KR100878979B1 - Light Emitting Diode of having Photonic Crystal Structure - Google Patents

Abstract

A light emitting diode having a photonic crystal structure is disclosed. In the light emitting diode using gallium nitride, a photonic crystal structure is provided on the substrate. Since the photonic crystal structure is formed between the substrate and the gallium nitride layer, there is no etching process of the p-type thin nitride film, so that the formation of an easy p-type ohmic electrode is realized. In addition, as the gallium nitride layer is grown on the curved sapphire, it is possible to promote horizontal growth and minimize defects in the thin film to prevent a decrease in light efficiency.

Description

Light Emitting Diode of having Photonic Crystal Structure

1A to 1D are cross-sectional views illustrating the formation of a light emitting diode having a photonic crystal structure according to a preferred embodiment of the present invention.

2 is a plan view illustrating a photonic crystal structure of a light emitting diode according to a preferred embodiment of the present invention.

3 is a graph showing a photonic bandgap when a substrate made of sapphire forms holes in a rectangular array with a depth of 0.1 μm.

4 is a full wave simulation result for confirming whether light extraction efficiency is increased by a light emitting diode according to an exemplary embodiment of the present invention.

Explanation of symbols on the main parts of the drawings

100 substrate 140 photonic crystal pattern

160: buffer layer 180: undoped semiconductor layer

200: lower contact layer 210: n-type cladding layer

220: active layer 230: p-type cladding layer

240: upper contact layer

The present invention relates to a light emitting diode, and more particularly to a light emitting diode having a photonic crystal structure.

In general light emitting diodes, the internal quantum efficiency is almost 100% (I. Schnitzer et al., Appl. Phys. Lett. 78, 3379), but the external quantum efficiency out of the actual device is 3% to 30% or less (MG Craford, Semiconductor and semimetals, 64 (Academic press, 2000)). This is due to the total reflection caused by the difference in refractive index at the interface between the device and the air when the light produced in the multi-quantum well structure comes out of the device. Due to this physical phenomenon, the light extraction efficiency of the conventional light emitting diode is very low.

In order to overcome this problem, the surface roughness is formed on the p-type gallium nitride through a non-selective etching process without a mask so that when the light traveling upward reaches the rough surface, the incident angle is smaller than the critical angle so that the light is easily directed to the outside of the light emitting diode. There is an example of increasing the light extraction efficiency by allowing it to exit. In addition, p-type gallium nitride was dry-etched using a mask to form protrusions, and when the protrusions proceeded upward, the incident angle was smaller than the critical angle so that light could easily escape to the outside of the light emitting diode, thereby increasing light extraction efficiency. There is an example.

In addition, US Pat. No. 6,842,403 also provides a photonic bandgap photonic crystal by changing the refractive index of the p-type gallium nitride surface of the light emitting diode to change the mode of light trapped inside the gallium nitride compound. By allowing the light to escape from the outside without being trapped inside, it improves the light extraction efficiency of the light and controls the light from the light emitting diode side. But the paper Appl. Phys. According to Lett 87, 203508 (2005), the structure is required to etch the p-type gallium nitride thin film, so that the area for forming the p-type ohmic layer is reduced, and the series resistance is increased.

In addition, X. A. Cao reported a problem that p-type gallium nitride thin film is converted to n-type by plasma damage when dry etching the p-type thin film (Appl. Phys. 75, 2569 (1999)). This may cause problems in forming the ohmic electrode of the semiconductor light emitting diode and injecting the current uniformly, and may reduce the luminous efficiency of the device. On the other hand, since the structures are formed only on the p-type gallium nitride, the space for extracting light trapped in the side is limited only to the thickness of the p-type gallium nitride thin film, so that the increase in the actual light extraction efficiency is not significant.

 Japanese Laid-Open Patent Publication No. 2006-165583 increases the light extraction efficiency by forming an uneven structure between the substrate and the gallium nitride layer. In addition, Japanese Laid-Open Patent Publication No. 2004-153090 improves light extraction efficiency by forming irregularities on a sapphire substrate and inserting a layer having a refractive index similar to gallium nitride or gallium nitride thereon. However, the above patents can increase the light extraction efficiency of light by suppressing internal reflection by using a simple uneven structure, there is a problem that can not overcome the low light efficiency still can not extract the light emitted to the side.

An object of the present invention for solving the above problems is to provide a light emitting diode having a photonic crystal structure, p-type gallium nitride thin film is not converted to n-type and can easily form an ohmic electrode to increase the light extraction efficiency have.

The present invention for achieving the above object, a substrate; An n-type lower contact layer formed on the substrate; An active layer formed on the lower contact layer; And an upper contact layer formed on the active layer, and a photonic crystal pattern having a regular pattern formed thereon is provided on the substrate.

The object of the present invention, the substrate; A photonic crystal pattern having a regular pattern formed on the substrate; An undoped semiconductor layer filling a hole and an upper portion of the photonic crystal pattern; An n-type lower contact layer formed on the undoped semiconductor layer; An n-type clad layer formed on the lower contact layer; An active layer formed on the n-type cladding layer; A p-type cladding layer formed on the active layer; And a p-type upper contact layer formed on the p-type cladding layer, wherein the regularly arranged circular holes of the photonic crystal pattern have a rectangular or hexagonal arrangement. Is it achieved through the provision of?

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

Example

1A to 1D are cross-sectional views illustrating the formation of a light emitting diode having a photonic crystal structure according to a preferred embodiment of the present invention.

Referring to FIG. 1A, a mask pattern 120 is formed on a substrate 100. The substrate 100 is composed of gallium nitride, SiC, GaAs, Si, ZnO or sapphire. In addition, the mask pattern 120 preferably uses chromium or photoresist.

First, the entire surface of the substrate 100 is coated with chromium, and then a photoresist is coated on the chromium. Subsequently, a photoresist pattern is formed using a conventional photolithography process, and a mask pattern 120 formed of chromium is formed using the formed photoresist pattern as an etching mask. The mask pattern 120 may be formed through wet etching. Nitric acid or the like may be used as the etching solution during wet etching. In addition, wet etching is performed until a portion of the surface of the substrate 100 is exposed.

Subsequently, when the mask pattern 120 is formed, the photoresist pattern remaining on the mask pattern 120 is removed.

In addition to the formation of the mask pattern 120 through the above-described process, the mask pattern 120 may be formed through another method. That is, the photoresist may be applied onto the substrate 100, and the mask pattern 120 may be formed through a conventional photolithography process.

Referring to FIG. 1B, the photonic crystal pattern 140 is formed using the mask pattern 120 as an etching mask. The photonic crystal pattern 140 is preferably formed through anisotropic dry etching. In addition, the etchant during dry etching is BCl ₃ Or Cl ₂ . When the photonic crystal pattern 140 is formed, the mask pattern 120 existing thereon is removed.

Referring to FIG. 1C, a buffer layer 160 is formed in spaced spaces between upper and lower portions of the protruding photonic crystal pattern 140. In addition, an undoped semiconductor layer 180 is formed on the buffer layer 160. For example, when the substrate 100 is sapphire, the buffer layer 160 is formed of gallium nitride. The buffer layer 160 formed of gallium nitride serves as a crystalline seed through which the undoped semiconductor layer 180 may epitaxially grow. Therefore, the undoped semiconductor layer 180 is provided on the buffer layer 160. If the buffer layer 160 has gallium nitride, the undoped semiconductor layer 180 is made of gallium nitride.

The lower contact layer 200 is provided on the undoped semiconductor layer 180. The lower contact layer 200 is configured of an n type. In addition, when the undoped semiconductor layer 180 is composed of gallium nitride, the lower contact layer 200 has a configuration including gallium nitride.

In addition, the buffer layer 160 and the undoped semiconductor layer 180 may be omitted. That is, the lower contact layer 200 may be formed directly on the photonic crystal pattern 140.

In addition, when the buffer layer 160 and the undoped semiconductor layer 180 include gallium nitride, these are Ga _1-xy In _x Al _y N (0 ≦ x, y ≦ 1, x + y <1). It is preferably formed on the substrate 100, the lower contact layer 200 is preferably made of n-type Al _x Ga _y In _Z N (0 ≤ x, y, z ≤ 1).

Referring to FIG. 1D, an n-type cladding layer 210, an active layer 220, a p-type cladding layer 230, and an upper contact layer 240 are sequentially formed on the formed lower contact layer 200. In addition, an n-type electrode 250 is formed on the lower contact layer 200, and a p-type electrode 260 is formed on the upper contact layer 240.

In addition, the n-type cladding layer 210 and the p-type cladding layer 230 may be omitted.

The n-type cladding layer 210 has Al-gallium nitride, and the active layer 220 has an Al _x Ga _y In _Z N (0 ≤ x, y, z ≤ 1) barrier layer and Al _x Ga _y In _Z It is preferable that a single or multi-quantum well structure of the well layer consisting of N (0 ≦ x, y, z ≦ 1) is made. In addition, the p-type cladding layer 230 has Al-gallium nitride, and the n-type cladding layer 210 and the p-type cladding layer 230 have gallium nitride and Al _x Ga _{1 - x} N (0 ≦ _x ≦ 1). It may be formed of a superlattice layer consisting of).

Photonic crystals refer to a structure of a lattice composed of two or more dielectrics that are spatially repeated at half-wavelength intervals of emitted light. Typically, when light propagates through a space having a half wavelength separation distance, strong interference occurs, and the characteristics of the wave are changed. That is, when light passes through the photonic crystal, the photonic crystal is recognized as a new optical medium. Therefore, in the case of using the photonic crystal, artificial manipulation and control of the light is possible, and in the optical device, light traveling in the horizontal direction can be controlled in the vertical direction. Therefore, the light efficiency of the light emitting diode which is an optical element is maximized.

Meanwhile, in the light emitting diodes illustrated in FIGS. 1A to 1D, an indium gallium nitride layer is used as a well layer, and gallium nitride is used as a barrier layer as a multi-quantum well structure for emitting green, blue, or ultraviolet light. This is because the bandgap of indium gallium nitride can change from 0.7 eV (infrared) to 3.4 eV (ultraviolet) through visible light by the amount of doping of indium.

2 is a plan view illustrating a photonic crystal pattern of a light emitting diode according to a preferred embodiment of the present invention.

Referring to FIG. 2, the photonic crystal pattern includes a circular hole 300 and a peripheral part 320 surrounding the hole. In FIG. 1D, the holes correspond to portions etched in the patterned structure on the sapphire substrate 100, and the periphery portions correspond to portions not etched on the surface of the substrate 100.

The circular hole 300 is filled with the undoped semiconductor layer 180 or with the lower contact layer 200. That is, the photonic crystal is formed by the material filling the hole 300 and the substrate remaining by etching.

In addition, the photonic crystal pattern formed on the substrate 100 may be a rectangular array or a hexagonal array. Depending on the photonic crystal pattern, the photonic bandgap of the formed photonic crystal is changed, and the distance a between adjacent holes and the radius r of the hole are also changed.

3 is a graph showing a photonic bandgap when a substrate made of sapphire forms holes in a rectangular array with a depth of 0.1 μm.

Referring to FIG. 3, the periphery of the patterned sapphire substrate is embedded with a gallium nitride layer. At this time, the refractive index of sapphire is 1.7, and the refractive index of a gallium nitride layer is 2.4. In FIG. 3, Frequency represents a distance a between the patterns / emission wavelength λ, and Ratio represents a distance a between the radius r of the pattern and the pattern.

If the ratio is 0.3 and the wavelength is 460 nm (blue emission), then the Freqency region is 0.615 ~ 0.635, and the preferred period and the radius of the hole obtained through the calculation are 287 ± 5 and 86 ± 2, respectively. In this way, the photonic crystal can extract the light extraction efficiency by vertically controlling the light trapped horizontally when a specific period and the radius of the hole is implemented for a specific wavelength can increase the light efficiency.

4 is a full wave simulation result for confirming whether light extraction efficiency is increased by a light emitting diode according to an exemplary embodiment of the present invention.

Referring to FIG. 4, a hole formed on the sapphire substrate has a depth of 0.1 μm, and holes of a cubic array are formed, and a distance between the holes is 290 nm and a radius is 87 nm. As a simulation method, a finite difference time domain calculation tool using a genetic algorithm was used. Thus, the result of the increase in the light extraction efficiency of the photonic crystals with and without photonic crystals can be predicted.

In FIG. 4, the light emitting diode according to the present invention shows light in which light is doped in the vertical and horizontal components, respectively, into the air inside the light emitting diode. It can be seen that the more the intensity of light is inclined toward the red, and the less the intensity of the light is toward the blue. White is also the strongest part of light. As shown in FIG. 4, in the case of the structure in which the photonic crystal is formed on the sapphire substrate, it can be seen that the amount of light guided by the vertical component (white region of the vertical component) is greatly increased.

This is because the photonic crystal converts light trapped laterally vertically. This value was measured quantitatively in the graph below. In FIG. 4, green represents the intensity of light guided downward, and blue represents the intensity of light guided upward. Therefore, when measured quantitatively, it can be confirmed through the graph that the intensity of light guided downward is increased by three times or more compared with the structure (b) without the photonic crystal. Therefore, the light extraction efficiency is greatly increased by using the photonic crystal.

According to the present invention as described above, since the active layer or the p-type gallium nitride thin film of the single or multiple quantum well structure of the photonic crystal structure is not damaged by dry etching, it is possible to solve the problem of deterioration of the electrical and optical characteristics of the conventional light emitting diode. It is possible to implement a high efficiency and high output light emitting device. That is, in the conventional case, when the photonic crystal structure is formed on the lower contact layer, the p-type gallium nitride layer is damaged by etching, or the conductivity type is changed to n-type, but as disclosed in the present invention, This problem can be solved when forming the photonic crystal structure directly on the substrate.

Although described above with reference to a preferred embodiment of the present invention, those skilled in the art will be variously modified and changed within the scope of the invention without departing from the spirit and scope of the invention described in the claims below I can understand that you can.

Claims (8)

Board; An n-type lower contact layer formed on the substrate; An active layer formed on the lower contact layer; And An upper contact layer formed on the active layer, A photonic crystal pattern having a regular pattern is provided on the substrate, The photonic crystal pattern has circular holes arranged regularly and the arrangement of the circular holes has a rectangular or hexagonal arrangement, The circular hole is filled with a material constituting the lower contact layer, the light emitting diode having a photonic crystal structure, characterized in that the interval of the half wavelength of the light emitted. delete delete The light emitting diode of claim 1, wherein the substrate is sapphire and the lower contact layer is made of gallium nitride. Board; A photonic crystal pattern having a regular pattern formed on the substrate; An undoped semiconductor layer filling a hole and an upper portion of the photonic crystal pattern; An n-type lower contact layer formed on the undoped semiconductor layer; An n-type clad layer formed on the lower contact layer; An active layer formed on the n-type cladding layer; A p-type cladding layer formed on the active layer; And A p-type upper contact layer formed on the p-type cladding layer, wherein the regularly arranged arrangement of the circular holes of the photonic crystal pattern has a rectangular or hexagonal arrangement, and the circular holes are half waves of emitted light. A light emitting diode having a photonic crystal structure having a long gap. delete The method of claim 5, wherein the photonic crystal pattern, Applying chromium on the substrate, Forming a photoresist pattern on the coated chromium, A mask pattern made of chromium is formed by using the formed photoresist pattern as an etching mask, And a photonic crystal pattern formed by etching the substrate using the mask pattern to form a photonic crystal pattern. The light emitting diode of claim 5, wherein the light emitting diode having the photonic crystal structure further comprises a buffer layer for growth of the undoped semiconductor layer on the photonic crystal pattern.

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