Materials for Next-Generation Power Electronics Sokrates T. Pantelides, Vanderbilt University, DMR 0907385

1. Materials for Next-Generation Power ElectronicsSokrates T. Pantelides, Vanderbilt University, DMR 0907385 Outcome: Collaborative research between Vanderbilt University (S. T. Pantelides and L.C. Feldman), Auburn University (S. Dhar and J. . Williams), the University of Tennessee at Knoxville (G. Duscher) and Fisk University (Q. Lu) has resulted in a deeper understanding of the oxidation of silicon carbide and has led to the development of superior oxide-silicon carbide interfaces that have the potential to revolutionize next-generation 4H-SiC based power electronic switches. Impact: The material processing advances made in this work, once suitably transferred to 4H-SiC power MOSFET fabrication technology, will lead to significantly lower energy losses and higher energy efficiencies in next-generation high voltage power conversion systems. Explanation: Owing to a wide-band gap of 3.2 eV, 4H-SiC is the emerging semiconductor technology for high voltage, high temperature power electronics. State-of-the-art SiC power MOSFETs are significantly superior power switches compared to their Si based counterparts for voltage ratings above 1000 V. However, current 4H-SiC MOSFETs do not perform at their theoretical potential due to the low channel mobility associated with atomic scale defects and disorder at the SiO2/4H-SiC interface-which forms the heart of the MOSFET. This research is focused at understanding various atomic scale defects and investigating various nanoscale interfacial chemical modification processes that would lead to an enhancement of the channel mobility.

2. A Potential Breakthrough in Channel MobilitySokrates T. Pantelides, Vanderbilt University, DMR 0907385 The industry-standard method for obtaining acceptable channel mobility in 4H-SiC MOSFETs is interfacial nitridation. This process incorporates nitrogen at the SiO2/4H-SiC interface and leads to the passivation of interfacial electron traps. As a consequence of trap passivation, the maximum channel mobility is around ~50 cm2 V-1 s-1 which is still more than a factor of 10 lower than the bulk mobility of 4H-SiC. Recently, phosphorus doped SiO2[1] was shown to increase the mobility substantially by Okamoto and coworkers in Japan, but this is accompanied by device instability. In this work a modified phosphorus process was developed which significantly improves the stability while achieving a substantially higher mobility compared to the state-of-the-art. Phosphorus process (unstable) Modified Phosphorus process (stable) State-of-the-art nitridation Maximum channel mobility in 4H-SiC MOSFETs versus total number of interface traps obtained by varying the interface passivation process. The modified phosphorus process developed in this work demonstrates 50 % higher mobility than the state-of-the-art nitridation process (S. Dhar and J. R. Williams, Auburn University)

3. Understanding the oxidation of SiCSokrates T. Pantelides, Vanderbilt University, DMR 0907385 Identification of the oxidant is a prerequisite for understanding any oxidation mechanism. Using Si-face SiC as an example, Postdoc Xiao Shen at Vanderbilt and PI S. T. Pantelides investigated the diffusion of both molecular and atomic oxygen at the oxide/semiconductor interface through quantum-mechanical calculations and found that the last oxide layer at the interface blocks the diffusion of molecular oxygen by a 4 eV barrier (upper right) while allows the diffusion of atomic oxygen (the barrier is 1.2 eV, lower right). As the result, only atomic oxygen is an effective oxidant. These findings are useful for improving the efficiency of oxidation processes of materials. Ebarrier= 4 eV Ebarrier= 1.2 eV

4. Hyperfine active defects in SiC electronicsSokratesT. Pantelides, Vanderbilt University, DMR 0907385 Point defects limit the performance of several SiC devices including diodes and MOSFETs. For defects which have an odd number of electrons, their hyperfine activity is a key signature to allow for microscopic identification. Blair Tuttle (Associate Professor at Penn State Behrendand Visiting Professor at Vanderbilt), has used first-principles calculationsto help identify a nitrogen vacancy complex observed by P. Lenahan, a colleague at Penn State. Notice in figure the spin density (yellow) is mainly localized on the silicon atoms of the vacancy. This feature is consistent with EPR observations. Thomas Aichinger, Patrick M. Lenahan, Blair R. Tuttle, and DethardPeters, Appl. Phys. Lett. vol. 100, 112113 (2012). Atomic structure and the spin density of a N, vacancy defect in SiC. Blue = Si, Brown = C, White = N, Yellow = iso-contour for spin density (courtesy of B. Tuttle)

5. SiC-SiO2 Interface and Its Influence on MobilitySokrates T. Pantelides, Vanderbilt University, DMR 0907385 SiC as a power electronic material exhibits many advantages, but SiC MOSFETs show rather low mobilities compared to the mobility of bulk SiC. The interface between SiC and SiO2 is generally considered to be the cause for the reduced mobility. The exact structure and chemistry of this interface is still under debate. Gerd Duscher at the University of Tennessee Knoxville used atomic resolution Z-contrast imaging and electron energy-loss spectroscopy to investigate samples synthesized at Auburn University by J. R. Williams. It was concluded that the previously observed interface layer is due to the miscut and does not exhibit any stoichiometric change. The structure of the interface which is limiting the device performance may be caused by the steps and facets at the interface introduced by the miscut. These results were possible through focus series with Z-contrast imaging using instrumentation at ORNL. A B Images by Gerd Duscher

6. N bonding at the SiC/SiO2 interfaceSokrates Pantelides, Vanderbilt University, DMR 0907385 Side View H | Si-N-O-Si 399.5 eV X-ray photoemission spectroscopy was carried out by Yi Xu and Len Feldman (Rutgers University, participants in the NSF grant) on oxidized SiC followed by 2 hours anneal in NO gas, after etching the oxide in order to probe the interface. The data are consistent with all N atoms bonded to Si atoms, suggesting either an epitaxial SiON layer as proposed by Shirasawa et al. or a disordered Si-C-O-N bonded interlayer as proposed by the Vanderbilt team (S. Wang et al). OH | Si-N-Si H | SiO-Si-N-Si-OSi 399.5 eV 398.3 eV T. Shirasawa, et al. Phys. Rev. Lett.98, 136105 (2007); Phys. Rev. B 79, 241301(R) (2009) S. Wang, S. Dhar, A. C. Ahyi, A. Franceschetti, J. R. Williams, L. C. Feldman, and S. T. Pantelides, Phys. Rev. Lett. 98, 026101 (207)

7. Materials Science Teacher WorkshopsSokrates T. Pantelides, Vanderbilt University, DMR 0907385 Blair Tuttle (Associate Professor at Penn State Behrendand Visiting Professor at Vanderbilt), who is a participant in the research program using first-principles calculations, has organized Materials Workshops geared toward high school science teachers. Researchers from this NSF grant have given presentations on their SiC related research. The workshops are organized each year and cover themes including Materials for Energy Applications and Materials in Electronics. The full-day workshops have been attended by ~10 participants each year. Photos from Workshops in 2011 and 2012 (courtesy of B. Tuttle)

8. Materials Science Teacher Workshops Sokrates T. Pantelides, Vanderbilt University, DMR 0907385 SorrieCeesay, Masters student at Fisk University, did a Masters thesis on the oxidation/nitridation of SiC , graduated in May 2012. He gave an oral presentation at the Spring MRS meeting in San Francisco, April 2012.

9. Materials Science Teacher Workshops Sokrates T. Pantelides, Vanderbilt University, DMR 0907385 Participants at the team meeting at the Cree Technical Center, 11/21/2011