Crystallization of amorphous silicon (a-Si) thin films is important for fabricating thin film transistor (TFTs) and thin film solar cells [1]. Rapid heating is an attractive method with low thermal budget for fabricating these devices at a low cost. We have developed a simple heating method using a 2.45 GHz microwave associated with carbon powders [2,3]. Carbon powders are heated by microwave absorbance and samples are subsequently heated by the heat conduction. Temperature of carbon powders was increased above 1000 ^oC at 15 s and further increased to 1163 ^oC at 26 s by 1000 W microwave irradiation [3]. In this paper, we report crystallization of thin silicon films by the microwave heating. 

2.Experimental procedure

2.4, 5.5, 10, 19, 50-nm-thick a-Si films were formed on 500-μm-thick 1.8x1.8-cm²-sized quartz substrates by plasma sputtering method. 100-nm-thick SiO_xfilms were subsequently formed on the surface of 2.4-nm-thick a-Si film for preventing from evaporation of silicon during heat treatment. The samples were completely covered with carbon powders of 1 g. Then, microwave heating at 1000 W for 22 s was carried out using a commercial 2.45 GHz microwave oven. 

3.Results and Discussion

Analysis of optical reflectivity resulted in slightly decrease in film thickness by microwave heating from as-deposited 2.4, 5.5, 10, 19, and 50 nm to 1.8, 4.2, 6.0, 18, and 49 nm, respectively. Figure 1 shows Raman scattering spectra for the central points of the microwave heated silicon films with thicknesses between 1.8 and 49 nm as well as the initial 50-nm-thick amorphous silicon film. Analysis of the spectra was conducted with three Gaussian components of amorphous, nano-crystalline, and crystalline structures, respectively, to estimate the amorphous, nano-crystalline, and crystalline volume ratio [4]. Although, the initial a-Si film had a broad peak around 480 cm^-1 associated only with the amorphous phonon mode, the 22-s-microwave-heated silicon films had sharp peaks associated with the crystalline silicon phonon mode appeared between 515 and 518 cm^-1. The crystalline volume ratio was monotonously increased from 0.18 to 0.85 as film thickness increased from 1.8 to 49 nm. They also had broad tail spectra indicated by the arrows associated with the nano-crystalline phonon mode around 500 cm^-1, whose intensity increased from 0.07 to 0.49 as the film thickness decreased from 49 to 1.8 nm. Large crystalline grain growth was limited by a thin film thickness. Moreover, photoluminescence which is attributable to the direct energy transitions in nano-crystalline silicon and small crystalline grains was observed for the microwave heated 1.8-nm-thick silicon film by 355 nm YAG laser irradiation. The present results demonstrate a capability of formation of high density of nano-crystalline particles by the microwave heating, which will be useful for application to fabrication of quantum-effect-nano-crystalline-thin-film soalr cells.

References

[1] S. Uchikoga, N. Ibaraki, Thin Solid Films 383 (2001) 19.

[2] M. Hasumi, T. Nakamura, S. Yoshidomi, and T. Sameshima, Jpn. J. Appl. Phys. 53 (2014) 05FV05-1-6.

[3] S. Kimura, M. Hasumi, K. Ota, and T. Sameshima, Ext. Abstr. Solid State Devices and Materials, C-7-6 (2015).

[4] Z. Iqbal and S. Veprek: J. Phys. C 15 (1982) 377.

Fig.1 Raman scattering spectra of initial 50-nm thick a-Si film and microwave heated silicon films with thicknesses of 1.8, 4.2, 6.0, 18, and 49 nm.