Warren J. MoberlyChan

Catalytic hydride vapour phase epitaxy growth of GaN nanowires.

Catalytic growth of GaN nanowires by hydride vapour phase epitaxy is demonstrated. Nickel-gold was used as a catalyst. Nanowire growth was limited to areas patterned with catalyst. Characterization of the nanowires with transmission electron microscopy, x-ray diffraction, and low temperature photoluminescence shows that the nanowires are stoichiometric 2H-GaN single crystals growing in the [0001] orientation when grown on sapphire, with occasional stacking faults along the c-axis as the only defect type observed in most of the wires. A red shift observed in the photoluminescence was too large to be explained by the minor strain observed alone, and was only marginally affected by temperature, suggesting a superposition of several factors.

Cryo-FIB for Thinning Cryo-TEM samples and Evading Ice During Cryo-Transfer

When a Gatan Alto 2500 cryo stage is attached to an FEI-DualBeam DB235 FIB/SEM, the process of FIBsectioning sample preparation for TEM can be extended to Cryo-FIB-TEM prep [1]. The possibility now exists to use FIB for submicron site-specific sectioning inside biological materials such as cells. The practice remains less productive than cryo-ultramicrotomy; however, using the SEM inside the DB235 enables focusing the prep on nanometer-scale features in the sample. The devil in the details is not the fire (heat of either of the dual beams) but rather the ice (frost) that can occur at each step of the cryo-transfer-process. Until an allencompassing cryo-fast-freeze-prethin-FIB-SEM-TEM single tool is available, cryo-transfer is a necessary evil. To test the cryo-FIB-prep viability we have pushed the limits of cryo-transfer. A cryo sample is plunge-frozen at Albany, cryo-transferred to Harvard and into (and out of) the cryo FIB using a modified specimen holder, then cryo transferred back to Albany for cryo-TEM [2]. After several cryo-transfers from one holder to another, frost is difficult to avoid. However, with a goal to study submicron features, the cryo-DB235 enables one to ignore many ice boulders. FIB ion milling (with 30kV Ga ions) provides etching rates of ice that are >100 times that of Si [3]. Because the high-energy ions can penetrate deep into ice-based materials, care must be taken to avoid ion-induced artifacts [1]. However, the grazing incident ion beam used in this work for thinning limits damage, and FIB can be performed without generating heat and without crystallizing vitreous ice [2].

The Effect of Dopant Additions on the Microstructure of Boron Fibers Before and After Reaction to MgB2

Boron fibers made by a commercial chemical vapor deposition (CVD) process have been used as precursors for the formation of magnesium diboride (MgB2) superconducting wires. Prior to a reaction with magnesium, the addition of dopants such as carbon and titanium to the boron fiber has been shown to enhance the superconducting properties of MgB2. These dopants also influence the kinetics of the reaction with magnesium. In this study, the effect of carbon dopant additions on the microstructure of boron fibers was investigated using powder x-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Additionally, bundles of boron fibers were pressure infiltrated with molten magnesium and reacted at elevated temperatures. The microstructure and microchemistry of the fiber-metal interfaces were investigated by TEM and energy dispersive x-ray analysis (EDS).

The Effect of Excess Carbon on the Crystallographic, Microstructural, and Mechanical Properties of CVD Silicon Carbide Fibers

Silicon carbide (SiC) fibers made by chemical vapor deposition (CVD) are of interest for organic, ceramic, and metal matrix composite materials due their high strength, high elastic modulus, and retention of mechanical properties at elevated processing and operating temperatures. The properties of SCS-6{trademark} silicon carbide fibers, which are made by a commercial process and consist largely of stoichiometric SiC, were compared with an experimental carbon-rich CVD SiC fiber, to which excess carbon was added during the CVD process. The concentration, homogeneity, and distribution of carbon were measured using energy dispersive x-ray spectroscopy (SEM/EDS). The effect of excess carbon on the tensile strength, elastic modulus, and the crystallographic and microstructural properties of CVD silicon carbide fibers was investigated using tensile testing, x-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

Imaging Defects in Nanometer-scale Semiconductor Crystals: Statistical Nucleation Events are Few in Small Crystals, but can Control Growth

Defect control in semiconductor devices is as important for present-day nanometerdiameter fibers and devices as for single crystal silicon. TEM images 3 types of growth defects in small fibers: dislocations, stacking faults, and twins. (Point defects are not easily imaged by TEM. Grain boundaries are rare as the nanometer-scale devices are smaller than even a single grain wants to be.) Near-by free surfaces enable most dislocations to grow out of small-diameter crystals. However, stacking faults and twins may occur in these zincblende/wurtzite/diamond-based crystal structures, with interdependence to the growth orientation. Small crystals have been processed by many methods [1]. Ideal catalysts (neither consumed nor altered during growth) can limit location and diameter of crystal; and CVD may grow semiconductor phases at low temperature without damaging neighboring microdevices.

TEM of epitaxial thin films controlled by planes extending (near) normal to interface; with application to two methods to reduce crystal orientations in polycrystalline magnetic media

Abstract This work is a study of heteroepitaxial interfaces as applied to multilayer-thin films for magnetic information storage media. With a goal to develop a film crystallography that optimizes the alignment of magnetic dipoles to coincide with the write/read signal of the recording head, two TEM observations have elucidated a better understanding of what controls heteroepitaxial interfaces. The classical approach to establishing “lattice matching” of interfaces is to model the top plane of atoms of the substrate and then align the next plane of atoms in the subsequently deposited film, i.e. plane A and plane B should “match” (with minimal misfit), with both planes being parallel to the interface. Such mechanism is valid for idealized slow MBE growth where the planes remain atomically flat. However, most film deposition conditions quickly violate this atomically flat configuration. Here growth on a roughened interface is shown to be controlled by the matching of planes that extend (normal or near-normal) across the interface. A second classical observation is the nucleation of bi-crystals, which naturally increases the number of crystal orientations in subsequent films. However, this work exhibits two cases of reducing orientations! One case has a 3-D isotropically oriented cubic film followed by a hexagonal film with 2-&-1/4-D isotropy, and a second case where a 2-D random cubic film is followed by a hexagonal film with 1-&-1/2-D isotropy. The understanding and control of these heteroepitaxial interfaces enables reduction of film orientations to enhance properties, such as 100 Gigabit per-square-inch magnetic recording.

Characterization of a Ceramic-Metal-Ceramic Bond: Chemical Vapor Deposited (CVD) Silicon Carbide Joined by a Silver-Based Active Brazing Alloy (ABA)

Chemical vapor deposited (CVD) silicon carbide (SiC) ceramic material was joined to itself via an air stable, silver-based active brazing alloy (ABA). The microstructure and microchemistry of the interface was characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Results were compared to previous studies on the active alloy brazing of sintered silicon carbide using higher copper alloys.