Band gap of GaN films grown by molecular-beam epitaxy on GaAs and GaP substrates

There has been controversy concerning the energy gap of the cubic (zinc-blende) form of the wide-gap semiconductor GaN. Measurements are reported of the band gaps of both hexagonal (wurtzite) and cubic thin films deposited by a modified molecular-beam-epitaxy process on (001) GaAs and GaP substrates. The important difference from conventional MBE lies in the method of supplying nitrogen to the growing film. Here a rf nitrogen plasma source operating at 13 MHz is used. The structure of the films was monitored by x-ray diffraction and controlled by the addition of an As beam which results in growth of the cubic form—otherwise films grow with the hexagonal structure. The band gaps were measured at room temperature by optical reflectivity, as evidenced by the sharp reduction in interference oscillations as the photon energy approached the band edge, and confirmed by the observation of band-edge photoluminescence. The results can be summarized as Eg=3.42±0.02 eV for the hexagonal and 3.22±0.02 eV for the cubic form. The observation of films containing mixed hexagonal and cubic phases, which may have led to earlier errors in band-gap measurements, is also reported.