Electron transport mechanisms in thin boron-doped diamond films

Abstract

Electron transmission spectroscopy is used to examine the effect of transport distance, diamond nanostructure, and electron affinity on the cold emission characteristics of thin nanocrystalline diamond films. Energy distribution and intensity measurements are taken from films having different thicknesses (∼0.15, 2, and 4 μm) and surface properties (hydrogenated, cesiated), and two distinct transmission regimes are identified that exhibit fundamentally different characteristics. In measurements taken at sufficiently high beam energy Eo, electrons are transported through the conduction band of the diamond and emitted at a low-affinity surface, with transmission yields generally greater than 1. In this regime, the dependence on Eo results from the finite escape depth of the conduction-band electrons, which is determined to be ∼1 μm for these films based on a Monte Carlo analysis of the incident electron range. In measurements taken at lower values of Eo, electrons are generated outside of this escape range and are unable to reach the surface via conduction-band transport. In this regime, the transmission data are dominated by a much broader, low-intensity distribution, and the transmission yields are substantially lower than 1. The transmission is furthermore completely insensitive to changes in the surface properties of the diamond. Based on the nanostructure of the films, electrons are most likely transported along grain boundaries that propagate through the films.