Morphology analysis of sponge-like Si-SiO₂ nanocomposites using energy-filtered electron tomography and electron holographic tomography

Due to the possibility of band-gap engineering by quantum confinement, Si nanosponge structures embedded in SiO₂ formed by spinodal decomposition of metastable silicon-rich silicon oxide are promising absorbers for 3^rd generation solar cells. High-temperature annealing of thermodynamically metastable, silicon-rich oxide SiO_x with x < 2 leads to phase separation of elemental Si from stoichiometric SiO₂. While this phase separation results in disconnected Si nanoclusters for 1.2 ≤ x < 2, percolated Si nanostructures with a sponge-like morphology are observed for x < 1.2 [1].
To visualize the sponge-like morphology in SiO_x films for x around 1 after thermal treatment, energy-filtered transmission electron microscopy (EFTEM) imaging, EFTEM tomography, and electron holographic tomography (EHT) [2] were carried out. To this end, 200 nm thick SiO_x layers were prepared on p type (100) Si wafers by magnetron sputtering in Ar plasma from two simultaneously operating Si and SiO₂ targets. During subsequent annealing, samples were heated up to 1150 °C. Sponge-like nanostructures were investigated by EFTEM imaging using an image-corrected FEI Titan 80-300 microscope equipped with a GIF 863. For EFTEM tomography, a tilt series between ±70° was acquired in a Philips CM200 FEG microscope with a GIF 678, and for EHT, a tilt series from -74° to +79° was recorded in an image-corrected FEI Tecnai TF20 microscope. Tomographic reconstruction of the Si 3D morphology was performed with the Weighted Simultaneous Iterative Reconstruction Technique [3].
Valence-band plasmon energy-loss imaging is an appropriate approach to visualize the Si morphology in phase-separated Si-SiO₂ nanocomposites [4]. As an example, Figure 1 shows the Si plasmon EFTEM images (E_loss = 17 eV) of a SiO_x≈1 layer decomposed into Si and SiO₂ after thermal treatment at 1100 °C for 3 min (left) and 3 h (right). As indicated by the selected area electron diffraction patterns, coarsening of the Si nanostructure is accompanied by Si crystallite growth. Although Si plasmon EFTEM imaging can show the Si phase distribution in a planar projection, it does not provide 3D information. Therefore, EFTEM tomography was applied, revealing that a spinodal sponge-like morphology of Si is only partially visible in a volume of ca. (30 nm)³ (Figure 2). However, in a larger volume of ca. (140 nm)³ - as demonstrated by applying EHT on a needle-shaped specimen prepared by FIB - both isolated nanoparticles and percolated Si nanostructures with a sponge-like morphology are observed (Figure 3).

[1] T. Müller et al., Appl Phys Lett 85 (2004) 12.
[2] D. Wolf et al., Curr Opin Solid St M 17 (2013) 126.
[3] D. Wolf et al., Ultramicroscopy 136 (2014) 15.
[4] D. Friedrich et al., Appl Phys Lett 103 (2013) 131911.

Acknowledgements
The authors kindly acknowledge TEM sample preparation by Annette Kunz and Martina Missbach.