This work focuses on the use of near-IR Evanescent-Wave Cavity Ring-Down Spectroscopy (EW-CRDS) to detect sub-gap defect states in hydrogenated amorphous silicon (a-Si:H) thin films. This material is often employed as a model system for thin film research and has applications in microelectronics and photovoltaics. The EW-CRDS technique allows for time-dependent absorption measurements in both s- and p-polarization with a resolution of 33 ms and a minimum detectable optical loss of 10-7 using a silica optical resonator, or 'folded cavity'. The emphasis is placed on the role of hydrogen in the deposition process with respect to defect kinetics. Most research efforts in this field have mainly focused on detecting changes in Si-H bonding configurations from exposure to H or D radicals, not the influence on the defect concentrations. It is established that a folded cavity can be stripped of an a-Si:H thin film without affecting the sensitivity and accuracy of future EW-CRDS experiments. Newly manufactured cavities are shown to contain almost 10 times the specified amount of OH impurity (~ 0.5 ppm OH by weight) due to a manufacturing error. During earlier experiments the defect evolution during growth of the a-Si:H film was monitored, as well as the effect of quantified atomic H fluxes in the range of 0.042 - 0.200 Ls-1 incident on the surface of the film. The defect density is shown to increase during both growth and H dosing cycles and the defects reversibly 'heal' when the radical fluxes are terminated. In this work, these results are thoroughly scrutinized with the aim of resolving inconsistencies found between growth and H dosing data in a previous analysis. These discrepancies originate from a faulty film thickness determination, which is subsequently resolved with the aid of E-field calculations. For new experiments, the use of Real-Time Spectroscopic Ellipsometry (RTSE) or interferometry is recommended to monitor optical constants and thickness of the growing film. A kinetic model for the observed phenomena is proposed, based on direct insertion of atomic H into weak Si-Si bonds. It is found that surface processes contribute negligibly to the optical loss, while H-induced defect formation through insertion progresses into the bulk with a penetration depth of ~ 10 - 20 nm. The kinetic model can account for all observed features in the EW-CRDS H dosing data with the assumption that surface processes can be neglected. It is shown that, in the previous analysis of EW-CRDS growth data, it is wrongly assumed that all defects are created at the surface of the film.
Atomic hydrogen induced defect kinetics in a-Si:H: a review of data obtained by evanescent wave cavity ring-down spectroscopy
Peeters, F. J. J. (Author). 31 Dec 2008
Student thesis: Master