Page 17 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Photodetection Basics

10 Chapter One

exact match with commonly available 0.88-mm GaAlAs LEDs. This response
greatly reduces the need for optical filtration with interference filters or IR-
transparent black plastic molding for visible light suppression. This character-
istic of being “blind” to interfering wavelengths is made good use of in detectors
of silicon carbide (SiC). It is a high bandgap material that produces detectors
with sensitivity in UV and deep blue ranges. They are useful in UV photome-
try and for blue flame detection. Silicon carbide photodetectors have been made
available by Laser Components, which offer a peak sensitivity at 275nm and
a response that is very low above 400nm. At the peak the responsivity is
0.13A/W. Detectors fabricated from chemical vapor deposited diamond have
also been described (Jackman 1996) for use from 180 to 220nm. Sensitivity
throughout the visible spectrum is insignificant. Last, a large range of lead
sulphides, selenides, and tellurides are used for infrared detection in the 3- to
10-mm region.
For wavelengths below about 350nm the normal borosilicate (Pyrex) glass
used for detector windows becomes absorbing, and alternatives such as fused
silica or synthetic sapphire must be considered. These are transparent to
approximately 0.2mm and 0.18mm, respectively, depending on their purity and
fabrication methods. Many manufacturers offer a choice of window materials
for the same detector. At even shorter wavelengths, silicon can still be useful
for detection, but the window must be dispensed with altogether. Some
manufacturers provide windowless photodiodes in sealed, airtight envelopes.
However, once the envelope is opened, maintaining the low electrical leakage
properties of the photodiode under the attacks of atmospheric pollution and
humidity is difficult. They gradually become much noisier. This should there-
fore be considered only as a last resort or where alternative protection can be
provided. At these short wavelengths, air is itself becoming absorbing, necessi-
tating vacuum evacuation of the optical path. This is the origin of the term
vacuum ultraviolet region.
We have calculated the ideal responsivity of photodiodes assuming that the
photon is absorbed in the depletion region. Another significant reduction in per-
formance arises from photons that are reflected from the surface of the diode,
never penetrating into the material, let alone reaching the depletion region. The
fraction of energy lost in this way is given by the power reflection coefficients
of the Fresnel equations (Fig. 1.6). For the simplest case of normal incidence of
light from air into the material of refractive index n, the power reflectivity is
2
((n - 1)/(n + 1)) .
Detector semiconductors usually exhibit high refractive indices. For silicon
with n ª 3.5 the power reflectivity is 31 percent, leaving only 69 percent to pen-
etrate into the detector material. To reduce this problem, detectors are
often treated with antireflection (AR) coatings. For example, a one-quarter-
wavelength (l/4) layer of silicon nitride (Si 3 N 4 with n = 1.98) can reduce the
reflected power to less than 10 percent across the visible and near infrared and
essentially to zero at a fixed design wavelength. For special uses, three or four
photodiodes can be assembled to achieve very high absorption efficiency across

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.