38 research outputs found
Fabrication and Characterization of Controllable Grain Boundary Arrays in Solution Processed Small Molecule Organic Semiconductor Films
We have produced solution-processed thin films of
6,13-bis(triisopropyl-silylethynyl) pentacene with grain sizes from a few
micrometers up to millimeter scale by lateral crystallization from a
rectangular stylus. Grains are oriented along the crystallization direction,
and the grain size transverse to the crystallization direction depends
inversely on the writing speed, hence forming a regular array of oriented grain
boundaries with controllable spacing. We utilize these controllable arrays to
systematically study the role of large-angle grain boundaries in carrier
transport and charge trapping in thin film transistors. The effective mobility
scales with the grain size, leading to an estimate of the potential drop at
individual large-angle grain boundaries of more than one volt. This result
indicates that the structure of grain boundaries is not molecularly abrupt,
which may be a general feature of solution processed small molecule organic
semiconductor thin films where relatively high energy grain boundaries are
typically formed. This may be due to the crystal Transient measurements after
switching from positive to negative gate bias or between large and small
negative gate bias reveal reversible charge trapping with time constants on the
order of 10 s, and trap densities that are correlated with grain boundary
density. We suggest that charge diffusion along grain boundaries and other
defects is the rate determining mechanism of the reversible trapping.Comment: 12 pages, 11 figure
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Fundamental Mechanisms of Interface Roughness
Publication quality results were obtained for several experiments and materials systems including: (i) Patterning and smoothening of sapphire surfaces by energetic Ar+ ions. Grazing Incidence Small Angle X-ray Scattering (GISAXS) experiments were performed in the system at the National Synchrotron Light Source (NSLS) X21 beamline. Ar+ ions in the energy range from 300 eV to 1000 eV were used to produce ripples on the surfaces of single-crystal sapphire. It was found that the ripple wavelength varies strongly with the angle of incidence of the ions, which increase significantly as the angle from normal is varied from 55° to 35°. A smooth region was found for ion incidence less than 35° away from normal incidence. In this region a strong smoothening mechanism with strength proportional to the second derivative of the height of the surface was found to be responsible for the effect. The discovery of this phase transition between stable and unstable regimes as the angle of incidence is varied has also stimulated new work by other groups in the field. (ii) Growth of Ge quantum dots on Si(100) and (111). We discovered the formation of quantum wires on 4° misoriented Si(111) using real-time GISAXS during the deposition of Ge. The results represent the first time-resolved GISAXS study of Ge quantum dot formation. (iii) Sputter deposition of amorphous thin films and multilayers composed of WSi2 and Si. Our in-situ GISAXS experiments reveal fundamental roughening and smoothing phenomena on surfaces during film deposition. The main results of this work is that the WSi2 layers actually become smoother during deposition due to the smoothening effect of energetic particles in the sputter deposition process
Pressure-dependent transition from atoms to nanoparticles in magnetron sputtering: Effect on WSi2 film roughness and stress
We report on the transition between two regimes from several-atom clusters to
much larger nanoparticles in Ar magnetron sputter deposition of WSi2, and the
effect of nanoparticles on the properties of amorphous thin films and
multilayers. Sputter deposition of thin films is monitored by in situ x-ray
scattering, including x-ray reflectivity and grazing incidence small angle
x-ray scattering. The results show an abrupt transition at an Ar background
pressure Pc; the transition is associated with the threshold for energetic
particle thermalization, which is known to scale as the product of the Ar
pressure and the working distance between the magnetron source and the
substrate surface. Below Pc smooth films are produced, while above Pc roughness
increases abruptly, consistent with a model in which particles aggregate in the
deposition flux before reaching the growth surface. The results from WSi2 films
are correlated with in situ measurement of stress in WSi2/Si multilayers, which
exhibits a corresponding transition from compressive to tensile stress at Pc.
The tensile stress is attributed to coalescence of nanoparticles and the
elimination of nano-voids.Comment: 16 pages, 10 figures; v3: published versio