Electrical detection of trapped charge displacements using an ultra-thin SOI percolation transistor

Electrical detection of solid-state charge qubits requires ultrasensitive charge measurement, typically using a quantum point contact or single-electron-transistor, which imposes strict limits on operating temperature, voltage and current. A conventional FET offers relaxed operating conditions, but the back-action of the channel charge is a problem for such small quantum systems. Here, we discuss the use of a percolation transistor as a measurement device, with regard to charge sensing and backaction. The transistor is based on a 10nm thick SOI channel layer and is designed to measure the displacement of trapped charges in a nearby dielectric. At cryogenic temperatures, the trapped charges result in strong disorder in the channel layer, so that current is constrained to a percolation pathway in sub-threshold conditions. A microwave driven spatial Rabi oscillation of the trapped charge causes a change in the percolation pathway, which results in a measurable change in channel current. © The Electrochemical Society.

Single sublattice endotaxial phase separation driven by charge frustration in a complex oxide.

Complex transition-metal oxides are important functional materials in areas such as energy and information storage. The cubic ABO3 perovskite is an archetypal example of this class, formed by the occupation of small octahedral B-sites within an AO3 network defined by larger A cations. We show that introduction of chemically mismatched octahedral cations into a cubic perovskite oxide parent phase modifies structure and composition beyond the unit cell length scale on the B sublattice alone. This affords an endotaxial nanocomposite of two cubic perovskite phases with distinct properties. These locally B-site cation-ordered and -disordered phases share a single AO3 network and have enhanced stability against the formation of a competing hexagonal structure over the single-phase parent. Synergic integration of the distinct properties of these phases by the coherent interfaces of the composite produces solid oxide fuel cell cathode performance superior to that expected from the component phases in isolation.

Multi-frequency operation of a mems vibration energy harvester by accessing five orders of parametric resonance

The mechanical amplification effect of parametric resonance has the potential to outperform direct resonance by over an order of magnitude in terms of power output. However, the excitation must first overcome the damping-dependent initiation threshold amplitude prior to accessing this more profitable region. In addition to activating the principal (1st order) parametric resonance at twice the natural frequency ω0, higher orders of parametric resonance may be accessed when the excitation frequency is in the vicinity of 2ω0/n for integer n. Together with the passive design approaches previously developed to reduce the initiation threshold to access the principal parametric resonance, vacuum packaging (< 10 torr) is employed to further reduce the threshold and unveil the higher orders. A vacuum packaged MEMS electrostatic harvester (0.278 mm3) exhibited 4 and 5 parametric resonance peaks at room pressure and vacuum respectively when scanned up to 10 g. At 5.1 ms-2, a peak power output of 20.8 nW and 166 nW is recorded for direct and principal parametric resonance respectively at atmospheric pressure; while a peak power output of 60.9 nW and 324 nW is observed for the respective resonant peaks in vacuum. Additionally, unlike direct resonance, the operational frequency bandwidth of parametric resonance broadens with lower damping. © Published under licence by IOP Publishing Ltd.

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.

Dependence of dye regeneration and charge collection on the pore-filling fraction in solid-state dye-sensitized solar cells

Solid-state dye-sensitized solar cells rely on effective infiltration of a solid-state hole-transporting material into the pores of a nanoporous TiO 2 network to allow for dye regeneration and hole extraction. Using microsecond transient absorption spectroscopy and femtosecond photoluminescence upconversion spectroscopy, the hole-transfer yield from the dye to the hole-transporting material 2,2′,7,7′-tetrakis(N,N-di-p- methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) is shown to rise rapidly with higher pore-filling fractions as the dye-coated pore surface is increasingly covered with hole-transporting material. Once a pore-filling fraction of ≈30% is reached, further increases do not significantly change the hole-transfer yield. Using simple models of infiltration of spiro-OMeTAD into the TiO2 porous network, it is shown that this pore-filling fraction is less than the amount required to cover the dye surface with at least a single layer of hole-transporting material, suggesting that charge diffusion through the dye monolayer network precedes transfer to the hole-transporting material. Comparison of these results with device parameters shows that improvements of the power-conversion efficiency beyond ≈30% pore filling are not caused by a higher hole-transfer yield, but by a higher charge-collection efficiency, which is found to occur in steps. The observed sharp onsets in photocurrent and power-conversion efficiencies with increasing pore-filling fraction correlate well with percolation theory, predicting the points of cohesive pathway formation in successive spiro-OMeTAD layers adhered to the pore walls. From percolation theory it is predicted that, for standard mesoporous TiO2 with 20 nm pore size, the photocurrent should show no further improvement beyond an ≈83% pore-filling fraction. Solid-state dye-sensitized solar cells capable of complete hole transfer with pore-filling fractions as low as ∼30% are demonstrated. Improvements of device efficiencies beyond ∼30% are explained by a stepwise increase in charge-collection efficiency in agreement with percolation theory. Furthermore, it is predicted that, for a 20 nm pore size, the photocurrent reaches a maximum at ∼83% pore-filling fraction. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.