Light emitting diodes refer to inorganic semiconductor devices that emit light generated by recombination of electrons and holes, and are used in a variety of fields such as displays, vehicle lamps, general lighting devices, and the like.
Light emitting diodes can be classified into lateral type light emitting diodes, vertical type light emitting diodes, and flip-chip type light emitting diodes according to locations of electrodes or a connection type between the electrodes and an external lead.
The lateral type light emitting diodes can be fabricated relatively easily and thus are the most widely used in the art. Such a lateral type light emitting diode includes a growth substrate at a lower side thereof instead of being separated therefrom. As the growth substrate of the light emitting diode, a sapphire substrate is most widely used in the art and has a problem of difficulty in discharge of heat from the light emitting diode due to low thermal conductivity of the sapphire substrate. As a result, the light emitting diode has an increased junction temperature and decreased internal quantum efficiency, and becomes unsuitable for high current driving.
The vertical type and flip-chip type light emitting diodes have been developed in order to solve the problems of the lateral type light emitting diodes. Generally, as compared with a typical lateral type light emitting diode, the vertical type light emitting diode has good current spreading performance by adopting a structure in which a p-electrode is located at a lower side, and also exhibits good heat dissipation efficiency by adopting a support substrate which has higher thermal conductivity than sapphire. Furthermore, in the vertical type light emitting diode, an N-plane is subjected to anisotropic etching by photo enhanced chemical (PEC) etching or the like to form a rough surface, thereby significantly improving upward light extraction efficiency. However, in the vertical type light emitting diode, a total thickness of epitaxial layers is thin as compared with a luminous area, thereby providing difficulty in current spreading. That is, since the vertical type light emitting diode fabricated using an electrically conductive nitride layer is configured to allow direct flow of electric current between an upper electrode and a lower electrode formed at a lower side of the nitride layer, it is difficult to achieve uniform current spreading over the entirety of the active region. To solve this problem, an insulation material is provided to a location of a p-electrode corresponding to an n-electrode pad to prevent electric current from directly flowing from the n-electrode pad to the p-electrode. However, with this structure, the light emitting diode has a problem of cracking between the p-electrode and the insulation material during a process of removing the substrate.
On the other hand, the vertical type light emitting diode includes a lower semiconductor layer and an upper semiconductor layer formed of different conductive type semiconductors, and requires electrodes respectively connected to the upper and lower semiconductor layers. Thus, in fabrication of the vertical type light emitting diode, there is a need for a process of separating a growth substrate from the semiconductor layers.
Generally, in order to prevent damage to the semiconductor layers upon separation of the growth substrate, a metal substrate is bonded to the semiconductor layer at an opposite side to the growth substrate before separation of the growth substrate from the semiconductor layer. Then, the growth substrate is separated from the semiconductor layer by laser lift-off, chemical lift-off, or stress lift-off. The metal substrate is bonded to the semiconductor layer via a separate bonding layer, which serves to bond the metal substrate to the semiconductor layer while being cooled from a certain bonding temperature or more to room temperature.
On the other hand, the metal substrate and the semiconductor layer, for example, a gallium nitride-based semiconductor layer, have different coefficients of thermal expansion, thereby causing a bowing phenomenon wherein the semiconductor layer is bent while being cooled from the bonding temperature to room temperature. Such a bowing phenomenon becomes severe upon separation of the growth substrate over a large area. When the growth substrate is separated over a large area, there is a high possibility of damage to the semiconductor layer due to the bowing phenomenon, thereby making it difficult to separate the growth substrate at the wafer level. In order to prevent damage to the semiconductor layer caused by the bowing phenomenon, the growth substrate is separated from individual light emitting diodes after the wafer is divided into unit diodes. As a result, a conventional method of fabricating a vertical type light emitting diode requires a complicated process and high manufacturing costs.
Furthermore, since the typical vertical type light emitting diode has an upper metal electrode placed on a light emitting plane, the upper metal electrode can shield light emitted from the active layer. As a result, the vertical type light emitting diode can suffer from deterioration in light extraction efficiency.
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