Jan 10, 2013
Ions measure roughness of buried interfaces
The performance of functional multilayers depends strongly on the quality of the interfaces within the structure. Here quality refers to roughness and interdiffusion, which are studied typically using X-ray reflectivity (XRR) and transmission electron microscopy (TEM). In these techniques, high-energy photons or electrons are used to look deep in the multilayers at interfaces buried under the surface. In XRR, specularly reflected waves carry encrypted information on the layers and interfaces, whereas in TEM results can be seen directly on the screen, but only in a tiny cross-sectional window.
Now, as it turns out, properly designed secondary ion mass spectrometry (SIMS) is also able to comprehend roughness/interdiffusion in nanoscopic multilayers by collecting and simple model-processing direct elemental depth distributions (depth profiles). The approach retains many advantages of XRR and TEM, while avoiding their major drawbacks. The system does not depend on a material's optical properties – as XRR does. It is destructive, but, unlike TEM, it does not require sample preparation and can analyse laterally over much larger areas.
The SIMS set-up, which has been put through its paces by researchers at Argonne National Laboratory, has great potential to become a standard characterization tool to provide all essential structural and chemical information for a diverse range of applications in nanotechnology.
In their study, the team used an advanced custom-made time-of-flight (TOF) SIMS instrument called SARISA to sputter depth profile a |MgO/ZnO|×8/Si multilayer grown by atomic layer deposition (ALD). SARISA features a unique gentle dual-beam approach to depth profiling (gentleDB), which combines a raster-scanned orthogonally incident direct current Ar+ beam of a few hundred eV for layer-by-layer shaving the multilayer, with an obliquely incident pulsed Ar+ beam for TOF SIMS analysis in the centre of revealed sub-surface, cyclically following every shaving increment.
These depth profiles were analysed by a well established mixing-roughness-information model, which yielded a 1.5 nm nanolayer interfacial roughness within the MgO/ZnO multilayer. Further analysis suggested that the 1.5 nm roughness corresponds to native/jig-sawed interfacial roughness rather than to interfacial interdiffusion during the ALD growth. This finding was cross-validated using XRR.
Back in the lab, the group is now using gentleDB SIMS to characterize a novel class of materials synthesized by ALD: multilayers of metal-sulfide binaries and Cu2ZnSnS4, and metal-oxide/metal-sulfide heterojunctions. The results will help to better explain the fundamentals of ALD and address application needs in modern photovoltaics.
The researchers published their work in the journal Nanotechnology.
About the author
The study was performed collaboratively between the Surface Chemistry Group and the ALD Research and Development Group at ANL. Sergey Baryshev, Jeffrey Klug and Qing Peng are postdoctoral researchers who performed SIMS, XRR and ALD work, respectively. Alexander Zinovev and Emil Tripa are scientists who make the SARISA facility versatile and routinely operable. Jeffrey Elam and Igor Veryovkin are principal investigators developing, respectively, ALD and SIMS instrumentation and methodology at ANL. All were involved in discussions of the experimental data. This work was jointly supported by the US Department of Energy and NASA.