A four-contact measurement method has been preferred to a two-contact method for the resistivity measurements of conductive materials because it can exclude the contact resistance between two different materials [1]. Although in real measurements with a four-contact method, the contact resistance appears due to its systematic error. Accordingly a universal four-contact method was proposed [2].
We demonstrated an advantage of the universal four-contact method and applied it to ZnO nanowires and carbon nanotubes (CNTs) experimentally. To verify that the method can exclude the influence of contact resistance, we selected some electrode metals for ohmic contacts and Schottky contacts considering their work functions.
The work function of ZnO nanowire, shows n-type property, is 4.95eV and that of SWNTs, p-type property, varied depending on its geometry known to a chiral vector [3]. Generally, Ti/Au electrode which has 4.33eV work function acts as ohmic contact and Pt, 5.65eV, acts as Schottky contact for ZnO nanowires [4]. On the contrary Ti/Au acts as Schottky contact and Pt acts as ohmic contact for SWNTs because of their electronic types.
In this experiment, the independence of the contact resistance irrespective of the Schottky or ohmic contact relative to metal work functions was shown. To exclude the properties of individual nanowires and nanotubes, Ti/Au and Pt were separately evaporated and four-contact electrodes were made on each ZnO nanowires or carbon nanotubes after e-beam lithography process. Thanks to the advantage of the universal four-contact method, the discrepancy of the contact resistance of Ti/Au and Pt electrode which acts as ohmic and Schottky contact could be diminished.
This method can be applied to universal resistivity measurements for various nanowires and nanotubes.
[1] Low Level Measurement Handbook 6th, Keithley Instruments, Inc.
[2] Wenhua Gu and Kyekyoon Kim, Appl. Phys. Lett. 89, 253102 (2006).
[3] Chum-Wei Chen and Ming-Hsien Lee, Nanotechnology 15, 480 (2004).
[4] S. Halas and T Durakiewicz, J. Phys.: Condens. Matter 10 10815 (1998).
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