Omnidirectional reflectors with novel low-refractive-index material for light-emitting diodes

Solid state lighting technology requires high brightness light-emitting diodes (LEDs) with high light-extraction efficiency. A very promising approach to improve the light-extraction efficiency of devices is to integrate a high reflectivity omnidirectional reflector (ODR) with low-refractive-index (low-n) material onto LED chips. ODRs are defined as reflectors with reflectivity close to 100% at any incident angle for both polarizations. Low-n material is a new class of optical thin film material, which has great potential in application of not only ODRs but also other optical filters.; In this dissertation, internal ODRs with low-n material are discussed. Two kinds of ODRs are reviewed. One is a triple-layer ODR, which consists of a semiconductor, a quarter-wavelength thick low- n layer, and a reflective metal layer such as Ag or Al. The other one is a dielectric-multilayer ODR, which consists of a thick low- n material and a one-dimensional photonic crystal. Both reflectors have high reflectivity at any incident angle in the semiconductor ambient. Low-n material is the key component for the ODRs.; A novel low-n material with refractive index as low as n = 1.05, has been demonstrated to have advantages over conventional optical thin films. A single-pair distributed Bragg reflector using such low-n material is demonstrated to have enhanced reflectivity in the visible spectrum. A graded-index anti-reflection coating with index matching condition enabled by the low-n material is demonstrated to have reflectivity lower than 0.3% over wide spectrum range and angle of incidence.; A conductive low-n material is obtained by oblique-angle deposition of indium-tin oxide (ITO). Such material has a much lower refractive index than conventional transparent dense ITO material while still having good conductivity. A GalnN with triple-layer ODR using a conductive low-n material is successfully fabricated and incorporated into an LED that is shown to have enhanced light-output.; Finally, a new type of 3-dimensional reflector, a pyramid reflector, is demonstrated to provide an additional light-escape cone to the LED die so that the light-extraction efficiency can be significantly enhanced. GaN-based LED with pyramid reflector is demonstrated to have 13.9% enhancement of light output compared with the conventional LEDs.