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A multi-scale model for localized and mobile electroluminescence in carbon nanotube field-effect transistors

University of British Columbia

A multi-scale model of a carbon nanotube (CN) field-effect transistor (FET) has been developed that captures the novel electroluminescence (EL) behaviour observed in experimental devices. First, the electronic properties of the CN are derived from the nearest-neighbour tight-binding approximation. The electrostatics of a coaxially-gated CNFET are then rigorously established. In simulations, the open boundaries of this structure are closed by null-Neumann conditions. Based on an asymptotic analysis, limits of validity for the null-Neumann boundary condition are established, and it is shown that one may improve the accuracy of results by including a length of contact in the simulation domain. Next, charge transport is included in the model to address EL. Transport will be diffusive in the long CN channels (greater than or equal to 10 μm) found in experimental devices, however quantum tunnelling currents will be present through the Schottky-barriers at the interface between the contacts and the CN. A solution to the 1D effective-mass Schrödinger wave equation determines the thermionic emission and tunnelling currents in a region near the CN-contact interface. These currents establish boundary conditions for the 1D drift-diffusion and continuity equations, which describe charge transport in the long channel, where a finite and field-dependent mobility is assumed. The charge and electrostatics are solved self-consistently to obtain carrier density distributions on the CN surface. When both electrons and holes are present in the CN, they are assumed to recombine with an empirically determined rate, resulting in photon emission. Simulations of this system exhibit four experimentally observed EL phenomena. First, a light-spot that is localized to ~ 5 μm may be positioned longitudinally on the CN surface by varying the gate bias, and this mobile light-spot increases in intensity as it approaches the CN-contact interface before decaying. When the height of the Schottky-barrier is reduced, a persistent, fixed-position light-spot appears at a CN-contact interface. Finally, when a charge defect is introduced on the CN surface, a fixed-position light-spot appears under certain bias conditions where the injection of one type of carrier is favoured. These EL effects are predicted by the proposed model without resorting to any further physical explanations.

Text

2010-01-12

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http://hdl.handle.net/2429/18073

Electrical and Computer Engineering

University of British Columbia

UBCV

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