KR101499951B1 - Red light emitting device and method of making the same - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention [0002] The present invention relates to a light emitting device, and more particularly, to a red light emitting device and a manufacturing method thereof capable of improving luminous efficiency and reliability of the light emitting device. The present invention provides a method of manufacturing a red light emitting device, comprising the steps of: forming a multi-layered semiconductor layer on a substrate; Forming a mask defining a light extracting structure pattern on the semiconductor layer; Forming an electrode mask defining a protection region having an electrode shape on the mask; Forming a light extracting structure using the mask; And removing the mask and forming an electrode on the protection region.

LED, light emitting device, electrode, light extraction structure, etch.

Description

TECHNICAL FIELD The present invention relates to a red light emitting device and a method of manufacturing the same,

BACKGROUND OF THE INVENTION 1. Field of the Invention [0002] The present invention relates to a light emitting device, and more particularly, to a red light emitting device and a manufacturing method thereof capable of improving luminous efficiency and reliability of the light emitting device.

Light emitting diodes (LEDs) are well-known semiconductor light emitting devices that convert current into light. In 1962, red LEDs using GaAsP compound semiconductors were commercialized. GaP: N series green LEDs and information communication devices As a light source for a display image of an electronic device.

The extraction efficiency of such a semiconductor LED is usually only a few percent due to the high refractive index difference between air and epoxy or semiconductor, which is the medium for observing light.

For example, in the case of a red LED, considering the refractive index relation of the GaP layer located at the upper layer and the epoxy introduced for surface protection, the critical angle is about 25 °, and there is no absorption loss in the material, refraction), and if there is no mirror such as a DBR (Distributed Bragg-Reflector) on the lower layer, the extraction efficiency that can be detected in the epoxy region is about 4.7%.

The rest of the light is trapped inside the LED by the total reflection and is absorbed by the GaAs substrate through the lower layer or the intensity of the light is gradually reduced by the medium which can give an absorption loss like a quantum well, It is not included in the light extraction efficiency and disappears.

Therefore, a method for improving the light extraction efficiency of the LED is required.

Disclosure of Invention Technical Problem [8] The present invention provides a red light emitting device including a light extracting structure capable of improving light extraction efficiency, have.

According to a first aspect of the present invention, there is provided a method of manufacturing a red light emitting device, comprising: forming a semiconductor layer having a multilayer structure on a substrate; Forming a mask defining a light extracting structure pattern on the semiconductor layer; Forming an electrode mask defining a protection region having an electrode shape on the mask; Forming a light extraction structure at a specific etching depth (? / N) or more using the mask; And forming an electrode on the protection region by removing the mask, wherein two different period sections having a peak value of light extraction efficiency are displayed according to a pattern period of the light extracting structure, And the pattern of the extracted structure is formed in a period within a period section having a large value among the two different period sections. Here, lambda is an emission wavelength of the red light emitting device and n is the refractive index of the semiconductor layer.

According to a second aspect of the present invention, a red light emitting device includes: a reflective layer disposed on a conductive substrate; A multi-layered semiconductor layer located on the reflective layer; A light extracting structure formed on the semiconductor layer, the light extracting structure being formed at a portion other than a protective region where electrodes are to be formed, at a specific etch depth (? / N); Wherein the light extracting structure has two peaks in which the light extraction efficiency has a peak according to a pattern period of the light extracting structure, And is formed in a period within a cycle section having a larger value in the other two cycle sections. Here, lambda is an emission wavelength of the red light emitting device and n is the refractive index of the semiconductor layer.

The present invention can improve light extraction efficiency by introducing a light extracting structure into a light emitting device. In addition, the light extraction structure can solve the problem of junction between the semiconductor region and the electrode interface at the same time without affecting the light extraction efficiency at all.

Accordingly, it is possible to increase the light extraction efficiency and extend the optimum period by increasing the etch depth of the light extracting structure, and to solve the peeling phenomenon between the semiconductor layer into which the light extracting structure is introduced and the electrode interface. When the optimum period of the light extracting structure is extended as described above, the light extracting structure can be easily formed by a conventional method such as photolithography.

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

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. Rather, the intention is not to limit the invention to the particular forms disclosed, but rather, the invention includes all modifications, equivalents and substitutions that are consistent with the spirit of the invention as defined by the claims.

It will be appreciated that when an element such as a layer, region or substrate is referred to as being present on another element "on," it may be directly on the other element or there may be an intermediate element in between . It will be appreciated that if a portion of a component, such as a surface, is referred to as " inner ", it means that it is farther from the outside of the device than other portions of the element.

Further, relative terms such as " beneath " or " overlies " are used herein to refer to a layer or region relative to a substrate or reference layer, Can be used to describe the relationship.

It will be appreciated that these terms are intended to encompass different orientations of the device in addition to those depicted in the Figures. Finally, the term 'directly' means that there are no intervening elements in the middle. As used herein, the term " and / or " includes any and all combinations and all combinations of related items noted.

Although the terms first, second, etc. may be used to describe various elements, components, regions, layers and / or regions, such elements, components, regions, layers and / And should not be limited by these terms.

The light extraction efficiency of the semiconductor light emitting device is determined by the refractive index difference between the semiconductor light emitting layer in which light is generated and the medium (air or epoxy) that ultimately observes light. Since the semiconductor medium usually has a high refractive index (n > 2, the refractive index is denoted by n), the light extraction efficiency is usually only a few percent.

For example, in the case of a blue light emitting device based on gallium nitride (GaN; n = 2.46), the extraction efficiency through the upper part of the device is only about 9%, assuming that the external material is epoxy (n = 1.4). The remaining light is trapped by the total reflection process inside the device and is lost by an absorption layer such as a quantum well layer.

Looking at the ratio of light emission to the incident angle, the light corresponding to the total reflection angle occupies about 80% of the total light amount, so that the light extraction enhancement effect is virtually unexpected unless light belonging to this region can be extracted. Therefore, it is concluded that a structure capable of extracting light corresponding to the total reflection angle is inevitably required to improve the extraction efficiency of the light emitting device. At this time, one of the functions that can perform this role is photonic crystal.

From a mechanical point of view, the phenomenon of light transmission is that light travels through matter with different refractive indices. As with object motion in mechanics, the law of conservation of momentum is always followed by the movement of light (where the momentum of light is represented by the wavenumber vector (k = 2πn / λ)).

In other words, when the light travels through different media, the plane momentum component of the interface must be preserved. By substituting this with the total reflection phenomenon, the momentum conservation phenomenon can be understood more clearly.

Total reflection always occurs when light travels from a medium with a high refractive index to a medium with a low refractive index. Since the light in a medium having a high refractive index already has a large momentum, it has a momentum (plane component) which can not be obtained at any angle within a medium having a low refractive index at a certain incident angle or more. Since the momentum of a plane component must be preserved for the transmission of light, the only way that light can choose is the reflection process. Here, the minimum incident angle at which the momentum of the plane component can not be preserved corresponds to the critical angle.

The photonic crystal helps to add or subtract the momentum components generated by its periodicity to the light of total reflection angle, which can not preserve the momentum, and to extract it outward (see FIG. 1). This is the principle of improving the extraction efficiency of the light emitting device through the photonic crystal.

This is the same concept as the diffraction principle of light in the spectroscope. The magnitude of the momentum changes according to the period of the photonic crystal, and thus the diffraction efficiency of the light corresponding to the total reflection angle changes.

The extraction efficiency of photonic crystals is closely related to the structural parameters such as etch depth, filling factor, and lattice structure in addition to photonic crystal cycle. Therefore, it is very important to design and apply an optimal photonic crystal structure in order to obtain a high extraction efficiency improvement effect in a semiconductor light emitting device.

In addition to the internal structural parameters related to the photonic crystal, the degree of enhancement of the extraction efficiency by the photonic crystal is influenced by the refractive index of the background material filling the light emitting device and the reflectance characteristics of the reflective mirror in the light emitting device.

According to Snell's law, the higher the refractive index of the semiconductor material, the greater the critical angle, which means that the amount of light confined by the total reflection process inside the device increases. FIG. 2 is a graph showing the extraction efficiency according to the refractive index of the semiconductor light emitting region, assuming that the external background material is air.

At this time, the light emitting surface is a planar structure in which no light extracting structure is introduced. The graph shows that the refractive index of the light emitting layer and the extraction efficiency are inversely proportional to each other. This means that the relative extraction efficiency increase ratio can be increased when the light extracting structure is introduced to the surface of the device. Further, since the diffraction effect by the light extracting structure is proportional to the refractive index, it is possible to obtain a larger extraction effect by introducing the light extracting structure into the surface of the element of the high refractive index layer.

As a result, the relative extraction efficiency of the light emitting device through the light extraction structure can be improved by using an organic material having a low refractive index, gallium arsenide (GaAs) having a higher refractive index than gallium nitride (GaN) It can be expected that it will be more prominent.

The degree of improvement of the extraction efficiency by the light extraction structure is also closely related to the characteristics of the reflection mirror at the bottom of the device. The light extraction structure can be extracted to the outside through the process of combining with the light extraction structure even if the light having the incident angle corresponding to total reflection is extracted. However, since the diffraction efficiency can not be 100%, part of the light is always reflected back into the device. Therefore, the degree of improvement of the extraction efficiency depends on the characteristics of the lower reflection mirror.

There are three types of reflective mirrors at the bottom. A method of introducing a refractive index layer (a side-view type GaN light emitting element) lower than the refractive index of the light emitting layer, a method of introducing a metal layer (vertical GaN LED, metal-bonding red LED), a method of increasing the reflectance by a combination of dielectrics different in refractive index DBRs LED).

Each reflective mirror has a characteristic inherent to the angle of incidence. The reflective mirror method introducing a low refractive index layer exhibits a low reflectance with respect to the incident angle in the vertical direction, but exhibits an ideal reflectance of 100% by the total reflection process above a critical angle.

The metal-layer reflective mirror exhibits excellent overall reflectance for all angles of incidence, but it can not approach 100% reflectivity due to the high absorption rate of the metal material. In the case of a general Bragg reflector (DBR) mirror, the reflectance characteristic is very good near the incident angle in the vertical direction, but when it deviates from this angle, the reflectance drops sharply.

In order to obtain a high extraction efficiency improvement effect through the light extracting structure, the reflection mirror at the bottom should maintain a high reflectance not only in the vertical direction but also at an incident angle deviating therefrom.

As shown in FIG. 3, it can be seen that the reflection mirror of the metal layer mirror or the low refractive index layer is advantageous for improving the extraction efficiency by introducing the light extracting structure, as shown in the graph of the incident angle and the reflectance characteristic according to the kind of mirror.

Although the present invention can be applied to a general LED structure irrespective of the light emitting layer material, an embodiment of the red AlGaInP light emitting device will be described in order to limit the scope of the discussion.

The red AlGaInP-based light emitting device can be divided into two types according to the type of the lower reflection mirror. One of them is a structure as shown in FIG. 4, and includes DBRs (Distributed AlAs / AlGaAs (or AlGaInP / AlInP) layers deposited sequentially on a GaAs substrate 10 in quarter- Bragg-Reflectors) mirror structure 20.

A plurality of AlGaInP n-type semiconductor layers 40 and p-type semiconductor layers 50 are located on the upper and lower sides of the AlGaInP light emitting layer 30 on the mirror structure 20.

The other is a structure in which the GaAs substrate is removed and the metal reflection mirror 21 and the Si substrate 11 are bonded to each other as shown in Fig. A plurality of AlGaInP n-type semiconductor layers 41 and p-type semiconductor layers 51 are located on the upper and lower sides of the AlGaInP light emitting layer 31 also on the metal mirror 21.

The DBRs mirror structure 20 has a disadvantage in that the reflection characteristic is lower than that of the metal mirror 21, while having a process advantage that can include a reflection mirror by a single growth process. On the other hand, the joining structure of the metal mirror 21 has an advantage that the excellent reflection characteristic of the metal mirror 21 can be utilized although a joining process is added. Especially, as described above, when introducing the light extracting structure onto the surface, the extraction efficiency characteristics can be greatly improved.

Comparing the locations where the light extracting structure is introduced, the DBRs structure is formed in the p-type doped GaP layer 52 and the metal junction structure is formed in the n-type doped AlGaInP layer 42 . It is advantageous that the depth of the light extracting structure does not extend to the quantum well layer in order to prevent degradation of the internal quantum efficiency. Considering this, the DBRs mirror structure 20 has a depth of up to about 10 mm, and the metal mirror 21 junction structure has a margin of up to about 2 mm.

The degree of improvement of extraction efficiency through photonic crystal is closely related to the structural factor of photonic crystal. In particular, the period of the photonic crystal and etching depth are the most important factors. 3D-FDTD calculations were performed to investigate the efficiency variation of photonic crystal cycle and etching depth.

The calculated structure is a GaN-based light emitting device structure, and a 100% reflective mirror is assumed at the bottom. As shown in FIG. 6, it can be seen that the extraction efficiency is maximized in a specific period. When the maximum period is referred to as the optimum period, the optimum period may vary depending on the refractive index of the semiconductor light emitting layer and the refractive index of the external background material.

In general, the optimal period shifts in the longer direction as the refractive index constituting the photonic crystal becomes smaller, and vice versa as the refractive index contrast increases. Referring to the graph of FIG. 6, it can be seen that the optimum period also correlates with the etch depth. As the etch depth increases, the optimum period tends to move slightly in the longer direction.

As shown in FIG. 7, this phenomenon can be more clearly understood by plotting the tendency of the efficiency change depending on the period and etching depth in the surface graph.

As the etching depth increases, the extraction efficiency of the structure with a period of 1 mm or more steadily increases. When the etching depth reaches 900 nm, the optimum period appears at two points.

There is an optimum period in which the extraction efficiency is maximized, and the phenomenon that the optimum period shifts according to the etching depth is a general result regardless of the semiconductor material. Even in the case of a red AlGaInP-based light emitting device, there is a possibility that the lithography process can utilize a longer-directional cycle easier by increasing the etching depth.

As described above, it has been confirmed that the extraction efficiency of the light emitting device increases with the photonic crystal etching depth, and the optimum period can also be shifted in the long direction. In order to utilize these characteristics, it is advantageous that the thickness of the layer into which the light extracting structure in the light emitting element is introduced is sufficiently large. For example, the vertical GaN-based light emitting device or the red AlGaInP-based light emitting device that etches the n-GaN layer satisfies the above-described characteristics.

Therefore, it is possible to increase the extraction efficiency according to the etching depth and to move the optimum period. However, if the etch depth of the light extracting structure is increased as described above, peeling may sometimes occur between the semiconductor layer and the electrode interface, as shown in Figs. 8A to 8D.

8A is an electron micrograph showing the interface between the light extracting structure 70 and the electrode 60 in which the degree of surface deflection is much smaller than the wavelength of the light emitting layer. In this case, no special separation phenomenon is observed between the light extracting structure 70 and the electrode 60 interface. However, if the etch depth of the light extracting structure is increased, the electrode deposition region can not fill the space between the light extracting structures, resulting in peeling as shown in FIG. 8B.

Looking closely at the cross section, this phenomenon can be more clearly observed. As shown in FIG. 8C, a minute gap is observed between the light extracting structure 70 and the electrode 60 layer. If a current is applied to the light emitting device under such a condition, excessive heat is generated, and the electrode 60 may be peeled or deformed as shown in FIG. 8D, which may adversely affect the electrical characteristics of the device.

That is, increasing the etch depth of the light extracting structure will definitely help increase the optical extraction efficiency, but it can sometimes cause serious downtime problems due to the junction problem between the semiconductor material and the electrode layer.

In consideration of the above-described characteristics, in order to prevent an electrical problem due to peeling of the electrode layer while increasing the etching depth, a step of not introducing a light extracting structure into the electrode layer deposition region is required.

Figs. 9 to 15 show the flow of the process for the metal bonding structure of the AlGaInP-based light emitting device.

To summarize the process, first, as shown in Fig. 9, an AlGaInP-based p-type semiconductor layer, a light emitting layer, and an n-type semiconductor layer are sequentially formed on a GaAs substrate (not shown). As a specific example, a p-GaP layer 310, an AlGaInP light emitting layer 320, an n-AlInP layer 330, and an n-GaAs layer 331 are formed in this order.

In the light emitting device structure thus formed, the substrate is removed, and the Si substrate 100 and the metal mirror 200 are attached. At this time, an etching barrier layer (not shown) for wet etching located on the n-GaAs layer 331 ) May be added). The metal mirror 200 may include Ag or Al.

On the other hand, when the substrate is not removed, a multiple reflection layer (DBR) in which AlAs / AlGaAs or AlGaInP / AlInP semiconductor layers alternately are formed on a GaAs substrate can be formed.

The GaAs substrate or the Si substrate 100 described above may be all conductive and advantageously have p-type conductivity.

10, an etching mask layer 400 is formed on the n-GaAs layer 331 through a photolithography process and a photoresist PR is formed on the mask layer 400. Then, A first mask 500 defining the light extracting structure is formed of the same material, and the first mask 500 is used to etch the mask layer 400.

At this time, the mask layer 400 may be formed of an oxide such as SiO ₂ or SiN _x or a nitride-based material.

11, the mask layer 400 is formed of a second mask 410 having a pattern in which a light extracting structure is defined, wherein the first mask 500 is removed.

Thereafter, as shown in FIG. 12, an electrode mask 510 having the same or larger size as that of the n-type electrode to be formed later is formed on the second mask 410. This electrode mask 510 can protect the electrode formation region.

Next, as shown in FIG. 13, the n-GaAs layer 331 and the n-AlInP layer 330 are etched using the second mask 410 to form the light extracting structure 700.

Then, as shown in FIG. 14, the second mask 410 is removed, and the n-type electrode 600 is formed in the region secured by the electrode mask 510.

Next, the n-GaAs layer 331 can be removed, and a vertical light emitting device structure as shown in FIG. 15 is formed.

16, a light extracting structure 700 is formed in a portion except for the region where the n-type electrode 600 is formed. Since the light can not escape through the region of the electrode 600, the light extracting structure at the lower end of the region of the electrode 600 is practically meaningless.

Therefore, this selective region etch light extraction structure can solve the problem of junction between the semiconductor region and the electrode interface at the same time without affecting the light extraction efficiency at all.

Meanwhile, the light extracting structure 700 may include a general lattice (Triangular Lattice) having a periodicity and a quasicrystal (Pseudorandom, random structure) having no periodicity. have. At this time, the period of the light extracting structure 700 may have a value of the emission wavelength (?) To 5000 nm.

The extraction efficiency of the light extracting structure 700 increases according to the etch depth. For the increase of the etch depth, a structure having a period smaller than a specific value (emission wavelength: ~?) Has a specific etch depth (? The refractive index of the semiconductor layer tends to saturate at or above a certain level, whereas the structure of a period larger than the specific value (?) Shows a tendency of steadily increasing efficiency even at a specific etching depth (~? / N) or more. Thus, the optimal period for a particular etch depth appears in two cycles with different values. That is, in the light extracting structure 700, the optimum period moves in the long period direction according to the etching depth.

Considering such a phenomenon, the depth of the light extracting structure 700 can reach the maximum thickness of the semiconductor layer 330 in which the light extracting structure 700 is formed from? / N to 500 nm to 2 占 퐉 .

As described above, by increasing the etch depth of the light extracting structure 700, the light extracting efficiency can be increased and the optimum period can be extended, and at the same time, the peeling phenomenon between the semiconductor layer into which the light extracting structure is introduced and the electrode interface can be solved have. When the optimum period of the light extracting structure 700 is extended as described above, the light extracting structure 700 can be easily formed by a conventional method such as photolithography.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, It is natural to belong to the scope.

Figs. 1A and 1B are schematic views showing the principle of photonic crystal.

2 is a graph showing the extraction efficiency according to the refractive index.

3 is a graph showing an incident angle and reflectance along the middle of the reflection mirror.

4 is a cross-sectional view showing an example of a red light emitting element.

5 is a cross-sectional view showing another example of the red light emitting device.

6 is a graph showing the extraction efficiency according to the photocrystalline etching depth.

FIG. 7 is a graph showing the result of FIG. 6 as a two-dimensional surface graph.

8A to 8D are photographs showing the shapes of the electrodes formed on the light extracting structure.

9 to 15 are cross-sectional views showing one embodiment of a step of manufacturing a red light emitting device.

16 is a photograph showing a light emitting surface of a red light emitting element.

Claims (13)

In a method of manufacturing a red light emitting device, Forming a multi-layered semiconductor layer on a substrate; Forming a mask defining a light extracting structure pattern on the semiconductor layer; Forming an electrode mask defining a protection region having an electrode shape on the mask; Forming a light extraction structure at a specific etching depth (? / N) or more using the mask; Removing the mask and forming an electrode on the protection region, The light extracting structure has two different periods with a peak value of light extraction efficiency according to a pattern period of the light extracting structure. The pattern of the light extracting structure is formed in a period within a period having a large value among the two different periods Wherein the light-emitting layer is formed on the substrate. Here, lambda is an emission wavelength of the red light emitting device and n is the refractive index of the semiconductor layer. The semiconductor device according to claim 1, wherein the multi-layered semiconductor layer includes a double n-type semiconductor layer, Wherein the protective region is formed on the double n-type semiconductor layer. The method of claim 1, wherein the depth of the light extracting structure is 500 nm to 2 占 퐉. 2. The method of claim 1, further comprising the step of removing the substrate and forming a reflective metal on a surface of the substrate on which the substrate is removed. The method according to claim 1, further comprising forming a multiple reflection layer (DBR) on the substrate, wherein AlAs / AlGaAs or AlGaInP / AlInP semiconductor layers are alternately arranged. The method for manufacturing a red light emitting device according to claim 1, wherein the semiconductor layer is an AlGaInP-based semiconductor layer. A reflective layer disposed on the conductive substrate; A multi-layered semiconductor layer located on the reflective layer; A light extracting structure formed on the semiconductor layer, the light extracting structure being formed at a portion other than a protective region where electrodes are to be formed, at a specific etch depth (? / N); And an electrode positioned on the protection region, The light extracting structure has two different periods with a peak value of light extraction efficiency according to a pattern period of the light extracting structure. The pattern of the light extracting structure is formed in a period within a period having a large value among the two different periods Wherein the red light emitting element is a red light emitting element. Here, lambda is an emission wavelength of the red light emitting device and n is the refractive index of the semiconductor layer. 8. The red light emitting device according to claim 7, wherein the reflective layer is a reflective metal or a multiple reflective layer. 9. The red light emitting device according to claim 8, wherein the multiple reflection layer is formed of AlAs / AlGaAs or AlGaInP / AlInP semiconducting layers alternately. 8. The red light emitting device according to claim 7, wherein the protection region is formed on the double n-type semiconductor layer included in the semiconductor layer of the multi-layer structure. 8. The red light emitting device according to claim 7, wherein the substrate is a GaAs substrate or a Si substrate. The red light emitting device according to claim 7, wherein a depth of the light extracting structure is 500 nm to 2 占 퐉. 8. The red light emitting device according to claim 7, wherein the period of the light extracting structure is an emission wavelength of the semiconductor layer to 5000 nm.

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