Germanium Possessing Facet-Specific Trap States and Carrier Lifetimes

Previously, the notable differences in the band structure and changes in bond length and bond distortion between the semiconducting and metal-like planes of germanium have been used to understand the facet-dependent electrical conductivity properties of germanium wafers. To gain further insights into the appearance of electrical facet behaviors, impedance measurements were performed on the Ge{111}, {110}, and {100} wafers. Impedance data and several conductivity-related parameters were used to produce a diagram showing the amount of trap states and the trap state energies. The trap states are found within the germanium band gap with a facet-specific distribution of energies. Compared to the {100} and {110} wafers, the Ge{111} wafer has the lowest trap state density in the probed voltage range. This is consistent with its best conductivity property, as trap states hinder the direct excitation of electrons to the conduction band. Carrier lifetime can also be obtained from the impedance data. The {111} surface generally has the shortest carrier lifetime, which is related to its high electrical conductivity. Interestingly, diffuse reflectance and ultraviolet photoelectron spectral (UPS) measurements yield the smallest Schottky barrier between Ag and the most conductive Ge{111} surface, showing this approach to understanding electrical facet effects can still be useful despite its inadequacy to account for the facet-specific conductivity behaviors of Si wafers. However, one should not rely solely on the experimentally determined Schottky barriers to explain electrical facet effects.