Page 125 - Photonics Essentials an introduction with experiments

Light-Emitting Diodes

Light-Emitting Diodes 119

flected back into the interior of the LED because of the textured na-
ture of the interface. On the second or third encounter, the photon will
find itself in the escape cone and be emitted. By texturing the surface,
we can increase its emissivity, which is a measure of how easy it is for
light to cross an interface. This idea of using a textured surface to lo-
calize the emitted photons near the interface until they can cross the
interface within the escape cone dictated by the index contrast has
been applied to LED design. Using this method, LEDs with external
emitting efficiencies greater than 30% have been measured.
A bright, shiny, smooth surface such as polished metal has lower
emissivity than a rough surface of the same material. The typical
semiconductor wafer used as a substrate for a LED is highly planar
and polished, and consequently has a low-emissivity surface. Emissiv-
ity is proportional to absorbance. Light incident on a semiconductor
with a highly textured surface is more likely to be absorbed than if
the surface is a smooth low-emissivity structure. High-emissivity sur-
face treatment is also used to prepare solar cells with absorption bet-
ter than that presented by a smooth, planar surface.
Schnitzer and coworkers at UCLA have shown how this concept can
be turned into reality. They covered the surface of an LED with glass
spheres, and then they sandblasted the surface, using the spheres as
a mask. The result was to transfer the pattern of the spheres into the
surface of the LED. The result of this is shown in Fig. 6.11. The effect
of texturing the surface triples (!) the external efficiency. The inven-
tion of this technique is a key event that has changed thinking about
the application of LEDs to lighting applications.
Light is an electromagnetic wave, just like a radio wave. Radio en-
gineers have long understood that the most efficient way to emit radio
waves is to use an antenna. Radio antennas do not have flat polished
surfaces like the top surface of most semiconductor wafers from which
LEDs are made. The external emission efficiency of LEDs could be
further improved by implementing a photon antenna on the surface of
the LED, using principles learned from radio antenna design.
The interest in improving the external emission efficiency of LEDs
is motivated in large part by the challenge of demonstrating a light
source more efficient than a tungsten light bulb, the overall power ef-
ficiency of which is about 10%. There is an active area of research to
improve the performance of LEDs by modifying the matrix element
(defined as the probability that a radiative transition will occur) in
Eq. 6.3 in a way that increases the radiative recombination rate. The
improvement in the quantum efficiency by such a change can be esti-
mated from Eq. 6.8. The improvement is achieved by building a mi-
croresonant cavity around the emitting region. The dimensions of the
cavity are chosen to be a fixed multiple of the light for which one

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