Gallium Arsenide (GaAs) - Courses

448 A. Borghesi and G. Guizzetti
by Spitzer et al. [8J and Pikhtin and Yas'kov [14J and were obtained with
exceptionally perfect single crystals grown by a modified wet-hydrogen
transport process. All crystals had very high resistivities and three different
thicknesses (0.0212,0.0724, and 0.362 cm).
The 0.8-0.2-/lm region straddling the fundamental band gap has been
measured more recently by ellipsometry on surfaces containing native oxides
that are mathematically "removed" in the analysis, or samples are chemically
polished to remove this oxide and then put into an inert atmosphere while
the measurement of the ellipsometric parameters !/J and d are performed
[24]. We have included these values and at the ends of this spectral region
have overlapped other data for comparison.
The optical properties from the fundamental absorption edge (0.53 /lm)
to about 0.08 pm can be determined by KK analysis of normal-incidence
reflectance data. The R data between 0.53 and 0.21 /lm obtained with a sensitive
double-beam spectrophotometer by Stokowski and Sell [19J provide
a considerable improvement in sensitivity over the older measurements reported
by Philipp and Ehrenreich [16J and Seraphin and Bennett [23]. The
samples used in Stokowski and Sell [19J were grown by the Czochralski
method, were sliced parallel to a {111} face, and a weak ( '" 0.2%) brominemethanol
solution was used as a polishing etch. The absolute-reflectance values
were accurate to within ± 3% of the reflectivity. The data were about
10% higher than those in Philipp and Ehrenreich [16J, in agreement within
1% with those calculated from n at 0.4733 /lm as measured by Dean et al.
[11]. In the 0.21-0.08-/lm range no other R data are available except those
from Philipp and Ehrenreich [16].
Absorption and !"eflection measurements in the 0.08-0.03-/lm range [18,
20J and only absorption measurements in the 0.08-0.008-/lm range [17J
were performed with synchrotron radiation and with a typical resolution of
2 A over the whole spectral range. The crystalline samples for transmission
were obtained by flash evaporation and samples of at least three different
thicknesses between 500 and 1500 A were used. The thicknesses were determined
with a Tolansky interferometer with an accuracy of about 10% and
also during evaporation with a calibrated quartz-crystal monitor. The absolute
values determined for the absorption coefficient were affected by an
error of less than 20%, which arose mainly from uncertainties in the thicknesses.
The R measurements [20J were performed under nearly normal incidence
[15J on polished bulk samples, etched in bromine-methanol solution.
Since the incoming and reflected intensities were not determined simultaneously,
no accurate absolute values of R were obtained (± 50%); therefore
the measured reflectivities were scaled so as to coincide around the first
maximum at 0.059 /lm with the ones calculated by the KK analysis transforms
from II data. Some discrepancies between scaled Rand R from KK
analysis may be due to a structural difference between the thin films used
to measure T and the bulk samples for R. Also, there may be some influence
of the surface treatment and of light scattering. Larger discrepancies be-