  We present transport measurements of silicon MOS split gate structures with
and without Sb implants. We observe classical point contact (PC) behavior that
is free of any pronounced unintentional resonances at liquid He temperatures.
The implanted device has resonances superposed on the point contact transport
indicative of transport through the Sb donors. We fit the differential
conductance to a rectangular tunnel barrier model with a linear barrier height
dependence on source-drain voltage and non-linear dependence on gate bias.
Effects such as Fowler-Nordheim (FN) tunneling and image charge barrier
lowering (ICBL) are considered. Barrier heights and widths are estimated for
the entire range of relevant biases. The barrier heights at the locations of
some of the resonances for the implanted tunnel barrier are between 15-20 meV,
which are consistent with transport through shallow partially hybridized Sb
donors. The dependence of width and barrier height on gate voltage is found to
be linear over a wide range of gate bias in the split gate geometry but
deviates considerably when the barrier becomes large and is not described
completely by standard 1D models such as FN or ICBL effects.
