Chemical routes to GeSi (100) structures for low temperature Si-based semiconductor applications

Article abstract of DOI:10.1063/1.2437098

The authors describe very low temperature (350-420 °C) growth of atomically smooth Ge films (0.2-0.4 nm roughness) directly on Si(100) via gas-source molecular beam epitaxy. A carefully tuned admixture of (Ge H3) 2 C H2, possessing unique pseudosurfactant properties, and conventional Ge2 H6 provides unprecedented control of film microstructure, morphology, and composition. Formation of edge dislocations at the interface ensures growth of virtually relaxed monocrystalline Ge films (～40-1000 nm thick) with a threading dislocation density less than 105 cm-2 as determined by etch pit measurements. Secondary ion mass spectroscopy showed no measurable carbon incorporation indicating that C desorbs as C H4, consistent with calculated chemisorption energies.

Full text of DOI:10.1063/1.2437098

Chemical routes to Ge ∕ Si ( 100 ) structures for low temperature Si-based
semiconductor applications
M. A. Wistey, Y.-Y. Fang, J. Tolle, A. V. G. Chizmeshya, and J. Kouvetakis
Citation: Applied Physics Letters 90, 082108 (2007); doi: 10.1063/1.2437098
View online: http://dx.doi.org/10.1063/1.2437098
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/90/8?ver=pdfcov
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APPLIED PHYSICS LETTERS 90, 082108 ͑2007͒
Chemical routes to Ge/Si„100… structures for low temperature Si-based
semiconductor applications
M. A. Wistey, Y.-Y. Fang, J. Tolle, A. V. G. Chizmeshya, and J. Kouvetakisa͒
Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287
͑Received 16 November 2006; accepted 3 January 2007; published online 23 February 2007͒
The authors describe very low temperature ͑350–420 °C͒ growth of atomically smooth Ge films
͑0.2–0.4 nm roughness͒ directly on Si͑100͒ via gas-source molecular beam epitaxy. A carefully
tuned admixture of ͑GeH3͒2CH2, possessing unique pseudosurfactant properties, and conventional
Ge2H6 provides unprecedented control of film microstructure, morphology, and composition.
Formation of edge dislocations at the interface ensures growth of virtually relaxed monocrystalline
Ge films ͑ϳ40–1000 nm thick͒ with a threading dislocation density less than 105 cm−2 as
determined by etch pit measurements. Secondary ion mass spectroscopy showed no measurable
carbon incorporation indicating that C desorbs as CH4, consistent with calculated chemisorption
energies. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2437098͔
Elemental Ge exhibits many device advantages over
pure Si. Its smaller bandgap is attractive for photodetector
and modulator applications in the 1.3–1.6 ␮m range.1–3
Transistors based on Ge should also provide greater speed
performance due to higher carrier mobilities. Since the
manufacturing infrastructure for Si–Ge technologies is well
established, the direct growth of Ge on Si could produce new
classes of opto- and microelectronic systems, but such
growth has been problematic. The conventional formation of
mismatched ͑4%͒ Ge on Si proceeds via the Stranski-
Krastanov mechanism yielding islands ͑after deposition of
3–4 ML͒ rather than relaxed, continuous layers. For thick
films a high roughness is obtained and dislocation densities
of ϳ108 cm−2 are observed,4 reducing mobility in electronic
devices and also increasing dark current in photodetectors.
Low temperature ͑TϽ375 °C͒ chemical vapor deposi-
tion ͑CVD͒ of GeH4 has produced Ge layers directly on
Si͑100͒ possessing fairly smooth surfaces with occasional
pits and dislocation densities that appear to be too high for
certain applications.5 Higher temperature growth ͑T
Ͼ400 °C͒ produces rougher layers that display classic strain
and defect induced cross-hatched patterns and increases the
propensity of microcrack formation upon cooling6 making
the approach incompatible with back end complementary
metal-oxide semiconductor processes. A more recent method
utilizes graded buffers of Si1−xGex in which the Ge content is
gradually increased up to 100% to relieve the misfit strain
with the substrate. Typically 10 ␮m is required to achieve
acceptable levels of defects ͑ϳ106 cm−2͒ and an expensive,
complicated chemical mechanical polishing step is necessary
to produce a smooth surface.7
the pseudomorphic growth and suppress the propagation of
dislocation cores throughout the layer. During subsequent
growth, the compound promotes self organized assembly of
thick films with flat and defect-free surfaces. In the case of
͑GeH3͒2CH2, the driving force for its deposition reactions is
the facile elimination of robust CH4 by-products.
The Ge films were grown directly on Si by gas source
molecular beam epitaxy ͑MBE͒ at 350–420 °C and
5ϫ10−5 Torr using appropriate stock mixtures of
͑GeH3͒2CH2 and Ge2H6 at a flow rate of ϳ0.08 SCCM
͑SCCM denotes cubic centimeter per minute at STP͒. The
RCA cleaned Si ͑100͒ wafers were sonicated for 5 min in
methanol, blow dried by N2, and inserted through a load lock
into the chamber at a base pressure of 5ϫ10−10 Torr. The
substrates were then heated at 650 °C to remove volatile
contaminants until the chamber base pressure was restored.
The remaining native Si oxide was subsequently removed by
flashing to 1150 °C. The Ge on Si films were characterized
for morphology, microstructure, purity, and crystallographic
properties by atomic force microscopy ͑AFM͒, Rutherford
backscattering ͑RBS͒, secondary ion mass spectrometry
͑SIMS͒, cross sectional transmission electron microscopy
͑XTEM͒, and high resolution x-ray diffraction ͑HR-XRD͒.
The threading defect concentrations were estimated using an
etch pit density ͑EPD͒ technique.7
Initially, we systematically varied the molar ratio of the
͑GeH3͒2CH2 and Ge2H6 reactants over a broad range from
1:2 to 0:1 and found that the specific concentration of
͑GeH3͒2CH2 dramatically affected the deposition properties,
as well as the film composition and morphology. For ex-
ample, growth using molar ratios from 1:2 to 1:10 produced
discontinuous layers with rough surfaces dominated by is-
lands of variable size and shape. RBS C-resonance revealed
significant carbon contamination primarily in samples grown
using molar ratios of 1:2 and 1:5, suggesting that in highly
concentrated mixtures the pseudosurfactant action of
͑GeH3͒2CH2 is compromised. Further reduction of the com-
pound molar ratio into the range of 1:10–1:15 yielded high-
purity, carbon-free films with smooth surfaces and thickness
up to ϳ1 ␮m at growth rates of ϳ5 nm/min. A control ex-
periment using pure Ge2H6 was also conducted, which pro-
duced films with rms roughness of ϳ16 nm, as expected.
Collectively, our experiments indicate that the pseudosurfac-
These issues have prompted us to develop an alternative,
more straightforward approach based on purpose built mo-
lecular precursors specifically designed to tailor the surface
reactions at the growth front. In this work we employ highly
reactive compounds such as digermylmethane ͓͑GeH3͒2CH2͔
with built-in “pseudosurfactant” capabilities to promote
layer-by-layer growth of relaxed Ge films devoid of delete-
rious threading dislocations. In the initial growth stages the
deposition of the molecules proceeds via formation of edge
dislocations, which heal the interfacial strain associated with
a͒
Electronic mail: jkouvetakis@asu.edu
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0003-6951/2007/90͑8͒/082108/3/$23.00 90, 082108-1 © 2007 American Institute of Physics
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082108-2
Wistey et al.
Appl. Phys. Lett. 90, 082108 ͑2007͒
FIG. 1. AFM of Ge films grown using pure Ge2H6 ͑a͒ and
͑GeH3͒2CH2 /Ge2H6 in 1:15 ratio ͑b͒ at 420 °C indicating rough and smooth
surface layer growths, respectively.
tant properties of ͑GeH3͒2CH2 are enhanced and the film
quality is optimized for reactant ratios close to 1:15 and
growth temperatures in the 350–420 °C range.
AFM studies indicate that of all Ge samples grown un-
der these conditions possess atomically flat surfaces.
Figure 1 shows an image of a 400 nm Ge film exhibiting a
rms of ϳ0.21 nm for 5ϫ5 ␮m2 areas. Note that a Ge film of
comparable thickness grown at the same conditions using
pure Ge2H6 ͓Fig. 1͔ shows a high roughness of ϳ16 nm.
Random and channeling RBSs indicate pure Ge materials
perfectly aligned with the substrate. In most samples the ra-
FIG. 3. ͑Color online͒ ͑Top left͒ SIMS of a Ge film deposited using
͑GeH3͒2CH2 /Ge2H6 in 1:15 ratio ͑region Ge1͒ followed by in situ growth of
pure Ge2H6 ͑region Ge2͒. ͑Right͒ Corresponding XTEM bright field image
showing the complete Ge layer which is free of penetrating defects and
displays a smooth surface. Arrow and dotted line indicate the switchover
point in the growth. ͑Bottom͒ SIMS profile of an 800 nm Ge film showing
no measurable C contamination across the layer.
tio of the aligned versus the random peak heights ͑␹ ͒,
min
which measures the degree of crystallinity, decreases from
10% at the interface to 5% across the layer indicating a dra-
matic reduction in dislocation density. HR-XRD scans in-
cluding ͑224͒ and ͑004͒ reciprocal space maps showed that
all Ge films are virtually strain-free which is consistent with
our observation of atomically smooth surfaces. Several thin
samples ͑Ͻ80 nm͒ revealed a slight tensile strain which is as
high as −0.04% at room temperature ͑a topic of ongoing
studies͒. XRD also indicated nominal mosaic spreads
of ϳ0.14° and lateral correlations ͑grain size͒ of ϳ0.25 ␮m.
XTEM in phase-contrast and Z-contrast modes was used
to characterize the microstructure and morphology of the
films. The bright field images revealed essentially no pen-
etrating threading defects over an extended lateral range of
2 ␮m, independently of the layer thickness ͑Fig. 2͒. The film
surface in all cases was atomically flat. High resolution im-
ages in ͓110͔ projection near the Ge/Si heterojunction indi-
cated sharp, commensurate interfaces and revealed the pres-
ence of periodic edge dislocations confined to the plane of
the interface. The arrows in Fig. 2͑c͒ show the location of
well-separated stress-relieving dislocation cores. The dis-
tance between the cores corresponds to an alignment of 29 Si
rows with every 28 Ge rows, which closely matches the ratio
of the lattice constants between Si and Ge
͓5.657 Å͑Ge͒/5.431 Å͑Si͒=1.04 and 29/28=1.035͔. This
alignment perfectly accommodates the lattice mismatch be-
tween Si ͑100͒ and Ge͑100͒ leading to layer-by-layer growth
of stress-free films. The edge dislocations can also be seen in
the Z-contrast images ͑not shown͒ which further confirm the
presence of a well defined interface between Si and Ge. Elec-
tron energy loss nanoanalysis across the interface corrobo-
rates the Z-contrast interpretation showing a chemically
abrupt transition between Si and Ge, with no elemental in-
termixing or measurable levels of impurities above the usual
background. EPD was used to determine threading disloca-
tion densities in selected films with a thickness up to
800 nm. A typical sample was etched for 200 s at an average
rate of ϳ2 nm/s using a mixture of 700 ml CH3COOH,
70 ml HNO3, 4 ml HF, and 270 mg I2. The etched samples
were examined by an optical microscope and the resulting
pits were counted under 1000ϫ magnification. No disloca-
tions were detected within the first 300–400 nm over areas
of 85ϫ64 ␮m2. At 500 nm the dislocation density values
were found to be below ϳ105 cm−2 and this increased
abruptly towards the interface region. This is consistent with
XTEM images that show no threading defects throughout the
upper portion of the film but a substantial pileup near the
interface.
SIMS was used to determine the Ge, C, and O atomic
profiles of representative samples with variable thicknesses
grown at 350–420 °C. In all cases the SIMS profiles show
that the C content is at the detection limit, below
3ϫ1017 cm−3 ͑bottom of Fig. 3͒. In view of the relatively
high concentration level of C in the reaction mixture
͑ϳ3 at.% with respect to Ge͒ this finding confirms the com-
FIG. 2. Diffraction contrast micrographs of Ge layers with 50 and 800 nm
thicknesses, respectively. Note the lack of penetrating defects and the very
smooth surface morphologies in both cases. HR XTEM image of the
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Ge/Si͑100͒ heterointerface. Edge dislocations indicated by arrows.
plete elimination of CH4 from the ͑GeH3͒2CH2 molecules,
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082108-3
Wistey et al.
Appl. Phys. Lett. 90, 082108 ͑2007͒
according to the reaction ͑GeH3͒2CH2͑gas͒→CH4͑gas͒
+2Ge͑solid͒+2H2͑gas͒. We note that the O content through
the film thickness was also at background levels. Minor C
and O contamination peaks, commonly observed in CVD
grown materials, can be also seen at the Ge/Si interface.
These C and O signatures vary from sample to sample indi-
cating that C ͑in particular͒ may not be related to any mea-
surable decomposition of ͑GeH3͒2CH2 in the vicinity of the
interface. However, some C detected at impurity levels could
be due to side reactions of the precursor with the reactive Si
surface. Nevertheless, the fact that oxygen levels track those
of carbon in all of our samples cannot be explained in terms
of the precursor decomposition.
FIG. 4. Low T, low P reaction of ͑GeH3͒2CH2 with H-covered Ge surface.
Sequence shows intact adsorption of GeH3 –CH2 –GeH3, surface decompo-
sition, and release of CH4. Bond enthalpies relative to gas phase
͑GeH3͒2CH2 ͑prior to reaction͒ are indicated in eV.
To confirm that ͑GeH3͒2CH2 does not deposit C we con-
ducted a control experiment in which an initiation layer of
ϳ250 nm was first grown on Si using the 1:15 reactant ratio.
The remainder of the film was then deposited in situ by
switching the gas source to pure Ge2H6 yielding a total layer
thickness of 800 nm. We note that the growth rate during the
latter stage using pure Ge2H6 is four to five times higher than
that using the 1:15 mixture. XTEM and AFM examinations
of the full sample thickness revealed a complete, continuous,
and monocrystalline layer with a flat surface ͑rms ϳ0.4 nm͒.
SIMS showed a constant Ge content throughout and the typi-
cal C and O impurity peaks located at the interface ͑Fig. 3͒.
In addition, small and distinct C and O signals can be seen
slightly above the noise at the switchover point, 250 nm
above the Si/Ge interface. These minor impurities likely
originate from exposure of the surface to the reactor ambient
during the ϳ1 h switchover. XTEM showed that the micro-
structure throughout the growth transition region is indistin-
guishable from the bulk material indicating that the layer-by-
layer growth is uninterrupted in the absence of ͑GeH3͒2CH2.
This demonstrates conclusively that the carbon from the pre-
cursor is continuously eliminated and does not accumulate
on the Ge surface. Furthermore this two step process yields
high growth rates, Ͼ20 nm/min, making the method viable
for large scale fabrication.
Recent growth studies of Ge on Si suggest that conven-
tional Sb or As surfactants alter the free energy of the surface
and promote layer-by-layer growth far beyond the critical
thickness.8 These surfactants remain on the surface where
they mediate chemisorption interactions reducing reactant
diffusion, and lower surface tension. In contrast, our two step
Ge growth described above demonstrates that any C origi-
nating from the precursor desorbs as CH4 and thus cannot
act as a conventional surfactant. Instead, the effective surfac-
tant mechanism likely involves the formation of surface
pseudosurfactant intermediate groups composed of
–GeH2–CH2–GeH2– ͑or –GeH2–CH3͒ submonolayers,
which are analogous in function to a conventional surfactant
͑Fig. 4͒. These groups are continuously replenished at the
growth front by the incident ͑GeH3͒2CH2 flux and serve as
site holders for the organized assembly of smooth films and
circumvent unintentional incorporation of residual As/Sb
surfactants at these low growth temperatures.
complexes was then calculated and the most plausible ex-
ample consistent with the absence of gas phase reactions at
low temperatures ͑350–400 °C͒ and pressures ͑10−5 Torr͒ is
shown in Fig. 4. Here the intact adsorption of ͑GeH3͒2CH2
produces a strained methylene-bridged –GeH2–CH2–GeH2–
intermediate complex localized at the growth front. The bond
strain due to the C/Ge size difference, then leads to either
immediate release of methane ͑net gain of −1.04 eV͒ or con-
version to the GeH2–CH3 complex shown in Fig. 4, with a
net gain of −0.52 eV. The latter intermediate then finally
desorbs methane with an energy gain of −0.52 eV. In either
case two –GeH3 groups are deposited as building blocks for
layer-by-layer growth. Thus, the net reaction enthalpy for the
deposition of Ge and release of CH4 is found to be
−1.13 eV which represents a strong driving force as de-
scribed by the equation ͑GeH3͒2CH2͑gas͒+2͓−H͔͑surface͒
→2͑−GeH3͒͑surface͒+CH4͑gas͒. We note that the presence
of excess metastable surface complexes is also likely respon-
sible for the “surface poisoning” as discussed earlier for
deposition of high molar ratios ͑1:2–1:5͒. Finally, the ten-
dency of the C atoms to form strong C–H terminated leaving
groups ultimately prevents the incorporation of C into the
Ge, particularly at our low T and P conditions.
In conclusion, we find that digermylmethane with
built-in pseudosurfactant capabilities enables growth of
atomically flat ͑rms 0.21 nm͒ and stress-free Ge films di-
rectly on Si. High growth rates are achieved at 350–420 °C
using Ge2H6 and produce perfectly pure films with threading
dislocation densities below 105 cm−2 ͓orders of magnitude
less than that attainable from the best competing process
available for Ge/Si͑100͒ structures͔.
This work was supported by the AFOSR MURI,
FA9550-06-01-0442 and NSF ͑DMR-0526604͒.
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