BAND-GAP OF GAN FILMS GROWN BY MOLECULAR-BEAM EPITAXY ON GAAS AND GAPSUBSTRATES

There has been controversy concerning the energy gap of the cubic (zin c-blende) form of the wide-gap semiconductor GaN. Measurements are rep orted of-the band gaps of both hexagonal (wurtzite) and cubic thin fil ms deposited by a modified molecular-beam-epitaxy process on (001) GaA s and GaP substrates. The important difference from conventional MBE l ies 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 add ition of an As beam which results in growth of the cubic form-otherwis e films grow with the hexagonal structure. The band gaps were measured at room temperature by optical reflectivity, as evidenced by the shar p reduction in interference oscillations as the photon energy approach ed the band edge, and confirmed by the observation of band-edge photol uminescence. The results can be summarized as E(g)=3.42+/-0.02 eV for the hexagonal and 3.22+/-0.02 eV for the cubic form. The observation o f films containing mixed hexagonal and cubic phases, which may have le d to earlier errors in band-gap measurements, is also reported. (C) 19 95 American Institute of Physics.