Iron doped GaN layers were grown on (110) Si substrates by ammonia molecular beam epitaxy (MBE) using solid elemental iron as a source. Specular films with concentrations up to 1x10(2) cm(-3), as determined by secondary ion mass spectroscopy, were grown, unlike a limited incorporation of Fe into GaN by metal-rich rf plasma MBE. The Fe concentration in the film showed an exponential dependence on the inverse of source temperature with an activation energy of 3.4 eV, which agrees well to the reported value for the sublimation of Fe. A 1.5 gm thick GaN film with a sheet resistance of 1 G Omega/sq. was obtained by compensating unintentional residual donors with a small Fe concentration of 1x10(17) cm(-3). X-ray diffraction rocking curves indicated high crystalline quality, very similar to an undoped film, showing that the Fe incorporation required to obtain the semi-insulating film properties did not affect the structural properties of the film. The low-temperature PL spectra of highly resistive and semi-insulating Fe:GaN in the range of 10(17)-10(18) cm(-3) show dominant exciton emissions and enhanced donor-acceptor-pair (DAP) emissions, implying that Fe ions contribute to the DAP transition between donor levels and Fe-related acceptor levels, possibly compensating the residual donors to achieve the semi-insulating electrical properties.