Hysteresis in volume fraction of clusters incorporated into a-Si:H films deposited by SiH₄ plasma chemical vapor deposition

Suppressing cluster incorporation into hydrogenated amorphous silicon films deposited by SiH₄ discharge plasma is the key to better film stability, because cluster incorporation contributes significantly to the formation of SiH₂ bonds which are closely related to light-induced degradation of the films. Here, we report hysteresis in the deposition rate and the volume fraction of clusters incorporated into a-Si:H films deposited by SiH₄ plasma CVD; the deposition rate and the volume fraction show looping behaviors when sequentially changing the discharge power, because clusters formed in the previous condition tend to remain in plasma and affect significantly the deposition rate and the volume fraction. The hysteresis is also shown as a function of SiH* emission intensity, being proportional to the radical generation rate due to electron impact dissociation of SiH₄. By utilizing the hysteresis phenomenon, the volume fraction of clusters in films can be reduced significantly. We propose a model of plasma containing clusters, in which clusters play a key role in the hysteresis, namely the nonlinear behavior of the deposition rate and the volume fraction of clusters in films. Eventually, we deposited a-Si:H films of low cluster incorporation at a high deposition rate of 0.09 nm/s utilizing the hysteresis phenomena.