#
Percolated Si:SiO_{2} Nanocomposites: Oven- vs. Millisecond Laser-Induced Crystallization of SiO_{x} Thin Films

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Results

^{−2}. Compared to the as-deposited sample, the oven-treated SiO${}_{0.6}$ film did not show a significant change in bulk composition. The fitted bulk O content of 39.4% conformed to the bulk O content of the as-deposited layer within the accuracy of measurement and homogeneity of sample deposition, resulting in SiO${}_{0.65}$. The surface oxygen peak increased after oven treatment. The protective SiO${}_{2}$ surface layer apparently grew in thickness to $150\times {10}^{15}$ cm

^{−2}during the oven treatment, i.e., by about 50%. Furthermore, the laser-treated silicon oxide film did not show a change in the bulk composition (Table 1). Its O content of 38.4%, resulting in SiO${}_{0.63}$, was the same as for the other two films within the experimental accuracy. As mentioned before, the oxygen peak superimposed on the high-energy edge of the SiO${}_{0.6}$ bulk layer was stronger than in the as-deposited and oven-treated one. Moreover, a peak at the high-energy edge of the SiO${}_{0.6}$ Si signal was unambiguously apparent (Figure 1). At the same time, a minimum was seen next to the oxygen peak towards lower energy. Both features were described by a thin Si-rich layer, referred to as Intermediate 1, underneath the protective SiO${}_{2}$ surface layer with an areal density of $100\times {10}^{15}$ cm

^{−2}and a low O content of 10%. To improve the fitting of the low-energy slope of the SiO${}_{0.6}$ layer Si signal, an additional Si-rich second intermediate layer with an areal density of $50\times {10}^{15}$ cm

^{−2}and an oxygen content of 10% was introduced between the bulk SiO${}_{0.6}$ and the silica substrate. This analysis indicated that contrary to oven treatment, laser processing led to the formation of Si-rich interlayers at both interfaces of the SiO${}_{0.6}$ bulk layer.

^{−1}to 600 cm

^{−1}, where the F${}_{2g}$ crystal vibration of crystalline Si (c-Si) and the broad line of a-Si were observed [25,26,27,28,29] (Figure 2).

^{−1})) with the peak maximum at 485 cm

^{−1}. It corresponded to that of a-Si [27,30] and indicated a phase separation of SiO${}_{0.6}$ into a-Si and SiO${}_{2}$ occurring already during the deposition and not only after high-temperature processing, as predicted and reported in the literature for this material system. After oven tempering, the spectrum had a sharp strong line at slightly less than 521 cm

^{−1}with an extended low-energy shoulder. Fitting this Raman spectrum with one Breit–Wigner function (BWF, asymmetry factor q = −7.2) and two Gaussians gave line positions (relative integral intensities) of 519 cm

^{−1}(60%), 500 cm

^{−1}(6%) and 482 cm

^{−1}(34%). This is the typical Raman signature of so-called micro-crystalline silicon, consisting of nanocrystalline and amorphous Si fractions [31,32]. The sharp peak at 519 cm

^{−1}was attributed to the F${}_{2g}$ phonon mode of the nanocrystalline silicon fraction (c-Si). Its FWHM of $7.9$ cm

^{−1}was two- to three-times larger than the natural linewidth of single-crystalline silicon at room temperature reported in the literature (≈$3.5$ cm

^{−1}) [33,34] or that of a reference Si wafer sample ($2.8$ cm

^{−1}). The shoulder peak with the intensity maximum at 482 cm

^{−1}represented the a-Si fraction in the sample, and the intermediate line at 500 cm

^{−1}was attributed to defective Si (def-Si), i.e., wurtzite-type or near-surface Si [32,35,36,37]. Laser treatment of the SiO${}_{0.6}$ resulted in a structure, the Raman spectrum of which exhibited only one single strong peak at first glance (Figure 2). The best line fitting results (r${}^{2}$ = 0.996) were obtained assuming a BWF (FWHM = 5 cm

^{−1}) and a Gaussian line with Raman shifts (relative integral intensities) of 517 cm

^{−1}(86%) and 470 cm

^{−1}(14%).

^{−1}and $5.0$ cm

^{−1}with such a correlation from the literature [24] gave values of approximately 8 $\mathrm{nm}$ and > 15 $\mathrm{nm}$ for the oven- and laser-treated samples.

^{−1}, which was also seen in the other diffraction patterns, originated from the amorphous SiO${}_{2}$ substrate and matrix. Therefore, the as-deposited film was X-ray amorphous and did not exhibit a crystalline phase. Oven treatment led to the evolution of three diffraction peaks at about 3.19 nm

^{−1}; 5.22 nm

^{−1}; 6.11 nm

^{−1}, which corresponded to the (111), (220) and (311) lattice planes of Si [42]. The observed linewidths were broad and showed an increase with increasing scattering vector. After laser treatment, diffraction peaks at the same positions were observed, but with higher intensity, as well as narrower linewidth. The increased intensity can be correlated with an increased scattering volume of well-ordered lattice planes (i.e., a higher density of crystallites in the layer) and the smaller linewidth results from larger crystallite sizes, compared to the oven-treated sample.

## 3. Discussion

#### 3.1. Structural and Compositional Homogeneity

#### 3.2. Comparison of Structural Properties

^{−1}) and laser- (4 cm

^{−1}) treated samples. A decrease in the resonance frequency of the collective lattice motion can be caused by phonon confinement or by an increased mean lattice spacing compared to the reference due to the presence of strain [24,44,45,46,47,48] or by an increased temperature [29]. The grain sizes derived by EFTEM and complementarily by XRD analysis yield structure sizes too large for phonon confinement, since for this effect, a typical size below 10 nm has to be reached [4]. Sample heating during Raman measurement was excluded, since no dependence of the applied laser power on the Raman signal was found.

^{−1}to 4 cm

^{−1}would suggest an expanded lattice, due to a tensile stress of 1.7 GPa to 3.6 GPa [49,50]. Such stress would lead to a lattice expansion of 0.39–1.38%. Hence, Raman results seem to contradict the XRD results. However, due to the measurement geometries applied, XRD probes the out-of-plane Si lattice distances, whereas Raman in the applied 180${}^{\circ}$ scattering geometry is sensitive to the in-plane Si geometries. During heat treatment, relaxation of stresses at Si-SiO${}_{2}$ interfaces takes place, which is reduced and eventually inhibited during cooling. The thermal expansion coefficient for the SiO${}_{2}$ substrate ($\alpha \approx $ $0.5\times {10}^{-6}$ K

^{−1}[51]) is much smaller than the thermal expansion coefficient for Si ($\alpha \approx $ $2.6\times {10}^{-6}$ K

^{−1}[52,53]). It follows that the contraction of the Si phase in the thin film is inhibited mainly by the relatively lower contraction of the SiO${}_{2}$ substrate. This in-plane tensile stress results in an in-plane expansion of the lattice. To compensate the in-plane expansion, a compression of the out-of-plane Si lattice components follows.

#### 3.3. Origin of Different Structure Sizes

^{−1}[54,55]; O: $1\times {10}^{-16}$ $\mathrm{c}$$\mathrm{m}$${}^{2}$ $\mathrm{s}$

^{−1}[55]) for the temperature applied during oven treatment. Similarly, Si self-diffusion by self-interstitial or vacancy transport is limited to $1\times {10}^{-17}$ $\mathrm{c}$$\mathrm{m}$${}^{2}$ $\mathrm{s}$

^{−1}[55] at 950 ${}^{\circ}\mathrm{C}$. The diffusion of O atoms in silicon by interstitial transport, on the other hand, reaches diffusion constants of $1\times {10}^{-11}$ $\mathrm{c}$$\mathrm{m}$${}^{2}$ $\mathrm{s}$

^{−1}at 950 ${}^{\circ}\mathrm{C}$, which is five orders of magnitude larger than any other regarded diffusion possibilities. In addition to bulk diffusion, Si and O atoms can diffuse along grain boundaries, which is about 4–8 orders of magnitude faster [56]. Approaching the melting point of a material, the diffusivity of grain boundary diffusion converges to the bulk diffusivity [55]. Hence, the relevant transport for the coarsening of the Si:SiO${}_{2}$ nanocomposite is the one of O atoms in the silicon matrix by interstitial transport and the transport of Si and O atoms along grain boundaries.

^{−2}is applied to the sample, taking the optical laser power and dwell time into account. When regarding the absorption of the laser wavelength by the SiO${}_{0.6}$ layer, 21 J cm

^{−2}are absorbed by the material. In contrast, an energy of 210 J cm

^{−2}is needed to heat the substrate and the thin film, about $0.1$ J cm

^{−2}is due to the SiO${}_{0.6}$ film. It is straightforward that a more evolved nanostructure requires more energy to be formed, i.e., it requires either longer processing times or higher temperatures. Therefore, since the processing time during laser treatment is much shorter, a higher temperature must have been achieved.

^{−1}, just below the melting point of silicon [59]. This represents an increase by three orders of magnitude, not sufficient to explain the observed coarsening of the laser-treated Si:SiO${}_{2}$ nanocomposite structure. A further rise in temperature, resulting in a process occurring in the liquid state, leads to a sudden increase of the diffusion of O atoms in the Si-phase to $3\times {10}^{-4}$ $\mathrm{c}$$\mathrm{m}$${}^{2}$ $\mathrm{s}$

^{−1}[59], i.e., by five orders of magnitude. A similar rise upon melting is expected for the diffusion along grain boundaries. This now fully conforms with the experimental findings. We therefore conclude that during laser treatment, growth and enhanced phase separation occurs in the liquid state of silicon. The temperature window can now be assumed to range from the melting temperature of a-Si to that of SiO${}_{2}$, i.e., from 1200 ${}^{\circ}\mathrm{C}$ [60,61] to 1705 ${}^{\circ}\mathrm{C}$ [62,63], since a breakdown of the general, percolated morphology would be expected for a system consisting of two liquids.

#### 3.4. Formation of Interface Layers

## 4. Materials and Methods

#### 4.1. Sample Deposition and Processing

#### 4.2. Characterization

^{−2}(0.1 mW laser power) in order to avoid any thermally- or photo-induced transformation of Si. The collected Raman-scattered light was dispersed by an 1800 mm

^{−1}holographic grating and recorded with a liquid nitrogen cooled CCD detector.

## 5. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Rutherford backscattering spectrometry (RBS) spectra of SiO${}_{0.6}$ layers on SiO${}_{2}$: as-deposited (

**a**), after conventional oven (

**b**) and after laser treatment (

**c**). The energy range between 1 keV and $1.7$ keV, which shows traces of Ar and W, was omitted for clarity. Squares represent measured data and lines fitted curves. In (

**a**), single-layer elemental profiles are additionally shown, i.e., red for Si and blue for O. The roman letters (I, II, III) indicate the major RBS feature types explained in the text.

**Figure 2.**Raman spectra of SiO${}_{0.6}$ layers on SiO${}_{2}$: as-deposited (

**a**), after conventional oven (

**b**) and after laser treatment (

**c**). Grey squares represent measured spectral data points and solid lines fitted spectra. Dashed lines indicate the expected positions of the TO-like band associated with a-Si and the F${}_{2g}$ phonon mode of c-Si.

**Figure 3.**(

**i**) XRD patterns of SiO${}_{0.6}$ layers on SiO${}_{2}$: as-deposited (a), after conventional oven (b) and after laser treatment (c). The broad peak at about $2.4$ nm

^{−1}originated from the a-SiO${}_{2}$ matrix and silica substrate. Measurement data are shown by grey squares. The solid lines represent fitted curves. (

**ii**,

**iii**) The determined diffraction peak width is plotted vs. its position; the linear regression was made for Williamson–Hall analysis to retrieve crystallite size D and micro-strain $\u03f5$ for the oven- (

**ii**) and laser-treated (

**iii**) sample.

**Figure 4.**High-resolution TEM image of SiO${}_{0.6}$ layers: as-deposited (

**a**) and after conventional oven treatment (

**b**).

**Figure 5.**High-resolution TEM image of laser-crystallized SiO${}_{0.6}$ layers. (

**a**) Si (111) lattice fringes expand across the whole field of view. The matrix surrounding the crystal is assumed to be SiO${}_{2}$. (

**b**,

**c**) show enlargements of diagonal opposite areas of (

**a**).

**Figure 6.**Cross-sectional Si plasmon-loss filtered TEM images (E${}_{loss}$ = 17 eV) of SiO${}_{0.6}$ layers: as-deposited (

**a**), after conventional oven (

**b**) and after laser treatment (

**c**).

**Table 1.**Overview of RBS results of SiO${}_{0.6}$ layers on SiO${}_{2}$, as-deposited, oven- and laser-treated. Atomic fraction (at. %), areal density (${\rho}_{area}$) of silicon and oxygen, the resulting x of ${\mathrm{SiO}}_{\mathrm{x}}$ fraction and thickness obtained by TEM, ${d}_{TEM}$, as well as spectroscopic ellipsometry (SE), ${d}_{SE}$, for individual layers observed.

Sample | Si | O | x of SiO_{x} | TEM Thickness | SE Thickness | ||
---|---|---|---|---|---|---|---|

at. % | ${\mathit{\rho}}_{\mathbf{area}}/{10}^{15}\frac{1}{{\mathbf{cm}}^{2}}$ | at. % | ${\mathit{\rho}}_{\mathbf{area}}/{10}^{15}\frac{1}{{\mathbf{cm}}^{2}}$ | ${\mathit{d}}_{\mathit{TEM}}/\mathbf{nm}$ | ${\mathit{d}}_{\mathit{SE}}/\mathbf{nm}$ | ||

as-deposited | |||||||

surface | 33.3 | 33.3 | 66.6 | 66.5 | 2 | 14 | 15 |

bulk | 61.0 | 1691 | 38.9 | 1079 | 0.64 | 500 | 509 |

oven-treated | |||||||

surface | 33.3 | 50 | 66.6 | 99.8 | 2 | 39 | 40 |

bulk | 60.5 | 1663 | 39.4 | 1084 | 0.65 | 501 | 482 |

laser-treated | |||||||

surface | 33.3 | 50 | 66.6 | 99.8 | 2 | 28 | 29 |

intermediate 1 | 90.0 | 90 | 9.9 | 9.9 | 0.11 | 27 | 24 |

bulk | 61.5 | 1648 | 38.4 | 1031 | 0.63 | 479 | 446 |

intermediate 2 | 90 | 45 | 10 | 5 | 0.11 | 24 | 10.7 |

**Table 2.**Treatment parameters and selected structure properties of as-deposited, oven- or laser-treated SiO${}_{0.6}$ layers. CVF, crystalline Si volume fraction.

As-Deposited | Oven | Laser | |
---|---|---|---|

treatment realization | |||

exposition time | - | 270 min ≡ 16,200 s | $17\times {10}^{-3}$ s |

temperature | - | 950${}^{\circ}\mathrm{C}$ | n/a |

resulting structure properties | |||

bulk composition | SiO${}_{0.64}$ | SiO${}_{0.65}$ | SiO${}_{0.63}$ |

EFTEM structure size | 2 nm | 10 nm | 30 nm |

XRD grain size | n/a | 11 nm | 22 nm |

Raman CVF | 0% | 72% | 92% |

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Schumann, E.; Hübner, R.; Grenzer, J.; Gemming, S.; Krause, M. Percolated Si:SiO_{2} Nanocomposites: Oven- vs. Millisecond Laser-Induced Crystallization of SiO_{x} Thin Films. *Nanomaterials* **2018**, *8*, 525.
https://doi.org/10.3390/nano8070525

**AMA Style**

Schumann E, Hübner R, Grenzer J, Gemming S, Krause M. Percolated Si:SiO_{2} Nanocomposites: Oven- vs. Millisecond Laser-Induced Crystallization of SiO_{x} Thin Films. *Nanomaterials*. 2018; 8(7):525.
https://doi.org/10.3390/nano8070525

**Chicago/Turabian Style**

Schumann, Erik, René Hübner, Jörg Grenzer, Sibylle Gemming, and Matthias Krause. 2018. "Percolated Si:SiO_{2} Nanocomposites: Oven- vs. Millisecond Laser-Induced Crystallization of SiO_{x} Thin Films" *Nanomaterials* 8, no. 7: 525.
https://doi.org/10.3390/nano8070525