William M. Vetter

Direct evidence of micropipe-related pure superscrew dislocations in SiC

A set of powerful x-ray imaging techniques using white-beam synchrotron radiation have been developed and applied to clearly reveal and map micropipes in SiC crystals at a “magnified” level. The experimental results and the corresponding simulations demonstrate explicitly that the micropipes are pure superscrew dislocations (SSDs). Moreover, these techniques provide accurate descriptions of the detailed structure of the SSDs, including the spatial distribution of the strain fields, the magnitudes of the Burgers vectors, the dislocation senses, and the surface relaxation effects.

Recombination-enhanced defect motion in forward-biased 4H–SiC p-n diodes

The generation and evolution of defects in 4H–SiC p-n junctions due to carrier injection under forward bias have been investigated by synchrotron white beam x-ray topography, electroluminescence imaging, and KOH etching. The defects are Shockley stacking faults with rhombic or triangular shapes bound by partial dislocation loops with dislocation lines along Peierls valleys (〈11-20〉) or along the intersection of the basal plane containing the fault and diode surface. The Burgers vector of all bounding partials was of 1/3〈10-10〉-type. Among six possible types of partial dislocations with these properties, only two were observed in the volume of the epitaxial structure. One was tentatively identified as 30° carbon-core [C(g) 30°] and second as 30° silicon-core [Si(g) 30°] partial dislocation. Only one of them [proposed to be the Si(g) 30° partial] have been observed to move and emit light under forward bias. The other type of bounding dislocation [C(g) 30°] remained stationary during current injection. Low a...

Superscrew dislocation contrast on synchrotron white-beam topographs: an accurate description of the direct dislocation image

A kinematic (geometrical) diffraction simulation model has been developed to provide understanding of direct dislocation images on synchrotron white-beam X-ray topographs, and has been successfully applied to illustrate the contrast formation mechanisms involved in images of micropipe-related superscrew dislocations in silicon carbide crystals. The coincidence of the simulations with the contrast features of the superscrew dislocation images, recorded using a series of synchrotron topography techniques, shows that this model is capable of revealing the detailed diffraction behavior of the highly distorted region around the dislocation core and determining the quantitative characteristics of the dislocations. The simulation technique is thus demonstrated to be a simple but efficient method for interpretation of synchrotron topographs, and may be applied to explain the topographic contrast characters of general crystal defects.

Contribution of x-ray topography and high-resolution diffraction to the study of defects in SiC

A short review is presented of the various synchrotron white beam x-ray topography (SWBXT) imaging techniques developed for characterization of silicon carbide (SiC) crystals and thin films. These techniques, including back-reflection topography, reticulography, transmission topography, and a set of section topography techniques, are demonstrated to be particularly powerful for imaging hollow-core screw dislocations (micropipes) and closed-core threading screw dislocations, as well as other defects, in SiC. The geometrical diffraction mechanism commonly underlying these imaging processes is emphasized for understanding the nature and origins of these defects. Also introduced is the application of SWBXT combined with high-resolution x-ray diffraction techniques to complete characterization of 3C/4H or 3C/6H SiC heterostructures, including polytype identification, 3C variant mapping, and accurate lattice mismatch measurements.

Hexagonal voids and the formation of micropipes during SiC sublimation growth

Hexagonal voids observed in sublimation grown SiC boules were examined using optical microscopy, atomic force microscopy (AFM), scanning electron microscopy, KOH etching, and synchrotron white-beam x-ray topography. Voids formed at imperfections in the attachment layer between the seed and crucible cap. They are platelet-like in shape with lateral sizes between 50 and 750 μm and thickness along the c axis between 5 and 25 μm. Growth steps were observed on the void facets closest to the seed and evaporation steps were observed on void facets closest to the growth surface, providing evidence for void movement during crystal growth. AFM images revealed that growth steps nucleate at a void sidewall, flow across the bottom of the void, and terminate in a trench-like depression. KOH etching of waters between the void and seed revealed dislocations lining up along the trace of the void path, often with higher densities corresponding to the location of the trench. X-ray topographs showed a random distribution of ...

Synchrotron white beam topography characterization of physical vapor transport grown AlN and ammonothermal GaN

Structural defects in AlN single crystals grown by the sublimation method and GaN single crystals grown by the ammonothermal method are characterized by synchrotron white-beam X-ray topography in conjunction with optical microscopy. AlN platelets are either of (1120) or (0001) type depending on the growth conditions. Dislocation densities of the order of 10 3 cm -2 or lower are observed in some crystals. X-ray topographs reveal the presence of growth sector boundaries, inclusions, and growth dislocations that indicate slight impurity contamination. The 2H crystal structure of GaN single crystals obtained by the ammonothermal method was verified by Laue X-ray pattern analysis. GaN crystals grown are of the order of 1 mm in size and are either (0001) platelets or [0001] prismatic needles. Generally, prismatic needles are characterized by lower degree of mosaicity than (0001) platelets.

Nucleation of threading dislocations in sublimation grown silicon carbide

The structural defects in sublimation-grown silicon carbide layers have been investigated by transmission electron microscopy, atomic force microscopy, x-ray topography, and KOH etching. Nucleation of two-dimensional islands on damage free surfaces of high quality Lely seeds led to formation of stacking faults at the initial stages of growth. The location and number of stacking faults correlates with threading dislocation density. Also, the growth rate is shown to have a pronounced effect on the threading dislocation densities. Elementary screw dislocation density has been observed to increase from 20 cm−2 to 4×103 cm−2 for growth rates increasing from 0.02 to 1.5 mm/h. Growth on seeds miscut 5° off the c axis resulted in screw dislocation densities almost two orders of magnitude lower than on axis growth. The results are interpreted as due to SiC stacking disorder at the initial stages of growth.

Enlargement of Step-Free SiC Surfaces by Homoepitaxial Web-Growth of Thin SiC Cantilevers

Lateral homoepitaxial growth of thin cantilevers emanating from mesa patterns that were reactive ion etched into on-axis commercial SiC substrates prior to growth is reported. The thin cantilevers form after pure stepflow growth removes almost all atomic steps from the top surface of a mesa, after which additional adatoms collected by the large step-free surface migrate to the mesa sidewall where they rapidly incorporate into the crystal near the top of the mesa sidewall. The lateral propagation of the step-free cantilevered surface is significantly affected by pregrowth mesa shape and orientation, with the highest lateral expansion rates observed at the inside concave corners of V-shaped pregrowth mesas with arms lengthwise oriented along the 〈1100〉 direction. Complete spanning of the interiors of V’s and other mesa shapes with concave corners by webbed cantilevers was accomplished. Optical microscopy, synchrotron white beam x-ray topography and atomic force microscopy analysis of webbed regions formed ...

Basal plane slip and formation of mixed-tilt boundaries in sublimation-grown hexagonal polytype silicon carbide single crystals

Optical microscopy, synchrotron white beam x-ray topography (SWBXT), and high resolution x-ray diffraction (HRXRD) were used to study the distribution of basal plane dislocations in bulk 4H silicon carbide crystals grown by the physical vapor transport method. An etch pit array was observed on the silicon face of KOH-etched off-cut wafers. The arrays were aligned parallel to each other and perpendicular to the off-cut direction. The etch pits were oval-shaped, which is characteristic of basal plane dislocations. Corresponding array images have been observed by SWBXT. Based on the characteristic distribution, the etch pit arrays are interpreted as the slip traces of high temperature deformation during the growth process. Thermoelastic stress is proposed as the plausible cause of the deformation. In addition, basal plane dislocation pileups were found in the proximity of polygonized threading edge dislocation arrays. SWBXT and HRXRD were used to study the misorientation related to such dislocation structure...

Assessment of Polishing-Related Surface Damage in Silicon Carbide

The subsurface damage generated by the polishing of silicon carbide crystals was investigated by measuring dislocation densities in sublimation grown SiC layers and through the use of high-resolution X-ray diffraction. Physical vapor transport growth on silicon carbide seeds, with a typical polishing finish using I μm diamond paste, leads to the nucleation of threading edge dislocations of density on the order of 10 7 cm -2 and threading screw dislocations of density on the order of 10 6 cm 2 . Chemical mechanical polishing lowered the dislocation density by four orders of magnitude for threading screw dislocations and two orders of magnitude for threading edge dislocations. Controlled high temperature hydrogen etching was used to determine the depth of damage produced by mechanical polishing and it was found to be 700 ± 300 A. Diffuse scattering from mechanically polished, chemical mechanically polished, and hydrogen etched SiC crystals were quantified by triple axis high-resolution X-ray diffraction, A consistent trend of decreasing diffuse scattering intensity was observed in mechanically polished, chemical mechanically polished, and hydrogen etched surfaces. Root mean squared (rms) roughness measurements of the surface finishes, obtained with atomic force microscopy, were in agreement with the high-resolution X-ray diffraction results. The mechanically polished surfaces had an rms roughness that was two to three times larger than the chemical mechanically polished surfaces.