Durability and photo-electric characteristics of a mille-feuille structured amorphous selenium (a-Se)-arsenic selenide (As2Se3) multi-layered thin film

A mille-feuille structured amorphous selenium (a-Se)-arsenic selenide (As2Se3) multi-layered thin film and a mixed amorphous Se-As2Se3 film is compared from a durability perspective and photo-electric perspective. The former is durable to incident laser induced degradation after numerous laser scans and does not crystallise till 105 of annealing, both of which are improved properties from the mixed evaporated film. In terms of photo-electric properties, the ratio between the photocurrent and the dark current improved whereas the increase of the dark current was higher than that of As2Se3 due to the unique current path developed within the mille-feuille structure. Implementing this structure into various amorphous semiconductors may open up a new possibility towards structure-sensitive amorphous photoconductors. © 2013 Elsevier B.V.

The influence of 1D, meso- and crystal structures on charge transport and recombination in solid-state dye-sensitized solar cells

We have prepared single crystalline SnO2 and ZnO nanowires and polycrystalline TiO2 nanotubes (1D networks) as well as nanoparticle-based films (3D networks) from the same materials to be used as photoanodes for solid-state dye-sensitized solar cells. In general, superior photovoltaic performance can be achieved from devices based on 3-dimensional networks, mostly due to their higher short circuit currents. To further characterize the fabricated devices, the electronic properties of the different networks were measured via the transient photocurrent and photovoltage decay techniques. Nanowire-based devices exhibit extremely high, light independent electron transport rates while recombination dynamics remain unchanged. This indicates, contrary to expectations, a decoupling of transport and recombination dynamics. For typical nanoparticle-based photoanodes, the devices are usually considered electron-limited due to the poor electron transport through nanocrystalline titania networks. In the case of the nanowire-based devices, the system becomes limited by the organic hole transporter used. In the case of polycrystalline TiO2 nanotube-based devices, we observe lower transport rates and higher recombination dynamics than their nanoparticle-based counterparts, suggesting that in order to improve the electron transport properties of solid-state dye-sensitized solar cells, single crystalline structures should be used. These findings should aid future design of photoanodes based on nanowires or porous semiconductors with extended crystallinity to be used in dye-sensitized solar cells. © 2013 The Royal Society of Chemistry.

Nanoscale-accuracy transfer printing of ultra-thin AlInGaN light-emitting diodes onto mechanically flexible substrates

The transfer printing of 2 μm-thick aluminum indium gallium nitride (AlInGaN) micron-size light-emitting diodes with 150 nm (±14 nm) minimum spacing is reported. The thin AlInGaN structures were assembled onto mechanically flexible polyethyleneterephthalate/polydimethylsiloxane substrates in a representative 16 × 16 array format using a modified dip-pen nano-patterning system. Devices in the array were positioned using a pre-calculated set of coordinates to demonstrate an automated transfer printing process. Individual printed array elements showed blue emission centered at 486 nm with a forward-directed optical output power up to 80 μW (355 mW/cm 2) when operated at a current density of 20 A/cm2. © 2013 AIP Publishing LLC.

Structural tunability and switchable exciton emission in inorganic-organic hybrids with mixed halides

Room-temperature tunable excitonic photoluminescence is demonstrated in alloy-tuned layered Inorganic-Organic (IO) hybrids, (C12H 25NH3)2PbI4(1-y)Br4y (y = 0 to 1). These perovskite IO hybrids adopt structures with alternating stacks of low-dimensional inorganic and organic layers, considered to be naturally self-assembled multiple quantum wells. These systems resemble stacked monolayer 2D semiconductors since no interlayer coupling exists. Thin films of IO hybrids exhibit sharp and strong photoluminescence (PL) at room-temperature due to stable excitons formed within the low-dimensional inorganic layers. Systematic variation in the observed exciton PL from 510 nm to 350 nm as the alloy composition is changed, is attributed to the structural readjustment of crystal packing upon increase of the Br content in the Pb-I inorganic network. The energy separation between exciton absorption and PL is attributed to the modified exciton density of states and diffusion of excitons from relatively higher energy states corresponding to bromine rich sites towards the lower energy iodine sites. Apart from compositional fluctuations, these excitons show remarkable reversible flips at temperature-induced phase transitions. All the results are successfully correlated with thermal and structural studies. Such structural engineering flexibility in these hybrids allows selective tuning of desirable exciton properties within suitable operating temperature ranges. Such wide-range PL tunability and reversible exciton switching in these novel IO hybrids paves the way to potential applications in new generation of optoelectronic devices. © 2013 AIP Publishing LLC.

Controlled amine functionality in self-assembled monolayers via the hidden amine route: chemical and electronic tunability.

A synthetic strategy for fabricating a dense amine functionalized self-assembled monolayer (SAM) on hydroxylated surfaces is presented. The assembly steps are monitored by X-ray photoelectron spectroscopy, Fourier transform infrared- attenuated total reflection, atomic force microscopy, variable angle spectroscopic ellipsometry, UV-vis surface spectroscopy, contact angle wettability, and contact potential difference measurements. The method applies alkylbromide-trichlorosilane for the fabrication of the SAM followed by surface transformation of the bromine moiety to amine by a two-step procedure: S(N)2 reaction that introduces the hidden amine, phthalimide, followed by the removal of the protecting group and exposing the free amine. The use of phthalimide moiety in the process enabled monitoring the substitution reaction rate on the surface (by absorption spectroscopy) and showed first-order kinetics. The simplicity of the process, nonharsh reagents, and short reaction time allow the use of such SAMs in molecular nanoelectronics applications, where complete control of the used SAM is needed. The different molecular dipole of each step of the process, which is verified by DFT calculations, supports the use of these SAMs as means to tune the electronic properties of semiconductors and for better synergism between SAMs and standard microelectronics processes and devices.

Oxide electronics for imaging and displays

Despite material weaknesses, considerable progress has been made in designing large area systems such as displays and imaging arrays. This talk will address the various large area technologies, and in particular, review amorphous oxide semiconductors and associated design approaches, along with driving schemes for displays, imaging and other applications. © 2013 IEEE.

On the variation of the 2DEG charge density with the density of the surface donor traps in AiGaN/GaN transistors

Gallium nitride (GaN) has a bright future in high voltage device owing to its remarkable physical properties and the possibility of growing heterostructures on silicon substrates. GaN High Electron Mobility Transistors (HEMTs) are expected to make a strong impact in off line applications and LED drives. However, unlike in silicon-based power devices, the on-state resistance of HEMT devices is hugely influenced by donor and acceptor traps at interfaces and in the bulk. This study focuses on the influence of donor traps located at the top interface between the semiconductor layer and the silicon nitride on the 2DEG density. It is shown through TCAD simulations and analytical study that the 2DEG charge density has an 'S' shape variation with two distinctive 'flat' regions, wherein it is not affected by the donor concentration, and one linear region. wherein the channel density increases proportionally with the donor concentration. We also show that the upper threshold value of the donor concentration within this 'S' shape increases significantly with the AIGaN thickness and the Al mole fraction and is highly affected by the presence of a thin GaN cap layer. © 2013 IEEE.

Optimising the dynamic performance of an all-wide-bandgap cascode switch

A SPICE simulation model of a novel cascode switch that combines a high voltage normally-on silicon carbide (SiC) junction field effect transistor (JFET) with a low voltage enhancement-mode gallium nitride field effect transistor (eGaN FET) has been developed, with the aim of optimising cascode switching performance. The effect of gate resistance on stability and switching losses is investigated and optimum values chosen. The effects of stray inductance on cascode switching performance are considered and the benefits of low inductance packaging discussed. The use of a positive JFET gate bias in a cascode switch is shown to reduce switching losses as well as reducing on-state losses. The findings of the simulation are used to produce a list of priorities for the design and layout of wide-bandgap cascode switches, relevant to both SiC and GaN high voltage devices. © 2013 IEEE.

Superradiance: exploiting quantum phase transition in the real world

Superradiance (SR), or cooperative spontaneous emission, has been predicted by R. Dicke before the invention of the laser. During the last few years one can see a renaissance of both experimental and theoretical studies of the superradiant phase transition in a variety of media, ranging from quantum dots and Bose condensates through to black holes. Until recently, despite of many years of research, SR has been considered as a phenomenon of pure scientific interest without obvious potential applications. However, recent investigations of the femtosecond SR emission generation from semiconductors have opened up some practical opportunities for the exploitation of this quantum optics phenomenon. Here we present a brief review of some features, advantages and potential applications of the SR generation from semiconductor laser structures

Atom probe tomography for advanced metallization

In microelectronics, the increase in complexity and the reduction of devices dimensions make essential the development of new characterization tools and methodologies. Indeed advanced characterization methods with very high spatial resolution are needed to analyze the redistribution at the nanoscale in devices and interconnections. The atom probe tomography has become an essential analysis to study materials at the nanometer scale. This instrument is the only analytical microscope capable to produce 3D maps of the distribution of the chemical species with an atomic resolution inside a material. This technique has benefit from several instrumental improvements during last years. In particular, the use of laser for the analysis of semiconductors and insulating materials offers new perspectives for characterization. The capability of APT to map out elements at the atomic scale with high sensitivity in devices meets the characterization requirements of semiconductor devices such as the determination of elemental distributions for each device region. In this paper, several examples will show how APT can be used to characterize and understand materials and process for advanced metallization. The possibilities and performances of APT (chemical analysis of all the elements, atomic resolution, planes determination, crystallographic information...) will be described as well as some of its limitations (sample preparation, complex evaporation, detection limit, ...). The examples illustrate different aspect of metallization: dopant profiling and clustering, metallic impurities segregation on dislocation, silicide formation and alloying, high K/metal gate optimization, SiGe quantum dots, as well as analysis of transistors and nanowires. © 2013 Elsevier B.V. All rights reserved.

Dopant compensation in HfO2 and other high K oxides

The theory of doping limits in semiconductors and insulators is applied to the case of wide gap oxides, crystalline, or amorphous, and used to explain that impurities do not in general give rise to gap states or a doping response. Instead, the system tends to form defect complexes or undergo symmetry-lowering reconstructions to expel gap states out of the band gap. The model is applied to impurities, such as trivalent metals, carbon, N, P, and B, in HfO2, the main gate dielectric used in field effect transistors. © 2014 AIP Publishing LLC.

In situ intercalation dynamics in inorganic-organic layered perovskite thin films.

The properties of layered inorganic semiconductors can be manipulated by the insertion of foreign molecular species via a process known as intercalation. In the present study, we investigate the phenomenon of organic moiety (R-NH3I) intercalation in layered metal-halide (PbI2)-based inorganic semiconductors, leading to the formation of inorganic-organic (IO) perovskites [(R-NH3)2PbI4]. During this intercalation strong resonant exciton optical transitions are created, enabling study of the dynamics of this process. Simultaneous in situ photoluminescence (PL) and transmission measurements are used to track the structural and exciton evolution. On the basis of the experimental observations, a model is proposed which explains the process of IO perovskite formation during intercalation of the organic moiety through the inorganic semiconductor layers. The interplay between precursor film thickness and organic solution concentration/solvent highlights the role of van der Waals interactions between the layers, as well as the need for maintaining stoichiometry during intercalation. Nucleation and growth occurring during intercalation matches a Johnson-Mehl-Avrami-Kolmogorov model, with results fitting both ideal and nonideal cases.