Edward K. Sanchez

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 ...

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.

Stacking faults created by the combined deflection of threading dislocations of Burgers vector c and c+a during the physical vapor transport growth of 4H–SiC

Observations have been made, using synchrotron white beam x-ray topography, of stacking faults in 4H–SiC with fault vectors of kind 1/6⟨202¯3⟩. A mechanism has been postulated for their formation which involves overgrowth by a macrostep of the surface outcrop of a c-axis threading screw dislocation, with two c/2-height surface spiral steps, which has several threading dislocations of Burgers vector c+a, with c-height spiral steps, which protrude onto the terrace in between the c/2-risers. Such overgrowth processes deflect the threading dislocations onto the basal plane, enabling them to exit the crystal and thereby providing a mechanism to lower their densities.

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.

Nonuniformities of electrical resistivity in undoped 6H-SiC wafers

Chemical elemental analysis, temperature-dependent Hall measurements, deep-level transient spectroscopy, and contactless resistivity mapping were performed on undoped semi-insulating (SI) and lightly nitrogen-doped conducting 6H-SiC crystals grown by physical vapor transport (PVT). Resistivity maps of commercial semi-insulating SiC wafers revealed resistivity variations across the wafers between one and two orders of magnitude. Two major types of variations were identified. First is the U-shape distribution with low resistivity in the center and high in the periphery of the wafer. The second type had an inverted U-shape distribution. Secondary-ion-mass spectrometry measurements of the distribution of nitrogen concentration along the growth axis and across the wafers sliced from different locations of lightly nitrogen-doped 6H–SiC boules were conducted. The measured nitrogen concentration gradually decreased along the growth direction and from the center to the periphery of the wafers. This change gives ri...

Sense determination of micropipes via grazing-incidence synchrotron white beam x-ray topography in 4H silicon carbide

Computer modeling using the ray-tracing method has been used to simulate the grazing-incidence x-ray topographic images of micropipes in 4H silicon carbide recorded using the pyramidal (11−28) reflection. Simulation results indicate that the images of micropipes appear as white features of roughly elliptical shape, canted to one side or other of the g vector depending on the dislocation sense. Observed images compare well with the simulations, demonstrating that the direction of cant provides a simple, nondestructive, and reliable way to reveal the senses of micropipes. Sense assignment has been validated using back-reflection reticulography.

Characterization of 100 mm Diameter 4H-Silicon Carbide Crystals with Extremely Low Basal Plane Dislocation Density

Synchrotron White Beam X-ray Topography (SWBXT) studies are presented of basal plane dislocation (BPD) configurations and behavior in a new generation of 100mm diameter, 4H-SiC wafers with extremely low BPD densities (3-4 x 102 cm-2). The conversion of non-screw oriented, glissile BPDs into sessile threading edge dislocations (TEDs) is observed to provide pinning points for the operation of single ended Frank-Read sources. In some regions, once converted TEDs are observed to re-convert back into BPDs in a repetitive process which provides multiple BPD pinning points.

The Nucleation and Propagation of Threading Dislocations with c-Component of Burgers Vector in PVT-Grown 4H-SiC

Studies of threading dislocations with Burgers vector of c+a have been carried out using synchrotron white beam X-ray topography. The nucleation and propagation of pairs of opposite sign threading c+a dislocations is observed. Overgrowth of inclusions by growth steps leads to lattice closure failure and the stresses associated with this can be relaxed by the nucleation of opposite sign pairs of dislocations with Burgers vector c+a. Once these dislocations are nucleated they propagate along the c-axis growth direction, or can be deflected onto the basal plane by overgrowth of macrosteps. For the c+a dislocations, partial deflection can occasionally occur, e.g. the a-component deflects onto basal plane while the c-component continuously propagates along the growth direction. One factor controlling the details of these deflection processes is suggested to be related to the ratio between the height of the overgrowing macrostep and that of the surface spiral hillock associated with the threading growth dislocations with c-component of Burgers vector.

Basal plane dislocation multiplication via the Hopping Frank-Read source mechanism in 4H-SiC

Synchrotron white beam x-ray topography (SWBXT) observations are reported of single-ended Frank-Read sources in 4H-SiC. These result from inter-conversion between basal plane dislocations (BPDs) and threading edge dislocations (TEDs) brought about by step interactions on the growth interface resulting in a dislocation comprising several glissile BPD segments on parallel basal planes interconnected by relatively sessile TED segments. Under stress, the BPD segments become pinned by the TED segments producing single ended Frank-Read sources. Since the BPDs appear to “hop” between basal planes, this apparently dominant multiplication mechanism for BPDs in 4H-SiC is referred to as the “Hopping” Frank-Read source mechanism.

Basal Plane Dislocation Multiplication via the Hopping Frank-Read Source Mechanism and Observations of Prismatic Glide in 4H-SiC

In this paper, we report on the synchrotron white beam topographic (SWBXT) observation of “hopping” Frank-Read sources in 4H-SiC. A detailed mechanism for this process is presented which involves threading edge dislocations experiencing a double deflection process involving overgrowth by a macrostep (MP) followed by impingement of that macrostep against a step moving in the opposite direction. These processes enable the single-ended Frank-Read sources created by the pinning of the deflected basal plane dislocation segments at the less mobile threading edge dislocation segments to “hop” from one slip plane to other parallel slip planes. We also report on the nucleation of 1/3< >{ } prismatic dislocation half-loops at the hollow cores of micropipes and their glide under thermal shear stress.