Morphology studies of Si-SiO₂ nanocomposites using energy-filtered transmission electron microscopy

Due to significant band gap widening by quantum confinement, Si nanosponge structures embedded in SiO₂ formed by spinodal decomposition of metastable silicon-rich silicon oxide are promising, advanced Si absorbers for 3^rd generation solar cells. In thermodynamically metastable, silicon-rich oxide, i.e. SiO_x with x < 2, high-temperature annealing results in the formation of Si precipitations in a stoichiometric SiO₂ matrix. Phase separation of SiO_x films with 1.2 ≤ x < 2 (Si excess of up to 40 at.-%) leads to disconnected Si nanoclusters, whereas for x < 1.2 (Si excess larger than 40 at.-%) phase separation results in percolated Si nanostructures with a sponge-like morphology [1].

To reveal such a sponge-like morphology in sputter-deposited SiO_x films for x ~ 1 after activation by rapid thermal annealing (RTA), energy-filtered transmission electron microscopy (EFTEM) imaging as well as EFTEM tomography were carried out and the results were compared with kinetic Monte-Carlo (KMC) simulations. To this end, 200 nm thick SiO_x layers were prepared at room temperature on p type (100) Si wafers by magnetron sputtering in Ar plasma from two simultaneously operating Si and SiO₂ targets. During subsequent RTA in Ar or Ar + 5 % H₂ ambient, samples were heated up to 1150 °C and annealed for 30 s. Sponge-like nanostructures were investigated by EFTEM imaging using an image-corrected FEI Titan 80-300 microscope equipped with a Gatan Imaging Filter 863. For EFTEM tomography, a tilt series between ±70° was acquired in steps of 2° in a Philips CM200 FEG microscope with Gatan Imaging Filter 678. The tilt series alignment, i.e. the correction of residual displacements, was carried out using the IMOD software [2], while the tomographic reconstruction of the Si 3D morphology was performed with the Weighted Simultaneous Iterative Reconstruction Technique [3].

The contrast in zero-loss filtered high-resolution TEM images is caused by the coherent superposition of unscattered and elastically scattered electrons within the thin TEM lamella, and hence, related to the projected atomic structure. Consequently, Bragg-oriented Si nanocrystals larger than a minimum size are visible in Figure 1 and indicate phase separation of the SiO_x film with subsequent crystallization during RTA. Since large amorphous Si precipitates crystallize preferably, high-resolution TEM images allow the determination of a maximum Si structure size. For small Si structures, however, phase separation is also possible without crystallization. Such amorphous precipitates as well as not Bragg-oriented Si nanocrystals cannot be observed with high-resolution electron microscopy, leading to an underestimation of the Si phase fraction. Therefore, EFTEM analysis is a suitable alternative for Si morphology studies. In particular, valence-band plasmon energy-loss imaging is an appropriate approach, since the Si plasmon peak is, except the zero-loss peak, the most intense feature in the electron energy-loss spectrum. It has a narrow energy distribution of a few eV, and thus, allows to distinguish the Si phase from the SiO₂ compound [4]. As shown in Figure 2, the expected sponge-like Si morphology in phase-separated SiO_x has been proven by Si plasmon imaging, which particularly shows that Si filaments have diameters of a few nanometers with a narrow size distribution. This finding is in excellent agreement with large-scale simulations based on KMC (Figure 3). Although Si plasmon EFTEM images can show the Si phase distribution in a planar projection, they do not provide three-dimensional information. For example, a superposition of Si nanodots cannot be distinguished from a sponge-like morphology in a 2D projection. Therefore, EFTEM tomography was applied, revealing that the separation of silicon into percolated nanostructures is not obvious (Figure 4). Coarsening of the Si sponge accompanied by a loss of percolation may be the reason. Indeed, longer annealing at 1100 °C for 3 h leads to separated non-spherical nanoclusters [5]. However, an underestimation of the thinnest, presumably amorphous, Si filaments cannot be excluded. Prolonged electron irradiation during acquisition of the EFTEM tilt series might have an influence on the sponge-like morphology, too.

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