M. Härting

Ab initio pseudopotential study of vacancies and self-interstitials in hcp titanium

By means of an ab initio plane-wave pseudopotential method, monovacancy, divacancy and self-interstitials in hcp titanium are investigated. The calculated monovacancy formation energy is 1.97 eV, which is in excellent agreement with other theoretical calculations, and agrees qualitatively with published experimental results. The relaxation of the atoms around a single vacancy is observed to be small. Two divacancy configurations, the in-plane and the off-plane, have also been shown to be equally stable. With regards to the interstitials, of the eight configurations studied, two (octahedral and basal octahedral) have relatively lower formation energies and are, thus, the most likely stable configurations. We find small energy differences between them, suggesting their possible co-existence. It is also observed that the tetrahedral configuration decays to a split dumbbell configuration, whereas both the basal tetrahedral and the basal pseudocrowdion interstitials decay to the basal octahedral configuration. Using the nudged elastic band method (NEB), we determine a possible minimum energy path (MEP) for the diffusion of self-interstitial titanium atoms from an octahedral site to the nearest octahedral site. The energy barrier for this migration mechanism is shown to be about 0.20 eV.

The Influence of Strain on Point Defect Dynamics

Stress migration of point and open-volume defects is an important problem in a wide variety of applications, ranging from semiconductor technology and basic metallurgical problems to ion implantation and surface modification. Often the driving force of the drift is the local stress field of other defects in the material, although other sources of residual stress as well as external loads play an important role. This paper concentrates on the behaviour of non-equilibrium vacancies in a non-uniformly strained material. Illustrative models of one-dimensional problems are developed and compared to available experimental results in different ion implanted materials.

Nanoparticle composites for printed electronics.

Printed Electronics is a rapidly developing sector in the electronics industry, in which nanostructured materials are playing an increasingly important role. In particular, inks containing dispersions of semiconducting nanoparticles, can form nanocomposite materials with unique electronic properties when cured. In this study we have extended on our previous studies of functional nanoparticle electronic inks, with the development of a solvent-based silicon ink for printed electronics which is compatible with existing silver inks, and with the investigation of other metal nanoparticle based inks. It is shown that both solvent-based and water-based inks can be used for both silver conductors and semiconducting silicon, and that qualitatively there is no difference in the electronic properties of the materials printed with a soluble polymer binder to when an acrylic binder is used.

Influence of growth temperature on the microcrystallinity and native defect structure of hydrogenated amorphous silicon

The microstructure of hydrogenated amorphous silicon grown by hot-wire chemical vapour deposition (HW-CVD) on glass substrates, at different substrate temperatures ranging from 300 to 500 °C, has been studied using X-ray diffraction and positron annihilation techniques. In previous studies it has been shown that recrystallization is accompanied by a relaxation of the defect structure with an increase in the free volume at the positron annihilation site. The object of this work is to relate the initial defect configuration to the degree of order in the structure, which has been characterized through its radial density function giving accurate estimates of the nearest-neighbour separation and bond angles.

Stress in hydrogenated amorphous silicon determined by X-ray diffraction

Abstract The residual strain in an a-Si:H layer has been directly determined with X-ray diffraction techniques from variations in the diffraction angle of the first amorphous peak using CuKα radiation. The layer was deposited by HW-CVD on glass substrates at a growth temperature of 300 °C, and is known from previous studies to be highly disordered. It was found to have an average compressive stress of 750 MPa, using the c-Si lattice parameter as a reference, and typical values of the elastic constants for a-Si:H, increasing strongly towards the surface.

Interfacial and Network Characteristics of Silicon Nanoparticle Layers Used in Printed Electronics

In printed electronics the use of semiconducting silicon nanoparticles allows more than the simple printing of conductive materials. It gives the possibility of fabricating robust and inexpensive, active and reactive components like temperature sensors which are shown as an example. In our approach high quality silicon nanoparticles with stable, essentially oxide-free surfaces are used to replace the pigment in water-based graphic inks, which on curing have unique semiconducting properties, arising from the transport of charge through a percolation network of crystalline silicon nanoparticles. In this study scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) were employed to investigate the mesoscale structure of the particle network and, more importantly the structure of the interface between particles. An intimate contact between lattice planes of different particles was observed, without the presence of an intervening oxide layer.

Characterisation of RF-sputtered platinum films from industrial production plants using slow positrons

Platinum films, used in thin film technology, produced by radio-frequency sputter deposition on aluminium oxide substrates under different conditions, have been studied by positron beam and other techniques, before and after production annealing. The defect structure in the layers has been characterised using both positron lifetime and Doppler-broadening spectroscopy, and compared with X-ray studies of crystallinity and texture.

Probing the structure of iron at extreme conditions by X-ray absorption near-edge structure calculations

We present the K-edge X-ray absorption near edge spectra of hexagonal-closed packed iron at pressure and temperature conditions relevant to Earths mantle conditions. The calculated spectra have been obtained using the first-principles scheme based on the continued-fraction approach and norm-conserving pseudopotentials. The atomic configurations used for the X-ray absorption near edge spectroscopy calculations were obtained from classical molecular dynamics simulations, using an optimized embedded-atom potential. We compare our calculated spectra to recently available experiment results (R. Boehler, H.G. Musshoff, R. Ditz, G. Aquilanti, and A. Trapananti, Rev. Sci. Instrum. 80 (2009), pp. 045103–045108) and identify the main features of the spectra that may indicate onset of melting in iron.

A novel mode of current switching dependent on activated charge transport

We demonstrate a fully printed transistor with a planar triode geometry, using nanoparticulate silicon as the semiconductor material, which has a unique mode of operation as an electrically controlled two-way (double throw) switch. A signal applied to the base changes the direction of the current from between the collector and base to between the base and emitter. We further show that the switching characteristic results from the activated charge transport in the semiconductor material, and that it is independent of the dominant carrier type in the semiconductor and the nature of the junction between the semiconductor and the three contacts. The same equivalent circuit, and hence similar device characteristics, can be produced using any other material combination with non-linear current-voltage characteristics, such as a suitable combination of semiconducting and conducting materials, such that a Schottky junction is present at all three contacts.

Light induced changes in the defect structure of a-Si:H

The effect of light-soaking, using a simulated daylight spectrum (colour temperature 5800 K) on the crystallinity, defect structure, and total hydrogen concentration of a-Si:H grown by HW-CVD is being investigated in an ongoing project. In this article, positron beam based electron momentum spectroscopy is applied to monitor the evolution of the open-volume defect structure after each illumination stage. The results indicate an initial increase in free volume at dangling-bond complexes on illumination, with no indication of a change in defect concentration or generation of larger microvoids. On further illumination, there is a reduction in the low electron momentum fraction, which is not accompanied by a similar change in the free-volume. This can be interpreted as a reconfiguration of the hydrogen in the dangling-bond complex, followed by its release into a mobile state.