Page 249 - Chalcogenide Glasses for Infrared Optics

AMI Infrar ed Crystalline Materials 225

for the crystal 10 to 14 in long. All purification runs follow the same
procedure, always zoning in the same direction relative to the rod.
On completion of a run, the crystal is removed and its surface sand-
blasted. The crystal is evaluated relative to resistivity using a conduc-
tion method. A dc value of 0.1 A is run through the crystal. An electric
probe with contacts exactly 1 cm apart is applied to the surface to
measure and record the voltage drop across each centimeter segment,
1 cm at a time for the full length of the crystal. The numbers are con-
verted to resistivity. After measurement, the crystal surface is
acid-etched prior to the next run. The process is repeated 8 to 10 times,
depending on the beginning purity of the silicon bar, until the mea-
sured resistivity falls within the range of the specification. Then the
crystal is ready to be grown in single-crystal form. The top of the
crystal is cut off to remove the impurities, and the ends are switched
so the growth will be in the same direction relative to the crystal as in
the purification process. The crystal is attached to the pedestal for
rotation during growth. The crystal growth is controlled by the
operator observing the process through the window. The growth rate
again is very slow. On completion, the crystal resistivity is measured to
ensure all regions meet the resistivity specification, 10,000 to 30,000 Ω·cm.
On meeting the standard, the crystal is ready to be sawed into slices
that will be polished and sized in thickness and diameter. AMI
developed a good paveway silicon business having substantial sales
in the period from 1984 to 1992. Eventually, European competition at
lower cost created a paveway market in which AMI could no longer
compete. Even though government policy usually calls for domestic
sources for critical materials, government production programs often
ignore the policy and put price first.

9.5 Silicon as an Infrared Optical Material
Semiconductor materials have free carriers of current, both positive
(holes) and negative (electrons). Intrinsic levels depend upon
temperature and the bandgap of the material. Silicon has a bandgap
of only 1.04 eV (electronvolts) which means intrinsic carriers are ther-
mally generated at relatively low temperatures. However, at room
5
temperature, the intrinsic resistivity of silicon is 2 × 10 Ω·cm. Infrared
radiation interacts with free carriers and is absorbed. The magnitude
of absorption increases with increased wavelength to at least the
power of 2. With increase in temperature, mostly the impurity atoms
in silicon contribute free carriers, referred to as extrinsic, reducing the
resistivity. TI results showed that free carrier absorption at room
8
temperature for N-type silicon, 1 Ω·cm resistivity or less, was signifi-
cant at 10 µm. Silicon produced in the normal Cz process was never
pure enough to have a resistivity over 25 to 50 Ω·cm. When heated,
N-type silicon with a resistivity of 50 Ω·cm begins to show free carrier