It has been shown that a ferroelectric material integrated into the gate stack of a transistor can create an effective negative capacitance (NC) that allows the device to overcome "Boltzmann tyranny". While this switching below the thermal limit has been observed with Si-based NC field-effect transistors (NC-FETs), the adaptation to 2D materials would enable a device that is scalable in operating voltage as well as size. In this work, we demonstrate sustained sub-60 mV/dec switching, with a minimum subthreshold swing (SS) of 6.07 mV/dec (average of 8.03 mV/dec over 4 orders of magnitude in drain current), by incorporating hafnium zirconium oxide (HfZrO or HZO) ferroelectric into the gate stack of a MoS 2D-FET. By first fabricating and characterizing metal-ferroelectric-metal capacitors, the MoS is able to be transferred directly on top and characterized with both a standard and a negative capacitance gate stack. The 2D NC-FET exhibited marked enhancement in low-voltage switching behavior compared to the 2D-FET on the same MoS channel, reducing the SS by 2 orders of magnitude. A maximum internal voltage gain of ∼28× was realized with ∼12 nm thick HZO. Several unique dependencies were observed, including threshold voltage (V) shifts in the 2D NC-FET (compared to the 2D-FET) that correlate with source/drain overlap capacitance and changes in HZO (ferroelectric) and HfO (dielectric) thicknesses. Remarkable sub-60 mV/dec switching was obtained from 2D NC-FETs of various sizes and gate stack thicknesses, demonstrating great potential for enabling size- and voltage-scalable transistors.

Dielectric constants, k, of Zr͑Hf͒ silicate alloy gate dielectrics obtained from analysis of capacitance-voltage curves of metal-oxide-semiconductor capacitors with 3-6 at. % Zr͑Hf͒ are significantly larger than estimates of k based on linear extrapolations between SiO 2 and compound silicates, Zr͑Hf͒SiO 4. Analysis of infrared spectra of Zr silicate alloys with 3-16 at. % Zr indicates increases in the coordination of Zr to O atoms from 4 to approximately 8 with increasing Zr content. The major contributions to enhancements in k in these low Zr͑Hf͒ content alloys are explained by a transverse infrared effective charge that scales inversely with increasing Zr-O bond coordination.

The local bonding of Zr, Si, and O atoms in plasma-deposited, and post-deposition annealed Zr silicate pseudobinary alloys ͓(ZrO 2) x (SiO 2) 1Ϫx ͔ was studied by x-ray photoelectron spectroscopy ͑XPS͒ and Auger electron spectroscopy ͑AES͒. Systematic decreases in XPS binding energies, and increases in AES kinetic energies with alloy composition x are consistent with an empirical chemical bonding model based on electronegativity equalization; however, there are significant departures from the predicted linear composition dependencies of that model. Deviations from linearity in the XPS compositional dependencies are correlated with dipolar network atom fields as determined from ab initio calculations. The nonlinearities in the x dependence of Zr MVV and O KVV AES spectral features are determined primarily by oxygen-atom coordination dependent shifts in valence band offset energies. The energy spread in the compositional dependence of binding energies (ϳ1.85 eV) for the XPS Zr 3d 5/2 and Si 2p features combined with x-ray absorption spectroscopy data indicates that the conduction band offset energies between the Si substrate and Zr silicate dielectrics are alloy composition independent. Changes in O 1s XPS features in alloys with xϳ0.3 to 0.6 as function of annealing temperature are consistent with a previously identified chemical phase separation that occurs after 60 s anneals at 900°C in a nonoxidizing ambient, Ar.

This article presents a comprehensive spectroscopic study of chemical phase separation in zirconium silicate alloys, (ZrO 2) x (SiO 2) 1Ϫx , using Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy, extended x-ray absorption fine structure spectroscopy, and near edge x-ray absorption spectroscopy. These measurements are complemented by measurements of x-ray diffraction and high resolution transmission electronic microscopy imaging. This combination has been applied to Zr silicate alloys, providing the first comprehensive comparisons of the relative sensitivities of these spectroscopic techniques applied to micro-and nanoscale chemical phase separation of high-k dielectric alloys.

The internal structures of various (ZrO2)x(SiO2)1-x alloys (x ≤ 0.5) were investigated. A remote plasma enhanced-metal organic chemical vapor deposition (RPEMOCVD) process was used to deposit films with varying alloy composition on Si(100) substrates. This study indicates that for the glassy silicate phase, g-ZrSiO4, a glass transition temperature, Tg, exists between 800°C and 900°C at which phase separation into the end-member components, SiO2 and ZrO2, occurs.