Serial and Parallel Si, Ge, and SiGe Direct-Write with Scanning

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Serial and Parallel Si, Ge, and SiGe Direct-Write with Scanning Probes and Conducting Stamps Stephanie E. Vasko,†,‡ Adnan Kapetanovic,† Vamsi Talla,†,§ Michael D. Brasino,† Zihua Zhu,|| Andreas Scholl,^ Jessica D. Torrey,† and Marco Rolandi*,† Department of Materials Science and Engineering, ‡Department of Chemistry, and §Department of Electrical Enginering, University of Washington, Seattle, Washington 98195, United States Environmental Molecular Sciences, Pacific Northwest National Laboratory Sciences, Richland, Washington 99352, United States ^ Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States

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ABSTRACT: Precise materials integration in nanostructures is fundamental for future electronic and photonic devices. We demonstrate Si, Ge, and SiGe nanostructure direct-write with deterministic size, geometry, and placement control. The biased probe of an atomic force microscope (AFM) reacts diphenylsilane or diphenylgermane to direct-write carbon-free Si, Ge, and SiGe nano and heterostructures. Parallel direct-write is available on large areas by substituting the AFM probe with conducting microstructured stamps. This facile strategy can be easily expanded to a broad variety of semiconductor materials through precursor selection. KEYWORDS: Silicon nanostructure, germanium nanostructure, SiGe, heterostructures, nanopatterning ecent advances in epitaxial transfer1
3 and vapor
liquid
solid growth (VLS)4
6 have demonstrated precise materials integration in semiconductor nanostructures. These nanostructures will likely play a key role in the future of electronic and photonic devices.7 Advantages of multimaterial nanostructures range from strain induced gain in charge carrier mobility8 to composition dependent band gap engineering.9 Epitaxial transfer produces high-performance electronic and photonic devices. However, epitaxial transfer still involves traditional semiconductor fabrication, which may not be the best option when performance is not the only design driver. Among these, solid-state semiconductor photovoltaic applications would greatly benefit from a less demanding materials integration alternative.10 VLS growth offers a cost-effective wide library of available materials ranging from Si, Ge, Si1-xGex11,12 and III
Vs.13 While geometry and placement control has been greatly improved,14 obtaining deterministic growth for effective on-chip integration is still challenging. An intriguing strategy is to localize growth using scanning probes instead of a catalyst particle. This strategy allows precise control growth placement as well as growth direction. In this fashion, metallic and semiconductor features have been demonstrated via scanning tunneling microscope chemical vapor depositon (STM-CVD).15
17 However, low throughput and high vacuum conditions required during growth limit such nanostructures to laboratory devices. The atomic force microscope (AFM) can fabricate nanostructures in ambient conditions.18
24 Several AFM techniques such as dip-pen nanolithography (DPN)25,26 and high-field carbon direct-write27,28 have been successfully demonstrated with greatly increased throughput.

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DPN affords fast writing at large scale with multiple self-aligned parallel tips.29 High throughput large-scale carbon direct-write can be performed at 1 cm s
1 tip speeds27 or by using microstructured conducting stamps that mimic multiple parallel tips.30 Recently, we have demonstrated that AFM high field direct-write can write sub-30 nm carbon-free Ge nanostructures as a faster, more-approachable alternative to STM-CVD.31,32 Here, we show that this direct-write strategy can pattern broad variety of semiconductor nanostructures. Si, Ge, and SiGe heterostructures can be fabricated both with single AFM tips as well as conducting stamps (Figure 1). Individual tips offer nanometer resolution in small areas, while conducting stamps afford low-cost high-throughput large-area nanofabrication. For highresolution direct-write, a biased AFM tip traces desired shapes on the substrate while both are immersed in an inorganic liquid precursor (Figure 1a). A quartz cell designed for imaging in fluid (∼1 mL) is used to contain the precursor. The electric field between the tip and the substrate (>109 V m
1) initiates the chemical reaction that deposits the desired material onto the substrate. 32 As previously demonstrated, Ge direct-write is accomplished from diphenylgermane (DPG, Gelest).31,32 For Si direct-write, we choose the Si equivalent of DPG, diphenylsilane (DPS, Sigma-Aldrich) (Figure 1b). This family of precursors with aromatic groups is particularly suited for the AFM directwrite reactions. For these precursors, the most energetically Received: March 5, 2011 Revised: May 1, 2011 Published: May 16, 2011 2386

dx.doi.org/10.1021/nl200742x | Nano Lett. 2011, 11, 2386–2389

Nano Letters

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Figure 1. (a) Schematic of the probe sample geometry and deposition process for AFM direct-write. A moderate sample bias (∼12 V) induces a large electric field (>109 V m
1). Electrons tunnel from the tip into the precursor molecule, where X = Ge or Si (b), and cause the fragmentation of the precursor and deposition of the desired product. (c) In the parallel patterning scheme, this reaction occurs at each of the multiple nanoscale asperities present on the stamp, yielding an array of deposited structures.

Figure 2. Tapping mode topographic AFM images of the following: (a) False-colored Ge (green) and Si heterostructure PacMan. Ge pacman written at þ12 V (sample) and 1 μm s
1, Ge eyeball written at þ11 V (sample) and 1 μm s
1, Si pellet written at þ12 V (sample) and 5 μm s
1. (b) Ge and Si crossbars, Ge lines written at þ16 V (sample) and 1 μm s
1, Si lines written at þ12 V (sample) and 1 μm s
1. (c) Detailed view of a crossbar node from the previous image. (d) Approximately 70 nm wide SiGe lines written at 10 V and 100 μm s
1. Height scale from dark brown to bright yellow 10 nm (a), 5 nm (b
d). Scale bar = 500 nm (a), 500 nm (b), 300 nm (c), 1 μm (d). Panels b and d were enhanced with 45 virtual illumination.

favorable fragmentation route under high-field and electron bombardment involves the two phenyls and the two hydrogens leaving as benzene.32 This leaves a Si or Ge radical or ion free for deposition onto the substrate. The direct-write product should thus be free of any carbon contamination, which is often a problem when using organometallic precursors. Ge and Si heterostructures are routinely produced at a rate of 1 μm sec
1 (Figure 2a
c). Complex architectures made of Si and Ge nanostructures can be easily fabricated. This is accomplished

in a single direct-write session without the need of tip
sample realignment. First, Ge direct-write is perfomed in DPG (Figure 2a, Pac-Man). Second, the reaction volume is flushed with an excess (>20 mL) of DPS to ensure minimal precursor cross-contamination. Third, Si direct-write is performed from DPS (Figure 2a, Pac-Dot). This is quite a general procedure that can produce Si
Ge heterostructures (Figure 2b,c). Cross-bar architetures with Si and Ge nanoribbons intersecting each other are easily obtainable. Close analysis of the junctions shows that the second material (Si) is written on the top of the first one (Ge) when the direct-write patterns are crossed (Figure 2c). Complex Si
Ge nanoribbons junction devices may thus be fabricated using AFM direct- write. To further the variety of materials, proof-of-concept SiGe nanostructures were made (Figure 2d). SiGe direct-write is available because DPS and DPS are mutually soluble and can be easily introduced in the reaction volume. To ensure high-quality of the Si
Ge direct-write patterns, we carefully perform careful chemical analysis of patterns deposited from filtered DPS and DPG (Whatman filter, 0.2 μm). Chemical composition is first determined using time-of-flight secondary ion mass spectroscopy (ToF-SIMS; Figure 3a
i).27,31 To facilitate sample location, DPS, DPG, and 1:1 DPG/DPS microscopic pads are fabricated via AFM direct-write (Figure 3a,d,g). Since a Si nanostructure on a Si wafer does not provide chemical contrast, samples were prepared on a thin W film. From the ToF-SIMS scans, germanium (Figure 3a
c) and silicon (Figure 3d
f) are found only in structures nanofabricated from DPG and DPS, respectively. For the sample prepared from DPS/ DPG, both semiconductors are present in the SIMS spectra (Figure 3g
i). However, it was not possible to determine if the Si/Ge ratio matches the solution stoichiometry. This will require carful refinement of these measurements. As purity is essential for electronic grade semiconductors, we carefully evaluate carbon contamination of the patterns. To this end, we employ near-edge X-ray absorption fine structure (NEXAFS) as recorded by a photoemission electron microscope (PEEM; Figure 3j).27,28,31 NEXAFS spectra were obtained with ca. 0.3 eV energy resolution from stacks of PEEM images taken at successive X-ray energies. Here, we estimate the relative carbon content from the C K-edge spectra of patterns prepared from DPG, DPS, and n-octane for comparison.31 From this data, it is 2387

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Nano Letters

Figure 3. Representative AFM images of Si, Ge, and SiGe (a,d,g) pads (scale bars 2.5 μm, height scale = 10 nm) and ToF-SIMS images (scale bars 2.5 μm, 256  256 pixels, 40 scans) of Si, Ge, and SiGe nanostructures, respectively. Silicon ion count (from dark brown to yellow, b, e,h) and Germanium-74 ion count (c,f,i). Data was collected after cleaning with a 1 keV O2þ beam to remove surface contamination. NEXAFS spectra acquired at the onset of the C K-edge (j, 285.7 eV) on Si pads written from DPS (green), Ge pads written from DPG (red), and pads written with n-octane precursor (dotted black) to verify C content of Si and Ge structures.

clear that Ge-fabricated nanostructures are completely carbon free and that Si-fabricated structures contain, at most, carbon traces. The Ge result is not totally suprising. Ge has no carbon solubility in a broad-temperature range. On the other hand, carbon readily dissolves in silicon up to the formation of silicon carbide. The lack of carbon on the Si structures confirms that the phenyls on the precursor molecules are indeed good leaving groups for this chemistry. From this result, it is envisionable that other diphenyl- or triphenyl-type organometallic precursors may be used to expand the composition of available nanostructures. Exciting possibilities include using group III and group V precursors to create compound semiconductors. Furthermore, these precursors may be added in small quantities to DPS or DPG to dope nanostructures during growth. To further these possibilities, future work will also be aimed at the visualization of the direct-write material crystal structure using transmission electron microscopy. This will allow correlating crystallinity with growth parameters, precursors, and composition. Throughput is a recurring concern of scanning probe based fabrication strategies. Carbon direct-write is feasible on large areas at velocities as high as 1 cm s
1.27 However, we have not yet found appropriate precursors to replicate such velocities in semiconductor direct-write. An alternate strategy to increase throughput is to mimic multiple tips working in parallel by using microstructured conductive stamps. Parallel patterning via stamping has been demonstrated for oxide33 and carbonaceous features.30 Results from organic precursor direct-write are particularly encouraging for our efforts, given the similar high-field reactions involved during deposition. For rapid demonstration of this concept, we built a simple stamping machine by taking

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Figure 4. Gold-coated PDMS stamp mounted in the AFM quartz liquid cell (a). Tapping mode AFM images of stamp master (b). Si structures fabricated on the Si substrate with þ31 V (sample) pulse for 2 min (c). Ge structures fabricated on the silicon substrate with a þ31 V (sample) pulse for 2 min (d). Scale bar = 1 cm (a), 10 μm (b
d). Height scale from dark brown to bright yellow = 200 nm (b), 20 nm (c,d).

advantage of the precise position control of the AFM platform (Figure 4a). This approach follows the same philosophy of polymer pen lithography25 and eliminates the need for the fabrication of a complex stamping device. In our approach, 2 mm  4 mm flexible stamps are mounted onto the liquid cell, which is used during AFM direct-write (Figure 4a). The liquid cell provides a contained environment for the condensation of an inorganic meniscus between the stamp and the substrate. The stamp, fabricated using previously developed CD molding,33 consists of an array of 200 nm tall parallel lines at 1.4 μm pitch (Figure 4b). A conductive substrate is mounted on the AFM piezo stage. The liquid cell/substrate environment is purged of water by flushing dry N2 at 3 L min
1 for 30 min. Then either a DPG or DPS meniscus is formed between the stamp features and the surface by saturated vapor at 3.5 L min
1 for one hour. Nanoscale features are deposited applying a stamp-sample bias (31 V on the substrate for 2 min). Using this technique, large arrays of both silicon (Figure 4c) and germanium nanostructures (Figure 4d) are easily fabricated. Further inspection of the samples reveals patterns spanning areas of several millimeters square. Since there is no fundamental limitation for how large stamps can be fabricated, improved alignment abilities will allow this process to be easily expanded to the wafer scale. Furthermore, stamps can be fabricated using molds of any shape or feature size. This will allow for the parallel direct-write of semiconductor nanostructures with the correct geometry and placement for desired applications. In conclusion, we have demonstrated that Si, Ge, and SiGe nanostructures can be easily fabricated with AFM and microcontact printing direct-write with commercially available precursors. AFM direct-write merges the deterministic placement control of nanolithography with the chemical control of bottomup growth in a single process. High throughput fabrication has been demonstrated using conducting nanostructured stamps to 2388

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Nano Letters produce large arrays of Ge and Si nanostructures. This proof-ofconcept demonstration lays the groundwork for the expansion of this technique to extensive library of CVD precursors. By exploiting this library, the direct-write and stamping approaches can provide a cost-effective approach to a wide variety of materials for devices. Experimental Section. Si (100) substrates (Addison Engineering, B-doped, resistivity F = 0.02 Ω cm, native oxide thickness ≈ 2 nm) were cleaned by previously published methods.31 Nanostructure direct-write was performed on a Veeco Multimode V AFM with a fluid cell operated in contact mode with Sb-doped Si cantilevers (Nanoworld, F = 0.02
0.025 Ω cm, spring constant k ≈ 0.2 N m
1). A bias was applied to the sample (þ6 to þ10 V) and/or the tip (
12 to 0 V) via the Nanoscope IV controller. The force set point was the same as for imaging in contact mode (∼1
10 nN). Patterning at lower or higher forces did not affect the patterns as long as contact with the sample was maintained during writing. After patterning, the samples were rinsed in acetone, methanol, and isopropanol. Tapping mode imaging was performed on the same AFM with Veeco Probes TESP Si (Sb) doped cantilevers (F = 0.01
0.025 Ω cm, k = 40 N m
1, resonant frequency ν ≈ 300 kHz). Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) was performed at the Environmental Molecular Sciences Laboratory at Pacific Northwest National Laboratory with a TOF.SIMS5 spectrometer (ION-TOF GmbH, Germany). A 25 keV Biþ ion beam, which was focused into a diameter of ∼250 nm with a current of 0.6 pA at 20 kHz repeating rate, was used to image the area of interest. C K-edge NEXAFS spectra were collected at beamline 7.3.1 (PEEM-2) of the Advanced Light Source at the Lawrence Berkeley National Laboratory. Detector dark current was subtracted and the spectra were normalized to unity from 5 to 10 eV before the edge jump to extract chemical information that was encoded in the energy dependent absorption of the sample. Nonenergy-dependent effects caused by differences in illumination and local work function were thus removed. Stamps used for parallel patterning were fabricated via previous methods33 and were coated with a 5 nm Cr adhesion layer and a 50 nm Au conductive layer 50 nm using a Balzers PLS 500 e-beam evaporator. Voltage was applied to the stamp using a using a Tektronix PS283 DC power supply. After deposition, the printed substrates were washed with isopropanol and annealed at 550 C to remove trace organics.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The AFM direct-write and the parallel stamping work were supported from the National Science Foundation under CHE1012419, a 3M Untenured Faculty Grant, and the University of Washington. The AFM direct-write work was also supported from an INTEL gift. Additionally, the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 (Advanced Light Source) is gratefully acknowledged. A portion of the research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.

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’ REFERENCES (1) Ko, H.; Takei, K.; Kapadia, R.; Chuang, S.; Fang, H.; Leu, P. W.; Ganapathi, K.; Plis, E.; Kim, H. S.; Chen, S.-Y.; Madsen, M.; Ford, A. C.; Chueh, Y.-L.; Krishna, S.; Salahuddin, S.; Javey, A. Nature 2010, 468, 286. (2) Yoon, J.; Jo, S.; Chun, I. S.; Jung, I.; Kim, H.-S.; Meitl, M.; Menard, E.; Li, X.; Coleman, J. J.; Paik, U.; Rogers, J. A. Nature 2010, 465, 329. (3) Paniccia, M.; Rao, R. M.; Yee, W. M. J. Vac. Sci. Technol., B 1998, 16, 3625. (4) Chen, R.; Tran, T.-T. D.; Ng, K. W.; Chuang, L. C.; Sedgwick, F. G.; Hasnian, C.-C. Nat. Photonics 2011, 5, 170. (5) Greytak, A. B.; Lauhon, L. J.; Gudiksen, M. S.; Lieber, C. M. Appl. Phys. Lett. 2004, 84, 4176. (6) Wu, Y.; Xiang, J.; Yang, C.; Lu, W.; Lieber, C. M. Nature 2004, 430, 61. (7) Soref, R. IEEE J. Sel. Top. Quantum Electron. 2006, 12, 1678. (8) Tromp, R. M.; Ross, F. M. Annu. Rev. Mater. Sci. 2000, 30, 431. (9) Mooney, P. M.; Chu, J. O. Annu. Rev. Mater. Sci. 2000, 30, 335. (10) Lill, M.; Schroder, B. Appl. Phys. Lett. 1999, 74, 1284. (11) Meyerson, B. S. Proc. IEEE 1992, 80, 1592. (12) Potapov, A. V.; Orlov, L. K.; Ivin, S. V. Thin Solid Films 1998, 336, 191. (13) Kuykendall, T.; Pauzauskie, P. J.; Zhang, Y.; Goldberger, J.; Sirbuly, D.; Denlinger, D.; Yang, P. Nat. Mater. 2004, 3, 524. (14) Tian, B.; Xie, P.; Kempa, T. J.; Bell, D. C.; Lieber, C. M. Nat. Nanotechnol. 2009, 4, 824. (15) Laracuente, A.; Bronikowski, M. J.; Gallagher, A. Appl. Surf. Sci. 1996, 107, 11. (16) Kent, A.; Shaw, T.; von Molnar, S.; Awschalom, D. D. Science 1993, 1249. (17) Ehrics, E. E.; Yoon, S.; de Lozanne, A. L. Appl. Phys. Lett. 1988, 53, 2287. (18) Wei, Z.; Wang, D.; Kim, S.; Kim, S.-Y.; Hu, Y.; Yakes, M. K.; Laracuente, A. R.; Dai, Z.; Marder, S. R.; Berger, C.; King, W. P.; de Heer, W.; Sheehan, P. E.; Riedo, E. Science 2010, 11, 1373. (19) Pellegrino, L.; Yanagisawa, Y.; Ishikawa, M.; Matsumoto, T.; Tanaka, H.; Kawai, T. Adv. Mater. 2006, 18, 3099. (20) Germain, J.; Rolandi, M.; Backer, S. A.; Frechet, J. M. J. Adv. Mater. 2008, 20, 4526. (21) Rolandi, M.; Quate, C. F.; Dai, H. Adv. Mater. 2002, 14, 191. (22) Tseng, A. A.; Notargiacomo, A.; Chen, T. P. J. Vac. Sci. Technol., B 2005, 23, 877. (23) Knoll, A. W.; Pires, D.; Coulembier, O.; Dubois, P.; Hedrick, J. L.; Frommer, J.; Duerig, U. Adv. Mater. 2010, 22, 3361. (24) Pires, D.; Hedrick, J. L.; De Silva, J.; Frommer, J.; Gotsmann, B.; Wolf, H.; Despont, M.; Duerig, U.; Knoll, A. W. Science 2010, 328, 732. (25) Braunschweig, A. B.; Huo, F.; Mirkin, C. A. Nature Chem. 2009, 1, 353. (26) Bellido, E.; de Miguel, R.; Ruiz-Molina, D.; Lostao, A.; Maspoch, D. Adv. Mater. 2010, 22, 352. (27) Rolandi, M.; Suez, I.; Scholl, A.; Frechet, J. M. J. Angew. Chem., Int. Ed. 2007, 46, 7477. (28) Suez, I.; Rolandi, M.; Backer, S. A.; Scholl, A.; Doran, A.; Okawa, D.; Zettl, A.; Frechet, J. M. J. Adv. Mater. 2007, 19, 3570. (29) Salaita, K.; Wang, Y.; Fragala, J.; Vega, R. A.; Liu, C.; Mirkin, C. A. Angew. Chem. 2006, 45, 7099. (30) Martinez, R. V.; Losilla, N. S.; Martinez, J.; Huttel, Y.; Garcia, R. Nano Lett. 2007, 7, 1846. (31) Torrey, J. D.; Vasko, S. E.; Kapetanovic, A.; Zhu, Z.; Scholl, A.; Rolandi, M. Adv. Mater. 2010, 22, 4639. (32) Vasko, S. E.; Jiang, W.; Chen, R.; Hanlen, R.; Torrey, J. D.; Dunham, S.; Rolandi, M. Phys. Chem. Chem. Phys. 2011, 13, 4842. (33) Albonetti, C.; Martinez, J.; Losilla, N. S.; Greco, P.; Cavallini, M.; Borgatti, F.; Montecchi, M.; Pasquali, L.; Garcia, R.; Biscarini, F. Nanotechnology 2008, 19, 435303.

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