In this work we present the
results of nanostructural studies and electrical characterization of Au/Ge
films. Ultra thin films of gold were grown on amorphous germanium substrates.
These films were fabricated under very high vacuum condition using thermal
evaporation technique. The nanostructure of the films were determined using
the atomic force microscopy method. The thickness of the films range from
2A to 100 A and [the average cluster size ranges from 5nm to 100nm]. It
is found that the size of the gold clusters depend on substrate temperature.
In this communication we present the Atomic Force Microscopy data, and
the results of the electrical resistance and its dependence on the temperature.
Analysis of the data shows that these nano sized films are insulators
In the rich literature of the
physics of disordered metals, the structural and electrical properties
of ultra thin (near mono layer) films of metals are shown to exhibit
interesting behavior because of their two dimensional (2D) nature. Such
materials exhibit high resistance greater than h/4e2 (~30kW/square).
They display phenomenon related to disorder, such as strong localization
effects, Universal conductance fluctuation and suppression of superconductivity.
It is also shown that Quantum Size Effects (QSE)" arise when the thickness
of the film is comparable to the length scales of the system such as the
electron mean free path and the de Broglie wavelength. The recurrent experimental
and technical inquiries in the literature about thin films is centered
at electrical properties of such materials when the thickness is
comparable to the important quantum mechanical length scales of the system
. For application in molecular electronics, the technical problems
include fine tuning material processing of thin films of metals for molecular
electronics. One of the experimental problems in this case is the ability
to obtain very thin films, free of percolation effects and granularity.
In this case such film can be used as electrodes to the molecule under
investigation. The man goals this project include production such
film, investigation of bonding of thiols and biologically active
molecules to gold and measurement of electron transport through the adsorbed
molecules, using in-situ apparatus. In this communication we describe the
procedure we followed to prepare the films as well as some of our results
from the measurement of electrical conductivity.
Fisher Scientific Premium microscope
slides, 1mm thick, are cut with a diamond knife into 1cm x 1cm squares
were cleaned with ethyl alcohol, and masked with aluminum foil, as shown
above. Four symmetrically placed silver pads were sputtered using Kurt
Lester sputtering system. The Kurt Lesser Sputtering system consists of
a high RF voltage, a pumping system, water cooled electrodes and high purity
Four copper wires (40 gauge
) are attached to the silver pads, using Ted Pella Inc. fast drying
silver point (Lot #0378), for in situ measurement of the sheet resistance
The substrate was mounted on a low temperature insert, where the four
copper wires are soldered to the sample measurement platform that run to
the measuring electronics via a vacuum feed through.
For most of our measurements the pressure in the chamber was maintained
at 2-5 x10-8 torr using nitrogen cooled diffusion pump. A Dycor
mass spectrometer was used to identify molecules that may be outgassed
from the sample-platform system as well as the from the chamber itself.
In most cases, within the resolution of the spectrometer, the vacuum
was clean from such molecules as water vapor, nitrogen, carbon dioxide
and other molecules.
Evaporation of Germanium and Gold Films
The samples were prepare using thermal evaporation technique under ultar
high vacuum condition ( 2 -5x 10-8 torr) . The temperature
of the substrate is monitored using CY7 series silicon diode sensor. .
Two sources were used for evaporating Ge and Au in succession, without
breaking the vacuum. A quartz crystal thickness monitor was used to determine
the thickness of Ge and Au in conjunction with the DC circuit
that is used for in situ measurement of the DC sheet resistance.
The electronics is prepared to measure the sheet resistance using
a high impedance Keithley 614 (K614) electrometer. The current through
the sample is monitored using Keithley 485 (K485) Pico ammeter, A 6.0 Volt
battery was used as a source. The temperature was monitored using a diode
temperature sensor using a LakeShore DRC93A temperature controller.
The evaporation rate was tuned to 5A/s for Ge and 0.1 to 0.5 A/s for
Au. The thickness monitor and sample assembly is shown in picture
After evaporation of Ge, the Inficon apparatus was always reset to measure
the thickness of Au. During the evaporation of Au the current
flow through the DC circuit is monitored. That way we established
the formation of monolayers of Au on the on top of the Ge. Once such condition
is established the evaporation of Au is halted.
||The nano-structure of our samples was identified by the Atomic Force
Microscope (AFM) in contact mode. In this mode a sharp probe on a cantilever
with a spring constant of less than 1 N/m is brought in direct contact
with the surface and the repulsive force between the tip and the surface
The thickness of the samples mostly correspond to the thickness obtained
during the film growth. The samples contain features of several sizes
ranging from 10 nm-1000 nm. More measurements are needed to
identify the chemical composition of these features
We measured the electrical Resistance(in this case the sheet resistance)
of ultra thin gold films deposited on amorphous germanium, as a function
of temperature and thickness. Several samples have been measured.
From the room temperature resistance value we estimated the electrical
resistivity to be 10-3 W-cm. Such
high resistivity led us to speculate that these materials are "semiconducting".
For typical semiconductors such as Si and Ge r(T)
=Cexp(-Eg/2kT). Where C is the resistivity when the temperature is very
large, Eg is the energy gap, that is the minimum energy needed to
excite electrons from the valence band to the conduction band, and k is
the Boltzmann constant.
In almost all samples we studied the materials showed semiconducting
behavior. We also made a qualitative estimate of the "energy gap"
associated with the activation behavior by plotting ln R as a function
of 1/T shown in Figures . This gap depends on the thickness of the sample,
and ranges from 10 mev for high thickness to 100 mev for low thickness.
This suggests that the system would undergo an insulator to metal transition
as the thickness of the gold is increased.
This analysis provided us with a qualitative measure of the semiconducting
behavior.. However, we believe that the conduction mechanism in our sample
can not be described by assuming traditional doped semiconductors because
the resistivity is too high.