Piezoelectric quantization in GaInN thin films and multiple quantum well structures

Details of the electronic bandstructure in pseudomorphic Ga1-xInxN/GaN single heterostructures (0 < x < 0.22) are studied. In photocarrier modulated reflectance strong modulation of the density of states (Franz-Keldysh oscillations) is found due to a piezoelectric field of about 0.6 MV/cm in the strained layer. No excitons are expected to form in the presence of this field. Studying the composition dependence we determine a piezoelectric coefficient partial derivative\P\/partial derivative epsilon(LZ) = 0.46 C/m(2) and extrapolate a spontaneous polarization in GaN \P-eq\ = 3.9 mC/m(2). Photoreflection indicates the presence of localized tail states 50 - 100 meV below the bandgap which are well explained by the Franz-Keldysh effect involving k non-conserving transitions in the large electric field. Luminescence is found to originate in these electric field induced states. The derived bandgap energies can be approximated by an interpolation yielding bowing parameters b = 2.6 eV (photoreflection) and b = 3.2 eV (luminescence) for pseudomorphic films with 0.07 less than or equal to x less than or equal to 0.22. These findings may affect interpretation of device performance.