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  • ItemOpen AccessAccepted version Peer-reviewed
    CMOS compatible electrode materials selection in oxide-based memory devices
    (AIP Publishing, 2016-07-14) Zhuo, VYQ; Li, M; Guo, Y; Wang, W; Yang, Y; Jiang, Y; Robertson, J
    Electrode materials selection guidelines for oxide-based memory devices are constructed from the combined knowledge of observed device operation characteristics, ab-initio calculations, and nano-material characterization. It is demonstrated that changing the top electrode material from Ge to Cr to Ta in the Ta2O5-based memory devices resulted in a reduction of the operation voltages and current. Energy Dispersed X-ray (EDX) Spectrometer analysis clearly shows that the different top electrode materials scavenge oxygen ions from the Ta2O5 memory layer at various degrees, leading to different oxygen vacancy concentrations within the Ta2O5, thus the observed trends in the device performance. Replacing the Pt bottom electrode material with CMOS compatible materials (Ru and Ir) further reduces the power consumption and can be attributed to the modification of the Schottky barrier height and oxygen vacancy concentration at the electrode/oxide interface. Both trends in the device performance and EDX results are corroborated by the ab-initio calculations which reveal that the electrode material tunes the oxygen vacancy concentration via the oxygen chemical potential and defect formation energy. This experimental-theoretical approach strongly suggests that the proper selection of CMOS compatible electrode materials will create the critical oxygen vacancy concentration to attain low power memory performance.
  • ItemOpen AccessAccepted version Peer-reviewed
    Multi-band magnetotransport in exfoliated thin films of CuxBi2Se3
    (IOP Publishing, 2018) Alexander-Webber, JA; Huang, J; Beilsten-Edmands, J; Cermak, P; Drasar, C; Nicholas, RJ; Coldea, AI; Alexander-Webber, Jack Allen [0000-0002-9374-7423]
    We report magnetotransport studies in thin (<100nm) exfoliated films of CuₓBi₂Se₃ and we detect an unusual electronic transition at low temperatures. Bulk crystals show weak superconductivity with Tc~3.5K and a possible electronic phase transition around 200K. Following exfoliation, superconductivity is supressed and a strongly temperature dependent multi-band conductivity is observed for T<30K. This transition between competing conducting channels may be enhanced due to the presence of electronic ordering, and could be affected by the presence of an effective internal stress due to Cu intercalation. By fitting to the weak antilocalisation conductivity correction at low magnetic fields we confirm that the low temperature regime maintains a quantum phase coherence length Lφ>100nm indicating the presence of topologically protected surface states.
  • ItemOpen AccessAccepted version Peer-reviewed
    Deep Subthreshold TFT Operation and Design Window for Analog Gain Stages
    (Institute of Electrical and Electronics Engineers (IEEE), 2018) Cheng, X; Lee, S; Nathan, A; Cheng, X [0000-0001-7798-4747]; Lee, S [0000-0002-5523-8476]; Nathan, A [0000-0002-2070-8853]
    The intrinsic gain (Ai), transconductance efficiency (gm/IDS) and cut-off frequency (fT) are analysed for thin film transistors (TFTs) operating in the deep subthreshold region to assess the impact of device variations on the design window for analogue circuits. Results suggest that subthreshold operation could improve the Ai and gm/IDS at the cost of reduced fT and increased sensitivity to bias and process variations. Interestingly, a less steep subthreshold slope (SS) could, to some extent, compensate for these shortcomings, which is in contrast to the general thinking of pursuing a steeper SS in TFT fabrication.
  • ItemOpen AccessPublished version Peer-reviewed
    Understanding and Controlling Cu-Catalyzed Graphene Nucleation: The Role of Impurities, Roughness, and Oxygen Scavenging
    (American Chemical Society, 2016-12-27) Braeuninger-Weimer, P; Brennan, B; Pollard, AJ; Hofmann, S; Braeuninger-Weimer, Philipp [0000-0001-8677-1647]; Hofmann, Stephan [0000-0001-6375-1459]
    The mechanism by which Cu catalyst pretreatments control graphene nucleation density in scalable chemical vapor deposition (CVD) is systematically explored. The intrinsic and extrinsic carbon contamination in the Cu foil is identified by time-of-flight secondary ion mass spectrometry as a major factor influencing graphene nucleation and growth. By selectively oxidizing the backside of the Cu foil prior to graphene growth, a drastic reduction of the graphene nucleation density by 6 orders of magnitude can be obtained. This approach decouples surface roughness effects and at the same time allows us to trace the scavenging effect of oxygen on deleterious carbon impurities as it permeates through the Cu bulk. Parallels to well-known processes in Cu metallurgy are discussed. We also put into context the relative effectiveness and underlying mechanisms of the most widely used Cu pretreatments, including wet etching and electropolishing, allowing a rationalization of current literature and determination of the relevant parameter space for graphene growth. Taking into account the wider CVD growth parameter space, guidelines are discussed for high-throughput manufacturing of "electronic-quality" monolayer graphene films with domain size exceeding 1 mm, suitable for emerging industrial applications, such as electronics and photonics.
  • ItemOpen AccessPublished version Peer-reviewed
    Controlling nanowire growth through electric field-induced deformation of the catalyst droplet.
    (Springer Science and Business Media LLC, 2016-07-29) Panciera, Federico; Norton, Michael M; Alam, Sardar B; Hofmann, Stephan; Mølhave, Kristian; Ross, Frances M; Hofmann, Stephan [0000-0001-6375-1459]
    Semiconductor nanowires with precisely controlled structure, and hence well-defined electronic and optical properties, can be grown by self-assembly using the vapour-liquid-solid process. The structure and chemical composition of the growing nanowire is typically determined by global parameters such as source gas pressure, gas composition and growth temperature. Here we describe a more local approach to the control of nanowire structure. We apply an electric field during growth to control nanowire diameter and growth direction. Growth experiments carried out while imaging within an in situ transmission electron microscope show that the electric field modifies growth by changing the shape, position and contact angle of the catalytic droplet. This droplet engineering can be used to modify nanowires into three dimensional structures, relevant to a range of applications, and also to measure the droplet surface tension, important for quantitative development of strategies to control nanowire growth.
  • ItemOpen AccessAccepted version Peer-reviewed
    Bandwidth enhancement in multimode polymer waveguides using waveguide layout for optical printed circuit boards
    (OSA, 2016-08-09) Chen, J; Bamiedakis, N; Vasil'Ev, P; Penty, RV; White, IH; Bamiedakis, Nikolaos [0000-0003-1981-1623]; Vasilyev, Petr [0000-0002-4680-910X]; Penty, Richard [0000-0003-4605-1455]; White, Ian [0000-0002-7368-0305]
    © 2016 OSA. Dispersion studies demonstrate that waveguide layout can be used to enhance the bandwidth performance of multimode polymer waveguides for use in board-level optical interconnects, providing >40 GHz×m without the need for any launch conditioning.
  • ItemOpen Access
    Room temperature sputtering of inclined c- axis ZnO for shear mode solidly mounted resonators
    (AIP Publishing, 2016) Rughoobur, G; Demiguel-Ramos, M; Mirea, T; Clement, M; Olivares, J; Díaz-Durán, B; Sangrador, J; Miele, I; Milne, WI; Iborra, E; Flewitt, AJ; Rughoobur, G [0000-0002-1693-4532]; Iborra, E [0000-0002-1385-1379]
    ZnO films with a c-axis significantly inclined away from the surface normal were grown by a remote plasma sputtering technique at room temperature. The films were used to make solidly mounted resonators (SMRs) operating in shear mode at a resonant frequency of 1.35 GHz. Control of the ZnO microstructure was achieved using a polycrystalline AlN seed layer which can be added on top of a sputtered acoustic mirror to give a complete SMR device. The ZnO was reactively sputtered in an atmosphere of argon and oxygen from a zinc target. The c-axis of the ZnO was estimated to be at an angle of ∼45° to the surface normal. SMRs were measured to have quality factors (Q) of up to 140 and effective electromechanical coupling coefficients of up to 2.2% in air. Although an inclined c-axis can be achieved with direct growth onto the acoustic mirror, it is shown that the AlN seed layer provides higher coupling coefficients and narrower inclination angular distribution. The responses of the devices in liquids of different viscosities (acetone, water, and AZ5214E photoresist) were measured. The shear mode Q decreased by 45% in acetone, 72% in water, and 92% in AZ5214E.
  • ItemOpen Access
    Design and modelling of solidly mounted resonators for low-cost particle sensing
    (IOP Publishing, 2016) Villa-López, FH; Rughoobur, G; Thomas, S; Flewitt, AJ; Cole, M; Gardner, JW; Flewitt, Andrew [0000-0003-4204-4960]
    This work presents the design and fabrication of Solidly Mounted Resonator (SMR) devices for the detection of particulate matter (PM2.5 and PM10) in order to develop a smart low-cost particle sensor for air quality. These devices were designed to operate at a resonant frequency of either 870 MHz or 1.5 GHz, employing zinc oxide as the piezoelectric layer and an acoustic mirror made from molybdenum and silicon dioxide layers. Finite element analysis of the acoustic resonators was performed using COMSOL Multiphysics software in order to evaluate the frequency response of the devices and the performance of the acoustic mirror. The zinc oxide based acoustic resonators were fabricated on a silicon substrate using a five mask process. The mass sensitivity of the acoustic resonators was estimated using a 3-D finite element model and preliminary testing has been performed. The theoretical and observed mass sensitivity were similar at ca. 145 kHz/ng for the 870 MHz resonator when detecting PM2.5 suggesting that SMR devices have potential to be used as part of a miniature smart sensor system for airborne particle detection.
  • ItemOpen Access
    An Approach to Simultaneously Test Multiple Devices for High-Throughput Production of Thin-Film Electronics
    (Institute of Electrical and Electronics Engineers (IEEE), 2016) Kumar, A; Flewitt, AJ; Flewitt, Andrew [0000-0003-4204-4960]
    New generation of thin-film transistors (TFTs), where the active material is amorphous oxide, conjugated polymer, or small molecules, have the advantage of flexibility, high form factor, and large scale manufacturability through low cost processing techniques, e.g., roll-to-roll printing, screen printing. During high-throughput production using these techniques, the probability of defects being present increases with the speed of manufacturing and area of devices. Therefore a high-throughput and low cost testing technique is absolute essential to maintain high quality of final product. We report a Simultaneous Multiple Device Testing (SMuDT) approach which is up to 10 times faster and cost effective than conventional testing methods. The SMuDT approach was validated using circuit simulation and demonstrated by testing large scale indium gallium zinc oxide (IGZO) TFTs. A method to ‘bin’ the tested devices using Figure of Merit was established.
  • ItemOpen Access
    Co-catalytic absorption layers for controlled laser-induced chemical vapor deposition of carbon nanotubes.
    (American Chemical Society (ACS), 2014-03-26) Michaelis, F Benjamin; Weatherup, Robert S; Bayer, Bernhard C; Bock, Maximilian CD; Sugime, Hisashi; Caneva, Sabina; Robertson, John; Baumberg, Jeremy J; Hofmann, Stephan; Weatherup, Robert [0000-0002-3993-9045]; Baumberg, Jeremy [0000-0002-9606-9488]; Hofmann, Stephan [0000-0001-6375-1459]
    The concept of co-catalytic layer structures for controlled laser-induced chemical vapor deposition of carbon nanotubes is established, in which a thin Ta support layer chemically aids the initial Fe catalyst reduction. This enables a significant reduction in laser power, preventing detrimental positive optical feedback and allowing improved growth control. Systematic study of experimental parameters combined with simple thermostatic modeling establishes general guidelines for the effective design of such catalyst/absorption layer combinations. Local growth of vertically aligned carbon nanotube forests directly on flexible polyimide substrates is demonstrated, opening up new routes for nanodevice design and fabrication.
  • ItemOpen Access
    On-chip temperature-compensated Love mode surface acoustic wave device for gravimetric sensing
    (AIP Publishing, 2014) Liu, Q; Flewitt, AJ; Flewitt, Andrew [0000-0003-4204-4960]
    Love mode surface acoustic wave (SAW) sensors have been recognized as one of the most sensitive devices for gravimetric sensors in liquid environments such as bio sensors. Device operation is based upon measuring changes in the transmitted (S21) frequency and phase of the first-order Love wave resonance associated with the device upon on attachment of mass. However, temperature variations also cause a change in the first order S21 parameters. In this work, shallow grooved reflectors and a “dotted” single phase unidirectional interdigitated transducer (D-SPUDT) have been added to the basic SAW structure, which promote unidirectional Love wave propagation from the device's input interdigitated transducers. Not only does this enhance the first-order S21 signal but also it allows propagation of a third-order Love wave. The attenuation coefficient of the third-order wave is sufficiently great that, whilst there is a clear reflected S11 signal, the third-order wave does not propagate into the gravimetric sensing area of the device. As a result, whilst the third-order S11 signal is affected by temperature changes, it is unaffected by mass attachment in the sensing area. It is shown that this signal can be used to remove temperature effects from the first-order S21 signal in real time. This allows gravimetric sensing to take place in an environment without the need for any other temperature measurement or temperature control; this is a particular requirement of gravimetric biosensors.
  • ItemOpen Access
    The feasibility of building large scale optical switches using a novel MZI-SOA hybrid approach
    (OSA, 2014) Cheng, Q; Wei, JL; Wonfor, A; Penty, RV; White, IH; Cheng, Qixiang [0000-0001-8671-2102]; Wonfor, Adrian [0000-0003-2219-7900]; Penty, Richard [0000-0003-4605-1455]; White, Ian [0000-0002-7368-0305]
    For the first time, the feasibility of nanosecond large-scale optical switches is demonstrated using a novel MZI-SOA hybrid approach. In a filter-free recirculating loop, the potential performance of up to 128x128 port switch is demonstrated.
  • ItemOpen Access
    High speed plastic optical fiber data links using LEDs
    (2014) Wei, JL; Li, X; Geng, L; Bamiedakis, N; McKendry, JJD; Xie, E; Ferreira, R; Gu, E; Dawson, MD; Penty, RV; White, IH; Penty, Richard [0000-0003-4605-1455]
    Plastic optical fiber (POF) data links have been widely viewed as a cost- and energy- effective solutions for short-reach Gigabit data communications such as in-home and automotive networks. POF also has excellent compatibility with low-cost light-emitting-diodes (LEDs). In this paper, we demonstrate a 4 Gb/s single micro-LED (μLED) PAM-16 link and a record 4.7 Gb/s multiple μLEDs PAM-8 link by 1) use of avalanche photodiodes (APD) in combination with high order modulation formats; and 2) illumination of multiple μLEDs. Both digital implementation based on FPGA and analogue implementation are considered for real-time transmitters.
  • ItemOpen Access
    MIMO DWDM system using uncooled DFB lasers with adaptive laser bias control and postphotodetection crosstalk cancellation
    (Institute of Electrical and Electronics Engineers (IEEE), 2014) Zhu, J; Ingham, JD; Von Lindeiner, JB; Wonfor, A; Penty, RV; White, IH; Wonfor, Adrian [0000-0003-2219-7900]; Penty, Richard [0000-0003-4605-1455]; White, Ian [0000-0002-7368-0305]
    A proof-of-principle demonstration of a multiple input-multiple output (MIMO) dense wavelength division multiplexing (DWDM) system is reported. It uses standard uncooled distributed feedback (DFB) lasers with intensity modulation-direction detection (IM-DD), in which the temperature of each laser is allowed to drift independently within a 50°C temperature range. A feedback-based laser bias control algorithm is introduced to guarantee acceptable wavelength spacing and a post-photodetection minimum mean square error (MMSE) decoder is applied to cancel the inter-channel crosstalk. The relative sensitivity of the MIMO receiver in both a random laser temperature drift scenario and a worst case scenario are investigated by simulations in MATLAB. Experimental results for a 40-channel × 12.5 Gb/s DWDM system transmitting over 28 km of single-mode fiber with worst possible wavelength distribution prove the feasibility of the technique.
  • ItemOpen Access
    Experimental comparison of antenna clustering strategies in MIMO distributed antenna systems
    (IEEE, 2014) Gordon, GSD; Crisp, MJ; Penty, RV; White, IH; Gordon, George [0000-0002-7333-5106]; Crisp, Michael [0000-0002-3548-9235]; Penty, Richard [0000-0003-4605-1455]; White, Ian [0000-0002-7368-0305]
    In this paper the effect of partitioning arrays of transmitting antennas into spatially separated clusters on the condition number and capacity of MIMO wireless systems is examined using experimental channel measurements of an indoor MIMO-enabled distributed antenna system (DAS). It is first shown for a 3 _ 2 MIMO system that distributing the transmit antennas into spatially separated clusters provides an improvement in channel conditioning (_1dB) and hence capacity, in line with previous findings. Next, a configuration with 6 transmit antennas and 2 receive antennas is examined and it is found that when it is operated as a 6 _ 2 MIMO system, it makes negligible difference to the conditioning of the channel whether the transmit antennas are grouped into 3 clusters of 2 antennas or 2 clusters of 3 antennas. The capacity varies by only a small amount (_%1) between the two clustering schemes, which can be accounted for by environment-specific signal-tonoise ratio (SNR) effects. It is then concluded that for the two typical indoor DAS scenarios tested here, if sufficient transmit diversity is provided (i.e. there are a relatively large number of transmit antennas compared to receive antennas), greater spatial distribution through increased clustering provides diminishing performance improvements. Given the typically lower installation cost of less distributed arrangements, they may then be a preferred option in commercial DAS deployments.
  • ItemOpen Access
    Demonstration of the feasibility of large-port-count optical switching using a hybrid Mach-Zehnder interferometer-semiconductor optical amplifier switch module in a recirculating loop.
    (The Optical Society, 2014-09-15) Cheng, Q; Wonfor, A; Wei, JL; Penty, RV; White, IH; Cheng, Qixiang [0000-0001-8671-2102]; Wonfor, Adrian [0000-0003-2219-7900]; Penty, Richard [0000-0003-4605-1455]; White, Ian [0000-0002-7368-0305]
    For what we believe is the first time, the feasibility of large-port-count nanosecond-reconfiguration-time optical switches is demonstrated using a hybrid approach, where Mach-Zehnder interferometric (MZI) switches provide low-loss, high-speed routing with short semiconductor optical amplifiers (SOAs) being integrated to enhance extinction. By repeatedly passing signals through a monolithic hybrid dilated 2×2 switch module in a recirculating loop, the potential performance of high-port-count switches using the hybrid approach is demonstrated. Experimentally, a single pass switch penalty of only 0.1 dB is demonstrated for the 2×2 module, while even after seven passes through the switch, equivalent to a 128×128 router, a penalty of only 2.4 dB is recorded at a data rate of 10 Gb/s.
  • ItemOpen Access
    Bandwidth studies on multimode polymer waveguides for ≥ 25 Gb/s optical interconnects
    (Institute of Electrical and Electronics Engineers (IEEE), 2014) Bamiedakis, N; Chen, J; Penty, RV; White, IH; Bamiedakis, Nikolaos [0000-0003-1981-1623]; Penty, Richard [0000-0003-4605-1455]; White, Ian [0000-0002-7368-0305]
    Multimode polymer waveguides constitute a promising technology for use in board-level optical interconnects. However, the continuous improvements in high-speed performance of VCSELs raise important questions about their ability to support such high data rates due to their inherent highly-multimoded nature. Thorough experimental studies on the bandwidth of a 1.4 m long multimode spiral waveguide are presented in this paper, indicating a bandwidth-length product of at least 35 GHz×m even in the case of an overfilled launch. No significant transmission impairments are observed for spatial input offsets, while error-free (BER<10⁻¹²) data transmission over the 1.4 m long spiral waveguide is demonstrated at 25 Gb/s.
  • ItemOpen Access
    A thermalization energy analysis of the threshold voltage shift in amorphous indium gallium zinc oxide thin film transistors under simultaneous negative gate bias and illumination
    (AIP Publishing, 2014) Flewitt, AJ; Powell, MJ; Flewitt, Andrew [0000-0003-4204-4960]
    It has been previously observed that thin film transistors (TFTs) utilizing an amorphous indium gallium zinc oxide (a-IGZO) semiconducting channel suffer from a threshold voltage shift when subjected to a negative gate bias and light illumination simultaneously. In this work, a thermalization energy analysis has been applied to previously published data on negative bias under illumination stress (NBIS) in a-IGZO TFTs. A barrier to defect conversion of 0.65–0.75 eV is extracted, which is consistent with reported energies of oxygen vacancy migration. The attempt-to-escape frequency is extracted to be 106−107 s−1, which suggests a weak localization of carriers in band tail states over a 20–40 nm distance. Models for the NBIS mechanism based on charge trapping are reviewed and a defect pool model is proposed in which two distinct distributions of defect states exist in the a-IGZO band gap: these are associated with states that are formed as neutrally charged and 2+ charged oxygen vacancies at the time of film formation. In this model, threshold voltage shift is not due to a defect creation process, but to a change in the energy distribution of states in the band gap upon defect migration as this allows a state formed as a neutrally charged vacancy to be converted into one formed as a 2+ charged vacancy and vice versa. Carrier localization close to the defect migration site is necessary for the conversion process to take place, and such defect migration sites are associated with conduction and valence band tail states. Under negative gate bias stressing, the conduction band tail is depleted of carriers, but the bias is insufficient to accumulate holes in the valence band tail states, and so no threshold voltage shift results. It is only under illumination that the quasi Fermi level for holes is sufficiently lowered to allow occupation of valence band tail states. The resulting charge localization then allows a negative threshold voltage shift, but only under conditions of simultaneous negative gate bias and illumination, as observed experimentally as the NBIS effect.
  • ItemOpen Access
    Analysis of amorphous indium-gallium-zinc-oxide thin-film transistor contact metal using Pilling-Bedworth theory and a variable capacitance diode model
    (AIP Publishing, 2013) Kiani, A; Hasko, DG; Milne, WI; Flewitt, AJ; Flewitt, Andrew [0000-0003-4204-4960]
    It is widely reported that threshold voltage and on-state current of amorphous indium-gallium-zinc-oxide bottom-gate thin-film transistors are strongly influenced by the choice of source/drain contact metal. Electrical characterisation of thin-film transistors indicates that the electrical properties depend on the type and thickness of the metal(s) used. Electron transport mechanisms and possibilities for control of the defect state density are discussed. Pilling-Bedworth theory for metal oxidation explains the interaction between contact metal and amorphous indium-gallium-zinc-oxide, which leads to significant trap formation. Charge trapping within these states leads to variable capacitance diode-like behavior and is shown to explain the thin-film transistor operation.
  • ItemOpen Access
    Device and circuit-level performance of carbon nanotube field-effect transistor with benchmarking against a nano-MOSFET.
    (Springer Science and Business Media LLC, 2012-08-19) Tan, Michael Loong Peng; Lentaris, Georgios; Amaratunga Aj, Gehan; Amaratunga, Gehan [0000-0002-8614-2864]
    The performance of a semiconducting carbon nanotube (CNT) is assessed and tabulated for parameters against those of a metal-oxide-semiconductor field-effect transistor (MOSFET). Both CNT and MOSFET models considered agree well with the trends in the available experimental data. The results obtained show that nanotubes can significantly reduce the drain-induced barrier lowering effect and subthreshold swing in silicon channel replacement while sustaining smaller channel area at higher current density. Performance metrics of both devices such as current drive strength, current on-off ratio (Ion/Ioff), energy-delay product, and power-delay product for logic gates, namely NAND and NOR, are presented. Design rules used for carbon nanotube field-effect transistors (CNTFETs) are compatible with the 45-nm MOSFET technology. The parasitics associated with interconnects are also incorporated in the model. Interconnects can affect the propagation delay in a CNTFET. Smaller length interconnects result in higher cutoff frequency.