Microcrystalline silicon thin film deposition from silicon tetrafluoride: Isolating role of ion energy using tailored voltage waveform plasmas

The energy of ions during plasma enhanced chemical vapor deposition (PECVD) is known to impact material quality, but isolating this effect from other process parameters (plasma density, pressure) in a capacitively coupled plasma (CCP) is not straightforward. In this work, we utilize a novel radio-frequency (RF) excitation technique - tailored voltage waveforms (TVW) - as a solution to achieve ion flux-energy decoupling through the electrical asymmetry effect. This makes it possible to independently study the impact of ion energy on material deposition. We study the impact of ion energy - more precisely the maximum ion bombardment energy (IBEmax) before collisions - on the PECVD of hydrogenated microcrystalline silicon (mu c-Si:H) thin films from an SiF4/H-2/Ar chemistry. Through structural and electronic analysis, we find that the variation of IBEmax directly translates into material quality, even at relatively high process pressure. Better material properties (crystalline grain features, material density and photoelectronic response) are obtained for films deposited with moderate values of IBEmax around 45-55 eV. Above this range, a deterioration in material quality is observed, presumably due to more effective bulk atomic displacements induced by the silicon-containing ions. These results are consistent with the performance of single-junction mu c-Si:H solar cell devices using these materials as active layers. Optimum performance is obtained for devices with an absorber layer deposited using a plasma excitation resulting in IBEmax in the range of similar to 45-55 eV. The variation in device performance is mainly due to changes in the open circuit voltage.