Structural properties of polycrystalline silicon films prepared at low temperature by plasma chemical vapor deposition

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Abstract
Evolution with thickness of the structure of the polycrystalline silicon (poly‐Si) films prepared at 300 °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 (dd4 flow rate, substrate temperature, and rf power) dependence is weak, and a linear region with steeper (or more moderate) slopes (300–500 nm≤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° in region 2, while they disappear in region 3. The FT‐IR and SIMS measurements for typical samples (Ts = 300 °C, 300 Pa) indicate that the grain boundaries are passivated by hydrogen in the bonding configurations of Si—Hn (n=1–3) and its concentration is approximately 3 at. %. The residual fluorine in the film is found to be much fewer (6×1019 cm−3) than hydrogen. It is found that the density of unpassivated dangling bonds indicates a low value of 1.1×1017 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.