The efficiency of Cu(In,Ga)Se2 (CIGS)-based solar cells could be continuously increased up to 22.6% by employing alkali metal dopants like Na, K, Rb, and Cs. The alkali metals are supplied to the CIGS layer from the glass substrate during deposition, from precursor layers or by a post deposition treatment. The alkali metal distribution in CIGS is not homogenous. Independently of the alkali metals used, their concentration at grain boundaries is much higher than that inside the grains. In this contribution, we discuss thermodynamic limitations for alkali metals in CIGS and show that in higher concentrations they are responsible for secondary phase separation. Applying the concept of immiscibility of phases for alkali metals in CIGS, we suggest how segregation at grain boundaries, formation of clusters in CIGS grains, sporadic formation of microstructures in the CIGS layer (hotspots, nodules), and separation of secondary phases with ordered structures can be interpreted.

To achieve the first demonstration of non-polar a-plane gallium nitride (GaN) epitaxy on (0 1 0) gallium oxide substrates by metal organic chemical vapor deposition (MOCVD), a low temperature AlGaN nucleation layer was engineered. Specific low temperature AlGaN growth parameters were necessary because the gallium oxide substrate begins to decompose at ∼600 °C in the ambient of H2. To achieve a smooth GaN epitaxial surface, low V/III molar ratio, and low pressure were required. To characterize the GaN film, AFM along with an orientation-dependent crystal tilt mosaic study by X-ray diffraction was performed. We effectively reduced threading dislocation density by applying in situ SiN interlayers grown by MOCVD. The oxygen contamination in the GaN film was found to originate from the substrate decomposition during GaN growth and can be reduced more than 10 times by using GaN buffer layer grown under N2 ambient.

An ultra-violet (UV) curable ink jet 3D printed material was characterized by an inverse finite element modeling (IFEM) technique employing a nonlinear viscoelastic–viscoplastic (NVEVP) material constitutive model; this methodology was compared directly with nanoindentation tests. The printed UV cured ink jet material properties were found to be z-depth dependent owing to the sequential layer-by-layer deposition approach. With further post-UV curing, the z-depth dependence was weakened but properties at all depths were influenced by the duration of UV exposure, indicating that none of the materials within the samples had reached full cure during the 3D printing process. Effects due to the proximity of an indentation to the 3D printed material material-sample fixing interface, and the different mounting material, in a test sample were examined by direct 3D finite element simulation and shown to be insignificant for experiments performed at a distance greater than 20 µm from the interface.

Room-temperature fracture along the (111) plane of silicon is probed at the micron-scale using chevron notched cantilever beams that enable stable crack growth before unstable fracture in successful tests. The main experimental observation is that a growing crack can extend and arrest at different stress intensity factor values within the same specimen. The present data thus provide evidence of variations in the effective Si fracture toughness along the path of a growing crack. This effect could be explained by variations in the extent of limited cracktip plasticity along the crack path. The present work also shows that the microscopic chevron notch test is, from an experimental point of view, an inconvenient method to probe the fracture toughness of silicon because it is difficult with silicon to nucleate a crack at the chevron tip at loads low enough to allow for subsequent stable crack growth.

Complex oxides and semiconductors exhibit distinct yet complementary properties owing to their respective ionic and covalent natures. By electrically coupling complex oxides to traditional semiconductors within epitaxial heterostructures, enhanced or novel functionalities beyond those of the constituent materials can potentially be realized. Essential to electrically coupling complex oxides to semiconductors is control of the physical structure of the epitaxially grown oxide, as well as the electronic structure of the interface. Here we discuss how composition of the perovskite A- and B-site cations can be manipulated to control the physical and electronic structure of semiconductor—complex oxide heterostructures. Two prototypical heterostructures, Ba1−x Sr x TiO3/Ge and SrZr x Ti1−x O3/Ge, will be discussed. In the case of Ba1−x Sr x TiO3/Ge, we discuss how strain can be engineered through A-site composition to enable the re-orientable ferroelectric polarization of the former to be coupled to carriers in the semiconductor. In the case of SrZr x Ti1−x O3/Ge we discuss how B-site composition can be exploited to control the band offset at the interface. Analogous to heterojunctions between compound semiconducting materials, control of band offsets, i.e., band-gap engineering, provides a pathway to electrically couple complex oxides to semiconductors to realize a host of functionalities.

The purpose of this paper is to propose a critical assessment of Young’s modulus determination of coated materials using Impulse Excitation Technique (IET). In this technique, the coated substrate is excited by an impulse and the acoustic vibrations are recorded. The frequency of the first bending mode is then used in a mechanical model to obtain the Young’s modulus of the coating. The existing models are based on two different theories: the flexural rigidity of a composite beam and the Classical Laminated Beam Theory (CLBT). The aim of the present paper is to assess the accuracy (trueness and precision) of the technique. For this, different models proposed in the literature are compared with a finite element model of the specimen for various conditions. The trueness and precision of models were evaluated and the best model was identified. Then a detailed uncertainty budget is performed to identify and quantify the most influent factors on the global uncertainty.

A resistive switching (RS) phenomenon, namely reversible transitions between the low and high resistance states after forming process, is caused by the formation and rupture of a conductive filament. We confirmed that conductive filaments including a quantum point contact (QPC) in Pt/NiO/Pt RS cells were formed by semiforming, the first step of the forming process. In this study, we examine correlation between microscopic structures in NiO layers and forming characteristics in the Pt/NiO/Pt cells. The appearance condition of the quantized conductance is considered to be associated with the composition ratio of O to Ni of either equivalent to or larger than a critical value. Furthermore, we proposed a RS model based on the forming characteristics especially obtained from the RS cells with different size. Defects which act as the source of a conductive filament including a QPC by semiforming may be randomly distributed in a NiO layer according to Poisson statistics.

Epitaxial layers of insulating binary lanthanide oxides have been considered as potential alternative to conventional SiO2 for gate dielectric application in future Si-based MOSFET devices, which was investigated in more detail for epitaxial Gd2O3 and Nd2O3 as model systems. Additionally, the ability to integrate epitaxial dielectric barrier layers into Si structures can usher also in a variety of novel applications involving oxide/silicon/oxide heterostructures in diverse nanoelectronic and quantum-effect devices. Although epitaxial layers of such ionic oxides with excellent structural quality can be grown using molecular beam epitaxy, they often exhibit poor electrical properties such as high leakage current density, flat band instability, poor reliability etc. owing to the presence of electrically active charge defects, generated either during the oxide layer growth or typical subsequent CMOS process steps. Based on the origin and individual character of these defects, we review various aspects of defect prevention and passivation which lead to a significant improvement in the dielectric properties of the heterostructures.

The large volume production of flexible electronics by solution based roll-to-roll (R2R) manufacturing technologies is a promising upscaling strategy for the organic electronics industry. Typical optoelectronic devices like organic light emitting diodes (OLEDs) consist of a complex stack of functional layers. Solution deposition of these structures eliminates the need for expensive vacuum processing. This contribution presents approaches for solution based R2R production methods of functional OLED layers on flexible polymer substrates. The development of a R2R line with two slot-die coating stations is discussed which can deposit two uniform layers consecutively in a single run (“tandem coating”) at web speeds up to 30 m/min. Furthermore, it offers the unique feature that there is no contact between the rollers and the top side of the substrate where the functional coating is deposited. Thereby, an important source of particle contamination and other damage to the device is eliminated. In addition to continuous deposition, stripe and intermittent coating techniques have been developed, allowing the production of patterned layers. Finally, examples will be shown of OLEDs where two functional materials are deposited by R2R processing from solution.

This article is a review of research on nanostructured high-entropy alloys with emphasis on those made by the severe plastic deformation methods of mechanical alloying and high-pressure torsion. An example of thin film refractory metal alloys made by magnetron sputtering is also presented. The article will begin with a discussion of the seminal research of B.S. Murty and co-workers who first produced nanocrystalline high-entropy alloys by mechanical alloying of powders. This will be followed by a listing of research, in mostly chronological order, of mainly 3d transition metal alloys made nanocrystalline by mechanical alloying. Research on the well-studied Cantor alloy, from the literature and the author’s laboratory will be presented. The author’s and co-worker’s research on a low-density high-entropy alloy with single-phase fcc or hcp structure and an extremely high strength (hardness)-to-weight ratio will be described.

Organometal trihalide perovskite solar cells (PSCs) have sparked a frantic excitement in the scientific community because they can achieve high power conversion efficiencies (PCEs) even when fabricated by low-cost solution-processing technologies. However, the poor stability of PSCs has seriously hindered their commercialization. Among various kinds of PSCs, carbon-based PSCs without hole transport materials (C-PSCs) seem to be the most promising for addressing the stability issue because carbon materials are stable, inert to ion migration, and inherently water-resistant. Concurrent with the steady rise in PCE of C-PSCs, great progresses have also been attained on the device stability and scaling-up fabrication of C-PSCs, which have well signified the possible commercialization of PSCs in the near future. In this review, we will summarize these progresses with a view of exposing the promising prospect. We start by collating recent stability testing results of C-PSCs with reference to those of HTM-PSCs. Then, we update the research status on large-scale C-PSCs and their associated scalable fabrication technologies. Finally, we identify main issues to be addressed alongside future research directions.

Grain boundary segregation can reduce the driving force for grain growth in nanocrystalline materials and help retain fine grain sizes. However, grain boundary segregation is enthalpically driven, and so a stabilized nanocrystalline state should undergo a disordering process as temperature is increased. Here we develop a Monte Carlo-based simulation that determines the minimum free energy state of an alloy with a strong tendency for grain boundary segregation that considers both different grain sizes and a large solute configuration space. We find that a stable nanocrystalline alloy undergoes a disordering process where grain boundary segregated atoms dissolve into the adjacent grains and increase the grain size as a function of temperature. At a critical temperature, the single crystal state becomes the most preferred. Using this method, we are able to determine how the grain size changes as a function of temperature and produce equilibrium phase diagrams for nanocrystalline alloys.

Graphene, a two-dimensional (2D) crystalline material exhibits unique electronic, optical, and mechanical properties which makes it a promising candidate for optomechanical and optoelectronic devices. The giant plasmonic activity of graphene sheets enables low-dimensional confinement of light and enhanced light–matter interaction leading to significant enhancement of optical forces which may give rise to large mechanical deformations on account of ultralow mass density and flexibility of graphene. The multilayer stack and heterostructures of 2D materials provide access to a spectrum of guided modes which can be used to tailor the optical forces and mechanical states of graphene sheets. Here, we study the optical forces arising from the coupling of guided modes in layered structures of graphene sheets. We obtain the mechanical deformation states corresponding to each guided mode and demonstrate that the optical forces can be adjusted by changing the interlayer spacing as well as the chemical potential of graphene layers. Our results can be used for various designs of graphene-based optomechanical devices.

Composite magnetoelectrics implemented as thin film heterostructures are discussed in view of their applicability as highly sensitive magnetic field sensors. Here, either PZT or AlN served as piezoelectric component. The magnetostrictive phase consisted of layer systems based on FeCo or (Fe90Co10)78Si12B10. All functional layers were deposited with thicknesses of a few micrometers on Si cantilever structures with typical lateral dimensions of 25 mm by 2.2 mm. Magnetoelectric coefficients as large as 6900 V/cm Oe and a limit of detection as low as 1 pT/(Hz)1/2 were measured. Currently, the best result demonstrates a detection limit of 500 fT/(Hz)1/2 at 958 Hz frequency using a set of two sensors for external noise suppression. A frequency conversion technique is proposed to broaden the applicability of resonant magnetoelectric sensors to a wider frequency range. Finally, the achieved sensor performance is evaluated with regard to typical magnetic field amplitudes in medical applications.