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Nanostructure and Optical Properties of the Silicon Thin Films

material in different structural forms in the same device, it is possible to improve its conversion efficiency, since the absorption of light occurs in broader wavelength range. As a result of various quantum effects the optical energy gaps alter from 2.2 eV for amorphous Si to 1.1 for single-crystalline material, while nano-crystallites cover gaps between these two values. The relation of the nanostructure of the silicon thin films to the absorption coefficient was investigated. A series of thin silicon films with different degree of crystallinity were prepared by decomposition of silane gas, diluted with hydrogen, in radiofrequency discharge. The crystallite size, shape and the portion of crystalline phase were investigated by high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), Raman Spectroscopy (RS) and X-ray powder diffraction (XRD). The absorption coefficient (α ) was measured UV-VIS-transmitance and reflectance measurements. HRTEM and SAED were performed on Philips CM200 FEG microscope operated at 200 kV. Although in particular samples RS and XRD results did not reveal any crystalline bands/lines, HRTEM images showed crystallites with different sizes and shapes (FIG. 1). SAED patterns contained diffractions spots superimposed to amorphous halo, thus further indicating presence of both amorphous and crystalline phases in studied thin silicon films. The filtering of the HRTEM images was made to accomplish better recognition of the crystallites and insight to their arrangement (Fig. 2). The high local amount of crystallites observed in HRTEM images has confirmed the presence of crystalline phase in all investigated materials. The silicon has cubic structure, space group Fd3m, and the Raman spectrum of the microcrystalline Si is characterized by one intensive sharp band at 521 cm-1. The broad band around 480 cm-1 is attributed to amorphous phase [1]. The volume fractions of the crystalline phase were estimated from the ratio of the integrated intensities of crystalline and amorphous band after deconvolution of the spectra, and the crystallite sizes were calculated from the position and full with at half-maximum of the crystalline band (TABLE 1). This determination of the crystallinity from RS measurements is uncertain due to different cross-section factors of the Raman scattering for crystalline and amorphous silicon, so the results were compared with XRD measurements. The weight fraction of amorphous phase and average crystallite sizes were calculated using modified Rietveld refinement of XRD patterns. For the sample with higher amount of crystalline phase the calculated weight fraction of crystalline phase was 25.2 % and average crystallite size 11.1 nm, thus confirming the results estimated from Raman spectra. In FIG.3 are compared energy distribution of absorption coefficient (α ) for amorphous and nano-crystalline samples with similar crystal fraction, similar density but different sizes of crystals. This illustrates the possibility of tailoring the optical absorption and in that way increases the solar cells efficiency in various multilayer structures.

Thin Si films; nanostructure; HRTEM; SAED; Raman spectroscopy

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