KR20130123592A - Light emitting diode of having sloped band gap structure - Google Patents

Abstract

Disclosed is a light emitting diode having a band slope layer formed on an area in contact with a multi-quantum well layer which emits light. The band slope layer is formed on the upper and lower parts of the multi-quantum well layer. The band slope layer has a lower band gap than that of a barrier layer composing the multi-quantum well layer and has a value greater than the band gap of the well layer. When a forward bias is applied, excited electrons and holes are bounded in the well layer by tunneling the barrier layer. As a result, internal quantum efficiency can be increased continuously even when a high current is applied.

Description

Light Emitting Diode of Having Sloped Band Gap Structure

The present invention relates to a light emitting diode, and more particularly to a light emitting diode having a band structure inclined on both sides of the multi-quantum well structure.

Light emitting diodes are devices that generate light by application of a specific electric field. The luminescence operation is caused by the combination of electrons in the conduction band and the hole in the valence band, and a multi-quantum well structure is adopted for smooth recombination of electrons and holes.

Recently, gallium nitride-based light emitting diodes have been commercialized, and white light has been realized through the introduction of fluorescent materials. The gallium nitride light emitting diode has a phenomenon in which external quantum efficiency (EQE) is reduced at high current density or high voltage. This is referred to as "Efficiency Droop".

Efficiency droop occurs because of electron overflow, Auger recombination, and low hole injection concentration in multi-quantum well structures. In particular, the former excess is considered to be the most important factor among the above-mentioned causes. That is, due to the high current density in the multi-quantum well structure in which the barrier layer and the well layer are alternately formed, the electrons are not completely confined to the well layer, and the excess phenomenon that exceeds the barrier layer occurs. Due to the occurrence of excess phenomena, the electrons are not sufficiently recombined with the holes even at high current densities, and may be a cause of heat generation through scattering in a multi-quantum well structure.

In order to prevent excessive electrons, an electron blocking layer (EBL) having a high band gap is generally used. For example, an AlGaN layer is introduced into the cladding layer in a structure in which the barrier layer is made of GaN and the well layer is made of InGaN. The AlGaN layer acts as an electron blocking layer, prevents excess electrons and reduces efficiency of electron and hole density, thereby improving efficiency. However, despite the introduction of the electron blocking layer, the efficiency droop phenomenon is not completely eliminated and only causes the effect to be somewhat alleviated. This is because the blocking of electrons is not achieved even when the electron blocking layer is introduced, and the amount of electrons entering the multi-quantum well structure as the current density or voltage increases is relatively higher than the amount of holes.

Another factor causing the excess of electrons is the Quantum Confined Stark Effect (QCSE). This means the effect on the absorption spectrum or emission spectrum of the quantum well when an external electric field is applied. That is, when an electric field is applied to the outside, the electrons fall in the conduction band, and the valence band in the holes increases. In addition, in the conduction band inclined by the electric field, the electrons move to a region of low energy level. On the contrary, holes in the valence band move to areas with high energy levels. That is, a phenomenon in which the region of high density of electrons and the region of high density of holes do not coincide with each other in the well layer occurs, which reduces the recombination rate of electrons and holes. Electrons that do not recombine with holes cause excess electrons and deepen efficiency droop. This QCSE is generated when GaN is grown on the C plane of the sapphire substrate and InGaN / GaN, which is a multi-quantum well structure, is generated. That is, a piezoelectric field, a kind of piezoelectric phenomenon, is generated in the InGaN layer having a mismatch between the GaN layer and the lattice constant. This electric field acts as an external electric field that generates QCSE.

In order to solve the above problem, a method of using growth of a GaN layer having a non-polar growth crystal plane is proposed. However, in order to have a non-polar growth crystal plane, the surface of the sapphire substrate on which the single crystal growth is based should have a non-polar plane.

As described above, various methods are proposed to solve the excess phenomenon of electrons. However, even if one method is used or all the suggested methods are used, the excess of electrons is not completely eliminated, and the effect droop is continuously generated.

Thus, the emergence of techniques for improving the charge transport mechanism and preventing efficiency droop through the introduction of new stack structures will still be required.

An object of the present invention for solving the above problems is to provide a light emitting diode that is constrained electrons and holes to the well layer through the tunneling of the injected electrons and holes, thereby improving efficiency.

The present invention for solving the above problems, n clad layer for supplying electrons; A first band inclined layer formed on the n clad layer and having a changed band gap; A multi-quantum well layer formed on the first band inclined layer and performing a light emitting operation; A second band inclined layer formed on the multi-quantum well layer and having a band gap changed; And a p-clad layer formed on the second band inclined layer and supplying holes.

According to the present invention described above, a first band inclined layer is formed between the n-type n clad layer and the multi-quantum well layer, and a second band inclined layer is formed between the p-type p clad layer and the multi-quantum well layer. . Each band bevel layer has a bandgap above the well layer in the multi-quantum well layer, but lower than the barrier layer. Thus, even if a forward bias is applied, electrons or holes are constrained to the well layer through tunneling rather than moving over the barrier layer. Therefore, even if the current density increases, the internal quantum efficiency does not decrease, but increases continuously. This reduces the internal quantum efficiency.

1 is a cross-sectional view showing a laminated structure of a light emitting diode according to a preferred embodiment of the present invention.
FIG. 2 is a diagram illustrating a band gap of the light emitting diode of FIG. 1 according to a preferred embodiment of the present invention.
3 is a graph showing the characteristics of a light emitting diode manufactured according to a preferred embodiment of the present invention.

The present invention is capable of various modifications and various forms, and specific embodiments are illustrated in the drawings and described in detail in the text. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. Like reference numerals are used for like elements in describing each drawing.

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 to which this invention belongs. Terms such as those defined in commonly used dictionaries are to be interpreted as having a meaning consistent with the contextual meaning of the related art and are to be interpreted as either ideal or overly formal in the sense of the present application Do not.

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

Example

1 is a cross-sectional view showing a laminated structure of a light emitting diode according to a preferred embodiment of the present invention.

Referring to FIG. 1, the light emitting diode according to the present exemplary embodiment includes an n cladding layer 110, a first band gradient layer 120, a multi-quantum well layer 130, and a second band gradient layer formed on the substrate 100. 140 and the p clad layer 150.

The n clad layer 110 may be any semiconductor as long as the Fermi level may be increased by implantation of the dopant. For example, when the n clad layer 110 includes GaN, the n clad layer 110 may be formed by using a Group 4 element as a dopant. In addition, when the n clad layer 110 includes GaN, Si is preferably used as the dopant. In addition, the n clad layer 110 may include ZnO. When the n clad layer 110 includes ZnO, a dopant may be selected to serve as a donor for forming an n-type semiconductor. The n clad layer 110 supplies electrons in the light emission operation.

The first band inclined layer 120 is formed on the n clad layer 110. Preferably, the first band inclination layer 120 is of n type. Therefore, when the n clad layer 110 includes GaN, the first band inclination layer 120 may also include GaN, and a Group 4 element may be used as a dopant. Therefore, the first band inclination layer 120 also has n-type conductivity.

However, the concentration of the material for performing the bandgap increases from the surface in contact with the n-clad layer to the upper portion of the first band inclination layer 120. For example, when the base material of the first band inclination layer 120 is GaN, the concentration of In is increased toward the top. This means that the band gap decreases as the first band inclination layer 120 moves upward. That is, in the case of GaN, the band gap is 3.4 eV at room temperature, and in InN, the band gap is 0.65 eV at room temperature. Therefore, as the concentration of In increases, the band gap tends to decrease. However, the band gap at the top of the first band slanting layer 120 may be equal to or greater than the band gap of the well layer of the multi-quantum well layer 130.

The multi-quantum well layer 130 is provided on the first band inclination layer 120. The multi-quantum well layer 130 has a structure in which a barrier layer and a well layer are alternately formed. For example, when the barrier layer is GaN, the well layer is composed of InGaN. This is the result of selecting In as a material for performing bandgap control. Therefore, if the material layers having a large band gap and a relatively small band gap are alternately formed, the multi-quantum well layer 130 of the present invention may be formed.

In particular, the band gap in the well layer is preferably equal to or less than the uppermost band gap of the first band inclined layer 120.

The second band slope layer 140 is provided on the multi-quantum well layer 130. The second band inclined layer 140 has a p-type conductivity. In addition, if the second band inclination layer 140 is formed based on GaN, a Group 2 element may be used as the dopant. For example, Mg is used as the dopant. In addition, the second band inclination layer 140 has a tendency to decrease the concentration of the material for performing the band gap from the portion in contact with the multi-quantum well layer 130 toward the top. That is, when the second band inclination layer 140 includes GaN, the second band inclination layer 140 has the highest concentration of In at the portion in contact with the multi-quantum well layer 130. In addition, the concentration of In decreases toward the top. Therefore, the band gap at the portion in contact with the multi-quantum well layer 130 has the lowest value, and the band gap increases toward the top. The band gap at a portion in contact with the multi quantum well layer 130 is preferably greater than or equal to the band gap of the multi quantum well layer 130.

The p clad layer 150 is provided on the second band inclined layer 140. The p-clad layer 150 may be any material as long as the Fermi level can be lowered toward the valence band by implantation of a dopant. Therefore, the p clad layer 150 may include GaN or ZnO. If the p clad layer 150 contains GaN, a Group 2 element is used as the dopant. Therefore, Mg is preferably used as the dopant. Holes are generated in the p-clad layer 150, which is supplied to the multi-quantum well layer 130.

In FIG. 1, a first band inclined layer 120 is provided between an n cladding layer 110 and a multi-quantum well layer 130 having characteristics of an n-type semiconductor. The first band inclination layer 120 has the characteristics of an n-type semiconductor, and the closer to the multi-quantum well layer 130, the smaller the band gap. However, the band gap of the first band inclined layer 120 in contact with the multi-quantum well layer 130 is set to be equal to or greater than the band gap of the well layer.

In addition, a second band inclined layer 140 is formed between the multi-quantum well layer 130 and the p-clad layer 150 having the characteristics of the p-type semiconductor. The second band inclined layer 140 has a characteristic of a p-type semiconductor, and the closer the band is to the multi-quantum well layer 130, the smaller the band gap. However, the band gap of the second band inclined layer 140 in contact with the multi-quantum well layer 130 is set to be equal to or greater than the band gap of the well layer.

FIG. 2 is a diagram illustrating a band gap of the light emitting diode of FIG. 1 according to a preferred embodiment of the present invention.

2, the thermal equilibrium state due to the bonding of the semiconductor materials is ignored. Thus, Fermi levels between dissimilar membranes are inconsistent and only trends in changes in conduction and valence bands are shown. In addition, the distortion of the band due to the application of the electric field is also ignored. However, the tendency of the band in each laminated structure is shown. 2, the energy level of the conduction band is represented by Ec, and the energy level of the valence band is represented by Ev. Ec and Ev may be different for each stacked structure. However, the energy level Ec of the conduction band is described as representing the energy level of each successive laminated structure, and the energy level Ev of the valence band is also described as representing the energy level of each successive laminated structure.

First, the conduction band of the n clad layer 110 is in a smooth state. This shows a state where the dopant is uniformly doped throughout the n clad layer 110. If a forward bias is applied, electrons from the Fermi level move to the conduction band. Thus, the electrons in the conduction band appear to be excited. In addition, electrons are generated in the conduction band in the first band inclination layer 120 according to the application of the bias. The electrons in the conduction band have a characteristic of moving to a low energy level. Therefore, the electrons of the conduction band are concentrated in the region in contact with the multi-quantum well layer 130. The first band inclined layer 120 in contact with the multi-quantum well layer 130 has a low band gap. Thus, electrons in the conduction band are concentrated along the inclined energy band.

The multi-quantum well layer 130 has a structure in which a barrier layer and a well layer are alternately formed. Of course, the barrier layer is formed at a portion in contact with the first band inclination layer 120.

When no band inclination layers are provided, when a forward bias is applied, electrons in the conduction band of the n clad layer having a high energy level overflow the barrier layer and move to the conduction band of the well layer. That is, the quantum confinement effect is generated. In the process of overflowing the barrier layer and quantum confinement, electrons having energy above the level of the well layer and the barrier layer move directly to the p clad layer without being confined to the well layer. This is a factor for lowering the luminous efficiency.

In FIG. 2, the band gap of the first band slanting layer 120 has a level lower than that of the barrier layer. Thus, when forward bias is applied, electrons in the conduction band of the first band inclined layer 120 are constrained to the well layer through tunneling to the barrier layer. Of course, electrons with high energy levels can overflow the barrier layer, but this is not the dominant phenomenon.

The electrons tunneled through the barrier layer using the low bandgap of the first band slope layer 120 are constrained in the well layer.

In addition, it is assumed that the valence band of the p-clad layer 150 is also smooth. This means that the dopant is distributed in a substantially uniform state without local concentration of the dopant in the p clad layer 150. However, the Fermi level of the p clad layer 150 is moved in the valence band direction. When a positive bias is applied, holes are formed in the valence band of the p-clad layer 150. Holes in the valence band move to the second band inclination layer 140 according to the application of an electric field.

In addition, the second band inclined layer 140 in contact with the p clad layer 150 has a tendency to decrease the band gap closer to the multi-quantum well layer 130. When a positive bias is applied, holes are formed in the valence band. In addition, holes in the valence band have a characteristic of moving to a high energy state. Therefore, the holes of the second band inclined layer 140 are concentrated in the valence band in contact with the multi-quantum well layer 130.

Holes in the second band gradient layer 140 tunnel through the barrier layer of the multi-quantum well layer 130 by the electric field due to the applied forward bias. It is quantum confined to the well layer through tunneling.

Electrons and holes defined in the well layer through the above-described process are confined through tunneling without overflowing the barrier layer. Electrons and holes confined in the well layer may perform light emission through recombination.

In addition, the band gap of the first band inclination layer 120 and the second band inclination layer 140 may be linearly reduced toward the multi-quantum well layer 130. In addition, the first band inclination layer 120 and the second band inclination layer 140 are reduced from the multi-quantum well layer 130 to a predetermined area, and the smooth band from the predetermined area to the multi-quantum well layer 130 is maintained. It may be.

If the band inclination layer is not introduced as in the related art, electrons and holes should be constrained to the well layer by overflow of electrons and holes. Therefore, electrons of high energy state mean that energy must be lost through scattering or the like. Both scattering is a factor in generating thermal energy in the crystal structure. This means that the internal quantum efficiency is lowered.

On the other hand, in this embodiment, both tunneling is used. The scattering of particles is minimized through the tunneling of the protons according to the application of the electric field, and the protons are confined into the well layer while maintaining energy. Therefore, the internal quantum efficiency is increased. This means that the number of electrons and holes recombined in the well layer increases with the application of the electric field.

In addition, in the present embodiment, it is preferable that the first band inclined layer and the second band inclined layer contacting the multi-quantum well layer have a band gap greater than or equal to that of the well layer. This is to avoid the phenomenon that the light generated by the recombination in the well layer is reabsorbed in the first band slope layer and the second band slope layer. In addition, it is preferable that the lowest bandgap of the first band inclination layer or the second band inclination layer has a value below the bandgap of the barrier layer of the multi-quantum well layer.

3 is a graph showing the characteristics of a light emitting diode manufactured according to a preferred embodiment of the present invention.

Referring to FIG. 3, the n clad layer 110 has a thickness of 1500 nm, and Si is used as the dopant. The concentration of dopant to be doped is 2 * 10 ¹⁸ cm ^-3 . In addition, the first band inclination layer 120 formed on the n clad layer 110 has a thickness of 30 nm, and Si is used as the dopant. In is used as a factor for adjusting the band gap. In particular, the concentration of indium from the portion in contact with the n clad layer 110 to the multi-quantum well layer 130 is linearly changed. That is, the fraction of indium is changed from 0 to 0.185 relative to Ga. Thus, the end of the first band-gradient layer 120 is in contact with the multiple quantum well layer 130 has a compositional formula of In Ga _{0 .185 0 .815} N.

The multi-quantum well layer 130 is provided on the first band inclination layer 120. The barrier layer consists of GaN and is 10 nm thick. In addition, the well layer is composed of In _{0 .185} Ga _{0 .815} N, a thickness of 3nm. The multi-quantum well layer 130 is formed by alternating a barrier layer and a well layer, a barrier layer is provided at a portion in contact with the n clad layer, and a barrier layer is also provided at the uppermost layer of the multi-quantum well layer 130. The number of repetitions of the barrier layer and the well layer in the multi-quantum well layer 130 is five times. Thus, six barrier layers and five well layers are formed.

The second band slope layer 140 is provided on the multi-quantum well layer 130. The second band inclined layer 140 includes GaN, and Mg is used as the dopant. In addition, In is included for bandgap adjustment. The thickness of the second band inclination layer 140 is 30 nm, and the fraction of In is set at 0.185 to 0 from the surface in contact with the multi-quantum well layer 130 to the top thereof. The fraction of In represents the fraction of In relative to Ga.

The p clad layer 150 is formed on the second band inclined layer 140. The p clad layer 150 includes GaN, Mg is used as the dopant, and the thickness is 200 nm. The concentration of the dopant included in the p clad layer 150 is 1 * 10 ¹⁸ cm ^-3 .

The characteristic of the above-described light emitting diode is represented by FIG. 3.

For comparison with the above-described light emitting diode, a light emitting diode in which the first band slope layer and the second band slope layer are omitted is provided. That is, a light emitting diode comprising an n clad layer, a multi-quantum well layer and a p clad layer is provided. Its characteristic is represented by in FIG.

Referring to FIG. 3, a light emitting diode having no band inclination layer has a high internal quantum efficiency (IQE) at an initial stage in which a current is supplied. This is because electrons and holes are confined into the well layer at an appropriate level at a relatively low voltage. However, when the current density increases, electrons and holes overflow due to the supply of high energy, and the number of cases where the electrons and holes are not confined into the well layer increases. This leads to a decrease in efficiency. Therefore, as the current density increases, the internal quantum efficiency decreases.

On the other hand, when the band inclination layer is provided, it has a low internal quantum efficiency at a low current density. This is due to the fact that the bandgap at the distal end of the band ramp layer is lower than the bandgap of the barrier layer, and low energy is supplied to tunnel it. On the other hand, as the current density increases, the internal quantum efficiency increases. This is because electrons and holes penetrate through the barrier layer due to high energy and are confined to the well layer.

As a result, when the current density is 400 A / cm ² or more, the light emitting diode having the band gradient layer has a higher internal quantum efficiency than the conventional light emitting diode. In addition, even if the current density is continuously increased, the internal quantum efficiency is continuously increased. This means that the existing efficiency droop phenomenon that the internal quantum efficiency decreases with increasing current density is solved.

In the present invention described above, a first band inclined layer is formed between the n-type n clad layer and the multi-quantum well layer, and a second band inclined layer is formed between the p-type p clad layer and the multi-quantum well layer. Each band bevel layer has a bandgap above the well layer in the multi-quantum well layer, but lower than the barrier layer. Thus, even if a forward bias is applied, electrons or holes are constrained to the well layer through tunneling rather than moving over the barrier layer. Therefore, even if the current density increases, the internal quantum efficiency does not decrease, but increases continuously. This reduces the internal quantum efficiency.

110: n cladding layer 120: first band inclined layer
130: multi-quantum well layer 140: second band inclined layer
150: p cladding layer

Claims (7)

An n clad layer for supplying electrons;
A first band inclined layer formed on the n clad layer and having a changed band gap;
A multi-quantum well layer formed on the first band inclined layer and performing a light emitting operation;
A second band inclined layer formed on the multi-quantum well layer and having a band gap changed; And
And a p-cladding layer formed on the second band inclined layer and supplying holes. The method of claim 1, wherein the first band inclination layer has an n-type conductivity type, the band gap of the portion in contact with the multi-quantum well layer has a value less than the band gap of the barrier layer of the multi-quantum well layer. Light emitting diode. 3. The light emitting diode of claim 2, wherein the band gap of the portion in contact with the multi quantum well layer has a value equal to or greater than the band gap of the well layer of the multi quantum well layer. 4. The light emitting diode of claim 3, wherein a band gap of the first band inclined layer decreases closer to the multi-quantum well layer. The method of claim 1, wherein the second band inclined layer has a p-type conductivity type, the band gap of the portion in contact with the multi-quantum well layer has a value less than the band gap of the barrier layer of the multi-quantum well layer. Light emitting diode. The light emitting diode according to claim 5, wherein the band gap of the portion in contact with the multi quantum well layer has a value greater than or equal to the band gap of the well layer of the multi quantum well layer. 7. The light emitting diode of claim 6, wherein a band gap of the second band sloped layer decreases closer to the multi-quantum well layer.

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