Deposition of a-Si:H thin films using tailored voltage waveform plasmas: impact on microstructure and stability

Exciting processing plasmas using non-sinusoidal, “Tailored” voltage waveforms (TVWs), have recently been shown to be effective to separately control the maximum ion bombardment energy (IBE) and the ion flux on each electrode in the capacitively coupled plasma (RF-CCP) processes. In this work, we use it to deposit hydrogenated amorphous silicon (a-Si:H) thin films from hydrogen-diluted silane by low temperature plasma-enhanced chemical vapor deposition. The impact of using TVWs on the material's structural and electronic properties is examined. Excessively low IBE can lead to a high Si-H₂bonded hydrogen content within the deposited films, which results in a deterioration of the material stability upon light-soaking, detectable at a microstructure level. A low content of hydrogen bonded in a Si-H₂configuration and a low sub-gap density of states was observed in the film deposited using a “sawtooth-down” type of waveform. Such excitation also produced the a-Si:H films with the best transport properties (majority and minority carrier μτ-products and the ambipolar diffusion length) and stability under light-soaking.