2.3.2.2 Wafer Crystal Defects

Defects in SiC include open-core dislocations (called micropipes); low-angle boundaries; and conventional dislocations. Micropipe defects are considered as preventing the commercialization of many types of SiC devices, especially high-current power devices[53].

• Dislocations and Micropipes: Most discussions about micropipe formation mechanisms revolve around Frank's theory [54], which associates the micropipe with a superscrew dislocation that possesses a large Burgers vector, several times the unit cell dimension. The high stress along the center core of this superscrew dislocation causes preferential sublimation during the growth process and, consequently, the hollow core nature of the defect. These hollow core screw dislocations typically run parallel to the growth direction through the entire SiC boule. The screw dislocation content of micropipes is indicated by the existence of growth spirals originating at micropipes visible in stress birefringence [55], and by the results of synchrotron white beam X-ray topography (SWBXT) studies [56]. Despite the different fundamental and technological reasons for micropipe formation, commercial SiC wafers have shown steady improvement in the micropipe density over the past five years, leading to wafers with less than 15 micropipes per square cm of wafer area.
• Low-Angle Grain Boundaries: Low-angle boundaries near the crystal periphery tend to form with the growth of large-diameter crystals grown under non optimized process conditions. In SiC substrates, low-angle boundaries are visible between the magnitude of this burgers as void-like linear crystallographic features extending radially inward from the wafer edge and generally following low-index planes. They can sometimes extend through the entire thickness of the wafer. Recent work has resulted in a dramatic reduction in these defects, current research and development substrates up to diameters of 100 mm are now produced without these low angle grain boundary defects.

Figure 2.4: A map of micropipe defect count on a 3 inch semi-insulating 4H-SiC substrate [57], showing an average micropipe density of only 3/cm$^2$ and 96% of the area to be micropipe-free.

Table 2.4: Roadmap for SiC material [57].

	2002	2003	2006
micropipe density[/cm$^2$]	15	3	0.1
available area [mm$^2$]	8	16	40
semiconducting wafer size [inches]	3	3	4
semi-insulating wafer size [inches]	2	2-3	3

T. Ayalew: SiC Semiconductor Devices Technology, Modeling, and Simulation