Mary E. Vickers

Electron-beam-induced strain within InGaN quantum wells: False indium “cluster” detection in the transmission electron microscope

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.

Thick lead-free ferroelectric films with high Curie temperatures through nanocomposite-induced strain

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.

Determination of the indium content and layer thicknesses in InGaN/GaN quantum wells by x-ray scattering

We have determined the indium content and the layer thicknesses in an InGaN epilayer and InGaN/GaN quantum well structures by high-resolution x-ray diffraction (XRD) using the (002) reflection. The thickness of the total repeat (an InGaN well plus a GaN barrier) in the superlattice is easily determined from the spacing between the satellite peaks in an omega/2theta scan. Measurement of the individual thickness of InGaN and GaN layers and the indium content is less straightforward, since for multilayer structures the peak positions are influenced by both the indium content and the thickness ratio of the GaN to the InGaN layer. Thus, several different models may give reasonable fits to data collected over a limited range (about 1° omega/2theta either side of the (002)) showing only lower-order (−3 to +3) satellite peaks. Whenever possible, we have collected data over a wide range (about 4° omega/2theta) and determined the thickness ratio by examination of the relative intensities of weak higher-order satell...

In-plane imperfections in GaN studied by x-ray diffraction

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.

Composite Cu/Fe/MgB2 superconducting wires and MgB2/YSZ/Hastelloy coated conductors for ac and dc applications

We discuss the results of a study of MgB2 multifilamentary conductors and coated conductors from the point of view of their future dc and ac applications. The correlation between the slope of the irreversibility line induced by neutron irradiation defects and in situ structural imperfections and the critical temperature and critical current density is discussed with respect to the conductor performance and applicability. We debate the possible origin of the observed anomalous decrease of ac susceptibility at 50 K in copper clad in situ powder-in-tube MgB2 wires. Different conductor preparation methods and conductor architectures, and attainable critical current densities are presented. Some numerical results on critical currents, thermal stability and ac losses of future MgB2 multifilamentary and coated conductors with magnetic cladding of their filaments are also discussed.

Perpendicular Local Magnetization Under Voltage Control in Ni Films on Ferroelectric BaTiO3 Substrates

High-resolution magnetoelectric imaging is used to demonstrate electrical control of the perpendicular local magnetization associated with 125 nm-wide magnetic stripe domains in 100-nm-thick Ni films. This magnetoelectric coupling is achieved in zero magnetic field using strain from ferroelectric BaTiO3 substrates to control perpendicular anisotropy imposed by the growth stress. These findings may be exploited for perpendicular recording in nanopatterned hybrid media.

Nanopillar Spin Filter Tunnel Junctions with Manganite Barriers

The potential of a manganite ferromagnetic insulator in the field of spin-filtering has been demonstrated. For this, an ultrathin film of Sm0.75Sr0.25MnO3 is integrated as a barrier in an epitaxial oxide nanopillar tunnel junction and a high spin polarization of up to 75% at 5 K has been achieved. A large zero-bias anomaly observed in the dynamic conductance at low temperatures is explained in terms of the Kondo scattering model. In addition, a decrease in spin polarization at low bias and hysteretic magneto-resistance at low temperatures are reported. The results open up new possibilities for spin-electronics and suggest exploration of other manganites-based materials for the room temperature spin-filter applications.

Determination of the composition and thickness of semi-polar and non-polar III-nitride films and quantum wells using X-ray scattering

There is increasing interest in III-nitride films and multiple quantum well structures grown in non-polar or semi-polar orientations for application in light-emitting devices. We describe a method to obtain the compositions and the thicknesses of layers within III-nitride quantum well or superlattice structures grown in non-polar or semi-polar orientations, based on X-ray scattering. For each new crystallographic orientation considered, new axes were obtained and both the lengths and angles of these new axes calculated relative to the original conventional reference axes. These angles provide the coefficients of the matrix to transform the elastic constants published in the conventional setting (as used for polar c-plane oriented III-nitrides) into the appropriate new values. The new characteristic lengths and new elastic constants are then put into the general equation that relates the composition of a fully strained layer to the experimentally measured out-of-plane alloy d-spacing. Thus we have (a) dete...

The effect of wafer curvature on x-ray rocking curves from gallium nitride films

The full width at half maximum (FWHM) of x-ray rocking curves is often used as a measure of the crystalline quality of thin films. In this paper, the effects of wafer curvature on the x-ray rocking curves (ω-scans) obtained from a 2in. 0001-oriented sapphire wafer and an epitaxial 0001-oriented GaN film grown on such a wafer are discussed. Beam height reduction can limit curvature-related effects for symmetric and asymmetric reflections; these effects are generally significant for low dislocation density GaN films. Due to the shape of the area illuminated by the x-ray beam in the skew symmetric geometry, beam width reduction is instead appropriate, but this still gives an underestimate of ω-FWHM values obtained with an open detector and an overestimate of ω-FWHM values obtained with an analyzer. Therefore, skew symmetric ω-FWHM values from curved samples are often unreliable. Additionally, Williamson–Hall analysis using ω-scans is rendered unreliable in the presence of significant curvature. Variations in...

GaN-InGaN Quantum Well and LED Structures Grown in a Close Coupled Showerhead (CCS) MOCVD Reactor

A Close Coupled Showerhead MOCVD reactor has been used to grow GaN-InGaN quantum well (QW) structures and LEDs using two different growth regimes, one in which the wells are grown at a lower temperature than the barriers and the other in which both are grown at the same temperature. In general the optical quality of single temperature multi-quantum wells (MQWs) are superior to that of structures grown at two temperatures but the latter have been proved easier to optimise for operation at longer wavelengths. Improved emission wavelength uniformity has been noted for the single temperature structures and XRD and low temperature PL measurements of the latter have indicated that high quality structures have been achieved. Time resolved PL has shown that the radiative carrier lifetime increases with the indium content of the quantum wells making longer wavelength structures more susceptible to competition from non-radiative recombination.