KR100467041B1 - III-Nitride compound semiconductor light emitting device - Google Patents

Abstract

PURPOSE: A nitride semiconductor light emitting device is provided to increase outside quantum efficiency by directly forming a surface lattice without an etch mask by a photoelectrochemical etch process. CONSTITUTION: A plurality of nitride semiconductor layers generate light by recombination by electrons and holes, including an n-type electrode that is electrically connected to an active layer containing gallium and nitrogen, an n-type AlxGayln1-x-yN(0<=x<=1, 0<=y<=1, x+y<=1) layer epitaxially grown before the growth of the active layer, and an n-type AlxGayln1-x-yN(0<=x<=1, 0<=y<=1, x+y<=1) layer. The n-type AlxGayln1-x-yN(0<=x<=1, 0<=y<=1, x+y<=1) layer has a surface exposed by etching to cut a device and form an n-type electrode. A surface lattice(54) is formed on the exposed surface except an n-type electrode region by a photoelectrochemical etch process.

Description

Nitride Semiconductor Light Emitting Device {III-Nitride compound semiconductor light emitting device}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nitride semiconductor light emitting device, and more particularly to a nitride semiconductor light emitting device having a high external quantum efficiency. The nitride semiconductor referred to in the present invention refers to Al _x Ga _y In _1-xy N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, x + y ≦ 1).

The external quantum efficiency of the light emitting device is greatly influenced by the structural shape of the light emitting device and the optical properties of the constituent materials. Since a semiconductor constituting a general light emitting device has a higher refractive index than an external environment (epoxy or air layer), the majority of photons generated by the combination of electrons and holes stays inside the device. These photons pass through various paths such as a thin film, a substrate, and an electrode before escaping to the outside, and the absorption reduces the external quantum efficiency.

Particularly, in the nitride semiconductor light emitting device, due to the low conductivity of p-type GaN, a conductive film having a constant thickness is formed in a large area of the upper layer for efficient current diffusion, and the external quantum efficiency is increased by absorption of photons by the conductive film. Deteriorates a lot. In addition, a sapphire substrate is used despite the high lattice mismatch because there is no substrate. Since sapphire is an electrical insulator, it is impossible to form a metal electrode electrically connected to n-type GaN on the back side of the device. Etching is performed to expose the n-type GaN to form a metal electrode. Due to the limitation of the device fabrication, there are many limitations to increase the external quantum efficiency by modifying the shape of the device.

The technique of surface lattice formation has been studied in the series of AlGaInAs, AlGaInP, etc., and many applications are commercialized. However, in the case of GaAs, for example, the refractive index (n) is 3.5, which is much higher than the external environment of air (n = 1) or epoxy (n = 1.5), so that only a small amount of photons escape to the outside.

The maximum critical angle at which photons can escape to the external environment is closely related to the refractive index of the material forming the light emitting device. The maximum critical angle for escape from the semiconductor into the air is determined by the relation θ c = arcsin (1 / n). Is the maximum critical angle, and n is the refractive index of the semiconductor. According to this relationship, the maximum critical angle at which photons escape from GaAs into the air is as small as 16 °. Due to the limitation of the maximum critical angle, the amount of photons generated in the actual active layer escapes to the outside is very small, about 2%.

Various techniques have been proposed to overcome this limitation, and the most effective among them is to modify the shape of the light emitting device or to form surface grids on the surface. U.S. Patent No. 6,657,90B2 (name of the invention: LED having angled sides for increased side light extraction) discloses the construction of a light emitting device in the form of a hexagonal structure having a trapezoidal shape, and U.S. Patent No. 6504180B1 (name of the invention). Method of manufacturing surface textured high-efficiency radiating devices and devices obtained therefrom discloses the formation of surface gratings by wet and dry etching directly above or below an active layer where photons are generated.

However, the method of increasing the external quantum efficiency by forming the surface grid is difficult to apply to the nitride semiconductor light emitting device, for the following reasons.

First, the lattice mismatch is also large with the sapphire substrate, and the lattice mismatch between the different nitride semiconductors, such as AlN, GaN, and InN, is large, and therefore, the thickness of the p-type GaN forming the uppermost layer of the device is limited. As the p-type GaN grows thicker, crystal defects due to lattice mismatch become more prominent, which increases the absorption of photons, which is undesirable. In general, the thickness of a p-type GaN layer does not exceed 200 nm, and in such a thin case, surface lattice formation is impossible.

Second, sapphire used due to the absence of the substrate is an insulator, the crystal lattice is very difficult and the surface lattice formation is very difficult because the material is very stable.

Although the nitride semiconductor is transparent and has a relatively low refractive index (GaN, n = 2.5), the maximum critical angle at which photons can escape (θc = 24.6 ° in the case of GaN) is known to be excellent in optical characteristics, but photons that actually disappear inside Is more than 70%.

Currently, in the case of a nitride semiconductor light emitting device, many technologies are being developed to increase the external quantum efficiency. The most representative technique is flip chip technology (US Patent No. 6573537B1), and a p-type semiconductor layer, which is a top layer forming a nitride semiconductor light emitting device. There is a technique for increasing the surface roughness (US Pat. No. 6,443,3B1), a technique for forming a wave pattern on the surface of the p-type semiconductor layer (US Pat. No. 6,420,735B2).

In the above-described technique, a separate pattern is formed to form a surface lattice to form an etch mask layer through a photo process, and form a surface lattice through dry or wet etching. In this method, there are many restrictions on the shape or size of the surface grid to be formed. In particular, it is very difficult to reduce the size of the surface lattice or increase the density because of the limitation of the minimum line width in the photographic process.

1 is a cross-sectional view illustrating a conventional nitride semiconductor light emitting device.

Briefly describing the manufacturing process, first, the nitride semiconductor light emitting device includes a buffer layer 30, a lower contact layer 31 made of n-type nitride semiconductor, and a lower clad layer made of n-type nitride semiconductor (on the insulating substrate 20). 32), an active layer 33 made of nitride semiconductor, an upper cladding layer 34 made of p-type nitride semiconductor, and an upper contact layer 35 made of p-type nitride semiconductor are sequentially grown.

Next, a transparent electrode layer 51 in ohmic contact with the upper contact layer 35 is formed, and the upper contact layer 35, the upper cladding layer 32, the active layer 33, and the lower cladding layer 32 are formed. The n-type ohmic metal electrode layer 52 is formed on the lower contact layer 31 after exposing the lower contact layer 31 by mesa etching. Subsequently, a bonding pad 53 is formed on the transparent electrode layer 51.

Since sapphire is usually used as the substrate 20, since an electrode is not formed on the back surface of the substrate 20, metal electrodes 51 and 52 are formed on the surface of the substrate 20. Light is emitted to the outside through the thin transparent electrode 51.

In the structure of FIG. 1, it is difficult to change the shape, and because the upper contact layer 35 of the uppermost layer must be thin, it is very difficult to form a surface grid on the surface. If the upper contact layer 35 is formed relatively thick (> 1um), it is easy to form a surface lattice, but current growth technology makes it impossible to grow thick p-type AlGaInN having good crystallinity and grows thick. In this case, not only the power consumption increases due to the increase of the resistance, but also the photons generated in the active layer 33 are absorbed more by the upper contact layer 35, thereby decreasing the luminance of the device. Due to this limitation, it is not possible to form a surface lattice on the layer of the upper contact layer 35, which is the upper surface of the active layer 33, which is a light emitting part.

The technical problem to be achieved by the present invention is to provide a sufficient free space for the cutting process when cutting between light emitting devices, the fine and dense surface irregularities beyond the constraints of the photo process in a new method in the outer space of the chip It is to provide a nitride semiconductor light emitting device having a high brightness by forming a.

1 is a cross-sectional view illustrating a conventional nitride semiconductor light emitting device;

2 to 7 are views for explaining the nitride semiconductor light emitting device according to the present invention.

<Description of Reference Numbers for Main Parts of Drawings>

20: insulating substrate 30: buffer layer

31: lower contact layer 32: lower clad layer

33: active layer 34: upper clad layer

35: upper contact layer 40: UV

41: etching solution 44: DC power

51: transparent electrode layer 54: surface grid

According to an aspect of the present invention, a nitride semiconductor light emitting device includes: a lower contact layer formed on an insulating substrate and formed of an n-type nitride semiconductor; An upper contact layer consisting of an active layer made of nitride semiconductor and a p-type nitride semiconductor sequentially stacked on a predetermined region of the lower contact layer surface; An n-type ohmic contact metal layer formed on a predetermined region of an exposed surface of the lower contact layer that is not covered by the active layer; And a transparent electrode layer formed on the upper contact layer, wherein a surface grid is formed on an exposed surface of the lower contact layer that is not covered by the active layer and the n-type ohmic contact metal layer.

The surface lattice may be formed by photochemical etching, and the etching solution used at this time may include KOH, ammonia, or hydrochloric acid, and the light used is preferably ultraviolet light.

Hereinafter, with reference to the accompanying drawings, preferred embodiments of the present invention will be described in detail. In the drawings, the same reference numerals as in FIG. 1 denote components that perform the same function, and a repetitive description thereof will be omitted.

The following examples are only presented to understand the content of the present invention, and those skilled in the art may make many modifications within the technical spirit of the present invention. Accordingly, the scope of the present invention should not be construed as being limited to these embodiments.

2 is a cross-sectional view illustrating a nitride semiconductor light emitting device according to an embodiment of the present invention, and FIG. 3 is a plan view of FIG. 2.

2 and 3, the present invention exposes the lower contact layer 31 by mesa etching the upper contact layer 35, the upper cladding layer 32, the active layer 33, and the lower cladding layer 32. After the n-type ohmic metal electrode layer 52 is formed on the lower contact layer 31, the surface lattice 54 is formed on the exposed surface of the lower contact layer 31. do. That is, the present invention is characterized in that the surface grid 54 is formed on the outer portion of the chip excluding the light emitting portion.

Although the bonding pads 53 are generally formed on the transparent electrode layer 51 as shown in FIG. 1, the bonding pads 53 may be formed to directly contact the upper contact layer 35 by removing a portion of the transparent electrode layer 51 as shown in FIG. 2. The cladding layers 32 and 34 are not necessarily present but are present to improve the performance of the device.

The net amount of photons dissipated from the inside of the device to the outside of the device is closely related to the area where the surface grid is formed, the shape, size, and density of the surface grid. The larger the area where the surface lattice is formed, the higher the probability of escape of photons that disappear inside, but the size of the part that can form the surface lattice is limited due to the limitation of the device size. The size of each of the surface grids 54 needs to be larger than 1/4 of the central wavelength generated in the device, and the larger the density, the more photons that escape.

4 is a conceptual diagram showing that light escapes to the outside by the surface lattice 54. It can be seen that the light generated in the active layer 33 can effectively escape to the outside by forming irregularities on the edge portion except for the light emitting portion of the device.

FIG. 5 is a view for explaining the principle of photochemical etching occurring at a portion where the n-type nitride semiconductor layer 31 and the etching solution 41 are in contact with each other.

When ultraviolet light 40 is irradiated to the n-type nitride semiconductor layer 31, electrons and holes are formed on the surface of the n-type nitride semiconductor layer 31 by this light energy. At this time, the formed electrons move into the semiconductor and holes move to the semiconductor surface.

For example, when the n-type nitride semiconductor 31 is GaN, the holes moved to the semiconductor surface combine with GaN on the surface to separate gallium (Ga) molecules and nitrogen (N) molecules, and etching occurs. The etching principle contributed by the hole is expressed by the following equation. That is, 2GaN + 6H ⁺ ---> 2Ga ³⁺ + N ₂ . The DC power supply 44 applied thereto serves to speed up the etching rate by assisting the movement of the holes.

The etching rate is accelerated by the presence of lattice defects. In general, GaN semiconductors have lattice defect densities of 1 × 10 ⁶ cm ⁻³ to 1 × 10 ¹⁰ cm ⁻³ . Therefore, when the above photochemical etching is performed, the surface of the semiconductor is unevenly etched to cause surface irregularities.

6A is a photograph of AFM (Atomic Force Microscope) after etching for 2 minutes using a KOH solution of KOH: DI (deionized water) = 500 g: 1500 cc as an etching solution 41. As the amount of KOH increases, the etching rate increases. The root mean square (rms) value of surface flatness is about 7 nm.

6B is an AFM photograph after etching for 9 minutes. The root mean square (rms) value of surface flatness is about 500 nm. Therefore, it can be seen that as the etching time increases, the surface unevenness increases, and accordingly, the light output also increases.

7 is an applied current-luminance graph comparing the conventional case and the present invention. In the case of the present invention, there is a slight difference depending on the size and shape of the surface irregularities, but the average increase in brightness of about 30 to 60% compared to the existing technology.

According to the present invention, since the surface lattice is formed in the outer free space of the chip, a large formation area of the surface lattice can be ensured. In addition, since the lower contact layer 31 may be somewhat thick, it is easy to form a surface grid.

Unlike the photolithography process in which a separate mask pattern is required, the present invention forms a surface lattice without an etching mask through photochemical etching, and thus does not have a great restriction on the shape, size, and density of the surface lattice.

As a result, according to the present invention, the external quantum efficiency of the nitride semiconductor light emitting device is increased.

Claims (4)

A nitride semiconductor light emitting device comprising a plurality of nitride semiconductor layers epitaxially grown using a substrate, The plurality of nitride semiconductor layers generate light by recombination of electrons and holes, and an n-type Al _x Ga _y In _1-xy N epitaxially grown prior to the growth of the active layer containing gallium and nitrogen and the active layer (0 ≦ x ≦ 1 , 0≤y≤1, x + y≤1) layer, and n-type Al _x Ga _y In _1-xy N (0≤x≤1, 0≤y≤1, x + y≤1) layer An n-type electrode in contact, The n-type Al _x Ga _y In _1-xy N (0≤x≤1, 0≤y≤1, x + y≤1) layer is used to expose the surface exposed by etching for device cutting and formation of n-type electrode. have, A nitride semiconductor light emitting device, characterized in that the surface grid is formed by photochemical etching on the exposed surface except for the region where the n-type electrode is formed. delete The nitride semiconductor light emitting device of claim 2, wherein the etching solution used in the photochemical etching comprises KOH, ammonia, or hydrochloric acid. The nitride semiconductor light emitting device of claim 2, wherein the light used in the photochemical etching is ultraviolet light.