NANO
LETTERS
Semiconductor Nanowires Prepared by
Diffraction-Mask-Projection
Excimer-Laser Patterning
2004
Vol. 4, No. 5
895-897
Thomas Ho1 che,*,† Rico Bo1 hme, Ju1 rgen W. Gerlach, Frank Frost,
Klaus Zimmer, and Bernd Rauschenbach
Leibniz-Institut für Oberflächenmodifizierung e.V., Permoserstrasse 15,
D-04303 Leipzig, Germany
Received February 24, 2004; Revised Manuscript Received March 22, 2004
ABSTRACT
Substrate-adhered semiconductor nanowires can be prepared by locally confined laser ablation of high-quality thin films. Wurtzitic GaN films
with thicknesses of several tens of nanometers were deposited by ion-beam-assisted molecular beam epitaxy on 6H-SiC. Diffraction-maskprojection laser ablation (at a wavelength of 248 nm) was used to convert the thin film into a well-ordered arrangement of parallel, substrateadhered nanowires. The ablation profile generated by the stripe-patterned phase mask was shown to sharpen upon multipulse application.
Hence, structural widths of the remaining semiconductor banks below 200 nm can be achieved. Beyond substrate adherence, the introduced
methodology makes the preparation of growth-direction-independent nanowire orientations feasible.
Semiconductor nanowires (NWs) are essentially 1D structures with unique optical and electrical properties.1 Therefore,
NWs have attracted increasing interest for device use in fieldeffect transitors,2 light-emitting diodes,3 sensors,4 and so
forth. Given a sufficiently small cross section of the NW,
quantum confinement effects can be observed. For example,
in comparison to the bulk material, narrowing the wire
diameter increases its band gap.
Various approaches for growing semiconductor nanowires,
including physical and chemical vapor deposition, laser
ablation, template-assisted growth, or supercritical solution
synthesis, are known. Even though there have been many
recent reports on the fabrication of nanowires obtained by
either one of these techniques, nanowire assemblies are hard
to realize because of difficulties encountered in handling
individual wires in a reproducible and economical way.
Moreover, the crystallographic orientation of the NW axis
cannot be freely chosen.
In this letter, a highly promising method for the preparation
of well-ordered, substrate-adhered assemblies of nanowires
is proposed. The proposed technique is essentially based on
the deposition of high-quality thin semiconductor films followed by a patterning step consisting of the locally confined
laser ablation of the film. In general, there are two ways to
accomplish laser patterning, namely, interference patterning
and scaling-down diffraction-mask projection. Although the
former method has been applied to semiconducting thin
* Corresponding author. E-mail: [email protected].
† Also affiliated with 3D-Micromac AG, Max-Planck-Strasse 22b,
D-09114 Chemnitz, Germany.
10.1021/nl049703v CCC: $27.50
Published on Web 04/16/2004
© 2004 American Chemical Society
films,5-7 the latter approach was applied only to bulk
materials using nanosecond excimer lasers8 and thin metal
films using UV-laser pulses in the subpicosecond range.9
Using ion-beam-assisted molecular beam epitaxy,10 an
approximately 20-nm-thick GaN film was deposited on
conventionally polished (0001) 6H-SiC (substrate temperature: 650 °C; growth rate: 0.65 nm‚min-1). GaN films
cannot be patterned by the lithographic structuring of a resist
followed by a conventional wet-chemical etching process
because of the high chemical stability of GaN.11 In the
present approach, the GaN thin film was exposed to spatially
resolved ablation using phase-mask projection instead.8 For
this purpose, a KrF excimer laser (LPX 220i, Lambda
Physics, operated at a wavelength of 248 nm and incorporated into a Series 7000 laser workstation by Exitech Ltd.)
was used to illuminate a phase mask. Phase gratings with a
binary profile especially designed for the optical setup were
used to suppress the zeroth diffraction order and to allow
only (first-order diffraction to pass the objective. The phase
gratings (possessing a period of 22 µm and a depth of ∼250
nm) were etched into fused silica by reactive ion etching
(700 eV, 0.2 mA/cm2, CHF3) after structuring a resist mask
by excimer laser ablation. Utilizing a 15× demagnifying
Schwarzschild-type reflection objective (NA ) 0.28), we
found that the two diffracted beams imaged onto the sample
surface generate an interference pattern with a sine-shaped
intensity profile within an area of ∼130 µm × 150 µm. This
Fourier-filtered image of the mask can be deployed to
machine even larger areas in a step-and-repeat process using
a computer-controlled x-y-z positioning stage. Well-defined
Figure 1. Scanning electron micrograph of the ∼20-nm-thick wurtzitic GaN thin film on the (0001) 6H-SiC substrate after the application
of (a) just one laser pulse of 900 mJ/cm2, (b) two pulses at a fluency of 900 mJ/cm2, and (c) five pulses at 650 mJ/cm2. (a, b) White gallium
droplets can be seen to decorate the rim of gray GaN banks on a blank SiC substrate. (c) Those droplets have been entirely removed by a
dip in dilute HCl.
spatial etching of the GaN film was achieved by the variation
of the average laser fluence and the pulse number in the range
of 0.5 to 2 J/cm2 and 1 to 100 pulses, respectively.
As can be seen in Figure 1a, just one laser pulse of 900
mJ/cm2 was sufficient to thermally decompose the GaN thin
film, resulting in Ga droplets at positions where the laser
fluence exceeded the decomposition threshold of the film
(∼550 mJ/cm2), whereas nonirradiated areas remained unaffected, as can be concluded from the unchanged surface
microstructure. The width of the GaN banks (∼490 nm after
patterning with only one pulse) can be reduced by the
repeated application of laser pulses, as shown in Figure 1b.
(The average nanowire width has been reduced to 370 nm.)
A further reduction of the substrate-adhered nanowire width
(to 235 nm) is observed after applying five pulses (Figure
1c). In this micrograph, residual gallium droplets have
already been removed by a dip in HCl.
Scanning force microscopy (Figure 2) clearly proved that
the locally confined thermal decomposition of GaN to
gallium and volatile nitrogen was in fact selective (i.e., the
laser fluence was chosen such that the ablation threshold of
GaN (∼0.55 J‚cm-2) was exceeded, whereas those for SiC
(∼20 J‚cm-2) were not yet reached). Hence, the underlying
substrate was not impaired.
Whereas the width of the nanowire can be reduced by
ablation-profile sharpening caused by multipulse application,
the separation of individual wires can be reduced to about
375 nm by utilizing a Schwarzschild reflection objective with
a demagnification of 36×.
Beyond the results presented above, excimer-laser mask
projection opens up new avenues for nanoprocessing. First,
the application of a shorter wavelength (e.g., 157 nm) and a
Schwarzschild reflection objective with a larger numerical
aperture has the potential, when combined with multipulse
896
Figure 2. Scanning force microscopy image of the ∼20-nm-thick
wurtzitic GaN thin film on the (0001) 6H-SiC substrate after the
application of one laser pulse of 650 mJ/cm2.
irradiation, to decrease the nanowire width well below 100
nm because multipulse ablation does result in a sinn(x) ablation profile. Second, contacting the surface-adhered wires is
also very much facilitated in comparison to the very sophisticated techniques currently under development for nonlocated
nanowires. Either circuit paths are formed on the SiC substrate prior to GaN thin-film deposition or the contacts are
applied after the patterning of the GaN film by lithographic
means. Finally, the introduced technology is generally orientation-independent (i.e., GaN thin films with the crystalline
axis running either perpendicular (as found on (0001) 6H-SiC
substrates) or parallel (preferred on (100) γ-LiAlO2 substrates)
to the surface can be patterned along the surface normal
whatever the twisting angle about this normal might be).
Nano Lett., Vol. 4, No. 5, 2004
Moreover, using mask-projection laser patterning, not only
parallel banks but also crossed banks can be generated in
the thin film because the interference of first-order reflections
enables crossed banks (e.g. square or hexagonal patterns of
dots) to be fabricated. Such patterning would require only
the preparation of a suitable mask as described above. In
contrast to optical patterning based on the interference of
two (or more) laser beams, mask projection is easier to
perform (once a suitable mask is prepared), and the reproducibility of the results is much better.
In general, the described methodology is applicable in
cases where the band gap of a thin film under-runs the
corresponding value of the substrate because under such
conditions corresponding ablation thresholds are suited for
the patterning process. Besides GaN on SiC or sapphire,
suitable heterostructures would include ZnO on sapphire or
more generally all kinds of semiconductors on glasses, glass
ceramics, or dielectrics.
Acknowledgment. T.H. is indebted to Dr. Frank Heyroth
(Martin-Luther-University, Halle) for assistance in the operation of the SEM (Philips ESEM XL 30 FEG).
Nano Lett., Vol. 4, No. 5, 2004
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