A light emitting diode (LED) is typically composed of semiconductor layers with high indexes of refraction. For example, a group III nitride based LED can have layers with refractive indexes that are typically higher than two. A typical LED emits light uniformly in different directions. However, only some of the light exits the LED structure. A large fraction of emitted light is totally internally reflected and trapped in the semiconductor structure. This light trapping leads to light absorption of the semiconductor layers and contact, which in turn leads to a low light extraction efficiency for the LED.
Many approaches propose to improve light extraction efficiency through surface roughness and the shaping of the LED device. Shaping may be a straightforward and effective approach to increasing the light extraction efficiency of an LED. One approach discusses an LED in a shape of a truncated inverted pyramid, where four faces of an AlGaInP/GaP LED chip are mechanically fabricated to form the truncated inverted pyramid. Using this shape, the external quantum efficiency of the LED was increased. Another approach uses an etching process to fabricate a substrate with inclined faces. In this approach, LED epitaxial layers are selectively grown over the etched regions to obtain a multi-incline light emitting structure without using a mechanical fabrication process.
The use of various LED shapes have been proposed. One approach uses polyhedron chips (rhomboidal and triangular) with parallel bases. For example, FIG. 1A shows a horizontal cross section of a rhomboidal-geometry chip 2 according to the prior art, along with photon trajectories. As illustrated, the chip 2 has a plane deformation angle, αh≠90°. The photons that travel parallel to the horizontal plane inevitably escape since each internal reflection reduces the incidence angle by αh. An optimal deformation angle at which only a couple of flights are required for most photons to escape is close to αh=2Θc(ns, ne), where Θc is the escape cone, ns is the refractive index of the semiconductor materials, and ne is the refractive index of the surrounding environment (e.g., air). If, in addition, the sidewalls of the chip are slanted similarly, most of the photons generated would find escape cones regardless of their traveling directions. Increased light extraction efficiency (e.g., up to 120% compared with a rectangular geometry) was shown using a statistical tracing of the photon trajectories. One proposal describes a technique for the fabrication of geometrically deformed chips by slanted sawing of wafers.
FIG. 1B shows a schematic vertical cross section of a truncated inverted pyramid LED 4 according to the prior art. The geometrically deformed LED 4 is formed from an epitaxial AlGaInP structure wafer bonded to a thick GaP substrate. By using a beveled dicing blade, chips with sidewall angles of 35° with respect to the vertical were fabricated. The geometry of the LED 4 is seen to improve light extraction by redirecting totally internally reflected photons from the sidewalls to the top surface or from the top surface to the sidewalls at small incidence angles.