Direct measurement of the propagation velocity of defects using coherent X-rays

Abstract

The properties of artificially grown thin films are often strongly a ected by the dynamic relationships between surface growth processes and subsurface structure. Coherent mixing of X-ray signals promises to provide an approach to better understand such processes. Here, we demonstrate the continuously variable mixing of surface and bulk scattering signals during realtime studies of sputter deposition of a-Si and a-WSi2 films by controlling the X-ray penetration and escape depths in coherent grazing-incidence small-angle X-ray scattering. Under conditions where the X-ray signal comes from both the growth surface and the thin film bulk, oscillations in temporal correlations arise from coherent interference between scattering from stationary bulk features and from the advancing surface. We also observe evidence that elongated bulk features propagate upwards at the same velocity as the surface. Furthermore, a highly surface-sensitive mode is demonstrated that can access the surface dynamics independently of the subsurface structure.A key objective for understanding surface dynamics during thin film growth is the ability to monitor nanometre-scale surface fluctuation dynamics in real time 1-3 . These fluctuations of roughness and density occur on timescales that rarely exceed a few seconds, and take place in environments that are inaccessible to most high spatial resolution probes. For example, scanning probe microscopy is widely used to study interfacial reactivity in non-vacuum environments 4 , but is limited by its inability to probe surfaces in real time during deposition; electron microscopy is mainly limited to high-vacuum environments and low magnetic fields 5,6 . X-rays have the potential to overcome these challenges owing to their highly penetrating nature and sensitivity to nanometrescale features. Observation of subsurface structures in real time during film growth appears to be even more challenging, and has rarely been attempted 7 . Bulk signals are sometimes observed as unwanted background in grazing-incidence surface X-ray scattering experiments, but there have been few attempts to quantitatively understand the features responsible for such signals 8,9 .The interaction of surfaces with nanometre-scale buried defects and the formation of bulk defects at a growing surface are integral to many industrial processes. For example, misfit dislocations nucleate at free surfaces and buried interfaces in strained-layer epitaxial growth of layers for photonic devices 10 ; motion and ordering of oxygen vacancies in complex oxide materials for ferroelectric memory depend on the surface conditions during growth 11-13 ; and voids in electrochemically deposited layers used for interconnects in electronic circuits are introduced by surface processes during deposition 14 .The use of X-ray scattering techniques to probe in situ realtime processes has largely been restricted to well-ordered crystalline structures and to statistical averages of disorder, owing to limitations in the spectral brightness of X-ray sources. A fu...

Concentration and velocity profiles in a polymeric lithium-ion battery electrolyte

“…The electrolyte velocity measurements were conducted using XPCS 26,27 in the heterodyne modality, [28][29][30][31] as illustrated in Fig. 1(b).…”

Co-GISAXS technique for investigating surface growth dynamics

“…Since scattering at exit angles above the Yoneda peak shows increased contributions from near-surface scattering which can lead to interference between surface and near-surface scattered waves [20], we focus on the scattered intensity at the Yoneda wing and below it. The g 2 (q || ,t) results are fitted with Eq.…”

“…On the other hand, in heterodyne experiments the scattered beam is made to interfere with a static or quasi-static reference and the intensity fluctuations of the resulting beam are studied [19]. Under conditions in which significant scattering from the bulk film is observed, we have discovered that heterodyning can occur between the bulk and surface signals during Co-GISAXS studies of thin film growth [20]. However, we focus here on experimental conditions giving homodyne behavior.…”

Early Stage Growth of Amorphous Thin Film: Average Kinetics, Nanoscale Dynamics and Pressure Dependence

“…B is the baseline, which is controlled close to 1 (varied from 0.9 to 1.1 in the fittings). The compressed exponential result suggests nonlinear dynamics [27] which will be discussed further in Section IV.…”