Caractérisation De Nanofils De Silicium Par Microscopie Électronique En Transmission

The goal of this thesis was to better understand and control nanowire growth by the Vapour Liquid Solid method, bymeans of structural, chemical and electronic characterization of nanowires using transmission electron microscopy.A first subject of study was the behavior of the gold catalyst particle. We have shown that control can be obtainedover gold diffusion from the catalyst particle over the nanowire sidewall, by changing the growth parameters, whichin turn influence the nanowire sidewall properties. The quantity of absorbed silane at the nanowire sidewall seemsto be the important parameter, that depends both on silane partial pressure and temperature and also an effect ofnanowire size was observed.A second topic was the crystallographic properties of the wires. The growth direction and sidewall facetingwere studied. In general the crystalline quality of wires grown with a gold catalyst using silane was high. If othercatalyst materials or precursor gasses were used more defects were observed. Especially in nanowires grown witha Ni catalyst and with a gold catalyst in combination with TMA, many vertical twin defects are present, as observedin an FIB prepared wire section sample. Therefore the different grains can superpose in the TEM image/DP, whichcan be falsely interpreted as a wurtzite hexagonal crystal structure. Clear evidence shows that in the observednanowires no hexagonal phase (with long range order) was present, but a high twin defect density can mimic ahexagonal structure on the length of a few atomic planes.A third theme was active dopant detection using off-axis electron holography. We show for the first time dopingcontrast in thin (60 nm) nanowires with axially modulated doping. Using off axis electron holography the electricalproperties of nanowires with axially modulated doping were studied, with the goal to determine the active dopantconcentration. We show that active dopant detection is possible in nanostructures such as nanowires. Dopingconcentrations as low as 10 at. cm−3 can be detected with respect to intrinsic silicon in nanowires as thin as60 nm. Potential simulations have been performed to better understand the experimental results. Comparisonof experiments and simulations shows that the surface properties of these nanostructures, and more specificallythe amount of charge present at or near the surface, are extremely important for the potential distribution in thestructure. The estimated amount of charge present at the wire-oxide interface is -1x10 e.c. (electron charges) cm−2,which is in excellent agreement with the amount of charge estimated from transport measurements.