Carina Höglund

Origin of the Anomalous Piezoelectric Response in Wurtzite ScxAl1-xN Alloys

The origin of the anomalous, 400% increase of the piezoelectric coefficient in Sc(x)Al(1-x)N alloys is revealed. Quantum mechanical calculations show that the effect is intrinsic. It comes from a strong change in the response of the internal atomic coordinates to strain and pronounced softening of C33 elastic constant. The underlying mechanism is the flattening of the energy landscape due to a competition between the parent wurtzite and the so far experimentally unknown hexagonal phases of the alloy. Our observation provides a route for the design of materials with high piezoelectric response.

B4C thin films for neutron detection

Due to the very limited availability of He-3, new kinds of neutron detectors, not based on 3He, are urgently needed. Here, we present a method to produce thin films of (B4C)-B-10, with maximized de ...

Wurtzite structure Sc1−xAlxN solid solution films grown by reactive magnetron sputter epitaxy: Structural characterization and first-principles calculations

AlN(0001) was alloyed with ScN with molar fractions up to ∼22%, while retaining a single-crystal wurtzite (w-) structure and with lattice parameters matching calculated values. Material synthesis was realized by magnetron sputter epitaxy of thin films starting from optimal conditions for the formation of w-AlN onto lattice-matched w-AlN seed layers on Al2O3(0001) and MgO(111) substrates. Films with ScN contents between 23% and ∼50% exhibit phase separation into nanocrystalline ScN and AlN, while ScN-rich growth conditions yield a transformation to rocksalt structure Sc1−xAlxN(111) films. The experimental results are analyzed with ion beam analysis, x-ray diffraction, and transmission electron microscopy, together with ab initio calculations of mixing enthalpies and lattice parameters of solid solutions in wurtzite, rocksalt, and layered hexagonal phases.

Cubic Sc1-xAlxN solid solution thin films deposited by reactive magnetron sputter epitaxy onto ScN(111)

Reactive magnetron sputter epitaxy was used to deposit thin solid films of Sc1-xAlxN (0 andlt;= x andlt;= 1) onto MgO(111) substrates with ScN(111) seed layers. Stoichiometric films were deposited ...

Electronic structure of GaN and Ga investigated by soft x-ray spectroscopy and first-principles methods

The electronic structure and chemical bonding of wurtzite-GaN investigated by N 1s soft x-ray absorption spectroscopy and N K, Ga M1, and Ga M2,3 emission spectroscopy is compared to that of pure G ...

Topotaxial growth of Ti2AlN by solid state reaction in AlN∕Ti(0001) multilayer thin films

The formation of Ti2AlN by solid state reaction between layers of wurtzite-AlN and α-Ti was characterized by in situ x-ray scattering. The sequential deposition of these layers by dual magnetron sputtering onto Al2O3(0001) at 200°C yielded smooth, heteroepitaxial (0001) oriented films, with abrupt AlN∕Ti interfaces as shown by x-ray reflectivity and Rutherford backscattering spectroscopy. Annealing at 400°C led to AlN decomposition and diffusion of released Al and N into the Ti layers, with formation of Ti3AlN. Further annealing at 500°C resulted in a phase transformation into Ti2AlN(0001) after only 5min.

Neutron Position Sensitive Detectors for the ESS

The European Spallation Source (ESS) in Lund, Sweden will become the worlds leading neutron source for the study of materials. The instruments are being selected from conceptual proposals submitted by groups from around Europe. These instruments present numerous challenges for detector technology in the absence of the availability of Helium-3, which is the default choice for detectors for instruments built until today and due to the extreme rates expected across the ESS instrument suite. Additionally a new generation of source requires a new generation of detector technologies to fully exploit the opportunities that this source provides. The detectors will be sourced from partners across Europe through numerous in-kind arrangements; a process that is somewhat novel for the neutron scattering community. This contribution presents briefly the current status of detectors for the ESS, and outlines the timeline to completion. For a conjectured instrument suite based upon instruments recommended for construction, a recently updated snapshot of the current expected detector requirements is presented. A strategy outline as to how these requirements might be tackled by novel detector developments is shown. In terms of future developments for the neutron community, synergies should be sought with other disciples, as recognized by various recent initiatives in Europe, in the context of the fundamentally multi-disciplinary nature of detectors. This strategy has at its basis the in-kind and collaborative partnerships necessary to be able to produce optimally performant detectors that allow the ESS instruments to be world-leading. This foresees and encourages a high level of collaboration and interdependence at its core, and rather than each group being all-rounders in every technology, the further development of centres of excellence across Europe for particular technologies and niches.

A solid phase reaction between TiCx thin films and Al2O3 substrates

TiCx thin films were deposited on Al2O3 substrates at 900°C by using a multiple cathode high current pulsed cathodic arc. The Ti:C pulse ratio and, hence, the composition was varied from C rich to Ti rich. It is found that the Al2O3 substrate is decomposed and reacts with the TiCx film to incorporate significant amounts of O and Al in the growing film. When the stoichiometry is suitable, epitaxially oriented Ti2AlC MAX phase with significant O incorporated is formed. The results indicate that Al2O3 is not an ideal substrate material for the growth of transition metal carbides and MAX phase thin films.

Electronic structure investigation of the cubic inverse perovskite Sc3AlN

The electronic structure and chemical bonding of the recently discovered inverse perovskite Sc3AlN, in comparison to those of ScN and Sc metal, have been investigated by bulk-sensitive soft-x-ray e ...

Electronic structure and chemical bonding anisotropy investigation of wurtzite AlN

The electronic structure and the anisotropy of the Al - N π and σ chemical bonding of wurtzite AlN has been investigated by bulk-sensitive total fluorescence yield absorption and soft x-ray emission spectroscopies. The measured N K, Al L 1 , and Al L 2,3 x-ray emission and N 1s x-ray absorption spectra are compared with calculated spectra using first principles density-functional theory including dipole transition matrix elements. The main N 2p - Al 3p hybridization regions are identified at 1.0 to -1.8 eV and -5.0 to -5.5 eV below the top of the valence band. In addition, N 2s - Al 3p and N 2s - Al 3s hybridization regions are found at the bottom of the valence band around -13.5 eV and -15 eV, respectively. A strongly modified spectral shape of Al 3s states in the Al L 2,3 emission from AlN in comparison to Al metal is found, which is also reflected in the N 2p - Al 3p hybridization observed in the Al L 1 emission. The differences between the electronic structure and chemical bonding of AlN and Al metal are discussed in relation to the position of the hybridization regions and the valence band edge influencing the magnitude of the large band gap.