5-4-5 SiC Crystal Dislocation Defects
Table 5.2 summarizes the major known dislocation defects found in present-day commercial 4H- and 6H-SiC wafers and epilayers . Since the active regions of devices reside in epilayers, the epilayer defect content is clearly of primary importance to SiC device performance. However, as evidenced by Table 5.2, most epilayer defects originate from dislocations found in the underlying SiC substrate prior to epilayer deposition. More details on the electrical impact of some of these defects on specific devices are discussed later in Section 5.6.
The micropipe defect is regarded as the most obvious and damaging “device-killer” defect to SiC electronic devices .A micropipe is an axial screw dislocation with a hollow core (diameter of the order of a micrometer) in the SiC wafer and epilayer that extends roughly parallel to the crystallographic c-axis normal to the polished c-axis wafer surface . These defects impart considerable local strain to the surrounding SiC crystal that can be observed using X-ray topography or optical cross polarizers . Over the course of a decade, substantial efforts by SiC material vendors has succeeded in reducing SiC wafer micropipe densities nearly 100-fold, and some SiC boules completely free of micropipes have been demonstrated . In addition, epitaxial growth techniques for closing SiC substrate micropipes (effectively dissociating the hollow-core axial dislocation into multiple closed-core dislocations) have been developed . However, this approach has not yet met the demanding electronic reliability requirements for commercial SiC power devices that operate at high electric fields .
Even though micropipe “device-killer” defects have been almost eliminated, commercial 4H- and 6HSiC wafers and epilayers still contain very high densities (>10,000 , summarized in Table 5.2) of other less-harmful dislocation defects. While these remaining dislocations are not presently specified in SiC material vendor specification sheets, they are nevertheless believed responsible for a variety of nonideal device behaviors that have hindered reproducibility and commercialization of some (particularly high electric field) SiC electronic devices . Closed-core axial screw dislocation defects are similar in structure and strain properties to micropipes, except that their Burgers vectors are smaller so that the core is solid instead of a hollow void . As shown in Table 5.2, basal plane dislocation defects and threading edge dislocation defects are also plentiful in commercial SiC wafers .
As discussed later in Section 184.108.40.206.2, 4H-SiC electrical device degradation caused by the expansion of stacking faults initiated from basal plane dislocation defects has hindered commercialization of bipolar power devices . Similar stacking fault expansion has also been reported when doped 4H-SiC epilayers have been subjected to modest (~1150°C) thermal oxidation processing . While epitaxial growth techniques to convert basal-plane dislocations into threading-edge dislocations have recently been reported, the electrical impact of threading-edge dislocations on the performance and reliability of highelectric field SiC devices remains to be fully ascertained . It is also important to note that presentday commercial SiC epilayers still contain some undesirable surface morphological features such as “carrot defects” which could affect SiC device processing and performance .
In an exciting initial breakthrough, a Japanese team of researchers reported in 2004 that they achieved a 100-fold reduction in dislocation density in prototype 4H-SiC wafers of up to 3 in. in diameter . While such greatly improved SiC wafer quality offered by this “multiple a-face” growth technique should prove highly beneficial to electronic (especially high-power) SiC device capabilities, it remains uncertain as of this writing as to when this significantly more complex (and therefore expensive) growth process will result in commercially viable mass-produced SiC wafers and devices.