Papers by Author: Yi Chen

Abstract: The influence of substrate surface scratches on the quality of CVD grown 4H-SiC homo-epitaxial layers has been studied using a combination of post-growth Monochromatic Synchrotron X-ray Topography (MSXT) and KOH etching. MSXT observations suggest that the scratches on the substrate surface act as dislocation nucleation centers during the growth. When the scratch is along the off-cut direction, only TED-TED pairs are generated. As the inclination of the scratch to the off-cut direction increases, an increasing number of TED-BPD pairs are generated. A model is presented for a possible mechanism for the nucleation of dislocations at scratches.

Abstract: Comparative studies of defect microstructure in 4H-SiC wafers have been carried out using photoluminescence (PL) imaging and grazing-incidence Synchrotron White Beam X-ray Topography. Images of low angle grain boundaries on the PL images correlate well with SWBXT observations, and similar correlation can be established for some micropipe images although the latter is complicated by the overall level of distortion and misorientation associated with the low angle grain boundaries and the fact that many of the micropipes are located in or close to the boundaries. This validation indicates that PL imaging may provide a rapid way of imaging such defect structures in large-scale SiC wafers.

Abstract: Electron-hole recombination activated Shockley partial dislocations bounding expanding stacking faults and their interactions with threading dislocations have been studied in 4H-SiC epitaxial layers using synchrotron x-ray topography. The bounding partials appear as white stripes or narrow dark lines in back-reflection X-ray topographs recorded using the basal plane reflections. Such contrast variations are attributable to the defocusing/focusing of the diffracted X-rays due to the edge component of the partial dislocations, which creates a convex/concave distortion of the basal planes. Simulation results based on the ray-tracing principle confirm our argument. The sign of the partial dislocations can be subsequently determined.

Abstract: Etching of 4H-SiC wafers in molten KOH as a method for micropipe and dislocation density analysis was investigated. The obtained results were correlated with those of the synchrotron white beam x-ray topography. Heavily nitrogen-doped SiC shows a significantly different etching behavior in comparison with the low-doped material. This complicates identification of different types of threading defects. In particular, it is difficult to separate Threading Screw Dislocations (TSD) from Threading Edge Dislocations (TED). Depending on the level of doping and thermal history of the crystal, some of the etch pits emerging due to the 1c screw dislocations can be as large as those due to the micropipes.

Abstract: Threading edge dislocations (TEDs) in a 4H-SiC epitaxial layer are investigated using high-resolution synchrotron topography. Six types of TED image are confirmed to correspond to the Burgers vector directions by a comparison of computer simulated images and observed topography images in crystal boundaries. Using a mapping method, a wide spatial distribution of the six types of TED is examined in a quarter section of a 2-inch wafer.

Abstract: The density and sense distribution of elementary threading screw dislocations in a physical vapor transport grown 3-inch 4H silicon carbide wafer have been studied. The density of TSDs ranges between 1.6×103/cm2 and 7.1×103/cm2 and the lowest density is observed at positions approximately half radius off the wafer center. The dislocation sense of elementary threading screw dislocations can be readily revealed by the asymmetric contrast of their images in grazing-incidence x-ray topographs using pyramidal plane reflections. The circumferential and radial distributions of the sense of elementary threading screw dislocations have been studied and no clear trends are observed in either distribution.

Abstract: Grazing-incidence synchrotron topography studies of micropipes (MPs) and closed-core threading screw dislocations (TSDs) have been carried out and the results compared with ray-tracing simulations. Simulations indicate that both MPs and TSDs appear as roughly elliptically shaped white features which are canted to one side or the other of the g-vector depending on the dislocation sense and which have asymmetric perimeters of dark contrast which are greatly enhanced on the side towards which the feature is canted (again depending on the dislocation sense). For MPs, observations are generally consistent with this although the cant of the features is more obviously discerned than the asymmetry in the perimeter contrast. Sense assignment for MPs has been validated using back-reflection reticulography. For TSDs, observation are again generally consistent with the simulations although the smaller feature size and the variability in the line direction of the TSDs make the asymmetry of perimeter contrast a more obvious and reliable way to determine the dislocation sense than the sense of cant. TSD dislocation senses so obtained were validated using back-reflection images of same-sign and opposite-sign pairs.

Abstract: A review is presented of the current understanding of the dislocation configurations observed in PVT-grown 4H- and 6H-SiC boules and CVD-grown 4H-SiC homoepitaxial layers. In both PVT-grown boules and CVD-grown epilayers, dislocation configurations are classified according to whether they are growth dislocations, i.e., formed during growth via the replication of dislocations which thread the moving crystal growth front, or result from deformation processes (under either mechanical or electrical stress) immediately following growth, during post growth cooling, i.e., behind the crystal growth front or during device operation. Possible formation mechanisms of growth defects in the PVT grown boules, such as axial screw dislocations and threading edge dislocation walls are proposed. Similarly, possible origins of growth defect configurations in CVD-grown epilayers, such as Frank faults bounded by Frank partials, BPDs and TEDs, are also discussed. In a similar way, the origins of BPD configurations resulting from relaxation of thermal stresses during post-growth cooling of the PVT boules are discussed. Finally, the susceptibility of BPD configurations replicated into CVD grown epilayers from the substrate towards Recombination Enhanced Dislocation Glide (REDG) is discussed.

Abstract: Thick (> 25 µm) 4H n+ epitaxial layer growth was performed on 4H n+ substrates utilizing chlorine containing etch chemistries in a hot wall CVD system. Optimization of the n+ epitaxial layer growth was achieved by varying C/Si ratio and N2 flow. Desired epitaxial layers have doping levels > 5x1018 cm-3, epitaxial surface roughness <10 nm on a 20x20 µm area and overall micropipe density reduction. To confirm the conversion of micropipes into closed core screw dislocations, microscopic examination of the epitaxial and wafer surfaces was carried out after KOH etching. Grazing incidence x-ray topography (XRT) as well as cross sectional XRT and microscopy were also performed. The cross sectional evaluation showed that the dissociation of the micropipes occurs very close to the epitaxy/wafer interface.

Abstract: The interactions between basal plane dislocations (BPDs) and threading screw and edge dislocations (TSDs and TEDs) in hexagonal SiC have been studied using synchrotron white beam x-ray topography (SWBXT). TSDs are shown to strongly interact with advancing basal plane dislocations (BPDs) while TEDs do not. A BPD can cut through an individual TED without the formation of jogs or kinks. The BPDs were observed to be pinned by TSDs creating trailing dislocation dipoles. If these dipoles are in screw orientation segments can cross-slip and annihilate also potentially leaving isolated trailing loops. The three-dimensional (3D) distribution of BPDs can lead to aggregation of opposite sign edge segments leading to the creation of low angle grain boundaries (LAGBs) characterized by pure basal plane tilt of magnitude determined by the net difference in densities of the opposite sign dislocations. Similar aggregation can also occur against pre-existing prismatic tilt boundaries made up of TED walls with the net difference in densities of the opposite sign dislocations contributing some basal plane tilt character to the LAGB.