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Transport in Graphene Nanoelectronic Devices - Effects Due to Gold Deposition on the Surface of Graphene. Vandenham, Joshua 2011

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Transport in Graphene NanoelectronicDevicesEffects Due to Gold Deposition on the Surface ofGraphenebyJoshua VandenhamA THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFBACHELOR OF SCIENCEinThe Faculty of Science(Physics)THE UNIVERSITY OF BRITISH COLUMBIA(Vancouver)June 2011c Joshua Vandenham 2011AbstractGraphene is a unique material with extraordinary properties, in both its me-chanical structure and strength as well as its superb electronic transport. Ithas become an important focus of physical and chemical research, and manypractical applications that take advantage of its properties have been foundas well. Since graphene is a single atomic layer all charge carriers involvedin conduction through it are always in contact with the surface. Thus, sur-face effects are expected to play a very important role in determining thecharacteristics of a sheet of graphene. In this project, graphene nanoelec-tronic devices are fabricated, and the effects on their transport propertiesof depositing minute amounts of gold onto their surfaces are investigated.iiTable of ContentsAbstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiTable of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiiList of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vList of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viAcknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . viiDedication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1.1 Physical Properties and Interest . . . . . . . . . . . . 11.1.2 Applications . . . . . . . . . . . . . . . . . . . . . . . 22 Graphene Nanoelectronics . . . . . . . . . . . . . . . . . . . . 32.1 Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.1 Mechanical Exfoliation . . . . . . . . . . . . . . . . . 42.1.2 Device Design . . . . . . . . . . . . . . . . . . . . . . 62.1.3 Electron-Beam Lithography . . . . . . . . . . . . . . 62.1.4 Bonding . . . . . . . . . . . . . . . . . . . . . . . . . 142.2 Characterization Measurements . . . . . . . . . . . . . . . . 152.2.1 Method . . . . . . . . . . . . . . . . . . . . . . . . . . 152.2.2 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 193 Gold Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . 223.1 Theory and Motivation . . . . . . . . . . . . . . . . . . . . . 223.2 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.2.1 Liquid Nitrogen Cooling System . . . . . . . . . . . . 253.2.2 Rotating Slit Shutter . . . . . . . . . . . . . . . . . . 263.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28iiiTable of Contents4 Conclusions and Future Work . . . . . . . . . . . . . . . . . . 324.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . 32Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34AppendixFabrication Details . . . . . . . . . . . . . . . . . . . . . . . . . . . 36ivList of Tables3.1 Work Functions . . . . . . . . . . . . . . . . . . . . . . . . . . 233.2 Gold Evaporation Parameters . . . . . . . . . . . . . . . . . . 28vList of Figures1.1 Graphene Energy Dispersion . . . . . . . . . . . . . . . . . . 22.1 A Simple Device . . . . . . . . . . . . . . . . . . . . . . . . . 42.2 Graphene on Silicon chip . . . . . . . . . . . . . . . . . . . . 52.3 Graphene on Silicon Oxide, magnified . . . . . . . . . . . . . 62.4 Lithography: Step 1 . . . . . . . . . . . . . . . . . . . . . . . 72.5 Lithography: Step 2 . . . . . . . . . . . . . . . . . . . . . . . 72.6 Lithography: Step 3 . . . . . . . . . . . . . . . . . . . . . . . 82.7 Lithography: Step 4 . . . . . . . . . . . . . . . . . . . . . . . 92.8 Lithography: Step 5 . . . . . . . . . . . . . . . . . . . . . . . 102.9 Lithography: Step 6 . . . . . . . . . . . . . . . . . . . . . . . 112.10 Lithography: Step 7 . . . . . . . . . . . . . . . . . . . . . . . 122.11 A chip after 1st stage is complete . . . . . . . . . . . . . . . . 132.12 A chip after 2nd stage is complete . . . . . . . . . . . . . . . 142.13 An image of bonds to a device . . . . . . . . . . . . . . . . . 152.14 The lightbulb annealer . . . . . . . . . . . . . . . . . . . . . . 162.15 The end of the dunker stick . . . . . . . . . . . . . . . . . . . 172.16 The effect of applying a voltage to the back gate . . . . . . . 182.17 A Dirac Peak before Annealing . . . . . . . . . . . . . . . . . 182.18 The Same Peak after Annealing . . . . . . . . . . . . . . . . . 192.19 An Inverted and Scaled Dirac Peak . . . . . . . . . . . . . . . 213.1 The Effects of Few-Monolayer Gold Depositions . . . . . . . . 233.2 The Effects of Sub-Monolayer Gold Depositions . . . . . . . . 243.3 The Undoing of Gold Deposition Effects due to Temperature 243.4 The New Evaporator Cooling System . . . . . . . . . . . . . . 253.5 The New Evaporator Shutter . . . . . . . . . . . . . . . . . . 273.6 Dirac Peak before Gold Deposition . . . . . . . . . . . . . . . 293.7 Dirac Peak after Gold Deposition . . . . . . . . . . . . . . . . 293.8 Inverted and Scaled Peak after Gold Deposition . . . . . . . . 30viAcknowledgementsI would like to thank Dr. Joshua Folk, my supervisor, for giving me theopportunity to experience real research in a fascinating field, as well as forhis guidance throughout the year.I would also like to thank (in no particular order) Julien Renard, MarkLundeberg, Aryan Navabi, Dennis Huang, and Ali Khademi, who are allother members of Dr. Folk’s research group. They were all very willing toprovide instruction on equipment, explain graphene physics, provide advice,and otherwise share their experiences with all of the things that were newto me.Finally, I would like to thank Dr. Rob Kiefl for providing general butvery useful advice regarding the presentation of research, in both writtenand oral form.viiDedicationTo my parents, who have supported me in every way possible throughoutmy time at UBC.viiiChapter 1Introduction1.1 GrapheneGraphene, the material studied in this project, is novel for a wide variety ofreasons.Graphene is a single atomic sheet of carbon which exhibits a honeycomblattice molecular structure. Being only one atom ‘thick’, graphene can besaid to be the thinnest material known to the world. In 2004, A. Geim and K.Novoselov performed groundbreaking experiments involving graphene thatbrought the material to the attention of physicists and engineers aroundthe globe, kickstarting the plethora of research undergone since then [1, 2].They were awarded the 2010 Nobel Prize in Physics for their achievement.1.1.1 Physical Properties and InterestThe properties of graphene make it of great interest for fundamental physicalresearch [2, 3].Graphene has an extraordinarily high conductance, and this is a mainmotivator for graphene research. Expectedly then, it also exhibits highelectron mobility, and, in an ideal situation, perfect charge homogeneity.However, situations are often not ideal and much research has been devotedto improving the quality of obtainable graphene, through both new synthesistechniques and alternate methods of device fabrication [4–8].Graphene’s energy dispersion can be described by the following equationE(k) =  √∆2 +(¯hvFk)2 (1.1)where vF is the Fermi velocity and ∆ is the difference in energy be-tween the two triangular sublattices that comprise the honeycomb lattice ofgraphene. If ∆ = 0, as is normally the case, the energy dispersion becomeslinear:E(k) =  ¯hvFjkj: (1.2)11.1. GrapheneThis is the reason graphene does not have a bandgap (See Figure 1.11).Figure 1.1: The linear energy dispersion of graphene. Note the lack of abandgap.1.1.2 ApplicationsGraphene’s properties are not only interesting, but useful as well, and it iscurrently used in a wide variety of practical applications [9–16]. Graphene-based materials are utilized for their combination of strength and flexibil-ity. The desirable electrical properties of graphene are most often exploitedin nanoelectronics, such as graphene-based transistors, discussed further inChapter 2. Graphene-based optical displays are an interesting example ofusing both the mechanical and electrical properties of the substance.1Figure adapted from a talk slide of group member Dennis Huang.2Chapter 2Graphene NanoelectronicsGraphene nanoelectronics refer to any electrical device that incorporatesgraphene, typically built on the nanometre scale. These include a widevariety of objects of varying size, complexity, and purpose, including: Simple devices, such as the ones used in this project. These devices’only purpose is to allow a piece of graphene to be electrically con-tacted, so that electrical measurements of the graphene flake can beperformed, usually to characterize the electrical properties of the flake.An example can be seen in Figure 2.1. Complex devices, also fabricated for research purposes. Often onerequires a graphene flake of a precise size (and therefore etching of theflake is required) or wishes to incorporate additional features such astop gates. These devices are made in order to perform more preciseor sophisticated measurements. 2 Practicalpurposedevices, suchasonesoutlinedinSection1.1.3. Graphenefield-effect transistors, such as the ones studied in [9, 10] are likely themost prominent example.2For example, see [17], where a top gate is used to tune the bandgap of bilayer graphene.32.1. FabricationFigure 2.1: An example of a simple device. The graphene flake can beseen in the centre, and it is contacted by eight gold leads. This number issimply to ensure there are a variety of leads to perform 4-probe resistancemeasurements with, and in case some leads fail there isn’t a danger of fallingbelow four.2.1 FabricationThe fabrication of graphene nanoelectronics is a many-step process, outlinedbelow. Quantitative specifics (such as equipment settings and other values)can be found in the Appendix.2.1.1 Mechanical ExfoliationIn order to build a graphene device one must first obtain a monolayer flakeof graphene on a substrate.3 This is done through mechanical exfoliation.Silicon wafers (which naturally form a silicon oxide layer on their surface)are patterned with gold bonding pads and alignment marks and then bro-ken into individual chips. Then, thin pieces of graphite (which molecularlyare large stacks of graphene sheets) are attached to adhesive tape and thetape is pressed down onto the chip. When it is peeled away, some of thegraphite remains on the tape while some remains on the chip. Since thebonds between carbon atoms within a layer of graphene are very strong, butthe inter-layer bonds between sheets are very weak, the graphite splits easilyand leaves a few layers of graphene behind. One must then examine the chipunder a microscope and search for graphene monolayers (as opposed to bi-3This step is described for completeness, but was never performed by me. It is done inbatches, and I was able to simply take chips from these batches when needed.42.1. Fabricationor multi-layer flakes). One can tell the thickness of a flake by its colour rela-tive to the substrate. In this sense, obtaining a good quality and good-sizedgraphene flake is left to chance. Microscope images of a graphene flake onthe typical substrate can be seen in Figures 2.2 and 2.3.Figure 2.2: A microscope image of part of a chip. Gold bonding pads can beseen at the top and bottom of the image, and circular gold alignment marksare also present. A monolayer graphene flake is circled. Thicker flakes canbe seen nearby.52.1. FabricationFigure 2.3: A monolayer graphene flake on a Silicon Oxide substrate. Thecontrast between them is not high, as the graphene is only one atom thick.2.1.2 Device DesignOnce an appropriate flake has been identified, the device must be designed.An image of the area surrounding the flake is taken and imported into De-signCAD after an alignment process. Then, one can literally draw wiresfrom the pads towards the graphene flake at will. Typically, when there isspace, one ends up with a spider-like pattern, as in Figure 2.11. Also, it isgenerally a good idea to design multiple paths from a pad to the destina-tion, so that if one small area of the wire is ruined the pad is still electricallyconnected to where it should be.2.1.3 Electron-Beam LithographyPreparationOnce the design is complete it is time to perform the actual lithography.This is done in a cleanroom setting to avoid allowing particles that mayotherwise land on the surface of the chip to interfere.First, the chip is cleaned in acetone and dried (Figure 2.4). Then, it isplaced in a spinner and a few drops of a polymer called PMMA are depositedon top. PMMA consists of long molecular chains. The spinner then spinsthe chip at a set angular frequency for a set duration. This spreads thePMMA over the chip in a particular known thickness, so that the exposure62.1. Fabricationrate of the Scanning Electron Microscope (SEM) to be used in a future stepcan be set accordingly. After the PMMA is spun onto the chip it is baked.(Figure 2.5)Figure 2.4: You begin with a (cleaned) chip that simply has graphene restingon top.Figure 2.5: PMMA is spread over the entire chip using a spinner, and thenit is baked.72.1. FabricationThe process is repeated a second time using a slightly different variety ofPMMA. The two varieties have different strengths, so that when an area isexposed to the electron beam (see next section) the extent of the destructionof the polymers is different, resulting in ‘trenches’ with a different width neartheir top than their bottom.Lithography with Scanning Electron MicroscopeThe PMMA-coated chip is then placed into the vacuum chamber of theSEM. The design file created earlier is loaded into the computer. Focussingof the electron beam and alignment of the sample stage must be done byhand. However, once the beam is focussed and the chip is aligned (via thegold alignment marks) the computer performs the actual exposure (Figure2.6).Figure 2.6: The chip is placed inside the SEM and the associated computerprogram provides the instructions necessary to guide the electron beam anddraw out the pre-designed pattern.The electron beam breaks up the long polymer chains of the PMMA inthe areas exposed (Figure 2.7).82.1. FabricationFigure 2.7: The polymers of PMMA that were exposed to the beam havebeen broken down into smaller molecules, whereas the unexposed PMMAretains its original structure.After the exposure is complete, the chip is removed from the SEM’svacuum chamber and placed in a weak solvent. This solvent can dissolvethe broken-down PMMA molecules but not the intact long strands. Thus,‘trenches’ in the PMMA are left where the polymer was exposed to theelectron beam. (See Figure 2.8)92.1. FabricationFigure 2.8: The entire chip is immersed in a solvent that is potent enough todissolve the broken down chunks of PMMA but not the intact long polymers.This leaves ‘trenches’ in the PMMA in the exposure pattern.Evaporation and LiftoffAfter the use of the SEM the chip is placed in the top of a metal evaporator,which consists of a chamber that can be pumped to near vacuum containingtungsten boats used to vaporize metals. Gold is placed in one of the boatsand heated until it evaporates. The gold atoms deposit themselves uniformlyover the chip (Figure 2.9). The rate of deposition can be observed via acrystal monitor in the evaporation chamber, and thus specific thicknesses ofgold can be deposited.102.1. FabricationFigure 2.9: The chip is placed in a metal evaporator and gold is evaporated,which coats the entire chip in a thin layer of gold, filling the trenches.After the gold deposition the chip is placed in hot acetone, which is asolvent that can dissolve the long strands of PMMA that were not exposedto the electron beam. With the PMMA gone, the gold that sat on top of itfloats away from the chip. This may need to be encouraged with the use of asonicator, a device that sends pulses through the beaker containing the chip,in the hopes that any gold that did not already float away will be knockedaway. After this and a rinse in more acetone all of the unwanted gold is gone,leaving the situation in Figures 2.10 and 2.11. The entire process must bedone twice. The second time there is more precision throughout the process,due to closer alignment marks and a thinner electron beam. One is finallyleft with a contacted graphene flake, as in Figure 2.12. The chip is nowready to be bonded.112.1. FabricationFigure 2.10: The chip is immersed in a stronger solvent, which is able todissolve the intact PMMA. The gold that was deposited on top of the PMMAthen floats away from the chip, whereas the gold that was deposited intothe trenches is attached to the graphene/silicon oxide and remains as thedesired nanowires.122.1. FabricationFigure 2.11: A chip after the first stage of lithography and evaporation iscomplete. Wires approach the graphene flake but do not contact it. Rather,they connect to a new set of small pads, with their own alignment marks.This is done so the mold for the wires close to the graphene can be createdwith more precision.132.1. FabricationFigure 2.12: A chip after the second stage of lithography and evaporation iscomplete. Small wires now connect the small pads directly to the graphene,and so the large bonding pads are finally connected to the flake.2.1.4 BondingThe final stage in producing a measurable device is bonding. First, thebottomside of the chip is scratched in order to expose the silicon beneaththe silicon oxide layer. With the silicon exposed, the chip is glued to a chipcarrier using silver paste. In this way, the silicon beneath the silicon oxidethat the graphene sits on can also be electrically contacted, which is essentialfor the measurements described in the next section.After the chip is in its carrier, macroscopic gold threads are attached topads on the carrier and to the gold pads of the device, electrically connectingthem. This is done using a bonder (pictured in Figure X). The graphenedevices can now be accessed by macroscopic measurement apparatus via thechip carrier. See Figure 2.13 for an image of the bonding wires.142.2. Characterization MeasurementsFigure 2.13: An image of bonds to a device. The bonding wires are firstattached to the chip carrier pads, and then drawn over to the device bondingpads, where they are melted together to form electrical contact.2.2 Characterization MeasurementsAs discussed above, the principle purpose of the simplest graphene nano-electronic devices is to facilitate electrical measurements of the grapheneflake(s) involved. These measurements can characterize the flake in a fewimportant ways, as discussed below.2.2.1 MethodPreparationThere are three important steps to take before a proper measurement canbe made. First, the device should be annealed. This is a process where thedevice is heated in the presence of forming gas (a mixture of hydrogen andnitrogen). Figure 2.14 shows the setup. A low temperature anneal (200 C)is useful for removing water from the surface of the device due to exposure tothe air. A higher temperature anneal (400 C) can burn away PMMA residueleft on the graphene as well. Annealing immediately before a measurement152.2. Characterization Measurementscan drastically improve the results due to its ability to eliminate unwantedsurface effects (See the difference between Figures 2.17 and 2.18).Figure 2.14: The lightbulb annealer used in this project. The chip is placedon the copper sheet in the bottom right, which surrounds a projector-bulb.The voltage to the bulb can be adjusted, which adjusts the amount of heatit releases and therefore the temperature of the chip, which is constantlymeasured. The chip and heat source can be encased in a glass tube and putinto vacuum or into forming gas.After annealing, the sample is quickly placed in a dunker stick, which isthen sealed and pumped down to near vacuum. This is to avoid undoingwhat the annealing accomplished and letting water settle on the grapheneagain. See Figure 2.15 for an image.162.2. Characterization MeasurementsFigure 2.15: The end of the dunker stick, showing the pins that connect tochip carriers. Wires for all 32 possible leads run through the stick to themeasurement devices. The stick is placed in a sheath to seal it and pumpdown to vacuum.A small amount of helium is then let into the dunker stick, and the stickis then immersed in liquid nitrogen. The helium serves to carry thermalenergy from the graphene. The liquid nitrogen would not be able to coolthe sample very quickly without it. Since helium is inert there is no dangerof it affecting the measurements. Measurements are done at 77 K to reducenoise.MeasurementTheprinciplemeasurementperformedonadeviceisameasurementofdeviceresistance vs. a changing back gate voltage. Resistance is measured usinga Lock-In Amplifier, with either a 2-probe or a 4-probe setup. All of thedata presented here was taken using a 4-probe setup to ensure only grapheneresistance and not contact resistance was being measured.The resistance of a substance depends in part on its charge carrier densityn. If there are more electrons (or holes) in the graphene flake, current willflow more easily. One can control n via the application of a voltage to thepreviously mentioned back gate. When no voltage is applied, the chargecarrier density in the graphene is unchanged. However, applying a voltageto the back gate causes charge of the appropriate sign to build up in thesilicon. This induces charge of the opposite sign in the graphene (See Figure2.16). These extra charges can also serve as charge carriers. So in theory,as one increases VBG one increases n, which in turn lowers the resistance R172.2. Characterization Measurementsof the graphene flake.Figure 2.16: The effect of applying a voltage to the back gate of a device.Charge builds up in the silicon due to the voltage, which in turn inducesopposite charge in the graphene. These new charges can easily serve ascharge carriers, increasing the effective charge carrier density n of the flake.10009008007006005004006050403020100Figure 2.17: A measurement of R (in ohms) vs. VBG (in volts), withouthaving annealed the device prior to measurement.182.2. Characterization Measurements25002000150010006050403020100Figure 2.18: Another measurement of R (in ohms) vs. VBG (in volts) ofthe same device measured in the previous figure, this time after having an-nealed the device prior to measurement. Note how the peak shifts drasticallytowards the 0V point.2.2.2 AnalysisThe peaks obtained when performing the described measurements are calledDirac peaks. Their shape contains much information regarding the charac-teristics of the graphene.InhomogeneityThe ‘inhomogeneity’ is not a numerical property of a graphene flake, but asense of how inhomogeneous a flake is can still be obtained from these mea-surements. It was mentioned above that, in theory, increasing VBG shouldincrease the charge carrier density n, and therefore reduce the resistanceR of the graphene flake. However, as can be seen in both Figure 2.17 and2.18, the R of the graphene actually increases when VBG is increased. Theexplanation is simple: there are excess charges or charge traps present in theflake due to inhomogeneity. These may come from water on the surface fromthe brief time the devices are exposed to air, or, more often, from chargetraps in the silicon oxide. The oxide is not uniform, crystalline SiO2, butmay have areas of SiO3 or SiO, which introduce electrons or holes to thegraphene. Thus, when jVBGj is increased, it does introduce more electrons(or holes) to the graphene flake, but at first these only serve to cancel outthe excess holes (or electrons) that were already present in the flake andable to carry charge. Therefore increasing VBG actually serves to reduce nand therefore increase R.However, as VBG continues to increase, eventually a turning point is192.2. Characterization Measurementsreached. This happens when the new electrons (or holes) have finally com-pletely cancelled out the intrinsic holes (or electrons). From that pointforward, adding even more electrons does serve to increase n, and so R de-creases as expected. This is what causes the peak structure observed inmeasurements.The location of the Dirac peak (i.e. the value of VBG where R is max-imal) can therefore tell us ‘how inhomogeneous’ the flake is. A perfectlyhomogeneous flake would have its peak at 0 V. The further the peak is fromthat point, the more charges there are to overcome, and so the more in-homogeneous the graphene is. One can notice that the peak shifts muchcloser to 0 V if annealing is done before measurement (See Figures 2.17 and2.18). This is because the annealing process removes the water and residuefrom the surface of the flake, which contributed to inhomogeneity. Howeverit doesn’t effect the inhomogeneity of the substrate, which is why the peakdoesn’t move all the way to 0 V.The FWHM (the width of the peak at half its maximum value) is anotherway to read off the amount of charge inhomogeneity in the device. A thinnerpeak corresponds to less charge inhomogeneity.MobilityPerhaps the most important characteristic of a graphene flake is its electronmobility. Mobility is a measure of how easily electrons (or holes) can travelthrough a solid. As discussed Chapter 1, one of the properties that makesgraphene so interesting is its very high mobility.Electron mobility  is defined as follows =  ne (2.1)It is expectedly related to the conductance  (since conductance is alsoa measure of how easily electrons can travel through a solid), as well as thecharge carrier density n and the elementary charge e.One can measure the mobility of a graphene flake by manipulating thedata that gives the Dirac peaks above. First, it was asserted that VBG isproportional to n (if the peak is shifted to 0 V). Thus, after scaling thehorizontal axis one can plot the data against n rather than VBG.Secondly, R is related to the resistivity  of the graphene through theflake’s aspect ratio, which in turn is equal to 1 . Thus, after scaling thevertical axis and inverting the values one can obtain a plot of  vs. n, as inFigure 2.19.202.2. Characterization MeasurementsFrom the definition above, =  en (2.2)and therefore the slope of a graph of  vs. n, multiplied by e is themobility  of the flake.A perfect flake will exhibit a ‘V’ shape, with two linear portions of thesame slope, as in theory the electron mobility and hole mobility of grapheneare equal. However, in a flake with inhomogeneity, the graph of  vs. n willnot necessarily be linear near the peak, nor have equal slopes on either side ofthe peak when it does become linear. This can be seen in the example shownin Figure 2.19, which is not a particularly homogeneous flake. However,despite this the electron mobility (calculated from the positive slope) of thisflake was quite good, with   4000cm2Vs .3.02.52.01.5x10-3 3210-1 x1012 Figure 2.19: The same dataset shown in the previous two figures, but withthe axes manipulated in order to extract the mobility. The horizontal axisis now charge carrier density n, and the vertical is now 1=R. The mobilitycan then be obtained from the slope of the linear sections.21Chapter 3Gold DepositionThe main experiment done to the devices fabricated in this project involvethe deposition of sub-monolayer amounts of gold onto a flake of grapheneand an investigation of the effects this has on electronic transport.3.1 Theory and MotivationGraphene is comprised of a single atomic layer, so all charge carriers involvedin conduction are automatically always in contact with the surface of thematerial, unlike in traditional conductors, where many travel through thematerial’s body. This makes surface effects an extremely important aspectto consider in terms of graphene conduction, as they are especially able toharm transport properties.However surface effects could also potentially be utilized for interestingor valuable purposes. For example, there exist predictions and experimentsthat suggest depositing small amounts of gold atoms onto the surface ofgraphene can induce strong spin-orbit interaction in the electrons within[18]. This means that as an electron moves within the graphene its spin isquickly rotated. This is interesting in its own right, but is also the basis for‘spintronic’ transistors.Furthermore, studies show that various amounts of gold deposition ontothe surface of graphene can drastically effect its mobility and inhomogeneity.For example, Y. Ren et al. show that the deposition of few-monolayeramounts of gold on the surface of graphene shifts the Dirac peak of thesample [19]. This is explained by the differing work functions of gold andgraphene 3.1. Since the work function of gold is higher, it takes more energyto remove an electron from a gold atom than a carbon atom in graphene.Thus, it is energetically favourable for some of the electrons in graphene tobe donated to the gold atoms. This has the effect of reducing the numberof negative charge carriers, shifting the Dirac peak in the positive direction(See Figure 3.1).223.1. Theory and MotivationGraphene 4.6Gold 4.8Table 3.1: The differing work functions of graphene and gold [19]. Thedifference means that electrons will favour being attached to gold overgraphene, which causes the graphene to donate some of its charge carri-ers, shifting the Dirac peak.Figure 3.1: Results from [19], where few-monolayer amounts of gold weredeposited on the surface of graphene. The Dirac peak shifts to the right asmore gold is deposited. This is explained by the differing work functions ofgold and graphene.The effects of depositing a much smaller amount of gold appear to bevery different [20]. The Dirac peak is shifted in the opposite direction, andthe mobility is decreased (See Figure 3.2).233.1. Theory and MotivationFigure 3.2: Results from [20]. Submonolayer (on the order of 1000ths ofmonolayers) gold was deposited at cryogenic temperatures. a) The effect onthe location of the Dirac peak. Gold amounts are measured by the amountof time the sample was exposed to gold that was evaporating at a very lowrate. The peak shifts the opposite direction of the previous experiment,so something more complicated must be happening. b) The mobility as afunction of gold deposited.It has also been observed that the effects shown in Figure 3.2 are irre-versably undone once the sample is brought to room temperature (See Figure3.3). This can make measurement and reproducability difficult without theproper equipment.Figure 3.3: The effects shown in Figure 3.2 are undone when the sample isbrought to room temperature, and do not reappear if recooled.243.2. MethodThe apparent discrepancy of the work in [19] and [20] might be ex-plained by the vastly differing amounts of gold deposited on the graphene.This project does its own investigation of the effects of submonolayer golddeposition on the surface of graphene nanoelectronic devices.3.2 MethodIn order to deposit an appropriate amount of gold with the sample at anappropriate temperature, a few modifications to the metal evaporator hadto be made.3.2.1 Liquid Nitrogen Cooling SystemIn order to keep the sample at 77 K during the evaporation, a new stagefor the sample had to be constructed. The result is pictured in Figure 3.4.The chip is attached to the bottom of the copper plate, which is in thermalcontact with copper tubes leading outside the evaporator. Liquid nitrogencan be pumped through the tubes to cool the plate, and ultimately thesample. It possible to flow the liquid nitrogen through at a steady ratethroughout the evaporation.Figure 3.4: The cooling system for the remodelled evaporator. The sampleis attached to the bottom of the copper plate. Liquid nitrogen is pumpedthrough the tubes from outside the chamber while depositing gold. Thiskeeps the sample at 77 K throughout the process.253.2. Method3.2.2 Rotating Slit ShutterA new shutter had to be designed and built for the evaporator. The orig-inal shutter was simply a metal plate that could be moved to either coverthe sample or expose it to the evaporating gold. This is sufficient for largeevaporations, such as those done when fabricating a device. Once the evapo-ration rate is steady, the shutter is opened and the amount deposited simplydepends on how long it is left open and the evaporation rate. Evaporationin this way usually takes 10 to 20 minutes.However, in order to deposit an amount of gold equivalent to  0.1% of amonolayer (roughly 1 m˚A) the shutter would have to be opened and closedfar more quickly and precisely than was possible.The new shutter allows for depositions of this magnitude. It shields thesample from the evaporating metal regardless of its position, except for asmall slit of width w = 1:6mm. With this design, rather than having simplyand ‘on’ and ‘off’ state, the slit can be quickly passed over the sample toproduce a very low effective exposure time. Once the slit has passed thesample is again covered (See Figure 3.5).263.2. MethodFigure 3.5: The new evaporator shutter. TOP: The chip will be alignedroughly with the visible hole in the evaporator lid when everything is puttogether. BOTTOM: The shutter can be rotated to quickly pass the slitover the sample.The effective exposure time t can be calculated as follows.t = wv (3.1)where w is the slit width and v is the velocity of the slit as it passes overthe sample. Furthermore,v = dT (3.2)T =  ! (3.3)where d is the total distance the slit moves (or at least the part of the slitthat will pass over the small graphene device) when the shutter is rotated273.3. Resultsan angle  and T is the time it takes for the shutter to rotate that sameangle  at a rate ! rad/s. Howeverd =  R (3.4)where R is the distance from the pivot point of the shutter to the sample.Thereforev =  R ! = R! (3.5)and sot = w!R (3.6)Then, the thickness h that is evaporated ish = rt = rw!R (3.7)where r is the rate of evaporation. Therefore, using the parameters inTable 3.2, one can calculate that h  6:52 m˚A.r 0.5 ˚A/sw 1.6 mmR 11.5 cm! 1 rad/sTable 3.2: The parameters used to deposit a submonolayer amount of goldonto a graphene nanoelectronic device. w and R are fixed by the geometryof the shutter. r is as low a rate as one can evaporate at and be sure onlygold is being evaporated. ! is estimated. One improvement to the systemwould be to have a rotary encoder measure ! rather than trying to time itby hand.3.3 ResultsThe results of 4-probe resistance vs. back gate voltage measurements for aparticular graphene device are reproduced below. Figure 3.6 shows the re-sults before gold deposition4, and figure 3.7 shows the results of an identical4Figure adapted from a graph produced by Ali Khademi, a group member283.3. Resultsmeasurement (same flake and contact probes) after gold was deposited. Aclear shift in the location of the Dirac peak can be observed.Figure 3.6: The result of the back gate voltage scan before gold was de-posited. Note the position of the peak at slightly less than 40 V. Theelectron mobility was calculated to be  2000cm2Vs from this data.500040003000200010000Device Resistance (Ohms)6040200-20-40Back Gate Voltage (Volts)Figure 3.7: The result of a back gate voltage scan similar to the previousfigure, done with the same contacts of the same flake, after a submonolayeramount of gold was deposited onto the graphene at 77 K. The peak hasshifted roughly 20 V compared to the previous measurement.Applying the methods discussed in Chapter 2, the mobility can be cal-culated from a modified form of this data (Fig 3.8). Most of the data lies tothe left of the peak, and so the electron mobility cannot be calculated very293.3. Resultsprecisely. Nevertheless, an estimate of  2000cm2Vs can be obtained. This isthe same estimate as before gold deposition. 0 0.0005 0.001 0.0015 0.002 0.0025 0.003-7 -6 -5 -4 -3 -2 -1  0  11/R (S)n (x10^12)Conductance vs. Charge Carrier DensityFigure 3.8: The data presented in Figure 3.7 manipulated to allow for amobility calculation. Most of the data in on the hole side, so the estimateof electron mobility is not precise.Disclaimer These results are very preliminary. Firstly, only one devicewas tested in this way, though it was tested using a variety of its probesand the results are consistent between those measurements. Secondly, it ispossible that water is the cause of these shifts, as opposed to the depositedgold. This is suspected because, due to the nature of the experiment, thedevice cannot be annealed immediately before measurement. It was an-nealed before being placed in the evaporator chamber (which is filled withnitrogen gas when not pumped to vacuum), but after the gold deposition ithad to be immediately placed in the dunker stick for measurement (whichwas then also pumped to vacuum, and cooled to 77 K). In theory, the chipshould not be exposed to water while in the evaporator, and so effectivelyit is only exposed to air for roughly twice the time that a regular chip is303.3. Resultsbetween annealing and measurement. Therefore, it is unlikely water couldaccount for the entire change, but it does likely play some role. The effectsof annealing after gold deposition have not yet been tested. Depending onthe temperature it may or may not seriously effect the deposited gold.31Chapter 4Conclusions and FutureWork4.1 ConclusionsThe results in Section 3.3 suggest that gold deposited in the amounts dis-cussed shifts the Dirac peak of the graphene flakes in the positive direction,as in [19]. This suggests the different-work-functions explanation to be valid.However, the amount of gold and the conditions in which it was depositedare much more similar to those in [20], yet the results do not compare atall. This may be due to the fact that the sample was briefly heated to roomtemperature before being re-cooled for measurement. As shown in Figure3.3, the effects of gold deposition are irreversably undone when the sampleis heated. Thus, it is reasonable to suggest that the gold deposited in thisexperiment initially produced results similar to those of [20] but, since thesample had to be heated due to apparatus restrictions, the effects were lost.The effects therefore resemble those in [19], yet far more gold was depositedin that experiment, and the Dirac peak experienced no shift from it’s orig-inal point after reheating in [20]. This ambiguity requires much furtherexperimental work to resolve.4.2 Future WorkFurther testing of the effects of gold deposition is certainly required to reacha true conclusion.First, the effect of a gentle anneal to desorb water after the gold hasbeen deposited should be investigated. Does it change the effect the goldhas? Does it change the results significantly at all, suggesting water playeda large role? Or do the results remain similar, suggesting water involvementwas indeed negligible, as with a normal measurement.Second, the effects of varying thicknesses of gold should be investigated.Does twice the gold produce twice the change? Also, if a gold is deposited,324.2. Future Workand the chip is then measured, can another layer of gold be deposited eventhough the previous layer was twice brought to room temperature from 77K? That is, would the results be different if twice the gold was deposited inthe initial deposit at 77 K as opposed to over two deposits? Unfortunately,as important a fact this is to understand, a large number of finished deviceswould be required to fully investigate it, and time did not allow for thatmany devices to be fabricated.33Bibliography[1] Novoselov, K. S. et al. Electric field effect in atomically thin carbonfilms. Science 306 666-669 (2004)[2] Geim, A. K. & Novoselov, K. S. The rise of graphene. Nature Mater.6, 183-191 (2007)[3] Rao, C. N. R. et al. Some novel attributes of graphene. J. Phys. Chem.Lett. 1, 572-580 (2010)[4] Stankovich, S. et al. Synthesis of graphene-based nanosheets via chem-ical reduction of exfoliated graphene oxide. Carbon 45, 1558-1565(2007)[5] Hernandez, Y. et al. High-yield production of graphene by liquid-phaseexfoliation of graphite. Nature Nanotechnol. 3, 563-568 (2008)[6] Wei, D. et al. Synthesis of N-doped graphene by chemical vapor de-position and its electrical properties. Nano Lett. 9, 1752-1758 (2009)[7] Zheng, M. et al. Metal-catalyzed crystallization of amorphous carbonto graphene. Appl. Phys. Lett. 96, 063110 (2010)[8] Dean, C. R. et al. Boron nitride substrates for high-quality grapheneelectronics. Nature Nanotechnol. 5, 722-726 (2010)[9] Lin, Y. et al. Operation of graphene transistors at gigahertz frequen-cies. Nano Lett. 9, 422-426 (2009)[10] Lin, Y. et al. 100-GHz transistors from wafer-scale epitaxal graphene.Science 327 662 (2010)[11] Stoller, M. D. et al. Graphene-based ultracapacitors. Nano Lett. 8,3498-3502 (2008)[12] Stankovich, S.et al. Graphene-basedcompositematerials. Nature 442,282-286 (2006)34[13] Kim, K. S. et al. Lagre-scale pattern growth of graphene films forstretchable transparent electrodes. Nature 457, 706-710 (2009)[14] Schedin, F. et al. Detection of individual gas molecules adsorbed ongraphene. Nature Mater. 6, 652-655 (2007)[15] Schlapbach, L. & Zuttel, A. Hydrogen-storage materials for mobileapplications. Nature 414, 353-358 (2001)[16] Katz, H. E. et al. A soluable and air-stable organic semiconductorwith high electron mobility. Nature 404, 478-481 (2000)[17] Zhang, Y. et al. Direct observation of widely-tunable bandgap in bi-layer graphene. Nature 459, 820-823 (2009)[18] Varykhalov A. et al. Electronic and Magnetic Properties of Quasifree-standing Graphene on Ni. Phys. Rev. Lett. 101, 157601 (2008)[19] Ren, Y. et al. Controllingthe electrical transport propeties of grapheneby in situ metal deposition. Appl. Phys. Lett. 97, 053107 (2010)[20] McCreary, K. M. et al. Effect of cluster formation on graphene mobil-ity. Phys. Rev. B 81, 115453 (2010)35Fabrication DetailsBelow are some quantitative fabrication details. 1st stage lithography wires are typically designed to be 4 m wide, and2nd stage wires are typically 500nm to 1 m. Spinners are set to revolve at 5000rpm for 45 seconds. This spreadsthe PMMA to a layer  100nm thick. For contrast, the oxide layerthat forms on the surface of the silicon chip is  300nm. Chips with PMMA are baked at 180◦C for 10 minutes. The electron beam current of the SEM is typically 170 pA for 1ststage and 20 pA to 60 pA for 2nd stage. In either case, other computer-decidedsettingssuchasbeamdwelltimeensurethatthetotalexposureamount is 180 units/unit, for all patterning. The ‘weak’ solvent used to dissolve the broken-down PMMA is a mix-ture of methyl isobutyl ketone (MIBK) and isoproponal alcohol (IPA). When evaporating, a small layer of chromium is actually depositedbefore the gold. This helps the gold make a good contact with thechip. For 1st stage, 2 - 5 nm of chromium are deposited before 75 -100 nm of gold. Deposition rate for the gold is typically 1.2 to 1.5 ˚A/s.Chromium deposition rate varies but is smaller. For second stage only0.5 nm of chromium are deposited, with 50 - 100 nm of gold following. For liftoff the chip is placed in 80◦C acetone for 10 minutes, thensonicated for a few seconds if necessary. The chip is then rinsed withacetone as it is removed from the beaker and then set in IPA foranother 10 minutes. The chip and chip carrier must be heated to 100◦C for 10 minutes andthen 200◦C for 50 minutes in order for the silver paste to properlybond the two together. Annealing is done in forming gas at pressures of 20 mmHg to 60 mmHg.36

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