3D Si-SiO₂ nano-networks formed by diode laser-induced liquid- and solid-state decomposition of SiO_x

Thin films of nano-structured crystalline silicon (nc-Si) are potential absorber and supporting layers for next-generation Si solar cells. As one candidate, Si-SiO₂ nanocomposites with percolated nc-Si have been fabricated by rapid thermal annealing (RTA) of sputter-deposited SiO_x films (x≈1). A percolated silicon network has been formed by solid state phase separation into nc-Si and SiO₂ [1, 2].
In the present study, SiO_0.6 layers of ~500nm thickness are grown on quartz by ion beam sputter (IBS) as well as by reactive magnetron sputter (RMS) deposition. Formation of percolated Si-SiO₂ nanocomposites is achieved by two different modes of thermal treatment: (i) Furnace annealing at 950°C and (ii) scanning laser processing. In case (ii), a diode laser with dwell times in the ms range, power densities of ~30 kW/cm², a wavelength of λ= 808nm and a line focus of 100µm x 11mm is applied. This process is ~106 times faster than isothermal treatment and ~103 times faster than RTA. Another advantage of this method is the usability of temperature sensitive substrates and maintaining homogeneous processing.
Rutherford backscattering spectra of as-deposited and processed SiO_0.6 reveals a compositional change in thin surface and interface layers, but no significant change in the bulk composition. Raman spectroscopy and X-ray diffraction show that the crystallinity of the nc-Si is higher for the laser-treated sample.
High resolution- and energy-filtered transmission electron microscopy (HTEM, EFTEM) show additionally, that in both cases the as-deposited SiO_0.6 is transformed into a percolated nanocomposite consisting of amorphous SiO₂ and nc-Si.
In more detail, laser processing of IBS-deposited layers leads to isotropic morphologies self-similar to furnace-annealed samples, but scaled up by a factor of ~5. This is explained by a phase separation in the liquid state and the solid state, respectively, which cause diffusion coefficients differing by several orders of magnitude.
During the deposition by RMS, phase separated filament-like morphologies form. Here, furnace annealing leads to enhanced phase separation accompanied by crystallization. In contrast laser processing erases the as-deposited filaments and produces isotropic morphologies similar to IBS-deposited and laser-processed samples

[1] Friedrich, D. et al. Sponge-like Si-SiO₂ nanocomposite - Morphology studies of spinodally decomposed silicon-rich oxide. Appl. Phys. Lett. 103, 131911 (2013).
[2] Ilday, S. et al. Multiscale Self-Assembly of Silicon Quantum Dots into an Anisotropic Three-Dimensional Random Network. Nano Lett. 16, 1942–1948 (2016).