Recently, a light emitting diode (LED) is spotlighted as a light emitting device. Since the LED can convert electric energy into light energy with high efficiency and long life span of about 5 years or more, the LED can remarkably reduce the energy consumption and repair and maintenance cost. In this regard, the LED is spotlighted in the next-generation lighting field.
Such an LED includes a light emitting semiconductor layer including a first conductive semiconductor layer, an active layer and a second conductive semiconductor layer, in which the active layer generates light according to current applied to the first and second conductive semiconductor layers.
The LED may be classified into a lateral type LED and a vertical type LED.
According to the lateral type LED, the first conductive semiconductor layer, the active layer and the second conductive semiconductor layer are formed over a growth substrate, and the second conductive semiconductor layer, the active layer and the first conductive semiconductor layer are partially removed such that a portion of the first conductive semiconductor layer can be exposed to form an electrode layer. Thus, the light emitting area may be reduced so that the light efficiency may be degraded.
In addition, according to the lateral type LED, since the first conductive semiconductor layer, the active layer and the second conductive semiconductor layer are formed over the growth substrate having low thermal conductivity, the heat dissipation is not easy.
In contrast, according to the vertical type LED, a first electrode layer is formed over the first conductive semiconductor layer and a second electrode layer is formed under the second conductive semiconductor layer, so it is not necessary to remove the active layer to form the electrode layer. Thus, the light emitting area may not be reduced, so that the light efficiency may be improved as compared with that of the lateral type LED.
In addition, according to the vertical type LED, heat is transferred through the second electrode layer, so the heat dissipation is easy.
Although the vertical type LED has the advantages, several requirements sill remain in the vertical type LED and the lateral type LED.
First, in the LED, the light efficiency has to be increased by preventing a current flowing between the first and second electrodes from being concentrated over a specific region and allowing the current to be widely spread to increase the light emitting area.
Second, in the LED, since the second conductive semiconductor layer has relatively high sheet resistance due to low carrier concentration and mobility, a transparent current injection layer including ITO or ZnO is required to form an ohmic contact interface. However, the current injection layer including ITO or ZnO forms a schottky contact interface instead of the ohmic contact interface through the following processes such as deposition and heat treatment processes. Accordingly, the current injection layer forming the ohmic contact interface is required.
Third, in the vertical type LED, when the light emitting semiconductor layer is bonded to the conductive support substrate through a wafer bonding process, cracks or breakages may be caused in the light emitting semiconductor layer due to the difference in a thermal expansion coefficient between the light emitting semiconductor layer and the conductive support substrate. Accordingly, the wafer bonding process has to be performed at a temperature of less than 300° C. to minimize the thermal stress, so that it is difficult to perform the subsequent processes.