Tan SL, Soong WM, Green JE, Steer MJ, Zhang S, Tan LJJ, Ng JS, Marko IP, Sweeney SJ, Adams AR, Allam J, David JPR (2013) Experimental evaluation of impact ionization in dilute nitride GaInNAs diodes, Applied Physics Letters 103 (10)
The anomalous behavior of impact ionization in dilute-nitride GaInNAs photodiodes with a range of nitrogen content below 4% is investigated. The ratio of hole- and electron-initiated ionization coefficients, k = ²/±, is enhanced by a factor up to
Gopee V, Thomas O, Hunt C, Stolojan V, Allam J, Silva SRP (2016) Carbon Nanotube Interconnects Realized through Functionalization and Sintered Silver Attachment, ACS Applied Materials and Interfaces 8 (8) pp. 5563-5570 American Chemical Society
Carbon nanotubes (CNTs) in the form of interconnects have many potential applications, and their ability to perform at high temperatures gives them a unique capability. We show the development of a novel transfer process using CNTs and sintered silver that offers a unique high-temperature, high-conductivity, and potentially flexible interconnect solution. Arrays of vertically aligned multiwalled carbon nanotubes of approximately 200 ¼m in length were grown on silicon substrates, using low-temperature photothermal chemical vapor deposition. Oxygen plasma treatment was used to introduce defects, in the form of hydroxyl, carbonyl, and carboxyl groups, on the walls of the carbon nanotubes so that they could bond to palladium (Pd). Nanoparticle silver was then used to bind the Pd-coated multiwalled CNTs to a copper substrate. The silver?CNT?silver interconnects were found to be ohmic conductors, with resistivity of 6.2 × 10?4 ©m; the interconnects were heated to temperatures exceeding 300 °C (where common solders fail) and were found to maintain their electrical performance.
Allam J, Sutton R, Sjjad M, Litvinenko K, Siddique S, Wang Z, Yang Q-H, Brwon T, Loh W (2011) Reaction, diffusion and dissociation of excitons on carbon nanotubes, OSA Technical Digest (CD) The Optical Society
We study exciton reactions on carbon nanotubes in the regime of many, few and one exciton per nanotube, and demonstrate classic 1-D reaction-diffusion behaviour. Dissociation occurs when exciton spacing is less than the exciton length.
Gopee V, Thomas O, Hunt C, Stolojan V, Allam J, Silva SRP Carbon Nanotube Interconnects Realized through Functionalization and Sintered Silver Attachment, AMER CHEMICAL SOC
Allam J, Sajjad MT, Sutton R, Litvinenko K, Wang Z, Siddique S, Yang Q-H, Loh WH, Brown T (2013) Measurement of a reaction-diffusion crossover in exciton-exciton
recombination inside carbon nanotubes using femtosecond optical absorption, Physical Review Letters American Physical Society
Exciton-exciton recombination in isolated semiconducting single-walled carbon
nanotubes was studied using femtosecond transient absorption. Under sufficient
excitation to saturate the optical absorption, we observed an abrupt transition
between reaction- and diffusion- limited kinetics, arising from reactions
between incoherent localized excitons with a finite probability of ~ 0.2 per
encounter. This represents the first experimental observation of a crossover
between classical and critical kinetics in a 1D coalescing random walk, which
is a paradigm for the study of non- equilibrium systems.
Roither J, Pichler S, Kovalenko MV, Heiss W, Feychuk P, Panchuk O, Allam J, Murdin BN (2006) Two- and one-dimensional light propagations and gain in layer-by-layer-deposited colloidal nanocrystal waveguides, APPLIED PHYSICS LETTERS 89 (11) ARTN 111120 AMER INST PHYSICS
Tan SL, Zhang S, Soong WM, Goh YL, Tan LJJ, Ng JS, David JPR, Marko IP, Adams AR, Sweeney SJ, Allam J (2011) GaInNAsSb/GaAs Photodiodes for Long-Wavelength Applications, IEEE ELECTRON DEVICE LETTERS 32 (7) pp. 919-921 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
Litvinenko KL, Murdin BN, Allam J, Pidgeon CR, Zhang T, Harris JJ, Cohen LF, Eustace DA, McComb DW (2006) Spin lifetime in InAs epitaxial layers grown on GaAs, PHYSICAL REVIEW B 74 (7) ARTN 075331 AMERICAN PHYSICAL SOC
Tan LJJ, Soong WS, Tan SL, Goh YL, Steer MJ, Ng JS, David JPR, Marko IP, Chamings J, Allam J, Sweeney SJ, Adams AR (2009) Dark current mechanisms in InxGa1-xAs 1-yNy, IEEE Proceedings of LEOS Annual Meeting Conference pp. 233-234 IEEE
In order to extend the photo response of GaAs to optical telecommunication wavelengths, In and N can be incorporated into GaAs to yield a perfect lattice match of InxGa1-xAs1-yNy with GaAs with a bandgap that strongly decreases with increasing N composition. The potential usage of such a material as photodetectors and photovoltaic applications has been reported.In this work, we investigate the dark current mechanisms in the InxGa1-xAs1-yNy material.
Tan SL, Tan LJJ, Goh YL, Zhang S, Ng JS, David JPR, Marko IP, Allam J, Sweeney SJ, Adams AR (2010) Reduction of dark current and unintentional background doping in InGaAsN photodetectors by ex situ annealing, Proceedings of SPIE - Posters Session 7726 SPIE
InGaAsN is a promising material system to enable low-cost GaAs-based detectors to operate in the telecommunication spectrum, despite the problems posed by the low growth temperature required for nitrogen incorporation. We demonstrate that InGaAsN p+-i-n+ structures with nominal In and N fraction of 10% and 3.8%, grown by molecular beam epitaxy (MBE) under non-optimal growth conditions, can be optimized by post growth thermal annealing to match the performance of optimally grown structures. We report the findings of an annealing study by comparing the photoluminescence spectra, dark current and background concentration of the as-grown and annealed samples. The dark current of the optimally annealed sample is approximately 2 ¼A/cm2 at an electric field of 100 kV/cm, and is the lowest reported to date for InGaAsN photodetectors with a cut-off wavelength of 1.3 ¼m. Evidence of lower unintentional background concentration after annealing at a sufficiently high temperature, is also presented.
Litvinenko KL, Nikzad L, Allam J, Murdin BN, Pidgeon CR, Harris JJ, Cohen LF (2007) Spin dynamics in narrow-gap semiconductor epitaxial layers, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM 20 (6) pp. 461-465 SPRINGER
O'Rourke C, Allam J, Boehringer K, Klaedtke A, Hamm J, Hess O (2005) Coherent interactions and long term evolution of ultrafast transients in a semiconductor laser, 2005 IEEE LEOS Annual Meeting Conference Proceedings (LEOS) pp. 823-824 IEEE
Ng JS, Soong WM, Steer MJ, Hopkinson M, David JPR, Chamings J, Adams AR, Sweeney SJ, Allam J (2007) GaInNAs lattice-matched to GaAs for photodiodes, Conference Proceedings - International Conference on Indium Phosphide and Related Materials pp. 347-349
We present optical and electrical characterization data obtained from bulk GaInNAs (lattice-matched to GaAs) diodes with varying GaInNAs composition. Good lattice-matching to GaAs, low reverse dark current and long wavelength absorption were achieved simultaneously, without the aid of post-growth annealing and use of antimony during the growth. © 2007 IEEE.
Litvinenko KL, Nikzad L, Pidgeon CR, Allam J, Cohen LF, Ashley T, Emeny M, Zawadzki W, Murdin BN (2008) Temperature dependence of the electron Lande g factor in InSb and GaAs, PHYSICAL REVIEW B 77 (3) ARTN 033204 AMER PHYSICAL SOC
Litvinenko KL, Murdin BN, Allam J, Pidgeon CR, Bird M, Morris K, Branford W, Clowes SK, Cohen LF, Ashley T, Buckle L (2006) Spin relaxation in n-InSb/AlInSb quantum wells, NEW JOURNAL OF PHYSICS 8 PII S1367-2630(06)15617-7 IOP PUBLISHING LTD
Bhattacharyya S, Henley SJ, Mendoza E, Gomez-Rojas L, Allam J, Silva SRP (2006) Resonant tunnelling and fast switching in amorphous-carbon quantum-well structures, NATURE MATERIALS 5 (1) pp. 19-22 NATURE PUBLISHING GROUP
Buchacher T (2017) Polarisation dynamics in ferroelectric materials.,
Ferroelectric materials have established themselves as indispensable in key applications such as piezoelectric transducers and energy storage devices. While the use of ferroelectrics in these fields dates back more than 50 years, little progress has been made to extend applications of ferroelectrics into new fields. To a large extend the observed slow progress is not caused by a lack of potential applications, but to by the inherent complexity associated with a structural phase transition, combined with strong coupling of polarisation, strain and temperature, and the strong modification of the phenomena by material defects.
This thesis takes a look at prospective applications in energy storage for pulse power applications, solid state cooling and non-volatile random access memory and identifies key issues that need to be resolved. The thesis delivers time-domain based approaches to determine ferroelectric switching behaviour of bulk materials and thin films down to sub-ns time scales. The approach permitted study of how information written to a ferroelectric memory decays as a result of multiple non-destructive read operations. Furthermore simultaneous direct measurements of temperature and ferroelectric switching established a direct link between the retarded switching phenomenon observed in ferroelectrics and temperature changes brought by the electrocaloric effect. By comparison with analytical models and numerical simulation a large localised temperature change on the scale of individual domains is postulated. It implies a much larger coupling between switching and local temperature than has been previously considered. In extension of the model the frequency dependence of polarisation fatigue under bipolar conditions is explained by the occurrence of large temperature gradients in the material.
Anguita JV, Cox DC, Ahmad M, Tan Y, Allam J, Silva SRP (2013) Highly transmissive carbon nanotube forests grown at low substrate temperature, Advanced Functional Materials 23 (44) pp. 5502-5509 Wiley
Despite the "darker than black" association attributed to carbon nanotube forests, here is shown that it is also possible to grow these structures, over heat-sensitive substrates, featuring highly transmissive characteristics from the UV to infrared wavelengths, for forest heights as high as 20 ¼m. The optical transmission is interpreted in terms of light propagation along channels that are self-generated by localized bundling of tubes, acting as waveguides. A good correlation is shown between the distribution of diameter sizes of these sub-wavelength voids and the transmission spectrum of the forests. For the shorter visible and near-UV wavelengths, this model shows that light propagates by channeling along individual vertical voids in the forests, which elucidates the origin for the widely-reported near-zero reflectance values observed in forests. For the longer infrared wavelengths, the mode spreads over many nanotubes and voids, and propagates along a "homogeneous effective medium". The strong absorption of the forest at the shorter wavelengths is correlated in terms of the stronger attenuation inside a waveguide cavity, according to the » attenuation dependency of standard waveguide theory. The realization of this material can lead to novel avenues in new optoelectronic device design, where the carbon nanotube forests can be used as highly conducting "scaffolds" for optically active materials, whilst also allowing light to penetrate to significant depths into the structure, in excess of 20 ¼m, enabling optical functionality. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Tan S, Hunter C, Zhang S, Tan L, Goh Y, Ng J, David J, Marko I, Sweeney S, Adams AR, Allam J (2012) Improved optoelectronic properties of rapid thermally annealed dilute nitride GaInNAs photodetectors, Journal of Electronic Materials 41 (12) pp. 3393-3401 IEEE
We investigate the optical and electrical characteristics of GaInNAs/GaAs long-wavelength photodiodes grown under varying conditions by molecular beam epitaxy and subjected to postgrowth rapid thermal annealing (RTA) at a series of temperatures. It is found that the device performance of the nonoptimally grown GaInNAs p-i-n structures, with nominal compositions of 10% In and 3.8% N, can be improved significantly by the RTA treatment to match that of optimally grown structures. The optimally annealed devices exhibit overall improvement in optical and electrical characteristics, including increased photoluminescence brightness, reduced density of deep-level traps, reduced series resistance resulting from the GaAs/GaInNAs heterointerface, lower dark current, and significantly lower background doping density, all of which can be attributed to the reduced structural disorder in the GaInNAs alloy.© 2012 TMS.
Buchacher T, Rokosz M, Dorey RA, Allam J, Gregory A (2017) Electrocaloric induced retarded ferroelectric switching, Applied Physics Letters 110 (2) AIP Publishing
Ferroelectric switching in bulk materials, at modest electric fields, is a relatively fast process, occurring on time scales of microseconds and less. A secondary retarded switching phenomenon also occurs on time scales of seconds and has previously been attributed to defect induced elevated energy barriers between polarisation states. As ferroelectric switching is a thermally activated process the barrier heights are also affected by temperature which is not constant in ferroelectric materials due to the electrocaloric effect. Here an additional EC induced retardation mechanism is proposed whereby EC induced temperature changes repeatedly temporarily prevent further FE switching during cooling cycles.
Anguita JV, Ahmad M, Haq S, Allam J, Silva SRP (2016) Ultra-broadband light trapping using nanotextured decoupled graphene multilayers, Science Advances 2 (2) e1501238 American Association for the Advancement of Science
The ability to engineer a thin two-dimensional surface for light trapping across an ultra-broad spectral range is central for an increasing number of applications including energy, optoelectronics, and spectroscopy. Although broadband light trapping has been obtained in tall structures of carbon nanotubes with millimeter-tall dimensions, obtaining such broadband light?trapping behavior from nanometer-scale absorbers remains elusive. We report a method for trapping the optical field coincident with few-layer decoupled graphene using field localization within a disordered distribution of subwavelength-sized nanotexturing metal particles. We show that the combination of the broadband light?coupling effect from the disordered nanotexture combined with the natural thinness and remarkably high and wavelength-independent absorption of graphene results in an ultrathin (15 nm thin) yet ultra-broadband blackbody absorber, featuring 99% absorption spanning from the mid-infrared to the ultraviolet. We demonstrate the utility of our approach to produce the blackbody absorber on delicate opto-microelectromechanical infrared emitters, using a low-temperature, noncontact fabrication method, which is also large-area compatible. This development may pave a way to new fabrication methodologies for optical devices requiring light management at the nanoscale.