MOCVD of MoS2: Nucleation Density, Lateral Growth Rate, and Coverage Dependence on Growth Variables

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.