Influence of AlN spacer and GaN cap layer in GaN heterostructure for RF HEMT applications

Abstract

The influence of GaN cap layer and AlN spacer layer on mobility, carrier concentration, bandgap, crystal structure, inherent strain and stress is investigated for three widely used GaN heterostructure viz. Al0.3Ga0.7N/GaN, Al0.3Ga0.7N/AlN/GaN and GaN/Al0.3Ga0.7N/AlN/GaN on 6H-SiC substrate. The growth process is kept identical with less than 1% standard deviation for all three samples in order to investigate the true effect of GaN cap and AlN spacer layers on heterostructure properties. The GaN heterostructure properties have been extensively studied by Hall measurement, high-resolution x-ray diffraction, Raman and photoluminescence spectroscopy. The interdependency among structural, mechanical, electrical and optical properties of GaN heterostructure due to incorporation of AlN space and GaN cap layer is established. Furthermore, presented characterization results for all three heterostructures are identical with semiconductor device physics-based simulation. The carrier mobility and concentration were found least in conventional AlGaN/GaN heterostructure. It has been observed a pronounced increase of inherent tensile stress and carrier concentration by introducing AlN spacer layer between AlGaN and GaN, whereas carrier mobility increased by employing GaN cap layer on the top of the heterostructure.