The III-nitrides include the semiconductors AlN, GaN and InN, which have band gaps spanning the entire UV and visible ranges. Thin films of III-nitrides are used to make UV, violet, blue and green light-emitting diodes and lasers, as well as solar cells, high-electron mobility transistors (HEMTs) and other devices. However, the film growth process gives rise to unusually high strain and high defect densities, which can affect the device performance. X-ray diffraction is a popular, non-destructive technique used to characterize films and device structures, allowing improvements in device efficiencies to be made. It provides information on crystalline lattice parameters (from which strain and composition are determined), misorientation (from which defect types and densities may be deduced), crystallite size and microstrain, wafer bowing, residual stress, alloy ordering, phase separation (if present) along with film thicknesses and superlattice (quantum well) thicknesses, compositions and non-uniformities. These topics are reviewed, along with the basic principles of x-ray diffraction of thin films and areas of special current interest, such as analysis of non-polar, semipolar and cubic III-nitrides. A summary of useful values needed in calculations, including elastic constants and lattice parameters, is also given. Such topics are also likely to be relevant to other highly lattice-mismatched wurtzite-structure materials such as heteroepitaxial ZnO and ZnSe.

A new class of 'powder-in-tube' Mg-B superconducting conductors has been prepared using two different methods: an in situ technique where an Mg + 2B mixture was used as a central conductor core and reacted in situ to form MgB 2 , and an ex situ technique where fully reacted MgB 2 powder was used to fill the metal tube. Conductors were prepared using silver, copper and bimetallic silver/stainless steel tubes. Wires manufactured by the in situ technique, diffusing Mg to B particles experienced ∼25.5% decrease in density from the initial value after cold deformation, due to the phase transformation from Mg + 2(β − B) → MgB 2 all with hexagonal structure.A comparative study of the intergranular current and grain connectivity in wires was conducted by AC susceptibility measurements and direct four point transport measurements. Using a SQUID magnetometer, magnetization versus magnetic field (M-H ) curves of the round wires before and after sintering and reactive diffusion were measured at 5 K and in magnetic fields up to 5 T to define the J cmag . The direct current measurements were performed in self field at 4.2 K. A comparison between zero-field-cooled (ZFC) and field-cooled (FC) susceptibility measurements for sintered Ag/MgB 2 , and reacted Cu/Mg + 2B conductors revealed systematic differences in the flux pinning in the wires which is in very good agreement with direct high transport current measurements.

Ferroelectric materials are used in applications ranging from energy harvesting to high-power electronic transducers. However, industry-standard ferroelectric materials contain lead, which is toxic and environmentally unfriendly. The preferred alternative, BaTiO(3), is non-toxic and has excellent ferroelectric properties, but its Curie temperature of ∼130 °C is too low to be practical. Strain has been used to enhance the Curie temperature of BaTiO(3) (ref. 4) and SrTiO(3) (ref. 5) films, but only for thicknesses of tens of nanometres, which is not thick enough for many device applications. Here, we increase the Curie temperature of micrometre-thick films of BaTiO(3) to at least 330 °C, and the tetragonal-to-cubic structural transition temperature to beyond 800 °C, by interspersing stiff, self-assembled vertical columns of Sm(2)O(3) throughout the film thickness. The columns, which are 10 nm in diameter, strain the BaTiO(3) matrix by 2.35%, forcing it to maintain its tetragonal structure and resulting in the highest BaTiO(3) transition temperatures so far.

InGaN quantum wells have been found to be extremely sensitive to exposure to the electron beam in the transmission electron microscope (TEM). High-resolution TEM images acquired immediately after first irradiating a region of quantum well indicates no gross fluctuations of indium content in the InGaN alloy. During only a brief period of irradiation, inhomogeneous strain is introduced in the material due to electron beam damage. This strain is very similar to that expected from genuine nanometer-scale indium composition fluctuations which suggests there is the possibility of falsely detecting indium-rich “clusters” in a homogeneous quantum well.

Large thermal changes driven by a magnetic field have been proposed for environmentally friendly energy-efficient refrigeration 1 , but only a few materials which suffer hysteresis show these giant magnetocaloric effects 2-11 . Here we create giant and reversible extrinsic magnetocaloric effects in epitaxial films of the ferromagnetic manganite La 0.7 Ca 0.3 MnO 3 using strain-mediated feedback from BaTiO 3 substrates near a first-order structural phase transition. Our findings should inspire the discovery of giant magnetocaloric effects in a wide range of magnetic materials, and the parallel development of nanostructured bulk samples for practical applications.2 Magnetocaloric (MC) effects may be parameterized as adiabatic changes of temperature, or isothermal changes of entropy or heat, and have long been used to achieve millikelvin temperatures in the laboratory 12 . More recently, the discovery of giant MC effects near room temperature has led to suggestions for household and industrial cooling applications 1 . However, these giant MC effects arise in only a few materials [2][3][4][5][6][7][8][9][10][11] (Table 1), where strongly coupled magnetic and structural degrees of freedom produce magnetic phase transitions that are accompanied by changes in crystal symmetry 2-10 or volume 11 . It is therefore interesting to explore whether giant MC effects in magnetic materials can be created-rather than merely tuned 16 -via strain. (Table 1). By exploiting a first-order structural phase transition in BaTiO 3 (BTO) substrates, we create giant and reversible MC effects in epitaxial films of LCMO via the entropic interconversion of ferromagnetic and paramagnetic phases, whose coexistence 17,18 we reveal using photoemission electron microscopy (with magnetic contrast from x-ray magnetic circular dichroism) and ferromagnetic resonance.These extrinsic MC effects arise due to a strain-mediated feedback mechanism near the rhombohedral-orthorhombic transition in BTO at ~200 K, i.e. well away from LCMO C T at which the small intrinsic MC effects are seen. 3At temperature T, the isothermal entropy change ) (H S  of a magnetic material due to applied magnetic field H may be obtained via the Maxwell relationprovided that thermally driven changes in measured magnetization M arise due to changes in the magnitude and not the direction of the local magnetization (μ 0 is the permeability of free space, the prime indicates the dummy variable of integration). The Clausius-Clapeyron equation:represents a nominally equivalent indirect method for evaluating S  across first-order phase transitions in terms of the corresponding change in spontaneous magnetization 0 M  and the field-induced shift in transition temperature T 0 . Equations 1 and 2 follow from thermodynamics and do not depend on microscopic details. X-ray diffraction (XRD) of room-temperature LCMO//BTO reveals that the film reflections are weak and broad, and confirms the presence of 90° BTO domains (Fig. 1).The relative population of BTO twins varies between substrates, wi...

Carbon nanotube (CNT) fibers consist of a network of highly oriented carbon nanotube bundles. This paper explores the ingress of liquids into the contiguous internal pores between the bundles using measurements of contact angles and changes in fiber dimensions. The resultant effects on the internal structure of the fiber have been examined by WAXS and SAXS. A series of time-resolved experiments measured the influence of the structural changes on the electrical resistivity of the fiber. All organic liquids tested rapidly wicked into the fiber to fill its internal void structure. The local regions in which the nanotube bundles are aggregated to give a bundle network were broken up by the liquid ingress. For the range of organic penetrants examined, the strength of the effects on structure and electrical resistivity was correlated, not only with the degree to which the liquid reduced the nanotube surface energy, but also with the Hansen affinity parameters. The fact that liquid environments influence the electrical performance of these fibers is of significance if they are to replace copper as power and signal conductors, with added implications regarding the possible ingress of external insulating materials, and possibly also sensing applications.

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We have studied a series of GaN films grown with a range of dislocation densities by atomic force microscopy (AFM), transmission electron microscopy (TEM) and high resolution x-ray diffraction (HRXRD). The (002), (004), (006), (105), (204), (302), (100), (110), (200) and (300) reflections were measured as reciprocal space maps (RSMs) or scans in ω and ω/2θ. The latter 4 in-plane reflections were measured using a low, or glancing, incident angle with respect to the film surface. We have used a variety of different methods to try and obtain reliable measurements for mosaic tilt, twist, crystallite size and microstrain both in- and out-of plane. From (hk0) data in-plane twist angles were measured ranging from 0.37° to 0.078° and in-plane microstrains from 3.5 × 10−4 to 1.8 × 10−4. The improvements in the quality of the GaN layers relate to the increased island coalescence time, which reduces in particular the number of edge-type threading dislocations. The first three samples had a much larger tilt ∼0.09° than the last three ∼0.04°. However, the latter samples were bowed, so results from a single measurement on the (002) peak are too large. Beam restriction on the (002) or an extrapolation from several (00l) reflections gives more reliable results. The values obtained for in-plane crystallite size are in general variable or unreliable. For some samples the sizes are considered to be too large to be accessible by XRD; in most cases the peak broadening is dominated by tilt or twist or microstrain and the results are sensitive to assumptions about the peak shape. For the samples with smaller measurable crystallite sizes, the (hk0) peaks were too weak to measure reliably. The cell parameters showed more compressive strain with fewer dislocations. The trends observed by HRXRD are consistent with AFM and TEM results.