Nanowires of GaN have been grown by vapor phase deposition in templates.
Single crystalline GaN nanowires (hexagonal wurzite structure) of approximate diameter 14 nm and several micron in length were fabricated in an anodic alumina
membrane of around 50 mm thickness [53]. The four Raman lines of the nanowires in the range 531 to 745 cm
À1 agree with the phonon vibration frequencies of
the crystalline wurtzite GaN. The growth was carried out in an alumina membrane
prepared from 5 N aluminum. The deposition was performed at 1273 K from a
vapor of Ga 2 O with NH 3 gas. The gaseous Ga 2 O was prepared from heating in situ
a mixture of Ga and Ga 2 O 3 . A highly ordered array of such GaN using a similar
growth procedure to that above has been reported. The growth of the nanowires
was catalyzed by electrodeposition of In in the pores from InCl 3 electrolyte [54].
These wires with typical diameter 50 nm and 40–50 mm length have visible PL in
the region 360–540 nm at room temperature.
Aligned carbon nitride nanotubes have been fabricated in an alumina membrane
using electron cyclotron resonance (ECR) plasma-assisted CVD of C 2 H 2 [55]. The
membranes have a thickness of @50–80 mm and pore diameter of @250 nm. To
promote the flow of ionic fluxes through the nanochannels of the alumina template a negative dc bias voltage was applied to the graphite substrate holder.
Si nanowires have been grown in templates using a direct vapour–liquid–solid
route [56]. The wires were grown in a 60 mm thick and 200 nm pore diameter alumina membrane. A thin layer (< 1 mm) of Au is electrodeposited to form the catalyst. The growth was carried out using a 5% mixture of SiH 4 in H 2 in an isothermal low pressure reactor at 500
C. The nanowire has a defect-free core of Si with a
2–3 nm oxide layer and is capped with Au at both ends. It has a growth direction of
[100].
Application of CVD techniques, particularly plasma assisted CVD, to grow
aligned nanotubes of complex semiconductor or oxides in alumina templates is a
new tool. It is expected that in future this technique will find more applications.
21.4
Scanning Probe Based Anodic Oxidation as a Tool for the Fabrication of
Nanostructures
It is more than a decade since it was realized that the scanning probe microscope
(SPM), like the scanning tunneling microscope (STM) and the atomic force microscope (AFM) could be used to create structures at the nanometer scale on a
surface. It has also been demonstrated that even atoms can be manipulated using
an STM [57]. There were reports of STM tip induced modification of resist as early
as 1988 [58]. The field of SPM-based nanolithography is an established procedure
to create patterns at nanometer level that can be transferred onto an underlying
substrate, just like any other lithography mask in optical or e-beam lithography.
This can lead to fabrication of nanoelectronic devices. The biggest advantage of the
SPM-based lithography (despite its low writing speed) is that the same SPM can be
used for writing the pattern and to image it after the writing process. Early reviews
21 Nanolithography and Nanomanipulation
706
Single crystalline GaN nanowires (hexagonal wurzite structure) of approximate diameter 14 nm and several micron in length were fabricated in an anodic alumina
membrane of around 50 mm thickness [53]. The four Raman lines of the nanowires in the range 531 to 745 cm
À1 agree with the phonon vibration frequencies of
the crystalline wurtzite GaN. The growth was carried out in an alumina membrane
prepared from 5 N aluminum. The deposition was performed at 1273 K from a
vapor of Ga 2 O with NH 3 gas. The gaseous Ga 2 O was prepared from heating in situ
a mixture of Ga and Ga 2 O 3 . A highly ordered array of such GaN using a similar
growth procedure to that above has been reported. The growth of the nanowires
was catalyzed by electrodeposition of In in the pores from InCl 3 electrolyte [54].
These wires with typical diameter 50 nm and 40–50 mm length have visible PL in
the region 360–540 nm at room temperature.
Aligned carbon nitride nanotubes have been fabricated in an alumina membrane
using electron cyclotron resonance (ECR) plasma-assisted CVD of C 2 H 2 [55]. The
membranes have a thickness of @50–80 mm and pore diameter of @250 nm. To
promote the flow of ionic fluxes through the nanochannels of the alumina template a negative dc bias voltage was applied to the graphite substrate holder.
Si nanowires have been grown in templates using a direct vapour–liquid–solid
route [56]. The wires were grown in a 60 mm thick and 200 nm pore diameter alumina membrane. A thin layer (< 1 mm) of Au is electrodeposited to form the catalyst. The growth was carried out using a 5% mixture of SiH 4 in H 2 in an isothermal low pressure reactor at 500
C. The nanowire has a defect-free core of Si with a
2–3 nm oxide layer and is capped with Au at both ends. It has a growth direction of
[100].
Application of CVD techniques, particularly plasma assisted CVD, to grow
aligned nanotubes of complex semiconductor or oxides in alumina templates is a
new tool. It is expected that in future this technique will find more applications.
21.4
Scanning Probe Based Anodic Oxidation as a Tool for the Fabrication of
Nanostructures
It is more than a decade since it was realized that the scanning probe microscope
(SPM), like the scanning tunneling microscope (STM) and the atomic force microscope (AFM) could be used to create structures at the nanometer scale on a
surface. It has also been demonstrated that even atoms can be manipulated using
an STM [57]. There were reports of STM tip induced modification of resist as early
as 1988 [58]. The field of SPM-based nanolithography is an established procedure
to create patterns at nanometer level that can be transferred onto an underlying
substrate, just like any other lithography mask in optical or e-beam lithography.
This can lead to fabrication of nanoelectronic devices. The biggest advantage of the
SPM-based lithography (despite its low writing speed) is that the same SPM can be
used for writing the pattern and to image it after the writing process. Early reviews
21 Nanolithography and Nanomanipulation
706