Bottom-up assembly of metallic germanium

TL;DR

This paper presents a bottom-up method to create homogeneous, high-density metallic germanium with precisely controlled 3D doping profiles by stacking 2D phosphorus layers, surpassing traditional doping techniques.

Contribution

It introduces a novel stacking approach for 3D doping in germanium, enabling high-density, low-resistivity metallic layers with atomic precision.

Findings

Successfully achieved 3D homogeneous doping in germanium

Demonstrated high electron densities of 10^19 to 10^20 cm^-3

Validated the uniformity of free electrons via multiple advanced techniques

Abstract

Extending chip performance beyond current limits of miniaturisation requires new materials and functionalities that integrate well with the silicon platform. Germanium fits these requirements and has been proposed as a high-mobility channel material,[1] a light emitting medium in silicon-integrated lasers,[2,3] and a plasmonic conductor for bio-sensing.[4,5] Common to these diverse applications is the need for homogeneous, high electron densities in three-dimensions (3D). Here we use a bottom-up approach to demonstrate the 3D assembly of atomically sharp doping profiles in germanium by a repeated stacking of two-dimensional (2D) high-density phosphorus layers. This produces high-density (10^19 to 10^20 cm-3) low-resistivity (10^-4 Ohmcm) metallic germanium of precisely defined thickness, beyond the capabilities of diffusion-based doping technologies.[6] We demonstrate that free electrons from distinct 2D dopant layers coalesce into a homogeneous 3D conductor using anisotropic quantum interference measurements, atom probe tomography, and density functional theory.