ABSTRACT
The investigation of the optical constants (e.g., the refractive index n and the extinction coefficient κ) has been performed in the mid-infrared spectrum for various silicon nitride (SiNx) configurations. By exploiting the transfer matrix method formulation, photometric measurements of transmission and reflection have been used for iteratively calculating the optical parameters of interest. To ensure the reliability of the n and κ, the same material from which such parameters were extracted was deposited for three different thicknesses, e.g., 600, 200, and 100 nm. While the former is optically characterized, the remaining two are used for validation purposes. For each experimental/calculated comparison, the average (made over the whole considered spectrum interval) of the relative error never exceeds 1.5%, which ensures the correctness of the given n and κ. For the sake of completeness, a detailed analysis of the intrinsic limitations arising from the very nature of the method will also be conducted.
ABSTRACT
In this contribution, a rigorous numerical calibration is proposed to characterize the excitation of propagating mechanical waves by interdigitated transducers (IDTs). The transition from IDT terminals to phonon waveguides is modeled by means of a general circuit representation that makes use of Scattering Matrix (SM) formalism. In particular, the three-step calibration approach called the Thru-Reflection-Line (TRL), that is a well-established technique in microwave engineering, has been successfully applied to emulate typical experimental conditions. The proposed procedure is suitable for the synthesis/optimization of surface-acoustic-wave (SAW) based devices: the TRL calibration allows to extract/de-embed the acoustic component, namely resonator or filter, from the outer IDT structure, regardless of complexity and size of the letter. We report, as a result, the hybrid scattering parameters of the IDT transition to a mechanical waveguide formed by a phononic crystal patterned on a piezoelectric AlN membrane, where the effect of a discontinuity from periodic to uniform mechanical waveguide is also characterized. In addition, to ensure the correctness of our numerical calculations, the proposed method has been validated by independent calculations.
ABSTRACT
Multichannel analysis of graphene nanoribbons (GNR), often required for describing applications to practical devices, constitutes a heavy computational task, even in a simplified framework like that provided by discrete or nearest neighbour models. Scattering (S) matrix techniques, widely used for quantum transport in low dimensional systems and for the computation of electromagnetic fields, is shown here to provide a powerful formal platform for the analysis, and, in principle, the synthesis, of GNR multiport circuits. Periodic modes, solutions of GNR waveguides, are demonstrated to obey charge conservation and reciprocity constraints corresponding to unitary and symmetry properties of the S-matrix, under proper normalization conditions. We propose a systematic use of this approach to deal with problems such as scattering by lattice defects, the presence of external applied fields, crossing GNRs and T-junctions.
ABSTRACT
We present a generalization of the self-consistent analysis of carbon nanotube (CNT) field effect transistors (FETs) to the case of multi-wall/multi-band coherent carrier transport. The contribution to charge diffusion, due to different walls and sub-bands of a multi-wall (mw) CNT is shown to be non-negligible, especially for high applied external voltages and 'large' diameters. The transmission line formalism is used in order to solve the Schrödinger equation for carrier propagation, coupled to the Poisson equation describing the spatial voltage distribution throughout the device. We provide detailed numerical results for semiconducting mw-nanotubes of different diameters and lengths, such as current-voltage characteristics and frequency responses.
