M.J. Mills

Ion‐assisted pulsed laser deposition of cubic boron nitride films

Ion‐assisted pulsed laser deposition has been used to produce films containing ≳85% sp3‐bonded cubic boron nitride (c‐BN). By ablating from a target of hexagonal boron nitride (h‐BN), BN films have been deposited on heated (50–800 °C) Si(100) surfaces. The growing films are irradiated with ions from a broad beam ion source operated with Ar and N2 source gasses. Successful c‐BN synthesis has been confirmed by Fourier transform infrared (FTIR) spectroscopy, high‐resolution transmission electron microscopy (TEM), selected‐area electron diffraction, electron energy‐loss spectroscopy, and x‐ray diffraction. The films are polycrystalline and show grain sizes up to 300 A. In addition, Rutherford backscattering, elastic recoil detection, and Auger electron spectroscopies have been used to further characterize the samples. The effects of varying ion current density, substrate growth temperature, growth time, and ion energy have been investigated. It is found that stoichiometric films with a high c‐BN percentage ca...

Room temperature deformation and mechanisms of slip transmission in oriented single-colony crystals of an α/β titanium alloy

Abstract Primary creep at low homologous temperatures and low stresses has been widely reported in α/β Ti alloys. Creep in these alloys is dependent on microstructure, with the colony microstructure showing the least creep resistance. There exists a Burgers orientation relationship between the α and the β phase, which has been assumed to allow for easy slip transmission across the α/β interfaces. Constant strain rate and creep tests were performed on single-colony crystals of a near-α alloy oriented for slip along different prismatic slip systems in the α phase. A distinct anisotropy in the deformation behavior of different colony crystals is found. The origin of this anisotropy is due to the relative misalignment of the slip systems between the α and the β phases which results in the formation of residual dislocations during slip transmission. A simple model is presented based on the accumulation of residual dislocations at both the α/β interface and the α matrix, which provides insight into the mechanism of slip transmission, strain hardening and primary creep of these colony structures.

Deformation and creep modeling in polycrystalline Ti-6Al alloys

Abstract This paper develops an experimentally validated computational model for titanium alloys accounting for plastic anisotropy and time-dependent plasticity for analyzing creep and dwell phenomena. A time-dependent crystal plasticity formulation is developed for hcp crystalline structure, with the inclusion of microstructural crystallographic orientation distribution. A multi-variable optimization method is developed to calibrate crystal plasticity parameters from experimental results of single crystals of α-Ti–6Al. Statistically equivalent orientation distributions of orientation imaging microscopy data are used in constructing the polycrystalline aggregate model. The model is used to study global and local response of the polycrystalline model for constant strain rate, creep, dwell and cyclic tests. Effects of stress localization and load shedding with orientation mismatch are also studied for potential crack initiation.

Phenomenological and microstructural analysis of room temperature creep in titanium alloys

Primary creep is the dominant mode of deformation during creep of titanium alloys at room temperature. Based on a study of both Ti-6Al and Ti-6Al-2Sn-4Zr-2Mo, it is shown that the transient creep behavior can be described by a power law of the form {var_epsilon} = At{sup a}, while the strain-rate-sensitive Hollomon law, {sigma} = K{var_epsilon}{sup n}{dot {var_epsilon}}{sup m}, represents the constant strain rate behavior of titanium alloys reasonably well. A simple analytical result is derived to relate these two expressions. Using this solution, the long time creep response has been predicted reasonably well from the constant strain rate results for the two alloys studied. Relative to other metals, it is shown that titanium alloys exhibit exceptionally low values of strain hardening. Optical microscope observations of slip line evolution have been used to relate the deformation mechanisms to the macroscopic behavior. Operative slip systems, as well as dislocation distributions and morphologies, are also presented for the first time following creep of a single-phase {alpha} microstructure in Ti-6Al.

Measurements of antiphase boundary and complex stacking fault energies in binary and B-doped Ni3Al using TEM

Abstract The fourfold dissociation of superdislocations in Ni3Al has been recorded on images that were formed using second-order reflections, and the dissociation distances of the Shockley partial dislocations that bound a complex stacking fault (d CSF) have been measured. Comparisons with computer simulated (2g · 5g) images highlight the presence of supplementary intensity peaks when d CSF is less than 3.0 nm. These simulations also indicate that the distance between the weaker pair of intensity peaks can be used to measure d CSF if the experimental observations are corrected for the image shift that occurs during the microscopy. Experimental observations confirm the presence of the supplementary peaks, and measured values of d CSF were found to be larger in binary Ni3Al than in the boron-modified alloy. These measured values have been corrected through comparison to image simulations, and the corresponding complex stacking fault and antiphase boundary energies have been calculated using anisotropic elas...

Control and elimination of nucleation-related defects in GaP/Si(001) heteroepitaxy

GaP films were grown on offcut Si(001) substrates using migration enhanced epitaxy nucleation followed by molecular beam epitaxy, with the intent of controlling and eliminating the formation of heterovalent (III-V/IV) nucleation-related defects—antiphase domains, stacking faults, and microtwins. Analysis of these films via reflection high-energy electron diffraction, atomic force microscopy, and both cross-sectional and plan-view transmission electron microscopies indicate high-quality GaP layers on Si that portend a virtual GaP substrate technology, in which the aforementioned extended defects are simultaneously eliminated. The only prevalent remaining defects are the expected misfit dislocations due to the GaP–Si lattice mismatch.

Microstructure of cubic boron nitride thin films grown by ion‐assisted pulsed laser deposition

A microstructural study of boron nitride films grown by ion‐assisted pulsed laser deposition is presented. Fourier transform infrared spectroscopy, electron‐energy‐loss spectroscopy, and electron‐diffraction measurements indicate that within the ion‐irradiated region on the substrate, the film consists of a high fraction of the cubic phase (cBN) with a small amount of the turbostratic phase; outside the irradiated region, only the turbostratic phase is detected. Conventional and high‐resolution electron microscopic observations show that the cBN is in the form of twinned crystallites, up to 40 nm in diameter. Particulates, formed by the laser ablation process, reduce the yield of cBN in the irradiated regions by shadowing local areas from the ion beam. The films exhibit a layered structure with an approximately 30‐nm‐thick layer of oriented turbostratic material forming initially at the silicon substrate followed by the cBN. The observations of oriented turbostratic material and twinned cBN crystallites a...

Deformation mechanisms and microtensile behavior of single colony Ti-6242Si

The constant strain rate deformation behavior of individual a ‐b colonies of the titanium aeroengine alloy Ti‐6Al‐2Sn‐4Zr‐ 2Mo‐0.1Si (compositions in wt.%) has been investigated through microsample tensile testing techniques. The mechanical behavior and deformation mechanisms observed were found to depend strongly on colony orientation. A near-Burger’s orientation relationship (OR) has been observed for the a (hcp) and b (bcc) phases in these colonies, grown utilizing a vertical float zone technique. The Burger’s OR is believed to be responsible for the observed anisotropy in the critical resolved shear stress for the three a:3e11-20e{10-10} prism slip systems. A more pronounced anisotropy was observed in the critical resolved shear stress (CRSS) for the three a:3e11-20e{0001} hcp basal slip systems. Scanning Electron Microscopy (SEM) investigations of the slip line morphology of the deformed samples indicates an inhibition of b-lath shearing for colonies oriented for basal slip, while frequent shear offsets of the b phase by slip lines were observed in the samples oriented for prism slip. The effects of the Burger’s OR, slip line spacing and elastic interactions stresses arising from the a ‐b interface are considered in explaining the observed anisotropy in CRSS.

Polarization-Induced pn Diodes in Wide-Band-Gap Nanowires with Ultraviolet Electroluminescence

Almost all electronic devices utilize a pn junction formed by random doping of donor and acceptor impurity atoms. We developed a fundamentally new type of pn junction not formed by impurity-doping, but rather by grading the composition of a semiconductor nanowire resulting in alternating p and n conducting regions due to polarization charge. By linearly grading AlGaN nanowires from 0% to 100% and back to 0% Al, we show the formation of a polarization-induced pn junction even in the absence of any impurity doping. Since electrons and holes are injected from AlN barriers into quantum disk active regions, graded nanowires allow deep ultraviolet LEDs across the AlGaN band-gap range with electroluminescence observed from 3.4 to 5 eV. Polarization-induced p-type conductivity in nanowires is shown to be possible even without supplemental acceptor doping, demonstrating the advantage of polarization engineering in nanowires compared with planar films and providing a strategy for improving conductivity in wide-band-gap semiconductors. As polarization charge is uniform within each unit cell, polarization-induced conductivity without impurity doping provides a solution to the problem of conductivity uniformity in nanowires and nanoelectronics and opens a new field of polarization engineering in nanostructures that may be applied to other polar semiconductors.

Characterization of Metamorphic GaAsP/Si Materials and Devices for Photovoltaic Applications

GaAsyP1-y anion-sublattice compositionally graded buffers and device structures were grown directly on Si(100) substrates by way of a high-quality GaP integration layer, yielding GaAsP target layers having band gaps of photovoltaic interest (1.65-1.8 eV), free of antiphase domains/borders, stacking faults, and microtwins. GaAsyP1-y growths on both Si and GaP substrates were compared via high-resolution X-ray diffractometry of the metamorphic buffers and deep-level transient spectroscopy (DLTS) of p+-n diodes that are lattice matched to the final buffer layer. Structural analysis indicates highly efficient epitaxial relaxation throughout the entire growth structure for both types of samples and suggests no significant difference in physical behavior between the two types of samples. DLTS measurements performed on GaAsP diodes fabricated on both Si and GaP substrates reveal the existence of identical sets of traps residing in the n-type GaAsP layers in both types of samples: a single majority carrier (electron) trap, which is located at EC - 0.18 eV, and a single minority carrier (hole) trap, which is located at EV + 0.71 eV. Prototype 1.75-eV GaAsP solar cell test devices grown on GaAsyP1-y/Si buffers show good preliminary performance characteristics and offer great promise for future high-efficiency III-V photovoltaics integrated with Si substrates and devices.