[Paper Review] Enhanced Electron Transport in Thin Copper Films via Atomic-Layer Materials Capping

David Guzman, Alejandro Strachan|arXiv (Cornell University)|May 3, 2018

Surface and Thin Film Phenomena3 citations

TL;DR

This study uses first-principles DFT and non-equilibrium Green’s functions to show that capping thin Cu(111) films with graphene or MoS₂ reduces resistance per unit length by 20% and 13%, respectively, due to suppressed surface scattering on atomically roughed surfaces. In contrast, stanene and h-BN capping increase resistance, while surface oxidation reduces conductance by a factor of 10.

ABSTRACT

Using first-principles calculations based on density functional theory and non-equilibrium Green's functions, we characterized the effect of surface termination on the electronic transport properties of nanoscale Cu slabs. With ideal, clean (111) surfaces and oxidized ones as baselines we explore the effect of capping the slabs with graphene, hexagonal boron nitrate, molybdenum disulfide and stanene. Surface oxide suppresses balistic conductance by a factor of 10 compared to the ideal surface. Capping the ideal copper surface with graphene slightly increase conductance but MoS$_2$ and stanene have the opposite effect due to stronger interactions at the interface. Interestingly, we find that capping atomistically roughed copper surfaces with graphene or MoS$_2$ decreases the resistance per unit length by 20 and 13%, respectively, due to reduced scattering. The results presented in this work suggest that two-dimensional materials can be used as an ultra-thin liner in metallic interconnect technology without increasing the interconnect line resistivity significantly.

Motivation & Objective

• To investigate how two-dimensional capping materials affect electron transport in nanoscale copper films.
• To determine whether 2D materials can serve as ultra-thin, effective diffusion barriers without increasing resistivity in Cu interconnects.
• To quantify the impact of surface roughness and oxidation on electron scattering and conductance in Cu-based hybrid systems.
• To evaluate the role of interfacial interactions and Fermi level alignment in modulating transport properties.

Proposed method

• First-principles density functional theory (DFT) calculations were used to model the electronic structure of Cu(111) slabs capped with graphene, h-BN, MoS₂, and stanene.
• Non-equilibrium Green’s functions (NEGF) were employed to compute electron transport properties, including conductance and resistance as a function of length.
• The conductance was analyzed via transmission spectra and local bond current analysis to identify scattering mechanisms at interfaces.
• The one-dimensional resistivity (ρ₁D) and effective mean free path (λₛ𝒹) were extracted from linear fits of resistance vs. length data.
• Surface defects were modeled using 25% vacancy concentrations to simulate realistic, atomically roughed copper surfaces.
• Oxidized and ideal (111) Cu surfaces were used as baseline references to compare capping effects.

Experimental results

Research questions

• RQ1 How does capping a clean Cu(111) surface with graphene, h-BN, MoS₂, or stanene affect its electronic conductance?
• RQ2 To what extent does surface oxidation reduce electron transport in thin Cu films compared to ideal surfaces?
• RQ3 How do interfacial interactions influence electron scattering and resistivity in 2D-material-capped Cu systems?
• RQ4 Can graphene or MoS₂ capping reduce the resistance per unit length in defective (rough) Cu films compared to bare films?
• RQ5 What is the effective electron mean free path due to surface defects in bare and hybrid Cu systems?

Key findings

• Surface oxidation reduces conductance by a factor of 10 compared to ideal, clean Cu(111) surfaces.
• Capping ideal Cu(111) surfaces with graphene increases conductance slightly due to a 0.75 eV upward shift in the Fermi level relative to graphene’s Dirac point.
• Capping with MoS₂ and stanene reduces conductance by ~30% due to strong interfacial interactions increasing surface scattering.
• For defective Cu surfaces with 25% vacancies, graphene capping reduces resistance per unit length by 20% compared to bare Cu.
• MoS₂ capping reduces resistance per unit length by 13% on defective Cu, while stanene increases it by ~35%.
• The effective electron mean free path due to surface defects increases from 10 nm in bare Cu to higher values in Cu/Gr and Cu/MoS₂ hybrids, indicating reduced scattering.