A light-emitting device includes a substrate, a first nitride semiconductor stack formed on the substrate, a nitride light-emitting layer formed on the first nitride semiconductor stack, a second nitride semiconductor stack formed on the nitride light-emitting layer, and a first transparent conductive oxide layer formed on the second nitride semiconductor stack. The second nitride semiconductor stack includes a plurality of hexagonal-pyramid cavities formed in an upper surface of the second nitride semiconductor stack. The plurality of hexagonal-pyramid cavities of the second nitride semiconductor stack are filled with the first transparent conductive oxide layer, and a low-resistance ohmic contact is generated at the inner surfaces of the plurality of hexagonal-pyramid cavities so as to decrease the operation voltage and improve light-emitting efficiency of the light-emitting device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting device, and more particularly, to a high efficiency light-emitting device.

2. Description of the Prior Art

Semiconductor light-emitting devices have been applied widely in optical display devices, traffic signals, data storing devices, communication devices, illumination devices, and medical apparatuses.

The conventional nitride LED includes a thin metallic layer on a top surface of the LED, such as material of the Ni/Au group, regarded as a transparent conductive layer. However, part of LED light still cannot travel through metal. Light generated by the LED is absorbed by the thin metallic layer and the light transmittance is decreased. In order to have a good transmittance, the thickness of the thin metallic layer is limited to be within several tens to several hundreds of angstroms. Although the thickness of the thin metallic layer is limited, the thin metallic layer merely has transmittance of visible light in the range of 60%˜70%, and the light-emitting efficiency of the LED is still low.

U.S. Pat. No. 6,078,064, which is included herein by reference, discloses an LED structure. The surface of LED includes a transparent conductive oxide layer formed on a p-type contact layer of a high carrier concentration. Generally, the transparent conductive oxide layer has a high transmittance of more than 90%. Therefore, the thickness of such layer can be thicker and the current spreading is better, such that the brightness and light-emitting efficiency of the LED are improved. Note that the transparent conductive oxide layer must contact with the p-type contact layer of a high carrier concentration more than 5×1018cm−3, so as to form a better ohmic contact.

Taiwan Patent No. 144,415, which is incorporated herein by reference, discloses a method for forming a reverse tunneling layer. An N+ reverse tunneling contact layer is formed between a transparent oxide electrode layer and a semiconductor light-emitting layer to achieve the purpose of forming a good ohmic contact so as to improve the light-emitting efficiency of the LED and decrease the operation voltage.

In addition, Y. C. Lin also disclosed a related method in the paper “InGaN/GaN Light Emitting Diodes with Ni/Au, Ni/ITO and ITO p-Type Contacts” (Solid-State Electronics Vol. 47 Page 849-853). He disclosed that a thin metallic layer was formed on a p-type contact layer of a nitride LED, and then a transparent conductive oxide layer was formed on the thin metallic layer. This method can efficiently reduce the contact resistance between the p-type contact layer and the transparent conductive oxide layer. However, the transmittance is still decreased by the thin metallic layer and thus the light-emitting efficiency of the LED is still affected by the thin metallic layer.

Therefore, the present invention aims to improve the brightness of an LED, to solve the contact resistance issue occurring between such a contact layer and transparent conductive oxide layer, and to simplify the process complexity.

SUMMARY OF THE INVENTION

It is therefore an object of the claim invention to provide a light-emitting device with high transmittance to solve the above-mentioned problems.

The claimed invention discloses a light-emitting device. The light-emitting includes a substrate, a first nitride semiconductor stack formed on the substrate, a nitride light-emitting layer formed on the first nitride semiconductor stack, a second nitride semiconductor stack formed on the nitride light-emitting layer having a plurality of hexagonal-pyramid cavities on the surface of the second nitride semiconductor layer opposite to the nitride light-emitting layer, wherein the hexagonal-pyramid cavity extends downward from the surface of the second nitride semiconductor layer, and a first transparent conductive oxide layer formed on the second nitride semiconductor stack. The plurality of hexagonal-pyramid cavities of the second nitride semiconductor stack are filled with the first transparent conductive oxide layer, such that a low-resistance ohmic contact is generated between the transparent conductive oxide layer and the inner surfaces of the plurality of hexagonal-pyramid cavities.

In general, if the second nitride semiconductor stack is p-type material and its surface opposite to the nitride light-emitting layer is flat smooth, and parallel to the substrate surface, the transparent conductive oxide layer cannot directly form a good ohmic contact with the p-type nitride semiconductor stack and thereby increases the operation voltage.

In contrast, the claimed invention provides a plurality of hexagonal-pyramid cavities in the surface of the p-type nitride semiconductor stack opposite to the nitride light-emitting layer, wherein the hexagonal-pyramid cavity extends downward from the surface of the second nitride semiconductor layer, and then forms a transparent conductive oxide layer over the surface, wherein the transparent conductive oxide layer contacts not only the flat region of the surface of the p-type nitride semiconductor without cavity area (hereinafter “flat outer surface”), but also the inner surfaces of the hexagonal-pyramid cavities (below called “cavity inner surface”). The surface energy state of the flat outer surface differs from that of the cavity inner surfaces. The difference between the surface energy states is contributed by the difference of the crystal directions as well as the difference in the surface energy potential between the flat outer surface and the cavity inner surfaces. If the transparent conductive oxide layer is directly formed on the flat outer surface of the p-type nitride semiconductor stack, the interface between the transparent conductive oxide layer and the flat outer surface has a higher potential barrier leading to a higher contact resistance. However, when the transparent conductive oxide layer contacts with the cavity inner surfaces, since the lower potential barrier between interface of the cavity inner surface and the transparent conductive oxide layer, a good ohmic contact can be formed. Therefore, the p-type layer does not need a high carrier concentration as mentioned in the previous prior art. The operation voltage of the device can be reduced to the level as the conventional Ni/Au based LED.

When operation current is applied, current is first spread through the transparent conductive oxide layer, then flowing into the p-type nitride semiconductor stack mainly through the lower resistance contact area of the cavity inner surfaces contacting with the transparent conductive oxide layer, and finally flowing to the light-emitting layer to generate light.

Furthermore, the other advantages of the claimed invention of the hexagonal-pyramid cavities are that the hexagonal-pyramid cavities can effectively reduce both the total reflection effect on the device surface and the light absorption effect of the p-type nitride semiconductor stack. Thus the light-emitting efficiency can be further enhanced. Besides, the light transmittance of the transparent conductive oxide layer is better than that of the conventional thin metallic layer. Constantly, the claimed invention can greatly improve the light-emitting efficiency of the device and can provide the device a low operation voltage.

DETAILED DESCRIPTION

Please refer toFIG. 1, which is a diagram of a light-emitting device based on the present invention. The light-emitting device includes a sapphire substrate10; a nitride buffer layer11formed on the sapphire substrate10; an n-type nitride semiconductor stack12formed on the nitride buffer layer11, wherein the surface of the n-type nitride semiconductor stack12far from the substrate includes a first surface and a second surface; a nitride multiple quantum-well structure light-emitting layer13formed on the first surface of the nitride multiple quantum-well structure light-emitting layer13; a p-type nitride semiconductor stack14formed on the nitride multiple quantum-well structure light-emitting layer13, wherein the surface of the p-type nitride semiconductor stack14far from the nitride multiple quantum-well structure light-emitting layer13includes a plurality of hexagonal-pyramid cavities141; a transparent conductive oxide layer15formed over the p-type nitride semiconductor stack14and the hexagonal-pyramid cavities141, wherein the material of the transparent conductive oxide layer15contacts the cavity inner surfaces1411; an n-type electrode16formed on the second surface of the n-type nitride semiconductor stack12; and a p-type electrode17formed on the transparent conductive oxide layer15.FIG. 2is a diagram of the p-type nitride semiconductor stack14having the plurality of hexagonal-pyramid cavities141.

The contact resistance formed between cavity inner surfaces1411and the transparent conductive oxide layer15is lower than the contact resistance formed between the flat outer surface140of the p-type nitride semiconductor stack14and the transparent conductive oxide layer15.

The shape and angle of the hexagonal-pyramid cavities141structures depend on the physical crystal property of nitride, such as the crystal property of nitride. Take C-(0001) sapphire substrate for example. Each angle between each adjacent pyramid surface is about 120 degrees substantially, and the pyramid surfaces includes a (10-11) or (11-22) lattice surface group.

The method for forming the hexagonal-pyramid cavities141includes at least one step or more than one step, as shown in the following.

1. Surfactant, such as Si or Mg, can be provided for transforming the crystal nucleation of the hexagonal-pyramid cavities141so as to form the hexagonal-pyramid cavities141on the surface of the p-type nitride semiconductor stack14or inside the p-type nitride semiconductor stack14when the initial layers of the hexagonal-pyramid cavities141grow.

2. The initial layers of the hexagonal-pyramid cavities141grow between the epitaxial temperature 700° C. and 950° C. for transforming the crystal nucleation so as to form the hexagonal-pyramid cavities141on the surface of the p-type nitride semiconductor stack14or inside the p-type nitride semiconductor stack14.

3. The initial layers of the hexagonal-pyramid cavities141grow in a rich nitrogen ambiance for transforming the crystal nucleation so as to form the hexagonal-pyramid cavities141on the surface of the p-type nitride semiconductor stack14or inside the p-type nitride semiconductor stack14.

4. After the p-type nitride semiconductor stack14is formed, the surface of the p-type nitride semiconductor stack14can be etched by performing a chemical wet etching process, such as with high temperature H3PO4, to form the hexagonal-pyramid cavities141.

5. A smaller hexagonal-pyramid cavity can be formed first by epitaxial growth. After that, a larger hexagonal-pyramid cavity141can be formed by performing a chemical wet etching process on the smaller hexagonal-pyramid cavity, so as to improve the light-emitting efficiency. If the hexagonal-pyramid cavity141is formed by epitaxial growth directly, stress can occur on the edge of the hexagonal-pyramid cavity141so that an epitaxial defect occurs to decrease the epitaxial quality and affect the electrical properties of the LED. However, if the smaller hexagonal-pyramid cavity is formed first by epitaxial growth and then etched by a chemical wet etching process so as to make the smaller size hexagonal-pyramid cavity larger and deeper, this could avoid a damage of the hexagonal-pyramid cavity141epitaxial layers.

The density of the present invention hexagonal-pyramid cavities141can be within the range of 1×107cm−2to 1×1011cm−2. Please refer toFIG. 3, which shows the best density range of the present invention hexagonal-pyramid cavities141. FromFIG. 3, when the density of the hexagonal-pyramid cavities141increases from 1×108cm−2to 2×109cm−2, the brightness increases from 117 mcd to 150 mcd. This indicates that increasing the density of the hexagonal-pyramid cavities141can improve the brightness of the LED.

The diagonal length of the upper side of the hexagonal-pyramid cavity141is within the range of 10 nm to 1 μm. Please refer toFIG. 4, which shows the best range of the diagonal length of the hexagonal-pyramid cavity141. FromFIG. 4, when the diagonal length of the hexagonal-pyramid cavity141increases from 122 nm to 168 nm, the brightness can increase from 128 mcd to 173 mcd, which implies a larger hexagonal-pyramid cavity betters the brightness of the LED.

The depth of the present invention hexagonal-pyramid cavities141can be within the range of 10 nm to 1 μm. Please refer toFIG. 5, which shows the best range of the depth of the hexagonal-pyramid cavities141. FromFIG. 5, when the depth of the hexagonal-pyramid cavity141increases from 60 nm to 125 nm, the brightness increases from 130 mcd to 150 mcd. That is, a deeper hexagonal-pyramid cavity141can increase the brightness of the LED.

Note that the bottom of the hexagonal-pyramid cavity141should be above the light-emitting layer13. If the bottom of the hexagonal-pyramid cavity141extends to the light-emitting layer13, the electrical properties of the LED would be poor.

Moreover, the transparent conductive oxide layer15should be thick enough to fill and cover around the hexagonal-pyramid cavities141so that the perimeter of each hexagonal-pyramid cavity141contacting with the transparent conductive oxide layer15is continuous, not discontinuous or broken. Otherwise, current may not pass into the nitride semiconductor stack14through the low resistance contact of the inner surfaces of the hexagonal-pyramid cavities141contacting with the transparent conductive oxide layer15, and thus the operation voltage will be increased.

Please refer toFIG. 6, which is a table of average depth of the hexagonal-pyramid cavities141, thickness of the transparent conductive oxide layer15and operation voltage. This example is a nitride LED having the hexagonal-pyramid cavities141, which have an average depth of 150 nm. Suppose that different thickness of the transparent conductive oxide layer15, 70 nm and 220 nm, are respectively formed on the nitride semiconductor stack14. The operation voltage of an LED with a 70 nm transparent conductive oxide layer15is about 3.6V when the current is 20 mA. However, the operation voltage of an LED with a 220 nm transparent conductive oxide layer15is about 3.3V in the same condition. This implies that when the thickness of the transparent conductive oxide layer15is enough, the operation voltage can be reduced.

The transmittance of the transparent conductive oxide layer15is more than 50% when a wavelength of light is within the range of 300 nm to 700 nm. The transparent conductive oxide layer15can be formed by an electron beam evaporator, a sputter, a thermal coater, or any combination of such. While forming the transparent conductive oxide layer15, the best way is to fill the hexagonal-pyramid cavities141so that the area of the low resistance contact is increased to efficiently reduce the operation voltage of the LED.

In addition, after the transparent conductive oxide layer15fills the hexagonal-pyramid cavities141, the surface of the transparent conductive oxide layer15does not have the property of the hexagonal-pyramid cavities141. In other words, the refractive index difference of materials below and above the hexagonal-pyramid cavities141should be maximized, such that the light-extraction effect can be improved. Therefore, the refractive index of the transparent conductive oxide layer15should be between the refractive indexes of nitride material and package material. Preferably, the absolute value of refractive index difference of the transparent conductive oxide layer15and the nitride material is higher than that of the transparent conductive oxide layer15and the package material.

FIG. 7shows a comparison graph of light intensity vs. operation current for three kinds of LEDs, wherein LED-A is an LED having the present invention hexagonal-pyramid cavities141and the transparent conductive oxide layer15, LED-B is an LED having a thin metallic layer but no hexagonal-pyramid cavities141, and LED-C is an LED having a conductive oxide layer but no hexagonal-pyramid cavities141. FromFIG. 7, LED-B has an imperfect light-emitting property and the brightness is lower because the light transmittance of the thin metallic layer is lower than that of the conductive oxide layer. LED-C, having the conventional thin metallic layer replaced with the transparent conductive oxide layer has excellent light transmittance, and thereby improves the light-emitting effect and enhances the light-emitting efficiency. However, LED-A takes advantage of hexagonal-pyramid cavities141to increase total light-emitting area, and reduce light loss caused by the total reflection effect and light absorption of the semiconductor stack over the light-emitting layer. Therefore, LED-A can greatly increase brightness and light-emitting efficiency.

FIG. 8shows a comparison graph of positive current vs. voltage for three kinds of LEDs, wherein LED-A is an LED having the present invention hexagonal-pyramid cavities141and the transparent conductive oxide layer15, LED-B is an LED having a thin metallic layer but no hexagonal-pyramid cavities141, and LED-C is an LED having a conductive oxide layer but no hexagonal-pyramid cavities141. FromFIG. 8, the operation voltage of LED-B having the thin metallic layer is the lowest one of the three LEDs. Due to imperfect ohmic contact, the operation voltage of LED-C having the conductive oxide layer is very high. For example, when the current is 20 mA, the operation voltage of LED-C is more than 5V. Nevertheless, the operation voltage of the LED-A having the conductive oxide layer and the hexagonal-pyramid cavities141can be reduced to similar to that of LED-B. Therefore, the present invention can provide better performance.

Please refer toFIG. 9, which is a diagram of a second embodiment of the light-emitting device according to the present invention. The second surface of the n-type nitride semiconductor stack12of the light-emitting device further comprises an n-type electrode contact area121and a non-electrode contact area122. The n-type electrode16is formed on the n-type electrode contact area121. The non-electrode contact area122further includes a high-efficiency light-emitting surface. A rough surface or a plurality of hexagonal-pyramid cavities is formed by performing an etching process or epitaxial growth on the high-efficiency light-emitting surface. In this embodiment, the light-emitting device includes a rough surface123. Because of the rough surface123of the non-electrode contact area122, lateral light reflected between the substrate10and the n-type nitride semiconductor stack12can be reduced, so that the lateral light can be emitted effectively for increasing the light-emitting efficiency of the LED.

Please refer toFIG. 10, which is a diagram of a third embodiment of the light-emitting device according to the present invention. The light-emitting device further comprises a second transparent conductive oxide layer18formed on the rough surface123and the non-electrode contact area122; also, the second transparent conductive oxide layer18contacts with the n-type electrode16, such that current spreading at the second transparent conductive oxide layer18is better. Moreover, the light-emitting efficiency is improved if the refractive index of the second transparent conductive oxide layer18is between the refractive indexes of nitride material and package material.

In the above-mentioned embodiments, a transparent conductive oxide layer can be formed between the n-type electrode16and the n-type electrode contact area121of the second surface of the n-type nitride semiconductor stack12.

In the above-mentioned embodiments, the transparent conductive oxide layer can be taken as the n-type electrode.

In the above-mentioned embodiments, the n-type electrode contact area121can further include a plurality of hexagonal-pyramid cavities.

In the above-mentioned embodiments, the sapphire substrate10has an off angle between 0 and 10 degrees. The sapphire substrate10can be substituted by a substrate made of a material selected from a group consisting of GaN, AlN, SiC, GaAs, GaP, Si, ZnO, MgO, MgAl2O4, and glass.