D. N. Buckley

Anodic formation and characterization of nanoporous InP in aqueous KOH electrolytes

The anodic behavior of highly doped (>10 18 cm -3 ) n-InP in aqueous KOH was investigated. Electrodes anodized in the absence of light in 2-5 mol dm -3 KOH at a constant potential of 0.5-0.75 V (SCE), or subjected to linear potential sweeps to potentials in this range, were shown to exhibit the formation of a nanoporous subsurface region. Both linear sweep voltammograms and current-time curves at constant potential showed a characteristic anodic peak, corresponding to formation of the nanoporous region. No porous region was formed during anodization in 1 mol dm -3 KOH. The nanoporous region was examined using transmission electron microscopy and found to have a thickness of some 1-3 μm depending on the anodization conditions and to be located beneath a thin (typically ∼40 nm), dense, near-surface layer. The pores varied in width from 25 to 75 nm and both the pore width and porous region thickness were found to decrease with increasing KOH concentration. The porosity was approximately 35%. The porous layer structure is shown to form by the localized penetration of surface pits into the InP, and the dense, near-surface layer is consistent with the effect of electron depletion at the surface of the semiconductor.

An Investigation by AFM and TEM of the Mechanism of Anodic Formation of Nanoporosity in n-InP in KOH

The early stages of nanoporous layer formation, under anodic conditions in the absence of light, were investigated for n-type InP with a carrier concentration of ∼3 X 10 18 cm -3 in 5 mol dm -3 KOH and a mechanism for the process is proposed. At potentials less than ∼0.35 V, spectroscopic ellipsometry and transmission electron microscopy (TEM) showed a thin oxide film on the surface. Atomic force microscopy (AFM) of electrode surfaces showed no pitting below ∼0.35 V but clearly showed etch pit formation in the range 0.4-0.53 V. The density of surface pits increased with time in both linear potential sweep and constant potential reaching a constant value at a time corresponding approximately to the current peak in linear sweep voltammograms and current-time curves at constant potential. TEM clearly showed individual nanoporous domains separated from the surface by a dense ∼40 nm InP layer. It is concluded that each domain develops as a result of directionally preferential pore propagation from an individual surface pit which forms a channel through this near-surface layer. As they grow larger, domains meet, and the merging of multiple domains eventually leads to a continuous nanoporous sub-surface region.

Investigation of stress and morphology in electrodeposited copper nanofilms by cantilever beam method and in situ electrochemical atomic force microscopy

Both stress and atomic force microscopy (AFM) measurements were carried out in situ during potentiostatic electrodeposition of copper on gold in 0.05moldm−3 CuSO4 in 0.1moldm−3 H2SO4 with and without additives. With no additives, compressive stress generally developed initially and films subsequently underwent a compressive-to-tensile (C-T) transition. With increasing negative potential, the time for the C-T transition decreased rapidly as the rate of coalescence of nuclei (measured by AFM) increased rapidly. This is consistent with models that attribute the C-T transition to increasing tensile stress due to coalescence of nuclei. Furthermore, at a potential of −75mV (Cu∕Cu2+), where AFM showed very little coalescence of nuclei, no C-T transition was observed, again consistent with these models. The nucleation density measured by AFM increased from 2.7×107cm−2 at −75mVto2.5×109cm−2 at −300mV. Stress measurements with a combination of three additives [1×10−3moldm−3 Cl−, 8.82×10−5moldm−3 polyethylene glycol...

Vacancy-Induced Plastic Deformation in Electrodeposited Copper Films

A tensile stress developed in polycrystalline copper films during room-temperature aging was computed using a diffusion equation for excess vacancies migrating to the grain boundaries. This theory is based on an assumption that a free volume created by the arrival of excess vacancies at the grain boundaries of thin copper films is instantly eliminated and this action introduces a biaxial tensile stress in the plane of the film. The tensile stress was calculated as a function of aging time, grain size, and excess vacancy concentration and it was found that it could exceed the yield stress of copper. This result suggests that plastic deformation could occur in electrodeposited copper films during room-temperature aging, consistent with experimental observations.

An Isothermal Annealing Study of Spontaneous Morphology Change in Electrodeposited Copper Metallization

A systematic investigation of the effect of annealing time and temperature on the incubation period for spontaneous morphology change (SMC) in electrodeposited copper metallization is reported. Based on an additivity principle derived by Mittemeijer for transformations with an Arrhenius-type dependence of kinetics on temperature, it is shown that the remaining incubation time for SMC at room temperature following annealing at a given temperature should be linearly related to the annealing time. By repeatedly scanning atomic-force microscopy images at room temperature, the time at which SMC occurred was determined for films annealed for various times at temperatures ranging from 32 to 60°C. At each temperature studied, the remaining incubation time at room temperature was found to decrease approximately linearly with increasing annealing time, thus experimentally verifying the behavior predicted by the additivity principle. An Arrhenius plot of the measured rates of decrease showed good linearity and yielded a value of 0.48 eV for the activation energy. This is consistent with a vacancy diffusion mechanism for the process occurring during the incubation period, and supports our proposed mechanism for SMC.

Wind energy storage technologies

Energy storage is widely recognised as a key enabling technology for renewable energy and particularly for wind and photovoltaics. Distributed generation could also help, but the location of wind resources and consumption are almost mutually exclusive. The main thrust of the US DoE Energy Storage Programme (

Grain-Size Effect on a Plasma-Based Copper Etch Process

615 M) is in the direction of batteries and CAES. Advanced battery storage (electric vehicles (EV), fl ow batteries, second-life EV batteries) has the potential to reduce the need for grid infrastructure as it is not topographically, geologically and environmentally limited. This chapter includes a brief examination into the energy storage techniques currently available to assist large-scale wind penetrations. These are pumped-hydroelectric energy storage (PHES), underground pumped-hydroelectric energy storage (UPHES), compressed air energy storage (CAES), battery energy storage (BES), fl ow battery energy storage (FBES), fl ywheel energy storage (FES), supercapacitor energy storage (SCES), superconducting magnetic energy storage (SMES), hydrogen energy storage system (HESS), thermal energy storage system (TESS) and fi nally, EVs. The objective was to identify the following for each: how it works; advantages; applications; cost; disadvantages and future potential. A brief comparison was then completed to indicate the broad range of operating characteristics available for energy storage technologies. It was concluded that PHES/UPHES, FBES, HESS, TESS and EVs are the most promising techniques to undergo further research. The remaining technologies will be used for their current applications in the future, but further development is unlikely.

Origins of Cracking in Highly Porous Anodically Grown Films on InP

The effect of the copper films microstructure on the Cl 2 plasma-based copper etch process has been studied. Copper films were sputter deposited and annealed at different temperatures. The films grain size increased and the resistivity decreased with the annealing temperature. Under the same plasma exposure condition, the copper conversion rate and the CuCl x reaction product formation rate increased monotonically with the grain size. At the same time, the Cl content and the porosity of CuClx increased with the grain size. The roughness of the etched Cu surface also increased with the original copper grain size. These observations were explained by diffusion mechanisms of Cl and Cu atoms in the plasma-copper reaction process as well as microstructures of CuCl x and the original copper film.

Stress Evolution in Electrodeposited Copper Metallization during Room-Temperature Aging

The nature of the surface films grown during the anodization of InP in an aqueous (NH 4 ) 2 S electrolyte has been investigated. The previously reported cracking of these films is explicitly demonstrated to occur ex situ and not during the electrochemical treatment. The films have been identified as In 2 S 3 and are shown to have a columnar morphology. The measured film thickness varies linearly with the charge density passed, and comparison between experimental measurements and theoretical estimates for the thickness indicates a porosity of 70-80% for the In 2 S 3 film. Film cracking is attributed to shrinkage during drying of the highly porous film and does not necessarily imply stress in the wet as-grown film.

Deconvolution of the Potential and Time Dependence of Electrochemical Porous Semiconductor Formation

Current silicon technology applies copper films as an interconnect material, which is deposited by an electrodeposition method at room temperature. Immediately after deposition, these copper films are often structurally unstable and can undergo a room-temperature aging process. The microstructure of copper films grown by various thin film deposition techniques, including vapor deposition, sputtering, and electrodeposition, is known to change during room temperature storage. For example, both recrystallization and grain growth have been reported.