MPhil Thesis Presentation
Modeling and Simulation of Dopant Induced Effects on The Characteristics of Carbon Nanotube Based Devices

by Mr Zubair AHMED

 :  15 Aug 2013 (Thu)
 :  10am
Venue  :  Room 2463, 2/F (Lifts 25-26), HKUST

Examination Committee
Prof Kevin J CHEN, ECE/HKUST (Chairman)
Prof Man Sun CHAN, ECE/HKUST (Thesis Supervisor)
Prof Zhi Yong FAN, ECE/HKUST
Carbon Nanotubes (CNT) are seen as viable replacement for silicon technology based on its small dimensions and ballistic transport. CNT based FETs outperform silicon MOSFETs in sub-10nm regime. Within CNTFETs, doped CNTFET has three orders better on-to-off current ratio compared to Schottky Barrier CNTFET but still dopant incurred effects in 1D CNTs remain enigmatic due to lack of reproducible results. Considering doping by adsorption mechanism, an added dopant in CNT leads to quantum wave-function interactions, dependent on dopant location which disrupts pristine eigen-states. It also causes attractive/ repulsive forces on π bonds of CNT leading to induced strain. Electrostatic as well as spatial position and strain effects of dopants are the subject matter of this thesis. 
We utilized CNT sparse geometry together with first principle simulations to quantize CNT transport characteristics versus dopant position. Model of electrostatic dipole variation with dopant location was used for predicting conductance degradation in n-type device due to dopant proximity to metal interface. Dopant stationed away from depletion region gave least contact resistance and improvement faded away, though better than un-doped case, as dopant was placed away from interface. 
Dopant generated strain effect in source/drain regions of CNTFET and its predicament on device current was investigated employing Density functional theory (DFT) for geometry optimization and numerical simulation for idea qualification. DFT showed that adsorbed dopants provoke strain and numerical results discerned ~25% current over-estimation. A surface-potential based model was presented incorporating dopant incited strain effect. It bode well with numerical results in On-state but suffered from band-to-band tunneling (BTBT) in sub-threshold regime. 
BTBT, a ramification of dopant electrostatics at interface of doped/un-doped CNT, allowed source confined carriers to tunnel into channel valence band degrading current on-off ratio. Poisson equation was used to model interface electrostatics neglecting triangular potential approximation based model with constant tunneling length. We then presented a complete compact model for doped CNTFET with negligible error at both on and off-state for realistic device behavior.
*** ALL ARE WELCOME !! ***