Improvement of Field-Effect Mobilities in TFTs: Surface Plasma Treatments Vs Stack Dielectric Structures

Abstract

Thin Film Transistors were deposited with different insulator/semiconductor interfaces using silicon nitride (a-SiNX) as bulk dielectric, to determine what interface modification technique (plasma treatment or interface layer deposition) is the most suitable for improving their electrical characteristics and stability. One of the samples has only a-SiNX and was used as control sample. Three samples were treated with H2, N2 and O2 plasmas prior to the amorphous silicon deposition. Another two samples were prepared using other materials such as silicon carbide (a-SiCX) or silicon oxide (a-SiOX) as interface layers, building a-SiNX/a-SiCX or a-SiNX/a-SiOX stacks. Results show that oxidizing plasma treatment increases drastically the threshold voltage (VTh) from 6.23V to about 11.2V. N2 and H2 plasma treatments are effective in lowering the leakage OFF current, but only H2 plasma leads to a decrease of VTh (from 6.23V to 4.98V). N2 plasma treated TFTs present an increase of VTh (from 6.23V to 6.91V) and are less stable to voltage bias stress than H2 plasma treated TFTs. In all samples submitted to interface plasma, an improvement of ∝FE was observed, except in the case of O2 plasma. Devices made with an intermediate a-SiOX layer present lower Vth and ∝FE. The best improvement was achieved in samples with a-SiNX/a-SiCX stack layers, achieving ∝FE = 0.56 cm 2 /V.s, which represents an increase of ~ 50% over conventional TFT.