High-quality superlattice structures of GaN/AlGaN were grown on (0001) sapphire substrates by molecular beam epitaxy. The threading dislocation density was reduced by growing low-temperature AIN layers in between the high-temperature GaN. In addition, in situ monitoring of the growth rate was achieved using pyrometric interferometry. Cross-sectional transmission electron microscopy of the superlattice structures revealed abrupt interfaces between GaN/AlGaN and excellent layer uniformity. We observed intersubband absorption at wavelengths as short as 1.52 muM in the GaN/AlGaN material system. A range of intersubband absorption peaks was observed between 1.52 and 4.2 muM by varying the well thickness and barrier Al content. In addition, the distribution of the built-in electric field between the well and barrier layers was also found to affect the intersubband transition wavelength.