Ultrafast intersubband transitions in GaN/AlGaN heterostructures at ∼1.55-μm wavelength

We present a study of the electron relaxation times in different GaN/AlGaN multiple quantum well heterostructure samples. In particular, a comparative study of room temperature intersubband electron scattering lifetimes in a variety of single and coupled quantum well samples is presented for different excitation powers and wavelengths. All samples were grown by molecular beam epitaxy on sapphire substrate. Typical quantum well widths of ∼10 to 15 Å were used together with AlN bulk and superlattice barriers with an AlN-mole fraction of ∼0.65 and ∼0.9, respectively. In absorption measurements, the single quantum well samples show intersubband absorption peaks ranging from ∼1.4 to ∼1.7 μm wavelength. Two absorption peaks at ∼1.75 and ∼2.45 μm were measured for an asymmetric coupled double quantum well structure. The intersubband electron scattering lifetimes were determined with time resolved pump-probe measurements using pump and probe beams with a pulse width of ∼130 fs and wavelengths of ∼1.55 and ∼1.7 μm, respectively. For the single quantum well samples, intersubband scattering lifetimes of ∼260 to ∼300 fs have been measured. For the upper excited state of the coupled double quantum well, a lifetime of ∼230 fs has been found.