Atomic scale characterization of graphene p-n junctions for electron-optical applications

TL;DR

This study uses atomic-scale microscopy to analyze graphene p-n junctions, revealing non-idealities affecting electron-optical applications and providing benchmarks for future device improvements.

Contribution

The paper provides detailed atomic-scale characterization of graphene p-n junctions, linking geometric and doping profiles to electron-optical performance, and suggests methods to improve junction quality.

Findings

Natural graphite gates improve junction geometry.

Lateral interface roughness limits Veselago focusing.

Doping profile non-linearity hampers carrier collimation.

Abstract

Graphene p-n junctions offer a potentially powerful approach towards controlling electron trajectories via collimation and focusing in ballistic solid-state devices. The ability of p-n junctions to control electron trajectories depends crucially on the doping profile and roughness of the junction. Here, we use four-probe scanning tunneling microscopy and spectroscopy (STM/STS) to characterize two state-of-the-art graphene p-n junction geometries at the atomic scale, one with CMOS polySi gates and another with naturally cleaved graphite gates. Using spectroscopic imaging, we characterize the local doping profile across and along the p-n junctions. We find that realistic junctions exhibit non-ideality both in their geometry as well as in the doping profile across the junction. We show that the geometry of the junction can be improved by using the cleaved edge of van der Waals metals such as graphite to define the junction. We quantify the geometric roughness and doping profiles of junctions experimentally and use these parameters in Nonequilibrium Green's Function based simulations of focusing and collimation in these realistic junctions. We find that for realizing Veselago focusing, it is crucial to minimize lateral interface roughness which only natural graphite gates achieve, and to reduce junction width, in which both devices under investigation underperform. We also find that carrier collimation is currently limited by the non-linearity of the doping profile across the junction. Our work provides benchmarks of the current graphene p-n junction quality and provides guidance for future improvements.