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quartz modulator po arizer2
window - / /
Figure 2. Schematic of RDS/MOCVD setup.
where Rl, and R, are the reflectances for light polarized
in the as-noted crystallographic direction. Further
details are published elsewhere [5, 6].
The wafers were cleaned using a method similar to
that described by Fitzgerald et al. . The wafers were
first etched in 1NH40H:1H202:10H20 at room
temperature for 1 min. This removes about 0.2 pm. The
wafer is then rinsed in deionized H20 for 2 min followed
by I min in 1HCI:IH202:10H20 and a 2-mi rinse in
flowing DI H20. The wafers are then blown dry with a
stream of N2 gas. The HCI-based solution also etches Ge
at 0.2 pm/min. The samples are then spin-dried and
mounted on molybdenum carriers. The carriers are
heated radiatively with a low thermal mass resistance
heater. The temperature of the sample is determined by
optical pyrometry. The MOCVD reactor pressure was
fixed at 70 torr with a H2 carrier flow of 6 slm.
From a surface science point of view, the hot interior
surfaces of a typical MOCVD reactor are an infinite
source of impurities. Our reactors are used routinely to
grow III-V compounds of Ga, In, Al, As, and P. Hence, if
the last material grown in the reactor is, for example,
GalnP, a subsequent anneal of Ge(100) in this reactor
will contaminate the surface with, primarily, P and In.
Therefore, the results in section 3.1. were achieved using
a freshly cleaned reactor. For section 3.2., we used clean
molybdenum susceptors in a GaAs-contaminated
reactor. This contamination yields a finite partial
pressure of As upstream of the substrate.
After annealing, the samples are quenched to room
temperature and transferred quickly to the analytical
chamber. The quench rate is between 200 and
300'C/min and the total transfer time is less than 10 min.
The STM images were recorded at a sample bias of
-3V and a tunneling current of InA. With the exception
of tilting and artificial illumination to enhance the
contrast, the STM are as recorded.
All the Ge(100) surfaces examined in this study
exhibited a 1x2 or 2x1 reconstruction to some degree.
Schematics of these two reconstructions are presented in
Fig. 3. The arsenic-free Ge(100) and As:Ge(100) results
are very similar to those found for UHV- or MBE-
prepared surfaces. The Ge dimer bonds form a 2x1 array
with the bond axis parallel to the  step edge. With
the addition of As to this surface, the surface symmetry
becomes 1x2, comprised of As dimers with bond axes
perpendicular to the step edges. In the following, we
will briefly show the results for the clean Ge(100) and
As:Ge(100) phases, then compare and contrast with the
results for samples annealed in AsH3.
3.1 Clean Ge(100)
The two principal contaminants of most Ge surfaces
are carbon and oxygen. Exposing the surface to H2 at
elevated temperatures readily removes oxygen. The free
energy for the reaction
GeO2+2H_ ea Ge+2H20
goes through zero at about 500 K . Hence at typical
growth temperatures, there is virtually no germanium
oxide on the surface of the growing crystal and indeed
none is found with AES after annealing in H2.
Prolonged annealing times in H2 at elevated
temperatures are required to remove C; the mechanism by
which this occurs is not known. However, C is easily
removed by annealing the sample in a partial pressure of
AsH3 or PH3. Presumably, the atomic hydrogen from the
dissociation reaction of the hydride molecule quickly
reacts with the C contaminants to produce the stable,
volatile species CH4.
0 0 0 0 0 0
o o o o O 0
0 0 0 000
0O O O
O O O O O Q
Oo 0 0 0 0
0-0 0-0 0 -0
0-.0 0 0-0 00
(011) o0 0-0 0-0 0-0
-O O- 00-0-O
0-00-0 0 -00O
O O O O
O 0 O O
O O O O
O O O 0
0- O O
Figure 3...Summary of Ge(100) or As:Ge(100) surface
phases. The Ge or As dimer bonds are represented by
an array of dumbbells. For the 2x1 reconstruction, the
dimer rows run perpendicular to the step edges
(represented by thick vertical line segments). If the
terrace reconstruction determines the GaAs sublattice
orientation, then the 1x2 As:Ge(100) surface phase
should produce GaAs with B-type (011) steps.
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NCPV preprints for the 2. world conference on photovoltaic solar energy conversion, article, September 1, 1998; Golden, Colorado. (digital.library.unt.edu/ark:/67531/metadc707815/m1/71/: accessed January 22, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.