Graphene Device Tutorial.docx

Graphene Device Tutorial.docx

《Graphene Device Tutorial.docx》由会员分享，可在线阅读，更多相关《Graphene Device Tutorial.docx（33页珍藏版）》请在冰豆网上搜索。

GrapheneDeviceTutorial

GrapheneDeviceTutorial

Introduction

ThepurposeofthistutorialistopresenthowtheATK-SEpackageincombinationwiththeVirtualNanoLab（VNL）canbeusedtoinvestigateanano-scaletransistor.ForthetransistorstructurewewilluseaGrapheneJunctionDeviceinspiredbythepaper[2],whereATKwasusedtoinvestigatethepropertiesofasimilarsystem.

Figure1:

Thedevicesystemconsideredinthistutorialconsistsofaz-shapedGrapheneNanoRibbon,ontopofadielectricandcontrolledbyametallicgate.Thecontourplotillustratestheelectrostaticpotentialthroughthesystem.

ThesystemisillustratedinFigure1.Itconsistsofa3regionsandformametal-semiconductor-metaljunction.Byapplyingagatepotentialtothecentralregionthesystemcanfunctionasafieldeffecttransistor.Wewillcalculatethefollowingpropertiesofthesystem.

TransmissionSpectrum

Temperaturedependentconductance

Temperatureinducedcurrent

Conductanceasfunctionofgatepotentialandtemperature

Currentasfunctionofbias,gatepotentialandtemperature

Note

WheneverpossiblewewillusetheVNLforsettingupandAnalysingtheresults.TofamilarizewithVNLwerecommendrunningtheVNLQuickTour.TheAtomistixToolKitSemiEmpirical（ATK-SE）istheunderlyingcalculationengineforthistutorial.Acompletedescriptionofalltheparameters,andinmanycasesalongerdiscussionabouttheirphysicalrelevance,canbefoundintheATKReferenceManual.

CalculatingtheTransmissionspectrum

TableofContents

∙LaunchingVNL

∙Buildingagraphenejunctiondeviceusingthecustombuilder

∙StartingtheScriptGeneratortool

∙ChoosingthecalculatorparametersintheScriptGeneratortool

∙SelectingphysicalpropertiesfromtheScriptGenerator

∙Savingacalculation

∙Runningthecalculation

∙Examiningtheresults—theTransmissionSpectrum

∙Plottingthe3-Delectro-staticpotential

LaunchingVNL

InordertolaunchVirtualNanoLab（VNL）,dothefollowing

∙Windows:

PressthestartbuttonandchooseQuantumWise→VirtualNanoLab2009.06

∙Linux:

Startaterminalwindow,andtypethefollowingatthecommandprompt:

vnl&

Ineithercase,VOLEisstartedanditsmaincomponent,theVNLMainWindowappears:

Buildingagraphenejunctiondeviceusingthecustombuilder

TosetuptheatomiccoordinateswewillusetheCustombuildertool.Tostartupthetool,left-clickontheicon

ontheToolbar.Thecustombuilderwindownowopens.

Thetoolcanbecustomizedtoproducedifferentconfigurationsthroughabuilderscript.AbuilderscriptisaPythonprogramwithsomeinputparametersandfunctionsforsettingupatomiccoordinatesaccordingtotheinputparameters.TheparameterscanbespecifiedthroughthecustombuilderGUI.ThetoolcomeswithanumberofbuilderscriptswhichareavailablefromtheBuildersmenu.NowselecttheGrapheneJunctionBuilder.

AfterselectingtheGrapheneJunctionBuilder,theleftcolumnofthetoolwillhaveamenuwithanumberofparametersandintherightcolumnthereisa3-Dviewofthestructurewhichcorrespondtothesettingoftheparameters.

ThestructureisaDeviceConfigurationconsistingoftwoelectrodesandacentralregion.Theelectrodesareperiodicstructures,andtheirunitcellsarevisualized.Thecentralregionseamlesslyconnecttheleftelectrodewiththerightelectrode

Nowchangethesettingsofthebuilderparametersaccordingtothefigurebelow

Thenewsettingsoftheparametershaveelongatedtheelectrodesandthecentralregion,aswellasenabledadielectricregionandagatepotentialbelowthestructure.Thedielectricregionisvisualizedasatransparentbrownregionandthegatepotentialisametallicregionbelowthis.

Therequiredlengthoftheelectrodecelldependsonthecomputationalmodelemployed.Strictlyspeaking,thelengthofthecellinthethec-directionshouldbelongerthantheinteractiondistanceofthemodel.Formostmodelsalengthof~7Åsuffice.

Note

Informationaboutthedifferentparameterscanbeobtainedbyleft-clickingthehelpshowbuttonintheinfofieldoftheleftcolumn.

StartingtheScriptGeneratortool

ThenextstepistosetupaTransmissionSpectrumcalculationforthegraphenejunction.First,locatethedragicon

positionedintheright-bottomoftheDatabasewindow.Left-clickanddrag-and-droptheiconontheScriptGeneratortool

locatedontheVNLToolbar.ThewindowoftheScriptGeneratortoolthenopenswiththeGrapheneJunctiondisplayedintheScriptGenerator3Dviewerindicatingthatthisistheactiveatomicconfigurationofthetool.

TheDatabasetoolisnolongerneeded,sotosavespaceonyourdesktop,closethetoolbypressingCtrl+Q

ChoosingthecalculatorparametersintheScriptGeneratortool

OurnexttaskistosetupacalculationfortheGrapheneJunctionandspecifythephysicalpropertiesthatshouldbecalculated.Thefirstthingwewilldo,istoselectthecalculatorthatwillbeused.Todothis,selectthecheckboxlabeledDeviceExtendedHuckellocatedundertheCalculatorsview.Asaresultofthisselection,theSimulationparametersviewdisplaysaparameter“tree”.Byselectingindividualitemsinthetree,wecanchangeandfine-tunevariouspropertiesofthecalculator.Inthefollowingwewillsetupappropriateparametersforthecalculation.

Themainwindowcontainstheparametersthatwemostfrequentlychange.TospeedupthecalculationwewilllowerthenumberofkpointsintheCdirectionto50,thisshouldbesufficient.Wewillalsochangetheelectrontemperatureto300Kelvin,increasingtheelectrontemperaturemakesconvergenceeasier.Thewindowshouldnowlooklikethis

InthenextstepwechangethebasissetforthecalculationtouseparametersspecificallyfittedtogiveagooddescriptionforGraphite[4].Tothisendleft-clickontheBasisSetlineandselecttheCerda.Carbon[Graphite]andCerda.Hydrogen[C2H4]basisset.ThedefaultvalueoftheVacuumlevelisadjustedsuchthateventhoughtheCarbonandHydrogenparametersarefittedtodifferentreferencesystems,theycanbeusedtogether.Thewindowshouldnowlooklikethis

ThenextpartwesetupistheboundaryconditionsforthePoissonsolver,thereforeleft-clickonthePoissonSolverline.IntheC-directiontheboundaryconditionfortheelectrostaticpotentialisdeterminedbytheelectrostaticpotentialintheelectrodes,correspondingtoaDirichletboundarycondition.ForthetwootherdirectionswewillchooseaNeumannboundarycondition.TheNeumannboundaryconditioncorrespondstoazeroelectricfieldattheboundaryofthecomputationalbox,itgivesthebestmodelforstudyingtheeffectofthegatepotential.Thisisthelastcalculatorparameterwewillsetandthewindowshouldnowlooklikethis

SelectingphysicalpropertiesfromtheScriptGenerator

ThePhysicalpropertyviewdisplaysthepropertiesthatcanbecalculatedfortheGrapheneJunctionwiththeextended-Device-Hückelcalculator.Wewillenableall3analysischoices,i.e.theElectronDifferenceDensity,ElectrostaticDifferencePotential,andTransmissionSpectrum.AswewillseeinthefollowingfromtheTransmissionSpectrumwecanobtaintheconductanceandcurrentofthedevice.

Asaresult,asub-treenamedPhysicalpropertiesisaddedtotheparametertreeintheSimulationparametersview.left-clickonthePhysicalpropertieslineandspecifythenameoftheNetCDFdatafileas"z-a-z-6-6.nc".YourScriptGeneratorwindowshouldnowlookasfollows:

FinallywewillchangethedefaultsettingsfortheTransmissionSpectrumCalculationtomakeacalculationfor200pointsintheenergyrange[-2,2]eV.left-clickontheTransmissionSpectrumlineandspecifyparametersaccordingly,asillustratedbelow

Savingacalculation

Wearenowdonesettingupthecalculation.Thenextstepistosavethecalculation.YoudothisbypressingtheSaveAsbuttonintheScriptGeneratorsFilemenu,whichsavesthedefinedcalculationsettingasaPythonscriptonyourdisk.Selectthename“z-a-z-6-6.py”andleft-clickSave.Now,shifttotheVNLMainWindow.Noticethatthefilez-a-z-6-6.pyyousavedisdisplayedinthebrowserviewoftheMainWindow.

Runningthecalculation

Oncethatyourcalculationhasbeenstoredasascriptondisk,thescriptcanbeexecutedwiththeJobManagertool.Todothis,locatethefileinthebrowserviewoftheMainWindow,andthendrag-and-dropthisfileontotheJobManagericon

locatedintheToolbar.

TheJobManagerwindowthenappears

Tip

Incases,whereyouarenotinterestedinsavingthecalculation,youcouldjustaswellhavedrag-and-droppedthecalculationdirectlyfromtheScriptGeneratortooltotheJobManagertoolbyusingthedragicon

Tostarttheexecutionofthecalculation,presstheStartontheJobManagerwindow.Asaresult,alogwindowappearsdisplayingandupdatinginformationregardingthejobexecutionstate.Thefirstpartofthecalculationistheself-consistentpart.Thejobtakes10-40minutesdependingonthetypeofyourcomputer.

Thejobwillfirstperformaself-consistentcalculationforeachoftheelectrodes.Thetheself-consistentHartreepotentialisusedasboundaryconditionforaself-consistentcalculationforthecentralregionoftheDeviceConfiguration.Finally,thetransmissionspectrumofthestructurewillbecalculated.Oncethejobhasfinished,thelogwindowcontainsthefollowinglines:

NanoLanguageScriptfinishedsuccessfully

+-------------------------------------------------------------

|NanoLanguageScriptexecutionfinished

+-------------------------------------------------------------

Examiningtheresults