Gallium Arsenide (GaAs) - Courses

468 Roy F. Potter
[19] are listed in Table V. Room temperature was not stated. Archer [20]
also measured the optical properties over a reduced spectral range with less
spectral detail. Cardona et al. [21] prepared crystalline and amorphous Ge
films on heated and room-temperature KCI substrates, respectively. Samples
were floated off and picked up with a copper mesh. Transmission measurements
were done with synchrotron radiation. The data in Table V were read
from a graph. Room temperature was not stated.
Lukirskii et al. [22] have measured the unpolarized reflectance for Ge as
a function of angle of incidence at four x-ray wavelengths (since e < 1, this
gives a type of external attenuated total reflection). Samples were deposited
on glass substrates. Room temperature was not specified. Note that the angles
as given in Lukirskii et al. [22] are grazing angles, i.e., nl2 -
0.41 flm (1.5-3eV). When the extinction coefficient is low, the agreement in
n between the two sets is quite good (better than 0.25% for 0.83 flm > A >
0.74 flm). As k becomes larger, the discrepancy increases to about 1% at 2 eV
and 4% at 3 eV. At the longer wavelengths Archer's data [20] are about 4%
below those of Aspnes and Studna [19], coming into reasonable agreement
(less than 1% difference) for 0.54 flm > A > 0.36 flm. Thus, the data of Aspnes
and Studna [19] for 0.82 flm > A > 0.2 flm appear to be the best choice for
refractive-index values. Data of Philipp and Taft [17] appear to be too low.
In the spectral region of interband transitions the extinction coefficient
becomes quite large (k > 4), corresponding to absorption coefficients of the
order of 10 6 cm - 1. Reflectance-type measurements must be used. However,
some overlap with transmittance measurements exists for comparison. Measurements
of Dash and Newman [15] extend to 0.69 flm, which permits
comparison with Potter [18J as well as with Aspnes and Studna [19]. The
former appears to be low by about a factor of 2, whilte the agreement with
the latter is within 10-20%, which, given sample treatment and technique
differences, must be considered good. As k becomes larger, Potter's data [18J
come closer to agreeing with those of Aspnes and Studna [19]. For 0.62 flm >
A> 0.4 flm, Potter's data [18J are consistently larger than the data of Aspnes
and Studna [19J by 10-20%. Again, it seems that the data of Aspnes and
Studna [19J would be the choice for values of k.
For values of n overlap regions occur for the data of Briggs [1], Potter
[18J, and the Sellmeier fit. Given that the data of Briggs [lJ and Potter [18]
were taken at 20°C or more, the agreement is good. Below about 2 flm, the
Sellmeier results would increasingly deviate from the measured values. That
the reflectance method agrees with the angle-of-deviation method better than
0.25% in this spectral region establishes the compatibility of the two methods.