| dc.description.abstract | Scaling towards increasing miniaturization, as per the so-called Moore’s law, has governed
electronics technology by the CMOS route over the past decades. 2D materials atomically thin
crystalline layers are emerging as a potential replacement for Si in the “beyond Moore” era of
CMOS scaling. Molybdenum disulfide (MoS2) has emerged as a promising two-dimensional
(2D) material due to its unique electronic, optical, and mechanical properties, making it suitable
for various advanced device applications. However, achieving high-quality, large-area
monolayer MoS2 with controlled growth characteristics remains challenging. Using MoS2 as a
case study, this thesis looks at the impact of vapor deposition parameters on the nucleation and
growth of islands with the aim of controlling grain sizes and, hence, grain boundary defect
density in the deposited atomic layers.
A central goal of this research is to address the limitations associated with grain size scaling in
transition metal dichalcogenides (TMDs). The two phenomena that need to be controlled are
nucleation, its rate and resulting density of domains, and the lateral velocity of the islands so
nucleated, which in turn determines size and time to coverage. These factors are central to
achieving larger, high-quality monolayer films within a practical timeframe. Given classical
models present in the literature to treat the nucleation and lateral velocity of isolated islands on
the one hand and the models to treat surface coverage on the other, a framework is assembled
to explain the growth process. Both the nucleation and growth rate are exponentially dependent
on the supersaturation. The nucleation density must be reduced considerably to allow for large
grain growth, implying that the surface supersaturation needs to be low. This reduced
supersaturation reduces the lateral growth velocities, leading to considerable time scales for
growth. The barrier to attachment of the adatoms is identified as the rate-limiting step. Thus,
this barrier to attachment must be reduced to enable the growth of large grains of MoS2. This
enables faster lateral growth rates and coverage.
Following the analytical model, the fundamental parameters influencing nucleation and grain
growth are explored through controlled experiments. The effect of the various parameters, such
as temperature, pressure, and flow rates of gases, on the mode of growth and quality of the
film has been thoroughly studied. A substrate with full monolayer coverage of MoS2 was
achieved in under 3 minutes, indicating rapid growth kinetics. The precursor's partial pressure - identified as a critical factor influencing growth kinetics – was reduced further to better
control nucleation and grain growth. Additionally, the analytical framework developed in this
thesis is applied to analyze data from a previous report by our group. The findings highlight
the critical role of the energy barrier to attachment in the MoHC-substrate combination. A
potential solution to mitigate the issue of slow lateral growth is proposed by suggesting ways
to reduce this attachment barrier.
Since the attachment barrier limits grain size, sapphire substrates were chosen for their
potential to promote epitaxial and aligned growth; through this work, we investigate the impact
of atomic steps and edges of sapphire on nucleation sites and grain orientation, which
significantly influence growth repeatability and epitaxial alignment. While the transition from
SiO2 to sapphire was driven by the desire for improved epitaxial growth and grain alignment,
the inconsistency in achieving repeatable results remains a critical challenge. We examine the
impact of sapphire substrates on the chemical vapor deposition (CVD) growth of MoS2,
focusing on the statistical variation observed in epitaxial alignment and grain growth. The
atomic steps and crystallographic edges of sapphire play a pivotal role in determining the
nucleation sites, leading to variations in MoS2 grain size, orientation, and overall film quality.
The reconstruction of sapphire surfaces during high-temperature growth leads to step and
terrace width variations. Additionally, differences in local supersaturation and the S/Mo ratio
result in diverse shapes and morphologies of growing MoS2 islands, which impact the
repeatability of the growth process. This study provides valuable insights into the factors
contributing to these statistical fluctuations by examining the interaction between substrate
characteristics and growth outcomes. An alternative approach - annealing the wafer at high
temperatures for an extended period prior to growth - can help create well-defined steps with
uniform spacing across the entire wafer, improving consistency in the growth process. The
findings highlight key considerations for optimizing the CVD process and improving the
reproducibility of epitaxial MoS2 growth on sapphire substrates.
The results of this study provide a systematic approach to controlling MoS2 grain
characteristics by isolating and tuning process parameters, thereby achieving improved grain
sizes within practical time constraints. This research offers valuable insights into the
engineering of 2D materials for enhanced device integration and represents a step forward in
achieving scalable, high-quality MoS2 layers tailored for specific electronic applications. | en_US |
