Intersubband optical absorption and electron relaxation rates in GaN/AlGaN coupled double quantum wells

Recent interest in intersubband (IS) transit ions in semiconductor heterostructures With large band offset has been fueled by attempts to extend the wavelength range of IS-based optical devices to the fiber-optics wavelength range around similar to 1.55 mum. GaN/AlGaN-based heterostructures are of particular interest due to their large effective electron mass and large longitudinal optical phonon energy, Both are essential to achieve ultrafast electron relaxation at large transition energies. IS absorption in GaN/AlGaN single and coupled double quantum wells (DQWs) has been measured. The samples were grown by molecular beam epitaxy Oil sapphire substrate and with a large (0.65 or 0.9) AlN-mole fraction in the barriers. Peak absorption wavelengths as short as 1.35 mum and 1.52 mum were measured for a symmetric DQW of 12 Angstrom wide wells coupled by a 10 Angstrom wide barrier, which also showed evidence of excited-state anti-crossing. As expected. asymmetric DQWs displayed no such anti-crossing, and the ground-state anti-crossing energies were found to be much smaller - as a result of the comparatively large effective electron mass - than the energy broadening of individual transitions. The asymmetric DQWs displayed peak absorption wavelengths between 1.5 and 2.9 mum. The electron relaxation time, attributed to longitudinal optical phonon scattering has been measured by pump-probe technique as 240 fs for a coupled DQW sample.