Short-range order controlled amphoteric behavior of the Si dopant in Al-rich AlGaN

AlGaN alloys with high Al content offer the possibility to create deep ultraviolet (UV) light sources emitting at wavelengths ≤ 240 nm with enhanced quantum efficiency. However, pushing the limits of the band gap energies towards higher values leads to problems with n-type doping of AlGaN alloys with the standard choice of Si donors when the Al content surpasses ∼80 %. This is due to the formation of the so-called negative Si DX center where the Si-N bond is ruptured, followed by a displacement of the N atom. In this paper, we show that the amphoteric nature of the Si dopant in AlGaN alloys is fundamentally controlled by the local environment and the ordering of the Ga and Al atoms in the vicinity of the Si atom. Our conclusions are based on advanced characterization sensitive to the local environment of defects and impurities: positron annihilation spectroscopy and X-ray absorption spectroscopy. Electronic structure calculations complete the picture. As our experiments show that the presence of Ga in the vicinity of Si controls the donor character of the latter in high Al content AlGaN, we propose that spatial ordering of these atoms could allow efficient n-type doping at even higher Al contents, including AlN.