Daniel Queen

From amorphous to nanocrystalline: the effect of nanograins in amorphous matrix on the thermal conductivity of hot-wire chemical-vapor deposited silicon films

We have measured the thermal conductivity of amorphous and nanocrystalline silicon films with varying crystalline content from 85 K to room temperature. The films were prepared by the hot-wire chemical-vapor deposition, where the crystalline volume fraction is determined by the hydrogen (H2) dilution ratio to the processing silane gas (SiH4), R  =  H2/SiH4. We varied R from 1 to 10, where the films transform from amorphous for R  <  3 to mostly nanocrystalline for larger R. Structural analyses show that the nanograins, averaging from 2 to 9 nm in sizes with increasing R, are dispersed in the amorphous matrix. The crystalline volume fraction increases from 0 to 65% as R increases from 1 to 10. The thermal conductivities of the two amorphous silicon films are similar and consistent with the most previous reports with thicknesses no larger than a few μm deposited by a variety of techniques. The thermal conductivities of the three nanocrystalline silicon films are also similar, but are about 50-70% higher than those of their amorphous counterparts. The heat conduction in nanocrystalline silicon films can be understood as the combined contribution in both amorphous and nanocrystalline phases, where increased conduction through improved nanocrystalline percolation path outweighs increased interface scattering between silicon nanocrystals and the amorphous matrix.

Effect of magnetic Gd adatoms on the transport properties of ultrathin gold films

Ultrathin two-dimensional gold films have been grown on an amorphous Ge underlayer by quench condensation at low temperature, followed by adsorption of magnetic Gd atoms and nonmagnetic Y atoms. The resulting electrical transport as a function of temperature and composition has been investigated in situ. Gold films of different sheet resistances R have been used for the Gd and Y adsorption platform. The temperature and thickness dependence of the conductance G G=1 /R indicates that the Au films cross from a strongly localized regime, where conductivity is through hopping and where electron correlation effects are expected to be strong, to a weakly localized regime. The system is shown to be sensitive to different added electronic states, in that adding Gd or Y increases G, but much less than adding the same amount of Au for all initial G values. No difference is observed down to 5 K between added Gd and Y, showing that there is no effect of the Gd magnetic moments on electrical transport. The absence of magnetic localization and dominance of adding electronic states over added electronic potential disorder in this quench-condensed ultrathin system is discussed and attributed to the intrinsically high electronic concentration of Au.