Page 61 - Photonics Essentials an introduction with experiments

Photodiodes

Photodiodes 55

tor and the air, Fresnel’s equations show that the reflected light can
be reduced to zero. however this will occur only at one wavelength of
interest. To accomplish this, the intermediate antireflection layer
must have an index of refraction that is equal to the geometric mean
of the air and the semiconductor. That is,
n AR = 1 · 3 .5 1.9 (3.31)
And the thickness must be equal to one-quarter the optical wave-
length in the antireflection layer:

/n
Thickness = t = (3.32)
4
Silicon nitride (n = ~2) is often used for antireflection coatings. If we
wished to make such a coating on a silicon photodiode to minimize re-
flection at
= 1 m, the optimum thickness of the coating would be
about 1200 Å. In practice, the coating may be deposited while the
diode is operating under illumination at the wavelength of interest, so
that the optimum thickness can be determined directly. Bear in mind
that the antireflection coating will reduce reflections only at the de-
sign wavelength; they increase reflection loss at other wavelengths
(see Fig. 3.10).
In order to eliminate recombination as an issue, the photons must
all be absorbed within a minority carrier diffusion length of the junc-
tion. In addition, it is equally important to eliminate defects that may
act as recombination centers. There is higher density of recombina-
tion centers at the semiconductor/air interface. The effect of surface
recombination can be mitigated by passivation of the surface. This
can be accomplished by the same coating used to reduce reflections.
The manufacture of silicon and GaAs has reached such a state of ex-
cellence that the presence of defects can be ignored for photodiode ap-
plications. In addition, it is straightforward to design the photodiode
device so that all of the photons can be collected.
However, because of the long absorption length for photons in sili-
con, photocarriers will have to diffuse to the junction over substantial
distances. This feature means that high quantum efficiency can be
achieved in silicon photodiodes at the cost of degradation in the speed
of response. This trade-off is not present in direct band gap photodi-
odes like GaAs because the speed of response is usually not limited by
photocarrier diffusion.
This discussion of antireflection coatings is pertinent because it ap-
plies to the way most commercial photodiodes are manufactured. How-
ever, this is not the only way to achieve lower reflection losses. The flat,
shiny surface of a typical semiconductor like silicon or InP is a low-

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