Growth of silicon nanowires
Intrinsic Si nanowires were synthesized by catalytic vapor
liquidsolid chemical vapor deposition [36] for which a 2 nm
thick gold layer was deposited as catalyst on thermally oxidized Si
wafers. Hydrogen diluted silane gas (H2/SiH4: 170/30 sccm) in a
total pressure of 15 mbar was admitted as the precursor molecule
at 400 C. The diameter and length of asgrown nanowires were
~10e60 nm and ~5 mm, respectively. Dispersions of Si nanowires in
2propanol solution were prepared by sonication for subsequent
randomoriented nanowire deposition.
2.2. Fabrication of dual layer nanowirepolythiophene transistors
A series of bottomgate bottomcontact FETs were fabricated
with channel length (L) of 20 mm and width (W) of 500 mm. The
channel layer consisted of a randomoriented network of Si nano
wires by dropcasting, covered with a solutionprocessable poly
thiophene semiconductor as the host matrix. Source and drain
contact patterns (Cr/Au: 5/50 nm thick) were de ned by photoli
thography, thermal evaporation and liftoff onto heavily doped n
type Si substrates with thermally grown SiO2 (200 nm thick) as
dielectric layers. A network of dispersed Si nanowires was drop
casted by a volumecontrolled single channel pipette. Repeating
the nanowire deposition process (at about 10 min intervals to allow
for solvent drying) can control the density of nanowire loading on
device substrates from N ¼ 0 to 0.4 (average number of nanowires
per 10 10 mm2). Substrates with nanowires were baked at 60 C
overnight and cleaned by oxygen plasma to remove residual sol
vent/contaminates completely.
After diluted HF dipping (HF/H2O: 1/40, 50 s) of native oxide
shell on Si nanowires, the surface of SiO2 and loaded nanowires
were treated with hydrophobic selfassembled monolayers (SAMs)
of noctadecyltrichlorosilane (ODTS, 90%, Aldrich) to enhance
polymer chain stacking. A conjugated semiconductor solution of