Page 264 - Chalcogenide Glasses for Infrared Optics

AMI Infrar ed Crystalline Materials 239

1 2 3 4 5 6 7891 2 3 4 56 7891 2 3 4 56 7891 2 3 4 56 7891 2 3 4 56 7891
1
9
8
7
6
5
4
3
2
100 9
8
7
6
5
4
3
2
10 9 8 7 6 5 4
Absorption at 10.6 mm (cm –1 ) 1.0 3 2 9 8 7 6 5 4 3

0.1
8 2 9
7
6
5
4
3
2
0.01 9
8
7
6
5
4
3
2
0.001
0.001 0.01 0.1 1.0 10 100
Resistivity (Ω·cm)
FIGURE 9.20 Free carrier absorption at 10.6 µm in N-type gallium arsenide as a
function of resistivity.

a decrease in resistivity front to back of 1 to 0.35 Ω·cm in the 8-in plate
length. Such a fall is consistent with an increase in dopant concentra-
tion as a melt is slowly frozen down the length of the plate, such as
found in the zone refining purification process of high-purity materials.
Classical methods 22,23 may be used to calculate the absorption by
infrared, the long wavelengths of microwave and radio frequencies. Such
methods have been used before by the author in dealing with the depth
18
of mechanical damage in GaAs. The classical expressions all show the
absorption process increasing with the wavelength squared. However, for
24
GaAs the value is reported to be 3 not 2; the effect is much stronger than
predicted with classical expressions. When our measured values were
reexamined, we found our calculated power value was 2.8, not quite 3.