The absorption properties in hetero-polarization GaN/AlxGa1-xN (x = 0.06) q
uantum well structures are studied in reflection, photoreflection, and phot
oluminescence excitation spectroscopy and compared with the results of band
structure calculations. Above the energy of the main luminescence transiti
ons we observe three distinct absorption thresholds. From Franz-Keldysh osc
illations in the absorption spectra we directly derive the value of the act
ing electric field within the barriers. Upon this field strength we base a
calculation of the electronic band structure and interband transition energ
ies. The results suggest that the observed absorption edges are the AlGaN b
and edge and two quantized levels involving the crystal-field spilt-off hol
e.