Growth and characterization of AlGaN/AlN/GaN/AlGaN double heterojunction structures with AlGaN as buffer layers

High electron mobility transistors (HEMTs) structures with GaN, Al0.025Ga0.975N and Al0.04Ga0.96N high resistivity (HR) buffers were grown on sapphire by metal organic chemical vapor deposition (MOCVD). The structural and electrical properties of these three samples were investigated and compared. By increasing Al composition of AlGaN buffer, full width at half maximum (FWHM) values of (0002) and (10-12) x-ray rocking curves for buffer increase, indicating higher threading dislocation density. Room temperature noncontact Hall measurements were performed, and the measured 2DEG mobility was 1828 cm2/V s for GaN buffer, 1728 cm2/V s for Al0.025Ga0.975N buffer, and 1649 cm2/V s for Al0.04Ga0.96N buffer, respectively. Combining the theoretical calculation with the experiments, it was demonstrated that the decrease of mobility was attributed to higher dislocation density in sample with higher Al composition of AlGaN buffer. Devices were fabricated and it was found that the double heterojunction (DH) HEMT with Al0.025Ga0.975N buffer could effectively reduce the buffer leakage current.