Silicon carbide possesses oustanding properties such as a wide band-gap, high thermal conductivity, good chemical stability and high saturated electron drift velocity that constitute such a significant improvement over conventional semiconductor materials that many potential applications are envisaged. However, while some applications have already been realized, issues relating to crystalline defects remain a barrier to the successful realization of several others. The central focus of this thesis is to study defect structures in SiC bulk crystals, epilayers and devices using synchrotron x-ray topography as well as other characterization techniques. The goals of the studies are to understand the behavior and nucleation mechanisms of various defects and their mutual interactions in SiC bulk crystals, epilayers and devices so as to design strategies to mitigate their negative effects by either reducing their densities or completely eliminating them. The following in-depth studies have been carried out:
I. Chemical vapor deposition growth and characterization of silicon carbide homo-epitaxial layers. Homo-epitaxial silicon carbide layers were grown by low pressure chemical vapor deposition using halide precursors. Growth processes were carried out using different growth parameters, e.g., temperatures and gas flow rates. Thermodynamic process was studied using a simple equilibrium model. The surface morphology and defect structures in the grown epilayers were investigated using scanning electron microscopy, optical microscopy, high-resolution x-ray diffraction and x-ray topography.
II. Investigation of the interactions between basal plane dislocations and threading dislocations and the formation mechanisms of low angle grain boundaries. The interactions between basal plane dislocations and threading dislocations as well as the formation mechanisms of low angle grain boundaries were studied using synchrotron white beam x-ray topography. The threading screw dislocations act as effective pinning points for the basal plane dislocations, while the threading edge dislocations do not. Threading edge dislocation walls act as obstacles for the glide of basal plane dislocations and aggregation of basal plane dislocations is observed. Interactions between the distributions of basal plane dislocations can induce the formation of low angle grain boundaries. Two cases were observed: (1) where edge type basal plane dislocations of opposite sign are observed to aggregate together to form basal plane tilt boundaries and (2) where similar aggregation occurs against pre-existing prismatic tilt boundaries thereby contributing some basal plane tilt to the boundaries.
III. Investigation of the interaction between advancing Shockley partial dislocations and threading dislocations in silicon carbide bipolar devices. The interaction between the advancing Shockley partial dislocations and threading dislocations in silicon carbide bipolar devices was studied and a novel interaction mechanism was proposed. The advancing Shockley partial dislocations are able to cut through threading edge dislocations without generating any trailing dislocation segments. However, when the advancing Shockley partial dislocation encounters a threading screw dislocation, a prismatic stacking fault is nucleated via the cross-slip and annihilation of partial dislocation dipole. This configuration is sessile and precludes the further formation of double stacking faults which would be even more detrimental to device performance than the single stacking faults.
IV. Determination of the dislocation sense of micropipes. The dislocation sense of micropipes has been studied. The ray-tracing method has been carried out to simulate the (11-28) grazing-incidence synchrotron white beam x-ray topographic images of micropipes. The white elliptical features are canted to one or the other side of the reflection vector, depending on the handedness of the micropipes, which provides a simple, non-destructive and reliable way to reveal their senses.
V. Determination of the dislocation sense of closed-core threading screw dislocations. The handedness of closed-core threading screw dislocations was studied. Ray-tracing simulation indicates that the sense of the threading screw dislocations can be revealed based on the position of the thickened dark contrast in their x-ray topographic images. “Small Bragg angle” back-reflection x-ray topography has been carried out to validate our argument. Complete agreement between the results from grazing-incidence x-ray topography and “small Bragg angle” back-reflection x-ray topography proves that either method provides a simple, non-destructive and unambiguous way to reveal the sense of closed-core threading screw dislocations.
VI. Determination of the character of threading edge dislocations in silicon carbide. X-ray topographic images of threading edge dislocations were studied. Six types of threading edge dislocations were first observed in x-ray topography and their Burgers vectors can be determined from a single (11-28) grazing-incidence x-ray topograph, based on the simulation using ray-tracing method. Their Burgers vectors can be further verified by an additional equivalent (-12-18) reflection and a dislocation half loop containing two threading edge dislocation segments. The threading edge dislocation arrays are tending to orient along <1-100> directions and the Burgers vectors of the individual threading edge dislocations are perpendicular to the arrays, which is expected to minimize the total strain energy.
VII. Determination of the core-structure of Shockley partial dislocations. The corestructure of Shockley partial dislocations was studied. (Abstract shortened by UMI.)
|School:||State University of New York at Stony Brook|
|School Location:||United States -- New York|
|Source:||DAI-B 69/12, Dissertation Abstracts International|
|Keywords:||Bulk crystals, Defects, Epilayers, Silicon carbide, Single crystals, X-ray topography|
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