Variability and Reliability of Graphene Field-Effect Transistors with CaF2 Insulators

TL;DR

This study demonstrates that epitaxial CaF2 can serve as a stable, atomically flat gate insulator in graphene FETs, showing comparable hysteresis and stability to traditional oxides, with potential for high-performance applications.

Contribution

First use of 2nm epitaxial CaF2 as a gate insulator in GFETs, with comprehensive variability and stability analysis showing promising device performance.

Findings

CaF2 exhibits low hysteresis below 0.01 V at EOT ~1 nm

Device-to-device variability is low with similar transfer characteristics

Hysteresis is mainly due to thermally activated border defects

Abstract

Graphene is a promising material for applications as a channel in graphene field-effect transistors (GFETs) which may be used as a building block for optoelectronics, high-frequency devices and sensors. However, these devices require gate insulators which ideally should form atomically flat interfaces with graphene and at the same time contain small densities of traps to maintain high device stability. Previously used amorphous oxides, such as SiO2 and Al2O3, however, typically suffer from oxide dangling bonds at the interface, high surface roughness and numerous border oxide traps. In order to address these challenges, here we use for the first time 2nm thick epitaxial CaF2 as a gate insulator in GFETs. By analyzing device-to-device variability for over 200 devices fabricated in two batches, we find that tens of them show similar gate transfer characteristics. Our statistical analysis of the hysteresis up to 175C has revealed that while an ambient-sensitive counterclockwise hysteresis can be present in some devices, the dominant mechanism is thermally activated charge trapping by border defects in CaF2 which results in the conventional clockwise hysteresis. We demonstrate that both the hysteresis and bias-temperature instabilities in our GFETs with CaF2 are comparable to similar devices with SiO2 and Al2O3. In particular, we achieve a small hysteresis below 0.01 V for equivalent oxide thickness (EOT) of about 1 nm at the electric fields up to 15 MV/cm and sweep times in the kilosecond range. Thus, our results demonstrate that crystalline CaF2 is a promising insulator for highly-stable GFETs.