. The method of claim 4, wherein And a ratio of the thickness of the first sub quantum well layer to the thickness of the second sub quantum well layer is 15/35.

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