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...

The surface topological evolution during the growth of indium nitride (InN) by plasma-assisted atomic layer epitaxy (ALEp) on gallium nitride (GaN) (0001) substrates was studied using in situ real-time grazing incidence small-angle x-ray scattering (GISAXS) for 180, 250, and 320 °C growth temperatures. The GISAXS data reveal that the ALEp growth of InN on GaN in this temperature range proceeds in a Stranski–Krastanov mode, in which the 2D–3D transition occurred after 2.3 monolayers for 180 °C, 1 monolayer for 250 °C, and 1.5 monolayers for 320 °C. The corresponding initial island center-to-center distances were 7.4, 11.6, and 11.7 nm. Additionally, island coarsening was observed to increase with temperature. After 200 growth cycles, the mean island diameters were 3.9, 5.6, and 7.0 nm, and the mean island center-to-center distances were 8.6, 13.7, and 17.1 nm for 180, 250, and 320 °C growth temperatures, respectively. For the 320 °C growth, the mean island shape was observed to gradually evolve from relatively mounded to cylindrical. These results are supported by atomic force microscopy and specular x-ray reflectivity.

Linear-regime Ar bombardment of Si produces symmetrical ripple structures at ion incidence angles above 45° measured off-normal (Madi 2009 J. Phys.: Condens. Matter 21). In the nonlinear regime, new behaviors emerge. In this paper, we present experimental results of ion bombardment that continues into the nonlinear regime until pattern saturation at multiple ion incidence angles, showing the evolution of their grazing incidence small-angle x-ray scattering (GISAXS) spectra as well as atomic force microscopy topographs of the final, saturated structures. Asymmetric structures emerge parallel to the direction of the projected ion beam on the sample surface, constituting a height asymmetry not found in the linear regime. We then present simulations of surface height evolution under ion bombardment using a nonlinear partial differential equation developed by Pearson and Bradley (2015 J. Phys.: Condens. Matter 27 015010). We present simulated GISAXS spectra from these simulations, as well as simulated scattering from a sawtooth structure using the FitGISAXS software package (Babonneau 2010 J. Appl. Crystallogr. 43 929-36), and compare the simulated spectra to those observed experimentally. We find that these simulations reproduce many features of the sawtooth structures, as well as the nearly-flat final GISAXS spectra observed experimentally perpendicular to the sawtooth structures. However, the model fails to reproduce the final GISAXS spectra observed parallel to the sawtooth structures.

Detailed quantitative measurement of surface dynamics during thin film growth is a major experimental challenge. Here X--ray Photon Correlation Spectroscopy with coherent hard X--rays is used in a Grazing--Incidence Small--Angle X--ray Scattering (i.e. Co--GISAXS) geometry as a new tool to investigate nanoscale surface dynamics during sputter deposition of a--Si and a--WSi2 thin films. For both films, kinetic roughening during surface growth reaches a dynamic steady state at late times in which the intensity autocorrelation function g2(q,t) becomes stationary. The g2(q,t) functions exhibit compressed exponential behavior at all wavenumbers studied.The overall dynamics are complex, but the most surface sensitive sections of the structure factor and correlation time exhibit power law behaviors consistent with dynamical scaling.

In situ synchrotron x-ray studies were employed to develop a fundamental understanding of the low temperature atomic level processes (ALPs) for GaN substrates to develop in situ methods for preparation of epitaxy ready surfaces. An emulated gallium flash-off (GFO) ALP, followed by a hydrogen clean ALP, and a subsequent nitridation ALP are studied as a function of temperature and number of cycles. The results demonstrate that ideal GFO ALP results are achieved at a higher temperature, 500 °C, and that only ten GFO ALP cycles are needed to remove the surface oxide and result in an ordered GaN surface. Continued GFO ALP cycles at 500 °C roughen the GaN surface. GFO ALP executed at 400 °C only roughens the surface, while executing the GFO ALP at 250 °C causes uneven surface features presumably due to the incomplete removal of the oxide. The hydrogen clean ALP generally roughens the surface at all three temperatures after 30 cycles of the GFO ALP. Further, the nitridation ALP executed after 30 cycles of the GFO ALP, at any of the above temperatures, has little effect since the surface of the GaN has been roughened beyond recovery. These results provide insight into optimal GaN substrate surface preparation at temperatures consistent with the low temperature atomic layer epitaxy process.