Thermal Annealing of Light-Induced K Centers in Hydrogenated Amorphous Silicon Nitride
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THERMAL ANNEALING OF LIGHT-INDUCED K' CENTERS IN HYDROGENATED AMORPHOUS SILICON NITRIDE
E. D. TOBER', E. SIGARI", J. KANICKI'", and M. S. CROWDER"
"Department of Physics, University of California, Davis, CA 95616 "*IBM,Storage Systems Products Division, 5600 Cottle Rd., San lose, CA 95193 "-IBM, Research Division, T. J. Watson Research Center, Yorktown Heights, NY 10598 Abstract
The thermally-induced decay of light-induced, paramagnetic neutral silicon dangling bonds (K0 centers ) in hydrogenated amorphous silicon nitride thin films is monitored using electron spin resonance. The nitride films are of gate-quality and nitrogen-rich and are deposited at two different temperatures (250 and 400 °C). The kinetics for isothermal annealing of the light-induced K° states is dependent upon sample deposition temperature and is observed to follow a stretched exponential dependence, exp
{-
(t/-t)P), upon annealing time (t).
The stretched ex-
ponential factor, Pi, shows a non-linear dependence upon annealing temperature including temperature independent regimes. Thermal annealing is thermally activated with an apparent activation energy of -0.4 eV and is independent of deposition temperature. These results indicate that annealing is a dispersive process which involves hopping and multiple trapping or trap controlled hopping in the thermal annealing of light induced K° centers in amorphous SiN 1 .6:H. Introduction
Hydrogenated amorphous silicon nitride (a-SiN,:H) is commonly used as a gate dielectric in a-Si:H based devices. Many of these devices (such as TFT's in LCD displays [1-3]) operate while exposed to ultra-violet light. Exposure to ultraviolet light has been found to induce paramagnetic defects in the bulk of the nitride [4-8]. These defects have been identified as being singly occupied (neutral) silicon dangling bonds (Ko centers) [9]. The light-induced K0 centers are metastable and act as trapping centers for electrons and holes. The trapping of charge in the nitride has the potential to adversely affect the device performance. Krick et. al. [10] found that such centers could be partially removed by annealing at elevated temperatures. We have examined the isothermal annealing kinetics of the light-induced electron spin resonanace signal (LESR) as a function of annealing time. The time dependence for the decay of KO centers has been observed to follow a stretched exponential dependence on annealing time (t), exp{ - (t/r)#), with a themally activated r [II]. This behavior is found to depend upon the deposition temperature of the nitride films.
Mat. Res. Soc. Symp. Proc. Vol. 219. @1991 Materials Research Society
130
Experimental
The hydrogenated, amorphous silicon nitride thin films were produced by plasma-enhanced chemical vapor deposition onto fused quartz substrates held at 250 and 400°C. The resulting nitride is of gate-quality and is nitrogen rich with a N to Si ratio of 1.6. The films deposited at 250 and 400'C arc 1225 nm thick and contain 38 and 31% at. hydrogen, respectively. The samples were cut in
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