1. Germanium Nanowire Growth and Device Applications
Lauren Klein, Tong Wang, Eric Garfunkel
Department of Chemistry, Rutgers University, Piscataway, NJ
Nanowire Growth and High-k Dielectric Layer Characterization by Rutherford Backscattering Spectrometry (RBS, 2MeV He)
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.
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 3-aminopropyltriethoxysilane (APTES) in methanol for 10 minutes. When subsequently submerged in a colloidal solution of Au, the wafers become uniformly coated with Au nanoparticles.
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.
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. 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
VLS growth in LPCVD reactor
a): before and b): after 10 cycles of atomic layer deposition of HfO2 on Ge nanowires grown on Si (100) substrate.
c): after 10 cycles of HfO2 ALD on Ge nanowires grown on Si (111) substrate.
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. Ge Hf c) b)
d) Ge nanowires grown on SiO2 at 360°C and 3.2 torr germane partial pressure for 5 min.
Epitaxial Growth on Si and Ge
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. a) b) d)
Ge areal density: 434E15 atoms/cm2 (equivalent thickness: 98.4 nm)
Ge coverage: 33.5% Ge O Si a)
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. Au
Single-Nanowire FETs: Two-Probe and Four-Probe Transport Measurement
Passivation of GeNW
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
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.
In addition to the high-k deposition noted here, we have begun to explore wet chemical passivation of our nanowires.
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.
In order to achieve a more robust wet-chemical passivation, we have functionalized our wires using 1,8-octanedithiol. We intend to further explore hydrogermylation and ammonia sulfide passivation schemes.
Current work involves FTIR and XPS studies of these and other passivation chemistries. a) c)
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. c) d) b) d) b) c)
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.
Schematic of GeNW transistor
4-terminal GeNW device
4-terminal GeNW schematic
References:
McIntyre, P. Chidsey, C. J. App. Phys , 100,
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, Dai, H. J. Am. Chem. Soc ,
Dai, H. Appl. Phys. 2006, 84
Han, S. J. Am. Chem. Soc. 2001, 123,
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.
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
b) Top view and c) side view of Ge nanowires grown on ITO
Crystallinity and epitaxy evident from XRD spectra
Ge lattice constant = /- 0.01Å
Bulk value = 5.657Å