MSE Seminar Series: Balaji Raghothamachar
Friday, March 4, 2016
1:00 p.m.-2:00 p.m.
Room 2110, Chemical and Nuclear Engineering Building
Department of Materials Science & Engineering
Stony Brook University
Materials Development of Wide Bandgap Semiconductors for Power Electronics
Wide band gap (WBG) semiconductors (silicon carbide, gallium nitride, aluminum nitride, gallium oxide and diamond at various stages of development) are the drivers for the next generation of power electronics that are integral to the Smart Grid, many clean energy technologies and consumer electronics. WBG semiconductor power switching devices, especially those made on SiC and GaN, promise transformative advances in electrical power switching systems because of superior electrical and thermal properties of these materials compared to silicon. The presence of defects such as dislocations, grain boundaries, stacking faults, homogeneous and inhomogeneous strains limits the performance and reliability of devices fabricated from these materials. Therefore, highly perfect, defect-free, large area native single crystal substrates/templates of WBG semiconductors are preferred for controlled growth of active device layers. Through the application of synchrotron radiation based X-ray topography (XRT) techniques, we can obtain detailed configurations of defect structures in these crystals that shed light on their origins and evolution thereby enabling the development of strategies for their elimination or minimization. The application of XRT to the complete analysis of the distribution, character and origins of grown-in c-axis screw dislocations, deformation induced basal plane dislocations, and grown-in threading edge dislocations in PVT-grown 4H-SiC substrates will be discussed. In addition, dislocation behavior during CVD growth of homo-epitaxial layers on offcut substrates will be presented with insights into the relaxation processes involving the creation of interfacial dislocations and dislocation half loop arrays. Similar analysis of defect distributions in PVT-grown AlN as well as hydride vapor phase epitaxy (HVPE)-grown GaN and ammonothermal-grown GaN will also be presented.
Synchrotron white beam X-ray topography (SWBXT) and synchrotron monochromatic beam X-ray topography (SMBXT) are two XRT modes that enable non-destructive, high resolution imaging of structural defects in as-grown boules, large-diameter wafers and epilayers. This technique in conjunction with ray tracing simulations of defect images and in complement with TEM, SEM, optical microscopy and other characterization techniques plays a key role in the investigation of defect formation mechanisms and processing-microstructure-properties correlation in all kinds of crystalline materials (semiconductors, oxides, proteins, etc.). The ability of SWBXT for studies of dynamical material processes and the enhanced capabilities of new techniques such as Stress Mapping Analysis by Ray Tracing (SMART) and Rocking Curve Imaging (RCI) derived from XRT will also be demonstrated.