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 WP4.1: LT-UHV 4 STM probes systems under an SEM planar testing

Lead participant: Krakow (P10) 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 self-organized 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 pad-substrate 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 nano-islands 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 notfound4) 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:

R2 D2 pp=U

I= 1

πσ sln ( s−r

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]:

R3 D2 pp=U

I= 1

πσ b(1

r− 1

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] 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).

[2] 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).

[3] 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).

[4] 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).

[5] 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 hosting a Tomonaga-Luttinger liquid. Nat. Phys. 7, 776–780 (2011).

[6] Hofmann, P., Wells, J. W.: Surface-sensitive conductance measurements, J. Phys.: Condens. Matter 21, 013003 (2009).

[7] 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, 1180-1184 (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).