| dc.description.abstract | The prospects of using Graphene and MWCNT as a channel material for RF transistors
and interconnects, respectively, have recently garnered much attention. E orts
are being made for improvements at the material and device fronts. The development
of a technology platform for devices based on these materials demands consideration of
two crucial factors - 1) comprehensive understanding of time evolution of electro-thermal
transport, so that the associated mechanisms can be modelled for deterministic operation
of devices at di erent time scales (frequency); 2) understanding the physical limits
of operation by assessment of reliability against electro-thermal transport induced instantaneous
or catastrophic failure and long-term material degradation. In this work, a
comprehensive investigation of time-evolution of electro-thermal transport is presented,
and its implications on operational reliability is explored. Various time-scales and associated
events discussed in the thesis are shown in figure 1. When an electron enters a
channel, it undergoes collision at the rate estimated by Fermi's Golden Rule. The state
of collision is a near-adiabatic process, which lasts for few pico-seconds, during which
the collision-induced thermal energy remains localised to the site of collision. Gradually,
the collision-induced thermal energy or heat diffuses in the entire device, due to
which the device enters a state of electro-thermal non-equilibrium. The state is prone to
catastrophic events like EOS/ESD. The non-equilibrium state is succeeded by a state of
electro-thermal equilibrium, during which the temperature and excess energy transients
settle down, and further elongated operation results in degradation of the material. Given
the low-dimensional nature of the materials chosen for this thesis and their extraordinary
electrical and thermal properties, the devices so fabricated demand deployment of novel
techniques for the investigation of time-evolution of transport and assessment of reliability.
A measurement setup employing a synergetic utilization of a transmission line
pulse generator, DC SMUs, current sensor, high frequency oscilloscope and a Raman spectrometer is developed for such investigations. To comprehend the temporal nature of
transport, it's time evolution is investigated during the process of redistribution of thermal
energy after inelastic collisions. The process, governed by diffusion of heat across
the device, happens at the time scale corresponding to characteristic thermal diffusion
time of the device, which, owing to high thermal conductivity of graphene, MWCNT
and high thermal mass of dielectric substrate, is of the order of nano-seconds. The time
response is captured during a state of electro-thermal non-equilibrium, which exists in
a window between near-adiabatic and steady state of the device. The response is interpreted
by studying the corresponding effects on metal-graphene contact resistance,
conductivity near the Dirac point, conduction through higher sub-bands and inner shells
of a MWCNT. The parameters were found to show a temporal behaviour during the
electro-thermal non-equilibrium state, followed by a saturation during the steady state.
Having developed an understanding of electro-thermal transport through graphene and MWCNT, an assessment of operational reliability is done for electro-thermal transport induced
instantaneous or catastrophic failures and long-term material degradation. The instantaneous
failures correspond to damage during energy redistribution (non-equilibrium
state) or post energy redistribution (equilibrium or steady state) across the device. The
former failure conditions are emulated by electro-static discharge (ESD), while the latter
corresponds to electrical over-stress (EOS). The degradation of material is studied at a
longer time scale, while capturing the changes in chemical state of the material. The reliability
assessment is done while considering the possible heat sinks in the device - metal
contacts and substrate. Breakdown behaviour corresponding to shell-by-shell ablation
and defect-by-defect unzipping are investigated as possible failure routes in a MWCNT
and graphene, respectively.
In graphene, the phonon transport due to high electric-field was found to impart a transient
nature to contact resistance and conduction near Dirac point [1], both of which have
previously been assumed to be time-independent parameters. Their transient nature was
captured during the non-equilibrium state of the transport, which, in turn, was found to
persist for few nano-seconds. Such a transient behaviour of contact resistance highlights
the elusive role of metal-graphene interface in dissipation of excessive heat in 2D-material
based devices, which has been captured here by monitoring it at the time-scale of thermal
diffusion time. The role of contacts in removing heat is further substantiated by
breakdown voltage, which was found to scale with contact resistance. The electric-field
induced breakdown was found to follow two different pathways. A field-dependent defectby-
defect breakdown was observed at high electric-fields, particularly under electro-static
discharge (ESD) and electrical over-stress (EOS) conditions. At low electric-fields, a
time-dependent defect-by-defect breakdown was captured, where the oxidation of defects
(captured through Raman map) was dictated by time for which the electric-field was
applied. The graphene channel, instead of showing a complete and abrupt breakdown,
was found to fail in steps, captured through discrete fall in current through the channel.
Overall, the breakdown was found to follow a cascade of two-step process -(1) excessive
scattering and eventual heat dissipation at the defect site and (2) oxidation at the defect
site, resulting in emergence of more defects [2].
A similar set of investigations were carried out on MWCNT-based interconnects. Interestingly,
under high-fields, the electro-thermal transport was found to cause shell-by-shell
breakdown of a MWCNT [3]. The current due to electro-thermal transport through a suspended MWCNT was found to increase with time due to rise in temperature of the
cold electrode, followed by saturation due to attainment of equilibrium [4]. The tubes
supported by a polar dielectric were found to break at higher voltages than the suspended
tubes, which, in turn, is attributed to remote Joule heating of the polar dielectric via
surface polar phonons [5,6,]. Moreover, at high currents, in addition to melting of the hot
contact, flow of heat from this end to the cold contact, through a bundle of MWCNTs,
was found to cause a melt of cold contact as well [7]. Further, a controlled technique to
anneal the contacts and channel is explored, which enables a lever to achieve a precise
control on resistance of the MWCNT-based devices at the time scale of nano-seconds and
provides insights into the process of current annealing [8].
In a nutshell, the mechanism behind time-evolution of electro-thermal transport through
graphene and MWCNT-based devices has been investigated, and its manifestation as a
potential reliability and aging issue has been explored. It is found that the electrical
transport is a time-function of intrinsic heating (scattering) of the device. A maximum
change of 50% in metal-graphene contact resistance was captured over a time-span of 8
ns. Further, it is found that the low-dimensional nature of 1D and 2D materials amplifies
the effect of various interfaces and defects on the electro-thermal transport through the
devices based on these materials. Consequently, their reliable operation demands engineered
interfaces and low-defect density, for efficient phonon transport across the interface
and delayed oxidation of the lattice, respectively. Contrary to bulk semiconductors,
which break only above a critical field, the time-dependent failure behaviour of graphene
has been discovered, which precludes the existence of failure threshold and manifests as
a potential defect-assisted aging issue for graphene and other 2D material-based devices | en_US |
