1. Field of the Invention
The present invention relates to a semiconductor light emitting device, and more particularly, to a vertical semiconductor light emitting device having improved light extraction efficiency and a method of manufacturing the same.
2. Description of the Related Art
A light emitting diode (LED) is a semiconductor device that can emit light of various colors due to electron-hole recombination occurring at a p-n junction between p-type and n-type semiconductors when current is supplied thereto. Such an LED is advantageous over a filament-based light emitting device in that it has a long lifespan, low power usage, superior initial-operation characteristics, and high vibration resistance. These factors have continually boosted the demand for LEDs. Notably of late, a great deal of attention has been drawn to group III nitride semiconductors that can emit light in a blue/short wavelength region.
Nitride semiconductor crystals, constituting a light emitting device using the group III nitride semiconductor, are grown on a specific growth substrate such as a sapphire or SiC substrate. However, in case of using an insulating substrate such as a sapphire substrate, there are significant limitations on the arrangement of electrodes. That is, a nitride semiconductor light emitting device according to the related art generally has electrodes arranged in a horizontal direction, such that current flow is narrow. Due to such a narrow current flow, an operation voltage Vf of the light emitting device is increased to thereby deteriorate current efficiency and be susceptible to electrostatic discharge.
In order to solve these problems, research into a semiconductor light emitting device having a vertical electrode structure is being conducted. After a light emitting structure for a light emitting diode is formed on a sapphire growth substrate using Metal Organic Chemical Vapor Deposition (MOCVD) equipment or Molecular Beam Epitaxy (MBE) equipment, a first conductivity type ohmic-contact electrode structure is formed on an upper nitride semiconductor layer, which is disposed on the uppermost portion of the light emitting structure. Then, a conductive substrate, separately prepared in addition to the growth substrate, is subjected to a wafer solder bonding process, and the growth substrate is removed, and thus, a light emitting diode having a semiconductor structure is manufactured.
In general, a vertical semiconductor light emitting device has a structure in which electrodes having different polarities are formed on upper and lower surfaces of a light emitting structure including an n-type semiconductor layer, an active layer and a p-type semiconductor layer, and is advantageous in terms of electrostatic discharge as compared with a horizontal semiconductor light emitting device. However, in order to achieve a sufficient current diffusion effect even in the vertical semiconductor light emitting device, the electrodes are required to be formed across a large area. As the area of the electrodes is increased, extraction efficiency of light emitted from the light emitting structure is reduced.
The light emission efficiency of the light emitting device is determined by internal quantum efficiency and light extraction efficiency (or external quantum efficiency). Particularly, light extraction efficiency is determined based on optical factors of the light emitting device, i.e., refractive indices of the individual elements of the light emitting structure, the flatness of an interface between the elements or the like. The internal quantum efficiency of the light emitting device reaches approximately 100%, whereas the external quantum efficiency thereof is significantly low.
This is due to a total reflection of light caused by a difference in the refractive index at an interface between the light emitting device and air when light produced in the light emitting device is emitted outside of the light emitting device. That is, since a semiconductor layer constituting the light emitting device has a refractive index larger than that of external air or the substrate, a critical angle determining an incident angle range in which light can be emitted is reduced. As a result, a considerable amount of light emitted from the active layer is totally reflected internally such that it may travel towards a side of the light emitting device, and accordingly, a certain amount thereof may be lost or emitted in an undesirable lateral direction. When the light produced within the light emitting device strikes the surface of the light emitting device, if the incident angle thereof is larger than the critical angle, the light is reflected internally without being extracted to the outside.
Furthermore, the light emitted from the active layer is extracted to the outside after passing through the n-type semiconductor layer in an isotropic direction. As shown in FIG. 1, light (a) emitted from the active layer in a substantially vertical direction may be extracted along a short path, whereas light (b) emitted from the active layer in an inclined direction may follow a long path and be considerably absorbed by the semiconductor layer. As a result, the light extraction efficiency of the light emitting device is further reduced.