V. A. Coleman, S. Venkatesan, P. N. K. Deenapanray, H. H. Tan, and C. Jagadish

1
POROUS ALUMINA AS A TEMPLATE FOR THE DESIGN OF
NOVEL SEMICONDUCTOR NANOSTRUCTURES
V. A. Coleman, S. Venkatesan, P. N. K.
Deenapanray, H. H. Tan, and C. Jagadish Departmen
t of Electronic Materials Engineering, Research
School of Physical Sciences and Engineering, The
Australian National University, Canberra, ACT
0200, Australia
INTRODUCTION
Nanostructured materials, in particular
semiconductors, are set to become the next
generation of devices. Although it is currently
possible to fabricate devices from nano-scale
materials such as semiconductor quantum dots in
order for the true potential of these materials
to be realised there is a need to push towards
size, shape and distribution uniformity. Porous
alumina offers one such avenue towards achieving
this goal. Produced by a simple, inexpensive
bench-top electrochemical anodisation process,
it is characterised by a hexagonally close-packed
ordered array of pores, the size and density of
which can be controlled by changing the anodising
conditions and pre/post anodising treatment.
Goal is to produce a template that can be used
to produce an ordered array of gold nano-dots on
a surface. Gold acts as a catalyst for the
growth of semiconductor nanowires and dots.
FIGURE 3 SEM image of the porous alumina
barrier layer after removal of aluminium. (sample
partially gold coated to prevent charging)
Underlying pores
Barrier layer (gold coated)
GOLD EVAPORATION ONTO GaAs
Gold (50nm) was deposited through the a
free-standing porous template onto chemically
cleaned oxide free GaAs (111)B S-I substrates
(Fig. 1(g), (h)). Whilst the gold did transfer
through the template onto the surface, upon
template removal agglomeration of gold nano-dots
occurred, with the gold appearing to be mobile
under the electron beam. These results are
currently under further investigation.
FIGURE 1 Schematic diagram showing (a)
preliminary oxidation (b) removal of first oxide
layer to create ordered texture for subsequent
pore growth (c) final oxidation to form ordered
porous layer (d) removal of aluminium (e) barrier
layer removal and pore widening (f) adhesion of
porous template to substrate (g) deposition of
gold through the template to form gold nanodots
(h) removal of porous template (i) 1. epitaxial
growth of semiconductor nanowires (i) 2.
epitaxial growth of nanowires with quantum dot
layer
EXPERIMENTAL

• High Purity, 0.25 mm thick aluminium foil was
  purchased from Goodfellow
• The foil was cut into 1cm2 pieces and
  chemically degreased (boiling TCE,
  isopropanol, DI H2O, N2 blow-dry) ready for
  anodisation
• Anodiastion was conducted in an electrochemical
  pulsed oxidation cell
• 0.3M oxalic acid was used as an electrolyte
• Anodisation carried out for 1 hour at 40V DC
  using a 6ms pulse cycle and duty load of 2/3
• Following anodisation, the porous layer is
  etched in 5 phosphoric acid
• Remaining aluminium may be etched in acidified
  cupric chloride solution

FIGURE 4 SEM image of agglomerated Au nano-dots
on GaAs (111)B formed by evaporation through a
porous alumina template
FIGURE 2 SEM image of a porous alumina layer
after four oxidation steps showing large domain
of pore order
CONCLUSIONS/FUTURE WORK
We have successfully fabricated free standing
porous alumina templates with ordered pore
domains 1mm in size. The pores are typically
60nm in size. Currently more work needs to be
conducted into pattern transfer in order to
realise the growth of ordered semiconductor
nanostructures, such as those illustrated in
Figure 1 (i).
PORE ORDERING
TEMPLATE FORMATION
Although pores can be achieved after one
anodisation cycle, to get good ordering, a number
of anodisation cycles must be conducted. After
each cycle, the previously made porous layer is
etched off with phosphoric acid (see Fig. 1. (a)
and (b)). We find that after four cycles, well
ordered pores are formed with domains of 1mm in
size. Pore size is 60nm (though this can be
controlled) An SEM image of a large ordered
domain of pores is shown in figure 2.
After the fourth oxidation step, the porous
alumina can be turned into a free standing mask
(Fig 1. (c) (e)). This is achieved by first
protecting the porous layer and then etching the
aluminium from the back side in acidified cupric
chloride soultion. Figure 3 shows an SEM image
of the barrier layer (bottom of alumina pores)
after aluminium removal. The barrier layer is
removed by etching in phosphoric acid. A free
standing template is then formed which may be
applied to a substrate for pattern transfer (Fig
1. (f)).
REFERENCES