Quaternary alloy ScAlGaN: A promising strategy to improve the quality of ScAlN

ScAlN is an emerging ultrawide bandgap semiconductor for next-generation radio frequency electronic devices. Here, we show that the material quality of ScAlN grown by molecular beam epitaxy can be drastically improved by alloying with Ga. The resulting quaternary alloy ScAlGaN exhibits a single-phase wurtzite structure, atomically smooth surface, high crystal quality, sharp interface, and low impurity concentration. Most significantly, oxygen impurity incorporation in ScAlGaN is found to be three to four orders of magnitude lower compared to that for ScAlN grown on AlN templates utilizing a similar Sc source. We further demonstrate that ScAlGaN/GaN superlattices exhibit clear periodicity with sharp interfaces. Moreover, GaN high electron mobility transistors with high sheet electron density and high mobility have been realized using ScAlGaN as a barrier. This work provides a viable approach for achieving high-quality Sc-III-N semiconductors that were not previously possible and further offers additional dimensions for bandgap, polarization, interface, strain, and quantum engineering.