Novel Properties of a-SI:H Prepared by Dual Ion Beam Sputtering

ABSTRACTFilms of a-Si:H were deposited by dual ion beam sputtering using a new configuration in which both the argon and hydrogen beam sources are directed at the silicon target. This geometry also permits independent control of the hydrogen and argon energy and particle flux. Infrared absorption mealurents show that even for high hydrogen concentrations, the 2000 cm-1 Si-H stretching band is dominant. This result is in contrast with the more conventional configuration in which the H soyrce is directed at the substrate, resulting in films with dominant 2100 cm-1 mode. This suggests that the precursors resulting in H-incorporation are different for the two configurations. In fact, IR reflectance and SIMS analysis of the silicon sputtering target reveal hydrogen is incorporated, peaking at about 30 Å below the target surface. A strong increase in the photo and dark dc conductivity occurs as the hydrogen ion enery is reduced below 30 eV, suggesting the importance of preventing high energy back-scattered H ion bombardment of thS film. At a H ion energy of 8eV, the values are 2x10-5 (AM1) and 2x10-9 (ohm-cm-1), respectively. Spectroscopic ellipsometry measurements of films reveal a Si-Si bond packing greater than that of low Hcontent a-Si prepared by LPCVD even up to H contents as high as 24%. Above 25% a microstructural transition is observed, verified by SEM, resulting in an increase in the density of voids, (which appears to be responsible for a sudden drop in the hydrogen-induced compressive stress) and accompanied by a shift in the dominant stretching mode energy.