Towards the insulator-to-metal transition at the surface of ion-gated nanocrystalline diamond films

TL;DR

This study explores the combined effects of boron doping and ionic gating on nanocrystalline diamond films, revealing challenges in achieving a true insulator-to-metal transition due to surface disorder and scattering.

Contribution

It introduces a combined doping and gating approach to investigate the insulator-to-metal transition at diamond surfaces, highlighting limitations caused by surface disorder.

Findings

Boron doping enhances maximum hole density achievable electrostatically.

Ionic gating shifts conductivity towards quantum critical regime.

Surface disorder and scattering prevent full metallic transition.

Abstract

Hole doping can control the conductivity of diamond either through boron substitution, or carrier accumulation in a field-effect transistor. In this work, we combine the two methods to investigate the insulator-to-metal transition at the surface of nanocrystalline diamond films. The finite boron doping strongly increases the maximum hole density which can be induced electrostatically with respect to intrinsic diamond. The ionic gate pushes the conductivity of the film surface away from the variable-range hopping regime and into the quantum critical regime. However, the combination of the strong intrinsic surface disorder due to a non-negligible surface roughness, and the introduction of extra scattering centers by the ionic gate, prevents the surface accumulation layer to reach the metallic regime.