1. Field of the Invention
The present invention relates to a method of fabricating a vertical structure nitride semiconductor light emitting device, and more particularly, to a method of fabricating a vertical structure nitride semiconductor light emitting device that can allow the vertical structure nitride semiconductor light emitting device to be fabricated with a polygonal cross section having five or more sides such that the amount of light emitting from a side surface of the light emitting device is increased to improve the light extraction efficiency and to decrease the heat generation due to an total internal reflection.
2. Description of the Related Art
In general, a nitride semiconductor, which is a III-V semiconductor crystal, such as GaN, InN, AlN or the like, is widely used in a light emitting device (LED) that can emit a single wavelength of light, especially, blue light.
Since a nitride semiconductor light emitting device in particular is fabricated using a sapphire substrate or the like satisfying a lattice matching condition for a crystal growth, it has a horizontal structure in which two electrodes connected with p-type and n-type nitride semiconductor layers are arranged nearly in parallel with an upper surface of a light emitting structure. A horizontal structure nitride semiconductor light emitting device (LED) according to the related art is shown in FIG. 1A.
Referring to FIG. 1A, the horizontal structure nitride semiconductor LED according to the related art is configured to include a sapphire substrate 11, an n-type nitride semiconductor layer 12, an active layer 13, a p-type nitride semiconductor layer 14 and an ohmic contact layer 15 sequentially deposited on the sapphire substrate, an n-side electrode 16 disposed on a partially exposed upper surface of the n-type nitride semiconductor layer 12, and a p-side electrode disposed on an upper surface of the ohmic contact layer 15.
However, the horizontal structure nitride semiconductor LED has several drawbacks as follows.
First, a flow of current toward the p-side electrode 17 from the n-side electrode 16 via the active layer 13 is formed narrow in a horizontal direction. Such a narrow current flow causes the forward bias (Vf) of the horizontal structure nitride semiconductor LED to increase, so that the current efficiency is deteriorated.
Also, in the horizontal structure nitride semiconductor LED, since an increase in the current density increases the amount of heat generation and a low thermal conductivity of the sapphire substrate suppresses a smooth heat radiation, a mechanical stress may occur between the sapphire substrate and the nitride semiconductor light emitting structure due to the increase of heat generation, so that the device become instable.
Additionally, in order to form the n-side electrode 16 in the horizontal structure nitride semiconductor LED, it is required that the partial upper surface of the n-type nitride semiconductor layer 12 be exposed by partially removing the active layer 13 and the p-type nitride semiconductor layer 14 by an area larger than the area of the n-side electrode 16. To this end, the light emitting area is decreased and the light emitting efficiency according to a ratio of brightness to device size is deteriorated.
To improve the aforementioned drawbacks of the horizontal structure nitride semiconductor LED, development of a vertical structure nitride semiconductor LED is actively performed, in which the sapphire substrate is removed using a laser lift-off process.
FIG. 1B is a perspective view of a vertical structure nitride semiconductor LED according to the related art. Referring to FIG. 1B, the related art vertical structure nitride semiconductor LED includes an n-type nitride semiconductor layer 12, an active layer 13, a p-type nitride semiconductor layer 14, a high reflective ohmic contact layer 15 and a conductive support substrate 18. To form the above vertical structure nitride semiconductor LED, the n-type nitride semiconductor layer 12, the active layer 13, and the p-type nitride semiconductor layer 14 are sequentially formed on a sapphire substrate and thereafter the sapphire substrate is removed using a laser lift off (LLO) process. At this time, the n-type nitride semiconductor layer 12 serves as the uppermost layer and an upper surface of the n-type nitride semiconductor layer 12 is used as a light emitting surface. A transparent electrode layer 19 for improving the current diffusion can be optionally formed on the upper surface of the n-type nitride semiconductor layer 12. Also, an n-side electrode 16 is formed on the upper surface of the n-type nitride semiconductor layer 12 or an upper surface of the transparent electrode layer 19, and is supplied with current through a wire bonded thereto.
Such vertical structure nitride semiconductor LEDs have generally a rectangular cross section. To this end, when light generated from the active layer 13 travels toward a side surface of the LED, a difference in refractive index between the nitride constituting the LED and outer air restricts an incident angle allowing light to emit outside the LED. As shown in FIG. 2, the light generated at a point of the active layer 13 of the LED can pass through the side surface of the LED and emit to an outside only when it travels at an angle smaller than an incident angle θ. The light traveling at an angle larger than the incident angle is totally reflected toward an inside of the LED. Since the totally reflected light cannot emit from the inside of the LED to the outside, the light extraction efficiency of the LED is deteriorated, so that brightness decreases. In addition, since the light that does not emit to the outside of the LED is dissipated as heat inside the LED to increase the exothermic amount of the LED, the inner temperature is elevated to change the characteristic of the LED and to shorten the lift span.