[Show abstract][Hide abstract] ABSTRACT: We report on factors affecting the performance of a broadband, mid-IR absorber based on multiple, alternating dielectric / metal layers. In particular, we investigate the effect of interface roughness. Atomic layer deposition produces both a dramatic suppression of the interface roughness and a significant increase in the optical absorption as compared to devices fabricated using a conventional thermal evaporation source. Absorption characteristics greater than 80% across a 300 K black body spectrum are achieved. We demonstrate a further increase in this absorption via the inclusion of a patterned, porous anti-reflection layer.
[Show abstract][Hide abstract] ABSTRACT: We present results from measurements on double period structures of
alternating dielectric and thin metal layer coated with micro-patterned
anti-reflection layer to improve absorption in mid-infrared range. We
examine the effect on performance of patterns' period and the
correlation with the effective medium theory. We find that the numerical
results agree with the measured absorption spectra. We also investigate
the limit of pattern feature size to achieve performance suggested by
effective medium theory.
[Show abstract][Hide abstract] ABSTRACT: Studies on phase separation of mixtures of tetranitro zinc-
phthalocyanine (tn-ZnPc) and [6,6]-phenyl-C61-butyric acid
methyl ester (PCBM) were performed in which we controlled the
evaporation rate of the solvent (chloroform). Phase-contrast AFM
analysis reveals that slowing down the evaporation rate of the solvent
facilitates the nucleation of the donor component, and the two
components phase-separate. The size of the molecular agglomerates and
single small particles decreases for slow solvent evaporation and the
density of small particles per unit area increases by an order of
magnitude over the range studied.
[Show abstract][Hide abstract] ABSTRACT: We report on observations of unstable growth on GaAs(001) surfaces
nanopatterned with grooves of varying length/width aspect ratios. For
homoepitaxial growth at temperatures near 500^o, we find that
ridges build up at the upper long edges of grooves oriented along .
No ridges form at the long edges of grooves oriented ; instead
cusps form at the bottoms of such grooves. Most interestingly, we find
that the evolution of ridge heights during growth breaks into two
distinct branches, with the separation occurring at a groove length of
7.5 ± 2.5 μm for growth at 525^o, and at a length
which is an order of magnitude smaller than this for growth at
460^o. These observations indicate the presence of very
large, thermally-activated characteristic lengths which governs the
evolution of the topography during growth.
Journal of Crystal Growth 03/2013; · 1.55 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The adsorption and diffusion behavior of a Ga adatom on the GaAs (001)‐c(4 × 4)-heterodimer surface were studied by employing ab initio density functional theory (DFT) computations in the local density approximation. Structural and bonding features of the c(4 × 4)-heterodimer reconstruction surface were examined. A comparison with the c(4 × 4)-ss reconstruction was performed. Minimum energy sites (MES) on the c(4 × 4)-heterodimer surface were located by mapping the potential energy surface for a Ga adatom. Barriers for diffusion of a Ga adatom between the neighboring MES were calculated by using top hopping- and exchange-diffusion mechanisms. We proposed two unique diffusion pathways for a Ga adatom diffusing between the global minimums of two neighboring unit cells. Signature differences between electronic structures of top hopping- and exchange‐diffusion mechanisms were studied for relevant atoms. We observed a higher diffusion barrier for exchange mechanism compared to top hopping.
[Show abstract][Hide abstract] ABSTRACT: We investigate elastic buckling as a driving force for roughening of polystyrene, a model resist, during plasma etching. Force curve measurements of the effective modulus of etched polystyrene films, along with modeling which accounts for adhesive forces and surface corrugation, show that an extremely stiff modified layer results from Ar-ion etching, with the modulus increasing with incident energy. The results are in good quantitative agreement with analysis based on the observed dominant corrugation wavelength, and buckling theory providing the corrugation is taken into account.
[Show abstract][Hide abstract] ABSTRACT: We propose a periodic multilayer structure of dielectric and metal interlayers to achieve a near-perfect broadband absorber of mid-infrared radiation. We examine the influence of four factors on its performance: (1) the interlayer metal conductance, (2) the number of dielectric layers, (3) a nanopatterned antireflective layer, and (4) a reflective metallic bottom layer for backreflection. Absorption characteristics greater than 99% of the 300 K and 500 K blackbody spectra are found for the optimized structures. Incident angle and polarization dependence of the absorption spectra are examined. We also investigate the possibility of fabricating a nanopatterned antireflective layer to maximize absorption.
[Show abstract][Hide abstract] ABSTRACT: We present the results of kinetic Monte Carlo simulations of homoepitaxial growth on a patterned substrate in the presence of an extra barrier to a diffusing adatom crossing of steps from above (Ehrlich-Schwoebel barrier) on topographically patterned surfaces. Our results indicate that over a wide range of Ehrlich-Schwoebel barrier heights, incident atom fluxes, and temperatures that multilayer islands or "growth mounds" grow in arrangements which are directed by the topographical pattern. Our simulations indicate that a series of arrangements should form as the temperature is changed due to a competition between the temperature-dependent mound size and the pattern period. We compare these predictions with experimental observation of directed mound assembly on nanopatterned GaAs(001).
Physical Review B 02/2012; 0854218515(85). · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Fullerene/porphyrin mixtures are of great interest in bulk
heterojunction organic solar cells. Here we study the morphology of the
phase separation which occurs when [6,6]-phenyl-C61-butyric
acid methyl ester (PCBM) and tetranitro-zinc phthalocyanine (tn-ZnPc),
are deposited onto silicon (111) substrates, including the individual
domain length scales, shapes and wetting angles. tn-ZnPC forms small
clusters on the Si surface with a contact angle of approximately 15 ,
while PCBM forms compact clusters on broad (˜0.5 um diameter)
``wetting-layer'' disks, with the cluster contact angle of ˜19 .
Interestingly, a 50% mixture shows topography qualitatively similar to
that for PCBM, but with a larger contact angle of 22 , indicating that
the mixture wets the interface less than either pure component alone.
[Show abstract][Hide abstract] ABSTRACT: We present results from measurements on periodic multilayer structure of
alternating dielectric and thin metal layers to achieve a broadband
absorber of mid-infrared radiation. We examine the effect on performance
of a back-reflective metallic bottom layer, surface roughness at
interfaces, the metal conductance, the thickness of dielectric layers,
and a patterned anti-reflective layer. We determine optimum structure
parameters for absorption of a 500 K-black body spectrum, and find that
the numerical results agree well with the measured absorption spectra.
We also investigate the possibility of fabricating a patterned
anti-reflective layer to further increase the absorption.
[Show abstract][Hide abstract] ABSTRACT: Low energy electron microscopy images of Si(111) surfaces during sub monolayer deposition of Co or Ni at temperatures near 700°C reveal a phase separation into a disordered "ring cluster" phase and the ordered 7 × 7 reconstruction. For Co/Si(111) we find that temperature-driven phase separations at a fixed dose are reversible and occur at a fixed surface coverage. The shape of the phase boundary reveals a repulsive interaction between Co-containing ring clusters in the disordered phase. For Ni/Si(111), dissolution into the bulk prevents reversible separations between surface phases. At coverages close to 1/7 ML an ordered close-packed ring cluster phase forms for Co/Si(111), but not for Ni/Si(111). The phase disappears irreversibly on heating, due to the nucleation and growth of stable silicide islands.
Surface Review and Letters 01/2012; 05(06). · 0.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The collective effects in a periodic array of plasmonic double-antenna meta-molecules are studied. We experimentally observe that the collective behavior in this structure substantially differs from the one observed in their single-antenna counterparts. This behavior is explained using an analytical dipole model. We find that in the double-antenna case the effective dipole-dipole interaction is significantly modified and the transverse long-range interaction is suppressed, giving rise to the disappearance of Wood's anomalies. Numerical calculations also show that such suppression of long-range interaction results in an anomalous spatial dispersion of the electric-dipolar mode, making it insensitive to the angle of incidence. In contrast, the quadrupolar mode of the antenna pair experiences strong spatial dispersion. These results show that collective effects in plasmonic metamaterials are very sensitive to the design and topology of meta-molecules. Our findings envision the possibility of suppressing the spatial dispersion effects to weaken the dependence of the metamaterials' response on the incidence angle.
[Show abstract][Hide abstract] ABSTRACT: We examine the Wood's anomaly in arrays of nanorod pairs and find an apparent strong coupling between longitudinal plasmon modes and an oscillating dipole moment perpendicular to the plane of the substrate. OCIS codes: (160.3918) Metamaterials; (260.2030) Physical optics; (260.5740) Resonance
[Show abstract][Hide abstract] ABSTRACT: The authors show that poly(4-vinylpyridine) (P4VP) resist eliminates plasma-induced surface roughening for dry etch process conditions (100% Ar, 90% Ar/C4F8) that produce significant roughness in a wide variety of other polymers. In hot-embossed patterned structures, P4VP also shows no sidewall striations and line edge roughness after plasma etching, in contrast to other polymers investigated in this work. The mechanism underlying the retention of smooth surfaces for P4VP was investigated based on the observation that plasma-induced surface roughness in polystyrene (PS) has been linked to wrinkling caused by the formation of a thin, dense, ion-damaged layer. By x-ray photoelectron spectroscopy and in situ ellipsometry analysis, the authors studied two possible mechanisms that would suppress wrinkling in plasma-exposed P4VP: softening of the ion-damaged layer by nitrogen addition and stiffening of the polymer underlayer by VUV modification. While the authors report that the elastic modulus of the ion-damaged layer is reduced in Ar plasma-exposed PS when nitrogen is added to the gas discharge, the ion-damaged layer of P4VP showed no significant change relative to PS. However, by examining only the VUV radiation effect of the Ar discharge on P4VP, evidence of VUV cross-linking was observed. Mechanical stiffening of the VUV cross-linked layer is likely to suppress wrinkling in P4VP when an ion-damaged layer is formed during normal plasma exposure and explain the lack of surface and line edge roughness in the vinylpyridine material.
Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures 01/2011; 29. · 1.36 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Photolithographic patterning of organic materials and plasma-based transfer of photoresist patterns into other materials have been remarkably successful in enabling the production of nanometer scale devices in various industries. These processes involve exposure of highly sensitive polymeric nanostructures to energetic particle fluxes that can greatly alter surface and near-surface properties of polymers. The extension of lithographic approaches to nanoscale technology also increasingly involves organic mask patterns produced using soft lithography, block copolymer self-assembly, and extreme ultraviolet lithographic techniques. In each case, an organic film-based image is produced, which is subsequently transferred by plasma etching techniques into underlying films/substrates to produce nanoscale materials templates. The demand for nanometer scale resolution of image transfer protocols requires understanding and control of plasma/organic mask interactions to a degree that has not been achieved. For manufacturing of below 30 nm scale devices, controlling introduction of surface and line edge roughness in organic mask features has become a key challenge. In this article, the authors examine published observations and the scientific understanding that is available in the literature, on factors that control etching resistance and stability of resist templates in plasma etching environments. The survey of the available literature highlights that while overall resist composition can provide a first estimate of etching resistance in a plasma etch environment, the molecular structure for the resist polymer plays a critical role in changes of the morphology of resist patterns, i.e., introduction of surface roughness. Our own recent results are consistent with literature data that transfer of resist surface roughness into the resist sidewalls followed by roughness extension into feature sidewalls during plasma etch is a formation mechanism of rough sidewalls. The authors next- - summarize the results of studies on chemical and morphological changes induced in selected model polymers and advanced photoresist materials as a result of interaction with fluorocarbon/Ar plasma, and combinations of energetic ion beam/vacuum ultraviolet (UV) irradiation in an ultrahigh vacuum system, which are aimed at the fundamental origins of polymer surface roughness, and on establishing the respective roles of (a) polymer structure/chemistry and (b) plasma-process parameters on the consequences of the plasma-polymer interactions. Plasma induced resist polymer modifications include formation of a thin (∼1–3 nm) dense graphitic layer at the polymer surface due to ion bombardment and deeper-lying modifications produced by plasma-generated vacuum ultraviolet (VUV) irradiation. The relative importance of the latter depends strongly on initial polymer structure, whereas the ion bombardment induced modified layers are similar for various hydrocarbon polymers. The formation of surface roughness is found to be highly polymer structure specific. Beam studies have revealed a strong ion/UV synergistic effect where the polymer modifications introduced at various depths by ions or ultraviolet/UV photons can interact. A possible fundamental mechanism of initial plasma-induced polymer surface roughness formation has been proposed by Bruce etal [J. Appl. Phys. 107, 084310 (2010)]. In their work, they measured properties of the ion-modified surface layer formed on polystyrene (PS) polymer surfaces, and by considering the properties of the undamaged PS underlayer, they were able to evaluate the stressed bilayer using elastic buckling theory. Their approach was remarkably successful in reproducing the wavelength and amplitude of measured surface roughness introduced for various ion bombardment conditions, and other variations of experimental parameters. Polymer material-dependent VUV modifications introduced to a depth
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 01/2011; 29(1):010801-010801-35. · 1.36 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report on the effect of arrays of Au nanopillars of controlled size and spacing on the spectral response of a P3HT: PCBM bulk heterojunction solar cell. Prototype nanopillar-patterned devices have nearly the same overall power conversion efficiency as those without nanopillars. The patterned devices do show higher external quantum efficiency and calculated absorption in the wavelength range from approximately 640 nm to 720 nm, where the active layer is not very absorbing. The peak enhancement was approximately 60% at 675 nm. We find evidence that the corresponding resonance involves both localized particle plasmon excitation and multiple reflections/diffraction within the cavity formed by the electrodes. We explore the role of the attenuation coefficient of the active layer on the optical absorption of such an organic photovoltaic device.
[Show abstract][Hide abstract] ABSTRACT: We measure, simulate, and analyze the optical transmission through arrays of Ag nanorod pairs and U-shaped nanostructures as a function of polarization and angle of incidence. The bianisotropic nature of the metamaterials is exhibited in data and in simulations, and we argue that the electric field rather than the magnetic field excites the low frequency "magnetic" mode. We also observe spatial dispersion in the form of frequency shifts as a function of incident angle which we attribute to coupling effects between neighboring structures. A simple model based upon coupled electromagnetic dipoles is found to provide a qualitative description for the main features observed in the spectra.
[Show abstract][Hide abstract] ABSTRACT: The Ga adatom adsorption and diffusion processes on the GaAs(001)-c(4×4) surface were studied using ab initio density-functional-theory computations in the local density approximation. Two distinct sets of minima and transition sites were discovered for a Ga adatom relaxing from heights of 3 and 0.5 Å from the surface. These two sets show significant differences in the interaction of the Ga adatom with surface As dimers. An electronic signature of the differences in this interaction was identified. We computed the energetic barriers to diffusion for various adsorption sites. From these, we propose three pathways for diffusion of a Ga adatom on this surface which indicate anisotropic diffusion along different directions.
[Show abstract][Hide abstract] ABSTRACT: The uncontrolled development of nanoscale roughness during plasma exposure of polymer surfaces is a major issue in the field of semiconductor processing. In this paper, we investigated the question of a possible relationship between the formation of nanoscale roughening and the simultaneous introduction of a nanometer-thick, densified surface layer that is formed on polymers due to plasma damage. Polystyrene films were exposed to an Ar discharge in an inductively coupled plasma reactor with controllable substrate bias and the properties of the modified surface layer were changed by varying the maximum Ar <sup>+</sup> ion energy. The modified layer thickness, chemical, and mechanical properties were obtained using real-time in situ ellipsometry, x-ray photoelectron spectroscopy, and modeled using molecular dynamics simulation. The surface roughness after plasma exposure was measured using atomic force microscopy, yielding the equilibrium dominant wavelength λ and amplitude A of surface roughness. The comparison of measured surface roughness wavelength and amplitude data with values of λ and A predicted from elastic buckling theory utilizing the measured properties of the densified surface layer showed excellent agreement both above and below the glass transition temperature of polystyrene. This agreement strongly supports a buckling mechanism of surface roughness formation.
Journal of Applied Physics 05/2010; · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We describe a near perfect broad band absorber based on a laterally nanostructured multilayer material. We present calculations of the structure that demonstrates over 98% absorption over a wide frequency range around the 300 K black body spectrum. We also show that a nanostructured metamaterial allows us to tailor the index of refraction using effective medium theory, with further improvement of the absorption characteristics to over 99% over the same frequency range. The absorber can be adapted for use in any frequency range and any source type. These materials may have applications in energy harvesting and scattered light control.