Band Alignment of Sc_xAl_1–xN/GaN Heterojunctions

ScAlN is an emergent ultrawide-band-gap material with both a high piezoresponse and demonstrated ferroelectric polarization switching. Recent demonstration of epitaxial growth of ScAlN on GaN has unlocked prospects for new high-power transistors and nonvolatile memory technologies fabricated from these materials. An understanding of the band alignments between ScAlN and GaN is crucial in order to control the electronic and optical properties of engineered devices. To date, there have been no experimental studies of the band offsets between ScAlN and GaN. This work presents optical characterization of the band gap of molecular beam epitaxy grown ScxAl1–xN using spectroscopic ellipsometry and measurements of the band offsets of ScxAl1–xN with GaN using X-ray photoemission spectroscopy, along with a comparison to first-principles calculations. The band gap is shown to continuously decrease as a function of increasing ScN alloy fraction with a negative bowing parameter. Furthermore, a crossover from straddling (type-I) to staggered (type-II) band offsets is demonstrated as Sc composition increases beyond approximately x = 0.11. These results show that the ScAlN/GaN valence band alignment can be tuned by changing the Sc alloy fraction, which can help guide the design of heterostructures in future ScAlN/GaN-based devices.