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Articles 1 - 13 of 13

Full-Text Articles in Physics

The Density Factor In The Synthesis Of Carbon Nanotube Forest By Injection Chemical Vapor Deposition, R. W. Call, C. G. Read, C. Mart, T. -C. Shen Jan 2012

The Density Factor In The Synthesis Of Carbon Nanotube Forest By Injection Chemical Vapor Deposition, R. W. Call, C. G. Read, C. Mart, T. -C. Shen

T. -C. Shen

Beneath the seeming straight-forwardness of growing carbon nanotube(CNT) forests by the injection chemical vapor deposition(CVD) method, control of the forest morphology on various substrates is yet to be achieved. Using ferrocene dissolved in xylene as the precursor, we demonstrate that the concentration of ferrocene and the injection rate of the precursor dictate the CNT density of these forests. However, CNT density will also be affected by the substrates and the growth temperature which determine the diffusion of the catalyst adatoms. The CNT growth rate is controlled by the temperature and chemical composition of the gases in the CVD ...


Low-Temperature Charge Transport In Ga-Acceptor Nanowires Implanted By Focused-Ion Beams, S. J. Robinson, C. L. Perkins, T. -C. Shen, J. R. Tucker, T. Schenkel, X. W. Wang, T. P. Ma Jan 2007

Low-Temperature Charge Transport In Ga-Acceptor Nanowires Implanted By Focused-Ion Beams, S. J. Robinson, C. L. Perkins, T. -C. Shen, J. R. Tucker, T. Schenkel, X. W. Wang, T. P. Ma

T. -C. Shen

Ga-acceptor nanowires were embedded in crystalline Si using focused-ion beams. The dc current-voltage characteristics of these wires after annealing are highly nonlinear at low temperatures. A conductance threshold of less than 50mV is observed independent of Ga+ dosage and implant beam overlap. These features suggest a Coulomb blockade transport mechanism presumably caused by a network of Ga precipitates in the substrate. This granular scenario is further supported by measurements of gated nanowires. Nanowires with metallic conductance at low temperatures could be achieved by reducing the current density of the focused-ion beams.


Electron Transport In Laterally Confined Phosphorus Δ-Layers In Silicon, S. J. Robinson, J. S. Kline, H. J. Wheelwright, J. R. Tucker, C. L. Yang, R. R. Du, B. E. Volland, I. W. Rangelow, T. -C. Shen Jan 2006

Electron Transport In Laterally Confined Phosphorus Δ-Layers In Silicon, S. J. Robinson, J. S. Kline, H. J. Wheelwright, J. R. Tucker, C. L. Yang, R. R. Du, B. E. Volland, I. W. Rangelow, T. -C. Shen

T. -C. Shen

Two-dimensional electron systems fabricated from a single layer of P-donors have been lithographically confined to nanometer scale in lateral directions. The electronic transport of such quasi-one-dimensional systems with and without a perpendicular magnetic field was characterized at cryogenic temperatures. Experimental data fit well with two-dimensional weak localization and interaction theory when the phase coherence length is shorter than the smaller dimension of the confinement. Below a transition temperature the wire conductance saturates.


Low Temperature Silicon Epitaxy On Hydrogen Terminated Si(100) Surfaces, J. -Y. Ji, T. -C. Shen Jan 2004

Low Temperature Silicon Epitaxy On Hydrogen Terminated Si(100) Surfaces, J. -Y. Ji, T. -C. Shen

T. -C. Shen

Si deposition on H terminated Si(001)-2×1 surfaces at temperatures 300–530K is studied by scanning tunneling microscopy. Hydrogen apparently hinders Si adatom diffusion and enhances surface roughening. The post-growth annealing effect is analyzed. Hydrogen is shown to remain on the growth front up to at least 10ML. Si deposition onto the H/Si(001)-3×1 surface at 530K suggests that dihydride units further suppress Si adatom diffusion and increase surface roughness.


Nanoscale Electronics Based On 2d Dopant Patterns In Silicon, T. -C. Shen, J. S. Kline, T. Schenkel, S. J. Robinson, J. -Y. Ji, C. L. Yang, R. R. Du, J. R. Tucker Jan 2004

Nanoscale Electronics Based On 2d Dopant Patterns In Silicon, T. -C. Shen, J. S. Kline, T. Schenkel, S. J. Robinson, J. -Y. Ji, C. L. Yang, R. R. Du, J. R. Tucker

T. -C. Shen

A nanoscale fabrication process compatible with present Si technology is reported. Preimplanted contact arrays provide external leads for scanning tunneling microscope (STM)-defined dopantpatterns. The STM’s low energy electron beam removes hydrogen from H terminated Si(100) surfaces for selective adsorption of PH3 precursor molecules, followed by room temperature Si overgrowth and 500 °C rapid thermal anneal to create activated P-donor patterns in contact with As+-implanted lines. Electrical and magnetoresistance measurements are reported here on 50 and 95 nm-wide P-donor lines, along with Ga-acceptor wires created by focused ion beams, as a means for extending Si device ...


Ultra-Dense Phosphorous Delta-Layer Grown Into Silicon From Ph3 Molecular Precursors, T. -C. Shen, J. -Y. Ji, M. A. Zudov, R. -R. Du, J. S. Kline, J. R. Tucker Jan 2002

Ultra-Dense Phosphorous Delta-Layer Grown Into Silicon From Ph3 Molecular Precursors, T. -C. Shen, J. -Y. Ji, M. A. Zudov, R. -R. Du, J. S. Kline, J. R. Tucker

T. -C. Shen

Phosphorous δ-doping layers were fabricated in silicon by PH3 deposition at room temperature, followed by low-temperature Si epitaxy.Scanning tunneling microscope images indicate large H coverage, and regions of c(2×2) structure. Hall data imply full carrier activation with mobility<40 cm2/V s when the surface coverage is ≲0.2 ML. Conductivity measurements show a ln(T) behavior at low temperatures, characteristic of a high-density two-dimensional conductor. Possible future applications to atom-scale electronics and quantum computation are briefly discussed.


Nanoscale Oxide Patterns On Si (100) Surfaces, T. -C. Shen, C. Wang, J. W. Lyding, J. R. Tucker Dec 1995

Nanoscale Oxide Patterns On Si (100) Surfaces, T. -C. Shen, C. Wang, J. W. Lyding, J. R. Tucker

T. -C. Shen

Ultrathin oxide patterns of a linewidth of 50 Å have been created on Si(100)‐2×1 surfaces by a scanning tunneling microscope operating in ultrahigh vacuum. The oxide thickness is estimated to be 4–10 Å. The morphology and spectroscopy of the oxide region are obtained. Hydrogen passivation is used as an oxidation mask. The defects caused by oxidation in the passivated region before and after the hydrogen desorption are compared and discussed. The multistep silicon processings by an ultrahigh vacuum scanning tunneling micropscope is thus demonstrated.


Nanometer Scale Patterning And Oxidation Of Silicon Surfaces With An Ultrahigh Vacuum Scanning Tunneling Microscope, J. W. Lyding, G. C. Abeln, T. -C. Shen, C. Wang, J. R. Tucker Aug 1994

Nanometer Scale Patterning And Oxidation Of Silicon Surfaces With An Ultrahigh Vacuum Scanning Tunneling Microscope, J. W. Lyding, G. C. Abeln, T. -C. Shen, C. Wang, J. R. Tucker

T. -C. Shen

Nanoscale patterning of the Si(100)‐2×1 monohydride surface has been achieved by using an ultrahigh vacuum (UHV) scanning tunneling microscope(STM) to selectively desorb the hydrogen passivation. Hydrogen passivation on silicon represents one of the simplest possible resist systems for nanolithography experiments. After preparing high quality H‐passivated surfaces in the UHV chamber, patterning is achieved by operating the STM in field emission. The field emitted electrons stimulate the desorption of molecular hydrogen, restoring clean Si(100)‐2×1 in the patterned area. This depassivation mechanism seems to be related to the electron kinetic energy for patterning at ...


Nanoscale Patterning And Oxidation Of H-Passivated Si(100)-2x1 Surfaces With An Ultrahigh Vacuum Scanning Tunneling Microscope, J. W. Lyding, T. -C. Shen, J. S. Hubaceck, J. R. Tucker, G. C. Abeln Jan 1994

Nanoscale Patterning And Oxidation Of H-Passivated Si(100)-2x1 Surfaces With An Ultrahigh Vacuum Scanning Tunneling Microscope, J. W. Lyding, T. -C. Shen, J. S. Hubaceck, J. R. Tucker, G. C. Abeln

T. -C. Shen

Nanoscale patterning of the hydrogen terminated Si(100)‐2×1 surface has been achieved with an ultrahigh vacuum scanning tunneling microscope.Patterning occurs when electrons field emitted from the probe locally desorb hydrogen, converting the surface into clean silicon. Linewidths of 1 nm on a 3 nm pitch are achieved by this technique. Local chemistry is also demonstrated by the selective oxidation of the patterned areas. During oxidation, the linewidth is preserved and the surrounding H‐passivated regions remain unaffected, indicating the potential use of this technique in multistep lithography processes.


Ion Irradiation Effects On Graphite With Scanning Tunneling Microscope, T. -C. Shen, R. T. Brockenbrough, J. R. Tucker, J. W. Lyding Jan 1991

Ion Irradiation Effects On Graphite With Scanning Tunneling Microscope, T. -C. Shen, R. T. Brockenbrough, J. R. Tucker, J. W. Lyding

T. -C. Shen

Scanning tunneling microscope is used to create local surface modifications by means of ion impact damage. Graphite has been used as a test case to demonstrate this local surface sputtering. Using 0.1-µs voltage pulse of - 30 to - 140 V applied to the sample in a rough vacuum of 10-2 Torr, a confined area of damage (typically about 100 Å in diameter) is usually obtained. The damaged area consists of several layers of terraces. Defects of the size of a few atoms can also be found. Electronic perturbations caused by defects can form superlattices with a spacing three times that ...


Bubbles Without Cores, T. C. Shen Jan 1988

Bubbles Without Cores, T. C. Shen

T. -C. Shen

The decay of a false vacuum in a theory without a true vacuum is studied. Using variational arguments, we find that for a class of potentials of the type aφi+bφi (a>0, b<0 and i2φ2—ηφ3+ λφ4 is presented to discuss the transition from a thin-wall bubble to a thick-walled one. The validity conditions of using these solutions to describe the false vacuum decay within the framework of the semiclassical approximation are discussed.


Higher-Dimensional Self-Consistent Solution With Deformed Internal Spaces, T. -C. Shen, J. Sobczyk Jan 1987

Higher-Dimensional Self-Consistent Solution With Deformed Internal Spaces, T. -C. Shen, J. Sobczyk

T. -C. Shen

We study a system of gravity and free massless scalar fields minimally coupled to gravity in a 7- dimensional background which is a direct product of a 4-dimensional Minkowski space and a 3- dimensional homogeneously deformed three-sphere. Compactification is caused by the vacuum energy of scalar fields. The effective potential as a function of two parameters (scale and deformation) is calculated numerically after dimensional regularization. We find the effective potential decreases rapidly toward negative infinity in both prolate and oblate directions. The classical curvature, however, can balance the quantum effect and yields three extrema. In addition to the round S ...


Symmetry Behavior Of The Static Taub Universe: Effect Of Curvature Anisotropy, T. C. Shen, B. L. Hu, D. J. O'Connor Jan 1985

Symmetry Behavior Of The Static Taub Universe: Effect Of Curvature Anisotropy, T. C. Shen, B. L. Hu, D. J. O'Connor

T. -C. Shen

Using the static Taub universe as an example, we study the effect of curvature anisotropy on symmetry breaking of self-interacting scalar field. The one-loop effective potential of a λφ4 field with arbitrary coupling (ξ) is computed by ζ-function regularization. It is expressed as a perturbative series in a small anisotropy parameter α measuring the deformation from the spherical Einstein universe with radius of curvature α. This result is used for analyzing the symmetry behavior of such a system as a function of the geometric (α,α) and field (ξ,λ) parameters. The result is also used to address the ...