Ultrathin AlN/GaN Heterojunctions by MBE for THz Applications

Abstract

ABSTRACTHigh electron mobility transistor (NEMT) based on AlN/GaN can be the source to generate terahertz (THz) radiation. The basic requirements are that a) the electron density in the 2DEG should be large enough for ensuring that the electron-electron scattering mean-free path is much shorter than the distance between the contacts to ensure a viscous, fluid-like flow, and b) the mobility should be high enough such that the possible plasma modes are not strongly damped. To show AlN/GaN HEMT is a qualified candidate for THz generation, we have calculated the plasma frequency for the two-dimensional electron gas (2DEG) in the GaN channel and compared it with other material, such as Si, GaAs and InAs. 3nm AlN has been grown on the GaN substrate by Nitrogen-source radio-frequency molecular-beam expitaxy (RFMBE). The growth temperature has been optimized and found to be 730C. Results from Hall measurement show the electron charge is ∼2.5*1013 cm−2 with the mobility of 530 cm2/Vs (room temperature) and 860 cm2/Vs (77K). A series AlN samples have been grown and studied by AFM, which shows the critical thickness of AlN on GaN is between 4nm and 5nm.