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.