Patent Application
20140014974
1. Technical Field
This present application relates to a light-emitting device, and more particularly to a light-emitting device having a patterned substrate and the method of manufacturing.
2. Background of the Disclosure
Recently, efforts have been devoted to improve the luminance of the light-emitting diode (LED) in order to apply the device to the lighting domain, and further procure the goal of energy conservation and carbon reduction. There are two major aspects to improve luminance. One is to increase the internal quantum efficiency (IQE) by improving the epitaxy quality to enhance the combination efficiency of electrons and holes. The other is to increase the light extraction efficiency (LEE) that emphasizes on the light emitted from light-emitting layer and therefore reducing the light absorbed by the LED structure.
Surface roughening technology is one of the efficient methods to enhance luminance, and a well-known method is to form a patterned substrate. The light emitted from the active layer on the patterned substrate is easily reflected back to the epitaxial stack because of total internal reflection (TIR) effect and absorbed by the epitaxial stack to generate heat. It causes both the poor light extraction efficiency and the heat dissipation. Nevertheless, the pattern on the substrate is usually formed deeper in order to compensate the light loss due to the internal total reflection effect, but the high aspect ratio of the deeper pattern on the substrate causes difficulty for subsequently epitaxial growth and adversely affects the epitaxial quality.
Another prior technique to roughen surface is to utilize mechanically polishing method to form a randomly distributed rough patterns on the substrate surface. By this method, it is hard to control the pattern dimension such as the depth or the width. Moreover, the epitaxy quality is poor for an epitaxial layer grown on the randomly rough surface.
One aspect of the disclosure proposes a light-emitting device having a patterned substrate. The patterned substrate benefits both quality of epitaxy layer and light extraction efficiency.
One aspect of the present disclosure provides a light-emitting device, comprising a patterned substrate having a plurality of cones, wherein a space is formed between two adjacent cones; and a light-emitting stack formed on the cones, wherein the light-emitting stack comprises a first conductivity-type semiconductor layer, an active layer, and a second conductivity-type semiconductor layer. Moreover the first electron and the second electron electrically connect to the first conductivity-type semiconductor layer and the second conductivity-type semiconductor layer individually. Furthermore, the plurality of cones formed on the patterned substrate by fixed and/or un-fixed spacing to isolate the multiple cones.
a-1b shows a light-emitting device in accordance with one embodiment of the present disclosure.
a shows the light extraction intensity measurement result of LEDs in accordance with the embodiments of the present disclosure.
b shows the output power measurement result of LEDs in accordance with the embodiments of the present disclosure.
a shows a light-emitting device (LED) in accordance with one embodiment of the present disclosure. The LED 100 comprises a growth substrate 101, an epitaxial stack 109, a first electrode 107, and a second electrode 108. The cross-section of the substrate 101 shows a plurality of cones 102. In the embodiment, the LED 100 comprises an intermediate layer 103 grown on the substrate 101, and an epitaxial stack 109 grown on the intermediate layer 103.
The intermediate layer 103 can be a buffer layer used to reduce the lattice mismatch between the substrate 101 and the epitaxial stack 109. The intermediate layer 103 can also be a single layer, multiple layers, or a structure to combine two materials or two separated structures where the material can be organic, inorganic, metal, semiconductor and so on, and the structure can be a reflection layer, a heat conduction layer, an electrical conduction layer, an ohmic contact layer, an anti-deformation layer, a stress release layer, a stress adjustment layer, a bonding layer, a wavelength converting layer, a mechanical fixing structure and so on.
The epitaxial stack 109 comprises a first semiconductor layer 104 with a first conductivity-type grown on the intermediate layer 103 which comprises non-doped semiconductor layer or doped semiconductor layer, an active layer 105 grown on the first semiconductor layer 104, a second semiconductor layer 106 with a second conductivity-type grown on the active layer 105. The first electrode 107 is formed on the first semiconductor layer 104 after etching the epitaxial stack 109 until a part of semiconductor layer 104 is exposed, and the second electrode 108 is formed on the second semiconductor layer 106. In another embodiment, the first electrode 107 is formed on one side of the growth substrate 101 opposite to another side attached to the epitaxial stack 109.
Substrate 101 comprises a plurality of cones 102 with a spacing 101a between two adjacent cones 102, wherein each cone 102 comprises a top 201, a bottom 202, and an inclined sidewall 203 between the top 201 and the bottom 202 as shown in
Referring to
As shown in
As mentioned above, the larger the maximum distance B between the arc 204 and the chord 205 of the arc 204, the larger the surface area of the cone 102 for diffusing the light and increasing the light extraction efficiency. But a larger distance B can hinder the epitaxial layer from growing on the space (not shown) between two adjacent cones 102, and can increase the probability of the light being absorbed between adjacent cones 102. In one embodiment, the maximum distance B between the arc 204 and the chord 205 of the arc 204 can be 0-0.5 nm, and in another embodiment, it is expected to be 0-0.2 nm considering the growth of the epitaxy layers. Thus the spacing S between two adjacent cones 102, the maximum distance B between the arc 204 and the chord 205 of the arc 204 and the bottom width D1 of the cone 102 form a relationship represented by a second ratio Q2=B/(D1+S), which is used for preventing light absorption between adjacent cones 102 and to ensure a sufficient growth time for growing the epitaxy layers. The second ratio Q2 can be around 0-0.2, and preferably to be 0-0.05.
In order to avoid the light absorption due to the light reflection inside the cones 102 of substrate 101 caused by the difference between refractive index between the intermediate layer 103 and the substrate 101, the top width D2 of the cone 102 is expected to be larger than 0. The larger top width D2 of the cone 102 implies a larger entrance for light to emit into cones 102, while the top width D2 of the cone 102 is between 0-(Wd/nintermediate) nm wherein the Wd is the major wavelength of the internal light and the nintermediate is the refractive index of the intermediate layer 103. In one embodiment, the top width D2 of the cone 102 is smaller than 0.1 nm. In order to guide the light to the epitaxial stack 109 through the top 201 before being absorbed within the cone 102, the cone 102 is designed to have an angle θ between the bottom 202 of the cone 102 and the chord 205 of the arc 204 between 40°-60°, preferably to be about 48°.
As described above, with consideration of the light extraction efficiency and the growth rate of the epitaxy layers, a ratio of the top 201 area to the bottom 202 area is designed to be less than 0.0064. Thus the bottom width D1 and the top width D2 of the cone 102 has a relationship represented by a third ratio Q3=(D2/D1) between 0-0.08, preferably between 0-0.03.
According to the light extraction intensity shown in
As shown in Table 1a, LEDs are designed with two different cone sizes designated as spec I and spec III. The LED of spec III has a patterned substrate with cone size having a first ratio Q1 of 0.13, and the LED of spec I has a patterned substrate of cone size having a first ratio Q1 of 0.25.
Furthermore, the quality of the epitaxy layers of the LEDs is verified by the factor of WHM (full width at half maximum) tested by XRD (X-ray diffraction) analysis. As shown in Table 1b, the LED of spec III has smaller XRD WHM value than that of the LED of spec I, which indicates the LED of spec III has better epitaxial quality. In sum, the LED of spec III not only has better lighting characteristics but also better epitaxy layer quality comparing with the LEDs of spec I.
It should be noted that the proposed various embodiments are not for the purpose to limit the scope of the disclosure. Any possible modifications without departing from the spirit of the disclosure may be made and should be covered by the disclosure.
