Electrical Characterization of ultra thin films of Gold Evaporated on amorphous Germanium Substrates

Tanina Bradley, Christopher Jessamey and Abebe KebedeDepartment of Physics, NC A&T State University, Greensboro, NC 27411

Myron Strongin and Yuguang CaiDepartment of Physics, Brookhaven National Laboratory, Upton NY 11793

INTRODUCTION

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 ( 30k).. 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 . From practical application, 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 the films can be used as electrodes to the molecule under investigation. In fact the objective of our project is to produce such films of gold for the investigation of bonding of thiols to gold, by looking at changes in the conductivity of the gold films. If this can be done right and reproducible, it will be an innovative and simple approach that may replace the use of STEM to measure the electrical conductivity of molecules.

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 .

PROCEDURE

SUBSTRATE PREPARATION

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 argon gas. 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.

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ABSTRACT 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 resistivity and its dependence on the temperature

RESULTS AND DISCUSSIONWe 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 Silicon and Germanium r(T) = r0exp(-Eg/2kT). Where r0 is the resistance when the temperature is very large, Eg is the energy gap, that is the energy needed to excite electrons from the valence band to the conduction band, and k is the Boltzmann constant. In Figures A and B we show representative plots of resistance versus temperature on linear scale and the log R versus 1/T plot for one of our samples. As can be seen the resistance shows activated behavior. In most samples we studied similar behavior was observed. We also made a qualitative estimate of the “energy gap” associated with the activation behavior from figures similar to Figure B. 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 may undergo an insulator to metal transition as the thickness of the gold is increased. Disordered two dimensional systems exhibit a unique conduction describe as Anderson localization in which case the conduction is due to hoping of electrons. According to Mott’s Variable Range Hoping (VRH) theory for the electrical conduction on the insulating side (x < xc) of a disordered system exhibiting metal to insulator transition , the conductivity is given by

SURFACE MORPHOLOGYThe 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 is measured. 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

ELECTRICAL CIRUIT FOR IN-SITU MEASUREMENT

The resistance was determined using the Van Der Pauw analysis for square sample.

DEPOSITION CHAMBER AND THICHNESS MONITOR

REISTIVITY VERSUS TEMPERATURE

FIGURTE A

FIGURE B

FIGURE C

SPECIMEN MOUNT

REFERNECES1. Disordered Electronic Systems, Patrick A. Lee and T.V.Ramarkrishnan, review of Modern Physics vol. 57 No. 2 April 19952. Conduction in Glasses Containing Transition Metal Oxidxes, N.F. Mott Journal of Non-Crystalline Solids 1 (1968) P1-173. Two Dimensional Electrical Conductivity in Quenched Condensed Metal Films, R. C. Dynes, J.P. Garno and J.M. Rowell, Physical Review Letter, Vol 40, No. 2, PP 479-4824. .Structural and Electrical Properties of Gold-Germanium Interfaces, B.Dwir anfd G. Deutscher, Physical Review B, Vol. 40, No. 17 , P11880 (1989)5. Current and Voltage Characteristics of Self Assembled Monolayers by Scanning Electron Microscopy, Supriyo Datta et. al. Physical Review Letters Vol. 79, No. 13, 1997 PP. 2530-2533

ACKNOWLEDGEMNTS

We Would like to acknowledge the Education and Outreach Program of Brookhaven National Laboratory, the North Carolina Alliance for Minority Participation and the support of the National Science Foundation. We would like to thank the Department of Physics of Brookhaven National Laboratory, Dr. Myron Stgrongin Dr. Yuguang Cai, and Mr. Francis Lobe as well as all members of the department.

€ σ=σ0exp(−T0/T)nFIGURE D

The VRH models predict that n =1/4 for three dimensional system, and n=13 for two dimensions. Whereas the n=1/2 is applicable for both. In this case the multibody coulomb interaction is included in the theory treated by Mott. Experimentally n is in the range of 1/4 n 1/2. We analyzed the electrical resistivity in log scale as a function of (1/T)n (n =1/2, n=1/3 and n=1/4), to see the applicability of VRH model to our data. We found out that for this sample best the n=1/4 fits well whereas the is a large deviation from n=1/2 and n=1/3. If such model is indeed applicable the dimensionality of this sample may be closer to three than two.

In some of our samples we observed that the resistivity is less temperature dependent at low temperature, as shown in Figure C. There are two possibilities for this to occur. The first possibility is that it can be caused by impurity conduction. Similar phenomenon is observed in known semiconductors Germanium doped with Antimony and compensated by copper impurities. For high purity samples adding copper impurities (p-type) decreases the resistance. Whereas it increases when copper impurities are added to the less pure sample. The mechanism of impurity driven conduction is known to be due to the formation of impurity bands (low impurities), and the presence of compensating minority impurities.

The second possibility is the significance of the percolation conductivity at low temperatures. The conduction comes from two contribution, the first is the conductivity of Ge/Au system and filamentary conduction due to well connected gold grains. At high temperature both contribute more or less the same to the conduction. But at low temperature the Ge/Au system becomes semiconducting, while the filaments stay metallic. This in effect shorts the increasing temperature dependent resistance of the Ge/Au system, leaving the impression that the system became metallic.

It is early to describe the observed electrical resistance of this sample as if to be caused by such impurity driven conduction. While more work is needed to understand this observation, we have not ruled out the possibility that it can be caused by experimental artifacts.

Current (K485)

Voltage (K614)1M

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6V

DRC93Temperature Controller

Infcon Thickness Monitor