Structural Analysis of Nanocrystals Embedded in Amorphous Si Films
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785 Mat. Res. Soc. Symp. Proc. Vol. 452 0 1997 Materials Research Society
EXPERIMENT The films investigated in this study were deposited by sputtering on glass (Coming 7059) and single-crystal silicon substrates. Two set of samples were prepared following the approach of direct and indirect growing of silicon nanocrystallites. The first set of samples (labelled hereafter as A) were deposited in pure hydrogen gas (99.9999%) at a pressure of 0.4 Torr and by varying the substrate temperature (Ts) between 50 °C and 250 'C. The thicknesses were compressed between 0.5 and 1.0 /m. The second set (labelled hereafter as B) were prepared by fixing the substrate temperature at 250 °C and by varying the hydrogen content (rH) of the mixed Ar-H 2 atmosphere from 0% to 75% of hydrogen; an annealing step for thermal crystallization was performed afterwards at 720 TC under N2 atmosphere and with a duration of 90 min. The thicknesses were between 0.9 and 1.4 Anm in this case. The size of the crystallites, stress and the crystalline fraction of the layers were determined from the analysis of the Raman spectra. The measurements were performed in a Jobin Yvon T64000 instrument by exciting the sample with an Ar laser working at a wavelength of 514.5 nm. The sample was mounted in the sample holder of a microscope and the light was collected in the backscattering configuration. The diameter of the spot on the sample was of about 0.7 ,Im and the power on the sample was of about 1 mW. X-Ray Diffraction (XRD) experiments and Transmission Electron Microscopy (TEM) observations were also performed as an independent way of measuring crystallite size and preferential orientations. Furthermore, the results are discussed in the light of Optical Transmittance (OT) measurements of the samples from 0.4 to 4.1 eV, performed to obtain the refractive index and the absorption coefficient and by applying a suitable model we determined the fraction of voids relative to the total volume. The hydrogen content of the layers was measured from the InfraRed (IR) absorption spectra recorded for the films deposited on silicon substrates. RESULTS AND DISCUSSION The Raman, XRD and TEM measurements of the samples investigated have shown evidence of variable proportion of crystalline fraction and/or crystalline size with T, (set A) or rH (set B). The Raman experimental spectra of all the samples presented both silicon amorphous and silicon crystalline bands. The Raman spectrum of the amorphous silicon is characterized by four main bands which were fitted with gaussian shapes centered at frequencies given approximately by 480 cmi', 380 cm-1 , 310 cm-n and 150 cm-1 [3]. The spectrum of bulk crystalline silicon is characterized by a lorentzian shaped band centered at 520 cm-r and having a width at half maximum of approximately 3 cm-1 . Nevertheless, we found the crystalline band of the Raman spectra of our samples to be highly asymmetric in the low frequency side. This is a clear indication of important confinement effects due to the small size of the crystallites as h
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