Metal-insulator transition in films of doped semiconductor nanocrystals

TL;DR

This paper theoretically derives the critical doping concentration for metal-insulator transition in ligand-free doped semiconductor nanocrystal films and experimentally investigates conduction mechanisms near this transition.

Contribution

It provides a theoretical expression for the critical concentration in nanocrystal films and experimentally explores conduction near the transition point.

Findings

Critical concentration $n_c$ derived for nanocrystal films.

Experimental conduction mechanism observed near half of $n_c$.

Localization length approaches three times nanocrystal diameter at high doping.

Abstract

To fully deploy the potential of semiconductor nanocrystal films as low-cost electronic materials, a better understanding of the amount of dopants required to make their conductivity metallic is needed. In bulk semiconductors, the critical concentration of electrons at the metal-insulator transition is described by the Mott criterion. Here, we theoretically derive the critical concentration $n_c$ for films of heavily doped nanocrystals devoid of ligands at their surface and in direct contact with each other. In the accompanying experiments, we investigate the conduction mechanism in films of phosphorus-doped, ligand-free silicon nanocrystals. At the largest electron concentration achieved in our samples, which is half the predicted $n_c$, we find that the localization length of hopping electrons is close to three times the nanocrystals diameter, indicating that the film approaches the metal-insulator transition.