Electrical Characterization of ultra thin films of Gold Evaporated on amorphous Germanium Substrates Tanina Bradley, Christopher Jessamey in addition to Abebe Kebede Department of Physics, NC A&T State University, Greensboro, NC 27411

Electrical Characterization of ultra thin films of Gold Evaporated on amorphous Germanium Substrates Tanina Bradley, Christopher Jessamey in addition to Abebe Kebede Department of Physics, NC A&T State University, Greensboro, NC 27411 www.phwiki.com

Electrical Characterization of ultra thin films of Gold Evaporated on amorphous Germanium Substrates Tanina Bradley, Christopher Jessamey in addition to Abebe Kebede Department of Physics, NC A&T State University, Greensboro, NC 27411

Contreras, Rodolfo, Food Editor has reference to this Academic Journal, PHwiki organized this Journal Electrical Characterization of ultra thin films of Gold Evaporated on amorphous Germanium Substrates Tanina Bradley, Christopher Jessamey in addition to Abebe Kebede Department of Physics, NC A&T State University, Greensboro, NC 27411 Myron Strongin in addition to Yuguang Cai Department of Physics, Brookhaven National Laboratory, Upton NY 11793 INTRODUCTION In the rich literature of the physics of disordered metals, the structural in addition to 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/) They display phenomenon related to disorder, such as strong localization effects, Universal conductance fluctuation in addition to 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 in addition to the de Broglie wavelength, . The recurrent experimental in addition to 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 as long as molecular electronics. One of the experimental problems in this case is the ability to obtain very thin films , free of percolation effects in addition to 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 as long as 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 in addition to reproducible, it will be an innovative in addition to 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, in addition to 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 in addition to argon gas. Four copper wires (40 gauge ) are attached to the silver pads, using Ted Pella Inc. fast drying silver point (Lot 0378), as long as 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 plat as long as m 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-plat as long as m 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 in addition to other molecules. ). ABSTRACT In this work we present the results of nanostructural studies in addition to 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 as long as ce microscopy method. The thickness of the films range from 2A to 100 A in addition to [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, in addition to the results of the electrical resistivity in addition to its dependence on the temperature RESULTS AND DISCUSSION 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 in addition to 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 in addition to 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 b in addition to to the conduction b in addition to , in addition to k is the Boltzmann constant. In Figures A in addition to B we show representative plots of resistance versus temperature on linear scale in addition to the log R versus 1/T plot as long as 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, in addition to ranges from 10 mev as long as high thickness to 100 mev as long as 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 as long as the electrical conduction on the insulating side (x < xc) of a disordered system exhibiting metal to insulator transition , the conductivity is given by SURFACE MORPHOLOGY 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 in addition to the repulsive as long as ce between the tip in addition to 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 as long as square sample. DEPOSITION CHAMBER AND THICHNESS MONITOR REISTIVITY VERSUS TEMPERATURE FIGURTE A FIGURE B FIGURE C SPECIMEN MOUNT REFERNECES 1. Disordered Electronic Systems, Patrick A. Lee in addition to T.V.Ramarkrishnan, review of Modern Physics vol. 57 No. 2 April 1995 2. Conduction in Glasses Containing Transition Metal Oxidxes, N.F. Mott Journal of Non-Crystalline Solids 1 (1968) P1-17 3. Two Dimensional Electrical Conductivity in Quenched Condensed Metal Films, R. C. Dynes, J.P. Garno in addition to J.M. Rowell, Physical Review Letter, Vol 40, No. 2, PP 479-482 4.Structural in addition to Electrical Properties of Gold-Germanium Interfaces, B.Dwir anfd G. Deutscher, Physical Review B, Vol. 40, No. 17 , P11880 (1989) 5. Current in addition to 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 in addition to Outreach Program of Brookhaven National Laboratory, the North Carolina Alliance as long as Minority Participation in addition to 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, in addition to Mr. Francis Lobe as well as all members of the department. FIGURE D The VRH models predict that n =1/4 as long as three dimensional system, in addition to n=13 as long as two dimensions. Whereas the n=1/2 is applicable as long as 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 in addition to n=1/4), to see the applicability of VRH model to our data. We found out that as long as this sample best the n=1/4 fits well whereas the is a large deviation from n=1/2 in addition to 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 as long as 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 in addition to 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 as long as mation of impurity b in addition to s (low impurities), in addition to 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 in addition to 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 underst in addition to this observation, we have not ruled out the possibility that it can be caused by experimental artifacts. Current (K485) Voltage (K614) 1MW R 6V DRC93Temperature Controller Infcon Thickness Monitor Strayer University-Baymeadows Campus FL www.phwiki.com

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