Energy Distribution of Interface Traps in High-K Gated MOSFETs

J.-P. Han, E. M. Vogel, *E.P. Gusev, *C. D'Emic, C.A. Richter, D. W. Heh, J. S. Suehle

National Institute of Standards and Technology, Semiconductor Electronic Division,
Gaithersburg, MD 20899, USA
*IBM Semiconductor Research and Development Center, T.J. Watson Research Center,
Yorktown Heights, NY 10598, USA

It's well known that the degraded carrier mobility has been one of the major challenges that have held back the implementation of high K gate dielectrics to replace SiO2 for the scaled CMOS technology. The causes of the degradation are not clearly understood. It has also been commonly observed for high-k gated MOSFET's, the n-channel carrier mobility tends to be much more severely degraded than it's p-channel counterpart[1]. One possible cause for this is that Dit is larger in the upper half of the bandgap than that in the lower half, because the former affects the n-channel MOSFET's while the latter affects the p-channel MOSFET's. The purpose of this work is to verify this hypothesis experimentally.
Charge pumping has been demonstrated as a powerful tool to characterize the interface trap density with high accuracy and sensitivity for small MOSFET's [2]. Recently, it has been used to study mean capture cross-sections of high-K gated MOSFET's [3]. However, the energy distribution of interface trap density in high-k gated MOSFET's has not been reported. In this paper, we use variable rise/fall-time charge pumping (CP) to determine the energy distribution of interface trap density (Dit) and capture cross-section of electrons/holes in high-k HfO2 gated nMOSFETs. Our results have revealed that the Dit is much higher in the upper half of the bandgap than that in the lower half of the bandgap. These results are consistent with the observation that n-channel mobilities are more severely degraded than p-channel mobilities when compared to conventional MOSFET's with SiO2 as the gate dielectric. The results were verified by capacitance-voltage (C-V) and ac conductance techniques. We conclude that the gross asymmetry of the Dit distribution of high K gated MOSFETs is at least partially responsible for the much more severe degradation of nMOSFET mobility than it's pMOSFET counterpart.

[1] B Guillaumot et al, IEEE-IEDM Tech. Dig. 355, 2002
[2] G. Groeseneken et al. IEEE Trans. Electr. Dev. 31, 42, 1984
[3] G-W. Lee et al. Appl. Phys. Lett. 81(11), 2050, 2002