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

Over the barrier, hot electron transport across 8 nm thick amorphous ${\mathrm{Al}}_{\mathrm{2}}{\mathrm{O}}_3$ 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 $V_{\mathrm{th}},$ 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 ${\mathrm{W–Al}}_{\mathrm{2}}{\mathrm{O}}_3$ interface ${\Phi }_B\text{=3.90±0.03\hspace{0.05em}}\mathrm{eV}$ and the dynamic dielectric constant ${\epsilon }_{\mathrm{if}}\text{=1.86±0.1,}$ which reflects the strong image force lowering of the barrier observed at both interfaces. A band offset between the ${\mathrm{Al}}_{\mathrm{2}}{\mathrm{O}}_3$ 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 ${10}^{12} {\mathrm{cm}}^{\text{−2}}$ range were deduced to lie in the oxide near the ${\mathrm{Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}\mathrm{–Si}$ 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 ${\mathrm{Al}}_{\mathrm{2}}{\mathrm{O}}_3$ was readily stressed to failure upon injecting sub-nano-Coulomb of charge at electron kinetic energies in the 4–6 eV range.