Experimental observation of sequential tunneling transport in GaN/AlGaN coupled quantum wells grown on a free-standing GaN substrate

A GaN/AlGaN multiple-quantum-well structure based on an asymmetric triple-quantum-well repeat unit was grown by molecular beam epitaxy, and its vertical electrical transport characteristics were investigated as a function of temperature. To minimize the density of dislocations and other structural defects providing leakage current paths, homoepitaxial growth on a free-standing GaN substrate was employed. The measured vertical-transport current-voltage characteristics were found to be highly nonlinear, especially at low temperatures, consistent with sequential tunneling through the ground-state subbands of weakly coupled adjacent quantum wells. Furthermore, different turn-on voltages were measured depending on the polarity of the applied bias, in accordance with the asymmetric subband structure of the sample repeat units.