Ab initio investigations of two-dimensional carrier gas at interfaces in GaN/AlN and GaN/AlN/Al₂O₃ heterostructures

The formation of a two-dimensional electron gas (2DEG) at the GaN (0001)/AlN interface holds significant implications for GaN-based high-voltage and high-frequency (RF) devices. Due to the promising results provided by the addition of a thin layer of AlN in metal-oxide-semiconductor channel high-electron-mobility transistor devices, this interface can be found in both the access region and near the dielectric gate. Recent ab initio simulations shed light on the crucial role played by spontaneous and piezoelectric polarizations within polar GaN and AlN crystals in driving the formation of the 2DEG. This study explores the underlying mechanisms behind the 2DEG formation and investigates the impact of fixed charges and additional layers, like Al2O3, on the carrier concentration. Consistent with the literature, our findings highlight the predominant role of polarizations within III-V materials in the formation of the 2DEG. Moreover, we examine the influence of fixed charges on the AlN surface, revealing their ability to accumulate or deplete the 2DEG, while maintaining charge conservation through the emergence of a new two-dimensional charge gas on the AlN surface. Additionally, we explore the effects of incorporating a beta-Al2O3 crystal layer on the GaN/AlN structure, finding that the 2DEG's carrier density is reduced, yet not entirely eliminated, while a significant positive charge concentration at the AlN/Al2O3 interface pins the Fermi level. This comprehensive investigation contributes to our understanding of microscopic phenomena in III-V heterostructures, paving the way for future advancements and applications in power electronics.