Modeling the Nanoscale World
NNIN/CNF 2005 Fall Workshop
Cornell University, Ithaca, NY
October 10-12th, 2005
Pristine Semiconducting  Silicon Nanowires
Abhishek Kumar Singh, Vijay Kumar[2,3], Ryunsuke Note, and Yoshiyuki Kawazoe
 Institute for Materials Research, Tohoku University, Aoba-ku Sendai 980-8577, Japan
 Dr. Vijay Kumar Foundation, 45 Bazaar Street, K. K. Nagar (West), Chennai 600 078, India
 Computational Sciences (RICS), National Institute of Advanced Industrial Science and Technology (AIST)
AIST Tsukuba Central 2, Umezono 1-1-1, Tsukuba 305-8568,
Silicon nanowires (SiNWs) based nanotechnology has established the importance of semiconducting materials at nanoscale. Remarkable development has taken place in this area recently, and the applications shown for the semi-conducting SiNWs go beyond the electronics [1,3] to biological and chemical sensors . Applications of SiNWs in optical and photonic devices  has touched the region which are almost forbidden for bulk silicon. We present the finding of semiconducting pristine silicon nanowires by using ab initio calculations . These nanowires are oriented along  direction with bulk Si core and are bounded by Si (100) and Si (110) planes in lateral directions. The geometrical reconstructions on the facets, are general and similar to one observed on Si surfaces and can be further extrapolated to any thickness. This can help in identi_cation and recognition of facets geometry of experimental SiNWs. The thicker nanowire show polytypism. Importantly, irrespective of the thicknesses, the nanowires are indirect band gap semiconductors and are important for the nanoscale devices. Structural resemblance to experimentally formed hydrogenated SiNWs  can help to find ways to remove hydrogen from the surfaces and get the pristine SiNWs. Doping could be another step to explore further possibilities of applications of these nanowires.
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 M. S. Guiksen, L. J. Lauhon, J. Wang, D. C. Smith, C. M. Lieber, Nature 415, 617 (2002).
 A. K. Singh, V. Kumar, R. Note, and Y. Kawazoe, submitted.
 D. D. D. Ma, C. S.Lee,F.C. K. Au, S. Y.Tong, and S. T. Lee, Science 299, 1874(2003).
Enhance the potential image contrast of scanning Kelvin probe microscopy (SKPM)
Zhitao Yang, Michael Spencer
School of Electrical and Computer Engineering, Cornell University, Ithaca, NY 14853
Non-contact AFM (atomic force microscopy) based scanning Kelvin probe microscopy (SKPM) is widely used to measure surface work functions and electrostatic potentials on nanoscale materials, devices and circuits. However, the accuracy of scanning Kelvin probe microscopy is suffered from a cantilever effect, which is due to non-trivial capacitance gradients associated with the cantilever and the surrounding potential domains around the local potential right below the probe tip. This cantilever effect deteriorates the contrast of the potential image. We introduce two approaches to eliminate this cantilever effect so that the accuracy of AM-SKPM can be improved. One approach is to fabricate a shielding layer on the sample, so that the surrounding potential domains can be shielded from the cantilever and their interactions are replaced by the interaction between the cantilever and the shielding layer, which potential can be controlled by adjusting the external bias. The local potential region over which we want to measure can still be accessed by the tip, through an aperture on the shielding layer. The true local surface potential can be extracted by solving a series of linear equations which are generated by measuring the local potential with different DC bias voltages on the shielding layer. The other approach to eliminate the cantilever effect is to modify the commercial SKPM probe by depositing a shielding layer on the tip-side of the cantilever and most part of the tip cone, while the tip end remains exposed. If the shielding layer is biased with a DC voltage or grounded, the AC signal on the cantilever and most part of the tip cone can be shielded from interacting with the sample. Experimental data shows both approaches can improve the potential measurement accuracy significantly, and the contrast of potential image has been enhanced. Once the cantilever elimination is eliminated, the finite size of the tip cone becomes the only source of the second-order measurement error. For both approaches we give solutions to reduce the cone effect hence the accuracy can be enhanced further. The first approach can be made versatile by using a micro-manipulator to hold a mobile aperture structure over the sample, while the second approach can be easily integrated into commercial probe fabrication process.
Modeling of ferromagnet semiconductor heterostructures in 1D
for the optimization of the spin transport efficiency
Abdel F. Isakovicand Kien-Man Ng
Physics Dept. LASSP,
This past decade has seen a rapid rise in the number of studies related to spin transport and dynamics on nanoscale in multilayers and heterostructures. This poster will address a need for the research efforts with combined computational and experimental techniques. It will be shown how self-consistent solving of a system of Poisson and Schrodinger equations and/or Poisson and DDE (drift-diffusion equations) is used to understand details of performance and the efficiency improvement of spin transport in ferromagnet-semiconductor heterostructures (FMSCH). These heterostructures, operating largely as a system of back-to-back diodes (Schottky and PIN) at modest voltages (~ 2 V) and at relatively small current densities (~ 1-100 A/cm2) have been used as a model system  in studies of feasibility of spin transport in FMSCH, where remarkable efficiencies were achieved that persist up to 295 K . Depending on the direction of transport of spin-polarized electrons, we distinguish between spin ejection (from semiconductor towards ferromagnet) and injection (from ferromagnet into semiconductor), and we present the results of modeling in both cases. The focus of our modeling is the role the kinetic energy of spin-polarized carriers plays in the efficiency of the spin transport. Our model explains the existence of the bias range with high and low efficiencies, noted in experiments, and clearly demonstrates a link between FMSCH design and spin transport efficiency. We find that the role of a variety of spin relaxation processes cannot be ignored, and propose a semiqualitative model that accounts for such processes. Among other results we include a finding of an imbalance between holes and electrons, which is a part of the efficiency problem, and a spatially dependent, and bias- and doping-controlled recombination that may work both in favor and against the spin transport processes.
 A. F. Isakovic et al. Phys. Rev. B 64, 1612304
 C. Adelman et al., Phys. Rev. B 71, 121301
Dopant Diffusion in Silicon-Germanium Alloys
Mohit Haran, James A. Catherwood and Paulette Clancy
and Biomolecular Engineering,
Ab initio calculations and Molecular Dynamics simulations with semi-empirical models have decoupled the twin influences of strain and the effect of the presence of germanium on the diffusivity of intrinsic defects and group V dopants (P, As, Ab) in strained Si/SiGe materials. The trends found for the group V dopants are assembled into a broad framework that reconciles boron and group V dopants and which will allow researchers to predict dopant diffusion in SiGe for other dopants. Molecular Dynamics of intrinsic defects in SiGe using Stillinger-Weber models shows that vacancy diffusivities increase linearly with increasing germanium content. In contrast, interstitial diffusivity remains constant till the concentration of Ge reaches 15% before falling with further increases in germanium content. Tensile strain causes an increase in interstitial-assisted dopant diffusivity and decreases vacancy-assisted diffusivity. This effect is opposed by the ‘chemical’ effect of Ge which reduces interstitial diffusivity, to an extent dependant on dopant size, while assisting vacancy diffusivity by lowering formation energies by ~0.2 eV.
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