Effect of hydrogen dilution on the deposition of carbon-rich a-SiC:H films by the electron cyclotron resonance method

The deposition of hydrogenated amorphous silicon carbide (a-SiC:H) films from a mixture of silane, acetylene and hydrogen gas using the electron cyclotron resonance chemical vapour deposition (ECR-CVD) process is reported. The variation in the deposition and film characteristics such as the deposition rate, optical band gap and IR absorption as a function of the hydrogen dilution is investigated. The deposition rate increases to a maximum value of about 250 Å min-1 at a hydrogen dilution ratio of about 20 (hydrogen flow (sccm)/acetylene + silane flow (sccm)) and decreases in response to a further increase in the hydrogen dilution. There is no strong dependence of the optical band gap on the hydrogen dilution within the dilution range investigated (10-60) and the optical band gap calculated from the E04 method varied marginally from about 2.85 to 3.17 eV. The room temperature photoluminescence (PL) peak energy and intensity showed a prominent shift to a maximum value of about 2.17 eV corresponding to maximum PL intensity at a moderate hydrogen dilution of about 30. The PL intensity showed a strong dependence on the hydrogen dilution variation.

Nature of disorder and localization in amorphous carbon

The electronic structure of amorphous diamond-like carbon is studied. Analysis of the participation ratio shows that π states within the σ-σ* gap are localized. The localization arises from dihedral angle disorder. The localization of π states causes the mobility gap to exceed the optical gap, which accounts for the low carrier mobility and the flat photoluminesence excitation spectrum. © 1998 Elsevier Science B.V. All rights reserved.

Doping mechanism in tetrahedral amorphous carbon

Doping in hydrogenated amorphous silicon occurs by a process of an ionised donor atom partially compensated by a charged dangling bond. The total energies of various dopant and dopant/bonding combinations are calculated for tetrahedral amorphous carbon. It is found that charged dangling bonds are less favoured because of the stronger Coulombic repulsion in ta-C. Instead the dopants can be compensated by weak bond states in the lower gap associated with odd-membered π-rings or odd-numbered π-chains. The effect is that the doping efficiency is low but there are not charged midgap recombination centres, to reduce photoconductivity or photoluminescence with doping, as occurs in a-Si:H.

Properties of tetrahedral amorphous carbon deposited by a filtered cathodic vacuum arc

The properties of a highly sp3 bonded form of amorphous carbon denoted ta-C deposited from a filtered cathodic vacuum arc (FCVA) are described as a function of ion energy and deposition temperature. The sp3 fraction depends strongly on ion energy and reaches 85% at an ion energy of 100 eV. Other properties such as density and band gap vary in a similar fashion, with the optical gap reaching a maximum of 2.3 eV. These films are very smooth with area roughness of order 1 nm. The sp3 fraction falls suddenly to almost zero for deposition above about 200 °C.

Doping mechanism in tetrahedral amorphous carbon

Doping in hydrogenated amorphous silicon occurs by a process of an ionized donor atom partially compensated by a charged dangling bond. The total energies of various dopant and dopant/bonding combinations are calculated for tetrahedral amorphous carbon. It is found that charged dangling bonds are less favored because of the stronger Coulombic repulsion in ta-C. Instead the dopants can be compensated by weak bond states in the lower gap associated with odd-membered π-rings or odd-numbered π-chains. The effect is that the doping efficiency is low but there are not charged midgap recombination centres, to reduce photoconductivity or photoluminescence with doping, as occurs in a-Si:H.

Surface atomic properties of tetrahedral amorphous carbon

The surface energy and surface atomic structure of tetrahedral amorphous carbon has been calculated by an ab-initio method. The surface atoms are found to reconstruct into sp2 sites often bonded in graphitic rings. Placing the dangling bonds on adjacent surface atoms lower their energy by π-bonding and this is the source of the low surface energy. The even lower surface energy of hydrogenated amorphous carbon (a-C:H) is due to the hydrogenation of all broken surface bonds. © 2005 Elsevier B.V. All rights reserved.