Influence of the grain structure on the Fermi level in polycrystalline silicon: A quantum size effect?

Abstract

It has been observed by several authors that metal-oxide-semiconductor devices with polycrystalline Si (poly-Si) gates behave differently depending on the doping species in poly-Si: the work-function difference between the silicon substrate and the gate (φPS) is higher when the gates are doped with arsenic than when they are doped with phosphorus. As a function of the doping devel, this difference becomes first noticeable at ∼1019 cm−3, and then it increases for heavier doped materials, reaching 120 meV near the dopant solubility limit. We believe that the different behavior of φPS can be explained by different grain textures at the poly-Si/SiO2 interface. Our transmission electron microscopy of the films indicates that while P-doped material consists of large (≊3000 Å) grains, As-doped poly-Si preserves its as-deposited columnar structure, even after a high-temperature anneal. Moreover, at the interface with the gate oxide an as-deposited microstructure with very small (≊100 Å) ‘‘embrionic’’ grains is preserved. On the basis of these observations, we suggest a model for the different behavior of φPS. The model is based on a quantum size effect which becomes important for such small grain dimensions at the interface in As-doped poly-Si. This effect drastically reduces the number of states available in the conduction band at low energies and thus forces a more complete filling of the impurity band. The resulting shift of the Fermi level provides a qualitative explanation for the observed puzzling difference between the work functions of As- and P-doped poly-Si.