Effects of ammonia and oxygen plasma treatment on interface traps in AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors

A HfO₂/Al₂O₃/AlN stack deposited in a plasma-enhanced atomic layer deposition system is used as the gate dielectric for GaN metal–insulator–semiconductor structures. Prior to the deposition, in situ remote plasma pretreatments (RPPs) with O₂, NH₃, and NH₃–O₂ (in sequence) were carried out. Frequency-dependent capacitance–voltage (C–V) measurements reveal that for all the RPP-treated samples, the interface trap density D_it was significantly reduced compared to the sample without RPP. In particular, the NH₃-treated sample exhibits an extremely low D_it of ∼10¹¹ cm⁻²·eV⁻¹ from E_C − 0.31 eV to E_C − 0.46 eV. To characterize the interface traps at deeper levels, the transfer pulsed I–V measurement was employed further. In the energy range of E_C − E_T > 0.54 eV, the interface trap charge density Q_it of various samples demonstrates the following order: NH₃ RPP < NH₃–O₂ RPP < O₂ RPP < without RPP. By adjusting multiple pulse widths, the D_it derived from the Q_it result matches the C–V measurement precisely. Moreover, X-ray photoelectron spectroscopy analysis of the Ga 3d core-level indicates that NH₃ RPP facilitates the conversion of Ga₂O into GaN at high RF power of 2500 W, whereas O₂ RPP mainly oxidizes GaN to relatively stable Ga₂O₃. Combined with the C–V and pulsed I–V measurements, it is confirmed that a strong positive correlation exists between Ga₂O and the interface traps, rather than Ga₂O₃.