st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Germanium Nanowires
and Germanium Nanoparticles Embedded into Hydrogenated Silicon Matrix
The Ha Stuchlikova1, Vladislav Drinek 2, Zdenek Remes 1, Jiri Stuchlik 1
1
Institute of Physics, Academy of Sciences of the Czech Republic v.v.i.,
Cukrovarnicka 10, 162 53 Praha 6, Czech Republic.
2
Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic v. v. i.
Rozvojova 135, 165 02 Praha 6, Czech Republic.
Abstract: We used several precursors for preparation of Germanium nanowires (GeNWs) and
Germanium nanopartilces (GeNPs) using various substrates and deposition methods such as
laser ablation (LA), low pressure chemical vapor deposition (LPCVD), plasma enhanced
chemical vapor deposition (PECVD) and evaporation. We applied GeNPs for modification
of a-Si:H and µc-Si:H based diode structures and demonstrate their performance by
volt-ampere characteristics.
Keywords: Nanowires. Quantum dots, Matrix, CVD, PECVD, LA
1. Inroduction
Nowadays, nanotechnology is driven by the recent
progress in fabrication and characterization of the low
dimensional nano-structures, such as nanodots, nanowires, nanotubes, nanorods, nanopillars, nanoribbons
nanoplatelets etc., made of materials and alloys with
unique properties. The nanostructures have been studied
and tested in detail for nanoelectronic systems as well as
nanooptics, nanomechanics and nanobiology. The forming
of Ge nanodots (QDs) and nanoparticles on the crystallographic pristine silicon surfaces under high have been
studied for many years [1,2]. Further studies were concentrated on deposition of multistructures with embedded
nanopartiles [3]. Dislocations, stress and dangling bonds
on the boundary between Ge and Si due to different lattice
constants can affect the quality of those structures and
change expected physical behaviour [4].
Our aim is to study hydrogenated amorphous and
microcrystalline silicon (a-Si:H, µc-Si:H) thin films with
embedded Ge nanoparticles. We expect that hydrogen
saturates dangling bonds on the boundaries of Ge
nanoparticles in amorphous or microcrystalline silicon.
The hydrogenated silicon thin film (Si:H) diode structures
with embedded Ge nanoparticles can be perspective for
effective solar energy conversion
2. Experimental
Germanium nanowires (GeNWs) were deposited by
Low Pressure Chemical Vapor Deposition (LPCVD) of
hexamethyldigermane
(GeMe3)2
(Aldrich,
tech.,
993-52-2) onto stainless steel (AISI 321). The reactor was
evacuated by a Pfeiffer Vacuum TCP 380 turbopump to a
base pressure about 10-4 Pa. The CVD procedure proceeded in a furnace (Thermolyne 21100) for about 40
minutes in the flow mode at temperature of 490 °C and
pressure of 90-100 Pa.
For the fabrication of ordered Ge nanoparticles we
used a lithography procedure with polystyrene spheres.
The substrate c-Si or Corning glass was cleaned by the
RCA1 process to get hydrophilic silicon surfaces. We
tested a 1:1:5 solution of NH4OH(25%):H2O2(30%):H20
at temperature 80 °C for 15 min. just before usage. A
monolayer of polystyrene spheres was deposited on the
hydrophilic substrates. We deposited Ge film of 10 – 20
nm by a conventional vacuum vapor deposition onto the
polystyrene mask. Subsequently, the polystyrene spheres
were removed from substrates during 2 min ultrasonic
bath in tetrahydrofuran or CH2Cl2. Then the substrates
were rinsed in deionised water [5,6]. In this way it was
possible to deposit Ge nanoparticles on the hydrophilic
clean insulating glass substrates or monocrystalline silicon.
The PECVD growth of a-Si:H and µc-Si:H was carried out at a fixed sample temperature (220 °C) and a
pressure of 70 Pa with 8 sccm SiH4 diluted in 50 sccm H2,
in the case of a-Si:H or 1.6 sccm SiH4 in the case of
µc-Si:H. The power of the RF plasma was 0.18 W.cm−2
and the frequency 13.56 MHz. To eliminate oxide contamination we used a high vacuum PECVD chamber with
the base pressure below 10−5 Pa prior introducing high
purity silane (5.0) and hydrogen (7.0) gases.
Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses were done on a Philips
XL30 CP instrument equipped with an EDX detector PV
9760. Accelerating voltage ranged from 5 to 25 kV, depending on the thickness of the deposited layer. Transmission electron microscopy (TEM) was carried out on a
JEOL JEM 3010 microscope operated at 300 kV (LaB6
cathode, point resolution 0.17 nm). Images were recorded
on a Gatan CCD camera with resolution 1024x1024 pixels using the Digital Micrograph soft-ware package. Deposits were transferred onto a holey-carbon-coated copper
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
grid by brushing the grid dipped in ethanol against the
substrate plate containing the deposit.
vacuum processes: PECVD, evaporation and laser ablation. These technological procedures are perspective for
in-situ processes without breaking vacuum.
3. Results and discussion
GeNW diameter ranged from 5 to 20 nm with length up
to 10 m (Fig. 1a). Statistic evaluation of HRTEM images
(not shown) revealed the uniform diameter thickness with
mean value 10 nm which is far below the Bohr radius
(<24.3 nm). After exposing to ambient atmosphere an
irregular thin shell (1-2 nm) GeOx on the surface of the
GeNWs (Fig 1b) was created.
Although the growth mechanism for Vapor-Liquid-Solid
(VLS) procedures has been satisfactory clarified [7], the
initial stage of the non-catalyst growth has not been
closely studied so far. In case iron substrates Mathur et al
[8] proposed an intermetallic concept of self-catalyst initial stage of GeNWs. They assumed formation of Fe-Ge
intermetallic nuclei on iron substrate at 325 °C using
Ge(C5H5)2 precursor. However, we succeeded to prepare
GeNWs onto W or Ta substrates, too. Therefore we deduced that another non-catalyst growth mechanism has to
be taken into account [9]. We suppose that various defects, irregularities, and imperfections (impurities, missing/extra atom(s) in the structure, crystal edges and faces,
grain boundaries) may serve as nucleation sites. Ge atoms, radicals, clusters etc. drifted in the gas phase settle
and dwell at those sites. This way concentration of the
species increases resulting in the growth of nanocrystal
seeds promoting (self-catalyzed) growth of nanowires.
Fig. 1. a) SEM image of GeNWs grown on a stainless
steel substrate b) HRTEM image of GeNWs elongated
along [110] direction with distinguished atoms in the
struture -there is an oxidized layer on the surface of the
GeNWs
We have tested formation of the ordered arrays of the
GeNPs using polystyrene spheres lithography. [5,6] When
using the polystyrene spheres, the wettability of the substrate surface is very important. Fig. 2 shows the arranged
polystyrene spheres on the surface of the oxidized mono
crystalline silicon. However, we have found that this
method is not suitable for thin film diode structures fabricated on ITO/glass substrates because of the ITO deterioration during cleaning process. Thus we have to focus on
Fig. 2. The polystyrene spheres lithography, a) arranged
polystyrene spheres on the surface of monocrystalline
silicon, b) AFM image of deposited GeNPs, c) side view
profile of GeNPs
First, we tested the fabrication of Ge nanoparticles on
the surface of monocrystalline Si substrate. On the p+ Si
substrate we deposited small-sized GeNPs (size about 10
nm – see Fig. 3a) by thermal decomposition (LPCVD) of
triethylgermane (Et3GeH Aldrich, tech.). In the cases of
Fig. 3b and 3c we tested the evaporation of Ge thin film
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
on the surface of a-Si:H or c-Si:H thin films deposited
by decomposition of silane in the glow discharge
(PECVD). To form on the surface of a-Si:H small
nanoparticles – GeQDs - we evaporated Ge thin films
with thickness 4 nm only. Obviously the surface quality of
a-Si:H thin films influences the clustering of the nanoparticles due to annealing and plasma treatment after the
evaporation Ge thin films. After this process we scanned
the surface (Fig. 4).
Fig. 5. NIP diode structure based on a-Si:H with one
monolayer of embedded GeNPs by evaporation and hydrogen treatment or by laser ablation of Ge target
Fig. 3. SEM pictures of GeNPs on the surfaces:
a) p+ monocrystalline silicon b) a-Si:H and c) µc-Si:H
thin films
Fig. 6. I-V characteristics of a-Si:H diode structures (see
Fig. 4.) with one monolayer of GeNPs made by laser ablation of Ge target (circles) and by Ge evaporation
(squares) before second subsequent a-Si:H deposition.
Fig. 4 AFM picture of a-Si:H surface after deposition of
4 nm Ge thin film and hydrogen plasma treatment
The NIP diode structures based on a-Si:H with one
monolayer of the embedded Ge nanoparticles are shown
in Fig.5. Theirs I-V characteristics are in Fig. 6. The rectifying properties are much better in the case of Ge evaporation when compared to the Ge nanoparticles made by
laser ablation of Ge target.
4. Conclusion
In this paper we have studied the formation of the Ge
nanoparticles embedded in hydrogenated amorphous silicon thin films. First, we have prepared the GeNWs by
LPCVD. Second, we have deposited Ge nanoparticle arrays by thermal evaporation through self-ordered polystyrene spheres. The disadvantage of these methods is the
contamination of the nanostructures, their surfaces and the
interface between the nanostructure and silicon matrix
when exposed to air. Nevertheless, it is possible to reduce
the contamination by the hydrogen plasma treatment before second subsequent deposition of undoped a-Si:H thin
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
film.
We have demonstrated a preparation of Ge nanostructures embedded in thin silicon films in vacuum chambers
avoiding contamination. A first attempt with deposition
using the laser ablation was not so successful. On the
other hand, using Ge thermal evaporation followed by the
annealing and hydrogen treatment we have shown the
deposition of the Ge nanoparticles on the nominally undoped (intrinsic) a-Si:H and prepared NIP diode structure
with GeNPs embedded in the i-layer with the standard
rectifying properties.
5. Acknowledgements:
We acknowledge the grant projects
M100101216, M100101217 and LH12236.
13-25747S,
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