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Nanocrystalline Silicon Films Deposited by RF PECVD for Bottom-gate Thin-film Transistors

Published online by Cambridge University Press:  01 February 2011

Mohammad Reza Esmaeili Rad ,
Czang-Ho Lee ,
Andrei Sazonov  and
Arokia Nathan
Mohammad Reza Esmaeili Rad
Affiliation:
[email protected], University of Waterloo, Electrical and Computer Engineering, 196 Westmount Rd. N. , unit 603, Waterloo, ON, N2L 3G5, Canada
Czang-Ho Lee
Affiliation:
[email protected], University of Waterloo, Electrical and Computer Engineering, 200 University Ave., Waterloo, ON, N2L 3G1, Canada
Andrei Sazonov
Affiliation:
[email protected], University of Waterloo, Electrical and Computer Engineering, 200 University Ave., Waterloo, ON, N2L 3G1, Canada
Arokia Nathan
Affiliation:
[email protected], University of Waterloo, Electrical and Computer Engineering, 200 University Ave., Waterloo, ON, N2L 3G1, Canada
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Abstract

Thin-film transistors (TFTs) in active-matrix organic light emitting diode (AMOLED) displays are required to supply high and stable driving current to OLEDs. Top-gate TFTs with nanocrystalline silicon (nc-Si) active layer have shown promise to render high mobility and stable driving current. However, to be compatible with current production facilities, bottom-gate TFTs are demanded. Currently, bottom-gate nc-Si TFTs show insufficient field effect mobility and exhibit driving current instability due to presence of amorphous incubation layer at the interface with gate dielectric. Our research is motivated by the need to eliminate the incubation layer. In order to do so, we studied nc-Si deposition process to find the RF PECVD deposition regimes which lead to minimum incubation layer.

We have deposited a set of undoped nc-Si films by 13.56 MHz PECVD at 250°C by varying RF power, reactor pressure, silane and hydrogen flow rates. Raman spectroscopy, constant-photocurrent method (CPM) and optical absorption have been used to measure film crystallinity, defect density and optical bandgap, respectively. Carrier transport in the films has been studied using dark conductivity, photoconductivity and conductivity activation energy measurements.

Our results reveal that silane and hydrogen flow rates are the most contributing factors to film characteristics. The results also indicate that the reactor pressure does not have a significant effect on the film crystallinity. However, CPM data confirm that to obtain lower defect density, medium or high deposition pressures are preferred. We obtained films with dark conductivity and Raman crystallinity in the order of 10-6-10-7S/cm and 60-80 %, respectively. Furthermore, we have deposited nc-Si films as thin as 20nm with 60% crystallinity, which is crucial for bottom-gate TFTs. Finally, four different sets of bottom-gate TFTs have been fabricated by changing gate dielectric compositions and changing [SiH4]/([SiH4] + [H2]) gas flow ratio. The device performance, relationship to the film structure and deposition process, and future improvements will be discussed in details.

Keywords

devicesplasma-enhanced CVD (PECVD) (deposition)Si
Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 910: Symposium A – Amorphous and Polycrystalline Thin-Film Silicon Science and Technology – 2006 , 2006 , 0910-A22-13
Copyright
Copyright © Materials Research Society 2006

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