Enhanced thermal stability of the two-dimensional electron gas in GaN/AlGaN/GaN heterostructures by Si3N4 surface-passivation-induced strain solidification

The plasma-enhanced chemical-vapor-deposited Si3N4 surface passivation layers with high rf (13.56 MHz) and low rf (380 KHz), were investigated, using the annealing experiment at 500degreesC in N-2 ambient, for their effectiveness in regard to the stability of the two-dimensional electron gas (2DEG) in GaN/AlGaN/GaN high-electron-mobility transistor structures. When the unpassivated sample was annealed, as observation using the Hall and high-resolution x-ray diffraction measurements, an irreversible degradation of the 2DEG behavior and a strain relaxation of the AlGaN epilayer occurred. The Si3N4 deposited with high frequency provided superior long-term stability for the sample and showed no change in the 2DEG conductivity and the strain state in AlGaN epilayer after 170 h of aging. However, a significant degradation after deposition and a partial recovery of the 2DEG behavior after annealing due to the Si3N4 densification, was found in sample passivated by low-frequency Si3N4. The enhancement of the long-term stability can be explained as a consequence of the strain solidification of the AlGaN epilayer induced by a denser Si3N4 passivation. (C) 2004 American Institute of Physics.