AlGaN/GaN high-electron-mobility transistors (HEMTs) are widely used in high-frequency and high-power applications owing to the high two-dimensional electron gas (2DEG) concentration. However, the microscopic origin of the 2DEG remains unclear. This hinders the development of device fabrication technologies, such as threshold voltage modulation, current collapse suppression, and 2DEG concentration enhancement technologies, as well as AlGaN/GaN sensors with very high sensitivity to polar liquids. To clarify the 2DEG microscopic origin, we studied the effects of gas molecules on AlGaN/GaN surfaces through various experiments and first-principles calculations. The results indicated that the adsorption of gas molecules on the AlGaN/GaN surface is an important phenomenon, clarifying the microscopic origin of the 2DEG. This study elucidates the properties of AlGaN/GaN heterojunctions and promotes the development of new fabrication technologies for AlGaN/GaN devices.

In polar crystals, strong long-range electric fields can distribute defect emission over multiple phonons, obscuring which atomic motions carry the lattice relaxation. AlN's strongly polar lattice and prevalent oxygen-related deep defect centers make it a demanding system for probing how local defect symmetry and electronic transition character select these phonons. Here, we identify axial ${({V}_{\mathrm{Al}}\text{\ensuremath{-}}{\mathrm{O}}_{\mathrm{N}})}_{\mathrm{a}}$ as the dominant oxygen-related deep center in AlN by resolving its 815 and $855\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ local vibrational modes (LVMs), and show that its two recombination pathways select different multiphonon relaxation coordinates. The free-to-bound charge-state transition has a large Huang-Rhys factor ($S=9.3$), with an effective phonon energy in the $815\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ LVM region, whereas the intracenter transition ($S=4.9$) lies higher at $877\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, indicating increased contribution from the AlN LO-phonon continuum. More broadly, this AlN case suggests a framework for deep centers in polar wide-band-gap semiconductors, pointing to how defect symmetry and transition character jointly partition lattice relaxation between localized and bulk phonon modes.