Hot carrier transport effects in Al2O3-based metal-oxide-semiconductor structures

Abstract

Over the barrier, hot electron transport across 8 nm thick amorphous Al2O3 layers embedded in metal-oxide-semiconductor (MOS) structures was investigated with ballistic electron emission microscopy (BEEM). The oxide field dependence of the BEEM threshold voltage Vth, which corresponds to the potential maximum of the barrier, was found to be dominated by image force and charge trapping/detrapping effects. The static barrier height at the W–Al2O3 interface ΦB=3.90±0.03 eV and the dynamic dielectric constant εif=1.86±0.1, which reflects the strong image force lowering of the barrier observed at both interfaces. A band offset between the Al2O3 and Si conduction bands of 2.78±0.06 eV was deduced. Electron trap levels at energies overlapping the Si band gap and of densities in the 1012 cm−2 range were deduced to lie in the oxide near the Al2O3–Si interface. Their occupancy is determined by the position of the interface Fermi level. For p-type substrates the traps were empty (filled) for positive (negative) applied bias. Local, electrical stressing increased the interface trap charge for n-type substrates, but had negligible consequences on p-type substrates. The Al2O3 was readily stressed to failure upon injecting sub-nano-Coulomb of charge at electron kinetic energies in the 4–6 eV range.