Positron annihilation lifetime spectroscopy (PALS) and Doppler broadening positron annihilation spectroscopy (DB-PAS) depth profiling demonstrate pronounced growth of vacancy clusters at the grain boundaries of as-deposited Al-doped ZnO films deposited as transparent conductive oxide (TCO) on $\mathrm{Cu}\left(\mathrm{In},\mathrm{Ga}\right)\mathrm{S}{\mathrm{e}}_2$ (CIGS) solar cells upon accelerated degradation at $85{}^{\circ }\mathrm{C}/85\%$ relative humidity. Quantitative fractions of positrons trapped either in the vacancy clusters at the grain boundaries or in Zn monovacancies inside the grains of ZnO:Al were obtained by detailed analysis of the PALS data using a positron trapping model. The time and depth dependence of the positron Doppler depth profiles can be accurately described using a planar diffusion model, with an extracted diffusion coefficient of $35\mathrm{n}{\mathrm{m}}^2/\mathrm{hour}$ characteristic for in-diffusion of molecules such as ${\mathrm{H}}_2\mathrm{O}$ and $\mathrm{C}{\mathrm{O}}_2$ into ZnO:Al TCO films via the grain boundaries, where they react with the ZnO:Al. This leads to increased open volume at the grain boundaries that imposes additional transport barriers and may lead to charge carrier trapping and nonradiative recombination. Simultaneously, a pronounced increase in series resistance and a strong reduction in efficiency of the ZnO:Al capped CIGS solar cells is observed on a remarkably similar timescale. This strongly indicates that these atomic-scale processes of molecular in-diffusion and creation of open volume at the grain boundaries play a key role in the degradation of the solar cells.

PhySH: Solar Cells, Positron Annihilation Spectroscopy, Grain Boundaries, Vacancies, Thin Films, Diffusion, Electrical Properties, Solid State Chemistry, Optoelectronics

In recent years, Raman spectroscopy has been used to visualize and analyze ferroelectric domain structures. The technique makes use of the fact that the intensity or frequency of certain phonons is strongly influenced by the presence of domain walls. Although the method is used frequently, the underlying mechanism responsible for the changes in the spectra is not fully understood. This inhibits deeper analysis of domain structures based on this method. Two different models have been proposed. However, neither model completely explains all observations. In this work, we have systematically investigated domain walls in different scattering geometries with Raman spectroscopy in the common ferroelectric materials used in integrated optics, i.e., ${\mathrm{KTiOPO}}_4$, ${\mathrm{LiNbO}}_3$, and ${\mathrm{LiTaO}}_3$. Based on the two models, we can demonstrate that the observed contrast for domain walls is in fact based on two different effects. We can identify on the one hand microscopic changes at the domain wall, e.g., strain and electric fields, and on the other hand a macroscopic change of selection rules at the domain wall. While the macroscopic relaxation of selection rules can be explained by the directional dispersion of the phonons in agreement with previous propositions, the microscopic changes can be explained qualitatively in terms of a simplified atomistic model.

Recent results have identified two-dimensional (2D) tellurene as a potential van der Waals (vdW) material for thermoelectric and optoelectronics applications owing to their pseudo-one-dimensional (anisotropic) behavior and structure. While hydrothermal synthesis is suitable for yielding potentially crystalline materials, it is well known for its incompatibility with manufacturing and scaling. There is an urgent need for synthesis of tellurene sheets in vdW layered form with the desired crystallinity as well as controlled crystalline anisotropy using more conventional and scalable processes. Here, we report on the synthesis of tellurene sheets onto a variety of surfaces of vdW-bonded 2D layered crystals in their trigonal layered form with controlled tellurene chain (anisotropy) direction using horizontal physical vapor deposition method. Our systematic tellurene growth studies on six different vdW surfaces show that GaS and GaSe surfaces enable highly crystalline and self-oriented tellurene sheets. Detailed cross-sectional transmission electron microscopy (TEM), angle-resolved Raman spectroscopy, scanning electron microscopy, and energy-dispersive spectroscopy measurements provide fundamental insights into the growth dynamics and characteristics of tellurene. First-principles calculations and cross-sectional TEM measurements suggest that tellurene chains are well-oriented along the GaSe armchair lattice direction owing to the much-reduced total energy of the system and a stronger degree of coupling across adjacent layers. Synthesized tellurene sheets exhibit remarkable structural anisotropy, as evidenced by angle-resolved Raman measurements, and the overall results open a clear pathway for layer-by-layer growth of tellurene sheets with controlled crystalline anisotropy using conventional synthesis techniques.

Dirac line node (DLN) semimetals are a class of topological semimetals that feature band-crossing lines in momentum space. We study the type-I and type-II classification of DLN semimetals by developing a criterion that determines the type using band velocities. Using first-principles calculations, we also predict that ${\mathrm{Na}}_3\mathrm{N}$ under an epitaxial tensile strain realizes a type-II DLN semimetal with vanishing spin-orbit coupling, characterized by the Berry phase, which is ${\mathbb{Z}}_2$ quantized in the presence of inversion and time-reversal symmetries. The surface energy spectrum is calculated to demonstrate the topological phase and the type-II nature is demonstrated by calculating the band velocities. We also develop a tight-binding model and a low-energy effective Hamiltonian that describe the low-energy electronic structure of strained ${\mathrm{Na}}_3\mathrm{N}$. The occurrence of a DLN in ${\mathrm{Na}}_3\mathrm{N}$ under strain is captured in the optical conductivity, which we propose as a means to experimentally confirm the type-II class of DLN semimetals.

Engineering exchange interaction between magnetic thin films is a key ingredient for achieving high-performance spintronics devices. In this work, the exchange coupling effect of bilayers consisting of perpendicularly magnetized tetragonal phase $\mathrm{SrRu}{\mathrm{O}}_3$ (SRO) and in-plane magnetized $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ has been investigated. We observed an intriguing antiferromagnetic exchange coupling at the interface of such heterostructures with orthogonal magnetic configuration. The unique exchange coupling coexists with the formation of domain walls in the SRO layer as inferred from the magnetometry and magnetotransport measurements, resulting in a positive exchange bias effect. The thickness and temperature dependence of the bias field further confirm the interfacial origin of the antiferromagnetic coupling effect.

Engineering of phononic resonances in ferroelectric structures is a new knob to realize novel multifunctional devices. Here we show the possibility of predictively designing and fabricating phononic nanoresonators utilizing combinations of MBE-grown insulating (${\mathrm{BaTiO}}_3$, ${\mathrm{SrTiO}}_3$) and metallic (${\mathrm{SrRuO}}_3$) oxides. We experimentally demonstrate the confinement of acoustic waves in the 100-GHz frequency range in a phonon nanocavity, and the the time and spatial beatings resulting from the coupling of two different hybrid nanocavities forming an acoustic molecule. Additionally, the direct measurement of Bloch-like oscillations of acoustic phonons is observed in a system formed by ten coupled resonators. Utilizing coherent phonon generation techniques, we study phonon dynamics directly in the time domain. The metallic ${\mathrm{SrRuO}}_3$ layer introduces a local phonon generator and transducer that allows for the spatial, spectral, and time-domain monitoring of the complex generated waves. Our results introduce ferroelectric cavity systems as a new realm for the study of complex wave localization phenomena at the nanoscale. These systems can be successfully designed and conceived using state of the art growth techniques that combine perovskite oxides possessing multifunctional properties.

Strain is a key parameter affecting the physical properties of heterostructured thin films and nanosized objects. Generally, the design of application-optimized hybrid structures requires good structural compatibility between the involved phases. However, when controlled appropriately, lattice mismatch can turn from a detrimental to a beneficial property, enabling further functionality tuning. Due to their large heterointerface, nanocolumnar composites are an ideal test bed for such strain engineering approaches, but coupling mechanisms at vertical interfaces are still poorly understood. In the present paper, we therefore present a detailed analysis of ultrathin Ni nanowires, with diameters between 1.7 nm and 5.3 nm, vertically epitaxied in a ${\mathrm{SrTiO}}_3$/${\mathrm{SrTiO}}_3$(001) matrix. Using a combination of x-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and x-ray absorption spectroscopy (XAS) measurements, we unveil peculiar structural features of this hybrid system. We show that the axial deformation of the nanowires depends on their diameter and that their radial strain differs sensitively from the value expected when considering the Poisson ratio. We also provide evidence for the existence of a relaxation mechanism consisting in a slight tilting of crystallographic nanowire domains which reduces the misfit at the Ni-${\mathrm{SrTiO}}_3$ heterointerface. This, in turn, induces significant structural disorder and results in a successive amorphization of the metallic phase upon diameter reduction of the nanowires.

We have clarified the origin of a magnetically dead interface layer formed in yttrium iron garnet (YIG) films grown at above 700 °C onto a gadolinium gallium garnet (GGG) substrate by means of laser molecular beam epitaxy. The diffusion-assisted formation of a Ga-rich region at the YIG/GGG interface is demonstrated by means of composition depth profiling performed by x-ray photoelectron spectroscopy, secondary ion mass spectroscopy, and x-ray and neutron reflectometry. Our finding is in sharp contrast to the earlier expressed assumption that Gd acts as a migrant element in the YIG/GGG system. We further correlate the presence of a Ga-rich transition layer with considerable quenching of ferromagnetic resonance and spin wave propagation in thin YIG films. Finally, we clarify the origin of the enigmatic low-density overlayer that is often observed in neutron and x-ray reflectometry studies of the YIG/GGG epitaxial system.

Metaloxide thin films were deposited on tilted-axes ${\mathrm{NdGaO}}_3$ substrates (TAS NGO) by pulsed laser deposition. A specific growth mode resulting in an inclination between crystallographic planes of the top layer and that of the bottom layer was commonly observed. A simple geometrical growth model, taking into account faceting of the surface of the bottom layer, explains the observed dependencies well. The matching of the top and the bottom layer is essentially three dimensional, with graphoepitaxial matching in the substrate plane. The three-dimensional graphoepitaxial (3DGE) growth mechanism seems to be quite common for deposition on TAS with tilt angles more than 5°. It was observed for eight of ten studied combinations of materials, including multilayer heterostructures, for four different deposition techniques, and on substrates with different predeposition treatment. The 3DGE growth was observed both with increase and decrease of the top layer tilt angle compared to the tilt angle of the bottom layer. Two different 3DGE dependencies can be distinguished in the high-angle range $(>{15}^{\circ })$: with a tendency towards standard growth above some threshold angle, and retaining 3DGE behavior until a tilt angle of 45° is reached, either by the top or by the bottom layer. In a simplified way the difference may be attributed to two different formation mechanisms: the first one generates the additional tilt when the growing grain overgrows another grain, seeded on the next step on the substrate surface, while for the second mechanism the inclination is formed when the grain is seeded over the step. The first type is better described by a tangent angular dependence, it is observed usually when a compressive strain is induced in the top layer. The second type follows a sine dependence, and is usually seen for a tensile-strained top layer.

While nucleation-limited transformation mechanisms are widely implicated in unstrained, undoped $\mathrm{V}{\mathrm{O}}_2$ nanoparticles, a direct link between nucleation barriers and hysteresis widths has not yet been established. Here, we investigate microscopic transformation of structural domains optically in hydrothermally grown $\mathrm{V}{\mathrm{O}}_2$ particles ∼0.5–46 μm in length, which are not elastically clamped to the substrate. We observe abrupt and generally complete transformation in individual particles, consistent with a nucleation-limited transformation mechanism. The forward and reverse transformation temperatures are not correlated, suggesting a range of potency of nucleation sites for both forward and reverse transformation in undoped particles, resulting in a hysteresis of 2.9–46.3 °C. Thus, the macroscopic hysteresis width in bulk $\mathrm{V}{\mathrm{O}}_2$ powders and dispersed particulate films is primarily attributable to a distribution of critical nucleation temperatures between different particles. These findings suggest that as $\mathrm{V}{\mathrm{O}}_2$ volume elements are scaled down for microelectronic applications, manipulation of nucleation sites via defect engineering may be required to control the degree of the $\mathrm{V}{\mathrm{O}}_2$ element reversibility.

Inelastic neutron scattering was used to measure phonon dispersions in a single crystal of ${\mathrm{FeGe}}_2$ with the C16 structure at 300, 500, and 635 K. Phonon densities of states (DOS) were also measured on polycrystalline ${\mathrm{FeGe}}_2$ from 325 to 1050 K, and the Fe partial DOS was obtained from polycrystalline ${}^{57}\mathrm{FeGe}_2$ at 300 K using nuclear resonant inelastic x-ray scattering. The dominant feature in the temperature dependence of the phonon spectrum is thermal broadening of high-energy modes. The energy shifts of the low- and high-energy parts of the spectrum were almost the same. DFT calculations performed with the quasiharmonic approximation gave results in moderate agreement with the experimental thermal energy shifts, although the isobaric Grüneisen parameter calculated from the quasiharmonic model was smaller than that from measurements. The thermal broadening of the phonon spectrum and dispersions, especially at high energies, indicates a cubic anharmonicity to second order that should also induce phonon shifts. We show that different anharmonic contributions cancel out, giving average phonon shifts in moderate agreement to calculations with the quasiharmonic approximation. The different parts of the large phonon contribution to the entropy are separated for ${\mathrm{FeGe}}_2$, showing modest but interpretable anharmonic contributions.

We investigated the temperature dependence of thermally excited spin current properties of ferromagnet Py $\left({\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}\right)$ by using a lateral spin-valve structure. The spin-dependent Seebeck coefficient at the Py/Cu interface was found to show a significant increase at low temperature while the charge Seebeck coefficient showed the ordinal reduction with decreasing temperature. This produces a crossover between the charge- and spin-dependent Seebeck coefficients, resulting in the thermal spin polarization greater than $100\%$ below 125 K. From the first-principles calculation specially developed for the Py/Cu interfaces, the spin-dependent Seebeck coefficient is found to be highly susceptible to the magnetic disorder-induced scattering at the interface. The calculation with considering the temperature dependence of the magnetic disorder provides a consistent description for the significant enhancement of spin-dependent Seebeck coefficient in the Py/Cu interface. Our demonstration shows the importance of interface disorder and paves the way toward the development of conventional ferromagnetic metal for the practical application of thermo-spin energy conversion.

Significantly high efficiency of the photoelectrochemical (PEC) water splitting process can be achieved by using two semiconductors in a tandem device. The smaller band gap (SBG) material in the device has a band gap of $\sim 1$ eV, whereas the larger band gap (LBG) material has a band gap of $\sim 2$ eV. However, a very limited number of LBG semiconductors have been explored and here we investigate systematically the quaternary chalcogenides of $A_2{BCX}_4$ type. We calculate the properties of the materials in six different crystal structures. Based on the criteria of thermodynamic stability, band gap, and good charge transport properties, we find a handful of potential LBG candidates from a pool of 1368 materials. Additionally, by extrapolating our analyses we also find a few SBG semiconductors, some of which are already known, e.g., CZTS/AgZTSe. This consolidates our approach for the LBG semiconductors and therefore invites experimental investigation of the candidates identified as efficient LBG semiconductors for the tandem devices.

Polymeric nitrogen with single bonds can be created from the molecular form at high pressure and due to large energy difference between triple and single bonds it is interesting as an energetic material. Its structure and properties are, however, still not well understood. We studied amorphous nitrogen by ab initio simulations, employing molecular dynamics and evolutionary algorithms. Amorphous nitrogen was prepared at a pressure of 120 GPa by quenching from a hot liquid, by pressure-induced amorphization of a molecular crystal, and by evolutionary search. All three amorphous forms were found to be structurally similar. We studied in detail the structural evolution of the system upon decompression from 120 GPa to zero pressure at 100 K. At pressures above 100 GPa, the system consists mainly of 3-coordinated atoms (80%) connected by single bonds while some short chains made of 2-coordinated atoms are also present. Upon decompression, the number of 3-coordinated atoms rapidly decreases below 60 GPa and longer chains are created. At 20 GPa the system starts to create also ${\mathrm{N}}_2$ molecules and the ultimate structure at $p=0$ contains molecules inside a polymeric network consisting dominantly of longer chains made of 2-coordinated atoms. Besides structure, we also study vibrational and electronic properties of the system and estimate the amount of energy that could be stored in amorphous nitrogen at ambient pressure.

We explore the layered palladium oxides ${\mathrm{La}}_2{\mathrm{PdO}}_4$, ${\mathrm{LaPdO}}_2$, and ${\mathrm{La}}_4{\mathrm{Pd}}_3{\mathrm{O}}_8$ via ab initio calculations. ${\mathrm{La}}_2{\mathrm{PdO}}_4$, being low spin $d^8$, is quite different from its high spin nickel analog. Hypothetical ${\mathrm{LaPdO}}_2$, despite its $d^9$ configuration, has a paramagnetic electronic structure very different from cuprates. On the other hand, the hypothetical trilayer compound ${\mathrm{La}}_4{\mathrm{Pd}}_3{\mathrm{O}}_8$ ($d^{8.67}$) is more promising in that its paramagnetic electronic structure is very similar to that of overdoped cuprates. But even in the $d^9$ limit (achieved by partial substitution of La with a 4+ ion), we find that an antiferromagnetic insulating state cannot be stabilized due to the less correlated nature of Pd ions. Therefore, this material, if it could be synthesized, would provide an ideal platform for testing the validity of magnetic theories for high-temperature superconductivity.

Perovskite oxynitrides are a promising class of materials for photocatalytic water splitting due to their small band gaps and suitably aligned band edges. Recently, epitaxially strained perovskite oxynitrides started to attract interest due to the possibility to engineer the anion order and ferroelectric distortions. However, contrary to perovskite oxides there have been no studies on the band-gap evolution in perovskite oxynitrides with epitaxial strain. Here, we investigate, using density functional theory calculations, the influence of epitaxial strain on the band gaps of two different perovskite oxynitrides and compare our results with perovskite oxides. As opposed to cubic oxides where both compressive and tensile strains narrow the band gap, we find that the anion order leads to nondegenerate N $2p$ bands already at zero strain, which leads to opposite and anion order-dependent evolutions of the band gap under compressive and tensile strain. Furthermore, we find that in perovskite oxynitrides polar distortions increase the band gap by up to 1 eV for 4% strain whereas for perovskite oxides the increase is only about 0.1 eV for the same amount of strain. The reason for this difference is the larger ionic radius and smaller electronegativity of nitrogen leading to larger ferroelectric distortions and a stronger dependence of the band gap on the distortion amplitude. These results imply that ferroelectric distortions are strongly detrimental to light absorption and that their suppression, for example, above a critical temperature, should result in a marked decrease in the band gap and enhanced absorption of visible light.

Nanoholes, drilled in the (001) surface of AlGaAs by local Al droplet etching, are shown to consist of faceted inner walls. The most prominent facets of the inverted pyramidlike nano-sized etch pits are the {111}A and {$1\overline{1}1$}B surfaces, which differ in their atomic surface terminations. In the [110] direction, the {111} facets change to {112} and/or {113}, which are both stepped surfaces with (111)A terraces. Etching-temperature-dependent data indicate that this facet transition seems kinetically hindered up to etch temperatures above 660 °C, at which point the walls along $\left[1\overline{1}0\right]$ have already evolved completely towards {$1\overline{1}1$}B facets. The redeposited ring material outside the nanohole develops facets with indices of (115) and higher, thereby forming relatively flat structures. The facets and their indices are unraveled by a combination of atomic force microscopy, scanning electron microscopy, and x-ray diffraction, which is performed on an ensemble as well as on a single hole using nanodiffraction. These results imply that this nanoconfined etching process can be largely understood in a vapor-liquid-solid scheme, which includes the bulk thermodynamics in the Al-Ga-As system, the surface energies of low index facets, and their etch rates and surface terminations.

In modern $\mathrm{GaAs}/{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ heterostructures with record high mobilities, a two-dimensional electron gas (2DEG) in a quantum well is provided by two remote donor $\delta$ layers placed on both sides of the well. Each $\delta$ layer is located within a narrow GaAs layer, flanked by narrow AlAs layers which capture excess electrons from donors but leave each of them localized in a compact dipole atom with a donor. Still excess electrons can hop between host donors to minimize their Coulomb energy. As a result they screen the random potential of donors dramatically. We numerically model the pseudoground state of excess electrons at a fraction $f$ of filled donors and find both the mobility and the quantum mobility limited by scattering on remote donors as universal functions of $f$. We repeat our simulations for devices with additional disorder such as interface roughness of the doping layers, and find the quantum mobility is consistent with measured values. Thus, in order to increase the quantum mobility this additional disorder should be minimized.