Study of nanocrystalline silicon films synthesized Below 100 degrees C by catalytic chemical vapor deposition.

Nanocrystalline silicon (nc-Si) films were synthesized by catalytic chemical vapor deposition at a low substrate temperature (100 degrees C) for use as an active layer in bottom-gate thin-film transistors. The hydrogen-dilution technique was employed to increase the crystalline volume fraction of the synthesized films. The incubation layer thickness was estimated to be 5.1 nm for a hydrogen-dilution ratio, R(H) (= [H2]/[SiH4]), of 54. When R(H) was increased from 64 to 74, the deposition rate decreased from 20 to 0.5 nm/min. In order to achieve a high deposition rate and high crystallinity near the interface region, we modulated R(H) through the film thickness. We also fabricated metal-insulator-semiconductor-insulator-semiconductor diodes from multilayer structures consisting of an nc-Si layer sandwiched between two silicon nitride layers. By analyzing the capacitance-voltage characteristics of these diodes, we found that the hysteresis and rectifying behavior of these diodes were affected by the the nc-Si layer thickness.

Optical and structural properties of nanocrystalline silicon potential well structure fabricated by cat-chemical vapor deposition at 200 degrees C.

We attempted to fabricate multi-layer, thin film structures by catalytic chemical vapor deposition (Cat-CVD) at a low temperature (200 degrees C). A 5-10-nm-thick nanocrystalline silicon (nc-Si) layer was positioned asymmetrically between two silicon nitride (SINx) layers. The compositions of the SiNx layers were varied between silicon-rich and nitrogen-rich. Each layer was deposited continuously in the Cat-CVD chamber without post-annealing. High-resolution transmission electron microscopy (HRTEM) revealed that the nc-Si layer grew in columns on the surface of the bottom SiNx layer, and the columnar structure extended up to a few nanometers of the top SiNx layer. In photoluminescence (PL) spectra, the overall intensity increased with the thickness of the nc-Si layer, but the primary peak position changed more sensitively relative to the composition of the SiNx layers. Capacitance-voltage (C-V) hysteresis was observed only when 10-nm-thick nc-Si layers were inserted between the nitrogen-rich silicon nitride (NRSN) layers. Under a bias voltage of 5 V, the current in the sample with a 10-nm-thick nc-Si layer was higher by at least two orders of magnitude than that in the sample with a 5-nm-thick nc-Si layer. The I-V curve was fitted well using both the Fowler-Nordheim and the Poole-Frenkel models for electric fields of magnitudes greater than 1.1 MV/cm, thereby implying that both mechanisms contribute to the increase in the leakage current.

Suppression of electrical breakdown in silicon nitride films deposited by catalytic chemical vapor deposition at temperatures below 200 degrees C.

Silicon nitride (SiN(x)) films for a gate dielectric layer of thin film transistors were deposited by catalytic chemical vapor deposition at a low temperature (< or = 200 degrees C). A mixture of SiH4, NH3 and H2 was used as a source gas. Metal-insulator-semiconductor (MIS) capacitor structures were fabricated for current-voltage (I-V) and capacitance-voltage (C-V) measurements. The breakdown voltage characteristics of the SiN(x) films were improved by the increase of NH3/SiH4 and H2/SiH4 mixing ratios and substrate temperatures. H2 treatment was attempted to improve the breakdown voltage further. A breakdown voltage as high as 6.6 MV/cm was obtained after H2 annealing at 180 degrees C. The defect states inside the SiN(x) films were analyzed by photoluminescence spectra. Silicon dangling bonds (2.5 eV) and nitrogen dangling bonds (3.1 eV) were observed. These defect states inside the SiN(x) films disappeared after H2 annealing. Flat band voltage shifts were observed in C-V curves, and their magnitudes decreased as the defect states inside the SiN(x) films decreased.