The effect of an a-SiNx:H gate insulator on the threshold voltage and field-effect mobility in amorphous silicon thin-film transistors has been investigated by using different compositions of a-SiNx:H, and the threshold-voltage shift is found to be reduced at an a-SiNx:H composition of x=1.0. Photoluminescence measurements reveal that the threshold-voltage shift for x<1.0 is due to electron trapping in the localized states of the a-SiNx:H layer, while that for x>1.0 is due to electron trapping in the deep states near the a-Si:H/a-SiNx:H interface, arising from lattice strain. Field-effect mobility decreases as x increases and is due to the increased number of deep states in the a-Si:H near the a-Si:H/a-SiNx:H interface.
We examined the effects of the deposition sequence on the threshold voltage and field-effect mobility in amorphous silicon thin-film transistors. When the nitrides are deposited after the a-Si:H(normal), the threshold voltages become more positive, and the field-effect mobilities are lower than with the inverted sequence. Photoluminescence measurements reveal that the tail states in the a-Si:H near the a-Si:H/a-SiN
x
interface for the normal sequence are wider than for the inverted sequence.
A double layer of plasma chemical vapor deposition SiO
x
N
y
and SiN
x
was applied to the gate insulator of an amorphous silicon (a-Si) thin-film transistor (TFT). When a thin SiN
x
layer is inserted between the a-Si of an a-Si TFT and the SiO
x
N
y
gate insulator, the density of trapped charges is found to decrease less than for a gate insulator with only SiN
x
, when the thickness of SiN
x
is decreased. No deterioration was observed in the switching characteristics of an a-Si TFT with a SiO
x
N
y
/SiN
x
gate insulator, compared with a-Si TFTs with SiN
x
gate insulators commonly used in liquid crystal displays. The density of trapped charges is related to the threshold voltage shift.
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