Enhancement of 2DEG Effective Mass in AlN/Al0.78Ga0.22N High Electron Mobility Transistor Structure Determined by THz Optical Hall Effect

We report on the free charge carrier properties of a two-dimensional electron gas (2DEG) in an AlN/AlxGa1–xN high electron mobility transistor structure with a high aluminum content (x = 0.78). The 2DEG sheet density Ns=(7.3±0.7)×1012 cm−2, sheet mobility μs=(270±40) cm2/(Vs), sheet resistance Rs=(3200±500) Ω/◻, and effective mass meff=(0.63±0.04)m0 at low temperatures (T=5 K) are determined by terahertz (THz) optical Hall effect measurements. The experimental 2DEG mobility in the channel is found within the expected range, and the sheet carrier density is in good agreement with self-consistent Poisson–Schrödinger calculations. However, a significant increase in the effective mass of 2DEG electrons at low temperatures is found in comparison with the respective value in bulk Al0.78Ga22N (meff=0.334 m0). Possible mechanisms for the enhanced 2DEG effective mass parameter are discussed and quantified using self-consistent Poisson–Schrödinger calculations.