Gallium Arsenide (GaAs) - Courses

466 Roy F. Potter
cut as polished prisms having included angles ranging between 5 and 18°.
The angle of minimum deviation was measured to obtain the refractive index.
Data of Icenogle et al. [4] at 297 K and some data of Edwin et al. [5] at
25°C, originally given in tabular form, are listed in Table V. The tabulated
data are plotted in Fig. 4.
The precision of the data of Icenogle et al. [4] was such that Barnes and
Piltch [6] used them as a basis for evaluating the coefficients of a temperaturedependent,
Sellmeier-type equation
n 2 = A + BA 2 /(J.? - C) + DA 2 /(A 2 - E).
Using the temperature-dependent values of n, they found the following
values for the coefficients:
A = -6.040 x 1O- 3 T + 11.05128,
B = 9.295 x 1O- 3 T + 4.00536,
C = -5.392 x 1O- 4 T + 0.5999034,
D = 4.151 x 1O- 4 T + 0.09145,
E = 1.51408T + 3426.5.
We have evaluated this expression for "room temperture" of 18°C (291 K),
over a large infrared wavelength range, and these values are included in
Table V. Some comparison with the results from the several experiments
outlined here can be made in the table. The Sellmeier equation represents
the accurate values for high-grade optical-quality germanium within 0.025%,
or better, over the infrared spectral range 2.5-14 p.m and can be extrapolated
to 2.0 and 40 p.m.
Many more measurements have been reported for this spectral range, but
in most cases the specimens were thin films, amorphous material, or heavily
doped. Evaporated films do not, in general, have properties representative
of bulk material, so such results are applicable only to material evaporated
or sputtered by a given recipe. Li [7] tabulates most such measurements
that have appeared in the literature in the past half-century. Li [7] also gives
a table of recommended values for the infrared at various temperatures.
In the infrared spectral region there are two contributions to the intrinsic
absorption properties of germanium. When the infrared radiation field interacts
with the crystal lattice, it is described as a photon-phonon interaction.
Since selection rules include the conservation of energy and wave vector or
momentum, the spectrum will include features related to the photon excitation
of multiple phonons. Such "phonon" bands have been observed for Ge.
Collins and Fan [8] reported absorption coefficients for phonon bands for
300 K. These have been read from a graph and are listed in Table V. Values
of Lax and Burstein [9] are comparable. More recently, Aronson et al. [10]
have measured the absorption coefficient for high-resistivity Ge ( '" 50 n em)
at 300 K. They used samples of different thickness measuring the transmit-