Copyright (c) 2013 Purdue University All rights reserved. http://docs.lib.purdue.edu/nanopub Recent documents in Birck and NCN Publications en-us Wed, 17 Apr 2013 01:42:14 PDT 3600 http://docs.lib.purdue.edu/nanopub/918 http://docs.lib.purdue.edu/nanopub/918 Mon, 15 Apr 2013 14:44:05 PDT Thin film dielectrics have broad applications, and the performance degradation due to charge trapping in these thin films is an important and pervasive reliability concern. It has been presumed since the 1960s that current transport in intermediate-thickness (IT) oxides (∼10–100 nm) can be described by Frenkel-Poole (FP) conduction (originally developed for ∼mm-thick films) and algorithms based on the FP theory can be used to extract defect energy levels and charging-limited lifetime. In this paper, we review the published results to show that the presumption of FP-dominated current in IT oxides is incorrect, and therefore, the methods to extract trap-depths to predict lifetime should be revised. We generalize/adapt the bulk FP current conduction model by including additional tunneling-based current injection. Steady state characteristics are obtained by a flux balance between contacts and the IT oxide. An analytical approximation of the generalized FP model yields a steady state leakage current J ∝ exp(−B√E)(1 − C√E − D/E), where B, C, and D are material-specific constants. This reformulation provides a new algorithm for extracting defect levels to predict the corresponding charging limited device lifetime. The validity and robustness of the new algorithm are confirmed by simulations and published experimental data.

]]> Sambit Palit et al. http://docs.lib.purdue.edu/nanopub/917 http://docs.lib.purdue.edu/nanopub/917 Mon, 15 Apr 2013 14:44:04 PDT Degradation and failure due to dielectric charging has been a dominant and pervasive reliability concern for RF-MEMS switches. Traditionally, the operational lifetime dictated by this degradation phenomenon is extrapolated from a series of measurements of time-dependent shifts in Capacitance-Voltage (C-V) characteristics under accelerated stress conditions. In this paper, we explain why the classical large-signal C-V methodology may lead to a pessimistic under-prediction of device lifetime. Using both simulations and experiments, we propose and verify a new small-signal characterization technique based on resonance characteristics of MEMS cantilever beams. This new technique overcomes the limitations of the classical approaches to accurately anticipate device lifetime and opens up the possibility of non-obtrusive, in-situ runtime monitoring of degradation in RF-MEMS switches. Moreover, since the technique is amenable to `parallel' implementation, it has the potential to be used both as an in-line process monitor as well as to reduce the overall time to technology qualification.

]]> Sambit Palit et al. http://docs.lib.purdue.edu/nanopub/916 http://docs.lib.purdue.edu/nanopub/916 Thu, 11 Apr 2013 13:32:01 PDT Actuation characteristics of electromechanical (EM) actuators have traditionally been studied for a few specific regular electrode geometries and support (anchor) configurations. The ability to predict actuation characteristics of electrodes of arbitrary geometries and complex support configurations relevant for broad range of applications in switching, displays, and varactors, however, remains an open problem. In this article, we provide four universal scaling relationships for EM actuation characteristics that depend only on the mechanical support configuration and the corresponding electrode geometries, but are independent of the specific geometrical dimensions and material properties of these actuators. These scaling relationships offer an intrinsic classification for actuation behavior of a broad range of EM actuators with vastly different electrode/support geometries. Consequently, the problem of analysis/ design of complex EM actuators is reduced to the problem of determining only five scaling parameters, which can be obtained from no more than three independent characterization experiments or numerical simulations.

]]> Sambit Palit et al. http://docs.lib.purdue.edu/nanopub/915 http://docs.lib.purdue.edu/nanopub/915 Tue, 22 Jan 2013 09:18:21 PST We report an interesting scaling trend in the switching time and the switching voltage of the organic ferroelectric copolymer PVDF-TrFE as a function of the device area. We have found that shrinking the lateral dimensions of the ferroelectric film results in a dramatic decrease in the switching time and the switching voltage. The phenomenological theory, that explains this abnormal scaling trend, involves in-plane interaction of the polymeric chains of the two-dimensional Langmuir-Blodgett (LB) films of the copolymer PVDF-TrFE interchain and intrachain coupling results in a weak power-law dependence of the switching field on the device area (E-SW alpha A(CH)(0.1)) which is ultimately responsible for the decrease in the switching time and switching voltage. For this scaling study we have used the organic ferroelectric copolymer as the top gate dielectric of a field-effect transistor structure with poly silicon nanowires as channel material. The gated channel area was varied by more than two orders of magnitude (0.04-5 mu m(2)) while the thickness of the ferroelectric copolymer film was kept constant at 100 nm. Our findings are believed to be of importance to both, the fundamental understanding of non-equilibrium processes in correlated condensed matter systems and the technological use of ferroelectric copolymers for non volatile memory applications. (C) 2012 Elsevier B.V. All rights reserved.

]]> Saptarshi Das et al. http://docs.lib.purdue.edu/nanopub/914 http://docs.lib.purdue.edu/nanopub/914 Tue, 22 Jan 2013 08:10:58 PST We have increased the ratio of s-polarization (normal incidence) to p-polarization photocurrent to 50% in a quantum dot-in-a-well based infrared photodetector form the typical s-p polarization ratio about 20%. This improvement was achieved by engineering the dot geometry and the quantum confinement via post growth capping materials of the Stranski Krastanov growth mode quantum dots (QDs). The TEM images show that the height to base ratio of shape engineered QDs was increased to 8 nm/12 nm from the control sample's ratio 4 nm/17 nm. The dot geometry correlates with the polarized photocurrent measurements of the detector. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4764905]

]]> Jiayi Shao et al. http://docs.lib.purdue.edu/nanopub/913 http://docs.lib.purdue.edu/nanopub/913 Fri, 11 Jan 2013 08:27:27 PST We report drastically different onset temperatures of the reentrant integer quantum Hall states in the second and third Landau level. This finding is in quantitative disagreement with the Hartree-Fock theory of the bubble phases which is thought to describe these reentrant states. Our results indicate that the number of electrons per bubble in either the second or the third Landau level is likely different than predicted.

]]> Nianpei Deng et al. http://docs.lib.purdue.edu/nanopub/912 http://docs.lib.purdue.edu/nanopub/912 Fri, 11 Jan 2013 07:39:18 PST Arrays of semiconductor nanowires are of potential interest for applications including photovoltaic devices and IR detectors/imagers. While nominally uniform arrays have typically been studied, arrays containing nanowires with multiple diameters and/or random distributions of diameters could allow tailoring of the photonic properties of the arrays. In this Letter, we demonstrate the growth and optical properties of randomly branched InSb nanowire arrays. The structure mentioned can be approximated as three vertically stacked regions, with average diameters of 20, 100, and 150 nm within the respective layers. Reflectance and transmittance measurements on structures with different average nanowire lengths have been performed over the wavelength range of 300-2000 nm, and absorbance has been calculated from these measurements. The structures show low reflectance over the visible and IR regions and wavelength-dependent absorbance in the IR region. A model considering the diameter-dependent photonic coupling (at a given wavelength) and random distribution of nanowire diameters within the regions has been developed. The diameter-dependent photonic coupling results in a roll-off in the absorbance spectra at wavelengths well below the bulk cutoff of similar to 7 mu m, and randomness is observed to broaden the absorbance response. Varying the average diameters would allow tailoring of the wavelength dependent absorption within various layers, which could be employed in photovoltaic devices or wavelength-dependent IR imagers.

]]> Asaduzzaman Mohammad et al. http://docs.lib.purdue.edu/nanopub/911 http://docs.lib.purdue.edu/nanopub/911 Fri, 11 Jan 2013 07:39:17 PST This paper presents the design and experimental validation of dc-dynamic biasing for >50x switching time improvement in severely underdamped fringing-field electrostatic MEMS actuators. The electrostatic fringing-field actuator is used to demonstrate the concept due to its robust device design and inherently low damping conditions. In order to accurately quantify the gap height versus voltage characteristics, a heuristic model is developed. The difference between the heuristic model and numerical simulation is less than 5.6% for typical MEMS geometries. MEMS fixed-fixed beams are fabricated and measured for experimental validation. Good agreement is observed between the calculated and measured results. For a given voltage, the measured and calculated displacements are typically within 10%. Lastly, the derived model is used to design a dc-dynamic bias waveform to improve the switching time of the underdamped MEMS actuators. With dynamic biasing, the measured up-to-down and down-to-up switching time of the actuator is similar to 35 mu s. On the other hand, coventional step biasing results in a switching time of similar to 2 ms for both up-to-down and down-to-up states.

]]> J. Small et al. http://docs.lib.purdue.edu/nanopub/910 http://docs.lib.purdue.edu/nanopub/910 Fri, 21 Dec 2012 10:34:34 PST Device-performances of 3.7 THz indirect-pumping quantum- cascade lasers are demonstrated in an InGaAs/InAlAs material system grown by metal-organic vapor-phase epitaxy. The lasers show a low threshold-current-density of ~420 A/cm² and a peak output power of ~8 mW at 7 K, no sign of parasitic currents with recourse to well-designed coupled-well injectors in the indirect pump scheme, and a maximum operating temperature of T_max~100 K. The observed roll-over of output intensities in current ranges below maximum currents and limitation of T_max are discussed with a model for electron-gas heating in injectors. Possible ways toward elevation of T_max are suggested.

]]> Kazuue Fujita et al. http://docs.lib.purdue.edu/nanopub/909 http://docs.lib.purdue.edu/nanopub/909 Thu, 20 Dec 2012 09:28:16 PST This article provides experimental evidence of one-dimensional behavior of silicon (Si) nanowires (NWs) at low-temperature through both transfer (Id−VG) and capaci- tance−voltage characteristics. For the first time, operation of Si NWs in the quantum capacitance limit (QCL) is experimentally demonstrated and quantitatively analyzed. This is of relevance since working in the QCL may allow, e.g., tunneling field-effect transistors (TFETs) to achieve higher on-state currents (Ion) and larger on-/off-state current ratios (Ion/Ioff), thus addressing one of the most severe limitations of TFETs. Comparison of the experimental data with simulations finds excellent agreement using a simple capacitor model.

]]> Ramon B. Salazar et al. http://docs.lib.purdue.edu/nanopub/908 http://docs.lib.purdue.edu/nanopub/908 Tue, 27 Nov 2012 06:59:33 PST Nanostructured electrodes (NEs) improve optical absorption and charge collection in photovoltaic (PV) devices. Traditionally, the electrodes have been designed exclusively for higher optical absorption. Such an optical design of the electrodes does not necessarily ensure better charge collection. Since the efficiency of organic PV (OPV) devices is hindered by the low carrier mobility of the organic semiconductors, the charge collection property of the NEs provides an interesting design alternative. The goal of this paper is the formulation of the essential design rules for NEs to improve charge collection in the low-mobility organic materials. We use detailed optoelectronic device simulation to explore the physics of NEs embedded in the organic semiconductors and quantify its effect on the performance gain of organic solar cells. Our analysis suggests that an optimum codesign of electrodes and morphology is essential for significant performance improvement (mainly through fill factor) in OPV cells.

]]> Biswajit Ray et al. http://docs.lib.purdue.edu/nanopub/907 http://docs.lib.purdue.edu/nanopub/907 Thu, 15 Nov 2012 14:08:45 PST A multiscale approach was adopted for the calculation of confined states in self-assembled semiconductor quantum dots (QDs). While results close to experimental data have been obtained with a combination of atomistic strain and tight-binding (TB) electronic structure description for the confined quantum states in the QD, the TB calculation requires substantial computational resources. To alleviate this problem an integrated approach was adopted to compute the energy states from a continuum 8-band k.p Hamiltonian under the influence of an atomistic strain field. Such multiscale simulations yield a roughly six-fold faster simulation. Atomic-resolution strain is added to the k.p Hamiltonian through interpolation onto a coarser continuum grid. Sufficient numerical accuracy is obtained by the multiscale approach. Optical transition wavelengths are within 7% of the corresponding TB results with a proper splitting of p-type sub-bands. The systematically lower emission wavelengths in k.p are attributable to an underestimation of the coupling between the conduction and valence bands.

]]> Parijat Sengupta et al. http://docs.lib.purdue.edu/nanopub/906 http://docs.lib.purdue.edu/nanopub/906 Thu, 15 Nov 2012 14:08:42 PST Porous crystalline Si nanowires (PC-SiNW) represent an attractive solution for enhancing the thermoelectric efficiency (ZT) of SiNWs by reducing the lattice thermal conductance (κl). A modified valence force field (MVFF) phonon model along with Landauer’s approach is used to analyze the ballistic κl in PC-SiNWs. A systematic study focusing on the influence of pore size, density, and distribution on the ballistic κl of PC-SiNWs is presented. The model predicts a maximum reduction of ~19%, ~23% and ~30% for 1, 2 and 3 pores, respectively with a constant removal of ~12% of the atoms in all the cases. The model also predicts a higher reduction of the ballistic κl as the pore separation increases, in the case of 2, 3 and 4 pores, for the same percentage of atoms removed (~12%) in all the cases. Thus, the presence of a high number of small, well-separated pores suppress κl strongly. This reduction in ballistic κl, in the coherent limit, is attributed to the reduction of the total number of phonon modes and smaller participation of phonon modes (in κl) with increasing number of pores.

]]> Abhijeet Paul et al. http://docs.lib.purdue.edu/nanopub/905 http://docs.lib.purdue.edu/nanopub/905 Thu, 15 Nov 2012 14:08:40 PST Modeling and simulation take an important role in the exploration and design optimization of novel devices. As the downscaling of electronic devices continues, the description of interfaces, randomness, and disorder on an atomistic level gains importance and continuum descriptions lose their validity. Often a full-band description of the electronic structure is needed to model the interaction of different valleys and nonparabolicity effects. NEMO 5 [1] is a modeling tool that addresses these issues and is able to provide insight into a broad range of devices. It unifies the capabilities of prior projects: multiscale approaches to quantum transport in planar structures in NEMO-IO [2], multimillion-atom simulations of strain and electronic structure in NEMO-30 [3] and NEMO-30-Peta [4], and quantum transport in nonplanar structures in OMEN [5]. NEMO 5 aims at becoming a community code whose structure, implementation, resource requirements and license allow experimental and theoretical researchers in academia and industry alike to use and extend the tool.

]]> Sebastian Steiger et al. http://docs.lib.purdue.edu/nanopub/904 http://docs.lib.purdue.edu/nanopub/904 Thu, 15 Nov 2012 14:08:09 PST In the last few years, evolutionary computing (EC) approaches have been successfully used for many real world optimization applications in scientific and engineering areas. One of these areas is computational nanoscience. Semi-empirical models with physics-based symmetries and properties can be developed by using EC to reproduce theoretically the experimental data. One of these semi-empirical models is the Valence Force Field (VFF) method for lattice properties. An accurate understanding of lattice properties provides a stepping stone for the investigation of thermal phenomena and has large impact in thermoelectricity and nano-scale electronic device design. The VFF method allows for the calculation of static properties like the elastic constants as well as dynamic properties like the sound velocity and the phonon dispersion. In this paper a parallel genetic algorithm (PGA) is employed to develop the optimal VFF model parameters for gallium arsenide (GaAs). This methodology can also be used for other semiconductors. The achieved results agree qualitatively and quantitatively with the experimental data.

]]> Mehdi Salmani-Jelodar et al. http://docs.lib.purdue.edu/nanopub/903 http://docs.lib.purdue.edu/nanopub/903 Thu, 15 Nov 2012 14:08:07 PST Abhijeet Paul et al. http://docs.lib.purdue.edu/nanopub/902 http://docs.lib.purdue.edu/nanopub/902 Thu, 15 Nov 2012 14:08:05 PST The science gateway nanoHUB.org is the world’s largest nanotechnology user facility, serving 167,196 users in 2010 with over 2,300 resources including 189 simulation programs. Surveys of nanoHUB users and automated usage analysis find widespread simulation use in formal classroom education, thereby connecting recent research more rapidly and closely to education. Analysis of 719 citations in the scientific literature by over 1,300 authors to nanoHUB.org resources documents use of simulation programs by new research collaborations, by researchers outside of the community originating the program, and by experimentalists. The publication and author networks reveal research collaborations and capacity building through knowledge transfer. Analysis of secondary citations documents the quality of the conducted research with an h-index of 30 after just 10 years of operation. Our analysis proves with quantitative metrics that impactful research can be conducted by an ever growing research community. We argue that HUBzero technology and the user- focused design and operation of nanoHUB.org are success criteria that can be transferred to other science gateways.

]]> Gerhard Klimeck et al. http://docs.lib.purdue.edu/nanopub/901 http://docs.lib.purdue.edu/nanopub/901 Thu, 15 Nov 2012 14:08:00 PST The thermoelectric power-factor (PF) and efficiency (ZT) of GaAs nanowires (NWs) can be improved by (i) choosing a proper wire growth and channel orientation, (ii) by applying uniaxial tensile stress, and (iii) suitable wire cross-section size. In this work we study the impact of these three factors on the PF and the ZT. Tensile stress, channel direction and cross-section size allows bandstructure engineering to tune the electronic conductance (G) and the Seebeck coefficient (S). [110] GaAs NWs grown on (111) surface provide maximum PF (3X) and ZT (1.3X) compared to [100]/(100) NWs, which can be attributed to the G enhancement induced by the L valley contribution under strain.

]]> Abhijeet Paul et al. http://docs.lib.purdue.edu/nanopub/900 http://docs.lib.purdue.edu/nanopub/900 Thu, 15 Nov 2012 14:07:55 PST nanoHUB.org is arguably the largest online nanotechnology user facility in the world. Just between July 2010 and June 2011 it served 177,823 users. 10,477 users ran 393,648 simulation jobs on a variety of computational resources ranging from HUBzero-based virtual execution hosts for rapid, interactive runs as well as grid-based resources for computationally-intense runs. We believe that as such our users experience a fully operational scientific “cloud”-based infrastructure even though it is not using “standard” computational cloud infrastructures such as EC2. In this paper we explore the use of standard computational cloud-based resources to determine whether they can deliver satisfactory outcomes for our users without requiring high personnel costs for configuration. In a science gateway environment, the assignment of jobs to the appropriate computational resource is not trivial. Resource availability, wait time, time to completion, and likelihood of job success must all be considered in order to transparently deliver an acceptable level of service to our users. In this paper, we present preliminary results examining the benefits and drawbacks of utilizing standard computational cloud resources as one potential venue for nanoHUB computational runs. In summary we find that cloud resources performed competitively with other grid resources in terms of wait time, CPU usage, and success in a multiple job submission strategy.

]]> Lynn K. Zentner et al. http://docs.lib.purdue.edu/nanopub/899 http://docs.lib.purdue.edu/nanopub/899 Thu, 15 Nov 2012 14:07:53 PST Novel device concepts and better channel materials than Si are required to improve the performance of conventional metal-oxide-semiconductor field-effect transistors (MOSFETs). The exploration of III-V semiconductors is mainly driven by the extremely high electron mobility of the materials. Recently, several researches have demonstrated that III-V high electron mobility transistors (HEMTs) can achieve high-speed operation at low supply voltage for applications beyond Si-CMOS technology. While the intrinsic device performance looks promising, current prototypes are dramatically influenced by high contact resistances. From a modeling point of view the understanding of the intrinsic device performance is now quite advanced, while the understanding of the contacts remains quite limited. Hence, a precise theoretical approach is required to model the contact characteristics. This work investigates the contact resistance physics of InAs HEMT transistors. The Nano-Electronic Modeling Tool (NEMO5) is used to solve the non-equilibrium Green’s function (NEGF) formalism which embeds Schrödinger and Poisson equations self-consistently. For this study a real-space effective mass approximation with a simple phonon scattering is utilized.

]]> Seung Hyun Park et al.