Nanocrystalline silicon TFT process using silane diluted in argon–hydrogen mixtures

We have investigated the effect of Ar dilution on the deposition process of intrinsic nc-Si:H (hydrogenated nanocrystalline silicon) thin films used as active layers of top-gate TFTs, in order to improve the TFTs performances. The nc-Si:H films were deposited by plasma enhanced chemical vapor deposition (PECVD) at low temperature (165 °C) and the related TFTs were fabricated with a maximum process temperature of 200 °C. During the nc-Si:H films deposition, the SiH4 fraction and the total flow of the diluting gases Ar + H2 mixture was kept constant, H2 being substituted by Ar. We have pointed out the active role played by the metastable states of excited Ar atoms in both the dissociation of SiH4 and H2 by quenching reactions in the plasma. The role of the atomic hydrogen during the film deposition seems to be promoted by the addition of argon into the discharge, leading to an increase of the deposition rate by a factor of about three and an enhancement of the crystalline quality of the nc-Si:H films. This effect is maximized when the Ar fraction in the Ar + H2 gases mixture reaches 50%. The evolution with Ar addition of the carriers mobility of the related TFTs is closely connected to the evolution of the crystalline fraction of the intrinsic nc-Si:H film. Mobilities values as high as 8 cm2 V−1 s−1 are obtained at the Ar fraction of 50%. For higher Ar fractions, the fall of the mobility comes with a degradation of the ID–VG transfer characteristics of the processed TFTs due to a degradation of the nc-Si:H films quality. OES measurements show that the evolution of the Hα intensity is closely connected to the evolution of the deposition rate, intrinsic films crystalline fraction and TFTs mobility, providing an interesting tool to monitor the TFTs performances.