ClnH6-nSiGe compounds for CMOS compatible semiconductor applications: Synthesis and fundamental studies

Article abstract of DOI:10.1021/ja0713680

We describe the synthesis of a new family of chlorinated Si-Ge hydrides based on the formula Cl_nH_6-nSiGe. Selectively controlled chlorination of H₃SiGeH₃ is provided by reactions with BCl₃ to produce ClH₂SiGeH₃ (1) and Cl ₂HSiGeH₃ (2). This represents a viable single-step route to the target compounds in commercial yields for semiconductor applications. The built-in Cl functionalities are specifically designed to facilitate selective growth compatible with CMOS processing. Higher order polychlorinated derivatives such as Cl₂SiHGeH₂Cl (3), Cl₂SiHGeHCl ₂ (4), ClSiH₂GeH₂Cl (5), and ClSiH ₂GeHCl₂ (6) have also been produced for the first time leading to a new class of highly reactive Si-Ge compounds that are of fundamental and practical interest. Compounds 1-6 are characterized by physical and spectroscopic methods including NMR, FTIR, and mass spectroscopy. The results combined with first principles density functional theory are used to elucidate the structural, thermochemical, and vibrational trends throughout the general sequence of Cl_nH_6-nSiGe and provide insight into the dependence of the reaction kinetics on Cl content in the products. The formation of 1 was also demonstrated by an alternative route based on the reaction of (SO₃CF₃)SiH₂-GeH₃ and CsCl. Depositions of 1 and 2 at very low temperatures (380-450°C) produce near stoichiometric SiGe films on Si exhibiting monocrystalline microstructures, smooth and continuous surface morphologies, reduced defect densities, and unusual strain properties.

Full text of DOI:10.1021/ja0713680

Published on Web 06/05/2007
ClnH6-nSiGe Compounds for CMOS Compatible
Semiconductor Applications: Synthesis and Fundamental
Studies
Jesse B. Tice, Andrew V. G. Chizmeshya, Radek Roucka, John Tolle,
Brian R. Cherry, and John Kouvetakis*
Contribution from the Department of Chemistry/Biochemistry, Arizona State UniVersity,
Tempe, Arizona 85287-1604
Received February 26, 2007; E-mail: jkouvetakis@asu.edu
Abstract: We describe the synthesis of a new family of chlorinated Si-Ge hydrides based on the formula
ClnH6-nSiGe. Selectively controlled chlorination of H3SiGeH3 is provided by reactions with BCl3 to produce
ClH2SiGeH3 (1) and Cl2HSiGeH3 (2). This represents a viable single-step route to the target compounds in
commercial yields for semiconductor applications. The built-in Cl functionalities are specifically designed
to facilitate selective growth compatible with CMOS processing. Higher order polychlorinated derivatives
such as Cl2SiHGeH2Cl (3), Cl2SiHGeHCl2 (4), ClSiH2GeH2Cl (5), and ClSiH2GeHCl2 (6) have also been
produced for the first time leading to a new class of highly reactive Si-Ge compounds that are of fundamental
and practical interest. Compounds 1-6 are characterized by physical and spectroscopic methods including
NMR, FTIR, and mass spectroscopy. The results combined with first principles density functional theory
are used to elucidate the structural, thermochemical, and vibrational trends throughout the general sequence
of ClnH6-nSiGe and provide insight into the dependence of the reaction kinetics on Cl content in the products.
The formation of 1 was also demonstrated by an alternative route based on the reaction of (SO3CF3)SiH2-
GeH3 and CsCl. Depositions of 1 and 2 at very low temperatures (380-450 °C) produce near stoichiometric
SiGe films on Si exhibiting monocrystalline microstructures, smooth and continuous surface morphologies,
reduced defect densities, and unusual strain properties.
Introduction
We have recently developed a CVD method that allows for
the first time the growth of Ge-rich (Ge g 50 atom %) device-
quality alloys on Si that cannot be obtained by conventional
routes.6-8 Our breakthrough technology utilizes previously
unknown designer Si-Ge hydrides such as the silyl-germyl
sequence of molecules [(H3Ge)xSiH4-x, x ) 1-4], which are
obtained in viable high-purity yields via straightforward syn-
thetic methodologies developed in our labs.6 The inherent
incorporation of the entire SiGe, SiGe2, SiGe3, and SiGe4
molecular cores of the H3GeSiH3, (H3Ge)2SiH2, (H3Ge)3SiH,
and (H3Ge)4Si compounds into the films provides unprecedented
control of morphology, composition, and strain. Our ongoing
work in this area addresses a critical need for precursors and
processes that yield high quality semiconductors under condi-
tions compatible with modern device fabrication and processing.
The chlorination of Si-Ge hydrides to form volatile and
highly reactive Cl-substituted analogues represents an unex-
plored area of precursor synthesis with tremendous potential in
modern semiconductor device development. The benefits of this
strategy are already evident by the widespread commercial use
of chlorosilanes in selective growth of high performance Si-
based electronic materials. In the topical SiGe alloy arena,1 new
materials with Ge-rich compositions are being vigorously
developed to enable higher speed in microprocessors,2 lower
power consumption in cell phones,3 silicon-based photonics,4
and more efficient solar cells.5 However, for selective growth
applications the currently available synthetic strategy, involving
multisource reactions of chlorosilanes and germanes, might not
provide compositional control and requires high-temperature
growth which is incompatible with CMOS-based processing of
small features on patterned wafers. In view of the increasing
demand for very low-temperature selective growth of Si1-xGex
for high frequency applications, new approaches to circumvent
the above challenges are highly desirable.
The successful application of the (H3Ge)xSiH4-x family of
compounds suggests that the synthesis of ClnH6-nSiGe ana-
logues (via controlled chlorination of their simplest constituent
member SiH3GeH3) should produce compounds such as ClSiH2-
GeH3 (1) and Cl2SiHGeH3 (2) (Figure 1) which are designed
to provide significant benefits from a selective area growth
(1) Mooney, P. M.; Chu, J. O. Annu. ReV. Mater. Sci. 2000, 30, 335.
(2) Luryi, S.; Xu, J.; Zaslavsky, A. Future Trends in Microelectronics: Up
the Nanocreek; Wiley: New York, 2007.
(3) Heftman, G.; Grossman, S.; Day, J. Power Electron. Technol. 2003, 44-
(6) Ritter, C. J.; Hu, C.; Chizmeshya, A. V. G.; Tolle, J.; Klewer, D.; Tsong,
I. S. T.; Kouvetakis, J. J. Am. Chem. Soc. 2005, 127 (27), 9855.
(7) Hu, C.-W.; Menendez, J.; Tsong, I. S. T.; Tolle, J.; Chizmeshya, A. V. G.;
Ritter, C.; Kouvetakis, J. Appl. Phys. Lett. 2005, 87 (18), 181903.
(8) Chizmeshya, A. V. G.; Ritter, C. J.; Hu, C.-W.; Tolle, J.; Nieman, R.; Tsong,
I. S. T.; Kouvetakis, J. J. Am. Chem. Soc. 2006, 128 (21), 6919.
46.
(4) Jalali, J.; Paniccia, M.; Reed, G. Silicon Photonics. IEEE MicrowaVe
Magazine 2006, 7, 1440.
(5) Green, M. A. Third generation photovoltaics: Ultra-high conversion
efficiency at low cost. Prog. PhotoVoltaics 2001, 9, 123.
9
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ClnH6 nSiGe Compounds
A R T I C L E S
-
temperature vapor pressure of ∼150 Torr. Due to its high
reactivity, compound 1 was characterized by physical methods
such as mass spectrometry, FTIR, and NMR, rather than
conventional elemental analyses. The mass spectra were ob-
tained by direct insertion and revealed an isotopic envelope
centered at 141 amu corresponding to the molecular ion, and a
fragmentation pattern consistent with the proposed molecular
structure ClSiH2GeH3. The IR spectra obtained from measure-
ments on gaseous samples showed the expected Si-H and
Ge-H stretching bands split into a set of doublets at 2183, 2169
and 2082, 2070 cm-1, respectively. The bending modes between
950 and 540 cm-1 are also split into doublets. A complete first
principles analysis of the vibrational properties indicated that
the observed IR spectra compare well with the simulated
analogues as described in detail in a later section. The NMR
spectra showed the GeH3 proton resonance as a triplet at 3.12
ppm, the SiH2 proton resonance as a quartet at 4.68 ppm, and
the 29Si resonance as a singlet at -24.61 ppm. The proton peaks
are shifted downfield with respect to the corresponding GeH3
(3.06 ppm) and SiH3 (3.40 ppm) signatures of SiH3GeH3,
consistent with the substitution of H on the Si atom by the
electron withdrawing Cl atom. Moreover a proton-decoupled
1H-29Si HMQC (heteronuclear multiple quantum coherence)
spectrum showed that the H atoms at 4.68 ppm are directly
attached to Si atom at -24.61 ppm which confirms the
molecular structure. These interpretations are corroborated with
our prior NMR studies of the related SiH4-x(GeH3)x family of
compounds.6
Figure 1. Structural representations of ClH2SiGeH3 and Cl2HSiGeH3
molecules. Si, Ge, Cl, and H are depicted as gold, blue, green, and white
spheres, respectively.
perspective compared to current approaches based on high-
temperature reactions of chlorinated silanes. In general, it has
been suggested that selective growth of Si or Si-Ge films can
be achieved by in situ use of chlorinating etchants derived from
these reactions. These etchants remove the amorphous material
typically deposited on masked features (in a blanket deposition
step) at a much higher rate than they remove crystalline material
grown on the oxide-free regions. Thus, they promote selective
growth on the unmasked areas around the patterned regions on
the wafer. The newly introduced ClSiH2GeH3 and Cl2SiHGeH3
compounds could also function as built-in cleaning agents for
the removal of amorphous material at relatively low tempera-
tures during in situ selective growth of a variety of device
structures on patterned Si wafers.
CVD reactions of halogenated Si compounds are described
by numerous patent applications to be problematic due to
contamination of the deposited material by residual Cl impurities
and undesirable particulates. In the new single sources, 1 and
2, the built-in composition and the high reactivity provide precise
control with regard to complete HCl elimination and simulta-
neously enable incorporation of the desired SiGe content in the
purest possible form. Finally, the larger mass of the halogenated
silylgermanes provides an increased kinetic stability allowing
growth to be conducted at even lower temperatures than the
corresponding SiH3GeH3 hydride.
NMR samples kept for several days showed no additional
signals due to impurities or decomposition byproducts, indicating
that the solutions of the compound remain stable and pure.
Multigram batches of liquid 1 kept at 22 °C also showed no
discernible decomposition over extended time periods, to form
HCl, chlorosilanes, or germanium hydride polymers, indicating
that the compound is remarkably robust in its pure physical
form. We note that an alternative synthesis of 1 was also
performed using PhSiH2GeH3 and HCl (eq 2).
This report describes the preparation and properties of the
new compounds ClSiH2GeH3 (1) and Cl2SiHGeH3 (2). Their
use to deposit stoichiometric SiGe films on Si at very low
temperatures (370-450 °C) is also described. En route to 1 and
2 we have produced an entire new series of multiply chlorinated
derivatives with intriguing bonding properties and potential
applications in low-temperature synthesis of Si-Ge optoelec-
tronic alloys.
PhSiH2GeH3 + HCl f ClSiH2GeH3 + C6H6
(2)
However, due to the similarity in boiling points, 1 could not be
separated by fractional distillation from the benzene byproduct.
Furthermore, the overall yield of 1 was found to be too low for
applications in materials synthesis, and this route therefore was
not pursued any further.
A complicating factor in the synthetic route describe by eq 1
is that overall yields of pure (SO3CF3)SiH2GeH3 are severely
limited. This can be traced to the low yields (maximum 25%)
obtained in forming PhSiH2GeH3 from the reaction of PhSiH2-
Cl and KGeH3. Furthermore, (SO3CF3)SiH2GeH3 is highly
reactive and therefore difficult to handle, making its purification
and storage for subsequent use challenging. In fact, the
compound has inexplicably and randomly exploded during
synthesis while following the same routine preparation and
isolation procedures.
Results and Discussion
(a) Synthetic Routes to ClH2SiGeH3 (1) and Cl2SiHGeH3
(2). The synthesis of ClH2SiGeH3 (1) was initially conducted
by reaction of CsCl with the triflate substituted silylgermane
(SO3CF3)SiH2GeH3 (eq 1).
(SO3CF3)SiH2GeH3 + CsCl f
ClSiH2GeH3 + Cs(SO3CF3) (1)
There are numerous examples of silane and germane halo-
Decane was used as the solvent, and the overall yield was
∼60%. The (SO3CF3)SiH2GeH3 starting material was prepared
by direct reaction of PhSiH2GeH3 and HSO3CF3 in the absence
of solvent at -35 °C. Pure ClH2SiGeH3 (1) was conveniently
isolated by vacuum distillation from the solid Cs(SO3CF3)
byproduct, as a colorless, pyrophoric liquid with a room-
genation reactions reported to date using a variety of methods.9-15
(9) Drake, J. E.; Goddard, N. Inorg. Nucl. Chem. Lett. 1968, 4, 385.
(10) Mackay, K. M.; Robinson, P.; Spanier, E. J.; MacDiarmid, A. G. J. Inorg,
Nucl. Chem. 1966, 28, 1377.
(11) Swiniarski, M. F.; Onyszchuk, M. Inorg. Synth. 1974, 15, 157. (b) Becker,
G.; Holderich, W. Chem. Ber. 1975, 108, 2484.
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A R T I C L E S
Tice et al.
BCl3 + SiH3GeH3 f 1/2B2H6 + Cl3SiGeH3
(5)
Our search for a viable, straightforward, and safe synthetic
methodology that is also scalable from an industrial perspective
to produce Si-Ge analogues has led to the development of a
single-step route utilizing the commercially available starting
materials BCl3 and SiH3GeH3. When BCl3 and SiH3GeH3 are
combined in a 1:3 molar proportion, we find that all of the BCl3
reactant is consumed and a stoichiometric amount of B2H6 is
produced according to eq 3.
The above results indicate that a distribution of multiple
chlorinations occurs with increasing the amount of BCl3 and
thus the activity of Cl in the reaction. This prompted us to
increase the BCl3 concentration even further in an attempt to
drive the third chlorination on the Si center. In principle, a 2:1
ratio of BCl3 to SiH3GeH3 is expected to yield the limiting Cl3-
SiGeCl3 perchlorinated product according to the reaction shown
by eq 6.
BCl3 + 3SiH3GeH3 f 1/2B2H6 + 3ClSiH2GeH3 (3)
The target ClSiH2GeH3 (1) compound was obtained by trap-
to-trap fractionation in significant yields (∼60-70%) and was
identified by spectroscopic methods as described above.
However, in addition to 1, the disubstituted derivative Cl2-
HSiGeH3 (2) was isolated and purified in small amounts as a
byproduct in the reaction mixture. The NMR spectra of the
mixture prior to separation showed that a ratio of 12:1 of
monochloro to dichloro is typically produced. The GeH3
NMR resonance of 2 was observed as a doublet at 3.25 ppm
and the SiH resonance as a quartet at 5.57 ppm which is
consistent with a second substitution of Cl on the Si site. The
1H-29Si HMQC spectrum also determined that the proton
(5.57 ppm) is bound to the Si atom (+9.77 ppm). The
integrated SiH/GeH3 peak intensity ratio is 1:3, as expected.
Compound 2 was further characterized by gas-phase IR and
mass spectrometry. These spectroscopic results combined
with DFT theoretical simulations of the IR spectra (described
below) confirm the proposed Cl2SiHGeH3 molecular
structure.
2BCl3 + SiH3GeH3 f B2H6 + Cl3SiGeCl3
(6)
We found however that in spite of numerous variations in
reaction conditions the 2:1 stoichiometry invariably produced
only higher order chlorinated derivatives in the ClnH6-nSiGe
sequence rather than the hexachloro product. The main products
were found to be the Cl2HSiGeHCl2 (4) and Cl2SiHGeH2Cl
(3) compounds while minor byproducts included compounds
ClSiH2GeH2Cl (5), ClSiH2GeHCl2 (6), and dichlorosilane (Cl2-
SiH2). The outcome of enhancing the chlorine activity in these
reactions was to substitute a maximum of two chlorines on the
Si center with additional Cl atoms bonding to the Ge site of
Cl2SiHGeH3. Table 1 summarizes the target and observed
products obtained with increasing the Cl activity as described
by eqs 3-6.
(c) Structural Trends of ClnH6-nSiGe via NMR. Among
the compounds synthesized, the mono- and dichloro species 1
and 2 are readily isolated as distinct volatile liquids. However,
the very close boiling points and high reactivity of the
polychlorinated derivatives 3-6 precluded their individual
separation by normal distillation or fractionation techniques.
Accordingly, their compositions were determined and their
structures were elucidated by extensive 1H and 29Si NMR
analyses such as 1H correlated spectroscopy (COSY) and 1H-
29Si HMQC techniques. The 1H NMR spectra showed the
expected multiplicity in various SiHx and GeHx peaks and 29Si
satellite peaks which exhibit 1JSi-H and 2JSi-H coupling constants
with nominal values of 200-300 and 10-50 Hz, respectively.
These techniques in combination with the coupling constants
are particularly useful because they unambiguously establish
the connectivities of the various H and Cl atoms to the Si-Ge
molecular core for each compound in the various product
mixtures.
(b) Higher Order Chlorination of H3SiGeH3. The presence
of Cl2SiHGeH3 (2) as a minor component in the above reaction
prompted us to explore stoichiometric reactions of BCl3 and
SiH3GeH3 in a 2:3 molar ratio to promote its formation as the
primary product (see eq 4).
2BCl3 + 3SiH3GeH3 f B2H6 + 3Cl2SiHGeH3
(4)
This reaction was carried out at 0 °C for 2 h and yielded
equal amounts of ClSiH2GeH3 (1) and Cl2SiHGeH3 (2) indicat-
ing that the second chlorination at the Si center is enhanced
under these conditions, as expected. When the chlorine activity
was further increased by combining the reactants in 1:1 molar
ratio (see eq 5), the Cl2SiHGeH3 (2) species was predominately
produced while the formation of ClSiH2GeH3 (1) was essentially
suppressed. Under these conditions, the reaction also produced
a substantial amount of a low-volatility liquid mixture which
consisted primarily of the polychlorinated Cl2SiHGeH2Cl (3)
and Cl2SiHGeCl2H (4) compounds. These were detected and
characterized by NMR spectroscopy. The NMR spectra also
showed trace amounts of two additional compounds which were
identified to be the ClSiH2GeH2Cl (5) and ClSiH2GeHCl2 (6)
species by their 1H and 29Si resonance signatures. We note that
the thermodynamically expected Cl3SiHGeH3 was not formed
under these conditions, although the reaction stoichiometry was
specifically tuned to promote its formation according to the
reaction shown by eq 5.
1
Table 2 lists the H and 29Si chemical shifts as well as the
Si-H and H-H J couplings derived from our measurements
for all chlorinated compounds listed in Table 1 including the
previously discussed 1 and 2 as well as the SiH3GeH3 reactant.
We note the following trends in the chemical shift data: (a)
For a fixed number of Cl atoms on the Si site, increasing the
number of Cl atoms on the Ge site enhances the shielding of
the SiHx protons resulting in a systematic upfield shift of the
latter. For example, in the sequence Cl2SiHGeH3, Cl2SiHGeH2-
Cl, and Cl2SiHGeHCl2, the Si-H shifts are 5.57, 5.35, and 5.15
ppm, respectively (steps of ∼0.2 ppm). Similarly for ClSiH2-
GeH3, ClSiH2GeH2Cl, and ClSiH2GeHCl2, the SiH2 shifts are
4.68, 4.53, and 4.38 ppm, respectively (steps of ∼0.1 ppm).
(b) By contrast, the GeHx shifts are independent of the number
of Cl atoms on the Si site and simply scale downfield in
proportion to the number of Cl atoms on Ge site (δGeHx: ∼3.2
ppm for “-GeH3”, 4.8 ppm for “-GeH2Cl”, and 6.0 ppm for
(12) Ward, L. G. L. Inorg. Synth. 1971, 13, 154.
(13) Ward, L. G. L.; MacDiarmid, A. G. J. Inorg. Nucl. Chem. 1961, 20, 345.
(14) Lobreyer, T.; Oeler, J.; Sundermeyer, W. Chem. Ber. 1991, 124, 2405.
(15) Van Dyke, C. H.; MacDiarmid, A. G. J. Inorg. Nucl. Chem. 1963, 25,
1503.
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ClnH6 nSiGe Compounds
A R T I C L E S
-
Table 1. Chlorination Products and Byproducts as a Function of Chlorine Activitya
BCl3:SiH3GeH3
target product
observed products
observed byproducts
1:3
2:3
1:1
2:1
ClSiH2-GeH3
Cl2SiH-GeH3
Cl3Si-GeH3
Cl3Si-GeCl3
ClSiH2-GeH3
Cl2SiH-GeH3
ClSiH2-GeH3, Cl2SiH-GeH3
Cl2SiH-GeH3
ClSiH2-GeClH2, Cl2SiH-GeClH2, ClSiH2-GeCl2H, Cl2SiH-GeCl2H
ClSiH2-GeClH2, Cl2SiH-GeH3, ClSiH2-GeCl2H
Cl2SiH-GeClH2, Cl2SiH-GeCl2H
a The first column lists the ratio of BCl3 to SiH3GeH3 reactants. All reactions were carried out at T ) 0 °C for 2 h.
Table 2. 1H and 29Si Shifts and Si-H and H-H J Coupling
Here we use this approach to study the idealized general
reactions involving the BCl3 and H3SiGeH3, which can be
written as depicted by eq 7.
Constants for ClnH6-nSiGe Compoundsa
compd
δ
1H (SiHx)
δ
1H (GeHx)
δ
29Si
1JSi
2JSi
3JH
-H
-H
-H
b
H3SiGeH3
3.52 (q)
4.68 (q)
5.57 (q)
3.18 (q)
3.12 (t)
3.25 (d)
4.86 (t)
4.82 (d)
6.12 (t)
5.97 (d)
-91.60 204.0
-24.61 231.9
7.2 3.8
3.3
nBCl3 + 3H3SiGeH3 f n/2 B2H6 + 3ClnH6-nSiGe (7)
ClSiH2GeH3
Cl2SiHGeH3
ClSiH2GeH2Cl 4.53 (t)
Cl2SiHGeH2Cl 5.35 (t)
ClSiH2GeHCl2 4.38 (d)
Cl2SiHGeHCl2 5.15 (d)
9.77 271.9 13.1 3.0
-24.26 244.8 3.5
This simple reaction model subsumes a number of side reactions
involving intermediate species such as BCl2H, BClH2, SiCl2H2,
GeH4, and GeH2. Of specific interest in this study are the
chlorination reactions on the Si center of H3SiGeH3 described
by eqs 8-10.
3.36 290.0 29.2 4.3
-22.8 7 308. 0 51.5 7.0
-1.12 259.7 39.4 4.4
a The assignments were made on the basis of the COSY and HMQC
experiments and were confirmed by the JSi-H (listed in Hz). b Reference
14.
first chlorination: BCl3 + H3SiGeH3 f
ClH2SiGeH3 + BCl2H (8)
“-GeHCl2” roughly independent of the number of Cl attached
to Si). (c) The trends in the 29Si shifts are more subtle, and our
study finds that for a fixed number of Cl atoms on the Si site
slight downfield shifts are associated with increasing Cl atoms
on the Ge site (for “ClH2Si-” molecules, δSi are -24.6, -24.3,
-22.9 for 0, 1, and 2 Cl atoms on the Ge site, respectively).
(d) Furthermore, this effect becomes more pronounced as the
number of Cl atoms on the Si site is increased (for “Cl2HSi-”
molecules, δSi are 9.8, 3.4, -1.1 for 0, 1, and 2 Cl atoms on
the Ge site, respectively). This suggests that chlorines bonded
to Ge systematically shield the Si nuclei as the number of Cl
atoms is increased. (e) Some notable trends of the coupling
constants are as follows: For a fixed number of chlorines on
the Si site, the 1JSi-H and 3JH-H values increase with increasing
second chlorination: BCl3 + ClH2SiGeH3 f
Cl2HSiGeH3 + BCl2H (9)
third chlorination: BCl3 + Cl2HSiGeH3 f
Cl3SiGeH3 + BCl2H (10)
In all three of these reactions, the unstable BCl2H product is
initially produced and assumed to rapidly convert to diborane
and BCl3 via the well-known disproportionation reaction15 (eq
11)
3(BCl2H)2 f B2H6 + 4BCl3
(11)
1
number of chlorines bonded to Ge. The JSi-H value increases
which we do not consider here further. As a first step toward
computing both the transition states involved in reactions 8-10,
and standard reaction free energies based on eq 7, we calculated
the equilibrium thermochemistry of all reactants and products.
The ground state structures, the zero-point energy (ZPE)
corrected electronic energies (E0 + ZPE), and thermally
corrected Gibbs free energies at 298 K (GTH), of all pertinent
reactants and products, are provided in Figures 2 and 3. All
structural optimizations were performed using “tight” force
convergence criteria. Figure 2 lists the structures and energies
of reactants and products (other than the title ClnH6-nSiGe
compounds) such as BCl3, B2H6, BHCl2, and H3SiGeH3
produced in the reactions shown by eqs 7-10. The correspond-
ing data for the title compounds ClnH6-nSiGe (n ) 1,...,4) is
provided in Figure 3. Note that this list does not include all
possible “trans” and “gauche” isomeric forms displayed by the
heterochlorinated ClH2SiGeH2Cl, Cl2HSiGeH2Cl, ClH2SiGeHCl2,
and Cl2HSiGeHCl2 derivatives. We find that the energy differ-
ence between isomers of the same compound is typically 10
kJ/mol but that their vibrational spectra are distinct. In this study,
we consider the lowest energy isomer in each case as shown in
Figure 3. The molecules are ordered according to the number
of Si-Cl bonds (followed by the number of Ge-Cl bonds). A
complete account of the role of isomers in the interpretation of
the thermochemistry and vibrational spectra of chlorinated
silylgermanes will be presented in a forthcoming publication.
with increasing the number of Si-Cl bonds in the SiH3GeH3,
ClSiH2GeH3, and Cl2SiHGeH3 sequence, while the opposite is
observed for the 3JH-H. The highest 3JH-H value corresponds to
the most chlorinated Cl2SiHGeHCl2 molecule. Similar com-
parisons can be made for the chlorinated disilane analogues
where similar trends are observed for the coupling constants
1JSi-H (197.8, 226.5, 267.7, 233.1, 277.2, 285.3) and 3JH-H (3.5,
2.8, 2.1, 2.6. 2.4, 4.0) corresponding to the compounds (SiH3-
SiH3, ClSiH2SiH3, Cl2SiHSiH3, ClSiH2SiH2Cl, Cl2SiHSiH2Cl,
and Cl2SiHSiHCl2),16 respectively.
Simulation Studies of ClnH6-nSiGe Molecules
(a) Reaction Thermochemistry. To complement the experi-
mental studies and elucidate the origin of the chlorination
reaction trends described above, we carried out a comprehensive
simulation study of the ClnH6-nSiGe molecules. Themochem-
istry analysis of the chlorination reactions was carried out using
the Gaussian03 program17 with a density functional theory
description of the electronic structure at the B3LYP/6-311N++G-
(3df,3pd) level. We have successfully used this approach in prior
work to accurately describe the bonding, structure, and vibra-
tional properties in related Si-Ge molecular hydride systems.6,8
(16) Sollradl, H.; Hengge, E. J. Organomet. Chem. 1983, 243, 257-269.
(17) Frisch, M. J. et al. Gaussian 03, Revision B.04; Gaussin Inc.: Wallingford,
CT, 2004.
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A R T I C L E S
Tice et al.
∆H, which provide a useful, but less rigorous, guide to
understanding the salient bonding trends. To obtain these bond
energies, we utilized atomization values which we calculated
from the most basic gaseous species such as H2, B2H6, BCl3,
SiH4, GeH4, SiCl4, GeCl4, and H3SiGeH3 at the B3LYP/6-
311G++(2d,2p) level of theory. Using this approach, we
obtained the following: ∆H[H-H] ) 110 kcal/mol, ∆H[B-
Cl] ) 104 kcal/mol, ∆H[B-H] ) 76 kcal/mol, ∆H[Si-H] )
87 kcal/mol, ∆H[Ge-H] ) 73 kcal/mol, ∆H[Si-Cl] ) 96 kcal/
mol, ∆H[Ge-Cl] ) 77 kcal/mol, and ∆H[Si-Ge] ) 47 kcal/
mol. Using these values, the gas-phase chlorination reaction
enthalpies ∆HBX based on eq 7 were calculated and are given
in Table 3. Note that these follow closely the trends obtained
using the full thermochemistry analysis (see ∆GTH in Table 3).
In particular, the ordering predicated by the number of Si-Cl
bonds formed is properly reproduced although the finer details
are missed.
Figure 2. Equilibrium structures of BCl3, BHCl2, B2H6, and H3SiGeH3.
Listed within the figure are the bond lengths (Å), zero-point energy (ZPE)
corrected electronic energies, and thermally corrected Gibbs free energies
at 298 K (energies in Hartree). Legend: Si, yellow; Ge, blue; Cl, green; B,
pink; H, white.
(b) Reaction Kinetics. The surprising absence of Cl3SiGeH3
in the reaction products in spite its apparent thermodynamic
stability prompted us to conduct a brief study of the transition
state structures associated with first, second, and third Si site
chlorination reactions given in eqs 8, 9, and 10, respectively.
The transition states (TS) were initially optimized at the B3LYP/
6-311G++(2d,2p) level using the Berny algorithm as imple-
mented in the Gaussian03 program (n.b., all transition state
thermochemistry calculations were carried out at 298 K). The
reaction of BCl3 and SiH3GeH3 (see eq 8) likely proceeds by
electrophilic attack of a silyl proton by BCl3 as shown in Figure
4 where the nucleophilic and electrophilic site reactivities of
BCl3 and H3SiGeH3 are compared on the basis of their self-
consistent molecular electrostatic potentials (MEP), obtained at
the B3LYP/6-311G++(3df,3pd) level. The latter are mapped
onto appropriate isodensity surfaces and color coded according
to local MEP value. Negative and positive regions are suscep-
tible to electrophilic/nucleophilic attack, respectively. Accord-
ingly, our calculations indicate that the electrophilic attack of
silyl protons by BCl3, as indicated in Figure 4a, is the most
likely reaction mechanism, due to the large MEP difference
associated with the reaction sites indicated. On the basis of a
plausible suggestion by Van Dyke and MacDiarmid we explored
numerous possibilities for the structure of the first chlorination
transition state complex15 involving four centered Si-H-B-
Cl configurations as shown in Figure 5. Our initial search using
the smaller but more efficient basis set yielded a TS configu-
ration with two soft modes: one associated with a four-member
transition state involving the transfer of Cl to Si and H to B,
and the other involving the internal rotation of the terminal
germyl group. Subsequent TS optimization using the full
B3LYP/6-311N++G(3df,3pd) prescription then produced a
final transition state configuration possessing only a single soft
mode (i297 cm-1). This indicates that the previously used small
set did not have the accuracy to provide the uniquely resolved
single soft mode. The latter represents the motion of the system
across the barrier leading to the chlorine-hydrogen exchange
on the Si site. The optimized structure of ClH2SiGeH3 (1) was
then used as a starting point for the second chlorination reaction
(eq 9) and the second chlorination product, Cl2HSiGeH3 (2),
was then in turn used to initialize the third chlorination
simulation. Using this approach, all three transition states were
located, and the results are summarized in Figure 5. The
A primary goal of this study is to elucidate, on energetic and
structural grounds, the chlorination trends in our compounds
shown in Table 1. We note that the list of compounds presented
in Figure 3 is augmented to include the Cl3SiGeH3 species
(which was expected but not observed) as well as the compounds
H3SiGeH2Cl, H3SiGeHCl2 with Cl atoms bound exclusively to
Ge. The latter have been tentatively observed but not on a
reproducible basis. This is presumably due to their relative
instability with respect to decomposition or additional chlorina-
tions at their Si sites to produce molecules containing at most
four chlorine atoms combined, as observed.
Collectively, the structures shown in Figure 3 reveal the
following general bonding trends: The Si-Ge bond has a
typical value ∼2.40 Å, which increases slightly in response to
the repulsion due to the number of chlorine atoms on Si and
Ge sites. The attachment of chlorine to Si and Ge also induces
systematic variations in the Si-H and Ge-H bonds, which are
found to lie in the ranges 1.471-1.482 and 1.533-1.580 Å,
respectively. Variations of comparable magnitude are found in
the Si-Cl and Ge-Cl bond lengths which exhibit values 2.053-
2.077 and 2.171-2.193 Å, respectively.
We calculated standard reaction free energies, ∆GRX at 298
K, following the generalized reaction given in eq 7, using the
thermally corrected free energies listed in Figures 2 and 3. Our
main results are provided in Table 3, which lists the reaction
free energies in order of decreasing magnitude. As can be seen
from this data, the relative stability of the chlorinated products
is directly related to the number of Si-Cl bonds, and not the
total number of bound chlorine atoms. This is consistent with
the experimentally observed order of chlorination in which the
first and second chlorination reactions occur exclusively on the
Si center, followed by progressive chlorination of the Ge site.
Note that the experimentally elusive H3SiGeH2Cl and H3-
SiGeHCl2 compounds are the least stable due to the absence of
Si-Cl bonds. In contrast, the most thermodynamically stable
derivative Cl3SiGeH3 was never obtained despite systematic
efforts to promote its formation. As we discuss below in detail,
this is likely due to slow reaction kinetics.
The key role of Si-Cl bonding in the chlorination thermo-
chemistry can be readily understood in terms of bond enthalpies,
9
7954 J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007

ClnH6 nSiGe Compounds
A R T I C L E S
-
Figure 3. Equilibrium structures for the 9 ClnH6-nSiGe molecules described in this study. Bond lengths are listed in Å, while the zero-point energy (ZPE)
corrected electronic energies and thermally corrected Gibbs free energies at 298 K are given in Hartrees. Legend: Si, yellow; Ge, blue; Cl, green; H, white.
a
Table 3. Standard Reaction Free Energies ∆GRX for Chlorination
Reaction Products and Reaction Enthalpies ∆HRX Obtained from
Bond Enthalpy Calculations
∆G
RX (kJ/mol)
∆H
BX (kJ/mol)
product
-50.6
-34.1
-26.9
-30.5
-14.9
-14.7
-12.8
-1.3
-79
-53
-59
-81
-27
-32
-38
-6
Cl3-Si-Ge-H3
Cl2H-Si-Ge-H3
Cl2H-Si-Ge-H2Cl
Cl2H-Si-Ge-HCl2
ClH2-Si-Ge-H3
ClH2-Si-Ge-H2Cl
ClH2-Si-Ge-HCl2
H3-Si-Ge-H2Cl
H3-Si-Ge-HCl2
H3-Si-Ge-H3
-5.5
-11
0
0.0
a Thermally corrected values at 298 K.
magnitude of the forward reaction barrier (indicated as a 298
K Gibbs free energy difference in the figure) increases
systematically through the chlorination sequence (33, 38, and
41 kcal/mol, respectively), while the free energy difference
between products and reactants decreases. The interatomic
distances of the “four center” configuration in the activated
Figure 4. Molecular electrostatic potential (MEP) interpretation of site
reactivity in the BCl3-H3SiGeH3 reaction: (a) The arrow connects the
reactive sites involved in the electrophilic attack of a silyl proton by BCl3.
(b) Nucleophilic attack of the Si site by the Cl. The arrow indicates a possible
attack site along the Si-Ge backbone, near the Si atom (in this frame the
H3SiGeH3 is oriented with the Si downward). The color bars indicate the
common range of the MEP (in atomic units) which has been mapped onto
isodensity contours of the BCl3 and H3SiGeH3 molecules. Note that the
MEP differences corresponding to the reaction sites shown (indicated by
arrowed brackets above the MEP scale bars) are larger for the electrophilic
process.
complex, and the corresponding soft mode frequencies (ωTS1
ωTS2, TS3) associated with the transition states, are given in
,
ω
Figure 5b. The displacement patterns associated with the barrier
crossing for the first and second chlorination soft modes involve
primarily a motion of the “four-center” constituents while the
rest of the complex remains essentially motion free. The normal
mode associated with the third chlorination reaction, however,
involves a similar motion of the entire four member complex,
but also includes an additional collective “rocking” type
movement of the two heavy constituent Cl atoms bonded to Si.
The latter may contribute to the slower kinetics associated with
9
J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007 7955

A R T I C L E S
Tice et al.
Table 4. Unscaled Harmonic Vibrational Frequencies of
ClH2SiGeH3 (1) and Cl2HSiGeH3 (2) Given in Parentheses (cm-1
)
and Corresponding Assignmentsa
ClSiH2GeH3
Cl2SiHGeH3
“Sil” (119) SiCl rocking
CV1(125)
CV2(189)
CV3(320)
CV4(448)
s SiCl2 rocking
Si-Cl2 scissor
Si-Ge stretch
s GeH2 wagging
a GeH3 rocking
s Si-Cl stretch
a Si-Cl stretch
BV1(333)
BV2(361)
BV3(471)
BV4(530)
BV5(702)
BV6(780)
BV7(840)
BV8(890)
BV9(950)
bV1(2142)
bV2(2146)
bV3(2224)
bV4(2236)
Si-Ge stretch
s SiH2/GeH2 wagging
a GeH3 + SiH2 rocking CV5(467)
Si-Cl stretch
CV6(520)
CV7(559)
CV8(746)
CV9(787)
CV10(808) a GeH3/SiH2 rocking
CV11(873) a GeH3 wagging
cV1(2113) s GeH3 stretch
cV2(2124) a GeH2 stretch
cV3(2132) a GeH stretch
cV4(2241) SiH stretch
a SiH3 rocking
s GeH3/SiH2 rocking
a GeH3/SiH2 rocking
a GeH3 wagging
s SiH2 wagging
s GeH3 stretch
a GeH3 stretch
s SiH stretch
s GeH3/SiH rocking
SiH wagging
Figure 5. Transition state data for the three chlorination reactions of the
Si site. (a) Structures of the reactant molecules, transition state complex,
and product molecules. Dashed lines in the TS configuration indicate the
bonds being “broken”. The thermally corrected Gibbs free energies (298
K) of the reactant and product states are given in kcal/mol relative to the
transition state, which is taken as the reference value (GTH ) 0). The first,
second, and third chlorination values are listed top to bottom, respectively.
(b) Four center TS configuration involving transfer of the Si-H proton to
the BCl3 molecule, and binding of chlorine to the Si site. Bond lengths for
the individual first, second, and third chlorination reactions are listed top
to bottom, respectively. The corresponding soft mode frequencies for the
a SiH stretch
a Given as s ) symmetrical, a ) asymmetrical, or “sil” for silent modes
with zero calculated intensity.
and symmetric Si-H counterparts (2220-2240 cm-1). The
corresponding low-frequency spectra (200-1200 cm-1) are
significantly more complex, and normal mode assignments are
given in Table 4. General assignments for both molecules, by
frequency range, are as follows: (i) 120-160 cm-1, Si-Cl
rocking modes; (ii) 320-350 cm-1, Si-Ge stretching; (iii) 520-
530 cm-1, Si-Cl stretches; (iv) 380-480 cm-1, asymmetric
GeH3/SiHx rocking vibrations; (v) 740-800 cm-1, symmetric
GeH3/SiHx rocking vibrations; and (vi) 800-950 cm-1, various
GeH3 and SiHx wagging vibrations. Among these assignments,
the group v GeH3/SiHx rocking modes are the most intense.
These have almost the same frequency in both molecules and
are designated as BV6 (780 cm-1) and CV8 (787 cm-1) in ClSiH2-
GeH3 (1) and Cl2SiHGeH3 (2) molecules, respectively, as shown
in Table 4.
A detailed comparison of the calculated and observed spectra
of compounds 1 and 2 is presented in Figure 6. On the basis of
our prior simulation work using comparable basis sets and model
chemistries, we have applied the 0.989 and 0.979 frequency
scale factors to the calculated the low- and high-frequency
spectra, respectively.6,8 As can be seen from the figure, the
calculated frequencies and intensities of both compounds match
their experimental counterparts very closely, suggesting that the
mode assignments listed in Table 4 are reliable. We note that
while a common scale factor is used here for both compounds,
a slight adjustment of the high-frequency factor (0.979) used
in Figure 5b could improve the individual agreement for each
compound further. We note that the simple harmonic vibrational
analysis presented here does not account for the significant
splittings (10-15 cm-1) observed for the vibrational bands in
the experimental spectra of both ClH2SiGeH3 and Cl2HSiGeH3.
We have investigated several possible origins for this splitting,
including dimeric interactions and roto-vib couplings. Test
calculations on (ClH2SiGeH3)2 dimers indicate that only a select
number of vibrational features are split, depending upon their
relative configurations, when molecules are dimerized. There-
fore, since dimerization does not split all peaks equally, we
dismiss this mechanism. In addition, the dimeric interaction
strengths are estimated to be only a few tenths of a kcal/mol,
and dimerization is therefore not likely to occur at room
temperature. We next examined the effects of roto-vib coupling
transitions state complexes are listed as ωTS1, ωTS2, and ωTS3
.
the third chlorination of the molecule. Furthermore, for the third
chlorination the thermodynamic driving force associated with
∆GTH (where ∆GTH ) GTH products - GTH reactants) is the
smallest (5 kcal/mol), and the reaction barrier is the largest (41
kcal/mol), while the first chlorination step involves a net reaction
energy gain of 11 kcal/mol and a barrier of 33 kcal/mol. In
combination, these observations suggest that the third chlorina-
tion is clearly less favorable than the first or second. Accord-
ingly, the Cl3SiGeH3 compound was never observed under our
reaction conditions. Careful analysis of the reaction products
did not reveal the presence of trichlorosilane (SiHCl3) which is
expected to be the most stable decomposition byproduct of Cl3-
SiGeH3, ruling out the transient formation of this compound
followed by its decomposition.
(c) Vibrational Properties of ClSiH2GeH3 and Cl2SiHGeH3.
The vibrational properties of all compounds described in Figures
2 and 3 were obtained as a byproduct of the thermochemical
analyses discussed above. Here we specifically describe the IR
spectra of only compounds ClSiH2GeH3 (1) and Cl2SiHGeH3
(2), which were fully isolated as pure volatile products and
utilized in proof of concept CVD applications. The vibrational
spectra of the remaining compounds will be reported elsewhere.
In previous investigations, we successfully used simulated
vibrational spectra to identify the various positional and
conformational isomers in
a
mixture of butane-like
GeH3SiH2SiH2GeH3 and GeH3GeH2SiH2GeH3 compounds.8
Here we use this approach to corroborate the molecular
structures (Figure 3) of the purified 1 and 2 as derived from
the NMR spectra.
The spectra of both 1 and 2 molecules were obtained using
the full B3LYP/6-311N++G(3df,3pd) prescription. The normal
modes exhibited positive definite vibrational frequencies, indi-
cating that the ground state structures are dynamically stable.
Symmetry analysis of the optimized structures indicates that
the molecules are asymmetric tops possessing Cs symmetry. In
general, the high-frequency spectra of both compounds contain
two primary features corresponding to asymmetric and sym-
metric Ge-H stretches (2120-2160 cm-1) and their asymmetric
9
7956 J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007

ClnH6 nSiGe Compounds
A R T I C L E S
-
Figure 6. Comparisons of the calculated and observed room-temperature IR spectra of molecules 1 and 2. Frequency scale factors of 0.989 and 0.979 were
applied to the calculated low- and high-frequency spectra, shown in parts a and b, respectively.
Pseudomorphic Growth of Si1-xGex
Proof-of-principle growth experiments were conducted to
investigate the applicability of compounds ClH2SiGeH3 (1) and
Cl2HSiGeH3 (2) to form semiconductor alloys at low temper-
atures. The growth studies were conducted on Si(100) substrates
in an ultrahigh vacuum (UHV) deposition system. The substrates
were prepared for growth by flashing at 1000 °C under UHV
(10-10 Torr) to remove the native oxide from the surface. The
gaseous precursors 1 or 2 were then reacted on the clean Si
surface to form films via desorption of HCl and H2 byproducts.
(a) ClH2SiGeH3 Growth Studies. The depositions of ClH2-
SiGeH3 were conducted at partial pressure and reaction tem-
peratures typically in the range of 10-5-10-6 Torr and 360-
Figure 7. Simulated rotational band contours of A-, B-, and C-type bands
obtained using calculated rotational constants for ClH2SiGeH3.
450 °C, respectively. The film composition was determined by
Rutherford backscattering (RBS) to be Si0.50Ge0.50 reflecting
precisely the Si/Ge ratio of the ClH2SiGeH3 precursor. The
heteroepitaxial character of the synthesized films was initially
investigated by random and channeling RBS, which indicates
that the material is perfectly aligned with the substrate. The
ratio of the aligned versus the random peak heights (ømin), which
measures the degree of crystallinity, decreases from 25% at the
interface to 10% at the surface indicating a typical defect “pile
at room temperature in these asymmetrical top molecules. Our
analysis revealed that roto-vib coupling produces splittings in
all bands, including the high-frequency Ge-H and Si-H modes
in both molecules. This can be explained by the relatively low
symmetry of the ClH2SiGeH3 and Cl2HSiGeH3 molecules,
which leads to A-type symmetries for all vibrational modes (e.g.,
no A′ or A′′ symmetry vibrations). As an example, using our
calculated rotational constants for the ClH2SiGeH3 molecule (A
up” near the interface while the upper portion of the film is
) 10.18 GHz, B ) 1.42 GHz, C ) 1.29 GHz), we have
computed the room-temperature rotational band profiles
corresponding to A-, B-, and C-type contours using an asym-
metrical top computer program. These are compared in Figure
7 which indicates that the possible profiles are quite distinct
and that the C-type contour closely matches to the observed
absorption band shapes with an apparent splitting of ap-
proximately ∼13 cm-1. The dichloro rotational constants give
rise to very similar splittings which further corroborate the origin
of these spectral features.
relatively defect-free. The film crystallinity was thoroughly
investigated by high-resolution cross sectional electron micros-
copy (XTEM) which revealed commensurate SiGe/Si interfaces
and perfectly monocrystalline epilayer microstructure. The TEM
samples showed very few defects penetrating through the layers,
which is consistent with the RBS dechanneling due to defect
pile-up near the interface. High-resolution X-ray diffraction (HR-
XRD) revealed perfectly aligned layers with low mosaic spread
and slightly strained microstructures as grown. A nominal XRD
relaxation of 75% was observed for a film thickness ∼80 nm.
9
J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007 7957

A R T I C L E S
Tice et al.
Atomic force microscopy (AFM) scans over areas 10 × 10 µm2
show that films with thickness of 80 and 100 nm display rms
values of 0.7 and 1.1 nm, respectively, indicating highly planar
surfaces. We note that no “cross hatched” surface patterns,
caused by misfit dislocations produced during strain relaxation,
were observed even for films annealed up to 700 °C. In fact,
the surface roughness either remained the same or improved
slightly with annealing indicating that the planarity of our
samples is thermally robust.
(b) Cl2HSiGeH3 Growth Studies. The deposition of Cl2-
HSiGeH3 (2) was conducted at 450 °C (10-5 Torr) and produced
films with thickness ranging from 10 to 80 nm at growth rates
approaching 1 nm/min, which is slightly higher than that
obtained from the deposition of 1. For films exceeding 10-20
nm in thickness, a quantitative analysis based on RBS showed
that the Si content can be as low as 43%. For thinner samples
(<10 nm), the film composition was derived from high-
resolution XRD data because the Si content was difficult to
extract from the substrate background using RBS. For thicker
samples, we observed a systematic reduction from the expected
(ideal) 50% stoichiometry when the deposition pressure is
reduced from 5 × 105 to 1 × 105 Torr. Silicon depletion is
likely enhanced under the lower pressure conditions due to facile
surface desorption of multichlorinated Si species, such as stable
SiH2Cl2 decomposition byproducts. The crystallographic align-
ment and surface morphology of these films were found to be
of comparable quality to those obtained from depositions of 1.
Nevertheless, their XTEM characterizations showed lower
threading dislocation densities of the layers, and superior
miscrostructural quality of the interface regions. This includes
flawless epitaxial registry and atomically abrupt transitions
between the film and the substrate, consistent with the absence
of elemental intermixing between the two materials at the low
growth temperatures.
Figure 8. (224) reciprocal space maps of the 10 nm thick sample and the
Si substrate, showing peaks 1 and 2 for the two layer structure. Note that
2 (corresponding to the layer adjacent to the interface as shown in the
schematic inset) falls directly below the Si (224) peak indicating perfect
lattice matching within the interface plane. Its elongation is due to the small
thickness of the layer. A more diffuse peak 1 is also clearly visible and
corresponds to the upper portion of the film. Note that peak 1 lies below
the relaxation line connecting the Si peak to the origin, indicating partial
relaxation (∼50%).
The strain state properties of the films were extensively
characterized using HR-XRD. Here we focus on two representa-
tive samples with layer thicknesses of ∼10 and 60 nm. Precise
in-plane and perpendicular lattice constants were determined
by high resolution (224) and (004) on- and off-axis reciprocal
space maps of the entire structure. In the case of the thinner
sample, the XRD data revealed that the growth produces two
discernible layers in which the initial layer adjacent to the
interface is fully strained to the Si substrate (a| ) aSi ) 5.4309
Å and a ) 5.5991 Å) and has a thickness of 5-7 nm (see
Figure 8). With elementary elastic strain relations18 (ꢀ ) -2C12/
C11ꢀ| where C11 and C12 are bulk SiGe elastic constants, and ꢀ
and ꢀ| are the perpendicular and parallel strains), the known
bowing behavior of bulk Si-Ge alloys, and the structural data
above, the composition of the initial alloy layer was determined
to be 45 ( 2% Ge and 55 ( 2% Si. The XRD data also showed
a diffuse, high mosaicity peak (peak 1 in Figure 8) indicating
that the upper portion of the film (grown following the initial
layer) is 50% relaxed and has lattice constants a| ) 5.4845 Å
and a ) 5.5654 Å corresponding to a composition of ∼47 (
2% Ge and 53 ( 2% Si. This indicates that this upper layer
possesses stoichiometries closer to the expected 50/50 value in
accordance with precursor Si-Ge content. In this diffusion-
limited low-temperature growth process, the tendency to strain-
match with Si can only be achieved by incorporation of excess
Figure 9. (Top) XTEM micrograph of the entire film thickness showing
flat surface and bulk region devoid of defects. (Bottom) High-resolution
micrograph of the interface region showing perfect pseudomorphic growth.
Si at the interface layer. Beyond a certain critical thickness
(∼50-60 Å under these conditions), the strain is gradually
relieved without any measurable defect formation, enabling the
full stoichiometry of the precursors to be incorporated.
In the case of the 60 nm thick film, we observe that the
growth also produces the same initial fully strained layer (∼10
nm thick) adjacent to the interface, followed by a transition to
a partially strained bulk film whose Si content approaches
nominal target values. Figure 9 shows an XTEM micrograph
of the entire film thickness in (100) projection. Note that the
film surface is atomically flat and the bulk material is devoid
of threading defects within a wide field of view (typically 2
µm). The high-resolution image of the interface region shows
(18) Brantley, W. A. J. Appl. Phys. 1973, 44, 534.
9
7958 J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007

ClnH6 nSiGe Compounds
A R T I C L E S
-
sodium benzophenone ketyl prior to use. The NMR spectra were
collected on an Inova 500 MHz spectrometer. Samples were dissolved
in deuterated benzene, and all nuclei were referenced either directly or
indirectly to the signal of TMS or the residual solvent peak as indicated.
Gaseous infrared spectra were obtained in 10-cm cells fitted with KBr
windows. Mass spectrometry data were obtained using a Leybold-
Inficon quadrupole mass spectrometer. Boron trichloride (Aldrich
99.95%) was distilled to remove residual HCl contaminants, and its
purity was confirmed by infrared and mass spectra prior to use. Lithium
tetrahydroaluminate (Aldrich), phenyl trichlorosilane (Aldrich), tri-
fluoromethane sulfonic acid (Alfa Aesar), cesium chloride (Aldrich),
and electronic grade germane gas (donated by Voltaix, Inc.) were used
as received. The starting materials include phenylsilane, which was
perfect commensuration and no indication of misfit dislocations
in spite of the large mismatch. This observation is consistent
with full pseudomorphic growth as determined by XRD. The
lack of defects can be explained by our observation of an
inherent compositional gradient which accommodates the strain
throughout the film. These mechanisms are unique to the low-
temperature growth protocol afforded by the specifically
designed precursor chemistry employed in this study.
Concluding Remarks
The synthesis of ClnH1-nSiGeH3 is achieved via selective
chlorination of SiH3GeH3 by BCl3 at low temperatures. The
ClH2SiGeH3 species is also formed via reaction of (SO3-
CF3)SiH2GeH3 and CsCl using a complex multistep reaction
route based on PhSiH2GeH3 low yield intermediates. Detailed
characterization of the products was conducted via a range of
spectroscopic and analytical methods including quantum chemi-
cal simulations of the reaction thermodynamics and kinetics.
Low-temperature depositions of ClH2SiGeH3 (1) and Cl2-
HSiGeH3 (2) have produced near stoichiometric SixGe1-x films
(x ) 0.43-0.55 depending on deposition conditions) possessing
the desired properties for semiconductor applications including
perfectly crystalline and epitaxial microstructures, smooth
morphologies, and unique strain/relaxed states.
We have previously utilized SiH3GeH3 and the butane-like
(GeH3)2(SiH2)2 compound to deposit Si0.50Ge0.50 films at low
temperatures. In both cases, we have obtained materials with
high quality morphological and microstructural properties.
Nevertheless, the use of SiH3GeH3 for low-temperature CMOS
selective growth of small size device features is not practical
due to the dramatic drop in growth rates below 475 °C
(negligible below 450 °C). In contrast, both ClH2SiGeH3 and
(GeH3)2(SiH2)2 yield growth rates at least 10 times higher than
those obtained from SiH3GeH3 in the 400-450 °C range, and
these compounds even display significant film growth at
temperatures below 400 °C. It should also be noted that the
film quality is comparable using either ClH2SiGeH3 or (GeH3)2-
(SiH2)2 to grow Si0.50Ge0.50. However, albeit (GeH3)2(SiH2)2
shows promise from a technical perspective, it represents a
demonstration in principle rather than a true proof of concept
from an industrial perspective. For instance, this compound is
difficult to synthesize in quantities greater than a few grams,
and its thermal stability in relation to storage and transportation
is not well understood. Thus, the ClH2SiGeH3 (1) and Cl2H2-
SiGeH3 (2) counterparts appear to be more practical candidates
for immediate deployment as a 50/50 SiGe source. Their
synthesis is much easier to scale up, and the starting materials
are readily available and inexpensive. The single step reaction
of BCl3 with H3SiGeH3 is conducted in a solvent free environ-
ment ensuring facile isolation and purification to produce
semiconductor grade material. Further, due to their high vapor
pressure, the compounds can be readily integrated with existing
commercial processes involving single-wafer low-pressure CVD
furnaces. Future work in this area will be focused on the use of
these compounds in selective deposition applications using
patterned substrates provided by ASM international.
prepared by reduction of phenyl trichlorosilane with LiAlH4, and phenyl
monochlorolsilane, which was prepared by the reaction of phenylsilane
with gaseous HCl. Phenylsilylgermane,14 germylsilyl trifluoromethane-
sulfonate,6 and germylsilane6 were prepared according to literature
procedures. All starting materials were checked by NMR spectroscopy
to verify their purities. Potassium germyl (KGeH3) was synthesized
via a modified literature preparation in monoglyme by reaction of
gaseous GeH4 with a finely dispersed sodium-potassium (80% K)
alloy. Note that the Teflon bar that was used to stir the GeH4/Na-K/
monoglyme solution was encapsulated in glass owing to the high
reactivity of Teflon with Na-K alloys.
Synthesis of ClnH6-nSiGe: Method A. (a) Reaction of CsCl with
(CF3SO3)SiH2GeH3. A 50 mL Schlenk flask was charged with n-decane
(15 mL) and (CF3SO3)SiH2GeH3 (0.60 g, 2.35 mmol). Cesium chloride
(0.42 g, 2.49 mmol) was added slowly via a powder addition funnel
at -25 °C to the flask, and the mixture was allowed to stir under N2
for 12 h at 22 °C. The volatiles were fractionally distilled through
U-traps held at -50, -110, and -196 °C under dynamic vacuum. The
-50 and -196 °C traps contained solvent and traces of GeH4. The
-110 °C trap retained pure, colorless 1 (0.186 g, 56% yield). Vapor
pressure: ∼147 Torr (23 °C). IR (gas, cm-1): 2183 (vs), 2171 (s),
2156 (vw), 2092 (vw), 2082 (vs), 2070 (s), 1076 (vw), 948 (m), 936
(m), 896 (m), 839 (vw), 830 (vw), 790 (s), 774 (vs), 764 (vs), 549
(m), 540 (m), 470 (w), 360 (vw). 1H NMR (500 MHz, C6D6): δ 4.68
(q, 2H, Si-H2), δ 3.12 (t, 3H, Ge-H3). 28Si NMR (500 MHz, C6D6):
δ -24.61. Mass spectrum: m/z isotopic envelopes centered at 141 (M+),
106 (H3GeSiH3+), 75 (GeH4+), 65 ClSiH2+, 36 (Cl+), 31 (SiH4+).
Caution: Mixtures of (SO3CF3)SiH2GeH3 haVe been found to explode.
Synthesis of ClnH6-nSiGe: Method B. (a) Reaction of 1:3 BCl3/
H3SiGeH3. Gaseous BCl3 (0.37 g, 3.2 mmol) and H3SiGeH3 (1.02 g,
9.6 mmol) were condensed into a 150 mL stainless steel cylinder at
-196 °C equipped with a high vacuum valve. The cylinder and its
contents were held at 0 °C for 2 h after which time the volatile contents
were distilled through U-traps held at -78, -95, and -196 °C. The
latter contained predominately B2H6 and unreacted H3SiGeH3 as
evidenced by gas IR. The -95 °C trap contained pure 1 as evidenced
by 1H NMR. The liquid in the -78 °C trap was analyzed by 1H NMR
(500 MHz, C6D6) and was found to be a mixture of 1 and 2 [δ 5.57 (q,
H, Si-H), δ 3.25 (d, 3H, Ge-H3)]. The integrated 1H peak intensities
of the Ge-H3 resonances indicated a ∼12:1 ratio of 1 to 2, which were
separated by fractional distillation through -78 to -95 °C traps. This
reaction yielded 5.70 mmol (∼60% yield) of 1 and 0.48 mmol (5%
yield) of 2.
(b) Reaction of 2:3 BCl3/H3SiGeH3. BCl3 (0.33 g, 2.8 mmol) and
H3SiGeH3 (0.45 g, 4.2 mmol) were condensed into a 150 mL stainless
steel cylinder and allowed to react at 0 °C for 2 h after which time
the volatiles were distilled through U-traps held at -78, -95, and
-196 °C. The contents of the -78 °C traps were shown to be a mixture
of 1 and 2 while those of the -95 °C trap were pure 1. The yields for
1 and 2 were found to be 1.5 mmol (36%) and 1.3 mmol (31%),
respectively. The -196 °C trap contained predominately B2H6 and
unreacted H3SiGeH3.
Experimental Section
General Methods. All manipulations were carried out under inert
conditions using standard high vacuum line and drybox techniques.
Dry, air-free solvents were distilled from either anhydrous CaCl2 or
9
J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007 7959

A R T I C L E S
Tice et al.
(c) Reaction of 1:1 BCl3/H3SiGeH3. BCl3 (0.70 g, 6.0 mmol) and
H3SiGeH3 (0.64 g, 6.0 mmol) were condensed into a 150 mL stainless
steel cylinder and allowed to react for 2 h at 0 °C. The volatiles were
then distilled through U-traps held at -35, -78, -110, and -196 °C.
The -196 and -110 °C traps contained B2H6 (∼3.0 mmol, 100% yield)
and minor concentrations of Cl2SiH2. The -78 °C trap contained pure
Cl2GeHSiHCl2. 1H NMR: δ 5.15 (d, H, J ) 7.0 Hz, Si-H), δ 5.97
(d, H, J ) 7.0 Hz, Ge-H). 29Si NMR: δ -1.12.
1
ClSiH2GeHCl2. H NMR: δ 4.38 (d, 2H, J ) 4.4 Hz, Si-H2), δ
6.12 (t, H, J ) 4.4 Hz, Ge-H). 29Si NMR: δ -22.87.
1
ClSiH2GeH2Cl. H NMR: δ 4.53 (t, 2H, Si-H2), δ 4.86 (t, 2H,
Ge-H2). 29Si NMR: δ -24.26.
2 as a clear, colorless liquid (1.5 mmol, 25% yield).
Cl2SiHGeH3 (2). Vapor pressure 40 Torr (23 °C), IR (gas, cm
-1
(d) Reaction of 2:1 BCl3/H3SiGeH3. Gaseous BCl3 (0.55 g, 4.7
mmol) and H3SiGeH3 (0.25 g, 2.35 mmol) were reacted for 2 h at -196
°C in the stainless steel cylinder reactor. The volatile contents were
then distilled through U-traps held at -35, -78, and -196 °C. The
-196 °C trap contained B2H6, unreacted BCl3, and a minor amount of
SiH2Cl2. The -78 °C trap contained traces of Cl2SiHGeH3 (5% yield).
The -35 °C trap contained the bulk product as a colorless low volatility
liquid (∼2.0 Torr vapor pressure) which consisted of ClSiH2GeHCl2,
Cl2SiHGeHCl2, ClSiH2GeH2Cl, and Cl2SiHGeH2Cl as evidenced by
extensive NMR studies (as described above for the 1:1 H3SiGeH3/BCl3
reaction). The 1H NMR resonance integration indicated that the
predominant species were Cl2SiHGeHCl2 and Cl2SiHGeH2Cl, while
ClSiH2GeHCl2 and ClSiH2GeH2Cl are present in very small amounts.
)
2194 (s,sh), 2099 (w), 2091 (s), 876 (m), 869 (m), 792 (m), 783 (m),
753 (vs), 748 (vs), 706 (vw), 577 (s), 536 (m), 460 (w). Mass
spectrum: m/z isotopic envelopes centered at 175 (M+), 141 (ClSiGeH5+),
106 (H3GeSiH3+), 100 (Cl2SiH2+), 75 (GeH4+), 65 ClSiH2+, 36 (Cl+),
1
31 (SiH4+). H NMR (500 MHz, C6D6): δ 5.57 (q, H, J ) 3.0 Hz,
Si-H), δ 3.25 (d, 3H, J ) 3.0 Hz, Ge-H3). 29Si (500 MHz, C6D6): δ
9.77. 2D 1H COSY confirmed that the 1H resonances were coupled to
1
1
each other. The H-29Si HMQC showed that the H resonance at δ
5.57 (q) was directly coupled to the 29Si resonance at δ 9.77.
Cl2SiHGeH2Cl (3), Cl2SiHGeHCl2 (4), ClSiH2GeH2Cl (5), and
ClSiH2GeHCl2 (6). The -45 °C trap contained a low volatility liquid
(∼2.0 Torr vapor pressure) which was characterized by NMR to be a
mixture of polychlorinated silylgermanes. The 29Si (500 MHz, C6D6)
spectrum consisted of four peaks at δ -22.87, -24.26, -1.12, and
We finally note that most reactions described above using method
B were conducted under 2 h intervals because longer times at the
temperatures employed resulted in intractable polymers rather than
molecular systems indicating that there are competing decomposition
reactions under these conditions.
1
3.36, and the H spectra (500 MHz, C6D6) showed eight peaks with
chemical shifts at δ 6.12 (t), 5.97 (d), 5.35 (t), 5.15 (d), 4.86 (t), 4.82
(d), 4.53 (t), and 4.38 (d). These data collectively suggested the presence
of four ClnH6-nSiGe compounds. A 1H-29Si HMQC (500 MHz, C6D6)
spectrum showed that the H atoms with resonances at δ 5.35, 5.15,
4.53, and 4.38 are bound to Si atoms with resonances at δ 3.36, -1.12,
Acknowledgment. This research was supported by the U.S.
Air Force Office of Scientific Research (AFOSR MURI,
FA9550-06-01-0442), Voltaix Corporation, and NSF (IIP 053950).
The XRD equipment used in this work was obtained by a grant
from the NSF (DMR-0526604)
1
-24.26, and -22.87, respectively. H 2D COSY experiments were
performed to establish connectivities. Cross-peaks were found to be
between the following pairs of [Si-Hx, GeHx] resonances: [δ 4.38 (d),
6.12 (t)], [δ 5.15 (d), 5.97 (d)], [δ 4.53 (t), 4.86 (t)], and [δ 5.35 (t),
4.82 (d)]. The corresponding Si-Hx/GeHx peak height ratios are 2:1,
1:1, 1:1, and 1:2, respectively. Accordingly, these can be assigned to
the compounds ClSiH2GeHCl2 (6), Cl2SiHGeHCl2 (4), ClSiH2GeH2Cl
(5), and Cl2SiHGeH2Cl (3), respectively.
Supporting Information Available: Complete ref 17. This
material is available free of charge via the Internet at
http://pubs.acs.org.
1
Cl2SiHGeH2Cl. H NMR: δ 4.82 (d, 2H, J ) 4.3 Hz, Ge-H2), δ
5.35 (t, H, J ) 4.3 Hz, Si-H). 29Si NMR : δ 3.36.
JA0713680
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7960 J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007