Study of Electronic Properties of III-Nitrides and Carbon Nanotubes by Electron Energy Distribution Analysis

Abstract

The energy distribution of electrons transported through intrinsic AlN films was directly measured as a function of the applied field and film thickness. The electron energy distribution featured kinetic energies higher than that of completely thermalized electrons. Transport through films thicker than 95 nm and applied field between 200 kV/cm - 350 kV/cm occurred as steady-state hot electron transport represented by a Fermi-Dirac/ Maxwellian energy distribution. At higher fields (470 kV/cm), intervalley scattering was evidenced by a second peak corresponding to the first satellite valley in AlN. Transport through 80 nm thick layers revealed the onset of quasi-ballistic transport. From these measurements, saturation velocities between 1.2 and 1.5x10 cm/s and a mean free path of 5.1 nm were determined under steady state conditions. Overshoots as high as five times the saturation velocity were observed and a transient length of less than 80 nm was deduced. Two field-emission states of single-walled carbon nanotubes were identified. The state yielding 10 times increased emission current was attributed to the presence of adsorbates on the nanotubes as confirmed by electron emission measurements at different background pressures. In the high current state, field-emitted electrons originated from states located up to 1 eV below the Fermi level, as determined by field-emission energy distribution measurements. This suggested that adsorbates introduced a resonant state on the surface which enhanced the tunneling probability of electrons. The adsorbed states were removed at high applied electric fields, presumably due to ohmic heating caused by large emission currents. This adsorption/desorption process was completely reversible. Using the Duke and Alferieff model, and a one-dimensional Fowler-Nordheim scheme, we demonstrated that adsorption enhances the field emission from single-walled carbon nanotubes through elastic resonance tunneling. As anticipated from this model, we observed FEED peak shifts towards lower energies and a symmetric peak shape in the energy distribution. The difference between the work function and the electronic binding energy of the non-perturbed state involved in the resonance was 0.3 eV ± 0.2 eV, thus the state lied close to the Fermi level of the carbon nanotubes.