Relation between conduction property and work function of contact metal in carbon nanotube field-effect transistors

Abstract

We have investigated the relation between the conduction property and the work function of the contact metal in carbon nanotube field-effect transistors (NTFETs). The conduction type and the drain current are dependent on the work function. In contrast to NTFETs with Ti and Pd contact electrodes, which showed p-type conduction behaviour, devices with Mg contact electrodes showed ambipolar characteristics and most of the devices with Ca contact electrodes showed n-type conduction behaviour. This indicates that the barrier height of the metal/nanotube contact is dependent on the work function of the contact metal, which suggests that the Fermi-level pinning is weak at the interface, in contrast to conventional semiconductors such as Si and GaAs. We have also demonstrated nonlinear rectification current-voltage characteristics in a nanotube quasi-pn diode with no impurity doping, in which different contact metals with different work functions are used for the anode and the cathode.

Schottky barriers in carbon nanotube-metal contacts

“…73,91 Nosho et al used Pd, Ti, Mg, and Ca to contact CNTs and observed an increase in the on-state current of the hole branch of the transfer characteristics with increasing metal work function. 92 However, Ca contacts for which a zero Schottky barrier height for electrons is expected, due to the low Ca work function of 2.9 eV, exhibited an electron current lower than expected. This indicates that either the work function of the Ca contact is far from that of a clean surface in vacuum, there is an oxide at the metal-CNT interface or there are additional tunneling barriers limiting the current.…”

Theoretical and experimental studies of Schottky diodes that use aligned arrays of single-walled carbon nanotubes

“…Even though the diode itself is not a main element of modern digital electronics, the physics of the diode structure is essential for many applications, including in optoelectronics [3]. Nanoscale diodes have been already demonstrated with carbon-based nanomaterials, such as graphene and individual nanotubes [4][5][6][7][8][9][10][11][12][13][14]. The work presented here focuses on diode structures made of parallel nanotube arrays, their rectification properties, and the physics of their electronic transport.…”

“…Electrodes defined by photolithography and liftoff were deposited directly on the arrays. One electrode was Pd(30 nm)/Ti(1 nm), providing the Ohmic contact [22][23][24] and the other electrode was Al(30 nm)/Ca(3 nm), providing the Schottky contact [9,25] to the s-SWNTs. Since the thickness of the deposited Ti layer was smaller than a typical SWNT diameter, the SWNTs had actual contacts to the Pd-bulk with the work function ~5.1 eV [22][23][24], making p-contacts.…”

“…Since the thickness of the deposited Ti layer was smaller than a typical SWNT diameter, the SWNTs had actual contacts to the Pd-bulk with the work function ~5.1 eV [22][23][24], making p-contacts. On the other hand, the Ca layer, with the work function ~2.9 eV [9,25], was sufficiently thick to coat completely the SWNTs and to make n-type contact. The channel lengths and channel widths were ~10 µm and ~15 μm, respectively.…”

Calculating the electronic transmission properties of semiconducting carbon nanotube Schottky diodes with increase in diameter

“…The performance of the semiconducting carbon nanotube Schottky diodes is much dependent on the Schottky barrier height built in the metallic-semiconducting carbon nanotube contacts. This barrier height depends on various material parameters, for example: the metal contact work function, the environment to which the device is exposed, the diameter of the nanotube and the interface structures [11][12][13][14][15][16]. Constantly advancement of the semiconducting carbon nanotube Schottky diode performance requires a good understanding of the metallic-semiconducting carbon nanotube contacts.…”