Light-emitting device

1. A light-emitting device, comprising:

a substrate structure, comprising a base portion having a surface and a plurality of protrusions regularly formed on the base portion;

a buffer layer covering the plurality of protrusions and the surface; and

III-V compound semiconductor layers formed on the buffer layer;

wherein one of the plurality of protrusions has a height not greater than 1.5 μm;

wherein the light-emitting device has a full width at half maximum (FWHM) of smaller than 250 arcsec in accordance with a (102) XRD rocking curve;

wherein the base portion comprises a first material and the plurality of protrusions comprises a second material which is different from the first material, and a refractive index of the second material is smaller than a refractive index of the first material; and

wherein the buffer layer comprises a third material which is different from the second material.

2. The light-emitting device according to claim 1 , wherein in a cross-sectional view, an included angle between a side surface of one of the plurality of protrusions and the surface of the base portion is less than 65 degrees.

3. The light-emitting device according to claim 1 , wherein one of the plurality of protrusions comprises a first portion and a second portion formed on the first portion, and the first portion is integrated with the base portion and comprises a material that is the same as the first material of the base portion.

4. The light-emitting device according to claim 3 , wherein the first material comprises sapphire.

5. The light-emitting device according to claim 3 , wherein the second material comprises SiO 2 .

6. The light-emitting device according to claim 3 , wherein the first portion of one of the plurality of protrusions has a height H 1 and the one of the plurality of protrusions has a height H; wherein a ratio of H 1 to H is between 1% and 30% both inclusive.

7. The light-emitting element of claim 1 , wherein an arrangement of the plurality of protrusions has a period and the period is between 1 μm and 3 μm both inclusive.

8. The light-emitting device according to claim 1 , wherein the buffer layer is conformally formed on the plurality of protrusion and the surface and the third material comprises aluminum nitride.

9. The light-emitting device according to claim 1 , wherein a thickness of the buffer layer is greater than 5 nm and not more than 50 nm.

10. The light-emitting device according to claim 1 , wherein a three-dimensional shape of one of the plurality of protrusions comprises a cone or a pyramid.

11. The light-emitting device according to claim 1 , wherein the FWHM is not less than 100 arcsec.