Study of the Electrical Properties of Low-Dimensional Materials Using Plane-Wave Computational Methods




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As transistor channel dimensions have continuously reduced, research efforts have been directed at finding alternative channel materials to overcome the limitations of Si-based planar Complementary Metal Oxide Semiconductor(CMOS) technology. Low dimensional materials like two-dimensional Transition Metal Dichalcogenides (TMDs) and one-dimensional nanostructures have emerged as promising candidates for future technology nodes due to the excellent electrostatic gate control they offer. In view of the great interest in low-dimensional materials we focus on the computational study of the electronic properties of these materials using a recently developed Bloch-wave based quantum transport solver. We focus our attention on improving the knowledge surrounding two important inputs to the Bloch-wave based quantum transport solver namely the dielectric constant (needed for accurate electrostatic analysis) and the empirical pseudopotentials used to build the Bloch-wave basis set. At first we compute the macroscopic dielectric constant for monolayer, bilayer and bulk TMDs, and hexagonal boron nitride (h-BN). Our calculations show that the electronic contribution dominates the dielectric response of 2H-TMDs and h-BN in both in-plane and out-of-plane directions whereas ionic contribution dominates the in-plane response for 1T-TMDs. The very high in-plane dielectric response can lead to enhanced polar optical scattering thereby negatively impacting its mobility. Next, we developed a general method for calibrating empirical pseudopotentials (EP) using ab-initio data. These generated EPs with the help of Virtual Crystal Approximation (VCA) are then used to study the device performance of gateall-around Si1−xGex alloy (x is alloy composition) nanowire based Field-Effect-Transistors. Our calculations suggest that the ON current increases monotonically with the increasing value of x for the h100i crystallographic orientation whereas it shows negligible variation for the h110i orientated nanowires. For a given alloy composition, the h100i crystallographic orientation seems to have superior performance compared against its h110i counterpart. Also, the performance gap quantified by the difference in the device ON current increases for higher values of x. The use of a virtual crystal approximation technique to study the Si1−xGex alloy nanowire overestimates the drive current for nanowires with sub 1 nm body sizes. Therefore, one must be careful when deriving the quantitative estimates about the device performance using VCA, especially for smaller width Si1−xGex alloy nanowires. However, VCA can still be used for obtaining qualitative behaviour of alloy nanowire devices for various alloy compositions, even at smaller widths.



Transition metal compounds, Pseudopotential method, Capacitors, Nanowires