Patrick G. Callahan

Electrically pumped continuous wave quantum dot lasers epitaxially grown on patterned, on-axis (001) Si

High performance III-V lasers at datacom and telecom wavelengths on on-axis (001) Si are needed for scalable datacenter interconnect technologies. We demonstrate electrically injected quantum dot lasers grown on on-axis (001) Si patterned with {111} v-grooves lying in the [110] direction. No additional Ge buffers or substrate miscut was used. The active region consists of five InAs/InGaAs dot-in-a-well layers. We achieve continuous wave lasing with thresholds as low as 36 mA and operation up to 80°C.

Low threading dislocation density GaAs growth on on-axis GaP/Si (001)

We report a systematic study of high quality GaAs growths on on-axis (001) GaP/Si substrates using molecular beam epitaxy. Various types of dislocation filter layers and growth temperatures of initial GaAs layer were investigated to reduce the threading dislocation densities in GaAs on GaP/Si. Electron channeling contrast imaging techniques revealed that an optimized GaAs buffer layer with thermal cycle annealing and InGaAs/GaAs dislocation filter layers has a threading dislocation density of 7.2 × 106 cm−2, which is a factor of 40 lower than an unoptimized GaAs buffer. The root-mean-square surface roughness was greatly decreased from 7.8 nm to 2.9 nm after the optimization process. A strong enhancement in photoluminescence intensity indicates that the optimized GaAs template grown on on-axis (001) GaP/Si substrates is a promising virtual substrate for Si-based optoelectronic devices.

Transmission scanning electron microscopy: Defect observations and image simulations

The new capabilities of a FEG scanning electron microscope (SEM) equipped with a scanning transmission electron microscopy (STEM) detector for defect characterization have been studied in parallel with transmission electron microscopy (TEM) imaging. Stacking faults and dislocations have been characterized in strontium titanate, a polycrystalline nickel-base superalloy and a single crystal cobalt-base material. Imaging modes that are similar to conventional TEM (CTEM) bright field (BF) and dark field (DF) and STEM are explored, and some of the differences due to the different accelerating voltages highlighted. Defect images have been simulated for the transmission scanning electron microscopy (TSEM) configuration using a scattering matrix formulation, and diffraction contrast in the SEM is discussed in comparison to TEM. Interference effects associated with conventional TEM, such as thickness fringes and bending contours are significantly reduced in TSEM by using a convergent probe, similar to a STEM imaging modality, enabling individual defects to be imaged clearly even in high dislocation density regions. Beyond this, TSEM provides significant advantages for high throughput and dynamic in-situ characterization.

Microstructural statistics for fatigue crack initiation in polycrystalline nickel-base superalloys

In advanced engineering alloys where inclusions and pores are minimized during processing, the initiation of cracks due to cyclic loading shifts to intrinsic microstructural features. Criteria for the identification of crack initiation sites, defined using elastic-plastic loading parameters and twin boundary length, have been developed and applied to experimental datasets following cyclic loading. The criteria successfully quantify the incidence of experimentally observed cracks. Statistical microstructural volume elements are defined using a convergence approach for two nickel-base superalloys, IN100 and René 88DT. The material element that captures the fatigue crack-initiating features in René 88DT is smaller than IN100 due to a combination of smaller grain size and higher twin density.

Three-dimensional texture visualization approaches: theoretical analysis and examples

Crystallographic textures are commonly represented in terms of Euler angle triplets and contour plots of planar sections through Euler space. In this paper, the basic theory is provided for the creation of alternative orientation representations using three-dimensional visualizations. The use of homochoric, cubochoric, Rodrigues and stereographic orientation representations is discussed, and illustrations are provided of fundamental zones for all rotational point-group symmetries. A connection is made to the more traditional Euler space representations. An extensive set of three-dimensional visualizations in both standard and anaglyph movies is available.

Recombination activity of threading dislocations in GaInP influenced by growth temperature

Room-temperature non-radiative recombination is studied at single dislocations in Ga0.5In0.5P quantum wells grown on metamorphic templates using cathodoluminescence and electron channeling contrast imaging. An analysis of the light emission intensity profiles around single dislocations reveals that the average recombination strength of a dislocation decreases by a factor of four and seven as a result of decreasing growth temperature of the GaInP quantum well from 725 to 675 and 625 °C, respectively. This reduction occurs despite little change in the diffusion length, precluding the prospect of inducing carrier localization by ordering and phase separation in GaInP at lower growth temperatures. These observations are rationalized by the premise that point defects or impurities are largely responsible for the recombination activity of dislocations, and the extent of decoration of the dislocation core decreases with temperature. Preliminary evidence for the impact of the Burgers vector is also presented. The...

EBSD Surface Topography Determination in a Martensitic Au-Cu-Zn Alloy

Au30Cu25Zn45 is a shape memory alloy with a composition that has been optimized to achieve a low hysteresis martensitic transformation, by increasing the degree of compatibility between the cubic austenite and the monoclinic martensite lattices [1]. This degree of lattice match has profound consequences for the multi-variant microstructure in that there is now no elastic energy penalty associated with the presence of variant-variant boundaries; this, in turn, can give rise to interesting curved martensite-austenite boundaries as well as the fact that the microstructure becomes completely irreproducible from one thermal cycle to the next. In our work, we attempt to verify a mathematical model [1] for the variant-variant geometries that occur during the transformation in Au30Cu25Zn45; in particular, we are interested in determining the complete transformation strain for each variant, including the out-of-plane component at the sample surface. Such a topographic measurement is made difficult because (a) the transformation temperature of the material is 50 C and (b) as stated above, every time the sample is transformed the martensite variant arrangement is completely different. Since crystallographic data needs to be collected along with the topography data to determine strain states and thus confirm the model, the topographic data needs to be collected simultaenously with the EBSD data. A novel technique that uses the background intensity of the EBSD patterns was used to achieve this result.

Reconstruction of Laser-Induced Surface Topography from Electron Backscatter Diffraction Patterns

We demonstrate that the surface topography of a sample can be reconstructed from electron backscatter diffraction (EBSD) patterns collected with a commercial EBSD system. This technique combines the location of the maximum background intensity with a correction from Monte Carlo simulations to determine the local surface normals at each point in an EBSD scan. A surface height map is then reconstructed from the local surface normals. In this study, a Ni sample was machined with a femtosecond laser, which causes the formation of a laser-induced periodic surface structure (LIPSS). The topography of the LIPSS was analyzed using atomic force microscopy (AFM) and reconstructions from EBSD patterns collected at 5 and 20 kV. The LIPSS consisted of a combination of low frequency waviness due to curtaining and high frequency ridges. The morphology of the reconstructed low frequency waviness and high frequency ridges matched the AFM data. The reconstruction technique does not require any modification to existing EBSD systems and so can be particularly useful for measuring topography and its evolution during in situ experiments.

Determination of Surface Topography from Laser Ablation using EBSD

In the last decade, serial sectioning with dual beam Focused Ion Beam-Scanning Electron Microscopes (FIB-SEM) has been used to successfully gather 3-D information from a variety of materials. The sizes of the 3-D volumes interrogated with this technique are limited by the milling rate of the Ga ion beams typically used. In order to collect larger volumes of data, a novel TriBeam system that combines ultrafast lasers with scanning electron microscopes has been developed [1]. The use of femtosecond laser ablation for machining allows for rapid machining of volumes of material approaching the mm scale. Machining with lasers results in a periodic surface roughness called the Laser-Induced Periodic Surface Structure (LIPSS); this surface roughness has a wavelength on the order of the wavelength of the machining laser [2]. A nickel sample was machined by laser ablation in the TriBeam system, and the resulting surface was scanned using electron backscatter diffraction (EBSD).