Effect of AlN and AlGaN Interlayers on AlScN/GaN Heterostructures Grown by Metal–Organic Chemical Vapor Deposition

AlScN/GaN heterostructures with their high sheet carrier density (n_s) in the two-dimensional electron gas (2DEG) have a high potential for high-frequency and high-power electronics. The abruptness of the heterointerface plays a key role in the 2DEG confinement, and the presence of interlayers (AlN, AlGaN) affects n_s and electron mobility (μ) and determines the sheet resistance (R_sh). AlScN/GaN heterostructures suitable for high-electron mobility transistors (HEMT) with and without a nominal AlN interlayer were grown by metal–organic chemical vapor deposition (MOCVD) and characterized electrically and structurally to gain a systematic insight into the unintentional formation and control of graded AlGaN interlayers by diffusion of atoms at the heterointerface. The AlN interlayer increases n_s from 2.52 × 10¹³ cm^–2 to 3.25 × 10¹³ cm^–2 and, as calculated by one-dimensional Schrödinger–Poisson simulations, improves the 2DEG confinement. The barrier growth temperature was varied from 900 °C to 1200 °C to investigate the effect of the thermal budget on diffusion. Growth at 900 °C reduces the thickness of the graded AlGaN interlayer and improves the 2DEG confinement, leading to R_sh of 211 Ω/sq, n_s of 2.98 × 10¹³ cm^–2, and μ of 998 cm²/(Vs).