ICMCTF 2024 Session MB4-MoM: 2D Materials: Synthesis, Characterization, and Applications

Hexagonal boron nitride (hBN) films are attractive for several emerging energy-related applications. Extensive previous research has focused on the growth and properties of either ultrathin hBN films with thicknesses up to a few monolayers or cubic BN films. The synthesis of wafer-scale hBN films with controlled thickness above ~10 nm with desired properties remains a challenge. Here, we present results of our ongoing systematic study of polycrystalline hBN films with thicknesses in a wide range of 50-6000 nm deposited by several variants of reactive magnetron sputtering with a radiofrequency (RF) driven discharge. We describe how the plasma discharge characteristics and, hence, resultant major film properties can be controlled by the magnetron source design, the confining magnetic field, and process parameters such as the working gas pressure (influencing landing neutral atom ballistics and energetics), substrate temperature (adatom mobility), and substrate bias (bombarding ion energy). Even without epitaxy, with substrates held close to room temperature, hBN films are polycrystalline, characterized by a FWHM of the major E_2g Raman vibrational mode (1370 cm^-1) in the range of 40 – 100 cm^-1, depending on deposition conditions. The FWHM reduces to ~30 cm^-1 when a higher deposition temperature of 600-800 ^oC is used. Interestingly, all as-grown films are polycrystalline (turbostratic, with asymmetrically stacked layers) rather than amorphous even for a high deposition pressure of 50 mTorr, characterized by low landing atom energetics. We also describe how these film growth and characterization experiments are guided by results of in-situ plasma diagnostics.

This work was performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA2734.

Two-dimensional (2D) materials like graphene and transition metal dichalcogenides (TMDs) have attracted significant attention due to their exceptional electrical properties, holding promise for next-generation nanoelectronics. However, integrating 2D materials into IC devices presents challenges, including precisely controlled synthesis methods, defect-free transfer processes, and back-end-of-line (BEOL) device integration.

In this talk, I will discuss advancements in selectively seeding growth of high-quality 2D materials on insulating substrates using a new precursor and advanced process. Additionally, an efficient and reliable method for the wafer-scale transfer of graphene and other 2D materials, ensuring integrity and cleanliness, will be presented. Finally, I will highlight the concept of a heterogeneously integrated 3D-IC, combining a 2D-based field-effect transistor (FET) with high-performance memory, showcasing the potential for BEOL and monolithic integration of 2D-based 3D-ICs.

Ti₃C₂T_x, belonging to the MXene family, exhibits distinctive characteristics that render it highly suitable for p-n diode/Schottky diode applications, particularly in conjunction with MoO₃ compounds. Ti₃C₂T_x - MoO₃ can offer exceptional thermal and mechanical stability, coupled with its elevated electrical conductivity and favorable chemical sensing ability, which makes these systems as an optimal choice for electronic device fabrication.

In this work thin films of Ti₃C₂T_x -MoO₃ bilayers are deposited using Pulsed Laser Deposition (PLD) technique, specifically tailored for studying the diode characteristics. The deposition process utilized a low laser energy density strategy, allowing the exploration of various compositions. The experiments employed a krypton fluoride (248 nm) excimer laser operating at a laser energy density of 1.5 J/cm2. First layer of Ti₃C₂T_x was deposited on sapphire substrate with substrate temperature 700°C, target to substrate distance as 5 cm and laser repetition rate of 10 Hz. The vacuum inside the chamber was 1.5 x 10^-5 mTorr. The MoO₃ was deposited on to the MXene layer using same energy density and base vacuum with substrate temperature of 400°C, and pulse repetition rate of 8 Hz. Here the oxygen partial pressure was maintained at 75 mTorr during deposition.

The compositional and morphological properties of Ti₃C₂T_x -MoO₃ films were investigated using X-ray diffraction, Field-emission scanning electron microscopy (FESEM), and EDAX. The transport characteristics show a systematic change from Schottky behavior to p-n junction diode characteristics as a function of varying deposition parameters. Variation in PLD energy density and substrate temperature were the main parameters which were varied. These results would be exhibited in this presentation. A novel non-Silicon based system is hence projected as a potential candidate for electronic and optoelectronic applications.