Comparative Discharge Diagnostic Study of Silane, Disilane, and Germane RF Discharges Using Optical Emission Spectroscop
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COMPARATIVE DISCHARGE DIAGNOSTIC STUDY OF SILANE, DISILANE, AND GERMANE RF DISCHARGES USING OPTICAL EMISSION SPECTROSCOPY AND MASS SPECTROMETRY H. CHATHAM AND P.K. BHAT, (GSI) Glasstech Solar, Inc., 80033 U.S.A.
12441 West 49th Avenue,
Colorado
Wheatridge,
ABSTRACT Measurements germane
and
rates
of
germane
dissociation result,
of the
of the depletions
disilane-germane and
rf
feedstock gases
discharges
monosilane
differ
indicate by
a
in
that the
factor
of
2
monosilanedissociation whereas
rates of germane and disilane differ by a factor of 0.8.
larger
germane concentrations
in monosilane-germane
discharges in
are required in
the As a
disilane-germane
than
order to achieve equal germanium content
in the resulting films.
INTRODUCTION Low pressure silane glow discharges have been extensively studied using a number of diagnostic
techniquesI.
These
studies have greatly increased
our understanding of the processes leading to the deposition of hydrogenated amorphous
silicon
studies
of
germane
glow
(a-Si:H).
In
monosilane-ogermane2'3 discharges
used
to
contrast,
there have
and
to our
none
produce
been
knowledge
hydrogenated
few diagnostic of
disilane-
amorphous
silicon-
germanium alloys (a-SiGe:H). These
alloys
optical band which
is
are of interest
gap is
advantageous
4
material .
in
solar cell applications because their
0.1 to 0.5 eV lower than that of a-Si:H (-
However,
because
of
the
these alloys suffer
increased in
light
general
1.78 eV),
absorption
of
the
from poorer properties
such as the photoconductivity and the density of states in the gap compared 4
with a-Si:H . diagnostic
In
an effort to improve these materials,
study of low pressure monosilane-germane
we have undertaken a
and disilane-germane
rf
glow discharges. We
have
depletion,
f,
concentration. 414
run
was
studied
the
dissociation
of
the
gases
by
measuring
the
of each gas as a function of rf power and fractional germane The optical also
emission intensity
monitored
as
a
function
of the SiH emission line at of
rf
power
and
germane
concentration.
RESULTS Diagnostic measurements were performed
in
an rf glow discharge
with load lock manufactured by GSI (see Figure 1). Mass spectra of the
Mat. Res. Soc. Symp. Proc. Vol. 118. ' 1988 Materials Research Society
system
32
INPUTGASMANIFOLD EXHAUST MECHAN PUMP
TURBO PUMP GT DETECNSTRPORTNIEWPPOR VACUUMS
V PAL
ROOTS
SGA
PROCMASS SEXTROME
BLWE
principle each
to
due
mass peaks power
rf
the
measuring
by
determined
were
obtained
was
concentration
germane
and
of these gases size
the
of
the
depletion of each gas at
The
the discharge.
in
the
variable
and Si 2 H6 + GeH4
+ GeH4
change
fractional
into a
through
The fractional depletions
Figure 2a.
in
are shown
gases
sampling
region
spectra for SiH4
Typical mass
conductance valve. mixtures
by
obtained spectrometer
mass
pumped
differentially
were
species
neutral
stable
Discharge deposition syst
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