D124
Journal of The Electrochemical Society, 159 (2) D124-D128 (2012)
0013-4651/2012/159(2)/D124/5/$28.00 © The Electrochemical Society
An Alternative Method to Grow Ge Thin Films on Si by
Electrochemical Deposition for Photonic Applications
M. J. Jawad,a M. R. Hashim,a N. K. Ali,b,z E. P. Co´rcoles,c and Maneea E. Sharifabadb
aSchool of Physics, Universiti Sains Malaysia, 11800-Penang, Malaysia,
bMaterial Innovations and Nanoelectronics Research Group, Faculty of Electrical Engineering, Ibnu Sina Institute for
Fundamental Science Studies, and cFaculty of Biomedical and Health Science Engineering Universiti Teknologi
Malaysia, 81310 Skudai, Johor, Malaysia
Germanium films several micrometers in thickness were electrochemically deposited on silicon wafers for the first time without
catalysts and at room temperature from a solution containing Ge species that have been electrochemically dissolved from Ge
target. The films were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy.
SEM images show that the deposited products presented different structures (Flower-like, spheres, and thin films) depending on
the current density. XRD reveals that the germanium electrodeposits were of polycrystalline structure and have the preferred
crystallographic growth orientation of (220). The grown films were deposited with nickel contact electrodes for characterization as
Metal semiconductor Metal (MSM) photodiodes. The current-voltage (I-V) measurements showed the ability to efficiently detect
both UV and visible photons. The low deposition temperature, the ease of thickness control, and the inherent advantage of spatial
selectivity of the electrodeposition process make this method a promising way to selectively grow high-quality germanium for
electronic device applications.
© 2011 The Electrochemical Society. [DOI: 10.1149/2.090202jes] All rights reserved.
Manuscript submitted August 23, 2011; revised manuscript received November 15, 2011. Published December 22, 2011.
In seeking new materials, researchers have focused on substitutes
that utilizes the new Ge nanoparticle material. Germanium nanocrys-
tals were prepared by electrochemical etching of Ge target followed
by electrochemical deposition of these Ge species on Si substrate.
This was followed by the fabrication of MSM Ni/Ge/Si photodetec-
tors. Surface morphology and crystal structure of the deposited Ge
films were investigated using scanning electron microscopy (SEM),
X-ray diffraction and Raman spectroscopy. I-V characteristics of the
fabricated photodetectors will also be discussed.
with superior electronic properties to complement and eventually re-
place Si.1,2 Indeed, Ge has regained some popularity in the field of
semiconductor electronics due to its high mobility and thus providing
an alternative to Si for high performance devices.3 Currently, light
detection in the UV spectral range technology utilizes wide-bandgap
materials. These include metal zinc oxide and magnesium zinc oxide,
III-V materials as well as Schottky-type TiO UV photodiodes. These
materials are grown on incompatible substances. Si and Ge-based op-
tical photodiodes are sensitive to visible and infrared radiation; their
responsivity in the UV region is low in its bulk structure since the
room temperature bandgap energy of Si is only 1.2 eV and 0.65 eV
for Ge. Contrary to bulk Si, Si nanoparticles exhibit behavior of direct
wide-bandgap materials. This makes it very sensitive to UV light and
transparent to visible light. Nayfeh et al.4 presented UV photodetectors
with superior efficiency created by deposition of Si nanoparticle films
on Si substrates. Ge has smaller electron and hole effective masses
and a larger dielectric constant than Si, the effective Bohr radius of
the excitons in Ge (∼25.3 nm) is larger than that in Si (4 nm). This
implies that the quantum confinement effects for Ge nanoparticles
can be seen at a much larger size,5 hence showing a larger shift of an
optical band gap (blue shift) than the Si nanoparticles. So far, to the
best of our knowledge, there is no report about utilizing elemental Ge
nanoparticles for UV/visible detection.
Germanium based thin films have been prepared mainly using a
variety of physical and chemical methods. Among popular physi-
cal methods are RF-sputtering,6 evaporation-condensation,7 chemical
vapor deposition,8 and electron beam evaporation.9 While these tech-
niques provide nanocrystal Ge and thin films, they all present some
difficulties besides high operating costs. Alternatively, cost effective
chemical solution methods have been widely used for the preparation
of freestanding colloid nanocrystals.10,11 However, it is still a chal-
lenge to synthesize Si and Ge nanocrystals using chemical methods,
mainly due to their strong covalent bonding and the need for high
temperature and pressure to promote crystallization.12
Experimental
Our method consists of two steps; firstly, the preparation of solu-
tion contains Ge species by electrochemical etching of the Ge target,
secondly, the electrochemical deposition of the Ge species on Si sub-
strate to produce Ge thin films. A piece of Ge (99.999% pure, 30 mm
× 10 mm × 5 mm) was immersed in a Teflon Beaker containing an
electrolyte mixture of HF (49%):Ethanol (95%), 1:4. The Ge target
was connected to the anode and a Pt wire immersed in the electrolyte
acted as a cathode. A current density of 300 mA/cm2 (electropol-
ishing regime) was applied for 3 hours. The substrates used for the
deposition of the Ge thin films were n-type silicon wafers with (100)
orientation and resistivity of 1–10 ꢀ cm. Prior to the deposition, the
2” Si wafer was cut into 4 pieces and cleaned using RCA cleaning
method. The Si piece was pressed to a Teflon cell at an open window
fit to quarter 2” Si wafer. Between the Si piece and the bottom of
the cell an O ring was used to seal the cell. The Si piece was con-
nected to the cathode and a Pt wire was immersed in the solution
as an anode. The solution was obtained during the first step (etching
step). Samples were prepared using different current densities (7.5 for
samples (a) and (b) and 5 and 2.5 mA/cm2 for samples (c) and (d),
respectively) for one hour at room temperature. After deposition, the
samples were washed several times with deionized water and dried
in ambient air. The structural properties of the samples were investi-
gated using scanning electron microscopy (SEM), X-ray diffraction
and Raman spectrometer (equipped with an argon ion (Ar+) laser with
λ = 514 nm).
Electrochemical deposition is an inexpensive and fast synthesis
of semiconductor thin films and nanostructures.13 There were few
reported studies in the last decade about germanium.14–16 Endres
et al.17–19 recently reported a nanoscale electrodeposition of Ge and
Si from ionic liquids. In this work we report for the first time the
fabrication of a new UV/visible photodetector using Ge-processing
Ni/Ge/Si MSM photodiodes were then fabricated. Prior to the de-
position of contact electrodes, samples were dipped in acetone and
methanol to clean the surface. A 300 nm thick Ni film was subse-
quently deposited onto the sample surface by DC sputtering to serve
as metal contacts. Metal masks were used to define the interdigitated
contact pattern. The fingers of the Ni contact electrodes were 230 μm
wide and 3.5 mm long with 400 μm spacing. The active area of the
whole device was 4×4 mm2. Photocurrent and dark current of the
z E-mail: nihad@fke.utm.my