AtMol deliverable reporting
4D1.1: Measurements of the surface conductance between
multiple metallic nanopads grown on semiconductor as a function
of their distance
Unit 1: Atomic scale interconnection machines
Lead participant: Krakow (P10)
WP4.1: LT-UHV 4 STM probes systems under
an SEM planar testing
Other participants: -
Person Months (Krakow): foreseen 26/real 26
Person Months (other participant): -
Start date: M 1
Planned end date: M 24
Real end date: M 36
Introduction:
Single molecule logic gates explored in AtMol inevitably requires that a single
molecule, or a molecular circuit is connected to external environment by a set of conductive
nanopads (nano-electrodes) interconnected by a network of atomic and/or molecular wires able
to make an electronic contact directly with the molecules. Therefore, in order to facilitate the
essential nanofabrication steps in the technology of such molecular electronic device &
circuitss, conductive 2D nanopads, islands, nanowires on semiconductor surfaces should be
constructed and characterized. In particular, it has to be established that they are sufficiently
electronically decoupled from the substrate, so that they could serve as interconnects. One way
of testing their functionality is by measuring the substrate surface conductance as a function of
the inter nanopad distance down to a nanometer range and compare with the parallel
measurements performed without the metallic pads, for example by direct surface conductance
measurements in a full 4-probe configuration as reported in AtMol year 2 by P11-Singapore
and published this year (Appl. Phys. Lett., 103, 083106 (2013)). In order to accomplish such a
goal, P10-Krakow used top STM-like contacts of the nanoprobe STM system navigated by a
high resolution Scanning Electron Microscope (SEM). The charge transport characterization
for 1D and 2D surface nanostructures were accomplished with nanoprobe STM tips capable of
forming well defined, Ohmic contacts by controlled approach to the nanometer size objects.
Deposition and self-assembling of noble metals on semiconductor surfaces have been
frequently studied for almost 3 decades. Despite of that, many aspects of such interfaces,
including quantum electronic properties of self-assembled metallic nanowires are only recently
explored. A spectacular example of such a system is Au on Ge(001) which after first publication
in 1988 [1], and some others in 2004, and 2005 [2,3] was not in a main stream of research until
recent letter by Schaefer and co-workers [4] in which formation of 1D electron liquid in selforganized gold chains on Ge(001) was described. Although detail understanding and theoretical
modelling of geometrical and electronic structure of the Au chains on Ge(001) are still under
discussions, gold on Ge(001) seemed to be an ideal system for measuring surface conductance
because of clear indications of the 1D and/or 2D confinement in the nanostructures produced
at the Au/Ge(001) interface [5,6]. Moreover, simultaneous appearance of the gold-induced
chain domains and clean Ge(001) reconstruction areas on the same surface [3] create
opportunity to use such conductive wires as interconnects to structures build on clean or
hydrogen passivated Ge(001).
In the Year 1 AtMol report, P10-Krakow had demonstrated that assembling of nanoscale,
atomically structured conductive pads on a Ge(001) semiconductor surface could be obtained
by a simple gold deposition at low temperature and a subsequent thermal annealing of
the Au/Ge(001) system at a temperatures above 720 K. However, to extend this work
towards atomic scale interconnects on a Ge(001) surface ready for an H passivation process,
P10-Krakow encountered 2 fundamental problems which precluded such possibility: (1)
the clean Ge(001) areas between Au nanowire pads and islands were heavily damaged
after the thermal treatment necessary for the regular wire/island formation and (2) gold from
the pads appeared to react and mix with the underlying Ge substrate causing that the padsubstrate interface was not well defined and its electronic structure could be heavily affected.
Therefore, P10-Krakow in Task 1-T3.2 demonstrated that the fabrication of metallic nanoislands and nanowires was possible for another noble metal, namely Ag deposited on a Ge(001)
surface. Furthermore, either by direct Ag deposition on the Ge(001)H surface and Ag anchoring
on residual dangling bonds, or by Ag deposition on bare reconstructed Ge(001) and a
subsequent passivation with atomic hydrogen, metal nanostructures could be constructed on the
Ge(001)H surface.
Taking the alternate route of depositing and assembling the metallic nano-pads by a growth
processes on a passivated surface, it must be clearly stated that the detail atomic scale process
of metal adsorption on a hydrogenated semiconductor surface is almost completely unknown.
However, it was shown theoretically that an Si(001)H can accommodate single or multiple
Ag atoms into the dangling bonds present on the surface [7]. The calculations show that for the
Ag atoms on a Si(100)H surface, it is energetically favourable to bind to a dangling bond rather
than form a cluster, or stay isolated on a perfectly passivated surface.
This AtMol year 3, P10-Krakow reports what is the first ever measurement of the surface
conductance of a clean and hydrogen passivated reconstructed Ge(001) by means of the 2-probe
UHV-STM system in contact with silver deposition-induced Ag nano-islands providing an
Ohmic contact to the substrate and navigated with a high resolution SEM microscope.
Experimental:
Starting with a Ge(001) surface where Ag nano-islands can be grown as described in AtMol
Task 1-T3.2, P10-Krakow is using an Omicron Nanoprobe system combining LT-STM 4-probe
system with high resolution SEM operating in UHV conditions. The system is equipped with
three UHV interconnected chambers with the base pressure in the low 10-10 mbar, or better. In
addition to the 4-probe STM unit, the system also consists of a low energy electron diffraction
(LEED) optics, a low temperature scanning tunnelling microscope (LT-STM), and a
hemispherical electron energy analyser allowing, in combination with the Gemini column, for
an Auger electron spectroscopy (AES) analysis as well as for scanning Auger microscopy
(SAM). All conductance measurements are done at room temperature.
The substrate is cut from a wafer of undoped n-type Ge(001) crystal with a resistivity of about
30 cm. The cleaning procedure consists of few cycles of 700 eV Ar+ sputtering at sample
temperature of 900 K (as measured by a pyrometer). After this procedure the Ge(001) surface
exhibits a mixed (2x1)/c(4x2) - 2 domain pattern as seen in Fig. 1.
Fig. 1: Clean Ge(001) as prepared by 700eV Ar+ sputtering and thermal annealing at 900 K.
a) STM image taken at room temperature (STM parameters: sample bias -2V and tunnelling
current 0.5 nA); b) LEED pattern showing mixed (2x1)/c(4x2) surface reconstruction at room
temperature; c) LT-STM image of clean Ge(001) surface taken at 4,5K (STM parameters:
sample bias -2V and tunnelling current 0.3 nA).
For the UHV two point conductance measurements with two STM tips and using the STM x,y
and z piezo motion per tip, those STM tips were mechanically connected to the selected silver
nano-islands separated by a distance “d” and the electrical resistance between the probes was
measured. A scheme of the measurements is shown in Fig. 2.
Fig. 2. A scheme of the 2 point-probe conductance measurements for the silver island on
Ge(001) surface (A). A scheme of the measured resistances (B).
P10-Krakow also investigated the influence of hydrogen passivation on assembly and stability
of the metal nano-islands and the first measurements were performed with the Ge(100)H
surface covered with the metallic islands of various sizes and with different separations between
them. Selected pairs of islands with varying inter-pad distance were chosen for measurements
of the surface conductance dependence as a function of the distance.
Description of the results:
Metallic islands on clean, well reconstructed Ge(001) surface were obtained by evaporation of
4 ML of silver at elevated temperature (around 675K). At this temperature growth of silver
follows a Volmer-Weber mechanism. After evaporation well shaped Ag nano-islands were
observed by the high resolution SEM of the nanoprobe under (Error! Reference source not
found.4) and confirmed by using independently an LT-UHV-STM (see 1D 3.3 report).
Fig. 4. SEM image of the 2 probes over Ag islands grown on the reconstructed Ge(001).
The observed Ag nano-islands ensure a good electronic contact between the STM tips and the
Ge surface and provide a stable contact area to the substrate. The measurements for clean,
reconstructed Ge(001) surface covered with silver islands are shown in Fig. 5. A sufficiently
large number of silver island pairs were chosen in order to explore a variation of the separation
distance from 150 nm to 7 m.
Following the work by Hofmann & Wells [9], 2-point probe (2pp) sample resistance could be
approximated by the solution of the Poisson equation written for an idealized situation, such as
for a homogeneous 2D or 3D conductor, a zero resistance of the probes and spherical/cylindrical
contact shapes. In such a case the measured resistance U/I is related to 2D sample conductivity
σs in the following way:
R22Dpp 
U
1
 sr 

ln 

I  s  r 
,
where s is the spacing between contacting tips, and r is the radius of the contacts (in this case
the size of Ag islands). For more realistic situations, it is difficult to separate this 2D part of the
resistance from the measured one because of the other resistance contribution in this junction
which is related to bulk (3D) resistance [9]:
R32Dpp 
U
1 1
1 

 

I  b  r s  r 
,
where σb is the 3D sample conductivity. For realistic conditions of the P10-Krakow
experiments, the radius of contact (the size of Ag islands ) is always significantly smaller than
the inter STM tips spacing (of the order of hundreds of nm), so r <<s and from the above
equations, R can be considered constant for 3D case, and is increasing slowly as ~ ln(s) for a
2D surface conductor.
From Fig. 5, it is clear that the measured dependence (resistance vs. separation distance) shows
2D behaviour of the conductance.
Fig. 5. Measurements of the resistance between silver islands on the clean Ge(001) surface obtained
using 2 point-probe method.
After performing the conductance measurements for clean Ge(001) surface covered with silver
nano-islands, the surface was passivated by hydrogen and the measurements were repeated. The
results of the 2-probe conductance measurements for H-passivated surface is shown on Fig. 6.
The separation distance in that case varied from 200 to 8500 nm. It is seen that the data for H-
passivated Ge(001) (resistance vs. distance) are best fitted by the curve characteristic for 2D
behaviour of the conductance.
Fig. 6. Measurements of the resistance between silver islands on H:Ge(001) surface obtained using 2
point-probe method.
Fig. 7. A comparison of the resistance dependences between silver islands on their separation
obtained for clean Ge(001) surface (black squares and red line) and H:Ge(001) surface (open circles
and green line) obtained using 2 point-probe method.
Conclusion: A comparison of the experimentally measured surface resistance between multiple
Ag nano-islands grown on clean and hydrogen Ge(001) surface as a function of their distance
is shown in Fig. 7. The difference is similar to the difference obtained on clean and H-passivated
Ge surface by four point-probe method performed without metallic pads as it is reported in
detail in the Deliverable 4-D1.2 report.
Ref. Publications:
[1]
[2]
[3]
[4]
[5]
[6]
[7]
Benson, J. A., Hansen, J. C., McEllistrem, M. T., Clendening, W. D., Tobin, J. G.: An
investigation of the Au/Ge(001) interface. Surf. Sci. 193, 37–46 (1988).
Wang, J., Li, M., Altman, E.: Scanning tunneling microscopy study of self-organized
Au atomic chain growth on Ge(001). Phys. Rev. B70, 233312 (2004).
Wang, J., Li, M., Altman, E.: Scanning tunneling microscopy study of Au growth on
Ge(001): Bulk migration, self-organization, and clustering. Surf. Sci.596, 126–143
(2005).
Schaefer, J., Blumenstein, C., Meyer, S., Wisniewski, M., Claessen, R.: New Model
System for a One-Dimensional Electron Liquid: Self-Organized Atomic Gold Chains
on Ge(001). Phys. Rev. Lett. 101, 236802 (2008).
Blumenstein, C., Schaefer, J., Mietke, S., Meyer, S., Dollinger, A., Lochner, M., Cui,
X.Y., Patthey, L., Matzdorf, R., Claessen, R.: Atomically controlled quantum chains
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Hofmann, P., Wells, J. W.: Surface-sensitive conductance measurements, J. Phys.:
Condens. Matter 21, 013003 (2009).
Kim, Gyu-Hyeong; Jeong, Sukmin, Binding of Ag Adatoms to Dangling Bond Defects
on a H-terminated Si(001) Surface, Journal of the Korean Physical Society, 56, 11801184 (2010).
[8]
M. Wojtaszek, M. Kolmer, S. Godlewski, J. Budzioch, B. Such, F. Krok, M. Szymonski,
“Multi-probe characterization of 1D and 2D nanostructures assembled on Ge(001)
surface by gold atom deposition and annealing” in “Advances in Atom and Single
Molecule Machines”, Springer Series on Advances in Atom and Single Molecule
Machine, ed. Christian Joachim (2012) ISBN 978-3-642-28171-6.
[9]
Hofmann, P., Wells, J. W.: Surface-sensitive conductance measurements. J. Phys.:
Condens. Matter 21,013003 (2009).