L. Schiavulli

Plasma deposition and characterization of photoluminescent fluorinated nanocrystalline silicon films

Fluorinated nanocrystalline silicon films, nc‐Si:H,F, have been deposited from SiF4–SiH4–H2 mixtures by means of the plasma enhanced chemical vapor deposition technique. The presence of fluorine atoms, which are effective etchant species, promotes selective etching giving nanocrystalline films. These materials, with grain size of 100–200 A, show a room temperature photoluminescence centered at 1.62 eV. Also, the widening of the optical energy gap (Eg=2.12 eV) is mainly due to the presence of nanocrystals rather than to the H content of 4.5 at. %.

Plasma deposition of amorphous SiC:H,F alloys from SiF4‐CH4‐H2 mixtures under modulated conditions

Fluorinated and hydrogenated amorphous silicon‐carbon alloys (a‐SiC:H,F) are produced by glow discharge decomposition of SiF4‐CH4‐H2 mixture. Small amount of CH4 in SiF4‐H2 mixture are enough to produce silicon carbon alloys having optical gap ranging between 1.8 and 2.6 eV. Materials, having 1.95 eV band gap and exhibiting optoelectronic properties typical of state of art a‐SiC:H, are deposited under plasma modulation conditions.

Optical constants of silicon films deposited by the r.f. glow discharge of SiCl4

Abstract The absorption coefficients and the refractive indices of silicon films deposited by using a glow discharge in an SiCl 4 -H 2 mixture were measured. Specimens were deposited with one deposition parameter being varied and the others kept constant. The dependence of the optical constants on the various deposition parameters was determined. The optical gap was calculated by applying Taucs analysis to the absorption data. The results are interpreted in terms of the hydrogen and chlorine contents and of the morphological changes in the silicon network.

Electrical properties of gold–polymer composite films

Electrical conductivity measurements in the range 50–300 K have been carried out on gold–fluorocarbon–polymer composite films. Such measurements have been performed on samples with different percentages in volume of gold, before and after thermal annealing up to 450 K. The experimental temperature dependence of conductivity, in the range 50–300 K, follows the exponential law σ=σ0 exp[−(T0/T)γ], where T0 and γ are constants. Best fit analyses carried out on experimental conductivity data, before and after thermal annealing, give values of γ that can be related to the gold content of the film and to the size of grains embedded in the polymeric matrix. Such results suggest that grain redistribution can take place after thermal annealing and that the grains tend to coalesce. Dependence of conductivity on temperature during heating and cooling processes is also discussed.

Deposition rate, ion bombardment and gap states density in glow discharge a-Si:H,F films

Abstract The optical transmission of hydrogenated and fluorinated amorphous silicon films has been measured by both photothermal deflection spectroscopy and classical transmittance. The density of defects and the Urbach energy are assumed as variable parameters. Their trends are discussed as a function of the deposition rate and of the effect of the ionic bombardment.

Optical absorption and electrical conductivity measurements of microcrystalline silicon layers grown by SiF4/H2 plasma on glass substrates

Abstract Microcrystalline silicon (μc-Si) films, having different crystalline fractions ( f c ) and thicknesses, were deposited on glass by plasma enhanced chemical vapor deposition (PECVD) starting from a SIF 4 /H 2 /He gas mixture. The absorption coefficient (α) in the energy range 0.8–2.0 eV was evaluated from the standard constant photocurrent method (S-CPM) and dark conductivity measurements have been performed. The S-CPM data elaboration was carried out according to the Favre approach, in order to take account for the bulk scattering. The true α(α true ) vs. energy trend was interpreted to evidence defect density and the crystalline-amorphous fraction of the samples. However, no thickness effect on the α true coefficient was observed, whereas the change in crystalline fraction resulted in different scattering effects.

Deposition of Photoluminescent Nanocrystalline Silicon Films by SiF 4 -SiH 4 -H 2 Plasmas

Visible photoluminescence at 1.62 eV has been observed at room temperature from fluorinated and hydrogenated nanocrystalline silicon (nc-Si:H,F) produced in a typical plasma enhanced chemical vapor deposition system. The use of SiF 4 -SiH 4 -H 2 mixture, because of the H 2 dilution and the presence of SiF 4 , favours the amorphous - crystalline transition through the etching process of the amorphous phase. The x - ray diffraction measurements give an average grain size of about 100 A. The presence of these nanocrystals shifts the absorption edge of the films towards higher energy. An energy gap of 2.12 eV is estimated, although the hydrogen content in the material is only 4.5 at. %. The temperature dependence of the photoluminescence behaves similarly to that of porous silicon.

Effects of the dopants on the electrical conductivity and hall mobility in Si: H, Cl films☆

Abstract The electrical conductivity was measured as a function of the temperature in films of hydrogenated and chlorinated silicon. Phosphine and diborane were added to the gas phase in well-controlled percentages. Differences in the behaviour of the activation energy of the conductivity are found according to the deposition conditions of the films. The Hall mobility varies with the doping and with the crystallite size, which in turn depends on the deposition rate.

Optical and electrical properties of glow discharge silicon films

Abstract Silicon films grown by r.f. glow discharge in a mixture of SiCl4 and hydrogen were characterized optically and electrically. It was possible to determine the optical constants as a function of the deposition temperature. Measurements of the dark conductivity as a function of the temperature permitted the calculation of the activation energies of various transport processes. The photoconductivity was studied as a function of the wavelength, the intensity of the incident light and the temperature.

Electronic properties of amorphous gallium arsenide deposited by reactive sputtering

Abstract Films of amorphous gallium arsenide(a-GaAs) have been deposited by reactive r.f. sputtering of monocrystalline GaAs target.The stoichiometry of the deposited films has been controlled by Rutherford Backscattering (RBS) and Energy Dispersive System (EDS).Conductivity has been measured as a function of the temperature and the density of gap states at the Fermi level was evaluated by Space-Charge-Limited-Currents (SCLC).