Horst P. Strunk

Photoluminescence from silicon nitride—no quantum effect

Silicon nitride compounds emit photoluminescence all over the visible range. Recent studies ascribed this luminescence to quantum-size effects within silicon nanocrystals that were either shown or assumed to form inside the silicon nitride matrix; the luminescence of the matrix itself was ignored. In contrast, observing the same luminescence even without the presence of silicon crystallites, our work identifies the silicon nitride matrix itself as responsible for the photoluminescence. All experimental observations are well explained by band tail luminescence from the silicon matrix. In contrast to the silicon nanocrystal approach, our model explains all aspects of the luminescence. As a consequence, we conclude that silicon nitride films are inappropriate if one aims at investigating photoluminescence from silicon nanocrystals within such a matrix.

The filamentary growth of metals

Abstract One-dimensional structures are starting to have impact in modern technological device design. A brief history of the fabrication of metal structures in such geometries is presented. At the dawn of modern science, in 1574, a recipe was published for the formation of “metallic grass”. This ancient recipe is reintroduced into modern technology using physical vapor deposition techniques to grow nanowhiskers. Defects in dewetting layers on various substrates are proven to act as nucleation sites for the whisker growth. This general approach is shown for Cu, Ag and Co nanostructures. The whiskers are of perfect crystal structure, devoid of defects and flaws and freestanding on the substrates. A phenomenological but energetically justified growth model is presented. The unique mechanical stability of these structures is shown by bending experiments. The theoretical limit of the mechanical strength is reached reproducibly.

Tuning the emission colour by manipulating terbium-terbium interactions: Terbium doped aluminum nitride as an example system

Terbium-terbium interactions in terbium doped semiconductors and insulators may lead to the so-called cross-relaxation process, which increases the D54 (green) emission of the terbium ions at the cost of the D53 (blue) luminescence intensity. This effect can generally be reduced by increasing the distance between an excited ion and the nearest ion in the ground state. A straightforward measure is to use a specimen with a decreased terbium concentration. The alternative is to increase the intensity of the excitation (either by photons or electrons) and thereby to reduce the population of terbium ions in the ground state. This paper works this process out with the example of AlN:Tb on the basis of a model and respective experimental results. As will be seen, stronger excitation causes in essence more Tb ions to be excited, thus less ions in the ground state which increases the distance between an excited and the nearest ground state ions. This hinders energy transfer between the terbium ions and thus counte...

Metal-induced crystallization of highly corrugated silicon thick films as potential anodes for Li-ion batteries.

Silicon has turned into one of the most promising anodes for high energy rechargeable Li-ion batteries. However, a huge volume expansion during alloying with Li always induces serious pulverization/delamination for microsized electrodes as well as undesired accumulation of solid electrolyte interphase (SEI). Many efforts have focused on various nanoengineering and binding strategies to construct integrated, robust ionic/electronic wiring networks but with a trade-off between active/inactive material ratio and performance retention. Here, we first apply a metal-induced crystallization (AIC) principle for immiscible metal/semiconductor systems (Si/Al bilayers in this work) to prepare microthick Si films consisting of a high density of isolated nanocolumns. This method furthermore brings about low temperature crystallization of initial amorphous Si and conformal coating of ion-conductive oxide to enhance the Li transport kinetics of bulk and interface. Both highly satisfactory capacity retention (1650 mAh/g after 500 cycles) and rate performance (∼1000 mAh/g at 8C) are achieved for such thick Si film anodes. This methodology can be used to prepare thick film samples with well-defined nanostructures but free of extra binder and conductive additives. It enables much higher area specific capacity than for inactive-component contained slurry samples and thin film samples. This postdeposition pore-creating can be extended to more alloying or conversion electrodes of thick films for high capacity Li/Na ion batteries.

Rare earth luminescence: A way to overcome concentration quenching

A model is developed to simulate the rare earth luminescence intensity in dependence of both the excitation rate and the dopant concentration. For low excitation rates, as in the case of photoluminescence investigations, concentration quenching is expected. In contrast for high excitation rates (as generally realized in cathodoluminescence experiments) concentration quenching can be suppressed and thus luminescence intensity increases with increasing dopant concentration. These results reconcile the recent photo- and cathodoluminescence results on GaN:Er presented by Chen et al. (APL 96, 181901, 2010)10.1063/1.3421535. Further experimental results indicate that the physical basis of the model is adequate.

Two modes of HVPE growth of GaN and related macrodefects

GaN films with thickness up to 3 mm were grown by halide vapour phase epitaxy method. Two growth modes were observed: the high temperature (HT) mode and the low temperature (LT) mode. Films grown in HT mode had smooth surface, however the growth stress was high and caused cracking. Films grown in LT mode had rough surface with high density of V-defects (pits), however such films were crack-free. The influence of growth parameters on the pit shape and evolution was investigated. Origins of pits formation and process of pit overgrowth are discussed. Crack-free films with smooth surface and reduced density of pits were grown using combination of the LT and HT growth modes.

Group Theory of Wannier Functions Providing the Basis for a Deeper Understanding of Magnetism and Superconductivity

The paper presents the group theory of best localized and symmetry-adapted Wannier functions in a crystal of any given space group G or magnetic group M. Provided that the calculated band structure of the considered material is given and that the symmetry of the Bloch functions at all the points of symmetry in the Brillouin zone is known, the paper details whether or not the Bloch functions of particular energy bands can be unitarily transformed into best localized Wannier functions symmetry-adapted to the space group G, to the magnetic group M, or to a subgroup of G or M. In this context, the paper considers usual as well as spin-dependent Wannier functions, the latter representing the most general definition of Wannier functions. The presented group theory is a review of the theory published by one of the authors in several former papers and is independent of any physical model of magnetism or superconductivity. However, it is suggested to interpret the special symmetry of the best localized Wannier functions in the framework of a nonadiabatic extension of the Heisenberg model, the nonadiabatic Heisenberg model. On the basis of the symmetry of the Wannier functions, this model of strongly correlated localized electrons makes clear predictions whether or not the system can possess superconducting or magnetic eigenstates.

The Reason why Doping Causes Superconductivity in LaFeAsO

The experimental observation of superconductivity in LaFeAsO appearing on doping is analyzed with the group-theoretical approach that evidently led in a foregoing paper (Krüger and Strunk in J. Supercond. 24:2103, 2011) to an understanding of the cause of both the antiferromagnetic state and the accompanying structural distortion in this material. Doping, like the structural distortions, means also a reduction of the symmetry of the pure perfect crystal. In the present paper we show that this reduction modifies the correlated motion of the electrons in a special narrow half-filled band of LaFeAsO in such a way that these electrons produce a stable superconducting state.

Eutectic nano-droplet template injection into bulk silicon to construct porous frameworks with concomitant conformal coating as anodes for li-ion batteries

Building porosity in monolithic materials is highly desired to design 3D electrodes, however ex-situ introduction or in-situ generation of nano-scale sacrificial template is still a great challenge. Here Al-Si eutectic droplet templates are uniformly injected into bulk Si through Al-induced solid-solid convection to construct a highly porous Si framework. This process is concomitant with process-inherent conformal coating of ion-conductive oxide. Such an all-in-one method has generated a (continuously processed) high-capacity Si anode integrating longevity and stable electrolyte-anode diaphragm for Li-ion batteries (e.g. a reversible capacity as large as ~1800 mAh/g or ~350 μAh/cm2-μm with a CE of ~99% at 0.1 C after long-term 400 cycles).

Photoluminescence from AlxIn1−xN layers doped with Tb3+ ions

AlxIn1−xN films (0 ≤ x ≤ 1) have been sputter deposited and annealed, both without and with terbium co-doping, to obtain a series of matrices whose band gap energies span the range from around 2 eV to 6 eV. The terbium green luminescence spectra (excitation wavelength 230 nm, i.e. 5.4 eV) are measured at room temperature as a function of the aluminium content (band gap route) and of the terbium concentration (concentration route). The green luminescence assumes a maximum of the integrated intensity at a band gap energy of 4.1 eV (x = 0.7) which can be argued to result from a resonant energy transfer from the host matrix into the Tb luminescence centres. Furthermore, the dependence of this maximum integrated intensity as a function of the Tb concentration, i.e. of the average distance between the Tb centres, suggests the energy transfer from the host material into the Tb luminescent centres to be due to dipole-dipole interaction via an exciton bound to the centre.