Alan Paussa

Electromechanical Piezoresistive Sensing in Suspended Graphene Membranes

Monolayer graphene exhibits exceptional electronic and mechanical properties, making it a very promising material for nanoelectromechanical devices. Here, we conclusively demonstrate the piezoresistive effect in graphene in a nanoelectromechanical membrane configuration that provides direct electrical readout of pressure to strain transduction. This makes it highly relevant for an important class of nanoelectromechanical system (NEMS) transducers. This demonstration is consistent with our simulations and previously reported gauge factors and simulation values. The membrane in our experiment acts as a strain gauge independent of crystallographic orientation and allows for aggressive size scalability. When compared with conventional pressure sensors, the sensors have orders of magnitude higher sensitivity per unit area.

Investigation of Strain Engineering in FinFETs Comprising Experimental Analysis and Numerical Simulations

This study combines direct measurements of strain, electrical mobility measurements, and a rigorous modeling approach to provide insights about strain-induced mobility enhancement in FinFETs and guidelines for device optimization. Good agreement between simulated and measured mobility is obtained using strain components measured directly at device level by a novel holographic technique. A large vertical compressive strain is observed in metal gate FinFETs, and the simulations show that this helps recover the electron mobility disadvantage of the (110) FinFET lateral interfaces with respect to (100) interfaces, with no degradation of the hole mobility. The model is then used to systematically explore the impact of stress components in the fin width, height, and length directions on the mobility of both n- and p-type FinFETs and to identify optimal stress configurations. Finally, self-consistent Monte Carlo simulations are used to investigate how the most favorable stress configurations can improve the on current of nanoscale MOSFETs.

A manufacturable process integration approach for graphene devices

We experimentally demonstrate DC functionality of graphene-based hot electron transistors, which we call Graphene Base Transistors (GBT). The fabrication scheme is potentially compatible with silicon technology and can be carried out at the wafer scale with standard silicon technology. The state of the GBTs can be switched by a potential applied to the transistor base, which is made of graphene. Transfer characteristics of the GBTs show ON/OFF current ratios exceeding 50.000.

Piezoresistive Properties of Suspended Graphene Membranes under Uniaxial and Biaxial Strain in Nanoelectromechanical Pressure Sensors

Graphene membranes act as highly sensitive transducers in nanoelectromechanical devices due to their ultimate thinness. Previously, the piezoresistive effect has been experimentally verified in graphene using uniaxial strain in graphene. Here, we report experimental and theoretical data on the uni- and biaxial piezoresistive properties of suspended graphene membranes applied to piezoresistive pressure sensors. A detailed model that utilizes a linearized Boltzman transport equation describes accurately the charge-carrier density and mobility in strained graphene and, hence, the gauge factor. The gauge factor is found to be practically independent of the doping concentration and crystallographic orientation of the graphene films. These investigations provide deeper insight into the piezoresistive behavior of graphene membranes.

An exact solution of the linearized Boltzmann transport equation and its application to mobility calculations in graphene bilayers

This paper revisits the problem of the linearized Boltzmann transport equation (BTE), or, equivalently, of the momentum relaxation time, momentum relaxation time (MRT), for the calculation of low field mobility, which in previous works has been almost universally solved in approximated forms. We propose an energy driven discretization method that allows an exact determination of the relaxation time by solving a linear, algebraic problem, where multiple scattering mechanisms are naturally accounted for by adding the corresponding scattering rates before the calculation of the MRT, and without resorting to the semi-empirical Matthiessens rule for the relaxation times. The application of our rigorous solution of the linearized BTE to a graphene bilayer reveals that, for a non monotonic energy relation, the relaxation time can legitimately take negative values with no unphysical implications. We finally compare the mobility calculations provided by an exact solution of the MRT problem with the results obtain...

Simulation of graphene nanoscale RF transistors including scattering and generation/recombination mechanisms

We present a Monte Carlo simulator for RF graphene FETs including the dominant scattering mechanisms and a simple model for band-to-band tunneling. We found that in state-of-the-art devices scattering is relevant and degrades the cut-off frequency compared to the predictions of ballistic models.

Low-field mobility and high-field drift velocity in graphene nanoribbons and graphene bilayers

In this paper we follow a semiclassical approach based on the Boltzmann Transport Equation (BTE) to simulate and compare with experiments the low-field mobility (μ) and the high-field drift velocity (vd) of graphene nano-ribbons (GNRs) and graphene bilayers (GbLs). It is found that remote phonons originating in the substrate have a large impact on the mobility, whereas their impact on the saturation velocity is smaller than predicted by recently proposed simplified model.

Simulation of the Performance of Graphene FETs With a Semiclassical Model, Including Band-to-Band Tunneling

We assess the analog/RF intrinsic performance of graphene FETs (GFETs) through a semiclassical transport model, including local and remote phonon scattering as well as band-to-band tunneling generation and recombination, validated by comparison with full quantum results over a wide range of bias voltages. We found that scaling is expected to improve the fT, and that scattering plays a role in reducing both the fT and the transconductance also in sub-100-nm GFETs. Moreover, we observed a strong degradation of the device performance due to the series resistances and source/drain to channel underlaps.

Comparison Between Pseudospectral and Discrete Geometric Methods for Modeling Quantization Effects in Nanoscale Electron Devices

This paper aims at comparing the pseudospectral method and discrete geometric approach for modeling quantization effects in nanoscale devices. To this purpose, we implemented a simulation tool, based on both methods, to solve a self-consistent Schrödinger-Poisson coupled problem for a 2-D electron carrier confinement according to the effective mass approximation model (suitable for FinFETs and nanowire FETs).

Pseudo-spectral method for the modelling of quantization effects in nanoscale MOS transistors

This paper presents a systematic comparison between the numerical efficiency of the pseudo-spectral (PS) and finite difference (FD) methods for the solution of eigenvalue problems related to both n and p-MOS transistors, with different geometries and carrier dimensionalities. Our results indicate remarkable advantages of the PS compared to the FD method in terms of CPU time.