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.
References
[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