Synthesis and Integration of 2D Materials and ALD Dielectrics
Restricted (Penn State Only)
- Author:
- Chen, Cindy
- Graduate Program:
- Materials Science and Engineering (PHD)
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 13, 2023
- Committee Members:
- John Mauro, Program Head/Chair
Mauricio Terrones, Outside Unit & Field Member
Suzanne Mohney, Major Field Member
Joshua Robinson, Chair & Dissertation Advisor
Raymond Schaak, Outside Field Member - Keywords:
- 2D Materials
Atomic Layer Deposition
Chemical Vapor Deposition
Semiconductors
Dielectrics - Abstract:
- Two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs) are considered as promising materials for augmenting conventional Si-based technology due to their atomically layered structure and tunable electronic and optical properties. Although promising for the continued scaling of transistors, 2D materials can also present several challenges that prevent the realization of their theoretical performance. First, 2D material synthesis is still not fully mature for industrial manufacturing, requiring extensive research and development to optimize towards wafer-scale, single-crystalline, and phase-pure synthesis of 2D materials. Second, 2D materials are highly susceptible to ambient instability, as the presence of surface defects can more profoundly impact the 2D material’s intrinsic electronic properties compared to that of bulk materials. Finally, integrating a suitable dielectric environment is crucial for enhancing the carrier transport properties of 2D semiconductors. The lack of out-of-plane bonding in 2D van der Waals (vdW) surfaces poses challenges for gate dielectric integration, as the chemical inertness leads to lower growth rates and non-uniform growth of dielectric layers on 2D materials. To improve 2D device performance, it is critical to address these challenges and develop 2D material synthesis and dielectric integration processes, with specific considerations of temperature requirement, scalability, reliability, and impact of dielectric environment on 2D electronic properties. In this work, we present the large-area, phase-selective growth of ambient-stable MoTe2 using hybrid physical chemical vapor deposition (HPCVD) and highlight the driving process parameters for phase control. We demonstrate a novel, multi-step growth method for the layer-by-layer growth of 2H-MoTe2, which enabled us to probe their layer-dependent optical properties. To improve the ambient stability of MoTe2, we employ ALD for BN encapsulation and demonstrate improved BN/MoTe¬2 oxidative stability up to 1 month in ambient conditions. We investigate both plasma-enhanced and thermal ALD routes for synthesizing BN and evaluating their impact on BN coverage on 2D material surfaces. It was observed that PEALD, which utilizes high-energy N2/H2 plasma, results in polycrystalline hBN, whereas thermal ALD with NH3 as the N precursor results in fully amorphous BN. We further demonstrate the wafer-scale, low-temperature (< 250 °C) thermal ALD synthesis of uniform, conformal amorphous boron nitride (aBN) thin films. ALD deposition temperatures between 125 and 250 °C result in stoichiometric films with high oxidative stability, yielding a robust dielectric strength of 8.2 MV/cm. Utilizing a seed-free ALD approach, we form uniform aBN dielectric layers on 2D surfaces and fabricate multiple quantum well structures of aBN/MoS2 and aBN-encapsulated double-gated monolayer (ML) MoS2 field-effect transistors to evaluate the impact of aBN dielectric environment on MoS2 optoelectronic and electronic properties. Our work in scalable synthesis of 2D materials and ALD dielectric integration paves a way towards realizing the theoretical performance of 2D materials for next-generation electronics.