Electronic Devices using Low-Dimensional van der Waals Materials and Their Heterojunctions

Abstract

As the state of the art in computation, communication and sensing advances, there is an ever-increasing search for novel materials and device architectures to satisfy the growing demands for more functionality, performance, smaller device sizes, speed and energy efficiency. Lower-dimensional materials - one-dimensional and two-dimensional materials and novel devices using them have been at the forefront of this search in recent years. The availability of many of these materials with various electronic and optical properties and the possibility to easily form their heterostructures has put them in this position. This work will present some electronic and optical devices using low-dimensional materials, addressing problems in high-performance and energy-efficient transistors and infrared optical detection.

Tellurium-based gate-all-around nanowire junctionless transistor: Tellurium is an element with a unique helical van-der Waal atomic structure, making it possible to make nanowires (one-dimensional) and sheets with atomic thickness (two-dimensional) without introducing surface defects and degrading mobility. Here, we explore gate-all-around junctionless transistor architecture, a suitable structure for sub-5nm technology node due to its excellent electrostatics, using the Tellurium nanowires. Further, we use h-BN, a two-dimensional insulator, as the gate dielectric. The combination of the unique material properties and the device architecture helped us fabricate p-type transistors with field-effect mobility of 570 cm^2/V.s, a high drive current of 216 mA/mm and an on-off ratio of 2X10^4.

Electrically self-aligned SnSe2/WSe2 heterojunction MOSFET/TFET: Tunnel FETs have been important devices in the search for energy-efficient transistors as they can have subthreshold-swing less than 60 mV/dec. Also, achieving self-aligned structures has been the one key factor behind the successful development of Silicon-based MOS technology. Achieving such a self-alignment using fabrication processes has been difficult for two-dimensional materials due to the lack of doping techniques. Using partially overlapping gates, we present an electrically defined self-aligned SnSe2/WSe_2 heterostructure device. The device operates as n-type and p-type transistors by applying appropriate gate biases. The device can also be used as a test platform to discern the properties of the heterojunction without confounding the effects of contact resistance. This device can be operated as a tunnel transistor as well. At room temperature, the device exhibits a subthreshold swing of less than 60 mV/dec.

High current double-channel 2D transistor for lower contact resistance: Transistors with high current drive are important for high performance as they offer high speed and larger fanout. However, getting a high current drive has been challenging in 2D transistors due to limitations arising from higher contact resistance. Also, getting higher current drives without increasing the device footprint on the chip is a major challenge. To this end, we present a vertically integrated double-channel transistor based on ultrathin 2D material to meet these challenges. The device structure effectively increases the contact area, thus reducing the contact resistance by about a factor of two without compromising the device footprint or electrostatic integrity. The proposed device structure achieves a high drive current of over 400 mA/mm.

Intraband hot-electron-based IR detection using 2D material heterostructure: Infrared radiation, being low energy, requires lower bandgap materials for the conventional photodetection mechanism of electron transition from the valence to the conduction band. However, most low band-gap materials, like black phosphorous, are known to be unstable and not friendly with standard fabrication processes. Here, we present infra-red detection using stable wide-bandgap material heterostructure and electron transition from one material's conduction band to the other's conduction band. We propose multiple device architectures to understand the mechanism of the device operation from their bias-dependent photo response and simple physical models. The device exhibits the responsivity of ~40 mA/W when excited with 1550 nm wavelength IR radiation. The device operates and exhibits a nearly flat response up to 1800 nm, and a signal above the noise floor could be observed up to 30 kHz.