STRUCTURAL-PROPERTIES OF POLYCRYSTALLINE SILICON FILMS PREPARED AT LOW-TEMPERATURE BY PLASMA CHEMICAL VAPOR-DEPOSITION

Evolution with thickness of the structure of the polycrystalline silicon (poly-Si) films prepared at 300-degrees-C has been studied by plasma decomposition of SiF4/SiH4/H2 source gases. The poly-Si films with varied thickness are characterized mainly by Raman spectroscopy, x-ray diffraction (XRD), and supplementarily by reflection high-energy electron diffraction, transmission electron microscopy, Fourier-transform infrared (FT-IR) spectroscopy, electron-spin resonance (ESR), and secondary-ion-mass spectroscopy (SIMS) measurements. The crystalline fraction of the film was calculated to be 87% by deconvoluting the Raman spectra. The grains indicated a strong <110> preferred orientation by XRD. The thickness (d) dependence of the diffracted (220) intensity is divided into three regions: an incubation region (d < 200 nm, region 1), a linear region (200 nm less-than-or-equal-to d < 300-500 nm, region 2) where the deposition parameter (SiF4 flow rate, substrate temperature, and rf power) dependence is weak, and a linear region with steeper (or more moderate) slopes (300-500 nm less-than-or-equal-to d, region 3) where the deposition parameter dependence is large. The measurements of the angular distribution of the <110> grains reveal that they contain slanting ones by more than 4-degrees in region 2, while they disappear in region 3. The FT-IR and SIMS measurements for typical samples (T(s) = 300-degrees-C, 300 Pa) indicate that the grain boundaries are passivated by hydrogen in the bonding configurations of Si-H(n) (n = 1-3) and its concentration is approximately 3 at. %. The residual fluorine in the film is found to be much fewer (6 x 10(19) cm-3) than hydrogen. It is found that the density of unpassivated dangling bonds indicates a low value of 1.1 x 10(17) cm-3 for the film with d = 280 nm by ESR measurements. The origin of the preferred orientation is also discussed on the basis of a model in which nucleation, ledge formation, and etching processes are considered.