At the end of scaling: 2D materials for computing and sensing applications

In the past half-century, the digital revolution completely changed the world we live in and the ways we experience it. Over this period, the underlying force supporting the continuous technological development has been the geometrical scaling of transistor dimensions, resulting in an exponential increase of the computation power density as well as a drastic decrease of chip cost. Approaching the end of the CMOS scaling era, the semiconducting industry faces enormous challenges to redefine its research priorities. Moreover, novel strict requirements on energy efficiency, system autonomy and remote sensing are posed by the development of the Internet of Things (IoT) networks. The quest for novel materials and device working principle is therefore more critical than ever. "More than Moore" devices aim to overcome the fundamental physical limitations of the MOSFET structure so to either replace or complement CMOS technology in a broad range of applications. Among this variegated research field, tunnel FETs have attracted a consistent interest in the last decades because of their steep turn on characteristic enabling the power supply scaling while maintaining a low standby power consumption. Several materials and TFET architectures have been demonstrated, with both promising results and great challenges in the quest for reducing the persisting performance gap with the established CMOS technology.

The aim of this thesis is to demonstrate the potential of two dimensional (2D) materials and heterojunctions for the realization of “More than Moore” devices as well as for high sensitivity sensors. The continuously expanding family of 2D materials offers a huge catalog of electronic, mechanical and optical properties. The transition metal dichalcogenide (TMDC) group in particular includes a set of promising semiconductors with relatively large, number of layer dependent, band gap. MoS2 FET with excellent performance have been reported, while WSe2 has the potential of becoming the base for a true 2D CMOS platform.

MOSFET devices were fabricated starting from black phosphorous and WSe2 in order to investigate respectively fabrication strategies characterized by a reduced air exposure and the deposition of high k dielectric on 2D flakes. We then investigated the opportunities offered by the deterministic transfer of 2D flakes realizing both 3D/2D and 2D/2D heterojunction devices. The first reported VO2/MoS2 devices have been demonstrated obtaining a good rectifying characteristic and excellent, temperature tunable photosensitivity. Prototypes of three terminal devices based on this junction open the possibility for true VO2 based field effect devices. Finally, a heterojunction TFET based on WSe2/SnSe2 is presented. Our devices exhibit an excellent turn on characteristic and a direct comparison with the built-in, same flake WSe2 FET show how they outperform the MOSFET realized in the same material system. Moreover, we propose a new device combining the advantages of both MOSFET and TFET in a unique structure.