4-Point Resistance Measurements of Individual Bi Nanowires
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4-Point Resistance Measurements of Individual Bi Nanowires Stephen B. Cronina, Yu-Ming Linb, Pratibha L. Gaif, Oded Rabinc, Marcie R. Blackb, Gene Dresselhausd, and Mildred S. Dresselhause a Department of Physics, bDepartment of Electrical Engineering and Computer Science, c Department of Chemistry, dFrancis Bitter Magnet Laboratory, and eon leave from the Massachusetts Institute of Technology, Cambridge, MA 02139 f DuPont Central Research and Development, Wilmington, DE 19880
ABSTRACT We have synthesized single crystal bismuth nanowires by pressure injecting molten Bi into anodic alumina templates. By varying the template fabrication conditions, nanowires with diameters ranging from 10 to 200nm and lengths of ~50µm can be produced. We present a scheme for measuring the resistance of a single Bi nanowire using a 4-point measurement technique. The nanowires are found to have a 7nm thick oxide layer which causes very high contact resistance when electrodes are patterned on top of the nanowires. The oxide is found to be resilient to acid etching, but can be successfully reduced in high temperature hydrogen and ammonia environments. The reformation time of the oxide in air is found to be less than 1 minute. Focused ion beam milling is attempted as an alternate solution to oxide removal.
INTRODUCTION The motivation for studying Bi nanowires is based on the unique properties of bulk Bi. First, Bi has very small effective masses, with mass components as small as 0.001me [1]. The small effective masses of Bi cause the effects of quantum confinement to be more pronounced since the energy of a quantized bound state is inversely proportional to the effective mass. Therefore, the effects of quantum confinement can be observed for nanowires of relatively large wire diameter. Second, bulk Bi has a very long mean free path, ~0.4mm at 4K and ~100nm at 300K [2]. Since the diameters of the nanowires are much smaller than the mean free path of the electrons, we expect the wires to exhibit 1D ballistic transport. Third, the low melting point of Bi (271oC) allows us to prepare the wires by pressure injection of molten Bi into a porous alumina template. It will be shown that this fabrication method yields extremely high quality crystalline nanowires. Finally, calculations of the transport properties predict that the Bi nanowires should have a very high thermoelectric efficiency [3]. As mentioned above, the effects of quantum confinement are pronounced in Bi because of its small effective masses. The change in band structure due to quantum confinement is shown schematically in figure 1. The dashed curves depict the band structure of bulk Bi, in which the T-point valence band overlaps with the L-point conduction band by 38meV [4], making bulk Bi a semimetal. Due to quantum confinement effects, the band edges split into subbands as shown by the solid curves. As the diameter decreases, the separation of the subbands gets larger, and eventually the lowest conduction subband and the highest valence C5.7.1
subband no longer overlap and the
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