Alan Elliot

Development of Nanopatterned Fluorine-Doped Tin Oxide Electrodes for Dye-Sensitized Solar Cells with Improved Light Trapping

Transparent conductors (TCs) are an important component of optoelectronic devices and nanoscale engineering of TCs is important for optimization of the device performance through improved light trapping. In this work, patterned periodic arrays of nanopillars and nanolines of pitch size of ~700 nm were created on fluorine-doped tin oxide (FTO) using nanoimprint lithography and reactive ion etching using environmentally friendly gases. The patterned FTO exhibits enhanced light trapping as compared to the unpatterned FTO, which agrees well with simulations based on Finite-Difference Time-Domain method for up to a distance of 4 μm. Dye sensitized solar cells (DSSCs) fabricated on the patterned FTO exhibited improved performance (fill factor and power conversion efficiency), which can be attributed to enhanced light absorption in the range 400-650 nm. Further, electrochemical impedance measurements revealed lower recombination resistance for the patterned FTO/TiO(2) electrode compared to the unpatterned FTO electrode/TiO(2) electrode as a result of better light capturing properties of patterned FTO. The direct fabrication of nanopatterns on TCs developed in the present study is expected to be a viable scheme for achieving improved performance in many other optoelectronic devices.

Atomic layer deposition of Al-doped ZnO/Al2O3 double layers on vertically aligned carbon nanofiber arrays.

High-aspect-ratio, vertically aligned carbon nanofibers (VACNFs) were conformally coated with aluminum oxide (Al2O3) and aluminum-doped zinc oxide (AZO) using atomic layer deposition (ALD) in order to produce a three-dimensional array of metal-insulator-metal core-shell nanostructures. Prefunctionalization before ALD, as required for initiating covalent bonding on a carbon nanotube surface, was eliminated on VACNFs due to the graphitic edges along the surface of each CNF. The graphitic edges provided ideal nucleation sites under sequential exposures of H2O and trimethylaluminum to form an Al2O3 coating up to 20 nm in thickness. High-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy images confirmed the conformal core-shell AZO/Al2O3/CNF structures while energy-dispersive X-ray spectroscopy verified the elemental composition of the different layers. HRTEM selected area electron diffraction revealed that the as-made Al2O3 by ALD at 200 °C was amorphous, and then, after annealing in air at 450 °C for 30 min, was converted to polycrystalline form. Nevertheless, comparable dielectric constants of 9.3 were obtained in both cases by cyclic voltammetry at a scan rate of 1000 V/s. The conformal core-shell AZO/Al2O3/VACNF array structure demonstrated in this work provides a promising three-dimensional architecture toward applications of solid-state capacitors with large surface area having a thin, leak-free dielectric.

Probing the Nucleation of

Ultrathin dielectric tunneling barriers are critical to Josephson junction (JJ) based superconducting quantum bits (qubits). However, the prevailing technique of thermally oxidizing aluminum via oxygen diffusion produces problematic defects, such as oxygen vacancies, which are believed to be a primary source of the two-level fluctuators and contribute to the decoherence of the qubits. Development of alternative approaches for improved tunneling barriers becomes urgent and imperative. Atomic layer deposition (ALD) of aluminum oxide is a promising alternative to resolve the issue of oxygen vacancies in the tunneling barrier, and its self-limiting growth mechanism provides atomic-scale precision in tunneling barrier thickness control. A critical issue in ALD of on metals is the lack of hydroxyl groups on metal surface, which prevents nucleation of the trimethylaluminum. In this work, we explore modifications of the aluminum surface with water pulse exposures followed by trimethylaluminum pulse exposures to assess the feasibility of ALD as a viable technique for JJ qubits. ALD films from 40 to 100 were grown on 1.4 to 500 of Al, and were characterized with ellipsometry and atomic force microscopy. A growth rate of 1.2 was measured, and an interfacial layer was observed. Because the interfacial layer thickness depends on the availability of Al and saturated at 2 nm, choosing ultrathin Al wetting layers may lead to ultrathin ALD tunneling barriers.

\hbox{Al}_{2}\hbox{O}_{3}

Atomic layer deposition (ALD) provides a promising approach for deposition of ultrathin low-defect-density tunnel barriers, and it has been implemented in a high-vacuum magnetron sputtering system for in situ deposition of ALD-Al₂O₃ tunnel barriers in superconductor-insulator-superconductor Josephson junctions. A smooth ALD-Al₂O₃ barrier layer was grown on an Al-wetted Nb bottom electrode and was followed with a top Nb electrode growth using sputtering. Preliminary low temperature measurements of current-voltage characteristics of the Josephson junctions made from these trilayers confirmed the integrity of the ALD-Al₂O₃ barrier layer. However, the <i>I</i>_c<i>R</i>_N product of the junctions is much smaller than the value expected from the Ambegaokar-Baratoff formula suggesting a significant pair-breaking mechanism at the interfaces.

in Atomic Layer Deposition on Aluminum for Ultrathin Tunneling Barriers in Josephson Junctions

Colloidal nanocrystals are attractive materials for optoelectronics applications because they offer a compelling combination of low-cost solution processing, printability, and spectral tunability through the quantum dot size effect. Here we explore a novel nanocomposite photosensitizer consisting of colloidal nanocrystals of FeS2 and PbS with complementary optical and microstructural properties for broadband photodetection. Using a newly developed ligand exchange to achieve high-efficiency charge transfer across the nanocomposite FeS2-PbS sensitizer and graphene on the FeS2-PbS/graphene photoconductors, an extraordinary photoresponsivity in exceeding ∼106 A/W was obtained in an ultrabroad spectrum of ultraviolet (UV)-visible-near-infrared (NIR). This is in contrast to the nearly 3 orders of magnitude reduction of the photoresponsivity from ∼106 A/W at UV to 103 A/W at NIR on their counterpart of FeS2/graphene detectors. This illustrates the combined advantages of the nanocomposite sensitizers and the high charge mobility in FeS2-PbS/graphene van der Waals heterostructures for nanohybrid optoelectronics with high performance, low cost, and scalability for commercialization.

Fabrication of Nb/Al2O3/Nb Josephson Junctions Using In Situ Magnetron Sputtering and Atomic Layer Deposition

Atomic layer deposition (ALD) provides a promising approach for deposition of ultrathin low-defect-density tunnel barriers, and it has been implemented in a high-vacuum magnetron sputtering system for in situ deposition of ALD-Al₂O₃ tunnel barriers in superconductor-insulator-superconductor Josephson junctions. A smooth ALD-Al₂O₃ barrier layer was grown on an Al-wetted Nb bottom electrode and was followed with a top Nb electrode growth using sputtering. Preliminary low temperature measurements of current-voltage characteristics of the Josephson junctions made from these trilayers confirmed the integrity of the ALD-Al₂O₃ barrier layer. However, the <i>I</i>_c<i>R</i>_N product of the junctions is much smaller than the value expected from the Ambegaokar-Baratoff formula suggesting a significant pair-breaking mechanism at the interfaces.

Printable Nanocomposite FeS2–PbS Nanocrystals/Graphene Heterojunction Photodetectors for Broadband Photodetection

Metal-Insulator-Metal tunnel junctions (MIMTJ) are common throughout the microelectronics industry. The industry standard AlOx tunnel barrier, formed through oxygen diffusion into an Al wetting layer, is plagued by internal defects and pinholes which prevent the realization of atomically-thin barriers demanded for enhanced quantum coherence. In this work, we employed in situ scanning tunneling spectroscopy (STS) along with molecular dynamics simulations to understand and control the growth of atomically thin Al2O3 tunnel barriers using atomic layer deposition (ALD). We found that a carefully tuned initial H2O pulse hydroxylated the Al surface and enabled the creation of an atomically-thin Al2O3 tunnel barrier with a high quality M-I interface and a significantly enhanced barrier height compared to thermal AlOx. These properties, corroborated by fabricated Josephson Junctions, show that ALD Al2O3 is a dense, leak-free tunnel barrier with a low defect density which can be a key component for the next-generation of MIMTJs.

Fabrication of

Vortex pinning by insertion of non-superconducting defects like BZO or BSO nanorods into the YBCO matrix is an effective means to enhance pinning since they self-assemble into columnar structures that provide strong pinning along the length of the flux-line. However, only limited control of their geometry is possible by current growth methods. To meet the requirements of applications that operate in magnetic fields of varying intensity or orientation, this work studies strain-mediated self-assembly of 3D pinning landscape through theoretical modeling as well as experimental exploration to achieve controllable growth BZO or BSO nanostructures in YBCO matrix films. The microstructure of BZO- and BSO-doped YBCO thin films was studied using transmission electron microscopy and the findings indicate that it is possible to produce a controllable defect landscape and improved critical current density with respect to different orientation of the magnetic field by manipulation of the strain relationships using vicinal substrates.

\hbox{Nb/Al}_{2}\hbox{O}_{3}/\hbox{Nb}

Atomic Layer Deposition (ALD) is a promising technique for growing ultrathin, pristine dielectrics on metal substrates, which is essential to many electronic devices. Tunnel junctions are an excellent example which require a leak-free, ultrathin dielectric tunnel barrier of typical thickness around 1 nm between two metal electrodes. A challenge in the development of ultrathin dielectric tunnel barriers using ALD is controlling the nucleation of dielectrics on metals with minimal formation of native oxides at the metal surface for high-quality interfaces between the tunnel barrier and metal electrodes. This poses a critical need for integrating ALD with ultra-high vacuum (UHV) physical vapor deposition. In order to address these challenges, a viscous-flow ALD chamber was designed and interfaced to an UHV magnetron sputtering chamber via a load lock. A sample transportation system was implemented for in situ sample transfer between the ALD, load lock, and sputtering chambers. Using this integrated ALD-UHV sputtering system, superconductor-insulator-superconductor (SIS) Nb-Al/Al2O2/Nb Josephson tunnel junctions were fabricated with tunnel barriers of thickness varied from sub-nm to ~1 nm. The suitability of using an Al wetting layer for initiation of the ALD Al2O3 tunnel barrier was investigated with ellipsometry, atomic force microscopy, and electrical transport measurements. With optimized processing conditions, leak-free SIS tunnel junctions were obtained, demonstrating the viability of this integrated ALD-UHV sputtering system for the fabrication of tunnel junctions and devices comprised of metal-dielectric-metal multilayers.

Josephson Junctions Using In Situ Magnetron Sputtering and Atomic Layer Deposition

Biaxially textured MgO templates have been successfully fabricated on several amorphous polymer films including Kapton tapes, polyimide, Poly(methyl methacrylate) (PMMA), and photoresist films using ion-beam-assisted-deposition (IBAD). With a Y2O3 buffer layer on polymer surfaces, roughening of the polymer surfaces due to preferential ion beam sputtering can be effectively reduced to meet the surface compatibility requirement for IBAD-MgO texturing. In-plane full-width-at-half-maximum (FWHM) of ~10.7 degrees and out-of-plane FWHM ~ 3.5 degrees have been obtained on homoepitaxial MgO films grown on top of the IBAD-MgO template. This method provides a practical route for fabricating epitaxial devices on polymers needed for flexible optoelectronics.