Helen M. Chan

Metalorganic Vapor Phase Epitaxy of III-Nitride Light-Emitting Diodes on Nanopatterned AGOG Sapphire Substrate by Abbreviated Growth Mode

Metalorganic vapor phase epitaxial (MOVPE) growth of GaN on nanopatterned AGOG sapphire substrates was performed, and characteristics of the light-emitting diode (LED) devices grown on patterned sapphire and planar substrates were compared. The nanopatterned sapphire substrates were fabricated by a novel process (AGOG) whereby aluminum nanomesas were epitaxially converted into crystalline Al2O3 via a two-stage annealing process. The GaN template grown on the nanopatterned sapphire substrate was done via an abbreviated growth mode, where a 15-nm thick, low-temperature GaN buffer layer was used, without the use of an etch-back and recovery process during the epitaxy. InGaN quantum wells (QWs) LEDs were grown on the GaN template on the nanopatterned sapphire, employing the abbreviated growth mode. The optimized InGaN QW LEDs grown on the patterned AGOG sapphire substrate exhibited a 24% improvement in output power as compared to LEDs on GaN templates grown using the conventional method. The increase in output power of the LEDs is attributed to improved internal quantum efficiency of the LEDs.

Role of segregating dopants on the improved creep resistance of aluminum oxide

Recent studies have demonstrated that p.p.m. levels of rare-earth dopant ions (e.g. Y, La, Nd) wield a beneficial and highly potent influence on the creep properties of alumina. In addition, codoping with ions of disparate sizes (Nd, Zr) resulted in even further enhancement of the creep behavior. In all cases, the dopant ions were found to strongly segregate to grain boundaries. Creep rates were not influenced by the presence of second phase precipitates, verifying that the creep improvement is a solid solution effect. In an attempt to clarify the exact mechanism(s) that controls creep behavior of the doped aluminas, various advanced characterization techniques have been applied including: secondary ion mass spectrometry, scanning transmission electron microscopy, orientation image microscopy, and extended X-ray absorption fine structure as well as atomistic computer simulation and studies of the creep kinetics. Although no definitive mechanism has been established, a logical explanation is that outsize ions segregate to more energetically favorable grain boundary sites, and improve creep resistance by blocking a few critical diffusive pathways. This mechanism is sufficiently general that it may be applicable to other ceramic systems.

DAMAGE-RESISTANT ALUMINA-BASED LAYER COMPOSITES

A new philosophy for tailoring layer composites for damage resistance is developed, specifically for alumina-based ceramics. The underlying key to the approach is microstructural control in the adjacent layers, alternating a traditional homogeneous fine-grain alumina (layer A ) for hardness and wear resistance with a heterogeneous alumina : calcium-hexaluminate composite (layer C ) for toughness and crack dispersion, with strong bonding between the interlayers. Two trilayer sequences, ACA and CAC , are investigated. Hertzian indentation tests are used to demonstrate the capacity of the trilayers to absorb damage. In the constituent materials, the indentation responses are fundamentally different: ideally brittle in material A , with classical cone cracking outside the contact; quasi-plastic in material C , with distributed microdamage beneath the contact. In the ACA laminates, shallow cone cracks form in the outer A layer, together with a partial microdamage zone in the inner C layer. A feature of the cone cracking is that it is substantially shallower than in the bulk A specimens and does not penetrate to the underlayer, even when the applied load is increased. This indicates that the subsurface microdamage absorbs significant energy from the applied loads, and thereby “shields” the surface cone crack. Comparative tests on CAC laminates show a constrained microdamage zone in the outer C layer, with no cone crack, again indicating some kind of shielding. Importantly, interlayer delamination plays no role in either layer configuration; the mechanism of damage control is by crack suppression rather than by deflection. Implications for the design of synergistic microstructures for damage-resistant laminates are considered.

Control of microchemical ordering in relaxor ferroelectrics and related compounds

Abstract A review is given of some of the ways to control microchemical domains (due to ordering on the B-sites) in relaxor ferroelectrics and related compounds. Depending on the system, ordering can he controlled by heat treatment, chemical composition and stoichiometry. Specific examples are given of both thermally- and chemically-induced ordering in relaxor ferroelectric and perovskite-related materials.

Control of calcium hexaluminate grain morphology in in-situ toughened ceramic composites

The influence of processing conditions on the morphology of calcium hexaluminate (CA6) grains in Al2O3: 30 vol% CaO·6Al2O3 (CA6) ceramic composites was investigated. Specimens were prepared by in-situ reaction sintering using precursor powders of alumina, and either calcium carbonate or calcium oxide. In some samples, 1 vol% anorthite glass was added as a sintering aid. X-ray diffraction was used to study the phase development in the as-calcined and sintered states. The resultant microstructures were characterized using both scanning electron microscopy (SEM), and imaging secondary ion mass spectrometry (SIMS). It was found that the CA6 grains developed a platelike morphology when CaCO3 was used as the starting calcium-rich powder. In contrast, samples prepared using CaO resulted in equiaxed CA6 grains. This result was observed to be independent of the anorthite glass addition. The findings are rationalized in terms of distinct CA6 reaction mechanisms, resulting from differences in the reactivity of the powders during the early stages of calcining.

Structural features of Y-saturated and supersaturated grain boundaries in alumina

Abstract Grain boundary segregation of Y in α -Al 2 O 3 and evolution of the structural environment around the Y atoms have been investigated using high resolution STEM and EXAFS. The stages of incorporation of Y atoms by α -Al 2 O 3 grain boundaries, on average, are characterized by three composition regimes: (I) dilute to saturated; (II) supersaturated [where the degree of supersaturation is determined by the nucleation barrier for Y 3 Al 5 O 12 (YAG)]; and (III) equilibrium with YAG precipitates. The average Y grain boundary concentration in equilibrium with YAG precipitates has been determined to be ∼1/4 equivalent monolayer, and the maximum supersaturation concentration has been determined to be ∼1/2 equivalent monolayer. EXAFS revealed that accompanying the supersaturation of grain boundaries with Y is an increasing Y–O nearest neighbor coordination number and, simultaneously, a significantly increased degree of ordering of Y with respect to Al ions beyond nearest neighbor O. This Y–Al distance is the same as that for Y absorbed on the free surface of α -Al 2 O 3 , and the same as that expected for the Y–Al distance when Y substitutes for Al with the Y–O distance relaxed to that in Y 2 O 3 . This compositional and structural information has led to a clearer picture of how the grain boundary segregated Y concentration influences grain boundary structure. For dilute Y concentrations, Y ions preferentially fill sites in the grain boundary core which have well defined order only within the nearest neighbor shell of oxygens. As the Y concentration increases, Y begins to occupy near-boundary sites, forming two near-boundary layers, each adjacent to a grain surface. The near-boundary layer has nearest neighbor ordering extending at least to nearest neighbor cations. Nucleation of the YAG phase leads to the depletion of Y from these partially ordered layers.

Control of texture in monolithic alumina

Abstract Conventional ceramic powder slips seeded with α-Al2O3 platelets were rheologically processed to align the seed crystals. An aqueous-based gel reaction was used to ensure that alignment was retained in the green body and throughout the drying procedure. Bulk samples were produced by laying-up the cast tapes, filter pressing to form a green compact and then drying and sintering. The degree of preferred orientation in the sintered samples was characterized by X-ray diffraction. It was found that the degree of texture was significantly enhanced relative to samples processed in the same manner, but with no platelet additions. A scanning electron microscopic study of the final microstructures showed that the grain size and grain morphology were sensitive to the initial distribution of platelets within the green tape, which in turn was dependent on the volume fraction and size of the platelets. Further, it was demonstrated that using this technique it is possible to achieve a specimen which exhibits an equi-axed grain structure and yet has a strong degree of preferred orientation. Finally, studies of indentation cracking showed that a pronounced texture inhibited long-range crack propagation, and instead promoted localized cracking.

Heteroepitaxial growth of bulk single-crystalPb(Mg 1/3 Nb 2/3 )O 3 –32 mol% PbTiO 3 from (111) SrTiO 3

SrTiO3 was investigated as an alternate seed material to grow Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT) ferroelectric single crystals by seeded polycrystal conversion. Fully dense polycrystalline samples of PMN–32 mol% PT doped with 3 vol% excess PbO were top-seeded with (111) SrTiO3 substrates. Annealing for 10 h at 1150 °C resulted in growth of PMN–32PT single crystals with sizes on the order of several millimeters. Orientation imaging microscopy confirmed that the grown crystal exhibited the same crystallographic orientation as that of the SrTiO3 seed. Elemental distributions analyzed using energy dispersive spectroscopy indicated that interdiffusi on of the relevant elements was negligible.

Dislocation structure of GaN films grown on planar and nano-patterned sapphire

Plane view and cross-section transmission electron microscopy (TEM) images were used to compare the density, character, and curvature of dislocations developed during metalorganic vapor phase epitaxy (MOVPE) of GaN on planar c-plane sapphire with those developed during growth on nano-patterned c-plane sapphire. Scanning electron microscopy (SEM) characterization of GaN films at different stages of growth for both types of substrates complemented the TEM investigation. GaN growth on wafers patterned with an array of submicron sapphire bumps exhibited relatively uniform nucleation and initial growth, as well as early island coalescence. It is suggested that this coalescence results in a relatively small fraction of dislocations with partial screw character at the surface of the films grown on the patterned substrate, and that this may be responsible for the improvements in carrier lifetime and device efficiency seen in earlier studies on similar sapphire substrates.

Thermal stability of Cu nanowires on a sapphire substrate

Cu nanowires with widths ranging from 110 to 300 nm were fabricated on a c-plane sapphire substrate using E-beam lithography and lift-off processes. Thermal annealing of these polycrystalline metal nanowires at 700 °C in an inert (nitrogen) atmosphere showed that for lines of width 160 nm or less, there was complete breakdown into widely spaced, individual beads in a short time (1 h). It was shown that the morphological changes were driven by reduction in the surface energy, with surface diffusion as the predominant transport mechanism. The spacing between the beads was approximately 1.8 times greater than the values predicted by Rayleigh instability theory for a free standing rod with equivalent radius. Based on thermodynamic and kinetic considerations, discrepancies between the experimental observations and the predictions of Rayleigh instability theory were attributed to the stabilization effect of the substrate.