Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session K17: Focus Session: Si, Ge and SiGe Nanostructures |
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Sponsoring Units: FIAP Chair: Leonid Tsybeskov, New Jersey Institute of Technology Room: Baltimore Convention Center 313 |
Tuesday, March 14, 2006 2:30PM - 3:06PM |
K17.00001: Dynamic studies of the growth of Si and Ge nanostructures Invited Speaker: By making observations in real time during the growth of nanostructures, it is possible to obtain kinetic data which help us to determine the physical phenomena controlling growth. In this presentation we will describe the growth of Si and Ge nanowires by the vapour-liquid-solid process. Observations were made in a transmission electron microscope during ultra high vacuum chemical vapour deposition of Si and Ge onto substrates covered by Au catalyst. We will show that wire length and diameter are controlled by the diffusion of the catalyst along the wire surfaces, and that this diffusion is driven by a coarsening process. We will compare nanowire growth with the formation of Ge quantum dots by self-assembly during strained layer epitaxy, in which surface diffusion of Ge driven by coarsening also plays an essential role in controlling the structures formed. We will describe spontaneous roughening of Si and Ge nanowire surfaces, a phenomenon which may have analogues in the formation of surface roughness in SiGe alloy thin films. We will finally discuss the control of wire structure for the fabrication of novel devices. [Preview Abstract] |
Tuesday, March 14, 2006 3:06PM - 3:18PM |
K17.00002: Silicon nanostructures grown by solid phase epitaxy Peter Hadley, Yann Civale, Lis Nanver Silicon nanocrystals can be grown by depositing amorphous silicon (a-Si) on a thin metal layer and then annealing this bilayer. The silicon diffuses through the metal and forms crystals at the metal substrate interface in a process called solid phase epitaxy (SPE). This process has been investigated for many metals (Au, Al, Pd, Ni, Cr, Fe, Co, Ti, V, Rh) and it is observed that the temperature where the significant diffusion takes place is roughly half the eutectic temperature for metals that form a eutectic composition with silicon and it is roughly half the melting temperature for the metals that form a silicide. Using this process, it is possible to deposit crystalline silicon at low temperature. In the past, uniform metal/a-Si bilayers were used. Here we describe experiments where contact windows were opened through silicon oxide before the bilayer was deposited and the metal was patterned before the annealing step. This results in the selective deposition of the silicon crystals. Devices made by this process will be described. [Preview Abstract] |
Tuesday, March 14, 2006 3:18PM - 3:30PM |
K17.00003: Patterning of Ge nanoparticles by focused electron beam Nan Jiang We demonstrate that Ge nano-particles can be precipitated in GeO$_{2}$ and GeO$_{2}$-SiO$_{2}$ efficiently by the high-energy focused electron beam. The dynamic process of nanoparticle precipitations in these oxides were observed from the TEM images in real time, and analyzed using time-resolved electron energy loss spectroscopy (TREELS). The composition and structure of the precipitated nanoparticles were determined using both high-spatial resolution EELS and diffraction techniques. It was found that the particles are pure Ge and the average size increases slightly with the irradiation time. The dependences of size of nanoparticles on dose and dose rate of electron beam were also statistically analyzed, which is important to optimize experimental conditions to control the size of patterned nanoparticles. The mechanism involving the nucleation and growth process on surfaces is responsible for the precipitation of Ge in GeO$_{2}$. This was obtained through the thickness dependence of precipitation process. The patterned nanoparticles were then fabricated using the scanning (STEM) mode in a single step process. We noted that the production of Ge nano-particles using this method is impressively efficient; less than a second is needed to create a large assembly of nanoparticles simultaneously under parallel illumination mode in TEM. Using the focused electron probe, a single particles can be formed within several tens milliseconds. [Preview Abstract] |
Tuesday, March 14, 2006 3:30PM - 3:42PM |
K17.00004: Near-infrared photoluminescence of germanium nanocrystal synthesized in inverse micelle Zhifeng Ren, Wenzhong Wang, Keda Wang, Daxing Wang In this work we study photoluminescence (PL) properties of Ge nanocrystals prepared via a low-temperature inverse micelle solvothermal route. The as-prepared nanocrystals have an average diameter of $\sim $ 24 nm. Visible- and near-IR PL is observed from the pure nc-Ge samples at room temperature. We present the PL results in the near-IR range. The PL was excited using either 488-nm or 632-nm laser lines. The luminescence signal was dispersed by a grating monochromator and then collected by a liquid-nitrogen cooled Ge detector. The lock-in technique was used with a light chopper at a frequency of 17 Hz. We found two peaks located at $\sim $1.37 eV (905 nm) and $\sim $1.45 eV (855 nm). Those are within the wavelength range for optical communications. Upon oxidation the relative intensity of the 1.45 eV peak was enhanced. And then the intensity decreased after hydrogen annealing. Meanwhile, the changes of 1.37 eV peak is negligible upon either oxidation or hydrogen annealing. The origination of the near-IR luminescence peaks is discussed. The 1.45 eV peak is more likely related with the surface states of the nc-Ge particles; while, the 1.37 eV peak is more likely originated from the bulk of the nc-Ge. [Preview Abstract] |
Tuesday, March 14, 2006 3:42PM - 3:54PM |
K17.00005: Strain driven organization of SiGe islands on ultra-thin Si cantilevers Clark S. Ritz, Frank S. Flack, Michelle M. Roberts, Donald E. Savage, Max G. Lagally Silicon/germanium 3-D islands have long been studied, both to improve understanding of growth mechanisms in strained epitaxial films and for potential applications as quantum dots (QDs). Self assembly of these QDs into ordered arrays is technologically important, and has been partially achieved through various forms of substrate strain patterning. We experimentally study the effects of substrate local compliancy on the organization of epitaxially grown Ge and Si(1-x)Ge(x) QDs. We form the QDs through the Stranski-Krastanov growth mode on single-crystal ultra-thin Si cantilevers with thicknesses on the order of 20nm. Films are deposited using molecular beam epitaxy and ultra-high vacuum chemical-vapor deposition (CVD) to grow islands on one or both sides of the cantilevers, respectively. Both types of samples show QD ordering along cantilever edges; however, CVD-grown films exhibit additional ordering. Angled SEM imaging shows a strong anti-correlation between the positions of islands grown on the top and bottom faces of the cantilevers. The crystallographic orientation of the cantilevers also plays a role, as islands order better when edges are aligned along [100] (the soft elastic direction). Results will be interpreted in the context of continuum elasticity theory. [Preview Abstract] |
Tuesday, March 14, 2006 3:54PM - 4:06PM |
K17.00006: Controlled Movement of SiGe/Si Microtubes in Suspension Minrui Yu, Robert Blick Microtubes are potential vehicles for chemical or drug delivery. By applying electric potentials on specially arranged electrodes, SiGe/Si microtubes suspended in liquids are moved in a controllable fashion to designated positions. Such manipulation mechanism is also applicable to smaller entities such as nanotubes. [Preview Abstract] |
Tuesday, March 14, 2006 4:06PM - 4:18PM |
K17.00007: Field Emission from a Nanomechanical Pillar Hyun Kim, Hua Qin, Robert Blick, Michael Westphall, Lloyd Smith We have measured the field emission in a nano-electromechanical structure in which a nanometre silicon pillar oscillates in between the source and drain electrodes. The device consists out of a mechanically flexible pillar with a length of some 200 nm and a diameter of some 50 nm and allows for mechanical resonant excitations at radio frequencies (10 $\sim$ 1000 MHz) and mechanical clocking of the field emitted electrons. Operation of the device at room temperature in the frequency range of 300 $\sim$ 400 MHz is presented. For AC and DC drive, the current shows a rich frequency dependent response. A modified Fowler-Nordheim field emission curve is observed and attributed to the effect of oscillating pillar excited by an alternating electric field. [Preview Abstract] |
Tuesday, March 14, 2006 4:18PM - 4:54PM |
K17.00008: Silicon Nanowire Devices Invited Speaker: Metal-catalyzed, self-assembled, one-dimensional semiconductor nanowires are being considered as possible device elements to augment and supplant conventional electronics and to extend the use of CMOS beyond the physical and economic limits of conventional technology. Such nanowires can create nanostructures without the complexity and cost of extremely fine scale lithography. The well-known and controllable properties of silicon make silicon nanowires especially attractive. Easy integration with conventional electronics will aid their acceptance and incorporation. For example, connections can be formed to both ends of a nanowire by growing it laterally from a vertical surface formed by etching the top silicon layer of a silicon-on-insulator structure into isolated electrodes. Field-effect structures are one class of devices that can be readily built in silicon nanowires. Because the ratio of surface to volume in a thin nanowire is high, conduction through the nanowire is very sensitive to surface conditions, making it effective as the channel of a field-effect transistor or as the transducing element of a gas or chemical sensor. As the nanowire diameter decreases, a greater fraction of the mobile charge can be modulated by a given external charge, increasing the sensitivity. Having the gate of a nanowire transistor completely surround the nanowire also enhances the sensitivity. For a field-effect sensor to be effective, the charge must be physically close to the nanowire so that the majority of the compensating charge is induced in the nanowire and so that ions in solution do not screen the charge. Because only induced charge is being sensed, a coating that selectively binds the target species should be added to the nanowire surface to distinguish between different species in the analyte. The nanowire work at Hewlett-Packard Laboratories was supported in part by the Defense Advanced Research Projects Agency. [Preview Abstract] |
Tuesday, March 14, 2006 4:54PM - 5:06PM |
K17.00009: Carrier transport in Ge nanowires / Si substrate heterojunction E.-K. Lee, B. Kamenev, L. Tsybeskov, S. Sharma, T.I. Kamins Semiconductor nanowires (NWs) attached to lattice-mismatched single-crystal substrates form quasi-one-dimensional (QOD) heterojunctions (HJs) where efficient structural relaxation might occur due to high surface-to-volume ratio. Current-voltage characteristics in Ge NW/(p+)Si samples with nearly micron-long Ge NWs exhibit metal-type conductivity with ohmic behavior and little conductivity temperature dependence. In contrast, Ge NW/(n+)Si samples display significant change in conductivity as a function of temperature with an activation energy up to 200 meV. In a narrow temperature interval near 150 K we observed current instabilities and oscillations for Ge NW/(n+)Si. At higher temperatures we find negative differential photoconductivity at low forward biases. Our experimental results are explained using a model of nearly ideal Si substrate/Ge NW hetero-interfaces. [Preview Abstract] |
Tuesday, March 14, 2006 5:06PM - 5:18PM |
K17.00010: A theoretical study of Si and Ge nanowires. A. Fazzio, J. T. Arantes, Cedric Rocha Le\~{a}o, Ant\^{o}nio J. R. da Silva We have performed a systematical \textit{ab initio} study of the electronic and structural properties of pure Ge and Si nanowires (NWs), as well as Ge nanowires doped with Mn atoms. All our results are obtained via total energy Density Functional Theory (DFT) calculations within the generalized gradient approximation and pseudopotentials. The growth directions for all wires were considered to be along the (110) direction, and they present a hexagonal cross section formed by the intersection of <111> \quad and <001> \quad planes, with all the dangling bonds saturated with H atoms. We have considered three different diameters of SiNW's of similar shapes: 2.01 nm, 2.67 nm e 3.61 nm. In particular, we analyze how their properties, such as dispersion relations, band gaps and structural properties, like nearest neighbor distances, vary with the diameter. Similar calculations were also performed for the Ge NWs. In this latter case we have also investigated Mn doping. We focused on tetrahedral interstitial and substitutional sites, and conclude that the most stable position for Mn is substitutional at the surface of the nanowire. [Preview Abstract] |
Tuesday, March 14, 2006 5:18PM - 5:30PM |
K17.00011: Single-hole tunneling and Coulomb blockade in the strain-relaxation-induced quantum ring Guohua Wang, D. Tambe, A. Zaslavsky, V. Shenoy, D. Syphers We fabricated an ultrasmall Si/SiGe strained vertical quantum dot in the Coulomb blockade regime, where strain relaxation in the SiGe quantum well creates a ring-like confinement potential.[1] The resonant tunneling characteristics contains features near the threshold due to single-hole tunneling and Coulomb blockade. The features are steps in one bias polarity and peaks in the other, which we attribute to the differences in the dynamic occupation of the dot due to double Si barrier asymmetry. The spacing of the features gives the charging energy of the quantum ring. When magnetic fields are applied parallel to the tunneling direction, the evolution of the single-hole tunneling features with magnetic fields reveals cusps arising from the angular-momentum transition of the single-particle ground state of the quantum ring in the magnetic field, and the periodicity of the cusps in magnetic field is consistent with the calculated one from h/e, the magnetic flux quantum. Unlike single-carrier tunneling into quantum dots, our data on the tunneling into a quantum ring show no evidence of a singlet-triplet transition. [1] J. Liu et al, Phys. Rev. Lett. 89, 096804 (2002). [Preview Abstract] |
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