Principal Investigator Tomas Palacios
Atomically thin layered two-dimensional transition metal dichalcogenides such as molybdenum disulfide (MoS2) have been proposed to enable aggressive minia- turization of FETs. We previously reported ultra-short channel MoS2 FETs with channel length down to 15 nm and 7.5 nm using graphene and directed self-assembly pattern technique, respectively. However, the power scaling in such devices suffers from the same issues as in CMOS technology. Obtaining a subthreshold swing (SS) below the thermionic limit of 60 mV/dec by exploiting the negative-capacitance (NC) effect in ferroelectric (FE) materials is a novel effective technique to allow for the reduction of the supply voltage and power consumption in field-effect transistors (FETs). Conventional ferroelectric materials, i.e., lead zirconate titanate, bismuth ferrite, and polymer ferroelectric di- electrics such as P(VDF)-TRFE are not technologically compatible with standard CMOS fabrication processes. On the other hand, fluorite-type doped HfO2 ferro- electric thin-films deposited by ALD offers the CMOS compatibility and scalability required for advanced electronic applications.
In this work, we demonstrate NC-MoS2 FETs by incorporating a ferroelectric doped HfO2 (Al:HfO2 or Si: HfO2 ) in the FET gate stack. Standard HfO2 has monoclinic crystal structure which can be transformed into orthorhombic phase by temperature, pressure, or doping. The electrical properties of the doped HfO2 thin-films can be tuned from dielectric to ferroelectric and even antiferroelectric by changing dopant type (Zr, Al, Si, Gd, Y, etc.), dopant fraction and/or capping layer. The ferroelectric nature of typical doped HfO2 thin film can be confirmed by the polarization measurement. Here, Si:Hf composition is kept fixed by controlling the 3DMAS/TEMAH pulses during the ALD. We observe steep SS in FETs when used these FE in the gate stack with carefully matched FE/DE bilayer. The NC-MoS2 FET built on a typical FE/DE bilayer showed a significant enhancement of the SS to 57 mV/dec at room temperature, compared with SSmin = 67 mV/dec for the MoS2 FET with only HfO2 as a gate dielectric.