S. Ha

Degradation of hexagonal silicon-carbide-based bipolar devices

Only a few years ago, an account of degradation of silicon carbide high-voltage p-i-n diodes was presented at the European Conference on Silicon Carbide and Related Compounds (Kloster Banz, Germany, 2000). This report was followed by the intense effort of multiple groups utilizing varied approaches and subsequent progress in both fundamental understanding of this phenomenon and its elimination. The degradation of SiC p-i-n junctions is now well documented to be due to the expansion of Shockley-type stacking faults in the part of the devices reached by the electron-hole plasma. The faults can gradually cover most of the junction area, impeding current flow and, as a result, increasing the on-state resistance. While in most semiconductors stacking faults are electrically inactive, in hexagonal silicon carbide polytypes (4H- and 6H-SiC) they form quantum-well-like electron states observed in luminescence and confirmed by first-principles calculations. The stacking-fault expansion occurs via motion of 30° sil...

Dislocation conversion in 4H silicon carbide epitaxy

Abstract The propagation of basal plane dislocations from off-axis 4H silicon carbide substrates into the homo-epitaxial layers has been investigated using chemical etching, optical microscopy, and transmission electron microscopy (TEM). The etch pit densities of threading edge and basal plane dislocations changed significantly across the epilayer/substrate interface. We have observed conversion of basal plane dislocations in the substrates into threading edge dislocations in the epilayers. TEM observation revealed that the threading dislocations in the epilayers are inclined from the c-axis toward the down-step direction. The conversion is interpreted as a result of the image force in the epilayers between flowing growth steps and basal plane dislocations. This effect can lead to an apparent improvement of the structural quality of epilayers compared to that of substrates.

Nucleation sites of recombination-enhanced stacking fault formation in silicon carbide p-i-n diodes

The morphology and nucleation sites of stacking faults formed during the forward operation of 4H silicon carbide p-i-n diodes were investigated using optical emission microscopy (OEM) and transmission electron microscopy (TEM). Partial dislocations bounding the stacking faults are mostly aligned to the 〈11–20〉 directions with Burgers vectors of the 1/3〈1–100〉 type. Arrays of dislocation half loops in the blocking layer serve as nucleation sites of double-rhombic stacking faults. The morphology of these stacking faults indicates that short basal plane segments associated with threading dislocations are the origin of rhombic stacking faults. All dislocations in a half-loop array have the same Burgers vector and nucleate on a single basal plane, which was evidenced by the merging of double-rhombic stacking faults. Most pre-existing basal plane dislocations within the blocking layer which are visible in OEM images dissociate to form stacking faults during the degradation. Basal plane dislocations aligned alon...

Core structure and properties of partial dislocations in silicon carbide p-i-n diodes

The electroluminescence, mobility, and core nature of partial dislocations bounding stacking faults in 4H silicon carbide p-i-n diodes were investigated using optical emission microscopy and transmission electron microscopy (TEM). The stacking faults developed and expanded in the blocking layer during high current forward biasing. Their bounding partial dislocations showed two distinct characteristics. Bright luminescent segments were mobile while dark invisible ones were stationary during biasing. TEM analysis of their Burgers vectors indicated that the mobile segments were silicon-core 30° partial dislocations while the immobile segments were carbon-core 30° ones.

Integrated process modeling and experimental validation of silicon carbide sublimation growth

A model that integrates heat and mass transfer, growth kinetics, anisotropic thermal stresses is developed to predict the global temperature distribution, growth rate and dislocation distribution. The simulated temperature and growth rate are compared with the experimental measurements. The time-depending growth process, e.g., the variations of the growth rate, the growth interface shape, and the thermal stresses with time in the growing crystal are studied using the integrated model. The resolved shear stress and the von Mises stress are used to predict the dislocation density. The effects of geometric configuration and design parameters on the growth of crystal are also discussed.

Origin of domain structure in hexagonal silicon carbide boules grown by the physical vapor transport method

Transmission electron microscopy (TEM), high-resolution X-ray di!raction, and KOH etching have been used to study the dislocation structure of 4H SiC crystals grown by the physical vapor transport method. Many of the etch pits on the Si(0 0 0 1) surface form arrays extending along the S11 100 T directions. Plan view conventional and high-resolution TEM show that the arrays consist of pure edge dislocations threading along the c-axis with identical Burgers vectors of the a/3S 112 1 0T type. The dislocation arrays constitute low angle [0 0 0 1] tilt boundaries, i.e., [0 0 0 1] is the common axis lying in the boundary. Typical values of the misorientation are in the 60}200 arcsec range. Evidence is presented that such boundaries can form by polygonization of the threading edge dislocations, which have been introduced into SiC crystals by prismatic slip. ( 2000 Published by Elsevier Science B.V.

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...

Cross-sectional structure of carrot defects in 4H–SiC epilayers

Surface morphology of carrot defects in 4H–SiC epilayers is described based on optical microscopy and molten potassium hydroxide etching. Its crystallographic structure is investigated using cross-sectional transmission x-ray topography. A threading screw dislocation in substrate serves as the nucleation source of a carrot. At the beginning of the epitaxial growth, the source dislocation is deflected toward the step-flow direction where a basal plane dislocation and a vertical planar defect nucleate together. The vertical planar defect fills the space between the basal plane dislocation and the deflected threading dislocation. This structure forms characteristic morphology on the epilayer surface, such as groove, shallow trench, etch line of the groove, and hexagonal and oval etch pits at each end of the groove.

Morphology of basal plane dislocations in 4H-SiC homoepitaxial layers grown by chemical vapor deposition

The morphology of basal plane dislocations (BPDs) in 4H-SiC homoepitaxial layers has been investigated by plan-view transmission x-ray topography and molten KOH etching. Three types of BPDs are distinguished based on their morphologies. These include interfacial dislocations, curved dislocations, and circular loop dislocations around micropipes. Their characteristics are studied in detail and possible sources of their formation during epitaxy are discussed.

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.