1. Field of the Invention
Exemplary embodiments of the present invention relate to a light emitting diode (LED), and more particularly, to an LED having improved light extraction efficiency.
2. Description of the Related Art
Gallium nitride (GaN)-based LEDs are widely used for display elements and backlights. Further, LEDs may have lower electric power consumption and a longer lifetime as compared with conventional light bulbs or fluorescent lamps, so that their applications have been expanded into general illumination while substituting for conventional incandescent bulbs and fluorescent lamps. Particularly, various types of LED packages, which emit mixed-color light, e.g., white light, have been introduced into the marketplace. A white light LED package may be used in backlight units, for general illumination, or the like.
Since the luminous efficiency of an LED package may depend on the luminous efficiency of an LED, efforts have been continuously made for improving the luminous efficiency of the LED. Particularly, efforts have been made for improving the light extraction efficiency of the LED.
In general, GaN-based nitride semiconductors may be grown on a heterogeneous substrate such as sapphire or silicon carbide. Particularly, nitride semiconductor layers may be formed on a patterned sapphire substrate (PSS), and the PSS may generally be used to fabricate an LED using the nitride semiconductor layers. The PSS may improve light extraction efficiency by scattering the light emitted from an active layer toward the substrate. Further, luminous efficiency may be improved by forming a metal reflector on a bottom surface of a transparent substrate such as a sapphire substrate to reflect light that passes through the sapphire substrate.
However, until the light generated in the active layer is scattered on a surface of the sapphire substrate, the light may proceed for a considerable distance within the nitride semiconductor layers. Even after the light is scattered on the surface of the sapphire substrate, the light may also proceed to a light emission surface for a considerable distance. Accordingly, since a light path may be lengthened, light loss may occur, and a portion of the scattered light may be trapped in the nitride semiconductor layers by internal total reflection.
A reflection metal layer, which may be made of aluminum, formed on the bottom surface of the sapphire substrate, may exhibit a reflectance of about 80% throughout the almost entire wavelength region of visible light. This reflectance is relatively high, but light loss may still occur even when the reflection metal layer is formed.
Meanwhile, sapphire is an electrical insulator, which accordingly limits the structure of an LED to a lateral type. Accordingly, there has recently been developed a technique in which epitaxial layers, such as nitride semiconductor layers, are grown on a heterogeneous substrate such as a sapphire substrate, a support substrate is bonded to the epitaxial layers, and the heterogeneous substrate is then separated using a laser lift-off technique or the like, thereby fabricating a high-efficiency vertical type LED (e.g., see U.S. Pat. No. 7,704,763, issued to Fujii, et al.).
FIG. 1 is a sectional view illustrating a conventional LED.
Referring to FIG. 1, a conventional vertical-type LED is fabricated by sequentially forming a GaN-based n-type layer 23, a GaN-based active layer 25 and a GaN-based p-type layer 27 on a growth substrate (not shown), forming a p-electrode 39 on the p-type layer 27, flip-bonding the p-electrode 39 to a Si submount 41 using a bonding metal 43, removing the growth substrate, and then forming an n-electrode 37 on the exposed n-type layer 23. An n-electrode 45 is then formed on the bottom surface of the Si submount 41. Furthermore, in U.S. Pat. No. 7,704,763, the surface of the exposed n-type layer 23 is formed to be roughened using a dry or photo-enhanced chemical (PEC) etching technique.