Electrical characterisation of Carbon Nanotubes

							                                    Electrical characterisation of Carbon Nanotubes

                                René Geithner, Holger Mühlig, Frank Schmidl, and Paul Seidel

  Carbon Nanotubes (CNTs) are one-
  dimensional nanoscopic conductors with
  unique mechanical and electrical properties
  [1].
  The electrical measurements are done on
  single-walled carbon nanotubes (SWNTs),
  which are electrically connected with gold
  (fig. 1).


                                                                                        Fig. 3: Semiconducting CNT at room temperature
                                                                                        Source-Drain current as a function of source-drain voltage at
                                                                                        different gate voltages
                                                                                        Conductance depends on gate voltage.




  Fig. 1: SEM image of CNTs contacted with gold


   A carbon nanotube field effect transistor
  (CNT-FET) is realized with a back-gate
  (fig. 2). Source-Drain voltage (V SD) and
  gate voltage (VG) is applied and the
  resulting source-drain current (I SD) is                                              Fig. 4: Metallic CNT at room temperature
  measured.                                                                             Source-Drain current as a function of source-drain voltage at
                                                                                        different gate voltages
                                                                                        Conductance is almost independent from gate voltage.
                              VSD                                    V+
                                                                            VA          At low temperatures a gap appears for all
                                                                              AD-card
Voltage source
                                             Vg                      V-                 samples, where no source-drain current is
                                                                                        detectable although a source-drain voltage
                                                     Current-voltage converter
                                                                                        is applied (figs. 5, 6). Quantum effects
      Voltage divider 100:1
                                                                                        might be evidenced on a few samples [2].
                                                     Uds                    Ids         These CNTs show Coulomb-blockade
                                                             back -gate
                                                                                        characteristics, which means that the
                                                                                        conductance and the gap width oscillate
  Fig. 2: Measurement                setup        used     for   the      electrical
  characterisation of CNT                                                               with varying gate voltage (figs. 5 – 8).
                                                                                        Two types of measurements are done at
  Measurements at room temperature show                                                 low temperatures.
  conductance of CNTs, which is dependent                                               1. Source-drain current is measured by
  as well as independent from an applied                                                    varying source-drain voltage and
  gate voltage (figs. 3, 4). These results                                                  holding       gate  voltage     constant
  correspond to the picture of metallic and                                                 (figs. 5, 6).
  semiconducting CNTs. In the case of                                                   2. Source-drain current is measured by
  semiconducting       CNTs,    conductance                                                 varying gate voltage and holding
  variations about some order of magnitudes                                                 source-drain       voltage       constant
  are achieved (fig. 3).                                                                    (figs. 7, 8).
Fig. 5: Semiconducting CNT at temperature below 20mK
Source-Drain current as a function of source-drain voltage at    Fig. 8: Metallic CNT at temperature below 20m
different gate voltages                                          Source-Drain current as a function of gate voltage at constant
Conductance and gap width depends on gate voltage. CNT is        source-drain voltage VS D = 25mV
blocking above applied gate voltage VG = -5V.                    Conductance oscillates with varying gate voltage over the whole
                                                                 rang of applied gate voltage.


                                                                 For a metallic CNT the results of the two
                                                                 types of measurements are plotted together
                                                                 in a colourscale-graph. This shows so
                                                                 called “Coulomb-diamonds” (fig. 9).




Fig. 6: Metallic CNT at temperature below 20mK
Source-Drain current as a function of source-drain voltage at
different gate voltages
Conductance and gap width depends on gate voltage. CNT is
conductive over the whole range of applied gate voltage.




                                                                 Fig. 9: Conductance G = dIS D/dUS D as a function of source-
                                                                 drain voltage and gate voltage for a metallic CNT. G i     s
                                                                 normalized to the maximum conductance of a mesoscopic
                                                                 conductor G0 = e²/h (Landauer-Büttiker).


                                                                 Further work will concentrate on quantum
                                                                 effects and the appearing gap on other
                                                                 CNTs and the ir temperature depend
                                                                 behaviour.
Fig. 7: Semiconducting CNT at temperature below 20m
Source-Drain current as a function of gate voltage at constant
source-drain voltage VS D = 100mV
                                                                 [1] M. P. Anantram et al., Reports on Process in
Conductance oscillates with varying gate voltage. CNT is             Physics 69, 507 -561, 2006
blocking above applied gate voltage VG = -3V.                    [2] B. Babic, Electrical Characterization of
                                                                     Carbon Nanotubes grown by the Chemical
                                                                     Vapor Deposition Method, Diss., University of
                                                                     Basel, 2004