Resolving the Evolution of Atomic Layer-Deposited Thin-Film Growth by Continuous <i>In Situ</i> X-Ray Absorption Spectroscopy
Xiaohui Qu, Danhua Yan, Ruoshui Li, Jiajie Cen, Chenyu Zhou, Wenrui Zhang, Deyu Lu, Klaus Attenkofer, Darı́o Stacchiola, Mark S. Hybertsen, Eli Stavitski, Mingzhao Liu
Abstract
In situ synchrotron X-ray absorption near-edge structure characterization of thin-film titania growth by atomic layer deposition (ALD) over ZnO nanowires reveals persistent low-coordinated Ti motifs leading to a new picture of ALD growth. Through the design of growth and measurement cycles, Ti K-edge spectral data are continuously recorded so as to characterize the film evolution as a function of ALD cycle number and the surface changes within the time scale of the ALD cycle. A unified set of analysis tools is developed to interpret the time-series of spectral data. A prenucleation stage of growth, a transition region, and then a steady-state growth stage are observed with distinguishable features. Multivariate curve resolution analysis, that is physically constrained, demonstrates two specific spectral components with associated, time-dependent concentrations. The bulk-film component tracks the stages of growth. The surface and interface components, present throughout the stages of growth, reveal a significant coverage of relatively isolated or loosely networked tetrahedrally coordinated Ti atomic motifs. Finally, spectral signatures for the intra-cycle growth kinetics are reconstructed at a time resolution of ∼ 1 s and demonstrate that the transient Ti motifs on the growing surface stabilize within a few seconds of the Ti precursor pulse.