Changes in the electrical conductivity of thin (< 300 nm) silicon films following prolonged exposure to atmosphere, are reported. Both reversible (by annealing at 150 ºC under vacuum) and irreversible (annealing-resistant) effects are found to occur, which are larger in thinner films. The conductivity prefactor and thermal activation energy obey the Meyer-Neldel rule, although detailed behaviour depends on film thickness and microstructure. Irreversible changes may result from oxidation of thinner, more porous films, with water and/or oxygen adsorption and desorption responsible for reversible changes. The need to identify and account for these effects when discussing and formulating transport mechanisms in these materials is underlined.

Transient photoconductivity in microcrystalline silicon thin films prepared by VHF PECVD over a range of [silane]: [silane + hydrogen] concentrations γ from 3 to 6.3% has been measured as a function of temperature (210 K - 390 K) and laser pulse density (1013 – 1016 cm3). The behavior of highly crystalline films (γ ∼ 3%) bears certain similarities to n-type amorphous silicon, where the power law decay at long times is controlled by hole rather than electron kinetics. As γ approaches 6% the decay begins to resemble that of intrinsic amorphous silicon, consistent with structural studies which indicate a transition from crystalline to amorphous character. Transient photocurrent density of states spectroscopy on transitional films suggests a similar deep defect density to high-quality amorphous silicon, but a larger conduction band tail energy of some 30 meV and a tenfold increase in localised state density at a depth of about 0.4 eV.

Articles you may be interested inLow temperature InAlAs buffer layers using trimethylarsenic and arsine by metalorganic chemical vapor deposition Appl.

The ultrahigh vacuum technique of molecular-beam epitaxy (MBE) has been successfully employed in growing highly oriented polycrystal and single-crystal niobium oxide layers on z-cut LiNbO3 substrates. A new variant of the monoxide has been grown and characterized, and the potential advantages and limitations of MBE for growing the higher oxides, such as Nb2O5, are put forward. Niobium metal layers have also been grown on z-cut LiNbO3. The deposition of excellent quality niobium oxide on α-alumina (z-cut sapphire) has been demonstrated, providing an optical waveguiding structure of large differential refractive index. Crystallinity and compositional data are presented for all the above layers. The post-deposition oxidation of oxygen-deficient films to the pentoxide has been studied in detail, and system requirements for the in situ growth of Nb2O5 proposed.

We report on the use of coplanar transient photoconductivity and post-transit time-of-flight spectroscopy techniques in the study of carrier transport in microcrystalline silicon films prepared over a range of crystallinities. Coplanar samples are susceptible to post-deposition oxidation and reversible adsorption of atmospheric gases, which may alter the apparent density of states. Coplanar measurements suggest lower deep defect densities in more highly crystalline films, but this is due at least in part to an increased occupancy of these states. A comparison of results obtained using both techniques suggests anisotropic transport, with reduced band tailing (greater structural order) along the direction of film growth, a larger defect concentration around the column boundaries, and a higher defect density within the amorphous tissue than in optimised single-component amorphous silicon films.