Efficacy of Raman mapping over ellipsometric spectroscopy and XRD for characterization of structurally heterogeneous PLD nc-Si thin films

摘要

The present report demonstrates the efficacy of Raman mapping over X-Ray diffraction (XRD) and spectroscopic ellipsometry (SE) in structural characterization of nanocrystalline Silicon (nc-Si) films having spatial heterogeneity in crystallinity and structure. The Si films were deposited via pulsed-laser deposition (PLD) technique at room temperature and elevated substrate temperature (T-s) of 400 degrees C and 700 degrees C, under vacuum (similar to 10-6 mbar). XRD showed that all films were polycrystalline while SE studies showed that only at 700 degrees C, the film was nanocrystalline in nature otherwise largely amorphous at lower T-s. This discrepancy in results is well explained by Raman maps which confirmed the presence of micron sized structures composed of nc-Si embedded in otherwise uniform matrix comprised of amorphous Silicon (a-Si) for films fabricated at room temperature and 400 degrees C whereas at 700 degrees C the uniform background matrix is dominated by nc-Si. The real and imaginary parts of pseudo dielectric function (epsilon(1),epsilon(2)) versus energy spectra obtained for films grown at RT, 400 degrees C, and 700 degrees C were modeled and fitted to substrate/film/top SiO2 layered structure. The simulated (epsilon(1),epsilon(2)) patterns are in excellent agreement with the recorded spectra. From the SE studies the band gap and thickness of the films were estimated. Band gap energy was found to vary from 1.35 to 1.30 eV approximately with increasing T-s from room temperature to 400 degrees C, then gradually increases to 1.55 eV at 700 degrees C. The variation of other optical constants of the films as a function of T-s also reflects the structural transition from amorphous-like to nanocrystalline-like with increasing T-s.