Two-dimensional electron gases in strained quantum wells for AlN/GaN/AlN double heterostructure field-effect transistors on AlN

Double heterostructures of strained GaN quantum wells (QWs) sandwiched between relaxed AlN layers provide a platform to investigate the quantum-confined electronic and optical properties of the wells. The growth of AlN/GaN/AlN heterostructures with varying GaN quantum well thicknesses on AlN by plasma molecular beam epitaxy (MBE) is reported. Photoluminescence spectra provide the optical signature of the thin GaN QWs. Reciprocal space mapping in X-ray diffraction shows that a GaN layer as thick as ∼28 nm is compressively strained to the AlN layer underneath. The density of the polarization-induced two-dimensional electron gas (2DEG) in the undoped heterostructures increases with the GaN QW thickness, reaching ∼2.5 × 10¹³/cm². This provides a way to tune the 2DEG channel density without changing the thickness of the top barrier layer. Electron mobilities less than ∼400 cm²/Vs are observed, leaving ample room for improvement. Nevertheless, owing to the high 2DEG density, strained GaN QW field-effect transistors with MBE regrown ohmic contacts exhibit an on-current density ∼1.4 A/mm, a transconductance ∼280 mS/mm, and a cut off frequency $fT∼104 GHz$ for a 100-nm-gate-length device. These observations indicate high potential for high-speed radio frequency and high voltage applications that stand to benefit from the extreme-bandgap and high thermal conductivity of AlN.