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 fu¨r Oberfla¨chenmodifizierung 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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