Hydrogen dilution effect on the properties of coplanar amorphous silicon thin-film transistors fabricated by inductively-coupled plasma CVD

Abstract

The electrical and optical properties of the hydrogenated amorphous silicon (a-Si:H) films deposited by inductively-coupled plasma (ICP) chemical vapor deposition (CVD) with a variation of H/sub 2/ flow rate have been studied. The photosensitivity of a-Si:H is /spl sim/10/sup 7/ when the H/sub 2//SiH/sub 4/ ratio is between 3 and 8. With increasing H/sub 2//SiH/sub 4/, the SiH/sub 2/ mode infrared absorption has a minimum at a H/sub 2//SiH/sub 4/ ratio of 8. Coplanar a-Si:H thin-film transistors (TFT's) were fabricated using a triple layer of thin a-Si:H, silicon-nitride, and a-Si:H deposited by ICP-CVD using ion doping and low resistivity Ni silicide. After patterning the thin a-Si:H/silicon-nitride layers on the channel region, the gate and source/drain regions were ion-doped and then heated at 230/spl deg/C to form Ni silicide layers. The low resistive Ni silicide formed on the a-Si:H reduces the offset length between gate and source/drain, leads to a coplanar a-Si:H TFT. The TFT exhibited a field effect mobility of 0.6 cm/sup 2//Vs and a threshold voltage of 2.3 V at the H/sub 2//SiH/sub 4/ ratio of 8. The effect of H/sub 2/ dilution in SiH/sub 4/ on the coplanar a-Si:H TFT performance has been investigated. We found that the performance of the TFT is the best when the SiH/sub 2/ mode density in a-Si:H is the minimum. The coplanar TFT is very suitable for large-area, high density TFT displays because of its low parasitic capacitance between gate and source/drain contacts.