Surface electronic properties of diamond

TL;DR

This paper reviews the surface electronic properties of hydrogen-terminated diamond, highlighting how a conductive layer forms via transfer doping, enabling applications like ion-sensitive transistors with pH-dependent conductivity.

Contribution

It provides a comprehensive analysis of the electronic surface states of diamond and demonstrates the fabrication of pH-sensitive ISFET devices based on this understanding.

Findings

Conductive surface layer forms via transfer doping.

Holes in the layer have mobilities up to 350 cm2/Vs.

Surface pH influences hole density and device conductivity.

Abstract

Surface electronic properties of undoped hydrogen terminated diamond covered with adsorbates or in electrolyte solutions are summarized. The formation of a conductive layer at the surface of diamond is discussed based on Hall effect, conductivity, contact potential difference (CPM), scanning electron microscopy (SEM), and cyclic voltammetry data applied on homoepitaxially grown CVD diamond films with atomically smooth hydrogen terminated surfaces. Due to electron transfer from valence band states into empty states of the electrolyte (transfer doping), a highly conductive surface layer is generated. Holes propagate in the layer with mobilities up to 350 cm2/Vs. The sheet hole density is in the range 10^11 to 5x10^12 cm^-2, and dependents on pH of the electrolyte. Numerical solutions of the Schrodinger and Poisson equations reveal a 2D density of state (DOS) distribution. This has been utilized to manufacture ion-sensitive field effect transistors (ISFET). The drain source conductivity is pH dependent, with about 66 mV/pH. Application of potentials larger than the oxidation threshold of +0.7 V (pH 13) to +1.6 V (pH 1) gives rise to strong leakage currents and to partial surface oxidation.