Frequency- and doping-level influence on electric and dielectric properties of PolySi/SiO2/cSi (MOS) structures

Abstract

The electric and dielectric characteristics of PolySi/SiO2/cSi (MOS) structure, such as series resistance (Rs), dielectric constants (ɛ′) and (ɛ″), dielectric losses (tan δ), and the ac electric conductivity (σac), were studied in the frequency range 100 kHz–1 MHz for various doping levels and two thicknesses for the polysilicon layer (100 and 175 nm). The experimental results show that the C and G/ω characteristics are very sensitive to the frequency due to the presence of interface states. Series resistance Rs is deduced from C and G/ω measurements and is plotted as a function of the frequency for various doping levels. It is found to decrease with frequency and doping level. To determine $$~{\varepsilon ^\prime }$$ , e″, tan δ, and $${\sigma _{{\text{ac}}}}$$ , the admittance technique was used. An interesting behavior of the constants, $$~{\varepsilon ^\prime }$$ and e″, was noticed. The $$~{\varepsilon ^\prime }$$ values fit led to relations between $$~{\varepsilon ^\prime }$$ and the frequency, on one hand, and between $$~{\varepsilon ^\prime }$$ and the electric conductivity of the polysilicon layers on the other. These relations make it possible to interpolate directly between two experimental points for a given frequency. The analysis of the results shows that the values of $$~{\varepsilon ^\prime }$$ , e″, and tan δ decrease with increasing frequency. This is due to the fact that in the region of low frequencies, interfacial polarization occurs easily, and the interface states between Si and SiO2 contribute to the improvement of the dielectric properties of the PolySi/SiO2/cSi structures. The study also emphasizes that the ac electric conductivity increases with the increase in frequency and doping level; this causes to the reduction in series resistance.