engleski

Crystalline to amorphous phase in silicone thin films

Silicon thin films are most common used material in production of thin films solar cells. The efficiency of devices could be substantially improved by applying Si material in different structural forms, enabling in this way the efficient absorption of light in broader wavelength interval. Due to variety of quantum effects, the optical energy gaps are different and vary from 2.2 eV for amorphous Si to 1.1 for monocrystalline material. For nano-crystallites energy gap vary between these two values. A series of samples with different degree of crystallinity were prepared by deposition of silane in radiofrequency field. The fraction of the crystalline phase was investigated by Raman Spectroscopy (RS), X-ray powder diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and selected-area electron diffraction (SAED). The silicon has cubic structure, space group Fd3m with one Raman active mode of vibration. The microcrystalline Si is characterized by one intensive sharp band in Raman spectra. The position of this band is 520 cm-1, it is broadened and shifted to lower frequencies for crystallites smaller than 30 nm, and nanocrystallinity is also manifested by low energy shoulder [I. H. Campbell and P. M. Fauchet, Solid State Commun. 58 (1986) 739]. The broad band with maximum around 480 cm-1 is attributed to amorphous phase [T. D. Kang, Hosun Lee, S. J. Park, J Jang, Soonil Lee, J. Appl. Phys. 92 (2002) 2467]. The volume fraction of the crystalline phase can be estimated from the ratio of the integrated intensities of crystalline and amorphous band after deconvolution of the spectra. We observed crystalline band in RS of our samples and estimated different ratios to amorphous bands. The position and the full with at half-maximum of the crystalline band also varied in our samples indicating different nano sized crystallites. However, procedure of deconvolution of the Raman bands is not straight forward due to the partially overlapping of the bands. Moreover, quantitative determination of the crystallinity only from Raman measurements is doubt because of different cross-section factors of the Raman scattering for crystalline and amorphous silicon. XRD measurements showed broad amorphous band for all samples. In one sample the diffraction lines of (111), (220) and (311) reflections of silicon were clearly observed indicating considerably higher amount of crystalline phase than in the other samples. In two samples only (111) and (220) line were hardly observable, and one sample was completely X-ray amorphous. It is well known that XRD is less efficient than electron diffraction [J. M. Cowley Diffraction Physics, 2nd ed. North-Holland Amsterdam (1981)] especially at nano scale, so SAED had showed diffractions spot at all investigated samples. Amorphous halo was superimposed to crystalline spots in all SAED patterns. Furthermore, HRTEM images clearly showed crystallites in all samples, although the size and shapes of crystallites. The estimated amount of amorphous phase observed in HRTEM was considerably smaller than observed by RS or XRD. The results of all of applied methods in determination the crystalinity fraction are compared and their mutual compatibility are discussed.

Silicon thin films; amorphous; crystalline; solar cells

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