IMX Seminar Series - Computational Discovery and Experimental Validation of Rutile GeO2 and GeSnO2 Alloys: A New Family of Ultra-Wide-Band-Gap Semiconductors for Power Electronics

Semiconducting materials play a crucial role in modern society, from information technology and optical communications to renewable energy generation and vehicle electrification. In particular, ultra-wide-band-gap (UWBG) semiconductors, i.e., semiconductors with band gaps wider than the 3.5 eV value of GaN, are promising for higher efficiency, reduced size, and lower cost in high-power electronics applications. For example, materials such as diamond, cubic BN, β-Ga₂O₃, and AlGaN promise higher conversion efficiency and orders-of-magnitude improvements in power density compared to current technologies (Si, SiC, and GaN). However, none of the above UWBG materials offers all the desired properties needed for high-performance electronics and, despite decades of research, very few alternative UWBG semiconductors have been realized to date. There is therefore a pressing need to develop synergistic methods that combine predictive theory with experimental synthesis and validation in order to discover and design new UWBG semiconductors that can surpass the limitations of current technologies.
In recent years, our team has advanced the development of rutile GeO₂ and its alloys with SnO₂ as a novel family of UWBG semiconductors that can surpass the state of the art in power electronics. Our predictive atomistic calculations demonstrate that these alloys exhibit superior fundamental properties that overcome the limitations of current materials. Their band gaps span from 3.6 eV for SnO₂ to 4.68 eV for rutile GeO₂ [1]. They are predicted to exhibit ambipolar dopability, with Sb_Ge, As_Ge, Ta_Ge, H_i, and F_O acting as shallow donors, while Al_Ge and Ga_Ge acting as acceptors [2]. The predicted carrier mobilities are high [3], while the relatively light carrier effective masses prevent the formation of self-trapped polarons. The predicted thermal conductivity is also high and surpasses β-Ga₂O₃, a prediction that we verified experimentally in unoptimized polycrystalline bulk samples [4]. Overall, we find that the predicted Baliga figure of merit of rutile GeO₂ (i.e., a measure of the performance of materials in power electronics), modified to account for donor ionization, surpasses all known semiconductors, demonstrating its unique potential for energy-efficient power electronics [5].
Experimentally, we demonstrate the synthesis of single-crystalline GeO₂-based thin films and substrates, which are prerequisites for epitaxial devices. Using suboxide molecular-beam epitaxy (MBE), we demonstrate the stability of GeSnO₂ alloy thin films over their entire composition range [6], while the development and epitaxy on single-crystalline rutile GeO₂ substrates enables the epitaxy of single-crystalline thin films [7]. Overall, our work demonstrates the unique promise of rutile GeO₂-based materials for advancing the state of the art in power electronic devices.

[1] J. Appl. Phys. 126, 085703 (2019)
[2] Appl. Phys. Lett. 114, 102104 (2019)
[3] Appl. Phys. Lett. 117, 182104 (2020)
[4] Appl. Phys. Lett. 117, 102106 (2020)
[5] Appl. Phys. Lett. 118, 260501 (2021)
[6] Appl. Phys. Lett. 117, 072105 (2020)
[7] J. Vac. Sci. Technol. A 40, 050401 (2022)

Bio: Emmanouil (Manos) Kioupakis is a Professor of Materials Science and Engineering and the Karl F. and Patricia J. Betz Family Faculty Scholar at the University of Michigan. He obtained his PhD in Physics at the University of California, Berkeley, and his undergraduate degree in Physics at the University of Crete. He also held a postdoctoral appointment in Materials at the University of California, Santa Barbara.
Prof. Kioupakis’ research group focuses on developing and applying predictive materials-modeling methods to explain and predict the synthesis and functionalities of new semiconductor materials for electronics, optoelectronics, and energy applications. Highlights of his work include pioneering calculations of phonon-mediated quantum phenomena in materials, such as optical absorption in silicon and non-radiative recombination in LEDs, as well as uncovering fundamental insights on the structure-property relationships of modern nitride and oxide semiconductors. He has supervised the PhD research of 21 students of diverse backgrounds, who pursue independent careers in academia, national laboratories, and the microelectronics industry. The courses he teaches at the University of Michigan on the physics and thermodynamics of materials focus on incorporating active-learning techniques into the large-classroom setting. Among numerous positive educational outcomes, these methods have also been demonstrated to increase the retention of female students in Engineering. Prof. Kioupakis has received the U.S. National Science Foundation CAREER Award, the Jon R. and Beverly S. Holt Award for Excellence in Teaching, and the Engineering Class of 1938 Award, the highest honor to early-career faculty in Engineering at the University of Michigan.
During the academic year 2024-25, Prof. Kioupakis is on sabbatical leave from the University of Michigan and is hosted as a Visiting Professor by IMX and IEM in STI at EPFL. Prof. Kioupakis will be hosted by Prof. Nicola Marzari at the Laboratory of theory and simulation of materials (THEOS) and the NCCR MARVEL, and by Prof. Elison Matioli at the power and wide-band-gab electronics research laboratory (POWERLAB). During his stay at EPFL, Prof. Kioupakis aims to engage on a broad range of collaborative research activities on the science and engineering of semiconducting materials and devices, and he is looking forward to fruitful discussions and networking with the EPFL community.