V.O. Yukhymchuk, V.M. Dzhagan, V.P. Klad’ko, Raman/PL study of reversible photo-induced cubic-hexagonal transformation of nanocrystalline Si V.O. Yukhymchuk, V.M. Dzhagan, V.P. Klad’ko, M.Ya. Valakh V. Lashkaryov Institute of Semiconductors Physics, National Academy of Sciences of Ukraine, Kyiv, Ukraine Nanotechnology and Nanomaterials NANO-2015. Lviv

INTRODUCTION There are known a numerous photoinduced processes in Physics, Chemistry, Biology: - Photosynthesis in Biology is one of the most important process on the Earth, - A lot of chemical reactions may be enhanced by light, - Photocatalysis is to-day one of the hot-points in Physical Chemistry, - Application of Liquid Crystals and Photochromic materials are based on light-induced processes. Nanotechnology and Nanomaterials NANO-2015. Lviv

In Solid State and Semiconductors Physics: - Photoinduced crystallization of thin layers (is used also in MBE), - Optical storage in Amorphous Materials, - Optically induced transformation of defect (impurity) subsystems in crystals, - Rapid Light-Induced Annealing of implanted semiconductors in electronic technology. It is important: Light Action is more pronounced for non-ideal systems (defects, impurities, disordering, SURFACE). The last is especially important for nanocrystalline materials. . . NANOTECHNOLOGY and NANOMATERIALS NANO-2015. LVIV

SILICON CRYSTALLINE STRUCTURE Usually Si is characterized by Cubic (Diamond-like) Structure But at high press Si may be of Hexagonal- Type. Beside the X-ray Diffraction it was Identified by Raman Scattering of Nano- Indented Samples Structural transformation in Si results not only in a Change of Phonon Spectra but also in Change of Energy Band Structure. Hexagonal silicon manifests bright red PL Fig. 1 Fig. 2 We have used Raman Scattering, Photoluminescence, X-ray diffraction

SAMPLES AND EXPERIMENT a Two kinds of samples: Sample 1 was obtained by grinding of Si plates in a ball mill (sizes in nano- and micrometer range) – Fig. 3,a. Sample 2 – commercially available “NanoAmor” Si with average size less then 100 nm – Fig. 3,b b Fig. 3. SEM images of sample 1 (a) and sample 2 (b) Raman Experiments and PL: LabRam HR800 microRaman System with Ar+ and He-Ne+ lasers, 100x objective SEM : Mira 3 Tescan Microscope XRD Measurement: An ARL X’ tra instrument with high – temperature Chamber HTK 1200 N

Power excitation was varied from 0,001 up to 2,0 mW. Raman spectra results Fig. 4. Normalized Raman spectra of sample 1 Power excitation was varied from 0,001 up to 2,0 mW. Peak I – cubic microcrystals. Its downshift and broadening indicates heating effect. Peaks II, III – hexagonal phase (nanocrystals). Larger downshift indicates a higher temperature

Fig. 5. Normalized Raman spectra of sample 2. The frequencies of peaks II and III coincide with frequencies known for h-Si. They equal: 495 – 503 cm-1 and 514 – 518 cm-1 - Experiment 493 (E2g) and 516 (A1g) - Theory Differences between specimens 1 and 2 gives evidence that EFFECT IS REALISED ONLY FOR NANONOCRYSTALLINE Si !

Photo- stimulation is needed! DISCUSSION We conclude that a reversible cubic-hexagonal structural transition in nanocrystalline Si takes place due to laser-induced heating. But transition is not purely thermally induced. Photo- stimulation is needed! EVIDENCES: Coincidence of peaks II and III with frequencies of h-Si. 2. Effect was absent for high-temperature RS measurements (up to 600oC but low Pexc). At the same time the estimation of real temperature from Stokes/Antistokes intensity ratio and spectral position of peaks gave T = 100 – 200oC for microcrystalls T = 300 – 400oC for nanocrystalls 3. Unusually high (5 – 6 times) relation between intensities of 2TA/TO for sample 2 as compare with typical for c-Si. Such relation corresponds to T = 1000 – 1100oC for c-Si . But it is comparable with known for h-Si.

4. No structural transformation at pure thermal annealing (without light) was confirmed by XRD study in the range 20 - 1000oC (Fig. 6). Fig. 6. XRD patterns of sample 1 at different temperatures, namely 24, 500, and 1000°C. The inset shows the region around one of the reflections in detail.

5. And for the last – PL measurements ( Fig. 7). Fig. 7. Photoluminescence spectra of sample 1 for exc =514.5 nm and varied Pexc.

CONCLUSION A cubic–hexagonal structural transformation of indirect-gap Si to direct-gap is found by increasing excitation power density of laser light in Raman and PL study. Transformation is realized only for nanocrystalline Si and is thermo-light induced process.

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