Werner Puff

Defect structure of carbon rich a-SiC:H films and the influence of gas and heat treatments

A comprehensive study of carbon rich a-SiC:H films using optical absorption measurements, Fourier transform infrared spectroscopy, thermal desorption measurements, atomic force microscopy, and positron lifetime and Doppler-broadening techniques suggests that open volumes are formed in the films, due to incomplete breaking of the source molecule during film deposition. These open volumes are interconnected and can effectively trap gases from the ambient, during the film growth or after deposition. With increasing temperature the gases are desorbed from the internal surfaces of these open volumes and are released from the sample. This increases the areal density of the defects and is observable in positronium formation and annihilations of positrons with surface electrons. The growth of a nanocrystalline structure is observable upon annealing. At sufficiently high temperatures thermal breaking of Si–H and C–H bonds occurs and results in irreversible structural changes and film densification due to new C–C b...

EFFECT OF ANNEALING ON THE DEFECT STRUCTURE IN A-SIC:H FILMS

The annealing behavior of amorphous, hydrogenated silicon carbide films in the range 400–900 °C was studied by optical characterization methods, 15N hydrogen profiling, and defect profiling using a variable energy positron beam. The films were deposited in an electron cyclotron resonance chemical vapor deposition system using ditertiary butyl silane [SiH2(C4H9)2] as the monosource for silicon and carbon. As‐deposited films were found to contain large concentrations of hydrogen, both bonded and unbonded. Under rapid thermal annealing in a N2 atmosphere, the bonded hydrogen effuses giving rise to additional Si–C bond formation and to film densification. After annealing at high temperatures in N2, a marked decrease in the total hydrogen content is observed. After annealing in vacuum, however, the hydrogen effusion promotes void formation in the films.

A positron lifetime study of lanthanum and niobium doped Pb(Zr0.6Ti0.4)O3

A study of vacancy-related defects in lanthanum and niobium doped PbZr0.6Ti0.4O3 with dopant concentrations of 0–6 and 0–4mol%, respectively has been performed using positron annihilation spectroscopy X-ray diffraction, and photoelectron spectroscopy. Positron lifetime as well as coincidence annihilation radiation Doppler line broadening measurements were carried out. It was found that the samples exhibit vacancylike defects that act as positron traps. Two main defect lifetime components were found in both sample sets one at ≈150ps and one at ≈300ps. These defect trapping sites can be attributed to single oxygen vacancies and A-site vacancies, respectively. Doppler line broadening measurements, however, do not show significant changes as a function of dopant concentrations in terms of shape S and wing W parameters.

An investigation of point defects in silicon carbide

Positron lifetime measurement on bulk samples of single crystal SiC wafers have shown that both 6H and 4H polytypes exhibit a bulk lifetime of 150±2ps. All samples contained a second, defect-related lifetime component, ranging in value from 250 to about 300 ps with rather low intensities. The defect structure exhibited by the nanocrystalline samples is, not unexpectedly, much more complex.Positron beam experiments on Electron Cyclotron Resonance Chemical Vapour Deposited (ECR-CVD) SiC thin films showed that the positrons are very sensitive to changes in important film parameters as a function of the deposition conditions. It was found that the film density is lower than expected, probably due to hydrogen incorporation; variations in compositions among different films were detected through variations in theS parameters, and differences were observed in the electric field at the film-substrate interface due to hydrogen passivation of dangling bonds and different substrate resistivities.

Chemically sensitive free-volume study of amorphization of Cu60Zr40 induced by cold rolling and folding

With the aim to contribute to a microscopical understanding of the processes of solid-state amorphization, the chemically sensitive technique of background—reduced Doppler broadening of positron-electron annihilation radiation in combination with positron lifetime spectroscopy and microstructural characterization is applied to a free volume study of the amorphization of Cu60Zr40 induced by consecutive folding and rolling. Starting from the constituent pure metal foils, a nanosale multilayer structure of elemental layers and amorphous interlayers develops in an intermediate state of folding and rolling, where free volumes with a Zr-rich environment occur presumably located in the hetero-interfaces between the various layers or in grain boundaries of the Cu layers. After complete intermixing and amorphization, the local chemical environment of the free volumes reflects the average chemical alloy composition. In contrast to other processes of amorphization, free volumes of the size of few missing atoms occur...

On the contribution of vacancy complexes to the saturation of the carrier concentration in zinc doped InP

Positron annihilation spectroscopy on Zn‐doped InP has revealed the presence of a defect with a positron lifetime of ∼330 ps in samples in which the carrier concentration has saturated. This lifetime is attributed to a complex involving vacancies and Zn atoms. A model is proposed in which this complex has a (−/0) level near the bottom of the band gap, and undergoes a large inward lattice relaxation upon the transition to the neutral charge state, causing a reduction in the positron lifetime to ∼281 ps. This model explains the positron annihilation results on annealed samples and at low temperatures, and is supported by Hall effect measurements. The concentration of these complexes is less than 1017 cm−3. Therefore, these complexes cannot solely account for the observed discrepancy between the carrier concentration and the Zn concentration in very heavily Zn‐doped InP.

Charging-induced defect formation in LixCoO2 battery cathodes studied by positron annihilation spectroscopy

Charging-induced formation of vacancy-type defects in LixCoO2 battery cathodes was studied by the defect-specific techniques of positron lifetime spectroscopy and Doppler broadening of positron–electron annihilation radiation. The regime of reversible charging is dominated by vacancy-type defects on the Li+-sublattice the size of which increases with increasing Li+-extraction. Indication is found that Li+-reordering which occurs at the limit of reversible Li+-extraction (x = 0.55) causes a transition from two-dimensional agglomerates into one-dimensional vacancy chains. Degradation upon further Li+-extraction is accompanied by the formation of vacancy complexes on the Co- and anion sublattice.

Transmission of positrons with aβ+ energy distribution through thin films

The transmission through Al foils of isotropically implanted positrons from a22Naβ+ source has been measured. It is shown that the transmission is reasonably well-described using an exponential profile once backscattering is accounted for, except for thicknesses below approximately 7 mg/cm2. Below this thickness, the measured transmission is slightly less than that predicted by the exponential profile. Such a deviation has previously been observed for collimated positrons, suggesting that the implantation profile has no significant dependence on the spatial distribution of the incident positrons. This deviation is critical for the proper interpretation of positron lifetime experiments on thin films using a conventional positron lifetime spectrometer.

A positron annihilation study of defects in neutron transmutation‐doped float‐zone (Ar)‐Si

Annealing of defects introduced by neutron transmutation doping of float‐zone silicon has been investigated by positron lifetime spectroscopy and Doppler‐broadening measurements. It is shown that the main defects anneal out at about 150 and 500 °C. During annealing, the formation of bigger defect complexes can be seen.

Microstructure and vacancy-type defects of high-pressure torsion deformed Al-3 wt%Cu alloy

The influence of high-pressure torsion (HPT) on the microstructure and free-volume type defects of supersaturated Al-3 wt%Cu was studied. By means of HPT, grain refinement down to sizes of 100 nm could be achieved. Positron annihilation spectroscopy was applied for the characterization of deformation-induced defects and of vacancy-solute interaction. The chemical sensitive method of two-dimensional Doppler broadening spectroscopy reveals predominantly deformation-induced defects. In comparison to undeformed supersaturated Al-3 wt%Cu, the fraction of Cu-decorated free-volume type defects is strongly reduced and Al2Cu-precipitates with equilibrium structure occur rather than metastable semi-coherent Al-Cu precipitates. These precipitates stabilize the nanocrystalline structure and give rise to a rather different aging behaviour compared to the undeformed state.