Time-dependent degradation of AlGaN/GaN heterostructures grown on silicon carbide

The AlGaN/GaN heterostructure field-effect transistors (HFETs) were grown on 4H-SiC substrates by metal-organic chemical-vapor deposition (MOCVD) with a range of Al compositions (30-35%) and AlGaN barrier thicknesses. Films with higher strains exhibited a time-dependent degradation of the two-dimensional electron gas (2DEG) that varied from days to weeks. Atomic force microscopy (AFM) measurements of the degraded films revealed a hexagonal cracking pattern with an increase in the medium-scale surface roughness. The localized strain relaxation of AlGaN barriers and increased roughness of the AlGaN/GaN interface and AlGaN surface result in a broad shoulder at the lower angle of the AlGaN peak and a loss of satellite fringes in the (0006) reflection x-ray diffraction (XRD) curve. This degradation raises serious questions with regard to reliability and survivability of AlGaN HFETs and may complicate device fabrication.