Analysis of Abnormal Upturns in Capacitance–Voltage Characteristics for MOS Devices With High-$k$ Dielectrics

In this letter, we analyze the nonsaturating upturns of capacitance under strong accumulation bias in MOS capacitors with high-k dielectrics. By comparing the electrical properties of dielectric samples with and without HfO₂ and by varying the ambient temperature, it is found that the conduction through the shallow trap levels in the HfO₂ bulk produces not only a steady-state current but also a dynamic current, which, in turn, causes the upturn in capacitance. The addition of RC shunts to the conventional small-signal model is proposed to consider the dynamic leakage effect. The model's effectiveness is verified by fitting the measured impedance spectrum and the measured capacitance. We suggest that measuring at a high frequency of hundreds of megahertz eliminates the dynamic interaction by shallow trap levels, allowing gate capacitance to be successfully reconstructed.

I. Introduction

Hafnium-Based high- dielectrics, such as hafnium oxide (HfO₂) and hafnium silicates (HfSi_xO_y), have been widely studied as alternative gate dielectrics in MOSFET technology since conventional silicon dioxide (SiO₂) fails to meet leakage target with equivalent oxide thickness (EOT) scaling, owing to significant direct tunneling [1]. Using high- dielectrics increases the physical thickness effectively while maintaining the gate capacitance. Nevertheless, relatively small bandgaps and many shallow trap levels in the high- materials cause other conduction mechanisms such as trap-assisted tunneling (TAT) [2], Poole–Frenkel (PF) conduction mechanism [3], and tunneling-assisted PF emission [4]. These mechanisms may affect device performance and characterization methods.