Measuring CNT FETs and CNT SETsUsing the Agilent B1500A

Application Note B1500-1

Agilent B1500A Semiconductor Device Analyzer

IntroductionExotic carbon nanotube (CNT)structures have generated a greatdeal of interest in the scientificcommunity. There are many po-tential applications for CNTs, butmuch of the current focus in nano-technology research is centeredon electrical component struc-tures, such as the CNT field effecttransistor (FET) and the CNTsingle electron transistor (SET).

The Agilent 4155C and 4156Chave been the de facto industrystandard instruments for semi-conductor device characteriza-tion for many years. In addition,they are very effective tools fornanotechnology device character-ization. However, the new AgilentB1500A Semiconductor DeviceAnalyzer is also an excellentinstrument for analyzing theelectrical properties of nano-technology devices, and it hasadded benefits not found in the4155C and 4156C.

The content of this applicationnote is based on a study conduc-ted by Professor K. Matsumotoof Osaka University. In his studyProfessor Matsumoto focused onhow chirality determines theelectrical properties of CNT FETsand CNT SETs, using the 4156CPrecision Semiconductor Param-eter Analyzer as the measure-ment tool. The new B1500Awould have worked equally well

for this study, and the B1500Aoffers added benefits, amongwhich is an application librarythat contains the applicationtests necessary to perform thesemeasurements.

CNTs have also been studied asmaterials for ultra large scaleintegration (ULSI) interconnec-tion because they can supportextremely high current densities(1000 times larger than those of

Cu), and CNTs possess very highthermal conductivity (10 timesgreater than that of Cu). CNTs alsoshow promise as the electricalsource in field emission displays,and a close relative, the carbonnano-horn, shows potential as amaterial for constructing fuel cells.The B1500A is an excellent tool forcharacterizing these fundamentalCNT structures.

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CNTs can behave as either con-ductors or semiconductors.Behavior is dependent on theirchirality, which is determined bythe holding angle and diameterof the nanotube. The patterningof CNTs has been successfullycontrolled using patterned cata-lysts and chemical vapor deposi-tion under the influence of anelectric field.

Figure 1 shows a scanning elec-tron microscope image of a CNTFET structure, which exhibits 10times higher saturation perfor-mance.

Figure 2 shows two drain IVcurves through a CNT channel,taken by sweeping the drainvoltage. Depending on thechirality of the CNT, these de-vices can be made to show eitherCNT FET behavior (left) or resis-tive behavior (right). This illus-trates how critical the control ofchirality is to the electrical per-formance of the CNT device. Onevariation of the simple CNT isthe single wall CNT (SWCNT),which can be created by varyingthe deposition conditions. Thisapplication note focuses on theSWCNT, and all subsequentreferences to CNT devices in thisapplication note refer to SWCNTdevices.

CNT electrical properties are determined by chirality

Figure 1. Scanning electron microscope image of a CNT FET

Figure 2. Two drain IV curves showing CNC FET behavior (left) and resistive behavior (right).

VG=0 V

VG=10 V

VG=0 V

VG=2 V

Carbon Nanotube

DrainSource

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An instrument with stable low-voltage and low-current measure-ment capability is essential for thestudy and characterization ofnanotechnology devices. Thereare two essential factors neces-sary to establish a good nano-technology test environment: asuitable measurement instrument;and a low noise test environment.In addition to basic measurementperformance, user-friendlyhardware and software inter-faces are also important forachieving an extremely high levelof efficiency in research work.

Measurement instrumentsAgilent offers two instrumentsthat are well suited for nanotech-nology measurement, the Agilent4156C Precision SemiconductorParameter Analyzer and theAgilent B1500A SemiconductorDevice Analyzer, which are shownin Figures 3 and 4 respectively.

The 4156C has a fixed configura-tion, with four source/monitorunits (SMUs), two voltage moni-tor units (VMUs) and two voltagesource units (VSUs). Thesemeasurement resources satisfythe vast majority of parametriccharacterization needs. The4156C also has an interactivefront panel with a combinationof fixed hardware keys and softkeys, which provide an easy-to-use compact operating environ-ment.

The B1500A, in contrast, ismodular and supports a varietyof SMU measurement modules,as well as a capacitance mea-surement unit (CMU). Figure 5shows rear view of the B1500Awith measurement modulesinserted. SMUs supported by theB1500A include high-resolution,medium-power, and high-power.The B1500A also supports anatto-sense and switch unit (ASU).

B1500A modules can providethe following measurement andforcing capabilities: current

Combined low-current measurement and nanotechnology research capabilities

Figure 3. Agilent 4156C Precision Semiconductor Analyzer

Figure 4. Agilent B1500A Semiconductor Device Analyzer

Figure 5. Agilent B1500A rear view with measurement modules inserted.

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measurement resolution down to100 aA: (1 × 10-16 A); current forcecapability up to 1 A; voltagemeasurement resolution down to0.5 µV; and voltage force capabilityup to 200 V.

The CMU covers a range from 1 KHzto 5 MHz. Accurate CV and DCmeasurements are easily achiev-able for specific output pins byusing the switching capabilitiesof the SMUs and CMU.

The B1500A includes Agilent’sWindows®-based, user-friendlyEasyEXPERT software interfaceand a front touch panel, as shownin Figure 6. In addition, theB1500A comes with more than100 semiconductor-relatedapplication tests. Users canselect an application test andimmediately execute the test.The B1500A software will auto-matically calculate the desiredDUT parameters. Applicationtests can be run with or withoutmodification but, if modificationis required, users can treat anapplication test as a templateand easily customize it with afew simple mouse clicks.

When a unique nanotechnologyapplication test is required, anew test library can be createdand easily shared among testgroup members. Since theapplication test concept issolution-based (as opposed tohardware based), it is very easyfor other group members toutilize the new test library.Group members do not have tounderstand the details of themeasurement setup in order touse the application tests.

Combined low-current measurement and nanotechnology research capabilities(continued)

Figure 6. B1500A EasyEXPERT Windows-based, user-friendly software interface.

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When performing nanotechnologymeasurement and characteriza-tion, grounding and shielding areessential for obtaining accurate low-current and low-voltage measure-ments. Figure 7 shows an exampleof a shielding box around theSMU connections and deviceunder test (DUT). The DUT, whichin this example is a CNT FET,must reside in a compact shield-ing box that is properly groundedto the circuit common of the SMUsvia the outer ground shield of thetriaxial cable. The shielding boxprevents outside electrical noise,magnetic noise, and optical noisefrom reaching the DUT.

SMU outputs coming from thedevice analyzer are triaxialcables and are attached to theshielding box wall with triaxialconnectors. To minimize noise,the outer common ground of thetriaxial cable must be connectedto the shielding box, and the mid-dle guard line should be routedwith the center force line to apoint as close to the DUT as pos-sible. The measurement signalfrom the triaxial connector isrouted to the DUT using a low-noise coaxial cable. The middleguard signal of the triaxial cablemust be connected to the outerconductor of the coaxial cable, asshown in Figure 8. The guard con-nection inside the shielding boxprotects the measurement signalfrom noise sources that mightexist in the shielding box (suchas those generated by vibration).It also eliminates stray capacitanceeffects that can slow the measure-ment response of the system. Itis also desirable to surround theback gate with the guard, as shownin Figure 7, when applying a gatebias from the substrate.

To interface the triaxial cablewith the coaxial cable throughthe shielding wall the Agilent16495J connector plate can beused, as shown in Figure 9.

Low-noise test environment

Figure 7. Example of a shielding box protecting SMU connections and the DUT.

Figure 9. Agilent 16495J connector plate for interfacinga triaxial cable through a shielding wall.

1 2

3 4

5 6

7 8

9 10

11 12

INPUT

GNDU INTLK

Figure 8. Middle guard signal of a triaxial cable connected to the outerground shield of a coaxial cable.

Middle guard signal Outer ground shield12345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890

Center conductor Outerinsulation

Outerinsulation

Triaxial Cable Coaxial Cable

With special chemical treatment,CNT FETs can be made to exhibitSET performance even at roomtemperature. When a nanoscalemulti-dot CNT is divided by defects,the drain current exhibits oscilla-tion as shown in Figure 10.

The oscillation observed in the I-Vplots of the drain current illustratea Coulomb blockade within theCoulomb gap(V = e/C, e : electron charge 1.6 × 10-19)caused by changes in the numberof trapped electrons in the islandof the multi-dot structure. Theirregularity of these oscillationsmay come from the multi-dotstructure. Theoretical calculationsput the total capacitance of thisSET at about 0.2 aF (2E-19F) andthe Coulomb energy at 400 meV.This corresponds to an islandapproximately the size of asphere 1 nm to 2 nm in diameter.

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Single electron transistor measurement performanceThis Coulomb energy correspondsto a temperature of approximately5000° K, so it is possible to detectthe transfer of a single electron,even at room temperature,through the change in the SETperformance.

In Professor Matsumoto’s study,the number of measurementpoints in the IV curves of the SETwas 10,000. For a measurementwith this many points, the B1500Awould be a good choice becauseit is very easy to transfer thissort of data into other Windows-based applications. In addition, theB1500A software environment

Measurement of multiple IV curves using the Agilent B1500A Analyzersupports automated measurementsequencing and data handlingthrough a GUI utility, whicheliminates the need to write aspecial program to perform thistask.

Both the 4156C and the B1500Acan be controlled using AgilentFLEX commands and whatever

programming language the userprefers. Agilent provides aVXIplug&play driver for bothinstruments, which gives you aneasy to use, high-level program-ming interface for your softwareenvironment. The VXIplug&playdriver can be used with VisualBASIC, C/C++, National Instru-ment LabView, and Agilent VEE.

Figure 10. Oscillation observed in the I-V plots of a drain current illustrate a Coulombblockade within a Coulomb gap.

This type of behavior is espe-cially important for sensors, andit is one potential application forthis device.

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SummaryThis application note presents anelectrical measurement solutionfor CNT FET and CNT SETcharacterization using theAgilent B1500A and Agilent4156C analyzers. These instru-ments can measure uniqueelectrical performance in nano-technology devices, and they caneven detect a single electrontransfer at room temperature.

The Agilent B1500A Semiconduc-tor Device Analyzer and theAgilent 4156C Precision Semi-conductor Parameter Analyzerare shown to be suitable choicesfor the electrical measurementand study of nanotechnologydevices.

Agilent gratefully acknowledgesProfessor K. Matsumoto of OsakaUniversity and his group for theCNT FET and CNT SET data.

ReferencesK. Matsumoto, S. Kinoshita, Y.Gotoh, K. Kurauchi, T.Kamimura, M. Maeda, K.Sakamoto, M. Kuwahara, N.Atoda and Y. Awano Jpn. J. Appl.Phys. Vol. 42 (2003) pp. 2415-2418

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© Agilent Technologies, Inc. 2005Printed in USA April1, 20055989-2842EN