a-Si1 xCx Research Articles

A spherical distributed Bragg reflector based on a-Si1–xCx: H and a-SiO2 layers

A spherical distributed Bragg reflector (SDBR) for near-infrared applications is elaborated via plasma-enhanced chemical-vapor deposition. It consists of the alternating quarter-wave layers of amorphous hydrogenized silicon carbide (a-Si1–xC x : H) and amorphous silicon oxide (a-SiO2) deposited onto a glass microsphere. The reflection spectra of the SDBR are measured at different points of its surface. In all cases, the Bragg-reflection bands, as well as the wavelength range of their overlap, are detected, evidencing the presence of a complete photonic band gap in the SDBR obtained.

ОПТОЭЛЕКТРОННЫЕ СВОЙСТВА ТОНКИХ ПЛЕНОК ГИДРОГЕНИЗИРОВАННОГО АМОРФНОГО КРЕМНИЯ-УГЛЕРОДА И НАНОКРИСТАЛЛИЧЕСКОГО КРЕМНИЯ

Some parameters of thin films fabricated of hydrogenated amorphous silicon–carbon alloys a-Si1 xCx:H with x = 0 and 0.5 and nanocrystalline silicon (nc-Si) and serving as a basis for developing solar cells including a Schottky barrier and p–i–n and double p–i–n heterojunctions have been considered. In double p–i–n heterojunctions, a-SiC/a-Si/nc-Si, the p-layer was made from a-SiC:H and used as a “window”, and the n-layer was made from nc-Si. The current-voltage characteristics of solar cells of each type at their illumination are studied. The highest efficiency of 11.5% was found for solar cells with the double p–i–n heterojunctions in the case where a cell 1 cm2 in area was illuminated with light of a 100-mW/cm2 intensity.

The effect of relatively low hydrogen dilution on the properties of carbon-rich hydrogenated amorphous silicon carbide films

Carbon-rich hydrogenated amorphous silicon carbide (a-Si1–xCx:H) films were deposited by plasma enhanced chemical vapor deposition (PECVD) using silane, ethylene and hydrogen as gas sources. The effect of relatively low hydrogen dilution on the properties of as-deposited samples was investigated. A variety of techniques including Scanning Electron Microscope (SEM), Fourier transform infrared spectroscopy (FTIR), Raman scattering (RS), UV-VIS spectrophotometer and photoluminescence (PL) spectroscopy were used to characterize the grown films. The deposition rate decreases with hydrogen dilution. The silicon to carbon ratio increases slightly with the addition of hydrogen. The phenomenon can be attributed to the dissipation of power density caused by hydrogen dilution. Raman G peak position shifting to a lower wave number indicates that hydrogen dilution reduces the size and concentration of sp2 carbon clusters, which is caused by the etching effect by atomic hydrogen. The optical band gap, which is controlled by the sp2 carbon clusters and Si/C ratio, changes unmonotonously. The as-deposited samples exhibited a blue-green room-temperature (RT) PL well visible to the naked eye with UV excitation. The PL band can be attributed to the radiative recombination of electron-hole pairs within small sp2 clusters containing C=C and C-H units in a sp3 amorphous matrix.

The influence of radio frequency power on the characteristics of carbon-rich hydrogenated amorphous silicon carbide films

A series of carbon-rich hydrogenated amorphous silicon carbide (a-Si1–xCx:H) films were prepared at different radio frequency (RF) powers from silane-ethylene-hydrogen plasma. The effect of the RF power on the bonding configurations and microstructures has been investigated. The grown films were characterized by a collection of techniques including Scanning Electron Microscope, Fourier transform infrared(FTIR) spectroscopy, Raman scattering and photoluminescence spectroscopy. The deposition rate increases upon RF power due to the enhancement of chemical reactivity of plasma. The carbon to silicon ratio increases, for more C2H4 molecules decompose with the enhancement of RF power and more carbon atoms are bonded into the films. Raman G peak position shifts to a higher wavenumber, which indicates that the size and concentration of sp2 carbon clusters increase as the RF power becomes stronger. Blue-green photoluminescence is detected at room temperature. The PL band can be attributed to the existence of the amorphous carbon clusters in films with high carbon concentrations.

Selective dissolution of silicon in porous amorphous Si 1– x C x :H

We have investigated the properties of porous layers obtained by anodisation of hydrogenated amorphous silicon-carbon alloys (a-Si1–xCx:H) in HF electrolyte. Room-temperature photoluminescence of bulk and porous a-Si1–xCx:H samples for different carbon concentrations reveals that, at the same carbon content, the photoluminescence energy is much higher for porous a-Si1–xCx:H than for the bulk material. Infrared characterisation of the porous layers suggests that carbon concentration is higher in porous a-Si1–xCx:H than in the starting material, accounting for the luminescence blue shift. This carbon enrichment, due to selective Si dissolution during porous layer formation, has been confirmed and quantified by secondary ion mass spectroscopy. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Evidence for excitonic behavior of photoluminescence in polymer-like a-C:H films

Abstract To understand the dynamics of energy transfer and randomization of photoluminescence polarization in hydrogen-rich polymer-like amorphous carbon a-C:H films, time-resolved investigations of intensity and anisotropy decays have been performed recently. The intensity decay rates increase exponentially as a function of emission energy with a behavior very similar to that observed in wide band-gap C-rich a-Si1–xCx:H. In addition, in polymer-like carbon, the observation of a plateau of PL anisotropy in the 100–1000 ps range, is taken as strong evidence for the existence of a finite density of excitonic species in radiative recombination phenomena; it does not fit the phonon-assisted depolarization models proposed earlier. Polarization anisotropy decays and steady-state values are consistently interpreted using a dipole–dipole non-radiative energy transfer mechanism (Forster mechanism) with a characteristic depolarization time of 50 ps rather independent of the emission energy. The latter value is likely to be related to the density of radiative centers distribution estimated independently in the constant exciton radius approximation, rather than the result of hopping in an exponential distribution of tail states.

Effects of Electrode Spacing and Hydrogen Dilution on a-SiC:H and a-Si:H Layers

A series of hydrogenated amorphous silicon carbide (a-Si1–xCx:H) films was deposited by rf glow discharge deposition using various pressures, electrode spacings and hydrogen dilution ratios. We found that improvement of the structure by hydrogen dilution is more effective when a large electrode spacing is applied. In the case of undiluted a-SiC:H, the product of pressure and electrode spacing appears to be the important parameter. Dilution causes an increase of the photoconductivity. The band gap decreases but increases again for highly diluted samples. A striking result is that the Fourier transform infra-red spectroscopy (FTIR) bands assigned to CHX and SiHx increase upon dilution when a small electrode spacing is applied, although the hydrogen content is reduced. It is shown that this is due to an increase of the density of the films and to an increase of the amount of carbon built into the bulk instead of into voids. The combination of decreasing hydrogen content, void fraction and increasing amount of carbon atoms into the bulk explains the behaviour of the photoconductivity and band gap as a function of H2 dilution.

Relation between thermal equilibrium temperature and bandgap in undoped hydrogenated amorphous silicon-based alloys

Abstract This Letter demonstrates that there is a good correlation between the thermal equilibrium temperature T E and the optical gap E opt for undoped hydrogenated amorphous silicon (a-Si: H) and a-Si-based alloys: T E, is proportional to E opt for both wide-bandgap a-Si-based alloys, such as a-Si1–x Cx: H and a-Si1–x Nx: H, and narrow-bandgap a-Si1–xCx: H alloys. It is also found that the relaxation process of all thermally-induced metastable changes in these alloys follows a stretched exponential form.