Papers

AVS2001 Session SC+SS-TuA: Semiconductor Nanostructures and Processing

Tuesday, October 30, 2001 2:00 PM in Room 122

Tuesday Afternoon

Start	Invited?	Item
2:00 PM		SC+SS-TuA-1 Nanoparticles for Fabrication of Zero- and One-dimensional Quantum Objects L. Samuelson, M. Bjork, K. Deppert, J. Ohlsson, M.H. Magnusson, A. Persson, C. Thelander, R. Wallenberg (Lund University, Sweden) Quantum dots and quantum wires are of great interest for their possible use in quantum devices, such as single-electron transistors. In many cases these quantum objects are fabricated using different forms of self-organized epitaxial growth. We will report the use of aerosol techniques for fabrication of nanoparticles which are used as building blocks for quantum devices and which also allow us to controllably grow semiconducting, III-V, nanowhiskers. We will first describe our method for controlled fabrication of crystalline nanoparticles of metalli c and semiconducting nanoparticles. Then we will turn to description of the way we grow nanowhiskers or nanoneedles using size-controlled nanoparticles as catalytic seeds which control the dimension and the location of the nanowhiskers. Transmission electron microscope characterization of chemical and structural properties of nanoparticles and nanowhiskers will be presented. Finally we will discuss electrical data from quantum devices obtained via nanomanipulation of nanoparticles and nanowhiskers, a technique that has allowed ohmic electrical contacts as well as tunnel-injecting contacts to be formed to these quantum objects.
2:20 PM		SC+SS-TuA-2 Growth of Ag Nanowires on Atomically Flat Ag films Formed on GaAs(110) Surfaces H.B. Yu, C.-S. Jiang, C.-K. Shih (University of Texas at Austin) By using the scanning tunneling microscopy, we study the growth and evolution of Ag nanowires on atomically flat Ag films deposited onto GaAs(110) substrates. We show the ability to grow Ag nanowires with a well-defined width and very large aspect ratio (>150:1). For atomically flat Ag-film on GaAs(110), it has been shown that the surface has a quasi-periodic superstructure with long (L) and short (S) wavelength modulations arranged according to the Fibonacci sequence.^1,2 We find that for the Ag nanowires grown on such a surface, the width of the nanowires is quantized in units of L (1.7 nm) or S (1.3 nm) segments. Very long (> 1 micron) nanowires of such a well-defined width can be formed on the surface with its ends terminated at the edge of the voids on the Ag film. The formation mechanism and the electronic properties of such nanowires will be discussed. ¹A.R. Smith, K.-J. Chao, Q. Niu, and C.K. Shih, Science 273, 226 (1996). ²Ph. Ebert, K.-J. Chao, Q. Niu, and C.K. Shih, Phys. Rev. Lett. 83, 3222 (1999).
2:40 PM		SC+SS-TuA-3 Ge Nanoclusters Prepared from Solution with Chemically Tailored Surfaces B.R. Taylor (Lawrence Livermore National Laboratory); S.M. Kauzlarich (University of California, Davis); L.J. Terminello, A.W. van Buuren, C.F.O. Bostedt, T.M. Willey (Lawrence Livermore National Laboratory) Ge nanoclusters have been prepared by a solution reaction between the Zintl salt Mg₂Ge and GeCl₄ in refluxing diglyme¹ and triglyme.² The nanoclusters are produced in a range of sizes from 2 to 10 nm in diameter, and are quantum confined. The particles were characterized by high-resolution transmission electron microscopy and Fourier transform infrared spectroscopy. The shift in band gap of the nano clusters was measured by optical spectroscopy and X-ray photoelectron spectroscopy . ¹ Taylor, B. R.; Kauzlarich, S. M.; Lee, H. W. H.; Delgado, G. R. Chem. Mater., 1998, v. 10, 22-24. ² Taylor, B. R.; Kauzlarich, S. M.; Lee, H. W. H.; Delgado, G. R. Chem. Mater., 1999, v. 11, 2493-2500.
3:00 PM		SC+SS-TuA-4 X-Ray Absorption and Emission Studies of Diamond Nanoclusters T. van Buuren, J. Plitzko, C.F.O. Bostedt, N. Franco, L.J. Terminello (Lawrence Livermore National Laboratory) The conduction and valence band structure of bulk diamond and diamond nanoclusters have been measured using x-ray absorption and x-ray emission spectroscopies. The diamond nanoclusters are commercially available products from the Straus chemical corporation and are synthesized in a detonation wave from high explosives. X-ray diffraction and TEM show that the nanodiamond powder is crystalline and approximately 3.5 +/- 1.0 nm in diameter. The nanodiamond K-edge absorption and emission show the same spectral features as bulk diamond with low impurity levels. The C1s core exciton feature clearly observed in the K-edge absorption edge of bulk diamond is not observed in the nanodiamond spectra. A possible explanation for this is a broadening due to a distribution of particle size. The depth of the second gap in the nanodiamond spectra is shallower than that of bulk diamond. This effect has been observed previously and attributed to quantum confinement. We note that no blue shift measured in the position of nanodiamond conduction edge when compared to the bulk diamond contrary to a recent publication that has reported large conduction band shifts in CVD grown diamond nanoclusters.¹ Experiments are in progress to measure the nanodiamond conduction band edge from the EELS spectra acquired with a field emission TEM. We compare our conduction band data to the published measurements and comment on the differences. Soft x-ray emission measurements of the valence band structure of the diamond nanocluster will also be presented. The electronic structure of the nanodiamond will be compared to recent results on Si and Ge nanoclusters and the effects of reduced sizes on the electronic structure of group IV semiconductors will be discussed.² The work is supported by the US-DOE, BES Ma-terial Sciences under contract W-7405-ENG-48, LLNL. ¹ Y.K. Chang et.al. PRL 82, 5377 (1999). ² van Buuren et.al. PRL 80, 3803 (1998).
3:20 PM		SC+SS-TuA-5 AFM Tip-mediated Nucleation and Growth of Passivated Au Nanocrystal Islands M.D.R. Taylor, P. Moriarty (University of Nottingham, UK); M. Brust (University of Liverpool, UK) Recent molecular dynamics simulations by Luedtke and Landmann¹ have highlighted the rich dynamics associated with diffusion of Au clusters on surfaces. However, although there is a significant amount of work related to the diffusion of clusters formed in gas-aggregation (and related) sources, to date there have been no experimental studies of the dynamic properties of passivated Au nanoparticles² deposited onto a surface from a colloidal suspension. We report non-contact mode and tapping mode (including phase imaging) atomic force microscopy (AFM) observations of the evolution of close-packed layers of 1.5 nm diameter thiol-passivated Au clusters on SiO2. The morphology of 1.5 nm cluster overlayers differs dramatically to that observed for dodecanethiol passivated clusters of 6 nm diameter.³ Furthermore, during each NC-AFM scan dramatic morphological changes occur - faceted holes develop, islands appear and grow, steps change appearance - which we attribute to strong tip-cluster interactions. Dynamic force-distance spectroscopy curves indicate that instabilities in the feedback loop are responsible for significant mass transport during (nominally) non-contact mode AFM scanning. ¹ Luedtke WD and Landmann U 1999 Phys. Rev. Lett. 82 3835 ² Brust M, Walker M, Bethell D, Schiffrin DJ and Whyman R 1994 J. Chem. Soc., Chem. Comm. 801 ³ Fractal Aggregation of Polydisperse Au Nanoclusters on Silicon, M. D. R. Taylor, P. Moriarty and M. Brust, submitted.
4:00 PM		SC+SS-TuA-7 Chemically Enhanced Electron Beam Induced Micromachining of SiO₂ J.H. Wang, A.R. Guichard, D.P. Griffis, P.E. Russell (North Carolina State University) While material removal using chemically enhanced focused ion beam micromachining is well known, utilization of electron beam induced chemistry for material removal is relatively unexploited. If practical techniques can be developed utilizing electron beam induced chemistry for material removal, issues involving the implantation or "staining" by the Ga ion beam generally used for micromachining can be avoided. In this study, the utilization of XeF₂ for electron beam induced selective etching of SiO₂ is investigated. The influence of electron dose, electron beam energy and XeF₂ pressure is presented. An etch rate of 10nm/sec over a square micron has been achieved at a chamber pressure of 8x10^-6 Torr. At low electron beam doses, m aterial is deposited (rather that etched). This deposited material and micromachined features have been characterized by AFM, SEM and EDS. These results clearly demonstrate the ability to micromachine SiO₂ using XeF₂ enhanced electron beam induced etching.
4:20 PM		SC+SS-TuA-8 Surface Passivation Effects of Deposited Ge-Nanocrystal Films Probed with Synchrotron Radiation C.F.O. Bostedt, T. van Buuren (Lawrence Livermore National Laboratory); T. Moller (Hasylab at Desy, Germany); L.J. Terminello (Lawrence Livermore National Laboratory) Clusters and nanocrystals represent a new class of materials that exhibit promising novel properties. The production of these nanostructures in the gas phase gives control over not only the size of the nanoparticles, but also over surface passivation â€“ often not possible in other growth modes. The clusters are condensed out of supersaturated Germanium-vapor that is cooled down in a He-atmosphere and are subsequently deposited on a variety of substrates. Their surface is then subsequently passivated with different materials evaporated into the vacuum chamber. This approach allows us to probe in a controlled and dynamic fashion the effect of surface passivation on nanocluster properties. The clusters are spherical in shape and their sizes are determined by atomic force microscopy (AFM) and confirmed by transmission electron microscopy (TEM). X-ray absorption spectroscopy (XAS) was performed on thin films of Germanium (Ge) clusters. We find that the passivating agent strongly alters the electronic structure of the clusters. In general the absorption edge shifts to significantly higher energies compared to cluster films without surface passivation. This can be explained with a stronger confinement effect in the passivated films compared to unpassivated ones due to a reduction of the cluster-cluster interactions. C. Bostedt acknowledges a fellowship from the German Academic Exchange Service DAAD in the HSP-III program, N. Franco from the Spanish Education and Culture Office. The work is supported by the US-DOE, BES Material Sciences under contract W-7405-ENG-48, LLNL.
4:40 PM		SC+SS-TuA-9 Surface Nanostructuring by Ion Sputtering: The Early Stages F. Buatier de Mongeot, D. De Sanctis, C. Boragno, U. Valbusa (Universita' di Genova, Italy) Ion sputtering is commonly used in surface science as a standard sample cleaning procedure. Recently, we have demonstrated that prolonged exposure to an ion beam can lead to the formation of regular patterns on surfaces, which have a spatial periodicity in the nanometer range.¹ However, it is not trivial to understand how a random process, like the impingement of ions on the surface, can lead to a regular spatial organization.² In order to investigate this aspect, we studied the early stages of the process by a Variable Temperature Scanning Tunneling Microscope VT-STM. The ion flux was reduced in order to follow the time evolution of the surface self-organization, starting from the single-impact events and until the formation of ripples on an Ag(110) surface occurred. We fixed the ion impact angle to 70 deg from the normal and low substrate temperatures, in order to enhance the erosive contribution.¹ Under these experimental conditions, prolonged exposure to the ion beam leads to the formation of a regular ripple pattern parallel to the ion beam direction with a wavelength in the nm range. Surprisingly, in the early stages, after exposing the surface to an ion dose as low as 0.03 ML the surface morphology shows a well defined correlation along the crystallographic directions and only at higher fluences the correlation figure alignes with the ion beam. In this contribution we discuss the relevant parameters of this phenomenon. ¹ S.Rusponi, G.Costantini, F.Buatier de Mongeot, C.Boragno and U.Valbusa, Appl.Phys.Lett. , 75 (1999) 3318 ² G. Costantini, F. Buatier de Mongeot, C. Boragno and U. Valbusa, Phys. Rev. Lett. 86 (2001) 838.