Effects of multi-level Fermi pinning and aggregate surface area on Schottky barrier heights at metal/semiconductor interfaces

A theoretical analysis is presented for the combination of stoichiometry changes within a metal contact to the effective Schottky barrier height of a metal/semiconductor interface. Results are shown for a metal/n-CdTe interface, and the stoichiometry changes within the metal contact due to Cd-richness and Te-richness are considered. Current-transport, X-ray photoelectron spectroscopy and photoluminescence studies have shown that Te-rich surfaces exhibit barrier heights of 0.72 +/- 0.02 eV and Cd-rich surfaces exhibit barrier heights of 0.93 +/- 0.02 eV. Here, it is shown that for Te-rich coverage of above 40% area of the contact the effective barrier height lies within the experimental error of the measured value 0.72 +/- 0.02 eV. For Te-rich coverage of less than 40% area and greater than 1% of the contact, the effective barrier height may take a range of values between 0.74 eV to 0.83 eV. However, when the Te-rich coverage becomes less than 1% of the contact, the effective barrier height changes drastically from 0.83 eV to 0.93 eV. This aggregate area effect on the barrier height gives strong support to previous experimental observations on Sb and Au contacts on n-CdTe surfaces. A majority of the diodes fabricated on bromine methanol etched surfaces yielded the lower barriers of similar to 0.72 eV, and a few diodes yielded higher barriers of similar to 0.93 eV, although the preparation procedures were identical for all diodes.