Laser doping for ohmic contacts in n-type Ge

TL;DR

This paper demonstrates that Gas Immersion Laser Doping can create highly doped Ge layers, enabling low-resistance ohmic contacts at cryogenic temperatures, with doping level being the key factor for contact performance.

Contribution

It introduces a laser doping method to achieve ultra-high doping levels in Ge, significantly reducing contact resistance for low-temperature applications.

Findings

High doping levels above solubility limit improve contact quality.

Doped layer thickness has minimal effect on contact resistance.

At high doping, contact resistance dominates total resistance.

Abstract

We achieved ohmic contacts down to 5 K on standard n-doped Ge samples by creating a strongly doped thin Ge layer between the metallic contacts and the Ge substrate. Thanks to the laser doping technique used, Gas Immersion Laser Doping, we could attain extremely large doping levels above the solubility limit, and thus reduce the metal/doped Ge contact resistance. We tested independently the influence of the doping concentration and doped layer thickness, and showed that the ohmic contact improves when increasing the doping level and is not affected when changing the doped thickness. Furthermore, we characterised the doped Ge/Ge contact, showing that at high doping its contact resistance is the dominant contribution to the total contact resistance.