Advanced encapsulant structures
Graded-index encapsulants consisting of several layers with different refractive indices were demonstrated by Lee et al. (2004). The layer of highest refractive index is in contact with the semiconductor chip. The outer layers of the encapsulant have lower refractive indices. Extraction efficiencies exceeding those with a constant refractive index can be attained by using such refractive-index graded encapsulants.
Encapsulants containing mineral diffusers cause light to reflect, refract, and scatter, thereby randomizing the propagation direction and isotropizing the far-field distribution. For multi-color devices (e. g. multi-chip white LEDs), mineral diffusers uniformize the color distribution. Mineral diffusers are optically transparent substances, such as TiO2, CaF2, SiO2, CaCO3, and BaSO4, with a refractive index different from that of the encapsulant (Reeh et al., 2003).
Encapsulants containing nanoparticles with a high refractive index (e. g. titania, magnesia, yttria, zirconia, alumina, GaN, AlN, ZnO, ZnSe) have been proposed by Lester et al. (1998). Nanoparticles embedded into a host (usually a polymer) do not scatter light if they are uniformly distributed and if their size is much smaller than the wavelength. The refractive index of the of the nanoparticle-loaded encapsulant is given by
n = nhostVhost + nnanoVnano (11 1)
Vhost + Vnano
where Vhost and Vnano refer to the volume of the host and nanoparticles, respectively. For high loading factors, the refractive index of the encapsulant can significantly exceed that of the host, thereby enlarging the semiconductor escape cone and increasing the extraction efficiency.