Deposition of germanium nanowires from hexamethyldigermane: Influence of the substrate pretreatment

Article abstract of DOI:10.1149/1.3476288

Germanium nanowires (GeNWs) were synthesized by low pressure chemical vapor deposition of hexamethyldigermane (GeMe₃)₂ at 490°C and a pressure of 90-100 Pa. GeNWs of several nanometers in diameter and a few micrometers in length were deposited onto substrates made of stainless steel, Fe, Mo, Ta, W, Si, and SiO₂. The influence of surface pretreatment of the substrates (roughening of surface, grooves made by a diamond tip or Ge thermal evaporation) is discussed. GeNW deposits were studied using scanning electron microscopy, transmission electron microscopy, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared, Raman spectroscopy, and energy-dispersive X-ray analyses.

Full text of DOI:10.1149/1.3476288

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Journal of The Electrochemical Society, 157 ͑10͒ K218-K222 ͑2010͒
0013-4651/2010/157͑10͒/K218/5/$28.00 © The Electrochemical Society
Deposition of Germanium Nanowires from
Hexamethyldigermane: Influence of the Substrate Pretreatment
V. Dřínek,a,z R. Fajgar,a M. Klementová,b and J. Šubrtb
aInstitute of Chemical Process Fundamentals of the AS Cr, v.v.i., Rozvojova 2, CZ-165 02 Prague 6,
Czech Republic
bInstitute of Inorganic Chemistry of the AS CR, v.v.i., 250 68 Husinec-Řež, Czech Republic
Germanium nanowires ͑GeNWs͒ were synthesized by low pressure chemical vapor deposition of hexamethyldigermane ͑GeMe3͒
2
at 490°C and a pressure of 90–100 Pa. GeNWs of several nanometers in diameter and a few micrometers in length were deposited
onto substrates made of stainless steel, Fe, Mo, Ta, W, Si, and SiO2. The influence of surface pretreatment of the substrates
͑roughening of surface, grooves made by a diamond tip or Ge thermal evaporation͒ is discussed. GeNW deposits were studied
using scanning electron microscopy, transmission electron microscopy, high resolution transmission electron microscopy, X-ray
photoelectron spectroscopy, Fourier transform infrared, Raman spectroscopy, and energy-dispersive X-ray analyses.
© 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3476288͔ All rights reserved.
Manuscript submitted November 16, 2009; revised manuscript received July 15, 2010. Published August 19, 2010. This was Paper
2621 presented at the Vienna, Austria, Meeting of the Society, October 4–9, 2009.
Nowadays, the progress in microelectronics has been driven by
nanoscale technology for two- and one-dimensional structures.
Therefore, nano-objects ͓nanowires ͑NWs͒, nanotubes, nanorods,
nanopillars, nanoribbons, etc.͔ made of novel materials and alloys
with unique properties have been studied and tested in detail in
microelectronic systems. Gradually, such nano-objects are imple-
mented into silicon technology to update and enhance the power and
efficiency of microelectronic circuits, solar cell arrays, catalyst sys-
tems, etc.
substrate with ultrapure water to form a highly reactive SiOx super-
ficial surface layer. Evidently, the structural and morphology surface
conditions induce a generation of energetically favorable nucleation
sites on a substrate for the consecutive GeNW growth.
Our motivation to prepare GeNWs using a noncatalyst-assisted
low pressure chemical vapor deposition ͑LPCVD͒ procedure was ͑i͒
to avoid heteroatom seeds; ͑ii͒ to obtain GeNWs as thin as possible
to ensure establishing of small confined quantum effects; and ͑iii͒ to
explain the triggering mechanism of noncatalyst GeNW growth.
Germanium is considered to replace silicon in some applications
as it has
a higher electron and hole mobility ͑3900 and
1900 cm2/V s, respectively͒ than silicon ͑1500 and 450 cm2/V s,
respectively͒. Moreover, the Bohr radius of germanium is larger
͑24.3 vs 4.3 nm for silicon͒ and, therefore, quantum phenomena are
more readily established.1 Field emission transistors using germa-
nium nanowires ͑GeNWs͒ have already been reported.2-4
Experimental
Sample preparation.— GeNWs were deposited by LPCVD of
hexamethyldigermane ͑GeMe3͒2 ͑Aldrich, technical grade, 993-
52-2͒ onto stainless steel, Fe, Mo, Ta, W, Si, and SiO2 substrates.
The reactor was evacuated by a Pfeiffer Vacuum TCP 380 tur-
bopump to a base pressure of ϳ10−4 Pa. The CVD procedure pro-
ceeded in a furnace ͑Thermolyne 21100͒ for ϳ40 min in the flow
mode at 490°C and a pressure of 90–100 Pa.
Substrates and their pretreatment.— Various substrates with an area
of 1–2 cm2 were tested for the growth of GeNWs: commonly used
industrial steel, stainless steel ͑Goodfellow, AISI 310͒, tantalum ͑Al-
drich, 99.9 + %͒, tungsten ͑Aldrich, 99.9 + %͒, molybdenum ͑Ald-
rich, 99.9 + %͒, iron ͑Aldrich, 99.99 + %͒, Si͑100͒, and SiO2. The
material of the substrate was selected with respect to the formation
of the alloy with Ge at a corresponding temperature in the furnace
͑490°C͒. Gaudet et al.12 studied thermally induced reactions of the
germanium substrate with 20 transition metals and found that Fe
forms an intermetallic alloy with Ge under 500°C, Mo and Ta above
this temperature, and W does not form an alloy at all. Before the
deposition, the substrates were cleaned in an ultrasonic bath in ac-
etone ͑Aldrich͒. Both the untreated and pretreated substrates were
used for the deposition.
The most commonly used methods for the preparation of
GeNWs are the vapor–liquid–solid ͑VLS͒ technique and the seed-
assisted deposition processes using catalyst nanoparticles ͑mostly
Au͒.5-7 However, the seeds act as traps for free charge carriers and
therefore shorten their lifetime. The seed atoms could also migrate
through the circuit structure and degrade its electronic properties.
Although some attempts were made to manufacture NW microelec-
tronic structures using Au seed nanocolloids,8,9 the whole manufac-
turing process is complicated by the steps of introducing and remov-
ing Au nanoparticles from the fabricated NW structure.
A solution to this problem is based on a noncatalyst process that
avoids using heteroatom seeds. Some of them seem not to be con-
venient for standard microelectronic arrays because they anticipate
other chemicals as in electrochemical etching applying a HCl-
containing aqueous electrolyte.10 The best results of the noncatalyst
growth of GeNWs have been obtained by chemical vapor deposition
͑CVD͒ procedures.
The first approach relates to the idea of intermetallic sites serving
as initial centers of GeNW growth. Mathur et al.11 proposed an
intermetallic concept of the self-catalyst initial stage of GeNWs
grown onto iron substrates. They assumed the formation of Fe–Ge
intermetallic nuclei on an iron substrate during the initial steps of
CVD performed at 325°C using a Ge͑C5H5͒2 precursor. However,
this approach is correct if germanium forms an alloy with the sub-
strate material at the corresponding temperature.12
Except the steel, all metal substrates were sequentially polished
by a diamond paste with a mean diameter of grain ranging from 1 to
5 ␮m. Subsequently, half of each substrate was roughened by a
sandpaper ͓fine 800 diameter of grain ͑13 ␮m͒, medium ͑360
− 30 ␮m͒, and rough ͑80–200 ␮m͔͒. The surface roughness ͑pa-
rameter Ra͒ was determined by an instrument for the measurement
of surface texture ͑Surtronic 3+, Taylor Hobson͒. The parameter Ra
was 0.4 ␮m for Mo, Ta, and W substrates and 0.6 ␮m for the Fe
substrate. A few grooves several millimeters long were scratched on
Ta and Mo substrates by the diamond tip.
The second approach addresses a selection of a substrate and/or
an appropriate surface modification before CVD deposition. Park et
al. used a high porosity Si substrate13 and Kim et al.14 etched Si
Silicon substrates were etched with 40% HF to remove a super-
ficial thin SiOx layer formed on the surface and subsequently rinsed
by deionized water. To initiate the GeNW growth, Si and SiO2 sub-
strates were modified by Ge thermal evaporation. The substrates
z E-mail: drinek@icpf.cas.cz
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Journal of The Electrochemical Society, 157 ͑10͒ K218-K222 ͑2010͒
were placed 1 cm from the downstream end of the furnace and
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exposed for 20 min to thermally evaporated Ge generated by heating
a lump of Ge at 950–955°C inside the furnace.
Methods of characterization.— Scanning electron microscopy
͑SEM͒ and energy-dispersive X-ray ͑EDX͒ analyses were done on a
Philips XL30 CP instrument equipped with an EDX detector PV
9760. The accelerating voltage ranged from 5 to 25 kV, depending
on the thickness of the deposited layer.
Transmission electron microscopy ͑TEM͒ was carried out on a
JEOL JEM 3010 microscope operated at 300 kV ͑LaB6 cathode,
point resolution 1.7 Å͒. Images were recorded on a Gatan charge-
coupled device camera with a resolution of 1024 ϫ 1024 pixels
using the Digital Micrograph software package. Deposits were
transferred onto a holey carbon-coated copper grid by brushing the
grid dipped in ethanol against the substrate plate containing the de-
posit.
The Fourier transform infrared ͑FTIR͒ measurements of the de-
posits on tantalum substrates were performed on an FTIR spectrom-
eter Nicolet Impact 400 ͑Nicolet͒ equipped with a 30° specular re-
flectance accessory. Spectra were recorded in 256 scans with a
resolution of 2 cm−1
.
Raman spectra were measured using a dispersive Raman instru-
ment Nicolet Almega XR with an excitation wavelength of 473 nm
and a power of 10 mW. Spectra were collected in 32 expositions
with a resolution of 2 cm−1
.
The X-ray photoelectron spectra were measured on an ESCA 310
͑Gammadata Scienta͒ electron spectrometer equipped with a mono-
chromatized X-ray source of Al K␣ radiation ͑h␯ = 1486.6 eV͒
and a hemispherical electron analyzer. The pressure of residual
gases in the spectrometer chamber was kept below 10−7 Pa. Photo-
electrons emitted in the direction normal to the geometrical surface
plane of the sample from the spot of 3 ϫ 5 mm were analyzed. The
spectra were fitted using a Gaussian–Lorentzian line shape, a Shirley
background subtraction, and a damped nonlinear least-square proce-
dure. The linewidth of peaks ͓full width at half-maximum ͑fwhm͔͒
was held constant for all components in a particular spectrum.
Results and Discussion
Polished metal plates with an area of 1–2 cm2 were used as
substrates for the deposition of GeNWs. A very thick GeNW layer
was developed on the Fe substrate, whereas on the other metal sub-
strates, GeNWs formed fluffy looking deposits ͑Fig. 1-3͒ not recog-
nizable by the naked eye. LPCVD performed on cleaned ͑not pol-
ished͒ commonly used industrial and highly defined AISI 310
stainless steel substrates resulted in GeNW deposits similar to those
on the Fe substrates. LPCVD carried out on all substrates roughened
with a sandpaper led only to slightly denser deposits. On the con-
trary, on the freshly roughened steel substrates, thinner deposits
were observed.
Using a diamond tip, several long grooves were made on Ta and
Mo substrates. Although the grooves did not affect the GeNW
growth on the Mo substrate, on the Ta substrate ͑Fig. 1͒ a boundary
was clearly discernible between a thick deposit of GeNWs in the
groove and the low density deposit grown on scratches made by a
sandpaper.
Figure 1. SEM micrographs of GeNWs on tantalum substrate: ͑a͒ Boundary
between thick deposit of GeNWs in a groove ͓top, enlarged in ͑b͔͒ and low
density deposit of GeNWs clustered on scratches introduced by sandpaper
roughening ͓bottom, enlarged in ͑c͔͒.
Several experiments were performed to synthesize GeNWs on Si
and SiO2 substrates. The GeNW growth on untreated, plain sub-
strates failed. Subsequently, substrates modified with thermally
evaporated germanium were used. In this case, scarcely scattered
GeNW islands were observed ͑Fig. 2͒. As a superficial layer SiOx is
commonly formed on silicon due to oxidation, another experiment
was performed after etching the Si substrate in HF. However, the
deposition of GeNWs was not successful either ͑Fig. 3a͒. Neverthe-
less, the thermal evaporation of germanium onto the etched silicon
surface before the deposition led to a rapid growth of a brownish
deposit with homogeneous texture composed of GeNWs ͑Fig. 3b͒.
Independent of the type of substrate, the diameter of the GeNWs
ranged from 5 to 20 nm with a length of up to 10 ␮m. Nevertheless,
some small differences were found; GeNWs with the smallest diam-
eters ͑mean value of ϳ10 nm͒ were synthesized on the stainless
steel substrates ͑Fig. 4a͒. The bright-field TEM image shows
GeNWs of thickness with the mean value far below the Bohr radius
͑Ͻ20 nm͒. An irregular thin shell ͑1–2 nm͒ on the surface of the
GeNWs ͑Fig. 5a͒ is composed of amorphous germanium oxide as
concluded from the X-ray photoelectron spectroscopy ͑XPS͒ results
discussed below. Electron diffraction ͑Fig. 4b͒ confirmed that the
wires are composed of cubic germanium ͑space group Fd3m, a
= 5.623 Å͒.
Two types of GeNWs were observed using HRTEM: elongated
along the ͗110͘ and ͗111͘ directions ͑Fig. 5͒. The NWs elongated
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Journal of The Electrochemical Society, 157 ͑10͒ K218-K222 ͑2010͒
Figure 2. SEM micrographs of GeNWs on silicon substrate ͑unetched, Ge-
modified͒: ͑a͒ Island-looking structure of GeNWs scattered over the whole
untreated silicon area and ͑b͒ a detailed view of a single island.
Figure 3. SEM micrographs of GeNWs on silicon substrate ͑etched͒: ͑a͒
Unmodified by Ge evaporation and ͑b͒ modified by Ge evaporation.
͑15 cm−1͒; however, in very thin NWs ͑10 nm and less͒ as in our
case, quantum confinement phenomena should be taken into ac-
count. In addition, the Raman spectra show typical D and G peaks at
1361 and 1588 cm−1, respectively.19 Apparently, in the course of the
deposition, small graphitic domains were grown, which were built
into the island formations as seen on some SEM images ͑Fig. 1-3͒.
Carbon was also detected by EDX analysis ͑approximately Ge:C
= 1:1͒.
along the ͗110͘ direction ͑Fig. 5a͒ are composed of single-crystal
germanium without any defects, whereas the other type elongated
along the cubic ͗111͘ direction ͑Fig. 5b͒ contains a four-layer hex-
agonal polytype of Ge, which corresponds to the stacking sequence
ABCB ͑the cubic structure is ABC͒ also described in Ref. 15.
In the FTIR spectra ͑not shown here͒ of GeNW deposits, the
following significant peaks were absent: ͑i͒ ␯͑Ge–Ge͒ vibration is
IR inactive; ͑ii͒ Ge–O stretching ͑ϳ980 cm−1͒ was not detected as
the amorphous germanium oxide shell is very thin; and ͑iii͒ the
absence of the ␯͑Ge–C͒ peak ͑ϳ580–630 cm−1͒ revealed the total
separation of germanium and carbon. Carbon traces were present as
a graphitelike phase as shown in the Raman spectra ͑see below͒.
However, most of the carbon material was pumped off as gaseous
hydrocarbon molecules in the course of LPCVD.
The XPS analysis of GeNWs on the tungsten substrate showed
the presence of two chemical states of germanium. The 2p3/2 spectra
of Ge are composed of two subpeaks with a binding energy of
1218.5 and 1220.3 eV. The former represents elemental Ge and the
latter represents germanium oxide. This fact indicates that GeNWs
are oxidized and covered by an irregular thin shell of germanium
oxide ͑Fig. 5a͒.
In the Raman spectra of GeNWs prepared on both types of stain-
less steel, an additional peak at 236–241 cm−1 ͑10 cm−1 fwhm͒ is
present. The Raman spectra of some samples on W and Ta substrates
also showed this peak, distinguished as a shoulder on the TO Ge–Ge
peak at 290 cm−1. The low frequency region below 250 cm−1 is
occupied by other peaks ascribed to amorphous Ge.20 However, the
peak position of the closest longitudinal optical peak of the amor-
phous Ge is nearly 20 cm−1 in distance. Moreover, the fwhm values
of those peaks are rather wide ͑40–100 cm−1͒ to fit our peak. It
means that the origin of the peak must be different. Interesting re-
sults were obtained by Jang et al.21 who generated deformations in
Ge wafers using indentation tests. By applying various loads, many
narrow peaks at 205, 230, 250, and 264 cm−1 arose ͑fwhm of the
strongest peak at 205 cm−1 was 8 cm−1͒. After removing the loads,
the peaks disappeared in several tens of hours. They tentatively as-
signed the peaks to the Ge-IV phase, which is cubic with space
group Ia3, a = 6.932 Å.22 Similarly, our peak is placed at approxi-
mately 230 to 240 cm−1. The decrease in peak frequency reveals a
weakening of Ge–Ge bonds and the narrow fwhm of 10 cm−1 mani-
fests a good crystallinity of this new crystallographic phase. A pos-
sible candidate is also the hexagonal Ge observed by high resolution
transmission electron microscopy ͑HRTEM͒ ͑Fig. 5b͒. Nevertheless,
further studies are necessary.
In the Raman spectra ͑Fig. 6͒, the presence of germanium is
proved by an asymmetrical transverse optical ͑TO͒ Ge–Ge peak at
290 cm−1, which is a characteristic of crystalline germanium. The
asymmetry of the peak is caused by a low frequency shoulder, which
was identified after deconvolution, as the TO peak of the amorphous
germanium with a maximum at 275 cm−1 and an fwhm of 32 cm−1
.
Such asymmetrical broadening and shift of the TO peak were al-
ready predicted and established by several authors.16-18 The value of
the fwhm of the TO peak of crystalline germanium seems to be high
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Journal of The Electrochemical Society, 157 ͑10͒ K218-K222 ͑2010͒
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Figure 4. TEM observations of GeNWs prepared on stainless steel AISI 310
substrate: ͑a͒ Bright-field image and ͑b͒ corresponding electron diffraction
͑SAED͒.
Figure 5. HRTEM observations of GeNWs prepared on stainless steel AISI
310 substrate: ͑a͒ GeNW elongated along ͓110͔ direction ͑the amorphous
germanium oxide shell is indicated͒ and ͑b͒ GeNW elongated along ͓111͔
direction contain a four-layer hexagonal polytype of Ge.
Mechanism of GeNW growth.— Although the growth mecha-
nism for VLS procedures has been satisfyingly clarified, the initial
stage of the noncatalyst growth has not been closely studied so far.
An intermetallic concept of the self-catalyst initial stage of GeNWs
proposed by Mathur et al.11 is based on intermetallic nuclei forma-
tion. However, we also succeeded in preparing GeNWs on Mo, Ta,
and W substrates. Mo and Ta form intermetallic phase͑s͒ with ger-
manium at elevated temperatures of ϳ700 and 600°C,12 respec-
tively, which is far above the deposition temperature of 490°C used
in our experiments. Moreover, as tungsten does not form any inter-
metallic alloy with germanium,12 the elucidation of GeNW growth
in the scope of this theory is not interpretable. For those metals,
another triggering mechanism has to be considered.
bare substrate and adheres to it. It retains a limited mobility until it
anchors at a specific surface site. Atoms are gradually captured at
the site and form a nucleus nanocrystal.
Second, the surface density of those surface sites has to be ap-
propriately low to nucleate atoms. Otherwise, the incoming atoms
get stuck immediately, resulting in a growth of a regular layer.
The nature of the surface site is crucial in the consideration. It
can be a defect ͑for example missing/extra atom at the surface of the
substrate material, free bond͒, imperfections ͑crystalline edge, face,
and grain boundary͒, or impurities ͑a surface chemical group͒ to
which the atom can be physically bound due to van der Waals
forces. Knowledge of modifying the surface, e.g., what sort of de-
fects, impurities, etc. to generate, would be extremely useful for the
fabrication of NW systems. In our work, only specific modifications
of some substrates meet the demands ͑Ge evaporation, grooves
made by a diamond tip͒, whereas others fail ͑sandpaper roughen-
ing͒. Similarly, Mathur et al.11 tried to initiate the GeNW growth by
Noncatalyst mechanism.— Physicochemical properties of the
substrate surface have not been taken into consideration in the
theory brought up above. Some conditions have to be fulfilled for
the building up of seed nanocrystals on the surface.
First, the incoming atom from the gas phase impinges upon a
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Journal of The Electrochemical Society, 157 ͑10͒ K218-K222 ͑2010͒
Moreover, a better understanding of ͑self-͒catalyst processes
could help to induce specific surface defects or modifications to
initiate NW growth. This possibility opens a way to avoid heteroa-
tom catalysts in the production of NW systems.
Acknowledgment
We thank Dr. Z. Bastl from J. Heyrovský Institute of Physical
Chemistry and Electrochemistry for XPS analysis. This work was
supported by the Grant Agency of the Czech Republic ͑project no.
203/09/1088͒ and by the Ministry of Education of the Czech Repub-
lic ͑project no. AVOZ40320502͒.
Institute of Chemical Process Fundamentals assisted in meeting the pub-
lication costs of this article.
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