Discovering the extreme limits to semiconductor band gaps

The magnitude of the band gap has been traditionally applied as a criterion to distinguish semiconductors from insulators; materials with gaps less than 3 eV are typically semiconductors, while wider-gap materials tend to be insulators. However, the development of ultra-wide-band-gap (UWBG) semiconductors such as AlGaN, diamond, BN, and β-Ga₂O₃ has challenged this gap-based criterion for materials classification and gave rise to questions such as how far the band gap of semiconductors can increase while maintaining delocalized carriers for conductivity and what is the widest band gap semiconductor. By applying high-throughput density functional theory calculation, we develop a materials discovery strategy to identify new extreme-gap semiconductors. We discover that materials composed of light elements, crystallized in simple, densely packed structures, and having s-orbital characteristics of conduction/valence bands have large band gap (> 7 eV) but light carrier effective mass (m_e^* < 0.7 m_e m_h^* < 2 m_e) that enable shallow dopants and high mobility and suppress the formation of polarons. Among the UWBG compounds, we identify rutile GeO₂ is an UWBG (4.68 eV) semiconductor with material properties surpassing the state-of-the-art materials and demonstrate the first film growth of single crystalline r-GeO₂. Our work motivates further exploration of r-GeO₂ as well as other UWBG semiconductors with even wider gaps as alternative UWBG semiconductors that can advance the power electronics and optoelectronics technologies.