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Germanium Nanowire Growth and Device Applications Lauren Klein, Tong Wang, Eric Garfunkel Department of Chemistry, Rutgers University, Piscataway, NJ Introduction: Germanium nanowires were grown via the vapor-liquid-solid (VLS) method in a hot-wall chemical vapor deposition reactor. Directionality was obtained by epitaxial growth on single-crystal Si and Ge substrates, aided by appropriate etch chemistries and growth conditions. The structure, chemistry and properties of nanowires have also been investigated. The unstable native oxide on Ge nanowires results in poor electrical and optical properties. We are investigating various chemical passivation routes, including chlorination, alkylation, H-termination, and thiol methods, as well as more robust high-k dielectrics, such as HfO2, by atomic layer deposition. The nanowires and their coatings have been characterized by scanning and transmission electron microscopies, x-ray photoelectron spectroscopy, x-ray diffraction, and Rutherford backscattering spectrometry. Ge nanowires were also grown on a variety of other substrates for potential applications as transistors, solar cell and sensors. Nanowires are grown via a vapor-liquid-solid (VLS) mechanism in our chemical vapor deposition reactor. Germane gas is used as a precursor, and substrates are functionalized with 20nm gold nanoparticle catalyst seeds. The gold nanoparticles catalyze the decomposition of the GeH4 and forming a liquid eutectic with the germanium as it dissolves in the metal. As the influx of Ge to the particles continues, the gold-germanium solution becomes supersaturated leading to the eventual precipitation of the germanium nanowire from the particle. Most wires grow from the substrate with the catalyst particle remaining on top. We tune growth times, partial pressure, flow rate, and temperature to achieve optimal growth on various substrates. Epitaxial Growth on Si and Ge Substrate Preparation To functionalize single crystal silicon and germanium substrates with Au, wafers were first etched in a 2% HF solution to remove any native oxide, then in a 0.1 M solution of 3aminopropyltriethoxysilane (APTES) in methanol for 10 minutes. When subsequently submerged in a colloidal solution of Au, the wafers become uniformly coated with Au nanoparticles. a) 10 cycles of HfO2 ALD measured equivalent HfO2 thickness: ~ 12.6 Å Ge areal density: 21.6E15 atoms/cm2 (equivalent thickness: 49 Å) Ge coverage: 3.6% Si a): before and b): after 10 cycles of atomic layer deposition of HfO2 on Ge nanowires grown on Si (100) substrate. The Au-functionalized wafers are again submerged in the HF solution for an additional 2 minutes before immediately being loaded into the reactor chamber. SiO2 and ITO substrates are functionalized under similar conditions. c): after 10 cycles of HfO2 ALD on Ge nanowires grown on Si (111) substrate. Ge Hf c) b) ZnO substrates were prepared with gold nanoparticles through a slightly different chemistry. The wafers are soaked in a 0.1 M solution of 1,4-diaminobutane for 2 hours and treated with gold colloids for 30 min. d) Ge nanowires grown on SiO2 at 360°C and 3.2 torr germane partial pressure for 5 min. Growth on Oxides Dense, randomly-oriented nanowire growth is achieved on oxide substrates. We continue to explore substrate materials such as silicon dioxide, indium tin oxide, and zinc oxide for further technological applications, such as the solar cell shown to the right. Energy [keV] 200 400 600 800 1000 1200 1400 1600 1800 7,000 6,500 d) Ge areal density: 434E15 atoms/cm2 (equivalent thickness: 98.4 nm) Ge coverage: 33.5% 6,000 5,500 5,000 70116_Tong_GeNW_25D_BA_20kr2.AS1 Simulated H O Si Ge Au 4,500 Ge 4,000 Counts b) a) VLS growth in LPCVD reactor Nanowire Growth and High-k Dielectric Layer Characterization by Rutherford Backscattering Spectrometry (RBS, 2MeV He) 3,500 O 3,000 2,500 a) Si 2,000 1,500 a) A polymer/nanowire hybrid solar cell using n-type polypyrrole derivatives and p-type germanium nanowires. Wires grown on ITO and coated in a polymer matrix. 1,000 Au 500 0 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 Channel Passivation of GeNW a) Top and b) side view of epitaxially grown wires on germanium <111> substrate with schematic illustrating growth directions c) Top and d) side view of epitaxially grown wires on silicon <100> substrate with schematic Illustrating growth directions After 400C anneal, without gate bias The unstable native oxide of germanium results in an electrically poor interface, making germanium unsuitable for many device applications. In order to use germanium nanowires for future technological applications, the passivation problem must be resolved. a) c) 4 3 2 Ids (X 10 nA) _ a) Ge Nanowires grown on SiO2. Tapering of wires is evident due to uncatalyzed radial CVD growth at high temperatures. Wires are randomly oriented on oxide substrates. d) c) Single-Nanowire FETs: Two-Probe and Four-Probe Transport Measurement 1 0 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 -1 -2 b) -3 In addition to the high-k deposition noted here, we have begun to explore wet chemical passivation of our nanowires. -4 Vds (V) After 400C anneal, Vds=0.1 V 0.7 0.6 0.5 b) Ids (X 10 nA) _ Germanium surfaces can be passivated by hydrogen termination after exposure to dilute hydrofluoric acid. Similarly, surfaces can be chlorine terminated through exposure to hydrochloric acid. 0.8 0.4 d) 0.3 0.2 Ge nanowires on Si100, near Si111 Bragg peak Ge nanowires on Si100, near Si220 Bragg peak c) Si 220 Ge111 Si111 Ge 220 In order to achieve a more robust wet-chemical passivation, we have functionalized our wires using 1,8octanedithiol. We intend to further explore hydrogermylation and ammonia sulfide passivation schemes. a) As-fabricated unpassivated Ge nanowire FET exhibiting large hysteresis by sweeping gate voltage measured in air immediately after a rapid thermal anneal in N2 at 400°C for 5 min to obtain ohmic contact. Gate oxide is 400 nm thick and electrodes are 40 nm thick Ti. 0 -10 -5 0 5 Current work involves FTIR and XPS studies of these and other passivation chemistries. Si 311 Ge220 Ge311 Vg (V) Crystallinity and epitaxy evident from XRD spectra Ge lattice constant = 5.661 +/- 0.01Å Bulk value = 5.657Å Acknowledgements: D. Olaya, M. Gershenson, N. Zhitenev, L. Wielunski, T. Emge, A. Ermakov, C.L. Hsueh, J. Zhu, Y. Lu, G. Fanchini, M. Chhowalla, R. Lorber, H.W. Zimmass, S. Guha References: c) 4-terminal GeNW device d) 4-terminal GeNW schematic Ge311 b) Top view and c) side view of Ge nanowires grown on ITO 10 -0.1 b) Schematic of GeNW transistor Ge nanowires on Si100, near Si311 Bragg peak Au111 0.1 Applications: We have begun to explore four type of Ge nanowire (NW) devices: single NW transistors, multi-NW thin film transistors, multi-NWs as components of photovoltaic cells, and NW sensors. McIntyre, P. Chidsey, C. J. App. Phys. 2006, 100, 024318 Korgel, B, Hanrath, T. J. Am. Chem. Soc 2004, 126 Lieber, CM. J. Phys. D: Appl. Phys. 2006, 39, R387 Yang, P. Nano Letters 2006, 6, 973 Chabal, Y. Rivillon, S. Appl. Phys. Lett. 2005, 87, 253101 Dai, H. J. Am. Chem. Soc. 2004 126, 11602 -11611 Dai, H. Appl. Phys. 2006, 84 Han, S. J. Am. Chem. Soc. 2001, 123, 2422-2425