Dependence of the work-function difference between the polysilicon gate and silicon substrate on the doping level in polysilicon

We correlate the work-function difference φ
ps
⁰
between the polysilicon gate and the silicon substrate in an MOS system with the doping level and carrier concentration in polysilicon. Polysilicon was doped by ion implantation with arsenic and phosphorus. The doping level was varied from 10
¹⁹
to 10
²⁰
cm
^-3
. Hall measurements were used to determine the carrier concentration in polysilicon at a given doping level. The Hall mobility and resistivity as a function of doping level were also obtained. The work function difference φ
ps
⁰
was determined by capacitance-voltage measurements on polysilicon-SiO
2
-Si capacitors with different oxide thicknesses. When plotted against the doping level, the work-function difference had a maximum at a dopant concentration of ≈ 5 × 10
¹⁹
cm
^-3
, which corresponds to an electron concentration of 1.5 × 10
¹⁹
cm
^-3
. At higher doping levels the value of φ
ps
⁰
decreases. The results can not be fully understood in terms of the Si band structure.