KR20090074359A - Red light emitting device and method of making the same - Google Patents

Abstract

A red light-emitting device and a manufacture method thereof are provided to introduce a light-extracting structure to a light-emitting element in order to improve light-extracting efficiency as well as reliability. A multilayered semiconductor layer is formed on a substrate. A mask is formed on the semiconductor layer to define a light-extracting structure pattern. An electrode mask is formed on the mask to define an electrode-shaped protective region. A light-extracting structure(700) is formed by using the mask. The mask is removed to form an electrode(600) on the protective region. The area of the protective region is the same as or greater than the area of the electrode.

Description

Red light emitting device and method of making the same

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

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, along with GaP: N series green LEDs. It has been used as a light source for display images of electronic devices, including.

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

For example, in the case of red LEDs, considering the refractive index relationship between the GaP layer located at the top and the epoxy introduced to protect the surface, the critical angle is about 25 °, with no absorption loss in the material, resulting in multiple reflections. Without reflection and without a mirror such as the Bragg reflection layer (DBR) in 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 total reflection and absorbed by the substrate GaAs through the lower layer, or the light intensity is gradually decreased by a medium that can cause absorption loss such as quantum wells. It disappears without being included in the light extraction efficiency.

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

SUMMARY OF THE INVENTION The present invention provides a red light emitting device including a light extraction structure capable of improving light extraction efficiency, and a peeling phenomenon does not occur during electrode formation by the light extraction structure, and a method of manufacturing the same. have.

As a first aspect for achieving the above technical problem, the present invention provides a method of manufacturing a red light emitting device, comprising the steps of: forming a semiconductor layer of a multi-layer structure on a substrate; Forming a mask defining a light extraction structure pattern on the semiconductor layer; Forming an electrode mask on the mask defining a protective region having an electrode shape; Forming a light extraction structure using the mask; Removing the mask and forming an electrode on the protective region.

As a second aspect for achieving the above technical problem, the present invention, the reflective layer located on the conductive substrate; A semiconductor layer having a multilayer structure positioned on the reflective layer; A light extracting structure formed on the semiconductor layer and formed in a portion excluding a protection region in which an electrode is to be formed; And an electrode located on the protection region.

The present invention can improve the light extraction efficiency by introducing a light extraction structure to the light emitting device. In addition, the light extraction structure does not affect the light extraction efficiency at all, and at the same time solves the bonding problem between the semiconductor region and the electrode interface.

Therefore, by increasing the etching depth of the light extraction structure, the light extraction efficiency can be increased and the optimal period can be extended, and the peeling phenomenon between the semiconductor layer and the electrode interface into which the light extraction structure is introduced can be solved. As such, when the optimal period of the light extraction structure is extended, there is an effect that the light extraction structure can be easily formed by a conventional method such as photolithography.

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

While the invention allows for various modifications and variations, specific embodiments thereof are illustrated by way of example in the drawings and will be described in detail below. However, it is not intended to be exhaustive or to limit the invention to the precise forms disclosed, but rather the invention includes all modifications, equivalents, and alternatives consistent with the spirit of the invention as defined by the claims.

When an element such as a layer, region or substrate is referred to as being on another component "on", it will be understood that it may be directly on another element or there may be an intermediate element in between. . If a part of a component, such as a surface, is expressed as 'inner', it will be understood that this means that it is farther from the outside of the device than other parts of the element.

Furthermore, relative terms such as "beneath" or "overlies" are used herein to refer to one layer or region relative to one layer or region and another layer or region with respect to the substrate or reference layer, as shown in the figures. Can be used to describe the relationship.

It will be understood that these terms are intended to include other directions of the device in addition to the direction depicted in the figures. Finally, the term 'directly' means that there is no element in between. As used herein, the term 'and / or' includes any and all combinations of one or more of the recorded related items.

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 / or regions It will be understood that it should not be limited by these terms.

The light extraction efficiency of the semiconductor light emitting device is determined by the difference in refractive index between the semiconductor light emitting layer in which light is generated and the medium (air or epoxy) in which light is finally observed. 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%.

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

When considering the emission ratio of the light to the incident angle, since the light corresponding to the total reflection angle occupies approximately 80% of the total amount of light, the light extraction improvement effect is virtually impossible without extracting light belonging to this region. Accordingly, in order to improve the extraction efficiency of the light emitting device, it is necessary to conclude that a structure capable of extracting light corresponding to the total reflection angle is necessary. At this time, one of those that can perform this role is a photonic crystal.

From the mechanical point of view, the phenomenon of light transmission is the movement of light through materials with different refractive indices. As with the movement of objects in mechanics, the law of conservation of momentum is always followed by the movement of light (where the momentum of light is expressed as a wave vector (k = 2πn / λ)).

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

Total reflection always occurs when light moves from a high refractive index medium to a low one. Since light in a medium with high refractive index already has a great momentum, it has a momentum (planar component) that cannot be taken at any angle in the medium with a low refractive index above a certain angle of incidence. In order to transmit light, the momentum of planar components must be preserved, so the only path that light can select is the reflection process. Here, the minimum incident angle that cannot preserve the momentum of the planar component corresponds to the critical angle.

Photonic crystals help to extract externally by adding or subtracting the momentum component generated by the periodicity (periodicity) for the light of the total reflection angle that cannot preserve the momentum (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 amount of momentum varies depending on the period of the photonic crystal, and thus the diffraction efficiency of light corresponding to the total reflection angle changes.

In addition to the period of the photonic crystal, the extraction efficiency of light by the photonic crystal has a close correlation with structural variables such as the etching depth, the filling factor, and the lattice structure. Therefore, in order to obtain a high extraction efficiency improvement effect in the semiconductor light emitting device, it is very important to devise and apply the optimum photonic crystal structure.

In addition to the internal structural parameters related to the photonic crystal, the degree of extraction efficiency improvement due to 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 a semiconductor material, the larger the critical angle, and the greater the amount of light trapped inside the device by total reflection. FIG. 2 is a graph showing extraction efficiency according to the refractive index of the semiconductor light emitting region when the external background material is assumed to be air.

At this time, the light emitting surface is a planar structure in which no light extraction structure is introduced. Looking at the graph, it can be seen that the refractive index and the extraction efficiency of the light emitting layer are in inverse relationship. This means that the relative extraction efficiency increase ratio can be increased when the light extraction structure is introduced on the surface of the device. In addition, since the diffraction effect by the light extraction structure is proportional to the refractive index contrast, the introduction of the light extraction structure on the element surface of the high refractive index layer can obtain a larger extraction effect.

In sum, the relative extraction efficiency of the light emitting device can be improved by using the light extraction structure in the gallium arsenide (GaAs) and gallium phosphorus (GaP) semiconductors having a higher refractive index than the organic material or gallium nitride (GaN). It can be expected to be more pronounced.

The degree of extraction efficiency improvement due to the light extraction structure is closely related to the reflection mirror characteristics of the bottom of the device. The light extracting structure has been described above that even light having an incident angle corresponding to total reflection can be extracted to the outside through a combination process with the light extracting structure. However, since the diffraction efficiency cannot be 100%, part of the light is always reflected again and enters the device. Therefore, the degree of extraction efficiency improvement depends on the reflection mirror property at the bottom.

The reflecting mirror at the bottom can be divided into three types. The method of introducing a refractive index layer lower than the refractive index of the light emitting layer (side type GaN light emitting device), the method of introducing a metal layer (vertical GaN LED, metal-bonding red LED), and the method of increasing the reflectance by combination of dielectrics having different refractive indices (red DBRs LEDs).

Each reflecting mirror has its own characteristics depending on the angle of incidence. Reflective mirrors with a low index of refraction layer exhibit low reflectance for the angle of incidence in the vertical direction, but exhibit an ideal reflectance of 100% above the critical angle by total reflection.

The metal layer reflecting mirror shows an overall good reflectance for all angles of incidence, but due to the high rate of absorption of the metallic material it is inaccessible to reflectance of 100%. In the case of a general Bragg reflector (DBR) mirror, it exhibits a very good reflectance characteristic near the incidence angle in the vertical direction, but when it is out of this, the reflectance drops sharply.

In order to achieve a high extraction efficiency through the light extraction structure, the lower reflecting mirror must maintain a high reflectance not only in the vertical direction but also at an incidence angle beyond the vertical direction.

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

The contents to be developed through the present patent may be applied to a general LED structure regardless of the light emitting layer material, but in order to limit the scope of discussion, embodiments of a red AlGaInP series light emitting device will be described.

The red AlGaInP series light emitting device can be divided into two types according to the type of the bottom reflecting mirror. One of them is a structure as shown in FIG. 4, in which an AlAs / AlGaAs (or AlGaInP / AlInP) layer is sequentially deposited on a GaAs substrate 10 in a quarter-wave length thickness. Bragg-Reflectors) mirror structure 20.

On the mirror structure 20, a plurality of AlGaInP series n-type semiconductor layers 40 and p-type semiconductor layers 50 are positioned above and below the AlGaInP emission layer 30.

The other is a structure in which the GaAs substrate is removed and the metal reflecting mirror 21 and the Si substrate 11 are bonded to each other, as shown in FIG. 5. On the metal mirror 21, a plurality of AlGaInP series n-type semiconductor layers 41 and p-type semiconductor layers 51 are positioned above and below the AlGaInP light emitting layer 31.

While the DBRs mirror structure 20 has a process advantage of including a reflective mirror in one growth process, the reflectance characteristic is inferior to that of the metal mirror 21. On the other hand, the bonding structure of the metal mirror 21 is added, but the advantage of the excellent reflective properties of the metal mirror 21 can be used. In particular, when introducing the light extraction structure to the surface as described above, it is possible to greatly improve the extraction efficiency characteristics.

Comparing the position where the light extraction 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. . In order to prevent the degradation of the internal quantum efficiency, it is advantageous that the depth of the light extraction structure does not extend to the quantum well layer. Considering this, the DBRs mirror structure 20 has a depth of up to approximately 10 mm, and the metal mirror 21 bonding structure has a margin of up to about 2 mm.

The degree of extraction efficiency improvement through the photonic crystal is closely related to the structural factors of the photonic crystal. In particular, the period and etching depth of the photonic crystal are the most important factors. 3D-FDTD calculation was performed to investigate the efficiency change according to the period and etching depth of the photonic crystal.

The calculated structure is a GaN-based light emitting device structure, and 100% reflective mirror is assumed at the bottom. Looking at the results, as shown in Figure 6, it can be seen that the extraction efficiency is maximized in a particular period. When the maximum period is referred to as an 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 optimum period moves in the longer direction as the refractive index constituting the photonic crystal is smaller, and in the shorter direction as the refractive index contrast is larger. On the other hand, looking at the graph of Figure 6, it can be seen that the optimum period is also correlated with the etching depth. As the etching depth increases, the optimum period tends to move little by little in the long direction.

As illustrated in FIG. 7, this phenomenon can be more clearly understood when the trend of efficiency change according to a period and an etching depth is represented by a surface graph.

From the surface graph results, it can be seen that as the etching depth increases, the extraction efficiency of the structure having a period of 1 mm or more increases steadily, and when the etching depth reaches 900 nm, the optimum period appears at two points.

There is an optimal period in which the extraction efficiency is maximum, and the shift of the optimum period according to the etching depth is a general result regardless of the semiconductor material. In the case of the red AlGaInP series light emitting device, the etching depth is also increased, and thus there is a possibility of using a long-direction period in which a lithography process is easier.

As described above, it was confirmed that the extraction efficiency of the light emitting device increases with the depth of photonic crystal etching, and the optimum period can also move in the long direction. In order to utilize these characteristics, it is basically advantageous if the thickness of the layer into which the light extraction structure is introduced in the light emitting device is sufficiently large. For example, the vertical GaN-based light emitting device or the red AlGaInP-based light emitting device etching 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 etching depth of the light extraction structure is increased in this manner, a peeling phenomenon may sometimes occur between the semiconductor layer and the electrode interface, as shown in FIGS. 8A to 8D.

8A is an electron micrograph showing an interface between the light extraction structure 70 and the interface of the electrode 60 where the degree of surface curvature is much smaller than the wavelength of the light emitting layer. In this case, no special peeling phenomenon between the light extraction structure 70 and the electrode 60 interface is observed. However, when the etching depth of the light extraction structure is increased, peeling may occur as shown in FIG. 8B because the electrode deposition region does not fill between the light extraction structures.

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

In other words, the increase in the etching depth of the light extraction structure is a clear help to increase the optical extraction efficiency, but can sometimes cause serious power failure problem depending on the bonding problem between the semiconductor material and the electrode layer.

In view of the above-described characteristics, in order to increase the etching depth and prevent the electrical problem due to the electrode layer peeling, a process of not introducing a light extraction structure into the electrode layer deposition region is required.

9 to 15 show the flow of the process for the metal junction structure of the AlGaInP series light emitting device.

In summary, 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, the p-GaP layer 310, the AlGaInP light emitting layer 320, the n-AlInP layer 330, and the n-GaAs layer 331 are sequentially formed.

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 etch barrier layer for wet etching located on the n-GaAs layer 331 (not shown). ) May be added). The metal mirror 200 may include Ag or Al.

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

The GaAs substrate or the Si substrate 100 described above can all be conductive, and it is advantageous to have a p-type conductivity.

Afterwards, as shown in FIG. 10, an etch mask layer 400 is formed on the n-GaAs layer 331 through photolithography, and the photoresist PR is formed on the mask layer 400. A first mask 500 defining a light extraction structure is formed of the same material, and the mask layer 400 is etched using the first mask 500.

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

By the etching process, as shown in FIG. 11, the mask layer 400 is formed of a second mask 410 having a pattern defining a light extraction structure, and the first mask 500 is removed.

Thereafter, as shown in FIG. 12, the electrode mask 510 having the same or larger size as the shape of the n-type electrode to be formed later is formed on the second mask 410. The electrode mask 510 may 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 extraction structure 700.

Thereafter, 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 may be removed, and a vertical light emitting device structure as shown in FIG. 15 is formed.

By the above-described process, as shown in FIG. 16, the light extraction structure 700 is formed in a portion except for the region in which the n-type electrode 600 is formed. Since light cannot escape through the electrode 600 region, the light extraction structure at the bottom of the electrode 600 region is virtually meaningless.

Accordingly, the selective region etching light extraction structure can solve the problem of bonding between the semiconductor region and the electrode interface without affecting the light extraction efficiency at all.

Meanwhile, the light extraction structure 700 described above may include both general light crystal structures having a periodicity (Square Lattice, Triangular Lattice, Archimedean Lattice) and light crystal structures having no periodicity (Quasicrystal, Pseudorandom, random structure). have. In this case, the period of the light extraction structure 700 may have a value of the emission wavelength λ to 5000 nm.

The light extraction structure 700 increases the extraction efficiency according to the etching depth, and for the increase in the etching depth, a structure having a period smaller than a specific value (emission wavelength; λ) has a specific etching depth (˜λ / n; n While the efficiency tends to saturate at or above the refractive index of the semiconductor layer, a structure having a period greater than a specific value (λ) tends to increase steadily even at a specific etching depth (˜λ / n). Thus, for an etch depth, the optimal period appears in two cycles with different values. That is, the light extraction structure 700 moves in the long period direction of the optimum period according to the etching depth.

In consideration of such a phenomenon, the depth of the light extraction structure 700 may range from λ / n to the maximum thickness of the semiconductor layer 330 on which the light extraction structure 700 is formed, which is 500 nm to 2 μm. Can be.

As described above, by increasing the etching depth of the light extraction structure 700, the light extraction efficiency can be increased and the optimal period can be extended, and the peeling phenomenon between the semiconductor layer and the electrode interface into which the light extraction structure is introduced can be solved. have. As such, when the optimum period of the light extraction structure 700 is extended, the light extraction structure 700 may be easily formed by a conventional method such as photolithography.

The above embodiment is an example for explaining the technical idea of the present invention in detail, and the present invention is not limited to the above embodiment, various modifications are possible, and various embodiments of the technical idea are all protected by the present invention. It belongs to the scope.

1A and 1B are schematic diagrams showing the principle of photonic crystals.

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

3 is a graph showing an incident angle and a reflectance according to the midstream of the reflecting 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 a red light emitting element.

6 is a graph showing extraction efficiency according to photonic crystal 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 shape of the electrode formed on the light extraction structure.

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

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

Claims (13)

In the method of manufacturing a red light emitting device, Forming a semiconductor layer of a multilayer structure on the substrate; Forming a mask defining a light extraction structure pattern on the semiconductor layer; Forming an electrode mask on the mask defining a protective region having an electrode shape; Forming a light extraction structure using the mask; And removing the mask and forming an electrode on the protective region. The method of manufacturing a red light emitting device according to claim 1, wherein an area of said protective region is equal to or larger than said electrode. The method of manufacturing a red light emitting device according to claim 1, wherein the light extraction structure has a depth of 500 nm to 2 m. 2. The method of claim 1, further comprising removing the substrate and forming a reflective metal on the silicon substrate on the side from which the substrate is removed. The method of claim 1, further comprising forming a multi-reflective layer (DBR) on which the AlAs / AlGaAs or AlGaInP / AlInP semiconductor layers are alternately positioned. The method of manufacturing a red light emitting device according to claim 1, wherein the semiconductor layer is an AlGaInP series semiconductor layer. A reflective layer located on the conductive substrate; A semiconductor layer having a multilayer structure positioned on the reflective layer; A light extracting structure formed on the semiconductor layer and formed in a portion excluding a protection region in which an electrode is to be formed; And a electrode positioned on the protection region. The red light emitting device of claim 7, wherein the reflective layer is a reflective metal or a multi-reflective layer. 10. The red light emitting device of claim 8, wherein the multi-reflective layer has alternating AlAs / AlGaAs or AlGaInP / AlInP semiconductor layers. The red light emitting device of claim 8, wherein the reflective metal comprises Ag. 8. The red light emitting device of claim 7, wherein the substrate is a GaAs substrate or a Si substrate. The red light emitting device according to claim 7, wherein the light extraction structure has a depth of 500 nm to 2 m. 8. The red light emitting device of claim 7, wherein a period of the light extraction structure is from a light emission wavelength of the semiconductor layer to 5000 nm.

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