Stoichiometry-Engineered Binary Chalcogen Alloy Thin Films Enabled by Integrated Synthetic Approach

Abstract

Continued channel scaling has strengthened the need for atomically thin semiconductors in next-generation CMOS technologies. In this context, two-dimensional (2D) van der Waals (vdW) materials have attracted extensive attention, as their atomically thin bodies provide excellent electrostatic control, suppressed short-channel effects, and compatibility with stacked device architectures. While notable progress has been made in achieving high-performance n-type devices, securing equally competitive p-type channels remains challenging, motivating exploration of alternative chalcogen-based systems. Tellurium (Te), a quasi-1D vdW solid, has emerged as a strong p-type candidate owing to its high hole mobility and environmental stability. However, its narrow bandgap (~0.35 eV) fundamentally limits the Ion/Ioff ratio in transistor operation, highlighting the need for bandgap-widening while preserving Te’s advantageous transport characteristics. Alloying with selenium (Se), which has a wider bandgap and maintains structural compatibility with Te, offers an effective route to achieve this. In this thesis, we pursue composition-tunable Se-Te alloy thin films as p-type channel materials by employing two complementary deposition strategies: (i) co-sputtering and (ii) atomic layer deposition (ALD).

To provide a versatile experimental platform to investigate Se-Te alloy formation, co-sputtering enables systematic variation of stoichiometry, assessment of material post-annealing (MPA) treatment to probe recrystallization and ordering behavior, and evaluation of electronic transport under diverse process-dependent film conditions. This approach enables direct correlation of deposition parameters with Se incorporation and the resulting microstructural changes, along with their impact on the electrical characteristics of the films. These correlations clarify how alloy composition governs bandgap and hole-transport behavior, providing a basis for practical optimization of the Se-Te system (Chapter 2).

Building on these insights, ALD enables low-temperature Se-Te growth through reactivity-enhanced reactant dosing schemes that improve initial nucleation and surface reactions. Sequence-driven approaches further enhance surface coverage and contribute to more reliable film formation. In addition, sequence-level modulation – implemented through adjustments in precursor pulse duration and the inclusion of another chalcogen precursor – offers a practical route for fine stoichiometric tuning, allowing composition control that is more readily accessible with ALD (Chapter 3).

By integrating co-sputtering-derived insights on composition, microstructure, and transport behavior with ALD process design, we obtain SexTe1-x thin films exhibiting tunable bandgaps, suppressed off-currents, and competitive hole mobility. This combined synthetic framework supports low-temperature and composition-engineered Se-Te growth that is suitable for wafer-scale, stackable device fabrication. Furthermore, the demonstrated routes establish a practical foundation for deploying Se-Te alloys in advanced CMOS process flows, enabling their consideration as robust p-type channel materials for future transistor and integration architectures.