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anic Semiconductors for Light-emitting Diodes
E. Fred Schubert, Thomas Gessmann, and Jong Kyu Kim
Introduction
During the past 40 years, light-emitting diodes (LEDs) have undergone a signifi-
cant development. The first LEDs emitting in the visible wavelength region were
based on pound semiconductors with external efficiencies of only
%. Today, the external efficiencies of red LEDs based on AlGaInP exceed
50 %. AlGaInP semiconductors are also capable of emitting at orange, amber,
and yellow wavelengths, albeit with lower efficiency. Semiconductors based on
pounds can emit efficiently in the UV, violet, blue, cyan, and
green wavelength range. Thus, all colors of the visible spectrum are now covered
by materials with reasonably high efficiencies. This opens the possibility to use
LEDs in areas beyond conventional signage and indicator applications. In partic-
ular, LEDs can now be used in high-power applications thereby enabling the re-
placement of incandescent and fluorescent sources. LED lifetimes exceeding
i 105 pare favorably with incandescent sources (Z 500 h) and fluorescent
sources (Z 5000 h), thereby contributing to the attractiveness of LEDs.
anic LEDs are generally based on p-n junctions. However, in order to
achieve high internal quantum efficiencies, free carriers need to be spatially con-
fined. This requirement has led to the development of heterojunction LEDs con-
sisting of different semiconductor alloys and multiple quantum wells embedded
in the light-emitting active region. The light-extraction efficiency, which measures
the fraction of photons leaving the semiconductor chip, is strongly affected by the
device shape and surface structure. For high internal-efficiency active regions, the
maximization of the light-extraction efficiency has proven to be the key to high-
power LEDs.
This chapter reviews important aspects of anic LED structures. Section
introduces the basic concepts of optical emission. Ban