Fangzhen Wu

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.

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.

Quantitative Comparison Between Dislocation Densitiesin Offcut 4H-SiC Wafers Measured Using SynchrotronX-ray Topography and Molten KOH Etching

AbstractMolten KOH etching and x-ray topography have been well established as two of the major characterization techniques used for observing as well as analyzing the various crystallographic defects in both substrates and homoepitaxial layers of silicon carbide. Regarding assessment of dislocation density in commercial wafers, though the two techniques show good consistency in threading dislocation density analysis, significant discrepancy is found in the case of basal plane dislocations (BPDs). In this paper we compare measurements of BPD densities in 4-inch 4H-SiC commercial wafers assessed using both etching and topography methods. The ratio of the BPD density calculated from topographic images to that from etch pits is estimated to be larger than 1/sinθ, where θ is the offcut angle of the wafer. Based on the orientations of the defects in the wafers, a theoretical model is put forward to explain this disparity and two main sources of errors in assessing the BPD density using chemical etching are discussed.

Studies of the Origins of Half-Loop Arrays and Interfacial Dislocations Observed in Homoepitaxial Layers of 4H-SiC

Synchrotron x-ray topography and KOH etching studies have been carried out on n-type 4H-SiC offcut substrates before and after homoepitaxial growth to study defect replication and strain relaxation processes and identify the nucleation sources of both interfacial dislocations (IDs) and half-loop arrays (HLAs), which are known to have a deleterious effect on device performance. Two cases are reported. In one, they nucleate from short segments of edge-oriented basal plane dislocations (BPDs) in the substrate which are drawn into the epilayer. In the other, they form from segments of half-loops of BPD that are attached to the substrate surface prior to growth which glide into the epilayer. The significance of these findings is: (1) It is demonstrated that it is not necessary for a BPD to intersect the substrate surface in order for it to be replicated into the homoepitaxial layer and take part in nucleation of IDs and HLAs; (2) The conversion of the surface intersections of a substrate BPD half-loop into threading edge dislocations (TEDs) does not prevent it from also becoming involved in nucleation of IDs and HLAs. This means that, while BPD to TED conversion can eliminate most of the BPD transfer into the epilayer, further mitigation may only be possible by continued efforts to reduce the BPD density in substrates by control of temperature-gradient- induced stresses during their physical vapor transport (PVT) growth.

Characterization and Formation Mechanism of Six Pointed Star-Type Stacking Faults in 4H-SiC

Synchrotron white-beam x-ray topography (SWBXT) studies of defects in 100-mm-diameter 4H-SiC wafers grown using physical vapor transport are presented. SWBXT enables nondestructive examination of thick and large-diameter SiC wafers, and defects can be imaged directly. Analysis of the contrast from these defects enables determination of their configuration, which, in turn, provides insight into their possible formation mechanisms. Apart from the usual defects present in the wafers, including micropipes, threading edge dislocations, threading screw dislocations, and basal plane dislocations, a new stacking fault with a peculiar configuration attracts our interest. This fault has the shape of a six-pointed star, comprising faults with three different fault vectors of Shockley type. Transmission and grazing topography of the fault area are carried out, and detailed contrast analysis reveals that the outline of the star is confined by 30° Shockley partial dislocations. A micropipe, which became the source of dislocations on both the basal plane slip system and the prismatic slip system, is found to be associated with the formation of the star fault. The postulated mechanism involves the reaction of 60° dislocations of a/3 〈

Characterization of V-shaped Defects in 4H-SiC Homoepitaxial Layers

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Studies of the Origins of Half Loop Arrays and Interfacial Dislocations Observed in Homoepitaxial Layers of 4H-SiC

〉 Burgers vector on basal plane and pure screw dislocations of a/3 〈