Wave Plasma-enhanced Chemical Vapor Deposition Research Articles

Dependence of Optical Absorption in Silicon Nanostructures on Size of Silicon Nanoparticles

The amorphous silicon nanoparticles (Si NPs) embedded in silicon nitride (SiNx) films prepared by helicon wave plasma-enhanced chemical vapor deposition (HWP-CVD) technique are studied. From Raman scattering investigation, we determine that the deposited film has the structure of silicon nanocrystals embedded in silicon nitride (nc-Si/SiNx) thin film at a certain hydrogen dilution amount. The analysis of optical absorption spectra implies that the Si NPs is affected by quantum size effects and has the nature of an indirect-band-gap semiconductor. Further, considering the effects of the mean Si NP size and their dispersion on oscillator strength, and quantum-confinement, we obtain an analytical expression for the spectral absorbance of ensemble samples. Gaussian as well as lognormal size-distributions of the Si NPs are considered for optical absorption coefficient calculations. The influence of the particle size-distribution on the optical absorption spectra was systematically studied. We present the fitting of the optical absorption experimental data with our model and discuss the results.

Enhancement of electron field emission from carbon nanofiber bundles separately grown on Ni catalyst in Ni–Cr alloy

The enhancement of electron field emission was successfully obtained from carbon nanofiber (CNF) bundles separately grown on Ni grains in Ni–Cr alloy compared with that from CNF bundles on pure nickel. Ni grains less than 1 μm in size were separated by Cr grains in the alloy prepared by cosputtering with Ni pellets on a Cr plate without a lithographic patterning process. CNFs grew on Ni grains as bundles of 100 nm diameter and 1 μm length by helicon wave plasma-enhanced chemical vapor deposition, controlling the distance between adjacent CNF bundles by 500–1000 nm. The field emission characteristics of electrons from CNF bundles were evaluated and compared using current–voltage characteristic curves and a Fowler–Nordheim (F–N) plot converted using a work function value of 4.7 eV for CNFs. The field enhancement factor of FE characteristics obtained from the slopes in the F–N plot increased with the average distance defined as the space between adjacent CNF bundles. CNFs separately grown on a Ni–Cr catalyst are a candidate for effective two-dimensional field emitter devices.

Excitonic photoluminescence characteristics of amorphous silicon nanoparticles embedded in silicon nitride film

We have investigated the photoluminescence (PL) properties of amorphous silicon nanoparticles (a-Si NPs) embedded in silicon nitride film (Si-in-SiNx) grown by helicon wave plasma-enhanced chemical vapor deposition (HWP-CVD) technique. The PL spectrum of the film exhibits a broad band constituted of two Gaussian components. From photoluminescence excitation (PLE) measurements, it is elucidated that the two PL bands are associated with the a-Si NPs and the silicon nitride matrix surrounding a-Si NPs, respectively. The existence of Stokes shift between PL and absorption edge indicates that radiative recombination of carriers occurs in the states at the surface of the Si NPs, whereas their generation takes place in the a-Si NPs cores and the silicon nitride matrix, respectively. The visible PL of the film originates from the radiative recombination of excitons trapped in the surface states. At decreasing excitation energy (Eex), the PL peak energy was found to be redshifted, accompanied by a narrowing of the bandwidth. These results are explained by surface exciton recombination model taking into account there existing a size distribution of a-Si NPs in the silicon nitride matrix.