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SURFACE AND INTERFACE ANALYSIS, VOL. 26, 105È108 (1998)
Electron-stimulated Desorption Total Cross-section
Determination for Digermane on Si(100)
A. F. Aguilera,1,2 J. H. Campbell,3 J. H. Craig, Jr.2,3,* and K. H. Pannell1,2
1 Department of Chemistry, 500 W. University Ave., University of Texas at El Paso, El Paso, TX 79968-0513, USA
2 Materials Research Institute, 500 W. University Ave., University of Texas at El Paso, El Paso, TX 79968-6664, USA
3 Department of Physics, 500 W. University Ave., University of Texas at El Paso, El Paso, TX 79968-0515, USA
We have studied digermane-covered Si(100) using electron-stimulated desorption (ESD). Estimates are presented
for the total H(a) ESD removal cross-section for digermane-exposed Si(100) substrates at 85 K using electrons
incident at 150 eV energy. It is found that electron-enhanced deposition of Ge occurs only when physisorbed
digermane is present. Auger electron spectroscopy provided the means for determining the relative amounts of
germanium adsorbed on the Si(100) surface following digermane exposures, electron irradiation and surface reconstruction. It is found that two coverage regimes are important : initial dosing of digermane on Si(100) at 85 K
results in overlayers consisting of both physisorbed digermane and chemisorbed GeH (a) (x = 1, 2 or 3) species ;
x
and short anneals to 200 K following exposure of the Si(100) surface at 85 K lead to the presence of only chemisorbed GeH (a). The two coverage regimes exhibit di†erent ESD behavior. Two kinetic energy distribution (KED)
x
peaks are seen when physisorbed digermane is present, and only one when it is absent. The ESD signal decay curves
obtained from the two surfaces are also di†erent : the presence of physisorbed digermane results in a twocomponent exponential signal decay ; the absence of the physisorbed species results in a single-exponential decay.
The total H removal cross-section from the physisorbed digermane overlayer was determined to be
r ¿ 1.4 Â 10—15 cm2, while that from Si(100) with only adsorbed GeH present was found to be r ¿ 2.6 Â 10—16
x
cm2. Our results suggest that adsorbed GeH (a) species remain intact on the surface even when the Si(100) subx
strate is annealed to 200 K, indicating that hydrogen migration from surface GeH (a) to Si surface sites does not
x
occur at 200 K. ( 1998 John Wiley & Sons, Ltd.
Surf. Interface Anal. 26, 105È108 (1998)
KEYWORDS : ESD ; Si(100) ; electron-stimulated desorption ; silicon
INTRODUCTION
The e†ects of an electron beam on digermane overlayers
on Si(100) surfaces are interesting in part because
enhanced Ge deposition has been observed.1 The
observation of enhanced Ge deposition and e†orts to
characterize this phenomenon may lead to a better
understanding of Ge H adsorption and decomposition
2 6 Although we have examined
mechanisms on Si(100).
Ge H on Si(100) previously, and have demonstrated
6 deposition is enhanced under electron irradiathat2 Ge
tion, further studies are needed to better characterize
the interaction of the electron beam with adsorbed
Ge H . In this work, we report the H(a) removal cross2 6 from Ge H -covered Si(100) surfaces using an
section
2 6 upon the surface at 70¡ o† the
electron beam incident
surface normal and 150 eV kinetic energy.
Samples of p-silicon(100) with a resistivity of D0.1 )
cm were prepared and placed into an ultrahigh vacuum
system operating at D2 ] 10~10 Torr. Auger electron
* Correspondence to : J. H. Craig, Materials Research Institute, 500
W. University Ave., University of Texas at El Paso, El Paso, TX
79968-6664, USA. E-mail : jcraig=utep.edu.
Contract grant sponsor : National Science Foundation ; grant no. :
CHE8920120.
CCC 0142È2421/98/020105È04 $17.50
( 1998 John Wiley & Sons, Ltd.
spectroscopy (AES) analysis conÐrmed contaminantfree initial surface conditions. First, electron-stimulated
desorption (ESD) kinetic energy distributions (KEDs)
for positive hydrogen ions from digermane-dosed
Si(100) surfaces held at 85 K were obtained by methods
described in detail elsewhere.2 The H` ESD KEDs
exhibited bimodal peak shapes that were Ðtted by the
least-squares method to a model based upon the work
of Nishijima and Propst,3 as shown in Fig. 1. The
bimodal peak in Fig. 1(a) is suggestive of the presence of
two distinct binding states from which the desorbing
positive hydrogen ions originate,2 and is consistent with
the fact that no annealing step was used (i.e. physisorbed digermane was present on surface). Previous
temperature-programmed desorption (TPD) results
indicated the presence of physisorbed digermane even
for low exposures on Si(100) at 85 K. Work done
elsewhere4 indicates that for Ge H adsorption at 85 K
2 GeH
6
there is at least one chemisorbed
state (x \ 1, 2
or 3) on Si(100) that produces H at 590x K during TPD
experiments. Thus, we conclude2 that our bimodal H`
ESD KEDs arise from hydrogen ions desorbing via
ESD from molecularly adsorbed (physisorbed) Ge H
and directly from adsorbed germyl fragments (GeH 2) on6
x
the Si(100) surface. In a separate set of experiments,
ESD KEDs from adsorbed GeH fragments on Si(100)
x
were obtained after Ñashing the Si(100)
sample to 200 K
to remove molecularly adsorbed (physisorbed)
Received 20 May 1997
Accepted 29 August 1997
106
A. F. AGUILERA, J. H. CAMPBELL, J. H. CRAIG, JR. AND K. H. PANNELL
Figure 1. (a) An ESD KED profile showing two distinct states. (b) An ESD KED obtained after a 200 K anneal to remove physisorbed
Ge H , which exhibits only one state. The insets in (a) and (b) show fits of our decay data using Eqn (1). The inset to (a) shows a
2 6
double-exponential
fit to data, obtained from a Si(100) substrate on which Ge H (a) and GeH (a) are adsorbed. The inset to (b) required
2 6
x
only a single-exponential fit.
digermane. The H` ESD KEDs from Si(100) having
only adsorbed GeH (a) exhibit only one ESD KED
peak, as seen in Fig.x 1(b). The TPD results conÐrmed
that no physisorbed digermane molecules were present
when performing this second set of ESD measurements.
In this work we directly measure the H` ESD signal
decays by setting the energy window (pass energy) of
our Bessel Box energy analyzer5 to a constant energy
(the Bessel box energy resolution is D1 eV). A single
exponential decay, given by Eqn (1)
C
I(t) \ I ] a
b
D
1 [ exp([bt)
bt
(1)
can be used to Ðt the experimentally obtained H` ESD
signal decays as described elsewhere.6 In Eqn (1), I(t) is
the ion current at time t, I is the background signal, a
b
is a Ðtted amplitude parameter
and b is the decay conSURFACE AND INTERFACE ANALYSIS, VOL. 26, 105È108 (1998)
stant (b \ pJ/e). We note that Eqn (1) was derived in a
manner that included the spatial dependence of the electron beam current density proÐle within the beam
spot.6 A Faraday cup was used to obtain electron beam
current density proÐles. The experimentally determined
electron beam current density proÐle was found to be
well approximated by
J(r) \ J exp([r2/a2)
(2)
0
where J(r) is the spatially dependent current density, J
is the current density at the center of the electron-beam0
spot, r is the independent variable and a is the Gaussian
width of the spot. From the Ðts of the exponential decay
in Eqn (1) to our data (see insets to Fig. 1), it is quite
straightforward to extract the decay constant. Provided
that the relation eb \ J p holds true, we may then plot
eb vs. J to obtain a line0 of slope p, which in this case is
the total0 H(a) removal cross-section for the process.
( 1998 John Wiley & Sons, Ltd.
ESD OF DIGERMANE-EXPOSED Si(100)
We obtained experimental H` ESD signal decay
curves from Si(100) surfaces both with and without a
molecularly adsorbed Ge H overlayer, as shown in the
2 6
insets to Fig. 1. From single-exponential decay Ðts to
the experimental decays at various J , plots of eb vs. J
0
0
indicate that the H(a) removal cross-section for surfaces
with physisorbed digermane present are a factor of two
greater than for surfaces with no physisorbed digermane
present on the surface. However, it is known that
molecularly adsorbed Ge H coexists with GeH (a) on
2 K
6 even for relatively
x low
the Si(100) surface at 85
Ge H (g) exposures. Thus, in the ESD experiments
2 6
where no e†ort is made to remove physisorbed Ge H ,
2 6
our Si(100) surface is covered with a mixture of strongly
bound (chemisorbed) GeH species and more weakly
x
bound (physisorbed) Ge H molecules. Intuition sug2
gests that the ESD signal 6 would have contributions
from both GeH (a) and Ge H (a), and bimodal ESD
KEDs such as xthat shown 2 in6 Fig. 1(a) conÐrm this
suspicion. With two surface states contributing to the
ESD signal, it is appropriate to Ðt the experimental
ESD signal decay curves obtained in the presence of
physisorbed digermane with double-exponential decay
curves. Double-exponential decay Ðts were used for the
experimental decays obtained from surfaces having physisorbed digermane, and an example of such a Ðt is
shown in the inset to Fig. 1(a), where the dashed lines
represent the component single-exponential decay
curves that are summed to obtain the resultant best-Ðt
double-exponential decay. Plots of eb vs. J are made
using the decay constants b obtained from0 each component decay curve, and arei shown as the squares and
triangles in Fig. 2. Cross-sections deduced from the
best-Ðt straight lines to the points in Fig. 2 indicate that
p \ 1.4 ^ 0.45 ] 10~15 cm2 and p \ 2.3 ^ 1.5
1 10~16 cm2 are the values of the component
2
]
H(a)
removal cross-sections. We note that the value for p is
1
an unusually large cross-section, of the order of the geometrical cross-section. However, we have previously
observed that physisorbed digermane is very susceptible
to electron beam-induced decomposition.1
Figure 2. Values of e b obtained from exponential decay fits to
the experimental signal decays are plotted vs . the J value used to
obtain the experimental decay. Single-exponential0 decays were
used to fit ESD signal decays obtained following a short anneal to
200 K, and the resulting data are shown as circles. Squares and
triangles represent data obtained from double-exponential fits to
the ESD signal decays obtained with physisorbed digermane
present on the surface.
( 1998 John Wiley & Sons, Ltd.
107
We obtained several H` ESD signal decays following
Ñashing of the Si(100) to 200 K to remove the weakly
bound Ge H species, and found that a single2 6
exponential decay Ðt the experimental data quite nicely.
An example of such a Ðt to the data is shown in the
inset to Fig. 1(b). Using the relation eb \ J p and the
0
same analysis as was used above, we obtained the data
represented by circles in Fig. 2, for which the best-Ðt
least-squares straight line gives a slope of
p \ 2.8 ^ 0.4 ] 10~16 cm2. The observation made
above that ESD KEDs for GeH /Si(100) exhibit only
x
one ESD desorption state supports the use of the singleexponential Ðt to the decays in Fig. 1(b).
Our results indicate that there is an ESD process
occurring with high cross-section when physisorbed
digermane is present on the surface. The high crosssection process correlates well with the presence of physisorbed digermane, so it is reasonable to attribute this
H` ESD signal to removal of H(a) from physisorbed
Ge H species. The high cross-section process only
2 6 when the high-energy (D5 eV) KED peak is
occurs
present, so we assign the high-energy KED peak as
being due to H` desorbed from physisorbed digermane.
Just as the low-energy KED peak at D3.1 eV is always
present, so is the H` ESD process having a crosssection of p D 2.6 ] 10~16 cm2, where the two low
cross-section values are essentially the same within
experimental error and have thus been averaged.
The origin of the H` ESD signal decay having a
cross-section of p D 2.6 ] 10~16 cm2 is less certain.
Possible surface states from which this H` ESD signal
might arise include GeH (a) as well as various surface
silicon hydride species.7x The primary question is
whether hydrogen atoms associated with surface germyl
species migrate to silicon surface sites during the 200 K
anneal used to remove physisorbed digermane. We estimate that the 200 K anneal supplies up to 12 kcal
mol~1 towards the activation of surface processes such
as di†usion of hydrogen from GeH (a) species to neighx
boring silicon surface sites. If the activation
barrier is as
low as 9 kcal mol~1, as recently postulated by Russell
and Eckerdt,8 such di†usion would be expected to
occur and the post-annealed samples would actually be
exhibiting H(a) removal cross-sections for removal of
H(a) from surface silicon hydrides. Alternatively, if the
activation barrier is higher, or the efficiency of energy
transfer from the surface to the adsorbate is lower than
we estimate, the low-energy KED peak would originate
from electron-stimulated removal of H(a) from surface
GeH (a) species.
x ESD data suggest that the GeH (a) species
Our
remains intact on the surface during the 200x K anneal.
Support for this conclusion arises from several observations. The low-energy ESD KED peaks in Fig. 1
appear at the same energy within Ðtting errors both
before and after the 200 K anneal, and the low-energy
KED peaks reported here appear at slightly higher
energies than similar peaks reported for ESD from
Si H /Si(100).7 Additionally, the cross-section for the
2 6
low-energy
KED peak found in this work
(p D 2.6 ] 10~16 cm2) is somewhat larger than the
cross-section for the low-energy (dihydride) KED peak
found in the work done with disilane adsorbed on
Si(100).7 Finally, the monohydride state would be
expected to form before the dihydride, and the ESD
SURFACE AND INTERFACE ANALYSIS, VOL. 26, 105È108 (1998)
108
A. F. AGUILERA, J. H. CAMPBELL, J. H. CRAIG, JR. AND K. H. PANNELL
KED peak from the monohydride occurs near 5 eV and
is not present in our post-anneal ESD KEDs. Thus,
ESD from surface silicon hydrides does not appear to
be occurring.
In summary, previous work in our laboratory has
shown that molecularly adsorbed (physisorbed) Ge H
2 6
must be present on the Si(100) surface for electron irradiation to signiÐcantly enhance Ge atom deposition.
The work reported here shows that enhanced Ge deposition is the result of electron-induced dissociation of
physisorbed digermane. Following exposure of the
Si(100) surface at 85 K to digermane gas, our ESD
KED data strongly suggest that two distinct Ge-H
states exist on the surface, which we ascribe to GeH (a)
x
and Ge H (a). Finally, after annealing of Si(100) sur2 6
faces covered with digermane to 200 K, our ESD data
suggest that the only adsorbate remaining on the
surface is GeH (a).
x
Acknowledgements
This work was supported in part by the Science and Technology
Program of the National Science Foundation, grant no. CHE8920120.
The authors also wish to acknowledge support received from the NSF
Materials Research Center of Excellence at the university of Texas at
El Paso, cooperative agreement No. HRD-9353547. A.F.A. acknowledges support from the Fulbright Institute of International Education and Consejo Nacional de Ciencia y Tecnologia (Mexico).
REFERENCES
1. J. H. Campbell, J. Lozano, A. F. Aguilera, J. H. Craig, Jr. and K.
H. Pannell, Appl . Surf . Sci . 108, 345 (1997).
2. J. H. Campbell, M. V. Ascherl and J. H. Craig, Jr., J . Vac . Sci .
Technol . A 12, 2128 (1994).
3. M. Nishijima and F. M. Propst, Phys . Rev . B 2, 2368 (1970).
4. B. M. H. Ning and J. E. Crowell, Surf . Sci . 295, 79 (1993).
SURFACE AND INTERFACE ANALYSIS, VOL. 26, 105È108 (1998)
5. J. H. Craig, Jr. and W. G. Durrer, J . Vac . Sci . Technol . A 7,
3337 (1989).
6. B. Xia and S. C. Fain, Jr., Phys . Rev . B 50, 14565 (1994).
7. J. Lozano, J. H. Craig, Jr., J. H. Campbell and M. V. Ascherl,
Nucl . Instrum . Methods Phys . Res . B 100, 407 (1995).
8. N. M. Russell and J. G. Eckerdt, Surf . Sci . 369, 51 (1996).
( 1998 John Wiley & Sons, Ltd.
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