Semiconductor light emitting diodes
for illumination and
information technology applications
E. Fred Schubert
Rensselaer Polytechnic Institute
There are two
fundamental ways to reduce optical losses and increase the extraction
efficiency in semiconductor light-emitting diodes, namely (i) the employment of
structures with near-perfect transparency and (ii) the employment of optical
reflectors with near-perfect reflection characteristics. Such reflectors need
to be electrically conductive, have high reflectivity (³ 99 %), and
omni-directional characteristics. Although semiconductor distributed Bragg
reflectors (DBRs) have high reflectivity, the angles of high reflectivity are
restricted to a small cone near normal incidence. For typical semiconductor
DBRs, the high-reflectivity cone has a size of only ± 20 °. As
a consequence, DBRs lack high reflectivity for oblique angles of incidence.
A new type of omni-directional reflector (ODR) is presented which
consists of a semiconductor, a low-index dielectric layer perforated by an
array of microcontacts, and a metal layer. It is shown that this triple-layer
ODR can have normal incidence reflectivities > 99 % and omni-directional
characteristics. The performance characteristics of ODRs can be factor of ten
better than those of conventional DBRs. Experimental results on AlGaInP LEDs with
an integrated triple-layer ODR are presented including the external efficiency
of the devices.
High-performance LEDs are instrumental for daylight illumination
sources based on multiple-LED approaches. Dichromatic and trichromatic sources
are analyzed in detail including figures of merit such as the luminous source
efficiency, color temperature, and color rendering capabilities for a wide
range of primary emission wavelengths. Spectral power density functions of LEDs
are assumed to be thermally broadened and inhomogeneously broadened to a full
width at half maximum of several kT,
in agreement with experimental results. It is shown that the spectral width of
the emission bands is a critical parameter. It is further shown that multi-LED
white light sources have the potential for luminous efficacies greater than 300 lm/W and color rendering indices greater than 90.
Contour maps for luminous efficacies and color rendering indices for
trichromatic sources as a function of the three emission wavelengths are presented.